Photoinduced electron transfer retropinacol reaction of 4-(N,N-dimethylamino)phenyl pinacols in chloroform

UV irradiation of 1,2-bis[4-N,N-(dimethylamino)phenyl]ethane-1,2-diol (la) and 2,3-bis[4-(N,N-dimethylamino)phenyl]butane-2,3-diol (1b) in deaerated chloroform leads to central carbon-carbon bond cleavage (retropinacol reaction) forming 4-(N,N-dimethylamino)benzaldehyde (2a) and 4-(N,N-dimethylamino)acetophenone (2b), respectively, in high yields. Chemically induced dynamic nuclear polarization (CIDNP) and fluorescence quenching experimental results reveal that the reaction proceeds via a photoinduced electron transfer mechanism from the excited pinacols with a very fast dechlorination of the chloroform radical anion and fragmentation of the pinacol radical cation as crucial steps.