Alkenyl iodonium ions

Hydrolysis of the amide, followed by reductive ozonolysis furnished 44 (99.5 0.5 mixture of 44a and 44b), which can be alkenylated to 45 via the aldehyde 44b. Halolactonization could be achieved to the frans-disubstituted lactone 46 via the iodonium ion 51 (Fig. 2), being preferred to 52 because of lesser allylic 1,3-strain [24]. Methylation of 46 gave rise to a 3 1 1 mixture of 47-49, which can be easily separated by chromatography. Deprotection followed by oxidation furnished (-)-roccellaric acid (1) [25], (-)-dihydropro-tolichesterinic acid (4) and the dimethylated acid 50 in pure form. 

Ochiai et al.369 have developed a method for the synthesis of alkenyl(aryl)iodonium ions with the use of trimethylvinylsilanes and idosobenzene [Eq. (4.107)]. Product formation is stereospecific with retention of configuration. 

Detailed studies on the UV absorption spectra of alkenyl-A3-iodane 133 (X = Cl,Br,I) in acetonitrile solutions in the presence of tetrabutylammonium halides show the equilibrium formation of the iodate 134 (Sect. 2.1), in addition to the iodonium ion 132 coordinated by acetonitrile via hypervalent bonding. [Eq. (116)] [213]. The equilibrium constants are summarized in Table 5. Determinations of the equilibrium constants by UV spectra were carried out at a low concentration of the A3-iodane (<10 4 M), without considering the formation of the corresponding dimers. The magnitudes of the equilibrium constant K1 clearly decrease in the order Cl >Br >1, which reflects the differences in the stability of alkenyl-A3-iodane 133. 

The substitution of alkenyl iodonium salts by halides, using tetrabutylammonium salts, has been studied (Table 9.2). Exclusive inversion of configuration occurred in acetonitrile, so that -precursors gave solely Z-haloalkenes in high yield [35]. In marked contrast, complete retention occurred with cuprous and potassium halides in dichloromethane. Retention of configuration was also noted in reactions of / -substituted alkenyl iodonium salts for example, from /J-phenylsulphonyl decenyl phenyliodonium ion, cis products were formed exclusively in high yield [34],... 

The synthesis of alkenyl iodides probably involves the frax.v-addition of hypoiodous acid via iodonium ion intermediate followed by cis-climination (Eq. Ill)173). 

NIS-promoted iodocarbocychzation reaction of various functionalized 1,5-enynes occurs via a 5-endo diastereoselective process to form iodo-functionalized cyclopentenes (Scheme 6.22). Initially, the iodonium ion activates the alkynyl functionality through jt-coordination. Upon nucleophilic attack of the alkenyl moiety in an anti fashion, an iodocyclization reaction occurs and a carbocation is formed. Further, proton abstraction by the succinimide anion affords the iodo-functionalized cyclopentenes [26]. 

Vinyliodonium ions, 35 and 36, are hypervalent iodine species in which one or two alkenyl ligands are bound to a positively charged iodine(III) atom. Although they are reactive with nucleophilic reagents, they are less labile than alkynyliodonium ions, and stable halide salts of vinyliodonium ions can be prepared. The first vinyliodonium compounds [i.e. (a, / -dichlorovinyl)iodonium salts] were synthesized by the treatment of silver acetylide-silver chloride complexes with (dichloroiodo)arenes or l-(dichloroiodo)-2-chloroethene in the presence of water (equation 152). The early work was summarized by Willgerodt in 1914115. This is, of course, a limited and rather impractical synthetic method, and some time elapsed before the chemistry of vinyliodonium salts was developed. Contemporary synthetic approaches to vinyliodonium compounds include the treatment of (1) vinylsilanes and vinylstannanes with 23-iodanes, (2) terminal alkynes with x3-iodanes, (3) alkynyliodonium salts with nucleophilic reagents and (4) alkynyliodonium salts with dienes. 

An unambiguous example is the nucleophilic substitution of l-alkenyl(aryl)io-donium salts with halide ions (Scheme 9). l-Decenyl(phenyl)iodonium tetra-... 

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