Synthesis, photochemical behaviour and cis/trans isomerisation of 1-(2,4,6-triisopropylphenyl)-2-(2,4,6-tri-tert-butylphenyl)diphosphene

Abstract

Irradiation of (E)-1-(2,4,6-triisopropylphenyl)-2-(2,4,6-tri-tert-butylphenyl)diphosphene (1a) yields its thermally unstable cis-isomer 1b, the diastereoisomeric cyclisation products 4a and 4b and the cleavage product 5. A mechanism for the formation of these products is proposed. The activation parameters for the thermal isomerisation reaction 1b→1a are experimentally determined as ΔH^‡= 29.5 ± 1.4 kcal mol^–1 and ΔS^‡= 38 ± 6 cal mol^–1 K^–1(1 kcal = 4.18 kJ); ΔH^‡ is the first experimentally determined measure for the strength of the π-component of a phosphorus–phosphorus double bond.