Anionic ring-opening polymerization of small phosphorus heterocycles and their borane adducts: An ab initio investigation

The kinetics and thermodynamics of anionic ring-opening reactions of phosphiranes, phosphetanes, and phospholanes and their borane adducts have been studied by high-level ab initio procedures. For the free heterocycles, model propagation reactions involving nucleophilic attack by Me₂P ^- at the ring α-carbon were found to be feasible for the three- and four-membered rings, but not for the five-membered ring. For the borane adducts, nucleophilic attack by Me₂(BH₃)P^- was only facile for the three-membered ring, despite an increased thermodynamic tendency toward ring opening for the borane adducts of both the three- and four-membered rings. The formation constants of the borane adducts of methylphosphirane and methylphosphetane were K = 2.6 × 10¹³ L mol^-1 and K = 1.2 × 10²⁰ L mol^-1, respectively. A Marcus analysis of the ring-opening reactions of methylphosphirane, methylphosphetane, and their borane adducts showed that the release of ring strain and an "additional factor" contribute to rate enhancement compared with their strain-free analogues. The additional factor was larger for the three-membered rings than for the four-membered rings and was larger in the free heterocycles than in their borane adducts. The additional factor is complex in origin and appears to reflect an increase in the separation between the bonding and antibonding orbitals of the breaking bond on going from the three-membered rings to the four-membered rings, and on going from the free hetero-cycles to the borane adducts.