The photochemistry of some cycloalkanecarbaldehydes in inert solvents

Abstract

The photochemistry of the first four members of the series of cycloalkanecarbaldehydes in inert solvents with λca. 300 nm has been studied. The photochemistry of cyclopropanecarbaldehyde differs from those of the other cycloalkanecarbaldehydes. All aldehydes exhibit two primary processes, viz, α-cleavage which is a T₁ process, and intermolecular hydrogen abstraction, which proceeds by both the S₁ and T₁ states of the aldehyde. Hydrogen abstraction by the cycloalkyl radical from the formyl radical then yields cycloalkane and CO. With cyclopropanecarbaldehyde, the resulting α-hydroxycyclopropylmethyl radicals rearrange to yield 3-formylpropyl and (Z)-but-1-enyloxyl radicals. The concommittantly formed cyclopropylcarbonyl radical does not decarbonylate due to conjugative stabilization of the cyclopropyl–carbonyl bond. For the same reason, cyclopropanecarbaldehyde gives only little α-cleavage. The cycloalkanecarbonyl radical formed with the other three aldehydes easily decarbonylates to yield the cycloalkyl radical. Combination of the radical pairs yield 1,2-dicycioalkyl-2-hydroxyethanone (2) and dicycloalkylmethanol (13). In a subsequent reaction, triplet excited cycloalkanecarbaldehyde abstracts the α-hydroxymethine hydrogen of (2) and disproportionation of the radical pair thus formed yields cycloalkylmethanol (12) and dicycloalkylethanedione (3). No products resulting from a Norrish type II process could be detected. The proposed mechanisms for the formation of the photoproducts are based on triplet quenching, radical scavenging, and CIDNP experiments.