TY - JOUR

T1 - Understanding the Regioselectivity of Aromatic Hydroxylation over Divanadium-Substituted γ-Keggin Polyoxotungstate

AU - Skobelev, Igor Y.

AU - Evtushok, Vasiliy Yu

AU - Kholdeeva, Oxana A.

AU - Maksimchuk, Nataliya V.

AU - Maksimovskaya, Raisa I.

AU - Ricart, Josep M.

AU - Poblet, Josep M.

AU - Carbó, Jorge J.

PY - 2017/12/1

Y1 - 2017/12/1

N2 - The aromatic hydroxylation of pseudocumene (PC) with aqueous hydrogen peroxide catalyzed by the divanadium-substituted γ-Keggin polyoxotungstate TBA4[γ-PW10O38V2(μ-O)(μ-OH)] (TBA-1H, TBA = tetrabutylammonium) has been studied using kinetic modeling and DFT calculations. This reaction features high chemoselectivity and unusual regioselectivity, affording 2,4,5-trimethylphenol (TMP) as the main product. Then the computational study was extended to the analysis of the regioselectivity for other alkoxy- and alkylarene substrates. The protonation/deprotonation of TBA-1H in MeCN/tBuOH (1:1) was investigated by 31P NMR spectroscopy. Forms with different protonation states, [γ-PV2W10O40]5- (1), [γ-HPV2W10O40]4- (1H), and [γ-H2PV2W10O40]3- (1H2), have been identified, and the protonation equilibrium constants were estimated on the basis of the 31P NMR data. DFT calculations were used to investigate the oxygen transfer process from hydroperoxo species, [γ-PW10O38V2(μ-O)(μ-OOH)]4- (2) and [γ-PW10O38V2(μ-OH)(μ-OOH)]3- (2H), and peroxo complex [γ-PW10O38V2(μ-2:2-O2)]3- (3) toward the different positions in the aromatic ring of PC, anisole, and toluene substrates. Product, kinetic, and computational studies on the PC hydroxylation strongly support a mechanism of electrophilic oxygen atom transfer from peroxo complex 3 to the aromatic ring of PC. The kinetic modeling revealed that the contribution of 3 into the initial reaction rate is, on average, about 70%, but it may depend on the reaction conditions. DFT calculations showed that the steric hindrance exerted by peroxo complex 3 is responsible for the origin of the unusual regioselectivity observed in PC hydroxylation, while for anisole and toluene the regioselective para-hydroxylation is due to electronic preference during the oxygen transfer from the active peroxo species 3.

AB - The aromatic hydroxylation of pseudocumene (PC) with aqueous hydrogen peroxide catalyzed by the divanadium-substituted γ-Keggin polyoxotungstate TBA4[γ-PW10O38V2(μ-O)(μ-OH)] (TBA-1H, TBA = tetrabutylammonium) has been studied using kinetic modeling and DFT calculations. This reaction features high chemoselectivity and unusual regioselectivity, affording 2,4,5-trimethylphenol (TMP) as the main product. Then the computational study was extended to the analysis of the regioselectivity for other alkoxy- and alkylarene substrates. The protonation/deprotonation of TBA-1H in MeCN/tBuOH (1:1) was investigated by 31P NMR spectroscopy. Forms with different protonation states, [γ-PV2W10O40]5- (1), [γ-HPV2W10O40]4- (1H), and [γ-H2PV2W10O40]3- (1H2), have been identified, and the protonation equilibrium constants were estimated on the basis of the 31P NMR data. DFT calculations were used to investigate the oxygen transfer process from hydroperoxo species, [γ-PW10O38V2(μ-O)(μ-OOH)]4- (2) and [γ-PW10O38V2(μ-OH)(μ-OOH)]3- (2H), and peroxo complex [γ-PW10O38V2(μ-2:2-O2)]3- (3) toward the different positions in the aromatic ring of PC, anisole, and toluene substrates. Product, kinetic, and computational studies on the PC hydroxylation strongly support a mechanism of electrophilic oxygen atom transfer from peroxo complex 3 to the aromatic ring of PC. The kinetic modeling revealed that the contribution of 3 into the initial reaction rate is, on average, about 70%, but it may depend on the reaction conditions. DFT calculations showed that the steric hindrance exerted by peroxo complex 3 is responsible for the origin of the unusual regioselectivity observed in PC hydroxylation, while for anisole and toluene the regioselective para-hydroxylation is due to electronic preference during the oxygen transfer from the active peroxo species 3.

KW - aromatic hydroxylation

KW - DFT

KW - homogeneous catalysis

KW - hydrogen peroxide

KW - kinetic-modeling

KW - polyoxometalate

KW - pseudocumene

KW - vanadium

KW - SELECTIVE OXIDATION

KW - MECHANISM

KW - MOLECULAR-ORBITAL METHODS

KW - ARENES

KW - EPOXIDATION

KW - PSEUDOCUMENE

KW - METHYLTRIOXORHENIUM-CATALYZED OXIDATION

KW - 2-METHYLNAPHTHALENE

KW - ALKENES

KW - PHENOLS

UR - http://www.scopus.com/inward/record.url?scp=85036669850&partnerID=8YFLogxK

U2 - 10.1021/acscatal.7b02694

DO - 10.1021/acscatal.7b02694

M3 - Article

AN - SCOPUS:85036669850

VL - 7

SP - 8514

EP - 8523

JO - Topics in Catalysis

JF - Topics in Catalysis

SN - 1022-5528

IS - 12

ER -