Reduction, of aromatic compound

Just as aromatic rings are inert to oxidation under most conditions, they re also inert to catalytic hydrogenation under conditions that reduce typical 

CHAPTER 16 I Chemisti7 of Benzen Electrophilic Aromatic Substitution 

To hydrogenate an aromatic ring, it s necessary either to use a platinum catalyst with hydrogen gas at several hundred atmospheres pressure or to use a more powerful catalyst such as rhodium on carbon. Under these conditions, aromatic rings are readily reduced to cyclohexanes. For example, o-xylene yields 1,2-dimethylcyclohexane, and 4-teri-butylphenol gives 4-terf-butylcyclohexanol. 

At low concentration of ethoxide, the rate depends only on the allyhc halide concentration. 

Primary halides of the type ROCH2X apparently undergo SNl-type reactions, whereas most primary halides do not. Can you propose a resonance explanation for the ability of halides of the type ROCH2X to undergo SnI reactions  

The following chlorides (Ph = phenyl) undergo solvolysis in ethanol at the relative rates given in parentheses. How can you explain these results  

Hydrogenation of benzene under pressure using a metal catalyst such as nickel results in the addition of three molar equivalents of hydrogen and the formation of cyclohexane (Section 14.3). The intermediate cyclohexadienes and cyclohexene cannot be isolated because these undergo catalytic hydrogenation faster than benzene does. 

Benzene can be reduced to 1,4-cyclohexadiene by treating it with an alkali metal (sodium, lithium, or potassium) in a mixture of liquid ammonia and an alcohol. This reaction is called the Birch reduction, after A. J. Birch, the Australian chemist who developed it. 

Refer to Table 5.3 on page 156 for a quantitative idea of the stability of a benzyl radical. How much more stable (in kj/mol) is the benzyl radical than a primary alkyl radical How does a benzyd radical compare in stability to an ally radical  

Styrene, the simplest alkenydbenzene, is prepared commercially for use in plastics manufacture by catalytic dehydrogenation of ethylbenzene. How might you prepare stynene from benzene using reactions you ve studied  

Just as aromatic rings are generally inert to oxidation, they re also inert to catalytic hydrogenation under conditions that reduce typical alkene double bonds. As a result, it s possible to reduce an alkene double bond selectively in the presence of an aromatic ring. I or example, 4-phenyl-3-buten-2-one is reduced to 4-phenyl-2-butanone at room temperature and atmospheric pressure using a palladium cataly st. Neither the benzene ring nor the ketone carbonyl group is affected. 

Problem 16.22 flow would you prepare diphenylmclhane, (l h)2CIi2, from benzene and an acid 

In the same way that an aromatic ring activates a neighboring (benzylic) C H toward oxidation, it also activates a benzylic carbonyl group toward reduction. Thus, an aryl alkyl ketone prepared by Friedel-Crafts acylation of an aromatic ring can be converted into an alkylbenzene by catalytic hydrogenation over a palladium catalyst. Propiophenone, for instance, is reduced to propylbenzene by catalytic hydrogenation. Since the net effect of Friedel-Crafts acylation followed by reduction is the preparation of a primary alkylbenzene, this two-step sequence of reactions makes it possible to circumvent the carbocation 

A resonance-stabilized benzylic radical. The spin surface shows that the unpaired electron is shared by the ortho and para carbons in the ring. 

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Akhrem, A. A. Reshetova, I. G. Titov, Yu. A. 1972, Birch Reduction of Aromatic Compound, Plenum New York... 

The term Birch reduction was originally applied to the reduction of aromatic compounds by alkali metals and an alcohol in ammonia. In recent years many chemists have used the term to include all metal-ammonia reductions, whether an alcoholic proton source is present or not. The author prefers to use the term Birch reduction to designate any reduction carried out in ammonia with a metal and a proton donor as or more acidic than an alcohol, since Birch customarily used such a proton donor in his extensive pioneering work. The term metal-ammonia reduction is best reserved for reductions in which ammonia is the only proton donor present. This distinction in terminology emphasizes the importance of the acidity of the proton donor in the reduction process. 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

The reduction of aromatic compounds 1 by alkali metals in liquid ammonia in the presence of an alcohol is called the Birch reduction, and yields selectively the 1,4-hydrogenated product " 2. 

This procedure is illustrative of the general method of reduction of aromatic compounds by alkali metals in liquid ammonia known as the Birch reduction. The theoretical and preparative aspects of the Birch reduction have been discussed in excellent reviews,4-4... 

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. 

Whatever the best explanation may be, an indication that allylic alkali metal compounds or allylic carbanions do in fact form the less stable of the two possible acids on neutralization is found in the results of the reduction of aromatic compounds by dissolving metals.376The detection of a paramagnetic intermediate in a similar system and polaro-graphic evidence indicate a one electron transfer in the rate and potential determining step.878 376 The mechanism therefore involves ions (or organometallic intermediates) like the following ... 

