Cyanophenyl migration

Photorearrangement of 4-/>-cyanophenyl-4-phenylcyclohexene (69) took place mainly byp-cyanophenyl migration/67,70,74 The conclusion could then be drawn that the rearranging excited state is not electron deficient at the /3-carbon atom, since one would not expect a cyanophenyl group to migrate to a positive carbon. The/3-carbon was proposed to have odd electron character ... 

SCHEME 7. Competitive phenyl versus cyanophenyl migration in the Type-B rearrangement... 

In addition, it was found45 that both the endo and exo stereoisomers of the cyanophenyl phenyl bicyclo[3.1.0] ketone afforded the same 2,3- versus 3,4-phenyl as well as the phenyl over cyanophenyl migration preference. This provided further support for the planar zwit-terionic intermediate. 

Photorearrangements in the crystalline state773 usually afford products with very high selectivity (Special Topic 6.5). Whereas irradiation of 4,4,5-triarylcyclohex-2-enone (160) in benzene solution affords a 1 1 ratio of phenyl to /)-cyanophenyl migration to form 161 and 162, the former product is produced exclusively in the solid state (Scheme 6.71).755... 

These solution preferences do not hold in solid-state photolysis (see Scheme 8.5). Remarkably, in the case of the endo cyanophenyl reactant 20b there is a preferential cyanophenyl migration in total contrast to the solution behavior. In all cases much more migration to C-4 was observed than in solution. 

The occurrence of a preferential phenyl migration signifies that cavity effects in the mini-crystal lattice cavity override the electronic preference for cyanophenyl migration. 

Thus, this is one dramatic example of how a cavity can change the course of a reaction. However, every attempt to nnder-stand this cavity effect by compntationally constructing a mini-crystal lattice with diradical 1 (i.e., DRl) embedded and comparing this with a mini-lattice with diradical 2 (i.e., DR2) embedded, invariably led to a lower energy for the mini-crystal lattice with the p-cyanophenyl migration intermediate. Initially this failure was ascribed to what seemed to be crystal disorder. 

The second item of interest was the change in regiose-lectivity at 16% conversion. It was ascertained that at this point, each molecule now had an adjacent product molecule. In addition it was shown that the ratio of the two products (i.e., from phenyl and cyanophenyl migration) followed a simple kinetic expression, shown in Equation 8.3, where index 1 refers to stage 1 and index 2 refers to stage 2. 

Thus, a series of critical results arose from this observation. The solution photolysis of this reactant (i.e., 10) is an example of the well-known Type B Rearrangement and affords a preferential cyanophenyl migration in a 15 1 ratio as depicted in Equation 75.2. ... 

Inclusion Compound Phenyl Migration Intermediate 14 Cyanophenyl Migration Intermediate 13 / Energy E2q>erinient... 

Nevertheless, for a number of reactions, a rather interesting and important phenomenon was uncovered. The first case was that of the dimorphs of 4-phenyl-4-p-cyanophenylcyclohexenone 10. In the case of Crystal B, which initially exhibited only phenyl migration, sharply at 16% conversion, the regiose-lectivity changed to a 1 1 ratio of phenyl to cyanophenyl migration. Thus, there were two stages, one up to 16% conversion and the other beyond that point. ... 

FIGURE 75.4 Plot of phenyl to cyanophenyl migration ratio R vs, (R+l)/C. 

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