Bronsted add catalysts

Enantioselective Synthesis of Amines by Chiral Bronsted Add Catalysts... 

Quite recently, not only Lewis adds, but also Bronsted adds were found to be effedive catalysts for the three-component Mannich-type reactions in water with the aid of a surfadant. For example, Akiyama and co-workers1301 have reported that a combination of HBF4 and SDS is effedive for the readions of aldehydes, amines, and silyl enolates. We have found that dodecylbenzenesulfonic add (DBSA), a Bronsted add with a surfadant moiety, also catalyzes the reactions in water.1311 Furthermore, DBSA can be used for the dired Mannich-type reactions of aldehydes, amines, and ketones, without using silyl enolates as nucleophilic components (Eq. 8).1321... 

K. Manabe, S. Kobayashi, Remarkable Enhancement of Readivity by Bronsted Adds in Aldol Readions Mediated by Lewis Acids-Surfadant-Combined Catalysts in Water Tetrahedron Lett. 1999, 40, 3773-3776. 

Some metal salts and Bronsted add were reported to work as catalysts for the propargylic substitution reactions, see ... 

Considerable efforts have been made to understand ZSM-5-based catalysts for the selective oxidation of benzene to phenol by nitrous oxide. However, the nature of the active species remains unclear. The most important proposals for the active species are extraframework Fe species [101], Bronsted add sites [102] and Lewis and A1 sites [103, 104]. The activity is usually interpreted in terms of very small, possibly... 

The allylation of carbonyl compounds with allyltin reagents is still an active area of organotin chemistry from the methodological point of view, and also for synthetic applications. For completeness we should add several alternative techniques, such as the development of trifluoromethanesulphonic acid as a Bronsted acid catalyst for the allylation of aldehydes in water123, or the design of fluorous allyltin reagents for the platinum-catalysed allylation of aldehydes124. 

A procedure was studied by which enamines are produced by the condensation of the parent ketones and a secondary amine in the presence of molecular sieves. The molecular sieves are used to trap the water which is formed.[6] The enamine formation is acid catalyzed both BrOnsted adds and Lewis adds can be used as catalysts.[7]... 

One can thus conclude that the greater is the concentration of surface Bronsted add sites, the higher should be the catalytic activity. However, when calculating the specific activity, i.e. the catalytic activity per Bronsted acid site (Fig.4a), some specific features of each catalyst can be evidenced. For instance, every site of F34 appears very active for HEP dehydration, in spite of the low overall activity, which is due only to the low concentration of surface acid sites. Other catalysts, such as HBETA 20, show a low specific activity, but a rather good overall performance, possessing a relatively high number of Bronsted acid sites. Of course, also the strength of the acid sites could play a role. In order to evaluate this parameter, for each catalyst we calculated the ratio between the concentration of Bronsted acid sites measured after... 

There are various acidic and basic sites on surfaces. Coordinatively unsaturated cations act as electron pair acceptors and are called Lewis acid sites. The Bronsted add sites on catalyst surfaces are hydroxyl groups able to donate protons. The basic sites are, in most cases, oxygen anions, including oxygen from hydroxyl groups. 

In a sodium mordenite of unit cell composition Nag[Al8Si4o096]24 H2O the ideal ou assed hydrogen form should have 1.61 x 10 Brcinsted acid sites per gram of dry zeolite. The maximum reaction rate correspcmds with 1.7 xlO molecules of water added to the zeolite. This quantity of water is just suffident to convert the hydrogen mordenite to hydronium mordenite. However, for general validity of the water co-catalysis mechanism it is necessary to prove that maximum reaction rates should appear in other hydrogen zeolite catalysts when the water added is equal to the number of Bronsted add sites. 

In alcoholic solution, lactic ester, for example, methyl lactate can be produced in the presence of a Lewis add catalyst On the contrary, Bronsted acid catalysts such as ion-exchange resins selectively convert GLA/DHA to pyruvaldehyde dimethylacetal via acetahzation of PA. Strong Br0nsted add sites should thus be diminished to avoid this acetahzation Lewis acid sites are responsible for selective formation of methyl lactate [200-202]. However, the rate-determining step for the reaction is considered to be the first dehydration of GLA/DHA to PA, which is accelerated by weak Bronsted acid sites [203]. A bifunctional catalyst with Lewis acid sites and weak Bronsted acid sites, for example, a composite of carbon (weak Bronsted acid) and Sn-sihca (Lewis add) is reported as a fast and selective catalyst for lactic acid and... 

In 2003, we first demonstrated that l,l -bi-2,2 -naphthol (BINOL)-derived chiral LBBA (Lewis base and Bronsted add) bifunctional phosphine CP17 (LB = PPhs, BA = Ph-OH) could be used as an effective catalyst in asymmetric aza-MBH reaction of A-tosylimines with MVK and phenyl acrylate, affording the corresponding adducts in good yields with high ees (Scheme 2.119). The addition of molecular sieves increased chemical yields because they removed the ambient moisture that caused the decomposition of A-sulfonated imines. The asymmetric induction of this catalyst is comparable to that of the quinidine... 

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