Photochemical E-Z isomerization of meta-terphenyl-protected phosphaalkenes and structural characterizations

A series of phosphaalkenes, E-ArP=C(H)Ar′ (Ar = 2,6-Mes ₂C₆H₃, Ar′ = Ph (1a); Ar = 2,6-Mes ₂C₆H₃, Ar′ = p-C₆H ₄Br (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-C^Ar 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.