- Twofold C?H Activation-Based Enantio- and Diastereoselective C?H Arylation Using Diarylacetylenes as Rare Arylating Reagents
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C?H bond activation has been established as an attractive strategy to access axially chiral biaryls, and the most straightforward method is direct C?H arylation of arenes. However, the arylating source has been limited to several classes of reactive and bulky reagents. Reported herein is rhodium-catalyzed 1:2 coupling of diarylphosphinic amides and diarylacetylenes for enantio- and diastereoselective construction of biaryls with both central and axial chirality. This twofold C?H activation reaction stays contrast to the previously explored Miura–Satoh type 1:2 coupling of arenes and alkynes in terms of chemoselectivity and proceeded under mild conditions with the alkyne acting as a rare arylating reagent. Both C?H activation events are stereo-determining and are under catalyst control, with the 2nd C?H activation being diastereo-determining in a remote fashion. Analysis of the stereochemistry of the major and side products suggests moderate enantioselectivity of the initial C?H activation–desymmetrization process. However, the minor (R) rhodium vinyl intermediate is consumed more readily in undesired protonolysis, eventually resulting in high enantio- and diastereoselectivity of the major product.
- Hu, Panjie,Kong, Lingheng,Li, Xingwei,Wang, Fen,Zhu, Xiaolin
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p. 20424 - 20429
(2021/08/09)
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- Asymmetric Hydrogenation of Cationic Intermediates for the Synthesis of Chiral N,O-Acetals
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For over half a century, transition-metal-catalyzed homogeneous hydrogenation has been mainly focused on neutral and readily prepared unsaturated substrates. Although the addition of molecular hydrogen to C=C, C=N, and C=O bonds represents a well-studied paradigm, the asymmetric hydrogenation of cationic species remains an underdeveloped area. In this study, we were seeking a breakthrough in asymmetric hydrogenation, with cationic intermediates as targets, and thereby anticipating applying this powerful tool to the construction of challenging chiral molecules. Under acidic conditions, both N- or O-acetylsalicylamides underwent cyclization to generate cationic intermediates, which were subsequently reduced by an iridium or rhodium hydride complex. The resulting N,O-acetals were synthesized with remarkably high enantioselectivity. This catalytic strategy exhibited high efficiency (turnover number of up to 4400) and high chemoselectivity. Mechanistic studies supported the hypothesis that a cationic intermediate was formed in situ and hydrogenated afterwards. A catalytic cycle has been proposed with hydride transfer from the iridium complex to the cationic sp2 carbon atom being the rate-determining step. A steric map of the catalyst has been created to illustrate the chiral environment, and a quantitative structure–selectivity relationship analysis showed how enantiomeric induction was achieved in this chemical transformation.
- Sun, Yongjie,Zhao, Qingyang,Wang, Heng,Yang, Tilong,Wen, Jialin,Zhang, Xumu
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p. 11470 - 11477
(2020/08/10)
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- Convenient methods for the synthesis of a library of hemilabile phosphines
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A series of novel functionalized phosphines of hemilabile character, R 2P(CH2)nZ, have been prepared from diarylphosphines using several synthetic methodologies. The synthetic methods include the alkylation of lithium diar
- Jimenez, M. Victoria,Perez-Torrente, Jesus J.,Bartolome, M. Isabel,Oro, Luis A.
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experimental part
p. 1916 - 1922
(2009/12/28)
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- Asymmetric hydrogenation reactions mediated by a new class of bicyclic bisphosphinites
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The bicyclic alcohol (-)-4 was prepared from (-)-bicyclo[3.2.0]hept-2- en-6-one (-)-1 in 50% yield. The diol (-)-4 was coupled to selected chlorophosphines 6-12 to produce a series of bisphosphinites 13-19 in 89-95% yield. From these bisphosphinites were prepared the rhodium complexes 20-26 which were characterised by 31P NMR and used in situ for the asymmetric hydrogenation of α-enamides 27-29. Complexes 21, 23-25 proved to be the superior catalysts for the production of (R)-N-acetylphenylalanine (91, 84, 90 and 87.5% ee) from 27 and (S)-N-acetylalanine methyl ester (70, 72, 68 and 71% ee) from 28.
- Derrien, Nadine,Dousson, Cyril B.,Roberts, Stanley M.,Berens, Ulrich,Burk, Mark J.,Ohff, Manuela
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p. 3341 - 3352
(2007/10/03)
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- Selective asymmetric hydrogenation of dehydroamino acid derivatives using rhodium and iridium diphosphinite carbohydrate catalyst compositions
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A process and catalyst composition are provided for the highly efficient enantioselective hydrogenation of dehydroamino acid derivatives. The catalyst composition comprises rhodium or iridium and a diphosphinite carbohydrate ligand, wherein the phosphorous atoms are attached to aromatic groups substituted with electron-donating substituents. Also provided is a means to selectively produce α amino acids in either the L or the D form, based upon use of a sugar in the ligand with phosphinites attached in an absolute Right-Left or Left-Right configuration, respectively.
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- Ligand Electronic Effects in Asymmetric Catalysis: Enhanced Enantioselectivity in the Asymmetric Hydrocyanation of Vinylarenes
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The enantioselectivity of the nickel-catalyzed, asymmetric hydrocyanation of vinylarenes using glucosederived, chiral phosphinite ligands, L, increases dramatically when the ligands contain electron-withdrawing P-aryl substituents.The substrate and solvent also strongly influence the enantioselectivity, with the highest ee's (85-91percent for 6-methoxy-2-vinylnaphthalene (MVN)) obtained for the hydrocyanation of electron-rich vinylarenes in a nonpolar solvent such as hexane.Mechanistic studies suggest the catalytic cycle consists of an initial HCN oxidative addition or vinylarene coordination to "NiL", followed by insertion to form an (η3-benzyl)nickel cyanide complex, and irreversible reductive elimination of the nitrile.A kinetic analysis of the NiLa(COD) (La, P-aryl=3,5-(CF3)2C6H3) catalyzed hydrocyanation of MVN indicates that as the HCN concentration is increased the catalyst resting state shifts from NiLa(COD) to a complex containing both MVN and HCN, presumably the (η3-benzyl)nickel cyanide intermediate NiLa(η3-CH3CHC10H6OCH3)CN.A 31P NMR analysis of the intermediate NiLa(MVN) shows little ground state differentiation of the MVN enantiofaces and suggests that the enantioselectivity is determined later in the mechanism.Deuterium labeling studies suggest that electron-withdrawing P-aryl substituents increase the rate of reductive elimination of the product nitrile from the (η3-benzyl)nickel cyanide intermediate and, on this basis, a rationale for the ligand electronic effect is proposed.
- Casalnuovo, Albert L.,RajanBabu, T. V.,Ayers, Timothy A.,Warren, Timothy H.
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p. 9869 - 9882
(2007/10/02)
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