D. Armesto et al., UNEXPECTED OXADI-PI-METHANE REARRANGEMENT OF BETA,GAMMA-UNSATURATED ALDEHYDES, Journal of organic chemistry, 61(4), 1996, pp. 1459-1466
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.