Enantioselective Silver-Catalyzed Transformations - Chemical

Review pubs.acs.org/CR

Enantioselective Silver-Catalyzed Transformations Hélène Pellissier* Aix Marseille Univ., CNRS, Centrale Marseille, iSm2, Marseille, France ABSTRACT: This review collects the major progress in the field of enantioselective transformations promoted by chiral silver catalysts, covering the literature since 2008, well illustrating the power of these especially mild Lewis acid catalysts to provide novel asymmetric reactions.

CONTENTS 1. Introduction 2. Silver-Catalyzed Aldol-Type Reactions 2.1. Aldol Reactions 2.2. Nitroso-Aldol Reactions 3. Silver-Catalyzed Mannich Reactions 3.1. Vinylogous Mukaiyama−Mannich Reactions 3.1.1. With Hoveyda−Snapper Catalysts 3.1.2. With Other Catalysts 3.2. Other Mannich-Type Reactions 4. Silver-Catalyzed Michael Reactions 4.1. α,β-Unsaturated Carbonyl Compounds as Acceptors 4.2. Nitroalkenes as Acceptors 4.3. Other Acceptors 5. Silver-Catalyzed 1,3-Dipolar Cycloadditions 5.1. Formal 1,3-Dipolar Cycloadditions of Glycine Imino Esters and α,β-Unsaturated Carbonyl Compounds 5.2. Formal 1,3-Dipolar Cycloadditions of Glycine Imino Esters and Nitroalkenes 5.3. Formal 1,3-Dipolar Cycloadditions of Isocyanoacetates and α,β-Unsaturated Carbonyl Compounds 5.4. Other 1,3-Dipolar Cycloadditions 6. Silver-Catalyzed Alkynylations 7. Silver-Catalyzed Allylations 8. Silver-Catalyzed Cyclizations of Allenes 9. Silver-Catalyzed Aminations 10. Silver-Catalyzed Domino and Tandem Reactions 10.1. Domino and Tandem Reactions Initiated by a Michael Addition 10.2. Domino Reactions Initiated by an Aldol Reaction 10.3. Domino Reactions Initiated by a Cyclization 10.4. Domino Reactions Initiated by a Mannich Reaction © XXXX American Chemical Society

10.5. Miscellaneous Domino Reactions 11. Silver-Catalyzed Miscellaneous Reactions 12. Conclusion Author Information Corresponding Author ORCID Notes Biography Dedication Abbreviations References

A B B D F F F H K O O P Q Q

AL AM AO AP AP AP AP AP AP AP AQ

1. INTRODUCTION The importance of chirality in pharmaceuticals has made asymmetric catalysis the most important area of synthetic organic chemistry.1,2 In particular, the use of transition metals has become in the past few decades a powerful tool to perform reactions in a highly enantioselective fashion.3−17 Silver(I) is known to interact with π-donors, such as alkenes, alkynes, allenes, and aromatics, but also with n-donors, such as (thio)ethers, amines, and phosphines, making strong and stable complexes more easily than other metals related to the fact that Ag(+) is among the most soft acids.18 Moreover, the use of silver(I) is economical relative to other expensive transition metals such as gold and platinum. However, among coinage metals including copper, silver, and gold, silver has been the most neglected in the area of organic chemistry for a long time, probably because of its moderate Lewis acidity. Indeed, it is only in the 1990s that silver-catalyzed asymmetric reactions have emerged as important synthetic methods with the early reports of Ito, Hayashi, Grigg, and Yamamoto.19−24 These have led to the development of an increasing number of novel enantioselective silver-catalyzed transformations of many types.

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V X X AA AB AD AE AE AG AI

Received: September 20, 2016

AK A

DOI: 10.1021/acs.chemrev.6b00639 Chem. Rev. XXXX, XXX, XXX−XXX

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organostannane compounds. In order to improve this point, these authors later developed an alternative asymmetric aldol reaction occurring between benzaldehyde and alkenyl trichloroacetate derived from cyclohexanone based on the same catalyst system using only 5 mol % of Bu3SnOMe, which resulted in the formation of the corresponding aldol product in 82% yield, 84% de, and 95% ee.43,44 In 2009, the same authors applied this catalyst system to the asymmetric aldol reactions of alkenyl trichloroacetates 1a−1c with α-ketoesters 2a−2d.45 The process was promoted by a combination of 20 mol % of AgOTf, 10 mol % of (R)-BINAP, and 8 mol % of Bu2Sn(OMe)2 in the presence of methanol as superstoichiometric additive in THF at −20 °C. As shown in Scheme 1, the

Especially in the past nine years, chiral silver complexes have demonstrated their high efficiency as special mild Lewis acids, becoming catalysts of first choice for many types of asymmetric reactions generally performed under mild reaction conditions and through experimentally simple procedures. For example, the first enantioselective silver-catalyzed domino reactions including multicatalyzed ones have been only recently described to finally novel asymmetric silver-catalyzed Mannich reactions, cycloadditions, aldol reactions, Michael additions, alkynylations, allylations, etc. The goal of this review is to provide a comprehensive overview of the major developments in enantioselective silver-catalyzed transformations published since 2008, since this field was most recently reviewed by Yamamoto in that year.25 It must be noted that several specific reviews have also been dedicated to special silver-catalyzed reactions not especially asymmetric including only a few references dealing with chirality.26−36 Moreover, a book was published by Harmata in 2010, dealing with the general use of silver in organic chemistry, which covered the literature up to 2008.37 The review has been divided into 10 principal sections, dealing successively with silver-catalyzed asymmetric aldol-type reactions (section 2), silver-catalyzed asymmetric Mannich reactions (section 3), silver-catalyzed asymmetric Michael reactions (section 4), asymmetric 1,3-dipolar cycloadditions (section 5), asymmetric alkynylations (section 6), asymmetric allylations (section 7), asymmetric cyclizations of allenes (section 8), asymmetric aminations (section 9), asymmetric domino and tandem reactions (section 10), and miscellaneous asymmetric reactions (section 11).

Scheme 1. Aldol Reaction of α-Ketoesters and Alkenyl Trichloroacetates

2. SILVER-CATALYZED ALDOL-TYPE REACTIONS 2.1. Aldol Reactions

The catalytic asymmetric aldol reaction is a powerful method for synthesizing chiral β-hydroxy carbonyl compounds. A wide range of chiral catalyst systems have been successfully developed to promote this type of reaction including metals as well as organocatalysts.37−40 The first example of chiral silver-catalyzed asymmetric aldol-type reactions was reported in 1990 by Ito et al., who employed chiral ferrocenylphosphine− silver(I) complexes as efficient chiral catalysts in asymmetric aldol-type reactions of aldehydes with tosylmethyl isocyanide.19 Actually, the aldol products were not isolated since the aldol reaction was followed by cyclization to directly afford the corresponding chiral 5-alkyl-4-tosyl-2-oxazolines through a domino reaction. The catalyst system was used at only 1 mol % of catalyst loading, providing the cyclic domino products in enantioselectivities of up to 86% ee. In 1991, the same authors applied a related catalyst to the first silver-catalyzed asymmetric aldol-type reaction of tributyltin enolates with aldehydes, which allowed the corresponding simple aldol products to be achieved with enantioselectivities of up to 90% ee in combination with high yields (86−96%).20 In these reactions, the tributyltin enolates were generated from the corresponding enol acetates and Bu3SnOMe. Generally, the processes evolved through antidiastereoselectivity in contrast to those promoted by Lewis acids other than silver complexes.41,42 Later in 1997, Yamamoto et al. promoted these reactions by a combination of AgOTf (15 mol %) with BINAP (6 mol %) which led to the corresponding chiral β-hydroxy carbonyl compounds in enantioselectivities of up to 95% ee and moderate to good yields (33−83%).23 Even if these reactions provided good results, they presented the disadvantage of using a stoichiometric amount of toxic

reaction began by the reaction of alkenyl trichloroacetate 1 with Bu2Sn(OMe)2 to give tin enolate A along with methyl trichloroacetate. Then, enolate A added to the α-ketoester 2 enantioselectively in the presence of the chiral silver catalyst, affording the tin alkoxide of aldol product B, which was subsequently protonated with methanol, resulting in the formation of the final product 3 with regeneration of Bu2Sn(OMe)2. Under these simple reaction conditions, various chiral β-hydroxyketones 3a−3f bearing a tertiary carbon center were obtained in moderate to quantitative yields (38−99%), low to complete diastereoselectivities (4 → 98% de), and moderate to high enantioselectivities of 60−93% ee (Scheme 1). It was found that cyclic as well as acyclic alkenyl trichloroacetates were tolerated although the latter needed B


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reaction times longer than those required by cyclic substrates to produce satisfactory yields (24 h vs 4−5 h). Concerning the scope of α-ketoesters, the best results were reached with aryl αketoesters. The substituent at the para-position of methyl benzoylformate (R3 = aryl) was found to influence both the diastereo- and enantioselectivities of the reaction. For example, for p-methoxy derivative 2c (R3 = 4-MeOC 6H4), the enantioselectivity of the process reached 90% ee in combination with 66% de while p-bromo derivative 2b (R3 = 4-BrC6H4) provided only 61% ee and 6% de. This study represented the first example of an asymmetric aldol reaction of ketones using alkenyl esters as masked enolates. With the need to extend the scope of this methodology, the authors investigated the reactivity of classical ketones, such as acetophenone and cyclohexanone, under similar reaction conditions, albeit without success. In 2014, the same authors demonstrated that it was possible to completely avoid the use of toxic organotin compounds in the asymmetric aldol reactions of alkenyl trihaloacetates 4a−4c with aldehydes 5a−5g by employing N,N-diisopropylethylamine as a base in the presence of methanol.46 Indeed, in this case, the chiral silver catalyst, composed of 8 mol % of (S)BINAP combined with 16 mol % of AgOTf, reacted with methanol in the presence of this base to afford the corresponding (S)-BINAP-AgOMe, which actually constituted the true catalyst of the aldol reaction (Scheme 2). Next, the thus-generated chiral silver methoxide attacked the alkenyl trihaloacetate 4 to yield chiral silver enolate C. The following addition of this enolate to aldehyde 5 provided chiral silver alkoxide of aldol product D. Finally, the protonation of D with methanol resulted in the formation of chiral β-hydroxy ketone 6 with regeneration of the chiral silver methoxide. Under these novel environmentally benign conditions, the aldol condensation of a range of alkenyl trihaloacetates 4a−4d to various aldehydes 5a−5g led to the corresponding chiral α-alkyl-βhydroxyketones 6a−6m with up to 99% yield, 98% de, and 95% ee, as shown in Scheme 2. In the reaction of cyclic alkenyl trichloroacetate 4a (R1, R2 = (CH2)4, X = Cl) with various aromatic, heteroaromatic, and aliphatic aldehydes, the anti-aldol products 6a−6g were obtained as major diastereomers in low to quantitative yields (16−99%), low to good diastereoselectivities of 34−82% de, and moderate to high enantioselectivities of 65−94% ee. In the case of aromatic aldehydes, enantioselectivities of 89−94% ee were achieved with the presence of electrondonating as well as electron-withdrawing substituents on the phenyl ring. Lower diastereo- and enantioselectivities were obtained in the reactions of heteroaryl, α,β-unsaturated, and aliphatic aldehydes (34−48% de and 65−78% ee for products 6e−6g). In addition to mono- and bicyclic alkenyl trichloroacetates 4a and 4c, the authors demonstrated that 1-tetralonederived alkenyl trifluoroacetate 4d provided the corresponding anti-products 6l and 6m in high yields, diastereo- and enantioselectivities of 78−88%, 86−92% de, and 90−95% ee, respectively. The best result of the study was reached by using this type of substrates (product 6l: 88% yield, 92% de, 95% ee). On the other hand, the diastereoselectivity of the reaction was reversed when using acyclic (Z)-alkenyl trifluoroacetate 4b (R1 = Ph, R2 = Me, X = F, E/Z = 8:92), which led to products 6h and 6i as major syn-diastereomers with high diastereoselectivities of 90−98% de, good yields of 73−94%, and moderate to good enantioselectivities of 73−89% ee. To explain these results, the authors have proposed the cyclic transition state structures depicted in Scheme 2 that explained the formation of

Scheme 2. Aldol Reaction of Aldehydes and Alkenyl Trihaloacetates

the anti-aldol products from (E)-silver enolates and that of the syn-aldol products from (Z)-silver enolates. Other types of asymmetric silver-catalyzed aldol reactions have been developed by several groups, such as Mukaiyama aldol reactions of aldehydes with silyl enolates. For example, in 2000, Yamagishi et al. catalyzed these reactions by a combination of AgPF6 and BINAP with moderate enantioselectivities of up to 69% ee.47 In 2001, Yamamoto et al. reported higher enantioselectivities of up to 97% ee for major syndiastereomeric aldol products arisen from comparable reactions based on the use of 4-Tol-BINAP combined with AgTf.48,49 Later, in 2006, high enantioselectivities (96% ee) were also achieved by Hoveyda and co-workers in the Mukaiyama aldol reaction of silyl enolates with α-ketoesters by using another type of silver catalyst system, such as a chiral amino acid based ligand combined with AgF2.50 C


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On the other hand, in asymmetric direct aldol reactions,38 the aldol donor and acceptor are used directly, without needing the preformation or preactivation of the enolate species for enantioselective C−C bond formation. The in situ catalytic generation of active enolates from aldol donors is the initial step in direct aldol reactions, and consequently the scope of the aldol donor is usually limited to carbonyl compounds bearing α-protons of relatively high acidity. Although the electronegativity values of sulfur and carbon atoms are similar, αsulfanyl carbonyl compounds have an inherently more acidic αproton than the parent carbonyl compounds because the sulfide functionality stabilizes the α-carbanion through appreciable stereoelectronic effects.51 In this context, Kumagai and Shibasaki have developed enantioselective silver-catalyzed direct aldol reactions of α-sulfanyl lactones 7a and 7b with aldehydes 5a−5l.52 As shown in Scheme 3, the reactions were

carbamate or ester group leading to products 9h−9i in 82−86% de and 98−99% ee as well as to α-pyridyl-substituted aldehyde, which provided a comparably good result (product 9j, 86% de, 98% ee). In addition to five-membered lactones, six-membered substrate 7b produced remarkable results with 79−92% yields, >90% de, and 99% ee. In contrast, a seven-membered lactone only produced a trace amount of the corresponding aldol product. The utility of this novel methodology was demonstrated in the development of a total synthesis of viridofungin A and NA 808, which are serine palmitoyl transferase inhibitors. 2.2. Nitroso-Aldol Reactions

The asymmetric nitroso aldol reaction constitutes a powerful method to introduce a hydroxyl group at the α position of a carbonyl compound, an important transformation in organic synthesis.53,54 In 2003, Yamamoto and Momiyama demonstrated that a BINAP-derived silver catalyst promoted the regioselective asymmetric O-nitroso-aldol reaction of tin enolates with moderate to excellent enantioselectivities of 52−97% ee combined with general high yields (65−92%).55 Further studies by the same authors showed that the regio- and enantioselectivities of these reactions were dependent on the structure of the active catalytic silver complex.56 For example, by using a bimetallic silver triflate derived from a chiral biphosphine, it was possible to reverse the regioselectivity of the reaction to afford preferentially the N-aldol product with almost complete regioselectivity (N:O >99:1) and high enantioselectivities of up to 98% ee. With the aim of avoiding the use of toxic tin enolates, Yamamoto et al. recently developed enantioselective silver-catalyzed O-nitroso-aldol reactions with silyl enol ethers.57 Among a series of chiral phosphite ligands derived from BINOL, ligand 10 was selected as optimal when combined with AgBF4 in the presence of CsF as fluoride source at −78 °C. Under these conditions, various cyclic silyl enol ethers 11a−11j reacted with PhNO to yield the corresponding O-nitroso-aldol products 12a−12j with a very high regioselectivity (O/N ≥98:2), and moderate to excellent enantioselectivities of 64−99% ee (Scheme 4). It was found that, in the absence of the fluoride source, the reaction did not proceed at all. The generality of the process was shown with a variety of silyl enol ethers. Six-membered substrates derived from unsubstituted cyclohexanone (R = H, n = 1) provided the corresponding products 12a−12c in 72−85% yields and uniformly excellent enantioselectivities of 95−98% ee. Moreover, a tetrahydropyran derivative was a suitable substrate leading to product 12d in 66% yield and 97% ee. The reaction of five-membered silyl enol ether 11e also gave the corresponding product 12e with 90% ee and 41% yield, while a seven-membered substrate provided a lower enantioselectivity of 64% ee (product 12f). Furthermore, 2-substituted (R ≠ H) cyclohexanone derivatives yielded products 12g−12h in high yields (80−99%) and moderate to good enantioselectivities of 76−79% ee. On the other hand, a better result (84% yield and 92% ee for product 12i) was obtained in the reaction of a αtetralone derivative 11i. To complete this study, the authors also investigated the diastereoselectivity of the reaction of chiral silyl enol ethers (S)-11j and (R)-11j. As shown in Scheme 4, the reaction of (S)-11j with PhNO under the same conditions afforded (2R,3R)-12 as a single stereoisomer (>98% de, 99% ee) in 91% yield, while the enantiomeric substrate (R)-11j led to (2R,3S)-12j with a good diastereoselectivity of 82% de combined with 99% ee and 70% yield. These results showed

Scheme 3. Aldol Reaction of Aldehydes and α-Sulfanyl Lactones

performed in toluene at −20 °C in the presence of DBU as a base and a combination of 3 or 5 mol % of AgPF6 as precatalyst with chiral Biphep-type ligand (R)-8. They afforded a range of chiral, densely functionalized lactones 9a−9m in moderate to high yields (50−93%), good to high syn-diastereoselectivities (80 to >90% de), and general excellent enantioselectivities of 89−99% ee. In particular, α,α-nonbranched aldehydes 5a−5j, susceptible to side self-aldol condensations under strongly basic conditions, provided the corresponding aldol products 9a−9j in good yields and selectivities. This selectivity probably resulted from the preferential enolization of the α-sulfanyl lactones, because of the soft−soft interaction between silver and the αsulfanyl group. Moreover, aldehydes bearing alkyl substituents and those with oxygen-containing substituents were tolerated, giving the corresponding aldol products 9a−9f with high syndiastereoselectivities of up to >90% de, combined with high enantioselectivities of 98−99% ee. Remarkably, even the use of formaldehyde 5g was successful (93% yield, 89% ee), demonstrating that the process was not sensitive to moisture. The scope of aldehydes was also extended to those bearing a D


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Scheme 4. O-Nitroso-Aldol Reactions of Silyl Enol Ethers

Scheme 5. O- and N-Nitroso-Aldol Reactions of Alkenyl Trichloroacetates

that the stereochemical outcome of the O-nitroso-aldol reaction could be controlled by the catalyst regardless of the configuration of the silyl enol ether substrate at C3. In 2009, Yanagisawa et al. demonstrated that Bu2Sn(OMe)2 catalyzed the N-nitroso-aldol reaction between alkenyl trichloroacetates and PhNO in the presence of methanol.58 With the aim of developing an asymmetric version of this process, the same group performed these reactions in the presence of 5 mol % of a combination of AgOAc with a P-chiral ligand such as (R,R)-t-Bu-QuinoxP*.59 Under these conditions, the reaction of a range of alkenyl trichloroacetates derived from cyclopentanone, cyclohexanone, and cycloheptanone 13a−13d with nitrosobenzene derivatives 14a−14d led to mixtures of the corresponding chiral O-adducts 15a−15h and N-adducts 16a− 16h in variable yields of 28−81%, with generally high enantioselectivities of 90−99% ee, and with O/N ratios of 63/37 to 97/3 (Scheme 5). Indeed, the major products of the reaction were α-aminooxyketones 15a−15h, while the corresponding α-hydroxyamino ketones 16a−16h always constituted the minor products. The scope of the reaction could be extended to 1-tetralone derivatives 13e−13f for which nearly exclusive O-selectivity (O/N = 96:4 to >99:1) in addition to excellent enantioselectivity (97−99% ee) was observed (products 15i−15j). As in the asymmetric aldol reaction of α-ketoesters with alkenyl trichloroacetates (Scheme 1), the process evolved through the formation of a tin enolate

from the reaction of the alkenyl trichloroacetate 13 with Bu2Sn(OMe)2. Then, this enolate added to PhNO (14a, R1 = R2 = H) enantioselectively in the presence of the chiral silver catalyst, to afford the tin alkoxide of O- and N-aldol products, which were subsequently protonated with methanol, resulting in the formation of the final products with regeneration of Bu2Sn(OMe)2. To explain the stereoselectivity of the reaction, the authors proposed the transition state depicted in Scheme 5. Initially, the silver atom of the catalyst coordinates to the nitrogen atom of PhNO. The tin enolate then approaches the oxygen atom of PhNO, while avoiding steric repulsion from a tert-butyl group of the chiral ligand. Thus, aminooxylation E


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occurrs selectively at the Si face of the tin enolate to yield (S)α-aminooxyketone 15a.

