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 ).

Photoactivated silicon–oxygen and silicon–nitrogen heterodehydrocoupling with a commercially available iron compound

“…22 Poly(silyl ethers) are appealing compounds due their degradability in acidic and basic medium. 23 Molecules containing Si-N bonds such as silamines are well established as bases and silylating agents in organic syntheses, 24 while poly(silazanes) are sought after for their potential as ceramic precursors. 25 Although Si-O heterodehydrocoupling using 1 has not been reported, derivatives of 1 catalyzing similar transformations are known, most notably by Nakazawa and co-workers in Si-O and Si-S heterodehydrocoupling via Cp(CO)2FeMe.…”

Barium Siloxides and Catalysed Formation of Si−O−Si' Motifs

“…Ae complexes provide effective catalysts for these reactions. [16] They also enable the formation of silazanes through NÀH/HÀSi heterodehydrocoupling, [17][18][19][20] and that of borasiloxanes from BOÀH/HÀSi coupling. [6] Yet, metalc atalysts, and af ortioriA e-based ones, have seldom been implemented in the controlled production of siloxanes, despite the commercial importance of oligo-and polysiloxanes.…”

Contemporary Molecular Barium Chemistry

“…Later on, barium catalysis was implemented in polycondensations for the synthesis of relatively high molecular weight polycarbosilazanes, including ferrocene‐containing polymers . A stepwise approach via alternating introduction of ‐(NR)‐ and ‐(SiR' 2 )‐ motifs was also attempted, but failed due to the propensity of ‐(RN‐SiR' 2 )‐ units to generate stable four‐member cyclo ‐disilazanes …”