RADICAL CAGE EFFECTS IN ADOCOBINAMIDE (AXIAL-BASE-OFF COENZYME-B12) - A SIMPLE METHOD FOR TRAPPING [ADO..COII] RADICAL PAIRS, A NEW BETA-H ELIMINATION PRODUCT FROM THE RADICAL PAIR AND MEASUREMENT OF AN UNPRECEDENTEDLY LARGE CAGE-RECOMBINATION EFFICIENCY FACTOR, F(C) GREATER-THAN-OR-EQUAL-TO 0.94

A simple method is reported for trapping organometallic caged radical pairs in adocobinamide (axial-base-off coenzyme B-12) Co-C bond thermolysis using high, ca. 1 M, concentrations of the stable nitroxide free radical TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy). The product studies as a function of [TEMPO] resolve a literature controversy by showing that beta-H elimination is not concerted, but rather proceeds through an [Ado...Co(II)] caged pair. The method permits the first measurement of the thermal fractional cage-efficiency factor (F(c)) for adocobinamide, 0.94 less-than-or-equal-to F(c) less-than-or-equal-to 1.0. This result, one of the few precise F(c) values for metal-carbon homolysis in an organometallic complex and a rare F(c) value for a bioorganometallic cofactor, is unprecedentedly large in comparison to all literature bond homolysis F(c) values in solvents of comparable viscosity. It thereby serves to confirm the literature's prediction of sizable (greater-than-or-equal-to 0.5) F(c) values in large, massive organometallic radicals.