Symmetrical guanidines

Imidocarbonyl dichlorides that are functionalized with an electron-withdrawing group (e.g., 1) react with amines at room temperature or below, affording symmetrical guanidines.4 It was originally suggested that guanidines with less symmetrical substitution patterns could not be formed... 

Corey and Grogan recently developed a novel catalytic enantioselective Strecker reaction which utilized the readily available chiral C2-symmetric guanidine 19 as a bifunc-tional catalyst [12], The addition of hydrogen cyanide to achiral aromatic and aliphatic N-benzhydrylimines 18 gave N-benzhydryl-a-aminonitriles 20 (Scheme 7), which were readily converted to the corresponding amino acids with 6 N HCI. The use of N-benzyl- or N-fluorenylimines afforded products of poor enantiomeric purity. 

Corey and co-workers reported the use of a C2-symmetric guanidine catalyst (53) for the Strecker reactions of N-benzhydryl aldimines [65]. The catalyst is be-... 

Corey and Grogan reported an enantioselective Strecker reaction via the C2-symmetric guanidine 14 (Equation 10.28) [56]. They proposed the transition-state structure shown in Figure 10.14, where the imine is activated by hydrogen bonding. 

Corey et al. reported enantioselective Strecker reaction catalyzed by C2-symmetric guanidine (21) (Scheme 2.62) [122]. A guanidium salt, generated from (21) and HCN, served as hydrogen bond donor to the aldimine, generating hydrogen-bond activated aldimine (Figure 2.23). 

The first highly enantioselective reaction catalyzed by a chiral guanidine was reported by Corey and coworkers in 1999 [128], C2-symmetric guanidine 46 was shown to be a highly effective catalyst in the Strecker reaction of aromatic aldi-mines, affording the Strecker products in high yields and enantioselectivities (Scheme 10.43). 

The beneficial effect of the presence of a guanidine moiety in the catalyst structure was showcased by Corey, who developed the C2-symmetric guanidine catalyst 157 (Equation 23) [113], This chiral guanidine furnishes enan-tioenriched a-aminonitriles (50-88% ees) from a variety of aromatic imines. The enantioselection was believed to arise from the bifunctional action of the guanidine catalyst as an activator of both the imine and the cyanide, as depicted in the proposed transition state structure 158. 

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