Scheme 6. Three-Component Vinylogous Mukaiyama− Mannich Reaction of Aliphatic Aldehydes, o-Thiomethyl-panisidine, and Trimethylsiloxyfurans

3. SILVER-CATALYZED MANNICH REACTIONS 3.1. Vinylogous Mukaiyama−Mannich Reactions

3.1.1. With Hoveyda−Snapper Catalysts. The Mannich reaction,60 occurring between a Schiff base and a nucleophile, constitutes one of the most powerful reactions for the construction of nitrogen-containing products.61,62 Over the past two decades, the catalytic asymmetric Mannich reaction,63−69 which allows biologically important chiral β-amino carbonyl compounds and derivatives to be easily prepared,70 has been widely investigated on the basis of using either chiral organometallic catalysts or organocatalysts.71−75 In 1998, Lectka et al. demonstrated for the first time that a combination of a BINAP-derived ligand with AgSbF6 behaved as a promising asymmetric catalyst system (10 mol %) in the Mukaiyama− Mannich reaction of α-imino esters with enol silanes, providing enantioselectivities of 90 to >99% ee.76 Later, Hoveyda and Snapper found that another type of chiral catalyst generated in situ from an amino acid derived phosphine and AgOAc employed at only 1−5 mol % of catalyst loading, earlier introduced by these authors to successfully promote the asymmetric cycloaddition reaction of Danishefsky’s diene with arylimines,77 was also able to promote asymmetric Mannich reactions of silyl enol ethers with aldimines with enantioselectivities of 89−98% ee in combination with moderate to excellent yields (51−98%).78 The asymmetric vinylogous Mannich reaction, as a variant of the Mannich reaction, has attracted increasing attention owing to its ability to directly deliver complicated and highly functionalized chiral δ-amino compounds. In 2006, Hoveyda and Snapper reported excellent results (up to 98% ee) in asymmetric vinylogous Mannich reactions of aryl aldimines with siloxyfurans by using only 1 mol % of related phosphine ligands bearing different amino acid residues in combination with AgOAc.79 On the other hand, enantioselective vinylogous Mannich reactions of aldimines derived from alkyl-substituted aldehydes are less developed because they are generally achieved with diminished efficiency and diastereoselectivity than those of the more common arylsubstituted aldimines. In 2008, the same authors described a novel protocol for efficient three-component silver-catalyzed asymmetric vinylogous Mannich reactions of the more demanding alkyl-substituted aldimines, generated in situ from the corresponding aldehydes 5a−5h and o-thiomethyl-panisidine 17, with trimethylsiloxyfuran 18a (R2 = H).80 This process was catalyzed by 5 mol % of a combination of AgOAc and chiral tert-leucine-derived phosphine ligand 19 in THF as solvent at −78 °C. As shown in Scheme 6, a range of aliphatic aldehydes 5a−5h led regioselectively to the corresponding unsaturated lactones 20a−20g as almost single anti-diastereomers (>96% de) with exceptional general enantioselectivity of >98% ee combined with moderate to high yields (50−90%). The lowest yield of 50% was obtained in the reaction of t-Busubstituted aldehyde 5e (R1 = t-Bu). The process could also be applied for the first time to 5-methyl-substituted siloxyfuran 18b (R2 = Me) which provided by reaction with cyclohexylcarboxaldehyde 5a (R1 = Cy), and o-thiomethyl-panisidine 17, the corresponding product 20h as a single stereoisomer (>96% de, >98% ee) in 88% yield (Scheme 6). It must be noted that this remarkable study, allowing single stereoisomers to be obtained, could also be situated in section

10 dealing with silver-catalyzed enantioselective domino reactions. The efficiency and high degrees of enantiodifferentiation in the catalytic enantioselective vinylogous Mannich reactions described in Scheme 6 were not limited to alkyl-substituted aldimines. Indeed, these authors demonstrated that the same reaction conditions could be applied to the reaction between phenyl or alkynyl aldimines (21a, 21b or 23a, 23b) derived from o-thiomethyl-p-anisidine or o-anisidine with trimethylsiloxyfuran 18a, allowing the corresponding Mannich products 22a, 22b and 24a, 24b to be formed in good to excellent yields (70 → 98%) and remarkable diastereo- and enantioselectivities of up to >98% de and >98% ee, respectively (Scheme 7).80 Generally, o-thiomethyl-p-anisidine-derived imines 21a and 23a were more efficient than o-anisidine-derived imines 21b and 23b (>98% yield vs 89% yield for reactions with phenylsubstituted aldimines and 86% vs 70% yield for reactions with alkynyl-substituted aldimines, respectively). Whereas reactions in all cases proceeded with excellent diastereoselectivity (>98% de), processes involving substrates bearing an S-containing Naryl group were found to be more enantioselective than those involving substrates with an O-containing N-aryl group. Notably, in all cases, no trace of products arisen from αaddition was detected. Ketimines are less reactive than aldimines, and consequently, they have been much less investigated in asymmetric Mannich reactions. In this context, the same authors have developed silver-catalyzed diastereo- and enantioselective vinylogous Mannich reactions of trimethylsiloxyfuran 18a with α-ketimine esters 25a−25i.81 As shown in Scheme 8, this was achieved by employing closely related reaction conditions, which regioselectively led to the formation of a range of unsaturated chiral lactones 26a−26i arisen from the reaction of the corresponding N-aryl aromatic α-ketimine esters 25a−25i with trimethylsiloxyfuran 18a. These products bearing an N-substituted all-carbon quaternary stereogenic center were obtained as anti-diastereomers in 78 to >96% de, good to excellent yields of 72−95%, and generally high enantioselectivities of 87−94% ee, except when o-bromo-substituted α-ketimine ester 25i was employed as substrate, which led to the corresponding product 26i in only F


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Scheme 8. Vinylogous Mukaiyama−Mannich Reaction of αKetimine Esters and Trimethylsiloxyfuran

Scheme 7. Vinylogous Mukaiyama−Mannich Reactions of Phenyl/Alkynyl Aldimines Derived from o-Thiomethyl-panisidine/o-Anisidine and Trimethylsiloxyfuran

32% ee. The p-nitro unit in ketimine substrates was shown to be crucial to achieve high efficiency as well as diastereo- and enantioselectivities, while the latter were enhanced by the presence of a methoxy group ortho to the N-aryl group. To explain the stereoselectivity of the reaction, the authors have proposed the endo transition state E wherein the substrate bound in a manner to minimize interaction with the bulky tertbutyl amino acid substituent, and the ketimine’s sterically hindered aryl substituent was situated trans to the Si-based nucleophile. Another noteworthy feature was the Lewis base activation of the siloxyfuran by the amide terminus, which led to the selective formation of the anti-products (Scheme 8). In addition to chiral tert-leucine-derived phosphine ligand 19, the same authors have developed iso-leucine-derived phosphine ligand 27 to be combined with AgOAc and applied to the asymmetric vinylogous Mannich reactions of aryl aldimines 28a−28d with 5-methyl-2-trimethylsiloxyfuran 18b.82 As shown in Scheme 9, the process resulted in the formation of chiral products 29a−29d, bearing an O-substituted quaternary carbon stereogenic center, as the major regioisomers (γ/α = 85:15) arising from regioselective γ-addition. The α-addition side products could be easily oxidatively removed. The scope of the process was extended to variously (substituted) aryl aldimines, providing the corresponding products 29a−29d in moderate yields (38−64%), albeit with general and remarkable diastereo- and enantioselectivities of >96% de and >98% ee, respectively. These reaction conditions were applied to alkynyl aldimines 30a−30e, which afforded by reaction with 5-methylsubstituted trimethylsiloxyfuran 18b the corresponding γproducts 31a−31e in moderate yields (30−65%), high diastereoselectivities of 90 to >96% de, and moderate to high enantioselectivities of 75−94% ee (Scheme 9). It must be noted that the regioselectivity of the reaction of alkynyl aldimines was higher (γ/α = 98:2) than that of aryl aldimines (γ/α = 85:15).

In addition, this catalyst system was also used at a 5 mol % catalyst loading to promote enantioselective vinylogous Mannich reactions involving siloxypyrroles as another type of nucleophilic partner.82 As shown in Scheme 10, the silvercatalyzed Mannich reaction of N-Boc-2-(trimethylsiloxy)pyrrole 32 with a range of aryl and heteroaryl aldimines 33a−33g led to the corresponding chiral α,β-unsaturated δamino-γ-butyrolactams 34a−34g with an exceptional regioselectivity (γ-addition/α-addition = 98:2), moderate to excellent yields (62−97%), and almost complete anti-diastereoselectivity (>96% de), combined with good to excellent enantioselectivities of 83−98% ee. Additions to imines bearing a bromosubstituted aryl unit (33b−33c), an electron-withdrawing (33d), or an electron-donating unit (33e−33f) proceeded with similar degrees of efficiency and selectivity. The lowest yield of 62% was obtained in the case of thienyl-containing product 34g, which could be attributed to competitive chelation of the Ag-based complex with the S atom of the thienyl group. The scope of this methodology could be extended to alkyne-substituted aldimines 35a−35d, leading to the corresponding chiral alkynyl-substituted diamine products 36a−36d in 93−98% yields with exceptional γ-regioselectivity (γ-addition/α-addition = 98:2), excellent diastereoselectivity G


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(>96% de), and good to high enantioselectivities of 86−95% ee. As shown in Scheme 10, the acetylene group could bear an aryl (35a, 35b), an alkyl (35c), or a silyl substituent (35d). It must be noted that these alkynyl-substituted products can be converted to a variety of useful chiral derivatives. Hoveyda−Snapper silver catalyst derived from tert-leucinebearing phosphine 19 was previously used by Curti and Casiraghi to develop this type of reaction (Scheme 11).83

Scheme 9. Vinylogous Mukaiyama−Mannich Reactions of Aryl/Alkynyl Aldimines and 5-Methyl-Substituted Trimethylsiloxyfuran

Scheme 11. Vinylogous Mukaiyama−Mannich Reaction of N-Aryl Aldimines and N-Boc-2-(trimethylsiloxy)pyrrole

Indeed, the asymmetric vinylogous Mannich reaction of various aromatic, heteroaromatic, and aliphatic N-aryl aldimines 37a− 37h with N-Boc-2-(trimethylsiloxy)pyrrole 32 afforded the corresponding chiral α,β-unsaturated δ-amino-γ-butyrolactams 38a−38h with complete γ-regioselectivity (γ-addition/αaddition >99:1), moderate to quantitative yields (36−99%), moderate to excellent anti-diastereoselectivities of 74−98% de, and moderate enantioselectivities of 42−80% ee. These authors also described a three-component version of Mannich reactions between various alkyl-substituted aldehydes 5a−5g, o-thiomethyl-p-anisidine 17, and N-Boc-2(trimethylsiloxy)pyrrole 32 by using the same catalyst system at a 5 mol % catalyst loading.84 As shown in Scheme 12, the process afforded the corresponding vicinal chiral diamino carbonyl products 39a−39g in good yields (51−92%), with virtually complete γ-site and anti-selectivities combined with generally high enantioselectivities of 82−96% ee. The utility of the formed Mannich products was demonstrated in the synthesis of an unprecedented perhydrofuro[3,2-b]pyrrolone product, an aza analogue of naturally occurring (+)-goniofufurone (Scheme 12). Later, in 2015, Hoveyda reinvestigated the three-component Mannich reaction of cyclohexylcarboxaldehyde (R = Cy) with the same partners albeit in the presence of 5 mol % of iso-leucine-derived phosphine ligand 27 in combination with the same quantity of AgOAc, which led with almost complete regioselectivity (γ-addition:α-addition >98:2) to the corresponding Mannich product with anticonfiguration in 82% yield, >96% de, and 91% ee.82 3.1.2. With Other Catalysts. In addition to the early use of BINAP-based silver catalysts,76 and to that of Hoveyda− Snapper silver catalysts derived from amino acid bearing phosphines, other types of chiral silver phosphine catalysts have been successfully investigated in asymmetric Mannich reactions of various aldimines with trimethylsiloxyfuran. For example, Shi et al. have developed chiral phosphine Schiff base type ligands

Scheme 10. Vinylogous Mukaiyama−Mannich Reactions of Aryl/Alkynyl Aldimines and N-Boc-2(trimethylsiloxy)pyrrole

H


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Scheme 12. Three-Component Vinylogous Mukaiyama− Mannich Reaction of Aliphatic Aldehydes, o-Thiomethyl-panisidine, and N-Boc-2-(trimethylsiloxy)pyrrole

Scheme 13. Vinylogous Mukaiyama−Mannich Reaction of Aromatic N-Aryl Aldimines and Trimethylsiloxyfuran in the Presence of a Phosphine Schiff Base Ligand

Scheme 14. Vinylogous Mukaiyama−Mannich Reaction of Aromatic N-Aryl Aldimines with Trimethylsiloxyfuran in the Presence of a Phosphine-Oxazoline Ligand

to be used at 11 mol % of catalyst loading in combination with AgOAc (10 mol %) to promote the enantioselective Mannich reaction of aromatic N-aryl aldimines 40a−40h with trimethylsiloxyfuran 18a.85 Among them, ligand 41 possessing two electron-withdrawing groups (Cl) at the 2- and 3-positions of the benzene ring was selected as optimal, allowing the formation of a range of chiral Mannich products 42a−42h with complete regioselectivity in moderate to good yields (51− 91%), with moderate to good enantioselectivities of 38−81% ee, and in almost all cases as single anti-diastereomers (98% de), as shown in Scheme 13. Indeed, only using aldimine 40f having a 2,4-dimethoxy group on the benzene ring of Ar2 afforded the corresponding product 42f in 50% de, presumably due to the electronic effect. Moreover, the lowest yield (51%) and enantioselectivity (38% ee) were obtained in the reaction of aldimine 40a having a p-nitro substituent on the phenyl ring of Ar1. It was found that the use of benzyl alcohol as superstoichiometric additive was important to achieve better yield and diastereoselectivity as well as enantioselectivity. Chiral phosphine-oxazoline ligands have been introduced by these authors in combination with AgOAc as novel catalyst systems to promote comparable reactions.86 As shown in Scheme 14, better yields (76−95%) and enantioselectivities of 87−99% ee were obtained for a range of variously substituted

chiral anti-butenolides 42a−42k produced from the reaction of the corresponding aromatic N-aryl aldimines 40a−40k with trimethylsiloxyfuran 18a in the presence of 10 mol % of AgOAc combined with 10 mol % of axially chiral phosphine-oxazoline ligand 43 and 2,2,2-trifluoroethanol as superstoichiometric additive. The reaction was completely γ-regioselective, providing the anti-configured products with moderate to complete diastereoselectivity (34−98% de). To explain the I


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the same reaction conditions.88 Remarkably, the process afforded the corresponding fluorinated products 48a−48j in excellent yields (95−99%) regio- and diastereoselectively (>90% de), as shown in Scheme 16. The excellent enantiomeric

stereoselectivity of the process, the authors have the transition state depicted in Scheme 14, in which the siloxyfuran approached the activated complex from one face of the imine with minimization of steric repulsion. Later, the same authors reported the first catalytic asymmetric Mannich reaction of fluorinated aldimines with trimethylsiloxyfuran by using a closely related catalyst system.87 Indeed, using tert-butyl-substituted chiral phosphine-oxazoline ligand 44 in combination with AgOAc at a 10−11 mol % catalyst loading in the presence of a superstoichiometric amount of ethanol as additive allowed the asymmetric Mannich reaction of fluorinated aldimines 45a−45i with trimethylsiloxyfuran 18a to be achieved in good to quantitative yields (70− 99%), good anti-diastereoselectivity of up to >90% de, and moderate enantioselectivities of 32−81% ee, as shown in Scheme 15. This ligand was found much more enantioselective

Scheme 16. Vinylogous Mukaiyama−Mannich Reaction of Chiral Fluorinated Aldimines and Trimethylsiloxyfuran in the Presence of a Phosphine-Oxazoline Ligand

Scheme 15. Vinylogous Mukaiyama−Mannich Reaction of Fluorinated Aldimines and Trimethylsiloxyfuran in the Presence of Another Phosphine-Oxazoline Ligand

than differently substituted chiral phosphine-oxazoline ligands, including the phenyl-substituted ligand 43. The best results were obtained for the reaction of fluorinated aldimines 45a− 45c bearing electron-rich aromatic groups, such as a 4methoxyphenyl group, which led to the corresponding products 46a−46c in quantitative yields, almost complete antidiastereoselectivity (>90% de), and moderate to good enantioselectivities of 68−81% ee. A lower enantioselectivity of 56% ee was obtained for the reaction of fluorinated aldimine 45e bearing an electron-poor aromatic group such as a 4chlorophenyl group. On the other hand, N-benzyl fluorinated aldimines 45g−45i gave lower enantioselectivities (32−42% ee) except in the case of aldimine 45f containing a CF3 group, which provided the corresponding product 46f in 74% ee. Notably, all the fluorinated aldimines gave the chiral products in good to excellent yields (70−99%) and diastereoselectivities (86 → 90% de). The synthetic utility of products 46a−46e was displayed by removal of the N-aryl groups by treatment with PhI(OAc)2 in methanol under mild reaction conditions, affording the corresponding free amine chiral products. As an extension of the precedent methodology, the authors have investigated the first asymmetric Mannich reaction of fluorinated aldimines 47a−47j bearing a chiral auxiliary, such as a (S)-1-phenylethyl group, with trimethylsiloxyfuran 18a under

excesses (87−98% ee) of the reaction were determined after removing the chiral auxiliary through hydrogenation and reduction of the double bond of butenolides 48a−48j followed by reaction with 4-bromobenzoic acid to give the corresponding fluorinated chiral amides 49a−49j. The authors have proposed the transition state model depicted in Scheme 16. To minimize steric interactions, the substrate was bound anti to the bulky tert-butyl substituent of the oxazoline of ligand 44. The catalyst-bound imine could react with the siloxyfuran through an endo-type addition, and consequently, the reaction of chiral fluorinated aldimine 47 with trimethylsiloxyfuran 18a in the presence of the catalyst system led to the (4R,5R) stereomer predominantly. This catalyst system was also applied by the same authors to develop the first asymmetric Mannich reactions between N-Boc aldimines and trimethylsiloxyfuran.89 As shown in Scheme 17, the reaction of a range of aryl, heteroaryl, as well as alkyl N-Boc aldimines 50a−50k with trimethylsiloxyfuran 18a afforded the corresponding chiral N-Boc-protected γ-butenolides 51a−51k in good yields (60−83%), and moderate to good anti-diastereoJ


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Scheme 17. Vinylogous Mukaiyama−Mannich Reaction of N-Boc Aldimines and Trimethylsiloxyfuran in the Presence of a Phosphine-Oxazoline Ligand

Scheme 18. Vinylogous Mukaiyama−Mannich Reaction of Aromatic N-Aryl Aldimines and Trimethylsiloxyfuran in the Presence of a Monophosphine Ligand

and enantioselectivities of 60−75% de and 63−86% ee, respectively. The synthetic utility of the products was demonstrated by the easy removal of the N-Boc-protecting group with trifluoroacetic acid at room temperature, providing the corresponding chiral free amines in excellent yields (92%). All the studies detailed above involved the use of chiral phosphine ligands having an additional nitrogen group. In 2013, Xu et al. decided to investigate the asymmetric Mannich reactions of aldimines with trimethylsiloxyfuran and other types of ligands, such as simple chiral monophosphines that did not bear an additional heteroatom.90 Among these novel chiral ligands prepared from (R)-BINOL, monophosphine 52 was selected as optimal in combination with AgOAc to promote the enantioselective Mannich reaction of various aromatic N-aryl aldimines 40a−40k with trimethylsiloxyfuran 18a. As shown in Scheme 18, the process afforded a range of chiral γ-butenolide derivatives 42a−42k in good to excellent yields (75−99%), and exceptional anti-diastereoselectivity of >98% de, combined with moderate enantioselectivities (53−78% ee). With the aim of extending the scope of this methodology, the authors investigated the reaction of an N-(2-methoxybenzene) aldimine derived from an alkyl aldehyde such as cyclohexylcarboxaldehyde. Unfortunately, the corresponding Mannich product was formed in 30% ee, 50% yield, and 88% de. This study demonstrated that a chiral monophosphine without an additional heteroatom-donating group could exhibit a good enantiocontrol and high catalytic performances in silvercatalyzed asymmetric Mannich reactions.

