Regiocontrol of the [2 + 2] Photodimerization in the Solid State Using Isosteric Resorcinols: Head-to-Tail Cyclobutane Formation via Unexpected Embraced Assemblies

Regiocontrolled head-to-head and head-to-tail [2 + 2] photodimerizations of (E)-methyl-3-(pyridin-3-yl)prop-2-enoate are achieved in the solid state using 4,6-di-X-res (where X = Cl, Br, I; res = resorcinol) as small-molecule templates. The components in each co-crystal form discrete, three-component supramolecular assemblies sustained by two O-H···N hydrogen bonds. Whereas the head-to-head photoproduct (rctt)-dimethyl-3,4-bis(pyridin-3-yl)cyclobutane-1,2-dicarboxylate forms using 4,6-di-X-res (X = Cl or Br), the head-to-tail regioisomer (rctt)-dimethyl-2,4-bis(pyridin-3-yl)cyclobutane-1,3-dicarboxylate forms using 4,6-di-I-res. The head-to-tail product is generated via unexpected embraced dimeric assemblies. The stereochemistry of the head-to-tail product is confirmed using single-crystal X-ray diffraction.