Side Chain Entropy and the Activation of Organocobalamins for Carbon-Cobalt Bond Homolysis: Synthesis, Characterization, and Thermolysis of the Neopentyl Derivative of a Unique Cobalamin Analog Lacking a c Side Chain

Inorganic Chemistry
Kenneth L. BrownEdward J. Valente

Abstract

Hydrodeamination of the c-amino derivative, 5, of cyanocobalamin (CNCbl) with hydroxylamine-O-sulfonic acid in aqueous base leads to an extensively rearranged product instead of the c side chain truncated derivative, 1, expected from simple deamination. The rearranged product (CNCbl-8-butanamide) crystallizes in the orthorhombic system, space group P2(1)2(1)2(1) with unit cell dimensions a = 16.041(11), b = 21.94(2), and c = 25.43(2) Å. It is devoid of substituents at corrin ring C(7) but quarternized at C(8) with an "upwardly" pseudoaxial methyl group and a d side chain expanded by one methylene group to a butanamide. The corrin ring of this rearranged derivative is significantly flatter (corrin ring fold angle 9.9 degrees ) than CNCbl itself (fold angle 18.0 degrees ). Conversion of CNCbl-8-butanamide to its neopentyl derivative (NpCbl-8-butanamide), a NpCbl analog which lacks a c acetamide side chain, permits a quantitative assessment of the influence of thermal motions of the c side chain on the entropy of activation for carbon-cobalt bond thermal homolysis in NpCbl. NpCbl-8-butanamide is shown to thermolyze homolytically to give products derived from the Np(*) radical quantitatively. The kinetics of the thermolysis of NpCb...Continue Reading

Related Concepts

hydroxylamine-O-sulfonic acid
Vitamin B 12
Cobalamins
Methyl Group
Carbene
Aqueous solution
Corrin
Acetamide

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