Unusual Secondary to Primary System Rearrangement via Ring Opening of 1-(3-Halopropylene)-4-methoxybenzenium Ions

Article abstract of DOI:10.1021/jo00146a007

The effect of electron-withdrawing groups on the ring opening of benzenium ions has been investigated by kinetics, equilibrium studies, and product analysis.The use of SnX4 (X = Br or Cl) compounds as highly ionizing solvents and as conveying agents of the X^- nucleophile works quite well. 1,2-Dihalo-3-(4-methoxyphenyl)propanes (2; X = Br or Cl) and 1,3-dihalo-2-(4-methoxyphenyl)propanes (3; X = Br or Cl) equilibrate at 100 deg C in SnX'4 (X' = Br or Cl) when X = X'; they undergo halogen exchange with partial rearrangement (2 -> 3 and 3 -> 2) when X differs from X'.This 2 -> 3 rearrangement is very unusual in benzenium ion chemistry and yields primary halide products.For X = X', the 3 to 2 equilibrium constant K is 10.1, whatever the halogen; the forward rate constants K₃₂ are 1.82x10^-5 and 1.86x10^-4 s^-1 for X = Br and Cl, respectively.For X different from X' (X = Br, X' = Cl), as well as for mixed compounds containing both halogens, the relative proportions of rearranged and unrearranged products have been examined in terms of the evolving reactions.Results indicate a mechanism involving 1-(3-halopropylene)-4-methoxybenzenium ions which, when they react with solvated halide ions, yield a mixture of 1,2- and 1,3-dihalides in equal amounts.This absence of ring-opening regioselectivity which contrasts with that of usual unsubstituted propylenebenzenium ions is attributed mainly to the electron-withdrawing effect of the halogen. Benzenium ion involvement in nucleophilic substitution can bring about significant rearrangement in secondary compounds containing electron-withdrawing groups.