Autocatalytic intermolecular versus intramolecular deprotonation in C-H bond activation of functionalized arenes by ruthenium(II) or palladium(II) complexes

Article abstract of DOI:10.1002/chem.201203813

The activation of the C-H bond of 1-phenylpyrazole (2) and 2-phenyl-2-oxazoline (3) by [Ru(OAc)₂(p-cymene)] is an autocatalytic process catalyzed by the co-product HOAc. The reactions are indeed faster in the presence of acetic acid and water but slower in the presence of a base K ₂CO₃. A reactivity order is established in the absence of additives: 2-phenylpyridine>2-phenyl-2-oxazoline>1-phenylpyrazole (at RT). The accelerating effect of added acetate ions reveals an intermolecular deprotonation after C-H bond activation by a cationic Ru^II center (S_E3 mechanism). The reactions of 1-phenylpyrazole and 2-phenyl-2-oxazoline first lead to the neutral cyclometalated complexes A ₂ and A₃ ligated by one acetate. The latter dissociate to the cationic complexes B₂⁺ and B₃⁺, respectively, and acetate. A slow incorporation of one or two D atoms into 2, 3, and 2-phenylpyridine (1) was observed in the presence of deuterated acetic acid. The reversibility of the C-H bond activation/deprotonation takes place from the cationic complexes B_n⁺ (n=1-3). They are also involved in oxidative additions to PhI, which are rate-determining and lead to the mono- and bis-phenylated products at high temperatures. A general mechanism is proposed for the arylation of arenes 1-3 catalyzed by [Ru(OAc) ₂(p-cymene)]. In contrast, the reaction of Pd(OAc)₂ with 2-phenylpyridine (1), is much faster: Pd(OAc)₂>[Ru(OAc) ₂(p-cymene)]. Since the kinetics is not affected by added acetates, the reaction proceeds through a CMD mechanism assisted by a ligated acetate (intramolecular process) and is irreversible. A bis-cyclometalated Pd ^II^Pd^II dimer D′₁ is formed whose bielectronic electrochemical oxidation leads to a [Pd^III^Pd ^III]²⁺ dimer, in agreement with the result of a reported chemical oxidation used in arene functionalizations catalyzed by Pd(OAc) ₂. Acetate aid: The activation of the C-H bond of 1-phenylpyrazole and 2-phenyl-2-oxazoline by [Ru(OAc)₂(p-cymene)] is an autocatalytic intermolecular process aided by free acetate (see figure). In contrast, activation by Pd(OAc)₂ proceeds through a concerted metalation-deprotonation (CMD) mechanism through an intramolecular and irreversible process that is assisted by ligated acetate. A cyclometalated dimeric Pd^II Pd^II complex is formed, the bielectronic electrochemical oxidation of which leads to a dimeric [Pd^III Pd ^III]²⁺ complex. Copyright