Vinyl shifts

Skeletal ring contraction steps of primary C7 and Cg rings are more probable than bicyclic intermediates (132b). Aromatization of methylcyclo-pentane indicated no carbonium mechanism with a nonacidic catalyst. Instead, Pines and Chen (132b) proposed a mechanism similar to that defined later as bond shift. This is a methyl shift. Two additional isomerization pathways characteristic of chromia have also been demonstrated vinyl shift (94) and isomerization via C3 and C4 cyclic intermediates (90a). These were discussed in Section III. 1,1-Dimethylcyclohexane and 4,4-dimethyl-cyclohexene gave mainly toluene over various chromia catalysts. Thus, both skeletal isomerization and demethylation activities of chromia have been verified. The presence of an acidic almnina support enhances isomerization dual function effects are thus also possible. 

Photolysis of the tosylhydrazone sodium salt (94) in diglyme gave, as the major product, 1,3-diene (95) by [l,2]-vinyl shift in the intermediate carbene. Vinyl migration occurs with retention of configuration and is postulated to occur in the singlet manifold. 

All these results were interpreted by a free-radical mechanism with the involvement of alkenes, and smaller (C3,C4) and larger (C7,C8) ring intermediates in aromatization. Skeletal isomerization was found to occur through vinyl shift and C3,C4 cyclic intermediates.202 Transition metals with the exception of Fe and Os, as well as Re, Co, and Cu, are active in aromatization of alkanes. Platinum,... 

When 4-anilino-l,1,1-trifluorobut-3-en-2-ones 17 are heated with phosphorus oxychloride, the resultant 2-(trifluoromethyl)quinolines 18 are the result of a 1,3-vinyl shift from nitrogen to carbon, followed by cyclization and dehydration.18 A wide variety of substitution patterns on the phenyl ring has been investigated. 

It is worth noting that both secondary argento cation intermediates could be in equilibrium through either an H or vinyl shift, depending on the nature of the R1 group (Scheme 3.12). 

This experimental result is, in fact, readily explained by the bond-shift mechanism described in the following paragraphs. A vinyl shift (Diagram 5) is predicted to be much easier... 

Theoretical and experimental studies revealed a mechanistic twist of the concerted [8+2] cycloaddition between dienylbenzo[c]furans and dimethyl acetylenedicarboxylate (DMAD). Thus, DFT calculation at the (U)-B3LYP/6-31+G(d) level had suggested a stepwise mechanism involving the formation of a zwitterionic intermediate for the [8+2] cycloaddition between DMAD and dienylbenzo[c]furans with electron-donating methoxy groups in the diene moiety. When no electron-donating substituents are present in the diene moiety of the dienylbenzo[c]furan, computational results indicated an alternative mechanism in which a [4+2] reaction occurs between the furan moiety and DMAD, and was followed by a [l,5]-vinyl shift <07JA10773>. 

Two types of rearrangements that bicyclo[4.2.1]nona-2,4,7-trien-9-yl cation [447] may undergo have been suggested by Leone et al. (1973). One of these involves 1,2-vinyl shift of the monoene bridge to the cationic centre to yield... 

Under similar conditions, cyclic acetals rearrange via a [1.2] vinyl shift, affording esters in excellent yields and enantiomeric excesses30. 

Determination of free enthalpy of activation of the rearrangement of the undeuterated radical to tropylium radical, sigmatropic 1,2-vinyl shifts not observed. ... 

A stepwise 1,7-vinyl shift was proposed to account for the reaction after carbene addition to make the cyclopropene. Deuterium labeling studies were consistent with this pathway. Interestingly, upon heating to 145°C the phenyl-substituted 5.2.0 tetraene isomerized to 2-phenylindene in what appears to be a cyclization of the cycloheptatriene moiety followed by opening of the bicyclo[2.1.0]pentene and a 1,5-hydrogen shift (Scheme 10.9). 

It now appears that benzofulvene, which gives naphthalene under conditions of flash vacuum pyrolysis,is an intermediate in the automerization of naphthalene. Thus benzofulvene labeled in the methylene position gave a 79 21 ratio of /3- and -labeled naphthalene consistent with competing aryl and vinyl shifts to carbene precursors to the naphthalenes (Scheme 11.6). 

In contrast to hydrogen shifts, 1,2-shifts of aryl and vinyl groups are quite facile. Eq. 11.72 shows the arrow pushing for a 1,2-vinyl shift. The reaction is fundamentally radical addition to an olefin, and then the reverse reaction. 

The 1,2-vinyl shift shown in Eq. 11.72 proceeds via a familiar structure, the cyclopro-pylcarbinyl radical we introduced in the context of free radical clocks (Table 8.7). In this case, the two species involved, the allylcarbinyl and cyclopropylcarbinyl radicals, are both discrete chemical entities that have been thoroughly characterized by EPR spectroscopy (Eq. 11.74). The equilibrium very strongly favors the ring-opened form, making the clock reaction, the opening of cyclopropylcarbinyl, essentially irreversible. 

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