Nucleophilic 1.2- methyl shift

Two possible mechanisms are proposed. Primarily the enol radical cation is formed. It either undergoes deprotonation because of its intrinsic acidity, producing an a-carbonyl radical, which is oxidized in a further one-electron oxidation step to an a-carbonyl cation. Cyclization leads to an intermediate cyclo-hexadienyl cation. On the other hand, cyclization of the enol radical cation can be faster than deprotonation, producing a distonic radical cation, which, after proton loss and second one-electron oxidation, leads to the same cyclo-hexadienyl cation intermediate as in the first reaction pathway. After a 1,2-methyl shift and further deprotonation, the benzofuran is obtained. Since the oxidation potentials of the enols are about 0.3-0.5 V higher than those of the corresponding a-carbonyl radicals, the author prefers the first reaction pathway via a-carbonyl cations [112]. Under the same reaction conditions, the oxidation of 2-mesityl-2-phenylethenol 74 does not lead to benzofuran but to oxazole 75 in yields of up to 85 %. The oxazole 75 is generated by nucleophilic attack of acetonitrile on the a-carbonyl cation or the proceeding enol radical cation. 

The mechanism involves the loss of water molecule from a protonated diol, followed by 1,2-nucleophilic shift of a group. Since the diol is symmetrical, protonation and loss of water take place with equal probability at either hydroxyl group. The resulting 3°-carbocation is relatively stable but 1,2-methyl shift generates an even more stable carbocation in which the charge is delocalized by heteroatom resonance (Scheme 2.11). 

Steps [2] Rearrangement of the 2° carbocation by a 1,2-methyl shift forms a more stable 3° and [3] carbocation. Nucleophilic attack of Br forms the product, a 3 alkyl halide. 

Nucleophilic attack at the methoxyl carbon of this compound liberated an open-chain acetal anion, which acted as a powerful nucleophile and attacked a further cyclopropane molecule in a Thorpe reaction [Eq. (28)]. Rearrangement then produced a A2-pyrroline (99) in an overall yield of 60%. On addition of acid, this A2-pyrroline aromatized with a simultaneous 1,2-methyl shift and formed the dihydropyrrolizines (100). A mixture of products is obtained because hydrolysis of the acetal function competes with the final ring closure reaction. 

When studying polymethylbornyl cations of type 136 Sorensen found the exo-3,2-methyl shift in these ions to occur much faster (by about 10 times) than the endo-3,2-methyl shift. The author offers the following explanation. The presence in tertiary 2-norbomyl ions of some overlap of the back-side part of the sp -orbital of C -6-exo-H with the empty p-orbital at causes the change in spatial orientation of the latter. As a result the exo substituent at C turns out to be more coplanar, and the endo one, less coplanar with the vacant p-orbital at II, which does explain the observed difference in the rates of the 3,2-shift. In more nucleophilic solu-... 

In view of these results the experiments using styrene were repeated at lower temperatures and the study extended to the more nucleophilic analogs, a-methyl styrene, tetraphenylethylene. No significant shifts of benzyl proton peaks were observed in any of these cases, even with a 500-Hz scale expansion. 

For the methyl-substituted ethylenes, i.e. in the absence of any steric effects, there is a roughly linear relationship between the chemoselectivity and the 13C nmr chemical shift of the most substituted carbon atom of the bromonium ions (Dubois and Chretien, 1978). This selectivity is therefore discussed in terms of the magnitude of the charge on the carbon atom and the relative hardness of the competing nucleophiles, according to Pearson s theory (Ho, 1977). However, this interpretation does not take into account the substituent dependence of the nucleophilic solvent assistance, which must play a role in determining this chemoselectivity. 

As illustrated in Scheme 5, ( )-31 directly gives cation 37 via phenyl participation, while (Z)-31 provides 38 more slowly via methyl participation. Cation 37 can further rearrange to more stable 38 by 1,2-hydride shift, but 38 cannot isomerize to less stable 37. As a result, ( )-31 can afford not only 33,35, and 36 but also 34, but (Z)-31 only gives 34 and 35 depending on the nucleophilicity of the solvent. The unrearranged product 32 is formed via inversion only from (Z)-3I in a more nucleophilic solvent. This must result directly from the SN2 reaction of (Z)-31. 

Alkylideneallyl cations can be described as resonance hybrids of 1-vinyl-substituted vinyl cations and allenylmethyl cations, and thus contain two reactive sites (the sp- and sp2-hybridized carbons) for nucleophilic addition (Scheme 1) (7,2). Hybridization affects the electronic and steric character of these reaction sites. The electronic property was deduced from the l3C NMR chemical shifts of alkylideneallyl cations measured under superacidic conditions (3) and also from the charge distribution calculated (4). The charge distributions are affected by substituents on the cation the sp2 carbon is more positive than the sp carbon when two methyl groups are introduced at the sp2 carbon. 

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