Anion radical intermediates Birch reduction

The Birch reduction has been used by several generations of synthetic organic chemists for the conversion of readily available aromatic compounds to alicyclic synthetic intermediates. Birch reductions are carried out with an alkali metal in liquid NH3 solution usually with a co-solvent such as THF and always with an alcohol or related acid to protonate intermediate radical anions or related species. One of the most important applications of the Birch reduction is the conversion of aryl alkyl ethers to l-alkoxycyclohexa-l,4-dienes. These extremely valuable dienol ethers provide cyclohex-3-en-l-ones by mild acid hydrolysis or cyclohex-2-en-l-ones when stronger acids are used (Scheme 1). 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

Perhaps it should be mentioned also the orientation of the Birch reduction which is strongly dependent on the nature of the aromatic substituents. Donor-substituted benzenes furnish predominantly 1-substituted 1,4-cyclohexadienes while acceptor-substituted analogues give 3-substituted 1,4-cyclohexadienes. The regioselectivities can be explained by the destabilizing d-d pairing in the intermediates from d-substi-tuted cyclohexadienyl radical anions leading to the 3-substituted products, and the... 

One of the solvated electrons is transferred into an antibonding 7t -orbital of the aromatic compound, and a radical anion of type C is formed (Figure 17.82). The alcohol protonates this radical anion in the rate-determining step with high regioselectivity. In the case under scrutiny, and starting from other donor-substituted benzenes as well, the protonation occurs in the ortho position relative to the donor substituent. On the other hand, the protonation of the radical anion intermediate of the Birch reduction of acceptor-substituted benzenes occurs in the para-position relative to the acceptor substituent. 

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). 

The alcohol acts as a source of protons, which react with the lone pairs. This reaction is called the Birch reduction. The final product is the cyclohexadiene in which the double bonds are unconjugated. If the alcohol is not present, then dimers may be obtained as the radical anions react with each other. In some substituted aromatic systems, both electrons are added before the ionic intermediate picks up any protons so that a dianion is formed initially. 

The first step of a Birch reduction is a one-electron reduction of the aromatic ring to a radical anion. Sodium is oxidized to the sodium ion Na. This intermediate is able to dimerize to the dianion. In the presence of an alcohol the second intermediate is a free radical which takes up another electron to form the carbanion. This carbanion abstracts another proton from the alcohol to form the cyclohexadiene. 

Rieke and Bales have reported further examinations of the naphthocyclobutene radical anion. Birch reduction of benzocyclopropene proceeds via the radical anion (517). No bicyclic intermediates such as those found in Birch reduction of other benzocycloalkenes are observed in this case. ... 

Draw the radical anion intermediate formed when methoxybenzene (anisole see Chapter 21, Section 21.2) is treated with sodium in ethanol and ammonia (Birch reduction), and draw all resonance structures. Based on stability of the radical anion product, suggest the final product of this reduction. 

Birch reduction (Section 13.11) The conversion of aromatic compounds into 1,4-cyclohexadienes through treatment with sodium in liquid ammonia-ethyl alcohol. Radical anions are the first formed intermediates. 

The ESR spectrum of the benzene radical anion 8, which is the prototype of the first intermediate in the Birch reduction, was reported in 1958. This spectrum was also reported in a matrix at 121 K. Calculations of the structures of the benzene radical cation 7 and the anion 8 suggest these are quite similar with significant bond alternation 7 C1C2 1.425, C2C3 1.358 8 C1C2 1.433, C2C3 1.366.38<= The radical anion of anisole was calculated to have highest electron density at the ortho position, and experimentally this was shown to be the preferred site for protonation. 

Alkali and alkaline earth metals dissolve in liquid ammonia with the formation of solvated electrons. These solvated electrons constitute a very powerful reducing agent and permit reduction of numerous conjugated multiple-bond systems. The technique, named for Birch provides selective access to 1,4-cydohcxiidicnes from substituted aromatics.8 In the case of structures like 21 that are substituted with electron-donating groups, electron transfer produces a radical anion (here 22) such that subsequent protonation occurs se lectively in the ortho position (cf intermediate 23) A second electron-transfer step followed by another protonation leads to com pound 24... 

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). ... 

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