Scheme 19. Mannich Reactions of 1,3-Dicarbonyl Compounds and N-Boc Aryl Aldimines in the Presence of a Ferrocenyl Phosphine Sulfur Ligand

3.2. Other Mannich-Type Reactions

(67−99%) and high enantioselectivities (86−91% ee) regardless of the electronic properties and steric hindrance of the phenyl ring of aldimines 55a−55g. A slightly lower enantioselectivity of 80% ee was obtained when the heteroaromatic aldimine 55h (Ar = 2-furyl) was used. The same methodology could be applicable to other β-dicarbonyl compounds, such as malonates and β-ketoesters 57a and 57b, as shown in Scheme 19. High isolated yields (86−90%) and good enantioselectivity of 78% ee were achieved for both products 58a and 58b. However, no valuable diastereoselectiv-

The use of silyl enol ethers as nucleophilic reagents in asymmetric Mannich reactions is not ideal from the standpoint of atom economy. In this context, in 2009 Zhou et al. introduced a novel silver catalyst system based on the combination of chiral ferrocenyl phosphine sulfur ligand 53 with AgOAc to achieve direct asymmetric Mannich reactions of acetylacetone 54 and N-Boc-protected aryl aldimines 55a−55h with enantioselectivities of up to 91% ee.91 As shown in Scheme 19, in the presence of only 3−3.3 mol % of catalyst, a range of chiral amines 56a−56g was produced in good yields K


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Scheme 21. Mannich Reaction of N-Tosyl Aryl Aldimines and a Glycine Schiff Base in the Presence of a Ferrocenyl Triazole Phosphine Ligand

ity (34% de) was observed in the asymmetric Mannich reaction between β-ketoester 57b and aldimine 55b, probably due to epimerization. Optically active α,β-diamino acids constitute an important class of products critical to a variety of biologically active molecules.70 In this context, the asymmetric Mannich-type reaction of glycine Schiff base 59 and derivatives with imines provided an efficient and convenient route for the preparation of these products. In 2003, Jørgensen et al. reported the first example of enantioselective copper-catalyzed Mannich reaction of imino glycine alkyl esters with imines performed in the presence of a chiral phosphine-oxazoline ligand.92 Ever since, other efficient catalyst systems have been developed for diastereo- and enantioselective Mannich reactions of glycine Schiff bases with aldimines by several groups. Among them, a combination of AgOAc with chiral ferrocenyl oxazoline phosphine ligand 60 employed at 3−3.3 mol % in THF at −25 °C was found efficient to promote the Mannich reaction of N-tosyl aldimines 61a−61g with glycine Schiff base 59 to give the corresponding α,β-diamino methyl esters 62a−62g as mixtures of syn- and anti-diastereomers in general with excellent yields of 92−99% and high enantioselectivities of up to 97% ee.91 The best results were achieved in the case of aliphatic aldimines 61f−61g which led to the corresponding synproducts 62f−62g with 80−86% de, 97−99% yields, and high enantioselectivities of 93−96% ee (Scheme 20). A series of

ee. The substitution pattern of the phenyl ring in the tosylimines was found to have an effect on the transformation. Therefore, the electron-withdrawing substituents in the ortho position (substrates 61b−61d) tended to produce higher syndiastereoselectivity (36−50% de) than those observed with unsubstituted tosylimines such as 61a (20% de); however, the 1-methyl group in substrate 61e gave a comparable diastereomeric ratio (16% de) to 61a. For these substrates, the enantioselectivities remained at as high a level as that of 61a. Three-electron-donating substituents had a slight, negative effect on the enantioselectivity of the reaction (substrates 61h, 61i), whereas four-electron-withdrawing substituents (products 62f, 62g) had almost no effect on the enantioselectivity. For these latter products 62f−62i, the syn/anti ratios were moderate (56/44 to 60/40). In 2014, Sansano et al. demonstrated that these reactions could also be promoted by chiral phosphoramidite−silver complexes.94 Indeed, when the Mannich reaction of a series of N-tosyl aryl aldimines 61a−61j with glycine Schiff base 59 was performed in toluene at room temperature in the presence of a combination of 5 mol % of AgOTf and the same quantity of chiral phosphoramidite ligand 64, it provided the corresponding Mannich products 62a−62j in moderate yields (30−70%), syn-diastereoselectivities (0−60% de), and variable enantioselectivities of 2−99% ee (Scheme 22). In addition to the result obtained from the use of nonsubstituted benzaldehyde N-tosyl aldimine 61a, which provided the corresponding syn-product in 96% ee, the presence of heteroatoms (Ar = 2-furyl) and phalogenated arenes (Ar = 4-FC6H4) in the aromatic part of imines 61f and 61i allowed the best enantioselection (92−99% ee) to be achieved. The authors have proposed the transition state depicted in Scheme 22, in which the freshly generated azomethine ylide was coordinated by a silver atom and the nucleophilic attack occurred at the tosyl imine whose arylsulfonyl group was situated far from the benzylidene moiety of the dipole. In this transition state, additional coordination between silver and sulfonamido group could not be ruled out. In addition to glycine Schiff base 59 employed as the usual partner of aldimines in asymmetric silver-catalyzed Mannich reactions, Zhou et al. have investigated the related reactions of

Scheme 20. Mannich Reaction of N-Tosyl Aldimines and a Glycine Schiff Base in the Presence of a Ferrocenyl Oxazoline Phosphine Ligand

(hetero)aromatic aldimines 61a−61e with electron-withdrawing and electron-donating substituents also underwent the desired transformation to give the corresponding Mannich products 62a−62e with high yields (92−98%) and good to excellent enantioselectivities (82−97% ee) for both diastereomers albeit with low diastereoselectivities of 2−22% de (Scheme 20). These reactions were later reinvestigated by Fukuzawa et al. by using another ferrocenyl chiral ligand, such as ferrocenyl triazole phosphine ligand 63, employed at 3 mol % of catalyst loading in combination with AgOAc.93 As shown in Scheme 21, the Mannich reaction of a range of N-tosyl aryl aldimines 61a− 61i with glycine Schiff base 59 performed in THF at room temperature afforded mixtures of syn- and anti-products 62a− 62i with moderate diastereoselectivities of 12−50% de but in 99% yield in all cases and high enantioselectivities of 85−98% L


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82% ee. α,α-Dicyanoolefin 65c (X = CH2) emerged as the best substrate for the asymmetric Mannich reaction since its reactions with a series of aldimines bearing various substituents on the aromatic ring provided the highest enantioselectivities of 59−82% ee. The process was found sensitive to both the steric and the electronic properties of the substituents on the phenyl ring of aldimines. Generally, phenyl substituents bearing electron-donating groups (Ar = 4-MeOC6H4) at the paraposition gave a higher enantioselectivity compared to those with an electron-withdrawing group (Ar = 4-FC6H4 or 4BrC6H4). The best result (96% yield, >90% de, 82% ee) was reached in the case of reaction between α,α-dicyanoolefin 65c (X = CH2) and p-methoxyphenyl aldimine. In 2012, Hui et al. reported asymmetric Mannich reactions of N-tosyl aldimines 61a−61k with oxazolones 68a and 68b promoted by a synergistic ion pair 69 consisting of a silver ion and a chiral BINOL-derived phosphate anion.95 Performed at room temperature in dichloromethane as solvent, these reactions led to valuable chiral quaternary α,β-diamino acid derivatives 70a−70n in moderate to high yields (58−95%) and good to high diastereo- and enantioselectivities of up to 92% de and 99% ee, respectively. As shown in Scheme 24, the best

Scheme 22. Mannich Reaction of N-Tosyl Aryl Aldimines and a Glycine Schiff Base in the Presence of a Phosphoramidite Ligand

Scheme 24. Mannich Reaction of N-Tosyl Aldimines and Oxazolones in the Presence of a Chiral Ion Pair Silver Catalyst

α,α-dicyanoolefins 65a−65d with N-Boc aryl aldimines 55a− 55e.91 These reactions were optimally promoted by a combination of 3 mol % of AgOAc and 3.3 mol % of chiral ferrocenyl oxazoline phosphine ligand 66 in diethyl ether at −25 °C. As shown in Scheme 23, they provided a range of chiral Mannich products 67a−67h in good yields (79−96%), and moderate to good anti-diastereoselectivities of 74 to >90% de, combined with low to moderate enantioselectivities of 25− Scheme 23. Mannich Reaction of N-Boc Aryl Aldimines and α,α-Dicyanoolefins in the Presence of a Ferrocenyl Oxazoline Phosphine Ligand

results were generally achieved by using (hetero)aryl aldimines 61a−61j which allowed enantioselectivities of 81−99% ee in combination with 75−95% yields for products 70a−70m. An alkyl aldimine 61k (R1 = n-Pr) led to the corresponding product 70n in only 58% yield with lower enantioselectivity (75% ee). Concerning aryl aldimines, those bearing electrondonating groups provided excellent enantioselectivities (98− 99% ee for products 70e−70f) while aldimines bearing electron-withdrawing groups at the para-position gave slightly lower enantioselectivities (82−91% ee for products 70h, 70k, 70l). With respect to 1-substituents and the 2-naphthyl group, high enantioselectivities of 94−95% ee were also obtained (products 70g, 70i, 70m), probably due to steric effects. M


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other hand, the catalytic system failed to promote the reaction of a ε-lactone (n = 3) to give product 72l, probably because of reluctant formation of the enolate. The utility of this methodology was shown by conversion of Mannich products into chiral trisubstituted aziridines. The latter could be further converted by treatment with Pd(OH)2 into important chiral α,α-disubstituted α-amino acid derivatives. In 2014, Yanagisawa et al. reported a novel example of enantioselective Mannich-type reaction of alkenyl trichloroacetates 1a−1d with N-aryl aromatic aldimines 40a−40k using a combination of AgOTf and (R)-Segphos as ligand.97 The reaction employed N,N-diisopropylethylamine as a base and 2,2,2-trifluoroethanol as superstoichiometric additive. It afforded optically active β-amino ketones 73a−73n in uniformly excellent enantioselectivities of 96 to >99% ee, as shown in Scheme 26. These chiral products were obtained with syn

In 2013, Kumagai and Shibasaki developed asymmetric silver-catalyzed Mannich-type reactions of α-sulfanyl lactones with N-Boc aldimines.96 These processes were promoted by a combination of 3 or 5 mol % of AgPF6 and chiral Biphep-type ligand (S)-8 in the presence of the same quantity of DBU as a base in toluene at −30 °C. Soft−soft interaction between Ag+ and the sulfanyl moiety afforded in situ chemoselective activation of α-sulfanyl lactones to catalytically generate the corresponding enolates by the synergistic action of the mild Brønsted base. The reaction of a range of aldimines 50a−50k with five-membered α-sulfanyl lactone 71a (n = 1) led to the formation of a range of chiral β-amino-α-methylthio lactones 72a−72i which bore two contiguous stereogenic centers. As shown in Scheme 25, these products were obtained in Scheme 25. Mannich Reaction of N-Boc Aldimines and αSulfanyl Lactones in the Presence of a Biphep-Type Ligand

Scheme 26. Mannich Reaction of N-Aryl Aromatic Aldimines and Alkenyl Trichloroacetates in the Presence of (R)Segphos Ligand

moderate to high yields (42−94%) and uniformly excellent enantioselectivities of 96−99% ee from the corresponding aryl, heteroaryl, and alkyl aldimines. Concerning the reaction of aromatic aldimines, the reactivity and diastereoselectivity were found dependent on the electronic nature of the substituents on the aromatic ring. Thus, aromatic imines bearing electrondeficient substituents produced the corresponding Mannich products 72a−72c in high diastereo- and enantioselectivities of >90% de and 99% ee, respectively. As the electron density of the aromatic ring increased, the diastereoselectivity of the reaction decreased steadily (86, 75, and 42% de for products 72d, 72e, and 72f). The face selection of the imines was compromised with electron-rich imines, presumably because the higher Lewis basicity of the imine nitrogen had a negative effect. The particularly low yield (42%) obtained in the reaction of p-MeO-substituted imine (product 72f) was ascribed to its intrinsic low electrophilicity. Even heteroaryl and alkyl aldimines were tolerated, providing the corresponding products 72h−72i in 97−99% ee. Whereas six-membered α-sulfanyl lactone 71b (n = 2) could be applicable as a pronucleophile, less reactivity or lower enantioselectivity was observed (49% yield for product 72k and 82% ee for product 72j). On the

selectivity in moderate to good yields of up to 81% via the chiral silver enolates generated in situ in THF at −30 °C. Remarkably, cyclic alkenyl trichloroacetates 1a−1c, such as cyclohexanone, cyclopentanone, and cycloheptanone derivatives, reacted with variously substituted N-aryl aromatic aldimines to provide the corresponding Mannich products 73a−73m with both remarkable diastereo- and enantioselectivities of >98 to >99% de and 98 to >99% ee, respectively. It is only in the case of an acyclic alkenyl trichloroacetate that the corresponding product 73n was obtained with a low diastereoselectivity of 38% de combined with a low yield of 24% but with still high enantioselectivity (96% ee) (Scheme 26). Only benzaldimines possessing an N-protective group N


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Inspired by these good results, the authors further investigated the related Michael addition of diphenylene glycine imino methyl ester 59 to α,β-unsaturated ketones 76.106 Using the same catalyst system in the presence of 20 mol % of DABCO as a base in THF at −40 °C, the Michael reaction of a range of aromatic α,β-unsaturated ketones 76 (R = aryl, heteroaryl) led to the corresponding chiral products 77 in high yields (82−93%) and enantioselectivities of 97−99% ee (Scheme 28). Notably, a ferrocenyl-substituted α,β-unsaturated

other than aromatic groups did not give satisfactory results. A plausible transition state for the reaction of cyclic alkenyl trichloroacetates is depicted in Scheme 26, in which the imine coordinates to a silver atom of the enolate to furnish a sixmembered cyclic structure. Accordingly, from the cyclic Eenolate, the syn-Mannich product is formed through this chairtype transition state.

4. SILVER-CATALYZED MICHAEL REACTIONS 4.1. α,β-Unsaturated Carbonyl Compounds as Acceptors

Scheme 28. Michael Reaction of α,β-Unsaturated Ketones and a Glycine Schiff Base

Michael-type reactions98 can be considered as one of the most powerful tools for the stereocontrolled formation of carbon− carbon as well as carbon−heteroatom bonds.99−104 A range of chiral metals and chiral organocatalysts have been applied to promote these fundamental reactions. It is only in 2005 that Kobayashi et al. reported the first enantioselective silvercatalyzed Michael addition of β-ketoesters to α,β-unsaturated ketones, performed in water in the presence of AgOTf and BINAP derivatives as ligands with promising enantioselectivities of up to 83% ee.105 Ever since, it must be recognized that still few publications have been reported dealing with the use of chiral silver catalysts in catalytic Michael additions of nucleophiles to various acceptors in an enantioselective fashion. As a recent example, Fukuzawa et al. have developed highly enantioselective silver-catalyzed conjugate additions of diphenylidene glycine imino methyl ester 59 to various arylidene and alkylidene malonates 74, providing the corresponding chiral Michael products 75 in moderate to quantitative yields of 36− 99% (Scheme 27).106 Performed in the presence of 5 mol % of

ketone was also compatible, providing the corresponding product in 84% yield and 90% ee. Moreover, the reaction of an aliphatic α,β-unsaturated ketone, such as methyl vinyl ketone (R = Me), yielded the corresponding product in 92% yield and 96% ee. It must be noted that this study represented the first enantioselective silver-catalyzed conjugate addition of glycine derivatives to α,β-unsaturated ketones. The use of heterogeneous chiral catalysts in organic synthesis allows catalyst recovery and reuse but also simplifies the separation of catalyst and products and even avoids metal contamination of the products. However, the development of this type of catalysts has lagged far behind advances in homogeneous chiral catalysts in terms of performance and scope of reactions.107,108 In this context, Kobayashi et al. have developed heterogeneous chiral nanoparticles related to their advantages, such as high stability, robustness, and reusability.109 Indeed, polymer-incarcerated Rh/Ag nanoparticles were prepared and further applied in the presence of chiral ligands to the asymmetric Michael addition of arylboronic acids 78 to α,β-unsaturated ketones 79 to give the corresponding Michael products 81. Ligand 80 employed at a loading of only 1 mol % was selected as the optimal ligand in these reactions when combined with 0.75 mol % of the bimetallic heterogeneous catalyst. Among α,β-unsaturated ketones investigated, 2-cyclohexenone provided the best results since, by reaction with a series of variously substituted arylboronic acids, it provided the corresponding Michael products in both high yields and enantioselectivities of 81−99% and 93−98% ee, respectively. Acyclic α,β-unsaturated ketones were also well tolerated, leading to the corresponding products in generally lower yields (70−91%) and enantioselectivities of 74−96% ee (Scheme 29). The catalyst system could be recycled several times by simple operations while retaining high yields and excellent enantioselectivities.

Scheme 27. Michael Reaction of Arylidene/Alkylidene Malonates and a Glycine Schiff Base

a combination of AgOAc and chiral P,S-ligand 63 in THF at room temperature and in the absence of a base in almost all cases, the process yielded the syn-products 75 as major diastereomers with 88−94% de and 90−99% ee in the case of arylidene malonates. An alkylidene malonate (R = Cy) led to the corresponding syn-product in 60% yield with a lower diastereoselectivity (60% de) with an excellent enantioselectivity of 97% ee. The scope of arylidene ethyl malonates was found to be large since aryl, heteroaryl, and ferrocenylsubstituted malonates were compatible. In the reaction of ferrocenyl-substituted malonate and alkylidene malonate, the presence of 25 mol % of Cs2CO3 as base allowed the reaction performance to be improved. O


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Scheme 29. Michael Reaction of α,β-Unsaturated Ketones and Arylboronic Acids

highly efficient methodology was developed by Fukuzawa et al. by using a combination of 5 mol % of AgOAc with 5.5 mol % of ferrocenyl triazole-based P,S-ligand 63,112 already employed by the same authors in enantioselective silver-catalyzed Mannich reactions between N-tosyl aryl aldimines and glycine Schiff base 59 (Scheme 21).93 As shown in Scheme 31, the application of Scheme 31. Michael Reaction of Nitroalkenes and a Glycine Schiff Base

In 2015, the same authors reported the synthesis of another bimetallic nanoparticle system from rhodium nanocomposites of polystyrene-based copolymers with cross-linking moieties and carbon black that was further applied in the presence of a chiral ligand as a catalyst system in asymmetric Michael additions of arylboronic acids 78 to α,β-unsaturated esters 82.110 In this case, the best results were achieved when the reaction was performed with chiral ligand 83 employed at a catalyst loading as low as 0.05 mol % in combination with only 0.25 mol % of the heterogeneous Rh/Ag catalyst. A range of βarylated products 84 derived from 3-aryl α,β-unsaturated esters were obtained in high yields of 68−95% and high enantioselectivities of 92−99% ee (Scheme 30). Furthermore,

this catalyst system to the reaction of glycine imino methyl ester 59 with various aryl nitroalkenes 85a−85j in the presence of 18 mol % of triethylamine (TEA) as a base in THF at −25 °C afforded the corresponding chiral Michael products 86a− 86j as single anti-diastereomers in high yields of 80−97% and excellent enantioselectivities of 95−99% ee. Uniformly excellent enantioselectivities were reached for a range of nitrostyrenes regardless of the electronic properties and position of the substituents on the phenyl ring. Even (E)-2-(2-nitrovinyl)naphthalene and 2-nitrovinylferrocene reacted to give the corresponding enantiopure products 86i−86j (99% ee). As an extension of the methodology, the same catalyst system was applied by these authors to promote asymmetric Michael additions of cyclic imino esters to aryl nitroalkenes.113 In this case, no additional base was required and the best results were achieved by performing the reactions in diethyl ether as solvent. As shown in Scheme 32, the process led to chiral densely functionalized 1-pyrroline derivatives 87 in both high yields and enantioselectivities of 82−98% and 83−98% ee, respectively, and to good to almost complete anti-diastereoselectivity (78−98% de). A wide range of 1-pyrroline esters 88 and aryl nitroalkenes 85 could be employed as substrates, including both electron-withdrawing and -donating substituted ones, and even heteroaryl- and ferrocenyl-substituted nitroalkenes. The p-nitrophenyl substituent was an exception, providing a low yield of 54% (vs 82−98%) of the corresponding conjugate product albeit with excellent enantioselectivity of 98% ee. To complete this study, the authors showed that promoting the reactions with CuOAc instead of AgOAc and in the presence of a chiral ferrocenyl oxazoline phosphine ligand allowed the reversed syn-diastereomers to be achieved in comparable very high enantioselectivities (94 → 99% ee) and high yields (69−86%).

