SPECTROSCOPIC INVESTIGATION ON STERICALLY HINDERED 1,2-DI-TERT-BUTYLETHENE DERIVATIVES - RESTRICTED CONFORMATIONAL EQUILIBRATIONS

The reactivity of two strained olefins, (E)-, 1, and (Z)-1,2-dimethyl- 1,2-di-tert-butylethene, 2, with electrophilic reagents has been exami ned for the first time. While stable epoxides are obtained when the ol efins above were reacted with MPBA, bromination of both 1 and 2 leads to an allylic bromide (6), then to a 2,3-di-tert-butyl-1,3-butadiene d erivative (7), and finally to the 1, 2-di(bromomethyl)-1,2-di-tert-but ylethene derivative 4 through addition-elimination steps. Compounds 4, as well as the similarly encumbered 1,2-diethyl-1,2-di-tert-butylethe ne, 3, exhibit relevant red-shifts of Raman stretching frequencies of the double bonds and downfield displacements of the chemical shifts of the vinylic carbons. Moreover, alkenes 3 and 4, at variance with 1 an d 2, show temperature-dependent NMR spectra due to hindered rotation o f the bromomethyl and ethyl groups, respectively. In both cases the ac tivation enthalpies for these processes have been calculated (Delta H double dagger = 12.4 and 16.3 kcal mol(-1), respectively), and evidenc es have been obtained in favour of a largely prevalent stepwise-like i nterconversion between the transoid and cisoid diastereomeric conforma tions, while the contribution, if any, of the ''cogwheel'' concerted m echanism is much less important.