Photoinduced CC-coupling reactions of rigid diastereomeric benzophenone-methionine dyads

The reactions of ketone/methionine systems are widely used as efficient and selective sources of biorelevant radical species. In this study, we address intramolecular variants of this couple with respect to its photosynthetic utility and as a mechanistic model of underlying elementary reaction steps of biological importance, especially with respect to the study of photoinitiated electron transport in complex peptides. The outcomes of this study are two-fold: (1) steady-state irradiation of sterically constrained benzophenone/methionine dyads afforded stable photocyclization products with high yield and product selectivity. (2) Mechanistic insights into the triplet-triggered product formation were obtained from an analysis of the flash photolysis results and the molecular structure of the stable product formed upon irradiation. Time-resolved experiments identified (net) hydrogen-atom transfer from the methionine as the mechanism of the triplet quenching and the resulting biradicals as the major precursor of the isolated stable product. Both the analyses of triplet quenching and stable-product formation in the diastereomeric pairs point to effects of chiral center configuration, i.e., significant stereoselectivity is observed for all elementary steps. The underlying stereochemical restraints were quantitatively addressed by means of molecular dynamics simulations.