Scheme 30. Michael Reaction of α,β-Unsaturated Esters and Arylboronic Acids

the catalyst could be successfully recovered and reused without significant loss of activity. In comparison to the previously developed heterogeneous catalyst 80 (Scheme 29), this catalyst system presented the advantages to provide higher enantioselectivities at very low catalyst loadings in metal and ligand. 4.2. Nitroalkenes as Acceptors

Nitroalkenes are also good Michael acceptors for the conjugate addition of glycine imino esters,111 leading to α-imino-γ-nitro ester products that can be transformed into biologically interesting α,γ-diamino acids through consecutive hydrolysis and reduction. It must be noted that relatively few asymmetric versions of these reactions have been reported. Among them, a P


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5. SILVER-CATALYZED 1,3-DIPOLAR CYCLOADDITIONS The 1,3-dipolar cycloaddition116 is a classic reaction in organic chemistry consisting of the reaction of a dipolarophile with a 1,3-dipolar compound that allows the production of various five-membered heterocycles.117−130 For example, azomethine ylides are versatile reactive 1,3-dipoles that undergo cycloaddition reactions with electron-deficient alkenes to afford functionalized pyrrolidines. A wide variety of chiral catalysts have been applied to promote asymmetric versions of this reaction, providing chiral proline derivatives, which are key chiral building blocks found in a number of natural products and pharmaceutically important compounds. Concerning enantioselective silver-catalyzed 1,3-dipolar cycloadditions, which generally afford endo-cycloadducts diastereoselectively, the first example was reported by Grigg et al., in 1995.21 It involved the reaction between ester-stabilized azomethine ylides with different activated olefins performed in the presence of stoichiometric amounts of AgOTf and a chiral prolinederived diphosphine ligand, providing the corresponding endocycloadducts in moderate to good yields (60−80%) and moderate enantioselectivities of up to 70% ee. It was later in 2002 that the first truly catalytic asymmetric reactions were reported by Zhang et al. using only 3 mol % of a combination of AgOAc with a chiral bis-ferrocenyl amide phosphine ligand.131 A complete endo diastereoselectivity and an excellent enantioselectivity of 97% ee combined with an 87% yield were obtained in the cycloaddition of a α-imino ester with dimethyl maleate. Inspired by this early work, several groups have developed other types of chiral ligands, such as aminophosphines, which in combination with AgOAc were successfully applied to promote 1,3-dipolar cycloadditions of α-imino esters with α,β-unsaturated esters with high enantioselectivities of up to 98% ee and yields of up to 98%.132−136 In 2007, good results (89% ee) were also achieved by Zhou et al. in comparable reactions catalyzed with chiral ferrocene-derived P,S ligands.137 The same year, Najera et al. investigated the formal 1,3-dipolar cycloadditions of azomethine ylides with other dipolarophiles, such as N-methylmaleimide, providing the corresponding bicyclic products with high endo diastereo- and enantioselectivities in reactions catalyzed by a combination of BINAP with AgClO4.138 Besides chiral phosphine ligands, in 2005 Jørgensen et al. introduced the use of cinchona alkaloids, such as hydrocinchonine, which were used in the presence of AgF in the 1,3-dipolar cycloaddition of azomethine ylides with acrylates to give the corresponding chiral cycloadducts in moderate enantioselectivities of up to 73% ee.139 In the period 2008−2014, different groups including those of Wang;140−148 Najera, Sansano, and Cossio;149−155 Kobayashi;156−159 Fukuzawa;160−163 Martin;164−167 Hu;168,169 Carretero and Adrio;170,171 Zhao;172 Gong;173 and Deng;174 among others,112,175−180 developed various other chiral silver catalysts based on bidentate ligands, such as biphosphines, aminophosphines, sulfur-containing phosphines, bisoxazolines, and diimines, but also monodentate ligands such as phosphoramidites, which were successfully applied to asymmetric 1,3-dipolar cycloadditions of azomethine ylides with a variety of activated olefins including malonates, maleimides, acrylates, nitroalkenes, and more complicated alkenes bearing electron-deficient groups. Since this period 2008−2014 has been recently covered by several reviews,121−130 it will not be detailed in this section, which will only focus on the period

Scheme 32. Michael Reaction of Nitroalkenes and 1Pyrroline Esters

4.3. Other Acceptors

Glycine imino methyl ester 59 could be added by the same authors to another type of Michael acceptors, arylidene diphosphonates 89.114 This process was previously successfully catalyzed by Wang et al. with chiral copper complexes.115 As shown in Scheme 33, the use of 5 mol % of a combination of Scheme 33. Michael Reaction of Arylidene Diphosphonates and a Glycine Schiff Base

AgOAc with ferrocenyl triazole-based P,S-ligand 63 in THF at −20 °C in the presence of 25 mol % of Cs2CO3 as a base yielded the corresponding chiral Michael products 90 as major syn-diastereomers in good to quantitative yields (64−99%) and uniformly high enantioselectivities of 90−98% ee. The syndiastereoselectivity of the reaction ranged from 52 to 62% de. The conditions were compatible with a series of 1-, 3-, and 4substituted benzylidene diphosphonates bearing electrondonating and electron-withdrawing substituents. Regardless of the electronic nature or position of the substituent, the corresponding syn-adducts were produced preferentially with high enantioselectivities. Substituent effects on the yields were also small, and products were obtained in high yields (70− 99%) except for the 1-methyl substrate, which provided the corresponding product in only 64% yield, probably due to steric hindrance. Moreover, the presence of heteroaryl substituents, such as a pyridylidene and 2-thienylidene, on the Michael acceptor allowed excellent results (96−98% ee) to be achieved. Q


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and ethanol at −20 °C, which allowed a range of chiral products 93 to be prepared in good to excellent yields of 80− 98%, moderate to almost complete exo-diastereoselectivities (60 → 96% de), and uniformly high enantioselectivities (90− 96% ee) regardless of the electronic nature substituents on the phenyl rings of the chalcones and aryl imino esters. The scope of this methodology could be extended to even more challenging and less reactive alkenes such as alkyl cinnamates. Indeed, by applying closely related conditions with the additional presence of K2CO3 as a base, the authors developed the first silver-catalyzed synthesis of cinnamate-derived chiral pyrrolidines 94 with exo-diastereoselectivity through the catalytic asymmetric cycloaddition of the corresponding methyl cinnamates 82 with glycine imino esters 92. These densely functionalized chiral products bearing four contiguous stereogenic centers were obtained in moderate to good yields (58− 86%), good to excellent exo-diastereoselectivities (80−98% de), and very high enantioselectivities (90−98% ee) (Scheme 34). As in the reaction of chalcones, the substrate scope of the two substrates was found wide, since a series of methyl cinnamates containing various groups on the phenyl ring (Ar2) as well as glycine imino esters derived from aromatic aldehydes bearing electron-deficient and electron-neutral substituents on the aryl rings reacted smoothly. Although the possibility of a concerted mechanism could not be excluded in the case of the base-free conditions (first reaction with chalcones), the authors have suggested that the cycloadditions evolved through a stepwise domino Michael/Mannich mechanism on the basis of experimental results (Scheme 34). In 2016, Oh et al. reinvestigated the formal 1,3-dipolar cycloaddition of chalcones with glycine imino esters in the presence of another type of chiral ligand combined with AgF at 10 mol % of catalyst loading in THF as solvent and tert-butanol as additive.182 When promoted by chiral multifunctional brucine diol 95 at −15 °C, the reaction of a range of aryl imino esters 92 with various chalcones 91 led to the exclusive formation of the corresponding endo-cycloadducts 96 in moderate to quantitative yields (50−99%) and low to excellent enantioselectivities (22−98% ee). Studying the substrate scope showed that the aryl substituent on the imino esters (Ar1) did not exert a significant effect on the observed enantioselectivities while the electronic nature of the chalcone’s β-aryl substituent (Ar3) detrimentally influenced the observed enantioselectivities of the formed pyrrolidines. For example, the lowest enantioselectivities of 22−55% ee were obtained for chalcones bearing a p-tolyl group, or an 1-, 3-, or 4-chloro-substituted phenyl group at the β-position. In this study, the authors have demonstrated that replacing AgF as metal source by CuOTf led to the exclusive formation of the enantiomer of the same endocycloadduct in good to quantitative yields (67−99%) and with high enantioselectivities (87−98% ee). The enantiodivergent outcomes in these reactions were assumed by the authors on the basis of the difference of ionic radii of the metals. While that of copper is estimated to be about 0.6 Å, the ionic radius of silver is around 1 Å, which resulted in the formation of 1:1 Cu:95 complex and 1:2 Ag:95 complexes. Likewise, the enantiodivergent cycloaddition of chalcones could be explained using the different molecularities of the catalysts (Scheme 35). Indeed, a smaller metal, such as Cu(I), formed a five-membered metallocycle with the tertiary amine moiety (N-19) and the tertiary alcohol (C-21) of the chiral ligand (L*), whereas a metal with relatively large ionic radii, such as Ag(I), complexed

2015−2016. Sections 5.1 and 5.2, dealing successively with asymmetric formal 1,3-dipolar cycloadditions of glycine imino esters with α,β-unsaturated carbonyl compounds and nitroalkenes as dipolarophiles, can be actually viewed as stepwise processes consisting of Michael additions followed by intramolecular Mannich (with α,β-unsaturated carbonyl compounds as dipolarophiles) or aza-Henzy (with nitroalkenes as dipolarophiles) reactions. 5.1. Formal 1,3-Dipolar Cycloadditions of Glycine Imino Esters and α,β-Unsaturated Carbonyl Compounds

In the past two years, novel chiral ligands as well as novel dipolarophiles have been investigated in asymmetric silvercatalyzed formal 1,3-dipolar cycloadditions. For example, Xia and Xu studied for the first time chalcones and alkyl cinnamates as dipolarophiles in reaction with glycine imino esters, in 2015.181 Among a series of chiral mono- and biphosphine ligands investigated in these reactions, aromatic amide-derived nonbiaryl atropisomer ligand Xing-Phos was selected as optimal to yield the corresponding chiral chalcone-derived pyrrolidines 93 bearing four contiguous stereogenic centers from chalcones 91 and glycine imino esters 92. As shown in Scheme 34, the process was performed with 5.5 mol % of this ligand combined with 2.5 mol % of AgF as precatalyst in a 1:1 mixture of THF Scheme 34. Formal 1,3-Dipolar Cycloadditions of Glycine Imino Esters and Chalcones/Methyl Cinnamates in the Presence of Xing-Phos Ligand

R


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Scheme 35. Formal 1,3-Dipolar Cycloaddition of Glycine Imino Esters and Chalcones in the Presence of a Multifunctional Brucine Diol Ligand

Scheme 36. Formal 1,3-Dipolar Cycloaddition of Glycine Imino Esters and Cyclopentene-1,3-diones in the Presence of a Ferrophos Ligand

with two ligands through the highly Lewis basic tertiary amine moiety (N-19). In 2015, Singh et al. reported an enantioselective silvercatalyzed desymmetrization of cyclopentene-1,3-diones 97 based on a formal 1,3-dipolar cycloaddition with glycine imino esters 98.183 Among a series of chiral ligands investigated, including (R)-BINAP, (R)-DTBM-Segphos, and various ferrophos ligands, ferrocenyl phosphine ligand 99 was found optimal to yield, at room temperature and at a low catalyst loading of 3 mol % in combination with 2 mol % of AgOAc, a series of chiral 5,5-fused bicyclopyrrolidines 100. These densely functionalized products bearing five contiguous stereogenic centers were produced in moderate to good yields (35−76%), moderate diastereoselectivities (50−60% de), and general high enantioselectivities (87−98% ee) (Scheme 36). Uniformly good results (58−76% yield, 50−66% de, and 87− 98% ee) were obtained from various azomethine ylides bearing

electron-donating as well as electron-withdrawing substituents on the aromatic group (R2). Even imino esters bearing a sterically bulky trisubstituted aromatic ring (R2 = 2,3-Me2-4MeOC6H2) and a naphthyl moiety smoothly produced the corresponding products in good yields (65 and 76%, respectively) and excellent enantioselectivities (91 and 98% ee), respectively. Azomethine ylides containing cinnamyl and furyl rings also underwent the reaction in good yields (69− 74%) and good to excellent enantioselectivities of 87% and 94% ee, respectively. Concerning the scope of cyclopentenediones, a wide range of electron-donating and electron-withdrawing substituents were well tolerated on the aromatic group R1, providing the corresponding products in good yields (35−68%) and excellent enantioselectivities of 94−96% ee. However, it was found that the presence of a 3-substituent on the aromatic ring of these substrates had a deleterious effect in comparison to a 4-substituent, which could be related to steric hindrance of the 3-substituent. In addition to substituted phenyl rings, the cyclopentenediones could bear a naphthyl (R1 = 2-Naph) and a vinyl group (R1 = vinyl), which gave the corresponding cycloadducts in moderate yields (68 and 63%, respectively), and excellent enantioselectivities of 95 and 94%, respectively, combined with a moderate diastereoselectivity of 50% de. The potential usefulness of this novel methodology was demonstrated by converting the bicyclic pyrrolidines into the corresponding enantiopure bicyclic pyrroles through oxidation with DDQ in toluene at room temperature without loss of enantioselectivity. Almost at the same time, these reactions were also developed by Wang et al. by using a (S)-TF-BiphamPhos-type ligand 101 in dichloromethane as solvent in the presence of a base such as triethylamine.184 Remarkably, the reaction of a wide range of cyclopentene-1,3-diones 97 with various glycine imino esters 102 afforded the corresponding chiral 5,5-fused bicyclic pyrrolidines 103 as almost single diastereomers (>90% de) in all cases and with both high yields (82−93%) and enantioselectivities (92 to >99% ee) (Scheme 37). Outstanding results (89−91% yield and 92−99% ee) were achieved in the reaction of non-α-substituted imino esters (R4 = H) bearing electron-deficient, electron-rich, and electron-neutral substituents on the phenyl ring (R3) with 2-benzyl-2-methylcyclopent4-ene-1,3-dione (R1 = Ph, R2 = Me). Additionally, imino esters S


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Scheme 37. Formal 1,3-Dipolar Cycloaddition of Glycine Imino Esters and Cyclopentene-1,3-diones in the Presence of a (S)-TF-BiphamPhos Ligand

Scheme 38. Three-Component Formal 1,3-Dipolar Cycloaddition of Ethyl Glyoxylate, Phenylalanine Ethyl Ester, and Maleimides in the Presence of (S)-BINAP

Later, Wang et al. developed a two-component version of related reactions promoted by 3 mol % of a catalyst generated in situ from AgOAc and (S)-TF-BiphamPhos-derived ligand 101 (Scheme 39).186 Actually, the reaction occurred between Scheme 39. Formal 1,3-Dipolar Cycloaddition of Glycine Imino Esters and N-(2-tert-Butylphenyl)maleimide in the Presence of an (S)-TF-BiphamPhos Ligand

bearing a cinnamyl group were also compatible, giving a high enantioselectivity (93% ee) with a lower yield (68%). It was worth noting that an alkyl imino ester (R3 = n-Bu) could also be applicable to the procedure, resulting in the desired product in 82% yield and 91% ee. Furthermore, several α-methyl/benzyl substituted imino esters (R4 = Me, Bn) remarkably led to the corresponding chiral bicyclic pyrrolidines bearing two quaternary stereogenic centers with both high yields (88−93%) and enantioselectivities (to >99% ee). In these latter cases of less reactive substrates, an inorganic base, such as Cs2CO3, was required as an additive and the reactions were performed at room temperature instead of −20 °C. Concerning the scope of the cyclopentenediones, it was found that the divergent substituents in the benzyl group did not display any significant effect on the catalytic activity and stereoselectivity, leading to products with 92−96% ee and 85−87% yield. High yield and excellent enantioselectivity were also obtained in the reaction of a naphthyl-substituted cyclopentenedione (84% yield and 94% ee). A cyclopentenedione combining methyl and allyl groups (R1 = vinyl, R2 = Me) and another one bearing an ethyl and a benzyl groups (R1 = Ph, R2 = Et) also provided excellent results (82−89% yield, 94−96% ee). On the other hand, Sansano et al. recently used (S)-BINAP as chiral ligand for Ag2CO3 at 5 and 2.5 mol % catalyst loadings in toluene at −10 °C to promote a three-component formal 1,3-dipolar cycloaddition between ethyl glyoxylate 104, phenylalanine ethyl ester 105, and variously N-substituted maleimides 106a−106f.185 As shown in Scheme 38, the imino esters generated in situ reacted with various N-protected maleimides to give the corresponding highly functionalized biologically important octahydropyrrolo[3,4-c]pyrrole derivatives 107a and 107c−107f as single endo-diastereomers in uniformly excellent yields of 94−98% and with good to high enantioselectivities (77−92% ee). The products bore four contiguous stereogenic centers. Even a nonprotected maleimide 106b (R = H) yielded the corresponding bicyclic product 107b in moderate yield of 65% albeit with a dramatically lower enantioselectivity (30% ee). A disadvantage of this process was related to the fact that it was limited to ethyl phenylalaninate as substrate.

glycine imino esters 102 and N-(2-tert-butylphenyl)maleimide 108 in dichloromethane at room temperature to yield the corresponding chiral bicyclic products 109 having four contiguous stereogenic centers in high yields (86−99%), having excellent enantioselectivities (90 to >99% ee), and as almost single diastereomers (>90% de). The study of the substrate scope of the α-nonsubstituted imino esters (R2 = H) showed that they could be substituted by aromatic, heteroaromatic, and aliphatic groups (R1), providing comparably excellent results (86−99% yield and 90 → 99% ee). Furthermore, more sterically hindered imino esters derived from α-substituted-α-amino acids (R2 ≠ H) were compatible. Indeed, alanine-derived imino esters (R2 = Me) were proven to be excellent substrates regardless of electronic properties of aromatic substitutes (R1), giving access to the corresponding endo-products in 99% yield and 98% ee. Notably, excellent results (97−98% yield and 97 → 99% ee) were also obtained with substrates with larger steric hindrance (R2 = Et, n-Bu, i-Bu, t-Bu, or Bn). To demonstrate the utility of this remarkable T


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methodology, the authors converted the products into 2Hpyrroles and polysubstituted pyrrole derivatives without loss of enantioselectivity, both of which constitute structurally important nitrogen-containing heterocycles. Closely related reactions were also investigated by Xia and Xu in the presence of Xing-Phos ligand as a chiral multifunctional ligand of AgF.187 In this case, the reaction was performed in toluene at −20 °C in the presence of a trace amount of water, which was shown to play an important role in the enhancement of the enantioselectivity. The reaction of a range of N-aryl-substituted maleimides 110 with various aromatic imino esters 92 led to the corresponding chiral pyrrolidines 111 having four contiguous stereogenic centers as single endo-diastereomers (>96% de) in good to excellent yields (83−99%) and moderate to excellent enantioselectivities of 65−98% ee, as shown in Scheme 40. The enantioselectivity of

