BINAP silver salt complexes

The remarkable affinity of the silver ion for hahdes can be conveniently applied to accelerate the chiral palladium-catalyzed Heck reaction and other reactions. Enantioselectivity of these reactions is generally increased by addition of silver salts, and hence silver(I) compounds in combination with chiral ligands hold much promise as chiral Lewis acid catalysts for asymmetric synthesis. Employing the BINAP-silver(I) complex (8) as a chiral catalyst, the enantioselective aldol addition of tributyltin enolates (9) to aldehydes (10) has been developed." This catalyst is also effective in the promotion of enantioselective allylation, Mannich, ene, and hetero Diels-Alder reactions. 

Silver salts or reagents have received much attention in preparative organic chemistry because they are useful catalysts for various transformations involving C-G and C-heteroatom bond formation.309 Especially, the silver(i)/ BINAP (2,2 -bis(diphenylphosphino)-l,T-binaphthalene) system is a very effective catalyst for a variety of enantio-selective reactions, including aldol, nitroso aldol, allylation, Mannich, and ene reactions. Moreover, silver salts are known to efficiently catalyze cycloisomerization and cycloaddition reactions of various unsaturated substrates. Recently, new directions in silver catalysis were opened by the development of unique silver complexes that catalyze aza-Diels-Alder reactions, as well as carbene insertions into C-H bonds. 

The asymmetric allylation of achiral aldehydes with a novel silver complex has recently been reported (Scheme 10-51) [90]. Initially, it was shown that the silver-promoted reaction of allyltributylstannane with benzaldehyde could be accelerated by triphenylphosphine. A survey of various chiral phosphine reagents and silver salts identified the combination of binap and AgOTf as optimal. The reaction of benzaldehyde and allyltributylstannane promoted by 5 mol% of the binap-AgOTf... 

The first example of the intermolecular AHR was reported by Hayashi et al. and involved the asymmetric arylation of 2,3-dihydrofurans using aryl triflates [18]. Although little or no enantiomeric excess was obtained when aryl iodide/silver salt combinations were used, the use of triflates along with the familiar Pd(OAc)2/ BINAP catalyst system resulted in the formation of the 2-aryl-2,3-dihydrofuran product 54, together with minor amounts of the 2, 5-dihydrofuran isomer 55. The rationale proposed by the authors for this outcome is shown in Scheme 12 it is hypothesized that addition of the catalytic complex to either face of the sub-... 

The normal neutral pathway (22 24 25 27) was ruled out by conducting the reaction with monodentate phosphine BINAP ligand mimics (Scheme 12.5). The products obtained were of low enantiomeric excess relative to reactions employing BINAP. The direct cationic pathway (24-> 26) was also eliminated due to the fact that the opposite stereochemistry was obtained under cationic conditions with the addition of silver salts. The switch in stereoselectivity in the presence of silver salts, moreover, indicates that oxidative insertion is not the enantioselective step. j8-Hydride elimination was also discounted as the enantioselective step due to the influence of the double-bond geometry of the starting material on the enantioselectivity of the cyclization. The proposed enantioselective step is the formation of the cationic intermediate 26 by an associative displacement (24-> 28-> 26). In the case of square planar pafladium(n) complexes, substitution chemistry can occur through associative processes. Axial coordination of the alkene would form the pentacoordinate pafladium(II) complex 28. Reports of isolated and characterized pentacoordinate palladium(II) species provide support for this proposed intermediate. 

Attempts to develop enantioselective protocols for the aza-Diels-Alder reaction were reported simultaneously with those described above. A first contribution in this area was the report by the. Mrgensen group,85 who studied the influence of salts of copper, silver, palladium, and zinc. Copper(I) perchlorate provides optimal yields and enantioselectivity, but complexes of BINAP (87) and Tol-BINAP (203) with AgSbF6, AgOTf, and AgC104 were able to catalyze the reaction, albeit with low enantioselectivity (Scheme 2.52). 

---