Amine aromaticity effects

Our recent studies on effective bromination and oxidation using benzyltrimethylammonium tribromide (BTMA Br3), stable solid, are described. Those involve electrophilic bromination of aromatic compounds such as phenols, aromatic amines, aromatic ethers, acetanilides, arenes, and thiophene, a-bromination of arenes and acetophenones, and also bromo-addition to alkenes by the use of BTMA Br3. Furthermore, oxidation of alcohols, ethers, 1,4-benzenediols, hindered phenols, primary amines, hydrazo compounds, sulfides, and thiols, haloform reaction of methylketones, N-bromination of amides, Hofmann degradation of amides, and preparation of acylureas and carbamates by the use of BTMA Br3 are also presented. 

Combined effect of BTMA Br3 and ZnCl2 in acetic acid provides a new excellent bromination procedure for arenes. That is, while such reactive aromatic compounds as phenols, aromatic amines, aromatic ethers, and acetanilides have been easily brominated by BTMA Br3 in dichloromethane in the presence of methanol, the reaction of arenes, less reactive compounds, with BTMA Br3 in dichloromethane-methanol did not proceed at all, even under reflux for many hours. However, arenes could be smoothly brominated by use of this agent in acetic acid with the aid of the Lewis acid ZnCl2 (Fig. 13) (ref. 16). 

For the cydative cleavage step, it turned out that aprotic conditions were definitely superior to the use of protic media. Thus, employing N,N-dimethylformamide as solvent at somewhat elevated temperatures furnished the desired compounds in high yields and excellent purities. Having established the optimized conditions, various phthalic acids and amines were employed to prepare a set of phthalimides (Scheme 7.51). However, the nature of the amine was seen to have an effect on the outcome of the reaction. Benzyl derivatives furnished somewhat lower yields, probably due to the reduced activities of these amines. Aromatic amines could not be included in the study as auto-induced ring-closure occurred during the conversion of the polymer-bound phthalic acid. 

Keywords Synergistic effect, Eschke test, Salt spray test, Vapour phase corrosion inhibitor, Weight loss test, Sulphurdioxide test, TU, Aliphatic amines, Aromatic amines. 

In contrast to aliphatic amines, aromatic amines hardly react with C02 [21a] because of their poorer basicity. However, in the presence of suitable auxiliary bases (B) (such as amidines or penta-alkylguanidine superbases), carbamate salts (BH)02CNRAr (R = H, alkyl) can be generated in solution, as supported by spectroscopic and reactivity data [29]. It has been shown that even tributylamine may be effective if a suitable alkali metal salt is also present in solution in the latter case, the N-arylcarbamate has been isolated as an alkali salt (Equation 6.3)... 

Although the reduction of nitro compounds is more widely applied in the aromatic series using procedures discussed in Section 6.5.1, p. 890, these are equally applicable to the reduction of primary or secondary nitroalkanes when these are readily available. a,/ -Unsaturated nitro compounds (arising for example from the Claisen-Schmidt reaction, Section 6.12.2, p. 1032) may be reduced to the saturated amine very effectively with sodium dihydrobis(2-methoxyethoxy)-aluminate ( Red-Al ). 

A study on how the difference in the aromaticity between (35) and (36) may affect the intrinsic barriers to proton transfer has been reported (Scheme 18).137 The intrinsic barriers for the deprotonation of the thiophene derivative by amines and OH- have been found to be somewhat higher than for the furan analogue. This result has been attributed to a combination of steric, inductive, and n -donor effects which overshadow the aromaticity effect. 

In the synthesis of a series of cyclic aromatic disulfide oligomers using oxidative coupling of dithiols with oxygen catalyzed by copper salts and an amine, an effective, easy, and rapid method for the synthesis of macrocyclic aromatic disulfide oligomer from 4,4/-oxybis(benzenethiol) by cyclodepoly-... 

