Alkaline‐Earth‐Catalysed Cross‐Dehydrocoupling of Amines and Hydrosilanes: Reactivity Trends,Scope and Mechanism |
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| Authors: | Clément Bellini Dr Vincent Dorcet Prof Dr Jean‐François Carpentier Dr Sven Tobisch Dr Yann Sarazin |
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| Affiliation: | 1. Organometallics: Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS—Université de Rennes 1, Campus de Beaulieu, Rennes, France;2. Centre de Diffractométrie des Rayons X, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS—Université de Rennes 1, Rennes, France;3. University of St Andrews, School of Chemistry, Purdie Building, North Haugh, St Andrews, UK |
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| Abstract: | Alkaline‐earth (Ae=Ca, Sr, Ba) complexes are shown to catalyse the chemoselective cross‐dehydrocoupling (CDC) of amines and hydrosilanes. Key trends were delineated in the benchmark couplings of Ph3SiH with pyrrolidine or tBuNH2. Ae{E(SiMe3)2}2 ? (THF)x (E=N, CH; x=2–3) are more efficient than {N^N}Ae{E(SiMe3)2} ? (THF)n (E=N, CH; n=1–2) complexes (where {N^N}?={ArN(o‐C6H4)C(H)=NAr}? with Ar=2,6‐iPr2‐C6H3) bearing an iminoanilide ligand, and alkyl precatalysts are better than amido analogues. Turnover frequencies (TOFs) increase in the order Ca<Sr<Ba. Ba{CH(SiMe3)2}2 ? (THF)3 displays the best performance (TOF up to 3600 h?1). The substrate scope (>30 products) includes diamines and di(hydrosilane)s. Kinetic analysis of the Ba‐promoted CDC of pyrrolidine and Ph3SiH shows that 1) the kinetic law is rate=kBa]1amine]0hydrosilane]1, 2) electron‐withdrawing p‐substituents on the arylhydrosilane improve the reaction rate and 3) a maximal kinetic isotopic effect (kSiH/kSiD=4.7) is seen for Ph3SiX (X=H, D). DFT calculations identified the prevailing mechanism; instead of an inaccessible σ‐bond‐breaking metathesis pathway, the CDC appears to follow a stepwise reaction path with N?Si bond‐forming nucleophilic attack of the catalytically competent Ba pyrrolide onto the incoming silane, followed by rate limiting hydrogen‐atom transfer to barium. The participation of a Ba silyl species is prevented energetically. The reactivity trend Ca<Sr<Ba results from greater accessibility of the metal centre and decreasing Ae?Namide bond strength upon descending Group 2. |
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| Keywords: | alkaline-earth metals barium density functional calculations cross-dehydrocoupling reaction mechanisms |
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