Reaction silyl migration mechanism

Transition metal carbonyls such as Co2(CO)8 and CoH(CO)4, formed in the reaction of R3SiH with dimer (but also Fe(CO)5 and M3(CO)i2 (M = Fe, Ru, Os)) have been found to be active catalysts for the hydrosilylation of olefins, dienes, unsaturated nitriles, and esters as well as for hydrosilylation C=0 and C=N bonds [56]. Hydrosilylation of phenylthioacetylenes in the presence of this catalyst is extremely regioselective [57]. Cobalt(I) complexes, e. g., CoH(X)2L3 (X = H, N), could be prospective candidates for investigation of the effectiveness of alkene hydrosilylation by trialkoxysilanes as well as dehydro-genative silylation [58]. Direct evidence for the silyl migration mechanism operative in a catalytic hydrosilylation pathway was presented by Brookhart and Grant [59] using the electrophilic Co cationic complex. 

A study of the mechanism of the reaction of 2-silylthiazole (65) with formaldehyde has concluded that the reaction occurs via the initial fast formation of an N-(silyloxymethyl)thiazolium-2-ylide (66) followed by a rate determining second addition of formaldehyde to give (67). This is followed by a fast 1,6-silyl migration and loss of a molecule of formaldehyde to give the final product (68) <96JOC1922>. 

This reaction bears a close similarity to the one discovered by J.F. Klebe who found that acid amides with dialkyldichlorosilanes yield disilaoxadiazines (2). The interesting intramolecular mobility of these compounds was studied by 11NMR spectroscopy and the mechanism of internal silyl migrations within their molecules arose some discussion in the literature (3,4). 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

As illustrated in Eq. (62), the photochemical isomerization of cyclotrisilene 48 to 88 is rationalized also by a mechanism including 1,2-silyl migration. Whereas three 1,2-silyl migration pathways are possible in this system (paths a-c), only path a leads to 88 via biradical 139 path b is an identity reaction and path c may lead to a minor product 138. 

Recently, Barton and coworkers investigated the mechanism of the 1,2-silyl migration in a related system through a combination of experiment and theory40. Pyrolysis of 12 at 600 °C cleanly produced a mixture of 12 and methylenedisilacyclopentene 13 (25%) (equation 12). A kinetic study of this reaction was conducted over the temperature range of 520-600 °C in a stirred flow reactor. The Arrehnius parameters for the first order formation of 13 were logA = 12.5 s-1 and Ea = 54 kcalmol-1. In the pyrolysis of a related all-carbon system 14, decomposition occurred at 550 °C but no isomerization to the methylene cyclopentene 15 was observed up to 700 °C (equation 13). 

Corey and coworkers reported the reactions of a-siloxyketones 128 with trimethylsi-lyllithiums in the presence of HMPA which gave the corresponding silyl enol ethers 130 (equation 86). The elimination of a-siloxy groups was proposed to occur via the 1,4-silyl migration followed by the Peterson elimination. In accord with this mechanism, the intermediate 129 was trapped by hydrolysis202. 

The generally accepted mechanism of the classic Danheiser annulation involves three basic steps the Lewis acid-catalyzed electrophilic combination of the a, 0-unsaturated ketone with the silylallene, a 1,2-sp-silyl migration, and a final cyclization step. This mechanism was first proposed by Danheiser in the original publication of the annulation and has been generally accepted but has never been formally investigated. A more detailed account of the reaction pathway is shown below. Treatment of the a,y5-unsaturated ketone 1 with TiCU produces a titanium complex existing as two resonance-stablized cations 26 and 27. Attack of the 2,3-7c-bond of the... 

These careful studies supported the mechanism shown for 8 —> 12. The first order kinetics in silylcarbinol were consistent with intramolecular C - O silyl migration. Reaction via 10 as a cyclic transition state, with the conjugate acid of the base remaining nearby, would account for the large negative entropy of activation. The Hammett studies indicated a transition state with buildup of carbanionic character in proceeding toward product. 

The reaction of silylborane with 1-halo-l-lithio-l-alkenes yields 1-boryl-l-silyl-l-alkenes via borate formation followed by 1,2-migration of silyl group (Equation (90)).76,240 The mechanism seems to be closely related to that proposed for the silaboration of isocyanide (Figure 2). Vinyl-substituted carbenoids, l-chloro-l-lithio-2-alkenes, react with silylpinacolborane to give l-boryl-l-silyl-2-alkanes in good yield (Equation (91)).241 This methodology is applied to the synthesis of l-boryl-l-silyl-2-cyclobutene.2 2 Similar reactions are carried out with other carbenoid... 

Figure 2 Plausible mechanism of the silyl group migration in the reaction of silylborane with carbenoid.

Products of the type (24) also result from enolizable ketones without the formation of silyl enol ethers if the reaction is carried out in the presence of tertiary phosphines. The proposed mechanism involves the betaine R3P—SiMe2 as the silylene transfer agent. In preventing a 1,3-hydrogen migration, the phosphine may well induce dimerization prior to oxasilacyclopropane formation. The dioxadisilacyclohexane (24) can be reduced with LiAIHU to give dimethylsilyl-substituted carbinols, so the reaction is of synthetic value (Scheme 34) (78JA7074). 

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