Birch reduction of arenes

The use of reducing metals nowadays is mainly restricted to acyloin and pinacol coupling reactions (see p. 53f.) and Birch reductions of arenes (A.A. Akhrcm, 1972 see p. 103f.) and activated C—C multiple bonds (see p. 103f.). 

Cathodic addition involves either the reduction of a 7T-system in an ECEC process (electrochemical, chemical, electrochemical, chemical) and the chemical reaction (C) of the intermediates with a carbon or heteroatom electrophile or the cathodic conversion of a C—X bond in an ECE process into an anion that adds to an electrophilic jr-system. The electrochemical Birch reduction of arenes... 

A special case involves complexes of cyclohexadienyl ligands, which may result from the addition of nucleophiles to r)6-arene complexes (Section 7.7) or hydride abstraction from complexes of readily available cyclohexadienes (Birch reduction of arenes) (Figure 7.14). In the latter case, it is within the chemistry of iron that such complexes find the widest... 

Reductions for which group 1 metals are used in organic chemistry can be divided into two broad classes those that involve addition of H2 across a tt bond, and those in which a bonds between non-H atoms are formed or broken. The conjugate reduction of a,j3-unsaturated ketones, the reduction of alkynes to trans-alkenes, and the Birch reduction of arenes belong to the first class. The reduction of C—X bonds, C—O bonds a to a carbonyl, and acyloin and pinacol condensations belong to the second class. 

These reactions of electrophiles with alkene complexes to abstract a hydride from the saturated carbon adjoining the coordinated ir-system can create a sequence for the functionalization of dienes that are derived from the Birch reduction of arenes. A commonly used example of this reaction is the abstraction of a hydride from (cyclohexadiene)Fe(CO)3 (Equation 12.71) by trityl cation as described by Fischer. Such hydride abstractions have been shown to occur by initial electron transfer, followed by H transfer in some cases. ... 

Birch also illustrated tire difference in selectivity between Wilkinson s catalyst and typical heterogeneous catalysts. Hydrogenation of 1,4-dihydrobenzenes (derived from the "Birch reductions" of arenes) in the presence of Wilkinson s catalyst forms the tetra-substituted alkene product, whereas hydrogenation with classic platinum and palladium heterogeneous catalysts causes disproportionation to form arene byproducts (Equation 15.6). Like the (3-keto alcohol in Equation 15.5, the ester in Equation 15.6 is not reduced. 

The complexes of cyclic dienes, such as 1,3-cyclohexadienes, are precursors to / -cyclohexadienyltricarbonyliron salts, which have been applied in origanic synthesis (p. 303). 1,4-Cyclohexadienes, available from the Birch reduction of arenes, are converted into the 1,3-diene complexes. The isomerization occurs through a transient f/ -allyliron hydride (p. 363). 

Reactions with Protic, ionic, Poiar Reagents. The reactions of radical anions with proton donors include the reduction of arenes, the well-known Birch reduction, as well as alkynes by alkali metals in liquid ammonia. Both reactions have synthetic utility and belong to the few radical ion reactions included in elementary textbooks. 

A clever application of this reaction has recently been carried out to achieve a high yield synthesis of arene oxides and other dihydroaromatic, as well as aromatic, compounds. Fused-ring /3-lactones, such as 1-substituted 5-bromo-7-oxabicyclo[4.2.0]oct-2-en-8-ones (32) can be readily prepared by bromolactonization of 1,4-dihydrobenzoic acids (obtainable by Birch reduction of benzoic acids) (75JOC2843). After suitable transformation of substituents, mild heating of the lactone results in decarboxylation and formation of aromatic derivatives which would often be difficult to make otherwise. An example is the synthesis of the arene oxide (33) shown (78JA352, 78JA353). 

Birch reductions of monosubstituted arenes yield 1,4-cyclohexadiene derivatives in which the alkyl group is a substituent on the double bond. With p-xylene, both methyl groups are double-bond substituents in the product. 

As early as 1969, Pedersen was intrigued by the intense blue colour observed upon dissolution of small quantities of sodium or potassium metal in coordinating organic solvents in the presence of crown ethers. Indeed, the history of alkali metal (as opposed to metal cation) solution chemistry may be traced back to an 1808 entry in the notebook of Sir Humphry Davy, concerning the blue or bronze colour of potassium-liquid ammonia solutions. This blue colour is attributed to the presence of a solvated form of free electrons. It is also observed upon dissolution of sodium metal in liquid ammonia, and is a useful reagent for dissolving metal reductions , such as the selective reduction of arenes to 1,4-dienes (Birch reduction). Alkali metal solutions in the presence of crown ethers and cryptands in etheric solvents are now used extensively in this context. The full characterisation of these intriguing materials had to wait until 1983, however, when the first X-ray crystal structure of an electride salt (a cation with an electron as the counter anion) was obtained by James L. Dye and... 

Reduction of arenes. Calcium in combination with amines, generally methylamine or ethylenediamine, reduces aromatic hydrocarbons to monoenes in generally high yield.1 Addition of r-butyl alcohol to this system effects reduction to nonconjugated dienes (Birch-type products). Ethylenediamine is essential in this case addition of a second amine is generally desirable for solubility reasons.2... 

The partial reduction of arenes can be achieved using the Birch reduction An alkali metal (lithium, sodium or potassium) is dissolved in liquid ammonia in the presence of the arene, an alcohol, such as 2-methylpropan-2-ol tert-buty alcohol) and a co-solvent to assist solubility. 

Although some cyclohexadienes are readily available, many can be obtained easily by Birch reduction, which involves reduction with solutions of alkali metals in liquid ammonia, a source of solvated electrons, in the presence of alcohol as a proton source.8-10 In previous years, the Bouveault-Blanc procedure, which uses sodium metal and alcohol in liquid ammonia, was frequently employed for direct reduction of aromatic esters however, it gave rise mainly to the corresponding substituted benzoic acid.11 Rabideau et al. reported a modified procedure 12 however, in our hands, this resulted in the reduction of the ester function to give benzoic acid. We have found that the Birch reduction of benzoic acid, followed by esterification, is an efficient procedure for the preparation of the corresponding 1,4-dihydro compound prior to the coordination of the arene to produce functionalized dimeric ruthenium-arene complexes.13... 

Reduction of pyridine with Na in a protic medium is interpreted as being analogous to the Birch reaction of arenes, i.e. a two-step, single-electron transfer involving the radical anion 132 followed by a 1,2- or 1,4-addition of hydrogen ... 

Birch reductions of monosubstituted arenes yield 1,4-cyclohexadiene derivatives in which the alkyl... 

Section 1111 An example of a reaction m which the ring itself reacts is the Birch reduction The ring of an arene is reduced to a nonconjugated diene by treatment with a Group I metal (usually sodium) m liquid ammonia m the presence of an alcohol... 

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