Lewis Acid promoted Oxidative Rearrangement of Tertiary Allylic Alcohols with the PhIO/TEMPO System

Abstract

A mild and environmentally friendly method for Lewis acid catalyzed oxidative rearrangement of tertiary allylic alcohols to β-disubstituted enones by the TEMPO/PhIO system is described. Bismuth triflate was found to be the most efficient catalyst for the majority of the substrates tested except for tertiary vinyl carbinols which could be transformed to enals in fair yields only when Re₂O₇ was used as catalyst. A plausible mechanism for this oxidative rearrangement is discussed.

iodosylbenzene - Lewis acids - oxidation - tertiary allylic alcohols - TEMPO

References and Notes

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General Procedure for Lewis Acid Catalyzed Oxidative Rearrangement of Tertiary Allylic Alcohols with TEMPO and PhIO (Table 2)
To a solution of tertiary alcohol (1 mmol) in CH₂Cl₂ (5 mL) were added PhIO (264 mg, 2 equiv) and TEMPO (15.6 mg, 0.1 equiv). For method B and C, powdered 4 Å MS was also added (0.08 and 0.13 g/mmol, respectively). The suspension was cooled to 0 ˚C and Lewis acid was added. Dissolution of PhIO is indicative of the end of the reaction. For method A, just after dissolution of PhIO, powdered NaHCO₃ was added and the stirring was continued for 10 min. In all cases, the reaction was poured onto a column of SiO₂ (20 g) and eluted with EtOAc-PE (1:6). The purity of each synthesized carbonyl compound was checked by NMR spectroscopy.

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Strong protic acids are known to depolymerize PhIO, see ref. 17b.

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In order to determine if Re₂O₇ was able to isomerize allylic alcohols in the lapse of the reaction time (1 h), it was added (8 mol%) to a cooled solution of 1-butyl-2-cyclopenten-1-ol in CH₂Cl₂. After 15 min, it was observed by TLC the disappearance of the starting material and formation of UV-absorbing, nonpolar products (dehydrated products).