Catalysis bifunctional proton transfer

In benzene solution mutarotation is catalysed by a range of bifunctional proton-transfer catalysts, including carboxylic acids and the nucleoside cytosine. Ab initio calculations on formic acid catalysis of the mutarotation of 2-hydroxytetrahydropyran indicate strong coupling of the double proton transfer to endocyclic C-O bond cleavage. ... 

Cox, M.M., Jencks, W.P. Concerted bifunctional proton transfer and general base catalysis in the methoxyaminolysis of phenyl acetate. J. Am. Chem. Soc. 1981, 103(3), 580-587. 

Another type of bifunctional catalysis has been noted with a,cn-diamines in which one of the amino groups is primary and the other tertiary. These substituted diamines are from several times to as much as 100 times more reactive toward imine formation than similar monofunctional amines. This is attributed to a catalytic intramolecular proton transfer. 

A major question has been that of bifunctional catalysis . For example, if a micelle contains both nucleophilic groups and groups which can transfer protons one might hope to achieve high rates of deacylation by having concerted nucleophilic attack and proton transfer (Scheme 6). Such concerted processes are well established in enzymic reactions, but evidence in... 

Jencks (1972) has concluded that concerted bifunctional acid-base catalysis is rare or nonexistent because of the improbability of meeting simultaneously at two sites on reactant and catalyst the conditions of the rule which he has proposed for concerted reactions. The rule states that concerted general acid-base catalysis of complex reactions in aqueous solution can occur only (a) at sites that undergo a large change in pAT in the course of the reaction, and (b) when this change in pAf converts 2m unfavourable to a favourable proton transfer with respect to the catalyst, i.e., the pAT-value of the catalyst is intermediate between the initial and final pAf-vadues of the substrate site. 

A catalytic process (often involving proton transfer) in which both functional groups of a bifunctional chemical species participate in the rate-controlling step. In such systems, the catalytic coefficient is larger than would be expected were only one functional group present. Bifunctional catalysis is not the same as two different catalysts acting in concert. 

Push-pull acid-base catalysis has been proposed to account for the proton switch mechanism which occurs in the methoxyaminolysis of phenyl acetate (Scheme 11.14) where a bifunctional catalyst traps the zwitterionic intermediate. A requirement of efficient bi-functional catalysis is that the reaction should proceed through an unstable intermediate which has p values permitting conversion to the stable intermediate or product by two proton transfers after encounter with the bifunctional catalyst the proton transfer with monofunctional catalysts should also be weak. 

Bronsted acid/base catalysis is the most common enzymatic mechanism, since nearly all enzymatic reactions involve a proton transfer. This means that nearly all enzymes have acidic and/or basic groups in their active site. In add catalysis, the substrate is protonated by one of the amino add residues at the active site (typically aspartic acid, glutamic acid, histidine, cysteine, lysine, or tyrosine). This residue itself must therefore be protonated at the readion pH (typically between pH 5 and 9), with a pKa just above this value. Conversely, in base catalysis, the pJCa of the deprotonating residue must be just below the physiological pH. Some enzymes can even carry out bifunctional catalysis, by protonating and deprotonating two different sites on the same substrate molecule simultaneously. 

An NMR kinetic study of a phosphine-catalysed aza-Baylis-Hillman reaction of but-3-enone with arylidene-tosylamides showed rate-limiting proton transfer in the absence of added protic species, but no autocatalysis.175 Brpnsted acids accelerate the elimination step. Study of the effects of BINOL-phosphinoyl catalysts sheds light not only on the potential for enantioselection with such bifunctional catalysis, but also on their scope for catalysing racemization. 

POMs are promising catalysts for acid, redox and bifunctional catalysis. In many structures, the transition metal addenda atoms such as Mo or W exist in two oxidation states, which results in different redox properties as determined by polarog-raphy. The exceptional ability of heteropolyanions to act as electron reservoirs has been demonstrated by the preparation and characterization of numerous reduced derivatives [32]. They also exhibit high solubility in polar solvents, which means that they can be used in homogeneous catalysis. The wide range of applications of heteropoly compounds are based on their unique properties which include size, mass, electron and proton transfer (and hence storage) abilities, thermal stability. 

