Autocatalytic Intermolecular versus Intramolecular Deprotonation in CH Bond Activation of Functionalized Arenes by Ruthenium(II) or Palladium(II) Complexes. Issue 23 (17th April 2013)

• Record Type: Journal Article
• Authors: Fabre, Indira; von Wolff, Niklas; Le Duc, Gaëtan; Ferrer Flegeau, Emmanuel; Bruneau, Christian; Dixneuf, Pierre H.; Jutand, Anny
• Is Part Of: Chemistry. Volume 19:Issue 23(2013)
• Page Start: 7595
• Page End: 7604
• Publication Date: 2013-04-17
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201203813
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The activation of the CH bond of 1‐phenylpyrazole (<bold>2</bold>) and 2‐phenyl‐2‐oxazoline (<bold>3</bold>) by [Ru(OAc)<sub>2</sub>(<italic>p</italic>‐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<sub>2</sub>CO<sub>3</sub>. 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<sup>II</sup> center (S<sub>E</sub>3 mechanism). The reactions of 1‐phenylpyrazole and 2‐phenyl‐2‐oxazoline first lead to the neutral cyclometalated complexes <bold>A<sub>2</sub></bold> and <bold>A<sub>3</sub></bold> ligated by one acetate. The latter dissociate to the cationic complexes <bold>B<sub>2</sub></bold><sup><bold>+</bold></sup> and <bold>B<sub>3</sub></bold><sup><bold>+</bold></sup>, respectively, and acetate. A slow incorporation of one or two D atoms into <bold>2</bold>, <bold>3</bold>, and 2‐phenylpyridine (<bold>1</bold>) was observed in the presence of deuterated acetic acid. The "reversibility" of the CH bond activation/deprotonation takes place from the cationic complexes <bold>B</bold><sub><italic><bold>n</bold></italic></sub><sup><bold>+</bold></sup> (<italic>n</italic>=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 <bold>1–3</bold> catalyzed by [Ru(OAc)<sub>2</sub>(<italic>p</italic>‐cymene)]. In contrast, the reaction of Pd(OAc)<sub>2</sub> with 2‐phenylpyridine (<bold>1</bold>), is much faster: Pd(OAc)<sub>2</sub>>[Ru(OAc)<sub>2</sub>(<italic>p</italic>‐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<sup>II</sup>^Pd<sup>II</sup> dimer <bold>D′<sub>1</sub></bold> is formed whose bielectronic electrochemical oxidation leads to a [Pd<sup>III</sup>^Pd<sup>III</sup>]<sup>2+</sup> dimer, in agreement with the result of a reported chemical oxidation used in arene functionalizations catalyzed by Pd(OAc)<sub>2</sub>.</p> </abstract>