Oxidation potentials of allylsilanes

Allylsilanes and benzylsilanes are more susceptible to anodic oxidation than tetraalklsilanes and arylsilanes. It should also be noted that the oxidation potentials of allylsilanes and benzyl silanes are much less positive than those of... 

Table 3. Oxidation potentials of allylsilanes and olefins Compound Ep (V vs. Ag/AgCl)...

Table 2 Oxidation Potentials of Allylsilanes and Related Compounds...

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

The C-Sn a orbital also interacts with a neighboring nonbonding p orbital of heteroatoms, and this interaction decreases the oxidation potential significantly [128,129]. It should be noted that the oxidation potentials of a-heteroatom-substituted tetraorganos-tannanes are less positive than those of carbon nucleophiles such as allylsilanes and enol sily ethers. Consequently, such carbon nucleophiles can be used for the electrochemical oxidation of a -heteroatom-substituted tetraorganostannanes to achieve the carbon-carbon bond formation. In fact, various intermolecular carbon-carbon bond formation reactions have been developed using a-heteroatom-substituted tetraorganostannanes [130-132]. 

Acyliminium ions. Oxidative destannylation of A/-(a-stannylalkyl) amides leads to reactive electrophiles, which can react with enol silyl ethers, allylsilanes, allylstannanes, and trimethylsilyl cyanide. The stannyl group lowers the oxidation potential of the amidic nitrogen, without which the acyliminium ions do not form under the oxidation conditions. 

Benzylsilanes and allylsilanes are easily oxidized anodically compared with alkylsilanes and arylsilanes. Benzylsilanes exhibit irreversible cyclic voltammetric waves. It is notable that their oxidation potentials (Ep) are markedly less positive than those of the unsilylated parent compounds owing to the a-jr interaction (Table 3)10a. It is interesting that a-trimethylsilylation of xylenes markedly decreases their oxidation potential while additional a -trimethylsilylation makes a little change (Table 3). It has also been reported that a a, a-interacting system (the neighboring C—Si bonds) in addition to a a-ir interaction caused a significant decrease of the oxidation potentials1013. 

In an attempt to investigate the synthetic potential of PET bond cleavage reactions Mariano has explored the PET bond cleavage reactions of allylsilanes using electron deficient iminium salts as the photooxidant [36]. AllyltrimethylsUane is an efficient quencher of the fluorescent iminium salt excited singlet state with a rate near diffusion control [36], The photoexdted iminium salt can oxidize the... 

The synthetic potential of the C-Si bond cleavage from allyl- and benzylsilane radical-cations produced by photoreaction of electron-deficient iminium salts has been extensively investigated [25]. When a benzylsilane or allylsilane is irradiated in the presence of an iminium salt such as 27, a one-electron oxidation of the silane to the excited iminium salt produces a radical/radical-cation pair. Subsequent C-Si bond dissociation from the silane radical-cation by the loss of TMS+ leads to an... 

Allylsilanes are known to be more easily oxidized than the corresponding unsilylated olefins. However, introduction of a fluoroalkyl group to the 2-position of an allylsilane causes a large increase in its oxidation potential as shown in Table 7. This effect is quite different from those observed with fluorinated chalcogeno compounds and amines (Tables 5 and 6). 

Silylated 1,3-butadienes, e.g. (161), are potentially attractive reaction partners in [4 + 2] cycloadditions since the adducts are prone to various function izations of the allylsilane. For example, addition product (162) gave allylic cohol (163) on hydrolysis/oxidation (Scheme 39). However, addition of diene (161) to methyl acrylate is inefficient and largely devoid of regio- and stereo-chemical control. 

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