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New advances in dual stereocontrol for

Cite this: DOI: 10.1039/c3cs60068h asymmetric reactions

Jorge Escorihuela,a M. Isabel Burgueteb and Santiago V. Luis*b

Received 19th February 2013 Achieving dual stereocontrol in asymmetric reactions using a single enantiomer for the building of the DOI: 10.1039/c3cs60068h chiral catalyst or auxiliary is a very important goal in enantioselective synthesis as it eliminates the need for having available the two enantiomers of the auxiliary or catalyst designed. Recent strategies and www.rsc.org/csr advances towards this goal during the last four years will be discussed throughout this review.

Introduction compounds, one enantiomer may exhibit favourable physio- logical activity, while the other may have no activity, inhibit Chirality represents an important phenomenon in chemical favourable physiological activity or, in some cases, exhibit sciences and in particular in the area of asymmetric synthesis, undesirable physiological properties. For example, the enantiomers in which the production and analysis of chiral molecules of chiral drugs such as omeprazole, ibuprofen and DOPA have been the subject of extensive and on-going research.1 A exhibit different pharmacological and pharmacokinetic activities wide variety of compounds exhibiting physiological activity, because their activity is based on the interaction with enzymes such as drugs, pesticides, and flavourings, are chiral. In those and receptors consisting of amino acids and other chiral biomolecules.2 In this regard, consideration of chirality is now an integral part of drug research and development and a Centro de Reconocimiento Molecular y Desarrollo Tecnolo´gico, the associated regulatory processes. Advances in chemical Departamento de Quı´mica, Universitat Polite´cnica de Vale`ncia, Camino de Vera s/n, 46022 Valencia, Spain technologies connected with the synthesis, separation, and

Downloaded by Georgetown University Library on 21/04/2013 21:25:34. b Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H Department of Inorganic and Organic Chemistry, Universitat Jaume I, analysis of pure enantiomers, together with administrative 12070 Castello´n, Spain. E-mail: [email protected] regulatory measures, have resulted in an increase in the number

Jorge Escorihuela was born in M. Isabel Burguete graduated in 1979 in Castello´n, Spain. He Chemistry at the University of graduated in Chemistry in 2003 Zaragoza in Spain and after a at the Jaume I University of stay at the University of Castello´n (Spain) and received a Pittsburgh (USA) under the grant from the Spain Ministry of supervision of Dr J. Rebek, she Education to pursue his PhD completed her PhD at the under the supervision of University of Valencia in 1989 Santiago V. Luis and M. Isabel under the direction of Dr F. Burguete. He received his PhD in Gavin˜a working on regulated Green Chemistry in 2009. His crown ethers and convergent doctoral research was focused on diacids. In 1989 she took an Jorge Escorihuela developing new chiral catalysts M. Isabel Burguete academic appointment at the derived from amino acids and University Jaume I in Castello´n their application in asymmetric reactions. After completing his (Spain) where she is currently Professor of Organic Chemistry. Her PhD, he joined Prof. Angel Maquieira’s group at Universitat main field of research is the development of new tools, in particular Polite´cnica de Vale`ncia as a research scientist in 2010. His homogeneous and supported catalysis approaches, in green and research interest is centred on the design of biosensors in silicon- sustainable chemistry. based materials and DNA microarrays.

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of newly registered chiral drugs containing only one of the attention has been paid to study the induction of both enantio- enantiomers. Therefore, the development of methods that can selectivities using a single chiral source by changing the reaction selectively produce each one of the enantiomers of a compound conditions. This chirality switching can be achieved in different with high enantiomeric excess is very important.3,4 ways, most often using different Lewis acids coordinated to Development of novel chiral ligands for an efficient asym- the chiral ligand, by modification of the solvent system or by metric transformation has become increasingly important in introducing structural features that modify the catalytic mecha- current asymmetric catalysis.5–7 Among the different types of nism or the structure of the catalytic site.12–15 chemical methods used to obtain optically active compounds, In many cases, experimental observations of this dual stereo- asymmetric catalysis is very attractive because it easily produces control are still an unknown phenomenon and the sense of the desired enantiomer from a minimal amount of chiral enantioselectivity cannot be rationalized. On the other hand, the source.8 From a practical point of view, this methodology is design of effective catalytic asymmetric methods to induce a often limited by the need for being able to obtain both switch in the enantioselectivity of a reaction still remains a enantiomeric products in pure form through the corresponding significant challenge.16 catalytic asymmetric transformations. This condition was tradition- ally fulfilled through the preparation of the two enantiomers of Dual stereocontrol using changes in catalyst substituents the chiral catalyst. However, many natural chiral sources are available in only one absolute configuration. The other enantiomer, The development of efficient synthetic chiral catalysts, includ- which is naturally rare, requires resolution or complicated ing chiral metal complex catalysts and chiral organocatalysts, is synthetic procedures. Therefore, it would be desirable to have at the center of research in asymmetric catalysis. In the first in hand available synthetic methodologies to obtain both case, one of the key parameters influencing the efficiency and enantiomers of a product using reagents from a single chiral asymmetric inductions observed is the structure of the chiral source, allowing a dual stereocontrol in the reaction under ligand. The chiral ligand modifies the reactivity and selectivity study through manipulation of the reaction conditions or the of the metal center in such a way that one of two possible non-chiral components of the catalyst.9 enantiomeric products is formed preferentially. A crucial current objective is the design and synthesis of new chiral catalysts, which Dual stereocontrol enable challenging and/or previously unknown asymmetric Since the report by Mosher and co-workers in 1972 on the transformations to occur highly efficiently and, if possible, in asymmetric reduction of unsymmetrical ketones with stoichio- a predictable way. Until recently, strategies for controlling enantio- metric chiral alkoxyaluminium hydrides,10,11 several metal- selectivity in metal-catalysed asymmetric reactions have depended based methodologies for enantiodivergent catalysis, in which largely on the design and application of chiral ligands that would the ligand, the central metal and the reaction conditions can be provide optimum steric or electronic interactions between the 17–22 tuned for this purpose, have been studied. Recently much catalyst and the substrate at the transition state. In 2008, Zhang and co-workers reported a reversal of enantio-

Downloaded by Georgetown University Library on 21/04/2013 21:25:34. selectivity for the palladium-catalysed asymmetric allylic sub- Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H Santiago V. Luis carried out his stitution with metallocene-based planar chiral diphosphine studies in Chemistry at the ligands just by modification of the substituents adjacent to the 23 University of Zaragoza in Spain diphenylphosphine group on the Cp rings. They found that and completed his PhD at the ligands with ester moieties adjacent to the diphenylphosphine University of Valencia in Spain group gave excellent enantioselectivity (99% ee, S-configuration) in 1983 working on mechanistic in this reaction. When a ligand with hydroxyl groups was used, studies with Dr F. Gavin˜a. After a high catalytic activity and moderate enantioselectivity were postdoctoral work at the Univer- obtained. However, in this case, the major isomer was shown sity of Pittsburgh (USA) under the to have the opposite R-configuration (Scheme 1). supervision of Dr J. Rebek, in the A dramatic switch of enantioselectivity was found by Hou, area of supramolecular chemistry, Wu and co-workers in the asymmetric Heck reaction controlled by 24 he obtained an academic appoint- benzylic substituents of P,N-ligands. These authors observed that the corresponding palladium complexes showed high catalytic Santiago V. Luis ment at the University Jaume I in Castello´n (Spain) where he is currently Professor of Organic Chemistry and Head of the research group in supramolecular and sustainable chemistry. His main areas of research involve supramolecular and biomimetic chemistry, in particular involving pseudopeptidic structures, and developing new tools from a sustainable and green chemistry perspective, with a special emphasis on catalysis and flow chemistry. For the last decade he has been coordinating the Spanish MSc and PhD Interuniversity Scheme 1 Reversal of enantioselectivity in the asymmetric allylic amination of Programs in sustainable chemistry. 1,3-diphenyl-2-propenyl acetate with benzylamine.

