Onium reactive species

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). 

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. 

The system for which the reactivity of carbenium and onium active species have been quantitatively compared is the polymerization of cyclic acetals [76] ... 

As discussed already, termination of ring-opening polymerization may proceed by (a) irreversible recombination with counterion and (b) irreversible chain transfer to polymer. Other sources of termination are also possible, depending on the system (c) reaction with other components of the system, solvent or impurities and (d) different reactions of more reactive species existing in equilibrium with stable onium species. 

Termination by Reactions of More Reactive Species Existing in Equilibrium with Stable Onium Species As already discussed, in the systems, in which unimolecular ring-opening of cyclic onium ion leads to highly stabilized carbocationic species, a concentration of the latter species in equilibrium with onium ions may be significant. This is, for example, the case of cationic polymerization of cyclic acetals, where carboxonium ions exist in equilibrium with their oxonium counterpart ... 

Polymerizations initiated via addition-fragmentation reactions can also be classified as an initiation process involving radicalic species. The principle of this class of reactions consists in the reaction of a photolytically formed radical with an allyl-onium salt generating a radical cation intermediate. These reactive species undergo a fragmentation giving rise to the formation of initiating cations. 

In principle, in the quaternary onium cation-catalyzed PTC reaction, the reactive species could be the free anion, the ion pair of the onium cation and anion, their complex aggregates, or a combination of all of these species. The behavior and structure of ion pairs and higher aggregates have been studied extensively using conductometric, spectro-photometric, spectroscopic, and magnetic resonance techniques [30]. In general, at low... 

In a PTC reaction catalyzed by quaternary onium salt involving the extraction of catalyst-anion ion pair, the kinetics is complicated by the reactive form of the reactant anion in the organic phase. From both physical and kinetic points of view, two types of ion pairs can be considered to exist, namely, the loose or solvent separated ion pairs and the tight or contact ion pairs. Since any form of the anion (free ion, catalyst-anion ion pair, or ion aggregates) could be the reactive species in the PTC reactions, it is worthwhile exploring the kinetics associated with the following two limiting cases of the reactive form of the anion. 

The third example is the work of H. Ito and C. E. Willson (Polym. Eng. Sci., 2, 1019, 1983). They made an immortal acidic catalyst by photoirradiation of a sensitizer (onium salt). In this reaction, removal of protective groups analogous to enzyme reactions took place. Incorporation of different kinds of reactive species in polymer matrix can yield diverse answers in response to different kinds of stimulations or questions. Approach from science to the arts seems to be possible in the near future. 

It is well established that the rhodium-carbene species generated upon activation of diazo compounds by rhodium complexes can undergo insertion into a X—H bond (X= C, Si, O, N) to form a new C—X bond under mild conditions (Scheme 3.58). This reaction involves formation of an onium yUde intermediate and subseqnent proton transfer [156]. The high reactivity of these onium ylide species, which can be trapped by an electrophile prior to the proton transfer leading to a second bond formation. 

Rhodium-Catalyzed Reactions via Zwitterionic Intermediates Diazo compounds are also known to undergo insertion into C—H bonds by action of a rhodium-based catalyst, giving rise to a zwitterionic species characterized by a similar reactivity to that of onium ylide species [170]. Recently, Hu et al. [171] have described that zwitterionic intermediates 150, obtained by carbene insertion into a C—H bond in indoles, can be trapped by imine 151 activated by a chiral Brpnsted acid. After optimization of conditions, three-component reactions carried out at -10 °C in toluene afforded the desired products 152 in high yields, >20 1 diastereoselectivities for the flnft-isomer, and 84-99% ee (Scheme 3.66). 

Cationic photopoiymerization is typically initiated by onium salts, for example, diaryliodonium salts Ar2C MtX or triarylsulfonium salts AtsS, MtK " (MtK = PF4", BFs", SbFs , etc.). The irradiation of photoinitiator generates a number of reactive species that subsequently react with solvent or monomer to give protonic acid HMtK . 

Figure 14.1 Pathways for generating the reactive onium carbanion species.

While structure 2 is an onium dication, it can not be considered a superelectrophile. Only if the electrophilic site(s) exhibit significantly increased reactivities due to interaction of the onium charge centers can the species be classified as distonic superelectrophiles. 

Among other distonic superelectrophiles described in the literature, there are carbo-onium dications. These include carbo-carboxonium dications, carbo-ammonium dications, and related ions. Despite the separation of charge in these superelectrophiles, some have been shown to have very high electrophilic reactivities. I. ike the carbodications described previously, the discussion here is limited to those systems that have been shown to have electrophilic reactivities greater than the related monocationic onium ions, as well as structural criteria supporting their designation as a distonic superelectrophilic species. 

Examination of the dication pairs 122/123 and 125/126 may also suggest a further useful definition of the distonic superelectrophiles the distonic superelectrophiles may be distinguished from onium dications that are without superelectrophilic activation (i.e., two isolated cationic centers) by comparison of the energies, reactivities, or structural/electronic criteria, of two closely related species. For example, it is expected that ammonium-carbenium dications 127 and 128 should vary considerably in terms of energy, due to the proximity of the charges. 

The mechanism of living copolymerisation in the presence of aluminium porphyrin coupled with quaternary onium salt was proposed to involve activation of the anion of the onium salt as the nucleophile by the metallopor-phyrin the structure of the active species was found to be six-coordinate aluminium prophyrin carrying one reactive axial ligand on both sides of its square planar AIN4 skeleton [188] ... 

In addition to covalent species and carbenium ions, the equilibria may involve onium ions, which are formed by reaction of carbenium ions with noncharged nucleophiles [Eq. (46a)]. This decreases the carbenium ions lifetime, and therefore the time available for isomerization to more stable and less reactive carbenium ions via hydride and alkyl anion shifts [Eq. (46b)]. Decreasing the probability of rearrangements by decreasing the carbenium ions lifetime is especially useful because such rearrangements can not be prevented by decreasing the polymerization temperature. 

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