Reduction Birch-type

Because of the highly negative reduction potentials ( —3.0 V vs. SCE) [32], the electroreduction of esters of aliphatic carboxylic acids to primary alcohols by direct electron transfer from the cathode is very difficult and the electrochemical Birch-type reduction of aliphatic esters in MeNH2 or liquid NH3 has not been reported until recently (Scheme 15) [33, 34]. This reaction is not a reduction by direct electron transfer from the cathode to the C=0 bonds of the ester but the reduction by a solvated electron. 

Scheme 15 Cathodic Birch-type reduction of aliphatic esters to alcohols R alkyl, yields 95%.

Cathodic reduction of arylsilanes in methylamine using LiCl as a supporting electrolyte in an undivided cell gives 1,4-cyclohexadiene derivatives. The reaction seems to proceed in a manner similar to the Birch-type reduction. The cathodic reduction in a divided cell provides desilylation products (equation 53)68. 

Na in f-PrOH is a single-electron Birch-type reduction (Chapter 25). You can t get much smaller than an electron)... 

Reductive aldol chemistry was investigated for the formation of 2,2-disubstituted-3-pyrrolines. Thus for example, Birch-type reduction of pyrrole 56 with lithium di-t-butylbiphenyl (LiDBB) in the presence of bis(methoxyethyl)amine (BMEA), quenching the unreacted LiDBB with 1,2-dibromoethane, treatment with magnesium bromide, and addition of isobutyraldehyde gave a >20 1 mixture of anti-57 and syn-SS <03OBC3749, 03TL1095>. 

Liquid ammonia (b.p. -33°C) is a solvent which is not encountered frequendy, but which does have several important general uses, in particular dissolving metal reductions ("Birch" type reductions) and most reactions involving lithium amide or sodium amide as bases. Ammonia gas from a cylinder is condensed directly into the flask (Fig. 14.5). 

The Birch-type reduction of 1,3-diphenylbenzo[c]furan (33) has been studied in detail by Smith and McCall (equation 110). ... 

A l,2-diphenyl-2,5-cyclohexadienyl rubidium complex isolated from the Birch-type reduction of 1,2-diphenylbenzene with mbidium metal in THF has been characterized. " ... 

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). ... 

Similarly, the Birch-type reduction of various tetrahydroisoquinoline A -oxides has been reported to generate secondary amines in good yields through cleavage of the central carbon to nitrogen bridge. ... 

Birch reduction is a standard reductive procedure of aromatic molecules to non-aromatic derivatives. Similar to the two-electron oxidation of phenol derivatives to the corresponding dienones, the Birch protocol is considered to involve reductive conversion of aromatic compounds to cyclohexadienes. When aromatic compounds were subjected to cathodic reduction in the presence of a proton source, Birch-type reduction products were produced (Scheme 5) [20 - 22]. 

Table 7 Birch-type reduction by electrochemical methodology...

An electrochemical methodology brings about Birch-type reduction. The reduction of BT was carried out in either aqueous or a mixed THF-water media, and mercury pool was employed as the cathode. It was found that BT is initially reduced to DHBT, which is subsequently reductively cleaved to 2-ethylbenzenethiol. Under all conditions, DHBT is the major product obtained by the transfer of a charge equivalent to 2 F/mol (Table 7) [153]. 

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