Zur photochemischen Decarbonylierung von Aldehyden in Lösung

Abstract

The photochemistry of the saturated aldehydes 9 and 12, and of the β,γ-unsaturated, homoallylic conjugated aldehydes of type 17 and 29 in solution has been investigated. The saturated compounds revealed competitive photochemical reactions such as decarbonylation according to equation (5) as the major reaction path, and intramolecular γ-hydrogen transfer with sec-cyclobutanol formation according to equation (4).
In the unsaturated aldehydes of type 17 and 29 photolytic decarbonylation is accelerated and occurs to the exclusion of other reactions. The aldehydic hydrogen is almost entirely transferred in an intramolecular process to the original formyl position. To a large extent, the reaction is insensitive to the presence of molecular oxygen or to the nature of the solvent. Further, photolytic decarbonylation could neither be effected by employing triplett sensitizers such as acetophenone or benzene, nor be quenched by irradiating in 1,3-pentadiene solution. Comparative rate studies of aldehyde conversion and product formation with compounds 29 and 30 showed no primary isotope effect.
Two mechanistic alternatives [equations (6) and (7)] are discussed for the photodecarbonylation of homoallylic conjugated aldehydes which cannot be distinguished on the basis of the data presented. The first is a unimolecular 1,2-rearrangement of hydrogen with synchronous liberation of carbon monoxide (6). The second (7) is a dissociation into a radical pair having close orbital overlap of its components which is not disengaged before hydrogen transfer and product formation. Such overlap is partially preformed in the ground and excited states of the starting aldehydes. The proposed radical pair (7) effects minimum reactivity towards radical scavengers and maximum spatial orientation for the hydrogen transfer process. Reversal in relative orientation within the pair of the radical components and/or dissociation of the portion of non-conjugated aldehyde present – both connected with a somewhat less pronounced “cage effect” – are tentatively held responsible for the formation of the minor product of type 19 which originates from 17 by a less than completely intramolecular hydrogen transfer [equation (8)].