Brpnsted adds

Conclusive evidence has been presented that surface-catalyzed coupling of alcohols to ethers proceeds predominantly the S 2 pathway, in which product composition, oxygen retention, and chiral inversion is controlled 1 "competitive double parkir of reactant alcohols or by transition state shape selectivity. These two features afforded by the use of solid add catalysts result in selectivities that are superior to solution reactions. High resolution XPS data demonstrate that Brpnsted add centers activate the alcohols for ether synthesis over sulfonic add resins, and the reaction conditions in zeolites indicate that Brpnsted adds are active centers therein, too. Two different shape-selectivity effects on the alcohol coupling pathway were observed herein transition-state constraint in HZSM-5 and reactant approach constraint in H-mordenite. None of these effects is a molecular sieving of the reactant molecules in the main zeolite channels, as both methanol and isobutanol have dimensions smaller than the main channel diameters in ZSM-S and mordenite. 

In 2007, AntiUa and coworkers described the Brpnsted add-catalyzed desymmetrization of me yo-aziridines giving vicinal diamines [75]. hi recent years, chiral phosphoric acids have been widely recognized as powerful catalysts for the activation of imines. However, prior to this work, electrophiles other than imines or related substrates like enecarbamates or enamides have been omitted. In the presence of VAPOL-derived phosphoric acid catalyst (5)-16 (10 mol%) and azidotrimethylsilane as the nucleophile, aziridines 129 were converted into the corresponding ring-opened prodncts 130 in good yields and enantioselectivities (49-97%, 70-95% ee) (Scheme 53). 

Noda A, Susan MA, Kudo K et al (2003) Brpnsted add-base ionic liquids as proton-conducting nonaqueous electrolytes. J Phys Chem B 107 4024-4030... 

Zeolites aie crystalline. Diffraction methods ate therefore the prime technique for structural characterization. Local stmctural features (such as Brpnsted add sites, non-frramewcHlr cation configurations etc.) that do not obey the full crystallographic symmetry are often present These contribute to the diffraction pattern only wealdy if they are dilute, or they t pear as disorder in the diffraction-averaged stracture. Such features must be probed by techniques such as solid state nmr or EXAFS that are sensitive to local enviromnent, but proper interpretations of such data must still be made in the context of the crystal structure provided by the di raction results. 

Also, Gong and co-workers reported a Brpnsted add catalyzed three-component asymmetric 1,3-dipolar cycloaddition reactions between aldehydes 213, amino esters 214, and dipolarphiles 215 by catalyst 216, providing pyrrolidines 217 in high yields with excellent enantioselectivity. Scheme 3.69 [86], The methodology introduced a concept that stereochemistry can be controlled by use of a chiral Br0nsted acid (BH), e.g., phosphoric acid. The chiral BH provided sufficient acidity... 

Johannes Nicolaus Brpnsted (1879-1947). Danish chemist. In addition to his theory of acids and bases, Brbnsted worked on thermodynamics and the separation of mercury isotopes. In some texts, Brpnsted adds and bases are called Brpnsted-Lowry adds and bases. Thomas Martin Lowry (1874-1936). Enghsh chemist. Brpnsted and Lowry developed essentially the same add-base theory independently in 1923. 

Br0nsted Acid Organocatalysis. The field of Brpnsted add organo-catalysis [151] is clearly dominated by chiral binol-derived phosphoric acids, which after the seminal reports of Akiyama et al. [152] and of Uraguchi and Terada [153] have become one of the most powerful types of organic catalysts [154]. Figure 2.33... 

Intrazeolitic MOCVD chemistry is relatively new. A synthetic chemistry for the preparation of encaged clusters from metd alkyl precursors (alkyl is often methyl) has been suggested by a few researchers. [220, 221] In the first step, a vapor phase precursor is introduced into the zeolite supercages. One of the methyl groups can then react with a Brpnsted add site, thereby produdng CH4 and a concurrent anchoring of the organometallic precursor to the zeolite internal surface (Eq. 4.16, Z represents the zeolite framework). 

In 1950, Derbyshire and Watos first considaed the possibility of electrophilic aromatic hydroxylation with electrophiUc hydroxyl (OH+) [41]. While in gas phase heterolytic cleavage of the 0—0 bond in HjOj molecule requires much more enagy than homolytic cleavage (ca. 300kcal/mol vs. 51 kcal/mol at 298 K), it becomes more favorable in polar solvents in the presence of Lewis or Brpnsted adds [42]. Similarly, peroxy acids (organic or inorganic) can provide electrophilic oxidizing species [42,43]. 

A plausible transition state (TS) to explain the origin of the activation and the asymmetric induction through a dualactivation mode by the catalyst has been envisioned by the authors (Fig. 9.3). The synergic effect provided by the involvement of both an achiral Brpnsted add and the chiral secondary amine 38 as the catalytic systan in this multicomponent process was crucial for the success of this reaction since neither by themselves could catalyze the formation of DHPM efficiently. Thus, the acid additive could be involved in the formation of... 

Lewis- and Brpnsted add-mediated Knoevenagel condensations are thought to follow a diverse type of mechanism (Scheme 13.3). The add-mediated enol formation with subsequent 1,2-addition to the activated aldehyde component provides a p-hydroxy carbonyl 4. Dehydration of this compound leads to the Knoevenagel condensation product 2. 

The Brpnsted definitions of acids and bases are not restricted to species in aqueous solution. In fact, Brpnsted add-base reactions sometimes take place in the gas phase. For example, in the reaction between HCl and NH3 gases, HCl acts as the Brpnsted acid, donating its proton to NH3, which, by accepting the proton, acts as a Br0n.sted base. The products of this proton transfer are the chloride ion (CF), and the ammonium ion (NH4), which subsequently combine to form the ionic solid ammonium chloride. 

Notably, the reaction is not prevented by the basic amine, suggesting that the initial gold-catalyzed cycUzation must be facile. 149 can then proceed through Lewis acid-or Brpnsted add-catalyzed formation of the N-acyl iminium ion 150, which cyclizes to yield the final product 147 in an excellent yield. 

Both nucleophilic and Brpnsted acid/base catalyzed pathways previously discussed were suggested as possible relevant mechanisms for the formation of enol esters 51. From a stereochemical point of view, the two mechanisms differ mainly in that the nucleophilic pathway would involve a diastereoseleclive protonation of the chiral zwitterionic enolate 47 (Scheme 3.22), while in the Brpnsted add/base mechanism, the catalyst may be considered as a chiral proton shuttle taking part in both deprotonation of diphenylacetaldehyde and enantioselective protonation of the prochiral enolate 52 (Scheme 3.23). Although a number of experiences have been conducted to gain insights into the mechanism, no clear-cut evidence was obtained to decide between these two relevant mechanisms. 

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