INTRAMOLECULARLY ALKYLATED COSALEN COMPLEXES - THERMOLYSIS AND PHOTOLYSIS OF COENZYME B-12 MODELS WITH A COBALT-TO-LIGAND CARBON BRIDGE

Product and kinetic studies on the anaerobic thermolysis and photolysi s of the cobalt-carbon bond in intramolecularly bridged organoCosalen compounds are presented. NMR, UV-Vis and electron spin resonance spect roscopic investigations reveal the formation of paramagnetic Co(II) co mplexes upon thermolysis or photolysis in solution. Solid state thermo lysis (studied by differential thermal analysis, thermogravimetry, mas s spectrometry and magnetochemistry) also leads to cobalt (II) product s. The fate of the carbon radical produced by homolytic cleavage of th e Co-C bond in solution depends on the reaction conditions used. In th e presence of a radical trap (tempo, pbn), carbon radical spin trappin g is the major pathway. On the other hand, a Co(II)salen complex carry ing an omega-alkenyl side chain was shown to be the major product when Cosalen(CH2)(4) (2) was subjected to prolonged photolysis in the abse nce of a radical trap. While the chemistries of intramolecularly bridg ed organoCosalen and open chain alkylCosalen complexes are qualitative ly quite similar, kinetic studies using the radical trap method reveal large quantitative differences. The bridged complex Cosalen(CH2)(3) ( 1) is quite unreactive towards thermolysis and photolysis, even at hig h trap concentration. Quantum yields (Phi) of Cosalen(CH2)(4) (2) and butylCosalen (4) were determined in toluene as a function of the conce ntration of tempo. For 2, Phi(max) is reached at 0.5 M of tempo, while for 4 Phi is already maximal at a very small excess of tempo (approxi mate to 0.0005 M); in both cases the complex concentration is ca. 10(- 4) M. These results are discussed in terms of the solvent-caged radica l pair concept. (C) 1998 Elsevier Science S.A.