REARRANGEMENT OF RHENIUM ALLYL VINYL KETONE COMPLEXES

The isomerization of parallel-perpendicular allyl vinyl ketone complex C5H5(CO)Re(eta(2),eta(2)-H2C=CHCH2COCH=CHCH2CMe3) (2) to the diastere omeric perpendicular-parallel complex C5H5(CO)Re(eta(2),eta(2)-H2C=CHC H2COCH=CHCH2CMe3) (6) occurred without formation of an unsaturated int ermediate trappable by either PMe3 or (CO)-C-13. The rearrangement of H5(CO)Re[eta(2),eta(2)-CH2=CHCH(CH3)COCH=CHCH2CM3] (8-exo) occurred wi th retention of stereochemistry at rhenium and with enantioface invers ion of both complexed alkenes. The kinetically formed parallel-perpend icular isomer 8-exo rearranged by sequential enantioface inversion of the vinyl pi-bond to give parallel-parallel intermediate 5(CO)Re[eta(2 ),eta(2)-CH2=CHCH(CH3)COCH=CHCH2CMe3] (12-exo) and then enantioface in version of the allyl pi-bond to form the perpendicular-parallel isomer 5(CO)Re[eta(2),eta(2)-CH2=CHCH(CH3)COCH=CHCH2CMe3] (9-exo). Enolizati on of allyl vinyl ketone complexes was observed, but is not required f or the isomerization. C-H insertion mechanisms involving a net hydroge n migration were excluded by deuterium labeling studies. A mechanism i n which rhenium migrates to the opposite enantiofaces of the alkene li gands via sigma-C-H complexes is proposed.