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Scheme 2 Reversal of enantioselectivity in the asymmetric Heck reaction of dihydrofurane. Scheme 4 Reversal of enantioselectivity in the aza-Henry reactions using sub- stituted nitroalkanes and guanidines as catalysts. activity and enantioselectivity in the asymmetric Heck reaction (Scheme 2), achieving a switch in the topicity of the major isomer through the use of ligands including or not additional catalyst structure revealed important selectivity trends as well substituents at the benzylic position. In particular, ligands with as an intriguing reversal of stereoselectivity for the reaction the benzylic position unsubstituted afforded products with between N-Boc-benzaldimine and nitromethane (Scheme 4). the (R)-configuration while ligands dimethylated at that position The topicity of the major isomer for the b-nitroamine products provided (S)-configuration products. Using density functional could be selectively reversed when a bisguanidine was used theory calculations in combination with an X-ray analysis of the rather than a monoguanidine. When performing the reaction Pd complexes, they provided a rational explanation for the reversal with the monoguanidine ligand, the (R)-product was obtained, of enantioselectivity. They concluded that these two ligands coordi- whereas in the presence of a bisguanidine ligand, the (S)-product nated with the Pd atom in a different conformation, thus, giving was isolated as the major enantiomer. They proposed that the place to two different seven-membered ring transition states. electrophile activation by the guanidinium salt played an In 2009, Maruoka and co-workers observed an enantioselectivity important role in the stereochemistry determining step. When switch in the direct asymmetric aminoxylation catalysed by a monoguanidine catalyst was used, the guanidinium ion binaphthyl-based chiral secondary amine ligands.25 The activated and oriented the imine via hydrogen bonds with the binaphthyl ligand having a 3,4,5-trifluorophenyl substituent imine nitrogen and carbonyl oxygen atoms. The phenyl group and a carboxyl activating group favoured the formation of on the guanidine catalyst backbone then blocked the bottom 2-aminoxy alcohols with the R absolute configuration with face of the imine, forcing the nitronate anion to attack the top moderate to good enantioselectivities (59–88% ee). Surprisingly, face of the imine. The same activation pattern occurs with the when using a binaphthyl-based amino sulfonamide with the same bisguanidine catalysts, but the additional guanidine group configuration, the corresponding S-2-aminoxy alcohol was obtained delivered the nitronate from the bottom face instead, taking in good yield with excellent enantioselectivity after reduction (up to advantage of both the electrophile-activating and nucleophile- 98% ee). They proposed a transition state model justifying the directing modes of catalysis.

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Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H inversion of enantioselectivity. In it, the nitrosobenzene, activated and directed by the carboxyl group, would approach the Re face of indole to N-tosylarylimines in the presence of axially chiral the s-trans-enamine initially formed from the (S)-binaphthyl ligand cyclometalated bidentate N-heterocyclic carbene palladium(II) having the 3,4,5-trifluorophenyl group to give the R-isomer. On the complexes was reported by Liu and Shi by adjustment of the other hand, the nitrosobenzene activated by the distal acidic proton 4-substituent at one of the aryl groups of the NHC–Pd(II) 27 of the triflamide ligand could approach only the Si face of the complexes (Scheme 5). They observed that the axially chiral corresponding s-cis-enamine, giving exclusively the S-isomer for the second catalyst (Scheme 3). An efficient reversal of enantioselectivity using tethered bisguanidine catalysts in the aza-Henry reaction has been reported by Lovick and Michael.26 They found that modifications of the

Scheme 5 Dual stereocontrol in the asymmetric Friedel–Crafts reaction of indole Scheme 3 Dual stereocontrol in the asymmetric aminoxylation catalysed by with a N-sulfonated imine catalysed by axially chiral cyclometalated bidentate binaphthyl-based chiral secondary amine ligands. NHC–palladium(II) complexes.

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Scheme 6 Alternative Re-face and Si-face attacking TSs for the Biginelli reaction between an enol and an imine thiourea using a sterically demanding phosphoric acid. Scheme 7 Reversal of enantioselectivity in the hydroformylation of styrene using Rh–phosphine phosphites.

catalyst (CH3 as the substituent) with an (R)-configuration afforded the desired product in the catalytic asymmetric Friedel–Crafts reaction with an (R)-configuration for the major enantiomer (up

to 54% ee), whilst the axially chiral catalyst bearing CF3 as the substituent with an (R)-configuration produced predominantly the (S)-enantiomer (up to 74% ee) in moderate to good yields. Organocatalytic enantioselective Biginelli and Biginelli- like reactions using chiral phosphoric acids derived from 3,30-disubstituted binaphthols were investigated by Luo, Gong and co-workers.28 They found that the size of 3,30-substituents of the catalyst was able to control the stereochemistry of the Biginelli reaction (Scheme 6). Most significantly, by appropriate tuning of the 3,30-disubstituents of the phosphoric acids, the Scheme 8 Enantioswitching in the catalytic asymmetric hydroboration using stereochemistry of the Biginelli reaction could be efficiently TADDOL-derived phosphites and phosphoramidites. reversed. They investigated the transition states by theoretical calculations to identify the key factors that control the enantio- selectivity. According to their data, the dual stereocontrol that the sense of the stereoinduction was dependent on rather originates from the two possible Si-face and Re-face attacking subtle features of the ligand (Scheme 8). For example, catalysts TSs. In general, for less sterically demanding systems, the Si-face employing a TADDOL phenylphosphite and those using the Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H attacking TS was more favoured, thereby giving the (R)-enantiomer closely related N-methylaniline-derived phosphoramidite of the as the major product. In contrast, the reaction catalysed by the same configuration gave opposite enantiomers of the product. 3,30-di(triphenylsilyl) phosphoric acid occurs through the Re-face The authors concluded that the different stereochemical outcomes attacking TS. In this case, for the Si-face attack, the phosphoric could reflect fundamental differences in catalyst structure, acid forms a weaker hydrogen bond with the enol, thereby causing reactivity, or reaction mechanism. theenoltobelessreactivethanthatintheRe-face attack. Also in 2010, a reversal of enantioselectivity was reported by In 2010, Pizzano and co-workers controlled the enantio- Hua and coworkers by tuning the conformational flexibility selectivity of the final products for the Rh-catalysed hydrogenation of phase-transfer catalysts for the enantioselective conjugate and hydroformylation of olefins by structural modifications of addition of 2-nitropropane to chalcone.31 When binol-derived a family of chiral 3,30-di-tert-butyl-5,50,6,60-tetramethyl-2,20- N-spiro quaternary ammonium salts were used as the phase- biphenol-derived phosphinephosphite ligands (Scheme 7).29 transfer catalysts in the conjugate addition of nitroalkanes In the hydroformylation of styrene, all ligands showed a good to chalcones and their analogues, an intriguing reversal of activity and gave conversions over 95%. The observed switch in enantioselectivity was observed. In this regard, the balance of product configuration was attributed to a different conformation of conformational rigidity and flexibility was proved to be an

the apical PPh2 group in the corresponding Rh(H)(P–OP)(CO)(olefin) indispensable feature of the chiral catalysts. The reversal of species. After performing computational and structural studies, enantioselectivity was found to be directly related to the nature they concluded that, at least for the hydrogenation, the chiral of the linker (Scheme 9). When using catalysts with short induction is predominantly associated to the phosphite fragment aliphatic linkers (n = 0), the reaction gave the (R)-adduct as the of the ligand. major enantiomer, whereas the (S)-product was the predominant An efficient enantioswitching was reported by Smith and enantiomer for catalysts with longer linkers (n =2). Takacs in the Rh-catalysed asymmetric hydroboration of In 2011, Yu and coworkers reported the application of highly b,g-unsaturated amides in the presence of TADDOL-derived active ruthenium(II) complex catalysts bearing an unsymmetrical phosphites and phosphoramidites.30 Interestingly, they found NNN ligand for the asymmetric transfer hydrogenation of ketones.32

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Scheme 9 Reversal of enantioselectivity by tuning the conformational flexibility of phase-transfer catalysts. Scheme 11 Reversal of the topicity for the addition of phenylboronic acid to cyclohexenone.

of arylboronic acids to chalcones.35 In the last case, they found that vinyl substituents of the ligands can afford a complete control of the stereoselectivity of the reaction. Both enantiomers of the products can be readily obtained with excellent ee values by changing the position of the substituents at the olefin fragment of the ligands (Scheme 11). The same could be achieved by changing the stereochemistry of the CQCbondoftheligands. Also in 2011, Xu, Wu and co-workers reported a diastereo- selective control in trapping of carbamate ammonium ylides with imines.36 The highly reactive protic ammonium ylides Scheme 10 Dual enantiocontrol in the asymmetric transfer hydrogenation of were successfully trapped by imine electrophiles in a controlled ketones. manner, and an enantioselective three-component Mannich-type reaction of a diazo compound, a carbamate, and an imine was developed using a dual catalysis strategy (Scheme 12). Interestingly, They found that alcohols were produced as the only products, the diastereoselectivity of the three-component reaction was and the reactions reached up to 100% conversion for the ketone found to be switchable simply by changing the steric nature substrates and final TOFs up to 180 000 h1. Interestingly, a of the 3,30-binol substituents. Both syn- and anti-a-substituted reversal of enantioselectivity was found in the asymmetric transfer a,b-diamino acid derivatives could be obtained in good yields hydrogenation of acetophenone. When the valine derived catalyst with high diastereoselectivity and enantioselectivity. Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H was used, the (S)-alcohol was formed in 96% yield and 56% ee, In 2012, Zhang and co-workers reported a dramatic switch whereas the catalyst derived from phenyl glycine gave a much in the stereoselectivity for the copper catalysed asymmetric better enantioselectivity (79% ee) of the antipode product conjugate addition of triethylaluminium to a,b-unsaturated (Scheme 10). Thus, the steric effect of the substituent on the 37 aromatic enones. They observed that the substituents at the chiral oxazolyl moiety clearly affected the enantioselectivity of 3,30,5,50-positions of the biphenyl backbone of the phosphor- the product. It is important to note, however, that this example amidite ligand showed a significant influence on the catalytic differs from others in this section: the original source of reaction. Ligands lacking substituents at the 3,30,5,50-positions chirality was not strictly maintained, being the ligands derived from different amino acids, although the configuration of the chiral center was kept unchanged. A completely switchable stereoselectivity in rhodium-catalysed asymmetric conjugate additions was reported by Liao and co-workers, in 2011.33 These authors synthesized several sulfoxide– olefin ligands and applied them to the Rh-catalysed conjugate addition of phenylboronic acid to cyclohexenone. The opposite absolute configuration of the product was obtained using the branched olefin ligands (R, up to 94% ee) relative to that from linear olefin ligands (S, up to 96% ee). According to structural information obtained using X-ray crystal-structure analysis, the authors concluded that the olefin geometry can completely reverse the absolute configuration of the product. In a more recent report, they studied the application of these ligands for Scheme 12 Diastereoselectivity switching in the trapping of carbamate ammonium the Hayashi–Miyaura reaction34 and the asymmetric 1,4-addition ylides with imines.