Reduction of aromatic compounds to dihydro derivatives by dissolved metals in liquid ammonia (Birch reduction) is one of the fundamental reactions in organic chemistry308. When benzene derivatives are subjected to this reduction, cyclohexa-1,4-dienes are formed. The 1,4-dienes obtained from the reduction isomerize to more useful 1,3-dienes under protic conditions. A number of syntheses of natural products have been devised where the Birch reduction of a benzenoid compound to a cyclohex-1,3-diene and converting this intermediate in Diels-Alder fasion to polycyclic products is involved (equation 186)308f h. 

Dihydroaromatics find diverse applications. The main way to prepare them is through Birch reduction of aromatic compounds (Birch 1944, Wooster and Godfrey 1937, Hueckel and Bretschneider 1939). Aromatic compounds are hydrogenated in diethyl ether or liquid ammonia, with alkali metals as reductants and alcohols as proton sources. 

A. A. Akhrem, I. G. Rshetova, and Y. A. Titov, Birch Reduction of Aromatic Compounds, IFGI/Plenum, New York, 1972. 

For a monograph, see Akhrem Reshotova Titov Birch Reduction of Aromatic Compounds Plenum New York, 1972. For reviews, see Rabideau Tetrahedron 1989, 5, 1579-1603 Birch Suhba Rao Adv. Org. Chem. 1972, 8,1-65 Kaiser Synthesis 1972,391-415 Harvey Synthesis 1970,161-172 House. Ref. 144, pp. 145-150,173-209 Huckcl Fortschr. Chem. Forsch 1966, 6, 197-250 Smith, in Augustine Reduction Techniques and Applications in Organic Synthesis Marcel Dekker New York, 1968, pp. 95-170. 

Mechanism and stereochemistry of reductions of ketones, 66 Mechanism of dehydrohalogenation, 292 Mechanism of hydrogenation, 111 Mechanism of reduction of aromatic compounds, 12... 

III. The Birch Reduction of Aromatic Steroids /II Mechanism of the reduction of aromatic compounds / 12 Factors influencing the rate of reduction / 14 Protonation of reduction intermediates / 17... 

Effect of Temperature and Cell Divider. Increase in temperature (runs 5 and 13) lowers current efficiency but does not affect product distribution. Using divider (runs 5 and 14) decreases current efficiency but shifts product distribution drastically in favor of octalin over hexalin. Similar results were obtained by Benkeser et al. (I) in their electrochemical reduction of aromatic compounds in methylamine. 

The deep blue solutions formed by dissolving alkali metals in ammonia do not rapidly generate the amide unless a catalyst is added.9 However, a hydrogen acceptor will also initiate the reaction and this forms the basis of the important Birch reduction of aromatic compounds (equation 2).10... 

Physical chemical studies of dilute alkali metal-ammonia solutions indicate the principal solution species as the ammoniated metal cation M+, the ammoniated electron e , the "monomer M, the "dimer" M2 and the "metal anion" M. Most data suggest that M, M2, and M are simple electrostatic assemblies of ammoniated cations and ammoniated electrons The reaction, e + NH3 - lf 2 H2 + NH2 is reversible, and the directly measured equilibrium constant agrees fairly well with that estimated from other thermodynamic data. Kinetic data for the reaction of ethanol with sodium and for various metal-ammonia-alcohol reductions of aromatic compounds suggest that steady-state concentrations of ammonium ion are established. Ethanol-sodium reaction data allow estimation of an upper limit for the rate constant of e + NH4+ 7, H2 + NH3. 

In view of the mechanism suggested above for the ethanol-sodium reaction, it seems likely that reduction of aromatic compounds by solutions of alcohols and alkali metals in liquid ammonia proceeds by a general mechanism involving a steady-state concentration of ammonium ion. Krapcho and Bothner-By (29) observed that the reduction of benzene and several substituted benzenes in lithium-alcohol-ammonia solutions,... 

A. J. Birch and G. Subba Rao (1972). Reduction by metal-ammonia solutions and related reagents , in Advances in Organic Chemistry. Ed. E. C. Taylor, New York Wiley-Interscience, Vol. 8, p. 1. See also A. A. Akhrem, I. G. Reshetova and Y. A. Titov (1972). Birch Reduction of Aromatic Compounds. New York IFI/Plenum. 

Cyclohexadienes are avaiable by the Birch reduction of aromatic compounds, and converted to 1,3-diene complexes by heating with Fe(CO)5. l-Methoxy-1,4-... 

An interesting example, and the one where these ideas were first applied,40 is the Birch reduction of aromatic compounds by sodium in liquid ammonia containing alcohol. These reactions seem to take place by two successive electron transfers, each followed by capture of a proton, i.e. 

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