Functional model oxygenations by nonheme iron complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 163

than iron. 77, 80-84 Strange to say, no nonheme iron complex has been used for
cleavage of catechols with molecular oxygen so long time.
In 1979, Funabiki et al. reported the oxygenative cleavage of 3,5-di- tert-
burylcatechol by a complex with molecular oxygen. 85 The reaction
was performed in anhydrous tetrahydrofuran at 25 °C under 1 atm by using
pyridine and bipyridine as ligands. Though the yield of the oxygenated product was
low compared with 3,5-di- tert -butyl-l,2-benzoquinone (DTBQ), it was first shown
that iron complexes could be potential catalysts for the oxygenase-like
oxygenations. In this first trial, a complex was used, considering the
importance of interaction of the complex with oxygen, but soon later it was shown
that the reaction is catalyzed by complexes similarly to -containing
oxygenases. 86 The main oxygenation products are shown in eq. 1. Isolation of an
intermediate monooxygenated compound, 2,4-di- tert -butylmuconic acid anhydride,
and its conversion to dioxygenated product, 3,5-di- -butyl-5-(carboxymethyl)-2-

furanone, indicated that the oxygen insertion proceeds stepwise rather than in one
step via a dioxetane intermediate. 86 In addition to these intradiol cleavage products,
extradiol cleavage products, i.e. 3,5-di- tert -butyl-5-(formyl)-2-furanone and 3,5-di-
tert

tert

formed, 86

-butyl-2-pyrone, were indicating that both intra- and extradiol

cleavages occur in this iron system using simple ligands.

The detailed studies in seeking for the better system with the higher
selectivity and catalytic activities revealed that in the series of mono- and bidentate
ligands pyridine and bipyridine are the most effective for oxygenations. 87 The
oxygenation proceeded catalytically in the presence of excess catechol, though the
catalytic efficiency was low because of the formation of DTBQ. This problem was
overcome by addition of 2,5-di- tert -butyl-hydroquinone that can convert DTBQ to
In the studies on mechanisms, the insertion of two oxygen atoms into
products was confirmed by experiments using in the anhydrous conditions. The
insertion of two atoms of oxygen was also shown recently by Yamahara et al .. 88
They synthesized a tridentate ligand (L = N -(2-hydroxyphenyl)- N -(2-pyridyl-
methyl)-benzylamine) having two N and one O atoms for coordination and isolated
catecholate complexes, and

Oxygenation of these complexes with gave products with two atoms of