Scheme 41. Formal 1,3-Dipolar Cycloaddition of Glycine Imino Esters and Nitroalkenes in the Presence of a Phosphoramidite Ligand

Scheme 40. Formal 1,3-Dipolar Cycloaddition of Glycine Imino Esters and Maleimides in the Presence of Xing-Phos Ligand tuted exo-4-nitroprolinates 112 bearing four contiguous stereogenic centers in moderate to high yields (33−92%), good to excellent enantioselectivities (84 to >98% ee), and variable diastereoselectivities of up to 86% de. These reactions were actually investigated in the presence of three types of precatalysts including AgOBz, AgOTf, and also Cu(OTf)2 for comparison. High chemical yields, exo-diastereoselectivities, and enantioselectivities were obtained in the reactions of aryl nitroalkenes performed with AgOBz as the precatalyst. On the other hand, AgOTf was recommended as the precatalyst for imines derived from aromatic aldehydes other than benzaldehyde (Ar ≠ Ph), while Cu(OTf)2 was more suitable for reactions involving α-substituted imino esters (R1 ≠ H). Concerning the use of AgOBz as precatalyst, the lowest diastereoselectivity (0% de) and yield (33%) were observed in the case of the reaction of a nitroalkene bearing an aliphatic substituent (R2 = cyclohexyl) which, however, yielded the corresponding product with an excellent enantioselectivity of 96% ee. On the other hand, the best result was obtained in the reaction of 4-tolyl-substituted nitroalkene, which afforded via reaction with a phenyl imino ester (Ar = Ph, R1 = H) the corresponding product to be produced in 92% yield, 86% de, and 98% ee. In general, in comparison with chiral copper catalysts, the silver complexes were more versatile and less sensitive to sterically congested substrates. Xing-Phos ligand was recently applied to this type of reactions by Xia and Xu.189 As shown in Scheme 42, in the presence of 5.5 mol % of this ligand combined with 2.5 mol % of AgF as precatalyst in THF at −20 °C, a series of aromatic nitroalkenes 85 (R3 = aryl) were found to react with aryl as well as alkyl imino esters 92 to give the corresponding highly substituted chiral 4-nitroprolinates 113 in uniformly high yields (82−99%) and enantioselectivities (88−99% ee) along with moderate to complete exo-diastereoselectivities of 68 to >98% de. The lowest enantioselectivity (77% ee) was obtained in the reaction of an alkyl nitroalkene (R3 = CH2Bn). In addition, an endo-selective version of these processes was described by Fukuzawa et al., in 2016.190 It involved the use of 5 mol % of AgOAc combined with the same quantity of ThioClickFerrophos ligand 63 in 1,4-dioxane at room temperature. The reaction of various aryl and heteroaryl nitroalkenes 85 with a range of (hetero)aryl imino esters 92 afforded the

the reaction was found to be sensitive to the electronic nature of substituents borne by the aryl rings (Ar1) of the imino esters. For example, 4-methylphenyl-substituted imino methyl ester afforded the corresponding product in only 65% ee, while the corresponding 4-halogen-substituted-phenyl imino methyl esters provided high enantioselectivities (90−91% ee). 5.2. Formal 1,3-Dipolar Cycloadditions of Glycine Imino Esters and Nitroalkenes

Nitroalkenes are known to be good Michael acceptors for the conjugate addition of glycine imino esters.111 In particular, the latter substrates have been employed as azomethine ylide precursors in enantioselective formal 1,3-dipolar cycloadditions with nitroalkenes evolving through domino Michael/aza-Henry reactions. A number of these reactions have been catalyzed by various types of chiral metal complexes and organocatalysts for the construction of chiral prolinates mainly obtained with the exo-configuration under the control of chiral metal catalysts. In 2015, Najera and Sansano reported one example of these reactions catalyzed by 5 mol % of a combination of chiral phosphoramidite ligand 64 with a silver precatalyst, such as AgOBz or AgOTf, in the presence of 5 mol % of triethylamine as base in toluene at room temperature.188 As shown in Scheme 41, the reaction of a series of aromatic glycine imino esters 102 with nitroalkenes 85 afforded the corresponding polysubstiU


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Scheme 42. Formal 1,3-Dipolar Cycloaddition of Glycine Imino Esters and Nitroalkenes in the Presence of Xing-Phos Ligand

Scheme 44. Formal 1,3-Dipolar Cycloaddition of Isocyanoacetates and α,β-Unsaturated Ketones

corresponding chiral pyrrolidines 113 as endo-cycloadducts in moderate to good yields of 51−86%, good to high diastereoselectivities (82−92% de), and enantioselectivities of 86−97% ee (Scheme 43). In the case of the reaction of an alkyl Scheme 43. Formal 1,3-Dipolar Cycloaddition of Glycine Imino Esters and Nitroalkenes in the Presence of a ThioClickFerrophos Ligand was responsible for the dual activation through hydrogen bonding interactions, as shown in Scheme 44. Shi et al. have reported the first example of a cinchona alkaloid derived squaramide/AgSbF6 cooperative catalytic system for the highly diastereo- and enantioselective formal 1,3-dipolar cycloadditions of isocyanoacetates 115 (R2 = H) and 114 (R2 = aryl, alkyl) with maleimides 118.192 As shown in Scheme 45, a range of chiral 1,3a,4,5,6,6a-hexahydropyrrolo[3,4-c]pyrrole derivatives 119 were prepared in moderate to high yields (50−98%), and generally excellent diastereoselectivities of >90% de, along with moderate to high enantioselectivities of up to 92% ee through cooperative catalysis with AgSbF6 and cinchona alkaloid derived squaramide 120. The highest enantioselectivities of 74−90% ee were achieved in the reaction of N-aryl maleimides with α-aryl isocyanoesters, while only 10−65% ee were obtained in the reaction of α-alkyl isocyanoesters or N-alkyl maleimides albeit combined with excellent diastereoselectivity (>90% de) and good yields (67− 85%). To explain the stereoselectivity of the domino Michael/ cyclization reaction, the authors have proposed the transitionstate model depicted in Scheme 45, in which one carbonyl group of the maleimide was hydrogen-bonded to the squaramide motif, while the α-proton of the isocyanoacetate was easily deprotonated by the quinuclidine nitrogen of the cinchona catalyst due to the activation of Ag(I) chelating to the terminal carbon of the isocyano group. A single hydrogen bond was then formed between the OH group of the enolized isocyanoacetate and the tertiary amine of the cinchona alkaloid. A weak hydrogen bond between the OR1 group of the enolized isocyanoacetate and the NH group in the squaramide moiety, as well as an interaction between Ag(I) and the other carbonyl group of the maleimide, could be formed concurrently, thus

imino ester (R = Cy), both lower yield and diastereoselectivity were obtained (47% yield and 76% de) albeit with high enantioselectivity (93% ee). 5.3. Formal 1,3-Dipolar Cycloadditions of Isocyanoacetates and α,β-Unsaturated Carbonyl Compounds

Escolano et al. have disclosed enantioselective formal 1,3dipolar cycloadditions of isocyanoacetates with α,β-unsaturated ketones evolving through domino Michael/cyclization reactions.191 The reaction of isocyanoacetates 114 (R2 = Bn) and 115 (R2 = H) with α,β-unsaturated ketones 79 performed in the presence of a combination of a chiral bifunctional cinchona alkaloid, such as cupreine 116, and AgNO3 provided the corresponding chiral functionalized 2,3-dihydropyrroles 117 in low to high yields (20−85%) and enantioselectivities of 16− 89% ee, as shown in Scheme 44. In this process, the two catalysts interacted cooperatively. AgNO3 increased the acidity of the pronucleophile, and the bifunctional cupreine catalyst V


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Scheme 45. Formal 1,3-Dipolar Cycloaddition of Isocyanoacetates and Maleimides

Scheme 46. Formal 1,3-Dipolar Cycloaddition of Isocyanoacetates and 2-(2-Aminophenyl)acrylates

forcing the isocyanoacetate enolate to attack the maleimide from the Re-face, thereby leading to the formation of two newly generated stereocenters with the (R,R)-configuration. Subsequently, a 5-endo-dig cyclization took place assisted by electrophilic silver isocyanide activation. The third stereocenter was formed as S-configuration after the cyclization step. In 2016, isocyanoacetates were also used as substrates by Xie et al. in enantioselective formal 1,3-dipolar cycloadditions with 2-(2-aminophenyl)acrylates.193 The reaction was cooperatively promoted by a combination of 5 mol % of AgNO3 and 6 mol % of chiral cinchona alkaloid 121 in acetonitrile at room temperature. It proceeded through the sequential Michael addition of metalated isocyanoacetates 115a−115d to acrylates 122a−122l, the intramolecular nucleophilic addition of the thus-formed enolates to the isocyano group, and the intramolecular attack of the nucleophilic amido to the thusproduced 2H-pyrrolidine intermediates. As shown in Scheme 46, a range of densely functionalized chiral cis-3a,8ahexahydropyrrolo[2,3-b]indoles 123a−123l could be achieved through this methodology in good to quantitative yields (73− 99%), and variable diastereo- and enantioselectivities of up to >90% and 90% ee, respectively. It must be noted that this core scaffold is present in a large array of biologically important natural products. The substrate scope of the process was found to be large since a wide range of functional groups, such as substituents on the phenyls and alkenes, various ester groups,

on both the acrylates and isocyanoacetates, and various amino protecting groups in acrylates, could be tolerated in the domino reaction, producing the tricyclic chiral products bearing up to four contiguous stereogenic centers in good yields albeit with generally low diastereoselectivities except for β-substituted 2(2-amidophenyl)-acrylates, which afforded the corresponding products as single diastereomers (>19:1 dr) in good enantioselectivities of 71−82% ee (products 123d and 123e). In 2015, Zhao et al. investigated for the first time the cyclization of allenoates with isocyanoacetates.194 These reactions were performed in the presence of a chiral silver catalyst generated in situ from Ag2O and cinchona alkaloid based phosphine ligand 124 in chloroform at −20 °C. When an unsubstituted isocyanide, such as methyl isocyanoacetate 115a, was reacted with various allenoates 125 (Scheme 47), it regioselectively afforded the corresponding chiral 3H-pyrroles 126 in good to high yields of 73−92% and enantioselectivities of 80−96% ee. Actually, these products arose from a [3 + 2] W


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Scheme 47. Formal 1,3-Dipolar Cycloadditions of Isocyanoacetates and Allenoates

Scheme 48. Inverse-Electron-Demand 1,3-Dipolar Cycloaddition of Isoquinolinium/Phthalazinium Methylides and Enecarbamates

carboxyl group (R1 = C6F5CH2), while the lowest one (77% ee) was obtained with an enecarbamate bearing a tert-butyl carboxyl group (R1 = t-Bu). Generally, enecarbamates with an electrondeficient substituent on the benzyl group (R1) provided the corresponding products in higher enantioselectivities than those with an electron-donating substituent. Moreover, the scope of the azomethine ylides was investigated by reacting optimal enecarbamate bearing a pentafluorobenzyl carboxyl group. Isoquinolinium methylides with electron-deficient substituents (R2), such as Cl or Br, at 4- or 5-positions were well tolerated to give the corresponding products in 80−99% yields and 95% ee. An isoquinolinium methylide with electronrich OMe group at the 5-position was also a suitable substrate, delivering the corresponding product in 82% yield and 91% ee. However, when the OMe substituent was situated at the 6position of the substrate, the corresponding product was achieved in much lower yield (33%) but with still high de and ee (>90% de and 91% ee). It is worth noting that a phthalazinium dicyanomethylide (X = N, R2 = Y = H) was also a suitable dipole, leading to the desired pyrrolophthalazine 131 in 91% yield, >90% de, and 95% ee.

cyclization followed by a 1,3-H shift (Scheme 47). The scope of this simple novel procedure was found broad since a wide variety of methyl allenoates bearing different alkyl groups were compatible, providing uniformly good results. To further extend the scope of this catalytic system, the reaction of allenoates 125 with substituted isocyanoacetates 114 was investigated under the same conditions. In this case, the direct [3 + 2] cyclization products 127 possessing an exocyclic olefin were obtained in moderate to high yields and diastereoselectivities of 58−90% and 72 to >90% de, respectively, in combination with high enantioselectivities of 82−96% ee (Scheme 47). A range of chiral heterocycles could be achieved from the reactions of benzyl- and methyl-substituted isocyanoacetates with variously alkyl-substituted ethyl and methyl allenoates.

6. SILVER-CATALYZED ALKYNYLATIONS Because of their versatile reactivities, alkynes and derivatives196,197 constitute key building blocks in organic chemistry.32,198−202 The nucleophilic addition of terminal alkynes to imines constitutes an alternative pathway for the production of propargylamines. In particular, chiral propargylamines are important building blocks for the synthesis of natural products, pharmaceuticals, and pesticides.203−205 While the asymmetric alkynylation of imines represents the most direct route to reach these important products, efficient methodologies still remain rare. Among them, Chen and Shi have developed a general enantioselective silver-catalyzed addition of aliphatic as well as alkynes 132 to a range of N-aryl aromatic aldimines 40 performed at room temperature in chlorobenzene in the presence of only 0.5 mol % of (R)-BINAP combined with 1 mol % of AgOTf. 206 As shown in Scheme 49, the corresponding chiral propargylamines 133 were obtained in uniformly high yields (83−95%) albeit with low to moderate

5.4. Other 1,3-Dipolar Cycloadditions

In 2016, the first catalytic asymmetric inverse-electron-demand 1,3-dipolar cycloaddition of isoquinolinium methylides 128 with enecarbamates 129 was reported by Feng et al. using chiral silver catalysis.195 This process was promoted by a combination of AgBF4 as precatalyst with chiral N,N′-dioxide ligand 130 in THF at 0 °C. It led to optically active pyrroloisoquinolines 131 (X = CH) as almost single diastereomers (>90% de) in moderate to quantitative yields (33−99%), and with moderate to high enantioselectivities of 77−95% ee, as shown in Scheme 48. The best enantioselectivities of 91−95% ee were obtained in the reaction of an enecarbamate with a pentafluorobenzyl X


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Scheme 49. Addition of Alkynes to N-Aryl Aromatic Aldimines

corresponding chiral oxazepine derivatives 136 in moderate to excellent yields of 44−96% and good to excellent enantioselectivities of 78−99% ee. A broad scope of alkynes including aryl acetylenes, heteroaryl acetylenes, and enynes were compatible, providing the highest enantioselectivities (generally 90−99% ee). Various substituents on the imine were also tolerated. The scope of the precedent methodology could also be extended to other conjugated alkynes, such as 1,3-diynes, as shown in Scheme 51.207 Indeed, terminal 1,3-diyne 137 reacted Scheme 51. Addition of a 1,3-Diyne to Seven-Membered Cyclic Imine Dibenzo[b,f ][1,4]Oxazepines

enantioselectivities of 23−76% ee. The lowest enantioselectivities of 23−34% ee were obtained in the reaction of aniline derivatives bearing electron-withdrawing or electron-donating groups at the para position (Ar2 = 4-MeO2CC6H4, 4-O2NC6H4, or 4-MeOC6H4), whereas higher enantioselectivities (37−76% ee) were obtained in the reaction of unsubstituted aniline derivatives (Ar2 = Ph). Impressively, the reaction tolerated a variety of terminal alkynes. Both electron-donating and electron-withdrawing-substituted phenylalkynes gave high yields (92−94%) combined with moderate enantioselectivities (43−56% ee), while 1-hexyne led to the corresponding product in 84% yield and 65% ee. On the other hand, internal alkynes were found not suitable for this transformation, presumably due to their lower reactivity compared with terminal alkynes. Interestingly in this process, the authors discovered that the ligand-to-silver precatalyst ratio played a crucial role in the reaction outcome, since no reaction occurred in the absence of ligand or in the presence of an excess of ligand. The extension of the silver-catalyzed asymmetric imine alkynylation to other challenging and unexplored imines, such as seven-membered cyclic imines, was achieved by Liu et al., in 2014.207 As shown in Scheme 50, the reaction of sevenmembered dibenzo[b,f ][1,4]oxazepines 134 with various alkynes 132 performed at 15 °C in 1,4-dioxane as solvent and promoted by a combination of 5 mol % of AgOAc and 10 mol % of chiral phosphoric acid 135 as ligand, yielded the

with differently substituted seven-membered dibenzo[b,f ][1,4]oxazepines 134 under the same reaction conditions to give the corresponding chiral densely functionalized oxazepine derivatives 138 in both moderate to high yields of 53−90% and enantioselectivities of 63−96% ee. This efficient process provided a facile access to optically active 11-substituted10,11-dihydrodibenzo[b,f ][1,4]oxazepine derivatives containing two carbon−carbon triple bonds, allowing easy subsequent transformations to be achieved. In 2011, Jarvo et al. developed a silver-catalyzed enantioselective propargylation reaction of aldimines 61 with allenyl boronic acid pinacol ester 139, generating the corresponding homopropargylic sulfonamide products 140 in moderate to quantitative yields of 40−99% and good to excellent enantioselectivities of 74−98% ee.208 As shown in Scheme 52, the reaction was promoted by a chiral silver catalyst in situ generated from 10 mol % of AgF and 12 mol % of chiral Walphos-type biphosphine (R,S)-141 at −20 °C. Concerning (hetero)aromatic imines that provided the best results, the presence of either electron-donating or electron-withdrawing groups on the aryl group (R) was tolerated, giving uniformly high enantioselectivities of 90−98% ee in combination with 70−99% yields. On the other hand, vinylic and aliphatic aldimines led to the corresponding products in lower yields and enantioselectivities of 40−47% and 74−89% ee, respectively (Scheme 52). To demonstrate the synthetic utility of this methodology, the authors further performed the silvercatalyzed intramolecular hydroamination of product 140a (R = Ph), which underwent 5-endo-dig cyclization resulting in an anti-Markovnikov adduct. Importantly, the cyclization reaction did not affect the newly formed stereogenic center, and provided an enantiomerically enriched 2-pyrroline in 99% yield and 97% ee, constituting a useful building block in the synthesis of bioactive pyrrolidines and alkaloids.

Scheme 50. Addition of Alkynes to Seven-Membered Cyclic Imine Dibenzo[b,f ][1,4]Oxazepines

Y


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62−89% and uniformly very high enantioselectivities of 90− 98% ee. Remarkably, alkylketimines gave results (72−88% yield, 96−98% ee) comparable to variously substituted diarylketimines (62−89% yield, 90−98% ee). To emphasize the utility of the pendant terminal alkyne of the formed products, the authors synthesized spirocyclic, alkenyl, and allyl derivatives through enyne ring-closing metathesis, Sonogashira cross-coupling, and reduction reactions that proceeded without loss of enantioselectivity. The scope of this methodology was extended to less reactive cyclic sulfamate ketimines 144a and 144b, which constitute a related class of N-sulfonylketimines that react with nucleophiles to provide sulfamidates. As shown in Scheme 53, the addition of allenyl boronic acid pinacol ester 139 to these substrates afforded the corresponding homopropargylic sulfamidates 145a and 145b in moderate to good yields (51−76%) and excellent enantioselectivity (98% ee). In addition to imines, allenyl boronic acid pinacol ester 139 has been added to ketones and α-ketoesters 146 in the presence of a chiral silver catalyst derived from AgF and chiral Walphos-type biphosphine 147.210 As shown in Scheme 54, the

Scheme 52. Addition of Allenyl Boronic Acid Pinacol Ester to Aldimines

Scheme 54. Additions of Allenyl Boronic Acid Pinacol Ester to Ketones and α-Ketoesters

Later in 2015, the same authors applied a related catalyst system using diastereomeric ligand (R,R)-141 albeit combined with AgPF6 as precatalyst instead of AgF to promote the first silver-catalyzed enantioselective propargylation of cyclic Nsulfonylketimines.209 As shown in Scheme 53, the reaction of allenyl boronic acid pinacol ester 139 with many aryl as well as alkyl cyclic N-sulfonylketimines 142 led to the corresponding homopropargylic products 143 in moderate to high yields of Scheme 53. Additions of Allenyl Boronic Acid Pinacol Ester to Cyclic N-Sulfonylketimines

reaction of methyl (hetero)aryl ketones (R = Me) led to the corresponding chiral tertiary alcohols 148 in moderate to high yields (48−95%) and enantioselectivities (71−90% ee), while α-ketoesters (R = CO2-t-Bu) provided the corresponding products 148 in 73−82% yields and slightly better enantioselectivities (86−91% ee). Remarkably, even a dialkyl ketone, such as methyl homobenzyl ketone, led to the corresponding alkynylated product in 60% yield and 71% ee. Z


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sterically bulky substituents on the phenyl ring required longer reaction times.

