Alkynes reaction with HTIB

HTIB [PhI(OH)OTs] has high reactivity toward alkenes and alkynes. Reactions ofHllB with alkenes afford v/c-ditosyloxyalkanes in moderate yield (Scheme 3.73) [225-227]. With cyclohexene and alkyl-substituted alkenes, this reaction proceeds as a stereoselective 5yn-addition, whereas phenyl-substituted alkenes and norbornene give products of skeletal rearrangements. 

The reaction of HTIB with alkynes generally affords two products alkynyl- (175) and alkenyl-(phenyl)iodonium (176) tosylates (Scheme 3.75) [234-238], Alkynyl(phenyl)iodonium tosylates 175 are major products in the reactions of terminal alkynes bearing bulky substituents (R = fcrf-butyl, scc-butyl, cyclohexyl, aryl, etc.), while alkenyliodonium tosylates 176 are formed from non-sterically hindered terminal alkynes or from internal alkynes. 

Reaction with Alkynes. Acetals of dihaloacetophenones are prepared from phenylethynes with 7V-halosuccinimide (NIS or NBS) and catalytic amounts of HTIB in methanol (eq 33). When an equimolar quantity of NIS is used in the presence of a catalytic amount of HTIB, a-diiodoacetophenone acetal Is formed in an excellent 3deld. If the ratio of NIS to phenyleth3me Is increased with concomitant increase in the amount of HTIB, the 3deld of the acetal decreases to 7 0% and methyl phenylglyoxylate is formed as a minor product. In this process, there is no evidence of formation of methyl phenylacetate, which is typically formed in the reaction of equimolar HTIB with phenylethjme in hot methanol. The acetal of methyl phenylglyoxylate is also prepared as the sole product by treatment of 2-lodo-l-phenylethyne with NIS and a catalytic amount of HTIB (eq 34). Terminal alkynes are also reported to react with HTIB in an ultrasound enhanced system to furnish arylsulfonates in a quick and simple one-step process (eq 35). ... 

The synthesis of the first alkynyliodonium tosylates was achieved by the treatment of terminal alkynes with [hydroxy(tosyloxy)iodo]benzene (HTIB) (equation 8)8,10,11. Such reactions are generally conducted with an excess of alkyne in chloroform at reflux, although they can be carried out at room temperature, and dichloromethane can be employed as solvent. This procedure is, however, restricted to terminal alkynes in which R is either an aryl group or a bulky alkyl group. With linear alkyl groups (i.e. R = n-Pr, n-Bu, fl-C5Hn), phenyl(/ -tosyloxyvinyl)iodonium tosylates are obtained instead (equation 9)8. In some cases (R = /-Pr, /-Bu), mixtures of alkynyl- and (/ -tosyloxyvinyl)iodonium tosylates are produced8. ter -Butylacetylene appears to be the optimum substrate for this approach and has been employed with a series of [hydroxy(tosyloxy)iodo]arenes for the synthesis of various aryl(ter/-butylethynyl)iodonium tosylates (equation 10)9. 

---