Atom Abstraction and Combination of the Resulting Radical with a Second Metal

Atom Abstraction and Combination of the Resulting Radical with a Second Metal 

Experimental evidence for this pathway is diverse. For example, the reactions occur with a 2 1 stoichiometry, but the rate law is second order (proportional to [M ] and [RX]). Products are generated with racemization at carbon, and the relative reactivity of alkyl halides follows the trend tertiary secondary primary Me. These relative reactivities parallel the stabilities of R instead of the reactivity as an electrophile in a nucleophilic substitution process. Furthermore the alkyl radical intermediate in this mechanism has been trapped, has undergone rearrangements, and has been detected directly by EPR. 

This reaction often occurs with Co(II) or Rh(II) systems, which are excellent one-electron reductants and poor two-electron reductants. For example, the 17-electron, d Co(II) complex KCo(CN)j reacts with alkyl halides by this path (Equation 7.9). These reactions display second-order kinetics, with the rate decreasing in the order RI RBr RCl. Only activated halides such as X-CHjCOjMe react. Alkyl tosylates do not.  

The rates of the atom absti action are influenced by the steric properties of the ligands. The rate of reactions of Co(dmgH)2L complexes are significantly retarded by increased steric bulk of the coordinated phosphine. Likewise, atom abstraction by 17-electron manganese-carbonyl radicals are slower for more hindered complexes and follow the 

The same process occurs with actinides and lanthanides for which only one-electron oxidation is possible. An example of such an oxidative addition is shown in Equation 7.10. Dissociation of an ether precedes the coordination of KX and halide abstraction. 

---