Vanadium-Based, Extended Catalytic Lifetime Catechol Dioxygenases: Evidence for a Common Catalyst

In 1999, a catechol dioxygenase derived from a V-polyoxometalate was reported which was
able to perform a record >100 000 total turnovers of 3,5-di-tert-butylcatechol oxygenation using O2 as the
oxidant (Weiner, H.; Finke, R. G. J. Am. Chem. Soc. 1999, 121, 9831). An important goal is to better
understand this and other vanadium-based catechol dioxygenases. Scrutiny of 11 literature reports of
vanadium-based catechol dioxygenases yielded the insight that they all proceed with closely similar
selectivities. This, in turn, led to a “common catalyst hypothesis” for the broad range of vanadium based
catechol dioxygenase precatalysts presently known. The following three classes of V-based compounds,
10 complexes total, have been explored to test the common catalyst hypothesis:  (i) six vanadium-based
polyoxometalate precatalysts, (n-Bu4N)4H5PV14O42, (n-Bu4N)7SiW9V3O40, (n-Bu4N)5[(CH3CN)xFeII·SiW9V3O40],
(n-Bu4N)9P2W15V3O62, (n-Bu4N)5Na2[(CH3CN)xFeII·P2W15V3O62], and (n-Bu4N)4H2-γ-SiW10V2O40; (ii) three
vanadium catecholate complexes, [VVO(DBSQ)(DTBC)]2, [Et3NH]2[VIVO(DBTC)2]·2CH3OH, and [Na(CH3OH)2]2[VV(DTBC)3]2·4CH3OH (where DBSQ = 3,5-di-tert-butylsemiquinone anion and DTBC = 3,5-di-tert-butylcatecholate dianion), and (iii) simple VO(acac)2. Product selectivity studies, catalytic lifetime tests,
electron paramagnetic resonance spectroscopy (EPR), negative ion mode electrospray ionization-mass
spectrometry (negative ion ESI-MS), and kinetic studies provided compelling evidence for a common catalyst
or catalyst resting state, namely, Pierpont's structurally characterized vanadyl semiquinone catecholate
dimer complex, [VO(DBSQ)(DTBC)]2, formed from V-leaching from the precatalysts. The results provide a
considerable simplification and unification of a previously disparate literature of V-based catechol
dioxygenases.