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Chiral Organosulfur Ligands/Catalysts with a Stereogenic Sulfur Atom: Applications in Asymmetric Synthesis Sylwia Otocka,†,‡ Małgorzata Kwiatkowska,† Lidia Madalińska,† and Piotr Kiełbasiński*,† †

Department of Heteroorganic Chemistry, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Łódź, Poland ‡ Department of Structural Biology, Medical University of Łódź, Ż eligowskiego 7/9, 90-752 Łódź, Poland ABSTRACT: Asymmetric synthesis, in which chiral organocatalysts or metal complexes with chiral ligands are used, has become the most valuable methodology for the preparation of enantiomerically pure organic compounds. Among such catalysts/ligands, a growing number constitute various organosulfur compounds. This Review provides comprehensive and critical information on the plethora of sulfur-based chiral ligands and organocatalysts used in asymmetric synthesis, which have been published within the last 15 years. However, it is confined to the presentation of only those chiral catalysts/ligands that possess a stereogenic sulfur atom and includes sulfoxides, sulfinamides, N-sulfinyl ureas, sulfoximines, and some related S-chiral derivatives.

CONTENTS 1. Introduction 2. Sulfoxides 2.1. Asymmetric Allylation of Aldehydes with Allyltrichlorosilane 2.2. Asymmetric Allylation of Hydrazones with Allyltrichlorosilane 2.3. Addition of Diethylzinc to Aldehydes and Enones 2.4. Reaction of Arylboronic Acids with Unsaturated Carbonyl Compounds 2.5. Reaction of Sodium Tetraarylborates with Unsaturated Compounds 2.6. Other Additions to Double Bonds 2.7. Allylic Substitution 2.8. Diels−Alder Cycloaddition 2.9. Other Asymmetric Reactions Catalyzed by Chiral Sulfoxides 2.9.1. Addition of the Cyano Group to Aldehydes 2.9.2. Aldol and Other Aldol-Type Reactions 2.9.3. Asymmetric Cyclopropanation 2.9.4. Asymmetric Aziridination of Unsaturated Aldehydes 2.9.5. Polydentate Sulfinyl Catalysts Bearing Prolinol Moiety 2.9.6. Asymmetric Aromatic C−H Coupling 3. Sulfinamides 3.1. Addition of Arylboronic Acids to Unsaturated Carbonyl Compounds and Related Reactions 3.2. Diels−Alder Reaction and Other Cycloadditions 3.3. Asymmetric Allylic Substitution 3.4. Pauson−Khand Reaction 3.5. Asymmetric Reduction of Ketimines © XXXX American Chemical Society

3.6. Enantioselective Aldol Reaction 3.7. Enantioselective Protonation 3.8. Enantioselective Transfer Hydrogenation of Ketones 3.9. Asymmetric Allylation of Acylhydrazones 3.10. Asymmetric Strecker Reaction 3.11. Asymmetric Diethylzinc Addition to Aldehydes 4. N-Sulfinyl Ureas 4.1. Aza-Henry Reaction 4.2. Addition of Thioacetic Acid to Nitroalkenes 4.3. Addition of Meldrum’s Acids to Nitroalkenes 4.4. Asymmetric Reduction of Enamines 5. Sulfoximines 5.1. Reactions of Allylic Substitution 5.2. Diels−Alder Cycloaddition 5.3. Aldol-Type Reactions 5.4. Hydrogenation Reactions 5.4.1. Asymmetric Reduction of Ketimines 5.4.2. Enantioselective Hydrogenation of Linear α,β-Unsaturated Ketones 5.4.3. Asymmetric Quinoline Hydrogenations 5.4.4. Rh-Catalyzed Asymmetric Hydrogenation of Olefins 5.5. Other Asymmetric Reactions Catalyzed by Chiral Sulfoximines 5.5.1. Carbonyl-ene Reactions 5.5.2. Enantioselective Halogenation of β-Oxo Esters 5.5.3. Enantioselective Diethylzinc Addition to Aldehydes 5.5.4. Asymmetric Biginelli Reaction 6. Related S-Chiral Ligands

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U U U U U V W W W X X Y Y Z AA AB AB AB AB AC AC AC AC AC AC AD

Received: August 4, 2016

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DOI: 10.1021/acs.chemrev.6b00517 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 6.1. Amino-Functionalized Sulfonimidamides 6.2. Chiral Sulfurous Diamide 7. Conclusions Author Information Corresponding Author ORCID Notes Biographies References

Review

Chiral, enantiomerically pure sulfoxides, particularly those possessing various additional functional groups, proved also efficient catalysts in asymmetric synthesis. Very recently an exhaustive overview by Trost and Rao came out that was devoted to the use of chiral sulfoxides as ligands in organometallic complexes for asymmetric synthesis.8 Another overview comprising a similar subject but with a particular stress laid on metal complexes with achiral sulfoxides was published by Dorta and co-workers.9 Although our Review also presents a number of chiral sulfoxide ligands (some examples have been deliberately omitted in order to avoid repetition of the discussion present in the reviews mentioned earlier), it has been complemented with the application of various types of enantiomerically pure sulfoxides as organocatalysts. This section is arranged according to the reactions of asymmetric synthesis in which chiral sulfoxides were applied.

AD AD AD AD AD AD AD AD AE

1. INTRODUCTION The selective preparation of chiral, enantiomerically pure compounds has recently become one of the main goals of organic synthesis. Among various methods used for this purpose, asymmetric catalysis is one of the most efficient and economical methods for obtaining such compounds. It is based on the use of either chiral organic catalysts or chiral ligands as complexes with various metals as indispensable reagents. Among these catalysts/ligands, a predominant number of compounds are heteroorganic derivatives, especially organosulfur and organophosphorus compounds. In recent years there has been huge interest in the exploration and use of new derivatives of these types. It has resulted in a great number of publications, which became a subject of various overviews.1−4 The aim of this Review is to present recent achievements in the field of chiral sulfur ligands/ catalysts, which have been published within the last 15 years. Although the Review contains mainly references that appeared after 2000, with a special emphasis put on those published after 2008, some earlier articles are also invoked to give a background to the subject discussed. However, to make it readable and of reasonable size, it is confined to the presentation of only those chiral enantiomeric catalysts/ligands that possess a stereogenic sulfur atom. Syntheses of particular ligands/catalysts are only briefly presented. The achievements in the area of asymmetric catalysis have been critically reviewed, which means that only the important and original results are mentioned and both advantages and drawbacks of particular catalysts/ligands are discussed. The paper comprises applications in asymmetric synthesis of various types of chiral enantiomeric sulfur derivatives containing a stereogenic sulfur atom (sulfoxides, sulfinamides, N-sulfinyl ureas, and sulfoximines) and is arranged according to the reactions in which they were used.

2.1. Asymmetric Allylation of Aldehydes with Allyltrichlorosilane

A range of aryl and hetaryl aldehydes 1 were reacted with allyltrichlorosilane in the presence of various enantiomerically pure sulfoxides as promoters to give enantiomerically enriched allyl alcohols 2 (Scheme 1) in variable yields and with different enantiomeric excesses depending on both the aldehyde and the sulfoxide used. Scheme 1. Asymmetric Allylation of Aldehydes

Thus, application of enantiomerically pure methyl para-tolyl sulfoxide 3a (Figure 1) as promoter and aldehydes 1a−c as reagents resulted in the formation of the corresponding allyl alcohols 2a in good yields (up to 88%) and with moderate enantiomeric excesses (42−55%).10 Liao and co-workers synthesized a series of enantiomerically pure sulfoxides 4a−i (Figure 1) exhibiting different electronic and steric properties and evaluated them as promoters in this reaction, using

2. SULFOXIDES Chiral, enantiomerically pure sulfoxides, which are easily accessible by a variety of methods,5 are well-known as powerful chiral auxiliaries in asymmetric synthesis. They were used in many asymmetric reactions in which high to excellent asymmetric inductions were observed, e.g., in carbon−carbon bond-forming reactions including 1,4-additions and cycloadditions.6 Such an ability of sulfoxides is explained in terms of the structural features of the sulfinyl group possessing a high configurational stability as well as important steric and stereoelectronic differences between the substituents at the sulfur atom (besides two organic substituents, there are the sulfinyl oxygen atom and free electron pair). Additionally, an interaction between the polarized S−O bond and an additional functional group present in the organic substituent may result in the stabilization of a reactive conformation which, in turn, properly controls the mode of the approach of a reagent.7

Figure 1. Enantiomeric sulfoxides 3 and 4. B


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configuration at the sulfinyl sulfur atom, exhibited a moderate catalytic activity: for 7, product 2a was obtained in 49% yield and with 47% ee; for 8, it was obtained in 45% yield and with 57% ee. The authors found that the chirality of the sulfoxide moiety played a crucial role in the stereochemistry of the reaction, as opposite enantiomers of the product were obtained in each case. No product of allylation was formed in the presence of compound 9.13 Scettri and co-workers tested catalytic properties of another group of chiral bidentate imino- and aminosulfoxides, i.e., 10− 12 (Figure 5), using aldehydes 1a and 1d as substrates. In

benzaldehyde 1a as reagent. The authors found that the substituents in the phenyl ring of the promoter played an important role in the enantioselectivity. For sulfoxides 4a, 4b, and 4d, the product was obtained with enantiomeric excess up to 59% and in moderate yield (up to 58%). The improvement of enantioselectivity and efficiency was observed when orthoalkoxyphenyl tert-butyl sulfoxides 4c and 4e−i were used (up to 89% ee and up to 69% yield). Low temperature was necessary to obtain high enantioselectivity. When an increased amount of allyltrichlorosilane (3.0 equiv) was used, the yield was improved to 72% after only 6 h, whereas the enantioselectivity was slightly lower (the product was obtained with 86% ee).11 In search of more efficient sulfoxide-based catalysts, Rowlands and co-workers synthesized a series of bis-sulfoxides 5 (Figure 2) and compared their activity with that of

Figure 5. Enantiomeric imino- and aminosulfoxides 10−12.

comparison with the monodentate (R)-methyl para-tolyl sulfoxide 3a, catalyst 10 showed an improvement in efficiency (from 35% to 58%−63% yield) but, conversely, deterioration of enantioselectivity (from 44% to 23−40% ee). No reaction was observed when catalyst 11 was used because of a rapid decomposition of the allylating agent, allyltrichlorosilane. More satisfactory results were obtained when ligand 12 and aldehyde 1d were used. Although conversion of the latter into the corresponding homoallylic alcohol 2d occurred in moderate yield (56%), the enantiomeric excess of the product was high (89% ee).14 Consequently, when the analogous tetradentate ligand, bissulfoxide 13 (Figure 6), was used, no reaction occurred due to the rapid decomposition of allyltrichlorosilane. Hence, a conclusion was drawn that it was necessary to replace the secondary amine groups in ligands 11 and 13 with tertiary moieties to produce efficient catalysts: monosulfoxide 12 and bis-sulfoxide 14, respectively (Figure 6). The highest enantioselectivity was achieved in the reaction with electronpoor aromatic 1a and heteroaromatic aldehydes 1b,d,e (up to 90% ee), as well as with aliphatic aldehyde 1g (72% ee). The yields of the resulting products did not exceed 66%.13 The analogous tetradentate imino-bis-sulfoxide 15 (Figure 6) proved to be inferior to catalysts 12 and 14 and led to products 2a,d,e,g,h in up to 65% yield and up to 70% ee.15

Figure 2. Enantiomeric bis-sulfoxides 5.

monodentate sulfoxide 3a. However, none of the sulfoxides 5 was better than the catalyst 3a. In the case of catalysts 5, where n = 1 and n = 3, the reaction did not proceed at all. When bissulfoxide 5 (n = 2) was used, the final product was obtained in only 3% yield and with enantiomeric excess of 40%.12 Interestingly, in contrast to sulfoxides 5, ortho-oxosubstituted aryl tert-butyl bis-sulfoxides 6a−c (Figure 3) with

Figure 3. Enantiomeric bis-sulfoxides 6.

varying tether lengths from one to three carbon atoms turned out to be very efficient catalysts. For example, bis-sulfoxide 6b with a two-carbon tether was the most stereoselective catalyst for the allylation of benzaldehyde 1a with allyltrichlorosilane, giving product 2a in high yield (77%) and with high enantiomeric excess (90%). The remaining two sulfoxides were less efficient: for 6a, product 2a was obtained in 74% yield and 86% ee; for 6c, it was obtained in 76% yield and 88% ee.11 In an earlier work, Rowlands and Barnes synthesized a series of other bidentate sulfoxides 7, 8, and 9 (Figure 4), which were applied in the same reaction, using benzaldehyde 1a as substrate. Among the three sulfoxides, only compounds 7 and 8, being a pair of diastereomers with an opposite absolute

2.2. Asymmetric Allylation of Hydrazones with Allyltrichlorosilane

The reaction of allyltrichlorosilane with hydrazones 16, leading to enantiomerically enriched amines 17 (Scheme 2), resembles the allylation of aldehydes described above. Interestingly, in many cases the same types of sulfoxides were used as chiral catalysts. Thus, Kobayashi and co-workers used in this reaction several chiral sulfoxides 3a−f (Figure 1). The best one was again (R)methyl para-tolyl sulfoxide 3a, which in the reaction of 16b with allyltrichlorosilane gave the desired product 17b in 80% yield and with 98% ee. With the increase of steric hindrance of the substituents in the catalyst, a decrease of the enantioselectivity of the reaction was observed (up to 69% ee, while catalyst 3c led to racemic products). The absolute configuration of the products depended on the absolute configuration of the sulfoxide.16 Fernandez, Khiar, and co-workers confirmed the result of the application of 3b as catalyst and compound 16b as

Figure 4. Enantiomeric oxazoline sulfoxides 7−9. C


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Figure 6. Enantiomeric tetradentate bis-sulfoxides 13−15.

