Substitution shifts, halide complexes

There is an alternative mechanism for halide replacement, following the sequence of nucleophile addition, protonation, and elimination of HX. In this pathway, the addition of the nucleophile need not be at the ipso position it can be ortho to halide leading to cine substitution or it can be at the meta or para positions, leading to tele substitution. The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with P a > 22 or so), protonation (which can occur at low temperature with even weak acids such as acetic acid), and hydrogen shifts in the proposed diene-chromium intermediates. Hydrogen shifts lead to an isomer that allows elimination of HX and regeneration of an arene chromium complex, now with the carbanion unit indirectly substituted for X (Scheme 40). ... 

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