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Alkaline‐Earth‐Catalysed Cross‐Dehydrocoupling of Amines and Hydrosilanes: Reactivity Trends, Scope and Mechanism
Author(s) -
Bellini Clément,
Dorcet Vincent,
Carpentier JeanFrançois,
Tobisch Sven,
Sarazin Yann
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201504316
Subject(s) - chemistry , reactivity (psychology) , nucleophile , medicinal chemistry , pyrrolidine , hydride , phenylsilane , metathesis , alkyl , ligand (biochemistry) , amine gas treating , oxidative addition , stereochemistry , catalysis , polymerization , hydrogen , organic chemistry , medicine , biochemistry , polymer , alternative medicine , receptor , pathology
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 Ph 3 SiH with pyrrolidine or t BuNH 2 . Ae{E(SiMe 3 ) 2 } 2 ⋅ (THF) x (E=N, CH; x =2–3) are more efficient than {N^N}Ae{E(SiMe 3 ) 2 } ⋅ (THF) n (E=N, CH; n =1–2) complexes (where {N^N} − ={ArN( o ‐C 6 H 4 )C(H)=NAr} − with Ar=2,6‐ i Pr 2 ‐C 6 H 3 ) bearing an iminoanilide ligand, and alkyl precatalysts are better than amido analogues. Turnover frequencies (TOFs) increase in the order Ca30 products) includes diamines and di(hydrosilane)s. Kinetic analysis of the Ba‐promoted CDC of pyrrolidine and Ph 3 SiH 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 ( k SiH / k SiD =4.7) is seen for Ph 3 SiX (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