Dioxygenase catalysis by d 0 metal–catecholate complexes containing vanadium and molybdenum with H 2(3,5-DTBC) and H 2(3,6-DTBC) substrates

Abstract

Facile synthetic dioxygenases, catalysts that can split O 2 and place both oxygen atoms selectively into 2 olefins to yield 2 epoxides or into 2 C–H bonds to yield 2 alcohols, remain a “Holy Grail” of oxidation catalysis. Recently, it was shown that [VO(3,5-DTBC)(3,5-DBSQ)] 2 (where 3,5-DTBC and 3,5-DBSQ are 3,5-di- tert-butylcatecholate and 3,5-di- tert-butylsemiquinone, respectively) is the catalytic-cycle resting state of a record catalytic lifetime catechol dioxygenase catalyst (100,000 total catalytic turnovers) for the substrate 3,5-di- tert-butylcatechol, H 2(3,5-DTBC) [C.-X. Yin, R.G. Finke, J. Am. Chem. Soc. 127 (2005) 9003–9013]. Herein we show that the precatalyst V(3,6-DTBC) 2(3,6-DBSQ) also gives dioxygenase products for the substrate H 2(3,5-DTBC), notably the same dioxygenase products in similar yields as seen for [VO(3,5-DTBC)(3,5-DBSQ)] 2. EPR studies show that the same g = 2.003–2.004 species are present in solution throughout the oxidation reaction for both [VO(3,5-DTBC)(3,5-DBSQ)] 2 and V(3,6-DTBC) 2(3,6-DBSQ). The similar products, product yields, and EPR spectra observed suggest that both [VO(3,5-DTBC)(3,5-DBSQ)] 2 and V(3,6-DTBC) 2(3,6-DBSQ) are feeding into the same catalytic cycle. In addition we have expanded the substrates to include H 2(3,6-DTBC), a substrate of interest for its higher symmetry, structurally simplified organic products and steric hindrance expected to favor the formation of monomeric metal–catecholate complexes. The precatalysts [VO(3,5-DTBC)(3,5-DBSQ)] 2 and V(3,6-DTBC) 2(3,6-DBSQ) were examined with H 2(3,6-DTBC) and found to give the same intradiol and extradiol dioxygenase products in the same yields. EPR studies of [VO(3,5-DTBC)(3,5-DBSQ)] 2 and V(3,6-DTBC) 2(3,6-DBSQ) with H 2(3,6-DTBC) show that the same spectra are observed throughout the oxidation, but at different reaction times. These EPR observations, along with the product studies, suggest that [VO(3,5-DTBC)(3,5-DBSQ)] 2 and V(3,6-DTBC) 2(3,6-DBSQ) have a common mechanistic cycle en route to the H 2(3,6-DTBC) dioxygenase products, albeit a mechanism different than that observed for H 2(3,5-DTBC) based on observed EPR spectra and differing product distributions. We also show that oxidation catalysis with [MoO(3,5-DTBC) 2] 2 follows primarily an oxidase path leading to the undesired benzoquinone autoxidation products for both H 2(3,5-DTBC) and H 2(3,6-DTBC) and with reaction times one to two orders of magnitude longer than seen for the vanadium precatalysts. The dramatic difference in product distribution and slower rate for [MoO(3,5-DTBC) 2] 2, along with its lack of a semiquinone ligand in at least this precatalyst, suggests the hypothesis that the d 0 vanadium(V) bonded to a semiquinone ligand in [VO(3,5-DTBC)(3,5-DBSQ)] 2 and V(3,6-DTBC) 2(3,6-DBSQ) is a necessary component of these dioxygenase precatalysts able to produce primarily dioxygenase products.