Aryl and Vinyl Shifts

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. 

A similar sequence is involved in the 1,2-aryl shift, and Eq. 11.73 shows a prototypical example. While we expect the reaction to be reversible, the case shown is driven to the right to form the more stable 3° radical. Such shifts presumably involve a cyclohexadienyl radical of the sort shown in the margin, but such a species is not detected and cannot be trapped. 

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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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). 

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