Enantioselective Cobalt-Catalyzed Transformations - Chemical

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Enantioselective Cobalt-Catalyzed Transformations

Chem. Rev. 2014.114:2775-2823. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/27/19. For personal use only.

Hélène Pellissier* and Hervé Clavier Aix Marseille Université, Centrale Marseille, CNRS, iSm2 UMR 7313 13397, Marseille, France 1. INTRODUCTION The catalysis of organic reactions by metals still constitutes one of the most useful and powerful tools in organic synthesis.1 Although asymmetric synthesis is sometimes viewed as a subdiscipline of organic chemistry, actually this topical field transcends any narrow classification and pervades essentially all chemistry. Of the methods available for preparing chiral compounds, catalytic asymmetric synthesis has attracted the most attention. In particular, asymmetric transition-metal catalysis has emerged as a powerful tool to perform reactions in a highly enantioselective fashion over the past few decades. Efforts to develop new asymmetric transformations focused preponderantly on the use of few metals such as palladium, CONTENTS rhodium, copper, iridium, or ruthenium. However, by the very fact of the lower costs and toxicity of cobalt catalysts in 1. Introduction 2775 comparison with other transition metals, ecological and 2. Cobalt-Catalyzed Cyclization Reactions 2776 economical cobalt-mediated transformations have received 2.1. Cycloaddition Reactions 2776 continuous ever-growing attention during the two last decades 2.1.1. Cyclopropanations 2776 that leads to exciting and fruitful research. This interest might 2.1.2. Aziridinations 2781 be explained by several reasons. Low rates for the β-hydride 2.1.3. [2+2+2] and [2+2+1] Cycloadditions 2782 elimination pathway are often observed with cobalt as 2.1.4. [4+2] Cycloadditions 2784 compared to other metals, palladium for instance; hence 2.1.5. [3+2] Cycloadditions 2786 cobalt-based catalytic systems are interesting alternatives in 2.1.6. Miscellaneous Cycloadditions 2787 established transformations. Cobalt catalysts also show an 2.2. Domino Reactions 2789 excellent tolerance to various functional groups, which makes 2.3. Miscellaneous Cyclization Reactions 2791 reaction scopes particularly wide. Moreover, cobalt has a high 3. Cobalt-Catalyzed Formations of Acyclic Comaffinity to carbon−carbon π-bonds, carbon−nitrogen π-bonds, pounds 2796 and carbonyl groups that was used to develop the Pauson− 3.1. Reduction Reactions 2796 Khand reaction, [2+2+2] cycloadditions, or the Nicholas 3.1.1. Reductions of Ketones and Carbonyl reaction, for example. These pioneering works have been Derivatives 2796 followed by the development of a large number of new cobalt3.1.2. Conjugate Reductions 2801 promoted transformations such as cyclopropanation through 3.2. Michael Reactions 2802 diazo transfer, various cycloadditions spanning from the [2+1] 3.3. (Nitro-)Aldol Reactions 2804 or [2+2] to the [6+4], or carbon−carbon bond-forming 3.4. Carbonyl-Ene Reactions 2805 reactions including carbon−hydrogen bond activation method3.5. Coupling Reactions between Alkenes and ologies. In the field of asymmetric cobalt-catalyzed reactions, an Alkynes 2806 impressive amount of novel methodologies has been developed 3.6. Hydrovinylation Reactions 2807 on the basis of the extraordinary ability of cobalt catalysts to 3.7. Ring-Opening Reactions 2809 adopt unexpected reaction pathways to new chiral cyclic as well 3.7.1. Hydrolytic Ring-Opening of Epoxides 2809 as acyclic products under relatively mild conditions. For 3.7.2. Ring-Opening of Epoxides by Nucleoexample, the first enantioselective cobalt-mediated [6+2] philes Other than Water 2810 cycloadditions, domino reactions, Pauson−Khand reactions, 3.8. Miscellaneous Formations of Acyclic Comaldol condensations, Michael additions, Nazarov reactions, pounds 2811 hydrovinylations of alkenes, ring-opening of epoxides, etc., have 4. Conclusion 2815 been described. The goal of this Review is to provide a Author Information 2815 comprehensive overview of the major developments in Corresponding Author 2815 enantioselective cobalt-catalyzed transformations published Notes 2815 since 1989, because this field was most recently reviewed by Biographies 2815 Pfaltz in a book chapter dealing with enantioselective catalysis Abbreviations 2816 References

2816 Received: July 26, 2013 Published: January 15, 2014 © 2014 American Chemical Society

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dx.doi.org/10.1021/cr4004055 | Chem. Rev. 2014, 114, 2775−2823

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with chiral cobalt and copper complexes.2−5 For the reader’s convenience, this Review has been divided into two parts. The first part deals with enantioselective cobalt-catalyzed cyclization reactions, while the second part includes asymmetric cobaltcatalyzed transformations giving rise to acyclic compounds.

optimal trans-selective cobalt complex was demonstrated to be cobalt(III) catalyst 1. As shown in Scheme 1, it induced the Scheme 1. trans-Selective Cyclopropanation of Aromatic Alkenes Catalyzed by a Salen Cobalt(III) Bromide Complex

2. COBALT-CATALYZED CYCLIZATION REACTIONS 2.1. Cycloaddition Reactions

Reactions that form multiple bonds, rings, and stereocenters are particularly important tools for the efficient assembly of complex molecular structures.6 Of the many families of reactions discovered over the past century, cycloaddition reactions hold a prominent place in the arsenal of synthetic methods currently available to organic chemists, and the research activity in this field shows no signs of abatement.7 Among the metals used to catalyze cycloadditions, cobalt has been found competent to promote enantioselectively the formation of carbo- and heterocycles of various ring sizes. 2.1.1. Cyclopropanations. Organic chemists have always been fascinated by the cyclopropane subunit,8 which has played and continues to play a prominent role in organic chemistry.9 Its strained structure, interesting bonding characteristics, and value as an internal mechanistic probe have attracted the attention of the physical organic community. While the cyclopropane ring is a highly strained entity, it is nonetheless found in a wide variety of naturally occurring compounds including terpenes, pheromones, fatty acid metabolites, and unusual amino acids.10 The cyclopropanation of olefins using the transition-metal-catalyzed decomposition of diazoalkanes is one of the most extensively studied reactions in the organic chemist’s arsenal.11 Indeed, the synthesis of cyclopropanes by transition-metal-mediated carbene transfer from aliphatic diazo compounds to carbon−carbon double bonds is not only a major method for the preparation of cyclopropanes most of the time exhibiting a trans-configuration, but is also among the best developed and most general methods available to the synthetic organic chemist.11e,12 Since the first enantioselective coppercatalyzed cyclopropanation reported by Nozaki and co-workers in 1966,13 numerous research groups have tried to find more efficient catalysts, and the most spectacular advances were reported by Aratani et al., who discovered through extensive evaluation of a large number of ligands a chiral (salicylaldiminato)copper(II) complex, which allowed enantioselectivities of up to 95% ee to be achieved.14 Ever since, other highly effective and stereocontrolled syntheses of functionalized cyclopropanes have been achieved, in particular, with catalysts based on copper,15 rhodium, and, more recently, ruthenium.16 Furthermore, cobalt complexes have been shown to be reactive catalysts for the α-diazoester decomposition, leading to a metal carbene that could convert alkenes into cyclopropanes. Although the early work in this area established that chiral cobalt(II) complexes were catalytically active, the low levels of diastereo- and enantiocontrol have limited their use in synthesis for a long time.17 One of the first successes in enantioselective intermolecular cobalt-catalyzed cyclopropanation reactions was a report by Nakamura et al. in 1978 on a chiral dioximatocobalt(II) complex derived from camphor, which allowed enantioselectivities of up to 88% ee to be achieved,18 but difficulties in catalyst homogeneity with chiral dioximato ligands inhibited additional studies. Later, Katsuki and coworkers introduced novel chiral salen cobalt(III) complexes to induce trans-selective cyclopropanation reactions.11g,19 The

decomposition of tert-butyl diazoacetate 2a in the presence of styrene derivatives 3 to generate the corresponding transcyclopropanes 4 with excellent diastereoselectivities of up to 94% de and enantiomeric excesses of 92−96% ee. In 1999, Yamada et al. demonstrated that chiral 3oxobutylideneaminatocobalt(II) complexes,20 such as 5 employed at 5 mol % of catalyst loading in THF as solvent at 40− 50 °C (Scheme 2), were quite effective in the same transselective reaction of monoaryl-substituted alkenes 3 with tertbutyl diazoacetate 2a.21 It was shown that the addition of a catalytic amount of N-methylimidazole (NMI) increased the rate of the reaction as well as the enantioselectivity. The process was limited to aryl-substituted alkenes, providing the corresponding chiral trans-cyclopropanes 4 in high yields (85−99%), good trans-diastereoselectivities (64−82% de), combined with excellent enantioselectivities (92−96% ee), whereas 1,1-disubstituted alkenes 6 led to the corresponding enantiopure trisubstituted cyclopropane derivatives 7 with low diastereocontrol (6% de). The authors have found that the diastereoselectivity in the cyclopropanation of styrene decreased to 66% de if methyl diazoacetate was used. A theoretical analysis of the reaction pathway by density functional method revealed that the axial donor ligand produced two prominent effects.22 One was that the activation energy for the formation of the cobalt carbene was reduced and that the activation energy for the cyclopropanation step was increased. The other was that the distance of the carbene carbon above the plane was shortened during the cyclopronation step. From these results, the axial donor ligand effects, enhancing reactivity and improving the diastereo- and enantioselectivities, in the 3-oxobutylideneaminatocobalt(II)catalyzed asymmetric cyclopropanation could be clearly explained. In relation with axial donor ligand effect, the same authors have shown that these highly enantioselective cyclopropanations could also be performed in environmentally friendly alcoholic and aqueous solvents.23 Indeed, the tetradentate ligand of the β-ketoiminatocobalt complex produces a rigid square planar structure around the cobalt atom, and the structure of the complex is almost independent from the solvent. Hence, the coordination of donor solvent on a vacant axial position would directly lead to activating the carbene carbon located at the other axial position. It is generally considered that metal−carbene carbon bonds in carbene complexes for cyclopropanation should be double-bonded; however, these authors have reported the theoretical and FT-IR 2776


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Scheme 2. trans-Selective Cyclopropanation of Alkenes Catalyzed by Cobalt(II) Complexes

major product in a moderate diastereoselectivity of 48% de and a high yield of 92%.25 Good enantioselectivities of 88% ee for the trans-product and 94% ee for the minor cis-product were obtained when performing the reaction at 25 °C in dichloromethane and using 5 mol % of catalyst loading. In addition to salen cobalt complexes and dinuclear cobalt complexes, cobalt(II) porphyrin complexes26 have been proved by Zhang et al. to be general and efficient catalysts for diastereo- and enantioselective cyclopropanation of alkenes.27 Indeed, cobalt(II) D2-symmetric porphyrins derived from chiral cyclopropanecarboxamide with tunable electronic, steric, and chiral

analyses revealing that the cobalt−carbon of the 3-oxobutylideneaminato or the salen−cobalt−carbene complexes was characterized as a single bond.24 On the other hand, dinuclear complexes, such as 8 (Scheme 2), which are coupled salen complexes, represent a new type of effective catalyst for cyclopropanation. This is due to the fact that the substrate molecules will invariably be subjected to chiral induction by the chiral backbone as they approach the complex platform. In 2005, Gao et al. applied this type of catalyst to the cyclopropanation of styrene 3a with ethyl diazoacetate 2b, providing the corresponding trans-cyclopropane 4a as the 2777


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Scheme 3. cis-Selective Cyclopropanation of Aromatic Alkenes Catalyzed by Cobalt(II) Complexes

68% ee). A major advantage of the cobalt catalytic system is the complete suppression of the diazoalkane dimerization (providing undesired corresponding alkenes), which constitutes a problem complicating the use of copper and most of the ruthenium and rhodium catalysts, necessitating the use of syringe pumps. In the last years, a novel type of highly modular and readily accessible pincer ligands, such as chiral bis(pyridylimino)isoindoles, was developed by Gade et al. to induce chirality in cobalt-catalyzed intermolecular cyclizations of aromatic alkenes 3 with ethyl diazoacetate 2b.32 Whereas the chirally modified pyridyl units acted as stereodirecting elements, the appropriate substitution pattern in the backbone provided a protective hedge for backside attack on the metal center. Their versatility as efficient stereodirecting ligands has been demonstrated by the obtention of high enantioselectivities of up to 94% ee for cyclopropanes 4 arising from the corresponding monosubstituted alkenes, as shown in Scheme 2. These high enantioselectivities combined with good to high trans-diastereoselectivities ranging from 88% to 92% de and high yields (92−97%) could be reached by using the optimized catalyst, [Co(tetraphenyl-carbpi)(OAc)] 10, at a catalyst loading of 2 mol % in toluene at room temperature. Not unexpectedly, this catalyst system was shown to be less efficient for 1,1-disubstituted alkenes 6, which provided the corresponding chiral trisubstituted cyclopropanes 7 in lower yields (71− 78%), and trans-diastereoselectivities of ≤50% de, as well as lower enantioselectivities ranging from 80% to 88% ee. These results constituted, however, the first study of chiral bis(pyridylimino)isoindole ligands in enantioselective catalysis with 3d-metal complexes. Later, other cobalt chiral bis(binaphthyl) porphyrin complexes, such as catalyst 11 (Scheme 2), have been developed by Gallo et al. to promote the cyclopropanation of mono- and disubstituted alkenes 3 and 6

environments, such as 9, were successfully investigated at 1 mol % of catalyst loading in toluene at room temperature in the cyclopropanation of styrene 3a with ethyl and t-butyl diazoacetates 2ab, providing the corresponding trans-cycloadducts 4 in high yields, diastereo-, and enantioselectivities, as shown in Scheme 2.28 It was shown that the use of DMAP as an additive allowed the enantioselectivities to be doubled and the production of the trans-isomer to be boosted, suggesting a significant trans influence of potential coordinating ligands on the metal center.29 A comparison between this catalytic system and iron, ruthenium, and rhodium porphyrins demonstrated that the common diazoacetate dimerization side reaction was minimized within the cobalt systems, thus providing higher yields in cyclopropanes. Moreover, cobalt-catalyzed cyclopropanations did not require the slow addition of diazo reagents, a practical protocol that is atypical when using metal catalysts other than cobalt. In 2007, the scope of this methodology was extended to a broad range of styrene derivatives bearing various substituents on the phenyl ring, such as methoxy, methyl, t-butyl, bromide, chloride, fluoride, acetate, and trifluoromethyl groups, furnishing the corresponding cyclopropanes in good yields, and in diastereo- and enantioselectivities of up to 100% de and 98% ee, respectively.30 Through comparative studies, these authors have demonstrated the superiority of cobalt over iron by performing the reactions with the same porphyrin ligand. Indeed, low to good yields (1−77%) associated with poor enantioselectivities (up to 28% ee) were obtained with the corresponding iron complex. In addition, these authors have found that similar reactions could be efficiently catalyzed by vitamin B12 derivatives such as aquocobalamin,31 which provided the corresponding cis-dominant cyclopropanes in excellent yields albeit with moderate enantioselectivities (up to 2778


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respectively), whereas cyclopropanes derived from α,βunsaturated ketones, esters, or amides were obtained in diastereoselectivities ranging from 90% to 98% de in combination with enantioselectivities of up to 97% ee. Moreover, asymmetric cyclopropanation with acceptor/acceptor-substituted diazo reagents remains, however, a major challenge because of their inherent low reactivity and perceived poor enantioselectivity. In 2008, Zhang et al. applied a family of cobalt(II) D2-symmetric porphyrins derived from chiral cyclopropanecarboxamide with tunable electronic, steric, and chiral environments in cobalt-catalyzed cyclopropanations of styrenes 3 with α-nitrodiazoacetate 14. 37 Among these cobalt complexes, catalyst 9 proved to be the optimal catalyst, producing the corresponding chiral cis-cyclopropane α-nitroesters 15 in good to excellent yields (51−98%), good to almost complete cis-diastereoselectivities (80% to >98% de), and good to high enantioselectivities ranging from 82% to 95% ee, as shown in Scheme 4. It must be noted that typically challenging

with ethyl diazoacetate 2b, giving the corresponding cyclopropanes 4 and 7, respectively.33 Good yields and low to moderate enantioselectivities of up to 71% ee were observed for the trans-major diastereomers combined with moderate to good diastereoselectivities of 32−68% de, whereas low to high enantioselectivities of up to 90% ee were obtained for the cisminor diastereomers. In 1999, Katsuki and co-workers succeeded in designing cisselective catalysts based on the salen scaffold such as cobalt complex 12.5c,34 The reaction of various aromatic mono- and disubstituted alkenes 3 and 6 with ethyl and t-butyl diazoacetates 2ab proceeded very well in the presence of NMI, providing the corresponding cis-cyclopropanes 4 and 7, respectively, in good to quantitative yields, good to excellent diastereoselectivities of up to 98% de, and excellent enantioselectivities of up to 99% ee. In this work, the authors have studied the catalytic efficiency of cobalt(II) salen complexes in comparison to the corresponding ruthenium(NO) salen complexes, and found that even if excellent enantio- and cis-selectivities were also achieved by using ruthenium catalysts, the yields in cyclopropanes were unsatisfactory because of self-coupling of the diazo compounds that occurred competitively, as was also mentioned by Zhang and co-workers for cobalt porphyrins.27 A drawback of this methodology was, however, the limitation of its scope to arylsubstituted alkenes, as shown in Scheme 3. In 2007, the same authors investigated other cobalt(II) complexes having chiral pentadentate salen ligands bearing an imidazole or pyridine derivatives as the fifth coordinating group in the same cyclopropanation reaction.35 Catalyst 13 bearing an imidazole was proved to be the most efficient catalyst to promote high cisdiastereoselectivity of 78−98% de in reactions of monosubstituted aromatic alkenes 3 in reaction with t-butyl diazoacetate 2a to give the corresponding cis-cyclopropanes 4, as shown in Scheme 3. Moreover, these products were obtained in excellent yields of 93% to quantitative, combined with enantioselectivities ranging from 93% to 96% ee (Scheme 3). The authors have shown that the reaction of disubstituted alkenes 6, such as α-methylstyrene, provided, however, moderate cis-diastereoselectivity of 10% de, moderate yield of 45%, albeit excellent enantioselectivity of 96−97% ee. These reactions were performed with 5 mol % of catalyst 13 in toluene at room temperature. While a number of catalytic systems worked exceptionally well with styrene derivatives and some electron-rich olefins, asymmetric cyclopropanation of electron-deficient olefins containing electron-withdrawing groups, such as α,β-unsaturated carbonyl compounds and nitriles, was proven to be a challenging problem presumably due to the electrophilic nature of the metal−carbene intermediates in the catalytic cycles. In this context, Zhang et al. investigated the asymmetric cyclopropanation of more challenging substrates, such as electron-deficient nonstyrenic olefins, with ethyl and t-butyl diazoacetates 2ab using Co(II) catalyst 9.36 Good to high yields (66−94%) and high trans-diastereoselectivities of up to 98% de were obtained for a range of formed trans-cyclopropanated products, making this catalyst one of the most selective for asymmetric cyclopropanation of olefins in general. The reactions were performed in toluene at room temperature in the presence of DMAP as an additive and 1 mol % of catalyst 9. It must be noted that generally the lowest diastereoselectivities and enantioselectivities were observed for the formation of 1,2cyclopropane cyanoesters (24−52% de, and 73−95% ee,

