Role of anions in the synthesis of cyclobutane derivatives via [2 + 2] cycloaddition reaction in the solid state and their isomerization in solution

Abstract

Trans-3-(4′-Pyridyl)acrylic acid (4-PA) is inert to photodimerization reaction both in solution and solid state. It is made photoreactive by forming salts with various acids. The anions of these salts play a key role in directing the packing of 4-PAH+ in the solid state. The anions CF 3CO2 -, Cl-, ClO4 -, and BF4 - direct the parallel alignments of 4-PAH+ in head-totail (HT) fashion and lead to the formation of HT-photodimer. On the other hand, bivalent anion SO4 2- directs parallel alignment of 4-PAH+ in head-to-head (HH) fashion and lead to the formation of HH-photodimer. The details of the anioncontrolled stereoselective syntheses of these two cyclobutane derivatives are presented. Interestingly, both cyclobutane compounds undergo isomerization from rctt-form to rctc-form in solution catalyzed by acid. (Figure presented) © 2011 American Chemical Society.