Mechanistic Studies into the Boron Enabled Intramolecular Photo-Ene Cyclization Reaction

A detailed mechanistic study of the boron enabled photo-ene cyclization reaction is presented. Experimental as well as computational methods were used to rationalize the divergent photo-ene reactivity observed with various 1,5-diene starting materials. Moreover, DFT studies have established a mechanism that relies upon two key electronic interactions in the chemoselectivity-determining step. Interestingly, spin contamination relative to the functional/basis set used was a key correlation observed while mapping the energy profiles for the crossed 2+2 and photo-ene reactions. To obtain energy profiles that agreed with experimental outcomes, minimizing spin contamination proved important. Furthermore, NBO analysis shows that the C-Bpin s-bond introduces stabilizing nonbonding interactions in the photo-ene reaction pathway while simultaneously introducing destabilizing interactions in the crossed [2+2] reaction pathway. It is hypothesized that the philicity of key triplet state intermediates is responsible for selectivity