Dataset Information

Mild and Regioselective Pd(OAc)₂-Catalyzed C-H Arylation of Tryptophans by [ArN₂]X, Promoted by Tosic Acid.

ABSTRACT: A regioselective Pd-mediated C-H bond arylation methodology for tryptophans, utilizing stable aryldiazonium salts, affords C2-arylated tryptophan derivatives, in several cases quantitatively. The reactions proceed in air, without base, and at room temperature in EtOAc. The synthetic methodology has been evaluated and compared against other tryptophan derivative arylation methods using the CHEM21 green chemistry toolkit. The behavior of the Pd catalyst species has been probed in preliminary mechanistic studies, which indicate that the reaction is operating homogeneously, although Pd nanoparticles are formed during substrate turnover. The effects of these higher order Pd species on catalysis, under the reaction conditions examined, appear to be minimal: e.g., acting as a Pd reservoir in the latter stages of substrate turnover or as a moribund form (derived from catalyst deactivation). We have determined that TsOH shortens the induction period observed when [ArN₂]BF₄ salts are employed with Pd(OAc)₂. Pd(OTs)₂(MeCN)₂ was found to be a superior precatalyst (confirmed by kinetic studies) in comparison to Pd(OAc)₂.

SUBMITTER: Reay AJ

PROVIDER: S-EPMC5557615 | biostudies-literature | 2017 Aug

REPOSITORIES: biostudies-literature

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Publications

Mild and Regioselective Pd(OAc)<sub>2</sub>-Catalyzed C-H Arylation of Tryptophans by [ArN<sub>2</sub>]X, Promoted by Tosic Acid.

Reay Alan J AJ Hammarback L Anders LA Bray Joshua T W JTW Sheridan Thomas T Turnbull David D Whitwood Adrian C AC Fairlamb Ian J S IJS

ACS catalysis 20170710 8

A regioselective Pd-mediated C-H bond arylation methodology for tryptophans, utilizing stable aryldiazonium salts, affords C2-arylated tryptophan derivatives, in several cases quantitatively. The reactions proceed in air, without base, and at room temperature in EtOAc. The synthetic methodology has been evaluated and compared against other tryptophan derivative arylation methods using the CHEM21 green chemistry toolkit. The behavior of the Pd catalyst species has been probed in preliminary mecha ...[more]

PMID: 28824821

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Project description:Direct C-H functionalization reactions have opened new avenues in catalysis, removing the need for prefunctionalization of at least one of the substrates. Although C-H functionalization catalyzed by palladium complexes in the presence of a base is generally considered to proceed by the CMD/AMLA-6 mechanism, recent research has shown that silver(I) salts, frequently used as bases, can function as C-H bond activators instead of (or in addition to) palladium(II). In this study, we examine the coupling of pentafluorobenzene 1 to 4-iodotoluene 2a (and its analogues) to form 4-(pentafluorophenyl)toluene 3a catalyzed by palladium(II) acetate with the commonplace PPh3 ligand, silver carbonate as base, and DMF as solvent. By studying the reaction of 1 with Ag2CO3/PPh3 and with isolated silver (triphenylphosphine) carbonate complexes, we show the formation of C-H activation products containing the Ag(C6F5)(PPh3)n unit. However, analysis is complicated by the lability of the Ag-PPh3 bond and the presence of multiple species in the solution. The speciation of palladium(II) is investigated by high-resolution-MAS NMR (chosen for its suitability for suspensions) with a substoichiometric catalyst, demonstrating the formation of an equilibrium mixture of Pd(Ar)(κ1-OAc)(PPh3)2 and [Pd(Ar)(μ-OAc)(PPh3)]2 as resting states (Ar = Ph, 4-tolyl). These two complexes react stoichiometrically with 1 to form coupling products. The catalytic reaction kinetics is investigated by in situ IR spectroscopy revealing a two-term rate law and dependence on [Pdtot/nPPh3]0.5 consistent with the dissociation of an off-cycle palladium dimer. The first term is independent of [1], whereas the second term is first order in [1]. The observed rates are very similar with Pd(PPh3)4, Pd(Ph)(κ1-OAc)(PPh3)2, and [Pd(Ph)(μ-OAc)(PPh3)]2 catalysts. The kinetic isotope effect varied significantly according to conditions. The multiple speciation of both AgI and PdII acts as a warning against specifying the catalytic cycles in detail. Moreover, the rapid dynamic interconversion of AgI species creates a level of complexity that has not been appreciated previously.

Dataset Information

Mild and Regioselective Pd(OAc)₂-Catalyzed C-H Arylation of Tryptophans by [ArN₂]X, Promoted by Tosic Acid.

Publications

Mild and Regioselective Pd(OAc)<sub>2</sub>-Catalyzed C-H Arylation of Tryptophans by [ArN<sub>2</sub>]X, Promoted by Tosic Acid.

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Dataset Information

Mild and Regioselective Pd(OAc)2-Catalyzed C-H Arylation of Tryptophans by [ArN2]X, Promoted by Tosic Acid.

Publications

Mild and Regioselective Pd(OAc)<sub>2</sub>-Catalyzed C-H Arylation of Tryptophans by [ArN<sub>2</sub>]X, Promoted by Tosic Acid.

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure

Tweets

Mild and Regioselective Pd(OAc)₂-Catalyzed C-H Arylation of Tryptophans by [ArN₂]X, Promoted by Tosic Acid.