Rapid transfer for selenium from tertiary phosphine selenides to tertiary phosphines

Abstract

Selenium transfer between PR₃Se and PR₃[R₃= MePh₂ or Ph₂(CH₂PPh₂)] in solution is rapid on the n.m.r. time scale, and a bimolecular process is indicated. Mixtures of Ph₂P(Se)C₂H₄(Se)PPh₂, Ph₂P(Se)C₂H₄PPh₂, and Ph₂PC₂H₄PPh₂ display simliar variable-temperature ¹H n.m.r. characteristics and their methylene-proton signals coalesce at elevated temperatures. The compound Ph₂P(Se)CH₂(Se)PPh₂ reacts immediately on mixing with Ph₂PCH₂PPh₂ to give exclusively Ph₂P(Se)CH₂PPh₂, whereas solutions of Ph₂P(Se)C₂H₄(Se)PPh₂ and Ph₂PC₂H₄PPh₂ equilibrate with Ph₂P(Se)C₂H₄PPh₂ in 1 : 1 : 2 proportions, respectively. The sulphur analogues behave similarly, but react much more slowly. Selenium reacts at ambient temperature with Ph₂P(S)CH₂PPh₂ to form only Ph₂P(S)CH₂(Se)PPh₂, but the action of sulphur on Ph₂P(Se)CH₂PPh₂ leads to a mixture of Ph₂P(Se)CH₂(Se)PPh₂, Ph₂P(Se)CH₂(S)PPh₂, and Ph₂P(S)CH₂(S)PPh₂. Controlling influences and synthetic implications of these reactions are discussed.