Phosphorus-31 and carbon-13 nuclear magnetic resonance studies of unsymmetrical tetra-alkyldiphosphanes R1R2PPMe2 and R1R2PPEt2

Abstract

A series of unsymmetrical tetra-alkyldiphosphanes of general formulae R¹R²PPMe₂ and R¹R²PPEt₂(with R¹, R²= Me, Et, Prⁱ, Bu^t) have been obtained by scrambling reactions between the corresponding symmetrical diphosphanes in CH₂Cl₂ solution and have been studied by n.m.r. spectroscopy in situ. Values of ³¹P and ¹³C chemical shifts, together with (P,P) and (P,C) coupling constants, are reported and discussed in relation to substituent effects and conformation. The parameter ¹J_PC+²J_PC is shown to be a good predictor of the preferred rotamer, and it is concluded that ¹J_PC and ²J_PC must be opposite in sign.