Making Photoreactive trans-3-(n'-Pyridyl)acrylicAcid(n=2, 3) with Head-to-Tail Orientation in the Solid State by Salt Formation

This work demonstrates that the photoinert trans-3(2'-pyridyl)acrylic acid (2 -PA) can be made photoreactive by S'alt formation with HCl, CF3CO2H, and H2SO4. All three salts undergo photodimerization in head -to -tail (HT) fashion resulting in the formation of the corresponding dimer, 2,4-bis(2'-pyridy1)-cyclobutane-1,3-dicarboxylic acid (HT-rctt-2,2'-BPCD), of which the former two salts, namely, [2-PAH]Cl center dot H2O and [(2-PAH)](CF3CO2) undergo single-crystal-to-single-crystal (SCSC) conversion. trans-3-(3'-Pyridyl)acrylic acid (3 -PA), on the other hand, is known to be photoreactive and undergoes photodinierization in head -to -head (HH) fashion producing the dimer H1,1-rctt-3,3'7, BPCD. The HH-orientation of 3 -PA can be flipped to HT by forming the C1O(4)(-) salt, which upon photodimerization produces HT-rctt-3,3'-BPCD. While HT-rctt-2,2'BPCD exhibits isomerization in the presence of acid in solution, both the HH- and HT-rctt-3,3'-BPCD were inert under similar conditions. Our work demonstrates how the noncovalent intermolecular interactions can play a crucial role in the stereoselective synthesis and also emphasizes that the position of the nitrogen atom in the pyridyl ring is vital for the isomerization to occur in solution.