Naphthalene Birch reduction

Many expensive reductions such as the Birch reduction of naphthalene to isotetralin, benzene to cyclohexene, with metallic sodium and liquid ammonia, or reduction with LiAlHa, can generally be carried out electrochemically at much lower cost and under safe conditions. Electrochemical processes allow fluorinations to be carried out without using fluorine gas. Conducting polymers have been made by electrochemical processes which operate under ambient conditions, and the polymer can be synthesized, doped and shaped in film form in a single step. 

The procedure described for the Birch reduction of naphthalene is a modification of the methods previously developed by Birch,5 Hiickel,2 and Grob.6 Apart from this reduction, no other practical approaches to isotetralin have become known. The scale employed in the present procedure is not mandatory to achieve optimum yields. Equally good yields were realized when the runs were halved or enlarged up to fourfold. In the latter case, however, the apparatus already reaches pilot plant dimensions. 

In one of the more frequently utilized Birch reactions (the reduction of alkyl/alkoxy-substituted naphthalenes), two reduction products are obtained as shown in Scheme 7.10. The acidity of the alcohol employed for protonation determines the ratio of these two products. For example, the ratio of the product hydrogenated in the substituted fused ring to the product hydrogenated in the unsubstituted fused ring was compared for methanol... 

This type of Birch reduction-alkylation also works in the naphthalene series36. Thus, the naphthalene carboxamides 10. when subjected to the standard Birch reduction-alkylation conditions, furnish a single set of diastereomers 11 (d.r. >95 5 by ]II and 13C NMR no experimental details given)36. 

PAHs and soils contaminated with PAHs are readily remediated by solvated electrons in NH3. Oligomeric reduced products are obtained. These reactions are slower than dehalogenation, as was demonstrated by the rapid formation of benzene, toluene, and naphthalene in Na/NH3 from their corresponding monochloro derivatives [24,28], Table 12 summarizes data on the destruction of pure PAHs. Soils contaminated with PAHs have been remediated to below detection levels. Mononuclear aromatics (benzene, toluene, anisole, and nitrobenzene) undergo ring reduction according to the well-known Birch reduction [11-18]. 

C=C bonds conjugated with carbonyl groups have been reductively lithiated with lithium and naphthalene in a reaction closely corresponding to the Birch reduction.9... 

Table 11 shows some representative results from the cathodic reduction of some aromatic hydrocarbons. These include cases with Ei j2 near the cathodic limit or in the discharge region of the SSE (benzene, toluene) and cases with Ex j2 at considerably more positive potential (naphthalene, anthracene again we must anticipate the discussion of reactivity and refer to Table 21). Reactions nos. 1, 2, 6, and 7 immediately demonstrate one difficulty with such studies in that the catholyte of a divided cell becomes strongly basic as electrolysis progresses. In sufficiently basic medium, the initial product, a 1,4-dihydro derivative (cf. the Birch reduction Birch and Subba Rao, 1972), will rearrange to a conjugated system which, in contrast to the 1,4-dihydro derivative, is further reducible to the tetrahydro product (nos. 1 and 6). In a non-divided cell the acid production at the anode balances the base production and thus only a little rearrangement occurs. It is therefore not a trivial problem to find out if the tetrahydro product is formed from the conjugated dihydro product, formed directly or by rearrangement [eqn (78)].

Naphthalene acts as a stable source of single electrons. It accepts an electron from sodium atoms to give a radical anion that can be drawn in various ways and provides the green solution. We have drawn it like a Birch reduction intermediate (pp. 628-9). 

Solvent effects Birch reduction. 1,2-Dimethoxyethane (Glyme) and Dimethyl ether (see Naphthalene-Sodium), Dimethylformamide. Dimethyl sulfone. Dimethyl sulfoxide. Diphenyl sulfoxide. Ethylene glycol. N-EthylmorphoUne. Hexamethylphosphoric triamide. Methylal. Methylene chloride. Methyl ethyl ketone. N-Methyl-2-pyrrolidone. Nitrometbane. Nitrosyl chloride. Phenetole. Tetrahydrofurane. Tetramethylene sulfone. Tetramethylene sulfoxide. Triethanolamine. Triethyl phosphate. Trifluoroacetic acid,... 

