HYDROGEN-ATOMS AS CONVENIENT SYNTHETIC REAGENTS - MERCURY-PHOTOSENSITIZED DIMERIZATION OF FUNCTIONALIZED ORGANIC-COMPOUNDS IN THE PRESENCE OF H2

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-degrees-C. The H atoms add to CH2 = CH-CH2X to give the intermediate radical CH3-(.CH)-CH2X, which dimerizes to give CH3CH(CH2X)-CH(CH2X)CH3. The saturated substrates CH3CH2CH2X undergo H abstraction to give CH3CH2(.CH)X as intermediates and CH3CH2CH(X)-CH(X)CH2CH3 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. H atoms are not sensitive to steric or polar effects. Radical fragmentation is avoided by using "high" pressures (1 atm). Intramolecular radical additions to C = C bonds and methyl group 1,2-shift were also seen in some cases. Exceptional product ratios are observed for cross-reactions involving hydroxyalkyl radicals where H-bonding favors the homodimers in certain cases. Several bond strengths of C-H bonds-alpha to CO were determined: EtCO2Me, 94.5; i-PrCO2Me, 92.7; cyclopentanone, 94.3; (i-Pr)2CO, 91.9 kcal/mol.