HIGH DIASTEREOSELECTIVITY IN THE CYCLIZATION OF 1,5-BIRADICALS: WHAT CAUSES SUCH SIZEABLE STERIC BARRIERS TO BIRADICAL COUPLING?

Article abstract of DOI:10.1016/0040-4039(91)80844-V

The high diastereoselectivities observed in the photocyclization of α-(o-ethylphenyl)acetophenones appear to reflect conformational equilibria in the triplet 1,5-biradical intermediates rather than steric barriers created during cyclization.The necessary biradical triplet -> singlet intersystem crossing is proposed to occur along the cyclization reaction coordinate since the orthogonality of the two singly-occupied p orbitals does not depress biradical lifetimes.