Unexpected Oxadi-π-methane Rearrangement of β,γ-Unsaturated Aldehydes

Abstract

The oxadi-π-methane rearrangement (ODPM) is considered to represent the normal photochemical behavior of β,γ-unsaturated ketones in the triplet excited π,π* state. However, the usual photoreactivity reported for the majority of β,γ-unsaturated aldehydes is decarbonylation. There are only two published reports of β,γ-unsaturated aldehydes that undergo the ODPM rearrangement. We now report efficient ODPM rearrangement in the triplet-sensitized irradiation of twelve cyclic and acyclic β,γ-unsaturated aldehydes, namely, 2,2-dimethyl-4,4-diphenyl-3-butenal (6), 1-methyl-3-phenyl-2-cyclohexene-1-carbaldehyde (7), 1-methyl-3-phenyl-2-cyclopentene-1-carbaldehyde (14a), 1-methyl-3-phenyl-2-cycloheptene-1-carbaldehyde (14b), 2,2-dimethyl-4-phenyl-3-butenal (18), 2-(3,4-dihydro-2-naphthyl)-2-methylpropanal (23), 3-(9-fluorenylidene)-2,2-dimethylpropanal (24), 5-cyclopentylidene-2,2-dimethyl-3-pentenal (27), 2,2,6-trimethyl-3,5-heptadienal (28), 2,2,4,4-tetraphenyl-3-butenal (35), 2-methyl-4,4-diphenyl-2-vinyl-3-butenal (36), and 4-methyl-2,2-diphenyl-3-pentenal (47). All of them afford the corresponding cyclopropyl aldehydes in 8−96% yield. Our results show that the ODPM rearrangement of aldehydes should be considered a normal photoreactivity of this type of compound. In one case (7), the formation of the corresponding 1,3-formyl migrated product was also observed. Aldehydes 35 and 47 undergo, in addition to the ODPM rearrangement, decarbonylation to the alkenes 37 and 51, respectively. The ODPM reaction takes place when the triplet energy from the sensitizer is efficiently transferred to the alkene moiety generating a T1 (3π,π*) excited state and, furthermore, when the biradical intermediates are stabilized by phenyl or vinyl substitution. Thus, 2,2,4-trimethyl-3-pentenal (46), in which these two requirements are not met, undergoes decarbonylation exclusively. Some structural factors that influence the efficiency of other di-π-methane processes, such as the di-π-methane (DPM) and azadi-π-methane (ADPM) rearrangements, are also operative in the ODPM rearrangement of aldehydes. Thus, diphenyl substitution on the central carbon of the β,γ-unsaturated aldehyde, as in 47, also promotes the ODPM rearrangement. In cases in which the competition between the ODPM and the DPM processes can occur, the selectivity observed depends on the relative stabilities of the 1,4-bridged biradical intermediates. Thus, aldehyde 36 yields the ODPM product exclusively, while 2-(2,2-diphenylvinyl)-2-methyl-4,4-diphenyl-3-butenal (43) affords the DPM product 44 only.