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Scheme 15 Enantioselective control in the Friedel–Crafts alkylation of indoles and pyrrole with b,g-unsaturated a-ketoesters catalysed by heteroarylidene- tethered bis(oxazoline) copper complexes.

whereas the ligand with an isopropyl group on the oxazoline Scheme 13 Reversal of enantioselectivity in the Cu-catalysed asymmetric con- jugate addition of triethylaluminium to a,b-unsaturated aromatic enones. ring produced the (R)-enantiomer in 99% yield with 9% ee. Interestingly, the predominant product for the BOX ligand with phenyl groups on the oxazoline rings was S-configured, the opposite configuration to that obtained using t-Bu-BOX by Jørgensen and co-workers.40 and py-BOX by Desimoni and co-workers.41 They proposed a transition state in which the b,g-unsaturated a-ketoester was coordinated to the copper center in an roughly tetrahedral geometry and the pyrrole attacking the b,g-unsaturated a-ketoester preferably from the Si face, leading to the formation of the predominant S-configured adduct (Scheme 15). Here, again, the configuration at the chiral ligand remains constant but the compound used as the chirality source is different. Liao and co-workers reported a switch in enantioselectivity in the palladium-catalysed asymmetric allylic etherification of racemic 1,3-diphenyl-2-propenyl acetate with aliphatic alcohols 42 Scheme 14 Reversal of enantioselectivity in the Pd-catalysed allylic alkylation of using chiral tert-butanesulfinylphosphine ligands. They controlled indoles using sulphur–MOP ligands. the absolute configuration of the product by the substituents position on the P-aryl group of the ligand. Similar trends were observed for the asymmetric allylic alkylation and amination Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H gave the product with S configuration, whereas those contain- of 1,3-diphenyl-2-propenyl acetate promoted by these chiral ing substituents at these positions (R = Me, Et, Ph) provided the sulfinylphosphine ligands (Scheme 16). product with R configuration (Scheme 13). Also in the same year, an efficient control of the enantio- selectivity in the Pd-catalysed allylic alkylation of indoles using sulphur–MOP ligands with different substituents was demon- strated by Hoshi, Hagiwara and co-workers (Scheme 14).38 They studied the effects of changing the sulphur substituent and among all the substituents studied, the 2-iPrPh induced the highest enantioselectivity, providing the allylation product in 92% ee (R configuration). Surprisingly, with the replacement of the aryl substituent on sulphur by its cyclohexyl alkylic counter- part, the enantioselectivity of the process was dramatically changed to give the opposite absolute configuration of the product (48% ee). In 2012, Fu and co-workers found an interesting reversal of enantioselectivity in the asymmetric Friedel–Crafts reaction of indole and the b,g-unsaturated a-keto butyric acid methyl ester.39 After screening of different ligands, they found that substituents on the bis(oxazoline) ring afforded a good enantioselectivity control. Thienyl methylidene tethered BOX ligands with phenyl or benzyl groups on the oxazoline ring afforded the alkylated product in Scheme 16 Enantioselective control in asymmetric allylic etherification of race- 99% yield with 65% and 7% ee (S enantiomer), respectively, mic 1,3-diphenyl-2-propenyl acetate with aliphatic alcohols with benzyl alcohol.

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The asymmetric addition of organozinc compounds to aldehydes has been extensively studied because it constitutes an efficient and simple reaction for obtaining chiral secondary alcohols, which are important building blocks in the synthesis of many biologically active compounds.43–45 Furthermore, it is also regarded as one of the benchmark reactions for exploring the catalytic potential of new ligands. Therefore, numerous efforts have been reported to develop new effective chiral ligands for asymmetric addition of diethylzinc to benzaldehyde.46–48 From the myriad of ligands Scheme 18 Reversal of enantioselectivity in the catalytic addition of Et2Zn to developed for this reaction, only a few have been reported to aldehydes using chiral 1,3-amino sulfonamide ligands. achieve a dual stereocontrol by tuning the ligand substituents. Alvarez-Ibarra, Collados Luja´n and co-workers in 2010 efficiently controlled the enantioselectivity of the addition product in the enantioselective addition of organozinc compounds to alde- hydes mediated by b-amino alcohols based on L-pipecolinic acid (homoproline) using different non-chiral substituents in the structure.49 They observed that when the reaction

was carried out at room temperature using an Et2Zn/C6H5CHO/ ligand molar ratio of 1.8 : 1 : 0.05, in toluene as solvent, an enantioselectivity of 82% (R) was obtained for the a-non- substituted alcohol. In contrast, an 83% ee for the S-enantiomer was observed for the ligand with a-diphenyl substitution (Scheme 17). The presence of other a-disubstitution patterns Scheme 19 Enantioselective control in the addition of dimethylzinc to alde- (a-dialkyl) did not cause the same effect on the asymmetric hydes catalysed by modular amino acid-L-proline dipeptides. induction. Theoretical studies based on Noyori’s 5/4/4 model and DFT methods allowed the authors to justify the effect of the 54 substituents at the C4 position of the piperidine ring on the acid-L-proline dipeptides. The steric bulk of the N-substituents stereocontrol of the reaction, showing that the trans-transoid- (R, see Scheme 19) played an important role in determining the anti-S transition state is the most favourably preorganised enantioselectivity in such a way that both enantiomers of the ground state. product could be obtained by their appropriate tuning. In 2010 and 2011, Hirose and co-workers prepared several Recently, a dual stereoselection for the same reaction was chiral 1,3-amino sulfonamide ligands derived from ()-cis-2- reported by De la Moya Cerero and co-workers by using a series benzamidocyclohexanecarboxylic acid and applied them as of easily accessible (1S)-ketopinic-acid derived hydroxyl 55 Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H ligands for the catalytic enantioselective addition of Et2Zn to amides. The authors explained the striking observed dual aldehydes.50,51 Through optimization of the structures and stereoselection on the basis of empiric models. The structures reaction conditions, they could define the best ligands being able for the active zinc catalysts and involved transition states were to quantitatively provide both enantiomeric secondary alcohols in supported by additional specific experimental structure–activ- good to excellent enantioselectivities (up to 94% and 98% ee for the ity tests (Scheme 20). They found that the sense of the stereo- (S)- and (R)-enantiomers, respectively) (Scheme 18). The authors selection exerted by these new ligands could be easily switched proposed an anti-6/4/4 TS, on the basis of the mechanistic studies by tuning the coordinative ability of the additional hydroxyl performed by Noyori and Yamakawa for 1,2-amino alcohols to group of the piperazine moiety. explain the observed enantioselectivity.52,53 Kang and Park have reported in 2012 a chirality switching for the enantioselective addition of dimethylzinc to aldehydes catalysed by modular chiral catalysts derived from various amino

Scheme 17 Enantioselective control in the diethylzinc addition to benzaldehyde Scheme 20 Dual stereoselection in the addition of diethylzinc to benzaldehyde mediated by homoprolinol derived b-amino alcohols. with the use of (1S)-ketopinic-acid derived hydroxyl amides.

