Homogeneous Bronsted acid

In some cases the effect of the nature of aromatic hydrogen substituent has been also observed (Table 48.2). These results contrast with those obtained using typical homogeneous Bronsted acid catalysts (e.g., sulphuric) for the acylation of the same substrate with acetic anhydride, under the same experimental conditions, where the yields (98%, 95%, 91% for R=N02, H, OMe, respectively) do not significantly depend on the nucleophile s substituent nature [23]. These data imderline the contribution of the heterogeneous catalyst. 

Like the AVADA and the AlkyClean processes, these two processes also replace the liquid acid/base catalysts with solid acids and bases [192]. Although the reaction mechanism for the heterogeneous acid-catalyzed esterification is similar to the homogeneously catalyzed one [207,208], there is an important difference concerning the relationship between the surface hydrophobicity and the catalyst s activity. This is especially true for fatty acids, which are very lipophilic compounds. One can envisage three cases First, if there are isolated Bronsted acid sites surrounded by a... 

Bhugun I, Lexa D, Saveant JM (1994) Ultraeffident selective homogeneous catalysis of the electrochemical reduction of carbon dioxide by an iron(O) porphyrin associated with a weak bronsted acid cocatalyst. J Am Chem Soc 116 5015-5016... 

The conventional method of preparation of these aromatic ketones is the homogeneous Friedel-Crafts acylation of aromatic hydrocarbons with carboxylic acid derivatives using Lewis acids (A1C13, FeCl3, BF3, ZnCl2, TiCl4) or Bronsted acids (polyphosphoric acid, HF). For this purpose, stoichiometric to excess amounts of the catalyst are required for the reaction to proceed (ref. 1). 

Acylation of aromatic compounds is a widely used reaction for the production of fine chemicals. The classic methodology for acylation is Friedel-Crafts, this uses stoichiometric amounts of Lewis acids, such as metal halides, but some Bronsted acids, such as polyphosphoric and sulphuric acid, can also be used as catalysts. Moreover, the reaction is carried out homogeneously and the catalyst must be destroyed to obtain the final products, generating a great amount of corrosive and toxic waste. 

It must be noted that the phenol/aldehyde reaction can be catalyzed by Bronsted acids (protonation of the carbonyl oxygen) as well as by Lewis acids (coordination of the carbonyl oxygen). In the latter case one Lewis centre (e.g. Al ) can accommodate and activate both the phenol and the aldehyde (cq. the benzyl alcohol, in the consecutive reaction). As a consequence, ortho-substitution is favoured [14,15]. The high 2,2 -dihydroxydiphenylmethane selectivity we obtained with homogeneous Al " -catalysis and with 7-alumina is consistent with these data. Additionally, the finding that the H - US - Y catalyzed toluene/formaldehyde-condensation gives a low 2,2 -selectivity, 19% [16], compared to the 32% we obtained with phenol, also indicates the hydroxyl-group plays a role. However, transalkylation, reported to lead to ortho-substitution in condensations of phenol with methanol on both zeolite- and non-zeolite Bronsted acid catalysts [17], can t be ruled out. 

This mechanism is entirely different from that of the homogeneous reaction catalyzed by Bronsted acid in nonaqueous solution. The latter mechanism, proposed by Bell et al. 56), is given below for comparison, with attention to the second-order dependence on the acid concentration ... 

CO and two of H2O and produces two moles of CO2. Homogeneous catalysis of this reaction can be accomplished with an iron carbonyl in conjunction with a Bronsted acid or base. Thus it appears that conditions might be found where the shift reaction itself can be eflFected homogeneously using metal complex catalysts. To this end, we have been examining the activity of various homogeneous catalysts for the shift reaction, and our investigations of metal carbonyl cluster complexes are summarized here. 

The use of homogeneous Bronsted and Lewis acids and bases was one of the main reasons for the production of high amounts of inorganic salts as byproducts in industrial syntheses. The ratio of mass unit by-product per mass unit of product is explained as an E-factor by Sheldon for several industry segments. For example, in the fine chemical industry and for the production of pharmaceuticals, sometimes 50-150 kg of by-products per kg desired product are formed. Therefore, new processes in the production of fine chemicals in order to reduce the high numbers of by-product formation are needed. 

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