Cope rearrangement steric factors

There are perhaps only two research areas in which the quantitative estimation of steric factors is routinely done, namely for hindered biphenyls (Cooke and Harris, 1967) and rotational or pseudorotation barriers in ethanes and cyclic compounds (Allinger et al., 1967). There are indications, however, that this interest is widening. Simonetta and Favini (1966) performed conformational calculations on the Cope rearrangement via chair or boat transition states and indicated that the orbital preference emphasized in a previous section should be supplemented with a steric factor. 

In addition to the major factors discussed above, a number of Cope equilibria can be interpreted only in terms of more subtle conformatimial factors which favor one isomer. Consider the two hydroxydicy-clopentadienes (56) and (57) in Scheme 6 at 140 C each is in equilibrium with its Cope rearrangement product (58) or (59). For the Mo-alcohol (56), the equilibrium ratio is 1 1, but in the endo isomer (57), where the hydroxy group suffers steric compression, the rearrangement product is favored by at least 4 1. In the diphenylcyclopropene (60), conjugation would appear to favor the starting material, but the... 

This study provided insight into mechanistic aspects of the anion accelerated oxy-Cope rearrangment. It also illustrates how control of terminal olefin stereochemistry in [3.3]-sigmatropic rearrangements can be used to establish vicinal stereochemistry. Note that both 59 (R=Me) and 71 give superb control. Only when there are overriding steric factors (70 and 72) does erosion of stereocontrol begin to occur. 

Thermal reactions of 1,2-divinylcyclobutanes have been reported by Berson and his colleagues, and again steric factors were found to have an inhibiting effect upon the Cope rearrangement of the cis-compounds. Thermolysis of... 

Apart from the Cope and Claisen-Cope rearrangements, a number of named reactions are to be found within the category of [3,3] shifts t reactions (6.106)-(6.111) are examples of some of these processes. The thio-Claisen rearrangement the sulphur analogue of reactions (6.106) and (6.107) is also well known. The all carbon atom counterpart of the Claisen rearrangement ((6.106) 0=CH2) was not discovered until it was realized that the final aro-matization step required the presence of a strong base to promote the necessary tautomeric change. In reactions (6.1 07)-(6.111) the influence of steric factors on the stereoselectivity of the reactions is apparent. In reaction (6.109),... 

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