Inner- and Outer-Sphere Roles of Ruthenium Phosphido Complexes in the Hydrophosphination of Alkenes

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• 作者：Roman G. Belli ; Krista M. E. Burton ; Stephanie A. Rufh ; Robert McDonald ; Lisa Rosenberg
• 刊名：Organometallics
• 出版年：2015
• 出版时间：December 14, 2015
• 年：2015
• 卷：34
• 期：23
• 页码：5637-5646
• 全文大小：441K
• ISSN：1520-6041

文摘

An inner-sphere synthetic cycle for the hydrophosphination of alkenes is proposed, based on observed [2 + 2] cycloaddition of a wide range of alkenes at a coordinatively unsaturated Ru鈺怭R₂ complex. Key intermediates in the cycle were prepared, and their reactions with various organic acid/base pairs were examined to identify both new ruthenium precursors and base cocatalysts that allow turnover of the proposed cycle. Two new cationic ruthenium indenyl phosphine complexes were isolated and structurally characterized. Although preliminary screening studies show the moderate activity of these and related neutral phosphido complexes for catalytic hydrophosphination of acrylonitrile by both HPPh₂ and HPCy₂, and comparable activity for the hydrophosphination of tert-butyl acrylate by HPPh₂, no activity was observed for the analogous hydrophosphination of 1-hexene. This is attributed to strong binding of the substrate phosphine to the unsaturated, planar Ru鈺怭R₂ fragment generated in situ, which inhibits the inner-sphere, alkene cycloaddition mechanism. An alternative, outer-sphere Michael addition process, involving a saturated complex with a strongly nucleophilic pyramidal Ru鈥揚R₂ ligand, is proposed to rationalize the observed selectivity for catalytic hydrophosphination of activated, but not simple, alkenes. Implications for further catalyst development are discussed.