Onium reaction mechanism

In decomposing oxonium ions the situation is quite different, i.e., the preference for alkene loss is much less emphasized and aldehyde (ketone) loss is gaining importance. The observed changes are in good agreement with the postulated mechanism of the onium reaction. [143,167] The alternative pairs of oxonium ion plus alkene and aldehyde plus carbenium ion may be formed with a preference for the first one, because APA is comparatively small (20-60 kJ mol" ) or even zero, e.g., for the acetone/isobutene pair [143,166,167]... 

Onium salts have been widely used as an acid generator for photo-, EB, and x-ray resist. In addition, aromatic polymers such as novolak and polyhydroxystyrene have been often used as a base polymer for EB and x-ray resist. The reaction mechanisms in a typical resist system have been investigated by pulse radiolysis [43,52,77-88], SR exposure [79,80,83-85], and product analysis [88]. Figure 6 shows the acid-generation mechanisms induced by ionizing radiation in triphenylsulfonium triflate solution in acetonitrile. The yields of products from electron beam and KrF excimer laser irradiation of 10 mM triphenylsulfonium triflate solution in acetonitrile are shown in Fig. 7 to clarify the... 

The fate of the onium carbanion Q+R incorporated into the organic phase depends on the electrophilic reaction partner. The most studied area in the asymmetric phase-transfer catalysis is that of asymmetric alkylation of active methylene or methine compounds with alkyl halides, in an irreversible manner. The reaction mechanism illustrated above is exemplified by the asymmetric alkylation of glycine Schiff base (Scheme 1.5) [8]. 

Many different photoinitiators based on onium -type compounds with anions of low nucleophilicity also have been described in the literature as effective catalysts for the polymerization of epoxides Thus, diaryliodonium salts diaryliodosyl salts triarylsulfonium salts and related compoundstri-phenylsulfoxonium saltsdialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenylsulfonium salts seem to be most suitable as photoinitiators for epoxy curing. Some of the principles of the reaction mechanism involving these initiators are discussed in detail in the following Sections. Various other onium photoinitiators such as diarylchloronium and diarylbromonium salts , thiopyrylium salts 3), triarylselenonium salts and onium salts of group Va elements >... 

T. Iwamoto, M. Akita, T. Kozawa, Y. Yamamoto, D. Werst, D.A. Trifunac, and D. Alexander, Radi ation and photochemistry of onium salts acid generators in chemically amplified resists, Proc. SPIE 3999, 204 213 (2000) A. Nakano, K. Okamoto, Y. Yamamoto, T. Kozawa, S. Tagawa, T. Kai, H. Nemoto, and T. Shimokawa, Deprotonation mechanism of poly(4 hydroxystyrene) and its deriva tives, Proc. SPIE 5753, 1034 1039 (2005) T. Kozawa, A. Saeki, and S. Tagawa, Modeling and simulation of chemically amplified electron beam, x ray, and EUV resist processes, J. Vac. Sci. Technol. B 22(6), 3522 3524 (2004) T. Kozawa, A. Saeki, A. Nakano, Y. Yoshida, and S. Tagawa, Relation between spatial resolution and reaction mechanism of chemically amplified resists for electron beam hthography, J. Vac. Sci. Technol. B 21(6), 3149 3152 (2003). 

The addition of a free radical to the carbon-carbon double bond of an allylic group that forms part of an onium ion can induce disintegration of the onium salt, thus giving rise to the release of an inert compound and a reactive radical cation. Allylic compounds employed for this purpose are presented in Chart 10.6, and the reaction mechanism for a typical case is presented in Scheme 10.14 [55]. 

The somewhat simplified reaction mechanism shown in Scheme 14.2 is based on the photogeneration of electron/hole pairs in PPP. While the holes react with triethylamine present in the system, the electrons remain in the polymer as delocalized anion radicals. They react with benzophenone to form the diphe-nylcarbinol anion, and the latter eventually reacts with CO2. The CO2 fixation is strongly enhanced by the presence of tetraethylammonium chloride. The soft onium cations are thought to stabilize the diphenylcarbinol anion, the precursor of the final product. 

The mechanism by which onium photoinitiators produce active centers is very complex and the details are still under investigation by a number of researchers (47,50-56). The generally accepted photoinitiation reaction mechanism for iodoninm salts is shown in Figure 2. Upon absorption of a photon the iodonium salt is promoted to an electronic excited state (denoted by an asterisk in the figure). 

