(Z)-1-Aryl-1-haloalkenes as intermediates in the Vilsmeier haloformylation of aryl ketones.

[reaction: see text] Vilsmeier reagents give (Z)-1-aryl-1-haloalkenes from aryl ketones bearing an electron-donating substituent at the ortho- or para-position. These haloalkenes are intermediates in the Vilsmeier haloformylation of the aryl ketones. Another reaction mechanistic pathway is thus available in certain Vilsmeier haloformylations, in competition with the commonly accepted route by way of an enaminoketone.