Photomechanochemical control over stereoselectivity in the [2+2] photodimerization of acenaphthylene

Tuning solubility and mechanical activation alters the stereoselectivity of the [2 + 2] photochemical cycloaddition of acenaphthylene. Photomechanochemical conditions produce the syn cyclobutane, whereas the solid-state reaction in the absence of mechanical activation provides the anti. When the photochemical dimerization occurs in a solubilizing organic solvent, there is no selectivity. Dimerization in H2O, in which acenaphthylene is insoluble, provides the anti product. DFT calculations reveal that insoluble and solid-state reactions proceed via a covalently bonded excimer, which drives anti selectivity. Alternatively, the noncovalently bound syn conformer is more mechanosusceptible than the anti, meaning it experiences greater destabilization, thereby producing the syn product under photomechanochemical conditions. Cyclobutanes are important components of biologically active natural products and organic materials, and we demonstrate stereoselective methods for obtaining syn or anti cyclobutanes under mild conditions and without organic solvents. With this work, we validate photomechanochemistry as a viable new direction for the preparation of complex organic scaffolds.

Automated summary

Tuning solubility and mechanical activation can alter the stereoselectivity of the [2 + 2] photochemical cycloaddition reaction. The solid-state reaction and the reaction in organic solvents yield the anti and syn products, respectively. DFT calculations show that the solid-state reaction proceeds via a covalently bonded excimer, driving anti selectivity. Under photomechanochemical conditions, the noncovalently bound syn conformer experiences greater destabilization, leading to the formation of the syn product. This work demonstrates the viability of photomechanochemistry for the preparation of complex organic scaffolds.