The Effect of Aromatic Tertiary Amines on the Copper(II) Catalyzed Decarboxylation of 3-Oxo-Glutaric Acid

Abstract

The kinetics of the copper(I1) catalyzed decarboxylation of 3-0x0-glutaric acid have been investigated in unbuffered solutions at 25 "C and 0.60 M ionic strength, both in the presence of aromatic tertiary amines, and with hydrated copper ions alone. The amines used were 2,2'-bipyridyl and 1,lO-phenanthroline. Equilibrium constants for the formation of the various copper complexes were determined with a copper ion selective electrode, particularly for the species CuAB, where A is the 3-0x0-glutarate ion, and B is an aromatic amine. The kinetic behavior was then interpreted as arising from the simultaneous decarboxylation of all the species present containing a 3-0x0-glutarate ligand. The resulting rate constants indicated that some kinetic effect was transmitted through the copper from the base to the 3-0x0-glutarate, but the effect was relatively small, and the bulk of the catalytic effect caused by the base arose from changes in equilibrium concentrations.Les cinttiques de la decarboxylation de I'acide 0x0-3 glutarique, catalyst par le cuivre(II), ont ete etudites dans des solutions non tamponees a 25 "C et a une force ionique de 0.60 M; ceci a la fois en presence d'amines aromatiques tertiaires et avec des ions hydrates du cuivre uniquement. Les amines utilisees sont la bipyridyle-2,2' et la phtnanthroline-],lo. Les constantes d'equilibre pour la formation des divers complexes du cuivre ont Cte determintes au moyen d'une electrode stlectives en cuivre, specialement dans le cas d'especes CuAB, ou A est l'ion 0x0-3 glutarate et B une amine aromatique. Le comportement cinetique a alors ete interpret6 en fonction d'une decarboxylation simultanee de toutes les especes presentes contenant le ligand 0x0-3 glutarate. Les constantes de vitesse qui en resultent, indiquent qu'un certain effet cinttique est transmis de la base vers 1'0x0-3 glutarate par l'intermediaire du cuivre; cet effet demeure cependant petit et I'importance de I'effet catalytique provoqut par la base provient de modifications dans les concentrations a I'tquilibre. Canadian Journal of Chemistry, 50, 1461 (1972) IntroductionThe decarboxylation of a-keto dicarboxylic acids is catalyzed by transition metal ions (l), and the reaction has been shown to occur through the spontaneous decomposition of the 1 : 1 complex of formula MA, where M2+ is a divalent metal ion, and A2-is the dianion of the keto acid. The catalytic effect of the metal ion may be appreciably enhanced when the 1 : 1 complexes of the metal ion are added instead of simply the hydrated metal ion (2-7). Presumably the decarboxylation now proceeds through a new reactive species, and it is shown in the present work that reasonably consistent rate constants are obtained, if it is assumed that this species is the 1 : 1 : 1 tertiary complex MAB, where B is the amine. Since aliphatic amines and phosphines are relatively ineffective in promoting decomposition, it is presumed that the delocalized n-systems of the aromatic amine are involved. The overall catalytic rate constant, kc,,, is defined by