Reaction three-centre elimination

An ab initio study on the unimolecular elimination reactions of methacrylonitrile has revealed a direct four-centre elimination of HCN and three-centre elimination of H2 channels.17 A methylcyanoethylidene intermediate has also been identified. 

Ion abundances for methane loss from metastable (CH3CD2CHD2)t ions have been found to be in the ratios 87 1 12 (CH4 CH3D CH2D2) after correction for the numbers of equivalent pathways [250]. The reaction is a 1, 2 elimination, so these results indicate both a large primary and a large secondary deuterium isotope effect (see Sect. 7.5.4). On the basis of these intramolecular isotope effects, a mechanism involving a non-classical transition state with a CCH three-centre bond has been proposed for this methane elimination [250]. 

The versatility of pyridoxal-P for providing stereoelectronically assisted bond cleavages is further illustrated by y replacement and y elimination-deamination reactions where mechanistic and stereochemical interest now focusses on the changes which occur at the three centres (C , and C ) in the a-amino acid substrate. Representative examples of this class of enzyme catalysed reactions are shown in Fig. 35. 

It is now necessary to consider to what extent deviations from smooth correlations of k /kj) with AG or AG in proton transfer reactions reflect departures from the Hammond postulate. The Hammond postulate may be expected to apply to reasonably closely related three-centre reactions of the type considered above, and in particular to their simplified representations by empirical potential-energy surfaces. It does not follow that it applies generally to more complex reactions, and it is now well established that for concerted ionic reactions failure of bond-making and bond-breaking to remain properly in step can lead in the transition state to charge-localizations at atoms that bear no charge in the reactants or products, with the consequence that substituents at these atoms show no correlation between their effects on reaction rates and equilibria [123-126]. This behaviour has been best characterized in E2 j8-eliminations,... 

Similar effects were observed in the structures of the lithium salts of ester enolates [43] studied by Seebach et al. (1985). Here too systematic differences in angles are observed compared with amide and ketone enolates, and there is a correlation between the bond angles and the difference in the two C-O bond lengths at the reaction centre for three compounds [43], consistent with incipient elimination of t-butoxide to give the ketene [44] (Ferretti et al., 1991). 

Density functional calculations, incorporating clusters with and without solvent coordination to lithium and/or copper, reveal that the 5 n2 transition state always features inversion and retention at the electrophilic and nucleophilic centres, respectively. This transition state (100) is such that the carbons of the three alkyl groups are in a different electronic and spatial environment thus, the formation of RR, rather than RR, is governed by the transition state (101) for the reductive elimination reaction of the Cu(II) intermediate. 

Oxidative additions are a special class of insertion reactions. In addition to the categories mentioned in Section 10, which covers this topic, insertions of alkylidenes, silylenes, etc., into M-H bonds fall into an ambiguous domain they are insertion reactions of the unsaturated species into the M-H bond, yet oxidative additions at the C, Si, etc., atom. A similar ambiguity exists regarding the reverse reactions, namely /i-hydride and a-hydride eliminations from element-alkyls compounds to yield hy-drido-olefin and hydrido-alkylidene complexes, respectively. The former reaction is a reverse insertion if the product is viewed as an olefin complex, but an oxidative addition if it is viewed as a three-membered metallocycle. The latter reaction is a reverse insertion if the alkylidene is viewed as neutral, but an oxidative addition of a C-H bond to the metal centre. The tautomerization of phosphorous acid and of dialkylphosphites ... 

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