Scheme 2. Asymmetric Allylation of Hydrazones

Figure 9. Enantiomeric ferrocenyl sulfoxides 22−24.

nylferrocene 24 gave (S)-17b in quantitative yield and with 82% ee. This led to a conclusion that the isopropanesulfinyl group confers higher chemical reactivity and better enantiomeric discrimination than the para-toluenesulfinyl group and similar or even much better reactivity and enantiomeric discrimination than the most popular tert-butanesulfinyl group.19

substrate and broadened the scope of catalysts by using C2symmetric bis-sulfoxides 5 (n = 0, 1) (Figure 2), 18, and 19a−e (Figure 7). Enantioselectivity of this process was highly

2.3. Addition of Diethylzinc to Aldehydes and Enones

Asymmetric addition of diethylzinc to aldehydes (particularly to benzaldehyde) is the transformation that is commonly used in the evaluation of chiral catalyst efficiency (Scheme 3). Various types of sulfoxides were also assessed in this way. Scheme 3. Asymmetric Addition of Diethylzinc to Aldehydes

Figure 7. Enantiomeric C2-symmetric bis-sulfoxides 18 and 19.

In 1993 Carreño and co-workers were the first to report the use of easily available β-hydroxyalkyl sulfoxides 26 and 27 (Figure 10) as chiral catalysts in this reaction, using

dependent on the catalyst concentration, hindrance of the sulfoxide substituents, and distance between the sulfur atoms. However, in general the results were to some extent disappointing. In no case was the catalyst efficiency better than that of 3a. The best catalyst was 5 (n = 1), which gave hydrazine 17b in 81% yield and 82% ee.17 In turn, Juaristi and co-workers employed in this reaction substrates 16c−g and chiral C2-symmetric bis-sulfoxides 20 and 21 (Figure 8) as catalysts and obtained the corresponding products 17c−g in up to 89% yield and with up to 76% ee.18

Figure 10. Enantiomeric β-hydroxyalkyl sulfoxides 26 and 27.

benzaldehyde 1a as reagent. On the basis of the results obtained, it was concluded that enantiomeric excess of the product was higher when the hydroxyl center in the chiral ligand was more sterically hindered. However, the highest enantiomeric excess of the product did not exceed 45%.20 Kiełbasiński and co-workers synthesized chiral hydroxy sulfoxides 28−30 and 32 and carboxy sulfoxide 31 (Figure 11), which were checked as catalysts in this reaction. Unfortunately, these compounds exhibited low catalytic activity. The desired product was obtained in moderate yields (up to 60%) and with low enantiomeric excess (up to 32%). The best results were achieved using compound 32.21 Another type of sulfoxide-based catalyst that was applied in this reaction was a large series of sulfinylferrocenes. Carretero and co-workers reported sulfinylferrocenes containing various substituents bonded to the ferrocene moiety, e.g., amino groups 33, dialkylamino groups 34, alkyl- and arylamido groups 35a−

Figure 8. Enantiomeric pyridinium bis-sulfoxides 20 and 21.

Fernandez, Khiar, and co-workers studied the allylation of N(benzoyl) isopropylhydrazone 16b with allyltrichlorosilane using the electronically rich ferrocenyl sulfoxides 22−24 (Figure 9) as catalysts. All the compounds proved to be very efficient and gave the product 17b in over 95% yield, but with different enantiomeric excesses. Thus, (R)-para-toluenesulfinylferrocene 22 and (R)-tert-butanesulfinylferrocene 23 produced (R)-17b with 26% ee only, while (S)-isopropanesulfiD


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Figure 11. Enantiomeric hydroxy 28−30, 32, and carboxy 31 sulfoxides.

Figure 12. Enantiomeric ferrocenyl sulfoxides 33−36.

authors: “The ferrocene scaffold plays a significant role in terms of rigidity and chirality. The use of an amine bearing an electron withdrawing substituent for the formation of a complex with the first equivalent of diethylzinc, and of a sulfide moiety as the Lewis base for coordination to the second diethylzinc are key factors for the success of this reaction.”24 Completely new types of tridentate sulfinyl catalysts were developed by Kiełbasiński and co-workers. The synthesis was based on an initial enzyme-catalyzed desymmetrization of 2,2′di(hydroxymethylphenyl) sulfoxide 44, followed by a three-step transformation of the resulting monoacetate 45 into a series of 2-hydroxymethylphenyl-2′-aminomethylphenyl sulfoxides 46 bearing various enantiomeric amine moieties. Among the amines used, there were open-chain primary amines, as in 46a− f, and cyclic secondary amines, aziridines, as in 46g−j (Scheme 4).25,26 The new compounds, containing two stereogenic centers, with one located on the sulfinyl sulfur atom and the other on a carbon atom of the amine moiety, were used as chiral catalysts in a number of asymmetric syntheses, which will be presented below and in a later part of the overview. Compounds 46a−f were first examined as catalysts in the diethylzinc addition to benzaldehyde and exhibited poor catalytic activityboth the yields and enantiomeric excesses of product 25a did not exceed 50%.25 However, when compounds 46g−j were applied as catalysts in the diethylzinc addition to various aldehydes, the corresponding products were obtained in up to 99% yield and up to 97% ee. The stereogenic centers located on the aziridine moieties exerted a decisive influence on the absolute configuration of the products catalysts 46h and 46i, bearing opposite enantiomers of the aziridines, gave opposite enantiomers of alcohols 25a (R1 = Ph), 25i (R1 = p-MeOC6H4), and 25j (R1 = n-Pr). However, the influence of the sulfinyl stereogenic center could not be neglected, because the reaction was stereoselective even if the catalyst bears an achiral aziridine moiety 46g; however, in this case both the yields and the enantiomeric excesses of the products were below 50%. It was concluded that the mode of chelation of the diethylzinc molecule must be different for the aziridine-containing ligands when compared with those containing open-chain amine moieties.26

d, and alkane- and arenesulfonamide groups 36a−f (Figure 12), and used them as catalysts in the asymmetric addition of diethylzinc to benzaldehyde. The authors obtained alcohol 25a in up to 93% yield, but the enantiomeric excess strongly depended on the substituent at the nitrogen atom. Moderate stereoselectivity was observed for the following catalysts: amine 33 and dialkylamine 34 (up to 93% yield and up to 28% ee) and amides 35a−d (up to 76% yield and up to 60% ee). More interesting results were obtained with sulfonamides 36a−f (up to 90% yield and up to 82% ee), with the best being 36b and 36c. It was concluded that the planar chirality of the ferrocene unit is the decisive chiral element involved in the reaction.22,23 Grach, Reboul, and Metzner synthesized enantiomerically pure ferrocenyl derivatives of sulfoxides 37−42, bearing an additional stereogenic center, and sulfides 43 (Figure 13),

Figure 13. Enantiomeric ferrocenyl sulfoxides 37−42 and sulfides 43.

which were used as catalysts in asymmetric addition of diethylzinc to benzaldehyde. Using ligands 37a−c, 41, 42a, and 43a,b, the desired product 25a was obtained in the yields exceeding 90%. The efficiency of the reaction decreased with increasing steric hindrance, especially of the substituent R1. In the case of sulfoxides 37−42 and 43c, the enantiomeric excess varied from 1 to 70%. Surprisingly, the highest enantiomeric excess (>90%) was observed when sulfide (!) 43b was employed. Hence, the following conclusion was drawn by the E


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Scheme 4. Chemoenzymatic Synthesis of Tridentate Ligands/Catalysts 46

Compounds 46h−j proved to be very efficient also in the conjugate addition of diethylzinc to enones (Schemes 5 and 6).

In this context, the additions of two other organozinc reagents to aldehydes should be mentioned. The first one was the addition of phenylethynylzinc, formed in situ from phenylethyne 51 and diethylzinc, to aldehydes 1a,i−m in the presence of catalysts 46h−j (Scheme 7), which gave the corresponding products 52 in up to 96% yield and with up to 98% ee.29

Scheme 5. Enantioselective Conjugate Addition of Diethylzinc to Chalcones

Scheme 7. Stereoselective Addition of Phenylethynylzinc to Aldehydes

Scheme 6. Enantioselective Conjugate Addition of Diethylzinc to 2-Cyclohexenone

The second one was the addition of allylzinc, produced in situ, to aryl aldehydes in the presence of 2-pyridylmethyl paratolyl sulfoxide 53 (Figure 15), which afforded the corresponding products 2 in up to 80% yield, but with up to 42% ee only (Scheme 8).30

Products 48 and 49 were obtained in up to 94% yield and with up to 95% ee. Also in these cases the stereogenic centers located on the aziridine moieties exerted a decisive influence on the absolute configuration of the products. The reaction required addition of Ni(acac)2; hence, the sulfoxides should be considered here as ligands.27 Very recently, Liao and co-workers used for the reaction shown in Scheme 5 chiral sulfoxide phosphine ligands as complexes with copper (II) triflate (instead of the nickel derivative used in Scheme 5). The conversions were almost 100%, but the isolated yields did not exceed 44%. The highest enantiomeric excess, 93%, was achieved for ligand 50b (Figure 14).28

Figure 15. Enantiomeric 2-pyridylmethyl sulfoxide 53.

Figure 14. Enantiomeric phosphinyl sulfoxides 50. F


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Scheme 8. Stereoselective Addition of Allylzinc to Aldehydes

Scheme 9. Stereoselective Conjugated Addition of Arylboronic Acids to 2-Cyclohexenone

2.4. Reaction of Arylboronic Acids with Unsaturated Carbonyl Compounds

The rhodium-catalyzed conjugate addition of arylboronic acids to unsaturated carbonyl compounds is a convenient procedure for the formation of a new C-aryl bond. In 2011 Xu and coworkers synthesized a new class of chiral sulfoxide−olefin hybrid ligands 54 and 55 (Figure 16) and used them to catalyze this reaction.31 Simultaneously, a paper by Wan and co-workers came out in which similar ligands were described.32 The authors chose such structures because of the fact that heteroatom−olefin hybrid ligands generally exhibit increased coordination ability to transition metals. Indeed, these ligands generally exhibited excellent catalytic activity and enantioselectivity in the rhodium-catalyzed asymmetric addition of a large variety of arylboronic acids 56a−l to cyclohexenone (Scheme 9). Ligands (54b−k) with a substituent R1 attached at the terminal position of the double bond showed much better enantioselectivity (up to 90% ee) while maintaining the same

high catalytic activity (90%−99% yield). The best results were achieved with ligands 54b, 54e, and 54k. When ligand 54l was used in the reaction, the product yield was very low (only 4%) with 48% ee, suggesting that a sterically very bulky R1 group would disfavor the coordination and induction. Ligands 54o and 54p seriously hampered the reaction, giving no or only a trace amount of products. The authors also checked the substitution effect (R1) in the central benzene ring moiety (54r−v). The introduction of methoxy groups (54s, 54t, 54u) was beneficial (99% yield, up to 95% ee).31 The highest

Figure 16. Enantiomeric sulfoxide−olefin hybrids 54 and 55. G


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of 60). In comparison to bisphosphines, bis-sulfoxides appeared to be better ligands in rhodium-catalyzed conjugate acid transfer of arylboronic acids to cyclic enones.37,38 In turn, Zhou, Li, and co-workers prepared other functionalized bis-sulfoxide ligands 61 (Figure 18) and used them in the

enantioselectivity of the reaction (99% ee) was observed with ligand 55b.32 Similar results were later obtained by Liao and co-workers, who additionally found out that moving of substituent R1 from the terminal position of the olefinic moiety to the neighboring carbon atom, like in ligands 54w and 54z, with preservation of the same absolute configuration at sulfur, caused reversal of the absolute configuration of product 57.33,34 In an earlier work the same group used also catalysts 50 (Figure 14), which proved to be very efficient and, in the case of catalyst 50c, gave products 57 in 99% yield and with 98% ee.35 Knochel and co-workers introduced special sulfoxide−alkene hybrids as a new class of chiral diastereomeric heterodentate ligands for the stereoselective conjugated addition of arylboronic acids to 2-cycloalkenones. Both diastereomers proved to be excellent ligands in Rh-catalyzed 1,4-addition reactions, furnishing chiral products in yields up to 99%, with up to 97% ee but with opposite stereoconfigurations. The latter has been taken as proof that coordination of the sulfinyl group represents a crucial factor for the formation of the reactive Rh complex. Scheme 10 shows the structures of the rhodium Scheme 10. Stereochemical Model Explaining the Opposite Configurations of the Addition Products