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Dual stereocontrol using changes in the nature of the metal center in coordination and organometallic catalysts During the last few decades, transition-metal catalysed reactions have played an important role in asymmetric organic synthesis by providing easy, selective and feasible processes to produce a wide variety of organic products.56 Among the different ways of achieving an efficient dual stereocontrol, the introduction of different central metals able to control the reaction enantioselectivity in a given specific way has attracted increasing attention. The use of two different alternative metals in combination with the same enantiomer of a ligand sometimes allows for the selective produc- Scheme 22 Inversion of enantioselectivity by changing the metal in the Henry tion of either product enantiomer in certain reactions. In the last reaction. few years, several metal-based systems illustrating this approach have been reported.57–63 In these systems, the unique character- istics of each metal ion, such as the atomic radius and its electronic active zinc(II) and copper(I) species (Scheme 22). Thus, the use properties, provide a mechanism for altering the coordination of dimethylzinc in the Henry reaction would generate a penta- pattern even with the same chiral ligand. This change would be coordinated methylzinc-derived transition state carrying the associated to the existence of different transition states leading to bidentate bisoxazolidine ligand, the activated nitronate anion, the switching in enantioselectivity. and the aldehyde substrate, to yield the (R)-enantiomer. In the In 2009, a new nucleophile, sulfonylimidate, was introduced case of CuOAc, a proton transfer from nitromethane to acetate, for the Mannich addition to imines by Kobayashi and co-workers, which is subsequently replaced with the substrate, is expected 64 including imines generated in situ. They reported that DBU- to generate a tetracoordinated Cu(I) transition state favouring catalysed reactions of sulfonylimidates with protected imines in the Re-face attack of the nitronate anion on the aldehyde and DMF provided the corresponding adducts in high yields and anti the formation of the (S)-enantiomer. selectivities (Scheme 21). Kinetic studies indicated that the rate A reversal of enantioselectivity in Cu–chiral bipyridine catalysed determining step is not the C–C bond formation but the substrate asymmetric ring-opening reactions of meso-epoxides with indole and deprotonation by DBU. Alkaline earth cations catalysed the aniline derivatives was observed by Kobayashi and co-workers. When reaction, with the choice of metal, in some cases, allowing performing the reaction with the chiral Cu–bipyridine complex in stereoselectivity reversal when combined with the appropriate water, the desired alcohol resulting from epoxide ring-opening was

solvent. Thus, the reactions catalysed by Mg(OtBu)2 in DMF obtained in 80% yield, with 96% ee, and the absolute configuration provided the adducts with high anti selectivity, while those of the product was 1S,2S. On the other hand, when the same chiral

catalysed by [Sr(HMDS)2]2 in THF gave syn selectivity. ligand was used in the presence of Sc(O3SC11H23)3,thesameproduct In the same year, Spangler and Wolf successfully achieved was also obtained in high enantiomeric excess (92% ee), but the Downloaded by Georgetown University Library on 21/04/2013 21:25:34. Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H an efficient switch in the enantioselective outcome of the nitroaldol absolute configuration was reversed (Scheme 23). They found from reaction of various aldehydes by replacement of dimethylzinc with X-ray crystal structural analysis that a square pyramidal structure 65 copper(I) acetate. When employing Me2Zn in the bisoxazolidine- for the Cu(II) complex and a pentagonal bipyramidal structure catalysed reaction, the formation of the corresponding (R)-hydroxy for the Sc(III) complex were formed, which could explain the nitroalkanes was favoured. By contrast, they predominantly reversal of the enantioselectivity.66 obtained the (S)-enantiomer when catalytic amounts of CuOAc An efficient diastereoselectivity switching using catalysts i were used. They assumed that this chiral switch could be derived from a common chiral ligand and either Sc(O Pr)3 or attributed to a different coordination sphere of the catalytically

Scheme 21 Dual stereocontrol in the Mannich addition of sulfonylimidates to Scheme 23 Enantioselectivity inversion in the Cu–bipyridine catalysed ring- imines. opening reactions of meso-epoxides with indole and aniline derivatives.

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Scheme 24 Dual enantioselective control in the catalytic asymmetric Mannich- type reaction of a-cyclopentanone and N-Boc imines.

i Scheme 26 Dual stereocontrol in the Friedel–Crafts alkylation of indoles with Er(O Pr)3 was reported by Nojiri, Kumagai and Shibasaki (Scheme 24). Maintaining the excellent reaction yields, the change b,g-unsaturated a-ketoesters using Ag and Sm complexes. in the metal center allowed obtaining very good anti/syn selectivities and enantioselectivities for the catalytic asymmetric Mannich-type leading to 94 and 85% ee using CuI, whereas the corresponding reaction of a-cyanoketones and N-Boc imines.67 Accordingly, each pyrrolidines with the opposite quaternary center at the 2-position metal afforded essentially a single and different estereoisomer. were obtained in 79 and 80% ee using the AgI derived catalytic These authors performed a comprehensive circular dichroism system. In 2011, Oh’s group also reported a stereodivergent study of the metallic catalysts providing important insights control in the asymmetric conjugate reactions of glycine (ket)- into the different assembly states of each catalytic system. imines with nitroalkenes using the same chiral catalyst derived Additionally, kinetic studies revealed clear ligand accelerations from a multidentate amino alcohol.69 Thus, respective syn and in both Sc(III) and Er(III) cases. In particular, the Er catalyst anti addition products were obtained with high diastereo- and exhibited a higher catalytic activity than the Sc counterpart, which enantioselectivities. was attributed to the different three-dimensional architectures of In 2010, Feng and co-workers reported an example of central the catalysts. metal controlled reversal of enantioselectivity in the asym- An efficient reversal of enantioselectivity by means of copper(I) metric Friedel–Crafts alkylation of indoles with b,g-unsaturated and silver(I) catalysts was developed by Oh and co-workers in 70 a-ketoesters. When 10 mol% of the Ag(I) catalyst was used, the 1,3-dipolar cycloaddition reactions using a brucine-derived (S)-isomer (82% yield and 85% ee) was isolated as the major amino alcohol ligand.68 Under the optimized conditions, in product. Alternatively, in the presence of the Sm(III) catalyst, the the presence of CuI and tertiary amines as bases, the (2R,4R,5S)- (R)-isomer was obtained (96% yield and 98% ee) (Scheme 26). endo product was obtained with a 98% yield and 96% ee. In Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H On the basis of their previous reports on Ag(I) complexes, and contrast, the reaction with AgOAc did not require a base, taking into consideration the low coordination number of Ag(I), thereby implying that acetate possibly plays this role. The they considered that a bidentate coordination of Ag(I) with two corresponding antipode, (2S,4S,5R)-endo product, was then oxygen atoms of the N-oxide is preferred for the Ag(I) complex. obtained with excellent enantioselectivity (Scheme 25). Exten- This favoured the indole attacking the b-Si face of the ester sion of the substrate scope was examined using iminoesters leading to the desired product with S configuration. For the derived from amino esters as well as different dipolarophiles. Sm(III) catalysed Friedel–Crafts reaction, they assumed a hexa- Good to excellent reversal of enantioselectivity was observed coordinated Sm(OTf) -derived transition state. In this the oxygen with the iminoesters derived from alanine and phenylalanine, 3 atoms of the N-oxide and the carbonyl oxygens coordinated to Sm(III) in a tetradentated manner. In this coordination, one N- oxide is located trans to the carbonyl oxygen of the ester, while other oxygens such as the two carbonyl oxygens of amides, another N-oxide, and the one at the R position of the ester were accommodated in the same plane. This arrangement resulted in the attack of the indole to the b-Re face of the Michael acceptor, which provides the access to R-configured indole derivatives. It is important to note that, in this case, best results for topicity reversal were obtained when a change in a non-chiral component of the ligand was simultaneously introduced. The diphenyl methyl amide favoured the formation of the (S) enantiomer while the 2,6-dimethyl phenyl amide favoured the (R) enantiomer. Scheme 25 Reversal of enantioselectivity in the 1,3-dipolar cycloaddition using A very particular reversal of enantioselectivity in the asymmetric a brucine-derived ligand. iron-catalysed hydrosilane reduction of ketones was observed

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Scheme 27 Reversal of enantioselectivity in the asymmetric iron-catalysed hydrosilane reduction of ketones using different metal additives. Scheme 29 Stereodivergent direct catalytic asymmetric Mannich-type reaction of a-methyl-a-isothiocyanato esters to ketimines.

by Nishiyama and co-workers in the presence of optically active (S,S)-bis(oxazolinylphenyl)amine iron catalysts.71 The authors Matsunaga, Shibasaki and co-workers were able to achieve found that when this complex was treated with various metal a dual stereocontrol in the asymmetric Mannich-reaction of additives before the addition of hydrosilane at 65 1C, the nature a-methyl-a-isothiocyanato esters with aryl and heteroaryl methyl of the metallic additive defined the topicity of the final product ketimines.73 These authors reported that the access to both (Scheme 27). Silver inorganic salts did not work efficiently as diastereomers could be obtained by a switch in the metal source. activators, whereas acetate and tert-butoxide salts produced the When they used Sr(II), anti-adducts were obtained in 84–97% ee and desired alcohol with 57% and 55% ee (R absolute configuration) 17 : 83–4 : 96 d.r. (syn/anti), and when the Mg(II) salt was employed, respectively. Cu and Mn powder did not show any catalyst syn adducts in 80–95% ee and 90 : 10–93 : 7 d.r. (syn/anti)were activation. Although Mg efficiently activated the complex to obtained (Scheme 29). By means of circular dichroism, they promote the reduction, giving 92% yield, it gave a low ee value observed differences between the aggregates of the two catalysts of 15% (R). Surprisingly, Zn powder efficiently promoted the which could explain the two stereodivergent pathways. catalysis to give 60% product yield and the product alcohol had an absolute configuration of S (44% ee). Dual stereocontrol using changes in the nature of the metallic Using the same family of chiral ligands, this group developed in precursor for the catalytic system 2011 a highly enantioselective Michael addition of 4-substituted An important feature in catalyst design is the source of the pyrazolones and 4-oxo-4-arylbutenoates. The reaction could produce Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H metal center used for the preparation of the corresponding both enantiomers of 4-substititued-5-pyrazolone derivatives in catalyst. In this section we have gathered a few recent examples in excellent enantioselectivities and diastereoselectivities. The which the dual stereocontrol of a given reaction can be performed respective enantiomers were obtained by changing the metal through changes in the nature of the metallic precursor used for center from Sc(OTf) to Y(OTf) (Scheme 28).72 To gain insight 3 3 the building of the catalytic species, even when using the same into the reaction mechanism, the authors investigated the element as the catalytic center. The nature of the precursor, thus, relationship between the ee value of the ligand and the product. can affect to a large extent the structure of the catalytic species Small nonlinear effects were observed for both catalytic and, accordingly, their stereochemical outcome. systems, thus suggesting that minor oligomeric aggregates of In the last few years, N-heterocyclic carbenes (NHCs) have been ligands and Sc(OTf) or Y(OTf) might exist in the reaction 3 3 recognized as useful ligands for the Cu-catalysed 1,4-addition of system. organometallic reagents to a,b-unsaturated compounds. Since the first publications by Alexakis and Mangeney et al. in 2001,74,75 a variety of NHC ligands have been reported for this catalytic applica- tion, but recently a few reports on the reversal of enantioselectivity for this asymmetric reaction have also been published. In 2009, Sakaguchi and co-workers observed a dramatic reversal of stereoselectivity in the Cu–NHC-catalysed conjugate addition of alkylzincs to cyclic enones by a simple change of the copper catalyst precursor in the presence of the same anionic tethered polydentate chiral NHC ligand.76 The most important result was that, using the same ligand derived from (S)-amino

