Iodonium ions, reactivity

The reactions involved are unimolecular, and the cyclohexenyl derivative 3 undergoes solely the spontaneous heterolysis while both spontaneous heterolysis and ligand coupling occur with the iodane 14. The relative contributions of the two reactions of 14 depend on the solvent polarity. The results summarized in Table I show that the iodonium ion and the counteranion are in equilibrium with the hypervalent adduct, X3-iodane. The equilibrium constants depend on the identity of the anion and the solvent employed, and the iodane is less reactive than the free iodonium ion as the k /k2 raios demonstrate. Spontaneous heterolysis of 3 occurs more than 100 times as fast as th t of the adduct 14 as observed in methanol the leaving ability of the iodonid group is lowered by association by more than 100 times. 

Under more basic conditions, a-elimination predominates and insertion of the carbene 40 to the solvent gives racemic 22. Non-basic and poorly nucleophilic conditions allow neighboring group participation to form the rearranged substitution product 23 with complete chirality transfer. The participation can be considered as an intramolecular nucleophilic substitution, and does occur only when it is preferable to the external reactions. Under slightly basic conditions with bases in HFIP, participation is allowed, and the weak base can react with the more electrophilic vinylic cation 21 (but not with the iodonium ion 19). A suitably controlled basicity can result in the formation of cycloalkyne 39, which is symmetrical and leads to racemization. These reactivities are illustrated in Scheme 6. 

Figure 7. The quinuclidine bromonium and iodonium ions are not reactive with...

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

Detailed studies on the solution structure of [hydroxy(mesyloxy)iodo]ben-zene and [hydroxy(tosyloxy)iodo]benzene 17 suggest that in aqueous solution iodosylbenzene 18 exists as a monomeric iodonium ion form 136, if the pH is < 2.3, and as a neutral species 137 at pH > 5.3 through mildly alkaline conditions [216]. The monomer 137 is soluble only to the extent of about 3 x 10"3 M. Based on these finding, we propose a structure of 138 as a reactive species in the reaction using a combination of (PhIO)n 18 and BF3-Et20. 

The difference in reactivity between alkylated and acylated pentenyl glycosides can be rationalised as follows the elctrophilic iodonium ion will add to the double bond of the pentenyl moiety to give a cyclic iodonium ion. Nucleophilic attack by the oxygen will lead to an oxonium ion intermediate which then forms an oxocarbenium ion and an iodo-tetrahydrofuran derivative. The aglycone oxygen will be of low... 

TV-Bromosuccinirnide (NBS) and NIS are the most widely used electrophiles to activate -pentenyl glycosides. Catalytic amounts of acids such as TfOH or trieth-ylsilyl triflate (TESOTf) are introduced to speed up the generation of bromonium or iodonium ions [43], The stereochemical outcome depends, as usual, on whether a participating group exists at C2. An alternative promoter is iodonium dicollidine perchlorate (IDCP), whose lower reactivity allows it to selectively activate certain disarmed n-pentenyl glycosides [44],... 

Silyl enol ethers are quite reactive towards IOB-boron trifluoride (or tetrafluoroboric acid) and can be considered as valuable starting materials for several reactions of synthetic importance. Of special interest is their use for carbon-carbon bond formation 1,4-diketones and unsaturated ketones are the products of such reactions further, they can be transformed to oc-hydroxy, methoxy or trifyloxy ketones. With tetrafluoroboric acid IOB forms a yellow solution containing the highly electrophilic Phi+ OH BF4 , stable up to 0°C. This species reacts readily with silyl ethers of several ketones, notably acetophenones, at —78°C, forming an unstable iodonium ion (ArCOCH2I+ Ph) which with another silyl ether affords 1,4-diketones. 

