Trans-silylation

Attack of excess methanol, in the presence of bases or acids, on trimethylsilyl-oxy ethers such as 13 or on dimethylsilyloxy compounds such as 50 during trans-silylations (cf the preceding section) probably also proceeds via transition states such as 73 or 74 to afford the free alcohols or phenols 11 and methoxytrimethylsi-lane 13a. 

Silylation-amination of 6-acetoxymethyl-5-deazapterine 265 with NH3, HMDS 2, and TsOH for 120 h at 155-160°C in an autoclave affords, after subsequent trans-silylation with boiling methanol, the diamino compound 266 in 74% yield [76]. Silylation-amination-cychzation of the substituted 4-quinolone 267 gives the alkaloid isoaptamine hydrochloride 268 in 51% yield [77, 78] (Scheme 4.30). 

Silylative Coupling (trans-Silylation) of Alkenes with Vinylsilanes. 207... 

To identify the stereochemical course of the protonation of the vinyl carbon, cis and trans silyl enol ethers derived from menthone were isomerized by use of a deuter-ated achiral proton source. Surprisingly, only the identical syn isomer was obtained from both the silyl enol ethers. Thus reaction of the cis isomer occurs via an anti Se mechanism whereas reaction of the trans isomer occurs via a syn Se mechanism. Interestingly, this cis silyl enol ether was isomerized more rapidly than the trans isomer. In the cis silyl enol ether, deuterium was located at a psewdo-axial position in the isomerized product. Therefore, the anti-S pathway can be explained by the product developing control via the product-like transition state assembly. The syn-S pathway for the trans silyl enol ether can be explained by substrate control via the favored intermediate. The relative contributions of the two pathways depend on the relationship between the free energies of their transition state assemblies (Sch. 8). 

The two reactions catalyzed by ruthenium complexes, i.e. silylative coupling (SC) (trans-silylation) catalyzed by I, II, V, and VI and cross-metathesis (CM) (catalysts III and IV) of vinyl- and allyl-substituted hetero(N,S,B)organic compounds with commercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxanes provide a universal route toward the synthesis of well-defmed molecular compounds with vinylsilicon functionality. 

Under the optimum conditions the reaction stereo- and regioselectively gives silylene-vinylene, siloxylene-vinylene [131], and silazanylene-vinylene [128, 129, 131] as well as silylene-vinylene-phenylene [132, 133] oligomers. This new, efficient route for preparation of organosilicon linear and cyclic oligomers as an extension of the silylative coupling trans silylation) of alkenes to dienes has recently been overviewed separately [131]. 

PhMeaSiLi, THE, 0°C, 3-6 h, 72-83% yield. Primary tosylates fail to react and tosylaziridines ring open to give trans silyl sulfonamides. ... 

The resulting alkoxide attacks the silicon, resulting in a trans-silylation. 

Summary Recent achievements in two catalytic reactions, i.e., silylative coupling and cross-metathesis of alkenes and dienes with vinyl-silicon compounds, which resulted in new synthetic routes to organosilicon molecular and macromolecular compounds are presented. The silylative coupling, also called dehydrogenative or trans-silylation and silyl group transfer, is catalyzed by metal complexes which either contain or initiate the formation of M-H and M-Si bonds, where M = Ru, Rh, Co and Ir. Cross-metathesis, which was developed very recently, proceeds in the presence of metallacarbenes, mainly those of rathenium (e.g., Grabbs catalyst). 

In the past decade two new catalytic reactions occurring between the same parent substances have been developed, namely silylative coupUng (trans-silylation) (Eq. 1) and cross-metathesis of alkenes (Eq. 2) with vinylsilicon compounds. 

The reaction of trans-silylation can be used for effective functionalization of 1,3,5-trivinylsilylbenzene, vinylcyclosiloxanes, vinylcyclosilazanes and vinyloctasilsesquioxane. 

The catalytic inactivity of metallacarbene species e.g. Schrock catalyst [32] and Grubbs complex RuCl2(PPh3)(CHPh) in metathesis of vinyl-trisubstituted silanes and siloxanes also supports such a mechanism. This reaction is also called homo(hetero)coupling or trans-silylation of olefins with vinylsilanes. 

A method for the preparation of halosilyl carbamates and isocyanate involves direct synthesis from amines via halosilyl carbamate intermediates [277]. A primary amine is converted to its carbamic acid salt, which is then treated with a silane containing > 2 halogen atoms bonded to Si. Gentle heating of the resulting halosilyl carbamate gives the isocyanate. In an alternative (exchange) procedure, the carbamic acid salt is treated with any halosilane to form a silyl carbamate, which is trans-silylated. 

Various imidazolylsilane derivatives are synthesized by a trans -silylation procedure between l-(trimethyl)imidazoles and MeSiCls (eq 23). ... 

The amide prepared by the trans-silylation of BSA with ClCH2SiMe2Cl has been shown not to be a AA-bis-silyl acetamide but to possess the intramolecularly co-ordinated five-membered heterocycle (59) derived from AA-bis(chloro-dimethylsilylmethyl) acetamide, with no Si—N bonds. The series of amides R CONR CCHaSiMegCl) were prepared similarly, and likewise are thought to involve intramolecular 0->Si co-ordination. ... 

Figure 5.2 shows the assumed transition state in the asymmetric aldol reaction with the activation of the aldehyde by the loaded tin complex of 176a. As tin(II) can adopt a fivefold coordination, it uses one of the vacant d-orbitals to coordinate with the aldehyde, thus activating it and forming the five-coordinated complex while still maintaining the rigid bicyclic backbone. The trans-silyl ketene acetal approaches the aldehyde from the 5i-face via an acyclic transition state, as postulated by the authors [91]. 

Hydrosilylation of vinylmethylsilane with chlorosilanes has been reported using catalytic amounts of tetracarbonyl(Ti -trimethylsilylethene)iron. A product mixture consisting of isomeric disilylethanes, monosilylethane, and disilylated ethene was obtained. Acrolein diethyl acetal can be hydrosilylated with triethylsilane in the presence of 1 mol% pentacarbonyliron to give the corresponding Markovnikov product (Scheme 4-335). Allyl alcohols afford a mixture of the allyl silyl ether and the alltyl silyl ether, whereas acrolein leads to a mixture of cis and trans silyl enol ethers. ... 

Benzothiazolyl-lithium (70), prepared from benzothiazole and n-butyl-lithium in THF at —80 C, reacts in situ with the appropriate chlorosilanes to afford mono- (71) and di-(2-benzothiazolyl)silanes (72) (see also these Reports, Vol. 2, p. 672). The latter are also formed in higher yields from (71) by a novel trans-silylation reaction. The assigned structures of the silylbenzothiazoles are in agreement with their observed n.m.r. spectra. The compounds are colourless, stable substances, of considerable reactivity. 

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