Figure 18. Enantiomeric bis-sulfoxides 61 and monosulfoxide 62.

above reaction. For comparison, ligand 62 (Figure 18), lacking one of the sulfinyl groups, was also synthesized and investigated. The source of rhodium was [Rh(C2H4)2Cl]2. Rhodium complexes of ligands 61a−c bearing the tertbutanesulfinyl group showed no catalytic activity. On the contrary, complexes of ligands 61d−f bearing the paratoluenesulfinyl group proved to be efficient catalysts for the title addition to give the corresponding products 57 in up to 98% yield and with 99% ee. Ligand 62 was ineffective in this reaction.39 Chiral, nonsymmetric (with different substituents at the sulfinyl sulfur atoms) bis-sulfoxide 65 (Figure 19) is an effective complexes of both diastereomeric ligands 58 and a tentative model, as proposed by the authors, explaining the stereochemical outcome of the reaction.36 Dorta and co-workers reported the application of rhodium(I) complexes with bis-sulfoxides 1,1′-binaphthalene-2,2′-diyl-bis(para-tolyl sulfoxide)(para-tolylbinaso) 59 and para-tolyl-Mebipheso 60 (Figure 17) as catalysts in the same reaction to give unprecedentedly good results. The resulting reaction products 57 were always formed in almost quantitative yield and with 99% ee. Moreover, these results were achieved using small amounts of catalysts (1.5 mol % of ligand 59 and 0.5−1 mol %

Figure 19. Enantiomeric bis-sulfoxide 65.

ligand in rhodium-catalyzed asymmetric reaction of arylboronic acids with chromenones 63 (Scheme 11). Enantioselective addition gave flavons 64 in 70% yield and with very good enantiomeric excess (92−95% ee).40,41 2.5. Reaction of Sodium Tetraarylborates with Unsaturated Compounds

In 2011 Liao and co-workers synthesized (R,R)-1,2-bis(tertbutanesulfinyl)benzene 66 (Figure 20) and used this simple ligand, which is easily accessible, in the asymmetric conjugate addition of sodium tetraphenylborate to N-substituted 2,3dihydro-4-pyridone 67 (Scheme 12). The appropriate adduct 68 was obtained in high yield (92%) and with excellent enantiomeric excess (99%).40,42

Figure 17. Enantiomeric bis-sulfoxides 59 and 60. H


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Scheme 11. Asymmetric Conjugate Addition of Arylboronic Acids to Chromenones

The same group accomplished a copper-catalyzed threecomponent borylstannation of aryl-substituted alkenes using chiral phosphino sulfoxide ligands 50c and 50j−m. (Scheme 14). The reaction is assumed to proceed via borylcupration of a substrate with a chiral ligand both controlling stereochemistry of the B−Cu addition and promoting transmetalation of the enantioenriched alkyl-Cu species. Moreover, the catalyst precursor [{(R)-50m-CuCl}2] was isolated and characterized by X-ray analysis, which allowed the authors to propose a stereochemical model of the reaction (Scheme 15). The appropriate adducts were obtained in up to 99% yield and with up to 96%.44

Figure 20. Enantiomeric bis-sulfoxide 66.

Scheme 12. Asymmetric Conjugate Addition of Sodium Tetraphenylborate to 2,3-Dihydropyridone

2.7. Allylic Substitution

Asymmetric allylic substitution, a palladium−chiral ligand complex-catalyzed replacement of the acetoxy group by the malonate moiety in diphenylpropenyl acetate 71 (Scheme 16), has been a subject of continuous interest for many years and has resulted in the development of numerous chiral ligands. Among them, there are a considerable number of S-chiral sulfur derivatives, particularly various types of sulfoxides. In an early work, Shibasaki and co-workers synthesized new chiral bis-sulfoxide ligand (S,S)-1,2-bis(para-toluenesulfinyl)benzene (BTSB) 73 (Figure 22), which had S,S-bidentate chelating donor atoms in the aromatic ring (and resembled ligands 65 and 66 shown earlier), and used it in the form of a palladium complex to catalyze this reaction. As a result, product (S)-72 was obtained in 64% yield and 65% ee. The monosulfoxide 74 (Figure 22) was less efficient.45 Liao and co-workers synthesized a new class of chiral bidentate ligands of type SO−S, containing tert-butanesulfinyl moiety 75 (Figure 23), and evaluated their catalytic properties in the title reaction. The authors investigated the influence of the substituents in the sulfide moiety, the steric properties of aromatic ring, and the temperature on the catalytic activity and enantioselectivity of these ligands. The size of the substituent at the sulfide sulfur atom had a great impact on the enantioselectivity: a smaller group (75b R3 = Me) gave a higher yield (98%) and lower ee (27%) than the larger groups, e.g., 75c, R3 = t-Bu (69% yield and 59% ee). Ligands 75d−f exhibited similar activities, and the asymmetric inductions were slightly enhanced with increased steric hindrance of the substituents at the sulfur atom (up to 97% yield and up to 70% ee). When a methoxy group was introduced at either the 3- or 6position of the core phenyl ring 75g and 75h, similar enantioselectivities (73% ee) and efficiencies (up to 96% yield) were observed. In the case of ligands 75i (R2 = Oi-Pr) and 75j (R2 = OCH2OMe), the product was obtained in a similar yield (98%) but with better enantiomeric excess (80% ee). When the temperature was lowered to −25 °C, ligands 75i and 75j gave products with lower efficiency (up to 37% yields) but nearly the same value of enantiomeric excess. In

2.6. Other Additions to Double Bonds

Liao and co-workers used phosphino sulfoxides, analogues of 50c, containing various alkyl substituents at phosphorus, namely, 50e−i, in a copper (I)-catalyzed asymmetric addition of pinacolyl diborane (pinB-Bpin) to N-Boc-imines (Scheme 13 Scheme 13. Asymmetric Pinacolboryl Addition of N-Bocimines

Figure 21. Enantiomeric phosphino sulfoxides 50.

and Figure 21). The resulting products, N-Boc amino boronic esters, were obtained in up to 96% yield and with up to 96% ee. In some cases a larger anion, namely, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF−), was used to improve the reaction outcome.43 I


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Scheme 14. Asymmetric Borylstannation of Alkenes

Scheme 15. Proposed Stereochemical Model of Enantioselective Borylstannation

Scheme 16. Asymmetric, Palladium-Catalyzed Allylic Substitution of Diphenylpropenyl Acetate

Figure 24. Enantiomeric pyrrolidine sulfoxide 76 and bis-sulfoxide 77.

Zhao and co-workers synthesized chiral pyridyl sulfoxides as ligands, which proved to be effective for Pd-catalyzed allylic substitutions of dimethyl 2-fluoromalonate, to furnish monofluorinated allylic compounds in 11−83% yield and with 18− 94% ee (Scheme 17). The most efficient ligand was compound 78 (Figure 25).49 Hiroi and co-workers developed sulfoxide ligands containing a phosphine moiety for use in the Pd-catalyzed asymmetric allylic alkylation. Use of benzyl sulfoxide ligand 79a (Figure 26) afforded product 72 in 53% yield and 74% ee. Interestingly, use of ligand 79b, deprived of the amide carbonyl group, gave the opposite enantiomer of product 72 in 73% yield and 57% ee.50 In turn, aminophosphine sulfoxide ligand 79c led to product 72 in 93% yield and 74% ee or, when the reaction was performed at −78 °C, in 68% yield and 93% ee.51 Liao and co-workers used another phosphinoamide ligand, 79d, in the reaction of unsymmetrical allyl acetate with indole, which resulted in the formation of the corresponding adduct 80 (Figure 27) in 84% yield, with 94% ee and up to 96% regioselectivity.52 Maerten, Baceiredo, and co-workers synthesized another type of phosphorus-containing sulfoxide ligands containing both phosphine diamide and sulfoxide moieties. The enantioselectivity of the title reaction was generally moderate (37−61% ee), with the best being for ligand 81a (Figure 28), thus having only one stereogenic center on the sulfinyl sulfur atom (conversion 73% and 74% ee).53 Toru and co-workers for the first time reported synthesis of chiral 1-phosphino-1′-sulfinylferrocenes 82 (Figure 29). These spatially and electronically varied sulfoxides were used as chiral ligands in the allylic alkylation of acetate 71 by dimethyl malonate (cf. Scheme 16) in the presence of [Pd(η3-C3H5)Cl]2 and N,O-bis(trimethylsilyl)acetamide (BSA). When ligand 82a was used, product 72 was obtained in 82% yield and with 58% ee. When p-methoxybenzene-82b and 1-naphthalenesulfinylferrocene 82d were employed, product 72 was obtained in very good yield (99%) but also with moderate enantiomeric excess (68% ee).54

Figure 22. Enantiomeric bis-sulfoxide73 and monosulfoxide74.

Figure 23. Enantiomeric sulfoxides 75.

comparison with ligands 75g or 75h, the asymmetric induction exerted by ligand 75k bearing two methoxy substituents was slightly higher (yield 98% and 77% ee).46 Interestingly, the same reaction was successfully catalyzed using ligands 50 to give product 72 in up to 100% yield and with up to 89% ee.47 It is noteworthy that the same ligands 50 were applied also in the stereoselective 1,4 addition of diethylzinc to enones (cf. Scheme 5) and in the 1,4-addition of arylboronic acids to enones (cf. Scheme 9). Skarżewski and co-workers reported the employment of pyrrolidine sulfoxide 76 and bis-sulfoxide 77 ligands (Figure 24) in the same Pd-catalyzed asymmetric allylic alkylation. Use of ligand 76 afforded product 72 in 71% yield and 88% ee. However, use of the corresponding bis-sulfoxide 77 did not lead to the desired product 72 (yield < 5%), indicating that the chiral sulfinyl moiety of 76 was not crucial for the reactivity or enantioselectivity.48 J


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Scheme 17. Palladium-Catalyzed Allylic Substitution with Dimethyl 2-Fluoromalonate

Figure 27. Product of asymmetric allylic alkylation with indole. Figure 25. Chiral pyridyl sulfoxide 78.

2.8. Diels−Alder Cycloaddition

The Diels−Alder cycloadditions are very often used as indicators of the efficiency and stereoselectivity of a chiral catalyst/ligand, because they lead to the simultaneous formation of several stereogenic centers (e.g., Scheme 18). Llera and co-workers synthesized a series of enantiomerically pure hydroxy sulfoxides 85 (Figure 30) and examined their efficiency as chiral ligands in the reaction shown in Scheme 18. In all cases the chemical yields and diastereoselectivities of products 84 were very high (88%−95% yield, with endo/exo ratio >90:10). The highest overall yield and stereoselectivity were achieved when a complex of magnesium iodide with hydroxy sulfoxide 85a or its enantiomer 85e were used as catalysts (95% yield, with endo/exo ratio >98:2 and 88% ee of the endo diastereomer).55 Another type of chiral sulfoxide ligands 86−90 (Figure 31), which proved to be useful in the above reaction, was developed by Hiroi and co-workers. These new ligands contained chiral sulfinyl moiety and 1,3-oxazoline ring, which in some cases contained an asymmetric carbon atom. The reactions were performed using ligands 86−90 and Lewis acids as catalysts. Among the latter, magnesium iodide proved again to be most effective. The use of 2-methoxy-1-naphthyl sulfoxide 87f afforded product (2S)-84 in 90% yield, 94:6 endo/exo ratio, and 92% ee. This high asymmetric induction was explained in terms of the double acceleration with the two chiral centers, that of the oxazoline moiety and of the sulfinyl group, respectively, because the loss of chirality on the oxazoline ((RS)-87a and 88) or of the sulfoxide (while transformed into a sulfone) resulted in a substantial decrease of enantioselectivity.56

Figure 28. Enantiomeric phosphinamido sulfoxides 81.

Figure 29. Enantiomeric 1-phosphino-1′-sulfinylferrocenes 82.

Scheme 18. Asymmetric Diels−Alder Cycloaddition of Cyclopentadiene with Dienophiles

Figure 30. Enantiomeric hydroxy sulfoxides 85.

Figure 26. Enantiomeric phosphine sulfoxides 79. K


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the presence of chiral titanium complexes, prepared in situ from Ti(i-PrO)4 and sulfoxides 92 and 93. Application of the chiral complexes gave product 91 in good yields (up to 95%) but with moderate enantiomeric excess (up to 60%). As might be expected, at lower temperature the reaction was slower but the enantiomeric excess increased (for (SS)-92 from 76% yield and 20% ee at 0 °C to 52% yield and 49% ee at −84 °C). Interestingly, when the epimer with the opposite absolute configuration at the sulfur atom was used, the Scheme 20. Asymmetric Aldol Reaction

Figure 31. Enantiomeric sulfoxides with 1,3-oxazoline ring 86−90.