More interestingly, the reaction conditions were applicable to benzophenones 149a−149e, which allowed their enantioselective propargylation to be reported for the first time. As shown in Scheme 54, the ability of the silver catalyst to distinguish between the two aromatic rings of benzophenones 149a−149e led to enantioenriched diaryl carbinols 150a−150e, which are present in a variety of drugs and medicinal agents. These products were generated in moderate to excellent yields (52− 95%) combined with high to excellent enantioselectivities (80− 97% ee). Various substituents were tolerated in the orthoposition including methyl group and halogens. In another context, Fan et al. have developed the first highly efficient asymmetric ring-opening of oxabenzonorbornadienes 151 with terminal alkynes 132, which was catalyzed by 5 mol % of a combination of AgOTf and Pd(OAc)2 in the presence of chiral Phanephos-derived ligand 152.211 As shown in Scheme 55, the reaction performed at 0 °C in DME as solvent led to the

7. SILVER-CATALYZED ALLYLATIONS Asymmetric allylation of carbonyl compounds constitutes a useful method to synthesize chiral homoallylic alcohols, which can be further easily converted into key β-hydroxy carbonyl compounds and derivatives.212−216 Early in 1996, Yamamoto and Yanagisawa introduced a combination of AgOTf with (R)BINAP as a novel catalytic system, which was demonstrated to promote highly enantioselective silver-catalyzed allylation of aldehydes with allylic stannanes, providing the corresponding chiral homoallylic alcohols in excellent enantioselectivities of up to 96% ee and high yields of up to 88%.22 More recently, Studer and Umeda investigated the asymmetric silver-catalyzed cyclohexadienyl transfer from 1,4-cyclohexadienyltributyltin 154 to various aromatic aldehydes 5a−5o using a combination of AgOTf and (S)-BINAP as catalyst system.217 As shown in Scheme 56, the reaction afforded the corresponding chiral 1,4-

Scheme 55. Ag/Pd-Catalyzed Ring-Opening Reaction of Oxabenzonorbornadienes with Alkynes

Scheme 56. Allylation of Aromatic Aldehydes with 1,4Cyclohexadienyltributyltin

corresponding chiral ring-opened products 153 in moderate to excellent yields of 54−95% combined with uniformly excellent enantioselectivities of 90−99% ee. This ligand was selected as optimal among a range of other ones including (R)-BINAP and derivatives, (R)-Synphos, (R)-MeO-Biphep, and (R)-Phanephos. Various substituted aryl acetylenes were proved applicable, providing the corresponding alkynylated products in high enantioselectivities of 90−99% ee. However, some adverse effects of the electronic or positional properties of the substituents on the reactions were observed. For example, strong electron-donating groups, such as methoxy on the 4- or 1-position of the phenyl ring (R3), were found to reduce the enantioselectivities of the reactions to 90% ee. On the other hand, the presence of electron-withdrawing substituents on the para-position of the phenyl ring of the alkyne allowed the best results to be achieved (87−95% yields, 95−99% ee). Notably, trimethylacetylene could also serve as a proper carbon nucleophile (R3 = TMS), and produced the corresponding product in 74% yield along with a high enantioselectivity of 97% ee. In contrast to aryl acetylenes, alkyl acetylenes were not tolerated, since no reaction occurred even with extended reaction times. Concerning the scope of the oxabenzonorbornadienes, those having electron-withdrawing groups and

cyclohexadienylphenylmethanols 155a−155o in good to quantitative yields of 80−99% and moderate to high enantioselectivities of 56−90% ee (determined for the corresponding oxidized products 156a−156o). Products 155a−155o were further submitted to oxidation by treatment with DDQ to give the corresponding biologically interesting arylphenylmethanols 156a−156o. Studying the scope of aromatic aldehydes, the authors obtained the best result (99% yield and 92% ee) in the reaction of 4-chlorobenzaldehyde. Another nice combination of high yield and enantioselectivity of 91% and 90% ee, respectively, was achieved in the reaction of 2-naphthaldehyde. Although reactions involving allylic stannanes as nucleophiles yielded the desired products in high enantioselectivities, they AA


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Scheme 58. Allylation of Phenyl N-Aryl Aldimines with Allyltrimethoxysilane

had the disadvantage tof being environmentally less benign organotin compounds. In 1999, to overcome this problem, Yamamoto et al. applied their catalyst system based on a combination of Tol-BINAP with AgF to promote enantioselective Sakurai-Hosomi-type allylation of aldehydes with allyltrimethoxysilane as allyl donor, which allowed the corresponding homoallylic alcohols to be obtained in enantioselectivities of up to 94% ee in good yields (70− 90%).218 More recently, Dudding and Mirabdolbaghi reinvestigated these reactions by using a combination of AgF with (R)-BINAP in methanol at −20 °C.219 As shown in Scheme 57, Scheme 57. Allylation of Aromatic Aldehydes with Allyltrimethoxysilane

pinacol allylboronate 164 reacted with a range of Csp3 intermediates, such as N,O-aminals 165a−165m, to give the corresponding homoallylamides 166a−166m in uniformly high yields of 88−99%. The process was performed in the presence of a preformed silver BINOL-derived phosphate 167 and indium(I) chloride at room temperature. It must be noted that, in the absence of the latter, the silver catalyst displayed only low reactivity and asymmetric induction. The reaction conditions were applied to a series of aromatic and heteroaromatic N,Oaminals that provided the corresponding homoallylamides 166a−166k in both excellent yields and enantioselectivities of 94−99% and 90−96% ee, respectively, while slightly lower yields and enantioselectivities (88−96% and 72−96% ee) were obtained in the case of aliphatic N,O-aminals 165l and 165m. The authors have proposed that the reaction evolved through an SN1 mechanism with iminium ion species F as a key intermediate, which supported the critical role of the chiral counteranion (Scheme 59). This study has demonstrated that boronates were dramatically more reactive and selective than classic silicon-based reagents, constituting the first highly enantioselective Hosomi−Sakurai reactions with Csp3 centers. Actually, in this study, the role of the silver chiral complex is to be a chiral anion donor; however, it was decided to maintain these results in the review. In 2014, Rios et al. developed a novel highly diastereoselective synthesis of highly functionalized alkyl-azaarene systems.223 The methodology included a synergistic catalysis event, involving activation of an alkyl azaarene with AgOAc, and Lewis base (DABCO) activation of a Morita−Baylis− Hillman carbonate. It led to the corresponding racemic allylated alkyl azaarenes with good yields and high diastereoselectivities (up to 88% de). Initial experiments have been done in order to develop an enantioselective version of this process. For example, the use of chiral ligands, such as (R)-BINAP or a cinchona alkaloid ligand, allowed the allylated products to be produced in moderate to high yields (54−90%) and high diastereoselectivity (>88% de), but with low to moderate enantioselectivities (30−50% ee).

the reaction of allyltrimethoxysilane 157 with various 1substituted benzaldehydes 158 led to the corresponding chiral homoallylic alcohols 159 in moderate to high yields (64−91%). The latter were subsequently submitted to a Mizoroki−Heck reaction to afford the corresponding C1-chiral 3-methyleneindan-1-ols 160 in good yields (70−76%) and moderate to good enantioselectivities (58−80% ee). It must be noted that the sterically demanding 1-trifluoromethylsulfonylated benzaldehyde did not react. This novel sequence constituted a new entry to chiral indanol derivatives which are key substructures within a number of biologically active compounds. Early in 2005, Yamamoto and Wadamoto reported asymmetric allylation reactions of ketones with allyltrimethoxysilane performed by using (R)-Difluorphos as chiral ligand in combination with AgF, providing the corresponding tertiary homoallylic alcohols with enantioselectivities of up to 96% ee.220 Encouraged by these results, the same authors later investigated the asymmetric allylation of aldimines 161a−161c with allyltrimethoxysilane 157, which is still challenging.221 In this case, the use of monophosphine ligands combined with AgF provided generally better results than diphosphine ligands, such as (R)-Difluorphos, in the synthesis of the corresponding chiral homoallylamines 162a−162c, except for imine 161c bearing an 4-PhOC6H4 group which formed in 82% ee (vs 60% ee). As shown in Scheme 58, when the reaction of various phenyl N-aryl aldimines 161a−161c and allyltrimethoxysilane 157 was catalyzed by 5 mol % of a combination of AgF with monophosphine ligand 163, it yielded the corresponding chiral homoallylamines 162a−162c in quantitative yields and moderate enantioselectivities of 60−77% ee. In 2011, a catalytic asymmetric borono variant of Hosomi− Sakurai reactions was reported by Kobayashi et al.222 Although nontoxic allyl boronates have been neglected as allylation agents, they present significant advantages in comparison with more nucleophilic silicon-based compounds, such as superior stability and unique reactivity and selectivity. For example,

8. SILVER-CATALYZED CYCLIZATIONS OF ALLENES The addition of heteroatoms onto an activated CC bond represents a useful method to gain access to a variety of AB


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Scheme 59. Allylation of N,O-Aminals with Pinacol Allylboronate

Scheme 60. Intramolecular Cyclizations of γ-Allenic Alcohols and Amines

functionalized molecules in an atom efficient manner. In particular, the intramolecular asymmetric transition-metalcatalyzed nucleophile addition to allenols and amino allenes constitutes a powerful tool for the synthesis of various chiral heterocycles.32 While several metals are capable of promoting these reactions, silver remains one of the most effective catalysts. In 2009, Hii et al. demonstrated that silver-catalyzed reactions of δ-allenols regioselectively afforded the corresponding vinyl-substituted tetrahydrofurans through 5-exo-trig cyclization, while the use of other Lewis acids, such as Sn(OTf)2 or Zn(OTf)2, favored the 6-exo-dig cyclization to give tetrahydropyran rings.224 Later, in 2012, an asymmetric version of this reaction was described for the first time by this group by using silver salts of chiral acids. As shown in Scheme 60, moderate enantioselectivities of up to 73% ee combined with almost quantitative yields were achieved in the intramolecular cyclization of γ-allenic alcohols 168a and 168b. This reaction was run at room temperature, yielding the corresponding chiral tetrahydrofurans 169a and 169b when it was promoted by chiral preformed TADDOL-derived phosphate silver catalyst 170.225 In the same study, the authors also reported the first asymmetric silver-catalyzed intramolecular hydroamination of γ-allenic amines 171a−171f to yield the corresponding chiral N-protected pyrrolidines 172a−172f. In this case, preformed phosphinate catalyst 173 was found much more active than phosphate catalyst 170, allowing conversions of 84−100% to be achieved in combination with moderate enantioselectivities of 46−68% ee, as shown in Scheme 60. Pyridine was used as an additive which resulted in an accelerative effect on the reaction (24 h vs 60 h). Moreover, the catalyst system was applicable to

different N-protecting groups, including sulfonamides, carbamates, and benzyl groups. In 2012, the first enantioselective intramolecular silvercatalyzed reaction of α-allenols 174 was developed by Hong et al.226 This process was promoted by a chiral preformed BINOL-derived phosphate silver catalyst in dichloromethane at −10 °C, leading to the corresponding chiral 2,5-dihydrofurans 175 in moderate to high conversions and enantioselectivities (44−68% and 41−93% ee). It evolved through kinetic resolution, providing the recovered enantioenriched substrates 174 with moderate to excellent enantioselectivities (53−99% ee) along with chiral cyclic products 175. The optimal chiral catalyst 176 was selected among a range of other silver complexes generated in situ from various types of ligands, such as BINAP, bisoxazolines, salen-type diimines, and monodentate phosphoramidites, combined with silver salts, such as AgNO3 and AgOTf, all of which provided low levels of enantioselectivity. The substrate scope showed that aryl-substituted αallenic alcohols could bear both electron-deficient and electronrich substituents, yielding the corresponding dihydrofurans with uniformly high levels of induction (82−90% ee), while the enantioenriched substrates were recovered in moderate to high enantioselectivities of 53−99% ee along with conversions of 44−56% (Scheme 61). Alkyl- and alkenyl-substituted α-allenic alcohols also underwent the reaction with generally a higher reaction rate (48−68% conversions), leading to the corresponding products in moderate to high enantioselectivities of 41−93% ee, while the recovered substrates were obtained in excellent enantioselectivities of 77−99% ee. It must be noted that, earlier in 2010, Mikami et al. developed intramolecular asymmetric hydroalkoxylation of γallenic alcohols to give the corresponding chiral dihydrofuran derivatives in the presence of a combination of a neutral dinuclear gold complex 177 with a chiral silver phosphate catalyst 178.227 As shown in Scheme 62, in the presence of only AC


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Scheme 61. Intramolecular Hydroalkoxylation of α-Allenic Alcohols through Kinetic Resolution

9. SILVER-CATALYZED AMINATIONS The electrophilic amination reaction constitutes a direct method to stereoselectively form C−N bonds, a fundamental process in both organic chemistry and biochemistry. Consequently, much progress has been made in the enantioselective α-amination of carbonyl compounds, such as aldehydes, ketones, α-ketoesters, α-cyanoesters, and other compounds, using azodicarboxylates as the nitrogen source since the pioneering work reported by Evans in 1997.228 In particular, the asymmetric α-amination of carbonyl compounds is an efficient route to important chiral α-amino acid derivatives.229,230 Early in 2000, Kobayashi et al. reported that AgClO4 combined with BINAP as chiral ligand facilitated the asymmetric amination of silyl enolates with azo diester compounds with enantioselectivities of up to 86% ee.231 More recently, Zhou et al. developed an efficient enantioselective silver-catalyzed α-amination of glycine Schiff bases 181a−181d with azodicarboxylates 182a−182d to give the corresponding chiral α,α-diamino carbonyl compounds 183a− 183g.232 When promoted by a combination of 3 mol % of AgOAc and 3.3 mol % of Taniaphos-type ligand 184 in toluene at −25 °C, the process afforded these products in excellent yields of 93−98% associated with moderate to excellent enantioselectivities of 75−98% ee (Scheme 63). The steric

Scheme 62. Intramolecular Hydroalkoxylation of γ-Allenic Alcohols through Au/Ag Catalysis

Scheme 63. α-Amination of Glycine Schiff Bases with Azodicarboxylates

2.5 mol % of this catalyst system, the reaction of a range of γallenic alcohols 179a−179i led to the corresponding chiral tetrahydrofurans 180a−180i in good to excellent yields (75− 98%) along with moderate to high enantioselectivities (70− 95% ee). Actually, in this study, the role of the silver chiral complex was proposed by the authors to be that of a chiral anion donor (Scheme 62); however, it was decided to present these results in the review. AD


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Scheme 64. α-Aminations of Alkenyl Trifluoroacetates with Azodicarboxylates

properties of both substrates were found crucial for the enantioselectivity of the reaction. For example, with increasing steric hindrance of the substituents in the glycine Schiff bases, the stereoselectivity decreased dramatically from 98% ee (for R2 = Me) to 75% ee (for R2 = t-Bu) in reaction with di-tert-butyl azodicarboxylate (R1 = t-Bu). In contrast, an improvement of enantioselectivities from 75% ee (for R1 = Et) to 98% ee (for R1 = t-Bu) was observed through increasing the steric hindrance of the substituents in azodicarboxylates. Actually, the best enantioselectivity of 98% ee was reached in the asymmetric amination reaction of di-tert-butyl azodicarboxylate (R1 = t-Bu) with benzophenone imine glycine methyl or ethyl ester (R2 = Me or Et). To explain their results, the authors proposed the mechanism depicted in Scheme 63, beginning with the deprotonation of glycine Schiff base 181 promoted by the chiral silver catalyst to generate reactive silver-bound azomethine ylide dipole G. Subsequently, G underwent enantioselective addition to azodicarboxylate 182, resulting in the formation of intermediate H, which reacted with acetic acid to give the final product 183 and regenerate the catalyst. In 2014, Yanagisawa et al. reported the asymmetric αamination of alkenyl trifluoroacetates 185a−185d with dialkyl azodicarboxylates 182a−182c promoted by a chiral silver catalyst generated in situ from 2 mol % of (R)-DTBM-Segphos and 2 mol % of AgOTf in the presence of CF3CH2OH.233 The reaction was supposed to evolve through a chiral silver enolate to afford various optically active α-hydrazino ketones 186a− 186f with quantitative yields and enantioselectivities of up to 97% ee (Scheme 64). Products 186b−186f, arising from the reaction of cyclic alkenyl trifluoroacetates, such as 1-tetralone derivatives (n = 1) and 1-benzosuberone derivatives (n = 2), were obtained with the best enantioselectivities of 90−97% ee. The reaction of acyclic alkenyl trifluoroacetates 187a and 187b also furnished the corresponding products 188a and 188b in quantitative yields, albeit with low enantioselectivities (17−31% ee) (Scheme 64). A plausible mechanism cycle was proposed by the authors, which began with the reaction between the chiral silver catalyst with CF3CH2OH in the presence of an appropriate base, such as azo diester 182 or α-aminated product 186 or 188, to generate the corresponding complex (R)-DTBM-Segphos-AgOCH2CF3, which was considered to be the true catalyst of the process. Subsequently, this chiral silver alkoxide attacked the alkenyl trifluoroacetate to yield chiral silver enolate I and CF3CH2OCOCF3. Then, chiral silver enolate I underwent amination with the azo diester to afford chiral silver amide of α-hydrazino ketone J. Finally, protonation of chiral silver amide J with CF3CH2OH resulted in the formation of the chiral final product and the regeneration of the chiral silver alkoxide. It must be noted that this study represented the first example of an enantioselective αamination catalyzed by a chiral silver alkoxide.

time-consuming protection−deprotection processes, as well as purification procedures of intermediates.234−236,248−261 The first example of asymmetric silver-catalyzed domino reaction was reported in 1990 by Ito et al., who employed chiral ferrocenylphosphine−silver(I) complexes as chiral catalysts to promote asymmetric domino aldol/cyclization reactions of aldehydes with tosylmethyl isocyanide.19 In the presence of only 1 mol % of catalyst loading, this reaction afforded the corresponding chiral 5-alkyl-4-tosyl-2-oxazolines in enantioselectivities of up to 86% ee. Ever since, a range of various enantioselective silver-catalyzed domino reactions, including multicomponent ones as well as multicatalyzed ones,31,262−275 have been successfully developed by several groups.

