Double methylene rotations

However, quantitative evaluation of the size of this preference depended on knowing the size of the secondary deuterium isotope effect on which C—C bond in 7b cleaves. With the seemingly reasonable assumption of a secondary isotope effect of 1.10 on bond cleavage, the experimental data led to the conclusion that double methylene rotation was favored over single methylene rotation by a factor of 50 in the stereomutation of 7b. Although the error limits on the measurements were large enough to allow the actual ratio to be much smaller, Berson wrote, There is no doubt that the double rotation mechanism predominates by a considerable factor. ... 

For the cis isomer, the ratio of racemization to epimerization was found to be 107 at 274.5 °C, and the cis isomer was found to undergo racemization 16.2 times faster than the trans isomer. The ratio of racemization to epimerization for the latter was found to be only 6.6 at 274.5 °C. Thus, these experiments confirmed the computational predictions that in 9b and 9c, double methylene rotations are faster than any process that leads to net single methylene rotation, and the preferred mode of double rotation is disrotation, rather than conrotation. ... 

Freedom of rotation about the double methylene bridge in the compound (7) (dimethyl 4,4 -(l,2-ethanediyl)bisben2oate [797-21-7]) destroys the rod shape of the molecule and prevents Hquid crystal formation. The stilbene derivative (8) (dimethyl 4,4 -(l,2-ethenediyl)bisben2oate [10374-80-8]) however, is essentially linear and more favorable for Hquid crystal formation. 

In 1975, Berson and co-workers disclosed the results of elegant experiments on the stereomutation of 7b, using deuterium atoms, instead of alkyl groups, as the stereochemical markers. From measurements of the relative rates of enantio-merization and trans cis isomerization of optically active 7b, Berson and co-workers found that there was, indeed, a preference for double over single methylene rotations. 

Both types of dynamics calculations gave the same type of results. The calculations found that the ratio of double-to-single methylene rotations is higher than the 1 1 ratio predicted by application of a TST model to the PES. Ratios of double-to-single rotations between 2.9 and 3.5 were obtained from the direct dynamics calculations of Doubleday et al., and a ratio of 4.7 was computed from the trajectory calculations performed by Carpenter and co-workers. ... 

While direct irradiation of , E-2,4-hexadiene 5 gives only , Z-2,4-hexadiene from singlet excited state, triplet-sensitized reaction gives both E,Z- and Z,Z-2,4-hexadienes. The singlet state reaction proceeds with just one terminal double bond rotation involving allyUc methylene or cyclopropane methylene diradical with just one double bond rotation, whereas the triplet excited state reaction proceeds with double double bond rotation [11]. 

Interest in the decomposition of cyclopropanes and the role of the trimethylene biradical in the decay mechanism has spanned more than three decades and has been fueled by two experiments that give apparently irreconcilable results. Experiments of S,S-trans-cyclopropane-l,2-d2 at 695 K indicate that isomerization via double-terminal rotation (i.e., con- and disro-tation of the terminal methylene groups) is at least 6 times more prevalent than isomerization via single-terminal rotation. Similar experiments with chiral... 

In view of the restrictions on the mode of approach of the radical to the double bond, significant strain develops at the transition state, and this requires rotation of the benzylic methylene group out of its preferred coplanar alignment. 

Dynamics calculations have also provided new approaches to the stereochemical modes through which cyclopropanes and trimethylene intermediates may be related. Full quantum dynamics calculations for the trimethylene diradical based on a reduced dimensionality model that followed wave packet densities and time constants for formation of products led to the conclusion that conrotatory and disrotatory double rotations of both terminal methylene groups are favored over a single rotation of just one by a 2.2 1 ratio." °... 

The reaction was studied theoretically by Hoffmann who concluded that it should involve a double rotation of the methylene groups.81 A study of the kinetics of racemization vs. isomerization of optically active trans-l, 2-dideuteriocyclopropane provided evidence for this process.82 Further experimental studies have made the interpretation of the details of this reaction more difficult.83 The most recent calculations84 have concluded that the conrotatory double rotation is slightly preferred. It is likely that substituents could have a major effect on the course of the reaction. 

The barrier for stereomutation is calculated to be close to 61 kcalmoT1 (CISD+Q/TZ + 2py 32,275 Conrotatory double rotation of the methylene groups is about 1 kcal mol-1 more favourable than single rotation of a methylene group. Calculations further reveal that dis-rotatory rotation is also possible with a barrier0.5 kcalmol-1 larger than conrotatory rotation (Figure 27). 

This led Stanley to propose the bimetallic cooperativity mechanism shown in Scheme 16. The key bimetallic cooperativity step involves rotation about the central bridging methylene group to give a H /CO double-bridged intermediate (or transition state) species, which directly leads to an... 

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