Automerization

JOC1537). The mechanisms of these transformations may involve homolytic or heterolytic C —S bond fission. A sulfur-walk mechanism has been proposed to account for isomerization or automerization of Dewar thiophenes and their 5-oxides e.g. 31 in Scheme 17) (76JA4325). Calculations show that a symmetrical pyramidal intermediate with the sulfur atom centered over the plane of the four carbon atoms is unlikely <79JOU140l). Reactions which may be mechanistically similar to that shown in Scheme 18 are the thermal isomerization of thiirane (32 Scheme 19) (70CB949) and the rearrangement of (6) to a benzothio-phene (80JOC4366). 

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

Thus far the only known example of tunneling exchange of heavy particles is automerization of cyclobutadiene [Dewar et al. 1984 Carsky et al. 1988],... 

The Cope rearrangement of hexa-l,5-diene does not allow for differentiation of starting material and product this is called a degenerate Cope rearrangement. Another example is the automerization of bicyclo[5,l,0]octa-2,5-diene 7 ... 

McMahon et al. contributed to this field not only by revealing the electronic spectrum of S-37111 but also by elucidating — on the basis of studies with labeled compounds — the complex mechanism of the automerizations and isomerizations of the three C3H2 species S-2, T-36 and S-37.69,112 A complete record, including high level calculations, was published by McMahon.69... 

It has been found by NMR that carbocations la-d and 3 are prone to undergo degenerate rearrangements [automerization reactions (26)] via 1,2-shifts of the vinyl and the methyl-substituted vinyl groups (Schemes 5, 6), with... 

The solid aromatic dianion salt of l,2-di-[ C][8]annulene has not been found to scramble the Cs even on heating to over 600 °C for 2hs. ° This behaviour of the cyclooctatetraene dianion is in contrast with that of neutral aromatic systems, which readily automerize in the gas phase. Apparently, when sufficient energy is applied... 

The H- and l3C-NMR study [86AG(E)842] shows that a rapid valence isomerization (101) <= (101a) takes place. For azete, the activation barrier of the automerization via a C2v transition state is 12.1 (MINDO/3) (80MI2),... 

The reaction involves a [1,3] sigmatropic migration of the carbon originating at C2 with transformation of the three-membered ring C3 C4- C5 to an allylie system. For that reason, the methylenecyclobutane system undergoes thermal automerization. 

The most fascinating classical example is the automerization of bulvallene,389 a degenerative Cope rearrangement, that lets every C atom in the molecule eventually occupy every possible position in the skeleton, even in the solid state.390... 

C]Naphthalene, investigated before and after heating in A1203 with benzene, gives identical 13C NMR spectra [405 a]. Thus no automerization (equal 13C enrichment on all... 

The only example of coherent chemical exchange associated with heavy particle transfer is automerization of cyclobutadiene ... 

The [(s-trans-diene)ZrCp2] complex (s-trans-1) equilibrates with the [(s-cA-diene)ZrCp2] isomer (x-cA-l) via a reactive high lying (r 2-butadiene) metallocene intermediate (2) [A(s-trans-1 s-cis-l, 283 K) = 22.7 0.3 kcal mol-1]. Syntheses of the (butadiene)zirconocene system carried out under kinetic control invariably led to pure s-trans-1, whereas a ca. 1 1 equilibrium of s-trans-1 and. v-ci.v-l was obtained under conditions of thermodynamic control.5,6 The cr,7i-structured s-cis-l isomer undergoes a dynamic ring-flip automerization process (see Scheme 2) that is rapid on the NMR time scale [AG futom = 12.6 0.5 kcal mol ].5... 

---