Hypervalent ions

Making the hypervalent ions is often the most difficult step in their study. Ion precursors are introduced into the ion source as a vapor. A flow of unreactive carrier gas over the sample, or if necessary a heated solids inlet probe, can be used to increase the flow of precursor. The ion source used in these experiments is a DC discharge that is typically set at 1200 V with 1 mA 

Therefore, secondary reactions are necessary to form the desired ions. In this situation, the halide ion and a second precursor molecule form a complex, and then the complex is stabilized by further collisions with either a third precursor molecule or an inert buffer gas M (reactions 2 and 3). 

Amazingly, the third-order kinetics of this type of reaction were first measured for li in 1928 this was the first study of anion reaction chemistry performed using a mass spectrometer. Some electrons are thermalized rather than captured by the precursor. This can lead to electron attachment to form odd-electron ions, while halide ion association forms even-electron ions. 

4 Torr and flow rate of 200 standard cm s was used. The buffer gas is He with up to 5% Ar added to stabilize the DC discharge source. Approximately 10 collisions with the buffer gas (and precursor molecules) in the flow tube ensure that the ions are cooled to room temperature.  

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The main purpose of this chapter is to review the development of hypervalent chalcogen chemistry after the book of "Chemistry of Hypervalent Compounds edited by Akiba (1999).5 Although several types of compounds may belong to hypervalent ions or molecules by the definition,3a such hypervalent chalcogen compounds (E = S, Se, and Te) will be mainly discussed here that contain CT-type linear bonds.15... 

The expressions nonclassical and hypervalent ion have also been used by some authors to describe distonic ions, but these are incorrect and thus should no longer be used. The term ylidion is limited to species where charge and radical are at adjacent positions. Thus, to describe the distance between charge and radical site, the terms a- (1,2-) distonic ion, p- (1,3-) distonic ion, y- (1,4-) distonic ion, and so forth are now in use [42,43]... 

One final method for deriving gas-phase thermochemistry of hypervalent ions does not involve gas-phase experiments at all. It requires measuring solid-state heats of formation of ionic salts and correcting to gas-phase values using calculated lattice energies for these salts. Since the lattice energies are in the vicinity of 1000 kJ/mol, 1% accuracy on these values is necessary to derive gas-phase thermochemistry with error limits of ca. 10 kJ/mol. This accuracy has yet to be met in practice. ... 

The nomenclature of onium ions derives from positively charged hypervalent ions such as ammonium (NH4 ) and hydroniwm (H3O+). 

A positively charged hypervalent ion of the non-metallic elements. Examples are the methonium ion CHs", the hydrogenonium ion Hg" and the hydronium ion HgO. Other examples are the oxonium, sulfonium, nitronium, diazonium, phosphonium and halonium ions. 

It includes a significant number of molecules with unusual electronic states (for example, ions, open shell systems and hypervalent systems). 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

The Gillespie-Nyholm rules usually can also be applied to other polyanionic compounds with hypervalent atoms. As an example, some polytellurides are depicted in Fig. 13.6. The Te

[Pg.136]

In recent years, a variety of hypervalent iodine reagents have been available. The versatility of these hypervalent organoiodine reagents in organic synthesis has been well recognized. Diaryliodonium salts constitute an important reagent class for the transfer of aryl groups. These iodonium ion salts have been used effectively in C-arylation of a variety of nucleopohiles.112 The arylation of the anion of nitroalkanes with diaryliodonium salts was already reported in 1963.113... 

It was found that treatment of a mixture of 120 and 121 with tris(diethylamino-sulfonium) trimethyldifluorosilicate [TASF(Et)] resulted in smooth addition-elimination to the naphthoquinone to form the y-alkylation product 125 (85 %). TASF(Et) is a convenient source of soluble, anhydrous fluoride ion [47]. It is believed that exposure of 121 to TASF(Et) results in fluoride transfer to generate a hypervalent silicate anion, as depicted in structure 124. The transfer of fluoride between TASF(Et) and 121 may be driven by stabilization of the anionic species 124 by delocalization of the carbon-silicon bond into the LUMO of the unsaturated ketone. 1,4-Addition-elimination of this species to the naphthoquinone 120 would then form the observed product. 