Scheme 19. Asymmetric Synthesis of Cyanohydrins

Figure 33. Enantiomeric tridentate sulfoxides 46.

stereochemistry of the reaction reversed and the stereoselectivity drastically changed. For example, for (SS)-93 the product (R)-91 was obtained in 95% yield and with 54% ee, while for (RS)-93, the product (S)-91 was isolated in 63% yield and with 22% ee. These results clearly indicated the “match− mismatch” effect resulting from the presence of two stereogenic centers in the molecules. They also confirm that the sulfoxide moiety was vital for the reactivity and stereoselectivity.57 2.9.2. Aldol and Other Aldol-Type Reactions. The tridentate sulfoxide derivatives 46a−d, which were presented in Scheme 4, were successfully applied either as ligands or

Figure 32. Enantiomeric sulfoxides with 1,3-oxazoline ring 92 and 93.

2.9. Other Asymmetric Reactions Catalyzed by Chiral Sulfoxides

Scheme 21. Asymmetric Nitroaldol (Henry) Reaction with Nitromethane

2.9.1. Addition of the Cyano Group to Aldehydes. The asymmetric synthesis of cyanohydrins continues to be an area of intense study due to the highly versatile nature of this structural motif. One of the most common synthetic procedures is the addition of cyanotrimethylsilane to aldehydes, performed in the presence of a chiral catalyst being a source of chirality (e.g., Scheme 19). Rowlands continued his interest in the synthesis and application of oxazoline-based sulfoxides and developed new chiral catalysts 92 and 93 (Figure 32). The reaction of benzaldehyde 1a with cyanotrimethylsilane was performed in L


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Mannich reaction, which proved that the presence of both stereogenic centers was vital for the stereoselectivity.61 Nevertheless, the most detailed studies were performed for the asymmetric Henry reaction (see Scheme 21). To this end, sulfoxide 46d was selected as a benchmark because of its high catalytic efficiency, and a wide series of its derivatives were synthesized and investigated as ligands. The changes made in the original structure of 46d comprised removal, substitution, or transformation of each functional group. All the new derivatives are collected below. Figure 34 presents the outcome of the asymmetric Henry reaction (see Scheme 21) performed with the use of benzaldehyde 1a as reagent and compounds 46d and 98−105 as ligands. The ligand numbers are accompanied by the values of the chemical yield (cy) and enantiomeric excess (ee) of product 91a.62 Inspection of Figure 34 indicates that the modification of the ligands had a considerable impact on the outcome of the reaction. Thus, protection of the hydroxy group (catalysts 98, 101, and 104) or removal of the stereogenic sulfinyl center (to produce sulfides 100−102 or sulfones 103−105) significantly lowered both the yield and the enantiomeric excess of the product. This might be taken as proof that the combination of the free hydroxy group and the stereogenic sulfinyl moiety constituted an essential condition for the catalytic efficiency of the ligands in spite of the fact that the main stereocontrol was exerted by the stereogenic center located on the amine moiety. Interestingly, replacement of the hydroxy group by the chiral amine moiety, to produce the diamino derivative 99, caused an enhancement in the chemical yield and preservation of the high enantiomeric excess of the chiral Henry product. Because the absolute configuration of both amino moieties in 99 was the same, the sulfinyl group was no longer a stereogenic center, which again proved the decisive role played by the stereogenic center located in the amine part of the catalyst. However, ligands 102 and 105, which bear two identical amino substituents but are deprived of the sulfinyl moiety, turned out to be inefficient as catalysts. This could be explained in terms of the particular coordinating ability of the sulfinyl group. All the findings allowed the authors to assume that the original catalysts 46 had a tridentate character because all three coordinating centers must simultaneously be present to make them efficient.62 2.9.3. Asymmetric Cyclopropanation. To extend the scope of applicability of the above tridentate ligands, an attempt was made at their use as catalysts for the asymmetric

catalysts in some asymmetric aldol-type condensations. Thus, the direct aldol reaction (Scheme 20) was performed in the presence of catalysts 46a−d (Figure 33) to give products 94 in Scheme 22. Asymmetric Aza-Henry Reaction with Nitromethane

up to 98% yield and with up to 96% ee. The best results were obtained using (RS,S)-46a and (RS,S)-46c. Interestingly, catalysts 46g−j, bearing aziridine moieties (Scheme 4), proved to be very inefficient, giving the product in both low yields and low ee values.58 The next reaction, in which ligands 46c,d were successfully used, was the nitroaldol (Henry) reaction (Scheme 21). Products 95 were obtained in up to 90% yield and up to 98% ee.59 In an analogous aza-Henry reaction (Scheme 22), the most efficient catalysts were sulfoxides 46a,b,c. Products 96 were obtained in up to 97% yield and up to 96% ee.60 In the next work, a possibility of the application of sulfoxides 46 as catalysts was for the first time checked in a threecomponent Mannich reaction (Scheme 23). The final results were in general satisfactory but dependent on the substrates and the catalyst used. The best ones were achieved for catalysts 46c and 46d and three products 97: when R2 = OH, R1 = CH2CH2Ph, the yield was 91%, the diastereomer ratio was 15:1, and the ee for the major diastereomer was 99%; when R2 = OH, R1 = p-NO2C6H4 with 85% yield, dr 20:1, 97% ee; when R2 = H, R1 = CH2CH2Ph with 82% yield and 95% ee.61 Because in all the cases presented (Schemes 5−7 and 20−23) the profound influence on the stereochemical course of the reactions was exerted by the absolute configuration of the amine moiety in the ligands/catalysts 46, an investigation was made to evaluate the importance of particular functional groups for the catalytic efficiency, particularly to examine whether the sulfinyl stereogenic center plays any role in the stereoinduction observed. Initial studies were performed for the asymmetric Scheme 23. Asymmetric Three-Component Mannich Reaction

M


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Figure 34. Influence of ligand structure on the asymmetric Henry reaction.62

the products depended on the absolute configuration of the stereogenic center located on the aziridine moiety.63

Scheme 24. Asymmetric Simmons−Smith Cyclopropanation of Allylic Alcohols

Scheme 25. Asymmetric Aziridination of α,β-Unsaturated Aldehydes

Simmons−Smith cyclopropanation of allylic alcohols. Notably, this was the first example of the application of these types of ligands in the asymmetric synthesis involving a carbenoid-like For an earlier report on the cyclopropanation of alkenes with ethyl diazoacetate mediated by a combination of salenruthenium and enantiomerically pure aryl alkyl sulfoxides, which gave the products in up to 92% yield, a cis/trans ratio up to 1:7.8 with up to 93% ee; see the paper by Nguyen and coworkers.64 2.9.4. Asymmetric Aziridination of Unsaturated Aldehydes. Another attempt to extend the scope of applicability of tridentate ligands 46 was their use as catalysts in the asymmetric aziridination of unsaturated aldehydes 108 (Scheme 25). Also in this case, the idea proved to be successful. The corresponding aziridines 109 were obtained in various yields, depending on the substituent R in substrate 108 (from 37 to 93%), although generally with very good diastereomer ratio (15:1) and enantiomeric excess of the major diastereomers (up to 92%). The most efficient catalysts were sulfoxides 46c,d bearing enantiomeric (S)- or (R)-1-(1′naphthyl)ethylamine moiety, respectively. Again, the stereogenic centers located in the amino moieties exerted a decisive influence on the absolute configuration of the products.65 2.9.5. Polydentate Sulfinyl Catalysts Bearing Prolinol Moiety. Successful application of tridentate sulfoxides bearing

Figure 35. Enantiomeric sulfoxide 46h.

intermediate (Scheme 24). This time, ligands 46g−j, bearing aziridine moieties, were used because of the application of diethylzinc as one of the reagents (cf. section 2.3 describing the reaction of diethylzinc with aldehydes27). The attempt occurred successful and brought some interesting findings. First, when starting from (E) substrates 106a−d, only trans products were obtained in up to 95% yield and with up to 94% ee. This finding was in agreement with the commonly accepted mechanism of this reaction, which indicates that the configuration of the starting alkene is retained in the product. The best result was obtained with substrate 106a and 46h as a ligand (Figure 35). Interestingly, the yield and enantioselectivity for the reaction of (Z)-cinnamyl alcohol 106e were moderate. As in previous cases, the absolute configuration of N


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Scheme 26. Synthesis of Prolinol-Containing Sulfinyl Catalysts

absolute configuration of the proline moiety exerted a decisive impact on the stereochemistry of these reactions, hence determining the absolute configuration of the products formed.66 2.9.6. Asymmetric Aromatic C−H Coupling. Yamaguchi, Itami, and co-workers utilized a palladium complex of an oxazoline diaryl sulfoxide−palladium/iron phthalocyanine 113 catalyst (Figure 36) in the enantioselective oxidative coupling of heterocycles with highly hindered arylboronic acids. The representative example using 2,3-dimethylthiophene and acid 56n gave the axially chiral biaryl 112 in 61% yield and with 61% ee (Scheme 27).67

additional functional groups provided a powerful stimulus to continue a search for other functionalized derivatives of this type. Hence, a new class of potential ligands/catalysts was synthesized that possess in their structure the prolinol moiety 110 and 111 (Scheme 26). The newly synthesized polydentate ligands, containing a stereogenic sulfinyl group, enantiomerically pure prolinol moieties, and free hydroxyl groups 110a,b, proved to be very efficient catalysts in a series of reactions of the asymmetric C− C bond formation, described earlier in this Review. For example, in the diethylzinc addition to aldehydes, as shown in Scheme 3, the product was obtained in 88% yield and 86% ee; in the aldol condensation, as shown in Scheme 20, the products were obtained in 44% yield and 84% ee; and in the threecomponent Mannich reaction as shown in Scheme 23, the products were formed in up to 94% yield, up to 20:1 diastereomer ratio, and up to 96% ee. Replacement of the central hydroxyl group in these ligands with the second prolinol

3. SULFINAMIDES Because of their ready availability in enantiomerically pure state and capability to form complexes with various metals, sulfinamides have recently piqued interest as chiral catalysts for asymmetric synthesis. For a recent highlight article, see ref 68, and for an overview of the applications of tertbutanesulfinamide and its derivatives in asymmetric synthesis, see ref 69. This part of the article will be focused on the application of various sulfinamides as ligands or organocatalysts and will be arranged according to the reactions in which they were used. Therefore, some reactions presented in the previous section will be recalled.

Figure 36. Enantiomeric palladium complex of an oxazoline diaryl sulfoxide−palladium/iron phthalocyanine 113.

3.1. Addition of Arylboronic Acids to Unsaturated Carbonyl Compounds and Related Reactions

Xu and co-workers reported in 2012 the use of ligands 114− 118 (Figure 37) in asymmetric reaction of arylboronic acids 56a and 56f with 2-cyclohexenone (cf .Scheme 9). Ligands 115a−d led to the corresponding products 57 in 99% yield and with 95% ee. To test the “match−mismatch”

moiety of the same absolute configuration gave new ligands 111a,b in which the sulfinyl group was not a stereogenic center anymore. Nevertheless, these ligands proved to be almost equally efficient as catalysts for the reactions investigated. The

Scheme 27. Enantioselective Oxidative Coupling of Heterocycles with Arylboronic Acids

O


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99% yield but with only 5% ee. However, ligands 118b−e afforded product 57 in 99% yield and 96% ee. Surprisingly, in the presence of ligand 118f, the reaction did not proceed.71 Simultaneously, Khiar and Fernandez used similar ligands of the type of 118 in the reaction of various cyclic and acyclic enones, and 118b was again the best and gave the products in up to 98% yield and with up to 99% ee.72 Du and co-workers used ligands 119 and 120 (Figure 38), which differed from ligands 116 in the position of the double bond, the length of the alkenyl chain, and the terminal substituents. The yield of products 57 reached 99%, while the ee values were up to 99%. Interestingly, it was found that irrespective of the diastereomeric ratio of the ligands the addition gave the products with almost the same conversions

Figure 37. Chiral sulfinamides 114−118.

effect, the authors compared ligands 114a and 114b. In the first case product 57 was obtained in 99% yield and with 95% ee, while in the second case it was obtained in 99% yield but with 85% ee. It was concluded that the substituents at C-1 and C-2 were relatively far away from the place where the rhodium atom is bound and that enantioselectivity of the reaction is mainly controlled by stereochemistry of the N-sulfinyl group. The

Figure 39. Chiral sulfinamides 125−130.

