Friday, September 15, 2017

A unified photoredox-catalysis strategy for C(sp3)– H hydroxylation and amidation using hypervalent iodine


A unified photoredox-catalysis strategy for C(sp3)H hydroxylation and amidation using hypervalent iodine 


Guo-Xing Li,a Cristian A. Morales-Rivera,b Fang Gao,a Yaxin Wang,a Gang He,a Peng Liu *b and Gong Chen *ac
 
aState Key Laboratory and Institute of Elemento-Organic Chemistry, College of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China. E-mail: gongchen@nankai.edu. cn
bDepartment of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA. E-mail: pengliu@pitt.edu
cDepartment of Chemistry, The Pennsylvania State University, 104 Chemistry Building, University Park, PA 16802, USA. E-mail: guc11@psu.edu
Chem. Sci. 2017, ASAP 
DOI: 10.1039/c7sc02773g
http://pubs.rsc.org/en/content/articlepdf/2017/sc/c7sc02773g?page=search

Abstract:






We report a unified photoredox-catalysis strategy for both hydroxylation and amidation of tertiary and benzylic CH bonds. Use of hydroxyl perfluorobenziodoxole (PFBlOH) oxidant is critical for efficient tertiary CH functionalization, likely due to the enhanced electrophilicity of the benziodoxole radical. Benzylic methylene CH bonds can be hydroxylated or amidated using unmodified hydroxyl benziodoxole oxidant BlOH under similar conditions. An ionic mechanism involving nucleophilic trapping of a carbocation intermediate by H2O or CH3CN cosolvent is presented.

Monday, September 11, 2017

Catalytic NH3 Synthesis using N2/H2 at Molecular Transition Metal Complexes: Concepts for Lead Structure Determination using Computational Chemistry





Abstract

While industrial NH3 synthesis based on the Haber–Bosch-process was invented more than a century ago, there is still no molecular catalyst available which reduces N2 in the reaction system N2/H2 to NH3. As the many efforts of experimentally working research groups to develop a molecular catalyst for NH3 synthesis from N2/H2 have led to a variety of stoichiometric reductions it seems justified to undertake the attempt of systematizing the various approaches of how the N2 molecule might be reduced to NH3 with H2 at a transition metal complex. In this contribution therefore a variety of intuition-based concepts are presented with the intention to show how the problem can be approached. While no claim for completeness is made, these concepts intend to generate a working plan for future research. Beyond this, it is suggested that these concepts should be evaluated with regard to experimental feasibility by checking barrier heights of single reaction steps and also by computation of whole catalytic cycles employing density functional theory (DFT) calculations. This serves as a tool which extends the empirically driven search process and expands it by computed insights which can be used to rationalize the various challenges which must be met.

Aqueous Au-Pd colloids catalyze selective CH4 oxidation to CH3OH with O2 under mild conditions

Aqueous Au-Pd colloids catalyze selective CH4 oxidation to CH3OH with O2 under mild conditions
Nishtha Agarwal1,Simon J. Freakley1,Rebecca U. McVicker1,Sultan M. Althahban2,Nikolaos Dimitratos1,Qian He1,David J. Morgan1,Robert L. Jenkins1David J. Willock1,Stuart H. Taylor1,Christopher J. Kiely1,2Graham J. Hutchings1,

1Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK.2Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015, USA.

http://science.sciencemag.org/content/early/2017/09/06/science.aan6515

Abstract
The selective oxidation of methane, the primary component of natural gas, remains an important challenge in catalysis. Using colloidal gold-palladium nanoparticles rather than the same nanoparticles supported on titanium oxide, we oxidized methane to methanol with high selectivity (92%) in aqueous solution at mild temperatures. Using isotopically labeled O2 as an oxidant in the presence of H2O2, we demonstrate that the methanol produced incorporated a substantial fraction (70%) of gas-phase O2. More oxygenated products were formed than H2O2 consumed, suggesting that the controlled breakdown of H2O2 activates methane which subsequently incorporates molecular oxygen through a radical process. If a source of methyl radicals can be established, then the selective oxidation of methane to methanol using molecular oxygen is possible.

Thursday, August 3, 2017

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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Tuesday, July 11, 2017

Rhodium(I)-Catalyzed Direct Carboxylation of Arenes with CO2 via Chelation-Assisted C-H Bond Activation

Rhodium(I)-Catalyzed Direct Carboxylation of Arenes with CO2 via Chelation-Assisted C-H Bond Activation 


Hajime Mizuno, Jun Takaya, and Nobuharu Iwasawa* 
Department of Chemistry, Tokyo Institute of Technology, O-Okayama, Meguro-ku, Tokyo 152-8551, Japan
dx.doi.org/10.1021/ja109097z |J. Am. Chem. Soc. 2011, 133, 1251–1253




ABSTRACT: Rh-catalyzed direct carboxylation of unactivated aryl C-H bond under atmospheric pressure of carbon dioxide was realized via chelation-assisted C-H activation for the first time. Variously substituted and functionalized 2-arylpyridines and 1-arylpyrazoles underwent the carboxylation in the presence of the rhodium catalyst and a stoichiometric methylating reagent, AlMe2(OMe), to give carboxylated products in good yields. The catalysis is proposed to consist of methylrhodium(I) species as the key intermediate, which undergoes C-H activation to afford rhodium(III), followed by reductive elimination of methane to give nucleophilic arylrhodium(I). This approach demonstrates promising application of C-H bond activation strategy in the field of carbon dioxide fixation.

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.