Sulfones, halo from alkenes

Trialkylboranes react rapidly and in high yields with a-halo ketones,a-halo esters, a-halo nitriles, and a-halo sulfonyl derivatives (sulfones, sulfonic esters, sulfonamides) in the presence of a base to give, respectively, alkylated ketones, esters, nitriles, and sulfonyl derivatives. Potassium tert-butoxide is often a suitable base, but potassium 2,6-di-tert-butylphenoxide at 0°C in THF gives better results in most cases, possibly because the large bulk of the two tert-buXy groups prevents the base from coordinating with the R3B. The trialkylboranes are prepared by treatment of 3 mol of an alkene with 1 mol of BH3 (15-16). With appropriate boranes, the R group transferred to a-halo ketones, nitriles, and esters can be vinylic, or (for a-halo ketones and esters) aryl. " °... 

Strongest evidence for a rapid equilibration of the a-halo sulfone with its a- and a -anions comes from reactions performed in D2O. If such reactions are interrupted before they are complete, the recovered a-halo sulfones are almost fully deuterated at both the a- and a -positions. Furthermore, the alkene products are also essentially fully deuterated, this time at the vinylic positions. When allowed to run to completion, this forms the basis of a very useful preparative method for deuterium-labeled alkenes (Scheme 8 and equation 24). Not only is a high percentage incorporation possible, but the source of deuterium is very cheap. 

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