and ee values. Hence, a conclusion was drawn that the sulfur chirality played a crucial role in the asymmetric induction.73 A rhodium-catalyzed 1,2-addition of arylboronic acids 56 to unsymmetrical α-diketones 121 proceeded in a highly regioand enantioselective manner in the presence of ligand 118b (Scheme 28). The corresponding α-hydroxyketones 122 were obtained in overall yield up to 98%. The regioselectivity of the process varied depending on the structures of the substituents and in some cases reached 98%. Enantiomeric excesses of the particular regioisomers were up to 99%.74 Xu and co-workers used structurally similar ligands, sulfinamides bearing alkenyl substituents, 114, 118, and 125− 130 (Figure 37 and 39), in the asymmetric reaction of arylboronic acids with α-ketoesters 123a,b (Scheme 29). The

Figure 38. Chiral sulfinamides 119 and 120.

authors confirmed this hypothesis in the next experiments. When ligand 117 was employed in the reaction, racemic product 57 was obtained in 25% yield.70 The reaction performed in the presence of ligand 118a gave product 57 in

Scheme 28. Asymmetric Addition of Arylboronic Acids to α-Diketones

P


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Scheme 29. Asymmetric Addition of Arylboronic Acids to α-Ketoesters

124 were later transformed into chiral 3-aryl-3-hydroxyindole derivatives.77 An interesting supplement to this reaction was recently reported by Xu and co-workers, who used similar ligands in the intramolecular addition of the arylboronic acid group to ketone moieties, as in 131 (Scheme 30). The most effective was ligand 125, which led to products 132 in up to 99% yield and with up to 98% ee.78 The same (e.g., 120b and 126) and some other analogous ligands (125, 134, and 135) (Figure 40) were applied in the addition of arylboronic acids to cyclic aldimines 136 (Scheme 31) and ketimines 138 (Scheme 32). In the case of the reaction of cyclic aldimine 136 with paramethoxyphenylboronic acid 56c (Scheme 31), the best results were obtained using ligands 133 - 135; product 137 was formed in 94−97% yield and 93−97% ee.79 The reaction of cyclic N-sulfonyl-α-iminoester 138 with para-methoxyphenylboronic acid 56c (Scheme 32) proceeded most efficiently in the presence of ligands 133−135: the yield of product 139 exceeded 93% and the enantiomeric excess was 90%.80 As a continuation of their research, Xu and co-workers elaborated a highly enantioselective rhodium-catalyzed arylation of cyclic diketimines 140 with arylboronic acids by employing ligand 125 (Scheme 33). The products, tetrasubstituted 1,2,5-thiadiazoline 1,1-dioxides, were obtained in up to 99% yield and with up to 99% ee.81 Treatment of N-benzylisatin 142 with selected arylboronic acids 56 (Scheme 34) in the presence of tetra-ortho-substituted biphenyl sulfinimines containing diphenylphosphine moieties 144 and 145 (Figure 41) gave 3-aryl-3-hydroxyoxoindoles 143 in moderate yields (30−78%) and enantioselectivities (38−73% ee). It was concluded that the absolute configuration of adduct 143 was controlled by the ligand axial chirality rather than the sulfur chirality.82

reaction was carried out in the presence of these ligands and gave α-hydroxycarbonyl compounds with quaternary carbon atom 124 as products with both yield and enantiomeric excess up to 98%. The best result was obtained in the presence of ligand 118b. In turn, in the reaction employing ligands 114, 126, and 129, products 124 were obtained with moderate enantiomeric excess (from 19% to 63%) and in yield up to 54%.75 Ligand 118b proved to be very efficient also in the 1,2addition of a variety of arylboronic acids 56 to heteroaryl αketoesters 123c−h. The yield of the corresponding adducts 124 Scheme 30. Asymmetric Intramolecular Addition of Arylboronic Acids to Ketones

varied from 53 to 94%, and ee always reached 95−98%.76 The same procedures proved successful in the arylation of 2-N-Bocaminoarylglyoxalates 123i−m. In this case, ligand 129 proved to be the most efficient and led to products 124 in up to 81% yield and with up to 98% ee. Interestingly, selected products

3.2. Diels−Alder Reaction and Other Cycloadditions

Optically stable and commercially available chiral sulfinamides can be easily transformed into sulfinyl imines through condensation with aldehydes and ketones. Ellman and coworkers utilized this feature and synthesized bis(sulfinyl)imidamidines 146a−f (SIAM) (Figure 42), having a more strongly chelating N-sulfinyl binding element, which were then

Figure 40. Chiral sulfinamides 133−135. Q


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Scheme 31. Asymmetric Addition of para-Methoxyphenylboronic Acid to a Cyclic Aldimine

Scheme 32. Asymmetric Addition of para-Methoxyphenylboronic Acid to Cyclic N-Sulfonyl-α-iminoester 138

Scheme 33. Asymmetric Arylation of Cyclic Diketimines

Scheme 35. Asymmetric Diels−Alder Cycloaddition of Cyclic Dienes with N-Acryloyloxazolidinones

Scheme 34. Stereoselective Addition of Arylboronic Acids to N-Benzylisatin

Cu(SbF6)2, and ligand 146a gave cycloadduct 148a (n = 1) in 96% yield, diastereomer ratio (dr) of 99:1, and 98% ee (Scheme 35). Modification of ligand SIAM (146b−e) did not improve reactivity and selectivity. Derivatives 146b and 146f led to products with inferior results (up to 76% yields and up to 97% ee). The authors examined the most efficient catalytic system, Cu(SbF6)2-SIAM 146a, using variously substituted substrates, i.e., imide derivatives of crotonic acid 147b (yield 76%, dr 98:2, and 97% ee), cinnamic acid 147c (58%, dr 95:5, and 94% ee) and fumaric acid 147d (85%, dr 97:3, and 96% ee). For the most reactive system, the amount of catalyst was decreased to 1 mol % without lowering efficiency and stereoselectivity. Less-reactive dienes such as cyclohexadiene led to cycloadduct 148a (n = 2) in only 50% yield, dr 98:2, and 90% ee.83,84 Another reaction, in which catalyst Cu(SbF6)2-SIAM 146a was used, was the cycloaddition of compounds 147 and butadienes 149 (Scheme 36). The best result was achieved with 2,3-dimethylbutadiene 149b and 147a as reagents. The appropriate product was obtained in 96% yield and 93% ee.84 Roglans, Verdaguer, Riera, and co-workers prepared a series of chiral bidentate N-phosphine-tert-butanesulfinamides 151a−i (PNSO) (Figure 43) and examined them as ligands in the intramolecular [2 + 2 + 2] cycloaddition of enediyne 152 (Scheme 37). The best result was achieved using complex [Rh(COD)2]BF4 with ligand 151a. Product 153 was obtained in 92% yield but with a low ee equal to 32%.85 Xia, Xu, and co-workers synthesized a series of aromatic amide-derived nonbiaryl atropoisomers with a diphenylphosphine group and multiple stereogenic centers, e.g., 154, called by the authors “Xing-Phos” (Figure 44), and used them in the silver-catalyzed asymmetric [3 + 2] cycloaddition of aldiminoesters 155 with nitroalkenes 156. The procedure proved to

Figure 41. Chiral biphenyl sulfinimines containing diphenylphosphine moieties 144 and 145.

Figure 42. Chiral bis(sulfinyl)imidamidines 146 (SIAM).

used as ligands in the asymmetric Diels−Alder reaction of cyclic and acyclic dienes with N-acryloyloxazolidinones 147a−d.83,84 For example, the reaction of cyclopentadiene with compound 147a in the presence of copper (II) hexafluorostiborate, R


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Scheme 36. Asymmetric Diels−Alder Cycloaddition of Butadienes with N-Acryloyloxazolidinones

Figure 45. Enantiomeric sulfinimine ligands bearing a phosphine substituent 158.

oxygen atomson the sulfinamide and tertiary amide groups− as well as the phosphine phosphorus atom coordinate to the silver center.86,87 As continuation of this work, the ligands were also used in the [3 + 2] cycloaddition of 155 with N-substituted maleimides, and the results were only slightly inferior to those obtained above (up to 99% yield, up to 98:2 dr, and up to 93% ee).88

Figure 43. Chiral bidentate N-phosphinyl-tert-butanesulfinamides 151.

Scheme 37. Stereoselective Intramolecular [2 + 2 + 2] Cycloaddition of Enediyne 152

Figure 46. Iridium complex of the sulfinyl imine ligand 159.

3.3. Asymmetric Allylic Substitution

Schenkel and Ellman prepared sulfinimine ligands bearing a phosphine substituent 158 (Figure 45) and transformed them Scheme 39. Stereoselective Hydrogenation of Olefins

Figure 44. Chiral sulfinamides 154 “Xing-Phos”.

Scheme 38. Asymmetric [3 + 2] Cycloadditions

into palladium complexes to use as catalysts in the allylic alkylation reaction (see Scheme 16). The best results were obtained with ligands 158b,c; product 72 was obtained with up to 96% ee. On the other hand, ligand 158a led to product 72 exhibiting only 49% ee, while with ligand 158d the reaction was completely nonselective. The authors presented an X-ray crystal structure of π-allyl Pd-bound 158c, confirming that the ligand binds palladium through phosphorus and nitrogen to form a six-membered ring chelate.89 Iridium complexes of the sulfinimine ligands (Figure 46) were used in the asymmetric hydrogenation of functionalized olefins (Scheme 39). The procedure required hydrogen pressure of 50−100 bar. Although conversions were usually

be highly efficient and enantioselective and gave substituted pyrrolidines 157 (over 25 examples!) in up to 95% yield, up to 99:1 dr, and up to 99% ee (Scheme 38); it confirmed the potential of hemilabile atropoisomers as chiral ligands in asymmetric synthesis. An X-ray analysis of the Ag-154a complex showed a binuclear silver structure in which two S


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Scheme 40. Asymmetric Pauson−Khand Reaction of Monosubstituted Alkynes with Norbornadiene

Scheme 42. Asymmetric Reduction of Ketimines by Trichlorosilane

Scheme 41. Asymmetric Pauson−Khand Reaction of Symmetrical Disubstituted Alkynes with Norbornadiene

(Scheme 41). The expected products 163 were obtained in up to 85% yield and with up to 94% ee.93 3.5. Asymmetric Reduction of Ketimines

Easily accessible chiral sulfinamides 164 and 165 (Figure 47) were synthesized by Sun and co-workers and used as the first highly efficient and enantioselective organocatalysts for asymmetric induction, relying solely on a stereogenic sulfur center. In the presence of 20 mol % of a catalyst, a broad range of N-arylketimines were reduced by trichlorosilane to produce amines 169 in various yields and enantioselectivities, depending on the catalyst and substrate used (Scheme 42). The least efficient were catalysts 164 and 165e (products 169 were

>99%, the high ee = 94% was achieved only with the use of complex 159.90 3.4. Pauson−Khand Reaction

Selected ligands, namely, 151a,f,i (Figure 43), were transformed into tetracarbonyl cobalt complexes with monosubstituted alkynes 160 and used in the Pauson−Khand reaction with norbornadiene (Scheme 40). Products 161 were obtained in up to 99% yield and up to 99% ee.91,92 Later on, the same group constructed other types of complexes 162, starting from N-diarylphosphine(borane) para-toluenesulfinamides and symmetrical disubstituted alkynes, and subjected them to the reaction with norbornadiene

Figure 48. (S)-phenylalanine derived sulfinamides 172.

obtained in up to 60% yield and up to 27% ee), while the best results were achieved using catalyst 165h (up to 98% yield and 93% ee).94 The corresponding bis-sulfinamides 167a,b

Figure 47. Enantiomeric sulfinamides 164−168. T


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excesses (up to 96%), with the most efficient being catalyst 173a. Interestingly, a dramatic difference in enantioselectivity

Scheme 43. Asymmetric Hydrosilylation of 3-Aryl-1,4Benzoxazines

Scheme 45. Enantioselective Protonation of Enol Silanes

exhibited a slightly better enantioselectivity than 165h.95 Interestingly, catalysts 166a,b, bearing an additional stereogenic center in the prolinyl moiety, gave similar results, simultaneously showing a strong stereochemistry match between the sulfur and carbon centers of chirality.96 Very recently, Khiar, Fernandez, and co-workers developed a new type of catalyst bearing three stereogenic centers, located on the sulfinyl sulfur, N-α-carbon, and phosphoryl phosphorus atoms 168. These compounds proved to be highly efficient catalysts in the reaction discussed, leading to a wide range of arylmethylamines in up to 97% yield and with over 99% ee.97

was observed for the tert-butanesulfinamide diastereomers 173a (96% ee) and 174a (53% ee), although in both cases the same prevailing enantiomer of the product was obtained. This was taken as proof that the chirality of the sulfinyl substituent, in addition to its acidifying nature, is important for stereoinduction. However, when a catalyst containing achiral secondary amino acids was used, the yield of the products drastically dropped, which clearly demonstrated the importance of the proline scaffold for good reaction efficiency.99 3.7. Enantioselective Protonation

Jacobsen and co-workers hypothesized that the combination of a sulfonic acid and sulfinamide catalyst containing a urea moiety could produce a chiral acidic species capable of effecting enantioselective protonation reactions. To this end, they synthesized a series of variously N-substituted tert-butanesulfinamides and used them in the enantioselective protonation of enol silanes 177 (Scheme 45). Properly selected sulfinamides, e.g., 175 and 176 (Figure 50), and an acidic component, 2,4dinitrobenzenesulfonic acid (2,4-diNBSA), proved to be effective catalysts for the reaction. Interestingly, sulfinamide 176d exhibited better stereoinduction than that containing a urea substituent 175 (86% ee vs 67% ee).100 In an earlier work, Jacobsen and co-workers used a similar reactive combination, i.e., catalyst 175 and ortho-nitrobenzenesulfonic acid (NBSA) in the Povarov reaction, a formal [4 + 2] cycloaddition of N-aryl imines and enamides, and they succeeded in the preparation of the corresponding products in up to 92% yield, up to 20:1 dr, and up to 99% ee (Scheme 46).101

Figure 49. N-prolyl sulfinamides 173 and 174.

