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DOI: 10.1055/s-2008-1078500
Lewis Acid promoted Oxidative Rearrangement of Tertiary Allylic Alcohols with the PhIO/TEMPO System
Publication History
Publication Date:
19 June 2008 (online)
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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.
Key words
iodosylbenzene - Lewis acids - oxidation - tertiary allylic alcohols - TEMPO
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References and Notes
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.
We have shown that allylic alcohols are rapidly oxidized, at 0 ˚C, with TEMPO/PhIO/Yb(OTf)3 system, see ref. 6b.
21Strong protic acids are known to depolymerize PhIO, see ref. 17b.
23In 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).