Scheme 28 Metal-directed switch in the enantioselective Michael addition of alcohols, Cu(OTf)2 preferentially yielded the S enantiomer in 0 pyrazolin-5-ones catalysed by chiral metal–N,N -dioxide complexes. the conjugate addition of Et2Zn to 2-cyclohexen-1-one, while

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Scheme 32 Dual enantioselective control in the NHC–Ir-catalysed reduction of Scheme 30 Cu–NHC-catalysed conjugate addition of alkylzincs to cyclic enones. ketones.

Cu(acac) produced the R enantiomer, thus achieving an effi- 2 using [IrCl(cod)]2 as the metallic precursor, although in this cient chiral switch (Scheme 30). A similar dual enantioselective case, isopropyl alcohol (IPA) was used as the reducing agent control was also observed in the reaction of 4,4-dimethyl-2-cyclo- (Scheme 32). hexen-1-one with Et2Zn. In 2010, the same authors were able to improve those results, Dual stereocontrol achieved by changes in the solvent through optimization of ligands and reaction conditions, in such Traditionally, the role of solvent is to dissolve the substrate and awaythatwithCu(OTf)2 the corresponding (S)-conjugate adducts reactant, but in some cases, a control in the reactivity or selectivity were formed in up to 99.5% ee, whereas Cu(acac)2 produced the of the reaction can be ascribed to the nature of the solvent. In this 77 adducts with opposite configurations with up to 86% ee. regard, Kanai, Koga and Tomioka reported in 1993 a dramatic In a further extension of their work, in 2012, they have reversal of the direction of enantiofacial differentiation for the shown that this reversal of the enantioselectivity could also be reaction of organocuprates with chalcones mediated by a chiral obtained by controlling the structure of the chiral ligand, thus phosphine.82 The reaction provided the S-andR-products in 78 achieving a double stereochemical control mechanism. diethyl ether and tetrahydrofuran (THF) respectively. This is In this regard, they compared the relative abilities between probably the most studied approach and different examples have (CH2)2- and CH2-bridged azolium salts, for the Et2Zn conjugate been reported in the last few decades.83–90 addition in the presence of catalytic amounts of Cu(OTf)2. A simple enantiodivergent catalytic Mannich-type reaction Under those conditions, the CH2-bridged ligand afforded 3-ethyl- employing conformationally flexible organocatalysts was reported cyclohexanone in almost quantitative yield and the (S)-enantiomer more recently, in 2010, by Sohtome, Nagasawa and co-workers,91 was isolated with 81% ee (Scheme 31). The use of the (CH2)2- when the reaction of N-Boc aldimines with malonates was bridged azolium ligand, just differing in a methylene group, carried out in nonpolar solvents, such as dichloromethane, produced the (R)-enantiomer (61% ee). To rationalize these toluene, chlorobenzene, and m-xylene, the (S)-product was 79 Downloaded by Georgetown University Library on 21/04/2013 21:25:34. Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H observations, the authors suggested that the Cu-catalysed-1,4- formed in 71–92% ee. In contrast, when the reaction was addition reaction might proceed through a mechanistic model carried out in acetonitrile, a reversal of the enantioselectivity similar to that proposed by Hoveyda and Jung for the NHC–Cu- was observed (Scheme 33). The reactions in acetonitrile gave 80 catalysed asymmetric allylic substitution reaction. the corresponding R products in 88–99% yield with 80–89% ee. Very recently, they also found that an Ir–NHC complex, Through the use of kinetic experiments the authors obtained the prepared from [IrCpCl2]2 and a similar chiral azolium compound, corresponding differential activation parameters and proposed catalysed the asymmetric hydrosilane reduction of acetophenone that the stereoselectivities of S-selective Mannich-type reactions 81 to form the (S)-alcohol as the major product. Surprisingly, the in nonpolar solvents are governed by the differences in the (R)-1-phenylethanol could also be obtained in 60% ee just by entropies of activation, whereas the stereodiscrimination

Scheme 31 Dual mechanism for the control of the stereoselectivity in the Scheme 33 Dual stereocontrol in organocatalysed Mannich-type reactions Cu-catalysed conjugate addition using azolium ligands. using solvent modification.

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Scheme 34 Dual stereocontrol in amine N-acetylation by changes in the Scheme 36 Dual stereocontrol in the 1,4-conjugated nitro-Mannich addition solvent. reaction achieved by modification of the solvent.

processes of R-selective reactions are governed by differences in More notable was the inverted configuration of the major product the enthalpies of activation. when the hydrogenation was carried out in non-coordinating In the same year, Jabbari performed a theoretical study of a solvents, such as dichloromethane or 1,2-dichloroethane, although stereoselective amine N-acetylation in order to rationalize the the achieved enantioselectivities for the S-configuration were origins of its stereoselectivity.92 When performing the reaction lower (up to 27% ee). Also in 2011, Anderson and co-workers reported an enantio- in nonpolar solvents, such as toluene, CHCl3, dioxane, cyclo- selective conjugated 1,4-addition of dialkylzinc to aromatic hexane, THF or CH2Cl2, the R form of the amide was produced preferentially. However, in polar solvents (MeOH, acetone, nitroalkenes catalysed by copper complexes. The choice of solvent dictated the formation of either the syn,anti or syn,syn CH3CN and DMSO), the amide with the S configuration was 94 the major product. This change in enantioselectivity by polar diastereoisomers. Lewis basic solvents such as THF, DME or solvents was found to originate from reduction of intra- acetone were found to promote the formation of the syn,anti- molecular hydrogen-bonding and a change in the conformation product, whereas less Lewis basic solvents (Et2O, TBME, CH2Cl2, i of the most stable diastereoisomeric intermediate (Scheme 34). Pr2O) promoted the formation of the syn,syn-diastereoisomer Another pronounced solvent effect was reported in 2011 by (Scheme 36). Haddad and co-workers in the catalytic asymmetric hydrogena- An interesting reversal of the induced topicity by changing the tion of heterocyclic ketone-derived hydrazones.93 These authors solvent for direct aldol reactions was reported by Messerer and 95 performed solvent screening in the Rh-catalysed asymmetric Wennemers using peptidic catalysts. They studied the effect of hydrogenation of t-butyl-2-(2H-pyran-3(4H,5H,6H)-ylidene)hydra- the solvent for the aldol reactions between cyclohexanone and zinecarboxylate in the presence of the Mandyphos ligand. They 4-nitrobenzaldehyde (Scheme 37). The aldol product was generally Downloaded by Georgetown University Library on 21/04/2013 21:25:34. Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H observed that alcohols had a largeeffectontheconversionand obtained with diastereoselectivities of approximately 1 : 2 in favour enantioselectivity of the reaction, the bulkier alcohols yielding of the anti-stereoisomer regardless of the solvent. The (R)-alcohol a diminished enantioselectivity and lower conversion (MeOH > was preferentially formed in pure DMSO or MeOH (up to 55% ee), EtOH > n-pentanol > 3-pentanol), due to increased steric constraints whereas the (S) isomer was preferentially formed in mixtures of at the metal center via solvent ligation (Scheme 35). Interestingly, water with DMSO or MeOH (up to 63% ee). They demonstrated water was found to play an important detrimental role in the that this reversal was due to a conformational switch between two enantioselectivity. The addition of 5% water in methanol different turn conformations of the catalyst. reduced the enantioselectivity from 85% ee to 26% ee. Very recently, in 2012, Zhang and co-workers reported a stereoselective control in the copper-catalysed conjugate addition

Scheme 35 Solvent effect for the asymmetric hydrogenation of heterocyclic Scheme 37 Reversal of alcohol configuration by changing the solvent in ketone-derived hydrazones. organocatalysed direct aldol reactions.