Scheme 44. Enantioselective Aldol Reaction

3.8. Enantioselective Transfer Hydrogenation of Ketones

Adolfsson and co-workers prepared chiral sulfinamides 178, derived from protected amino acids, and evaluated them as ligands for asymmetric catalysis in the metal-catalyzed enantioselective transfer hydrogenation of alkyl aryl ketones. The catalysts were generated in situ from sulfinamides 178 and arene complexes of rhodium and ruthenium, e.g., 179 (Figure 51). The catalytic reductions allowed for up to 77% conversion and led to the formation of chiral alcohols with up to 91% ee (Scheme 47).102

Wang, Sun, and co-workers achieved hydrosilylation of cyclic imines, namely, 3-aryl-1,4-benzoxazines 170, using (S)-phenylalanine-derived sulfinamide catalysts 172 (Figure 48) bearing stereogenic carbon and sulfur centers (Scheme 43). In the case of (SS,SC)-172, a broad range of chiral 3-aryl-3,4-dihydro-2H1,4-benzooxazine products 171 were obtained generally in 66− 98% yield and with 70−99% ee. It is worth noting that catalyst (RS,SC)-172, the diastereomer of (SS,SC)-172 with opposite stereochemistry on the sulfur atom, exhibited much lower enantioselectivity (product 171 obtained with 22% ee), indicating a strong match/mismatch effect between the two stereogenic centers.98

3.9. Asymmetric Allylation of Acylhydrazones

Both diastereomers of catalyst 175 (differing by the absolute configuration at the sulfinyl sulfur atom) were used in the indium-mediated asymmetric allylation of acylhydrazones 16c and 180 (Scheme 48). A great match−mismatch effect was observed. Thus, the (RS) diastereomer of 175 was highly efficientproducts 181 were obtained in up to 92% yield and up to 95% ee, while the (SS) diastereomer gave much worse results (75% yield and 26% ee).103

3.6. Enantioselective Aldol Reaction

Ellman and co-workers synthesized two series of enantiomerically pure diastereomeric N-prolyl sulfinamides, differing in the absolute configuration of the proline moiety, 173 and 174 (Figure 49), and demonstrated their utility for the enantioselective aldol reaction (Scheme 44). When catalysts 173 were used, all the products 94 were obtained in high yields (up to 91%) and with high enantiomeric

3.10. Asymmetric Strecker Reaction

A new class of chiral amide-containing sulfinamide organocatalysts was described by Khan and co-workers (simultaU


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Figure 50. Enantiomeric N-substituted tert-butanesulfinamides 175 and 176.

Scheme 46. Asymmetric Povarov Reaction

Scheme 48. Asymmetric Allylation of Acylhydrazones

Figure 52. Amide-containing sulfinamide 182.

Scheme 49. Asymmetric Strecker Reaction

Figure 51. Chiral sulfinamides 178 and ruthenium complex 179.

Scheme 47. Enantioselective Transfer Hydrogenation of Ketones

neously with the work of Wang and Sun, who developed analogous catalysts 172, Figure 4898) and proved to be highly efficient for the catalysis of the asymmetric Strecker reaction of N-benzhydryl and N-tosyl-substituted imines using EtOCOCN as a safe cyanide source. The most efficient was compound 182 (Figure 52), which catalyzed the reaction with a wide range of substrates to give the corresponding products in up to 90% yield and with up to 99% ee (Scheme 49).104

Figure 53. N-ortho-Hydroxyphenylmethyl sulfinamides 183.

3.11. Asymmetric Diethylzinc Addition to Aldehydes

A new type of chiral sulfinamides, bearing N-ortho-hydroxyphenylmethyl group 183 (Figure 53), was synthesized by Qin and co-workers and used in a catalytic enantioselective addition

of Et2Zn to aldehydes (cf. Scheme 3). The yields and enantiomeric excesses of products 25 varied depending on the catalyst structure and the aldehyde used. In the best cases, V


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Figure 54. Enantiomeric N-sulfinyl ureas 184−186.

Scheme 50. Asymmetric Aza-Henry Reaction with Nitroalkanes

the yields reached 95% and the enantiomeric excesses reached 96%.105

Scheme 51. Asymmetric Addition of Thioacetic Acid to Nitroalkenes

4. N-SULFINYL UREAS Urea- and thiourea-based hydrogen-bonding (H-bonding) organocatalysts have been developed for a variety of asymmetric nucleophilic additions. An important feature of these types of compounds is that they incorporate either an acidifying group or a chiral directing group. Ellman and coworkers were the first to introduce ureas and thioureas that incorporate the N-sulfinyl substituent, which is an acidifying moiety and simultaneously serves as a chiral controlling Starting from substrates 187a and 69j, product 188a was obtained in 64% yield and 95% ee, while starting from substrates 69j and 187b or 187c, products 188b and 188c, respectively, were obtained as the main diastereomers (dr 188:189 up to 93:7) in overall yield of up to 92% and with up to 96% ee. The most efficient catalyst was 186a.106 4.2. Addition of Thioacetic Acid to Nitroalkenes

Figure 55. Enantiomeric N-sulfinyl ureas 192 and 193.

Ellman and co-workers demonstrated that N-sulfinyl ureas 185a,b, 192, and 193 (Figure 54 and 55) used as organocatalysts promoted the first highly enantioselective addition of thioacetic acid to aromatic and aliphatic β-substituted nitroalkenes 190. The reaction was performed in cyclopentyl methyl ether (CPME) as solvent (Scheme 51). In this case, N-2,4,6-tri-isopropylbenzenesulfinyl urea 192 turned out to be the most selective catalyst in the addition of thioacetic acid to 2-arylo-1-nitroalkenes (up to 88% yield and up to 96% ee). Interestingly, diastereomeric N-2,4,6-tri-

element. They also established that the sulfinyl group was 2− 3 pK units more acidifying than the frequently used 3,5-bisCF3-phenyl group.106 Some applications of enantiomerically pure N-sulfinyl ureas and thioureas as chiral organocatalysts in asymmetric synthesis are exemplified below. 4.1. Aza-Henry Reaction

Ellman and co-workers used N-sulfinyl ureas 184−186 (Figure 54) as organocatalysts in the aza-Henry reaction (Scheme 50). W


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93% yield, up to 99:1 dr, and up to 98% ee (Scheme 52).109 In a subsequent paper the same group reported a catalytic

isopropylbenzenesulfinyl urea 193 not only gave inferior results (for 190a, conversion of 89% and 80% ee) but also led to the opposite enantiomer of product 191. This can be considered as proof that it is the chirality of the diaminocyclohexyl moiety that plays an important role in the stereoinduction observed. To sum up, it can be concluded that catalyst 192 possesses the ideal steric demand, acidity, and stereochemistry, among the other catalysts surveyed.107 The scope of the reaction was extended by the same group to the enantio- and diastereoselective thioacetic acid addition to cyclic α,β-disubstituted nitroalkenes, e.g., nitrocyclopentene, nitrocyclohexene, nitrocycloheptene, and variously substituted

Scheme 53. Asymmetric Reduction of Enamines with Trichlorosilane

Figure 56. Enantiomeric N-sulfinyl ureas 197 and 198.

3,4-dihydronaphthalenes. This time, diastereomeric catalyst 193 proved to be most effective to give the products in up to 98% yield, up to 99:1 dr, and up to 94% ee. Because the role of the chiral sulfinyl moiety in the stereochemistry of the reaction was not clear (vide supra), the authors decided to examine a series of catalysts that contained achiral replacements for the sulfinyl group, among others the sulfonyl moieties. None of them reached the selectivity of the original catalyst 193. This led to the conclusion that multiple factors contributed to asymmetric induction in sulfinyl urea catalysis, including the

enantioselective addition of α-substituted Meldrum’s acids 195 (R3 = Me, Et, CH2CH2Ph, Bn, and others) to α-substituted nitroalkenes 194 (R1 = H). Moreover, they also proved that the use of a catalyst with chirality residing only at the sulfinyl group (e.g., 198, Figure 56) did not diminish the yield (up to 98%) and enantioselectivity (up to 94% ee). The reaction was Scheme 54. Asymmetric Reduction of α-Fluoro-βEnaminoesters with Trichlorosilane

Scheme 52. Asymmetric Addition of Meldrum’s Acids to Nitroalkenes

acidity, steric size, electronics, solubility, and stereochemistry of the catalyst, and that the chirality of the sulfinyl group was very important to attain high enantioselectivity of the reaction.108 4.3. Addition of Meldrum’s Acids to Nitroalkenes

Addition of cyclohexyl Meldrum’s acid 195 (R3 = H, R4,R4 = cyclohexyl) to β- and α,β-disubstituted nitroalkenes was investigated by Ellman and co-workers using catalysts 184− 186, 192, 193, and 197 (Figures 54, 55, and 56). Interestingly, diastereomeric catalysts gave drastically differentiated selectivity, indicating a substantial matched−mismatched effect arising from the relative configurations of the sulfinyl and 1,2-diamine stereocenters. For example, the reaction of nitroalkene 194 (R1 = i-Bu, R2 = H) with 195 (R3 = H, R4 = Me) in the presence of catalyst 192 led to (R)-196 with 76% conversion and 87% ee, while in the presence of diastereomeric catalyst 193, (S)-196 was formed with 73% conversion and 64% ee. Compound 197 proved to be the best catalyst and led to products 196 in up to

Figure 57. Enantiomeric N-sulfinyl urea 202.

dependent on both the structure of Meldrum’s acid and the nitroalkene. The stereoselectivity was governed by the highly enantioselective protonation of the intermediary addition product.110 4.4. Asymmetric Reduction of Enamines

Sun and co-workers performed a highly enantioselective asymmetric reduction of β-amino nitroalkenes 199 with trichlorosilane in the presence of a series of S-chiral N-sulfinyl X


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reactions, particularly in the formation of new carbon−carbon bonds. The first overviews collecting the earliest achievements in this field came out in 2000114 and then in 2004,115 and they were followed by book chapters.116,117 Some of the most interesting examples are shown below and organized according to the types of reactions catalyzed.

Figure 58. Sulfoximines containing stereogenic sulfur atom 204.

5.1. Reactions of Allylic Substitution

ureas (Scheme 53). Among the ureas checked, the most efficient catalyst was 184a (Figure 54), which led to products 200 in up to 99% yield and with up to 97% ee.111 In a similar way, the asymmetric reduction of α-fluoro-βenaminoesters 201 was successfully implemented by Wang, Sun, and co-workers using trichlorosilane and catalyst 202 (Scheme 54 and Figure 57). The products, α-fluoro-βaminoesters 203, were obtained in up to 97% yield, up to 99:1 dr, and up to 78% ee.112

Bolm and co-workers synthesized chiral sulfoximines 204a−h (Figure 59) and used them as ligands in the reaction of allylic substitution, which was already shown earlier (see Scheme 16). Product 72 was obtained in 90% yield and with up to 65% ee, with the most efficient being ligand 204g.115,118

Figure 62. C-Phosphanylated sulfoximines 208 and 209.

