Vinyl cations 1,2-aryl rearrangements

Comparisons of product distributions in thermal and photochemical solvolyses show that the primary vinyl cation is not involved in thermolysis but is formed photochemically. The chirality probe approach using optically active 4-methylcyclohexylidenmethyl(aryl)iodonium tetrafluoroborate 19 was applied to the photosolvolysis.24 The rearranged product 4-methylcycloheptanone retained some optical activity, but the enantiomeric product in slight excess has a different structure depending on the iodoarene leaving group Arl of the substrate. The results indicate that the primary vinyl cation involved is not in a free, dissociated achiral form. 

To be really satisfactory, a Friedel-Crafts alkylation requires one relatively stable secondary or tertiary carbocation to be formed from the alkyl halide by interaction with the Lewis acid, i.e. cases where there is not going to be any chance of rearrangement. Note also that we are unable to generate carboca-tions from an aryl halide - aryl cations (also vinyl cations, see Section 8.1.3) are unfavourable - so that we cannot nse the Friedel-Crafts reaction to join aromatic gronps. There is also one further difficulty, as we shall see below. This is the fact that introduction of an alkyl substitnent on to an aromatic ring activates the ring towards fnrther electrophilic substitution. The result is that the initial product from Friedel-Crafts alkylations is more reactive than the... 

Long-lived vinyl cations have been reviewed.114 Vinyl cations (67) can be prepared by the fragmentation of alkenyl(aryl)iodonium friflates, giving triflate capture products, some of which may be rearranged.115 Otherwise vinyl cation research has been quiet this year. 

In suitable vinylic halides, nucleophilic photosubstitution occurs intramolecular- 295-297,324,330-332 por jnstance irradiation of 113 in methanol only yields benzofuran 114, and no vinyl ether product or 1,2-aryl rearrangement products are formed (equation 84a)296. Apparently, the reaction with an internal nucleophile is faster than the 1,2-aryl shift and also faster than the reaction with an external nucleophile. With the less stabilized vinyl cations derived from the a-methyl- and a-H-analogues of 113-OMe, 1,2-aryl shifts do... 

Other aspects of nucleophilicity toward vinyl cations are the site of capture of ambident ions and the easy intramolecular cyclization by o-methoxy and o-thiomethyl substituents on a P ring. The extensively studied P-aryl rearrangement across the double bond could be regarded as intramolecular substitution by the aryl ring, and data are available on the relative rate of rearrangement and capture by the solvent (69, 70). These topics are not discussed here for lack of space but should be addressed in a more complete discussion of the nucleophilicity. 

This general mechanistic scheme readily explains a number of experimental observations. For instance it is very clear why such ester shifts only ever take place between vicinal carbons [1], as it is only this arrangement that permits the formation of an alkene radical cation as intermediate. Intermolecular ester shifts are excluded for the same reason. Rearrangements of o-(acyloxy)aryl radicals (Scheme 7) [13, 14] and their vinyl counterparts would require the intermediacy of very high energy benzyne radical cations, as such no examples are known. Failed migrations between two secondary radicals (Scheme 8) may now be seen as being due not so... 

This reaction involves the elelctrophilic attack of an aryldiazonium cation on the enol carbon atom of an active methinyl compound to give an azo intermediate, which then hydrolyzes to form an arylhydrazone. Tautomerization of aryl hydrazone yields the vinyl hydrazine, which undergoes [3,3] sigmatropic rearrangement and subsequently cyclizes to give an indoline derivative which, upon evolution of ammonium ion, yields the indole derivative, as shown below. 

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