Date of Award
Doctor of Philosophy (PhD)
The structure of typical palladium complexes used as catalysts in organic synthesis is reviewed; particular attention is paid to the systematic study of palladium catalyzed vinylic substitution on a variety of aryl iodides and bromides (Heck-type vinylation). The general conditions of this process, i.e., a homogeneous catalyst operating at moderate temperature, which requires a proton acceptor to complete the catalyst cycle, are conducive to successful development of a polymer-bound catalyst. Using 4-picoline as a ligand under homogeneous Heck-type vinylation conditions. Morpholine is an adequate base to stabilize the catalytic picoline complex when aryl iodides are condensed with styrene. However, sodium bicarbonate in the presence of morpholine is required to effect the coupling reactions when aryl bromides are employed as the substrate. The following amines have been evaluated as ligands: 2-picoline, 4-picoline, 2-aminopyridine, benzyl 4-picolinate, 2-methoxypyridine, 3-chloropyridine and 2,2'-bipyridyl. Complexes derived from benzyl 4-picolinate exhibit higher catalytic activity than those derived from 4-picoline. Changing the ligand-to-palladium molar ratio from 1:1 to 3:1 for the bidentate ligands, bipyridyl and benzyl 2-(2-pyridyl) cinchonate, reduces catalytic activity two-fold. Transformation of the above mentioned catalyst systems into polymer-bound catalysts entailed synthesis and polymerization of the following monomers: 4'(3')-vinylbenzyl-4-oxypyridine, 4'(3')-vinylbenzyl-4-pyridinecarboxylate, and 4'(3')-vinylbenzyl-2-(2-pyridyl)cinchoninate. 4-vinylpyridine and each of the vinylbenzyl monomers was copolymerized with divinylbenzene to produce cross-linked insoluble resins with pendant ligand contents ranging from 1.57 to 8.27 meq/g. Treatment of the polymeric amine resins with sodium tetrachloropalladate produced polymer-bound palladium complexes capable of promoting the condensation of aryl halides with styrene. The initial rates of Heck type vinylation on the first cycle are nearly identical to those observed in the presence of a soluble catalyst complex. Thus, reactions in the presence of polymer-bound, heterogeneous catalysts are not subject to diffusional limitations. A significant decline in catalytic activity is observed when the catalyst is recycled a second or third time. Loss of catalytic activity can be traced to palladium leaching by an essential co-solvent, DMF or HMPA, and to occlusion of palladium in the precipitated olefin products.
Sun, Chia-hsing, "Palladium-Catalyzed Vinylic Substitution of Aryl Halides on Polymeric Nitrogen Supports." (1986). LSU Historical Dissertations and Theses. 4268.