Attack of Main Group Electrophiles on Alkyl Complexes Possessing d-Electrons

The rate expression (Equation 12.15) for electrophilic cleavage of the metal-carbon bond in CpFe(CO)jR (Equation 12.12) is consistent with a mechanism in which the equilibrium in Equation 12.14 precedes removal of the alkyl ligand from the iron. The rate of the reaction increases with ionic strength and solvent polarity, and these data are consistent with an ionic intermediate. The second-order dependence of the rate on HgX shows that the coordinated electrophile E in the intermediate is likely to be HgX with an HgX counterion.  

By one pathway, reductive elimination of R and the electrophile gives R-E with retention of stereochemistry at the metal-bound carbon (recall from Chapter 8 that concerted reductive elimination occurs with retention of configuration at this site). For example, the f/ireo-3,3-dimethylbutyl-l,2-(ij iron complex shown in Equation 12.16 reacts with mercuric chloride to form f/zreo-3,3-dimethylbutyl-l, 2- ij mercuric chloride. Similar results have been obtained for the reactions of t/u eo-PhCHDCHDFe(CO)2Cp and tra)is-(tlzreo-PhCHDCHD) 

W(CO)2(PEt3)Cp with as well as for the reactions of organometallic complexes with 

In some cases, reactions of electrophiles with metal-alkyl complexes possessing d-elec-trons appear to occur by mechanisms. - In this process, the electrophile can attack the frontside or backside of the alkyl group. Although retention of stereochemistry at the metal-bound carbon from frontside attack at the metal-carbon bond is often observed, inversion of stereochemistry from backside attack at this carbon has also been observed. 

By a third mechanism, the reaction occurs with loss of stereochemistry at the metal-bound carbon. This stereochemistry is observed if reaction with the electrophile leads to a relatively stable carbocation. For example, the reaction of HgX in Equation 12.18 forms racemic a-methylbenzyl chloride from the optically active a-methylbenzyl complex, presumably due to the stability of the benzylic cation. 

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