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

Abstract : Starting from Ph SiH, the barium precatalyst Ba[CH(SiMe ) ] ⋅(THF) was used to produce the disilazane Ph SiN(Bn)SiPh NHBn (4) by sequential N-H/H-Si dehydrogenative couplings with BnNH and Ph SiH . Substrate scope was extended to other amines and hydrosilanes. This smooth protocol gives quantitative yields and full chemoselectivity. Compound 4 and the intermediates Ph SiNHBn and Ph SiN(Bn)SiHPh were structurally characterised. Further attempts at chain extension by dehydrocoupling of Ph SiH with 4 instead resulted in cyclisation of this compound, forming the cyclodisilazane c-(Ph Si-NBn) (5) which was crystallographically authenticated. The ring-closure mechanism leading to 5 upon release of C H was determined by complementary experimental and theoretical (DFT) investigations. Ba[CH(SiMe ) ] ⋅(THF) and 4 react to afford the reactive Ba{N(Bn)SiPh N(Bn)SiPh } , 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-C H transfer to barium; this releases 5 and forms a transient [Ba]-Ph species, which undergoes aminolysis to regenerate [Ba]-N(Bn)SiPh N(Bn)SiPh . 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 ) than an alternative σ-metathesis pathway (ΔG =48.2 kcal mol ).