10. SILVER-CATALYZED DOMINO AND TANDEM REACTIONS A domino reaction has been strictly defined by Tietze as a process in which two or more bond-forming transformations occur based on functionalities formed in the previous step in which no additional reagents, catalysts, or additives can be added to the reaction vessel, nor can reaction conditions be changed.234−236 Domino reactions237−247 allow the synthesis of a wide variety of complex molecules including natural products and biologically active compounds to be economically achieved on the basis of one-pot processes avoiding the use of costly and

10.1. Domino and Tandem Reactions Initiated by a Michael Addition

It is important to note that this section does not include asymmetric formal [3 + 2] cycloadditions of azomethine ylides derived from glycine imino esters with α,β-unsaturated carbonyl compounds or nitroalkenes, which are supposed to evolve through domino Michael/Mannich or Michael/azaAE


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Henry reactions, respectively, since these transformations are collected in sections 5.1 and 5.2. Similarly, asymmetric formal [3 + 2] cycloadditions of isocyanoacetates with α,β-unsaturated carbonyl compounds, which evolve through domino Michael/ cyclization reactions have also been included in section 5.3 but could be situated in section 10 for the same mechanistic reasons. In the past few years, several types of enantioselective domino and tandem reactions initiated by Michael additions have been developed based on the use of silver catalysts sometimes combined with organocatalysts. Tandem catalyzed reactions refer to the synthetic strategies of modular combination of catalytic reactions into one synthetic operation, occurring one after the other and working in conjunction with each other with minimum workup or change in conditions in comparison to domino reactions defined by Tietze. A recent example of asymmetric tandem reactions based on sequential silver catalysis and organocatalysis was reported by Enders et al.276 It allowed the enantioselective synthesis of biologically interesting five-membered annulated hydroxycoumarins 189 with enantioselectivities of up to 99% ee to be achieved from the reaction between the corresponding 4-hydroxycoumarins 190 and aryl-substituted enynones 191. As shown in Scheme 65, the first step of the sequence involved the Michael addition of 4-hydroxycoumarins 190 to aryl-substituted enynones 191 catalyzed by 20 mol % of cinchona-derived primary amine 192 in THF at 4 °C in the presence of (S)-N-Boc alanine as chiral additive, yielding the corresponding Michael products. Studies revealed that THF, which was used as solvent in this first step, was inappropriate for the subsequent silver-catalyzed cyclization. Thus, before the second step, the solvent had to be changed to toluene prior to the addition of Ag2CO3, which was compatible to the organocatalyst. Then, a hydroxyalkoxylation afforded the final domino tricyclic products 189 in moderate to high yields (54−91%) and enantioselectivities of 70−99% ee. In the case of aryl-substituted enynones, the cyclization occurred through 5-exo-dig cyclization to give the corresponding domino chiral products in good to high yields and enantioselectivities irrespective of electronic and steric effects of substituents, although bulky substituents normally resulted in an increased reaction time in the cyclization step. Moreover, hydroxycoumarins bearing different substituents were also tolerated. In contrast, enynones bearing aliphatic substituents 193 led to the formation of the corresponding 6-endo-products 194 with comparable enantioselectivities of 89−90% ee, but combined with lower yields of 32−52% due to a less selective ring formation (Scheme 65, eq 2). Later, in 2015, the same authors reported a true domino Michael-initiated asymmetric reaction between alkyne-tethered nitroalkenes 195 and 5-pyrazolones 196 based on relay multicatalysis with cinchona-derived squaramide 197 and Ag2CO3.277 Indeed, in this case, it was not necessary to change the solvent to perform the second reaction (hydroalkoxylation), and consequently, the domino Michael/hydroalkoxylation reaction performed in dichloromethane at −20 °C directly provided chiral functionalized pyrano-annulated pyrazole derivatives 198 in moderate to excellent yields (48−95%) and high enantioselectivities (77−95% ee), irrespective of the steric or electronic nature of the substituents of alkynes (R1) which could be aromatic, heteroaromatic, and aliphatic groups (Scheme 66). Only the internal alkynes with bulky substituents on the 2-position (R1 = 2-BrC6H4, 2-ClC6H4, or 1-naphthyl) gave slightly lower yields (74−77%). Interestingly, in all examples, a clean cyclization to the 6-endo-derived products

Scheme 65. Tandem Michael/Hydroalkoxylation Reaction of 4-Hydroxycoumarins and Enynones

was observed. Moreover, different pyrazolinones led to similar results. The authors have proposed the relay catalysis concept to explain this process in which the first Michael addition was organocatalyzed by 197, and the second step was promoted by the silver catalyst. More recently, a closely related cinchona-derived squaramide 199 was combined with Ag2O by the same authors to catalyze the synthesis of multifunctionalized chiral five-membered spiropyrazolones 200.278 As shown in Scheme 67, these products were formed through domino Michael/Conia-ene reactions of 5-pyrazolones 196 with another type of alkynetethered nitroalkenes 201 in chloroform at −40 °C to room temperature. Notably, the process employed only 1 mol % of organocatalyst 199 and generally 3 mol % of Ag2O, which successively and respectively catalyzed the two steps of the domino reaction according to relay catalysis. Moderate to excellent yields and enantioselectivities of up to 99% and 99% AF


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influence the outcome of the reaction; however, bulky nitroalkenes led to lower yields of 27−54%. The application of nitroalkenes with internal alkynes bearing aliphatic substituents (R4 = Cy, n-Bu) was also feasible, although higher catalyst loadings in Ag2O (10 mol % instead of 3 mol %) had to be used to achieve comparable results.

Scheme 66. Domino Michael/Hydroalkoxylation Reaction of 5-Pyrazolones and Alkyne-Tethered Nitroalkenes

10.2. Domino Reactions Initiated by an Aldol Reaction

In 2011, Dixon et al. developed asymmetric domino aldol/ cyclization reactions of branched aliphatic as well as aromatic aldehydes 5 with isocyanoacetates 115 as well as α-substituted ones 114 by using a new class of chiral aminophosphine precatalysts derived from 9-amino(9-deoxy) epicinchona alkaloids, such as 202, in combination with Ag(I) salts, such as Ag2O.279 The corresponding chiral oxazolines 203 were obtained in 50−93% yields, and good to high diastereoselectivities (up to 98% de), along with good to excellent enantioselectivities (up to 98% ee), as shown in Scheme 68. Scheme 68. Domino Aldol/Cyclization Reaction of Aldehydes and Isocyanoacetates

Scheme 67. Domino Michael/Conia-Ene Reaction of 5Pyrazolones and Alkyne-Tethered Nitroalkenes

This protocol could be performed by mixing together the cinchona organocatalyst and Ag2O, which interacted cooperatively (Scheme 68). Moreover, the possibility of lowering the catalyst loading to 2 mol % of chiral aminophosphine and 0.5 mol % of Ag2O was demonstrated since, under these conditions, yields of 44−90% were obtained in combination with diastereo- and enantioselectivities of up to 86% de and 94% ee, respectively. Interestingly, when α-substituted isocyanoacetates were used, the opposite facial selectivity in the nucleophilic component was observed. As an extension of the methodology, the same authors very recently developed a short asymmetric synthesis of (−)-chloramphenicol, being the first one that relied on a catalytic enantio- and diastereoselective aldol reaction.280 As shown in Scheme 69, the key step of this strategy involved the silvercatalyzed domino aldol/cyclization reaction of p-nitrobenzalde-

ee, respectively, were achieved in combination with high diastereoselectivities of 78 to >90% de in the reactions of differently substituted pyrazolones and terminal alkynes. In general, the electronic nature of the substituent did not AG


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Scheme 69. Domino Aldol/Cyclization Reaction of pNitrobenzaldehyde and Isocyanoacetates and Synthesis of (−)-Chloramphenicol

methyl aryl ketones with either electron-withdrawing or electron-donating groups afforded the trans-configured oxazolines as well as methyl heteroaryl ketones. In addition to methyl aryl ketones, other alkyl aryl ketones, such as ethyl, n-propyl, and tert-butyl aryl ketones, led to the corresponding domino products with comparable efficiency. To illustrate the utility of this novel methodology, some of the oxazolines were easily converted through hydrolytic manipulation into amino acid derivatives. To explain the stereochemical outcome of the process, the authors proposed the transition state that is depicted in Scheme 70 in which the phosphorus and amide Scheme 70. Domino Aldol/Cyclization Reaction of Ketones and Isocyanoacetates

hyde 204 with isocyanoacetate 115e to give the corresponding trans-oxazoline 205 in 68% yield, along with a good diastereoselectivity of 84% de, and a high enantioselectivity of 93% ee (Scheme 69). These results were obtained when the reaction was catalyzed in the presence of 5 mol % of chiral cinchona alkaloid 206. Domino product 205 was further converted into the desired antibiotic through three supplementary steps. The scope of this reaction, catalyzed by other silver complexes derived from chiral cinchona alkaloids 202 and 207, was extended to various alkyl isocyanoacetates 115a− 115d, providing the corresponding oxazolines 208a−208e in comparable yields (56−80%), diastereoselectivities of 76−82% de, and and enantioselectivities of 78−87% ee (Scheme 69). In 2015, these authors applied a combination of Ag2O and chiral quinine-derived aminophosphine ligand 206, respectively used at loadings of 2.5 and 5 mol %, in AcOEt at −20 °C to related novel silver-catalyzed enantioselective domino reactions of isocyanoacetates with ketones instead of aldehydes.281 The domino process began with the aldol reaction of unactivated aryl alkyl ketones 146 with isocyanoacetates 115 to give the corresponding aldol products, which further cyclized to afford chiral oxazolines 209 possessing a fully substituted stereocenter in 55−84% yields and good to excellent diastereo- and enantioselectivities (70−92% de and 82−98% ee). Several

nitrogen atoms of the ligand, the oxygen atom of the ketone, and the terminal carbon atom of the isonitrile coordinated to a Ag(I) ion through a tetrahedral arrangement. Additional transition state stabilization was provided through hydrogen bonding of the protonated quinuclidine to the coordinated ketone oxygen atom. Notably, this interaction created a welldefined chiral pocket that could readily differentiate the enantiotopic faces of the bound ketone; unfavorable steric interactions forced the aryl group to be located away from the quinuclidine, and the attack of the enolate occurred preferentially to the Re face. An efficient enantioselective synthesis of densely functionalized dihydrofuran derivatives was recently developed by Singh et al. on the basis of the first example of asymmetric silvercatalyzed domino aldol/cycloisomerization reaction of ynones 210a−210k with cyclic 1,3-diketones 211a−211d.282 As shown in Scheme 71, the process was promoted by a chiral silver catalyst generated in situ from 10 mol % of AgOTf and 5 mol % of (R)-BINAP in dichloromethane at −60 °C, and provided the corresponding highly functionalized chiral dihydrofurans 212a−212m bearing an exocyclic double bond at the C2 position in low to excellent yields (26−95%) and moderate to excellent enantioselectivities (44−98% ee). A range of ynones AH


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Scheme 71. Domino Aldol/Cycloisomerization Reaction of Ynones and 1,3-Cyclohexanediones

Scheme 72. Domino Cyclization/Friedel−Crafts Reaction of o-Alkynylaryl Aldimines and Indoles

with various aryl-substituted terminals underwent the reaction with cyclohexanedione as well as heteroaryl-, naphthyl-, and nbutyl-substituted ynones. The presence of substituents on the 1,3-diketone was tolerated. When instead of an ester (EWG = CO2Et), which allowed 56−98% ee, a trifluoromethyl group (EWG = CF3) was used as an activating group in the ynone, the corresponding dihydrofuran 212m was obtained in 58% yield and 44% ee. Notably, the presence of an exocyclic double bond and a hydroxyl group in the domino products provides wide scope for further structural manipulation.

at room temperature at 10 mol % of catalyst loading in THF or AcOEt as solvent.286 The reaction involved 1-alkynylaryl ketones 218 as substrates which reacted through intramolecular cyclization to generate an ion pair comprised of the isobenzopyrylium intermediate K and chiral phosphate. The latter was subsequently reduced with Hantzsch esters 219a and 219b to afford chiral 1H-isochromenes 220a−220o in good to excellent yields (68−98%). When alkyl ketones (R1 = alkyl) were used as substrates, the best results were obtained when the reactions were performed in THF as solvent with the domino products obtained in 87−98% yields with enantioselectivities of 67−87% ee when an aryl group was introduced at the R2 position of ketones (products 220a−220f), while the presence of an alkyl group at this position led to comparable yield (89%) albeit combined with a lower enantioselectivity (22% ee) (product 220g, Scheme 73). On the other hand, the highest enantioselectivities (88−92% ee) associated with 68−90% yields were obtained in the reaction of aryl ketones (R1 = aryl) bearing an aryl substituent on the alkyne (R2) (products 220h−220n). The presence of an alkyl substituent at this position of aryl ketones provided a decreased enantioselectivity (49% ee) in 86% yield (product 220o). The utility of this novel methodology was demonstrated in the asymmetric synthesis of the 9-oxabicyclo[3.3.1]nona-2,6-diene framework, which is found in biologically active molecules. Another type of asymmetric domino reaction was reported by Yao et al. on the basis of relay catalysis arising from the combined use of 2.5 mol % of AgOAc with 3.75 mol % of chiral phosphoric acid 221.287 The reaction involved 3-alkynylacrylaldehydes 222a−222h and 2-hydroxystyrenes 223a−223h in DCE at room temperature, providing a mixture of the

10.3. Domino Reactions Initiated by a Cyclization

1,2-Dihydroisoquinolines and their derivatives constitute an important class of heterocyclic compounds found in numerous natural and pharmaceutical products.283,284 In this context, their asymmetric synthesis is particularly challenging. In 2012, You et al. reported a novel access to chiral multifunctional 1,2dihydroisoquinolines based on enantioselective silver-catalyzed reactions of 1-alkynylaryl aldimines 213a−213e with indoles 214a−214e.285 These domino cyclization/Friedel−Crafts processes were catalyzed by 10 mol % of chiral silver BINOL-derived phosphate 215 in toluene at 40 °C, leading to a range of domino products 216a−216i in low to excellent yields (26−95%) and low to high enantioselectivities (10−89% ee). As shown in Scheme 72, the best enantioselectivity of 89% ee was obtained for the reaction of substrate 213c containing an electron-withdrawing group. In addition to phenylsubstituted alkynes (R2 = Ph), alkyl-substituted alkynes (R2 = Cy or n-Bu) could also be well tolerated with good yields (80− 95%) but decreased enantioselectivities (10−54% ee for products 216e−216f). Concerning the nucleophilic partner, various substituted indoles bearing either electron-donating or electron-withdrawing groups reacted to give products 216f− 216h with moderate to high yields (26−95%) and moderate ee values (33−54% ee). Interestingly, N-methylindole 214e also gave the corresponding product 216i in 41% yield and 59% ee. In 2014, another type of asymmetric domino reactions was described by Terada et al. based on the use of chiral pentafluorophenyl-substituted silver phosphate 217 employed AI


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Scheme 73. Domino Intramolecular Cyclization/Reduction Reaction of o-Alkynylaryl Ketones

Scheme 74. Domino Cycloisomerization/Oxa-Diels−Alder/ Intramolecular Nucleophilic Substitution Reaction of 3Alkynylacrylaldehydes and 2-Hydroxystyrenes

corresponding chiral polycyclic products 224a−224n and 225a−225n. As shown in Scheme 74, the alkyne bond of substrate 222 was activated by Ag(I), initiating the cycloisomerization to afford a Ag(I)−pyrylium/chiral phosphate ionic pair L. Then, protonolysis of the C−Ag bond with AcOH regenerated AgOAc and yielded the key chiral pyrylium phosphate ionic pair M. Subsequently, the hydrogen bonding of the phosphate of this intermediate with the 2-hydroxystyrene substrate 223, led to an asymmetric oxa-Diels−Alder cycloaddition, leading to carbocation N. Finally, the latter was attacked internally by the phenolic hydroxyl group through a SN2 (attack at C1 position) or a SN2′ (attack at C3 position) mechanism, providing products 224 and 225 in 9−50 and 12− 67% yields, 15−92% ee, and 9−92% ee, respectively. The development of multimetallic catalytic systems and their application to asymmetric catalysis is an emerging area in modern organic synthesis.288−292 In 2009, Tanaka et al. reported a rare example of asymmetric domino reaction promoted by a combination of cationic rhodium(I) and silver(I) complexes.293 As shown in Scheme 75, the reaction of alkynylaryl aldehydes 226 with isatins 227 performed in the presence of 5 mol % of [Rh(cod)2]BF4, 10 mol % of AgBF4, and 5 mol % of chiral ferrocenyl ligand 228 in dichloromethane at room temperature with 5 mol % of triphenylphosphine as an

additive led to the corresponding densely functionalized tetrasubstituted alkenes 229 in moderate to excellent yields (52−96%) and uniformly excellent enantioselectivities (94 to >99% ee). Indeed, alkyl-, alkenyl-, and aryl-substituted 2alkynylbenzaldehydes provided excellent results in reaction with N-methyl-, N-phenyl-, and even N-H isatins. To explain the outcome of the process, the authors have proposed the mechanism depicted in Scheme 75 in which the exact roles of the two metals were not clarified. Previously, Porco et al. demonstrated that a cationic silver(I) complex could react with a 2-alkynylbenzaldehyde to form the corresponding benzopyrylium intermediate.294 On this basis, the authors tentatively proposed that AgBF4 could catalyze the formation of intermediate ketoaldehyde O, but when the reaction was performed with AgBF4 and in the absence of [Rh(cod)2]BF4, it was found that this intermediate was not produced but an unidentified mixture of products derived from the 2alkynylbenzaldehyde. This result suggested that rhodium(I) and silver(I) complexes cooperatively catalyzed the process AJ


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Scheme 75. Domino Cyclization/Oxa-Diels−Alder Reaction of Isatins and Alkynylaryl Aldehydes

Scheme 76. Domino Mannich/Cyclization Reaction of Ketimines and Isocyanoacetates

enantioselectivities (90−99% ee) while the diastereoselectivity ranged from 46−98% de. The lowest values of 46−50% de were obtained with aryl alkyl ketones bearing a methoxy group on the phenyl ring. However, these substrates afforded high enantioselectivities (98−99% ee). A chiral cinchona alkaloid derived squaramide 233 was combined by Shi and Zhao with AgOAc to cooperatively catalyze enantioselective domino Mannich/cyclization reactions of α-substituted isocyanoacetates 114a−114n with cyclic trifluoromethylated ketimines 234a−234g.296 In the presence of 5 mol % of this catalyst system in THF at 0 °C, the process afforded the corresponding chiral trifluoromethyl-substituted tetrahydroimidazo[1,5-c]quinazoline derivatives 235a−235q in good to quantitative yields of 76−99% along with a high diastereoselectivity (>88% de) and uniformly excellent enantioselectivities of up to 98% ee (Scheme 77). On the other hand, when the reaction conditions were applied to an alkyl-substituted isocyanoacetate (R1 = Bn), the corresponding domino product 235k was produced in only 58% ee albeit in excellent yield (98%). Concerning the scope of ketimines, they could bear electron-withdrawing, electron-donating, or electron-neutral substituents on the phenyl ring, providing comparable excellent results. Moreover, an N-unprotected ketimine 234g (R3 = H) was found to lead to the corresponding product 235q in 99% yield and 91% ee. On the other hand, replacing the trifluoromethyl group on the starting quinazolinones by a methyl group prevented the reaction, which indicated that the strong electron-withdrawing trifluoromethyl group was pivotal for the domino reaction to occur. The authors have proposed the transition state depicted in Scheme 77. The α-proton of isocyanoacetate 114 was easily deprotonated by the quinuclidine nitrogen of the organo-

with the exact role of AgBF 4 not clarified although demonstrated indispensable to reach good yields. Intermediate ketoaldehyde O, arising from domino cyclization/oxa-Diels− Alder cycloaddition, could then undergo an enantioselective intramolecular ketone hydroacylation through rhodacycle P to yield the final chiral tetrasubstituted alkene. 10.4. Domino Reactions Initiated by a Mannich Reaction

While a range of chiral catalysts including either metal-based or metal-free catalyst systems are know to successfully promote asymmetric domino Mannich/cyclization reactions of isocyanoesters with aldimines, the analogous asymmetric transformation of the significantly less reactive ketimines remains challenging in spite of its potential to provide a direct route to chiral imidazolines possessing vicinal stereogenic centers including a fully substituted β-carbon atom. In 2014, Dixon and Ortin reported the first enantioselective domino Mannich/ cyclization reaction of isocyanoacetates 115a and 115b with ketimines 230a−230o, which was based on the combined cooperative use of Ag2O with cinchona alkaloid derived aminophosphine organocatalyst 231.295 As shown in Scheme 76, a range of chiral imidazolines 232a−232s were prepared in good to excellent yields (62−98%) and uniformly excellent AK


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10.5. Miscellaneous Domino Reactions

Scheme 77. Domino Mannich/Cyclization Reaction of Cyclic Trifluoromethylated Ketimines and Isocyanoacetates

