Sequential Barium-Catalysed N-H/H-Si Dehydrogenative Cross-Couplings: Cyclodisilazanes versus Linear Oligosilazanes.
Starting from Ph3SiH, the barium precatalyst Ba[CH(SiMe3)2]2⋅(THF)3was used to produce the disilazane Ph3SiN(Bn)SiPh2NHBn (4) by sequential N-H/H-Si dehydrogenative couplings with BnNH2and Ph2SiH2. Substrate scope was extended to other amines and hydrosilanes. This smooth protocol gives quantitative yields and full chemoselectivity. Compound 4 and the intermediates Ph3SiNHBn and Ph3SiN(Bn)SiHPh2were structurally characterised. Further attempts at chain extension by dehydrocoupling of Ph2SiH2with 4 instead resulted in cyclisation of this compound, forming the cyclodisilazane c-(Ph2Si-NBn)2(5) which was crystallographically authenticated. The ring-closure mechanism leading to 5 upon release of C6H6was determined by complementary experimental and theoretical (DFT) investigations. Ba[CH(SiMe3)2]2⋅(THF)3and 4 react to afford the reactive Ba{N(Bn)SiPh2N(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-C6H5transfer to barium; this releases 5 and forms a transient [Ba]-Ph species, which undergoes aminolysis to regenerate [Ba]-N(Bn)SiPh2N(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).
Organometallics, Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes 1, Campus de Beaulieu, 35042, Rennes, France.
Clément
C
Bellini
Centre de Diffractométrie des Rayons X, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes 1, 35042, Rennes, France.
Thierry
T
Roisnel
Organometallics, Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes 1, Campus de Beaulieu, 35042, Rennes, France. jean-francois.carpentier@univ-rennes1.fr.
Jean-François
JF
Carpentier
School of Chemistry, University of St Andrews, Purdie Building, North Haugh, St Andrews, KY16 9ST, UK. st40@st-andrews.ac.uk.
Sven
S
Tobisch
Organometallics, Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes 1, Campus de Beaulieu, 35042, Rennes, France. yann.sarazin@univ-rennes1.fr.
Yann
http://orcid.org/0000-0003-1121-0292
Y
Sarazin
H−Si coupling; N−H coupling; barium; cyclisation; sequential chain extension; silazanes