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摘要
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 tBuNH(2). Ae{E(SiMe3)(2)}(2)(THF)(x) (E=N, CH; x=2-3) are more efficient than {N boolean AND N}Ae{E(SiMe3)(2)}(THF)(n) (E=N, CH; n=1-2) complexes (where {N boolean AND N}(-)={ArN(o-C6H4)C(H)=NAr}(-) with Ar=2,6-iPr(2)-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 3600h(-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 (k(SiH)/k(SiD)=4.7) is seen for Ph3SiX (X=H, D). DFT calculations identified the prevailing mechanism; instead of an inaccessible sigma-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-N-amide bond strength upon descending Group2.
- 出版日期2016-3-18
