Brpnsted acidity

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. 

J Metal nitrates in the presence of Lewis acids, Brpnsted acids and acid anhydrides... 

This reaction encompasses a number of interesting features (general Brpnsted acid/ Brpnsted base catalysis, bifunctional catalysis, enantioselective organocatalysis, very short hydrogen bonds, similarity to serine protease mechanism, oxyanion hole), and we were able to obtain a complete set of DFT based data for the entire reaction path, from the starting catalyst-substrate complex to the product complex. 

Solvent Class Dielectric Constant Brpnsted and/or Lewis Acid Brpnsted and/or Lewis Base Examples... 

The silver(I)-mediated electrocyclic ring opening of halocyclopropanes has been used to induce extensive skeletal rearrangements in gcm-dibromospiropentanes, providing rapid construction of naphthalenes and/or indenes (Scheme4.21 ).34 A variety of Lewis acids, Brpnsted acids, and solvent effects were carefully examined before optimal conditions were identified. It was found that subjection of spirocycle 60 to silver acetate in trifluoroacetic acid afforded rearrangement products 61 and 62 in moderate to good yields. The proposed mechanism of the reaction is illustrated in Scheme 4.21. 

Organocatalysts can be broadly classified as Lewis bases, Lewis acids, Brpnsted bases, and Brpnsted acids (for a review, see Seayad and List 2005). The corresponding (simplified) catalytic cycles are shown in Scheme 1. Accordingly, Lewis base catalysts (B ) initiate the catalytic cycle via nucleophilic addition to the substrate (S). The resulting complex undergoes a reaction and then releases the product (P) and the catalyst for further turnover. Lewis acid catalysts (A) activate nucleophilic substrates (S ) in a similar manner. Brpnsted base and acid catalytic cycles are initiated via a (partial) deprotonation or protonation, respectively. 

Identify each of the following terms (a) hydronium ion, (b) Br0nsted-Lowry theory, (c) proton (Brpnsted sense), (d) acid (Brpnsted sense), (e) base (Brpnsted sense), (f) conjugate, (g) strong acid or base, Qi) acid dissociation constant, i) base dissociation constant, (/) autoionization, k) pH, and (Z) K. ... 

Shibasaki and Groger developed lanthanide/alkali binapthoxide-based Lewis acid-Brpnsted base bifunctional catalysts [44]. One such example, the (R,R)-Ln-M-linked BINOL complex. 

As you might expect based on their definition of an acid, Brpnsted and Lowry defined a base as a proton acceptor. In the reaction taking place in Figure 8, ammonia, NH3, serves as the proton acceptor and is therefore a Brunsted-Lowry base. Ammonia also functions as a proton acceptor when it dissolves in water, as in the equation below. 

Transalkylation of alkylbenzenes, polyalkylbenzenes and other arenes can be brought about by a variety of catalysts including Lewis acids, Brpnsted acids and various zeolites and silicates with or without being doped with various transition metals or their oxides. There has been a particularly explosive growth in the volume of literature pertaining to the use of various natural and modified zeolites. Recent developments include the study and applications of shape-selective catalysis by zeolites. Much of the work is patented, and largely applies to industrial processes. 

Sample In the chemical reaction shown below, identify the Bronsted-Lowry acid, Brpnsted-Lowry base, conjugate acid, and conjugate base. 

Modification of H-ZSM-5 zeolites through solid-state reaction with ZnO was described by Yang et al. [32]. On the basis of XPS results they reported that, upon heat-treatment of a ZnO/H-ZSM-5 mixture, Zn ions migrated from the outer surface into the channels of the zeolite. This finding was supported by TPDA, IR (decrease of acidic Brpnsted sites upon solid-state reaction between ZnO and H-ZSM-5) and temperature-programmed reduction (TPR). The latter showed increased uptake and reducibility after thermal treatment of ZnO/H-ZSM-5 compared to ZnO. Zeolites Zn,H-ZSM-5 exhibited, after reduction in H2, pronounced selectivity in propane aromatization. 

Bromonium ion. See Halonium ion (R)- and (5)-2-Bromooctane, stereochemistry of hydrolysis of, 307—308, 319 A-Bromosuccinimide, reagent for allylic bromination, 371, 391 benzylic bromination, 415—416, 435 Brpnsted, Johannes, 134 Brpnsted acid. See Acidity Brpnsted base. See Basicity Brown, Herbert C., 228 Buckminsterfullerene, 410—411 1,3-Butadiene... 

