RESUMO
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). In benzene, divergent trends were observed: for the hydroxy compound 2 a lower lifetime of 790 ns was determined, whereas for 3 and 4 the lifetimes increased to 4.9 micros. Photolyses of the alpha-amino-substituted compounds 1 and 6 resulted in transient species with significant lower lifetimes (for 1 160 ns in benzene and 450 ns in acetonitrile; for 6 <100 ns in both solvents). The mesyloxy substrate 5 undergoes rapid C-O bond cleavage upon photolysis and no transient triplet species were detected. Computational (UB3 LYP/6-31G* and natural don orbital (NBO) analyses) results supported the assumption of a negative hyperconjugative interaction strongly stabilizing alpha-oxy-substituted over alpha-amino-substituted radicals.
RESUMO
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.
