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Donald L. Jameson,' Joseph J. Grzybowski,* Deb E. Hammels, Ronald K. Castellano, Molly E. Hoke, Kimberly Freed, Sean Basquill, Angela Mendel, and William J. Shoemaker
Cobaloximes have been prepared and studied extensively as model compounds for the coenzyme vitamin B12 (1, 2). These compounds, which exist at the interface of classical coordination chemistry, organometallic chemistry, and bioinorganic chemistry, offer the opportunity to expose students to these diverse areas. As part of an advanced laboratory course in organic and inorganic synthesis, we have developed a sequence of experiments (Scheme I) that demonstrate the scope of reactivity of this class of compounds.
The metal site of vitamin BIz consists of a cobalt atom coordinated by a tetrapyrrole macrocyclic ligand called a corrin (see structures below). One of the axial sites is occupied by a 5,6-dimethylbenzimidazole nucleotide. The key feature of the vitamins biochemical activity is its ability to form metal-alkyl (Co-C) bonds in the second axial site. The N4 macrocycle in the cobaloxime model complexes is made up of two monodeprotonated dioxime molecules linked at two points by hydrogen bonding. The result is an essentially planar macrocyclic ligand. The most common cobaloximes utilize the dimethylglyoxime ligand. The structure shows a typical cobaloxime model complex. A very large number of model complexes have been prepared in which both the neutral ligand and the alkyl ligand are varied (1, 2).
Alkyl or aryl derivatives of cobaloximes are routinely prepared by two general routes. A carbanionic reagent reacting with a Co(III) halide results in a nucleophilic displacement of the halide ion (eq 1). In the second method, Co(III) is converted to Co(I) using a variety of reducing agents (eq 2). In acidic or neutral solution, a cobalt hydride species exists, while under sufficiently basic conditions, the Co(I) species has significant anionic character and behaves as an exceedingly powerful nucleophile. Reaction of the Co(I) species with a wide variety of electrophiles generates an organocobaloxime (eq 3). In the organocobaloxime compounds, the cobalt formally exists in a +3 oxidation state, with the carbon ligand being assigned a charge of-1.
The reversal of reactivity displayed by this organometallic system has parallels in classical organic chemistry. Alkyl halides (carbon in a higher oxidation state) behave as electrophiles and readily undergo nucleophilic displacement of the...