Birch reduction of aromatic rings

CHAPTER 5 REDUCTION OF CARBONYL AND OTHER FUNCTIONAL GROUPS 

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Finally, in the Birch reduction of aromatic rings and subsequent hydrolysis, in all known cases the newly formed center of asymmetry has the more stable anti-configuration with respect to the neighboring center already present. This can be illustrated on the basis of the reduction of compounds (215) (Scheme 39), (216) (Scheme 54), and (217) [94]. 

Birch Reductions reduction of aromatic rings Organic Reactions 1976, 23, 1. Tetrahedron 1986, 42, 6354. Cornprehensice Organic Synthesis 1991, voJ. 8, 107. 

Metal-ammonia-alcohol reductions of aromatic rings are known as Birch reductions, after the Australian chemist Arthur J Birch who demonstrated their usefulness begin nmg m the 1940s... 

The elaboration of a method for the reduction of aromatic rings to the corresponding dihydrobenzenes under controlled conditions by A. J. Birch opened a convenient route to compounds related to the putative norprogesterone. This reaction, now known as the Birch reduction,is typified by the treatment of... 

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. 

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. 

The Birch reduction of aromatic hydrocarbons and ethers to the 2,5-dihydro derivatives proceeds most satisfactorily when the substitution pattern allows the addition of hydrogen to two unsubstituted positions in a para relationship. If this requirement is satisfied, better yields are obtained from more highly substituted aromatic rings than from (say) anisole itself, which affords a substantial amount (20%) of 1-methoxycyclohexene (c/. Scheme 1). Extra substitution presumably hinders protonation at the terminus of the dienyl anion (which would lead to a conjugated diene and overreduction). The utilization of anisole moieties as precursors to cyclohexenones has been of very limited value with many 1,2,3-substitution patterns and more densely substituted derivatives. Compounds (23) to (26), for example, have only been reduced by employing massive excesses (200-600 equiv.) of lithium metal,2 while the aromatic ring in (28) is completely resistant to reduction. ... 

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. 

There is no simple way to disconnect the TM shown below (dissonant charge pattern). However, the presence of a 1,6-dioxygenated compound suggests opening of a six-member ring. A variety of cyclohexene precursors are readily available via condensation and Diels-Alder reactions or via Birch reductions of aromatic compounds. 

Birch reduction 1,4-Reduction of aromatic rings using alkali metals dissolved in liquid ammonia as reducing agents. 60... 

Birch reduction. Reduction of aromatic rings by means of alkali metals in liquid ammonia to give mainly unconjugated dihydro derivatives. 

The Birch reduction is the organic reduction of aromatic rings by sodium in liquid ammonia invented by Arthur Birch. The reaction product is a 1,4-cyclohexadiene. The metal can also be lithium or potassium and the hydrogen atoms are supplied by an alcohol such as ethanol or tertbutanol. Sodium in liquid ammonia gives an intense blue color. 

For examples of Birch reductions, see Mander LN (1991) Partial reduction of aromatic rings by dissolving metals and by other methods. In Trost BM, Fleming I (eds) Comprehensive organic synthesis, vol 8. Pergamon, Oxford, p489 and references cited therein... 

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