Bronsted bifunctional catalysts

L-Proline is perhaps the most well-known organocatalyst. Although the natural L-form is normally used, proline is available in both enantiomeric forms [57], this being somewhat of an asset when compared to enzymatic catalysis [58], Proline is the only natural amino acid to exhibit genuine secondary amine functionality thus, the nitrogen atom has a higher p Ka than other amino acids and so features an enhanced nucleophilicity compared to the other amino acids. Hence, proline is able to act as a nucleophile, in particular with carbonyl compounds or Michael acceptors, to form either an iminium ion or enamine. In these reactions, the carboxylic function of the amino acid acts as a Bronsted acid, rendering the proline a bifunctional catalyst. 

Considerable effort has been devoted to the development of enantiocatalytic MBH reactions, either with purely organic catalysts, or with metal complexes. Paradoxically, metal complex-mediated reactions were usually found to be more efficient in terms of enantioselectivity, reaction rates and scope of the substrates, than their organocatalytic counterparts [36, 56]. However, this picture is actually changing, and during the past few years the considerable advances made in organocatalytic MBH reactions have allowed the use of viable alternatives to the metal complex-mediated reactions. Today, most of the organocatalysts developed are bifunctional catalysts in which the chiral N- and P-based Lewis base is tethered with a Bronsted acid, such as (thio)urea and phenol derivatives. Alternatively, these acid co-catalysts can be used as additives with the nucleophile base. 

Although dimeric Sharpless ligands as catalysts showed impressive results in related organocatalytic transformations, they provided only limited success in asymmetric MBH reactions (Scheme 5.12) [70]. These compounds are bifunctional catalysts in the presence of acid additives one of the two amine function of the dimers forms a salt and serves as an effective Bronsted acid, while another tertiary amine of the catalyst acts as a nucleophile. Whereas salts derived from (DHQD)2PYR, or (DHQD)2PHAL afforded trace amounts of products in the addition of methyl acrylate 8a and electron-deficient aromatic aldehydes such as 27, (DHQD)2AQN, 56, mediated the same transformation in ee up to 77%, albeit in low yield. It should be noted that, without acid, the reaction afforded the opposite enantiomer in a slow conversion. 

As stated above, the aromatization of short alkanes is carried out in presence of bifunctional catalysts, in where the dehydrogenating function is given by the metal component (Ga, Zn, Pt) and the H-ZSM-5 zeolite carries the acid sites. Although there is still some uncertainty concerning the initial activation of the alkane, probably both the metal and the zeolite acid sites are involved in this step. Metal sites can dehydrogenate the alkane to give the corresponding alkene, which can then be protonated on the Bronsted acid sites of the H-ZSM-5 zeolite to produce the carbocation. 

It was revealed that Rh/USY showed higher catalytic activity than commercial C0M0/AI2O3 catalyst in the hydrodesulfiirization of thiophene. We also studied the mechanism of hydrodesulfiirization of thiophene over RH/USY catalyst. As mentioned above, Rh/USY catalyst acts as bifunctional catalyst for the hydrodesulfiirization of thiophene, in which both Bronsted acid sites of USY and Rh in RH/USY catalyst act as active site. 

Some of the seminal studies of organocatalysts have been described, focusing on chiral Bronsted acid catalysts. Because this review is not comprehensive, there are a number of topics not covered in this chapter, such as carbene catalysts, bifunctional catalysts, and so on. 

Recent results are presented illustrating principal mechanistic differences between alkane isomerization in liquid acids and over solid acids, including bifunctional catalysts. Isotopic labeling shows that butane isomerization over solid acids proceeds preferentially as a bimolecular process, i.e. via a Cg intermediate, which subsequently decomposes, preferentially into two iso-Cn structures. Bronsted acid sites in zeolites form chemical bonds with metal clusters. The resulting metal-proton adducts function as "collapsed bifunctional sites". 