The process now known as reductive alkylation of rc-conjugated anions (quenching of anions) is as old as the preparation of the ions themselves5). The highly colored solutions obtained by the addition of alkali metals to solutions of aromatic hydrocarbons in ether were reacted with electrohpiles such as protons or alkyl halides (Scheme 2). The products of such a process are reduced hydrocarbons. The Birch reduction is one example oT such a process, reaction of an anion with an alkyl halide leading to an alkylated reduced hydrocarbon is another example 165). The complexity of the quenching experiments is demonstrated by the naphthalene radical anion 150-1581... 

Photo-Birch reductions. Irradiation of aromatic hydrocarbons in aqueous acetonitrile in the presence of sodium borohydride and 1,2-, 1,3-, or 1,4-dicyanobenzene with a high-pressure mercury arc results in reduction to dihydro derivatives. Phenahthrene is reduced to 9,10-dihydrophenanthrene (71%), anthracene to 9,10-dihydroanthracene (70%), and naphthalene to 1,4-... 

The preeminence of lithium and sodium in metal reductions has been challenged by the use of calcium in amines, developed by Benkeser. In the initial work, naphthalene was reduced to an 80 20 mixture of A -octalin (522) and A ( )-octalin (523) with lithium in diethylamine-dimethylamine. Replacing lithium with calcium gave a 77 23 mixture of 522 and 523 in 92% yield. This method has come to be called the Benkeser reduction. This has become an important modification because Birch reductions with sodium are... 

One of the most important applications of the Birch reduction is to convert aryl alkyl ethers into 1-alkoxycyclohexa-l,4-dienes which are then used as the starting materials in organic synthesis. Compared to the benzene series, the Birch reduction of naphthalenes may afford different products, depending on the position of the alkoxyl group. 

Synthesis of racemic (178) is shown in Scheme 37. Ketoester (177), synthesized from naphthalene-1,6-diol via 5-methoxy-2-tetralone, was converted into compound (180) by successive methylation at C-4, acetali-zation at C-3, reduction of the ester group to hydroxymethyl group, epoxidation of the C-5, C-6 double bond, and ring opening of the epoxide. Birch reduction of diol (180) with 18 equivalents of lithium in liquid ammonia followed by acid hydrolysis and subsequent methyl acetalization... 

Brase and coworkers [29] also reported highly enantioselective desymmetrization of rran5-bicyclo[4.4.0]decadienes 81-84, which were prepared through Birch reduction of naphthalene, epoxidation and epoxide-opening reaction (Figure 13.11). The position of C=C double bonds also differed from the previous report [25], Although a Mizoroki-Heck reaction of aryl iodide 81 with the usual chiral ligands such as BINAP, DIOP i(2R,3R)- or (25,35)-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane)... 

Electrides are synthetically useful and form the basis of so-called dissolving metal reactions, of which the Birch reduction of aromatic compounds is the paradigm, shown below for benzene and naphthalene ... 

The reduction of substituted naphthalene with CgK demonstrates the high selectivity of the reduction, as only the 1,4-dihydronaphthalene is obtained [29]. This method can be used as an alternative route to the Birch reduction and its... 

All the latter reactions lack practical utility. This is definitely not the case with the wealth of cyclization reactions that have emerged in the past decade. These enable the controlled dearomatization of benzenes and naphthalenes, which up to now have been the domain of Birch reductions. The new objective is to generate, mostly by metalation of an allylic or benzylic site, an organometallic species that is connected to an arene part by a carboxamide or sulfonamide type tether. Intramolecular attack on the arene ring followed by quenching of the enolate-like intermediate (e.g., 187 and 188) provides an annulated heterocycle for ftirther elaboration (Scheme 1-133). 

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