Metals in ammonia or amines

BIRCH hOCKEL - BENKESER Reduction Reduction of aromatics, unsaturated ketones conjugated dienes by alkali metals In ammonia or amines... 

Reductive desulfonylations and reductive eliminations can be performed employing alkali or alkaline earth metals in ammonia or low molecular weight amines. Lithium or sodium in ammonia or ethylamine are very effective systems... 

The less hindered f/ans-olefins may be obtained by reduction with lithium or sodium metal in liquid ammonia or amine solvents (Birch reduction). This reagent, however, attacks most polar functional groups (except for carboxylic acids R.E.A. Dear, 1963 J. Fried, 1968), and their protection is necessary (see section 2.6). 

The Birch reductions of C C double bonds with alkali metals in liquid ammonia or amines obey other rules than do the catalytic hydrogenations (D. Caine, 1976). In these reactions regio- and stereoselectivities are mainly determined by the stabilities of the intermediate carbanions. If one reduces, for example, the a, -unsaturated decalone below with lithium, a dianion is formed, whereof three different conformations (A), (B), and (C) are conceivable. Conformation (A) is the most stable, because repulsion disfavors the cis-decalin system (B) and in (C) the conjugation of the dianion is interrupted. Thus, protonation yields the trans-decalone system (G. Stork, 1964B). 

Single-bond cleavage with molecular hydrogen is termed hydrogenolysis. Palladium is the best catalyst for this purpose, platinum is not useful. Desulfurizations are most efficiently per-formed with Raney nickel (with or without hydrogen G.R. Pettit, 1962 A or with alkali metals in liquid ammonia or amines. The scheme below summarizes some classes of compounds most susceptible to hydrogenolysis. 

Although a wide variety of metals were claimed as active catalysts for formaldehyde hydrophosphination, platinum salts were preferred. Similarly, Group 10 metal salts were used to catalyze acrylonitrile hydrophosphination. Russian workers showed that Ni(II) or Co(II) salts in the presence of ammonia or amines would also catalyze the addition of phosphine to formaldehyde [6]. More recently, academic and industrial interest in these reactions was sparked by a series of papers by Pringle, who investigated late metal phosphine complexes as hydrophosphination catalysts. These and related studies are arranged below by substrate. 

Benzene and its homologs can be converted to the corresponding cyclo-hexadienes and cyclohexenes, and even cyclohexanes, by treatment with dissolving metals lithium, sodium, potassium or calcium in liquid ammonia or amines. Conversions are not complete, and the ratio of cyclohexadienes to cyclohexenes depends on the metal used, on the solvent, and on the presence of hydrogen donors (alcohols) added to the ammonia or amine [392, 393, 394]. 

A-Hydroxyphthalimide 178a (NHPI Chart 4), described for the first time in 1880, is a weak acid (plfa = 7), which forms highly colored salts with alkali metals, heavy metals, ammonia or amines. More precisely, the two crystalline forms of NHPI reported in the literature display different colors, colorless (white) and yellow, with the colors depending on the solvent in which the NHPI is prepared. The variations in physical properties, including color, have been attributed to synthetic problems , such as impurities, and contamination from a fluorescent compound that could not be isolated or characterized. 

These three emerging synthetic approaches appear to be of growing importance in the current literature. The first route concerns the deceptively simple oxidative addition of N H bonds to metal centres which is considered to be an essential step in the development of catalytic cycles for the addition of ammonia or amines to hydrocarbon substrates. A generalized cycle for the catalytic addition is illustrated in Scheme 6.1. 

One possible way to promote hydroamination is to activate the reacting ammonia or amine.288-291 Alkali metals were found to be useful catalysts. Even in the catalyzed addition of ammonia to simple alkenes, however, drastic reaction conditions are... 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

Whereas metal hydride is eliminated from the o--complex intermediates (cf. 309) in Chichibabin aminations, which results in the subsequent reaction of the metal hydride with ammonia or amines and the evolution of hydrogen, the o--complex intermediates can also be oxidized to the corresponding amino-N-heterocycles. Thus, 5-azacinnoline (320) can be aminated with primary and secondary aliphatic or benzyl amines in the presence of potassium hydroxide/potassium ferricyanide to give the corresponding amino derivatives 321 in up to 65% yield. The... 

A series of aminoethylpyrazines has been prepared in high yields by the reaction of 2-vinylpyrazine with ammonia or amines using methanolic acetic acid or metallic sodium as catalyst (730) 2-vinylpyrazine with acetamide and sodium at 120-130° gave 2-(2-acetamidoethyl)pyrazine (18) (731). 

The Birch reduction of aromatic hydrocarbons or of double bonds with alkali metals in liquid ammonia or amines [37] resembles in yield and selectivity the cathodic reduction with lithium halide as supporting electrolyte (for example, Ref. 38). 

Use of Alkali Metals in Ammonia. The reductive desulfonylation process with solutions of alkali or alkaline earth metals (Li, Na, and less frequently, Ca) in anhydrous ammonia or low molecular weight amines is solvent- and substrate-dependent and the outcome of the desulfonylation may be different depending on the reaction conditions employed. One of the main disadvantages of this... 

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