Hypervalent Iodine

Hypervalent Iodine Chemistry. 

Prof. Hinkle began his career at W&M performing stability/fragmentation research on alkenyl(aryl)iodonium salts, but we occasionally do test new substrates with electrophilic iodine (III) reagents

Iodine is a very large element. Unlike the smaller halogens, Iodine's size as well as great polarizability, iodine can adopt oxidation states such as -1, +3, and even +5. 

Our early work in the area of iodonium salts (iodine +3) showed the remarkable leaving group ability of the hypervalent iodine moiety found in the alkenyl(aryl)iodonium salts. Under very mild conditions, the salts shown below, (E)-1a-1c and (Z)-1a-1c, fragmented into three vinyl triflates as well as iodobenzene (Ar = Ph). We measured the kinetics of these reactions by 1H NMR spectroscopy. McNeil, A. J.; Hinkle, R. J.; Rouse, E. A.; Thomas, Q. A.; Thomas, D. B. "Vinyl Carbocations: Solution Studies of Alkenyl(aryl)iodonium Triflate Fragmentations," J. Org. Chem. 2001, 66, 5556-5565; (https://pubs.acs.org/doi/10.1021/jo015746%2B). 

Our most recent hypervalent iodine chemistry involved reaction of alkyne-diols containing a tethered phenyl moiety with Stang's reagent. It was published in 2017: Robert J. Hinkle, Sarah E. Bredenkamp, Robert D. Pike, Seong Ik Cheon, “Electrophilic Cyclization of Phenylalkynediols to Naphthyl(aryl)iodonium Triflates with Chelating Hydroxyls: Preparation and X-ray Analyses ,” J. Org. Chem. 2017, 82, 11781. ASAP published online 17 August 2017; (http://pubs.acs.org/doi/pdf/10.1021/acs.joc.7b01619). This reaction is highlighted in the Alkyne-Prins Cascade portion of this site.

We originally interpreted this fragmentation as evidence for the formation of a primary vinyl cation (Scheme 1, below), which had been unobserved in solution. Further work from our labs as well as a lab in Japan (Okuyama's lab) indicated that the primary vinyl cation (A) is, indeed, not necessary to interpret the results of this thermal reaction. The unrearranged products could be explained by both in-plane and out-of-plane SN2 reactions (Schemes 2/3) and rearranged products were generated by concerted migration of trans-B-substituents to afford secondary cations (B) followed by trapping with triflate (Scheme 4).  Hinkle, R. J. "Mikowski, A. M. "Kinetics of Stereoisomeric 2-Methyl-1-butenyl- (aryl)iodonium Triflates," Included in a special issue of ARKIVOC 2003, vi, 201-212, published 25 July 2003; (http://www.arkat-usa.org/ark/journal/2003/Varvoglis/AV-745A/745A.pdf).