The carbenium ion intermediate then eliminates the alkene by charge-induced cleavage of a C-C bond. The striking argument for a carbenium ion intermediate is presented by the influence of the y-substituent R on the competition of onium reaction and McL. If R = H, i.e., for propyl-substituted iminum ions, the products of both reactions exhibit similar abundance. If R = Me or larger or if even two alkyls are present, McL becomes extremely dominant, because then its intermediate is a secondary or tertiary carbenium ion, respectively, in contrast to a primary carbenium ion intermediate in case of R = H. The importance of relative carbenium ion stability for onium ion fragmentations (Chap. 6.11.2) will become more apparent when dealing with the mechanism of the onium reaction. 

The radiation-induced cationic polymerization in the presence of onium salts has attained practical importance for the EB curing of systems containing epoxides or vinyl ethers [22,23]. The chemical structures of typical compounds were presented in Table 3.23. THE does not play a role in this context, because of its very low propagation rate constant (kp 4 x 10 lmol s ). A reaction mechanism for the polymerization of vinyl ethers in the presence of an iodonium salt, as proposed by Crivello [22], is shown in Scheme 5.8. 

The tetra alkyl amm onium salts of [B Hg] , formed by ion-exchange reactions, have proven to be useful synthetic reagents because of their thermal and air stabihties. The stmcture of the [B Hg] ion has been determined by an x-ray study (66) and shown to have the 2013 styx stmcture, C2 symmetry. Mechanisms for the formation of this ion have been proposed (67). Tetraborane(lO) can be easily obtained from salts of [B Hg] (eq. 9). 

The first report of the use of bromine for the oxidation of sulphoxides appeared in 1966116. Diphenyl sulphone was isolated in 0.5-1% yield when the sulphoxide was treated with bromine in aqueous acetic acid for several hours. The yield was increased to about 5% by quenching the reaction with sodium carbonate. A kinetic study117 of a similar reaction involving dimethyl sulphoxide showed no significant yield improvement but postulated that the mechanism proceeds via an equilibrium step forming a bromosulph-onium type intermediate which reacted slowly with water giving dimethyl sulphone as indicated in equation (35). 

The reaction was originally used by Starks ( ) in his early studies of ptc mechanism. The reaction was monitored by gas-liquid chromatography and decane served both as solvent and internal standard. The reaction was run using a quaternary onium salt, Aliquat 336, and 18-crown-6 for calibration. The rates are shown in Table II. Please note that the relative rate of 1.00 corresponds to an absolute rate at 105 of 1.34 X 10 M-lsec . ... 

Nucleophilic substitutions are especially important for alkyl halides, but they should not be considered to be confined to alkyl halides. Many other alkyl derivatives such as alcohols, ethers, esters, and onium ions 3 also can undergo SN reactions if conditions are appropriate. The scope of SN reactions is so broad that it is impossible to include all the various alkyl compounds and nucleophiles that react in this manner. Rather we shall approach the subject here through consideration of the mechanisms of SN reactions, and then develop the scope of the reactions in later chapters. 

Since such reactions as a rule proceed under the action of nucleophilic reagents whose heteroatoms have the same nature as the onium heteroatom, the problem is whether these recyclizations are really isomeric, i.e., if the onium atom becomes the heterofunction (pathway a, Scheme 10), or if they are pseudoisomeric, i.e., the heterofunction has a heteroatom that belonged earlier to the reagent (pathway b, ANRORC mechanism). 

Another, important, application of the use of isotopic labeling to decide which of two stereoheterotopic ligands is involved in a classical chemical reaction is in the work of J. Sicher and his school concerning the mechanism of onium ion elimination 110>. As the result of extensive studies, it was concluded (Fig. 53) that elimi-... 

Reaction of triisopropylsilyl enol ether with a combination of iodosylbenzene 18 and trimethylsilyl azide at -15 °C gives directly the /J-azido triisopropylsilyl enol ether 38 in a high yield. A mechanism involving the reductive -elimination of a-iodanyl onium ion 37, probably produced by ligand exchange of in situ generated PhI(N3)OTMS with silyl enol ether, was proposed. Addition of azide to the resulting a,/l-unsaturated onium ion explains the formation of 38 [58,59]. 

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