The reactivity of two strained olefins, (E)-, 1, and (Z)-1,2-dimethyl-1,2-di-tert-butylethene, 2, with electrophilic reagents has been examined for the first time. While stable epoxides are obtained when the olefins 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 derivative (7), and finally to the 1, 2-di(bromomethyl)-1,2-di-tert-butylethene derivative 4 through addition-elimination steps. Compounds 4, as well as the similarly encumbered 1,2-diethyl-1,2-di-tert-butylethene, 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 and 2, show temperature-dependent NMR spectra due to hindered rotation of the bromomethyl and ethyl groups, respectively. In both cases the activation enthalpies for these processes have been calculated (Delta H double dagger = 12.4 and 16.3 kcal mol(-1), respectively), and evidences have been obtained in favour of a largely prevalent stepwise-like interconversion between the transoid and cisoid diastereomeric conformations, while the contribution, if any, of the ''cogwheel'' concerted mechanism is much less important.
Spectroscopic investigation on sterically hindered 1,2-di-tert-butylethene derivatives. Restricted conformational equilibrations
CHIAPPE, CINZIA;
1995-01-01
Abstract
The reactivity of two strained olefins, (E)-, 1, and (Z)-1,2-dimethyl-1,2-di-tert-butylethene, 2, with electrophilic reagents has been examined for the first time. While stable epoxides are obtained when the olefins 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 derivative (7), and finally to the 1, 2-di(bromomethyl)-1,2-di-tert-butylethene derivative 4 through addition-elimination steps. Compounds 4, as well as the similarly encumbered 1,2-diethyl-1,2-di-tert-butylethene, 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 and 2, show temperature-dependent NMR spectra due to hindered rotation of the bromomethyl and ethyl groups, respectively. In both cases the activation enthalpies for these processes have been calculated (Delta H double dagger = 12.4 and 16.3 kcal mol(-1), respectively), and evidences have been obtained in favour of a largely prevalent stepwise-like interconversion between the transoid and cisoid diastereomeric conformations, while the contribution, if any, of the ''cogwheel'' concerted mechanism is much less important.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
