- Custom Hydrosilane Synthesis Based on Monosilane
-
The omnipresence of silicon compounds with carbon substituents in synthetic chemistry hides the fact that, except for certain substitution patterns at the silicon atom, their preparation is often far from trivial. The challenge is rooted in the lack of control over nucleophilic substitution with carbon nucleophiles at silicon atoms with three or four leaving groups. For example, SiCl4 usually converts into intractable mixtures of chlorosilanes, typically requiring several distillation cycles to reach high purity. Accordingly, there is no universal approach to silanes with heteroleptic substitution. Here, using a bench-stable SiH4 surrogate, we introduce a general strategy for the on-demand synthesis of silicon compounds decorated with different aryl and alkyl substituents. Reliable protocols are the basis of the selective and programmable synthesis of dihydro- and monohydrosilanes; aryl-substituted trihydrosilanes are also accessible in a straightforward fashion. These otherwise difficult-to-access hydrosilanes are only three or fewer easy synthetic operations away from the SiH4 surrogate. Synthesizing silicon compounds with different carbon substituents from inorganic silicon precursors, i.e., basic silicon chemicals with hydrogen, halogen, or alkoxy substitution, is an intricate and often insoluble task. It is generally difficult to chemoselectively address one of these groups in chemical reactions, particularly when two or more of those are identical. Complicated separation and purification procedures are the result. The challenge of making these silicon compounds containing silicon–carbon bonds, typically hydro- and chlorosilanes, is accentuated considering their high demand in academia and industry. The present approach is a step forward in solving those limitations. It hinges on the stepwise decoration of the silicon atom of a liquid monosilane surrogate. Further development of this strategy and adjusting it to industrial needs could pave the way to easy access of an even more diverse manifold of silicon compounds for synthetic chemistry and material science. Oestreich and colleagues present an approach to the chemoselective stepwise preparation of hydrosilanes with the general formula R4–nSiHn where n = 1–3 and R can be different aryl and alkyl groups. The starting point is a bench-stable SiH4 surrogate with two Si–H bonds masked as cyclohexa-2,5-dien-1-yl substituents. A sequence of palladium-catalyzed Si–H arylation and B(C6F5)3-promoted deprotection and transfer hydrosilylation enables the programmable synthesis of hydrosilanes, even with three different substituents at the silicon atom.
- Yuan, Weiming,Smirnov, Polina,Oestreich, Martin
-
-
Read Online
- Catalytic and stoichiometric reactivity of β-silylamido agostic complex of Mo: Intermediacy of a silanimine complex and applications to multicomponent coupling
-
The reaction of complex (ArN=)2Mo(PMe3)3 (Ar = 2,6-diisopropylphenyl) with PhSiH3 gives the β-agostic NSi-H ...Msilyamido complex (ArNd)Mo(SiH2Ph) (PMe3)- (η3-ArN-SiHPh-H) (3) as the first product. 3 decomposes in the mother liquor to a mixture of hydride compounds, including complex {η3-SiH(Ph)-N(Ar)-SiHPh-H ... }MoH 3(PMe3)3 characterized by NMR. Compound 3 was obtained on preparative scale by reacting (ArN=)2Mo(PMe 3)3 with 2 equiv of PhSiH3 under N2 purging and characterized by multinuclear NMR, IR, and X-ray diffraction. Analogous reaction of (Ar′N=)2Mo(PMe3)3 (Ar′ = 2,6-dimethylphenyl) with PhSiH3 affords the nonagostic silylamido derivative (Ar′N=)Mo(SiH2Ph)(PMe3) 2(NAr′{SiH2Ph}) (5) as the first product. 5 decomposes in the mother liquor to a mixture of {η3-PhHSi- N(Ar′)-SiHPh-H ... }MoH3(PMe3)3, (Ar′N=)Mo(H)2(PMe3)2(η2- Ar′N=SiHPh), and other hydride species. Catalytic and stoichiometric reactivity of 3 was studied. Complex 3 undergoes exchange with its minor diastereomer 3′ by an agostic bond-opening/closing mechanism. It also exchanges the classical silyl group with free silane by an associative mechanism which most likely includes dissociation of the Si-H agostic bond followed by the rate-determining silane σ-bond metathesis. However, labeling experiments suggest the possibility of an alternative (minor) pathway in this exchange including a silanimine intermediate. 3 was found to catalyze dehydrogenative coupling of silane, hydrosilylation of carbonyls and nitriles, and dehydrogenative silylation of alcohols and amines. Stoichiometric reactions of 3 with nitriles proceed via intermediate formation of η2- adducts (ArN=)Mo(PMe3)(η2-ArN=SiHPh) (η2-NtCR), followed by an unusual Si-N coupling to give (ArN=)Mo(PMe3)(κ2-NAr-SiHPh-C(R)=N-). Reactions of 3 with carbonyls lead to η2-carbonyl adducts (ArN=) 2Mo(OdCRR0)(PMe3) which were independently prepared by reactions of (ArN=)2Mo(PMe3)3 with the corresponding carbonyl OdCRR′. In the case of reaction with benzaldehyde, the silanimine adduct (ArN=)Mo(PMe3)(η2-ArN=SiHPh)- (η2-O=CHPh) was observed by NMR. Reactions of complex 3 with olefins lead to products of Siag-C coupling, (ArN=)Mo(Et)(PMe 3)(η3-NAr-SiHPh-CH=CH2) (17) and (ArN=)Mo(H)(PMe3)(η3-NAr-SiHPh-CH=CHPh), for ethylene and styrene, respectively. The hydride complex (ArN=)Mo(H)(PMe 3)(η3-NAr-SiHPh-CH=CH2) was obtained from 17 by hydrogenation and reaction with PhSiH3. Mechanistic studies of the latter process revealed an unusual dependence of the rate constant on phosphine concentration, which was explained by competition of two reaction pathways. Reaction of 17 with PhSiH3 in the presence of BPh3 leads to agostic complex (ArN=)Mo(SiH2Ph)(η3-NAr-Si(Et)Ph-H) (η2-CH2=CH2) (24) having the Et substituent at the agostic silicon. Mechanistic studies show that the Et group stems from hydrogenation of the vinyl substituent by silane. Reaction of 24 with PMe 3 gives the agostic complex (ArN=)Mo(SiH2Ph)(PMe 3)(η3-NAr-Si(Et)Ph-H), which slowly reacts with PhSiH3 to furnish silylamide 3 and the hydrosilylation product PhEtSiH2. A mechanism involving silane attack on the imido ligand was proposed to explain this transformation.
- Khalimon, Andrey Y.,Simionescu, Razvan,Nikonov, Georgii I.
-
p. 7033 - 7053
(2011/06/25)
-