Ag/Pd Cocatalyzed Direct Arylation of Fluoroarene Derivatives with Aryl Bromides

Abstract

Diverse C−H functionalizations catalyzed by Pd employ Ag(I) salts added as halide abstractors or oxidants. Recent reports have shown that Ag can also perform the crucial C−H activation step in several of these functionalizations. However, all of these processes are limited by the wasteful requirement for (super)stoichiometric Ag(I) salts. Herein, we report the development of a Ag/Pd cocatalyzed direct arylation of (fluoroarene) chromium tricarbonyl complexes with bromoarenes. The small organic salt, NMe 4 OC(CF 3 ) 3 , added as a halide abstractor, enables the use of a catalytic amount of Ag, reversing the rapid precipitation of AgBr. We have shown through H/D scrambling and kinetic studies that a (PR 3 )Ag-alkoxide is responsible for C−H activation, a departure from previous studies with Ag carboxylates. Furthermore, the construction of biaryls directly from the simple arene is achieved via a one-pot chromium tricarbonyl complexation/C−H arylation/decomplexation sequence using (pyrene)Cr(CO) 3 as a Cr(CO) 3 donor.

Spatial Anion Control on Palladium for Mild C–H Arylation of Arenes

“…The bulky monodentate (CF3)3CO − , which does not participate in the six-membered CMD process, can be displaced from palladium by H2La (see Supporting Information) and act as a mild base 50 to remove H + after the C−H activation step, as demonstrated by the Larossa group. [51][52][53][54] The potential of the proposed spatial anion control to address challenging C−H functionalization reactions with palladium was examined by performing the non-directed arylation of arenes as limiting reactants. The constrained bis(carboxylic) acid H2La and palladium precatalyst 1a enabled catalytic C−H arylation of arene 3a with diaryl iodonium reagent 2a at ambient temperature, yielding 4a in 39% yield (Figure 2b, Figure S2).…”

Circular Discovery in Small Molecule and Conjugated Polymer Synthetic Methodology

“…Another report, inspired by a Au/Ag dual catalytic system, sought to discover a Pd/Ag system with catalytic amounts of both metals for direct arylation. 75 In this case, silver’s demonstrated ability to activate C–H bonds in electron-deficient arenes was utilized jointly with a Pd-mediated oxidative addition of aryl bromides. Both of these reports apply findings originating from mechanistic studies on a polymerization, showing cooperative discovery between small molecule and polymerization transformations.…”

Introduction to Spatial Anion Control for Direct C–H Arylation

“…Considering the points listed above, new palladium precatalysts and new arylation reagents 50 that contain nonafluoro- tert -butoxide anion, which was used earlier in C–H activation reactions by the Larossa group, 93 94 were developed to enable a direct C–H arylation reaction (Scheme 7b ). 35 The structure of the monodentate (CF 3 ) 3 CO – anion precludes the possibility for κ 2 -coordination or the formation of the 6-membered CMD transition state.…”

“…any strong acids or bases, which might prevent the coordination of spatially controlled carboxylates to Pd(II); d) strong ligands, such as pyridines or phosphines, which might block substrate binding sites on palladium; and e) silver(I) additives, which are very frequently used in nondirected C-H functionalizations but could influence the reactivity because of their potential varied roles in C-H activation and functionalization processes. 47,[86][87][88][89][90][91][92] Considering the points listed above, new palladium precatalysts and new arylation reagents 50 that contain nonafluoro-tert-butoxide anion, which was used earlier in C-H activation reactions by the Larossa group, 93,94 were developed to enable a direct C-H arylation reaction (Scheme 7b). 35 The structure of the monodentate (CF 3 ) 3 COanion precludes the possibility for  2 -coordination or the formation of the 6-membered CMD transition state.…”