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 ).