Scheme 4. cis-Selective Cyclopropanation of Alkenes with αNitrodiazoacetate Catalyzed by 9

substrates, such as aliphatic alkenes, were also successfully converted into the corresponding cyclopropanes with low to good diastereo- and enantioselectivities of 12−84% de and 75− 88% ee, respectively, in combination with moderate to high yields of 42−92%. It is interesting to note that, in this study, the cis-cyclopropanes were the major diastereomers generated in contrast with most of the other studies in which the major products exhibited a trans-configuration. In 2009, the same authors also reported the asymmetric cyclopropanation of aliphatic as well as aromatic alkenes 3 with another unusual diazo reagent such as succinimidyl diazoacetate.38 The reaction was catalyzed by 5 mol % of the same cobalt(II) D2-symmetric chiral cyclopropyl porphyrin 9 in toluene at room temperature with DMAP as substoichiometric additive, providing a range of chiral cyclopropane succinimidyl esters in moderate to high yields (30−90%) and remarkable trans-diastereo- and enantioselectivities of >98% de and 89− 98% ee, respectively. These results constituted the first asymmetric cyclopropanation of alkenes with succinimidyl diazoacetate, and, moreover, it must be noted that the resulted chiral products constituted valuable synthons for general synthesis of important chiral cyclopropyl carboxamide derivatives. Always in the context of asymmetric cobalt-catalyzed 2779


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Scheme 5. trans-Selective Cyclopropanation of Alkenes with Diazosulfones Catalyzed by a Cobalt(II)-Porphyrin

cyclopropanations of alkenes 3 with unusual diazo reagents, these authors have developed a closely related methodology for the trans-cyclopropanation of alkenes with α-cyanodiazoacetates such as tert-butyl α-cyanodiazoacetate.39 In this case, the reaction was preformed in hexane at −20 °C in the presence of 1 mol % of catalyst 9. Even higher enantioselectivities of up to 99% ee were achieved for the corresponding densely functionalized chiral cyclopropanes, possessing a myriad of potential synthetic and biological applications, in particular as precursors of chiral α-cyclopropyl-β-amino acids. Remarkably, a general almost complete trans-diastereoselectivity of >98% de was reached in all cases of substrates studied in combination with excellent yields (72−99%). It must be noted that this cobalt(II)-based system represented the first successful example of using this class of acceptor/acceptor-substituted diazo reagents for the asymmetric cyclopropanation process of aliphatic as well as aromatic alkenes. As another extension of this methodology, these authors developed the asymmetric cyclopropenation of terminal aromatic and related conjugated alkynes bearing varied steric and electronic properties with various acceptor/acceptor-substituted diazo compounds, such as α-cyanodiazoacetates and α-cyanodiazoacetamides, which provided the corresponding chiral trisubstituted cyclopropenes as single diastereomers in good to high yields (42−97%), and high enantiocontrol of the all-carbon quaternary stereogenic centers with enantioselectivities of 80−98% ee.40 In this case, the reactions were performed in trifluorotoluene as the solvent at room temperature or 40 °C with 1 mol % of catalyst 9. Under these reaction conditions, a remarkable degree of tolerance of this catalyst toward various functionalities, including CHO, OH, and NH2 groups, was demonstrated. In addition, the same authors have successfully developed cobaltcatalyzed asymmetric cyclopropanation of alkenes 3 with a range of other unusual diazo compounds such as diazosulfones 16.41 In this aim, they have designed a novel chiral porphyrin 17 with enhanced rigidity and polarity of chiral environment as a result of both intramolecular hydrogen-bonding interactions and the use of cyclic structures. The application of this chiral porphyrin as a cobalt ligand to promote the asymmetric cyclopropanation of a range of aromatic and electron-deficient aliphatic alkenes with various diazosulfones 16 provided the corresponding chiral cyclopropyl sulfones 18 in good to excellent yields of up to 99%, excellent trans-diastereoselectivities of >98% de in almost all cases of substrates studied, and general excellent enantioselectivities ranging from 90% to 97% ee with one exception of lower diastereo- and enantioselectivities of 58% de and 61% ee, respectively, in the case of acrylonitrile as alkene and N2CHTs as diazo reagent. Remarkably, this nice novel one-pot practical protocol was atypical because for many other catalytic cyclopropanation systems, a slow addition of the diazo compounds was necessary to avoid the competitive carbene dimerization side reaction. The results are collected in Scheme 5. Intramolecular versions of enantioselective cobalt-catalyzed cyclopropanation of alkenes have been recently developed by several groups. As an example, Katsuki et al. designed a series of novel chiral salen cobalt(II) catalysts that proved to be very efficient for the intramolecular cyclopropanation of various (E)2-alkenyl α-diazoacetates 19 in the presence of NMI.42 As shown in Scheme 6, the cyclopropanation of (E)-(aryl)allyl diazoacetates 19 into the corresponding chiral bicyclic products 20 proceeded in good to excellent enantioselectivities of up to 98% ee by using catalysts 21ab, while lower enantioselectivities

Scheme 6. Intramolecular Cyclopropanation of (E)/(Z)-2Alkenyl α-Diazoacetates

(68% ee) were obtained in the case of (Z)-(aryl)allyl diazoacetates. Moreover, to demonstrate the versatility of their novel catalyst system 10, Gade and co-workers have extended their precedent methodology (Scheme 2, catalyst 10) to the same intramolecular cyclopropanation of substrates 19, which provided products 20 as single diastereomers in good yields and good to high enantioselectivities ranging from 65% to 94% ee, as shown in Scheme 6.32 Furthermore, novel cobalt(II) catalyst 22 derived from a chiral cyclopropanecarboxamide containing two contiguous stereocenters was designed by Zhang et al. and subsequently investigated as promotor of an original asymmetric intra2780


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molecular cyclopropanation of a range of α-acceptorsubstituted allylic diazoacetates 23.43 This highly efficient novel methodology allowed for the first time the transformation of α-acceptor-substituted diazoacetates into enantioenriched 3oxabicyclo[3.1.0]hexan-2-one derivatives 24 bearing three contiguous stereocenters with multiple functionalities. As shown in Scheme 7, good to excellent yields of 62−99%

Scheme 8. trans-Selective Cyclopropanation of Styrenes with CF3CH2NH3Cl

Scheme 7. Intramolecular Cyclopropanation of α-AcceptorSubstituted Allylic Diazoacetates

95%), high diastereoselectivities of 84 to >98% de, and high enantioselectivities ranging from 84% to 94% ee. The scope of this methodology was extended to 1,1-disubstituted styrenes, which furnished the corresponding trisubstituted cyclopropanes in enantioselectivities of 87−97% ee, albeit in generally lower diastereoselectivities (34−80% de). It must be noted that catalyst 25 constituted the first of its kind that was active in asymmetric cyclopropanation with an in situ generated diazoalkane under extreme conditions (aqueous acidic, oxidative media). 2.1.2. Aziridinations. Aziridines are among the most fascinating intermediates in organic synthesis, acting as precursors of many complex molecules due to the strain incorporated in their skeletons. The high strain energy associated with the aziridine ring enables easy cleavage of the C−N bond. Therefore, aziridines can either undergo ringcleavage reactions with a range of nucleophiles or cycloaddition reactions with dipolarophiles, providing access to a wide range of important nitrogen-containing products.45 However, they are less widely used in synthesis than their oxygen counterparts, partly because there are fewer efficient methods for aziridination relative to epoxidation. This is particularly true when enantioselective methods are considered.46 Nitrogenatom transfer to alkenes is a particularly appealing strategy for the generation of aziridines because of the ready availability of olefinic starting materials and the direct nature of such a process. In addition to catalytic systems using copper, rhodium, or ruthenium,47 Zhang et al. showed that cobalt was able to promote the asymmetric aziridination of olefins 29 using diphenylphosphoryl azide 30 as the nitrene source, affording the corresponding N-phosphorylated aziridines 31.48 The reaction was carried out in the presence of D2-symmetric chiral porphyrins, such as 9, and was applied to a wide variety of styrenes, giving the corresponding enantioenriched aziridines 31 in good yields combined with moderate enantioselectivities of up to 53% ee, as shown in Scheme 9. A higher enantioselectivity of 71% ee was reached by using 20 mol % of the same catalyst in the presence of DMAP as an additive in dichloromethane as solvent, albeit combined with a low yield of 20%.

combined with excellent trans-diastereoselectivities of up to >98% de and high enantioselectivities ranging from 73% to 99% ee were obtained for the cyclopropanation of a range of cinnamyl diazoesters bearing a substituent at α-position, which could be an acceptor function such as a cyano, a nitro, or an ester group for the best results, but also a hydrogen or a methyl group, which provided enantioselectivities of 99% and 73% ee, respectively. In addition to cinnamyl diazoesters, a series of allylic α-cyanodiazoacetates were successfully converted into the corresponding chiral bicyclic products in 51−99% yields, high trans-diastereoselectivities (>98% de), and enantioselectivities ranging from 78% to 98% ee. There is still a scarcity of approaches for the enantioselective generation of trifluoromethyl-substituted cyclopropanes, which constitute important building blocks for drug discovery. In this context, Carreira et al. have developed a novel enantioselective cobalt-catalyzed route to these chiral products based on cyclopropanation of styrenes with in situ generated trifluoromethyl diazomethane.44 After screening another type of cobalt catalysts derived from (S,S)-1,2-cyclohexyldiamine and 2,3dihydroxybenzaldehydes, these authors have selected novel catalyst 25 bearing a combination of electron-donating and electron-withdrawing substituents on benzaldehydes as the optimal catalyst. As shown in Scheme 8, the cyclopropanation of a range of styrenes 26 with CF3CH2NH3Cl 27 in the presence of catalyst 25 provided the corresponding chiral transdisubstituted cyclopropanes 28 in moderate to high yields (49− 2781


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dienes, such as 2,3-dimethylbutadiene, which afforded the corresponding disubstituted aziridine in 53% yield and 87% ee. It must be highlighted that this work presented the first highly effective and enantioselective catalytic system for asymmetric aziridination of a broad range of olefins, without needing additional functionalities in the substrates for secondary binding interactions. 2.1.3. [2+2+2] and [2+2+1] Cycloadditions. In the past decade, the [2+2+2] cycloaddition reactions have evolved into a versatile member of the synthetic chemists’ toolbox for the preparation of functionalized arenes. In particular, transition metal-catalyzed [2+2+2] cycloadditions of unsaturated motifs, such as alkynes and alkenes, constitute the most atomeconomical and facile protocol for the construction of a sixmembered ring system.5a,50 Among them, enantioselective [2+2+2] cycloaddition is a fascinating protocol for the construction of chiral cyclic skeletons.51 In 1990, Lautens52 and Brunner53 independently reported almost at the same time the first highly enantioselective cobalt-catalyzed [2+2+2] cycloadditions performed in the presence of chiral phosphines. These processes occurring between norbornadiene 34 and acetylenic compounds 35 allowed the corresponding chiral monosubstituted deltacyclenes 36 to be synthesized in high yields and enantioselectivities of up to 98% ee. The effective catalysts used by these authors were obtained upon reduction of Co(acac)3 with Et2AlCl in the presence of chiral ligands such as (S,S)-Chiraphos or (+)-Norphos, respectively. Using (S,S)Chiraphos as chiral ligand, Lautens et al. reported yields of up to 85% in combination with enantioselectivities of up to 91% ee, while using (+)-Norphos, Brunner obtained a quantitative yield and an enantioselectivity of >98% ee for the deltacyclene arising from the reaction of norbornadiene with phenylacetylene. Later, Buono et al. demonstrated that these reactions could be catalyzed by a new catalytic system [CoI2/Zn/L*] with chiral organophosphorus bidentate ligands (L*) such as (S)-(+)-ValNOP (Scheme 11).54 As shown in Scheme 11, a range of variously monofunctionalized deltacyclenes 36 could be achieved from the corresponding acetylenic and propargylic compounds 35 in moderate to excellent yields and high to excellent enantioselectivities of up to 97% ee. The authors have shown that both yields and enantioselectivities were highly dependent on the reaction temperature, with the best results reached around 14 °C. The authors have proposed the two step process depicted in Scheme 11 to explain the formation of product 36. It involved the reversible formation of two diastereomeric five pentacoordinated cobalt intermediates 37 and ent-37, which could equilibrate according to the Berry pseudorotation,55 and Turnstile rotation mechanism.56 The low energy barriers for interconversion between 37 and ent-37 associated with a high energy difference of these highly substituted structures as a result of the substitution patterns could be responsible for the high enantioselectivity observed. The synthesis of axially chiral biaryls has attracted a lot of attention in the last decades due to the emergence of a large number of natural products containing structures with a stereogenic biaryl axis, chiral auxiliaries, and as ligands of catalytic systems. In this context, Gutnov and co-workers have studied cobalt-catalyzed asymmetric [2+2+2] cycloaddition of alkynes with nitriles such as 2-substituted 1-naphthonitriles.57 To induce this reaction, the authors have screened tartratederived and methyl-derived chiral cobalt(I) complexes, and it resulted that catalyst 39 was optimal among the others. Therefore, in the presence of 10 mol % of this catalyst in THF

Scheme 9. Aziridination of Styrenes with Diphenylphosphoryl Azide

Later, much better enantioselectivities of up to 94% ee were reported by the same authors in the asymmetric aziridination of a range of aromatic as well as aliphatic monosubstituted alkenes 3 with trichloroethoxysulfonyl azide 32 by using cobalt(II)chiral rigid and polar porphyrin 17.49 The process provided the corresponding chiral aziridines 33 in high yields (82−93%) and high enantioselectivities ranging from 80% to 99% ee in the case of monosubstituted aromatic alkenes as substrates, whereas monosubstituted aliphatic alkenes produced the corresponding aziridines in lower yields (26−42%) but with comparable high enantioselectivities ranging from 91% to 94% ee (Scheme 10). The scope of this methodology could be extended to aliphatic Scheme 10. Aziridination of Alkenes with Trichloroethoxysulfonyl Azide

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Scheme 11. [2+2+2] Cycloaddition of Norbornadiene with Alkynes

at 20 °C, the reaction of 2-substituted 1-naphthonitriles with 2 equiv of alkynes provided the corresponding 2-arylpyridines in low yields (3−11%) and moderate enantioselectivities of 40− 64% ee. When 2-substituted 1-naphthonitriles were reacted with dialkynes, the corresponding 2-arylpyridines were achieved in even lower enantioselectivities of 7−33% ee. On the other hand, the same authors have developed a novel route to axially chiral 1-aryl-5,6,7,8-tetrahydroquinolines on the basis of a cobalt-catalyzed [2+2+2] cycloaddition occurring between 1aryl-1,7-octadiynes and nitriles.58 As shown in Scheme 12, the

the authors have investigated the synthesis of biaryls possessing a five-membered ring annulated at the pyridine moiety, and they found that the size of the ring was too small to prevent the slippage of the rings around the biaryl axis due to the presence of the ring nitrogen, and consequently no enantioselectivity was observed in this case. In the past two decades, the Pauson−Khand reaction, which allows the preparation of a 2-cyclopentenone on the basis of a [2+2+1] cyclocarbonylation of an alkyne with an alkene, has attracted much attention from the synthetic chemistry community.59 Since the first discovery of the catalytic version,60 various catalytic versions including many other types of metal catalysts based on iron, nickel, titanium, zirconium, ruthenium, rhodium, and iridium have been applied, and the reaction pathways involving much milder conditions have been developed.61 Moreover, the scope of the olefin substrates has been widened by finding alkene equivalents such as dimethyl(pyridyl)(vinyl)silane, o-(dimethylamino)phenyl vinyl sulfoxide, and 2,3-disubstituted 1,3-butadiene. At present, the Pauson−Khand reaction is recognized as one of the most important arsenal in the synthesis of five-membered compounds including cyclopentenones.62 In 2000, Hiroi et al. reported the first example of a catalytic asymmetric synthesis of 2-cyclopentenone systems using a cobalt catalyst and chiral phosphines.63 As shown in Scheme 13, the intramolecular Pauson−Khand reaction of 1,6-enynes 43a−c under carbon monoxide atmosphere, using (S)-BINAP as most effective ligand of Co2(CO)8, led to the corresponding 2-cyclopentenone derivatives 44a−c in moderate to good yields and enantioselectivities of up to 90% ee. The scope of the reaction was extended to sulfonamides 43d−e, which provided under the same reaction conditions the corresponding bicyclic products 44d−e in comparable results with enantioselectivities of up to 94% ee (Scheme 13). Later, Gibson et al. investigated the mechanism of this reaction, isolating and identifying through X-ray crystallographic analysis an hexacarbonyldicobalt(0) complex in which BINAP binded to just one of the two

Scheme 12. [2+2+2] Cycloaddition of 1-Aryl-1,7-octadiynes with Nitriles

process was catalyzed by planar chiral (1-neomenthylindenyl)cobalt(cod) complex 39 under photochemical conditions, allowing the formation of various axially chiral 2-arylpyridines 42 to be achieved from the reaction of the corresponding 1naphthyldiynes 40 with a range of differently functionalized nitriles 41. Under optimized conditions, the enantioenriched tetrahydroquinolines 42 were formed in mostly good yields and good to high enantioselectivities of up to 94% ee. Moreover, 2783


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Scheme 13. First Intramolecular Pauson−Khand Reaction of 1,6-Enynes

