Hypervalent iodine reductive elimination

As described above, two fundamental modes of the reaction of organo-A3-iodanes involve ligand exchange, occurring at iodine(III) with no change in the oxidation state, and reduction of hypervalent A3-iodane to iodide, called reductive elimination. These processes are discussed in detail. 

Hypervalent aryl iodine reagents have also been used as the electrophile in direct ary-lation reactions. In these examples, the strongly oxidizing hypervalent iodine reagents are proposed to react with the intermediate Pd(II) species to generate a Pd(IV) intermediate. Subsequent reductive elimination of biaryl from this Pd(IV) intermediate is then believed to regenerate the active Pd(II) species (Equation 19.141). ... 

An unprecedented aminofluorination was reported by Liu et al. [86]. This reaction proceeds from 124 through another rare intramolecular trans-aminopaUadation and again with complete endo-selectivity to 125. In the presence of a hypervalent iodine(III), it then forms a Pd(IV) intermediate, which accommodates a fluoride for subsequent reductive elimination with retention of configuration. This mechanistic proposal was again arrived at from deuterium labeling studies. For chiral substrates, reasonable diastereoselectivities can be obtained from this reaction. 

Pyrroles almost exclusively add to acetylenes as Ai-centered nucleophiles however, when the acetylene is activated by a strong electron-withdrawing substituent, pyrrole can add to the triple bond as a C-centered nucleophile. " In their gold-catalyzed alkynylation of pyrrole with silyl-protected alkynyl benziodoxolone hypervalent iodine reagent 6, members of the Waser laboratory only saw C-alkynylated products (Scheme 10.2). Using this method, 1/f-pyrrole (1) could be C2-funtionalized to afford 7 in 62% yield, whereas Ai-TIPS pyrrole (8) was selectively transformed into C3-product 9 in 79% yield. In both cases, auration of the heterocycle led to an organogold(III) intermediate that reductively eliminated to afford the alkynylated products. 

A study, including computational results, of the role of copper enolate complexes in the a-arylation of enolates has shown that the reaction is likely to occur through oxidative addition of the iodoarene to form an aryl-copper(III) intermediate, (46), followed by reductive elimination to give the product. Copper(III) species are also likely to be involved in the meta-selective C-H arylation of acetanilides by diaryliodonium salts. However, radical species may be involved in the rhenium-catalysed electrophilic triflu-oromethylation of arenes using hypervalent iodine reagents. 

---