Onium reactivity

Here A, lA, and3A represent anthracene in the ground state, the first excited singlet state and first excited triplet state, respectively. In addition, I represents the onium salt initiation, while Rs and Rt correspond to the reactive centers formed by reaction of the onium salt with the excited singlet and triplet state anthracene, respectively. 

Organic cations (carbocations and onium ions) are important reactive intermediates in organic synthesis. From an experimental point of view, it is noteworthy that the manner in which we carry out reactions of organic cations is different from that for carbanions (Scheme 1). Usually, carbanions are generated and accumulated in a solution in the absence of electrophiles. After the generation process is complete, an electrophile is added to the solution of the pre-formed carbanion to achieve a desired transformation. In contrast, organic cations are usually generated in the presence of nucleophiles. This is probably... 

It is interesting to compare the nucleophilicity sequence found by Liotta and Grisdale (1975) for crown ethers, with the sequence recently reported by Landini et al. (1978) for onium compounds. For hexadecyltributylphos-phonium as the cation, the reactivity sequence (for n-octyl methanesulfonate) in chlorobenzene was CN- > > Cl- > Br- > I- > SCN- (Table 28), which... 

Table 5.4 Relative reactivities (normalized to iodide) for a series of nucleophiles under phase transfer, homogeneous dipolar aprotic, and homogeneous protic conditions, and the hydration number of the quaternary onium-anion ion pair [43]...

A general review of synthesis and reactivity of silica-immobilized onium catalysts, which deals mainly with phosphonium salts, is available [42]. 

At this time, only some of these particularities can receive an explanation. Onium bisphenolates are nonsolvated ion-pairs with reduced cation-anion interaction energy, and consequently are very reactive. Their low concentration in the organic phase easily explains the electrophilic nature of the chain ends, at least when the rate determining step is their transfer from the water into the organic phase ... 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

A number of S-, Se-, and Te-alkylated heterocyclic onium salts have been synthesized and their properties and reactivities reported [74JA7835 84M11 90HOU(E12b)676]. Until now, however, the corresponding per-... 

Two chapters in this volume describe the generation of carbocations and the characterization of their structure and reactivity in strikingly different milieu. The study of the reactions in water of persistent carbocations generated from aromatic and heteroaromatic compounds has long provided useful models for the reactions of DNA with reactive electrophiles. The chapter by Laali and Borosky on the formation of stable carbocations and onium ions in water describes correlations between structure-reactivity relationships, obtained from wholly chemical studies on these carbocations, and the carcinogenic potency of these carbocations. The landmark studies to characterize reactive carbocations under stable superacidic conditions led to the award of the 1994 Nobel Prize in Chemistry to George Olah. The chapter by Reddy and Prakash describes the creative extension of this earlier work to the study of extremely unstable carbodications under conditions where they show long lifetimes. The chapter provides a lucid description of modern experimental methods to characterize these unusual reactive intermediates and of ab initio calculations to model the results of experimental work. 

For adequate reaction rates, a high concentration of iodide anion is necessary. The cation portion of the salt appears to have little or no effect on catalytic activity or reaction selectivity. Inorganic iodides (such as potassium iodide) are the obvious first choice based on availability and cost. Unfortunately these catalysts have very poor solubility in the reaction mixture without added solubilizers or polar, aprotic solvents. These solubilizers (e.g., crown ethers) and solvents are not compatible with the desired catalyst recovery system using an alkane solvent. Quaternary onium iodides however combine the best properties of solubility and reactivity. 

The most important point about the alkyl halide reactivities in triphase catalysis is that the reactions which have the highest intrinsic rates are the most likely to be limited by intraparticle diffusion. The cyanide ion reactions which showed the greatest particle size and cross-linking dependence with 1-bromooctane had half-lives of 0.5 to 2 h and with benzyl bromide had half-lives of 0.13 to 0.75 h. The reactions of 1-bromooctane and of benzyl chloride which were insensitive to particle size and cross-linking had half-lives of 14 h and 3 h respectively. Practical triphase liquid/ liquid/solid catalysis with polystyrene-bound onium ions has intraparticle diffusional limitations. 

Substantial variations of the organic solvent used in triphase catalysis with polystyrene-bound onium ions have been reported only for the reactions of 1-bromo-octane with iodide ion (Eq. (4))74) and with cyanide ion (Eq. (3)) 73). In both cases observed rate constants increased with increasing solvent polarity from decane to toluene to o-dichlorobenzene or chlorobenzene. Since the swelling of the catalysts increased in the same order, and the experiments were performed under conditions of partial intraparticle diffusional control, it is not possible to determine how the solvents affected intrinsic reactivity. 

Dialkylhalonium ions are reactive alkylating agents. The alkylation of Jt-donor (aromatic and olefinic) and w-donor bases with dialkylhalonium ions has been studied.353 Alkylation of aromatics with dialkylhalonium ions was found to be not significantly different from conventional Friedel-Crafts alkylations, showing particular similarities in the case of alkylation with alkyl iodides. Alkylation of w-donor bases with dialkylhalonium salts provides a simple synthetic route to a wide variety of onium ions. 

The unique hydride abstraction property has been gainfully employed in developing novel synthetic reactions.530 Reactive hydrocarbons such as triphenylmethane, adamantane, and diamantane are readily fluorinated in the presence of nitrosonium ion in HF-pyridine media.537 In the presence of a suitable oxygen donor such as dimethyl sulfoxide, the nitrosonium ion can act as a nitrating agent538 [Eq. (4.152)]. The initially formed nitrito onium ion 223 transfer nitrates aromatics rather readily.245 The NO+-induced reactions are further reviewed in Chapter 5. 

The representative reaction system applied in asymmetric phase-transfer catalysis is the biphasic system composed of an organic phase containing an acidic methylene or methine compound and an electrophile, and an aqueous or solid phase of inorganic base such as alkaline metal (Na, K, Cs) hydroxide or carbonate. The key reactive intermediate in this type of reaction is the onium carbanion species, mostly onium enolate or nitronate, which reacts with the electrophile in the organic phase to afford the product. 

The exact pathway for generating the reactive onium carbanion species remains the subject of controversy, typically among Starks extraction mechanism (Scheme 1.2) and the Makosza interfacial mechanism (Scheme 1.3). 

The onium hydroxide then abstracts hydrogen from the acidic organic compound to give the reactive intermediate Q 1 R. ... 

Cation exchange from the metal cation to the onium carbanion improves the intrinsic reactivity of the latter due to formation of the naked anion . At the same time, the... 

Unlike the nucleophilic substitution reactions which generate stable onium halide after the reaction, nucleophilic additions to electrophilic C=X double bonds (X=C, N, O) provide rather basic onium anion species as an initial product. If the anion is sufficiently stable under the reaction conditions, onium anion will then exchange the counter ion for the other metal carbanion at the interface to regenerate the reactive onium carbanion Q+R. In another scenario, the basic onium anion may abstract the acidic hydrogen atom of the other substrate to provide Q 1 R directly. Such a reaction system ideally requires only a catalytic amount of the base although, in general, a substoichiometric or excess amount of the base is used to lead the reaction to completion. An additional feature of this system is the substantial possibility of a retro-process at the crucial asymmetric induction step, which might be problematic in some cases. 

---