Concerted exchange mechanism

Several authors suggested using the rate of H/D exchange between solid acids and methane as an indicator of the relative acidity of these catalysts.The first theoretical and experimental work in this field was published by van Santen and coworkers in Nature.For the theoretical part, the zeolite structure was represented by a small H3Si-OH-Al(OH)2-SiHs cluster, and for the experimental part, proton forms of Faujasite and MFI were chosen. The study concluded that there was a concerted exchange mechanism, in which no positive charge developed in the transition state (Fig. 3) with an activation energy of 150 20 kJ mol ... 

When [180]bicarbonate is a substrate, two labeled oxygen atoms enter the oxaloacetate, while the third appears in P . A concerted, cyclic mechanism could explain these results. However, study of kinetic isotope effects,291 use of a substrate with a chiral thio-phospho group,292 and additional lsO exchange studies293 have ruled out this possibility. A transient carboxyl phosphate (Eq. 13-53) is evidently an intermediate.294 295 The incorporation of the lsO from bicarbonate into phosphate is indicated by the asterisks. 

Since the products are the same chemical species as the reactants, the over-all reaction is substantially thermoneutral except for activation energy, the problem of energetics is thus side-stepped. The apparent ter-molecular reaction required by Eq. (43) is also no problem, as the dissolved molecules are essentially in constant collision with water molecules. Wilmarth, Dayton, and Flournoy questioned the adequacy of this concerted attack mechanism, however, as they believed that it would predict acid catalysis of the exchange reactions, as well as base catalysis. 

Fig. 11.1. Competing surface diffusion mechanisms on an fee (001) surface (adapted from Feibel-man (1990)) (a) intuitive mechanism in which adatom moves from one fourfold site to another by passing across bridge site, and (b) exchange mechanism in which diffusion occurs by concerted motion both of adatom and its subsurface partner.

Figure 2. Schematic showing (a) low probability of desorption for a protein adsorbed via several binding sites and (b) concerted cooperative mechanism of exchange between dissolved and adsorbed protein. (Reproduced with permission from Ref. 66. Copyright 1985 Plenum.)...

The differences in solvent exchange in HjO and DMF are also reflected in the exchange mechanisms. The activation volumes for water exchange are negative and, within experimental error, essentially constant for the different lanthanides this suggests a concerted associative (I ) mechanism. By contrast, the data for DMF exchange are more consistent with a gradual transition from an to a D mechanism between Tb " and Yb ". ... 

Figure 1 Formation of metal complexes in aqueous solution. Route I [(i) (vi) (iv)] represents a limiting mechanism, route Ila [(i) (ii) (iii) (iv)] a concerted water-exchange mechanism, and route Ilb [(i) (ii) (v) (iv)] a limiting 5n2 mechanism...

More possible is migration occurring by concerted two-center one-for-one exchange mechanism (3.69) or two-center cis pairwise exchange (3.70). 

The essential aspect of the u-bond metathesis mechanism is the concerted exchange of a metal-ligand a-bond with one of an incoming substrate where the reaction proceeds via a [2o-+2o-] cycloaddition, as illustrated in the transitions state in Figure 14.6. The key concept is that CT-bond metathesis is a one-step reaction with two u-bonds breaking (M-C and C -H) and two u-bonds forming (M-C and C-H). Therefore, there is no change involved in the oxidation state of the metal center. This reaction mechanism has been proposed for the d° and d°P metal compounds, and has been recently reviewed for transition metals. [58,59]... 

McDowell and Stirling194 studied electronic effects upon the reactivity of aryl vinyl sulfones towards amines. Rate constants for t-butylamine addition in ethanol at 25 °C were well correlated by the Hammett equation, with p = 1.59. Comparison of this with p values for H-D exchange mentioned above191 suggested considerable carbanionic character in the transition state, perhaps in a concerted mechanism. Rates of addition of amines to alkenyl, allenyl and alkynyl p-tolyl sulfones have also been measured195. 

Figure 3. Mitochondrial fatty acid oxidation. Long-chain fatty acids are converted to their CoA-esters as described in the text, and their fatty-acyl-groups transferred to CoA in the matrix by the concerted action of CPT 1, the acylcarnitine/carnitine exchange carrier and CPT (A) as described in the text. Medium-chain and short-chain fatty acids (Cg or less) diffuse directly into the matrix where they are converted to their acyl-CoA esters by a acyl-CoA synthase. The mechanism of p-oxidation is shown below (B). Each cycle of P-oxidation removes -CH2-CH2- as an acetyl unit until the fatty acids are completely converted to acetyl-CoA. The enzymes catalyzing each stage of P-oxidation have different but overlapping specificities. In muscle mitochondria, most acetyl-CoA is oxidized to CO2 and H2O by the citrate cycle (Figure 4) some is converted to acylcamitine by carnitine acetyltransferase (associated with the inner face of the inner membrane) and exported from the matrix. Some acetyl-CoA (if in excess) is hydrolyzed to acetate and CoASH by acetyl-CoA hydrolase in the matrix. Enzymes ...

A fundamentally different approach to the synthesis of 3-pyrrolines is evidenced in the annulation in Eq. 13.50 [58]. Ethyl 2,3-butadienoate 150 reacts with N-sulfony-limine 151 in the presence of triphenylphosphine under very mild conditions to give JV-protected 3-pyrroline 152 in 90% yield. The mechanism that has been postulated is related to that of the Baylis-Hillman reaction. Michael addition of triphenylphosphine to the allenyl ester generates a zwitterion that combines with the imine to give 153 in a non-concerted process. This is followed by ring closure, proton exchange and expulsion of triphenylphosphine to give 152. This annulation is successful only for aromatic or cinnamyl imines [59]. 

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