Indole Birch reduction

Birch reduction of indole with lithium metal in THF in the presence of trimethylsilyl chloride followed by oxidation with p-benzoquinone gave l,4-bis(trimethylsilyl)indoIe (106). This is readily converted in two steps into l-acetyl-4-trimethylsilylindole. Friedel-Crafts acylation of the latter compound in the presence of aluminum chloride yields the corresponding 4-acylindole (107) (82CC636). 

The ready electron availability of indoles means that they are rather easily (aut)oxidised in the flve-membered ring. Reductions can be made selective for either ring in acid solution, dissolving metals attack the hetero-ring, and the benzenoid ring can be selectively reduced by Birch reduction conditions. 

Even though a Birch reduction to give the requisite dihydronaphthalene for the Plieninger indolization is obviously successful, it is not an attractive or general method for providing these dihydronaphthalenes. In 1995, Kerr combined a high-pressure Diels-Alder cycloaddition of quinone mono ketals with 1,3-butadienes to give cycloadducts that readily aromatize to 1,4-dihydronaphthalenes (Scheme 3,... 

Because the reactions of related in -cyclohexadienyl complexes are synthetically valuable, the reactions of this ligand have been studied extensively. An outline of how this chemistry can be conducted on the Fe(CO)j fragment is shown in Equation 11.51. A variety of cyclohexadienes are readily available from Birch reduction of substituted aromatics. Coordination and abstraction of a hydride, typically by trityl cation, leads to cationic cyclohexadienyl complexes. These cyclohexadienyl complexes are reactive toward organolithium, -copper, -cadmium, and -zinc reagents, ketone enolates, nitroal-kyl anions, amines, phthalimide, and even nucleophilic aromatic compounds such as indole and trimethoxybenzene. Attack occurs exclusively from the face opposite the metal, and exclusively at a terminal position of the dienyl system. This combination of hydride abstraction and nucleophilic addition has been repeated to generate cyclohexa-diene complexes containing two cis vicinal substituents. The free cyclohexadiene is ttien released from the metal by oxidation with amine oxides. ... 

BERNTHSEN Acndine synthesis 32 BIGINELLI Pynmidone synthesis 33 BIRCH - HUCKEL - BENKESER Reduction 34 BISCHLER Benzomarine synthesis 35 BISCHLER - MOHLAU Indole synthesis 35 BISCHLER - NAPIERALSKI Isoquinoline synthesis 36... 

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). ... 

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