Scheme 15. Diels−Alder Reaction of a Dienamine with Enals

cobalts as precatalyst.64 In 2002, Buchwald and Sturla demonstrated that enantioselectivities of up to 75% ee could be achieved in comparable reactions by using chiral aryl bisphosphite ligands.65 In 2004, enantioselectivities of up to >91% ee combined with high yields of up to 95% were reported by Consiglio et al. employing (R)-MeO-BIPHEP as chiral ligand of Co2(CO)8 in the cyclocarbonylation of 4,4bis(carboethoxy)hex-6-en-1-yne.66 2.1.4. [4+2] Cycloadditions. The Diels−Alder reaction is a versatile reaction for the stereospecific construction of sixmembered rings. Its thermal uncatalyzed versions sometimes required harsh reaction conditions to be achieved, and consequently a number of methods have been developed to overcome this obstacle, such as transition-metal catalysis.67 It must be noted that there are only few excellent works that focused on asymmetric Diels−Alder reactions induced by chiral cobalt complexes. As an example, Kanemasa et al. have employed a cationic chiral aqua complex derived from a trans-chelating tridentate ligand, (R,R)-4,6-dibenzofurandiyl2,2′-bis-(4-phenyloxazoline) (DBFOX/Ph), and cobalt(II) perchlorate to induce the Diels−Alder cycloaddition of cyclopentadiene with 3-acryloyl-2-oxazolidinone.68 As shown in Scheme 14, the corresponding cycloadduct 45 was achieved in excellent yield of 97%, high endo-diastereoselectivity of 94% de, and remarkable enantioselectivity of 99% ee.

scope because only one diene and a few simple aldehydes have been investigated. A number of total syntheses of important natural products involve an asymmetric Diels−Alder reaction as key step, among them antibiotic (−)-platencin. This novel synthesis, reported by Nicolaou et al. in 2009, was based on a related enantioselective Diels−Alder cycloaddition of functionalized diene 50 with dienophile 51 catalyzed by a closely related chiral salen cobalt(III) catalyst 52, providing the corresponding densely functionalized cycloadduct 53 in excellent yield and enantioselectivity of 96% ee, as shown in Scheme 16.70 This key almost enantiopure product was further converted through nine steps into final (−)-platencin. In 2011, catalyst 52 was employed by Brimble et al. to promote a Diels−Alder cycloaddition to reach a chiral functionalized cyclohexene, which constituted a key intermediate in the asymmetric synthesis of a tetracyclic alkaloid methyllycaconitine analogue.71 As shown in Scheme 17, the reaction occurred between enal 54 and dienamine 55 to give the corresponding key cycloadduct 56 constituting the B ring of the natural product in good yield and enantioselectivity of 80% ee. Subsequent elaboration to form the A, E, and F rings was achieved by sequential Dieckmann, Mannich, and Wacker-type cyclizations to afford tetracyclic methyllycaconitine analogues. The asymmetric hetero-Diels−Alder reaction is one of the most efficient synthetic methodologies for the regio- and stereoselective construction of chiral six-membered heterocycles.72 In 1998, Wu et al. reported the enantioselective hetero-Diels−Alder cycloaddition of 1-(2-benzyloxyethyl)-3(tert-butyldimethylsilyl)oxy-1,3-butadiene with methyl glyoxylate catalyzed by 10 mol % of chiral salen cobalt(II) catalyst 57 (Scheme 18), which provided the corresponding cycloadduct in 75% yield with excellent endo/exo ratio of >99:1 albeit combined with a moderate enantioselectivity of 52% ee.73 In 2004, the same catalyst was applied by Jurczak et al. to induce the high-pressure (10−11 kbar) Diels−Alder cycloaddition of 1-methoxybuta-1,3-diene 58 with tert-butyldimethylsilyloxyacetaldehyde 59 to provide the corresponding cis-cycloadduct 60 in 52% yield with a cis-diastereoselectivity of 90% de and an enantioselectivity of 94% ee (Scheme 18).74 In this work, the authors have compared the catalytic efficiency of cobalt(II) catalyst 57 with the corresponding chromium(III)Cl salt under the same high-pressure conditions, and found higher

Scheme 14. Diels−Alder Reaction of Cyclopentadiene with 3-Acryloyl-2-oxazolidinone

Later, Rawal and co-workers designed highly efficient chiral salen cobalt(III) complexes such as 46, to promote the Diels− Alder cycloaddition between carbamate-substituted diene 47 and enals 48.69 The corresponding chiral cycloadducts 49 were formed in exceptional yields and enantioselectivities ranging from 85% to 98% ee, as shown in Scheme 15. Importantly, these reactions were conveniently carried out at 0 °C or even at room temperature, under an air atmosphere, and with minimal solvent. A drawback of the process was, however, its narrow 2784


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Scheme 16. Diels−Alder Reaction of a Dienamine with an Enal

enantioselectivities for the cobalt catalyst while lower yields and comparable cis-selectivity in comparison with the chromium catalyst. Earlier, Yamada and co-workers developed novel optically active cobalt(III) complexes as effective catalysts for the enantioselective hetero-Diels−Alder cycloaddition of various aryl and alkyl aldehydes 61 with 1-methoxy-[3-(tertbutyldimethylsilyl)oxy]-1,3-butadiene 62.75 Among these catalysts, cationic cobalt(III) triflate complex 63 was proved to be the most efficient to provide the corresponding chiral cycloadducts 64 in good to high yields (69−94%) and high general enantioselectivities ranging from 81% to 94% ee, as shown in Scheme 19. The addition of molecular sieves to the reaction was shown to improve both the yield and the enantioselectivity of the reaction. The authors have also investigated various other metal complexes of optically active 3-oxobutylideneaminato ligand. The use of titanium(IV), aluminum(III), copper(II), chromium(III), manganese(III), nickel(II), as well as oxovanadium(IV) complexes led to (almost) racemic cycloadducts obtained in low yields, whereas the corresponding cobalt(II) catalyst provided under the same nonoptimized reaction conditions a good enantioselectivity of 62% ee associated with an excellent yield of 96%. These authors reported a theoretical analysis of this reaction, which revealed the crucial role of the aldehyde coordination as an axial ligand on the spin states and Lewis acidity of the cobalt complexes, improving the enantioselectivity.76 Moreover, they showed that increasing the cationic character of the cobalt atom resulted in decreasing the activation energy, and that the spin transition between the triplet and singlet states occurred in the cobalt(III) catalytic cycle according to DFT study.

Scheme 17. Diels−Alder Reaction of a Dienamine with an Enal

Scheme 18. High-Pressure Hetero-Diels−Alder Reaction of 1-(2-Benzyloxyethyl)-3-(tert-butyldimethylsilyl)oxy-1,3butadiene with Methyl Glyoxylate

Scheme 19. Hetero-Diels−Alder Reaction of 1-Methoxy-[3-(tert-butyldimethylsilyl)oxy]-1,3-butadiene with Aldehydes

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Scheme 20. [3+2] Cycloaddition of Nitrones with Enals

2.1.5. [3+2] Cycloadditions. The 1,3-dipolar cycloaddition, also known as the Huisgen cycloaddition,77 is a classic reaction in organic chemistry consisting of the reaction of a dipolarophile with a 1,3-dipolar compound that allows the production of various 5-membered heterocycles.78 The transition state of the concerted 1,3-dipolar cycloaddition reaction is controlled by the frontier molecular orbitals of the substrates. Hence, the reaction of dipoles with dipolarophiles involves either a HOMO-dipole/LUMO-dipolarophile interaction (normal-electron-demand reaction), or a LUMO-dipole/ HOMO-dipolarophile reaction (inverse-electron-demand reaction), depending on the nature of the dipole and the dipolarophile. In some cases, when the frontier molecular orbital energies of the dipole and the dipolarophile are very similar, a combination of both modes of interactions can occur. These interactions can also be referred to as either exo or endo, where the endo transition state is stabilized by small secondary π-orbital interactions or via an exo-transition state lacking such a stabilization. However, steric effects can also be important factors for the endo/exo selectivity and override the secondary orbital interactions.79 Depending on the substitution pattern in the reacting partners, the stereochemical outcome of the process gives rise to either the endo- or the exo-cycloadducts. Moreover, the presence of a metal, such as a Lewis acid, in 1,3dipolar cycloaddition reactions can alter both the orbital coefficients of the reacting atoms and the energy of the frontier orbitals of both the 1,3-dipole and the dipolarophile, depending on the electronic properties of these reagents or the Lewis acid. In particular, the coordination of a Lewis acid to one of the two partners of the cycloaddition is of fundamental importance for asymmetric 1,3-dipolar cycloadditions, because the metal can catalyze the reaction. Furthermore, the Lewis acid may also have influence on the selectivity of the cycloaddition reaction, because the regio-, diastereo-, and enantioselectivity can all be controlled by the presence of a metal−ligand complex. A variety of enantioselective versions of this reaction have successfully used chiral cationic cobalt(III) complexes as chiral

catalysts. For example, Yamada et al. have employed highly efficient catalyst 65 for the enantioselective 1,3-dipolar cycloaddition reaction of α,β-unsaturated aldehydes 66 with nitrones 67.80 Even in the case of α-substituted α,β-unsaturated aldehydes, the corresponding cycloadducts 68 were obtained completely regioselectively, endo-selectively, and in good to high enantioselectivities of up to 92% ee. As shown in Scheme 20, the ratio of regioisomers 68/68′ was of >99:1 in almost all cases of substrates studied as well as the endo/exo ratio. Later, the same authors employed a closely related cationic cobalt(III) catalyst to promote the enantioselective 1,3-dipolar cycloaddition reaction of dihydrofuran with nitrones bearing an electron-withdrawing group, which afforded the corresponding cycloadducts in moderate to high yields (40−87%), high to excellent endo-selectivity (28% to >98% de), combined with low to good enantioselectivities ranging from 6% to 73% ee.81 In 2004, Kanemasa et al. employed a substituted cationic chiral aqua complex derived from a trans-chelating tridentate ligand, tetraphenyl-substituted (R,R)-4,6-dibenzofurandiyl-2,2′bis-(4-phenyloxazoline) (tetraphenyl-DBFOX/Ph), and Co(BF4)2 to induce at room temperature the enantioselective 1,3-dipolar cycloaddition of N-benzylideneaniline N-oxide 69 with α-bromoacrolein 70.82 As shown in Scheme 21, the corresponding cycloadduct 71 was obtained in 92% yield with remarkable diastereo- and enantioselectivities of >98% de and 98% ee, respectively. Moreover, Tang et al. reported the first example of enantioselective cycloadditions between various nitrones 72 and alkylidene malonates 73 in 2004.83 This reaction was induced by an in situ generated cobalt catalyst from Co(ClO4)2· 6H2O and chiral trioxazoline 74, which provided the corresponding chiral isoxazolidines 75 with both high enantioselectivities and high exo diastereoselectivities of up to >98% de and 98% ee, respectively (Scheme 22). Surprisingly, the authors found that, by simply changing the temperature of the reaction, both cis- and trans-cycloadducts could be prepared with high enantioselectivity. Indeed, performing the process at 2786


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Scheme 21. [3+2] Cycloaddition of N-Benzylideneaniline NOxide with α-Bromoacrolein

accounts for the absolute configuration of product (S,S)-77a arisen from the reaction of diphenylacetylene with 2-cyclohexenone has been proposed by the authors and is depicted in Scheme 23. The reaction was likely initiated by the reduction of cobalt(II) to cobalt(I) species 78 by zinc dust. Next, coordination of the alkyne in the equatorial position and the cyclic enone 76a with its si face in the axial position of the cobalt(I) center to form 79 followed by oxidative cyclization gave cobaltacyclopentene intermediate 80. Selective protonation at the α-carbon to the keto group of 80 generated intermediate 81. Next, carbonyl insertion into the cobalt− carbon bond formed cobaltalkoxide 82. Reduction of 82 by zinc dust provided intermediate 83 and regenerated the cobalt(I) species. Ultimately, the hydrolysis of 83 in air afforded final product 77a. 2.1.6. Miscellaneous Cycloadditions. In 1993, Lautens and co-workers reported the first enantioselective cobaltcatalyzed [4+2+2] cycloaddition of various 2-substituted buta1,2-dienes 84 with norbornadiene 34.85 This reaction was performed in the presence of 2 mol % of Co(acac)2 as precatalyst in combination with a chiral phosphine ligand and a reducing agent such as Et2AlCl (4 equiv). Among several chiral phosphine ligands investigated, such as (R)-Prophos, (S,S)Chiraphos, (S,S)-Me-BPE, (R,R)-iPr-BPE or (S,S)-Me-Duphos, (R)-Prophos was shown to be the most efficient, allowing enantioselectivities of up to 79% ee to be reached. As shown in Scheme 24, the reaction was performed in benzene and afforded the corresponding cycloadducts 85 in moderate to good yields of up to 66%. The yields were probably limited due to the competing polymerization of the dienes under the reaction conditions. On the other hand, very little change in the enantioselectivity of the reaction was observed whatever the nature of the substituent beared by the diene. The importance of optically active β-lactones as versatile chiral synthons underscores current research efforts for developing novel catalytic enantioselective methods, among which the asymmetric [2+2] cycloaddition between aldehydes

0 °C led to trans-products, whereas the corresponding cisproducts were produced when carrying out the reaction at −40 °C. On the basis of experimental studies, it was demonstrated that the reaction to form cis-isoxazolidines was reversible and subject to kinetic control at −40 °C. In the case of the reaction at 0 °C, the cycloaddition was subject to thermodynamic control, favoring the trans-isomers. In 2012, Cheng et al. reported an enantioselective cobaltcatalyzed reductive [3+2] cycloaddition of various alkynes with cyclic enones 76 to provide the corresponding chiral bicyclic tertiary alcohols 77 with high regioselectivity.84 As shown in Scheme 23, when the reaction was induced by the chiral cobalt complex in situ generated from CoI2 and (R,R,S,S)-Duanphos ligand in the presence of Zn as a mild reducing agent in 1,4dioxane as solvent, it allowed a range of chiral cycloadducts 77 to be achieved in good yields (50−76%) and high enantioselectivities of up to >99% ee. The process was initiated by the reduction of the cobalt(II) catalyst formed from CoI2 and (R,R,S,S)-Duanphos by zinc into the corresponding cobalt(I) species, which then coordinated the alkyne and the enone to undergo the cycloaddition. A reaction mechanism that

Scheme 22. [3+2] Cycloaddition of Nitrones with Alkylidene Malonates

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Scheme 23. [3+2] Cycloaddition of Alkynes with Cyclic Enones

this important versatile synthon was generated with an excellent yield and a complete enantioselectivity. In this study, the authors have screened metals other than cobalt for the desired bifunctional catalytic activity, finding that complexes derived from the same ligand and titanium(IV), iron(III), nickel(II), copper(II), and chromium(III) did not afford the desired βlactone 89. In 2008, Buono et al. described the first enantioselective cobalt-catalyzed [6 + 2] cycloaddition of cycloheptatriene 90 with alkynes, providing the corresponding chiral [4.2.1]-

and ketenes appears the most elegant. For reactive enolizable aliphatic aldehydes, such as benzyloxyacetaldehyde, catalysts having an excellent level of asymmetric induction are still needed. In this context, Lin et al. have designed a novel Lewis acid/Lewis base bifunctional catalyst 86 based on a covalent attachment of quinine to a salen cobalt(II) complex.86 This novel mixed chiral catalyst was shown to display a remarkable bifunctional catalytic activity in the enantioselective [2 + 2] cycloaddition of 2-benzyloxyacetaldehyde 87 with ketene 88 to give the corresponding β-lactone 89. As shown in Scheme 25, 2788


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afforded the absolute configuration of the novel chiral cycloadducts.

Scheme 24. First Cobalt-Catalyzed [4+2+2] Cycloaddition

2.2. Domino Reactions

According to Tietze, a domino reaction is strictly defined as a process in which two or more bond-forming transformations occur based on functionalities formed in the previous step, and, moreover, no additional reagents, catalysts, or additives can be added to the reaction vessel, nor can reaction conditions be changed.89 The use of domino reactions90 in organic synthesis is increasing constantly, because they allow the synthesis of a wide range of complex molecules, including natural products and biologically active compounds, in an economically favorable way by using processes that avoid the use of costly and time-consuming protection−deprotection processes, as well as purification procedures of intermediates.89,91 The forerunner in the cobalt-catalyzed domino processes was that developed by Vollhardt et al. with their excellent synthesis of steroids initiated by a [2+2+2] cycloaddition.92 Ever since, a number of cobalt-catalyzed domino reactions have been developed. Among them, an enantioselective cobalt-catalyzed multicomponent reaction initiated by a Diels−Alder cycloaddition was developed by Hilt et al. in 2006.93 As shown in Scheme 27, the process began with the Diels−Alder reaction of 1-boron-functionalized 1,3-diene 93 with alkyne 94, giving intermediate 95, which subsequently underwent an allylboration reaction with aldehyde 96 to finally provide the corresponding chiral multifunctionalized domino product 97 in moderate to good yields (57−87%) and moderate enantioselectivities of 71−78% ee. The product, incorporating a stereogenic quaternary center next to a stereogenic secondary alcohol functionality, was regio- and diastereoselectively produced by using a combination of CoBr2 with chiral ligand (S,S)-Norphos. In 2006, Sudalai and Paraskar developed a novel enantioselective cobalt-catalyzed domino reductive cyclization reaction of substituted γ-azido-α,β-unsaturated esters 98 to afford the corresponding γ-lactams 99 in high yields.94 As shown in Scheme 28, the process was induced by a catalytic amount of CoCl2 combined with chiral oxazoline 100 in the presence of BH4Na as the reducing agent. It provided γ-lactams 99 in 82−93% yields and moderate to excellent enantioselectivities of 51−98% ee. The scope of this methodology was extended to γ-cyano-α,β-unsaturated ester 101, which afforded under similar reaction conditions the corresponding δ-lactam 102 in almost quantitative yield and 86% ee (Scheme 28). The utility of these reactions was demonstrated by their applications to the total syntheses of (R)-baclofen, (R)-rolipram, and (R)-4fluorophenylpiperidinone, a key intermediate for (−)-paroxetine. Optically pure tetrahydroquinoline derivatives are widely used in organic synthesis and pharmaceutical chemistry due to their significant building blocks and intriguing biological activities. Traditional methodologies for the formation of these lichipins mainly include the Povarov reaction and the reduction of quinolines. As an alternative straightforward approach to afford tetrahydroquinolines, the tandem hydride transfer/cyclization process, which undergoes a zwitterionic intermediate formed by a 1,5-hydride transfer, has been developed in recent years. As an example, a highly enantioselective synthesis of tetrahydroquinolines was developed by Feng et al. via cobalt(II)-catalyzed domino 1,5-hydride transfer/cyclization reaction.95 As shown in Scheme 29, a chiral

Scheme 25. [2+2] Cycloaddition

bicyclononatrienes 91 in good yields and enantioselectivities of up to 92% ee.87 As shown in Scheme 26, these results were Scheme 26. [6+2] Cycloaddition of Cycloheptatriene with Alkynes

achieved by inducing the process with chiral phosphoramidites 92 based on 3,3′-disubstituted (R)-BINOL, which were selected as the most efficient ligands through the screening of a series of chiral bidentate phosphines and monodentate phosphoramidites. Later, these authors developed novel Pstereogenic triaminophosphines featuring an indoline or a 1,2,3,4-tetrahydroquinolidine pattern, which were further investigated as chiral ligands in the same cobalt-catalyzed [6+2] cycloaddition, albeit providing moderate enantioselectivities of up to 52% ee.88 A vibrational circular dichroism study 2789


