Alkenes reaction with HTIB

The reactions of HTIB with alkenes (Scheme 3.73) can be rationalized by a polar addition-substitution mechanism similar to the one shown in Scheme 3.70. The first step in this mechanism involves electrophilic flnfi-addition of the reagent to the double bond and the second step is nucleophilic substitution of the iodonium fragment by tosylate anion with inversion of configuration. Such a polar mechanism also explains the skeletal rearrangements in the reactions of HTIB with polycyclic alkenes [227], the participation of external nucleophiles [228] and the intramolecular participation of a nucleophilic functional group with the formation of lactones and other cyclic products [229-231]. An analogous reactivity pattern is also typical of [hydroxy(methanesulfonyloxy)iodo]benzene [232] and other [hydroxy(organosulfonyloxy)iodo]arenes. 

Reactions of alkenes with hypervalent iodine compounds lead mostly to vicinally functionalised alkanes. This is the case with PhI(OAc)2, PhIO, PhI(OH)OTs, PhI(OTf)0(TfO)IPh and other related reagents.230,231,233,239-247 poj. example, treatment of alkenes with PhI(OH)OTs, (HTIB), affords vie bis(tosyloxy)alkanes with a syn stereospecificity.239,241 n generally admitted that this reaction proceeds by the electrophilic attack of the hypervalent iodine species on the ethylenic double bond to afford a carbonium ion intermediate (140). This intermediate undergoes two consecutive Sn2 substitution reactions to eventually give the final products. (Scheme 5.17)... 

However, by analogy with the reactions of alkenes, an alternative mechanism via the formation of a pentacoordinate cyclic periodinane (145) is also plausible. Oxidative addition of HTIB or its analogs to the enol affords the periodinane. Ligand coupling yields a new elusive intermediate, an a-hydroxy-iodinane (146), which undergoes elimination of iodobenzene. (Scheme 5.21)... 

In combination with protic acid, NIS has been used for the preparation of a-iodo enones and j8, -dihalo enones (eqs 4 and 5 HTIB = [Hydroxy(tosyloxy)iodojbenzene). More reactive alkenes such as enol acetates react with NIS to afford a-iodo ketones. The reaction of enol ethers with NIS in the presence of an alcohol, affording iodoacetals, allows the synthesis of acetals by dehalogenation or mixed ketene acetals by elimination. ... 

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. 

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