Allylsilanes anodic

In contrast to the case of allylsilanes, anodic oxidation of disubstituted olefins provides in general four regioisomeric products because all the allylic carbon-hydrogen bonds can be cleaved. In the case of allylsilane, the cleavage of a C—Si bond takes place... 

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). 

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... 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

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 intramolecular coupling of enolethers with enolethers, styrenes, alkyl-substituted olefins, allylsilanes, and vinylsilanes was systematically studied by Moeller [69]. Many of these coupling reactions turned out to be compatible with the smooth formation of quaternary carbon atoms (Eq. 11) [70], which were formed diastere-oselectively and led to fused bicyclic ring skeletons having a ds-stereochemistry [71]. The cyclization is compatible with acid-sensitive functional groups as the allylic alkoxy group. Moeller has demonstrated in some cases that these reactions can be run without loss of selectivity and yield in a simple beaker with either a carbon rod or reticulated carbon as anode without potential control and a 6-V lantern battery as power supply [71]. 

Different nucleophiles such as methanol, allylsilanes, silyl enol ethers, trimethylsilyl-cyanide, and arenes can be used in this process [62]. When the sulfide itself contains an unsaturated or aromatic fragment and the process is carried out in the absence of a nucleophile, an intramolecular anodic sub-stitution/cyclization might occur [61-63]. Methyl esters of 2-benzothiazolyl-2-alkyl or aryl-acetic acid, oxidized in MeOH/Et4 NCIO4 or H2SO4 in the presence of CUCI2, form 2,2-dimethoxy products (Eq. 7) [64]. 

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. 

Anodic oxidation of allylsilanes in the presence of nucleophiles results in replacement of a trimethylsilyl group by a nucleophile which is introduced into the allylic carbon. Various oxygen and nitrogen nucleophiles such as alcohols, water, caboxylic acids, p-toluenesulfonic acid, carbamates or a sulfonamide can be employed in this reaction (equations 9—11)11 —13. 

It should be noted that in the case of geranyltrimethylsilane, the carbon-carbon double bond of the allylsilane moiety is oxidized selectively to give two regioisomeric products (equation ll)13. This is due to the electron-donating effect of the /i-silyl group. Since a methoxylated silane was formed in the anodic oxidation of an allylsilane as shown in equation 12, the reaction mechanism can be illustrated as in equation 1313. 

Silyl-l,3-dienes undergo anodic methoxylation in methanol to give 1,4-addition products with an allylsilane structure as intermediates. Therefore, they are further oxidized to give l,l,4-trimethoxy-2-butene derivatives as the final products. The products are easily hydrolyzed to provide the corresponding y-methoxy-a, /t-unsaUirated aldehydes. Since 1-trimethylsilyl-l,3-dienes are readily prepared by the reaction of the anion of l,3-bis(trimethylsilyl)propene with aldehydes or ketones, l,3-bis(trhnethylsilyl)propene offers a, /i-formylvinyl anion equivalent for the reaction with carbonyl compounds (equation 15)16. 

Anodic oxidation of a mixture of allylsilane and cyclic 1,3-diketones in the presence or absence of oxygen provides cyclic peroxides (equation 16) and dihydrofuran (equation 17) derivatives, respectively17,18. In these reactions, the cyclic diketones are discharged to... 

Organosilicon compounds bearing heteroatoms generally undergo anodic substitutions with the elimination of a silyl group in a manner similar to that observed for benzylsilanes and allylsilanes as shown in equation 20. 

Intramolecular anodic oxidation of allylsilane moiety with enol ether moiety gives the corresponding cyclized product 110 (equation 84)152. 

Recently, Yoshida and colleagues reported the anodic oxidation of monothioacetal 12 in the presence of allylsilane 13, and the results... 

Similarly, at a carbon anode in 1 1 MeOH-THF, anodic cyclization of allylsilane enol ether (XCIX) proceeded stereoselectively to give (C) [Eq. (63)]. The use of allyl silanes as the unsaturated nucleophilic component in such radical-cation cyclizations proved to be beneficial, though the exact mechanistic details remain somewhat speculative [147]. The method represents an improvement over earlier methods involving anodic cyclization of alkenyl-substituted enol acetates [148]. 

The anodic oxidation of heteroatom compounds having a silyl group and a stannyl group on the same a-carbon has been examined [133]. The anodic oxidation in the presence of the allylsilane led to the selective cleavage of the C-Sn bond and the introduction of the allyl group on the carbon [Eq, (32)]. Probably the C-Sn a orbital interacts more... 

Facile 6-elimination of the silyl group is also utilized in the intramolecular anodic olefin coupling reactions [159-161]. For example, the intramolecular anodic coupling of enol ether with allylsilane group has been reported [Eq. (44)]. This reaction seems to be quite useful for the construction of functionalized cyclic compounds because it leads to the regioselective formation of olefinic product via a facile 6-silyl elimination. 

The coupling of an allyl or acyl moiety onto carbon atoms is achieved by anodic oxidation of a-heteroatom substituted organostannanes or Oj -acetals in the presence of allylsilanes or silyl enol ethers. The reaction probably involves carbocations as intermediates that undergo electrophilic addition to the double bond [245c]. 

Moeller and coworkers [355-358] studied intramolecular anodic coupling of bisalkyl and bissilyl enol ethers, and enol ethers having a vinylsilane or allylsilane group. They found that cw-isomers about the ring juncture were preferentially or predominantly formed ... 

For example, the anodic oxidation of a silyl-substituted carbamate to generate a solution of N-acyliminium ion and the cathodic reduction of cinnamyl chloride in the presence of chlorotrimethylsilane to generate the corresponding allylsilane can be carried out simultaneously in a single electrochemical microflow cell under continuous flow conditions (Figure 5.10). The N-acyliminium ion, the anodic product, is allowed to react with the allylsilane, the cathodic product, to give the coupling product. 

The silyl group and ArS groups are effective for the oxidation of the benzylic position. The anodic oxidation of benzylic sulfides in the presence of allylsilanes takes place smoothly, giving rise to selective C-S bond cleavage and introduction of an allyl group on the benzylic carbon Eq. 11 [15]. 

The direct anodic coupling reaction using simple alkyl-substituted olefins as substrates usually generates a complex product mixture. The high reactivity of the intermediate alkyl radicals causes a lack of selectivity [20, 21]. An elegant way to achieve regiochemical control is the use of allylsilanes as reaction partners [21—25]. In the course of the cycUzatiOTi reaction, a distonic radical cation 9 is formed (Scheme 3), whereby the... 

Moeller KD, Hudson CM (1991) Intramolecular anodic olefin coupling reactions the use of allylsilanes. Tetrahedron Lett 32 2307-2310... 

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