Copper(II)-mediated autoxidation of tert-butylresorcinols.
Although copper(II)-mediated oxidation of phenols results in oxidative coupling rather than in oxygenation, it was recently reported that naturally occurring 5-alkylresorcinols undergo oxygenation in the presence of copper(II). To explore the generality of this reaction, the copper(II)-mediated autoxidation of 4-tert-butylresorcinol and 4,6-di-tert-butylresorcinol was investigated and was found to result in direct oxygenation at open activated positions and, at the tert-butyl-substituted positions, in oxygenation with competing loss of (as isobutylene) and 1,2-rearrangement of the tert-butyl group. 5-tert-Butyl-2-hydroxy-1,4-benzoquinone is the major product from both starting materials, and the final product mixture reflects, in part, coupling of metastable initially formed electrophilic and nucleophilic side products. Mechanisms that are consistent with the observed products and control reactions are proposed. The key step appears to be equilibration of a copper(II)-resorcinolate with a charge-transfer radical form that reacts regioselectively with O(2) as prescribed by resonance.
Ling
Ke-Qing
KQ
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, USA.
Lee
Younghee
Y
Macikenas
Dainius
D
Protasiewicz
John D
JD
Sayre
Lawrence M
LM
D000473:Alkanes / Q000737:chemistry*; D002384:Catalysis; D003300:Copper / Q000737:chemistry*; D007202:Indicators and Reagents; D015394:Molecular Structure; D010084:Oxidation-Reduction; D012118:Resorcinols / Q000737:chemistry*; D013237:Stereoisomerism
D000473:Alkanes; D007202:Indicators and Reagents; D012118:Resorcinols; D003300:Copper