Alpha-carbonyl substituent effect on the lifetimes of triplet 1,4-biradicals from Norrish-Type-II reactions.

Triplet 1,4-biradicals were generated by Norrish-Type-II hydrogen transfer from alpha-heteroatom-substituted beta-branched butyrophenones 1-6 and detected by laser flash absorption measurements. For three oxy-substituted compounds 2-4 (R(alpha)=OH, OCOMe, OCOOEt) comparable lifetimes were determined in acetonitrile (roughly 1.5 micros).

Substantial (2)h-magnetic isotope effects on the diastereoselectivity of triplet photocycloaddition reactions.

Magnetic isotope effects (MIE) on the diastereoselectivity of the triplet [2+2]-photocycloaddition of benzaldehyde with 2,3-dihydrofuran are described for four substrate combinations. The MIE results from H/D exchange at the reactive carbons of either the aldehyde or the alkene, respectively, but also from deuteration of the aromatic positions of benzaldehyde. Weaker isotope effects on the diastereoselectivity were detected for the reaction of alpha-deuterated propionaldehyde.