Dataset Information

Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)2 and Discovery of Fe(OTf)3 as a Highly Effective Cocatalyst.

ABSTRACT: Copper salts find widespread use in Pd-catalyzed oxidation reactions, and they are typically used as oxidants or redox-active cocatalysts. Here, we probe the origin of a dramatic acceleration effect of Cu(OTf)2 in the C-H/C-H aerobic oxidative coupling of o-xylene. NMR spectroscopic analysis of the PdII catalyst in the presence of Cu(OTf)2, together with other experimental and DFT computational studies of the catalytic reaction, show that Cu(OTf)2 activates the PdII catalyst for C-H activation via a non-redox pathway and has negligible impact on catalyst reoxidation. These observations led to the testing of other metal triflate salts as cocatalysts, the results of which show that Fe(OTf)3 is even more effective than Cu(OTf)2.

SUBMITTER: Wang D

PROVIDER: S-EPMC5485668 | biostudies-literature | 2017 Apr

REPOSITORIES: biostudies-literature

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Publications

Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)2 and Discovery of Fe(OTf)3 as a Highly Effective Cocatalyst.

Wang Dian D Stahl Shannon S SS

Journal of the American Chemical Society 20170418 16

Copper salts find widespread use in Pd-catalyzed oxidation reactions, and they are typically used as oxidants or redox-active cocatalysts. Here, we probe the origin of a dramatic acceleration effect of Cu(OTf)2 in the C-H/C-H aerobic oxidative coupling of o-xylene. NMR spectroscopic analysis of the PdII catalyst in the presence of Cu(OTf)2, together with other experimental and DFT computational studies of the catalytic reaction, show that Cu(OTf)2 acti ...[more]

PMID: 28399364

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Project description:A recent report established that the tetrabutylammonium (TBA) salt of hexavanadopolymolybdate TBA4H5[PMo6V6O40] (PV6Mo6) serves as the redox buffer with Cu(II) as a co-catalyst for aerobic deodorization of thiols in acetonitrile. Here, we document the profound impact of vanadium atom number (x = 0-4 and 6) in TBA salts of PVxMo12-xO40(3+x)- (PVMo) on this multicomponent catalytic system. The PVMo cyclic voltammetric peaks from 0 to -2000 mV vs Fc/Fc+ under catalytic conditions (acetonitrile, ambient T) are assigned and clarify that the redox buffering capability of the PVMo/Cu catalytic system derives from the number of steps, the number of electrons transferred each step, and the potential ranges of each step. All PVMo are reduced by varying numbers of electrons, from 1 to 6, in different reaction conditions. Significantly, PVMo with x ≤ 3 not only has much lower activity than when x > 3 (for example, the turnover frequencies (TOF) of PV3Mo9 and PV4Mo8 are 8.9 and 48 s-1, respectively) but also, unlike the latter, cannot maintain steady reduction states when the Mo atoms in these polyoxometalate (POMs) are also reduced. Stopped-flow kinetics measurements reveal that Mo atoms in Keggin PVMo exhibit much slower electron transfer rates than V atoms. There are two kinetic arguments: (a) In acetonitrile, the first formal potential of PMo12 is more positive than that of PVMo11 (-236 and -405 mV vs Fc/Fc+); however, the initial reduction rates are 1.06 × 10-4 s-1 and 0.036 s-1 for PMo12 and PVMo11, respectively. (b) In aqueous sulfate buffer (pH = 2), a two-step kinetics is observed for PVMo11 and PV2Mo10, where the first and second steps are assigned to reduction of the V and Mo centers, respectively. Since fast and reversible electron transfers are key for the redox buffering behavior, the slower electron transfer kinetics of Mo preclude these centers functioning in redox buffering that maintains the solution potential. We conclude that PVMo with more vanadium atoms allows the POM to undergo more and faster redox changes, which enables the POM to function as a redox buffer dictating far higher catalytic activity.

Dataset Information

Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)2 and Discovery of Fe(OTf)3 as a Highly Effective Cocatalyst.

Publications

Pd-Catalyzed Aerobic Oxidative Biaryl Coupling: Non-Redox Cocatalysis by Cu(OTf)<sub>2</sub> and Discovery of Fe(OTf)<sub>3</sub> as a Highly Effective Cocatalyst.

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