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)center dot(4-PAMe) and (C5I2Cl4)center dot 2(3-PAMe) were observed to be photoreactive. X-ray structural analysis revealed that molecules of C6I2C14 are stacked in a homogeneous face-to-face pi-pi Jr orientation 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)center dot(4-PAMe) and (C6I2Cl4)center dot 2(3-PAMe) undergo a regioselective and quantitative photoreaction to yield the head -to -head photoproduct, namely, (rctt)-dimethyl-3,4-bis(pyridine-4-y])cyclobutane-1,2-dicarboxylate and (rctt)dimethyl-3,4-bis(pyridine-3-yl)cyclobutane-1,2-dicarboxylate, respectively. X-ray analysis of (C6I2Cl4)center dot(2-PAMe) revealed the structural reason behind the observed photostability which is attributed to the large parallel displacement of the double bonds.