Aromaticity amalgams

Neta.1 Ama.lga.ms. Alkali metal amalgams function in a manner similar to a mercury cathode in an electrochemical reaction (63). However, it is more difficult to control the reducing power of an amalgam. In the reduction of nitro compounds with an NH4(Hg) amalgam, a variety of products are possible. Aliphatic nitro compounds are reduced to the hydroxylamines, whereas aromatic nitro compounds can give amino, hydra2o, a2o, or a2oxy compounds. 

Cesium forms simple alkyl and aryl compounds that are similar to those of the other alkah metals (6). They are colorless, sohd, amorphous, nonvolatile, and insoluble, except by decomposition, in most solvents except diethylzinc. As a result of exceptional reactivity, cesium aryls should be effective in alkylations wherever other alkaline alkyls or Grignard reagents have failed (see Grignard reactions). Cesium reacts with hydrocarbons in which the activity of a C—H link is increased by attachment to the carbon atom of doubly linked or aromatic radicals. A brown, sohd addition product is formed when cesium reacts with ethylene, and a very reactive dark red powder, triphenylmethylcesium [76-83-5] (C H )2CCs, is formed by the reaction of cesium amalgam and a solution of triphenylmethyl chloride in anhydrous ether. 

The exchange of aromatic protons can be effected in the absence of any -OH or —NH2 activating group during the course of a Clemmensen reduction in deuteriochloric and deuterioacetic acid mixture (see section Ill-D). This reaction has been carried out with various tricyclic diterpenes and is best illustrated by the conversion of dehydroabietic acid into its 12,14-d2-labeled analog (40 -+ 41).Amalgamated zinc is reportedly necessary for the exchange reaction since the results are less satisfactory when a zinc chloride-mercuric chloride mixture is used. 

The reaction of perfluoroalkyl iodides with calcium amalgam at -20 to 40 °C has been used to prepare perfluoroalkyl carbinols in 30-70% yields [/, 2] Aromatic aldehydes (equation 1) and ketones (equation 2) undergo this reaction, aliphatic aldehydes give high boiling mixtures The results are interpreted in terms of the intermediate formation of an organocalcium species... 

The reduction of iminium salts can be achieved by a variety of methods. Some of the methods have been studied primarily on quaternary salts of aromatic bases, but the results can be extrapolated to simple iminium salts in most cases. The reagents available for reduction of iminium salts are sodium amalgam (52), sodium hydrosulfite (5i), potassium borohydride (54,55), sodium borohydride (56,57), lithium aluminum hydride (5 ), formic acid (59-63), H, and platinum oxide (47). The scope and mechanism of reduction of nitrogen heterocycles with complex metal hydrides has been recently reviewed (5,64), and will be presented here only briefly. 

Treatment of dihalogeno aromatics with lithium amalgam, magnesium, zinc, etc.,... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

The Birch-type electrochemical reduction (460) (461) has been shown to proceed through the action of tetra-butylammonium amalgam in the steps (460)— (462)— (463), in contrast to a direct electron transfer from the electrode to the aromatics (Scheme 158) [548]. The preparative-scale reduction of anisole, of l,2,3,4-tetrahydro-6-methoxynaphthalene, and several aromatic steroids is performed in an H20-Bu4N0H-(Hg) system. The unique aspect of the reduction is the proposed formation of a tetrabutylammo-nium amalgam complex, BU4N (Hg) (465)... 

Aluminum. Reductions with aluminum are not commonly found, however, I have included the preparation of the amalgam. Aluminum can be used to reduce aromatic nitro compounds (Org, Syn., 52, 77(1972). 

Reduction of aromatic aldehydes to pinacols using sodium amalgam is quite rare. Equally rare is conversion of aromatic aldehydes to alkenes formed by deoxygenation and coupling and accomplished by treatment of the aldehyde with a reagent obtained by reduction of titanium trichloride with lithium in dimethoxyethane. Benzaldehyde thus afforded /ra/is-stilbene in 97% yield [206, 209]. 

Partial reduction of the aromatic ring is especially easy in anthracene-9-carboxylic acid which was reduced to 9,10-dihydroanthracene-9-carboxylic acid with 2.5% sodium amalgam in aqueous sodium carbonate at 10° in 80% yield [987]. Aromatic carboxylic acids with hydroxyl groups in the ortho positions suffer ring cleavage during reductions with sodium in alcohols and are converted to dicarboxylic acids after fission of the intermediate j8-keto acids. 

Sodium and sodium amalgam may be used for reduction of amides but the yields of amines are generally very low. Primary aromatic amides (benz-amides) were reduced at the carbonyl function with 3.3 equivalents of sodium in liquid ammonia and ethanol while in tert-butyl alcohol reduction took place in the aromatic ring giving 1,4-dihydrobenzamides [984. ... 

REDUCTION WITH ALUMINUM AMALGAM Reduction of Aliphatic-Aromatic Ketones to Pinacols [144]... 

Preparative scale reduction of oximes at a mercury or lead cathode in acid solution has been used in the conversion of the carbonyl function to amine. Originally, 30-50% sulphuric acid was used as solvent [195] but ethanol with dilute hydrochloric acid is usually satisfactory. Aliphatic and aromatic oximes give amines in 64-86% yields [196]. Aromatic ketoximes are also reducible in alkaline solution and acetophenone oxime has been converted to 1-phenylethylamine in a tri-potassium orthophosphate solution [197], The reduction of oximes in acid solution is tolerant of many other substituents as indicated by a number of examples [198, 199, 200. Phenylglyoxa monoxime in acid solution is however reduced at both the carbonyl and the oxime centres by sodium amalgam to yield 2-amino-1-phenylethanol [201]... 

Aluminum amalgam reduces aUphatic 53 aromatic nitro compounds to hydroxylamines in consistently good yields (equation 51) and this can be the method of choice for large-scale reductions. 

Indian workers described the synthesis of l,3,4-thiadiazolo[3,2-a]-5-triazine-5(//)-thiones (61) by a hetero Diels-Alder reaction between 2-(arylideneamino)-5-ethylthio-l,3,4-thiadiazoles (60) and aromatic isothiocyanates (Equation (2)) <94MI 410-0I>. Thiadiazoles can be reduced with sodium amalgam to the aldehyde thiosemicarbazone while lithium aluminum hydride will reduce mesoionic thiadiazoles all the way to the hydrazine <84CHEC-I(4)545>. 

Amalgamated zinc, 198 see also under Clemmenaen reduction Amides, 361 1074, 1075 see also Aliphatic acid amides and Aromatic acid amides... 

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