High-valent organometallic copper and palladium in catalysis

Abstract

Copper and palladium catalysts are critically important in numerous commercial chemical processes. Improvements in the activity, selectivity and scope of these catalysts could drastically reduce the environmental impact, and increase the sustainability, of chemical reactions. One rapidly developing strategy for achieving these goals is to use 'high-valent' organometallic copper and palladium intermediates in catalysis. Here we describe recent advances involving both the fundamental chemistry and the applications of these high-valent metal complexes in numerous synthetically useful catalytic transformations.

Figure 1

High-valent copper complexes (a) Early examples of isolable organometallic Cu^III complexes. (b) Cu^III intermediates of organocuprate reactions detected at −100 °C using rapid injection-NMR. [Et = ethyl, TMS = trimethylsilyl]

Figure 2

RI-NMR studies of the effect of Lewis bases on the reactivity of Cu^III complexes 8 and 9. (i) Pyridine-containing intermediate 8 is short lived and undergoes ligand exchange to form (CH₃CH₂)(CH₃)₃Cu^IIILi as well as C–C bond-forming reductive elimination. (ii) In contrast, dimethylaminopyridine-containing intermediate 9 is stable at −100 °C under analogous conditions.

Figure 3

High-valent copper complexes involved in carbon–heteroatom bond formation (a) Stoichiometric carbon–nitrogen bond-formation from an isolated organo-Cu^III [NHR₂ = pyridone, oxazolidinone, acetanilide]. (b) In situ observation of an organo-Cu^III intermediate in the coupling of aryl bromide 12 with pyridone. (c) In situ observation of an organo-Cu^III intermediate in the oxygenation of C–H bonds.

Figure 4

Early examples of Pd^III and Pd^IV organometallic complexes. [Ph = phenyl]

Figure 5

High-valent Pd complexes involved in carbon–halogen bond formation (a) Carbon–halogen bond-forming reductive elimination is thermodynamically unfavorable from most Pd^II species but not from high-valent Pd complexes like 19–22. (b) Select examples of Pd-catalyzed C–H halogenation reactions [L = L-type ligand, R = aryl, X= halogen, o-Ns = ortho-Nosyl, Tf = triflate, t-Bu = tert-butyl.]

Figure 6

Pd-catalyzed chlorination (a) Complex 23 is an efficient catalyst for the Pd-catalyzed C–H chlorination of benzo[h]quinoline. (b) The rate determining step (r.d.s.) of this Pd-catalyzed reaction is oxidation of 23 by N-chlorosuccinimide to form the Pd^III/III dimer 22. (c) High-valent Pd-catalyzed 1,2-arylchlorination (ii) is complementary to low-valent Pd-catalyzed reactions (i) of α-olefins.

Figure 7

Pd-catalyzed trifluoromethylation (a) Carbon–CF₃ bond-formation from Pd^II requires specialized phosphine ligands. (b) Using a high-valent Pd strategy, catalytic C–H trifluoromethylation has been developed via putative Pd^IV intermediate 28. [TES = triethylsilyl, Cy= cyclohexyl, Ac = acetyl, i-Pr = iso-propyl]

Figure 8

Oxidative bond-forming reactions catalyzed by Cu and Pd that exemplify similarities and differences between these two metals.