Hydrogen atoms as convenient synthetic reagents: mercury-photosensitized dimerization of functionalized organic compounds in the presence of molecular hydrogen

Abstract

Hydrogen atoms are generated by mercury photosensitization in an unexceptional apparatus that makes them readily available for organic chemistry on a preparatively useful scale at 1 atm of pressure and temperatures from 0-150°C. The H atoms add to CH 2 =CH-CH 2 X to give the intermediate radical CH 3 -(•CH)-CH 2 X, which dimerizes to give CH 3 CH(CH 2 X)-CH(CH 2 X)CH 3 . The saturated substrates CH 3 CH 2 CH 2 X undergo H abstraction to give CH 3 CH 2 (•CH)X as intermediates and CH 3 CH 2 CH(X)-CH(X)CH 2 CH 3 as final products. The reaction shows a tolerance for different functional groups, X, which may be an alkyl or fluoroalkyl chain or contain vinyl, epoxy, ester, ketone, nitrile, and silyl groups. Radical disproportionation products are also formed but are easily separated. H atoms attack the weakest C-H bonds of the substrates with high selectivity. In our earliest direct mercury photosensitization, Hg* often failed to attack the substrate C-H bonds to give dimers; the presence of H 2 strongly suppresses direct Hg* chemistry