ISSN:
0009-2940
Keywords:
Radicals
;
enol cation
;
Benzofurans
;
Oxazoles
;
Voltammetry, cyclic
;
Carbocations
;
α-carbonyl
;
Chemistry
;
Inorganic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The one-electron oxidation chemistry of three diaryl-substituted enols, 1,1-dimesityl-1-propen-2-ol (7), 1,2-dimesityl-1-propen-1-ol (8), and 2-mesityl-2-phenylethenol (9), has been examined. As a consequence of the pronounced steric shielding around the β-carbon, 7 does not react with nucleophiles upon addition of 2 eq. of oxidant, but cyclizes to yield benzofuran 18. Under identical conditions, the less shielded enols 8 and 9 react with methanol and other nucleophiles. Several mechanistic hypotheses for nucleophile incorporation are tested, thus allowing the second part of the α-umpolung reaction of ketones via enol cation radicals to be elucidated mechanistically. Fast-scan cyclic voltammetry studies (2000 V · s-1) show no reversible wave of the enol cation radicals. The corresponding methyl enol ether cation radicals, however, have a lifetime of several seconds at -10°C, and the kinetics of their reactions with acetonitrile and methanol was probed. The results obtained are consistent with fast deprotonation being the primary reaction of enol cation radicals. Under the given oxidative conditions the radicals formed are further oxidized. Consequently one-electron oxidation of enols provides an interesting entry to α-carbonyl carbocation chemistry.
Additional Material:
5 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/cber.19921250716
