Dissolving metals aromatic rings

Dissolving-Metal Reduction of Aromatic Compounds and Alkynes. Dissolving-metal systems constitute the most general method for partial reduction of aromatic rings. The reaction is called the Birch reduction,214 and the usual reducing medium is lithium or sodium in liquid ammonia. An alcohol is usually added to serve as a proton source. The reaction occurs by two successive electron transfer/proto-nation steps. 

Very little work has been done on reactions involving nucleophiles formed from hydrocarbons.124-142 The limitation on basicity of the carbanion, so that it does not react with solvent, has led to use of conjugated hydrocaibons, such as dienes or alkenes conjugated with aromatic rings. When initiated by dissolving alkali metal in liquid ammonia, complex mixtures are often produced on account of reduction processes,124 and regiochemistry and multiplicity of arylation in conjugated systems also create prob-... 

Birch reduction11 is the partial reduction of aromatic rings by solvated electrons produced when alkali metals dissolve (and react) in liquid amines. Typical conditions are sodium in liquid ammonia or lithium in methylamine. These electrons add to benzene rings to produce, probably, a dianion 57 that is immediately protonated by a weak acid (usually a tertiary alcohol) present in solution. The anions in the supposed intermediate 57 keep as far from each other as they can so the final product is the non-conjugated diene 58. It is important to use the blue solution of solvated electrons before it reacts to give hydrogen and NaNH2. 

L. N. Mander, Partial Reduction of Aromatic Rings by Dissolving Metals and by Other Methods, in Comprehensive Organic Synthesis (B. M. Trost, I. Fleming, Eds.), Vol. 8,489, Pergamon Press, Oxford, U. K., 1991. 

Na, or Li in liquid ammonia, for example) to reduce aromatic rings and alkynes. The dissolving metal reduction of enones by lithium metal in liquid ammonia is similar to these reactions—the C=C bond of the enone is reduced, with the C=0 bond remaining untouched. An alcohol is required as a proton source and, in total, two electrons and two protons are added in a stepwise manner giving net addition of a molecule of hydrogen to the double bond. 

For reduction, relevant data from polarographic and cyclic voltammetric experiments are summarized in Tables 1 and 2, respectively. For the results in Table 1 the variety of solvents and reference electrodes used makes comparisons difficult. It is clear, however, that even with the activation of a phenyl substituent (entries 6,7,9-14) reduction occurs at very cathodic potentials. In this context it is worth noting that in aprotic solvents at ca. — 3 V vs. S.C.E.) it becomes difficult to distinguish between direct electron transfer to the alkyne and the production of the cathode of solvated electrons. Under the latter conditions the indirect electroreductions show many of the characteristics of dissolving metal reductions (see Section II.B). Even at extreme cathodic potentials it is not clear that an electron is added to the triple bond the e.s.r. spectra of the radical anions of dimesitylacetylene and (2,4,6,2, 4, 6 -hexa-r-butyldiphenyl)acetylene have been interpreted in terms of equal distribution of the odd electron in the aromatic rings . 

Partial Reduction of Aromatic Rings by Dissolving Metals and by Other Methods... 

Extensive investigations have been made into further methods for the reduction of aromatic rings based on the use of dissolving metals in other solvents, especially the lower molecular weight amines (the Benkeser reduction), electrochemical methods (cathodic reductions), photochemical methods and the reaction of radical anions with silylating reagents rather than proton sources. The aim of much of this work has been to produce the normal Birch products more conveniently or cheaply, but very often the outcome has been quite distinct. The alternative method may then provide access to products which are not so easily obtained by the standard metal-liquid ammonia methodology. 

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