Synthetic Equivalents of 2-Lithiophosphinines: New Routes to 2-Functional Phosphinines

Two routes converting 2-bromophosphinines into 2-functional phosphinines are described. In the first one, a [2 + 4] cycloadduct between the 2-bromophosphinine, 2,3-dimethylbutadiene, and sulfur is first formed. On this adduct, a bromine to lithium exchange is performed by phenyllithium in THF at low temperature. This lithium derivative is converted into a functional derivative by reaction with an electrophile. Then, the 2-functional phosphinine is obtained by a combined reduction-cycloreversion using P(CH₂CH₂CN)₃ as the reducing agent at ca. 180 °C. A 2-(trimethylsilyl)- and a 2-(diphenylphosphino)phosphinine were thus prepared. In the second route, a bromine to lithium exchange is performed on a (2-bromo-phosphinine)pentacarbonyltungsten complex by phenyllithium in THF at −80 °C. The 2-functional phosphinine is recovered from its complex by heating with Ph₂PCH₂CH₂PPh₂ at 110 °C in toluene. A 2-iodo-, 2-(trimethylsilyl)-, and 2-[(diphenylphosphino)pentacarbonyltungsten]phosphinine were thus obtained. An intermediate [2-(ethoxycarbonyl)phosphinine]pentacarbonyltungsten complex readily adds water to give the corresponding l-hydroxy-l,6-dihydrophosphinine complex.