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Scheme 39 Reversal of enantioselectivity in the asymmetric alkylation of secondary phosphines.

Scheme 38 Reversal of enantioselectivity in the catalytic addition of Et2Zn to chalcone.

of diethylzinc to acyclic aromatic enones with phosphoramidite 96 ligands bearing a D2-symmetric biphenyl backbone (Scheme 38).

In this reaction, using Cu(OAc)2H2O as the catalyst precursor and

solvents such as toluene, Et2O, methyl t-butyl ether (MTBE) or diisopropyl ether (DIPE), the S-enantiomer was obtained (92% ee, 94% yield). Nevertheless, when THF was employed as the reaction solvent, the other enantiomer was obtained in excellent yield and enantioselectivity (R configuration, 99% ee, 96% yield). The authors attributed this switch in the enantioselectivity to the THF’s strong coordination ability, thus acting as a hemilabile

ligand and therefore modifying the structure of the catalyst. In Scheme 40 Enantioselective control for the desymmetrisation of bridged meso- the transition states involving THF as a solvent, the oxygen tricyclic succinic anhydrides. atom of THF should coordinate to the zinc ion being involved in the catalytic cycle. Additionally, the same authors have also reported an inversion of stereoselectivity through the control of In 2011, Chaubey and Ghosh reported a remarkable tempera-

the orientation of substituents on the D2-symmetric biphenyl ture effect for the desymmetrisation of bridged meso-tricyclic 105 backbone of phosphoramidites.97 succinic anhydrides using chiral oxazolidin-2-ones. They found that when the reaction was performed at 78 1C, no

Downloaded by Georgetown University Library on 21/04/2013 21:25:34. selectivity was observed; but as the temperature was increased to Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H Dual stereocontrol achieved by changes in the temperature of 28 1C the diastereoselectivity was improved (Scheme 40). A the reaction subsequent increase in temperature decreased the diastereo- The enantioselectivity can be improved by the modulation of selectivity until there was nearly no selectivity at around 0 1C. reaction conditions. In this context, reaction temperature, a With a further increase in temperature, a reversal as well as a parameter that can be altered easily, is an ideal variable in sharp improvement in the diastereoselectivity was seen until determining the levels of selectivity in both diastereo- and ca. 35 1C. This dual stereocontrol appeared to be kinetically enantioselective transformations. However, there are only a few driven at low temperature while the reversibility of the reaction reports of dual enantioselectivity being controlled by changing sets in at higher temperatures and thermodynamics governed the temperature.98–103 the reaction outcome. In 2009, Bergman and Toste et al. found a reversal of enantioselectivity in the asymmetric alkylation of chiral racemic Dual stereocontrol through introduction of additives or modificators secondary phosphines catalysed by ruthenium(II) catalysts by controlling this parameter.104 When the alkylation of methyl- The introduction of an additive into a reaction system may have phenylphosphine with benzyl chloride was performed at 45 1C, a dramatic effect on its reactivity or selectivity. In this regard, the corresponding chiral phosphineborane was isolated in 12% ee. additives such as water, acids and bases have been shown to Cooling the mixture to 0 1C led to a decrease in enantioselectivity influence either or both reactivity and enantioselectivity in (8% ee). Further reducing the temperature to 15 1C, however, catalysed transformations. In the previous sections we have led to a reversal of the enantioinduction. Finally, the alkylation considered some examples that could also fit well here. An at 38 1C afforded the product with opposite configuration in example should be the cases in which the solvent is modified by 80% ee (Scheme 39). A kinetic study suggested that the mecha- the addition of small amounts of a second solvent (i.e. water).95 nism of enantioselection changed as a function of temperature, Some of the accompanying examples could have also been giving rise to the observed reversal of induction. examined in other sections, but we have preferred to gather

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them here in order to better highlight the important role that specific additives can have on the control of a given reaction. In general, the main objective of introducing an additive is to modify the catalyst structure or that of the transition state, thereby adjusting reactivity or selectivity. In the last few years, several reports achieving enantioselectivity switching by the introduction of additives have been reported.106–112 In 2008, Dean and Hitchcock observed an efficient stereo- chemical control in the asymmetric addition of diethylzinc to aldehydes catalysed by (R,R)-hydrobenzoin mediated by the Scheme 42 Reversal of enantioselectivity obtained through the addition of an i 113 achiral amino alcohol in the asymmetric addition of diisopropylzinc to aldehydes absence or presence of Ti(O- Pr)4. These authors observed catalysed by chiral amino alcohols. that the diethylzinc addition to aldehydes in the absence of i Ti(O- Pr)4 favoured the formation of the (S)-enantiomer (up to i 85% ee), whereas the use of Ti(O- Pr)4 favoured the formation of the (R)-enantiomer (up to 78% ee). They proposed a tentative mechanism for these enantiodivergent pathways in which the formation of the opposite enantiomers was attributed to the different transition states mediated by either zinc or titanium. A similar inversion was reported by Mao et al. when using sulfamide–amine alcohol ligands for the catalytic asymmetric addition of diethylzinc to aldehydes. Under the reported con- ditions, the (S)-products were isolated in high yields and good enantioselectivities. But when they used the same conditions but in i 114 the presence of Ti(O- Pr)4,the(R)-products were obtained. Both examples could also be described in terms of enantioselectivity Scheme 43 Enantioselective control in the asymmetric alkylation of pyrimidine- switching associated to a change in the nature of the catalytic 5-carbaldehyde with diisopropylzinc using (2S,3S)-butane-2,3-diol as a ligand. metal center, although the situation is more complex, as seen in Scheme 41, also involving a change in the nature of the actual alkylating agent. More recently, the same group has reported how phenol Soai and co-workers found that the addition of achiral amino derivatives can be important stereocontrol additives for the alcohols reversed the enantioselectivity induced by chiral amino same reaction.116 When using (2S,3S)-butane-2,3-diol as alcohols during the autocatalytic addition reaction of diisopropyl- the ligand, (R)-pyrimidyl alkanol was obtained as the major zinc to 2-tert-butylethynylpyrimidine-5-carbaldehyde.115 They enantiomer, whereas mixtures of the chiral diol and phenol Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H performed kinetic studies and ab initio molecular orbital derivatives induced the formation of (S)-pyrimidyl alkanol calculations to gain information about the mechanistic origins (Scheme 43). Among all the diols used in the study, 2-naphthol

of this reversal of enantioselectivity (Scheme 42). The results (pKa = 9.3) was the most effective, followed by p-fluorophenol were initially interpreted as a consequence of interactions (pKa = 9.9) and p-cresol (pKa = 10.2). The overall set of data

between the two catalysts, which form a catalytically active allowed them to consider that the pKa value was not solely aggregate promoting the formation of the opposite enantiomer. responsible for the reversal phenomenon. The kinetic and enantioselectivity studies were consistent with In 2009, Abermil, Masson and Zhu reported that a simple the formation of dimeric catalytic species. achiral protic additive was capable of switching the final enantioselectivity of the b-isocupreidine and b-isocupreidine– amide catalysed enantioselective aza-Morita–Baylis–Hillman reaction between (E)-N-benzylidene-4-methoxybenzene sulfona- mide and methyl vinyl ketone with N-tosylimine.117 Under the original conditions, they obtained the (R)-enriched allylamine (in 85% yield and 96% ee), whereas the (S)-enriched allylamine was formed in 99% yield with 96% ee just by adding b-naphthol to the reaction mixture (Scheme 44). In the proline-catalysed a-amination of aldehydes induced by tertiary amine additives, Blackmond, Armstrong and co-workers observed,118 in 2010, a dramatic reversal of enantioselectivity. They studied the a-amination of propionaldehyde with diethyl azodicarboxylate using proline as a catalyst both in the absence Scheme 41 Proposed mechanisms for the enantiodivergent pathways in the and presence of catalytic amounts of the organic base DBU. The asymmetric addition of diethylzinc to aldehydes catalysed by (R,R)-hydrobenzoin. presence of DBU induced a reversal of product enantioselectivity

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Scheme 46 Diastereodivergent asymmetric sulfa-Michael additions of a- Scheme 44 Dual enantioselective control in the aza-Morita–Baylis–Hillman branched enones using a single chiral organocatalyst. reaction.

