Chemoselectivity Haloform reaction

Aldehydes may be converted to ( )-alkenyl halides by the reaction of CrCh with a haloform in THF. The highest overall yields for the conversion were with iodoform, but somewhat higher (E) (Z) ratios were observed with bromoform or chloroform. Other low-valent metals, such as tin, zinc, manganese and vanadium, were ineffective. As the examples in Table 19 indicate, the reaction is selective for the ( )-isomer, except in the case of an a,3-unsaturated aldehyde. In addition, the reaction with ketones is sufficiently slow for chemoselectivity to be observed for mixed substrates. 

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. 

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