Irradiation of trans- or cis-anethole in the presence of electron acceptors results in cis, trans isomerization, cyclodimerization, and (in the presence of oxygen) oxidative cleavage. The initial step in each of these reactions is photoinduced electron transfer to yield a singlet radical ion pair. Anethole cation radicals are the initial reactive intermediates formed in each reaction. Isomerization occurs via reverse electron transfer to generate triplet anethole which decays to a mixture of cis and trans isomers. Dimerization occurs via the [2 + 1] cycloaddition of a cation radical and a neutral which proceeds with retention of olefin configuration to yield a mixture of syn and anti head-to-head dimers. The syn/anti dimer ratio is sensitive to reaction conditions because of the lower stability of the syn vs anti dimer cation radical. Reaction of the monomer cation radical with O2 − or O2 results in formation of p-anisaldehyde. It is unlikely that an acyclic 1,4-cation radical is the precursor of any of the observed products.
ASJC Scopus subject areas
- Colloid and Surface Chemistry