Generation of simple enols in non-aqueous solution by fast double-bond migration of allylic alcohols with rhodium(I) and iridium(I) complexes

Abstract

The complexes [Rh(CO)(PPh₃)₃]ClO₄1 and [Ir(cod)(PhCN)(PPh₃)]ClO₄2(cod = cycloocta-1,5-diene), rapidly catalyse the double-bond migration of 2-ethylprop-2-en-1-ol 3 and prop-2-en-1-ol 4, respectively to generate a significant amount of the enols 2-methylbut-1-en-1-ol 5 and prop-1-en-1-ol 6 in the absence of a solvent and in CD₃COCD₃. Both enols 5 and 6 are quite stable and slowly undergo ketonization to the corresponding carbonyl compounds at room temperature in the absence of a solvent and in aprotic solvents. Detailed ¹H NMR spectral data at 300 MHz suggest that two isomers (Z and E) of compound 5 are simultaneously produced in the reaction of 3 and 1, the ratio of the isomers (major:minor) being ca. 6–9 : 1. Reaction of compound 4 with 2 also generates the Z(major) and E isomers (minor product) of 6. The Z isomer initially generated rapidly undergoes isomerization to give the E isomer in the presence of 2, while both the Z and E isomers relatively slowly undergo ketonization. Additional ¹H and ¹³C NMR and infrared spectral data for Me₂CCHOD and Me₂CDCHO are reported with some experimental details.