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Scheme 27. Three-Component Domino Diels−Alder/Allylboration Reaction

Scheme 28. Domino Reductive Cyclization Reactions

Scheme 29. Domino 1,5-Hydride Transfer/Cyclization Reaction

catalyst generated from L-proline-derived N,N′-dioxide 103 and Co(BF4)2·6H2O was applied to the asymmetric intramolecular hydride transfer-initiated cyclization reaction of a series of odialkylamino-substituted alkylidene malonate derivatives 104 to provide the corresponding biologically interesting tetrahydroquinolines 105 in high to excellent yields and high enantioselectivities of up to 90% ee. The mechanism of the process involved the formation of zwitterionic intermediate 106 through intramolecular hydride transfer, which subsequently cyclized to give the final product 105 (Scheme 29). A possible transition state model depicted in Scheme 29 was proposed by the authors to explain the absolute configuration of the products. In this model, the oxygens of N,N′-dioxide, amide oxygens, and the alkylidene malonate coordinated to cobalt(II) through a hexadentate manner. The carbanion preferred to attack the Re face rather than the Si face of the imine because the latter was strongly shielded by the nearby anthracenyl ring, which resulted in the S-configured product. Since the first catalytic domino Michael/aldol reaction reported by Noyori et al. in 1996,96 there have been numerous examples of domino reactions including a Michael addition. Among them, a number of enantioselective domino reactions initiated by a Michael addition have been promoted by chiral metal catalysts.91k,97 As an example, Feng et al. have reported an efficient asymmetric synthesis of 4H-chromene derivatives through a domino Michael/cyclization reaction of cyclohexane1,3-dione 107 with ethyl 2-cyano-3-arylacrylates 108.98 The reaction was induced by a chiral cobalt complex in situ

generated from Co(OAc)2·4H2O and a chiral salen ligand 109 derived from (R,R)-1,2-diphenylethane-1,2-diamine and 3,5-ditert-butylsalicylaldehyde. A series of additives including acids, bases, alcohols, and phenols were tested to improve the reactivity and enantioselectivity of the reaction, and the authors found that the enantioselectivities were improved by using 3,5dinitrosalicylic acid at 22.5 mol %. The corresponding chiral 2amino-5-oxo-5,6,7,8-tetrahydro-4H-chromene derivatives 110, having extensive biological and pharmacological activities, were 2790


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Scheme 30. Domino Michael/Cyclization Reaction

novel methodology is particularly adapted to enantioselective domino reactions, allowing a rapid and economic construction of highly functionalized chiral molecules from simple and readily available starting materials in one pot. A recent example of enantioselective domino reaction catalyzed by a combination of a chiral cobalt catalyst and an achiral organocatalyst was described by Oh and Kim in 2011.100 The process consisted of a domino aldol/cyclization reaction between aromatic as well as aliphatic aldehydes and methyl α-isocyanate 112, to give the corresponding chiral trans-oxazolines 113. By using a chiral cobalt complex in situ generated from CoI2 and brucine amino diol 114 in the presence of DBU as a base and with achiral thiourea 115, a range of chiral oxazolines 113 were achieved in good to high yields, trans-diastereo-, and enantioselectivities of up to >90% de and 98% ee, respectively, as shown in Scheme 31. The key of the success in this process lied in the strong anion-binding interaction between isocyanide 112 and thiourea 115, which potentially disturbed the intrinsic cobalt−isocyanide complexation (Scheme 31). The reaction was applicable to a range of aromatic, heteroaromatic, and aliphatic aldehydes with the lowest enantioselectivities found in the reactions of 2thiophenecarboxaldehyde (84% ee) and pivaldehyde (74% ee), while high diastereoselectivities were maintained (>90% de). Moreover, the limitation of the current cooperative catalysis lied in ortho- and meta-substituted benzaldehydes, for which low levels of enantioselectivity were obtained in the range of 20−50% ee but with always excellent diastereoselectivities of >90% de.

produced with moderate to good yields of up to 81% combined with good to high enantioselectivities of up to 89% ee, according to successive Michael addition, cyclization, protonation of imine intermediate 111, which regenerated the chiral salen-complex, and a final tautomerization, as depicted in Scheme 30. The results obtained with a wide range of ethyl 2cyano-3-phenylacrylates 108 showed that the electronic nature of the substituent in the aromatic ring (R1) had an obvious effect on the yield and enantioselectivity of the process. Generally, electron-withdrawing substituents provided better yields excepted for 3-methoxy phenyl than electron-donating substituents. Additionally, most of the substrates with substituents in the para position of R1 gave products with better enantioselectivities. Moreover, the 2-naphthyl substrate (R1 = 2-Naph, R2 = Et) also produced the corresponding domino product in good yield (64%) and enantioselectivity of 81% ee, while heteroaromatic substrate such as 2-thienyl derivative (R1 = 2-thienyl, R2 = Et) provided a lower enantioselectivity of 69% ee along with a yield of 75%. Although the concept of bifunctional asymmetric catalysts has been well established in transition-metal catalysis and organocatalysis, respectively, the recent emergence of cooperative catalysis between metals and small organic molecules has provided alternative ways of asymmetric reaction optimizations, where two distinctive catalysis modes are controlled by either one or two chiral (or achiral) components of the reaction. Furthermore, such a combination of multiple catalyst systems has opened new avenues for developing cooperative catalytic systems where the respective catalyst system alone fails to deliver sufficient catalyst reactivity and selectivity. In the past few years, an explosive number of multiple-catalyst systems for various organic transformations have been developed.99 This

2.3. Miscellaneous Cyclization Reactions

The prevalence of five-membered carbocycles in natural products and other bioactive compounds has provided a 2791


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Scheme 31. Domino Aldol/Cyclization Reaction

Scheme 32. Nazarov Reaction

Scheme 33. Baeyer−Villiger Reaction of 3-Aryl(Alkyl) Cyclobutanones

major impetus for the development of efficient methods for their construction. Over the years, the Nazarov reaction has been increasingly refined to meet this need. Most usually, this reaction involves the use of cross-conjugated dienones, treatment of which with a Lewis or Bronsted acid induces the formation of a pentadienyl cation that undergoes 4π electrocyclization to give an allyl cation, followed by proton migration to give, finally, a cyclopentenone.101 Somewhat surprisingly, it was not until the end of 2003 that catalytic asymmetric versions of the Nazarov cyclization began to surface in the literature.102 In addition to scandium, copper, nickel, and iron chiral complexes have been involved in enantioselective versions of the Nazarov reaction, and it is only recently that Itoh et al. investigated cobalt complexes derived from chiral pybox-type ligands.103 Indeed, these authors succeeded in demonstrating the cobalt-catalyzed asymmetric Nazarov reaction of divinyl ketones 116 using a chiral cobalt complex in situ generated from Co(ClO4)2·6H2O and (S,S)-ip-Pybox ligand. As shown in Scheme 32, the corresponding enantioenriched functionalized cyclopentenones 117 were achieved in moderate to good yields and enantioselectivities of up to 63% ee. In some cases of substrates, better enantioselectivities of up to 93% ee were reached by involving iron instead of cobalt catalysts. In 2001, Katsuki and Uchida found that chiral cationic salen cobalt(III) complex 118 was able to serve as an efficient catalyst for asymmetric Baeyer−Villiger reaction of 3-substituted cyclobutanones 119 using hydrogen peroxide as a terminal oxidant.104 As shown in Scheme 33, when the reaction was

performed in ethanol as the solvent at 0 °C in the presence of 5 mol % of catalyst loading, it afforded the corresponding chiral 3-aryl butyrolactones 120 in good yields and enantioselectivities ranging from 75% to 78% ee. Later, these authors have described novel salen cobalt(III) complexes bearing a chiral ethane-1,2-diamine moiety considered to take a square planar geometry.105 These cobalt catalysts were investigated to promote the same Baeyer−Villiger oxidation of 3-aryl as well as 3-alkyl cyclobutanones 119 into the corresponding chiral lactones 120 in the presence of hydrogen peroxide. Among a range of cobalt complexes investigated, all having a chiral binaphthalenediamine unit, catalyst 121 bearing electronwithdrawing F-atoms showed a good level of enantioselectivity ranging from 69% to 79% ee (Scheme 33). Although this process is actually a ring-expansion reaction and not a real cyclization reaction, it was decided to maintain it in this section because it afforded cyclic compounds. In 1999, Yokota et al. reported an asymmetric cyclization of a meso-diepoxide through hydration using chiral salen cobalt(III) complexes.106 As shown in Scheme 34, treatment of meso1,2:4,5-dianhydro-3-O-methylxylitol 122 by (R,R)-Jacobsen’s salen(Co) catalyst 123 led to the exclusive formation of the Denantiomer of 1,4-anhydro-3-O-methyl-D-arabinitol 124 in 78% 2792


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Scheme 34. Cyclization of a meso-Diepoxide

Scheme 35. Intramolecular Cyclizations of meso-Epoxy Alcohols

yield and excellent enantioselectivity of >99% ee. By using the (S,S)-enantiomer of catalyst 123, the corresponding Lenantiomer was formed in the same enantioselectivity and in even better yield of 88%. It must be noted that a remarkably low catalyst loading of 0.5 mol % was used in this process, which employed just water as reagent at room temperature. Application of this methodology to other meso-diepoxides, such as 1,2:5,6-dianhydro-3,4-di-O-methylallitol or 1,2:5,6-dianhydro-3,4-di-O-methylgalactitol, led, however, to mixtures of the corresponding optically active five- and six-membered cyclic compounds, which were, however, both obtained in excellent enantioselectivities (92−97% ee). In 1999, Jacobsen et al. reported the use of chiral salen cobalt(III) complexes to induce intramolecular desymmetrization of meso-epoxy alcohols 125a−c into the corresponding almost enantiopure bicyclic products 126a−c.107 As depicted in Scheme 35, treatment of epoxy alcohol 125a by chiral catalyst 123 under hydrolytic conditions led to bicyclic compound 126a in 96% yield and excellent enantioselectivity of 98% ee. Similarly, gem-bishydroxymethylcyclopentene oxide 125b cleanly cyclized under the same reaction conditions into the corresponding bicyclic ring system 126b in 86% yield and 95% ee. Moreover, meso-epoxy diol 125c underwent an exclusive 4exo ring closure to afford enantiopure oxetane 126c in 45% yield, while meso-epoxy diol 125d underwent a cobalt-catalyzed Payne rearrangement to provide 1,2-anhydrothreitol product 126d in 81% yield and 96% ee (Scheme 35). This novel powerful methodology allowed the synthesis of novel almost enantiopure cyclic and bicyclic ethers ranging from three- to seven-membered rings under mild conditions. Later, the same authors described an enantioselective intramolecular opening of 3-substituted oxetanes 127 catalyzed by chiral salen cobalt(III) complexes to afford the corresponding chiral functionalized tetrahydrofurans 128.108 When oxetanes 127 were activated by the same monomeric salen cobalt catalyst 123, they provided the corresponding tetrahydrofurans 128 in excellent yields and enantioselectivities of up to 99% ee, as shown in Scheme 36. The scope of the reaction of oxetanes with O-centered nucleophiles was examined with a variety of oxetanes bearing nucleophilic appendages. Thus, a series of substituted ethanol derivatives underwent ring-opening with high enantioselectivities ranging from 84% to 99% ee. Alkyl and phenyl substitution at the 3position of the oxetane was tolerated, affording products bearing quaternary stereocenters. Ring-opening of phenolic substrates provided enantioenriched dihydrobenzofurans. Transformation of organic halides into various organic compounds catalyzed by transition metals by means of oxidative addition has been recognized as an important tool

Scheme 36. Intramolecular Opening of Oxetanes

in organic synthesis. In this context, Cheng and co-workers have reported a highly efficient cyclization of o-iodobenzoates with aldehydes induced by cobalt bidentate phosphine complexes.109 An asymmetric version of this process was developed by using a cobalt complex of (S,S)-Dipamp 129 in the presence of zinc powder allowing the reduction of cobalt(II) to cobalt(I). As shown in Scheme 37, various aromatic aldehydes underwent cyclization with methyl 2iodobenzoate 130 in THF at 75 °C to provide the corresponding (S)-phthalides 131 in high yields (80−89%) and enantioselectivities ranging from 70% to 98% ee. This methodology has opened a novel route to these important chiral five-membered lactones which are present in a large number of biologically active compounds, which are also key 2793


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Scheme 37. Cyclization of o-Iodobenzoates with Aldehydes

Scheme 38. Iodolactonization Reactions

an o-metalated methylbenzoate complex 132 with both the ocarbon atom and the ester oxygen atom bonded to the cobalt(III) center. Coordination of the aldehyde molecule to the cobalt center adjacent to the o-metalated methyl benzoate group to give intermediate 133, followed by insertion of the cobalt−carbon bond to the aldehyde, which afforded cobalt− alkoxide intermediate 134. Intramolecular nucleophilic addition of the coordinated alkoxy group in 134 to the ester group gave

intermediates for the synthesis of natural products. The authors have shown that 2-iodobenzoates did not react under the same reaction conditions with aldehydes probably due to their lower reactivity relative to 2-iodobenzoates. To explain the results, the authors have proposed the mechanism depicted in Scheme 37 in which the reduction of cobalt(II) to cobalt(I) by zinc metal likely initiated the catalytic reaction. Oxidative addition of methyl 2-iodobenzoate 130 with the cobalt(I) species yielded 2794


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Scheme 39. Formation of Cyclic Propylene Carbonate through Kinetic Resolution of Propylene Epoxide with Carbon Dioxide

the final product 131 and a cobalt(III) species. The latter cobalt(III) species was reduced by zinc metal to regenerate the active cobalt(I) species. Halolactonization of unsaturated carboxylic acids, particularly the asymmetric version, is a powerful chemical process in synthetic organic chemistry, which can not only build small to large lactone rings but also functionalize olefinic double bonds. It has been proven that the stereochemistry of this reaction can be controlled by either chiral substrates or catalysts. While the substrate-controlled halolactonization has been investigated in detail, the generation of chirality by using a chiral catalyst remains to be further investigated. In this context, Gao et al. have developed a range of chiral salen cobalt complexes, which were investigated as chiral catalysts in the asymmetric iodolactonization of 5-substituted-4-pentenoic acid derivatives.110 By using catalyst 57, these authors have shown that moderate to good enantioselectivities of up to 74% ee were obtained for the iodolactones 135 arising from the corresponding (E)-5-substituted-4-pentenoic acid derivatives 136 (Scheme 38), whereas the (Z)-5-substituted-4-pentenoic acid derivatives led to the corresponding iodolactones in enantioselectivities lower than 12% ee. In this study, the authors have compared the catalytic efficiency of cobalt(II) salen complex 57 to the corresponding manganese(III), chromium(III), and aluminum(III) chiral complexes, and found that the latter catalysts provided much lower enantioselectivities than cobalt catalyst 57 (0−4% ee instead of 27% ee by using 57 under similar nonoptimized reaction conditions). Previously, the same authors have studied the enantioselective iodolactonization of 4-substituted-4-pentenoic acid derivatives 137 catalyzed by the

same complex, which afforded the corresponding iodolactones 138 in moderate to good enantioselectivities of up to 83% ee, as shown in Scheme 38.111 Indeed, a range of substrates having various substituents such as sterically bulky, electron-rich and electron-deficient, and aromatic and aliphatic groups reacted under these conditions, demonstrating that this protocol was amenable to a broad range of substrates. Introducing an electron-rich methoxy group into a phenyl substituent (R = pMeOC6H4) markedly decreased, however, the enantioselectivity of the reaction (22% ee). On the other hand, an electrondeficient Br instead of MeO groups tended to increase the enantioselectivity (73% ee). This influence was probably attributed to the different stability of the intermediate iodonium ion as affected by the electronic property of the substituents. On the other hand, asymmetric Darzens condensation of αhaloamides with benzaldehyde has been investigated by North et al., using a range of cobalt complexes of novel C1symmetrical salen chiral ligands derived from amino acids, such as (S)-alanine, (S)-phenylalanine, (R)-phenylglycine, and (S)-valine.112 Even if the corresponding epoxides were obtained in good to excellent yields (72−97%) as mixtures of cis- and trans-diastereomers, low to moderate enantioselectivities of up to 44% ee were observed for both of these two diastereomers. In 2007, the same authors reinvestigated these reactions by using chiral salen cobalt(II) complexes derived from diaminocyclohexane, which provided comparable enantioselectivities of up to 47% ee combined with moderate diastereoselectivities (14−42% de) and good to excellent yields (71−97%).113 These cobalt(II) complexes were demonstrated to give much better enantioselectivities than the corresponding 2795


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3. COBALT-CATALYZED FORMATIONS OF ACYCLIC COMPOUNDS

copper, titanium, oxovanadium, as well as nickel catalysts. Finally, several groups have studied the kinetic resolution of racemic terminal epoxides based on a coupling with carbon dioxide to give the corresponding chiral five-membered cyclic carbonates. Various chiral cobalt complexes have been investigated to promote this reaction, providing moderate to high enantioselectivities of up to 92% ee, albeit often combined with moderate conversions. For example, Jing et al. have synthesized multichiral cobalt(III) complexes of bis(1,1′-2hydroxy-2′-alkoxy-3-naphthylidene)-1,2-cyclohexanediamine (BINAD), which were further investigated as catalysts in the coupling of carbon dioxide with various terminal epoxides to provide the corresponding chiral cyclic carbonates.114 Enantioselectivities ranging from 6% to 89% ee in combination with yields of 5−33% were achieved when the reaction was performed in the presence of phenyltrimethylammonium tribromide as cocatalyst. Chiral heterobimetallic cobalt(II) salen complexes were also proven by Kim and co-workers to be able to provide chiral cyclic propylene carbonate in 25% yield and 89% ee from the reaction of propylene oxide with carbon dioxide.115 In this case, the authors have shown that using quaternary ammonium salts as cocatalysts led to lower enantioselectivities. More recently, comparable results were reported by Lu et al. by using multichiral cobalt(III) complex 139 in the presence of 200 equiv of ammonium salts as cocatalysts such as 140. The best result (35% yield, 92% ee) obtained under these conditions for the formation of propylene carbonate 142 from the reaction of propylene oxide 141 with carbon dioxide is shown in Scheme 39.116 The authors assumed that the nucleophilic cocatalyst played an important role in the product selectivity and enantioselectivity of the reaction. Therefore, in the presence of less than 1 equiv of this cocatalyst, the reaction did not afford the required cyclic chiral carbonate 142, albeit linear polypropylene carbonate 143. As shown in Scheme 39, the Co(III) complex initiated the coupling reaction by coordinating the epoxide, which was followed by attack by the cocatalyst, leading to the epoxide ring-opening and formation of a cobalt-bound alkoxide. The insertion of carbon dioxide into the cobalt−O bond formed a metal-bound carboxylate, which provided the production of the cyclic carbonate 142 via a backbiting pathway (Scheme 39). It must be noted that this process employed a very low catalyst loading of 0.05 mol %. In 2004, Yamada et al. described the incorporation of carbon dioxide in N,N-diphenylaminomethyloxirane induced by optically active chiral ketoiminatocobalt(II) complexes in the presence of a catalytic amount of a base such as trimethylsilyldiethylamine.117 Under optimal conditions, the corresponding cyclic carbonate was formed in 49% yield and 86% ee. In addition, reactions of terminal epoxides with carbon dioxide were performed in ionic liquids. For example, Jing et al.118 and Kim et al.119 have independently induced these reactions with chiral salen cobalt(III) complexes and immobilized chiral cobalt(II) salen catalysts, which provided moderate enantioselectivities of up to 60% and 65% ee, respectively. It must be noted that the treatment of terminal epoxides with carbon dioxide can also lead to the formation of polycarbonate products. The product selectivity in the formations of cyclic carbonates or acyclic polycarbonates can be controlled by altering the temperature, CO2 pressure, and the nature of the cocatalyst used or not and its loading. The selective formation of chiral acyclic polycarbonates through copolymerization of carbon dioxide with racemic terminal epoxides will be detailed in section 3.7.2.

