Mild and Efficient Nickel-Catalyzed Heck Reactions with Electron-Rich Olefins

Abstract

A new efficient protocol for the nickel-catalyzed Heck reaction of aryl triflates with vinyl ethers is presented. Mild reaction conditions that equal those of the corresponding palladium-catalyzed Heck reaction are applied, representing a practical and more sustainable alternative to the conventional regioselective arylation of vinyl ethers. A catalytic system comprised of Ni(COD)(2) and 1,1'-bis(diphenylphosphino)ferrocene (DPPF) in combination with the tertiary amine Cy(2)NMe proved effective in the olefination of a wide range of aryl triflates. Both electron-deficient and electron-rich arenes proved compatible, and the corresponding aryl methyl ketone could be secured after hydrolysis in yields approaching quantitative. Good functional group tolerance was observed matching the characteristics of the analogous Pd-catalyzed Heck reaction. The high levels of catalytic activity were explained by the intermediacy of a cationic nickel(II) complex potentially responsible for the successive β-hydride elimination and base promoted catalyst regeneration. Although these elementary reactions are normally considered challenging, DFT calculations suggested this pathway to be favorable under the applied reaction conditions.

Recent advances in homogeneous nickel catalysis

“…87 An example of the latter using Ni-catalysis is given by Skrydstrup and coworkers who demonstrated good selectivity for coupling of aryl triflates ( 64 ) and enol ethers ( 65 ), which following subsequent hydrolysis formed methyl ketones ( 66 ) (Figure 11a). 88 …”

Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: An Overview of Commonly Employed DFT Methods and Mechanistic Insights

“…The importance of cationic nickel intermediates in Ni(0)/Ni(II)-catalyzed Heck coupling of aryl triflates was deduced in a 2012 study (at the PBF (THF) B3LYP-D3//B3LYP/6-31G(d)(LACVP*) level of theory) by Norrby and Skrydstrup. 311 The facile dissociation of the triflate anion is suggested to result in formation of a cationic Ni(II) complex, which due to its electron-deficient nature can easily bind other ligands such as olefins. β-Hydride elimination was determined to be the rate-limiting step of the reaction.…”

NiCl2·6H2O: an efficient catalyst precursor for phosphine-free Heck and Sonogashira cross-coupling reactions

“…However, palladium is an expensive metal; description of commercial processes based on the Pd is less attractive for industrial scales. Therefore, development of palladium-free catalysts for the Heck reaction is attractive due to both economic and environmental concerns [13][14][15]. Also, most of the phosphorus ligands used in these reactions have drawbacks of difficulty of synthesis and poor thermal and air stability which are the main reasons for the growing interest in phosphine-free catalytic systems in coupling reactions.…”