1,3,2‐Diazaphospholenes Catalyze the Conjugate Reduction of Substituted Acrylic Acids

The potent nucleophilicity and remarkably low basicity of 1,3,2- diazaphospholenes (DAPs) is exploited in a catalytic, metal-free 1,4-reduction of free ,-unsaturated carboxylic acids. Notably, the reduction occurs without a prior deprotonation of the carboxylic acid moiety and hence does not consume an additional hydride equivalent. This highlights the excellent nucleophilic character and low basicity of DAP-hydrides. Func- tional groups such as Cbz group or alkyl halides which can be problematic with classical transition-metal catalysts are well tolerated in the DAP-catalyzed process. Moreover, the trans- formation is characterized by a low catalyst loading, mild reaction conditions at ambient temperature as well as fast reaction times and high yields. The proof-of-principle for a catalytic enantioselective version is described.