The effects of substituents and solvent polarity on photochemical [1,3] sigmatropic shifts. Experimental evidence in favor of the occurrence of sudden polarization in acyclic alkenes

Further experimental evidence regarding the occurrence of sudden polarization in acyclic alkenes is presented. It is shown that the yield of formation of the product derived from an intramolecular photochemical [1,3]-OH shift in 1 is dependent only on the polarity of the solvent employed. This result could be well explained in terms of a stabilization of the zwitterionic intermediate formed upon irradiation of 1 by reorientation polarization of the dipole solvent molecules. Besides this, it was found that replacement of the alkyl group at the terminal carbon atom of the C3–C9 exocyclic double bond in 1 by a phenyl substituent led to the occurrence of a photochemical [1,3]-H shift. This directive effect of the substituents at the exocyclic double bond could be well explained on the basis of the sudden polarization model.