UNPRECEDENTED AND REVERSIBLE COBALT-TO-CARBON ALKYL BOND REARRANGEMENT IN THE COENZYME-B12 MODEL COMPLEX C6H5CH2COIII[C2(DO)(DOH)PN]I - SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND MECHANISTIC STUDIES

Photolysis of the coenzyme B12 model complex PhCH2Co[C2(DO)(DOH)?? pn]I (1) leads to a high-yield, efficient synthesis of an unprecedented cobalt-to-carbon alkyl rearrangement product Co[C2(DO)(DOH)(pn)CH2Ph]I (2), (SP-5-15)-[2-[[3-[[2-(hydroxyamino)-1-methyl-2-(phenylmethyl)butylidene]amino]propyl]imino]-3-pentanone oximato(2-)-N,-N',N'',N''']iodocobalt. The novel product 2 is unequivocally characterized by X-ray crystallography, H-1 NMR, visible, and mass spectroscopy, and an elemental analysis. With pure 2 available, a thermal equilibrium between 1 and 2, K(eq) = 1.5 +/- 0.1 (69-degrees-C, benzene solvent), is shown to exist, thereby explaining the low (60%) yield of 2 from the thermolysis of 1 at 69-degrees-C. Thermolysis of 2 in the presence of TEMPO free radical trap quantitatively yields trapped benzylTEMPO and .Co(II)-[C2(DO)(DOH)pn]I. An Eyring plot of this reaction yields DELTA-H double-ended dagger = 26 +/- 2 kcal/mol, DELTA-S double-ended dagger = -6 +/- 7 cal/(mol.K), or DELTA-G double-ended dagger 298 = 27 +/- 3 kcal/mol; the DELTA-H double-ended dagger value plus appropriate corrections imply a low benzyl-carbon bond dissociation energy of 25 kcal/mol in 2. Also provided are a proposed mechanism for the formation of 2, a summary discussion detailing the significance of the results toward explaining a number of related, but poorly understood, literature reports, and a short list of some interesting but unanswered questions that form a basis for future research.