Silyl radical with lithium

The reduction of Cl3SiC6H2-2,4,6-Ph3 with lithium metal in the presence of trimethylsilyl chloride led to the isolation of a silafluorene compound that was formed by an intramolecular ring closure.100 Although the mechanism is unknown, it is believed that reduction affords a silyl radical, which may... 

In the reduction of conjugated dienes cis 1,4-addition of trimethyl-chlorosilane to give c -l,4-bis(trimethylsilyl)-2-butene is favored with sodium in THF, lithium naphthalide in THF, and with lithium in diethylether. It appears that the anion radical, which in nonionizing solvents should exist in a cis configuration, leads to the cis 1,4-addition of the silyl groups, whereas the dianions produced by further reduction lead to trans products (140). 

More recently Ito and Mikami observed" that the titanium reagent is not indispensable in the case of highly basic lithium enolates derived from cyclic ketones or silyl enolates, for which the reactions are very fast (ending in ca Is to 5 min) and afford high yields of the a-trifluoromethylation products. A tentative radical mechanism has been proposed for these processes" involving reaction of the Mef free radical with the enolate to give a radical intermediate which reacts with another MefI molecule to afford Mef and the a-trifluoromethylation product (equation 42). 

Lithium aluminium hydride reduction, with inversion of the chlorosilane formed yielded the hydrosilane ([a]D + 20°). Assuming a neat inversion in the reduction step, it appeared that the formation and trapping of the silyl radical occurred with 65% retention. 

Unlike carbanions and silyl anions, boryl anions (BR2"), in the form of boryllithiums, have been unknown until recently. In 2006, Japanese researchers reported the synthesis of a boryllithium via the reduction of a cyclic (diamino)bromoborane with lithium/naphthalene in THF, that is, Li+Np , where Np is the anion radical of naphthalene (Segawa, Y. Yamashita, M. Nozaki, K. Science 2006, 314, 113-115) ... 

Radical anions from aromatics which are intermediates in Birch-type reductions were prepared sonochemically. Pyridine, quinoline, and indole sonicated with lithium in THF in the presence of trimethylsilyl chloride yield the bis-TMS dihydroaromatics, which can be reoxidized, by air or benzoquinone, in a rapid and easy method to prepare silyl-substituted aromatics. The procedure was extrapolated to phenols (Eq. 6). ... 

Synthesis of Biaryls. When TBDPS protected 2-bromobenzyl alcohol is subjected to standard radical conditions, the initially formed aryl radical reacts with one phenyl ring of the TBDPS group at its ipso position, and the resulting intermediate further transforms into a silyl radical, which then reduced to the corresponding silane. After this phenyl migration, desilylation by methyl lithium eventually provides the biphenyl product (eq 10). Other phenyl silyl ethers of this type can also experience the same aryl migration and produce biaryls. 

The silyl radicals can easily and quantitatively undergo one-electron oxidation or reduction to form the corresponding cation and anion species (10). The silyl radical-based batteries have demonstrated remarkable efficiency with negligible loss of high power density. Importantly this electrochemical energy storage system employing silyl radical does not require any metals such as lithium (3). 

Silyl enol ethers have also been used as a trap for electrophilic radicals derived from a-haloesters [36] or perfluoroalkyl iodides [32]. They afford the a-alkylated ketones after acidic treatment of the intermediate silyl enol ethers (Scheme 19, Eq. 19a). Similarly, silyl ketene acetals are converted into o -pcriluoroalkyl esters upon treatment with per fluoro alkyl iodides [32, 47]. The Et3B/02-mediated diastereoselective trifluoromethylation [48,49] (Eq. 19b) and (ethoxycarbonyl)difluoromethylation [50,51] of lithium eno-lates derived from N-acyloxazolidinones have also been achieved. More recently, Mikami [52] succeeded in the trifluoromethylation of ketone enolates... 

---