Role of Anions in the Synthesis of Cyclobutane Derivatives via [2 + 2] Cycloaddition Reaction in the Solid State and Their Isomerization in Solution

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 CF3CO2–, Cl–, ClO4–, and BF4– direct the parallel alignments of 4-PAH+ in head-to-tail (HT) fashion and lead to the formation of HT-photodimer. On the other hand, bivalent anion SO42– directs parallel alignment of 4-PAH+ in head-to-head (HH) fashion and lead to the formation of HH-photodimer. The details of the anion-controlled 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.