Thursday, June 29, 2017

Molecular Adsorbates Switch on Heterogeneous Catalysis: Induction of Reactivity by N-Heterocyclic Carbenes

Molecular Adsorbates Switch on Heterogeneous Catalysis: Induction of Reactivity by N-Heterocyclic Carbenes



 Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster, Germany
 Institute for Solid State Theory and Center for Multiscale Theory and Computation, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
§Department of Chemistry, Graduate School of Science, Research Center for Materials Science (RCMS), and Integrated Research Consortium on Chemical Sciences (IRCCS), Nagoya University, Furo-cho, Chikusa, Nagoya, Aichi 464-8602, Japan
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/jacs.7b05112

Abstract

Abstract Image
We report the N-heterocyclic carbene (NHC)-induced activation of an otherwise unreactive Pd/Al2O3 catalyst. Surface analysis techniques demonstrate the NHC being coordinated to the palladium particles and affecting their electronic properties. Ab initio calculations provide further insight into the electronic effect of the coordination with the NHC injecting electron density into the metal nanocluster thus lowering the barrier for bromobenzene activation. By this NHC modification, the catalyst could be successfully applied in the Buchwald–Hartwig amination of aryl chlorides, bromides, and iodides. Various heterogeneity tests could additionally show that the reaction proceeds via a heterogeneous active species.

Tuesday, June 27, 2017

Catalytic Dehydrogenative C–C Coupling by a Pincer-Ligated Iridium Complex

Catalytic Dehydrogenative C–C Coupling by a Pincer-Ligated Iridium Complex

 Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
 Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/jacs.7b03433

Abstract

Abstract Image
The pincer-iridium fragment (iPrPCP)Ir (RPCP = κ3-2,6-C6H3(CH2PR2)2) has been found to catalyze the dehydrogenative coupling of vinyl arenes to afford predominantly
(E,E)-1,4-diaryl-1,3-butadienes. The eliminated hydrogen can undergo addition to
another molecule of vinyl arene, resulting in an overall disproportionation reaction with
1 equiv of ethyl arene formed for each equivalent of diarylbutadiene produced.
Alternatively, sacrificial hydrogen acceptors (e.g., tert-butylethylene) can be added to
 the solution for this purpose. Diarylbutadienes are isolated in moderate to good yields,
up to ca. 90% based on the disproportionation reaction. The results of DFT calculations
and experiments with substituted styrenes indicate that the coupling proceeds via
double C–H addition of a styrene molecule, at β-vinyl and ortho-aryl positions,
to give an iridium(III) metalloindene intermediate; this intermediate then adds a
β-vinyl C–H bond of a second styrene molecule before reductively eliminating product.
 Several metalloindene complexes have been isolated and crystallographically
characterized. In accord with the proposed mechanism, substitution at the ortho-aryl
positions of the styrene precludes dehydrogenative homocoupling. In the case of 2,4,6-trimethylstyrene, dehydrogenative coupling of β-vinyl and ortho-methyl C–H bonds
affords dimethylindene, demonstrating that the dehydrogenative coupling is not
 limited to C(sp2)–H bonds.

A Critical Assessment of the Direct Catalytic Oxidation of Methane to Methanol


A Critical Assessment of the Direct Catalytic Oxidation of Methane to Methanol

DOI: 10.1002/anie.201702550
Angew Chem International, AcceptedManuscript online: 23 June 2017


       DOI: 10.1002/anie.201702550

      Abstract

      Despite the emerging number of disparate approaches for the direct selective partial oxidation of methane, none of them has translated into an industrial process. The oxidation of methane to methanol is a difficult yet intriguing and rewarding task as it has the potential to eliminate the prevalent natural gas flaring by providing novel routes to its valorisation. This review considers the synthesis of methanol and methanol derivatives from methane by homogeneous and heterogeneous pathways. In establishing the severe limitations related to the direct catalytic synthesis of methanol from methane, we highlight the vastly superior performance of systems, which produce methanol derivatives or incorporate specific measures such as the use of multi-component catalysts to stabilise methanol. We thereby identify methanol protection as being indispensable in homogeneous and heterogeneous catalysis with regard to future research on this topic.