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. 

In addition to single bond fission it is also possible for two or more bonds to break simultaneously and for molecular fragments to be ejected. Both three- and four-centre transition structures are possible in molecular elimination processes, examples of the former being the elimination of H2 from CH2O and (a,a) elimination of HCl from CHCICF2. A four-centre (a,jS) transition structure must be involved for the elimination of HF from CHCICF2, and this competes with the three-centre elimination channel forming HCl (the ratio of three- to four-centre elimination is 0.87 0.13). 

The elimination of HCl from vinyl chloride (CH2CHCI), following photo-excitation at 193 nm (ArF laser) can occur via either a three-centre (a,a) or a four-centre (a,jS) transition structure. The HCl is observed to be both vibrationally (v < 7) and rotation-ally hot, with a bimodal rotational distribution (i.e. Ti 500 K and Ta = 10 000 K, for v < 4). The low rotation energy distribution results from four-centre elimination, whereas the hot distribution is from three-centre elimination. Low rotational excitation is expected with four-centre elimination, as the H atom has a small impact parameter for its movement towards the halogen atom. 

The energy difference is expected to be reduced and perhaps eliminated when the calculations are repeated utilizing the 4-3IG basis set because of Pople s experience with the C3H7 ion. The non-classical form should be preferentially stabilized by the improved representation of the atomic orbitals, which should enable calculations to give a better account of the long bonds and three-centre ring. 

The reductive elimination step (iv) is a three-centre mechanism, which creates the carbon-carbon bond, regenerates the catalyst and needs the R1 and R3 groups to be cis on the palladium. This may be the case when cis bidentate ligands are used188. On the other hand, a trans to cis isomerization may precede the reductive elimination, which operates through T-shaped Pd(II)189,190 or Pd(IV)191-193 intermediates. Finally, recent studies argued that a T-shaped three-coordinate species c -[PdR1R2L] may be formed directly by an associative transmetallation step. 

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 state of the system consisting of n electrons and two nuclei is now specified in terms of 3n + 6 spatial coordinates referred to the centre of mass plus three centre-of-mass coordinates. Three of the spatial coordinates are redundant and can therefore be eliminated. Various choices of redundant coordinates can be made we choose to... 

Methyl shikimate 116 has been made using a strategy with iodolactonisation at its heart.26 Note that shikimate has three chiral centres. Interestingly, these three centres are controlled in the synthesis by a centre that is finally destroyed. The first steps are iodolactonisation of 103 and subsequent elimination and epoxidation to give 109 as described above. 

The elimination of hydrogen from 1,1-difluoroethene occurs by a three-centre mechanism, a postulate that has been supported by calculations. The photodissociation of 2-chloro-l,l-difluoroethene shows the fast elimination of chlorine atoms. Photodissociation of 2-chloro-l,l-difluoroethene at 193 nm proceeds by elimination of HCl via a three or a four-centre pathway. Both CCl and CC double bond fission processes contribute to the decomposition of 1,1- and... 

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. 

A theoretical (RRKM) investigation of unimolecular dissociation of vinyl cyanide has indicated that rapid hydrogen scrambling precedes elimination of H2 and HCN, which occur preferentially via three-centre transition states. ... 

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,... 

It is unfortunately the case that when we incubate apoferritin with a certain number of iron atoms (for example as ferrous ammonium sulfate), the product, after elimination of non-protein-bound iron, does not have a homogeneous distribution of iron molecules which were able to (i) take up iron rapidly through the three fold channels, (ii) quickly transfer it and form a diiron centre on a ferroxidase site, and (iii) to transfer the iron inward to a nucleation site, where (iv) it will begin to catalyse iron oxidation on the surface of the growing crystallite, will accumulate iron much more rapidly, and in much greater amounts than molecules in which steps (i), (ii) and (iii) are slower, for whatever reasons (perhaps most importantly subunit composition, and the disposition of subunits of the two types H and L, one with regard to the other). This polydispersity makes the analysis of the process of iron uptake extremely difficult. 

In the case of polynuclear metal cluster SCO complexes in the solid state, there will be intra-cluster, as well as inter-cluster cooperativity. To eliminate inter-cluster effects totally, studies must be made in dilute solutions. Williams et al. have done just this for a dinuclear [Fe(II)2L3] helicate complex which does not contain a good superexchange pathway between the Fe(II) centre but, rather, three flexible bis-bidentate ligands. A very broad, two step, SCO was observed (LS-LS<->LS-HS<->HS-HS) and fitted to a model for negative cooperativity in which subtle structural changes around each Fe oc-... 

For a three-body system such as A + BC AB + C, there are 3 x 3 = 9 degrees of freedom in the problem. However, three degrees of freedom can be eliminated by considering the centre of mass coordinates. If we consider... 

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. 

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