3.1. Reduction Reactions

3.1.1. Reductions of Ketones and Carbonyl Derivatives. 3.1.1.1. Borohydride Reductions. The reduction of carbonyl compounds is one of the most fundamental reactions to achieve optically active alcohols from ketones.120 Sodium borohydride is the most conventional reducing agent due to its stability, high selectivity, and ease of handling. While optically active semicorrin cobalt(II) complexes were proved by Pfaltz et al. in 1989 to promote the highly enantioselective 1,4-reduction with sodium borohydride,121 no application to the 1,2reduction version was reported until 1995, when the group of Mukaiyama reported the first enantioselective borohydride 1,2reduction of ketones catalyzed by optically active cobalt complexes.122 As shown in Scheme 40, the reduction of a range of aromatic ketones 144 was successfully achieved by using premodified borohydride arising from NaBH4, tetrahyScheme 40. Borohydride Reductions of Aromatic Ketones

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drofurfuryl alcohol (THFA), and ethanol.123 When promoted by chiral (β-oxoaldiminato) cobalt(II) complex 145, it led to the corresponding alcohols 146 in high yields, often quantitative, and high enantioselectivities of up to 97% ee. In 2007, this process was reinvestigated by these authors who demonstrated that chloroform was not only the solvent of the reaction but also the activator for the cobalt complex.124 Indeed, a catalytic amount of chloroform was sufficient to induce the process in THF as solvent. In the same area, Yamada et al. have proposed a novel route to chiral ortho-fluorinated benzhyldrols 147 based on related enantioselective borohydride reduction of the corresponding ortho-fluorinated benzophenones 148.125 In this case, the process was catalyzed by even more sterically hindered chiral cobalt(II) complex 149, providing high yields and enantioselectivities of up to 97% ee, as shown in Scheme 40. The chelation between the fluorine atom and the carbonyl oxygen was shown to enhance the differentiation of the two aryl groups of the benzophenones during the enantioselective reduction. The scope of the process was successfully extended to aryl alkyl ketones. These processes allow novel routes to optically active benzhydrols, which are some of the most important frameworks of pharmaceutical compounds. In addition, the same authors have successfully developed the enantioselective borohydride reduction of various 1,3-dicarbonyl compounds based on the use of another chiral (βoxoaldiminato) cobalt(II) complex such as 150. Therefore, a range of almost enantiopure 1,3-diaryl-1,3-propanediols 151 were conveniently prepared from the corresponding symmetrical 1,3-diketones 152 in excellent yields and general excellent enantioselectivities of 96−99% ee, albeit with good diastereoselectivities of up to 80% de (Scheme 41).126 The further cyclization of these diols allowed an easy route to enantiopure C2-symmetrical cyclic amines to be achieved.127 By using the enantiomeric catalyst 153 in the presence of only 0.4 equiv of sodium borohydride, a highly chemo-, diastereo-, and enantioselective reduction of various asymmetrical 1,2-dialkyl3-aryl-1,3-diketones 154 provided the corresponding anti-aldoltype compounds 155 in moderate yields (41−48%) and excellent diastereo- and enantioselectivities of up to 99% de and 98% ee, respectively, as shown in Scheme 41.128 As another extension of this methodology, these authors have described the enantioselective reduction of 2-alkyl-3-aryl-3-keto esters 156 achieved in the presence of 4 mol % of the same catalyst 153 in the presence of a base, such as MeONa, to perform the process under dynamic kinetic resolution129 (Scheme 42), and in the presence of 1.2 equiv of sodium borohydride.130 As depicted in Scheme 42, a range of optically active anti-2-alkyl-3-hydroxy esters 157 were synthesized in high yields (82−93%), and high diastereo- and enantioselectivities of up to 92% de and 95% ee, respectively. Higher diastereoselectivities ranging from 98% to 99% de in combination with both excellent yields and enantioselectivities of 97−99% ee were reached in the application of the same conditions to the reduction of 2-substituted-1,3-diaryl-3hydroxypropanones 158 into the corresponding optically active 2-substituted-1,3-diaryl-3-hydroxypropanones 159 (Scheme 42).131 Moreover, the same authors have developed an atropoenantioselective borohydride reduction of biaryl lactones 160 evolving through dynamic kinetic resolution to afford the corresponding chiral opened biaryl products 161.132 As shown in Scheme 43, when the reaction was catalyzed by chiral β-

Scheme 41. Borohydride Reductions of 1,3-Diketones

Scheme 42. Borohydride Reductions of 2-Alkyl-3-aryl-3ketoesters and 2-Substituted-1,3-diaryl-3hydroxypropanones

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Scheme 43. Borohydride Reduction of Biaryl Lactones through Dynamic Kinetic Resolution

ketoiminatocobalt(II) complex 162, these products were achieved in high yields and enantioselectivities of up to 93% ee. A fast equilibrium between the atropo-isomers of the biaryl lactones 160-P and 160-M was demonstrated from HPLC analysis. Indeed, the biaryl axis in the lactones remained configurationally unstable and produced atropo-enantiomers in equilibrium. The chiral hydride could recognize one of these two atropo-enantiomers (160-M) and selectively attack it to afford the corresponding axially chiral biaryl compounds 161, which were configurationally stable. In 2003, the same authors investigated the enantioselective borohydride reduction of 2-substituted 3-ketoesters via dynamic kinetic resolution by using cobalt catalyst 150.133 In addition to remarkable enantioselectivities of 97−99% ee, and yields ranging from 68% to 97%, a general excellent diastereoselectivity of 99% de was obtained for the produced anti-2-substituted-3-hydroxy esters. Moreover, enantiomeric catalyst 153 was applied to the enantioselective borodeuteride reduction of p-methyl benzaldehyde, giving the corresponding primary alcohol in quantitative yield, isotopic purity of >95%, and enantioselectivity of 77% ee.134 Catalyst 150 was also involved in the enantioselective borohydride reduction of 2phenacylpyridine, providing the corresponding chiral amine in 94% yield and 92% ee.135 This process constituted the key step in the synthesis of sedamine. Another application of catalyst 150 was reported by the same authors to an efficient preparation of C2-symmetrical chiral ferrocenyl diols 163 through enantioselective borohydride reduction of the corresponding 1,1′-diacylferrocenes 164.136 As shown in Scheme 44, a range of enantiopure C2-symmetrical ferrocenyldiols 163 were achieved in high yields (69−94%) and moderate to excellent dl/meso ratio ranging from 80:20 to 99:1. The enantioselective reduction of N-diarylphosphinyl imines was also investigated by these authors, providing the corresponding chiral amines in good yields (81−97%) and enantioselectivities ranging from 77% to 99% ee when induced by closely related catalysts employed at less than 1 mol % of catalyst loading.137 The best results obtained for the reduction of N-diphenylphosphinyl imines 165 in chiral amines 166 with catalyst 145 are collected in Scheme 45. In their course of studying cobalt-catalyzed enantioselective borohydride reductions of various ketones,138 the same authors have also demonstrated that tetralone derivatives 167 could be reduced into the corresponding alcohols 168 by treatment with NaBH4 in the presence of chiral cobalt catalyst 145 in high yields and enantioselectivities of up to 91% ee under continuous-flow conditions (Scheme 46).139 In 2006, Yamada

Scheme 44. Borohydride Reduction of 1,1′-Diacylferrocenes

Scheme 45. Borohydride Reduction of NDiphenylphosphinyl Imines

and co-workers demonstrated on the basis of experimental and theoretical studies that the key reactive intermediate of borohydride reduction catalyzed by Schiff base−cobalt(II) complexes in chloroform was a dichloromethylcobalt hydride with a sodium cation, for example, that depicted in Scheme 46 for the present reaction.123d Thus, the initial cobalt(II) catalyst (145) was converted in chloroform into this corresponding dichloromethylcobalt hydride 169 with the sodium cation 2798


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Scheme 46. Borohydride Reduction of Tetralones under Continuous-Flow Conditions

Scheme 47. Borohydride Reductions of Aliphatic Ketones

intermediate. Indeed, chloroform was not only a suitable solvent, but it itself reacted as an activator with the cobalt complex to generate this essential reactive dichloromethylcobalt hydride with the sodium cation intermediate that catalyzed the borohydride reduction. Although the aryl carbonyl derivatives are suitable substrates to achieve a high enantioselectivity in borohydride reduction, the enantioselective reduction of aliphatic ketones still needed to be developed. In this context, the same authors have recently designed a novel in situ generated cobalt(III) complex 170 bearing a 1-chlorovinyl group.140 The authors have demonstrated that this active complex 170 was derived from catalyst 171 by treatment with sodium borohydride in 1,1,1-trichloroethane as the solvent. As shown in Scheme 47, catalyst 171 first generated the corresponding intermediate dichloroethyl−cobalt complex 172, which was further converted into the active cobalt complex 170 via elimination of hydrogen chloride due to the acidity of the terminal methyl group. The active complex 170 then underwent the classic mechanism of borohydride reduction of ketones.123d This in situ generated catalyst was found to provide moderate to high enantioselectivities of 61− 90% ee in the enantioselective reduction of various aliphatic ketones 173a−e including dialkyl ketones and 1-adamantyl ketones 173f−h into the corresponding alcohols 174a−h, along with moderate to excellent yields (16−97%). Very recently, the same authors demonstrated that the corresponding reusable and recyclable cobalt system was also efficient to induce chirality in comparable reactions.141 Finally, it must be noted that Kim and co-workers have investigated the catalytic activity of novel chiral salen cobalt complexes immobilized on mesoporous MCM-41 by grafting in the enantioselective borohydride reduction of aromatic ketones.142 These complexes were synthesized from 3-aminopropyltrimethoxysilane and 2,6-diformyl-4-tert-butylphenol through the multigrafting method, which presented the advantage that the ligand preferentially binded at locations on

the MCM-41 surface accessible for the substrate during the catalytic reaction. A relatively high enantioselectivity was obtained as compared to the corresponding homogeneous salen catalysts. 3.1.1.2. Hydrosilylations. Although asymmetric hydrogenation proved to be a successful strategy to reach optically active alcohols and amines, asymmetric 1,2-hydrosilylation of carbon−heteroatom bonds catalyzed by chiral transition metals complexes emerged as a desirable alternative to asymmetric hydrogenation due to the mild conditions and manipulative simplicity.143 In the past two decades, a variety of transition metal catalysts, especially those based on titanium, zinc, tin, copper, and iron, have been broadly exploited and applied in the relevant 1,2-hydrosilylation reactions with moderate to excellent enantioselectivities. On the other hand, the asymmetric hydrosilylation of ketones mediated by cobalt has received relatively moderate attention, since the pioneering works reported by Brunner and Amberger in 1991.144 In this work, in situ generated chiral cobalt(I)/pyridinyloxazoline complexes provided moderate enantioselectivities of up to 56% ee in the hydrosilylation of acetophenones with diphenylsilane. A breakthrough came in 2010 when Nishiyama 2799


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Scheme 48. Hydrosilylations of Alkyl Aryl Ketones

Scheme 49. Hydrosilylation of Alkyl Aryl Ketones

and enantioselectivities of up to 91% ee after subsequent hydrolytic workup. Variously substituted phenylmethylketones and naphthylmethylketones were successfully employed as substrates to establish the substrate scope. It was shown that electron-rich as well as electron-poor groups in the 3,4- or 5position had no effect on the productivity and enantioselectivity of the catalytic hydrosilylation. On the other hand, substitution at the aromatic ring in 2-position to the keto group led to a significant decrease in activity and enantioselectivity of the reaction, while 2,6-disubstituted acetophenones underwent no catalytic reduction. Whereas backbone substitution in the ligand had no significant effect on the catalytic performance, substitution at the chiral center in the wingtips of the pincers seemed to influence the catalyst stability and performance. Later, another asymmetric hydrosilylation of aryl alkyl ketones 176 was described by Chan et al.146 This process employed PhSiH3 as the hydride donor, and a cobalt catalyst in situ generated from a chiral dipyridylphosphine such as (S)-XylP-Phos and Co(OAc)2·4H2O. It constituted the first effective cobalt(II)-diphosphine-catalyzed hydrosilylation system, providing a range of chiral alcohols 179 in high yields and

and co-workers disclosed a highly efficient cobalt(II) complex of chiral bis(oxazolinylphenyl)amine 175, allowing enantioselectivities of up to 98% ee to be achieved.153 As shown in Scheme 48, a range of alkyl aryl ketones 176 were successfully converted into the corresponding alcohols 177 by reaction with (EtO)2MeSiH followed by hydrolytic work up in quantitative yields and high enantioselectivities in almost all cases of substrates studied. Indeed, with the exception of 1-acetyl naphthalene, which provided the lowest enantioselectivity of 60% ee, other ketones, such as variously substituted phenyl ketones or 2-acetyl naphthalene derivatives, gave higher enantioselectivities ranging from 87% to 98% ee. In the same context, Gade et al. have designed a novel family of chiral C2symmetric tridentate monoanionic N,N,N-pincer ligands based on the 1,3-bis(2-pyridylimino)isoindoline framework, which were further investigated as cobalt ligands to induce chirality in hydrosilylation of several aryl methyl ketones 176′ with tertiary silanes such as (EtO)2MeSiH.145 As shown in Scheme 48, tetracoordinated chiral cobalt alkyl complexes 178ab proved to be highly efficient promoters of this reaction, because the corresponding alcohols 179 were obtained in excellent yields 2800


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Scheme 50. Conjugate Additions of NaBH4 to α,βUnsaturated Carboxamides

moderate to excellent enantioselectivities of up to 94% ee after subsequent hydrolytic work up, as shown in Scheme 49. It must be noted that the reaction activities were dependent on the electronic nature of the substituents on the arene ring of the substrates. For example, only a trace of reaction occurred for acetophenone or p-methylacetophenone. Nonetheless, the aryl alkyl ketones embodying electron-withdrawing substituents on the phenyl group were apparently more conducive to both faster reaction rates and higher enantioselectivities. Another observation was that the positioning of the substituents on the phenyl ring of the ketones had a dramatic effect on the reaction outcomes. For example, acetophenone with an NO2 group substituted on either the para- or the meta-position resulted in a quantitative yield and high enantioselectivities (85−89% ee), while the corresponding sterically hindered ortho-substituted reagent led to a low yield (6%) and a moderate enantioselectivity of 51% ee. 3.1.2. Conjugate Reductions. 3.1.2.1. Conjugate Reductions Using Borohydrides. In 1989, Pfaltz and co-workers reported an enantioselective conjugate reduction of (E)-α,βunsaturated carboxylates with sodium borohydride induced by catalytic amounts (1 mol %) of chiral cobalt semicorrin complexes121 in situ generated from CoCl2 and chiral ligand 180 (Scheme 50).147 The corresponding chiral esters were achieved in high yields (84−97%) and enantioselectivities of 73−96% ee. Two years later, the same authors extended these reaction conditions to the highly enantioselective conjugate reduction of α,β-unsaturated carboxamides 181 with sodium borohydride promoted by chiral semicorrin cobalt catalysts in situ prepared from CoCl2 and the free ligands.148 Remarkably, enantioselectivities of up to 99% ee combined with essentially quantitative yields were achieved for the corresponding formed amides 182 by using cobalt complex of chiral bidentate nitrogen ligand 180, as shown in Scheme 50. Furthermore, a low catalyst loading of 0.12 mol % constituted a supplementary attractive attribute to this exceptional process. Among the various primary and secondary amides studied, the corresponding (Z)-isomers 183 were also examined under similar reaction conditions. As shown in Scheme 50, they led to the corresponding chiral amides 184 in comparable and remarkable enantioselectivities ranging from 93% to 97% ee and again in almost quantitative yields. Later, Yamada et al. reinvestigated these reactions by using chiral β-ketoiminato cobalt(II) complexes. For example, using catalyst 145 at 2 mol % of catalyst loading in tetrahydrofurfuryl alcohol as the solvent led to enantioenriched amides in moderate to excellent yields of 41−98% combined with moderate enantioselectivities ranging from 27% to 60% ee,149 while using 0.075−0.5 mol % of catalyst 185 (Scheme 50) provided chiral amides in good to high yields (69−99%) and better enantioselectivities ranging from 49% to 91% ee.150 It is reasonable to assume that a cobalt−enolate equivalent derived from the α,β-unsaturated carboxamide was generated as a reactive intermediate (Scheme 50). The latter was subsequently protonated by ethanol in an enantioselective manner to afford the final chiral carboxamide 182 or 184. In 2005, Reiser et al. introduced the use of more readily available chiral azabis(oxazoline) ligands in enantioselective conjugate reduction of α,β-unsaturated esters with sodium borohydride.151 Several differently substituted chiral ligands were screened, and the phenyl-substituted one 186 was selected as the most efficient ligand, allowing high to excellent enantioselectivities ranging from 92% to 97% ee to be achieved

in combination with high yields (85−89%) for the conjugate reduction of various aromatic as well as aliphatic α,βunsaturated esters 187 into the corresponding esters 188 (Scheme 51). Moreover, the scope of this methodology was successfully extended to the reduction of other Michael acceptors, such as γ-butyrolactones and α,β-unsaturated amides, which afforded the corresponding reduced products in good to high yields of 54−65% and 81−88%, respectively, in combination with enantioselectivities of up to 86% ee and 95% ee, respectively. Later, Fraile et al. reported a study on the recycling possibilities for chiral azabis(oxazoline)-cobalt complexes as catalysts for the enantioselective conjugate addition of NaBH4 to ethyl (E)-3-phenylbut-2-enoate 187b.152 They demonstrated that the best method for recycling was the use of liquid−liquid biphasic system. In this context, the use of a cobalt complex of chiral ditopic azabis(oxazoline) 189 in 1,3bis(2,2,2-trifluoroethoxy)propan-2-ol (BTFEP) as the solvent was shown to allow the conjugated reduction of adduct 187b into product 188b with enantioselectivities of up to 96% ee to be achieved in association with an excellent yield of 99% (Scheme 51). Moreover, the authors have found that this catalytic system was performant for five runs. 2801


