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Insights into the stability and structures of phosphine-boranes and their α-metalated derivatives

Lookup NU author(s): Dr Keith Izod, Dr Corinne Wills, Dr Ross Harrington, Professor Mike ProbertORCiD

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Abstract

The reaction between iPr2PCl and Ph2P(BH3)CH2Li gives the mixedphosphine/phosphine-borane Ph2P(BH3)CH2PiPr2 (1a) in good yield. Thermolysis of 1aleads to borane migration and the formation of Ph2PCH2P(BH3)iPr2 (2a) along with smallamounts of Ph2P(BH3)CH2P(BH3)iPr2 (3a) and Ph2PCH2PiPr2 (4a). Compound 3a may besynthesized directly from the reaction of 1a with BH3·SMe2, while 4a can be prepared cleanlyby heating 1a in methanol under reflux. Kinetic studies on the conversion of 1a to 2a revealthe reaction to be apparently first order in 1a, suggesting a dissociative process, and yield theactivation parameters ΔH⧧ = 63 ± 8 kJ mol−1, ΔS⧧ = −145 ± 24 J K−1 mol−1, and ΔG⧧ = 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 1a is disfavored,but that both the dissociative and associative mechanisms for borane migration operatesimultaneously. Metalation of 1a−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 thephosphine-borane groups stabilize the adjacent charge to a greater extent than the phosphine groups. This is supported by DFTcalculations, 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.

Publication metadata

Author(s): Izod K, Wills C, Anderson E, Harrington RW, Probert MR

Publication type: Article

Publication status: Published

Journal: Organometallics

Year: 2014

Volume: 33

Issue: 19

Pages: 5283-5294

Print publication date: 13/10/2014

Online publication date: 05/09/2014

Acceptance date: 28/08/2014

Date deposited: 31/10/2014

ISSN (print): 0276-7333

ISSN (electronic): 1520-6041

Publisher: American Chemical Society

URL: http://dx.doi.org/10.1021/om5005995

DOI: 10.1021/om5005995

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