Better results were obtained in the same laboratory when bissulfoximines 205 (Figure 60) were used as ligands. The yield of product 72 reached 99% with 93% ee. Decrease of the reaction temperature was beneficial and led to the increase of the product yield and ee. The highest enantioselectivity of the reaction was observed for ligand 205c.115,119 The first application of C-phosphanylated sulfoximines in asymmetric allylic substitutions was reported by Reggelin and co-workers. They prepared a series of enantiomerically pure sulfoximines 206 and 207 (Figure 61) and used them as ligands in the palladium-catalyzed asymmetric allylic alkylation of 1,3diphenylallyl acetate 71 (cf. Scheme 16) with the malonate anion. Only compound 206 and its sulfur epimer proved to be effective and led to product 72 in up to 99% yield and 95% ee. Interestingly, both epimers of 206 gave the same enantiomer of 72, which was taken as proof that the major source of enantioselectivity was the valine-derived C-stereogenic center, whereas the electron-rich imino nitrogen of the sulfoximine served as a rate enhancer.120 Another type of phosphorus-containing sulfoximine ligands, bearing the diphenylphosphine moiety at the β carbon atom with respect to the sulfoximine group (208 and 209, Figure 62), was developed by Gais and co-workers and used in the palladium-catalyzed asymmetric allylic alkylation of 1,3diphenylpropenyl acetate 71 with the malonate anion. The cyclic phosphanyl sulfoximines 209 proved to be very efficient ligands and gave products 72 in up to 98% yield and with up to 97% ee. On the contrary, the acyclic analogues 208 gave worse results.121,122 N-Phosphorylated sulfoximines, containing diazaphospholidine 210a,b or oxazaphospholidine 211a,b ring (Figure 63), bearing three stereogenic centers, and having absolute configurations as shown below, were used as ligands in various enantioselective palladium-catalyzed allylic substitutions (Scheme 55). In the case of the allylic aminations of 71 with pyrrolidine as the N-nucleophile, compounds 210 afforded product 212 in almost quantitative yields and with very good enantioselectivities (79−90% ee), regardless of the solvent or the ligand-to-metal ratio. At the same time, ligands 211 demonstrated rather poor enantioselectivities, with the highest ee value being only 58%. It was proven that the stereochemistry

Figure 59. Chiral sulfoximines 204.

5. SULFOXIMINES Sulfoximines containing stereogenic sulfur atom 204 (Figure 58) are attractive compounds in asymmetric synthesis due to

Figure 60. Bis-sulfoximines 205.

their stability in various reaction conditions and the possibility of functionalization of the nitrogen and carbon atoms. The first asymmetric reaction using a chiral sulfoximinebased reagent (an enantioselective reduction of an alkylphenyl ketone with stoichiometric amounts of a β-hydroxysulfoximine borane complex) was reported by Johnson and Stark in 1979.113 Since then a variety of sulfoximines have been widely used as catalysts or ligands in various types of asymmetric

Figure 61. C-Phosphanylated sulfoximines 206 and 207. Y


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Figure 63. N-Phosphorylated sulfoximines 210 and 211.

Similar results were obtained for the sulfonylation of 71: ligands

Scheme 55. Selected Enantioselective Palladium-Catalyzed Allylic Substitutions

210 led to quantitative yields of product 213 and 93−97% ee, while ligands 211 showed only moderate results, up to 60% ee. The same was observed for the allylation with dimethyl malonate; again ligands 210 proved to be highly efficient (up to

Figure 66. Sulfoximine-based ligands 219. Figure 64. C2-Symmetric bis-sulfoximines 205 and 214.

96% ee) and ligands 211 were much less effective (up to 50% ee).123

Scheme 56. Asymmetric Diels−Alder Cycloaddition of Cyclopentadiene with N-Acryloyloxazolidinones

Scheme 58. Asymmetric Mukaiyama Aldol Reactions

5.2. Diels−Alder Cycloaddition

Bolm and co-workers used various C2-symmetric bis-sulfoximines 205 and 214 (Figure 64) as ligands in the Diels−Alder reaction of cyclopentadiene and acryloyl-2-oxazolidinones 147 (Scheme 56). Of the series of sulfoximines assessed, compound 214 in combination with copper perchlorate as copper (II)

Figure 65. Bis-sulfoximine 217 and quinoline-based C1-symmetric sulfoximines 218.

Scheme 57. Asymmetric Cycloaddition of 1,3Cyclohexadiene with Ethyl Glyoxalate

of product 212 was predominantly determined by the phosphacycles and not, as hoped for, by the sulfur moiety.

Figure 67. C1-Symmetric aryl-bridged oxazolinyl sulfoximines 221. Z


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Scheme 59. Asymmetric Synthesis of Amides Using Vinylogous Mukaiyama Aldol Reaction

Scheme 61. Asymmetric Synthesis of Phosphonic γ(Hydroxyalkyl)butenolides

source was selected as the optimal catalytic system. Cycloadducts 215 were obtained as the main stereoisomers in up to 98% yield, 92:8 endo/exo ratio, and up to 84% ee.124 In a similar way, the cycloaddition of 1,3-cyclohexadiene with ethyl glyoxalate was performed using analogous bis-sulfoximine 217 (Figure 65). Product 216 was obtained in 98% yield, endo/ exo ratio 99:1, and with 98% ee (Scheme 57).125 The same reaction was carried out using quinoline-based C1symmetric sulfoximines 218 (Figure 65) as chiral ligands and afforded cycloadducts 216 in up to 98% yield, endo/exo ratio up to 99:1, and with up to 96% ee.126 5.3. Aldol-Type Reactions

double bond, and two different carbonyl groups, were obtained in up to 75% yield and up to 93% ee.130

Bolm and co-workers published a series of papers devoted to the Mukaiyama aldol reactions and developed a variety of sulfoximine-based ligands 219 (Figure 66). In the first publication, aminosulfoximines 219a−e were synthesized and used in the reaction between silyl enol ethers and α-keto esters (Scheme 58). Sulfoximine 219e proved to be the most efficient ligand, and the optimization of the reaction conditions allowed Scheme 60. Asymmetric Aldol Reaction of Cyclic Dienol Silanes with α-Ketoesters

Figure 68. C-Phosphanylated sulfoximines 228.

In a subsequent publication, Bolm’s group substantially extended the number of ligands 219 by the new examples f−m (and some others differing in the substituents at the sulfoximine Scheme 62. Asymmetric Iridium-Catalyzed Hydrogenation of Imines

the authors to obtain products 220 in up to 99% yield and up to 99% ee.127 In the next publication, the authors described the synthesis of novel C1-symmetric aryl-bridged oxazolinyl sulfoximines 221 (Figure 67). However, these derivatives proved to be relatively less efficient as ligands in the copper-catalyzed enantioselective Mukaiyama-type aldol reactions, giving products 220 in 33− 92% yield and with 9−84% ee. Only in the case of ligand 221, containing the L-(−)-norephedrine-derived oxazoline ring, was product 220 (R1 = Ph, R2 = H) obtained in 94% yield and with 94% ee.128 Simultaneously, Bolm and co-workers developed a procedure for the copper-catalyzed asymmetric vinylogous Mukaiyama aldol reaction,129 which was subsequently used in the synthesis of amides, starting from α-ketoesters 123 and achiral N,Osilylated ketene aminals 222 (Scheme 59). Again, the best ligand was aminosulfoximine 219e. The highly functionalized products 223, bearing a fully substituted stereogenic center, a

sulfur atom, which are not shown here) and applied them in the vinylogous Mukaiyama aldol reaction of α-ketoesters 123 with a number of cyclic dienol silanes 224 (Scheme 60). They performed detailed studies of the variation of the ligand backbones and succeeded in the adjustment of their structures to the requirements of the substrates. In this way, products 225 were obtained in up to 99% yield and up to 99% dr, with the AA


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anti diastereomers prevailing. The enantiomeric excess of the major diastereomers reached 99%, while that of the syn diastereomers was generally lower. The best ligands for this reaction were 219i,j,m.131

Figure 70. BINOL-derived N-phosphino sulfoximines 234.

Scheme 65. Asymmetric Hydrogenation of Olefins

Figure 69. Naphthalene-bridged P,N-type sulfoximines 229 and their iridium complexes 229A.

In a similar way, asymmetric synthesis of phosphonic γ(hydroxyalkyl)butenolides 227 was achieved by the same group (Scheme 61). They performed a reaction between a series of αketo phosphonates 226 with 2-trimethylsilyloxyfuran 224 (X =

iridium 229A in the presence of sodium tetrakis-3,5-bis(trifluoromethyl)phenylborate (NaBARF) (Figure 69), and used as catalysts to reduce a series of α,β-unsaturated ketones 230 (Scheme 63). The yields of products 231 were in the range

Scheme 63. Asymmetric Iridium-Catalyzed Hydrogenation of α,β-Unsaturated Ketones

Scheme 66. Asymmetric Copper-Catalyzed Carbonyl-Ene Reactions

O) using ligands 219 in the presence of copper salts. In this case, among a number of ligands checked, the best proved to be 219d,e, which gave products 227 in almost quantitative yields and with de and ee exceeding 99%.132 Scheme 64. Asymmetric Iridium-Catalyzed Hydrogenation of Quinolines 70−94%. The highest ee values (up to 94%) were achieved by using the complex that was obtained from ligand 229a.134 5.4.3. Asymmetric Quinoline Hydrogenations. The same ligands 229 and the same reaction procedure were used in the enantioselective iridium-catalyzed hydrogenation of quinolines 232 (Scheme 64). The reaction usually proceeded smoothly, resulting in up to 95% conversions. The highest enantiomeric excesses of products 233 were again obtained using the iridium complex of ligand 229a (up to 92%). In the case of ligands 229b−i, the ee values did not exceed 78%.135

5.4. Hydrogenation Reactions

5.4.1. Asymmetric Reduction of Ketimines. Another class of C-phosphanylated sulfoximines 228 (Figure 68) was developed by Bolm and co-workers and used in Ir-catalyzed asymmetric hydrogenations of acyclic N-aryl imines 168 (Scheme 62; cf. Scheme 42). Under optimized reaction conditions (sulfoximine 228d, 1.1 mol %; [Ir(COD)Cl]2, 0.55 mol %; H2, 20 bar; room temperature (rt)), full conversions were attained and products 169 were obtained with up to 98% ee.133 5.4.2. Enantioselective Hydrogenation of Linear α,βUnsaturated Ketones. Ligand 228d proved to be inefficient in the hydrogenation of α,β-unsaturated ketones. Therefore, new ligands, namely, naphthalene-bridged P,N-type sulfoximines 229, were synthesized, transformed into complexes with

Scheme 67. Asymmetric Halogenation of β-Oxo Esters

AB


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5.4.4. Rh-Catalyzed Asymmetric Hydrogenation of Olefins. Reetz, Bondarev, Gais, and Bolm synthesized BINOLderived N-phosphino sulfoximines 234 (Figure 70) and used them as ligands in the Rh-catalyzed hydrogenation of functionalized olefins 235. The corresponding products 236

Figure 73. Enantiomerically pure sulfonimidamides 244 and 245.

chlorinated with NCS using ligand 219e, product 240 was obtained with 91% ee.138 5.5.3. Enantioselective Diethylzinc Addition to Aldehydes. A new type of chiral sulfoximines derived from 3aminoquinazolinones 241 (Figure 71) was obtained and used

Figure 71. Chiral sulfoximines derived from 3-aminoquinazolinones 241.

Scheme 69. Solvent-Free Asymmetric Organocatalytic Aldol Reaction

were obtained in quantitative yields with up to >99% ee (Scheme 65). Interestingly, the absolute configuration of the products was governed by the chirality of the BINOL moiety, and the chirality of the sulfoximine moiety played a small role.136 5.5. Other Asymmetric Reactions Catalyzed by Chiral Sulfoximines

5.5.1. Carbonyl-ene Reactions. Selected aminosulfoximines 219 were applied as ligands in Cu-catalyzed carbonylene reactions (Scheme 66). The resulting hydroxy esters 237 and 238 were obtained with high enantiomeric excesses (up to

in a catalytic enantioselective addition of Et2Zn to aldehydes in the absence and presence of (i-PrO)4Ti (cf. Scheme 3). The yields and enantiomeric excesses of products 25 varied depending on the catalyst structure and the aldehyde used. In the best cases, the yields reached 99% and the enantiomeric excess was 91%.139 5.5.4. Asymmetric Biginelli Reaction. Finally, an attempt was made to introduce enantiopure sulfoximines to the field of organocatalysis. To this end, a series of sulfonimidoylcontaining thiourea-type compounds 242 (Figure 72) was synthesized and used as catalysts (in substoichiometric

Figure 72. Sulfonimidoyl-containing thiourea-type compounds 242.

91% ee) but in moderate yields (up to 63%). The best results were obtained using ligands 219a and 219m.137 5.5.2. Enantioselective Halogenation of β-Oxo Esters. Frings and Bolm developed a general protocol for the enantioselective halogenation of β-oxo esters (Scheme 67). All three chlorination, bromination, and fluorination reactions proceeded well. Starting from both cyclic and acyclic substrates, the corresponding products were formed in excellent yields but generally with low to moderate enantioselectivities (below 73% ee). Only in one case, when a cyclic substrate, (CH2)4, was

Figure 74. C2-Symmetric sulfurous diamides 246.

