Synthesis and structure of cyclic phosphine-boranes

In a search for cyclic phosphine-boranes, which can be expected to undergo ring opening polymerisation, or decomposition, to leave boron phosphide, the hydroboration reaction of ω-alkenyl-diphenylphosphines with boranes was investigated. Addition of one equivalent of borane (introduced as its tetrahydrofuran adduct) to phosphines H₂C=CH-(CH₂)_n-PPh₂ (n = 0, 1, 2) gave the corresponding phosphine-boranes H₂C=CH-(CH₂)_n-PPh₂BH₃ without any further cyclisation. With an excess of (thf)BH₃, hydroboration of the olefinic function occurred, but again no ring closure. Treatment of the alkenylphosphines with 9-borabicyclononane gave the expected cyclic phosphine-boranes (C₈H₁₄)B-(CH₂)_n-PPh₂ (n = 3, 4) in good yield as crystalline products. The structures of the two heterocycles have been determined by X-ray diffraction methods. They show the expected strain-free ring systems with short P-B bonds, comparable with the isoelectronic Si-C systems.