Methylcyclopentane (MCP) is a convenient probe molecule for interrogating the metal and acid sites of a bifunctional catalysts. For instance, metal clusters are formed in the cavities of zeolite Y by ion exchange, followed by calcination and reduction with hydrogen. Protons which act as Bronsted acid sites are formed during reduction of the metal ions. A monofunctional catalyst can be obtained by neutralizing these protons with NH3 or by secondary exchange with Na ions. With this acid-free form of such catalysts the ring-... 

Subsequent hydride transfer and metal catalyzed dehydrogenation steps lead to benzene. Qualitatively, the hydrocarbon conversion over bifunctional catalysts can thus be described as a reaction network using two types of sites. The reactions taking place on the Bronsted sites are similar to those in liquid acids, as described in the first part of this paper a second group of reactions takes place on the metal sites these steps are identical with those observed on the same metal in the absence of acid sites. Mills et al. devised a simple model... 

Some of the catalyst systems used in the asymmetric aldol reaction are also effective in related reactions. Thus, bifunctional catalysts and L-prohne-based organocatalysts have been used to good effect in the nitroaldol reaction and Mannich reaction. The latter process is also effectively catalysed by enantiomeri-cally pure Bronsted acids. Furthermore, much recent progress has been made in the development of a catalytic asymmetric Morita-Baylis-Hillman reaction using Lewis/Bronsted acid catalysts and bifunctional catalysts. 

The proposed mechanism for the isomerization of n-alkanes on bifunctional catalysts (60,61) is presented in Figure 14. From this mechanism an equilibrium between paraffins and olefins is established on the metal function. Then the olefins diffuse towards the Bronsted sites, where they become protonated and rearranged to give the branched carbenium ions. This, which is the rate controlling step, is followed by the desorption and hydrogenation, to yield the branched paraffins. 

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... 

Bronsted Base/Brensted Acid Bifunctional Catalysts... 

The design and synthesis of new tailor-made catalysts, optimized for specific reactions, requires an ability to control distances between catalytic sites and an understanding of the activity and interactions between the catalytic sites. This is also true for bifunctional catalysts (e.g., Pd/HZSM-5 catalysts for the hydrogenation of aromatics, where the presence of two different, nearby active sites is required for reaction. Here results for new NMR probe molecules are shown that can be used to estimate distances between catalytic sites. Using a combination of techniques including solid-state NMR the densitiy of Bronsted add sites in zeolite HY with diphosphine molecules was determined. ... 

The proposed mechanism presents the nucleophile undergoing a formal Bronsted base interachon with the guanidine catalyst, thus activating the hydro-cyanate for addition to the stabilized electrophile. In comparison to bifunctional catalysts, the guanidines are basic enough to activate and stabiUze both nucleophile and electrophile without assistance from other types of hydrogen-bond interachons. 

Bifunctional asymmetric catalysis, involving the synergistic activation of both acidic and basic sites in the substrates, has received great attention. The bifunctional catalysts discussed in this section generally contain a hydrogen-bond donor and a Bronsted basic moiety. 

Over the past decade, rapid growth has been achieved in organocatalytic asymmetric Diels-Alder and hetero-Diels-Alder reactions. Numerous organocatalysts such as chiral amines, guanidines, N-heterocyclic carbenes, Bronsted acids, and bifunctional catalysts have been successfully developed. The activation modes for these catalysts, such as imine-catalysis, enamine-catalysis, dienamine catalysis. 

Current efforts are focused on the preparation of bifunctional catalysts capable of directing the Lewis acid-catalyzed isomerization of alkyl glucoside intermediates to alkyl fmctosides, and their subsequent Bronsted acid-catalyzed dehydration to... 

The synthetic utility of the bifunctional catalysts in various organic transformations with chiral cyclohexane-diamine derived thioureas was established through the works of Jacobsen, Takemoto, Johnston, Li, Wang, and Tsogoeva. In the last decade, asymmetric conjugate-type reactions have become popular with cinchona alkaloid derived thioureas. The next section presents non-traditional asymmetric reactions of nitroolefins, enones, imines, and cycloadditions to highlight the role of chiral Bronsted base derived thiourea catalysts. 

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