Onium ions initiation

What is retained nowadays of the initial mechanism (Scheme 1) is the occurrence of a cationic intermediate. But bromine bridging is not general, and its magnitude depends mainly on the double bond substituents (Ruasse, 1990). For example, when these are strongly electron-donating, i.e. able to stabilize a positive charge better than bromine, / -bromocarbocations are the bromination intermediates. The flexibility of transition state and intermediate stabilization puts bromination between hydration via carbocations and sulfenylation via onium ions. 

Covalent acids, AH, co-initiators ROR, and -onium ions, e.g., EtsO+, are three types... 

The formation of 16 from 14 has been interpreted40 as occurring by an intramolecular displacement of the chlorosulfate group at C-2, by participation of the chlorine atom at C-l, to give initially a chlor-onium ion, as shown the departure of the chlorosulfate group is... 

Quaternary onium ions bound to silica gel were reported as phase transfer catalysts initially by Tundo 1I4) and by Rolla and co-workers115 . The short spacer chain catalyst 27 (1.0 mmol/g) was more active than the longer spacer chain phosphonium ion catalyst 28... 

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. 

Aromatic onium ions are a unique class of onium ions which are photoactive and decompose in the presence of light to generate radical-cations [58]. These radical-cations react with solvent or residual moisture to release protons that initiate cationic polymerization [Eq. (43)]. Thus, aromatic onium ions are used as cationic photoinitiators in both carbocati-onic olefin and ring-opening polymerizations, especially in photocuring and photolithographic processes. 

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

Dormant Species and Pseudocationic Propagation The majority of propagating chain ends in most cationic polymerizations initiated by protonic acids and/or cocatalyzed by Lewis acids do not exist as carbenium ions, but are instead dormant species. The two major types of dormant species are onium ions and covalent esters or halides. The covalent species are formed by reversible reaction of carbenium ions with nucleophilic anions onium ions are generated by reaction of carbenium ions with noncharged nucleophiles such as ethers, sulfides, and amines. Because the majority of propagating chain ends exist as dormant species, they are often the only species that can be detected spectroscopically ... 

When protonic acids are used as the initiators, nucleophiles do not complex oxyanions and therefore only form onium ions. The kinetic scheme of triflic acid initiated polymerizations of isobutyl vinyl ether in the presence of sulfides is presented in Eq. (80) [58,133]. The rate of polymerization described by Eq. (81) takes into account propagation by both carbenium kp ) and sulfonium ions ( /). 

The mechanistic details and roles of all constituents of the multicomponent initiating systems for new controlled/living carbocationic polymerization are also discussed in Section VI. At this stage it suffices to say that in both the new systems and conventional carbocationic polymerization, monomer is consumed by the repetitive electrophilic addition of growing carbocations whether or not in dynamic equilibrium with either covalent species or onium ions. 

Before going into details, let us make a brief statement that propagation in new controlled/living carbocationic systems has nearly the same mechanism as in the conventional systems discussed in Chapter 3, which consists of the electrophilic addition of carbenium ions to alkenes. The main difference is that carbenium ions are in dynamic equilibria with dormant species (covalent esters and onium ions). The correct choice of structures and concentrations of activators and nucleophilic additives as well as those of initiator allows for the preparation of polymers with predetermined molecular weights, low polydispersities, and controlled end functionality, including block copolymers (see Chapter 5). 

In the presence of monomers, all discussed initiators form the corresponding onium ions by alkylation or acylation, as shown schematically below, for the polymerization of cyclic ethers ... 

The nucleophilic site of heterocyclic monomer is a heteroatom. Reaction of heterocycle with a cation (e.g., initiation) leads to onium ion. 

In the random copolymerization process, both types of active species should be able to participate in the cross-propagation reactions. This imposes certain limitations on the choice of comonomers in the cationic polymerization of heterocyclic monomers. Onium ions, being the active species of these polymerizations, differ considerably in reactivity thus, as already discussed, oxonium ions initiate the polymerization of cyclic amines, whereas ammonium ions do not initiate the polymerization of cyclic ethers and the corresponding cross-propagation reaction would not proceed ... 

The high stability of the onium ions makra them immune to some reactions that cannot be avoided in the case of carbenium tons. Some onium tons, like tetraalkyl ammonium cations, do not even react with water, which is too wrak a nudeophile and cannot displace the amine ligand. This stability, in turn, has allowed the discovery of systems (monomers, initiators, solvents, temperature range) whidi behave as living systems,... 

Initiation with Stable Carbenium or Onium Ions and with Their Covalent Precursors ( Hidden Ions)... 

The utility of onium ions depends not only on the value of K and related rate constants of the equilibration involved in initiation but also on the rate constant of the subsequent propagation. Thus, in spite of the much higher nucleophilicity of ethws, cyclic acetals can be polymerized by trialkyloxonium salts, because of the... 

NMR studies of the polymerization of THF, oxepane, 3,3-dimethylthietane, 2-methoxy-2-oxo-l,3,2-dioxaphosphorinane and 2-methyl-2-oxazoline revealed that the structures of the growing qj ies are the corresponding onium ions these structures are given in Table 7 (earlier, the first reaction products, formed upon initiation, were observed in the polymerization of THF ). 

According to well-established facts the hydride anion can only be shifted or transferred to the carbenium ion the onium ions themselves are no hydride anion acceptors. Nevertheless, some of the onium ions exist in equilibrium with the correspondii carbenium ions (cf. Sect. 3.1) and in this isomeric form can abstract hydride anions. The classical experiments were performed by Jaacks in the polymerization of 1,3-diox-olane initiated by methoxymethylium perchlorate ... 

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