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 α-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 α-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.