Synthesis of 1,2-bis[(diorgano)phosphino]ethanes via Michaelis–Arbuzov type rearrangements

A three-step process for the synthesis of the bis(diorganophosphino)ethanes R2PCH2CH2PR2 where R=Et, Ph, iPr, Cy and tBu was examined. In the first step, diorganochlorophosphines were allowed to react with ethylene glycol in the presence of triethylamine at room temperature in THF solution. For R=Ph, iPr and Cy, the bisphosphinites R2POCH2CH2OPR2 were obtained in high yield. For R=Et, the bisphosphinite could not be isolated but may be formed in 80% mixtures with tetraethyldiphosphine, Et2PPEt2, as a minor component. The reaction of di-t-butylchlorophosphine with ethylene glycol occurs at temperatures greater than 130 °C giving di-t-butyl phosphine oxide, tBu2PH(O), as the only phosphorus-containing product. Thermolysis of the bisphosphinites R2POCH2CH2OPR2 (R=Ph, iPr and Cy) at 190–260 °C for 24 h gave the bisphosphine oxides, R2P(O)CH2CH2(O)PR2 in 9% (Ph), 90% (iPr) and 93% (Cy) yields. A DSC study of the thermal rearrangement of Cy2POCH2CH2OPCy2 to Cy2P(O)CH2CH2(O)PCy2 yielded an enthalpy of isomerization of −40.4±0.6 kcal mol−1. Reduction of the bisphosphine oxides, R2P(O)CH2CH2(O)PR2 (R=Ph, iPr and Cy) with trichlorosilane gave the bisphosphines, R2PCH2CH2PR2 in 80–85% yield. The overall yields of the bisphosphines R2PCH2CH2PR2 (R=iPr and Cy) in the three-step process were 61 and 75%, respectively, suggesting that this process should be an attractive synthetic pathway to these two bisphosphines.