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

2005-06-29T00:00:00Z (GMT) by Cindy-Xing Yin Richard G. Finke
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-<i>tert</i>-butylcatechol oxygenation using O<sub>2</sub> as the oxidant (Weiner, H.; Finke, R. G. <i>J. Am. Chem. Soc.</i> <b>1999</b>, <i>121</i>, 9831). An important goal is to better understand this and <i>other</i> vanadium-based catechol dioxygenases. Scrutiny of 11 literature reports of vanadium-based catechol dioxygenases yielded the insight that they all proceed with <i>closely similar selectivities</i>. 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, (<i>n</i>-Bu<sub>4</sub>N)<sub>4</sub>H<sub>5</sub>PV<sub>14</sub>O<sub>42</sub>, (<i>n</i>-Bu<sub>4</sub>N)<sub>7</sub>SiW<sub>9</sub>V<sub>3</sub>O<sub>40</sub>, (<i>n</i>-Bu<sub>4</sub>N)<sub>5</sub>[(CH<sub>3</sub>CN)<i><sub>x</sub></i>Fe<sup>II</sup>·SiW<sub>9</sub>V<sub>3</sub>O<sub>40</sub>], (<i>n</i>-Bu<sub>4</sub>N)<sub>9</sub>P<sub>2</sub>W<sub>15</sub>V<sub>3</sub>O<sub>62</sub>, (<i>n</i>-Bu<sub>4</sub>N)<sub>5</sub>Na<sub>2</sub>[(CH<sub>3</sub>CN)<i><sub>x</sub></i>Fe<sup>II</sup>·P<sub>2</sub>W<sub>15</sub>V<sub>3</sub>O<sub>62</sub>], and (<i>n</i>-Bu<sub>4</sub>N)<sub>4</sub>H<sub>2</sub>-γ-SiW<sub>10</sub>V<sub>2</sub>O<sub>40</sub>; (ii) three vanadium catecholate complexes, [V<sup>V</sup>O(DBSQ)(DTBC)]<sub>2</sub>, [Et<sub>3</sub>NH]<sub>2</sub>[V<sup>IV</sup>O(DBTC)<sub>2</sub>]·2CH<sub>3</sub>OH, and [Na(CH<sub>3</sub>OH)<sub>2</sub>]<sub>2</sub>[V<sup>V</sup>(DTBC)<sub>3</sub>]<sub>2</sub>·4CH<sub>3</sub>OH (where DBSQ = 3,5-di-<i>tert</i>-butylsemiquinone anion and DTBC = 3,5-di-<i>tert</i>-butylcatecholate dianion), and (iii) simple VO(acac)<sub>2</sub>. 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)]<sub>2</sub>, formed from <i>V-leaching</i> from the precatalysts. The results provide a considerable simplification and unification of a previously disparate literature of V-based catechol dioxygenases.