UNEXPECTED OXADI-PI-METHANE REARRANGEMENT OF BETA,GAMMA-UNSATURATED ALDEHYDES

The oxadi-n-methane rearrangement (ODPM) is considered to represent th e normal photochemical behavior of beta,gamma-unsaturated ketones in t he triplet excited pi,pi state. However, the usual photoreactivity re ported for the majority of beta,gamma-unsaturated aldehydes is decarbo nylation. There are only two published reports of beta,gamma-unsaturat ed aldehydes that undergo the ODPM rearrangement. We now report effici ent ODPM rearrangement in the triplet-sensitized irradiation of twelve cyclic and acyclic beta,gamma-unsaturated aldehydes, namely, 2,2-dime thyl-4,4-diphenyl-3-butenal (6), 1-methyl-3-phenyl-2-cyclohexene-1-car baldehyde (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-bute nal (36), and 4-methyl-2,2-diphenyl-3-pentenal (47). All of them affor d the corresponding cyclopropyl aldehydes in 8-96% yield. Our results show that the ODPM rearrangement of aldehydes should be considered a n ormal photoreactivity of this type of compound. In one case (7), the f ormation of the corresponding 1,3-formyl migrated product was also obs erved. Aldehydes 35 and 47 undergo, in addition to the ODPM rearrangem ent, decarbonylation to the alkenes 37 and 51, respectively. The ODPM reaction takes place when the triplet energy from the sensitizer is ef ficiently transferred to the alkene moiety generating a T-1 ((3) pi,pi ) excited state and, furthermore, when the biradical intermediates ar e 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 th e efficiency of other di-pi-methane processes, such as the di-pi-metha ne (DPM) and azadi-pi-methane (ADPM) rearrangements, are also operativ e in the ODPM rearrangement of aldehydes. Thus, diphenyl substitution on the central carbon of the beta,gamma-unsaturated aldehyde, as in 47 , also promotes the ODPM rearrangement. In cases in which the competit ion between the ODPM and the DPM processes can occur, the selectivity observed depends on the relative stabilities of the 1,4-bridged biradi cal intermediates. Thus, aldehyde 36 yields the ODPM product exclusive ly, while diphenylvinyl)-2-methyl-4,4-diphenyl-3-butenal(43) affords t he DPM product 44 only.