Solid state photodimerization of trans-2-(4-pyridyl)-4-vinylbenzoic acid via salt formation and isomerisation of cyclobutane compounds in solution

Several salts of trans-2-(4-pyridyl)-4-vinylbenzoic acid (HPVBA) obtained by reacting with inorganic acids and organic diamines are discussed in the context of their solid state packing and photoreactivity. The acids CF3CO2H, HClO4, HNO3, H2SO4 produced salts where H(2)PVBA(+) cations are oriented parallel in head-to-tail (HT) fashion, however, the trifluoroacetate salt did not undergo photodimerization. The head-to-tail (HT) parallel orientation was inversed to head-to-head (HH) orientation by incorporating excess H2SO4 during salt formation. Thus two stereoisomers viz. HT- and HH-dimers were obtained from salts derived from same salt-former. Salts were also made exploiting carboxylic acid functionality by employing ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane and it was found that only the salt obtained with 1,4-diaminobutane undergo photodimerization. In an analogy, it is shown that in spite of poor predictability, the strategy of salt formation works well and has significant impact in the synthesis of functional cyclobutane derivatives.