Hjehnencrantz, A. and Berg, U. (2002) New approach to biomimetic transamination using bifunctional [l,3]-proton transfer catalysis in thioxanthenyl dioxide imines. The Journal of Organic Chemistry, 67, 3585-3594. 

For the first, third, and fourth catalysts in (84) the two tautomers are chemically identical, and the same is true for ions such as HCOJ, HPO4", H2PO2, and H2ASO4, which have been reported to have an abnormally high catalytic activity in some reactions.It is clear that the effectiveness of this kind of catalyst is related to its particular electronic structure rather than to its acid-base properties, and the process is more appropriately described as tautomeric catalysis than as bifunctional or concerted acid-base catalysis. It is of interest that a theoretical treatment of some molecules in which acidic and basic groups form part of the same TT-electron system shows some parallelism between catalytic activity and the coupling constants of the molecular orbital theory moreover, a very general treatment of concerted proton transfers indicates that simple bifunctional acid-base catalysis is likely to be of importance only under very restricted conditions. ... 

A synchronous transfer of two protons, which in reaction (9.14) competes with a two-step process, is in some cases the predominant proton exchange mechanism. Such double proton migrations play an important role in many chemical and biochemical reactions in which the steric hindrances impeding proton transfer in a substrate molecule are removed thanks to the double proton exchange between substrate and enzyme [79]. The double proton transfers determine the mechanism of the bifunctional acid-base catalysis[80, 81]. The interest in the mechanism of double proton migrations in the H-bound complexes became especially keen after Lowdin [82] advanced in 1963 the hypothesis to the effect that it is precisely such processes in the DNA molecules that underlie the nature of spontaneous mutations. 

One aspect of asymmetric catalysis has become clear. Every part of the molecule seems to fulfill a role in the process, just as in enzymic catalysis. Whereas many of us have been used to simple acid or base catalysis, in which protonation or proton abstraction is the key step, bifunctional or even multifunctional catalysis is the rule in the processes discussed in this chapter.Thus it is not only the increase in nucleophilicity of the nucleophile by the quinine base (see Figures 6 and 19), nor only the increase in the electrophilicity of the electrophile caused by hydrogen bonding to the secondary alcohol function of the quinine, but also the many steric (i.e., van der Waals) interactions between the quinoline and quinuclidine portions of the molecule that exert the overall powerful guidance needed to effect high stereoselection. Important charge-transfer interactions between the quinoline portion of the molecule and aromatic substrates cannot be excluded. 

According to the available experimental data, it is impossible to distinguish between these mechanisms, but the second mechanism seems to be preferred [Scheme (7)] for, according to this Scheme, the reaction of amine addition proceeding through a cyclic transition state is completed in one step, whereas for the reaction to occur according to Scheme (2) or (6) it is additionally necessary to transfer the proton. Then, it is probable that the different mechanisms [Schemes (3) and (5)] may precede formation of one and the same transition state [Scheme (7)]. Note finally that the mechanism of bifunctional catalysis [Scheme (7)] is extremely popular in different reactions of nucleophilic substitution at the saturated carbon atom and reactions with participation of a carbonyl group32>. 

Nilsson and coworkers42 showed that addition of trifluoroacetic acid to enamines in dry pentane at 0 °C results initially in protonation at the ft carbon and proposed that the proton is transferred to the nitrogen atom by a bifunctional catalysis according to equation 19. They also showed that appropriate exchange resins can be used to favour selective protonation of enamines42c. 

An article is exclusively focused on outer sphere catalysts. In outer-sphere hydrogen catalysis, substrates such as ketone, imine, or A-heterocycle remain in the outer sphere. A hydride and a proton are transferred to these substrates by either a concerted or a stepwise path from catalysts such as Bullock s ionic hydrogenation catalysts, bifunctional catalysts in the tradition of Shvo and Noyori, and Stephan s frustrated Lewis pair catalysts. The outer-sphere pathways can use inexpensive metals and even non-metal catalysts, and lead to useful selectivity properties, particularly Noyori s asymmetric catalysis. ... 

---