In 2008, Hoveyda and Snapper developed enantioselective silver-catalyzed three-component domino imine-formation/azaDiels−Alder reactions evolving between aliphatic aldehydes 5a−5d, o-thiomethyl-p-anisidine 17, and Danishefsky diene 236.80 Performed in the presence of 5 mol % of a combination of AgOAc with chiral tert-leucine-derived phosphine ligand 19 in THF at 0 °C, the reaction led to the corresponding chiral dihydropiperidines 237a−237d in moderate to good yields (53−88%) and uniformly high enantioselectivities (90−95% ee), as shown in Scheme 78. The cycloadditions were effective Scheme 78. Three-Component Domino Imine-Formation/ Aza-Diels−Alder Reaction of Aliphatic Aldehydes, oThiomethyl-p-anisidine, and Danishefsky Diene

with aldimines generated in situ bearing an n-alkyl substituent (products 237a and 237b) as well as those carrying heteroatom-containing functional groups. However, azaDiels−Alder reactions with the latter class of substrates afforded the corresponding cycloadducts 237c and 237d in lower yields (53−66% vs 88%). With the aim of developing a novel total synthesis of biologically active natural alkaloid (−)-cephalotaxine, which contains a 1-azaspiro[4.4]nonane ring unit, Tu et al. have introduced asymmetric domino hydroamination/semipinacol rearrangement reactions of cyclobutanols 238a−238m promoted by a chiral silver catalyst 239 derived from a chiral phosphoric acid.297 As shown in Scheme 79, the use of 20 mol % of this preformed catalyst in carbon tetrachloride as solvent at 25 °C allowed a range of chiral azaspirocyclic products 240a−240m to be synthesized in uniformly high yields of 90− 99% combined with moderate enantioselectivities (55−82% ee). They arose from an intramolecular hydroamination of arylsulfonyl-protected substrates 238a−238m to give iminium intermediates Q and R, which subsequently underwent a semipinacol rearrangement to provide the final azaspirocycles 240a−240m. In 2013, (R)-BINAP was employed as ligand by Dudding and Mirabdolbaghi to promote silver-catalyzed asymmetric domino reactions between alkyl 2-formylbenzoates 241a−241i and allyltrimethoxysilane 157.298 As shown in Scheme 80, in the presence of 6−10 mol % of this ligand associated with the same quantity of AgF, the process yielded a series of chiral C3substituted phthalides 242a−242i in moderate yields of 52− 73% and low to good enantioselectivities of 33−86% ee. The

catalyst due to the interaction of Ag(I) with the isocyano group, resulting in a single H-bonding interaction between the OH group of the enolized isocyanoacetate and the tertiary amine and a weak hydrogen bonding between the OMe group of the enolized isocyanoacetate and the NH in the squaramide moiety. Simultaneously, the cyclic N-acyl ketimine was activated and oriented through hydrogen bonding with the NH and OH groups of the multi-hydrogen-bonding donor squaramide catalyst, thus forcing the isocyanoacetate enolate to be delivered through its Re face of the enol to the Si face of the imine moiety, leading to the formation of two stereogenic centers. Then, an intramolecular 5-endo-dig cyclization occurred to afford the final product. AL


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Scheme 79. Domino Hydroamination/Semipinacol Rearrangement Reaction of Alkyne-Tethered Cyclobutanols

Scheme 80. Domino Allylation/Transesterification Reaction of Alkyl 2-Formylbenzoates and Allyltrimethoxysilane

substrate scope showed that elongation of the n-alkyl chain (R1) of the starting unsubstituted (R2 = H) alkyl 2formylbenzoates improved the enantioselectivities (80−86% ee for R1 = Et, n-Hex, n-C12H25 vs 71% ee for R1 = Me). The methodology could also be applied to Merrifield resin bound substrate 241j, which led to the corresponding product 242j in 68% yield and 76% ee. To explain these results, the authors proposed the mechanistic cycle depicted in Scheme 80 in which a short-lived complex S was initially formed from the alkyl 2formylbenzoate 241, allyltrimethoxysilane 157, and the catalyst. Then, a fluoride-assisted transmetalation occurred to give a highly reactive Ag−allyl species that underwent allylation via T in a Re-stereofacial C−C bond-forming process, providing the Ag−alkoxy bound intermediate U. Finally, an intramolecular transesterification occurred to yield the product and regenerate the catalyst.

enantioselectivities (79−97% ee) (Scheme 81). It must be noted that donor−acceptor cyclopropene 244 was generated in situ by treatment of γ-phenyl-enoldiazoacetate 247 with Rh2(OAc)2. The scope of the process was extended to a range of nitrones, which led to the corresponding products with enantioselectivities generally >90% ee. A lower enantioselectivity of 82% ee was obtained with a nitrone bearing an electron-withdrawing substituent on the N-phenyl group (Ar1 = p-BrC6H4), but nitrones with an electron-donating substituent on the N-phenyl group with or without an electron-withdrawing substituent on the α-phenyl group showed higher enantioselectivities of up to 97% ee. Moreover, a 2-furyl nitrone (Ar1 = Ph, Ar2 = 2-furyl) gave the desired product in 95% yield and 79% ee. To explain these results, the authors have proposed the possible involvement of a silver carbene species in the cycloaddition process without excluding the possibility of a Lewis acid promoted pathway. The Friedel−Crafts reaction of aromatic compounds with aldehydes or ketones constitutes one of the most fundamental reactions in organic chemistry; however, its enantioselective catalytic version is still an unexplored field. The asymmetric Friedel−Crafts reaction of indoles provides a direct access to chiral indole derivatives that constitute privileged products in

11. SILVER-CATALYZED MISCELLANEOUS REACTIONS In 2013, Doyle et al. reported a highly diastereo- and enantioselective silver-catalyzed asymmetric formal [3 + 3] cycloaddition of nitrones 243 with donor−acceptor cyclopropene 244.299,300 The formal cycloaddition occurred with an exceptional stereocontrol in dichloromethane at −78 °C in the presence of 10 mol % of AgSbF6 combined with 12 mol % of chiral bisoxazoline 245 as ligand. It led, after subsequent deprotection by treatment with TBAF, to the corresponding chiral cis-disubstituted 3,6-dihydro-1,2-oxazine derivatives 246 as single cis-diastereomers with high yields (71−95%) and AM


Chemical Reviews

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loading of 0.5 mol % allowed a range of (R)-indole α-ketoesters 251 to be prepared in yields and enantioselectivities of up to 99% and 98% ee, respectively. In another context, Yamada et al. have reported enantioselective silver-catalyzed carbon dioxide incorporation into bispropargylic alcohols 252.303 The reaction was performed in the presence of a combination of AgOAc and chiral Schiff base ligand 253 in chloroform at 0 or 5 °C, providing chiral cyclic carbonates 254 in good to quantitative yields (58−99%) and moderate to high enantioselectivities (47−93% ee) (Scheme 83). The best enantioselectivities of 90−93% ee

Scheme 81. Formal [3 + 3] Cycloaddition of Nitrones and a Donor−Acceptor Cyclopropene

Scheme 83. Carbon Dioxide Incorporation into Bispropargylic Alcohols

medicinal chemistry.301 Only a few highly enantioselective processes based on chiral metal complexes have been reported for the asymmetric Friedel−Crafts alkylation of indoles with β,γ-unsaturated α-ketoesters. Among them, an efficient process was developed by Feng et al. by using a chiral silver catalyst in situ generated from 10 mol % of AgSbF6 and 10 mol % of chiral N,N′-dioxide ligand 248 in THF at −20 °C.302 As shown in Scheme 82, the reaction proceeded well for many differently

were obtained in the reaction of phenyl- and p-tolyl-substituted bispropargylic alcohols (R1 = Ph or 4-Tol). In the case of sterically hindered tert-butyl-substituted phenyl derivative (R1 = Ph, R2 = t-Bu), a lower enantioselectivity of 82% ee was obtained. Bispropargylic alcohols with variously substituted phenyl groups (R1 = 3-MeOC6H4, 4-F3CC6H4, 4-BrC6H4) were also compatible, but provided generally slightly lower enantioselectivities (79−87% ee) regardless of the electrondonating and electron-withdrawing substituents. In addition, an alkene-substituted alkyne (R1 = C(Me)CH2) smoothly reacted to give the corresponding product in 91% yield and 80% ee whereas a much lower enantioselectivity of 47% ee was obtained in the case of an alkyl-substituted alkyne (R1 = CH2OBn) albeit combined with an excellent yield of 97%. A cooperative bimetallic catalysis was applied by Shibasaki et al. to develop an asymmetric Conia-ene reaction of alkynetethered β-ketoesters.304 The catalyst system was constituted by a combination of AgOAc, and La(O-i-Pr)3, with chiral amidebased ligands 255 or 256. In this system, the enolate of the substrate was generated through activation of a carbonyl group by a hard Lewis acid such as lanthanum complex, which subsequently coupled with a silver-activated alkyne in an asymmetric environment. As shown in Scheme 84, an enantioselective Conia-ene reaction occurred between the alkyne moiety and the β-ketoester group of substrates 257 to afford the corresponding chiral cyclopentane derivatives 258 bearing an exocyclic olefin and two distinct α-carbonyl groups at a quaternary center. The best results were obtained by performing the reactions in ethyl acetate as solvent at 0 °C in the presence of triphenylphosphine as an additive. Generally, the products were achieved in good to quantitative yields (63− 100%) by using ligands 255 or 256. However, the reaction of a

Scheme 82. Friedel−Crafts Reaction of Indoles and β,γUnsaturated α-Ketoesters

substituted β,γ-unsaturated α-ketoesters 249 and indoles 250, independently of the electron-donating or electron-withdrawing character of the substituents, yielding the corresponding (S)-indole α-ketoesters 251 in moderate to high yields (51− 94%) and general high enantioselectivities (81−90% ee). Moreover, heteroaromatic and fused ring substrates were also compatible, giving the desired products with comparable enantioselectivity (84% ee). In this study, the authors also demonstrated the reversal of the enantioselectivity of this process through changing silver into samarium and by using a closely related ligand. Indeed, using a combination of a related N,N′-dioxide chiral ligand with Sm(OTf)3 at a low catalyst AN


Chemical Reviews

Review

Scheme 84. Conia-Ene Reaction of Alkyne-Tethered βKetoesters/Malonamate through Ag/La Catalysis

Scheme 85. Addition of Organoboronic Acids to Aldehydes through Ag/Rh Catalysis

yields (59−91%). Moreover, heteroaromatic boronic acids (Ar2 = 2-furyl or 2-thienyl) only reacted with an aromatic aldehyde bearing a π donor substituent (Ar1 = 4-MeOC6H4) with good to high yields (82−94%) albeit low enantioselectivities of 10− 27% ee. It must be noted that this study represented the first use of ultrasonic irradiation in asymmetric arylation. Finally, an asymmetric silver-catalyzed dearomatizing spirocyclization strategy was recently reported by Taylor and Unsworth, allowing the conversion of simple heteroaromatic compounds 261 containing ynone side chains into synthetically useful chiral functionalized spirocyclic products 262.306 This process was promoted by only 1 mol % of the silver salt 263 of a chiral phosphoric acid in chloroform at −10 °C and was insensitive to both air and moisture. As shown in Scheme 86,

β-ketoester bearing an electron-donating methoxy substituent (R1 = 4-MeOC6H4, R2 = OEt) provided a low yield (26%) but with a high enantioselectivity (93% ee). In all cases of substrates, including aliphatic β-ketoesters (R1 = alkyl) and a malonamate (R1 = NH2), the products were generated in uniformly high enantioselectivities (83−96% ee). The authors showed that no reaction occurred in the absence of one of the two metal catalysts, thus demonstrating that simultaneous activation of the 1,3-dicarbonyl group by the hard, Lewis acidic lanthanum and the alkyne group by the soft, Lewis acidic silver was crucial to promote the Conia-ene reaction. Remarkably, for some substrates a La/Ag catalyst loading as low as 0.5 mol % combined to 1 mol % of chiral ligand was sufficient to afford excellent results. In 2013, another multicatalyst system was developed by Ma and Song and further applied to asymmetric addition reactions of organoboronic acids to aldehydes.305 Indeed, these authors reported the synthesis of novel planar chiral N-heterocyclic carbene silver complex 259 derived from [2.2]paracyclophane. This was employed in combination with RhCl3 to promote the addition of aromatic boronic acids 78 to aryl aldehydes 5 to afford the corresponding chiral alcohols 260 in moderate to high yields (52−94%) and low to moderate enantioselectivities of up to 67% ee (Scheme 85). The reactions were performed at 40 °C in the presence of potassium fluoride as superstoichiometric additive in a 5:1 mixture of tert-butanol/ methanol as solvent. Moreover, the use of ultrasound irradiation allowed the yields to be enhanced due to higher catalytic activities. The reaction conditions were applicable to various aryl aldehydes and aromatic boronic acids bearing a wide variety of functional groups. In most cases, the reaction proceeded with notable efficiency with up to 94% yield by using only 3 mol % of catalyst loading. It was found that the electronic properties of the aryl aldehyde had an important effect on the enantioselectivity of the reaction. With electrondeficient aryl aldehydes, the catalyst system was more enantioselective (48−67% ee) while electron-rich aryl aldehydes provided lower enantioselectivities of 39−41% ee. In contrast, heteroaryl aldehydes, such as 2-furylaldehyde and 2-thienylaldehyde, led to the corresponding alcohols by reaction with various aromatic boronic acids in low enantioselectivities of 5−24% ee albeit with moderate to high

Scheme 86. Dearomatization of Aromatic Ynones through Spirocyclization

these simple reaction conditions were applicable to various subtrates including aryl-substituted ynones as well as a methylsubstituted ynone (R = Me). The best result (100% yield and 78% ee) was seen in the reaction of phenyl-substituted ynone (R = Ph, X = H). Furthermore, a quantitative yield combined with 72% ee was obtained in the reaction of a methylsubstituted ynone (R = Me, X = H).

12. CONCLUSION This review illustrates how much enantioselective silver catalysis has contributed to the development of various types of enantioselective highly efficient reactions. It updates the major progress in the field of enantioselective transformations AO


Chemical Reviews

Review

AUTHOR INFORMATION

promoted by chiral silver catalysts, illustrating the power of these especially mild Lewis acid catalysts to provide new reaction pathways, even if this field is still in its infancy. Especially in the past nine years, chiral silver complexes have become catalysts of first choice for many types of asymmetric reactions generally performed under mild reaction conditions and through experimentally simple procedures. Indeed, a steadily growing number of novel asymmetric silver-catalyzed reactions has been developed in the past decade, including aldol-type reactions, Mannich reactions, Michael reactions, formal 1,3-dipolar cycloadditions, alkynylations, allylations, and cyclizations of allenes, aminations, along with a wide range of domino and tandem reactions among other reactions, allowing new chiral cyclic as well as acyclic products to be achieved in high enantioselectivities. For example, highly enantioselective silver-catalyzed processes have been recently described for the first time, such as aldol reactions of ketones using alkenyl esters as masked enolates with 93% ee, conjugate additions of glycine derivatives to α,β-unsaturated ketones with 99% ee, cycloadditions of cinnamates with glycine imino esters with 98% ee, cyclizations of allenoates with activated isocyanides with 96% ee, inverse-electron-demand 1,3-dipolar cycloadditions of isoquinolinium methylides with enecarbamates with 95% ee, propargylations of cyclic N-sulfonylketimines and benzophenones with allenyl boronic acid pinacol esters with 98% ee, ring-openings of oxabenzonorbornadienes with terminal alkynes with 99% ee, Hosomi−Sakurai reactions of Csp3 centers with 96% ee, and intramolecular silver-catalyzed reactions of αallenols with 93% ee. Furthermore, it is only recently that a wide range of powerful asymmetric silver-catalyzed domino (and tandem) reactions have been reported for the first time. Among the best results are domino imine-formation/aza-Diels−Alder cycloaddition reactions of aldehydes, o-thiomethyl-p-anisidine, and Danishefsky diene, domino aldol/cycloisomerization reactions of ynones with cyclic 1,3-diketones, and domino aldol/cyclization reactions of ketones with isocyanoacetates, all providing enantioselectivities of up to 95−98% ee. This type of fascinating one-pot reactions have also been widely developed in recent years by using silver catalysts in combination with organocatalysts, also providing remarkable results. For example, enantioselectivities of up to 99% ee were reached in domino aldol/cyclization reactions of aldehydes with isocyanoacetates, domino Mannich/cyclization reactions of ketimines with isocyanoacetates, domino Michael/cyclization reactions of Naryl maleimides with isocyanoacetates, tandem Michael/hydroalkoxylation reactions of enynones with 4-hydroxycoumarins and of alkyne-tethered nitroalkenes with 5-pyrazolones, domino Michael/Conia-ene reactions of 5-pyrazolones with alkynetethered nitroalkenes, and domino cyclization/oxa-Diels−Alder cycloaddition reactions of isatins with alkynylaryl aldehydes. The excellent results reported in the past decade have demonstrated the high efficiency of silver catalysts in asymmetric catalysis, owing to their mild Lewis acidity and favorable redox potential, opening the way for developing new catalytic systems to perform reactions, such as C−C bond formations, C−heteroatom bond formations, or C−H functionalizations. Indeed, a bright future is undeniable for more sustainable novel and enantioselective silver-catalyzed transformations.

Corresponding Author

*Tel.: +33 4 91 28 27 65. E-mail: [email protected]. ORCID

Hélène Pellissier: 0000-0003-0773-5117 Notes

The author declares no competing financial interest. Biography Hélène Pellissier carried out her Ph.D. under the supervision of Dr. G. Gil in Marseille (France) in 1987. The work was focused on the reactivity of isocyanides. In 1988, she entered the National Center for Scientific Research (CNRS) as a researcher. After a postdoctoral period in Prof. K. P. C. Vollhardt’s group at the University of California, Berkeley, she joined the group of Prof. M. Santelli in Marseille in 1992, where she focused on the chemistry of diallylsilane and its application to the development of novel very short total syntheses of steroids starting from 1,3-butadiene and benzocyclobutenes.

DEDICATION In memory of Prof. Silviu Balaban. ABBREVIATIONS Ar aryl BINAP 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl BINOL 1,1′-bi-2-naphthol Biphep 2,2′-bis(diphenylphosphino)-1,1′-biphenyl Bn benzyl Boc tert-butoxycarbonyl Bt benzotriazole Bz benzoyl CB carbon black cod 1,5-cyclooctadiene CPME cyclopentylmethyl ether Cy cyclohexyl DABCO 1,4-diazabicyclo[2.2.2]octane DBU 1,8-diazabicyclo[5.4.0]undec-7-ene DCE 1,2-dichloroethane DDQ 2,3-dichloro-5,6-dicyanobenzoquinone de diastereomeric excess Difluorphos 5,5′-bis(diphenylphosphino)-2,2,2′,2′-tetrafluoro-4,4′-bis-1,3-benzodioxole DME dimethoxyethane DMF N,N-dimethylformamide DPP diphenylphosphinyl dr diastereomeric ratio DTBM-Segphos 5 , 5 ′ - b i s [ d i ( 3 , 5 - d i - t e r t - b u t y l - 4 methoxyphenyl)phosphino]-4,4′-bi-1,3-benzodioxole EDCI 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride ee enantiomeric excess EWG electron-withdrawing group Hept heptyl Hex hexyl L ligand Mes mesityl (2,4,6-trimethylphenyl) MS molecular sieves MTBE methyl tert-butyl ether Naph naphthyl AP


Chemical Reviews Ns Pent PG PI Pin Piv PMB QUINOX rt salen Segphos TADDOL TBAF TBDPS TBS TEA Tf THF TMS Tol Ts

Review

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nosyl pentyl protecting group polymer incarcerated pinacol pivaloyl p-methoxybenzoyl 2-(4,5-dihydro-2-oxazolyl)quinoline room temperature N,N′-ethylenebis(salicylideneiminato) (4,4′-bi-1,3-benzodioxole)-5,5′-diyl-bis(diphenylphosphine) α,α,α′,α′-tetraphenyl-2,2-dimethyl-1,3-dioxolane-4,5-dimethanol tetrabutylammonium fluoride tert-butyldiphenylsilyl tert-butyldimethylsilyl triethylamine trifluoromethanesulfonyl tetrahydrofuran trimethylsilyl tolyl 4-toluenesulfonyl (tosyl)

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