1.3.4.6- Tetraphenyl-1,5 -hexadiene

On addition of a second pair of phenyl substituents at C4 and C6, the interallylic distance further lengthens so that the contribution of structure C to the TS wave function is further enhanced. The ability of the second pair of phenyl substituents to increase the optimal value of R from 7 = 2.218 A to 7 = 2.649 A allows each of the phenyl substituents in l,3,4,6-tetraphenyl-l,5-hexadiene to provide more stabilization for the Cope TS than the pair of phenyl substituents in 1,3-diphenyl-1,5-hexadiene. This cooperative effect of the four phenyl groups in 1,3,4,6-tetraphenyl-1,5-hexadiene lowers AT/ for Cope rearrangement by, not twice, but by four times as much as the pair of phenyl substituents in 1,3-diphenyl-1,5-hexadiene. 

Another observation, which can be made from the results in Table 30.2, is that at the optimal values of R for the intermediate in the Cope rearrangement of 2-phenyl-1,5-hexadiene and for the TSs in the Cope rearrangements of 1,3-diphenyl-, and 1,3,4,6-tetraphenyl-1,5-hexadiene, ASjubs, = — 2A (jis,. Therefore, since +... 

Scheme 2.24. Activation volumes of the racemization of optically active 1,3,4,6-tetraphenyl-1,5-hexadiene and the mutual interconversion of the meso into the racemic diasteromer.

Examination of the activation energies for rearrangement of 2-phenyl-, 2,5-diphenyl-, 1,3-diphenyl-, 1,6-diphenyl-, 1,3,5-triphenyl-, and 1,3,4,6-tetraphenyl-1,5-hexadiene, all thermoneutral reactions to avoid thermodynamic considerations led Doering to conclude that neither model was appropriate. In the analyses, great care was taken to correct for the effect of phenyl on the stability of the starting material in each case. 

TriplienyI-2-benzyl-2H-thiopyran gekt entsprcchendin6-Thiono-l,2,4,6-tetraphenyl-hexadien-(2,4) iiber5. 2H-Chromen wird analog in 6-Oxo-5-prope,n-(2)-yliden-cyclohcxa-dien-(l,3) uberfiihrt3 und 2,2-Diphenyl-2H-l-benzothiopyran in 6-Thiono-5-[3,3-diphenyl-propen-(2)-yKden]-cyclohexadien-(l, 3)5. 

Irradiation of l,l,3,3-tetraphenyl-5-methyl-l,4-hexadiene gives the two products shown below. When the reaction is carried out by photosensitization, B is not formed. Suggest mechanisms for the formation of A and B. What other products might have been expected Can you rationalize their absence ... 

The first example of the rearrangement of a cyclopropyl radical to an allyl radical in solution was observed in the thermal decomposition of l-methyl-2,2-diphenylcyclo-propanecarbonyl peroxide The radical reacted by abstracting hydrogen from solvent or by rearranging to the l,l-diphenyl-2-methylpropenyl radical which dimerized to yield l,l,6,6-tetraphenyl-2,5-dimethyl-l,5-hexadiene (89). The proportion of dimeric product to that of cyclopropane is dependent on the solvent. If a good radical scavenger is used, such as chloroform, carbon tetrachloride or thiophenol, then only the unrearranged cyclopropane derivative is obtained. This is also the case when a radical trap such as iodine is added to a benzene solution. 

A volume expansion is expected for homolytic bond dissociations as already pointed out in the Introduction. This expectation has been confirmed for several homolytic bond cleavages showing positive activation volumes near AV - +10 cm mol [123]. The analysis of the pressure effect on the cleavage of azo compounds is however, complicated by the possibility of one- and two-bond scission processes [124]. The benzylic and benzhydrylic 1,4-shifts in the substituted pyr-idiminiumoxides (Scheme 2.29, entry (1)) [125] illustrate the utility of high pressure for the distinction between a pericyclic and dissociative mechanism comparable to the rearrangement of l,3,4,6-tetraphenyl-l,5-hexadiene which has already... 

On the other hand, the degenerate rearrangement of yn-l,3,4,6-tetraphenyl-l,6-hexadiene is equi-energetic. 

The reaction thus always leads to a six-membered ring, by 1,4-addition. Of the three double bonds concerned in the reaction, only one is retained in the product, and that at a different position. Rearrangements rarely occur in these reactions. Moreover, they are stereospecific on addition of maleic acid to a diene the adduct produced is a cw-dicarboxylic acid, and on addition of fumaric acid it is a frms-dicarboxylic acid. Substituents on the diene or philodiene are not wholly without effect on the course of the addition for example, 1,2,3,4-tetramethylbutadiene (3,4-dimethyl-2,4-hexadiene) reacts smoothly,33 but the tetraphenyl analog shows no tendency to add a philodiene. Bulky substituents at the 2,3-positions can completely suppress the addition, presumably because they prevent free rotation and thus disfavor formation of the con-... 

As expected, dienes with multiple substitution in the 1 and 4 positions, e.g., 2,4-dimethyl-2,4-hexadiene 18,4-methyl-1,3-hexadiene 19, and 1,1,4,4-tetraphenyl-1,3-butadiene 20, fail to react with maleic/ In these cases, only polymeric products are obtained. This is ascribable to the inability of the dienes to attain cisoid geometry. It must be remembered that these restrictions apply for acylic dienes only. 

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