Reactions of Organic Free Radicals with Metal Complexes

An analogcAis ambiguity holds for nucleophilic reactions. We have already seen one facet of (his problem in the oxidative addition of alkyl halides to metak 

Hayes and Cooper have de.scribed abstraction reactions from a metal alkyl by an electrophilic reagent that goes by an SET route. Instead of the normal p abstraction of hydride from an ethyl group, which occurs in the usual electrtqihilic abstraction, he finds a preference for a abstraction from a methyl group. Since H atom abstraction usually takes place at the weakest C—H bond, the metal substituent presumably weakens the a- more than the jS-C—H bonds of the alkyl. 

7 REACTIONS OF ORGANIC FREE RADICALS WITH METAL COMPLEXES 

The reactions of organic free radicals with metal complexes is much less well understood than the attack of electrophiles and nucleophiles. If the starting material is an 18e complex, the product will be a 17e or 19e species and therefore 

We saw an example of this process as part of larger mechanistic schemes in the radical-based oxidative additions of Section 6.3. We also saw typical radical rearrangements used to detect the presence of radical intermediates (e.g., Eq. 6.20). 

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The reactions of organic free radicals with metal complexes is much less well understood than the attack of electrophiles and nucleophiles. If the starting material is an 18e complex, the product will be a 17e or 19e species and therefore reactive, so the nature of the initial reaction product may have to be inferred from the final products. Addition to the metal is well recognized and is easiest to detect when the starting complex is 17e, so that the product becomes 18e. For example, organic radicals are known to react very rapidly with [Co (dmg)2py] as follows ... 

Electron paramagnetic resonance (EPR) spectroscopy [1-3] is the most selective, best resolved, and a highly sensitive spectroscopy for the characterization of species that contain unpaired electrons. After the first experiments by Zavoisky in 1944 [4] mainly continuous-wave (CW) techniques in the X-band frequency range (9-10 GHz) were developed and applied to organic free radicals, transition metal complexes, and rare earth ions. Many of these applications were related to reaction mechanisms and catalysis, as species with unpaired electrons are inherently unstable and thus reactive. This period culminated in the 1970s, when CW EPR had become a routine technique in these fields. The best resolution for the hyperfine couplings between the unaired electron and nuclei in the vicinity was obtained with CW electron nuclear double resonance (ENDOR) techniques [5]. 

We are here concerned with various organometallic reactions for which there is evidence that organic free radicals are implicated in the reaction pathway. Many of these arc formally two-electron oxidative additions or their retrogressions, the reductive eliminations (Section V,A). We shall focus attention on systems in which transition metal Group VIII complexes are involved. 

Controlled scission of the Cr-C bond in the presence of a substrate was used to determine the kinetics of reduction of a number of transition metal complexes with organic free radicals. The reaction scheme for Co(NH3)5py [65] is shown in Eqs. 53-55. 

This is one of several reactions of this type in which an organic negative radical-ion and its parent molecule react in the presence of an alkali metal. It is found, rather interestingly, that the rate coefficients depend on the nature of the metal. To account for this, it has been postulated that the metal is involved in a bridging role in the activated complex, e.g., dipy.. K" ". . dipy for the case of 2,2 -dipy-ridyl (dipy) A more extreme case of this association between the radical-ion and the ion of the alkali metal used to form it occurs in the reaction of benzophenone with its negative ion. The spectrum of (benzophenone)" in dme has many hyper-fine lines caused by the interaction of the free electron with the and, when the metal is sodium, the Na nuclei. When benzophenone is added, the structure, due to the proton interaction, disappears and only the lines associated with the sodium interaction remain. To account for this, it has been suggested that the odd electron moves rapidly over all the proton positions too fast for the lines characteristic of the electron in the different proton environments to be seen), but relatively slowly from one sodium nucleus to another. Seen another way, this means that the transfer of an electron from molecule to molecule is associated with the transfer of the cation . ... 

The primary product of the oxidation of organic compounds is hydroperoxide, which is known as an effective electron acceptor. Hydroperoxides are decomposed catalytically by transition metal salts and complexes with the generation of free radicals via the following cycle of reactions [1-6] ... 

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