Autoxidation-product-initiated dioxygenases: Vanadium-based, record catalytic lifetime catechol dioxygenase catalysis

In recent work, it was shown that V-containing polyoxometalates such as (n-Bu(4)N)(7)SiW(9)V(3)O(40) or (n-Bu(4)N)(9)P(2)W(15)V(3)O(62), as well as eight other V-containing precatalysts tested, evolve to a high activity, long catalytic lifetime (>= 30 000 - 100 000 total turnovers) 3,5-di-tert-butylcatechol dioxygenase, in which Pierpont's complex [VO(DBSQ)(DTBC)](2) (where DBSQ is 3,5-di-tert-butylsemiquinone and DTBC is the 3,5-di-tert-butylcatecholate dianion) was identified as a common catalyst or catalyst resting state (Yin, C.-X.; Finke, R. G. Vanadium-Based, Extended Catalytic Lifetime Catechol Dioxygenases: Evidence For a Common Catalyst. J. Am. Chem. Soc, 2005, 127 (25), 90039013). Herein, those findings are followed up by studies aimed at answering the following questions about this record catalytic lifetime 3,5-di-tert-butylcatechol dioxygenase catalyst: (1) What is the key to how V leaches from, for example, seemingly robust V-containing polyoxometalate precatalysts? (ii) What is the key to the sigmoidal, apparently autocatalytic kinetics observed? (iii) What can be learned about the underlying reactions that form [VO(DBSQ)(DTBC)](2)? (iv) Finally, do the answers to (i-iii) lead to any broader insights or concepts? Key findings from the present work include the fact that the reaction involves a novel, autoxidation-product-induced dioxygenase, that is, one in which the undesired autoxidation of the 3,5-di-tert-butylcatechol substrate to the oorresponding benzoquinone and H(2)O(2) turns on the desired dioxygenase catalysis via a V-leaching process which eventually yields Pierpont's complex, [VO(DBSQ)(DTBC)](2). Plausible reactions en route to [VO(DBSQ)(DTBO)](2) consistent with the kinetic data, the role of H(2)O(2), and the relevant literature are provided. The results provide a prototype example of the little observed but likely more general concept of an autoxidation-product-initiated reaction. The results also provide considerable simplification of, and insight into, the previously disparate literature of V-based 3,5-di-tert-butylcatechol dioxygenase catalysis.