Rh-Catalyzed Enantioselective Hydroalkenylative Cyclization of 1,6-Enynes Constructing All-Carbon Quaternary Stereocenters

We report the enantioselective construction
of all-carbon
quaternary
stereocenters (up to 99% ee) by the cationic Rh(I)/Segphos-catalyzed
hydroalkenylative cyclization of 1,6-enynes with α-substituted
acrylamides. This Rh catalysis proceeds with small excesses of alkenes
at room temperature in most substrates to yield the desired cyclization
products in high yields and enantioselectivity. Mechanistic studies
indicate acrylamide-substituted C–H bond activation processes:
for α-substituted acrylamides, the C–H bond is cleaved
by a cationic Rh(III) complex (rhodacyclopentene); in contrast, for
β-substituted acrylamides, the C–H bond is cleaved by
a cationic Rh(I) complex.