FORMATION OF ORGANOCOBALT PORPHYRIN COMPLEXES FROM REACTIONS OF COBALT(II) PORPHYRINS AND DIALKYLCYANOMETHYL RADICALS WITH ORGANIC SUBSTRATES - CHEMICAL TRAPPING OF A TRANSIENT COBALT PORPHYRIN HYDRIDE

Solutions of (tetrakis(p-methoxyphenyl)porphyrinato)cobalt(II) ((TAP)Co(II.)) and alkyl radicals (.C(CH3)(CH3)CN,.C(CH3)(CH(CH3)2)CN) formed from dialkylazo precursors react with alkenes and alkynes in CHCl3, DMF, and hydrocarbon solvents to form alkyl and vinyl complexes ((TAP)Co-R, (TAP)Co-C(R1)=C(R2)(R3)) and with alkyl halides and epoxides in DMF to form alkyl and beta-hydroxyalkyl complexes. These reactivity patterns are indicative of forming a transient cobalt(III) hydride ((TAP)Co-H) that adds with unsaturated organic substrates or, when deprotonated, reacts as a nucleophile ([(TAP)Co(I)]-) with alkyl halides and epoxides. Addition reactions of (TAP)Co-H with alkenes and alkynes generally occur with high regioselectivity corresponding to formation of the most stable organic radical (Markovnikov regioselectivity). Sterically encumbering substituents which inhibit (TAP)Co binding at the same carbon center can reverse the regioselectivity (anti-Markovnikov addition). Examples of organic group isomerization that correspond to 1,2(H,Co)-shifts for secondary alkyl complexes and 1,3(H,H)-shifts for vinyl derivatives are reported and discussed. High yields of regio- and stereospecific products using simple procedures suggest the synthetic utility of this versatile methodology in forming a wide range of organocobalt complexes.