Homogeneous Bronsted acid catalysts

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. 

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. 

Catalytic conversions in the monoterpene field have been reviewed recently [13-15]. There is an ongoing transition from conventional homogeneous catalysts (mineral acids, zinc halides) to solid Bronsted and Lewis acid catalysts. Thus, limonene can be alkoxylated with lower alcohols using zeolite H-Beta as the catalyst [16] at room temperature already, with high selectivity and conversion (Scheme 5.3). The alkoxy compounds are applied as fragrances with, dependent on the length of R, characteristic odors. 

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... 

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. 

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. 

Catalysis by solid acids is of paramount importance in industrial chmnistry, namely due to its application in catalytic cracking, one of the most important processes in the world. However, despite its enormous importance, only recently have practical and quantitative relationship between the acidity of the catalyst and its catalytic activity began to appear, unlike homogeneous acid catalysis, which has made use of the Brdnsted relations for many years. The difficulties to be overcome are of various nature but it was found by some of the authors that Bronsted-like relationships also apply to solid acid catalysts [1]. 

The use of Lewis and Bronsted acids as catalysts in oleic acid esterification reactions it was assessed. Although being homogeneous catalysts, both compoimds can be recovered and reused in the reactions via a... 

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