Novel tert-butyl migration in copper-mediated phenol ortho-oxygenation implicates a mechanism involving conversion of a 6-hydroperoxy-2,4-cyclohexadienone directly to an o-quinone

Copper mediated ortho-oxygenation of phenolates may proceed through the gen eration of a 6-peroxy-2,4-cyclohexadienone intermediate. To test this theor y, we studied the fate of sodium 4-carbethoxy2,6-di-tert-butylphenolate, wh ere the ortho-oxygenation sites are blocked by tert-butyl groups. Using the Cu(I) complex of N,N-bis(2-(N-methylbenzimidazol-2-yl)ethyl)benzylamine, i solation of the major oxygenated product and characterization by single-cry stal X-ray crystallography and NMR spectroscopy revealed it to be 4-carbeth oxy-3,6-di-tert-butyl-1,2-benzoquinone, resulting from a 1,2-migration of a tert-butyl group. The independently prepared B-hydroperoxide is transforme d by the Cu(I)- (or Cu(II)-) ligand complex to the same o-quinone. The obse rved 1,2-migration of the tert-butyl group appears to reflect an electron d emand created by rearrangement of the postulated peroxy intermediate. A mec hanism proceeding alternatively through a catechol and subsequent oxidation to the o-quinone seems ruled out by a control study demonstrating that the requisite intermediate to catechol formation would instead eliminate the 2 -tert-butyl group.