Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C – H activation

Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C–H activation

M. D. Marcinkowski, M. T. Darby, J. Liu, J. M. Wimble, F. R. Lucci, S. Lee, A. Michaelides, M. Flytzani-Stephanopolous, M. Stamatakis, E. C. H. Sykes

Nature Chem. 2018, ASAP
DOI: 10.1038/nchem.2915

Abstract:

The recent availability of shale gas has led to a renewed interest in C–H bond activation as the first step towards the synthesis of fuels and fine chemicals. Heterogeneous catalysts based on Ni and Pt can perform this chemistry, but deactivate easily due to coke formation. Cu-based catalysts are not practical due to high C–H activation barriers, but their weaker binding to adsorbates offers resilience to coking. Using Pt/Cu single-atom alloys (SAAs), we examine C–H activation in a number of systems including methyl groups, methane and butane using a combination of simulations, surface science and catalysis studies. We find that Pt/Cu SAAs activate C–H bonds more efficiently than Cu, are stable for days under realistic operating conditions, and avoid the problem of coking typically encountered with Pt. Pt/Cu SAAs therefore offer a new approach to coke-resistant C–H activation chemistry, with the added economic benefit that the precious metal is diluted at the atomic limit.

Mechanistic Basis for Efficient, Site-Selective, Aerobic Catalytic Turnover in Pd-Catalyzed C–H Imidoylation of Heterocycle-Containing Molecules

Key world: PdO2 adduct, CH activation

Stephen J. Tereniak and Shannon S. Stahl* 

Department of Chemistry, University of Wisconsin—Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/jacs.7b07359

Abstract

A recently reported Pd-catalyzed method for oxidative imidoylation of C–H bonds exhibits unique features that have important implications for Pd-catalyzed aerobic oxidation catalysis: (1) The reaction tolerates heterocycles that commonly poison Pd catalysts. (2) The site selectivity of C–H activation is controlled by an N-methoxyamide group rather than a suitably positioned heterocycle. (3) A Pd0 source, Pd2(dba)3 (dba = dibenzylideneacetone), is superior to Pd(OAc)2 as a precatalyst, and other PdII sources are ineffective. (4) The reaction performs better with air, rather than pure O2. The present study elucidates the origin of these features. Kinetic, mechanistic, and in situ spectroscopic studies establish that PdII-mediated C–H activation is the turnover-limiting step. The tBuNC substrate is shown to coordinate more strongly to PdII than pyridine, thereby contributing to the lack of heterocycle catalyst poisoning. A well-defined PdII–peroxo complex is a competent intermediate that promotes substrate coordination via proton-coupled ligand exchange. The effectiveness of this substrate coordination step correlates with the basicity of the anionic ligands coordinated to PdII, and Pd0 catalyst precursors are most effective because they selectively afford the PdII–peroxo in situ. Finally, elevated O2 pressures are shown to contribute to background oxidation of the isonitrile, thereby explaining the improved performance of reactions conducted with air rather than 1 atm O2. These collective results explain the unique features of the aerobic C–H imidoylation of N-methoxybenzamides and have important implications for other Pd-catalyzed aerobic C–H oxidation reactions.

Silicon-Tethered Strategies for CH Functionalization Reactions

Silicon-Tethered Strategies for CH Functionalization Reactions

M. Parasram and V. Gevorgyan

Acc. Chem. Res. ASAP, DOI: 10.1021/acs.accounts.7b00306
(http://pubs.acs.org/doi/pdf/10.1021/acs.accounts.7b00306)

Conspectus:

Selective and efficient functionalization of ubiquitous C−H bonds is the Holy Grail of organic synthesis. Most advances in this area rely on employment of strongly or weakly coordinating directing groups (DGs) which have proven effective for transition-metal-catalyzed functionalization of C(sp2)−H and C(sp3)−H bonds. Although most directing groups are important functionalities in their own right, in certain cases, the DGs become static entities that possess very little synthetic leverage. Moreover, some of the DGs employed are cumbersome or unpractical to remove, which precludes the use of this approach in synthesis. It is believed, that development of a set of easily installable and removable/modifiable DGs for C−H functionalization would add tremendous value to the growing area of directed functionalization, and hence would promote its use in synthesis and late-stage functionalization of complex molecules. In particular, silicon tethers have long provided leverage in organic synthesis as easily installable and removable/modifiable auxiliaries for a variety of processes, including radical transformations, cycloaddition reactions, and a number of TM-catalyzed methods, including ring-closing metathesis (RCM) and cross-coupling reactions. Employment of Si-tethers is highly attractive for several reasons: (1) they are easy to handle/synthesize and are relatively stable; (2) they utilize cheap and abundant silicon precursors; and (3) Si-tethers are easily installable and removable/modifiable. Hence, development of Si-tethers for C−H functionalization reactions is appealing not only from a practical but also from a synthetic standpoint, since the Si-tether can provide an additional handle for diversification of organic molecules post-C−H functionalization. Over the past few years, we developed a set of Si-tether approaches for C−H functionalization reactions. The developed Si-tethers can be categorized into four types: (Type-1) Si-tethers possessing a reacting group, where the reacting group is delivered to the site of functionalization; (Type-2) Si-tethers possessing a DG, designed for selective C(sp2)−H functionalization of arenes; (Type-3) reactive Si-tethers for C−H silylation of organic molecules; and finally, (Type-4) reactive Si-tethers containing a DG, developed for selective C−H silylation/hydroxylation of challenging C(sp3)−H bonds. In this Account, we outline our advances on the employment of silicon auxiliaries for directed C−H functionalization reactions. The discussion of the strategies for employment of different Si-tethers, functionalization/modification of silicon tethers, and the methodological developments on C−C, C−X, C−O, and C−Si bond forming reactions via silicon tethers will also be presented. While the work described herein presents a substantial advance for the area of C−H functionalization, challenges still remain. The use of noble metals are required for the C−H functionalization methods presented herein. Also, the need for stoichiometric use of high molecular weight silicon auxiliaries is a shortcoming of the presented concept.

 

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Computational Studies of Carboxylate-Assisted C − H Activation and Functionalization at Group 8 − 10 Transition Metal Centers

Computational Studies of Carboxylate-Assisted C−H Activation and Functionalization at Group 8−10 Transition Metal Centers

 

Davies, D. L.; Macgregor, S. A.; McMullin, C. L. Chem. Rev. ASAP

 

University of Leicester & Heriot-Watt University

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00839

Abstract:

 

Computational studies on carboxylate-assisted C−H activation and functionalization at group 8−10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C−H activation in terms of the “ambiphilic metal−ligand assistance” (AMLA) and “concerted metalation deprotonation” (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C−H bond being
activated (C(sp2)−H or C(sp3)−H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C−H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C−H functionalization based on carboxylate-
assisted C−H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C−H functionalization and applied computational chemistry are identified.

 

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