Stereoselective Chlorination and Bromination of Enamides and Enamines via an Electrostatic Attraction Effect Using (1,1-Diacetoxyiodo)benzene and a Halide Source

The
direct chlorination and bromination of (E)-enamines
and (Z)-enamides to the corresponding (Z)-configurated α-chloroenamines, α-bromoenamines, and
α-chloroenamides have been realized using NiCl2·6H2O or tetrabutyl ammonium bromide as a halide source and (1,1-diacetoxyiodo)­benzene
as an oxidant. The high stereoselective reactions which produce products
with only (Z)-configurations can be attributed to
the structure of the intermediates, the conformations of which are
controlled by the electrostatic attractions between the positively
charged nitrogen atoms and the oxygen atoms of the carbonyl group.
This type of electrostatic effect has never been reported in olefin
halogenations. For this reason, the three-membered bromonium ion is
only a minor intermediate in the enamine bromination pathway. These
methods open pathways to prepare α-chloroenamines and α-chloroenamides,
which are not accessible via the currently used methods.