Synthesis, characterization and stability of phosphonium phenolate zwitterions derived from a (diphenylphosphino)phenol derivative and oxiranes

Phosphonium phenolate zwitterions have been synthesized from 2,4-di-tert-butyl-6-(diphenylphosphino)phenol and five different oxirane derivatives. The reaction does not proceed at a detectable rate when the two reactants are combined in dichloromethane at room temperature. Despite the substantial ring strain, the reaction proceeds only with the addition of methanol, which acts as a hydrogen transfer shuttle, allowing a slow conversion to the desired zwitterions. The compounds have been fully characterized and single crystal X-ray crystallography has been performed on the methyloxirane and the phenyl glycidyl ether-derived zwitterion. The phosphonium phenolate units exhibit an ylidic bonding situation as evidenced by spectroscopic and crystallographic analysis. Glycidyl ethers were found to react faster than alkyl and aryloxiranes. Decomposition studies of the zwitterions showed high thermal stability in solution under ambient conditions. Under forced conditions (150 °C, 6 h), decomposition to the corresponding phosphine oxide and secondary aliphatic alcohols, the formally hydrogenated oxirane derivative, was observed.