Incorporating Ester Functionality within a Solid-State [2 + 2] Cycloaddition Reaction Based upon Halogen Bonding Interactions

The formation of cocrystals based upon 1,4-diiodoperchlorobenzene (C₆I₂Cl₄) as a halogen bond donor along with a series of isomeric pyridine vinyl esters, namely, (E)-methyl-3-(pyridine-X-yl)prop-2-enoate (where X = 4, 3, and 2) (4-PAMe), (3-PAMe), and (2-PAMe), has been achieved. In all cases, C₆I₂Cl₄ halogen bonds to a combination of pyridine nitrogen atoms as well as ester oxygen atoms to yield an extended solid. The cocrystals (C₆I₂Cl₄)·(4-PAMe) and (C₆I₂Cl₄)·2(3-PAMe) were observed to be photoreactive. X-ray structural analysis revealed that molecules of C₆I₂Cl₄ are stacked in a homogeneous face-to-face π-πorientation which then positions a pair of carbon-carbon double bonds (C=C) in the correct orientation to undergo a solid-state [2 + 2] cycloaddition reaction. Thus, upon exposure to ultraviolet light, both (C₆I₂Cl₄)·(4-PAMe) and (C₆I₂Cl₄)·2(3-PAMe) undergo a regioselective and quantitative photoreaction to yield the head-to-head photoproduct, namely, (rctt)-dimethyl-3,4-bis(pyridine-4-yl)cyclobutane-1,2-dicarboxylate and (rctt)-dimethyl-3,4-bis(pyridine-3-yl)cyclobutane-1,2-dicarboxylate, respectively. X-ray analysis of (C₆I₂Cl₄)·(2-PAMe) revealed the structural reason behind the observed photostability which is attributed to the large parallel displacement of the double bonds.