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

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• 作者：Keith Izod ; Corinne Wills ; Emma Anderson ; Ross W. Harrington ; Michael R. Probert
• 刊名：Organometallics
• 出版年：2014
• 出版时间：October 13, 2014
• 年：2014
• 卷：33
• 期：19
• 页码：5283-5294
• 全文大小：550K
• ISSN：1520-6041

文摘

The reaction between iPr₂PCl and Ph₂P(BH₃)CH₂Li gives the mixed phosphine/phosphine-borane Ph₂P(BH₃)CH₂PiPr₂ (1a) in good yield. Thermolysis of 1a leads to borane migration and the formation of Ph₂PCH₂P(BH₃)iPr₂ (2a) along with small amounts of Ph₂P(BH₃)CH₂P(BH₃)iPr₂ (3a) and Ph₂PCH₂PiPr₂ (4a). Compound 3a may be synthesized directly from the reaction of 1a with BH₃路SMe₂, while 4a can be prepared cleanly by heating 1a in methanol under reflux. Kinetic studies on the conversion of 1a to 2a reveal the reaction to be apparently first order in 1a, suggesting a dissociative process, and yield the activation parameters 螖H^{猝?/sup> = 63 卤 8 kJ mol鈥?, 螖S猝?/sup> = 鈭?45 卤 24 J K鈥? mol鈥?, and 螖G猝?/sup> = 106 卤 8 kJ mol鈥?, the negative entropy of activation conversely suggesting an associative process. DFT studies suggest that concerted migration of borane within a molecule of 1a is disfavored, but that both the dissociative and associative mechanisms for borane migration operate simultaneously. Metalation of 1a鈥?b>4a with nBuLi in the presence of tmeda gives the complexes [{Ph₂P(BH₃)}CHPiPr₂]Li(tmeda) (1b), [Ph₂PCH{P(BH₃)iPr₂}]Li(tmeda) (2b), [{Ph₂P(BH₃)}CH{P(BH₃)iPr₂}]Li(tmeda) (3b), and [Ph₂PCHPiPr₂]Li(tmeda) (4b), 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鈥揅(Ph) or P鈥揅(Pr) 蟽*-orbitals of the phosphine-borane substituents.}