Four-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. 

On the other hand, the formation of propargyl chloride has been explained by assuming the formation of species 94a which goes to product via a four-centre elimination reaction (equation 16). As an... 

If the decomposition is a unimolecular process then a loose, four-centred transition state has to be assumed. The log 15 value, though slightly high, is not unreasonable considering other four-centred elimination reactions such as, for instance, the hydrogen halide eliminations. However, the activation energy, determined experimentally, is higher than expected for such a reaction. 

C2H2F. In the reaction O + 1,1-C2H2F2, the OCH2CF2 intermediate is favoured and four-centre elimination of HF results [analogous to pathways (a), (b) and (c) in Fig. 14], giving an inversion ratio (Ni/Nq) that is closer to the statistical limit. 

For the allyl ether/Me3Si+ system the usual ether reaction , i.e. loss of an olefin via a four-centre elimination, is a minor process. The two major decomposition pathways occur through six-membered transition structures (reactions 115, 116) and, obviously, involve the allylic double bond. Quite remarkable is the methyl transfer 293- 295. 

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. 

Decomposition of methyl nitrite by concerted elimination (CH2O and HNO formation) and O-N bond dissociation (CH3O and NO formation) has been investigated by classical trajectories and statistical variational efficient microcanonical sampling-transition state theory. The dissociation, which exhibits an inverse deuterium isotope effect, is markedly faster than the four-centre elimination process. The UHF/AMl MO method has been used to study thermolyses of aroyl nitrites. ... 

One can postulate two principal routes to the formation of unsaturated carbenium ions from the dominant population of saturated ions. The first is a straightforward four-centre pyrolytic (and hence thermal and not catalytic) elimination of a saturated molecule from a saturated carbenium ion ... 

For isopropyl chloroformate decomposition in the gas phase at 240 °C these two reactions proceed at almost equal rates . The products can be accounted for in terms of a four-centre transition state (1) for the substitution reaction, and a six-centre state (2) for the elimination reaction... 

Bamford and Norrish observed that the free radical formation is the sole primary process in the photolysis of cyclohexanone, while step II is the major reaction occurring in the photolysis of 1-menthone. These results are rather difficult to interpret if reaction II occurs through a four-centred ring complex however, if a six-centred complex is involved, the consideration of the steric factors leads to a conclusion which is reconcilable with the results of Bamford andNorrish. The significance of steric factors (stereoelectronic requirements) appears from the fact that type II elimination is the major intramolecular path in the photolysis of ciy-2- -propyl-4-t-butyl cyclohexanone, while the photolysis of the tram compound yields the cis isomer as the major product The difference has been explained... 

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... 

This argument is illustrated in Scheme 3 and is equivalent to that put forward to explain syn addition of bromine to the / -silylstyrene (equation 45). Based on additional experiments Koenig and Weber were able to rule out some alternative mechanisms, such as a four-centred transition state or syn addition followed by anti elimination. 

DFT calculations have been reported of the copper-catalysed reaction of A-methyl indoles with amides, which result in amidation at the 2-position. The results show that a concerted metalation—deprotonation pathway does not explain the observed regioselectivity. Instead, a four-centre reductive elimination involving the transition state (80) is proposed. The reaction is likely to be completed by proton transfer from the amidated carbon atom to the butoxy ligand. Copper-catalysed reaction of azoles or polyfluoroarenes with sulfoximines may produce A-aryl sulfoximines. Owing to the mild reaction conditions, enantiopurity in the starting sulfoximine is retained in products such as (81). 

The kinetics and mechanism of the unimolecular gas-phase elimination of 2-(dimethylamino)ethyl chloride have been examined by using DFT methods to explain the enhanced reactivity in gas-phase elimination compared to the parent compound ethyl chloride. The TS located on the minimum energy path had a four-centred cyclic configuration comprising chlorine, hydrogen, and two carbon atoms and benefited from electron delocalization involving the dimethylamino substituent. 

Since no evidence for an initial cfj-isomer could be obtained, a transoid free-radical addition was preferred to a four-centre mechanism. No fluoro-trimethylsilane was formed in this reaction, which precludes the occurrence of the addition-elimination sequence proposed for reactions of the mercurial with fluoro-olefins. Perfluoropropyne behaves differently and its reaction with the mercurial yields mainly the substitution product 3,3,3-trifluoro-l-trimethylsilylpropyne (86%), an almost equal amount of fluorotrimethyl-sitane, and only 5% of a disilylpropene, CFa C SiMe3) CF SiMcs. The latter product was also obtained by treating hexafiuoropropene with the mercurial, but its geometry could not be firmly established and further developments are awaited to resolve the interesting mechanistic possibilities. 

C—I bond fission and four-centre HI elimination/ The marked change in the branching fractions for primary, secondary, and tertiary iodides is mainly a consequence of the change in threshold energy for HI elimination the effect of a-methyl substituents on the threshold energy has been interpreted in terms of the avoided intersection between ionic and non-polar potential energy surfaces. 

This mechanism fails, however, to explain autocatalysis by HCl, the catalysis by acetic acid in PVC/PVAC blends (see Section 15.11.1.2) and the catastrophic degradation induced by Lewis acids such as ZnCl2. It is probable, therefore, that molecular elimination of HCl via a four-centre transition state and HCl-catalyzed elimination via a six-centre transition state (Scheme 24) occurs simultaneously with the free radical route, the relative importance of radical or molecular elimination being dependent to some extent upon the conditions. The catalysis route in Scheme 24 is readily modified to explain acetic acid catalysis. ... 

---