Ca-X bonds

The broken bonds (boldface = dissociated fragment) BDEs (boldface= recommended data reference in parentheses) kcal/mol kj/mol Methods (reference in parentheses) References 

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The E2 elimination is a one-step process and neither a eationic nor a carbanionic intermediate is formed. Proton abstraction by the base and leaving group departure are concerted, although not necessarily synchronous. That is, Cp-H bond breaking may be ahead or behind Ca-X bond breaking (Scheme 12.2). 

Coupling constants are routinely used to determine the side-chain conformation of amino acids in peptides and proteins. Whereas proteins nowadays are almost exclusively studied as C- and N-labeled isotopomers, peptides usually have these isotopes in natural abundance, i.e. the magnetically active heteronuclei are highly diluted. Most amino acids contain a methylene group at the ji-position for which the X angle is determined by the conformation of the Ca—Cp bond. Two vicinal Jhh coupling constants can be measured Ha to and H to Usually... 

An examination of the EIEs in Table 30 shows that the trend in the EIEs cannot be explained by hyperconjugation alone. If these KIEs were determined by hyperconjugation alone, the /3-deuterium EIE would be expected to decrease as the substrate acquired more cationic character at the a-carbon, i.e. as the Ca—Cp bond shortened. However, this trend was not observed. As the leaving group improves along the series X = H, OH, F, OH2+, the a-carbon of the substrate becomes more planar, the Ca—Cp bond shortens and the Cp—H bond in the substrate becomes shorter, i.e. the amount of cationic character at the a-carbon increases and the secondary /3-deuterium EIE increases. This trend in the EIE is obviously not consistent with a model where hyperconjugation is the sole contributor to the EIE. 

A large number of NHC complexes tolerate moisture, air, and elevated temperatures [1, 2]. The stability of these compounds is, however, still limited. One of the most important decomposition route for these complexes is the reductive elimination of 2-aIkyl or 2-aryl substituted azolium salts from NHC complexes with alkyl or aryl groups in cA-position to the NHC ligand. This route can be considered as the reverse reaction of the previously discussed oxidative addition of C2-X bonds to transition metals and has been reviewed [94, 123]. 

The E2 mechanism is so called because the process is bimolecular and in solution consists of an attack by a base on the 3-hydrogen atom with synchronous splitting of the substituent X in the form of an anion. In heterogeneous catalysis, the most important feature is the timing of the fission of the two bonds Ca—X and C —H in the E2 or E2-like mechanism, these bonds are broken simultaneously. Because this can be achieved only by the action of two different centres, a basic one and an acidic one with both present on the surface, the kinetic distinction of the mechanism loses its original sense under these circumstances. 

It is well known from homogeneous reactions that the mechanism depends also on the strength of the Ca—X and C —H bonds and this applies also to heterogeneous catalysis. The double influence on the choice of mechanism, i.e. of the nature of the catalyst and of the reagent, has been graphically represented by Mochida et al. [66] (Fig. 3). They have... 

The rate of catalytic dehydrohalogenation is influenced by the structure of the reactants, but the extent of this effect varies from one catalyst to another with change of mechanism, i.e. with the timing of the fission of the Ca—X and Cp—H bonds. This is best seen from the published data on the deuterium kinetic isotope effect in Table 8. Their significance for the elucidation of the mechanism will be dealt with in Sect. 2.4.4 and here we can simply state that the value of the isotope effect depends on the nature of the catalyst. However, with a different reactant and within a series of related catalysts, kH/kD values independent of the catalyst were obtained (Table 9) [183],... 

Typical acidic catalysts are silica—alumina, transition metal sulphates or chlorides, calcium phosphate etc. They are characterised by low deuterium kinetic isotope effects and low stereoselectivity (see Tables 8,11 and 12). These results correspond to the E2cA or El mechanisms, between which a transition may be observed due to the influence of the structure of the reactants, i.e. according to the polarity of the Ca—X and Cp—H bonds. Again, the reactions of 1,2-dibromoethane and 1,1,2,2-tetrachloroethane yielded the evidence. The deuterium kinetic isotope effect on silica—alumina was 1.0 for the dibromo-derivative, which indicates a pure El mechanism, whereas for the tetrachloro-derivative, the value of 1.5 was found. 

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