Four-centre elimination processes

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. ... 

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. 

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). 

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... 

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