The Applications Of Chiral Polymeric Salen Ligands In Asymmetric Cyanation

An array of linear and crosslinked polymeric Salen ligands were synthesized and combined with V(V) and Ti(IV) to catalyze asymmetric O-acetyl cyanation and trimethylsilylcyanation of aldehydes. The relationship between the catalytic efficiency and the structure of polymer was also investigated in details.In the catalytic asymmetric O-acetyl cyanation of aldehydes, the linear polymeric Salen-V(V) complex (SV-1) showed the comparable catalytic activity and enantio-selectivity to the monomeric Salen-V(V) complex, and up to 94 % ee with full conversion was obtained at -42℃ with 5 mol % of catalyst. However, the corresponding linear polymeric Salen-Ti(IV) exhibited somewhat low catalytic activity. This problem successfully addressed by using a crosslinked polymeric Salen-Ti(IV) instead of its linear analogues. It was found that the catalytic activity of the crosslinked catalyst closely related with the degree of crosslinking, and the catalyst with the low degree of crosslinking tended to give higher catalytic activity and enantioselectivity. When the degree of crosslinking was 0.5 / 100, the catalyst exhibited the highest enantioselectivity. Thus, in the asymmetric O-acetyl cyanation of 4-fluoro- benzaldehyde, up to 91 % ee with full conversion was obtained at -20℃ with 2 mol % of catalyst. The crosslinked polymeric catalyst showed excellent recyclablity, for example, the catalyst with the degree of crosslinking in 25/100 did not showed any decrease in both activity and enantioselectivity even after six consecutive runnings.In the catalytic asymmetric trimethylsilylcyanation of aldehydes, the linear polymeric Salen-V(V) catalyst (SV-1) also showed good catalytic efficiency, and up to 91 % ee with 98 % conversion was obtained at -20℃ with 1.2 mol % of catalyst.Another salient feature of this polymeric catalyst was its easy recyclablity, even after 9 runs, the recovered catalyst still maintained good activity and enantioselectivity. While the linear polymeric Salen-Ti(IV) exhibited much lower catalytic activity than the corresponding monomelic catalyst. In comparison, the crosslinked polymeric Salen-Ti(IV) exhibited good catalytic activity in this reaction. Also, the catalyst with the 0.5 /100 degree of crosslinking showed the best catalytic activity, and up to 88 % ee with 95 % conversion was obtained in the asymmetric trimethylsilylcyanation of 4-fluorobenzaldehyde at 1 mol % of catalyst loadings.