Ruthenium-Catalyzed Direct Arylations with Aryl Pseudo Halides

Ruthenium-Catalyzed Direct Arylations with Aryl (Pseudo) Halides 

A phosphine ligand-free ruthenium-catalyzed direct arylation with aryl bromides as electrophiles was recently disclosed. Notably, the use of inexpensive RuCb (H20)n as catalyst allowed for economically attractive C—H bond functionahzations of 

The use of aryl tosylates as electrophiles is attractive, because they can be prepared from readily available phenols with less-expensive reagents than those required for synthesis of the corresponding triflates. Importantly, tosylates are more stable towards hydrolysis than triflates, yet significantly less reactive as electrophiles. As a result, protocols for traditional cross-coupHng reactions were only recently developed (see Chapter 2). In contrast, catalytic direct arylations with aryl tosylates were not reported until recently. Interestingly, a rathenium complex derived from heteroatom-substituted secondary phosphine oxide (HASPO) preligand 78 [40] allowed for direct arylations with both electron-deficient, as well as electron-rich aryl tosylates [41]. As pronucleophiles, pyridine, oxazoline and pyrazole derivatives could be efficiently functionalized. Selective mono- or diarylation reactions could be accomplished through the judicious choice of the 

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Unfortunately, experimental studies on the working mode of ruthenium-catalyzed direct arylations with organic (pseudo)halides are scarce. However, a beneficial effect of NaOAc on stoichiometric syntheses of ruthenacycles at ambient temperature was reported (see above) [49, 50], and suggested a cooperative depro-tonation/metalation mechanism [7, 30, 86] for the C-H bond activation step. Furthermore, recent computational DFT-calculations provided support for such a mechanistic rationale [87], Moreover, a transition state 83 was independently proposed to account for the high efficacy observed with (HA)SPO preligands in ruthenium-catalyzed direct arylation reactions (Scheme 31). 

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