Borane Adducts of Aromatic Phosphorus Heterocycles: Synthesis, Crystallographic Characterization and Reactivity of a Phosphinine-B(C 6 F 5 ) 3 Lewis Pair.

A phosphinine-borane adduct of a Me 3 Si-functionalized phosphinine and the Lewis acid B(C 6 F 5 ) 3 has been synthesized and characterized crystallographically for the first time. The reaction strongly depends on the nature of the substituents in the α-position of the phosphorus heterocycle. In contrast, the reaction of B 2 H 6 with various substituted phosphinines leads to an equilibrium between the starting materials and the phosphinine-borane adducts that is determined by the Lewis basicity of the phosphinine. The novel phosphinine borane adduct (6-B(C 6 F 5 ) 3 ) shows rapid and facile insertion and [4+2] cycloaddition reactivity towards phenylacetylene. A hitherto unknown dihydro-1-phosphabarrelene is formed with styrene. The reaction with an ester provides a new, facile and selective route to 1-R-phosphininium salts. These salts then undergo a [4+2] cycloaddition in the presence of Me 3 Si-C≡CH and styrene to cleanly form unprecedented derivatives of 1-R-phosphabarrelenium salts.