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Scheme 51. Conjugate Additions of NaBH4 to α,βUnsaturated Esters

3.2. Michael Reactions

Although first reported by Komnenos in 1883,154 the nucleophilic 1,4-addition of stabilized carbon nucleophiles to electron-poor olefins, generally α,β-unsaturated carbonyl compounds, is known as the Michael addition. More generally, Michael-type reactions can be considered as one of the most powerful and reliable tools for the stereocontrolled formation of carbon−carbon and carbon−heteroatom bonds,155 as has been demonstrated by the huge number of examples in which it has been applied as a key strategic transformation in total synthesis. It must be recognized that still few publications reported the use of chiral cobalt catalysts to achieve catalytic Michael additions of nucleophiles to electron-deficient alkenes in an enantioselective fashion. In 1984, pioneering works of Brunner et al. demonstrated that a catalytic system in situ generated from Co(acac)2 and (+)-1,2-diphenylethylenediamine as chiral ligand was able to induce the addition of methyl 1-oxo-2-indanecarboxylate to methylvinylketone with an optical induction of 66% ee.156 Unfortunately, attempts to improve optical yields of the Michael addition of 1,3-dicarbonyl compounds by involving other chiral ligands, such as alkaloid or salicylaldimine derivatives,156b,c proline-based ligands,157 or spirobiindane-containing ligands,158 were unsuccessful for many years. However, Pfaltz and co-workers demonstrated in 1998 that a cobalt-based system allowed a Michael addition of malonates to chalcone to be achieved with a high enantioselectivity of up to 89% ee.159 In this study, tert-butylsubstituted chiral bisoxazoline oxalamide ligands were found to provide these good enantioselectivities as a function of the steric hindrance of the malonate derivatives, however, in low chemical yields (12−17%). In 2006, Zhou et al. designed two novel chiral C2-symmetric spiro nitrogen-containing ligands including pyridine or quinolone units, 7,7′-bis(2-pyridinecarboxamido)-1,1′-spirobiindane (SIPAD), and 7,7′-bis(2-quinolinecarboxamido)-1,1′-spirobiindane (SIQAD).158 These ligands were combined with Co(OAc)2 to in situ generate the corresponding complexes, which were proved to be efficient catalysts in the enantioselective Michael addition of malonates to chalcones. The alkylation products were obtained in high yields (70−78%) albeit moderate enantioselectivities ranging from 47% to 57% ee. Later, Itoh et al. reported enantioselectivities of up to 95% ee for the asymmetric Michael addition of thiols to (E)-3-crotonoyloxazolidin-2-one 192 by using a catalytic system consisting of the combination of Co(ClO4)2·6H2O with (S,S)-ip-Pybox.160 The process was performed in THF at −20 °C in the presence of 4 Å molecular sieves, providing the corresponding Michael adducts 193 in moderate to high yields, as shown in Scheme 53. The best enantioselectivity of 95% ee was reached in the case of sterically hindered 2-methylbenzenethiol. In 2010, Matsunaga and Shibasaki reported an efficient example of bifunctional cooperative asymmetric catalysis using a homodinuclear bis-Co(III) Schiff base complex for the conjugate addition of β-ketoesters to nitroolefins.161 Using a dinucleating Schiff base, several combinations of metals, such as copper, palladium, nickel, manganese, zinc, or lanthanides, were investigated to identify the most powerful system promoting enantioselective Michael addition. More or less success was achieved because only a bis-nickel system gave rise to Michael adducts with good enantioselectivities of up to 74% ee. On the other hand, chiral bis-Co(III) complex 194 was found to efficiently promote the addition of a range of cyclic as well as acyclic β-ketoesters 195 to nitroalkenes 196, providing the

3.1.2.2. Conjugate Hydrosilylations. In 2010, Nishiyama et al. reported moderate to good enantioselectivities in the cobaltcatalyzed asymmetric conjugate hydrosilylation of enones with (EtO)2MeSiH by using chiral bis(oxazolinylphenyl)amine ligands.153 As shown in Scheme 52, the hydrosilylation of Scheme 52. Conjugate Hydrosilylation of α,β-Unsaturated Enones

enones 190 provided the corresponding reduced ketones 191 in high yields and enantioselectivities of up to 75% ee by using a combination of Co(OAc)2 with chiral ligand Bopa-dpm. In this study, the authors have selected this cobalt catalyst as the most efficient among a range of complexes including related nickel, copper, and iron. While nickel and copper acetates did not show the expected catalytic activity for the present reaction, cobalt(II) acetate complex strongly promoted the reaction in comparative high yields and with higher enantioselectivities than did iron. 2802


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interesting feature of this catalytic system was that it also worked in the absence of solvent, and the catalyst loading could be decreased to 0.1 mol %. Moreover, mechanistic studies and control experiments were carried out to confirm the intramolecular cooperative effect of the two cobalt centers. The postulated catalytic cycle of the reaction is depicted in Scheme 54. The authors assumed that the β-keto ester, for example 195a, coordinated to the sterically less hindered outer Co-metal center of complex 194. One Co-aryloxide (or Co-acetate) deprotonated the α-proton of β-keto ester 195a to generate a Co-enolate. Inner Co-metal center acted as a Lewis acid to activate the nitroalkene 196 in a manner similar to that observed in the monomeric Co-salen system. 1,4-Addition via bimetallic transition state followed by protonation afforded the final products 197 and regenerated the catalyst. In addition, these reaction conditions were also successfully applied to the enantioselective Michael addition of cyclic as well as acyclic βketoesters to alkynones, providing the corresponding chiral enones with remarkable results because a general diastereoselectivity of >94% de was obtained in all cases of substrates studied in combination with excellent yields ranging from 83% to 96% and enantioselectivities from 91% to 99% ee.162 In this study, catalyst screening allowed biscobalt complex 194 to be selected as the most efficient catalyst among a range of other dinuclear chiral complexes based on nickel, copper, zinc, samarium, lanthanum, and palladium. For example, moderate enantioselectivities were obtained by using the corresponding bisnickel catalyst, while rare-earth-metal complexes gave poor enantioselectivities, and bis-zinc or bis-copper provided racemic products. In addition, nucleophiles other than 1,3-dicarbonyl compounds, hydrides, or thiols have been studied to accomplish cobalt-catalyzed enantioselective Michael addition. For example, Feringa and de Vries have reported the addition of diethylzinc to chalcone mediated by a chiral cobalt complex generated from Co(acac)2 and chiral amino alcohols.163 In this study, the best enantioselectivity of 83% ee was achieved by using a (+)-camphor-based ligand. More recently, Nishimura and Hayashi studied the cobalt-catalyzed asymmetric conjugate alkynylation of α,β-unsaturated ketones.164 Having developed a cobalt-based catalytic system to achieve the conjugate addition o f s i ly la c e t y l e n es t o e n o n e s u si n g a bi d e n t at e diphenylphosphino(ethane) ligand, an asymmetric version was accomplished using a chiral biphosphine ligand 198, (2S,4S)-2,4-bis(diphenylphosphino)pentane. In this context, several chiral β-alkynylketones 199 were produced in good to high yields (53−93%) and enantioselectivities ranging from 79% to 91% ee starting from the corresponding α,β-unsaturated ketones 200 and (triisopropylsilyl)acetylene 201, as shown in Scheme 55. Soon after, this catalytic system was reinvestigated by these authors and applied to asymmetric addition of terminal alkynes to extended conjugate systems such as α,β,γ,δ-unsaturated carbonyl compounds.164b A thourough screening of chiral biphosphine ligands led one to identify (S,S)-Et-Duphos as the most efficient ligand providing the best enantioselectivities. As shown in Scheme 56, the addition of (triisopropylsilyl)acetylene 202 to aliphatic dienoates and dienamides 203 occurred exclusively in the δ-position, affording the corresponding 1,6-conjugate adducts 204 in good yields and high enantioselectivities of up to 99% ee. In addition, a α,β,γ,δunsaturated arene was found to be a good substrate in the alkynylation reaction because the corresponding Michael

Scheme 53. Michael Addition of Thiols to (E)-3Crotonoyloxazolidin-2-one

corresponding nitro-Michael adducts 197 in high to excellent yields (73% to >99%) with both good to excellent diastereoand enantioselectivities of up to >94% de and 99% ee, respectively, as shown in Scheme 54. It must be noted that the best results were obtained in the case of cyclic β-ketoesters as substrates. Indeed, lower diastereoselectivities of 54% de were obtained in cases of acyclic β-ketoesters, while high yield (73%) and excellent enantioselectivity of 96% ee were reached. An Scheme 54. Michael Addition of β-Ketoesters to Nitroalkenes

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lectivities ranging from 53% to 92% ee. The same authors have obtained even better results for the reaction of aromatic aldehydes with nitromethane by using chiral salen cobalt complexes such as 205 and 57 at a lower catalyst loading of 2 mol % in the presence of the same base, solvent, and temperature. As shown in Scheme 57, a range of chiral βnitroalcohols 206 were achieved in 36% to quantitative yields and enantioselectivities of 62−98% ee.168 In 2011, Duan et al. developed an approach to create an L-proline-functionalized cobalt-organic triangle to be used as size-selective homogeneous catalyst for asymmetric direct aldol reactions.169 This catalyst was generated by self-assembly through incorporating an L-proline moiety within a cobalto-helical triangle formed by assembling cobalt ions and two tridentate N2O units containing amide groups within a central benzene ring at the meta sites. Therefore, it included L-proline moieties as asymmetric catalytic sites and a helical-like cavity, and was proved to work as an asymmetric catalyst to prompt aldol reaction of ortho-, meta-, and para-nitrobenzaldehydes with cyclohexanone with size-, diastereo-, and enantioselectivity. Besides moderate yields ranging from 21% to 42%, the process afforded moderate to good diastereo- and enantioselectivities ranging from 0% to 83% de and 44% to 73% ee, respectively. On the other hand, enantioselective nitro-aldol (Henry) reaction of aromatic aldehydes with nitromethane was reported by Hong et al. in 2008.170 This efficient process was catalyzed by a newly designed self-assembled chiral dinuclear salen cobalt(II) complex 208 in situ prepared from the reaction of the corresponding salen cobalt(II) complex 207 with Co(OAc)2· 4H2O self-assembled through hydrogen bonding (Scheme 57). The reaction provided in the presence of DIPEA the corresponding chiral alcohols 206′ in high yields (65−99%) and enantioselectivities ranging from 81% to 96% ee, as shown in Scheme 57. The self-assembly through hydrogen bonding was confirmed by the X-ray structure and 1H NMR experiments. Later, the same authors developed a diastereo- and enantioselective nitro-aldol reaction of aliphatic as well as aromatic aldehydes with nitroalkanes other than nitromethane, such as nitroethane, nitropropane, and TBSOCH2CH2NO2, to afford the corresponding Henry products 209 bearing two stereocenters.171 When the process was promoted by novel

Scheme 55. Michael Addition of (Triisopropylsilyl)acetylene to α,β-Unsaturated Ketones

adduct was obtained in 68% yield and enantioselectivity of 88% ee. Control experiments proved the imperative need of zinc to form adducts and the influence of the geometrical structure of the starting material on both chemical and optical yields. Finally, it must be noted that Ganzmann and Gladysz have reported a low enantioselectivity of 33% ee combined with a yield of 78% in the Michael addition of dimethyl malonate to 2cyclopenten-1-one promoted by Werner salts of chiral tris(ethylenediamine)-substituted octahedral cation.165 3.3. (Nitro-)Aldol Reactions

In the history of organic synthesis, aldol reactions are among the most widely studied and extensively used for all of the approaches to carbon−carbon bond formation, most notably for rapid access to polyoxygenated compounds. The Henry reaction or nitro-aldol reaction is one of the most convenient reactions for direct carbon−carbon bond formation without any pretreatment to afford β-hydroxy-nitroalkanes from aldehydes and nitroalkanes. Since the first catalytic enantioselective version of this reaction reported by Shibasaki et al. based on the use of heterobimetallic lanthanide BINOL catalyst systems,166 various complex catalyst systems have been successfully developed. For example, Yamada et al. have found that chiral ketoiminato cobalt complexes efficiently catalyzed the enantioselective Henry reaction of aldehydes in the presence of a tertiary amine such as DIPEA.167 The most efficient catalyst 145 (Scheme 40) employed at 5 mol % of catalyst loading provided in dichloromethane as solvent low to quantitative yields (11−100%) and good to high enantiose-

Scheme 56. Michael Addition of (Triisopropylsilyl)acetylene to α,β,γ,δ-Unsaturated Carbonyl Compounds

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past decade.172 Recently, Reiser et al. reported the highly efficient use of simple L-proline as chiral ligand of cobalt to catalyze the enantioselective direct aldol reaction of a range of aromatic and aliphatic aldehydes with cyclic as well as acyclic ketones.173 The authors have found that the efficiency of these reactions was significantly higher as compared to the analogous classical proline-catalyzed processes as well as to other metal− proline complexes employed previously. Indeed, the use of combinations of L-proline with zinc and nickel chlorides provided lower enantioselectivities (≤81% ee for 92% ee with cobalt chloride), while manganese, iron, magnesium, or copper chlorides gave even lower enantioselectivities (37−75% ee). This novel protocol presented the advantage of being very simple through mixing inexpensive CoCl2 and L-proline in methanol at room temperature. In general, the best results for the formation of a number of chiral alcohols 211 were reached in the case of using cyclic ketones as the substrates with various aromatic and aliphatic aldehydes, providing good to excellent yields (50−93%), high diastereoselectivities of up to 96% de, and excellent enantioselectivities of up to 98% ee, as shown in Scheme 59. The scope of the methodology was extended to acyclic unsymmetrical ketones, which provided by reaction with aromatic aldehydes the corresponding chiral alcohols 212 in good to high yields (64−92%), moderate diastereoselectivities ranging from 34% to 66% de, combined with good to high enantioselectivities of 50−91% ee (Scheme 59). In some cases of substrates, the best enantioselectivities were reached by using DMSO instead of methanol as solvent. The authors have proposed that initially two molecules of L-proline were bound through the carboxylate groups to cobalt(II), giving rise to complex 213. The key catalytically active species could then function as the C2-symmetrical cobalt−proline complex 214. Moreover, during the aldol reaction, a pH value of 4−6 was measured, which was in agreement with the liberation of HCl during the catalyst formation.

Scheme 57. Henry Reactions of Aromatic Aldehydes with Nitromethane

3.4. Carbonyl-Ene Reactions

The enantioselective carbonyl-ene reaction promoted by a Lewis acid is a direct route to optically active homoallylic alcohols. The carbonyl-ene reaction constitutes one of the most convenient methods for carbon−carbon bond formation, which does not need any pretreatment of carbonyl compounds such as enolization, and the resulting homoallylic alcohols can be further transformed into more functionalized products by taking advantage of their carbon−carbon double bonds. A variety of chiral metal complexes have been widely investigated as Lewis acid catalysts; various centered metals (titanium, copper, scandium, chromium, cobalt), the design of chiral ligands (BINOL, bisoxazolines, Pybox, Schiff bases, etc), and their combinations have been tried for achieving a high catalytic ability and high enantioselectivity in the carbonyl-ene reaction. In 2001, chiral cationic cobalt(III) complexes were investigated by Yamada et al. as promotors for the enantioselective carbonyl-ene reaction of glyoxal derivatives 215 with a variety of alkenes.174 This reaction smoothly proceeded at −20 °C to afford the corresponding homoallylic alcohols 216 in good to high yields and high enantioselectivities of up to 94% ee when catalyzed by cobalt complex 217, as shown in Scheme 60. The authors have shown that, even in the presence of only 0.2 mol % of cobalt catalyst, the reaction occurred in high yield (98− 99%) and maintained high enantioselectivity (92−96% ee). The reaction temperature was shown to have an effect on both the enantioselectivity and the yield, because they were lower at

chiral [(bisurea-salen)cobalt] catalyst 210, it provided high anti selectivities of up to >96% de as well as excellent enantioselectivities of up to 99% ee, as shown in Scheme 58. The cooperative activation by H-bonds of urea and the Lewis acid cobalt center is shown in Scheme 58. It must be noted that comparable results were achieved in both cases of aromatic and aliphatic aldehydes, and, consequently, the use of this chiral urea-cobalt bifunctional catalyst has successfully extended the substrate scope of anti-selective Henry reactions to previously unexplored aldehydes. Moreover, the synthetic utility of this methodology was demonstrated by a concise asymmetric synthesis of (1R,2S)-methoxamine hydrochloride. The direct catalytic asymmetric aldol reaction is a powerful and atom-economical method for synthesizing chiral β-hydroxy carbonyl compounds. Many metals and also organocatalysts for reactions of aldehyde electrophiles have been developed in the 2805


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Scheme 58. Henry Reaction of Aromatic and Aliphatic Aldehydes with Other Nitroalkanes

Scheme 59. Aldol Reactions of Aldehydes with Cyclic and Acyclic Ketones

Scheme 60. Carbonyl-Ene Reaction of 1,1-Disubstituted Alkenes with Glyoxal Derivatives

Scheme 61. Carbonyl-Ene Reaction of Di- and Trisubstituted Alkenes with Ethyl Glyoxylate

room temperature and at 0 °C, while slightly improved at −40 and −60 °C instead of −20 °C. In 2007, Rawal et al. reinvestigated this type of reactions by using a more sterically hindered catalyst 218 in which bulky triisobutylsilyl substituents occupied the positions ortho to the phenolic oxygens.175 This complex catalyzed the reactions of various 1,1-disubstituted as well as trisubstituted alkenes with ethyl glyoxylate 219 at room temperature and using catalyst loadings as low as 0.1 mol %, which are ideal conditions. The processes provided the corresponding homoallylic alcohols 220 in excellent yields, diastereo-, and enantioselectivities of up to 98%, 92% de, and 98% ee, as shown in Scheme 61.

