Photochemical <i>E</i>−<i>Z</i> Isomerization of <i>m</i><i>eta</i>-Terphenyl-Protected Phosphaalkenes and Structural Characterizations

A series of phosphaalkenes, E-ArPC(H)Ar‘ (Ar = 2,6-Mes2C6H3, Ar‘ = Ph (1a); Ar = 2,6-Mes2C6H3, Ar‘ = p-C6H4Br (2a); Ar = 4-Br-2,6-Mes2C6H2, Ar‘ = Ph (3a); Ar = 4-Br-2,6-Mes2C6H2, Ar‘ = p-C6H4Br (4a)) have been prepared by phospha-Wittig reactions and characterized. Exposure of these materials either to room light over an extended period of time (days) or to UV light (hours) produced equilibrium mixtures of the E and Z isomers (1b−4b) as indicated by 1H and 31P NMR spectroscopy. The structures of compounds 4a and 4b were determined by single-crystal X-ray diffraction methods. Variable-temperature 1H NMR studies of 4b indicate hindered rotation about the P−CAr bond, with ΔH⧧ = 13.8 kcal/mol and ΔS⧧ = 1.3 eu. The electronic structures of E- and Z-PhPC(H)Ph have been examined using density functional theory.