Photoreduction of cycloalkanecarbaldehydes

Abstract

The photochemistry of cyclopropane- and cyclobutane-carbaldehyde in propan-2-ol with λ 300 nm has been studied. The primary chemical process upon irradiation of each of the two aldehydes in propan-2-ol is abstraction of the solvent methine hydrogen by both the ¹S and ¹T excited states of cyclopropanecarbaldehyde and only the ¹T excited state of cyclobutanecarbaldehyde. The resulting α-hydroxycycloalkylmethyl radicals (i) recombine with the 1-hydroxy-1-methylethyl radicals, (ii) abstract hydrogen from the solvent (and not from the 1-hydroxy-1-methylethyl radicals, as was demonstrated for cyclopropanecarbaldehyde using O-deuteriopropan-2-ol) and in the case of cyclopropanecarbaldehyde also from the parent aldehyde, and (iii) rearrange in the case of the cyclopropyl derivative into 4-oxobutyl radicals. Detailed mechanisms for the formation of the various photoproducts (nine identified for cyclopropane- and six for cyclobutane-carbaldehyde) also based on triplet quenching and radical scavenging experiments are proposed. The fate of the radical pair cyclo-C₄H₇CHOH and R· formed upon irradiation of cyclobutanecarbaldehyde in the presence of a hydrogen-donor RH seems to depend on the steric accessibility of the radical site of R·. For R = benzyl and α-hydroxybenzyl the (unlike) radicals mainly combine, whereas for R = cumyl and α-hydroxy-α-phenylbenzyl mainly cyclobutylmethanol is formed. For R = 1-hydroxy-1-methylethyl, radical combination and cyclobutylmethanol formation are of equal importance.