Sequential Barium-Catalysed N-H/H-Si Dehydrogenative Cross-Couplings: Cyclodisilazanes versus Linear Oligosilazanes

Starting from Ph₃ SiH, the barium precatalyst Ba[CH(SiMe₃ )₂ ]₂ ⋅(THF)₃ was used to produce the disilazane Ph₃ SiN(Bn)SiPh₂ NHBn (4) by sequential N-H/H-Si dehydrogenative couplings with BnNH₂ and Ph₂ SiH₂ . Substrate scope was extended to other amines and hydrosilanes. This smooth protocol gives quantitative yields and full chemoselectivity. Compound 4 and the intermediates Ph₃ SiNHBn and Ph₃ SiN(Bn)SiHPh₂ were structurally characterised. Further attempts at chain extension by dehydrocoupling of Ph₂ SiH₂ with 4 instead resulted in cyclisation of this compound, forming the cyclodisilazane c-(Ph₂ Si-NBn)₂ (5) which was crystallographically authenticated. The ring-closure mechanism leading to 5 upon release of C₆ H₆ was determined by complementary experimental and theoretical (DFT) investigations. Ba[CH(SiMe₃ )₂ ]₂ ⋅(THF)₃ and 4 react to afford the reactive Ba{N(Bn)SiPh₂ N(Bn)SiPh₃ }₂ , which was characterised in situ by NMR spectroscopy. Next, in a stepwise process, intramolecular nucleophilic attack of the metal-bound amide on the terminal silicon atom generates a five-coordinate silicate. It is followed by turnover-limiting β-C₆ H₅ transfer to barium; this releases 5 and forms a transient [Ba]-Ph species, which undergoes aminolysis to regenerate [Ba]-N(Bn)SiPh₂ N(Bn)SiPh₃ . DFT computations reveal that the irreversible production of 5 through such a stepwise ring-closure mechanism is much more kinetically facile (ΔG^≠ =26.2 kcal mol^-1 ) than an alternative σ-metathesis pathway (ΔG^≠ =48.2 kcal mol^-1 ).