PP-Rotation, P-Inversion and Metathesis in Diphosphines Studied by DFT Calculations: Comments on Some Literature Conflicts

Adam Molloy,Declan Gilheany,Goar Sánchez-Sanz

doi:10.3390/inorganics4040036

Abstract

The potential energy surface for internal rotation about the phosphorus–phosphorus bond was calculated at the PCMDCM/B3LYP/6-311++G(d,p) computational level for a set of eight symmetrical, unsymmetrical and P-stereogenic diphosphines; H4P2, Me4P2, (CF3)4P2, Ph4P2, Me2P–P(CF3)2, Me2P–PPh2, and the meso- and dl-isomers of Me(CF3)P–PMe(CF3) and MePhP–PMePh. Certain trends in the data were elucidated and compared with conflicting data from the literature regarding the relative population of anti and gauche rotational isomers. The pyramidal inversion barriers (stereomutation barriers in P-stereogenic cases) for the same set of diphosphines was estimated through the inversion transition states and also compared to literature values. Finally, the Me4P2 + (CF3)4P2 → 2Me2(CF3)2P2 metathesis reaction was also explored to evaluate its feasibility versus inversion. The finding of larger barriers in the metathesis than in the inversion rules in favour of an inversion mechanism for the stereomutation of P-stereogenic diphosphines.

Highlights

P-Stereogenic diphosphines of the form (R1 R2 P)2 exist in meso- and dl-diastereomeric forms in solution with separate Nuclear Magnetic Resonance (NMR) spectra observable by 1 H, 13 C and31 P NMR spectroscopy [1,2,3]
An apparent aggregation of diphosphines in high concentration polar solvents and the formation of unsymmetrical diphosphines in mixtures of symmetrical diphosphines supported an active metathesis pathway previously dismissed by LJM
This scrambling reaction typically results in a mixture of symmetrical and unsymmetrical diphosphines [8] but some polarized diphosphines were found to be “metathesis stable” and select mixtures yielded complete conversion to unsymmetrical diphosphine [3,9,10,11]

Summary

Introduction

P-Stereogenic diphosphines of the form (R1 R2 P) exist in meso- and dl-diastereomeric forms in solution with separate Nuclear Magnetic Resonance (NMR) spectra observable by 1 H, 13 C and31 P NMR spectroscopy [1,2,3]. An apparent aggregation of diphosphines in high concentration polar solvents and the formation of unsymmetrical diphosphines in mixtures of symmetrical diphosphines supported an active metathesis pathway previously dismissed by LJM. This scrambling reaction typically results in a mixture of symmetrical and unsymmetrical diphosphines [8] but some polarized diphosphines were found to be “metathesis stable” and select mixtures yielded complete conversion to unsymmetrical diphosphine [3,9,10,11]. The idea of a four-membered metathesis mechanism was invoked

Methods

Discussion

Conclusion