Scheme 68. Asymmetric Biginelli Reaction

AC


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general highly efficient. Therefore, the eventual selection of a ligand/catalyst should be made by the interested scientists with regard to its accessibility in the enantiomerically pure form. From the point of view of envisaged progress in this field, the following issues can be addressed. Although a possibility of inventing new classes of S-chiral ligands/catalysts cannot be excluded, there are not too many structures to be considered. As an example, S-chiral sulfodiimines could be taken into account. As a matter of fact, they have not been found among the ligands/catalysts thus far, which is most probably due to general difficulties to access them in enantiopure form. Interestingly, no records on “chiral sulfodiimines” or “optically active sulfodiimines” can be found in SciFinder. Nevertheless, even if they have been synthesized, it is difficult to predict whether they will be more efficient than the corresponding sulfoxides, sulfinamides, or sulfoximines. Organocatalysis, a particularly promising modern synthetic approach that is based on the use of metal-free organocatalysts, gives a possibility of their industrial applications. Nevertheless, because in the case of S-chiral organocatalysts such an approach seems to be still in its infancy, new reactions with broader substrate scopes must be explored in the future to make it more valuable and applicable. An interesting area, which should be taken into account, is to synthesize appropriate polymer-supported, thus immobilized, S-chiral sulfur-containing ligands/catalysts and use them as insoluble species in heterogeneous catalysis. This methodology should facilitate the workup of the reaction and enable an easy recovery and recycling of the catalysts. To sum up, many challenges remain, and one can await further developments in this field.

quantities) in the asymmetric Biginelli reaction, leading to a dihydropyrimidinone 243 (Scheme 68). Generally, the ee values of the product were low. However, when catalyst 242b was used in 10 mol % quantity and the reaction was performed at a low concentration of the urea substrate (0.025 mol/L), the desired product was obtained in 92% yield and with 44% ee.140

6. RELATED S-CHIRAL LIGANDS 6.1. Amino-Functionalized Sulfonimidamides

Enantiomerically pure sulfonimidamides 244 (both diastereomers with different absolute configurations at the sulfur atom, Figure 73) were synthesized and applied as organocatalysts in a solvent-free asymmetric aldol reaction of various aromatic aldehydes with cyclohexanone. For the diastereomer (SC,RS) the appropriate products were obtained in yields from to 22 to 84%, with the anti/syn ratio up to 98:2 and up to 98% ee (Scheme 69). The diastereomer with an opposite absolute configuration at the sulfur atom proved to be less effective, which was taken as proof that the stereogenic center on sulfur influenced the stereochemical result of the catalysis.141 In turn, the corresponding sulfonimidamides 245 were applied as ligands in the asymmetric copper-catalyzed Henry reaction of aromatic aldehydes with nitromethane (cf. Scheme 21). Depending on the aldehyde, the copper source, and the base used, products 95 were obtained in 50−93% yield and with up to 95% ee.142 6.2. Chiral Sulfurous Diamide

The first example of application of C2-symmetric sulfurous diamides 246 (Figure 74) as chiral ligands in transition metal catalysis was presented by Chen and co-workers. They used them for the enantioselective rhodium-catalyzed asymmetric 1,4-addition reactions of phenylboronic acid to α,β-unsaturated carbonyl compounds (cf. Scheme 9). High yields (up to 99%) and excellent enantioselectivities (up to 96%) were achieved for both cyclohexenone and cyclopentenone substrates.143

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Piotr Kiełbasiński: 0000-0002-0020-2492

7. CONCLUSIONS In this Review an attempt was made to provide comprehensive and critical information on the sulfur-based S-chiral ligands and organocatalysts used in asymmetric synthesis. Such derivatives are generally easily available, which makes them particularly useful in modern stereoselective transformations. The enantiomerically pure derivatives of variously substituted sulfoxides, sulfinamides, N-sulfinylureas, and sulfoximines or their metal complexes have proven to be highly efficient catalysts for a considerable number of the reactions of asymmetric synthesis. Although in certain cases the sulfur stereogenic center does not exert a decisive impact on the stereochemistry and stereoselectivity of a given reaction, its presence in the ligand/catalyst is always crucial. It either enhances the stereoselectivity or at least decides on the efficacy of the ligand/catalyst, which may be caused by specific interactions of the enantiomeric sulfinyl or sulfoximinyl moieties with a metal in organometallic complexes or directly with substrates to be reacted. However, it must be stressed that among the compounds discussed it is difficult to indicate a lead structure of a ligand/catalyst, because a large number of scattered data points makes the comparison of particular structure types practically impossible. Moreover, various types of S-chiral ligands/catalysts do not significantly differ in their stereoselectivities within the same reaction kinds, being in

Notes

The authors declare no competing financial interest. Biographies Sylwia Otocka (née Kaczmarczyk) was born in 1983 in Łódź, Poland. She graduated in chemistry from the Łódź University of Technology (2007). Subsequently, she joined Professor Piotr Kiełbasiński’s research group at the Department of Heteroorganic Chemistry, Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, where her research interest was focused on the development of new chiral heteroorganic catalysts/ligands via enzyme-promoted desymmetrization or kinetic resolution. She received her Ph.D. degree in Chemistry in 2014. Then she worked at the Bioorganic Chemistry Laboratory, Faculty of Pharmacy, Medical University of Łódź. Presently, she is working at the Department of Structural Biology, Medical University of Łódź. Małgorzata Kwiatkowska (née Albrycht) was born in Krosno, Poland. She received her M.Sc. degree in organic chemistry from the Łódź University of Technology in 1999. Her doctoral thesis on the application of heteroorganic compounds as substrates for enzymatic reactions and chiral catalysts for asymmetric synthesis was completed under the supervision of Professor Piotr Kiełbasiński in 2006. Her current scientific interests at the Department of Heteroorganic Chemistry, Centre of Molecular and Macromolecular Studies, Polish AD


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(10) Massa, A.; Malkov, A. V.; Kocovsky, P.; Scettri, A. Asymmetric Allylation of Aldehydes with Allyltrichlorosilane Promoted by Chiral Sulfoxides. Tetrahedron Lett. 2003, 44, 7179−7181. (11) Wang, P.; Chen, J.; Cun, L.; Deng, J.; Zhu, J.; Liao, J. Aryl tertButyl Sulfoxide-promoted Highly Enantioselective Addition of Allyltrichlorosilane to Aldehydes. Org. Biomol. Chem. 2009, 7, 3741− 3747. (12) Fulton, J. R.; Kamara, L. M.; Morton, S. C.; Rowlands, G. J. The Sulfinyl Moiety in Lewis Base-Promoted Allylations. Tetrahedron 2009, 65, 9134−9141. (13) Rowlands, G. J.; Kentish Barnes, W. Chiral Sulfoxides in the Enantioselective Allylation of Aldehydes with Allyltrichlorosilane. Chem. Commun. 2003, 2712−2713. (14) De Sio, V.; Acocella, M. R.; Villano, R.; Scettri, A. New Chiral Imino- and Amino-Sulfoxides as Activators of Allyl Trichlorosilane in the Asymmetric Allylation of Aldehydes. Tetrahedron: Asymmetry 2010, 21, 1432−1435. (15) Massa, A.; Acocella, M. R.; De Sio, V.; Villano, R.; Scettri, A. A Catalytic Asymmetric Allylation of Aldehydes with Allyl Trichlorosilane Activated by a Chiral Tetradentate Bis-Sulfoxide. Tetrahedron: Asymmetry 2009, 20, 202−204. (16) Kobayashi, S.; Ogawa, C.; Konishi, H.; Sugiura, M. Chiral Sulfoxides as Neutral Coordinate-Organocatalysts in Asymmetric Allylation of N-Acylhydrazones Using Allyltrichlorosilanes. J. Am. Chem. Soc. 2003, 125, 6610−6611. (17) Fernandez, I.; Valdivia, V.; Leal, M. P.; Khiar, N. C2-Symmetric Bissulfoxides as Organocatalysts in the Allylation of Benzoyl Hydrazones: Spacer and Concentration Effects. Org. Lett. 2007, 9, 2215−2218. (18) Garcia-Flores, F.; Flores-Michel, L.; Juaristi, E. Asymmetric Allylation of N-Benzoylhydrazones Promoted by Novel C2-Symmetric Bis-Sulfoxide Organocatalysts. Tetrahedron Lett. 2006, 47, 8235−8238. (19) Fernandez, I.; Valdivia, V.; Gori, B.; Alcudia, F.; Alvarez, E.; Khiar, N. The Isopropylsulfinyl Group: A Useful Chiral Controller for the Asymmetric Aziridination of Sulfinylimines and the Organocatalytic Allylation of Hydrazones. Org. Lett. 2005, 7, 1307−1310. (20) Carreño, M. C.; Garcia Ruano, J. L.; Maestro, M. C.; Martin Cabrejas, L. M. Catalytic Activity of Chiral β-Hydroxysulfoxides in the Enantioselective Addition of Diethylzinc to Benzaldehyde. Tetrahedron: Asymmetry 1993, 4, 727−734. (21) Kiełbasiński, P.; Albrycht, M.; Mikołajczyk, M.; Wieczorek, M. W.; Majzner, W. R.; Filipczak, A.; Ciołkiewicz, P. Synthesis of Chiral Hydroxythiolanes as Potential Catalysts for Asymmetric Organozinc Additions to Carbonyl Compounds. Heteroat. Chem. 2005, 16, 93− 103. (22) Priego, J.; Mancheño, O. G.; Cabrera, S.; Carretero, J. C. Aminosubstituted tert-Butylsulfinylferrocenes as a New Family of Chiral Ligands: Asymmetric Addition of Diethylzinc to Aldehydes. Chem. Commun. 2001, 2026−2027. (23) Priego, J.; Mancheño, O. G.; Cabrera, S.; Carretero, J. C. 2Amino-Substituted 1-Sulfinylferrocenes as Chiral Ligands in the Addition of Diethylzinc to Aromatic Aldehydes. J. Org. Chem. 2002, 67, 1346−1353. (24) Grach, G.; Reboul, V.; Metzner, P. Screening of Amino Sulfur Ferrocenes as Catalysts for the Enantioselective Addition of Diethylzinc to Benzaldehyde. Tetrahedron: Asymmetry 2008, 19, 1744−1750. (25) Rachwalski, M.; Kwiatkowska, M.; Drabowicz, J.; Kłos, M.; Wieczorek, W. M.; Szyrej, M.; Sieroń, L.; Kiełbasiński, P. EnzymePromoted Desymmetrization of Bis(2-Hydroxymethylphenyl) Sulfoxide as a Route to Tridentate Chiral Catalysts. Tetrahedron: Asymmetry 2008, 19, 2096−2101. (26) Leśniak, S.; Rachwalski, M.; Sznajder, E.; Kiełbasiński, P. New Highly Efficient Aziridine-Functionalized Tridentate Sulfinyl Catalysts for Enantioselective Diethylzinc Addition to Carbonyl Compounds. Tetrahedron: Asymmetry 2009, 20, 2311−2314. (27) Rachwalski, M.; Leśn iak, S.; Kiełbasiń s ki, P. Highly Enantioselective Conjugate Addition of Diethylzinc to Enones Using

Academy of Sciences, focuses on the chemoenzymatic synthesis of novel chiral heteroorganic compounds and their application in asymmetric synthesis as chiral organocatalysts or ligands for the complexes with transition metals. Lidia Madalińska was born in 1985 in Łódź, Poland. She graduated in chemistry from the University of Łódź in 2009. In the same year, she started doctoral studies at the Department of Heteroorganic Chemistry in the Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences. She is working in Professor Piotr Kiełbasiński’s research group as a Ph.D. student, focusing her attention on biocatalysis and biotransformations, particularly on the synthesis of new chiral heteroorganic catalysts/ligands via enzyme-promoted desymmetrization or kinetic resolution. Piotr Kiełbasiński was born in Łódź, Poland, in 1948. He graduated from the Technical University of Łódź in 1970. Since then he has been employed at the Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, where he obtained his academic degrees (Ph.D. in 1977 under the supervision of Professor Marian Mikołajczyk, D.Sc. in 2000, and professorship in 2009). He spent his postdoctoral stay at the University of Nijmegen, The Netherlands, working with Professor Binne Zwanenburg (1977−1978). He presently holds a position of Full Professor and Head of the Department of Heteroorganic Chemistry. His scientific interests comprise chemistry and stereochemistry of organosulfur and organophosphorus compounds, biocatalytic synthesis of chiral heteroorganic derivatives, and application of chiral heteroorganic ligands/catalysts in asymmetric synthesis. He is author or coauthor of 123 publications, including one book (Mikołajczyk, M.; Drabowicz, J.; Kiełbasiński, P. Chiral Sulf ur Reagents: Applications in Asymmetric and Stereoselective Synthesis; CRC Press: Boca Raton, FL, 1997) and a number of book chapters, among them in Patai’s series, in Comprehensive Organic Group Transformations, Houben-Weyl, and Science of Synthesis. He is also coeditor of one book: Enzymes in Action: Green Solutions for Chemical Problems; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2000, and two conference issues of Phosphorus, Sulfur and Silicon and the Related Elements (1991 and 2008). He has been co-organizer of several international conferences, mainly devoted to heteroorganic chemistry. He has supervised four Ph.D. theses; the next two are pending.

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