Haloform-chromium chloride

Treatment of an aldehyde with chromium(II) chloride and a haloform affords the (E)-haloalkene with high diastereoselectivity (Scheme 5.29) [42]. Here again, aldehydes are transformed selectively ketones are recovered unchanged. 

The use of a combination of diromium(II) chloride and a gem-dihalo compound is not such a readily accessible readion considering the availability of chro-mium(II) chloride, but shows high performance in reactions with aldehydes. High chemoselectivity and diastereoseledivity (T-selectivity) are observed. In particular, the combination of chromium(II) chloride and a haloform constitutes a very important method for the preparation of ( )-haloalkenes. 

Takai and coworkers showed that treatment of an aldehyde with chromium (II) chloride and haloform afforded -haIoaIkene with high diastereoselectivity (Scheme 8.29) [56]. Aldehydes were transformed selectively in the presence of... 

---