3.5. Coupling Reactions between Alkenes and Alkynes

Cobalt-catalyzed coupling reactions are very efficient for the elaboration of carbon−carbon bonds.176 In particular, asym2806


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metric reductive coupling involving alkynes as substrates is a competent method for the synthesis of highly regio-, stereo-, and enantioselective substituted alkenes. Various types of πcomponents, such as aldehydes, imines, epoxides, and ketones, have been employed in the enantioselective coupling with alkynes using several metal complexes (nickel, rhodium, or iridium). On the other hand, the asymmetric reductive coupling of alkynes with alkenes remains relatively less explored. As a recent example, Cheng et al. have developed an enantioselective synthesis of β-substituted cyclic ketones on the basis of a cobalt-catalyzed asymmetric reductive coupling of alkynes with cyclic enones 221.177 The process was induced by a chiral cobalt complex in situ generated from CoI2 and (R,R)-BINAP, providing regioselectively in the presence of zinc as the reducing agent the corresponding chiral β-alkenyl cyclic ketones 222 in good yields and enantioselectivities of up to 96% ee, as shown in Scheme 62. The scope of the reaction was

Scheme 63. Coupling of Alkynes with Oxa- and Azabenzonorbornadienes

Scheme 62. Reductive Coupling of Alkynes with Cyclic Enones

alkynes to oxabenzonorbornadienes without ring-opening, which was achieved by using a chiral phosphine-cobalt catalyst system. The authors have proposed the catalytic cycle depicted in Scheme 63 to explain the results. It is initiated by the reduction of cobalt(II) to cobalt(I) by zinc powder giving a cobalt(I) acetate 227, which undergoes the reaction with the terminal alkyne to form an alkynylcobalt(I) 228 and acetic acid. An approach of the alkynylcobalt 228 from the exo direction of the oxabenzonorbornadiene followed by syn-carbometalation generates an alkylcobalt(I) species 229. Protonation of alkylcobalt 229 with the terminal alkyne gives the alkynylation product and regenerates the alkynylcobalt intermediate 228.

large with comparable results for symmetrical as well as unsymmetrical alkynes, including electron-deficient ones. It is noteworthy, however, that the process was not suitable for terminal alkynes but instead led to facile homocyclotrimerization of the alkynes. The advantage of this reaction lies in the use of an air-stable catalyst, a mild reductive agent, and a simple hydrogen source (water). Later, Hayashi et al. reported the catalytic asymmetric addition of terminal alkynes such as silylacetylenes 223 to oxaand azabenzonorbornadienes 224, which provided the corresponding chiral 1,2,3,4-tetrahydro-2-alkynyl-1,4-epoxy(aza)naphthalenes 225.178 Among a series of chiral ligands including (S,S)-Chiraphos, (S,S)-BDPP, (R,R)-BINAP, (R,R)Dipamp, and (S,S)-Me-Duphos, ligand (R,R)-QuinoxP* 226 was selected as the most efficient to induce excellent enantioselectivity of up to 99% ee, in combination with high yields, as shown in Scheme 63. The authors have proposed the initial formation of a cobalt(I) acetate arising from the reduction by zinc powder of the starting cobalt(II) diacetate. The latter formed an alkynylcobalt(I) complex and acetic acid. An approach of this complex from the exo direction of the oxa(aza)benzonorbornadiene then allowed the final product to be achieved. It must be noted that this work constituted the first example of catalytic asymmetric addition of terminal

3.6. Hydrovinylation Reactions

The transition-metal-catalyzed codimerization of ethene with alkenes, also called the hydrovinylation reaction, is the addition of the elements of ethene (vinyl and hydrogen) across the double bond of a second alkene, offering a great potential for practical synthetic applications.179 Only moderate success has been reported in the first cobalt-catalyzed hydrovinylation reactions,180 and most of the time, this process is catalyzed by nickel or palladium complexes, which are limited to the use of monodentate ligands.181 Inspired by the work of Hilt et al. reported in 2001 dealing with the cobalt-catalyzed codimerization of a range of 1,3-dienes and alkenes,182 Vogt et al. have explored the asymmetric cobalt-catalyzed hydrovinylation of 2807


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Scheme 64. 1,4-Hydrovinylation of Substituted 1,3-Dienes with Ethene

Scheme 65. 1,4-Hydrovinylation of 1-Vinylcycloalkenes with Ethene

styrene with ethene, providing the corresponding chiral 3phenyl-1-butene.183 The activation of [CoX2(phosphine)] complexes by alkylating agents, especially Et2AlCl, afforded very active catalysts with unprecedented high selectivity for the formation of the expected codimer. Indeed, this product was obtained with more than 99% selectivity without trace of double bond isomerization. On the other hand, an enantioselectivity limited to 50% ee was obtained by using chiral bis(amido-phosphine) ligands. Following this lead, Sharma and RajanBabu reported the very efficient and highly

enantioselective hydrovinylation of a range of substituted unactivated linear 1,3-dienes 230 with ethene, providing exclusively the corresponding chiral (Z)-1,4-adducts 231 without any trace of the corresponding 1,2-regioisomers or any dimerization products.184 Among a limited set of chiral ligands that were explored, commercially available (R,R)-(2,2dimethyl-1,3-dioxalane-4,5-diylbismethylene)-bis-diphenylphosphine ((R,R)-DIOP) and (S,S)-2,4-bis-diphenylphosphinopentane ((S,S)-BDPP) were found to give the best results with almost quantitative yields in all cases of substrates studied and 2808


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kinetic resolution based on the use of Jacobsen’s chiral salen Co(III) complexes, such as catalyst 123 (and also chiral salen Cr(III) complexes), has emerged as a powerful and widely used method for resolving a wide range of terminal racemic epoxides, often affording both epoxides and their corresponding ringopened 1,2-diols in very high enantioselectivities. Alkyl-, halo alkyl-, aryl-, vinyl-, alkynyl-epoxides, including epoxides containing various functional groups, such as sulfone, ester, or dialkylphosphonate, are able to afford the corresponding chiral diols as well as chiral recovered epoxides in enantioselectivities of 99% ee (Scheme 66).187d,192,193 Most

remarkable enantioselectivities of up to >99% ee, as shown in Scheme 64. The reaction appeared to be quite general for dienes including (E)-1,3-pentadiene. Substrates with functional groups, such as benzyl ether, were tolerated, also providing excellent enantioselectivities (96−99% ee), but not unexpectedly, reacted sluggishly (40% yield). Finally, a diene with phenyl conjugation gave essentially a racemic product (98% ee. The ratios between the expected 1,4-hydrovinylation adducts and the undesired 1,2-hydrovinylation regioisomers were comprised between 85:15 and >99.5:0.5. For example, vinylcyclooctene among several other substrates gave exclusively the corresponding 1,4-adduct. The enhanced reactivity of the trisubstituted double bond in electrophilic reactions should make these enantiopure skipped 1,4-dienes valuable intermediates for synthetic applications.

Scheme 66. Hydrolytic Kinetic Resolution of Epoxides through Jacobsen Methodology

of the time, catalyst loadings as low as 0.01 mol % were sufficient to reach excellent results. It must be noted that examples of hydrolytic kinetic resolution of epoxides bearing two stereocenters remain still rare. For example, a resolved epoxypentenol was generated through this fashion in 48% yield and 98% ee and further employed as key intermediate in the total synthesis of (5S,7R)-kurzilactone.194 Another example was reported by Sudalai et al. who applied the same methodology to a series of benzyloxy- and azido-epoxides, affording a practical access to a wide range of enantiopure syn- or anti-alkoxy- and azido-epoxides along with the corresponding diols.195 In this study, the methodology was employed in a concise, enantioselective synthesis of bioactive molecules, such as (S,S)-reboxetine and (+)-epi-cytoxazone. In addition, efficient total syntheses of patulolide C and 11-epipatulolide C,196 and that of (+)-boronolide,197 have been independently achieved by Sharma and Babu and Kumar and Naidu, respectively, on the basis of this methodology. Several other biologically active products, such as cryptocarya diacetate,198 yene-polyol macrolide RK-397,199 and macroviracin A,200 have involved in their syntheses successful hydrolytic kinetic resolutions of epoxides bearing at least two stereocenters. Among the advantages of the hydrolytic kinetic resolution methodology are its broad applicability over a range of simple as well as functionalized terminal epoxides, high enantioselectivity, remarkable practical appeal, and low catalyst loading. Unsurprisingly, the generality and broad substrate specificity of hydrolytic kinetic resolution have been exploited for the production of a wide range of chiral synthons for natural products and bioactive compounds synthesis.201,202 Detailed mechanistic investigations of the hydrolytic kinetic resolution using monomeric salen metal catalysts have revealed secondorder kinetic dependence on catalyst concentration and point to a cooperative mechanism of catalysis.192a Various strategies have been explored for overcoming the entropic price of bringing two catalyst molecules together in the rate-limiting transition state and thereby enhancing catalytic efficiency in the

3.7. Ring-Opening Reactions

3.7.1. Hydrolytic Ring-Opening of Epoxides. Despite the increased industrial demand for enantiomerically pure compounds, to date only a few catalytic asymmetric processes have found commercial application,186 among them, rare exceptions are catalytic kinetic resolutions.187 Indeed, kinetic resolution as one of the most powerful tools in asymmetric catalysis has found wide applications in both academies and industry, complementing approaches such as asymmetric synthesis and classical resolution.188 Many reactions based on a kinetic resolution strategy have been achieved with high efficiency, among them are ring-opening reactions of racemic epoxides. Indeed, the hydrolytic kinetic resolution seems to be an ideal and simple methodology for the synthesis of enantiopure epoxides and diols.189 The process, developed by Jacobsen in 1997, was used to synthesize terminal epoxides along with the corresponding diols in virtually enantiomerically pure form.190 This methodology employed water as the sole reagent, small amounts of solvent, and often low loadings (0.2− 2 mol %) of recyclable chiral cobalt(III)-based complexes to afford the terminal epoxides and 1,2-diols in high yields and high enantioselectivities. An important number of building blocks for the synthesis of complex natural products and pharmaceuticals have been prepared on the basis of this methodology.191 Indeed, the Jacobsen methodology for enantioselective epoxide ring-opening by water or other nucleophiles is one of the most important recent developments in nonenzymatic catalytic kinetic resolution, especially the hydrolytic ring-opening of epoxides. Therefore, hydrolytic 2809


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obtained in the fluorous biphasic hydrolytic kinetic resolution of terminal epoxides when fluorinated anions were introduced. On the other hand, ring-opening of epoxides can also be performed through dynamic kinetic resolution. In this context, Kunz et al. have developed new composite materials, which ideally combined polymer functionalization with good masstransfer properties of monolithic carriers.216 This unique combination led to versatile materials for organic synthesis, which could be used in a flow-through mode. Based on these monolithic materials with different polymer functionalities, an example of dynamic kinetic resolution depicted in Scheme 68 consisted of a ring-opening of α-bromo epoxide 240 with water catalyzed by complex 241, providing the corresponding chiral bromo alcohol 242 in 76% yield and 91% ee.

hydrolytic kinetic resolution. In this context, catalysts derived from cyclic ligands that contained more than one metal center in close proximity to each other might display enhanced reactivity relative to conventional monomeric salen catalyst systems. Successful approaches identified to date include construction of covalently linked dimers. In each case of dimeric catalyst, enhanced reactivity relative to monomeric catalysts has been demonstrated.203 For example, a recyclable dimeric homochiral salen Co(III) complex 239 (Scheme 67) Scheme 67. Hydrolytic Kinetic Resolution of Epoxides Catalyzed by a Dimeric Salen Co(III) Complex

Scheme 68. Polymer-Supported Dynamic Kinetic Resolution of Epoxides

developed by Kureshy et al.,204 a chiral bimetallic Co(III) salencalix[4]arene hybrid,205 and a chiral macrocyclic dinuclear salen cobalt complex,206 both developed by Kleij et al., as well as various dimeric chiral salen cobalt complexes activated by InCl3, GaCl3, or BF3 and developed by Kim et al., have allowed remarkable enantioselectivities of up to 98% ee for the corresponding diols and >99% ee for the recovered epoxides to be obtained.207 Bimetallic chiral salen cobalt catalysts containing transitionmetal salts have also been demonstrated by Kim et al. to be highly efficient and enantioselective in hydrolytic kinetic resolutions of various epoxides.208 Enantioselectivities of up to 99% ee for the recovered epoxides combined with enantioselectivities of up to >85% ee for the corresponding ring-opened products and very high catalytic activity could be reached (40−50% yields). Another means for fixing or linking two or more Co(salen) units in close proximity to decrease the catalyst requirements by making the reaction of pseudo firstorder with respect to Co(salen) units was the discovery of oligomeric Co(salen) catalyst systems, which exhibited extremely high reactivities and enantioselectivities in the hydrolytic kinetic resolution of a variety of terminal epoxides under neat conditions with exceptionnally low catalyst loadings (0.01 mol %)192h,209 Despite these important advances, the discovery of easily recovered and recycled catalysts was needed. In this context, the immobilization of [(salen)Co(III)] complexes on various supports,210 such as polymers,211 gold colloids,212 mesoporous silica,213 or zeolite,214 was recently reported by several authors and its successful application to the hydrolytic kinetic resolution of epoxides, providing remarkable enantioselectivities of up to >99% ee. In addition, Pozzi et al. have demonstrated that the hydrolytic kinetic resolution of epoxides was feasible under fluorous biphasic conditions.215 It was shown that the nature of the counteranion had a dramatic effect on the catalytic activity of heavily fluorinated chiral salen cobalt(III) complexes. For example, excellent enantioselectivities of up to 99% ee for both the diols and the epoxides were

3.7.2. Ring-Opening of Epoxides by Nucleophiles Other than Water. Epoxides can also be resolved through ring-opening by nucleophiles other than water, such as amines,217 carbamates, imides, phenol derivatives,115,116,192b,218 alcohols,219 azides,192b,220 fluoride,221 carboxylic acids,222 or carbon nucleophiles, 223 allowing the access to various important chiral functionalized compounds.224 Among them, chiral β-amino alcohols are valuable intermediates in the synthesis of a variety of biologically active compounds and play a very significant role in asymmetric catalysis.225 Various efficient methods have been reported for their synthesis; noteworthy among them is the asymmetric ring-opening aminolytic kinetic resolution of racemic terminal epoxides with alkyl/arylamines by using different catalysts.187d,226 In particular, carbamates have given excellent results in the ringopening of epoxides through kinetic resolution. As an example, Bartoli et al. have used a chiral salen cobalt(III) complex 243, previously developed by Jacobsen’s group, to open terminal epoxides with carbamate NH2Boc, furnishing the corresponding Boc-protected 1,2-amino alcohols 244 in good yields and with exceptionally high enantioselectivity of >99% ee, as shown in Scheme 69.227 Selectivity factors were found to be >500 for all examples examined. This protocol was later extended to the enantioselective preparation of 5-substituted oxazolidinones, which have been shown to be valuable structural motifs of medicinally active drugs.228 More recently, Kureshy et al. have reported the use of highly efficient recyclable salen Co(III) complexes in ionic liquids in the cobalt-catalyzed kinetic resolution of aryloxy/terminal epoxides using carbamates as nucleophiles, providing high regio- and enantioselectivities of >99% ee for both the amino alcohols and the recovered epoxides.229 Chiral cobalt catalysts have also been applied to the enantioselective polymerization of monosubstituted epoxides where chiral racemic monomers are 2810


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248 to be achieved in enantioselectivities of up to 96% ee, as shown in Scheme 71. It must be noted that this highly isotactic poly(cyclohexene carbonate) constituted the first semicrystalline CO2-based polycarbonate.

Scheme 69. Kinetic Resolution of Epoxides through RingOpening with Carbamates

Scheme 71. Copolymerization of Cyclohexene Oxide with CO2

kinetically resolved during polymerization. This process provided two desirable products: enantiopure epoxides and stereoregular polyethers. In 2008, Coates and co-workers reported the first highly enantioselective polymerization catalyst for the kinetic resolution of monosubstituted epoxides.230 This chiral bimetallic cobalt(III)catalyst 245 exhibited high levels of activity and enantioselectivity of up to >99% ee for a range of ring-opened isotactic polyethers 246 possessing alkyl, aryl, and ether substituents, as shown in Scheme 70. It must be noted that the process employed bis(triphenylphosphine)iminium (PPN) acetate as a cocatalyst and a remarkable low catalyst loading of only 0.025 mol %.

