Insights into the Stability and Structures of Phosphine-Boranes and Their α‑Metalated Derivatives

The reaction between <i>i</i>Pr₂PCl and Ph₂P­(BH₃)­CH₂Li gives the mixed phosphine/phosphine-borane Ph₂P­(BH₃)­CH₂P<i>i</i>Pr₂ (<b>1a</b>) in good yield. Thermolysis of <b>1a</b> leads to borane migration and the formation of Ph₂PCH₂P­(BH₃)<i>i</i>Pr₂ (<b>2a</b>) along with small amounts of Ph₂P­(BH₃)­CH₂P­(BH₃)<i>i</i>Pr₂ (<b>3a</b>) and Ph₂PCH₂P<i>i</i>Pr₂ (<b>4a</b>). Compound <b>3a</b> may be synthesized directly from the reaction of <b>1a</b> with BH₃·SMe₂, while <b>4a</b> can be prepared cleanly by heating <b>1a</b> in methanol under reflux. Kinetic studies on the conversion of <b>1a</b> to <b>2a</b> reveal the reaction to be apparently first order in <b>1a</b>, suggesting a dissociative process, and yield the activation parameters Δ<i>H</i>^⧧ = 63 ± 8 kJ mol^–1, Δ<i>S</i>^⧧ = −145 ± 24 J K^–1 mol^–1, and Δ<i>G</i>^⧧ = 106 ± 8 kJ mol^–1, the negative entropy of activation conversely suggesting an associative process. DFT studies suggest that concerted migration of borane within a molecule of <b>1a</b> is disfavored, but that both the dissociative and associative mechanisms for borane migration operate simultaneously. Metalation of <b>1a</b>–<b>4a</b> with <i>n</i>BuLi in the presence of tmeda gives the complexes [{Ph₂P­(BH₃)}­CHP<i>i</i>Pr₂]­Li­(tmeda) (<b>1b</b>), [Ph₂PCH­{P­(BH₃)<i>i</i>Pr₂}]­Li­(tmeda) (<b>2b</b>), [{Ph₂P­(BH₃)}­CH­{P­(BH₃)<i>i</i>Pr₂}]­Li­(tmeda) (<b>3b</b>), and [Ph₂PCHP<i>i</i>Pr₂]­Li­(tmeda) (<b>4b</b>), respectively, which adopt similar structures in the solid state. Analysis of the crystal structures suggests that the phosphine-borane groups stabilize the adjacent charge to a greater extent than the phosphine groups. This is supported by DFT calculations, which show that the greatest delocalization of negative charge from the carbanion is into the P–C­(Ph) or P–C­(Pr) σ*-orbitals of the phosphine-borane substituents