Oxonium ions halides

It would be reasonable to assume that, in a solvent of high dielectric constant, such as nitromethane, the nitrile (84) is formed by direct attack of cyanide ion on an ion-pair (86) in which the bromide ion has the a-D orientation. Departure, assisted by metal ions, of the halide ion from 83 or 86, with possible assistance by the lone pair of the ring-oxygen atom, would lead to an oxonium ion (87) that could... 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

Alkyl halides react with diazines less readily than with pyridines. All the diazines are, nevertheless, more reactive toward methyl iodide than predicted by their pKa values and the Bronsted relationship. The significant although modest rate enhancements found are considered to arise from interactions between the two lone pairs on the nitrogen atoms this interaction is largest in pyridazine. Use of oxonium ions can convert the diazines into diquatemary salts. Quinoxalines and phenazines similarly yield diquatemary salts under forcing conditions. 

Often the requisite THF oxonium ion is generated in situ by using a combination of reagents based on the Meerwein syntheses of trialkyl oxonium salts (150). These combinations include epichlorohydrin or a reactive halide with a Lewis acid, a reactive halide with a metal salt, or sometimes just a Lewis acid alone. The epoxide portion is often referred to as a promoter. 

Tertiary Alkyl(Aryl)Sulfonium Ions. In contrast to tertiary oxonium ions, the tertiary sulfonium ions are stable and are prepared rather readily.167-169 They are even stable in aqueous solutions. Trialkylsulfonium ions are obtained by alkylation of dialkyl sulfides with alkyl halides. Trialkyloxonium and dialkylhalonium ions readily transalkylate basic dialkylsulfides. 

In the second step, nucleophilic attack of the halide ion on carbon of the oxonium ion gives 4-iodo-l-butanol. 

Tetrahydropyrans bearing halides at C4 can be accessed by a modified Taddei-Ricci reaction. The reaction involves condensation of allyltrimethylsilane, aldehydes and a cyclic acetal 371 in the presence of a Lewis acid to afford all ry -tetrahydropyrans 372 as a single diastereomer. The reaction proceeds via anion mediated ring closure of oxonium ion intermediate 373 (Scheme 89). This methodology was successfully applied to the synthesis of a model compound bearing all the structural motifs present in the eastern subunit of okadaic acid <1997TL2895>. 

Kinetics of the reactions of Me3Si+ with water (57) and alcohol (31,32) were investigated by ICR mass spectrometry. Proposed mechanisms involve formation of an oxonium ion as the primary product, which in the case of the methanol adduct is eventually decomposed to methoxy-silylenium ion according to Eq. (9). Reactions of halide transfer to silylenium ion [Eqs. (10) and (11)] have been studied by FT mass spectrometry (59) and by tandem mass spectrometry (44). Hydride transfer... 

Cationic Polymerization. Cationic polymerization is initiated by the transfer of a cation from the catalyst to the monomer. It allows a wider choice of monomers with double bonds, including carbonyls, cyclic ethers, and lactones. The ion may be within a carbonium or an oxonium ion. Friedel-Crafts halides, like AlCls or A CoHsJCL, are strong Lewis acids and initiate the polymerization directly. Weak Lewis acids need a... 

Allyl silanes will also attack carbonyl compounds when they are activated by coordination of the carbonyl oxygen atom to a Lewis acid. The Lewis acid, usually a metal halide such as TiCLj or ZnCl2, activates the carbonyl compound by forming an oxonium ion with a metal-oxygen bond. The allyl silane attacks in the usual way and the (3-silyl cation is desilylated with the halide ion. Hydrolysis of the metal alkoxide gives a homoallylic alcohol. 

Similarly, reactions of C B(OMe)2 4 with Hg(OAc)2 afforded C(FIgOAc)4 125 a series of tetra-mercuro-methanes, C(HgX)4 (X = C1, Br, I, CN, SCN) has been obtained from reactions of C Hg(OAc) 4 with halides or pseudohalides.126 In turn, the acetoxy compounds derived from Hofmann s base , previously thought to contain a C2 unit, are now believed to be a polymeric oxonium ion formed by condensation of C Hg(OH) 4. Treatment of this compound with acetic or trifluoroacetic acids afforded C Hg(02R) 4 (R = Me, CF3). Extensive spectroscopic studies of these derivatives have been carried out. Further reactions with... 

Silver salts are also utilized to perform nucleophilic additions to disulfide bonds to yield sulfenamides (146). If alkyl halides are treated with stoichiometric AgBF4 in dimethyl sulfoxide solvent (DMSO, solvent), the corresponding aldehydes/ ketones will form in good yields. This reaction is an alternative to the well-known Swern oxidation (147). In addition, silver can drive the formation of dialkylper-oxonium ions from alkyl halides, which then oxidizes sulfoxides or sulfides (148,149). In the presence of AgN03, sulfides can be oxidized into a-chloro sulfoxides by SO2CI2 (Fig. 36) (150). 

Sigma-bond electrophiles can be further subdivided into three types those where E is C, those where E is a heteroatom, and Brpnsted acids, in which E is H. Sigma-bond electrophiles of the first class include alkyl halides, alkyl sulfonates and other pseudohalides, oxonium ions (e.g., Me3COH2), and sul-fonium ions (e.g., Me3S+). Sigma-bond electrophiles of the first class may also... 

The reactions of alcohols with acid halides occur in two steps. In the first step, lone pairs of electrons on the oxygen atoms in alcohol molecules attract proton (H+) from the acid to form the oxonium ion. 

Alcohols react with hydrogen halides by nucleophilic substitution. The OH group is replaced by a halogen water is the byproduct. In the reaction mechanism, the first step involves formation of an oxonium ion by the Lewis acid-base reaction of the hydrogen ion of the hydrogen halide and alcohol oxygen. The rest of the reaction occurs by one of the nucleophilic substitution mechanisms depending on structure of the alcohol. In the Sn2 reaction, the next step involves displacement of the water molecule by halide ion to form the final products. In the Sn1... 

Ethers react with hydrogen halides to form an alkyl halide and an alcohol. The alcohol in turn can react to form a second molecule of alkyl halide and water. Thus in the presence of two mole-equivalents of hydrogen halide, an ether produces two moles of alkyl halide and one of water. The reaction mechanism is analogous to that of alcohols and hydrogen halides. The ether is protonated first to form an oxonium ion. In... 

Episulfonium ions (ESI) are three-membered sulfur-containing cationic species that have been postulated as intermediates in many reactions. Mechanisms of the reaction of l-alkoxy-2-(arylsulfanyl)alkyl halides with different nucleophiles in the presence of a Lewis acid appear to be more complex because of the possibility of forming either ESIs or oxonium ions (IOS) as intermediates (Figure 27). Dudley212 reported equilibrium structures and harmonic vibrational frequencies of two model compounds related to the intermediates of nucleophilic attack on l-alkoxy-2-(arylsulfanyl)alkyl halides... 

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