The four E/Z isomers of camphorquinone dioxime (cqdH2) were isolated as far back as 1903. Attempts to use these chiral ligands as components in catalysts promoting asymmetric organic transformations focused on the a-isomer (2Z, 3E) and d-isomer (2E, 3Z), which both coordinate to metals via N, O bonding. While complexes of the “normal” b-isomer (2E, 3E, exhibiting N, N bonding) have been reported, none have been well characterized and no “cobaloxime” complexes of these ligands have been reported. Cobaloxime complexes of camphorquinone dioxime can exist as three isomers. Preparation of Co(cqdH)2(4-Mepy)Br results in all three isomers, but the primary isomer has the cqdH gem-dimethyl groups anti in the complex. This stereochemistry is confirmed both by NMR spectroscopy and x-ray crystallography. Careful chromatography allows isolation of the other two isomers, whose structures may be tentatively assigned by NMR. Organometallic derivatives may be prepared by treatment with excess PhMgBr or, by sequential treatment of the BF2 bridged cobaloxime with NaBH4 and CH3I. Interestingly, the Co(cqdH)2(PPh3)Br complex gives rise to virtually a single isomer, possessing, in this case, cqdH gem-dimethyl groups cis about the cobalt. Structural, spectroscopic and electrochemical properties of the cqdH complexes will be compared to other cobaloximes.

Cobaloxime complexes of camphorquinone dioxime

ACS Spring 2022

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