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)4H 5PV14O42, (n-Bu4N) 7SiW9V3O40, (n-Bu4N) 5[(CH3CN)xFeII&middotSiW 9V3O40], (n-Bu4N)9P 2W15V3O62, (n-Bu4N) 5Na2[(CH3CN)xFeII&middot P2W15V3O62], and (n-Bu 4N)4H2-γ-SiW10V 2O40; (ii) three vanadium catecholate complexes, [V VO(DBSQ)(DTBC)]2, [Et3NH]2[V IVO(DBTC)2]&middot2CH3OH, and [Na(CH 3-OH)2]2[VV(DTBC)3] 2&middot4CH3OH (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. © 2005 American Chemical Society.