Scheme 45 Enantioselective control in the a-amination of propionaldehyde with diethyl azodicarboxylate using L-proline and DBU. Scheme 47 Effect of various acids on direct aldol reactions.

from 85% (R) to 46% (S) (Scheme 45). In a recent report, these Awano, Ohmura and Suginome observed that the stereo- authors performed computational studies and probed that the chemical course of the stereospecific Suzuki–Miyaura coupling nature of the intermediates in the a-amination of aldehydes of enantioenriched a-(acetylamino)benzylboronic esters with catalysed by prolinate salts supports an enamine carboxylate aryl bromides could be switched by the proper choice of acidic intermediate in the stereodetermining step.119 The corres- additives in the presence of a Pd–XPhos catalyst system.122 Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H ponding enamine carboxylate intermediates could be charac- These authors reported that highly enantiospecific, invertive C–C terized spectroscopically by 1H NMR. bond formation took place with the use of phenol as an additive In 2011, Melchiorre and co-workers reported an interesting to provide up to 99% ee of the (S)-product. In contrast, high chiral switching in the asymmetric conjugate addition of alkyl enantiospecificity with retention of the configuration (up to 98% thiols to a,b-disubstituted unsaturated ketones catalysed by ee for the (R)-enantiomer) was attained in the presence of i i means of a chiral quinuclidine derivative having a pendant Zr(O Pr)4 PrOH as an additive. They proposed that the intra- primary amine group.120 These authors observed that the molecular coordination of the amide oxygen atom to the boron choice of a properly designed acidic additive and the reaction center allowed the electrophilic attack of the palladium species on medium critically affected the sense of the diastereoselection, the boron-bound carbon atom only from the opposite side of the affording at will either the syn or the anti-product with high boronatom,leadingtotheformationofanopen-chaintransition enantioselectivity (Scheme 46). stateforthe(S)-product (Scheme 48). In this regard, the role of the i i An acid controlled diastereoselectivity in the asymmetric Zr(O Pr)4 PrOH additive would be the cleavage of this intra- aldol reaction of cycloketones with aldehydes using enamine- molecular coordination by its competitive coordination with the based organocatalysts was reported by Yang and co-workers in amide oxygen atom. The cleavage of the intramolecular coordina- 2011.121 These authors achieved this control with moderate to tion results in the formation of tricoordinated boron species, good enantioselectivity by changing the carboxylic acid used as suitable for the formation of a four-membered-ring transition an additive (Scheme 47). Thus, a chiral amine combined with state, in which the oxygen or halogen atom serves as a bridging TFA efficiently catalysed the asymmetric aldol reaction under group. Transmetalation via this cyclic transition state should solvent-free conditions to give an ee of 97% and syn/anti ratio of proceed with retention of configuration at the boron-bound 11/89, whereas in the presence of dicarboxylic acids, the reaction carbon atom through electrophilic attack of the palladium atom proceeded to give the aldol product in excellent yield (94%) with fromthesamesideastheboronatom,toaffordthe(R)-product. a syn/anti ratio as high as 83/2175 and good enantioselectivity In 2012, Maruoka and co-workers found an achiral-acid-induced (84% ee for the syn-product). switch in the enantioselectivity in the direct aldol reactions of

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Scheme 48 Dual enantiocontrol with the use of acid additives on the Suzuki– Miyaura coupling of a-(acetylamino)benzylboronic esters.

Scheme 50 Reversal of diastereoselectivity in the [4+2] cycloaddition of b- methylcyclohexenone and 3-phenylallylidenemalononitrile.

role in the reversal of enantioselectivities. A similar reversal of the product configuration was observed for the asymmetric, anti- selective Mannich reaction of a cyclic imino ester, when conducting the reactions in DMF in the absence of an acid additive or in the presence of 2,6-dinitrobenzoic acid (Scheme 49). Very recently, Chen and co-workers achieved a high reversal of diastereoselectivity in the amine-catalysed regioselective [4+2] cycloadditions of b-substituted cyclic enones and poly- conjugated malononitriles with the addition of primary-amines to the catalytic system.124 Endo cycloadducts were efficiently produced using a combination of 9-amino-9-deoxyepiquinidine Scheme 49 Enantioselectivity switching for the asymmetric aldol and Mannich and an acid derivative, whereas exo variants were obtained using reactions controlled by an acidic additive. 60-hydroxy-9-amino-9-deoxyepiquinidine (Scheme 50). Moreover, the corresponding products with the opposite configuration

Downloaded by Georgetown University Library on 21/04/2013 21:25:34. could be obtained using catalytic chiral amines derived from

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H various ketones and a-ketoesters catalysed by a chiral cis-diamine based trifluoromethanesulfonyl-amido organocatalyst.123 They natural quinine.

observed that the acid stoichiometry (C6F5CO2H) also has a significant effect on the reaction outcome. When performing Dual stereocontrol based on the counteranion in cationic metal the reaction in the absence of additive, high enantioselectivities catalysts were achieved. However, as the amount of acid added was The identity of the counteranion has been found to play an increased, higher diastereoselectivities were obtained but important role in the reactivity and stereoselectivity of cationic the enantioselectivity values diminished, with the opposite metal catalysts in asymmetric reactions.125–127 Nevertheless, enantiomer being obtained as the major one (Scheme 49). although the use of counteranions to control the reaction enantio- Interestingly, switching the solvent from MeOH to water also selectivity has attracted increased attention over the past years,128 resulted in a reversal of enantioselectivity. The authors the use of achiral counteranions for the reversal of an asymmetric proposed two transition state models (based on DFT calcula- induction has been rather limited.129–132 On the other hand, tions) to account for the observed absolute configuration of enantioselective reactions mediated by a chiral counteranion as both aldol products. In the absence of acid additives, the a-keto the only chirality source in a transition metal catalyst, were esters would be chelated and activated through the interaction reported only sporadically and with low success until 2007, when of the carbonyl group of the ketone with both the acidic an unprecedentedly high enantioselectivity was achieved.133 Tf-amide hydrogen atom and the acidic enamine NH hydrogen In 2012, Fan and co-workers observed a reversal of enantio- atom. The enamine moiety would then attack from the back selectivity induced by an achiral counteranion in the asymmetric side to furnish the aldol syn-product. On the other hand, in the hydrogenation of 2,4-disubstituted 1,5-benzodiazepines catalysed presence of acid additives and water, this interaction is clearly by cationic ruthenium diamine complexes.134 Under the optimized weakened, favouring an alternative transition state leading to conditions, both enantiomers of 2,4-diaryl-2,3,4,5-tetrahydro-1H- the reversal of the enantioselectivity. They concluded that the benzodiazepine derivatives were obtained with good to excellent two point chelation ability of a-keto esters played an important diastereoselectivity and excellent enantioselectivity by using the

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Scheme 53 Dual stereocontrol through ligand : metal stoichiometry in the dialkylzinc addition to benzaldehydes.

Scheme 51 Reversal of the sense of induction of the asymmetric hydrogena- tion reaction of 2,4-diaryl-1,5-benzodiazepines. nature of the substrate can have an important effect on the steric course of a given reaction, and, in the last few years, several reports have been published in which changing the substituents in the substrate allows controlling the selectivity of the reaction product.136–138 Those approaches, however, do not properly fit in this revision and will not be considered in more detail. The actual catalytic species present in a given system can be modified just by controlling the stoichiometry of the different components involved. In some instances, this can be achieved just by selecting the participation of either the thermodynamically or the kinetically favoured catalytically active species.139 The most simple example involves changing the ligand : metal ratio deter- mining the formation of catalytic metal complexes with very different coordination spheres.140–142 Thus, for instance, in the case of the dialkylzinc addition to benzaldehydes catalysed by chiral Ni–amino amide complexes, the use of 2 : 1 (ligand : metal) stoichiometries afforded the corresponding R enantiomers, Scheme 52 (S)-Proline/guanidinium salts cocatalysed synthesis of syn-aldols while the use of 1 : 1 stoichiometries provided the S enantiomers derived from cyclohexanone or cyclopentanone. as the predominant ones (Scheme 53). An interesting example reported by Ordo´n˜ez and co-workers, Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H in 2008, of a reversal of diastereoselectivity in the benzylation of the same enantiomer of the ligand but in the presence of different lithium enolates of chiral phosphonopropanoamides by changing achiral counteranions (Scheme 51). The authors justified this the base equivalents can be considered.143 They proposed that reversal of enantioselectivity based on the two different transition the reversal of diastereoselectivity can be attributed to the states depending on the ability of the counteranion to participate formation of aggregates of the dianion system and the excess in hydrogen bonding. of the lithium bases used (Scheme 54). Recently, Concello´n, del Amo and co-workers have reported An enantiofacial selectivity switch was also observed in the an interesting switch in diastereoselectivity in the proline-catalysed enantioselective amination reaction between N-Boc oxindole and asymmetric direct aldol reaction between a cycloalkanone and azodicarboxylate using bimetallic or monometallic Schiff base aromatic aldehydes through selecting the anion of an achiral catalysts.144 Matsunaga, Shibasaki and co-workers, in 2010, TBD derived guanidinium salt (Scheme 52).135 They observed observed that reactions using 1–2 mol% of bimetallic (R)–Ni(II)- that the tetraphenylborate salt of an achiral TBD-derived complex proceeded at 50 1Ctogive(R)-products in 99–89% yield guanidinium salt, used as cocatalyst with (S)-proline, provided high syn-diastereoselectivities and excellent enantioselectivities for the (R,R) product. When replacing this additive by guanidinium tetrafluoroborate, the preferential formation of the corresponding anti-products with very high enantiomeric excesses was observed.

