Reduction of aromatic ring

The hydrogenation of benzenoids to cyclohexane derivatives is very useful for various methodologies. Aromatic hydrocarbons require drastic conditions for reduction (for example PtOj/Hj/CHjCOOH Raney Ni/H /Pr/A, Rh-Al Oj/Hj). 

The heterocyclic compounds, viz. p)n idine, 2-phenylpyridine and 3-methylpyridine can be reduced to the corresponding hexahydro product by Sm-20% HCl in 90-95% yields. However in the reduction of4-aminopyridine by the above method, the 4-amino group is eliminated giving piperidine as the 

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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... 

Rhodium (2J) and ruthenium are excellent catalysts for the reduction of aromatic rings. It is with these catalysts that the best chance resides for preservation of other reducible functions (2,10,13,18,41,42,52). Rhodium (41) and ruthenium (45) each reduced methylphenylcarbinol to methylcyclohexyl-carbinol in high yield. Palladium, on the other hand, gives ethylbenzene quantitatively. Water has a powerful promoting effect, which is unique in ruthenium catalysis (36). 

Reduction of aromatic rings with lithium or calcium " in amines (instead of ammonia—called Benkeser reduction) proceeds further and cyclohexenes are obtained. It is thus possible to reduce a benzene ring, by proper choice of reagent, so that one, two, or all three double bonds are reduced. Lithium triethylborohy-dride (LiBEtsH) has also been used, to reduce pyridine derivatives to piperidine derivatives." ... 

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. 

Reduction of aromatic rings with lithium340 or calcium341 in amines (instead of ammonia)... 

Reduction of aromatic rings 5-15 Cyclization of dienes or diynes 5-18 Cyclization of unsaturated Grignard... 

Reduction of olefins and alkynes 5-10 Reduction of aromatic rings... 

Reduction of arenes.1 Raney nickel (Mozingo type) in combination with 2-propanol (reflux) effects reduction of aromatic rings in 2-18 hours. Naphthalene is reduced in 18 hours to tetralin (90% yield) and cis- and frans-decalin (10% yield). Anisole is reduced in 110 hours to cyclohexyl methyl ether (90% yield). Nitrobenzene is reduced quickly to aniline and then further to cyclohexylamine and cyclohexylisopropylamine. 

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. 

The mechanism follows the same course as the reduction of aromatic rings, but the vinyl anion is basic enough to deprotonate ammonia, so no added proton source is required. Vinyl anions are geometrically unstable, and choose to be E, negative charge in sp orbita,... 

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