A particularly useful variation of this reaction uses cyanide rather than HCN. a-Amino nitrilescan be prepared in one step by the treatment of an aldehyde or ketone with NaCN and NH4CI. This is called the Strecker synthesis and it is a special case of the Mannich reaction (16-19). Since the CN is easily hydrolyzed to the acid, this is a convenient method for the preparation of a-amino acids. The reaction has also been carried out with NH3 + HCN and with NH4CN. Salts of primary and secondary amines can be used instead of NH4 to obtain A-substituted and A,A-disubstimted a-amino nitriles. Unlike 16-52, the Strecker synthesis is useful for aromatic as well as aliphatic ketones. As in 16-52, the MeaSiCN method has been used 76 is converted to the product with ammonia or an amine.The effect of pressure on the Strecker synthesis has been studied. " °... 

The molecules able to be efficient H-atom donors to alkyl radicals are not restricted to hindered phenols. It has long been noted in the rubber industry that secondary aromatic amines are effective radical scavengers, and study of these materials has shown that they produce a range of coupling products that function in a way similar to the phenols. The problem of the intense colour of these by-products is limited when they are used in carbon-black-filled rubber but a major limitation in other polymers (Zweifel, 1998). The exceptions are the polyamides, in which aromatic amines have a greater stabilization efficiency than do hindered phenols. This may be related to the general observation in polyamides (e.g. nylon-6) that the amine-terminated polymer is more stable than the carboxylic acid-or methyl- terminated polymer. 

Secondary aromatic amines are effective antioxidants in the protection of saturated hydrocarbon polymers (polyolefins) against autooxidation. Their role in the stabilization of unsaturated hydrocarbon polymers (rubbers) is more complex depending on their structure, they impart protection against autooxidation, metal catalyzed oxidation, flex-cracking, and ozonation. The understanding of antioxidant, antiflex-cracking and antiozonant processes together with involved mechanistic relations are of both scientific and economic interest. 

The first example has been reported of the photofixation of COg in a non-biological system this involves the formation of 9,10-dihydrophenanthrene-9-carboxylic acid from irradiation of the phenanthrene-amine-COa system in DMSO or DMF.71 Both aliphatic and aromatic amines are effective, but the polarity of the solvent is important and the reaction does not occur in THF, dioxan, or n-hexane. Yields of carboxylic acids (up to 46%) have been reported and seemingly anthracene, pyrene, naphthalene, and biphenyl also undergo this reductive carboxylation. 

Vapor Phase Hydrogenations of Nitro Compounds. Catalytic hydrogenation of nitro compounds can be carried out in the vapor phase, provided the boiling point of the compound is low enough and the material is thermally stable. These two conditions effectively limit this process to aliphatic and relatively simple aromatic nitro compounds such as nitrobenzene or nitroxylene. Eady vapor-phase hydrogenation processes used fixed-bed catalysts. However, fluidized-bed catalytic vapor-phase hydrogenations, such as the one illustrated by the process for aniline, have become more common (see Amines, aromatic, aniline and derivatives). 

In other words, the aromatic carboxylic acid is activated by the pyridinyl triphosphonate cation so that the weakly basic aromatic amine can effectively attack the carbonyl center. The reaction has not been utilized commercially because the costs of recovering and regenerating triphenylphosphite far outweigh the cost advantage of using unmodified diacids. 

The latter is known as the elimination-addition mechanism of the nucleophilic aromatic substitution. The high reactivity of aryllithiums precludes the use of several types of functional groups such as aldehydes, ketones, esters, amides, amines, alcohols, phenols, nitro-compounds, carboxylic acids etc. However, properly protected alcohols, phenols or amines can effect the reaction. Various ether protecting groups, e.g. Tr, Me, /-Pr, Bn, 4-MeOBn, are employed in the former instances, whereas A, V-dibenzyl group is convenient for the amine protection. 

In a convenient one-pot process, easily accessed propargyl vinyl ethers and aromatic amines are effectively converted into tetra- and pentasubstituted 5-methylpyrroles, which can fnrther be transformed into 5-formylpyrroles via 2-iodoxybenzoic acid (IBX)-mediated oxidation (Binder and Kirsch 2006). The cascade reaction proceeds through a silver(l)-catalyzed propargyl Claisen rearrangement, an amine condensation, and a gold(l)-catalyzed 5-exo-dig heterocyclization, as shown in Schane 11.4. 

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