Author: Sit, Wing-nga
Title: Palladium (II)-catalyzed ethoxycarbonylation, arylation and arylcarboxylation reactions of 2-arylpyridines, oximes and oxazoline via ortho-selective C-H bond activation
Degree: Ph.D.
Year: 2010
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.
Language: English
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.
Rights: All rights reserved
Access: open access

Files in This Item:
File Description SizeFormat 
b23517426.pdfFor All Users11.92 MBAdobe PDFView/Open

Copyright Undertaking

As a bona fide Library user, I declare that:

  1. I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
  2. I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
  3. I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.

By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.

Show full item record

Please use this identifier to cite or link to this item: