A- silyl-substituted

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

The Peterson olefination reaction involves the addition of an a-silyl substituted anion to an aldehyde or a ketone followed by the elimination of silylcarbinol either under acidic (awP -elimination) or basic (syn-elimination) conditions to furnish olefins178. Thus, Peterson olefination, just like Wittig and related reactions, is a method for regioselective conversion of a carbonyl compound to an olefin. Dienes and polyenes can be generated when the Peterson reaction is conducted using either an ,/l-unsaturated carbonyl compound or unsaturated silyl derivatives as reaction partners (Table 20)179. 

Owing to such orbital interactions, a-silyl substitution causes the decrease in the LUMO level of the 7t-system and enhances the electron accepting ability of the 7i-system. Therefore, the reduction potentials of a-silyl-substituted 7t-systems are less negative than those of the parent jr-systems, although the magnitude of this effect is not large. 

The effects of silyl groups on the chemical behavior of the anion radicals generated by cathodic reduction is also noteworthy. It is well known that silyl groups stabilize a negative charge at the a position. Therefore, it seems to be reasonable to consider that the anion radicals of re-systems are stabilized by a-silyl substitution. The interaction of the half-filled re orbital of the anion radical with the empty low-lying orbital of the silicon (such as dx-pK interaction) results in partial electron donation from the re-system to the silicon atom which eventually stabilizes the anion radical. 

It has been known for some time that the basicities of a heteroatom decrease upon a-silyl substitution [12], For example, alkyl silyl ethers (R3Si-0-R ) are less basic than dialkly ethers. Silylamines are weak bases compared to alkylam-ines. This electron-withdrawing effect of silyl groups has been explained in terms of the interaction between low lying vacant orbitals such as 3d orbitals of silicon or a orbitals with the nonbonding p orbitals (lone pairs) of the heteroatom (Fig. 4). This interaction decreases the HOMO level which in turn lowers the basicity of the heteroatom. Such effect may also cause the increase of the oxidation potentials, but little study has been reported on the electrochemical properties of this type of compounds. 

Although little information has been available for the effect of a-silyl substitution on the electrochemical properties of heteroatom compounds, extensive studies have been carried out on the effect of /f-silyl substitution [10,13]. For the -substituted heteroatom compounds (substitution at the a carbon), the introduction of a silyl group results in a significant decrease of the oxidation potentials, although the magnitude depends upon the nature of the heteroatom. This effect is explained in terms of the interaction between the C Si a orbital and the nonbonding p orbital of the heteroatom (Fig. 5). This interaction raises the HOMO level which in turn favours the electron transfer. 

Acylsilanes are a class of compounds in which a silyl group is directly bound to the carbonyl carbon, and they have received considerable research interest from the point of view of both physical organic and synthetic organic chemistry [15]. Acylsilanes have a structure quite similar to the structure of a-silyl-substituted ethers a silyl group is attached to the carbon adjacent to the oxygen atom, although the nature of the C-O bond is different. Therefore, one can expect /1-silicon effects in the electron-transfer reactions of acylsilanes. 

Although oxidation potentials of aldehydes and ketones are generally very high, silyl substitution at the carbonyl carbon results in a significant decrease in the oxidation potential [16]. The decrease in the oxidation potentials is attributed to the rise of the HOMO level by the interaction of the C Si cr-bond and the nonbonding p orbital (lone pair) of the carbonyl oxygen (Fig. 9). In the case of a-silyl-substituted ethers, the rotation around the C-O bond is free and,... 

Gas-phase studies of a-silyl substituted carbenium ions show that these intermediates exist only in a very flat potential well (5, 7, 8, 9 ). They undergo fast 1,2-H or -alkyl shifts, producing the more stable silicenium or p-silyl substituted carbenium ions. 

Scheme3.7 Formation of a silyl-substituted butatrienylidene iron complex by reaction of [FeCI(Cp )(dppe)j with trimethylsilyl butadiyne.

The first X-ray structure of an a-silyl-substituted carbocation (33) is reported its pXR+ value is predicted to be 4.73 The trimesitylsilylium cation is proposed to be a nearly free, tricoordinate species.74 The dimethylsilylium cation undergoes isomer interconversion via (34), according to high-level calculations the most stable structure is... 

The effect of a-silyl substitution on the stability of a carbenium ion was qualitatively unclear for a long time. Early solvolytic studies by the groups of liaborn36 and Cartledge37 suggest a destabilizing effect of a-silyl substitution compared with alkyl. The measurement and interpretation of the kinetic a-silicon effect in solvolysis reactions is, however, often complicated by the fact that steric and ground state effects may play an important role and that, in addition, the rates of ionization often involve a contribution from nucleophilic solvent assistance. 

The different reaction course followed by the silyl- and alkyl-substituted allenes is fully consistent with the hyperconjugation model, which predicts that a /i-silyl group stabilizes carbenium ions better than a /3-alkyl group, and with previous conclusions that a-silyl substitution in carbocations is destabilizing relative to a-methyl substitution. The larger space requirements of the trimethylsilyl groups compared with methyl groups may also contribute to the kinetic stability of 413. 

The fact that a-silyl substitution leads to a significant stabilization of carbanionic species is well-known and has been exploited in synthetic chemistry. On the other hand, silyl anions themselves are in general much more stable than their carbon analogues. The stabilization of carbanions by silyl substituents in the a position has been measured by Brauman and coworkers37. The anions were generated via nucleophilic displacement reactions (equation 2) of a silyl group with F- (see also Section III.B). 

The electron affinities of a number of a-silyl substituted silyl and carbon radicals were determined in photodetachment experiments and confirmed by data obtained from ab initio calculations. The authors conclude in this study that the stabilization a carbanion experiences through a-silyl substitution is approximately 14-20 kcalmol-1 per silyl group that of a silyl anion is approximately 6-14 kcal mol-1. The larger stabilization in the carbanionic systems is readily explained by stronger hyperconjugation of the anionic carbon center with the silyl groups as compared to that of the silyl anion with a silyl group. 

Whereas photolysis of cyclohexasilanes usually results in silylene elimination and the successive formation of cyclopentasilanes and cyclotetrasilanes79, attachment of the Fp group to the ring as in 162 results in rearrangement to a silyl-substituted cyclopentasilane, 163, revealing that the presence of the transition metal alters the chemistry of these systems... 

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