Transition metal-alkyl homolytic bond

Olefin Formation Accompanying Homolytic Transition Metal-Alkyl Bond Dissociation... 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

Whereas homolytic ligand dissociation is not commonly observed for inorganic complexes, it has been identified as an important process in organometallic chemistry where it is favored by the characteristic weakness of transition metal-alkyl tr-bonds. Recent determinations yield metal alkyl band dissociation energies for CH3— Mn(CO)s (ca. 120 kJ/mol) and for several alkylcobalt complexes (ca. 80-100 kJ/mol) . Homolytic dissociation of such complexes results in the formation of free radicals and in the opening up of free radical catalytic pathways, e.g., for hydrogenation". Important biochemical examples of free radical catalytic mechanisms, initiated by the homolytic dissociation of a transition metal-carbon bond (i.e., the 5 -deoxyadenosy 1-cobalt bond of coenzyme 8,2) are provided by the coenzyme B,2-promoted rearrangements (see Section... 

Besides the /3-H elimination mechanism, chain transfers to aluminum and to the monomer can occur. Also, /3-alkyl abstraction has to be considered. The homolytic cleavage of the transition metal—carbon a-bond should only play a minor role (decomposition of the catalyst). 

It is only during the past ten years that reliable and widely applicable methods for determining homolytic metal-alkyl bond dissociation energies of stable or-ganometallic compounds in solution have been developed and that information about such bond dissociation energies has become available. Today about one hundred transition metal-alkyl bond dissociation energies have been determined, the majority for cobalt-alkyl complexes. Most of these have been from kinetic measurements. The scope, limitations and results of such determinations are discussed. 

This paper is concerned with certain aspects of the thermodynamics and kinetics of transition metal-alkyl homolytic bond dissociation processes, notably of stable, ligated complexes in solution (L M-R, where L is a ligand and R = alkyl, benzyl, etc.)(l ). The metal-alkyl bond dissociation energy of such a complex (BDE, strictly bond dissociation enthalpy) is defined as the enthalpy of the process represented by Equation 1. 

HALPERN Transition Metal—Alkyl Homolytic Bond Dissociation Processes 111... 

Two important modes of decomposition of transition-metal alkyls are a- and j5-elimination, while intramolecular reductive elimination, oxidative addition (e.g. y-elimination), and homolytic M—C bond fission are other pathways available. The formation of alkylidene complexes by a-elimination has been reviewed. The mechanism of a-elimination has received some attention recently but is not well worked out. The thermal decomposition of pentabenzyltantalum has the following stoicheiometry, although the nature of the Ta product is obscure ... 

Homolytic cleavage of the C—M bond is the most common process for thermal or photolytic decomposition of alkyl and aryl derivatives of the transition metals. In terms of a molecular orbital scheme, the process involves transfer of an electron from a bonding orbital to a C—M antibonding orbital. Electronic factors which broaden the energy gap between these orbitals should stabilize the compound toward homolytic decomposition. 

The penultimate unit effect may play a very important role in ATRR The rate constants of activation of monomeric and dimeric alkyl bromides with a CuBr-bpy (bpy=2,2 -bipyridine) complex as activator were determined. The ATRP relies on the reversible activation of a dormant alkyl halide through halogen abstraction by a transition metal complex to form a radical that participates in the classical free-radical polymerization figure (Fig. 2) prior to deactivation. In this equiUbrium, the alkyl radical (Pm ) is formed in an activated process, with a rate constant kact> by the homolytic cleavage of an alkyl halogen bond (Pm-Z) catalyzed by a transition metal complex in its lower oxidation state (Cu ). The relative values of fcact of the alkyl bromides were determined for CuBr/bpy catalyst systems in acetonitrile at 35°C. These systems followed the order EBriB (30) MBrP (3)>iBBrP (1) for monomeric initia-tors and MMA-MMA-Br (100) MA-MMA-Br (20) > MMA-MA-Br (5) > MA-MA-Br (1) for dimeric initiators. ... 

For a number of metal-alkyl complexes (M—R) the thermal stability decreases in the order R = Me > Et > Pr. In particular, the methyl complexes are often markedly more stable than the higher homologues. The lower stability of the ethyl and propyl complexes may be due in part to inductive effects and in part to steric weakening of the M-C bond. Another explanation, which is more plausible in some cases, is that the interaction discussed above gives rise to a transition state of lower energy than required for unaided homolytic dissociation, for example ... 

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