Lewis-Brpnsted acid , asymmetric protonation

In 1997, Yamamoto and co-workers described an elegant enantioselective protonation of the allyl tin derivative (Scheme 31.2). This methodology was based on the Lewis-acid-assisted Brpnsted acid strategy using a stoichiometric amount of Lewis-Brpnsted acid (LBA) B. This asymmetric protonation proceeds through a 8 2 mechanism. Indeed, the protonation of ( )-3-phenyl-2-butenyltrimethyltin la delivers the (5)-3-phenyl-l-butene 2 (Scheme 31.2). During... 

The absolute stereo-preference in the Diels-Alder reaction can be easily understood in terms of the most favorable transition-state assembly 5, in which an attractive donor-acceptor interaction favors coordination of the dienophile at the face of boron which is cis to the 2-hydroxyphenyl substituent. At this time, the conformation of a,y3-enal has a strong s-trans preference. We believe that the coordination of a proton of the 2-hydroxyphenyl group with an oxygen of the adjacent B-O bond in complex 5 plays an important role in asymmetric induction this hydrogen-bonding interaction via a Brpnsted acid would cause the Lewis acidity of boron and the jr-basicity of the phenoxy moiety to increase, and the transition-state assembly 5 would be stabilized. The jr-basic phenoxy moiety and the jr-acidic dienophile could then assume a parallel orientation at the ideal separation (3 A) for donor-acceptor interaction. In this conformation, the hydroxyphenyl group blocks the si face of the dienophile, leaving the re face open to approach by diene. 

For benzoic acid acceleration in Yb(OTf)3-catalyzed allylation of aldehydes in acetonitrile, (a) As-pinall, H.C. Greeves, N. Mclver, E. G. Tetrahedron Lett. 1998, 39, 9283. For acetic acid acceleration in Yb(fod)3-catalyzed ene reaction of aldehydes with alkyl vinyl ethers, ene reaction of aldehydes with alkyl vinyl ethers, (b) Deaton, M. V. Ciufolini, M.A. Tetrahedron Lett. 1993, 34, 2409. Yamamoto et al. reported Brpnsted acid-assisted chiral Lewis acids and Lewis acid-assisted Brpnsted acids which were used for catalytic asymmetric Diels-Alder reactions and protonations and stoichiometric asymmetric aza Diels-Alder reactions, aldol-type reactions of imines, and an aldol reaction, (c) Ishihara, K. Yamamoto, H. J. Am. Chem. Soc. 1994, 116, 1561. (d) Ishihara, K. Kurihara, H. Yamamoto, H. J. Am. Chem. Soc. 1996, 118, 3049. (e) Ishihara, K. Nakamura, S. Kaneeda, M. Yamamoto, H. J. Am. Chem. Soc. 1996, 118, 12854. (f) Ishihara, K. Miyata, M. Hattori, K. Tada, T. Yamamoto, H. J. Am. Chem. Sc c. 1994, 116, 10520. (g) Yamamoto, H. J. Am. Chem. Soc 1994, 116, 10520. (h) ishihara, K. Kurihara, H. Matsumoto, M. Yamamoto Ishihara, K. Kurihara, H. Matsumoto, M. Yamamoto, H. J. Am. Chem. Soc 1998, 120, 6920. 

Protons are the simplest and most easily available Lewis acids available to catalyze organic transformations, and the use of a chiral Brpnsted acid combines the potential of proton catalysis with asymmetric induction achieved through the choice of the proper counteranion. It is without doubt that chiral phosphoric acids have been the most successfully used chiral Brpnsted acids so far [62,64] and a variety of different applications have been reported over the last years. Scheme 6.26 gives a summarizing overview about only a few... 

The design for a direct catalytic asymmetric aldol reaction of aldehydes and unmodified ketones with bifunctional catalysts is shown in Figure 36. A Brpnsted basic functionality (OM) in the heterobimetallic asymmetric catalyst (I) could deprotonate the a-proton of a ketone to generate the metal enolate (II), while at the same time a Lewis acidic functionality (LA) could activate an aldehyde to give (III), which would then react with the metal enolate (in a chelation-controlled fashion) in an asymmetric environment to afford a P-keto metal alkoxide (IV). 

On the basis of ESl-MS observation as well as positive nmilinear effects of this system, we assumed that p-oxo-p-aiyloxy-trimer complex is the most enantiose-lective active species (Fig. 3). Therefore, Sm50(0-/Pr)i3 with a well-ordered structure would have beneficial effects for the formation of desired trimer species. Postulated catalytic cycle of the reaction based on the initial rate kinetic studies and kinetic isotope effect studies is shown in Fig. 4. In this catalyst system, both Cu and Sm are essential. We assume that the cooperative dual activation of nitroalkanes and imines with Cu and Sm is important to realize the syn-selective catalytic asymmetric nitro-Mannich-type reaction. The Sm-aryloxide moiety in the catalyst would act as a Brpnsted base to generate Sm-nitronate. On the other hand, Cu(ll) would act as a Lewis acid to control the position of iV-Boc-imine. Among possible transition states, the sterically less hindered TS-1 would be more favorable. Thus, the stereoselective C-C bond formation via TS-1 followed by protonation with phenolic proton affords syn product and regenerates the catalyst. 

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