3.8. Miscellaneous Formations of Acyclic Compounds

The Friedel−Crafts reaction of aromatic compounds with aldehydes or ketones constitutes one of the most fundamental reactions in organic chemistry; however, its enantioselective catalytic version is still an unexplored field. In 2003, a chiral salen cobalt(II) complex was applied by Jurczak et al. to induce the high-pressure Friedel−Crafts reaction of 2-methylfuran 249 with alkyl glyoxylates 250 to afford the corresponding enantioenriched furfuryl alcohols 251.233 As shown in Scheme 72, when catalyst 57 was employed, these alcohols were

Scheme 70. Polymerization of Epoxides

Scheme 72. Friedel−Crafts Reaction of 2-Methylfuran

Like most metal-catalyzed systems for the coupling reaction of carbon dioxide and epoxides, cobalt-based catalysts are able to afford both cyclic carbonates (Scheme 39, section 2.3) and polycarbonate products. The product selectivity of these two processes can be controlled by different parameters, such as temperature, carbon dioxide pressure, nature of cobalt catalyst used, and also by the use (or not) and nature of a nucleophilic cocatalyst and its relative loading.116 The copolymerization of monosubstituted epoxides with CO2 constitutes a powerful method for the synthesis of chiral polycarbonates.231 A highly enantioselective version of this process was recently developed by Lu et al.232 In this study, the authors investigated the enantioselective copolymerization of cyclohexene oxide with CO2 catalyzed by chiral dissymmetrical salen cobalt(III)NO3 complexes, such as catalyst 247 bearing bulky adamantyl and tert-butyl groups on the phenolate ortho positions, in the presence of bis(triphenylphosphine)iminium chloride (PPNCl) as nucleophilic cocatalyst. This methodology allowed the synthesis of the corresponding optically active polycarbonate

achieved in moderate to good yields and enantioselectivities of up to 76% ee. This work represented the first example of enantioselective Friedel−Crafts reaction catalyzed by a chiral salen-type complex. Chiral fluorinated organic compounds are well recognized as important materials in the field of biological and medicinal chemistry. Recently, several groups have shown that chiral catalysts of various metals, such as titanium, ruthenium, palladium, copper, nickel, or magnesium, could induce highly enantioselective α-fluorination of β-ketoesters. Despite these pioneering studies of enantioselective fluorination, the development of a new catalyst system was still required. In this context, Itoh et al. demonstrated the cobalt-catalyzed asymmetric αfluorination of cyclic β-ketoesters 252a−e with N-fluorobenzenesulfonimide (NFSI) in 2010.234 As depicted in Scheme 73, this reaction was promoted by a chiral cobalt complex prepared from Co(acac)2 and (R,R)-Jacobsen’s salen ligand 253, 2811


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Scheme 73. α-Fluorination and α-Chlorination of βKetoesters

comparable results by using the corresponding copper catalyst (91% yield, 81% ee). As in organic chemistry where a carbon attached to four different substituents is called an asymmetric carbon, in inorganic chemistry a metal complex can be asymmetric at the metal center as a function of its topology of coordination. For example, tetrahedral complexes bearing four different monodentate ligands or octahedral complexes with achiral bidentate ligands show this type of chirality, and they are called “chiral-at-metal”.236 So far, the preparation of enantiomerically pure “chiral-at-metal” catalysts is still very challenging, and therefore they have been scarcely applied to enantioselective transformations. In 2001, Soai and co-workers reported a rare example of “chiral-at-metal” catalysis with high enantiomeric excesses. Chiral octahedral cobalt complexes, such as (−)-546K[Co(edta)·2H2O], were able to induce highly enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde 259, affording the corresponding pyrimidyl alkanol 260 in quantitative yield and enantioselectivity of up to 94% ee (Scheme 75).237 Of note, the (+)-546-K[Co(edta)·2H2O] chiral Scheme 75. Addition of Diisopropylzinc to Pyrimidine-5carbaldehyde

providing the corresponding α-fluorinated products 254a−e in good yields and enantioselectivities of up to 90% ee. When employing an acyclic β-ketoester, such as ethyl 2-methyl-3-oxobutanoate 252f, as the substrate, the reaction afforded the corresponding α-fluorinated product 254f in both lower yield (64%) and enantioselectivity (71% ee). As an extension of this methodology, a chiral cyclic α-chlorinated product 255 could be produced from the corresponding cyclic β-ketoester 252g in 62% yield and 88% ee by using CF3SO2Cl as the source of chloride. In 2008, North et al. described the synthesis of novel C1symmetrical salen ligands, which were further investigated as chiral cobalt ligands in several asymmetric reactions under phase-transfer conditions including asymmetric alkylation of alanine derivative 256 with benzylbromide.112,235 As shown in Scheme 74, moderate to good enantioselectivities of up to 80% ee combined with high yields were achieved for the corresponding benzylated product 257 by using cobalt complex 258 as catalyst. In this study, the authors have obtained

complex gave the opposite enantiomer of the pyrimidyl alkanol with 91% ee. One inconvenience of this process was the use of 50 mol % of catalyst loading. This is of note because since these cobalt complexes are practically insoluble in toluene, the reaction occurs likely at the interface between the metal complex and the solvent. Therefore, the cobalt atom is not the true reactive center but is undeniably responsible for the enantioselectivity of the addition. The addition of cyanide to a carbonyl compound to form a cyanohydrin is one of the fundamental carbon−carbon bondforming reactions in organic chemistry.238 Cyanohydrins contain two functional groups: a nitrile and an alcohol (or protected alcohol), which can be readily further manipulated to produce a diverse range of 1,2-difunctional compounds, including many that are often found as components of important pharmaceuticals. Asymmetric cyanohydrin synthesis has come a long way over the last 100 years, with most progress having been made over the past decade by using chiral complexes of various metals. In this context, Belokon and coworkers have reported an original system involving the use of negatively charged complex ions in asymmetric cyanosilylation.239 An advantage of the achiral/chiral weakly coordinated anion ion-pair system is the ability to retain a greater Lewis acidity in the cation, when compared to the traditional metal complexes where the charge on the metal is compensated by the strong first-sphere ligand metal coordination. Thus, chiral metal cations, such as Na+ or K+, which are not usually

Scheme 74. Alkylation of an Alanine Derivative

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omatic, as well as aliphatic aldehydes. The lowest enantioselectivities (86% and 89% ee, respectively) were obtained in the cases of aldehydes bearing a 2-thienyl or a 1-naphthyl group in comparison with other aryl or heteroaryl groups. It is interesting to note that even an aliphatic aldehyde, such as cyclohexanecarbaldehyde, yielded the corresponding alcohol in 84% yield and 97% ee. Asymmetric hydrogenation of alkenes is one of the most prominent and well-established methods for the synthesis of enantiomers and has found numerous applications in the pharmaceutical, agrochemical, and fine-chemical industries. The vast majority of catalysts used are based on precious metals, with ruthenium, rhodium, and iridium being the most common.241 Replacing these expensive and toxic elements with more abundant and environmentally compatible first-row transition metals such as cobalt is attractive and an area gaining renewed attention. In this context, Ohgo et al. have investigated t he asymmetric h ydro genation of alkenes using dimethylglyoximatocobalt(II) complexes in the presence of quinine, which provided low to moderate optical yields (7− 49% ee),242 while enantioselectivities of up to 96% ee were reached by Paltz et al. in the asymmetric hydrogenation of α,βunsaturated esters catalyzed by in situ generated chiral cobalt semicorrin complexes.243 More recently, Chirik et al. employed enantiopure C1-symmetric bis(imino)pyridine cobalt complexes for the asymmetric hydrogenation of geminal-disubstituted olefins.244 Chiral C1-symmetric bis(imino)pyridine cobalt chloride, methyl, hydride, and cyclometalated complexes were investigated as catalysts for the enantioselective hydrogenation of a range of styrenes 265. Among these complexes, C1symmetric bis(imino)pyridine cobalt methylmetalated complex 266 in which one imine is anchored by a large 2,6diisopropylphenyl ring and the other is derived from enantiopure (S)-2-cyclohexyl ethylamine was proved to be the most efficient catalyst, providing the hydrogenated products 267 in low to excellent yields (5% to >98%) combined with moderate to excellent enantioselectivities of up to >98% ee, as shown in Scheme 78. It was noted that higher activity was observed for less hindered substrates, and that introduction of electron-donating and electron-withdrawing groups at the 4position of the styrene had little impact on the activity, but generally increased the enantioselectivity except for fluoro- and trifluoromethyl derivatives. The best results were achieved for phenylated alkenes with enantiomeric excesses of 80−98% ee, with the more sterically crowded olefins olefins producing higher selectivity, albeit with reduced activity. Importantly, the presence of coordinating functionalities on the olefin was not required for high enantiomeric excess. The Nicholas reaction is a versatile transformation involving the reaction of a cobalt carbonyl stabilized propargylic cation with different nucleophiles, such as alcohols, amines, thiols, phosphines, hydrides, as well as carbon nucleophiles in the

considered as likely candidates for Lewis acids in catalytic cycles, can become efficient asymmetric catalysts in combination with chiral weakly coordinated anions. For example, inert complex 261 readily prepared from tryptophan, salicylaldehyde, and K3[Co(CO3)3] was found to be the most efficient of a series of similar complexes studied in the asymmetric cyanosilylation of benzaldehyde with trimethylsilyl cyanide. The authors have studied the effect of additives, such as triphenylphosphine, indole, water, tert-butanol, etc. Therefore, it was demonstrated that in the presence of triphenylphosphine (0.1 mol %), the corresponding O-silylated mandelonitrile 262 was obtained in 85% yield and with an enantioselectivity of up to 77% ee, as shown in Scheme 76. It is interesting to note that, Scheme 76. Cyanosilylation of Benzaldehyde

in the presence of other cations, such as H+, Li+, Na+, Cs+, or NH4+, the reaction resulted in almost racemic product. Unfortunately, aldehydes other than benzaldehydes gave poor or no asymmetric induction. Among various organometallic reagents, organoboron reagents have gained much attention due to the advantages of air and moisture stability, low toxicity, and availability. Complexes of various metals, such as rhodium, palladium, platinum, nickel, copper, and iron, have been successfully used to catalyze the reaction of organoboronic acids to aldehydes. Despite the fact that a wide number of publications dealing with this reaction are available in the literature, only a few reports on asymmetric versions have been developed. As an example, the first enantioselective cobalt-catalyzed addition reaction of various phenylboronic acids 263 to substituted aldehydes was recently described by Cheng et al. to provide the corresponding biologically useful substituted diarylmethanols 264 in high yields and enantioselectivities ranging from 86% to 99% ee.240 As shown in Scheme 77, this process was induced by a cobalt catalyst in situ prepared from CoI2 and (R,R)-2,4-bisdiphenylphosphinopentane ((R,R)-BDPP) in the presence of a base such as K2CO3, and was proved to be compatible with a wide scope of phenylboronic acids and aromatic, heteroarScheme 77. Addition of Phenylboronic Acids to Aldehydes

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purpose was reported by Noyori et al., under catalytic hydride transfer conditions similar to those employed for the asymmetric hydrogenation of ketones.248 Recently, cobaltcatalyzed kinetic resolutions of secondary alcohols with molecular oxygen have been achieved. For example, Yamada et al. have reported good to high enantioselectivities of up to 96% ee for the aerobic kinetic resolution of various secondary benzylic alcohols 273 and 274 by employing chiral ketoiminatocobalt(II) complexes 271 or 272 as catalyst (Scheme 80).249 In this catalytic system, styrene was employed as the oxygen acceptor to be converted into the corresponding ketone. The use of another cobalt complex bearing a Schiff base ligand such as 275 allowed the kinetic resolution of α-hydroxy ketones 276 and α-hydroxy esters 277 to be achieved in high selectivity factors of up to 47 and 31.9, respectively (Scheme 81).250 Chiral cobalt catalysts have also been applied to achieve the resolution of other substrates, such as epoxides (section 3.6), and N-benzyl α-amino acids. In this context, the enantiomer of chiral Jacobsen’s salen cobalt catalyst 123 was employed by Gennari and co-workers to achieve a novel approach to the resolution of racemic N-benzyl α-amino acids in excellent yields and enantioselectivities of up to 99% ee by liquid−liquid extraction.251 As a result of the resolution by extraction, one enantiomer (S) of the N-benzylated α-amino acid predominated in the aqueous phase, while the other enantiomer (R) was driven into the organic phase by complexation to cobalt. The complexed amino acid (R) was then quantitatively released by a reductive (cobalt(III) into cobalt(II)) counter-extraction with aqueous sodium dithionite or L-ascorbic acid in methanol. The reductive cleavage allowed recovering the Co(II) complex in good yield, which could be easily reoxidized to Co(III) with air/AcOH and reused with essentially no loss of reactivity and selectivity. Investigation on the nitrogen substitution indicated that the presence of a single benzyl group on the amino acid nitrogen was important to obtain high enantioselectivity in the extraction process. A range of racemic N-benzyl α-amino acids, such as N-benzyl-threonine, N-benzyl-valine, N-benzyl-leucine, N-benzyl-phenylalanine, and N-benzyl-alanine, could be resolved under these conditions with enantioselectivities of 96%, 94%, 99%, 93%, and 66% ee, respectively. Moreover, the scope of this methodology was extended to the resolution of Nbenzyl β3-amino acids, such as N-benzyl β3-homophenylglycine, N-benzyl β3-homoalanine, and N-benzyl β3-homovaline, which were resolved in 93%, 93%, and 90% ee, respectively. Finally, Tokunaga et al. have shown that chiral salen cobalt complexes, such as the enantiomers of catalysts 57 and 123, could also allow the hydrolytic kinetic resolution of cis-2-tert-butylcyclo-

Scheme 78. Hydrogenation of Geminal-Disubstituted Alkenes

form of enol ethers, electron-rich aromatics, allyl silanes, allyl stannanes, and trialkyl aluminum reagents.245 Complexation of the precursor propargylic alcohols with dicobalt octacarbonyl proceeds smoothly at room temperature, and the complex formed is subsequently treated with a Lewis acid such as BF3· Et2O to generate a cation prior to addition of the nucleophile. Decomplexation is generally effected oxidatively, using cerium ammonium nitrate or iodine. Asymmetric versions of the Nicholas reaction have in general involved the use of chiral nucleophiles or chiral substrates. In 2008, Kann et al. reported the first asymmetric version of this reaction involving the use of racemic propargylic alcohols in conjunction with chiral ligands coordinated to cobalt, such as phosphoramidite ligands.246 As shown in Scheme 79, the treatment of propargylic alcohols 268 with cobalt carbonyl complex and then with 2 equiv of chiral pyrrolidine-substituted phosphoramidite ligand 269, followed by reaction with various nucleophiles in the presence of a Lewis acid such as BF3·OEt2, afforded, after decomposition by treatment with cerium ammonium nitrate, the corresponding Nicholas products 270. Yields and enantioselectivities fluctuated as a function of the alcohols 268 and the nucleophiles. This example illustrates perfectly the difficulty to associate high yields with high chiral induction. Kinetic resolutions based on the oxidation of a chiral secondary alcohol to a prochiral ketone have been of considerable interest because the later can be usually recycled into the racemic starting material by simple hydride reduction.187d,247 The first broadly applicable method for this Scheme 79. Enantioselective Nicholas Reaction

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Scheme 80. Kinetic Resolutions of Secondary Alcohols

Scheme 81. Kinetic Resolutions of α-Functionalized Secondary Alcohols

Notes

hexyl vinyl ether to be achieved with a good selectivity factor of 10.252

The authors declare no competing financial interest.

4. CONCLUSION This Review illustrates how much enantioselective cobalt catalysis has contributed to the development of various types of enantioselective ecological and economical reactions. It updates the major progress in the field of enantioselective transformations promoted by chiral cobalt catalysts, well illustrating the power of these green catalysts of lower costs to provide new reaction pathways, even if this field is still in its infancy. During the last 25 years, a steadily growing number of novel asymmetric cobalt-catalyzed reactions have been developed on the basis of the extraordinary ability of cobalt catalysts to adopt unexpected reaction pathways to new chiral cyclic as well as acyclic products under relatively mild conditions. For example, the first enantioselective cobaltmediated [6+2] cycloadditions, domino reactions, Pauson− Khand reactions, aldol condensations, Michael additions, Nazarov reactions, hydrovinylations of alkenes, and ringopening of epoxides have been described. Moreover, the ever-growing need for environmentally friendly catalytic processes prompted organic chemists to focus on more abundant first-row transition metals such as cobalt to develop new catalytic systems to perform reactions, such as C−C bond formations,253 C−heteroatoms bond formations,254 or C−H functionalizations.255 Therefore, a bright future is undeniable for more sustainable novel and enantioselective cobalt-catalyzed transformations.

Biographies

Hélène Pellissier carried out her Ph.D. under the supervision of Dr. G. Gil in Marseille (France) in 1987. The work was focused on the reactivity of isocyanides. In 1988, she entered the Center National de la Recherche Scientifique as a researcher. After a postdoctoral period in Professor K. P. C. Vollhardt’s group at the University of California, Berkeley, she joined the group of Professor M. Santelli in Marseille in 1992, where she focused on the chemistry of diallylsilane and its

AUTHOR INFORMATION Corresponding Author

application to the development of novel very short total syntheses of

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

steroids starting from 1,3-butadiene and benzocyclobutenes. 2815


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DUPHOS EDTA ee L MAO Mes MTBE Naph NCS NFSI NMI NMO NORPHOS PPN PYBOX QUINOX rt SALEN SIPAD

Hervé Clavier graduated from the Ecole Nationale Supérieure de Chimie de Rennes and received his M.Sc. in organic chemistry from the Université de Rennes where he completed his Ph.D. in 2005 under the supervision of Drs. Jean-Claude Guillemin and Marc Mauduit. He then joined the research group of Prof. Steven Nolan as a postdoctoral fellow and followed him to the ICIQ in Tarragona. In early 2009, he moved to the School of Chemistry at the University of St. Andrews to continue work with Prof. Nolan as a senior researcher, and in October 2009, he was appointed Chargé de Recherche (CNRS) at AixUniversité Université. His scientific interests include catalysis and new synthetic methodologies.

SIQAD TBS TEMPO Tf THF THFA TMS Tol Ts VALNOP

ABBREVIATIONS Acac BDPP BINAP

acetylacetone 2,4-bis-diphenylphosphinopentane 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl BINOL 1,1′-bi-2-naphthol BIPHEP 2,2′-bis(diphenylphosphino)-1,1′-biphenyl Boc tert-butoxycarbonyl BOPA bis(oxazolinylphenyl)amine BTFEP 1,3-bis(2,2,2-trifluoroethoxy)propan2-ol Bz benzoyl CAN ceric ammonium nitrate Tetraphenyl-carbpi (cyclopropaquinolinylideneimino)isoindole CHIRAPHOS 2,3-bis(diphenylphosphine)butane COD cyclooctadiene DBFOX 4,6-dibenzofurandiyl-2,2′-bis-(4-phenyloxazoline) DBU 1,8-diazabicyclo[5.4.0]undec-7-ene DCE dichloroethane de diastereomeric excess DIOP 2,3-O-isopropylidene-2,3-dihydroxy1,4-bis-(diphenylphosphino)butane DIPAMP 1,2-[(2-methoxyphenyl)phenylphosphino]ethane DIPEA diisopropylethylamine DKR dynamic kinetic resolution DMAP 4-(dimethylamino)pyridine DME dimethoxyethane DMSO dimethylsulfoxide DPPA diphenylphosphoryl azide (R,R,S,S)-DUANPHOS (1R,1′R,2S,2′S)-2,2′-di-tert-butyl2,3,2′,3′-tetrahydro-1H,1H′-(1,1′)biisophospindolyl

Xyl-P-Phos

1,2-bis(phospholano)benzene ethylenediamine tetraacetic acid enantiomeric excess ligand methylaluminoxane mesityl (2,4,6-trimethylphenyl) methyl-tert-butylether naphthyl N-chlorosuccinimide N-fluorbenzenesulfonimide N-methylimidazole N-methylmorpholine-N-oxide 2,3-bis(diphenylphosphino)-bicyclo[2.2.1]hept-5-ene bis(triphenylphosphine)iminium pyridine-bisoxazoline 2-(4,5-dihydro-2-oxazolyl)quinoline room temperature N,N′-ethylenebis(salicylideneiminato) 7,7′-bis(2-pyridinecarboxamido)-1,1′spirobiindane 7,7′-bis(2-quinolinecarboxamido)1,1′-spirobiindane tert-butyldimethylsilyl 2,2,6,6-tetramethylpipedinyloxyl trifluoromethanesulfonyl tetrahydrofuran tetrahydrofurfuryl alcohol trimethylsilyl tolyl 4-toluenesulfonyl (tosyl) N-diphenylphosphino-2(dipheny lphosph inoxymethyl)pyrrolidine 2,2′,6,6′-tetramethoxy-4,4′-bis[di(3,5dimethylphenyl)phosphino]-3,3′-bipyridine

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