Palladium Complexes of N‐Heterocyclic Carbenes as Catalysts for Cross‐Coupling Reactions—A Synthetic Chemist's Perspective

Abstract

Palladium-catalyzed C-C and C-N bond-forming reactions are among the most versatile and powerful synthetic methods. For the last 15 years, N-heterocyclic carbenes (NHCs) have enjoyed increasing popularity as ligands in Pd-mediated cross-coupling and related transformations because of their superior performance compared to the more traditional tertiary phosphanes. The strong sigma-electron-donating ability of NHCs renders oxidative insertion even in challenging substrates facile, while their steric bulk and particular topology is responsible for fast reductive elimination. The strong Pd-NHC bonds contribute to the high stability of the active species, even at low ligand/Pd ratios and high temperatures. With a number of commercially available, stable, user-friendly, and powerful NHC-Pd precatalysts, the goal of a universal cross-coupling catalyst is within reach. This Review discusses the basics of Pd-NHC chemistry to understand the peculiarities of these catalysts and then gives a critical discussion on their application in C-C and C-N cross-coupling as well as carbopalladation reactions.

Biaryl Phosphane Ligands in Palladium‐Catalyzed Amination

“…[70][71][72] Later the scope was improved by the application of chelating ligands, especially BINAP [73] and dppf. [74] Subsequent research using these and other ligand classes, notably monodentate phosphines including trialkylphosphines, [75][76][77] dialkylarylphosphines, [78] QPhos, [79] chelating phosphines including Josiphos-type ligands [80] and Xantphos [81] and other ligand classes such as Verkade's phosphatranes, [82] secondary phosphine oxides [83] and N-heterocyclic carbenes [84] have served to widen the substrate scope and resulted in milder reaction conditions. Synthesis of the dialkylbiaryl phosphine ligands is straightforward and completed in a single synthetic operation.…”

Platinum(ii) N-heterocyclic carbene complexes: coordination and cyclometallation.

“…[11][12][13][14] More recently, NHC ligands have been shown not to be innocent spectators, but reactive intermediates themselves, with a number of new products being formed. 15,16 Whilst most the studies on catalytically active systems have their emphasis on palladium based chemistry, 17 platinum complexes are also of interest. Amongst other uses, platinum complexes with NHC ligands have been used for the reductive cyclization of diynes and enynes 18 , the catalytic diboration of unsaturated molecules 19 and the tandem hydroborationcross coupling reaction.…”

Theoretical studies of iridium-mediated tautomerization of substituted pyridines

“…Carbenes are representative of such ligands [4e8] and have shown advantageous activity in reactions such as palladium-catalyzed CeC coupling [9,10] and ruthenium-catalyzed olefin metathesis reactions [11,12]. Syntheses of carbene complexes have been widely reported and extensively reviewed [13e18].…”