Palladium(II)-copper(I) cocatalyzed couplings of several (/ ,/ -dialkylvinyl)phenyliodo-nium triflates with 4-isopropoxy-3-tri- -butylstannyl-3-cyclobutene-l,2-dione (71) in DMF at room temperature have recently been described (equation 259)157. As observed in earlier studies of Cu(I)- or Pd(II)-promoted reactions of vinyliodonium salts, the vinyl ligands are introduced with retention of configuration. Since iodobenzene is a byproduct of ligand coupling, the production of 3-phenyl-4-isopropoxy-3-cyclobutene-1,2-dione might also be expected. However, the (/ ,/ -dialkylvinyl)iodonium ions are much more reactive... 

A reactive, cationic intermediate with a three-membered ring containing a halogen atom usually, a chloronium ion, a bromonium ion, or an iodonium ion. (p. 349)... 

Iodine isocyanate, preformed or made in situ from AgNCO and h, adds to alkeites - with the regio- and stneo-chemistry expected of reactions proceeding via cyclic iodonium ions. When the INCO is made in situ, a competing mechanism also occurs (except with the most reactive alkenes) in which the alkene complexes with the iodine, and the complex then reacts with the isocyanate ion to generate the same P-iodoisocyanate as obtained firom INCO direct The reaction can be carried out at -35 to +20 C in Et20, CHzCh, THF, pentane or excess alkene as solvent. Dichloromethane or ether are... 

For most iodinations the reactive species is not the iodonium ion. lodinations require iodine and an oxidant. The actual reactant may be an iodosulfuric acid or iodonitric acid species. 

Zuurmond, H M, van der Laan, S C, van der Marel, G A, van Boom, J H, Iodonium ion-assisted glycosylation of alkyl (aryl) 1-thio-glycosides regulation of stereoselectivity and reactivity, Carbohydr. Res., 215, C1-C3, 1991. 

With solvents such as methanol, acetone, acetic acid and acetonitrile, however, solvent pairticipation can occur . The intermediate bromonium ion is more reactive than the larger iodonium ion and therefore more susceptible to solvolysis, which affords products other than those of Br—Ng addition. 

In one reaction base is used and in the other it is not. When base is not used the product is the thermodynamic trans lactone 113 whereas with base then the kinetic cis lactone 111 is formed. The decision about stereochemistry is made when the iodine first attacks the double bond. But this is not the end of the story. Once again we might expect addition of I2 to be reversible (which means that the decision is not yet irrevocable) and so the selectivity will be down to which of the iodo-nium ions 114 or 115 cyclises. The influence of the base might be to change the rate determining step of the reaction. With a more reactive nucleophile to hand (carboxylate instead of carboxylic acid) we could imagine that the iodonium ion 114 would not have any time to equilibrate before nucleophilic attack occurs. 

Heck-type reactions with enol carboxylates (e.g., vinyl acetate) are generally complex. Most common are reactions in which vinyl acetate is employed as an ethylene equivalent (see Scheme 24). However, an example of a preparatively useful reaction with an intact acetate function is given in entry 44.The reaction of vinyl triflates with vinyl phosphonates affords the corresponding conjugate dienylphosphonates (entry 45).f A new access to reactive nonaflates via a one-pot nonaflation-Heck reaction was recently reported (entry 46). " This reaction sequence starts from silyl enol ethers and provides functionalized 1,3-dienes in a simple manner, lodonium salts can be used as RPd precursors (entry 47). It is notable that the palladium(O) insertion preferentially occurs inbetween the iodonium ion and the vinylic, rather than the arylic sp -hybridized carbon (entry 47). Some years ago, Jeffery used acetylenic halides to achieve (JiJ-enynoates and (Ji)-enynones in fair yields at room temperature (entry 48). ... 

In the view of the well-known reaction of thioglycosides with NBS (77-bromosuccinimide), which generates the unstable cationic species 31 in the course of formation of oxocarbenium ion 32 (Fig. 5.8), Konradsson et al. [5]. reported that 77-iodosuccinimide/trifluoromethanesulfonic acid induces the same reactivity with disarmed thioglycosides as substrates. (NIS/AgOTf or NIS/EtsSiOTf also proves to be an excellent source of iodonium ion.)... 

---