Summary of Cross-coupling Mechanisms

4h oilvl Hr p-McOCf.HjMgllr p-MeCX-TIl4-CHidl= l 1.-. ibl ) 

The traditional S 2 reaction is a concerted process, as illustrated in eq. (2.8). The electron pair of Nu hinds to the carbon at which displacement is occurring. simultaneously with the departure ol 1. . along with the electron pair ol the C — I, bond. In the transition slate for the reaction, iwo lobes ol an orbital of approximately p hybridization on (he central carbon overlap with orbitals of Nu and I. , respectively. A molecular orbital scheme would assign the iwo electron pairs noted above io the two lowest molecular orbitals (A and ID o the linear ihrce-center subsystem. Trout this picture, the observed inversion of configuration follows. Most commonly die 

The ion pair in eq. (2.10) could revert to reactants, undergo internal transfer of alkyl or halide from the anion to the carbocalion, or react with an external molecule of Grignard reagent. Since the medium is not an hospitable one for ionic species, the lifetime of the ion pair should be short and stereochemistry ntighl be partially preserved, with possible retention of configuration in the internal collapse or inversion in the attack by external reagent. 

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