Hypervalent iodine ligand coupling

H347). The reaction of Gilman s reagent, R2CuLi, also involves ligand coupling. There are many examples of hypervalent iodine compounds. 

The reactions of the /3-halogenovinyliodonium salts 24a,b with halide ions are quite slow, but proceed at 60°C to give a substitution product, (Z)-l,2-dihaloalkene 25, with complete retention of configuration (Scheme 22)." No inverted products were detected. As shown in Fig. 8 the rate of the reaction changes with halide concentration in the same way as for the /3-elimination. That is, the reaction must occur as an intramolecular process of the A -haloiodane intermediate as does the j8-elimination. Another characteristic of the reaction is the formation of the retained iodoalkene 26. The results are best accounted for by the ligand coupling mechanism within the hypervalent iodine intermediate The trends have been explained... 

Several mechanistic studies on the reactions of diaryliodonium salts with nucleophiles have been published. Ochiai and coworkers performed a mechanistic study on the phenylation of 3-keto ester enolates with diaryliodonium salts. An aryl radical trap was added to the reaction without affecting the outcome, which indicates that radical pathways are unlikely and the reaction occurs by direct coupling of the ligands on the hypervalent iodine center [889]. 

In the area of oxidation catalysis, several interesting reactions have been developed by Waser using a Koser-type hypervalent iodine 71 with an incorporated acetylene ligand (Scheme 16.17). Under catalysis with palladium hexafluoroacetylacetonate, nucleopalladation with both a phenol (e.g., 70) and an acid (e.g., 73) followed by oxidative C(sp )-C(sp) coupling led to new products 72 and 74. Initially, this reaction was developed for cyclization of alcohols or acids, respectively. Both five-and six-membered ring cyclization were successfully employed, and a total of 20 examples with 34-82% yield demonstrate the broad scope of this approach [58]. 

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