Photochemical [2+2] cyclization of helical phosphinamides in solution and in the solid state

Phosphole-based phosphahelicenes have been prepared which entail unprecedented phosphinamide functions. One of these diastereomeric phosphinamides displays an unusual propensity to intermolecular photochemical [2+2] cyclization leading to a bis phosphahelicene. The reaction affords the head-to-tail heterochiral diastereomer with total regio- and diastereoselectivity. The reaction has been observed not only in solution but also in the solid state, as a single-crystal-to-single-crystal process, under either X-ray or sunlight irradiation. The solid state arrangement of the starting compound strongly prefigures the geometry of the final dimeric species and accounts for the particular ease by which this reaction takes place. The second diastereomer of the phosphinamide does not undergo dimerization under analogous photochemical conditions, due to an unfavourable arrangement in the solid state.