Development of Green Asymmetric Organocatalytic Synthesis

Organocatalysts, which are less toxic than metal catalysis, as well as inexpensive, environmentally benign, and stable against moisture and oxygen compared to metal-based catalysts, have received considerable attention for being efficient and clean catalysts. With respect to green chemistry, the development of organocatalysis is a significant research subject for a sustainable society. This article reviews studies on the development of novel organocatalysts and the reactions achieved from using them. Focusing on the push-pull ethylene moiety, in which two electron-withdrawing groups (EWGs) were introduced, we proposed that the vinylogous amide proton (N-H) will lead to the design of organocatalysts. We have developed the diaminomethylenemalononitrile (DMM) organocatalysts, which are push-pull ethylenes having two cyano groups as EWGs, and proved that they are effective for highly stereoselective hydrophosphonylation with aldehydes. The catalytic ability of the DMM organocatalyst was demonstrated in the development of the first asymmetric hydrophosphonylation of ketones using organocatalysts. The DMM organocatalyst can be applied to the selective 1,4-addition asymmetric hydrophosphonylation of enones. In addition, we designed and synthesized novel organocatalysts bearing squaramide-sulfonamide motif as multiple hydrogen bond donors. Squaramide-sulfonamide organocatalysts efficiently catalyzed the asymmetric direct vinylogous aldol reactions of furan-2(5H)-one with aldehydes. Successively, we achieved the synthesis of gamma,gamma-disubstituted-delta-hydroxy-gamma-butenolide via asymmetric direct vinylogous aldol reaction of furanone derivatives using the squaramide-sulfonamide organocatalysts.