Mechanism of the Polyhomologation Reaction

Based on the preceding NMR spectra, a pre-equilibrium exists between the borane initiator and ylide that strongly favors formation of the zwitterionic complex 7. The preequilibrium is established immediately and complex 7 is stable at — IS C. These results are consistent with the proposed mechanism of a fast pre-equilibrium followed by a rate-limiting 1,2-migration step since homologation was observed only after warming above 0°C (Busch et al, 2002). 

The rate-determining step of the polyhomologation reaction is a 1,2-migration of an alkyl group from boron to carbon in the zwitterionic complex 7 (Busch et al, 2002). Structural analysis of zwitterionic complexes (X-ray crystallography and computational study) provides insight into the rearrangement. 

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