1,2-Vinyl migration

The one-step vinyl migration mechanism makes the same regiochemical prediction. The less substituted vinyl group should migrate, because the substituent(s) will stabilize the residual radical. 

Disulfones (58) showed no formation of the [M —104] + or m/z 180 either (equations 27a and 27b, respectively) but fragmented mainly via the well-known sulfone-sulfinate rearrangement. Vinyl migration predominates in all of the above rearrangements... 

Ring contraction by (Si - C) vinyl migration took place when the diazo-silacyclohexadiene 393 was heated in the presence of anhydrous CuS04 388>. The silafulvene 394 was not isolated, but it could be trapped by r-butanol, benzophenone or benzaldehyde to give 395,396a and 396b, respectively. Silane 395 was also obtained... 

The stepwise dehydrocyclization of hydrocarbons with quaternary carbon atoms over chromia was interpreted by Pines 94). Here a skeletal isomerization step prior to cyclization was assumed. This is not of a cationic type reaction, and the results were explained by a free radical mechanism accompanied by vinyl migration (Scheme IXA). Attention is drawn to the fact that... 

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. 

Double bond additions occur most readily with double bonds in the 3,4- or 5,6 positions, the former providing the path for vinyl migration and the latter yielding five- or six-membered rings depending upon molecular structure. 

Apparently, nonacidic chromia-alumina catalyzes C5-dehydrocyclization only when the new bond is formed between a primary carbon atom and the aromatic ring. We know that the isomerization of alkylbenzenes over nonacidic chromia-alumina involves free-radical intermediates and proceeds by phenyl or vinyl migration (41, 42). One can speculate that dehydrocycli-zation also has a free-radical mechanism here. 

Tomooka, K. Inoue, T. Nakai, T. Stereochemistry and mechanism of vinyl-migrating [1,2]-Wittig rearrangement of a-lithioalkyl vinyl ethers. Chem. Lett. 2000, 418-419. 

Methyl-3-phosphorylchroman-4-one 1106 is accessible via a tandem allyl-vinyl migration and cyclization reaction of the allylic phosphonate 1105 and ethyl salicylate (Equation 435) <2001SC2613>. 

Nitrones (268), derived from ketones, undergo a Beckmann-like rearrangement when treated with tosyl chloride in pyridine. Unlike the Beckmann rearrangement, however, the reaction is independent of the nitrone configuration, and shows a preference for vinyl migration. The consequence, for a 4-en-3-one derivative, is the formation of the unusual enamine-lactam (270). The hydroxyl-amino-O-tosylate derivative (269) is considered a likely intermediate (see also Part II, Chap. 2, p. 353). 

Compared with their cationic counterparts, only a few radical rearrangements are fast enough to appear in synthesis. An example is vinyl migration in which a vinyl group migrates via the formation of the methylcyclopropyl intermediate (Scheme 2.12). 

Ring contraction by (Si - C) vinyl migration took place when the diazo-... 

The di-TT-methane rearrangement of 1,4-pentadiene has been modeled using CAS-SCF/4-31G calculations. The results indicate that a singlet 1,3-diradical is the key intermediate. This species can be reached from the excited state via a Cl that involves vinyl migration. It consists of a vinyl group associated with the central carbon of an allylic system and is similar in structure to the Cl involved in alkene sigmatropic rearrangements (see p. 1093). The structure of the Cl is also consistent with the... 

This result can be rationalized in terms of the vinyl migration mechanism by noting that rearrangement will occur to give the more stable of the two possible 1,3-diradicals. The cyclopropane ring in the final product will then incorporate this terminus. 

Deuterium labeling experiments indicate that the formation of the Ti -ligand of 15 involves the electrophilic attack of the proton of the acid to the 5-carbon atom in the butadienyl ligand of 13, and the subsequent vinyl migration from the... 

Equations (5) and (6) presume that the cyclic Grignards 3 and 4 are intermediates in the formal 1,2-vinyl migrations. The subsequent finding (52) that the cyclopropylmethyl Grignard is preparable, though rapidly rearranging, supports this presumption. Further proof of its intermediacy is derived from experiments by Maercker (50, 50a) and by Hill (33), shown in Eqs. (7) and (8) respectively. In Eq. (7), cis-trans equilibration... 

An important observation above was that no spiro[4.4]nona-l,3,6-triene was found in the reaction, and it does not give the observed products upon pyrolysis. Instead, it gives an isoindane, presumably via a vinyl migration followed by a 1,5-hydrogen shift with log A = 11.9 - 21A00/23RT (Scheme 10.29). ... 

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