Supramolecular construction of a cyclobutane ring system with four different substituents in the solid state

Methods to form cyclobutane rings by an intermolecular [2 + 2] cross-photoreaction (CPR) with four different substituents are rare. These reactions are typically performed in the liquid phase, involve multiple steps, and generate product mixtures. Here, we report a CPR that generates a cyclobutane ring with four different aryl substituents. The CPR occurs quantitatively, without side products, and without a need for product purification. Generally, we demonstrate how face-to-face stacking interactions of aromatic rings can be exploited in the process of cocrystallization and the field of crystal engineering to stack and align unsymmetrical alkenes in CPRs to afford chiral cyclobutanes with up to four different aryl groups via binary cocrystals. Overall, we expect the process herein to be useful to generate chiral carbon scaffolds, which is important given the presence of four-membered carbocyclic rings as structural units in biological compounds and materials science.

one day after the slow evaporation of a clear solution of the photoproduct (35 mg) in ethanol (5 mL).
Single crystals of (BPE)·(8F) were obtained by combining a solution of 8F (25 mg) in toluene (1 mL) with a solution of BPE (14 mg, 1:1 molar ratio) in of toluene (1 mL). Single crystals suitable for X-ray diffraction studies were realized within 1 day after the slow evaporation of toluene. The photoreaction reached a quantitative yield within 30 hours of irritation, as determined by 1 H NMR spectroscopy. Single crystals of BPE-8F-cb, suitable for X-ray diffraction, were obtained within one day after the slow evaporation of a clear solution of the photoproduct (35 mg) in ethanol (3 mL).
Single crystals of (SBZ)·(8F) were obtained by combining a solution of 8F (25 mg) in toluene (1 mL) with a solution of SBZ (14 mg, 1:1 molar ratio) in of ethanol (1 mL). Single crystals suitable for X-ray diffraction studies were realized within 1 day after the slow evaporation of toluene. The photoreaction reached a quantitative yield within 30 hours of irritation, as determined by 1 H NMR spectroscopy. Single crystals of SBZ-8F-cb, suitable for X-ray diffraction, were obtained within one day after the slow evaporation of a clear solution of the photoproduct (34 mg) in a toluene:ethanol mixture (2 mL, 1:1, v:v).
Single crystals of (SBZ)·(9F) were obtained by combining a solution of 9F (25 mg) in toluene (2 mL) with a solution of SBZ (13 mg, 1:1 molar ratio) in of toluene (1 mL). Single crystals suitable for X-ray diffraction studies were realized within 1 day after the slow evaporation of toluene. The photoreaction reached a quantitative yield within 30 hours of irritation, yielding SBZ-9F-cb as a light brown solid, which was determined by 1 H NMR spectroscopy. Single crystals of [H-(SBZ-9F-cb)][p-TsO], suitable for X-ray diffraction, were obtained within one day after the slow evaporation of a clear solution of SBZ-9F-cb (20 mg) and p-TsOH (7.3 mg) in a dichloromethane:methanol mixture (2 mL, 1:1, v:v). Figure S10. Powder X-ray diffraction of powder of (BPE)·(8F) and simulated pattern from singlecrystal X-ray diffraction data. S14 Figure S11. Powder X-ray diffraction of powder of (SBZ)·(8F) and simulated pattern from singlecrystal X-ray diffraction data. Figure S12. Powder X-ray diffraction of powder of (SBZ)·(9F) and simulated pattern from singlecrystal X-ray diffraction data. Figure S13. Powder X-ray diffraction of powder of [H-SBZ-9F-cb][p-TsO] and simulated pattern from single-crystal X-ray diffraction data.

S4. Single Crystal X-ray Diffraction Data
Single crystal data was collected with a Bruker APEX II Kappa Diffractometer equipped with an Oxford Cryostream low temperature device using MoKα radiation (λ = 0.71073 Å) after having been secured to Mitegen magnetic mounts using Paratone oil. Data collection strategies to ensure maximum data redundancy and completeness were calculated using Apex2. 4 All calculation dealing with data collection, initial indexing, frame integration, Lorentz-polarization corrections and final cell parameter were again carried out by Apex2. 5 All structures were solved using ShelXT 6 and refined using ShelXL 7 in the Olex2 8 graphical user interface. Hydrogen atoms associated with carbon atoms were refined in geometrically constrained positions with isotropic thermal parameter Uiso(H) = 1.2 Ueq(CCH). Table S1. Crystal data and structure refinement for (SB)·(8F) and SB-8F-cb.

S5. Molecular Modeling
Electrostatic potentials for each functionalized alkene were calculated at ground state in gas phase using Spartan '18 V1.2.0 software. Equilibrium geometry was used for calculation without further constraints. All calculations were performed using the B3LYP/6-31G* density functional model.