Dual stereocontrol based on other factors Besides the main reaction parameters described above that can produce a reversal of the topicity of the major enantiomer, a variety of other parameters have been reported to cause this Scheme 54 Reversal of diastereoselectivity in the benzylation of lithium enolates dramatic effect. It is quite obvious that the steric and electronic of chiral phosphonopropanoamides by changing the base equivalents.

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Scheme 55 Enantioselective switch using bimetallic or monometallic Schiff base catalysts in the catalytic asymmetric amination of oxindoles.

Scheme 57 Dual stereocontrol by a ditopic organocatalyst with both catalytic sites located at the two sides of a rotary molecular motor.

both groups are located at different relative positions and provide final products with different stereochemistry. During one full rotary cycle, catalysts are formed that provide either the racemic mixture or preferentially the R or the S enantiomer of the chiral product. This provides the capacity to form either enantiomer of the product upon a proper selection of the Scheme 56 Dual enantiocontrol by adjusting the ligand-to-metal ratio on the position at the cycle.146 copper(I)-catalysed enantioselective alkynylation of a-imino esters. In 2012, a redox-reconfigurable copper complex derived from L-methionine and containing organocatalytic urea groups was synthesized by Canary and co-workers and applied to and 99–87% ee (Scheme 55). Reversal of enantiofacial selectivity the enantioselective addition of diethyl malonate to trans-b- was observed for the monometallic (R)-Ni-Schiff base catalyst, nitrostyrene (Scheme 58).147 This catalyst was able to yield giving the (S)-products in 98–80% ee. both enantiomers of the product by the right selection of the More recently, in 2010, Shao and Chan et al. also reported a oxidation state of the coordinated copper atom. Enantiomeric successful control of the enantioselectivity for the copper(I)- excesses of up to 72% (S) and 70% (R) were obtained in catalysed alkynylation of a-imino esters by adjusting the acetonitrile, for the Cu(II) and Cu(I) complexes, respectively. 145 Downloaded by Georgetown University Library on 21/04/2013 21:25:34. Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H ligand : metal ratio. They observed that a small excess of Facile interconversion between the copper redox states allowed ligand was detrimental to the enantioselectivity. This in con- easy access to both active helical forms of the complex and, trast to the usual observation suggests that an excess of chiral therefore, a precise control in enantioselectivity. The ability ligand is beneficial, most likely suppressing the background of the catalyst to invert the enantiomeric preference was reaction catalysed by an uncomplexed metal (Scheme 56). Thus, considered to depend on the helicity of the catalyst. The the ligand-to-metal ratio might have a decisive influence on the authors studied this system and observed that the CD spectra enantioselectivity of the reaction. Surprisingly, a fine tuning of gave significant mirror image ECCD character for the Cu(II) and the ligand-to-metal ratio was able to dramatically switch the Cu(I) complexes, consistent with the inversion of the helical product chirality. The positive nonlinear effect found agreed orientation of the chromophores. In this regard, this example with the presence and importance of heterochiral bis-(or higher) coordinated complexes in this catalytic asymmetric alkynylation of a-imino esters. The chiral tetrasubstituted helical alkene developed in 2011 by Wang and Feringa shown in Scheme 57 can behave as a light-driven molecular motor. Successive light irradiation and heat pulses drive the internal rotation around the carbon– carbon double bond to create alternative stereochemical arrangements for the catalytic fragments at both sides of this axis in a closed cycle. The compound is a ditopic organocatalyst containing a dimethyl amino pyridine moiety on one side of the double bond and a thiourea group at the other side and acts as an efficient catalyst for the conjugated addition of thiolates to Scheme 58 Dual stereocontrol by a redox-reconfigurable, ambidextrous asym- alkenones. Depending on the position at the heat/light cycle metric catalyst in the asymmetric conjugate addition reaction.

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Scheme 59 Dual stereocontrol based on the use of Brønsted or Lewis acid catalysts.

illustrates well the potential of an allosteric control of the stereochemical configuration of the active site for developing new and more optimized catalytic systems.148 Scheme 60 Cavity-induced enantioselectivity reversal in a chiral metal–organic The intramolecular acid catalysed aldol cyclization of 2,3,7- framework Brønsted acid catalyst. triketoesters formed from z-keto-a-diazo-b-ketoesters provides highly functionalized cyclopentanones with good diastereo- not always, resulting in a potential change in activity, selectivity selectivity in high overall yields via kinetically controlled and and enantioselectivity.157,158 A few cases have been described in stereodivergent catalytic processes. Thus, Lewis acid catalysis which the enantioselectivity of the reaction can be switched just gives high selectivity for the 1,2-anti tetrasubstituted cyclo- by the appropriate selection of the support. One example involves pentanones, whereas Brønsted acid catalysis produces the the use of polymer-supported Ti–TADDOLates as catalysts for the corresponding 1,2-syn diastereomers (Scheme 59).149 Doyle and Diels–Alder reaction. In this case, it was observed that, in co-workers, in 2012, proposed that the stereochemical outcomes general, the grafting process was accompanied by a decrease of the Brønsted and Lewis acid catalysed aldol reactions could be in the enantioselectivity of the reaction. The most remarkable rationalized by analysing the conformational intermediates en observation was, however, that when the chiral fragment was route to the respective products. Thus, activation of the central introduced in the matrix through the polymerization of the carbonyl by a Brønsted acid presumably resulted in the dipole corresponding TADDOL derivative containing polymerizable minimized orientation of the 1,2-diketo unit, which could then vinyl groups, the major enantiomer obtained had the opposite be attacked by the tethered Z-enol providing the 1,2-syn product. configuration to that of the major enantiomer obtained either in Lewis acid activation, however, has the ability to coordinate solution or with the analogous grafted polymer (Scheme 61).159–161 the 1,2-diketo unit locking the carbonyls in a syn-orientation, Strong effects of the substitution pattern of the ligand on the Downloaded by Georgetown University Library on 21/04/2013 21:25:34.

Published on 16 April 2013 http://pubs.rsc.org | doi:10.1039/C3CS60068H yielding the 1,2-anti diastereomer. outcome of the asymmetric Sharpless epoxidation of allylic Very recently, Lin and co-workers synthesized a pair of alcohols were observed by Garcı´a and co-workers (Scheme 62).162 highly porous chiral metal–organic frameworks from [Cu (car- 2 Starting from the same enantiomerically pure tartaric acid, the boxylate) ] secondary building units and chiral 3,30,6,60-or 4 esterification with different alcohols allowed obtaining 4,40,6,60-tetra(benzoate) ligands derived from 1,10-binaphthyl- the products in both enantiomeric forms. Additionally, they 2,20-phosphoric acid (Scheme 60).150 Both were active catalysts for Friedel–Crafts reactions between indole and imines. Inter- estingly, they found that these catalysed asymmetric reactions yielded the major enantiomers with the opposite chirality than those afforded by the corresponding homogeneous catalyst. Structural analyses and QM/MM calculations revealed that the flip of product handedness results from the chiral environment of the whole cavity. Heterogeneous chiral catalysts have attracted considerable attention from a wide range of scientific disciplines because of their fundamental and practical importance.151 Supported catalysts present some distinct advantages over the homoge- neous ones, in particular when their potential practical appli- cations are considered.152–154 When designing a supported system, the role played by the support in the activity, selectivity and enantioselectivity of the reaction must be carefully con- sidered.155,156 Immobilization on a polymeric matrix produces steric and electronic modifications of the catalyst, generally but Scheme 61 Dual stereocontrol using different polymeric matrices.

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diversity of reactions and catalytic systems considered, as well as the variety of parameters modified to achieve the dual stereocontrol. Actually, the exact knowledge of the mechanisms by which a change in a given parameter produces a switch in the order of the relative energies of the diastereomeric transi- tion states is still lacking in most instances. Much work is expected to be carried out, in this regard, in the next few years in order to rationalize some of the observed behaviours. In the meantime, however, the experimental data clearly reveal that the search for parameters which modification could control the topicity of the major enantiomer in a reaction using a single chirality source in the catalyst is a valid and practical alternative Scheme 62 Dual enantiocontrol in the asymmetric epoxidation of (E)-3-phenyl- to the development of two enantiomeric catalysts for this prop-2-en-1-ol using tartrate-derived ligands. purpose. This strategy, accordingly, represents an important methodology within the toolbox of synthetic organic chemists. attached amine groups to support the modified tartrate ligands onto a haloaryl-functionalized silsesquioxane moiety. This final Acknowledgements supported chiral tartrate ligand displayed reverse enantioselectivity in the asymmetric epoxidation of allylic alcohols with regard to the Financial support for our work has been provided by MINECO starting dimethyl tartrate ligand, both molecules having the same (CTQ2011-28903-C02-01) and GV (PROMETEO/2012/020). chiral sign. They interestingly observed that the enantioselectivity reversal observed for the POSS-functionalized material should be Notes and references ascribed to the size of the silsesquioxane fragment more than to the presence of the aromatic ring. 1 Asymmetric Synthesis-The Essentials, ed. M. Christman and Other examples of related heterogeneous and supported catalytic S. 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