Organocatalysts Lewis basic

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

A new, metal-free protocol involving (heteroaryl)oxazoline catalysts for the enantioselective reduction of aromatic ketones (up to 94% ee) and ketimines (up to 87% ee) with trichlorosilane has been developed. The reaction is characterized by an unusual, long-ranging chiral induction.The enantiodifferentiation is presumed to be aided by aromatic interactions between the catalyst and the substrate.360 Asymmetric reduction of A-arylketimines with trichlorosilane is catalysed by A-methyl-L-amino acid-derived Lewis-basic organocatalysts with high enantioselectivity (up to 92% ee) 61... 

Another class of bifunctional organocatalysts for the enantioselective aza-Morita-Baylis-Hillman reaction of imines (112) with enones (113) (Scheme 6) is based on BINOL (115). The efficiency of the catalysts proved to be mainly influenced by the position of the Lewis basic moiety attached to the BINOL scaffold. The activation of the substrate by acid-base functionalities and the fixing of conformation of the catalyst (115) are apparently harmonized to maximize the enantiocontrol (<95% ee) 52... 

Formamides as Lewis-Basic Organocatalysts in Hydrosilylation of Imines... 

Chiral N heterocyclic carbenes (NHCs), as Lewis basic organocatalysts, have been synthesized and applied to enantioselective organocatalytic reactions in recent years. Encouraged by Sheehan and Hunneman s first report of chiral thiazolium salts as NHC precursors for organocatalytic reactions [37], Leeper, Enders, Rovis, Glorius, Herrmann, and others have synthesized series of novel chiral NHCs with mono cyclic, bicyclic, or tricyclic backbones [38]. Recently, a series of bifunctional NHCs were synthesized and applied to aza BMH reaction of cyclopent 2 enone with... 

Some recent work has been directed towards the use of organocatalysts, in particular Lewis basic pipecolinic formamides, in the asymmetric hydrosilylation of N-arylimines. These catalysts function by activating the silane and exhibit broad substrate scope. For example formamide (3.186) effects enantioselective hydrosilylation of aryl-derived ketimines along with isopropyl-substituted imine (3.187) and a,P-unsaturated imine (3.188). 

The allylation of aldehydes provides a useful route to homoallylic alcohols. The most popular methods for achieving this in an asymmetric fashion are by the use of metal-based Lewis acid catalysts or using Lewis basic organocatalysts. ... 

A number of BINOL-based bifunctional organocatalysts, for example (7.171-7.173), containing both Bronsted acidic and Lewis basic sites have been used to good effect in the asymmetric MBH reaction. The amine-thiourea (7.171) promotes the MBH reaction of aliphatic aldehydes with 2-cyclohexenone with ees ranging from 80 to 94% while both the (pyridinylaminomethyl)BINOL (7.172) and phosphine (7.173) catalyse the aza-Bayhs-Hilhnan reaction of simple a,p-carbonyls such as MVK and phenyl acrylate with N-tosyl arylaldmines with similar levels of enantioselectivity. 

C. Laurence and J.-F. Gal, Lewis Basicity and Affinity Scales Data and Measurement, John WUey Sons, New York, 2010, is an outstanding reference with respect to both tabulated data and discussion of methodology. G. GUh and P. GUh, The Nature of the Hydrogen Bond, Oxford University Press, New York, 2009, provides an in-depth discussion of this topic. For an excellent review of superbases Superbases for Organic Synthesis Guanidines, Amidines, and Phosphazenes and Related Organocatalysts, Ishikawa, T., eds., Wdey, New York, 2009. 

B. Maji, M. Bieugst, H. Mayr, Angew. Chem. Int. Ed. 2011, 50, 6915-6919. N-Heterocychc carbenes organocatalysts with moderate nucleophUicity but extraordinarily high Lewis basicity. 

Sun and co-workers developed a novel Lewis basic organocatalyst 86 (Scheme 31), easily synthesized from commercially available L-pipecolinic acid. The catalyst 86 promoted the reduction of IV-aryl ketimines 85 with HSiCl3 2 in high yield and... 

With the increasing interest in developing catalysts able to be easily separated from the final product, many efforts have been devoted to the preparation of immobilized structures (for reviews on polymer-supported organocatalysts, see [201—204], for a more recent example, see also [205]). In this field, Kocovsky s group has also reported interesting Lewis base supported catalysts for the efficient asymmetric hydrosilylation of ketimines with silane 2. The first reported example was an N-methylvaline-derived Lewis basic formamide anchored to a polymeric support with... 

Sasai et al developed a bifunctional BINOL-derived organocatalyst (37) and reported an aza Morita-Baylis-Hillman reaction (Scheme 2.80) [148]. While the 3-pyridyl moiety functioned as a Lewis basic site, the diol moiety worked as a Bronsted acidic site. It is noted that introduction of N-isopropyl-N-3-pyridinylaminomethyl moiety at the 3-position is essential for attaining excellent enantioselectivity. They subsequently introduced 2-diphenylphosphinophenyl group onto the third position and successfully utilized it in the aza Morita-Baylis-Hillman reaction [149]. 

Wang Z, Ye X, Wei S, Wu P, Zhang A, Sun J. A highly enantioselective Lewis basic organocatalyst for reduction of iV-aryl imines with unprecedented substrate spectrum. Org. Lett. 2006 8 999-1001. 

S.C. PanandB. List s paper spans the whole field of current organocat-alysts discussing Lewis and Brpnsted basic and acidic catalysts. Starting from the development of proline-mediated enamine catalysis— the Hajos-Parrish-Eder-Sauer-Wiechert reaction is an intramolecular transformation involving enamine catalysis—into an intermolecular process with various electrophilic reaction partners as a means to access cY-functionalized aldehydes, they discuss a straightforward classification of organocatalysts and expands on Brpnsted acid-mediated transformations, and describe the development of asymmetric counteranion-directed catalysis (ACDC). 

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