Stereomutation of 1-Hydroxyallyl Cations. Thermal and Photochemical Isomerizations of Protonated Enones

Protonated trans-crotonaldehyde (2), trans-crotonic acid (4), trans-tiglaldehyde (6), trans-pent-3-en-2-one (8), and trans-3-methylpent-3-en-2-one (10), prepared by protonation of the analogous enones or acid in FSO₃H, isomerized to give the corresponding cis isomers 11,12,13,14, and 15, respectively, when irradiated at low temperatures. A photostationary state was set up between the cis and trans isomers of these hydroxyallyl cations, which in the case of the 2/11 and 4/12 systems, could be approached by starting with either the cis or trans forms. The cis isomers were all thermally unstable and quantitatively reverted to the corresponding trans isomers on heating. The rate constants for these stereomutations have been obtained in FSO₃H and FSO₃H/SbF₅ (4:1) media at +50 °C. In particular it was found that the rate constants for the isomerization of 11 → 2 and 13 → 6 were decreased by a factor of 10² on the addition of SbF₅ to FSO₃H, while a comparable change in medium for the isomerizations of 14 → 8 and 15 → 10 enhanced their rates of isomerization by a small amount. No deuterium incorporation could be detected when these photochemical or thermal isomerizations were carried out in FSO₃D or FSO₃D/SbF₅ (4:1). It is suggested that in FSO₃H the thermal isomerizations of 11 and 13 take place by a mechanism involving addition of a nucleophile to C(3) of the allyl cations, rotation and subsequent reionization. However, it would appear that the thermal stereomutation of 12, 14, and 15 in both FSO₃H and FSO₃H/SbF₅ and the stereomutation of 11 and 13 in FSO₃H/SbF₅ all proceed by formation and subsequent ring opening of a protonated oxete. Possible reasons for the operation of these two pathways and the implications of this work to the parent allyl cations are discussed.