Trico-ordinate phosphorus compounds as catalysts for the isomerization of (Z)-to (E)-azobenzene

The isomerization of (Z)-azobenzene to (E)-azobenzene is catalysed by trico-ordinate phosphorus compounds, ArnP(OR)3 –n where n= 0–2. The reaction is overall second order and for n= 1 and 2, the ρ values of –0.55 and –0.45, respectively, indicate a low degree of charge separation in the transition state. Solvent effects confirm this conclusion and relative rates for n= 0–3 suggest that the transition state is a hybrid of biphilic and polar components.