Onium ions synthesis

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

The remaining three chapters (Chapters 20-22) concentrate fully on onium ion chemistry. In Chapter 20, H.-J. Frohn and V. V. Bardin describe synthesis and multi-nuclear NMR studies of organoxenonium salts. Chapter 21 by R. S. Brown et al. focuses on the synthesis of chiral... 

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... 

We focused on two types of organic cations, W-acyliminium ions and alkoxycarbenium ions,2 because they are very popular in organic synthesis. A number of reactions involving such onium ions have been developed and widely utilized for the construction of organic molecules. 

Because propagation via onium ions competes at lower acid concentrations, the activated monomer mechanism is limited to the synthesis of lower molecular weight polyoxiranes. 

Synthesis and Characterization of Bissilylated Onium Ions of Group 15 Elements... 

As an extension of the amidoseleniation processes discussed previously, imidates (the third Ritter intermediate) may be utilized as precursors for the synthesis of cyclic amides (Scheme 67) and lactams (equation 52). Such combination of imidate and onium ion processes has much potential for further extension of Ritter-type chemistry. 

Figure 4. Straightforward synthesis of onium ion-tagged prolines.

NTs [17,18]. Simple copper(I) and copper(II) salts were found to be effective catalysts for the preparation of a wide range of racemic aziridines (Scheme 7). More recently, rhodium-based catalysts have been applied with success to the reaction of PhI=NNs with alkenes [19]. Along with Sharpless s recently-disclosed brom-onium ion-catalyzed reaction [3], the copper-catalyzed aziridination of olefins with PhI=NTs is probably the most general catalytic method devised thus far for the direct synthesis of racemic aziridines from alkenes. 

Onium Ions. Trialkyloxonium ions (R O A ) became the conventional initiators for the cationic ring-opening polymerization of all classes of heterocycles (cyclic acetals, ethers, sulfides, lactones, phosphates, and amines). They are prepared by two methods developed by Meerwin (38) and Olah (39). Another more general and convenient synthesis method was recently developed by Penczek et aL (40) ... 

No true living cationic vinyl polymers are known at present, although the papers of Skorokhodov on 1,2-dimethoxyethylene 1011 and of Higashimura on the I2-initiated polymerization of vinyl ethers, p-methoxystyrene (Sect. 15.3.1.1.3) and of N-vinyl carbazole 102) approach living features. It is possible, however, to prepare a vinyl polymer with an end-group that is inactive towards its own monomer but is able to convert a heterocyclic monomer into the onium ion, in this way starting the polymerization of a second monomer. The application of this approach for the synthesis of block copolymers is described below. 

The model illustrated in Figure 2 summarizes the overall mechanism for formation of epoxy polymer - clay nanocomposites. Upon solvation of the organoclay by the epoxide monomers, the gallery cations reorient from their initial monolayer, lateral bilayer, or inclined paraffin structure to a perpendicular orientation with epoxy molecules inserted between the onium ions. A related reorientation of alkylammonium ions has been observed previously for e-caprolactam intercalated clay intermediates formed in the synthesis of Nylon-6 -exfoliated clay nanocomposites (9). Thus, the ability of the onium ion chains to reorient into a vertical position in order to optimize solvation interactions with the monomer may be a general prerequisite for pre-loading the clay galleries with sufficient monomer to achieve layer exfoliation upon intragallery polymerization. 

The (di-f-butyl)silyl group has also proved effective for the conversion of cw-2,5-disubstituted THF derivatives to the corresponding fra s-2,5-disubstituted rings. Activation of the hydroxyl group followed by a 1,2-hydride shift generates the ox-onium ion at the C2 position. The di-f-butyloxysilane then delivers the hydride stereospeciflcally to form the traws-disubstituted product (eq 4). This motif is found in many natural products and a similar approach has been successfully applied to the synthesis of (+)-sylvaticin. ... 

The ionic radius of the onium ion is the principal feature of active site structure that affects activity. Alkyltrimethylammonium ions are more hydrated than alkyltributylammonium and alkyltributyl-phosphonium ions. With alkyl groups as large as n-butyl, it makes little difference whether the central atom is nitrogen or phosphorus. (The central atom does influence the stability, the ease of synthesis, and the cost of the catalyst however.)... 

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