Automerization reactions

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... 

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 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. 

Balaban et a/. have reported the catalytic automerization of [1- C]- or [3- C]-phenanthrene in the presence of a 1 1 mixture of AlCh-NaCl. They found that [l- C]phenanthrene transferred part of the label to C-3, but [3- C]phenanthrene transferred the label to two positions, i.e. mainly C-1 and to a lesser extent C-4, as shown in Scheme 16. Possible reaction mechanisms to account for these observations were also proposed. 

The Dewar thiophene 1 was oxidized to its S-Oxide. 19F NMR of the latter compound showed an automerization through the 1,3-shift of the S-oxide group.21 Later, the Dewar thiophene itself was found to automerize at much higher temperature.22 The lone pair on the sulfur atom was proposed to participate in the 1,3-shift. Lemal named it a pseudopericyclic reaction (Scheme 5). The bond order between the lone pair and the C—C double... 

The thermal automerization and rearrangement reactions of PAHs have been widely investigated during the past two decades (for examples see refs. [31 e, g, 62-64]). The main objective was to understand the processes of formation of aromatic hydrocarbons in fuel rich flames and the mechanisms of transformation of the PAHs that have been observed at these elevated temperatures. In most cases, thermally initiated rearrangement reactions in the carbon skeletons of PAHs require high enthalpies of activation resulting in low product selectivities and poor overall yields. Because the expected products are often more effectively prepared by conventional routes, this approach has been used as a synthetic tool only in a few cases, e.g. the synthesis of azulenes [65] and the rearrangement of bifluorenylidenes to benzenoid hydrocarbons [38]. 

In the preceding sections we have discussed the various methods for enumerating isomers. Graph-theoretical methods for enumerating isomerization reactions also exist and these will be briefly discussed now. The simplest isomerizations are automerizations [139], (topomerizar... 

The complete retention of the original T position in the product from the liquid-phase reaction indicates that automerization is slow in comparison writh the collision frequency in the condensed phase, in agreement wdth theoretical estimates (18) of the activation energy for Equation 7, ranging from 44 to 77 kcal mol While no direct information is provided by the decay experiments as to the specific mechanism of automerization (H vs. CH-group migration), the results nevertheless exclude any significant intervention of the nonclassical carbene structure... 

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