Effective Pincer Cobalt Precatalysts for Lewis Acid Assisted CO2 Hydrogenation

Effective Pincer Cobalt Precatalysts for Lewis Acid Assisted CO2 Hydrogenation

Ariana Z. Spentzos, Charles L. Barnes, and Wesley H. Bernskoetter*

University of Missouri

http://pubs.acs.org/doi/pdf/10.1021/acs.inorgchem.6b01454

Inorg. Chem. 

DOI: 10.1021/acs.inorgchem.6b01454

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Abstract:

The pincer ligand MeN[CH2CH2(PiPr2)]2 (iPrPNP) was employed to support a series of cobalt(I) complexes, which were crystallographically characterized. A cobalt monochloride species, (iPrPNP)CoCl, served as a precursor for the preparation of several cobalt precatalysts for CO2 hydrogenation, including a cationic dicarbonyl cobalt complex, [(iPrPNP)Co(CO)2]+. When paired with the Lewis acid lithium triflate, [(iPrPNP)Co(CO)2]+ affords turnover numbers near 30 000 (at 1000 psi, 45 °C) for CO2-to-formate hydrogenation, which is a notable increase in activity from previously reported homogeneous cobalt catalysts. Though mechanistic information regarding the function of the precatalysts remains limited, multiple experiments suggest the active species is a molecular, homogeneous [(iPrPNP)Co] complex.

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.