A series of benzisothiazolone derivatives 238 has been prepared from methylthiosalicylate 235 O60L4811>. The key cyclization step features the formation of a TV-acylnitrenium ion 237, generated by the hypervalent iodine reagent, phenyliodine(III)bis(trifluoroacetate) (PIFA). This ion cyclizes to benzisothiazol-3-one 238 upon intramolecular trapping of the thiol moiety. 

Figure 3.84 An illustration of the Pimentel-Rundle three-center MO model of hypervalency, showing equilibrium valence AO (xa-/b-Xc) overlap patterns for (a) 2pF—2pF—2pF NAOs of the trifluoride ion, F3 and (b) 2pF—lsp—2pF NAOs of the bifluoride ion, FHF-.

Linear triatomic anions The pioneering crystallographic studies of Odd Hassel140 on trihalides and related donor-acceptor species led to a far-reaching analysis of such Hassel compounds by Henry Bent.141 The triiodide ion (I3-, stable in aqueous solution) and other known linear trihalide XYZ- species also served as the prototype for Pimentel s incisive three-center MO analysis of hypervalency. We shall therefore begin with NBO/NRT investigation of a series of hypervalent and non-hypervalent triatomic anions in order to make contact with these classic studies. While ab initio studies add many quantitative details to the understanding of these species, the basic picture sketched by Bent and Pimentel is found to be essentially preserved. 

In analogy with (3.209), we can picture each hypervalent MFm species as being formed from a precursor normal-valent cation M"+Fm by n successive cu-additions of fluoride ions,... 

Figure4.49 Hypervalent complexes resulting from successive additions of fluoride ions to PtF42+ see (4.95a) and (4.95b) in the text.

In contrast to the cathodic reduction of organic tellurium compounds, few studies on their anodic oxidation have been performed. No paper has reported on the electrolytic reactions of fluorinated tellurides up to date, which is probably due to the difficulty of the preparation of the partially fluorinated tellurides as starting material. Quite recently, Fuchigami et al. have investigated the anodic behavior of 2,2,2-trifluoroethyl and difluoroethyl phenyl tellurides (8 and 9) [54]. The telluride 8 does not undergo an anodic a-substitution, which is totally different to the eases of the corresponding sulfide and selenide. Even in the presence of fluoride ions, the anodic methoxylation does not take place at all. Instead, a selective difluorination occurs at the tellurium atom effectively to provide the hypervalent tellurium derivative in good yield as shown in Scheme 6.12. 

Scheme 2. Equilibrium formation of the hypervalent adducts of the iodonium ion with bromide and their reactions. 

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. 

Aspects of bonding and structure/dynamics in selected carbonium ions were presented and discussed. These representative studies demonstrate the power of structural theory in the development of concepts that could lead to new and efficient processes, especially in the area of hydrocarbon chemistry and catalysis. There is no doubt that as newer theoretical and experimental techniques and models are introduced, they will be applied to the study of carbonium ions. A deeper understanding of structure/dynamics of hypervalent non-classical carbonium ions will not only deepen our knowledge of structural theory in chemistry, but could also help in the development of new processes and materials useful in our daily life. 

The parent compound and a set of monosubstituted bis(acylamino)diarylspiro-X4-sulfanes (360 X = H, Me, MeO, Cl, NO2) undergo hydrolysis to the corresponding sulfoxides (361). The probable mechanism involves rate-determining cleavage of one of the S—N hypervalent bonds in the spiro ring with simultaneous proton transfer to the nitrogen atom. The hydroxide ion which is formed thereby then attacks the sulfur atom in a fast step to form a diaryl(acylamino)hydroxy-k4-sulfane (362), which is converted into the sulfoxide (361) (Scheme 47). ... 

This review will concentrate on metal-free Lewis acids, which incorporate a Lewis acidic cation or a hypervalent center. Lewis acids are considered to be species with a vacant orbital [6,7]. Nevertheless, there are two successful classes of organocatalysts, which may be referred to as Lewis acids and are presented in other chapter. The first type is the proton of a Brpnsted acid catalyst, which is the simplest Lewis acid. The enantioselectivities obtained are due to the formation of a chiral ion pair. The other type are hydrogen bond activating organocatalysts, which can be considered to be Lewis acids or pseudo-Lewis acids. 

The step common to both of these reactions is electrophilic attack of a hypervalent iodine species at the a-carbon of the carbonyl compounds to yield an intermediate 3. Nucleophilic attack of methoxide ion or tosy-loxy ion with the concomitant loss of iodobenzene results in a-functionalized carbonyl compounds (Scheme 2). 

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