|Title:||Palladium (II)-catalyzed ethoxycarbonylation, arylation and arylcarboxylation reactions of 2-arylpyridines, oximes and oxazoline via ortho-selective C-H bond activation|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Organic compounds -- Synthesis
Transition metal catalysts
|Department:||Department of Applied Biology and Chemical Technology|
|Pages:||xix, 271 p. : ill. ; 30 cm.|
|Abstract:||Development of catalytic reactions involving carbon-hydrogen bond cleavage is currently one of the most attractive research subjects in organic synthesis. This thesis describes the C-C and C-O bond formation reactions via selective functionalization of C-H bonds. Our approach involves the reaction of Pd(OAc)2 with an arene C-H bond to form an organopalladium (II) (Ar-Pd) species, which would undergo further reaction with reactive radicals leading to C-C and C-O bond formations. In this work, 2-phenylpyridine was employed as a model substrate for the Pd(II) mediated ortho-C-H bond cyclometallation to form a palladacyclic complex, which has been characterized by X-ray crystallography. It was found that the palladacyclic complex would react with diethyl azodicarboxylate (DEAD) to give 2-(2-pyridyl)benzoate in 83 % yield. Subsequently, a catalytic coupling reaction of 2-arylpyridne with DEAD was developed with Oxone® or K2S2O8 as oxidant. The presence of radical scavengers (e.g. galvinoxyl) would significantly lower the product yield. This implies that the coupling reaction was mediated by EtOC(O)● radicals generated in situ from thermal decomposition of the DEAD. Biaryl formation is an important process in organic synthesis. Based on the findings of the radical-mediated cross-coupling reaction with DEAD, we considered that aryl radicals are potential coupling agents for direct C-H arylation reactions. It is known that aryl radicals can be generated from aryl acylperoxides. We found that treatment of the palladacyclic complex with benzoyl peroxide in a MeCN / AcOH (1 : 1 v/v) mixture at 100 °C afforded the desired biaryl product in 78 % yield. Later, Pd(OAc)2-catalyzed protocols for direct arylation of substrates such as arylpyridines, oximes, and oxazoline have been developed; good product yields and functional group tolerance were achieved. Various aryl acylperoxides containing substituents including F, Cl, Br, CF3, CN, NO2 and tBu are effective reagents for the arylation reaction. During the course of our arylation studies, we observed C-O bond formation in the reaction of benzoyl peroxide with the palladacyclic complex under anhydrous and acid-free conditions. Arylcarboxyl radical was generated by the thermal decomposition of aryl acylperoxide, which would further decarboxylate to aryl radicals under acid conditions. Treatment of the palladacyclic complex with anhydrous benzoyl peroxide in xylene at 130 °C afforded the ester product in 76 % yield exclusively. Later a catalytic protocol for arylcarboxylation of the aromatic C-H bond was developed. Noteworthy, arylcarboxylic esters are key intermediates for the synthesis of natural products and pharmaceuticals. We proposed that the reactions discussed above are initiated by directing group-assisted cyclopalladation, followed by the reaction of the palladacycle with radicals. This work demonstrated their radicals are unconventional coupling partners for the organopalladium-based cross-coupling reactions leading to C-C and C-O bond formation.|
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