Photochemical addition of methanol to stilbenes

Abstract

The photochemical addition of methanol to the alkenic bond of stilbene (1) involves two competitive pathways: viz. either direct addition of methanol to the alkenic bond or insertion by methanol of a carbene (13), emerging from a 1,2-H shift in (1). From the observed effect of substituents and the wavelength on the rates of these processes, the rearrangement pathway was concluded to involve excited (1) in a vibrationally unrelaxed S₁-state. The direct pathway might involve either a planar (ππ*) or a twisted excited S¹,-state of (1). The available experimental evidence precludes zwitterionic excited states from being involved in both pathways. The photochemical addition of methanol to the alkenic bond of 1,1-diphenylethene (3) follows a similar course. In this case a 1,2-Ph shift in excited (3) gives rise to carbene (13) and its insertion products.