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

Abstract

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 S₁ states of acyclic 1,3-dienes.