Mediation of conformationally controlled photodecarboxylations of chiral and cyclic aryl esters by substrate structure, temperature, pressure, and medium constraints.

Abstract

An aryl alkanoate, 2,4,6-trimethylphenyl (S)-(+)-2-methylbutyrate, whose ester group has a chiral center alpha to the carbonyl carbon and in which photo-Fries rearrangements are blocked by methyl substituents, undergoes facile photodecarboxylation under a variety of conditions and with complete retention of configuration. In fact, the decarboxylation process has many of the attributes of a symmetry-allowed suprafacial [1,3]sigmatropic rearrangement. The process requires concerted extrusion of carbon dioxide in a spiro-lactonic transition state, which has been investigated using high level DFT and CIS calculations: thermally less stable s-cis conformers in the ground and excited singlet states play an important role in determining the competitive efficiency of the process. Conformational control has also been imposed by substrate structure, solvent interactions, temperature, and applying external pressure, as well as using constraining media such as cyclodextrins and polyethylene films. The results are correlated with steady-state and dynamic fluorescence measurements at various temperatures in order to investigate further degrees to which ground and excited singlet state conformations affect the different photoreactivity channels available to the aryl esters.