Enantioselective Photochemical Carbene Insertion into C-C and C-H Bonds of 1,3-Diketones by a Guanidine-Amide Organocatalyst

A visible-light-induced enantioselective free carbene transfer reaction of alpha-diazoesters with 1,3-diketones is established by utilizing a chiral bifunctional guanidine-amide organocatalyst. Selective insertion of free carbene into the C-C or C-H bonds of 1,3-diketone could be achieved with alpha-diazoesters bearing different steric hindrances. The transformations enable the preparation of a series of 1,4-diketones with a quaternary carbon chiral center or chiral 2-substituted-1,3-diketones in good enantioselectivities. On the basis of the control experiments and DFT calculation, a stepwise triplet free carbene coupling pathway is suggested. A possible reaction mechanism is proposed to explain the origin of high enantioselectivities via hydrogen-bonds and electrostatic interactions of the catalyst.