Branching Out: Rhodium-Catalyzed Allylation with Alkynes and Allenes

Branching Out: Rhodium-Catalyzed Allylation with Alkynes and Allenes

Philipp Koschker and Bernard Breit

Albert-Ludwigs-Universität Freiburg, Germany

http://pubs.acs.org/doi/pdf/10.1021/acs.accounts.6b00252

Acc. Chem. Res. 

DOI: 10.1021/acs.accounts.6b00252

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Conspectus:

We present a new and efficient strategy for the atom-economic transformation of both alkynes and allenes to allylic functionalized structures via a Rh-catalyzed isomerization/addition reaction which has been developed in our working group. Our methodology thus grants access to an important structural class valued in modern organic chemistry for both its versatility for further functionalization and the potential for asymmetric synthesis with the construction of a new stereogenic center. This new methodology, inspired by mechanistic investigations by Werner in the late 1980s and based on preliminary work by Yamamoto and Trost, offers an attractive alternative to other established methods for allylic functionalization such as allylic substitution or allylic oxidation. The main advantage of our methodology consists of the inherent atom economy in comparison to allylic oxidation or substitution, which both produce stoichiometric amounts of waste and, in case of the substitution reaction, require prefunctionalization of the starting material. Starting out with the discovery of a highly branched-selective coupling reaction of carboxylic acids with terminal alkynes using a Rh(I)/DPEphos complex as the catalyst system, over the past 5 years we were able to continuously expand upon this chemistry, introducing various (pro)nucleophiles for the selective C−O, C−S, C−N, and C−C functionalization of both alkynes and the double-bond isomeric allenes by choosing the appropriate rhodium/bidentate phosphine catalyst. Thus, valuable compounds such as branched allylic ethers, sulfones, amines, or γ,δ-unsaturated ketones were successfully synthesized in high yields and with a broad substrate scope. Beyond the branched selectivity inherent to rhodium, many of the presented methodologies display additional degrees of selectivity in regard to regio-, diastereo-, and enantioselective transformations, with one example even proceeding via a dynamic kinetic resolution. Many advances presented in this account were driven by detailed mechanistic investigations including DFT-calculations, ESI-MS and in situ IR experiments and enabled the application of our chemistry for target-oriented syntheses demonstrated by several examples shown herein. In general, this research topic has matured over the past years into a viable option when synthesizing chiral compounds, from small molecules such as quercus lactones to complex target structures such as Homolargazole or Clavosolide A. This demonstrates the importance and utility of these coupling reactions, especially considering the ease with which carbon−heteroatom bonds can be built stereoselectively, with many of the product classes displaying motifs common in modern APIs.

Alkane CH functionalization and oxidation with molecular oxygen

 Paper highlights 3 strategies for partial oxidation of hydrocarbons using specific examples

 

Alkane C–H Functionalization and Oxidation with Molecular Oxygen

Dominik Munz and Thomas Strassner^*

Physikalische Organische Chemie, Technische Universität Dresden, 01069 Dresden, Germany

Inorg. Chem., 2015, 54 (11), pp 5043–5052

DOI: 10.1021/ic502515x

Publication Date (Web): March 30, 2015

Synopsis

General strategies for the partial oxidation of methane and lower alkanes by molecular oxygen are discussed on the basis of three examples. These are the functionalization of methane and propane by palladium bis(N-heterocyclic carbene) complexes in combination with vanadium cocatalysts, the functionalization of methane catalyzed by cobalt salts, and the stoichiometric reaction of methane with group XVII compounds.

Abstract

The application of environmentally benign, cheap, and economically viable oxidation procedures is a key challenge of homogeneous, oxidative alkane functionalization. The typically harsh reaction conditions and the propensity of dioxygen for radical reactivity call for extraordinary robust catalysts. Mainly three strategies have been applied. These are (1) the combination of a catalyst responsible for C–H activation with a cocatalyst responsible for dioxygen activation, (2) transition-metal catalysts, which react with both hydrocarbons and molecular oxygen, and (3) the introduction of very robust main-group element catalysts for C–H functionalization chemistry. Herein, these three approaches will be assessed and exemplified by the reactivity of chelated palladium (N-heterocyclic carbene) catalysts in combination with a vanadium cocatalyst, the methane functionalization by cobalt catalysts, and the reaction of group XVII compounds with alkanes.

Catalytic Mechanism and Efficiency of Methane Oxidation by Hg(II) in Sulfuric Acid and Comparison to Radical Initiated Conditions

Catalytic Mechanism and Efficiency of Methane Oxidation by Hg(II) in Sulfuric Acid and Comparison to Radical Initiated Conditions

Jack T. Fuller, III, Steven Butler, Deepa Devarajan, Austin Jacobs, Brian G. Hashiguchi, Michael M. Konnick, William A. Goddard, III, Jason Gonzales, Roy A. Periana, and Daniel H. Ess

ACS Catal. 2016, 6, 4312-4322
http://pubs.acs.org/doi/pdf/10.1021/acscatal.6b00226

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Abstract: 

Methane conversion to methyl bisulfate by HgII(SO4) in sulfuric acid is an example of fast and selective alkane oxidation catalysis. Dichotomous mechanisms involving C−H activation and electron transfer have been proposed based on experiments. Radical oxidation pathways have also been proposed for some reaction conditions. HgII is also of significant interest because as a d10 transition metal it is similar to d10 main-group metals that also oxidize alkanes. Density- functional calculations are presented that use both implicit and a mixture of implicit/explicit solvent models for the complete HgII catalytic cycle of methane oxidation to methyl bisulfate. These calculations are consistent with experiment and reveal that methane is functionalized to methyl bisulfate by a C−H activation and reductive metal alkyl functionalization mechanism. This reaction pathway is lower in energy than both electron transfer and proton-coupled electron transfer pathways. After methane C−H functionalization, catalysis is completed by conversion of the proposed resting state, [HgI(HSO4)]2, into Hg0 followed by Hg0 to HgII oxidation induced by SO3 from dehydration of sulfuric acid. This catalytic cycle is efficient because in sulfuric acid the HgII/Hg0 potential results in a moderate free energy barrier for oxidation (∼40 kcal/mol) and HgII is electrophilic enough to induce barriers of <40 kcal/mol for C−H activation and reductive metal alkyl functionalization. Comparison of HgII to TlIII shows that while C−H activation and reductive metal alkyl functionalization have reasonable barriers for TlIII, the oxidation of TlI to TlIII has a significantly larger barrier than Hg0 to HgII oxidation and therefore TlIII is not catalytic in sulfuric acid. Comparison of HgII to CdII and ZnII reveals that while M0 to MII oxidation and C−H activation are feasible for these first-row and second-row transition metals, reductive metal alkyl functionalization barriers are very large and catalysis is not feasible. Calculations are also presented that outline the mechanism and energy landscape for radical-initiated (K2S2O8) methane oxidation to methanesulfonic acid in sulfuric acid.