It was shown that solid-state ion exchange is also a suitable route to preparation of active acidic or bifunctional catalysts. Introduction of Ca or Mg into mordenite [21] or La " into Y-type zeolite, mordenite or ZSM-5 [22] by solid-state reaction yielded, after brief contact with small amounts of water, acidic zeolite catalysts which were, for instance, active in disproportionation and/or dealkylation of ethylbenzene or in cracking of n-decane [43]. The contact with water was essential to generate, after solid-state ion exchange, acidic Brpnsted centres (compare, for instance. Figure 2). In the case of solid-state exchange between LaClj and NH -Y an almost 100% exchange was achieved in a one-step procedure, and the hydrated La-Y reaction product exhibited a catalytic performance (selectivity in ethylbenzene disporportionation, time-onstream behaviour) comparable to or even better than that of a conventionally produced La-Y (96) catalyst [22,23]. In fact, compared to the case of NH -Y the introduction of La " " by solid-state reaction proceeded less easily and was frequently lower than 100% with H-ZSM-5 or H-MOR. 

With respect to the catalysts employed in conjugate additions, a big collection of efficient stable and environmentally friendly natural or newly designed chiral organocatalysts has already been developed. These catalysts are usually cheap to prepare and readily accessible in a range of quantities. They fall into four major classes Lewis bases, Lewis acids, Brpnsted bases, and Brpnsted acids [If]. The identification of the generic modes of activation of these catalysts has been crucial to the success of organocatalysis. 

As anticipated by the work of Wynberg [7], the presence of a protic group in the catalyst s structure, capable of activating and orienting the electrophile, had a positive impact on the enantioselectivity of the conjugate additions. On this basis, several Brpnsted acid/Brpnsted basic bifunctional catalysts has been applied in the sulfa-Michael addition. A list of them is reported in Figure 14.4. 

The combination of PdCOCOCFjjj, (/ )-C4-TunePhos (217), and ethyl sulfonic acid (Brpnsted acid, activator) catalysed the partial hydrogenation of 2,5-disubstituted... 

A review of formal aza-Diels-Alder reactions of imines with e-rich dienes and enones, in the presence of Lewis acids/Brpnsted acids/organocatalysts, has been presented. Bifunctional A-acyl aminophosphine catalysts (75) are effective asymmetric organocatalysts in the hetero-4- -2-cycloaddition of a-substituted allenoates with tosylaldimines to produce optically active tetrahydropyridines. The Brpnsted acid-catalysed aza-Diels-Alder reactions of cyclopentadiene with iminoacetates possessing two chiral auxiliaries yielded 2-azabicyclo[2.2.1]hept-5-ene cycloadducts with high exo-selectivities. ... 

MOF windows. MOF Cu(L-asp)bpeo.s displays a porous 3D structure with saturated mononuclear copper nodes. Its monoprotonation leads to a new MOF of formula Cu(l-asp)bpeo.5(HCl)(H20) (Figure 9a). Unlike homogeneous aspartate complexes, protonation takes place at one of the carboxylic groups of aspartate, which is still bound to copper, generating a particularly acidic Brpnsted acid inside the pores. Methanolysis of propylene oxide and (2,3)-epoxybutane using this material shows moderate yields, but turnover numbers (TONs) drop to zero with the bulkier (2,3-epoxypropyl)-benzene, indicative of pore size selectivity. 

Arrhenius acid, Brpnsted-Lowry acid, Lewis acid... 

Bentrup, U., Bruckner, A., Martin, A., and Lticke, B. Permanent blockade of in sitn generated acid Brpnsted sites of vanadyl pyrophosphate catalysts by pyridine dnring the partial oxidation of toluene. Chem. Commun. 1999, (13), 1169-1170. 

Once highly acidic Brpnsted acids are in hand, we can also use them as en effective Lewis acid catalysts for various organic transformations. This is simply because the metal attached to such a super conjugate base makes the metal site very... 

In Sect. 2.1, heterobimetallic transition metal (Cu)/rare earth metal (Sm) system with Lewis acid/Brpnsted basic properties was introduced. By suitably selecting metal combinations depending on the targeted reactions, variety of chiral bimetallic Lewis acid/Br0nsted base bifunctional catalysts could be created, such as aPd/La/la... 

Several ways how (asymmetric) organocatalysis can be classified have been reported [14]. One general strategy is a classification according to Lewis acids/Lewis bases and Brpnsted acids/Brpnsted bases with the general simphfied activation modes depicted in Scheme 6.16. 

Paterson et al. reported perhaps the simplest intramolecular oxyanion conjugate addition in the synthesis of the C1-C15 fragment of swinholide A [20, 21]. This particular cyclization constitutes one of the first examples of the less common endo conjugate addition to a dihydropyran. Cyclization of 13 under Lewis acid/Brpnsted basic conditions provided the racemic dihydropyrone 14 in good yield (61 %) (Scheme 3). Attempts at reaction optimization by changing solvent had little effect. 

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