Chlorinated Anilines as Molecular Templates to Achieve [2 + 2] Cycloaddition Reactions within Organic Cocrystals

The cocrystallization of either 2,3,5,6-tetrachloroaniline (C₆H₃Cl₄N) or 2,4,6-trichloroaniline (C₆H₄Cl₃N) with trans-1,2-bis-(4-pyridyl)-ethylene (BPE) results in a pair of three-component hydrogen-bonded cocrystals, namely 2-(C₆H₃Cl₄N)·(BPE) and 2-(C₆H₄Cl₃N)·(BPE). These cocrystals undergo up to a quantitative [2 + 2] cycloaddition reaction in the organic solid state upon exposure to ultraviolet light. Utilizing the ability of these chlorinated anilines to engage in both N-H···N hydrogen bonds along with homogeneous and face-to-face π-π stacking interactions ultimately positions BPE in a suitable location to photoreact and generate the stereoselective photoproduct rctt-tetrakis-(4-pyridyl)-cyclobutane (TPCB). The tendencies for these chlorinated anilines to form homogeneous π-stacks were investigated by means of density functional theory calculations with the goal to determine not only the overall strength but also the preference for this stacking pattern. In addition, a series of isostructural cocrystals were also achieved by incorporating two isosteric hydrogen-bond acceptors, namely 1,2-bis-(4-pyridyl)-acetylene (BPA) and azobipyridine (Azo), with these chlorinated anilines.