Palladium(II)-catalyzed organic transformations employing oxygen gas as the stoichiometric reoxidant
The organic transformations promoted by palladium(II) species have historically required either stoichiometric amounts of metal or the stoichiometric use of a chemical reoxidant, such as benzoquinone or Cu(II) salts. In recent years, several reports have described the use of oxygen gas as the stoichiometric reoxidant in Pd(II) chemistry. This advance has made the chemistry of Pd(II) more attractive because of reductions in cost and waste generation. This development has also encouraged researchers to probe the scope and limitation of Pd(II) chemistry using oxygen gas as the sole reoxidant of the metal. Three such organic transformations have been investigated and are herein described;An efficient system for the palladium-catalyzed oxidation of primary and secondary allylic and benzylic alcohols has been developed using O2 as the sole reoxidant of the palladium. The conditions have been applied to a wide variety of substrates producing the desired carbonyl compounds in moderate to excellent yields. The reaction has been shown to be effective on a 100 mmol scale with no change in reactivity or yield;Reaction conditions for the palladium(II)-catalyzed dehydrogenation of [beta]-dicarbonyl compounds have been pursued. The optimized conditions showed good results with benzyl substituted [beta]-dicarbonyl compounds, but simple aliphatic substituents proved relatively unreactive. By varying the reaction conditions low yields of unsaturated products from the aliphatic substituted substrates could be achieved;Several derivatives of olefinic amines have been subjected to palladium(II)-catalyzed cyclization conditions in a model system based on 2-(2-cyclopentenyl)ethanamine to form 5,5-bicyclic amine derivatives. The reactivity order for the derivatives was found to be tosylamide, formamide > urea, benzylcarbamate > acetamide > benzamide > trifluoroacetamide. The same derivatives were also examined for their efficiency in forming 6,5- and 5,6-bicyclic and 5-cyclic ring systems. The reactivity order was not observed to change with system, but only the tosylamide, formamide, benzylcarbamate and benzylurea were found to be synthetically useful for every system examined. Substrates that were intended to contain the carbonyl function of the derivative within the newly forming ring were found to be unreactive except in one case involving a biscyclization of a benzylurea derivative to form a tricyclic product.