Hydrogen‐Atom Transfer in Reactions of Organic Radicals with [Co^II(por)]^. (por=Porphyrinato) and in Subsequent Addition of [Co(H)(por)] to Olefins

Abstract

The mechanisms for hydrogen-atom transfer from the cyanoisopropyl radical (*)C(CH(3))(2)CN to [Co(II)(por)](*) (yielding [Co(III)(H)(por)] and CH(2)=C(CH(3))(CN); por = porphyrinato) and the insertion of vinyl acetate (CH(2)=CHOAc) into the Co-H bond of [Co(H)(por)] (giving [Co(III){CH(OAc)CH(3)}(por)]) were investigated by DFT calculations. The results are compared with experimental data. These reactions are relevant to catalytic chain transfer (CCT) in radical polymerization of olefins mediated by [Co(II)(por)](*), the formation and homolysis of organo-cobalt complexes that mediate living radical polymerization of vinyl acetate, and cobalt-mediated hydrogenation of olefins. Hydrogen transfer from (*)C(CH(3))(2)CN to [Co(II)(por)](*) proceeds via a single transition state that has structural features resembling the products [Co(H)(por)] and CH(2)=C(CH(3))CN. The separated radicals approach to form a close-contact radical pair and then pass through the transition state for hydrogen-atom transfer to form [Co(III)(H)(por)] and CH(2)=C(CH(3))CN. This process provides a very low overall barrier for the hydrogen-atom transfer reaction (DeltaG(double dagger) = +3.8 kcal mol(-1)). The reverse reaction corresponding to the addition of [Co(H)(por)] to CH(2)=C(CH(3))CN has a low barrier (DeltaG(double dagger) = +8.9 kcal mol(-1)) as well. Insertion of vinyl acetate into the Co-H bond of [Co(III)(H)(por)] also proceeds over a low barrier (DeltaG(double dagger) = +11.4 kcal mol(-1)) hydrogen-transfer step from [Co(III)(H)(por)] to a carbon atom of the alkene to produce a close-contact radical pair. Dissociation of the radical pair, reorientation, and radical-radical coupling to form an organo-cobalt complex are the culminating steps in the net insertion of an olefin into the Co-H bond. The computed energies obtained for the hydrogen-atom transfer reactions from (*)C(CH(3))(2)CN to [Co(II)(por)](*) and from [Co(H)(por)] to olefins, as well as the organo-cobalt bond homolysis energies correspond well with the experimental observations. The mechanism of alkene insertion into the Co-H bond of [Co(III)(H)(por)] is of general interest, because the species does not contain any cis-vacant sites to the hydride and the usual migratory insertion pathway is not available. The low barrier predicted here for the multistep insertion process suggests that (depending on the bond strengths) even for systems that do have a cis-vacant site, the radical-type insertion might compete with classical migratory insertion.

Co(porphyrin)-catalysed amination of 1,2-dihydronaphthalene derivatives by aryl azides

“…The obtained benzylic radical could be responsible for a hydrogen transfer reaction to cobalt(II) porphyrin yielding cobalt(III) hydride (J) and a naphthalene molecule. Then, the transfer of a hydrogen atom from complex J to the substrate forms another benzylic radical which, by reacting with complex I, affords the benzylic amine D. This mechanistic proposal is in accord with that already described [42] for the transfer of hydrogen atoms in the reactions of organic radicals with Co(II) porphyrin complexes.…”

Simple, Chemoselective, Catalytic Olefin Isomerization

“…In a similar way, AIBN functioned as a viable source of H-atom at elevated temperature (entry 12), in support of a radical mechanism for reversible HAT and analogous to the mechanism associated with Co-porphyrin/AIBN living radical polymerization. 30 , 31 Exclusion of either AIBN or the Co catalyst resulted in no isomerized product (see Supporting Information ).…”

“… 40 HAT is further supported by the absence of a proximal, empty valence for hydrometalation or cyclometalation on the putative Co-salen hydride complex. 31 The isomerization appears to be under kinetic, not thermodynamic control, since isopentenyl prenyl malonate 26 cycloisomerizes, whereas diprenyl malonate 27 does not, even though both species would give rise to the same radical intermediate subject to the same unimolecular cyclization rate. A tentative catalytic cycle based on these observations is shown in Figure 4 .…”

Redox-active ligands in catalysis

“…19,31 As in the case of 1, the relevant carbene intermediate, 5, is considered to have radical character and is able to react particularly readily with electron-poor olefins to give cyclopropanes. Cyclopropane ring formation is often a kinetically fast process 32 but especially so here, where ring closure is likely aided by the presence of a relatively weak Co-C bond.…”