Dimerization and Cycloaddition Reactions of Transient Tri-<i>tert</i>-butylphosphacyclobutadiene Generated by Lewis Acid Induced Isomerization of Tri-<i>tert</i>-butylphosphatetrahedrane

Abstract

Tri-<i>tert</i>-butylphosphatetrahedrane (<b>1</b>) is shown here to act as a synthon of isomeric tri-<i>tert</i>-butylphosphacyclobutadiene in the presence of a Lewis acid or transition-metal complex. When it is combined with a substoichiometric amount of triphenylborane, compound <b>1</b> forms a ladderane-type dimer of tri-<i>tert</i>-butylphosphacyclobutadiene in 72% isolated yield. Trapping of a generated intermediate was achieved by repeating the experiment in the presence of excess styrene (20 equiv) or ethylene (1 atm), and the corresponding [4 + 2] cycloadducts of tri-<i>tert</i>-butylphosphacyclobutadiene were isolated in 88% and 74% yields, respectively. The platinum complex (Ph₃P)₂Pt(C₂H₄) also reacts with <b>1</b> to form an orange η² complex of tri-<i>tert</i>-butylphosphacyclobutadiene in 80% isolated yield. Additionally, we report a novel method for generating a phosphinidenoid species via fluoride-induced trimethylsilyl fluoride elimination, leading to an improved preparative procedure for <b>1</b> (182 mg, 33% isolated yield).