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Mechanistic investigations in the photochemistry of acyclic conjugated dienes. Significance of the [1,5] sigmatropic shifts of hydrogen

par Rondelez, Daniel ;Boué, Stéphane
Référence Journal of the Chemical Society. Perkin Transactions 2, 6, page (647-649)
Publication Publié, 1976

Article révisé par les pairs

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Résumé :

The determination of initial quantum yields for product formation in the direct photolysis of the three interconvertible dienes, 4-methylpenta-1.3-diene and cis- and trans-2-methylpenta-1,3-diene, leads to revision of the mechanism formerly proposed by Crowley and brings more insight into the properties of the excited dienes in solution. Thus it is shown that cis-2-methylpenta-1,3-diene largely assumes an s-cis ground state conformation and that its S1, state makes efficient use of both the [1,5] hydrogen shift and the 1,4-cyclomerization co-ordinates for decaying to the ground state ; the two other dienes undergo unambiguously, though with a low quantum yield, an abnormal [1,5] shift of hydrogen which can only originate from a trans configuration and hence supports the doubly twisted (bicyclobutane-like) structure which we have previously assigned to at least one of the possible vibrationally relaxed S 1 states of acyclic 1,3-dienes.