title: Alkaline-Earth-Catalysed Cross-Dehydrocoupling of Amines and Hydrosilanes: Reactivity Trends, Scope and Mechanism.
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 Ph3 SiH 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=k[Ba](1) [amine](0) [hydrosilane](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.
authors: Organometallics: Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, 35042, Rennes, France.
authors: Clément
authors: C
authors: Bellini
authors: Centre de Diffractométrie des Rayons X, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, 35042, Rennes, France.
authors: Vincent
authors: V
authors: Dorcet
authors: Organometallics: Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, 35042, Rennes, France. jean-francois.carpentier@univ-rennes1.fr.
authors: Jean-François
authors: JF
authors: Carpentier
authors: University of St Andrews, School of Chemistry, Purdie Building, North Haugh, St Andrews, KY16 9ST, UK. st40@st-andrews.ac.uk.
authors: Sven
authors: S
authors: Tobisch
authors: Organometallics: Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes 1, Campus de Beaulieu, 35042, Rennes, France. yann.sarazin@univ-rennes1.fr.
authors: Yann
authors: Y
authors: Sarazin
mesh_terms: D000586:Amination; D000588:Amines / Q000737:chemistry*; D002384:Catalysis; D007700:Kinetics; D008024:Ligands; D008673:Metals, Alkaline Earth / Q000737:chemistry*; D008958:Models, Molecular; D058105:Polymerization; D012821:Silanes / Q000737:chemistry*
chemical_list: D000588:Amines; D008024:Ligands; D008673:Metals, Alkaline Earth; D012821:Silanes
keywords: alkaline-earth metals; barium; cross-dehydrocoupling; density functional calculations; reaction mechanisms