Onium ions preparation

Similarly, ketones are prepared via acyl carb-onium ions ... 

As discussed in Chapter 1, many electrophiles are stable in solvents and the presence of substances of moderate Lewis basicity. Even relatively strong electrophiles can be generated and handled under appropriate conditions.1 A wide variety of electrophilic carbocationic and onium ion salts have been prepared and characterized by spectroscopic methods, X-ray crystallography, kinetic studies, and other techniques. Several types of these electrophilic salts are even available commercially (such as varied onium, iminium, and carbocationic salts, etc.). Electrophiles are generally generated by the reactions of suitable precursors with Brpnsted or Lewis acids. They can be sometimes isolated, but for synthetic conversions they are more often generated in situ and reacted directly with nucleophilic reagents. 

Halonium ions are an important class of onium ions.43 The dialkylchloro, bromo, and iodohalonium ions can be prepared and even isolated as stable salts (i.e., 46), as shown by Olah et al. by reacting an excess of haloalkane with strong Lewis acid halides in solvents of low nucleophilic-ity (eq 14). In superacid solution, dialkylhalonium ions show enhanced alkylating reactivity.44 It is considered that this enhanced reactivity is due to further protolytic (or electrophilic) activation involving the non-bonded... 

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). 

The cations are prepared from 2,5-disilaheptanes, 3, by hydride transfer reactions. The transient silylium ion 4 undergoes an intramolecular reaction to the more stable cyclic onium ion 2. The precursor silanes, 3, are synthesized by salt metathesis reaction from compounds of the type Ph2ELi, where E is the relevant Group 15 element, and the silylchloride 5. The salts 2 [B(C6Fs)4]" are isolated after washing with pentane as white to yellow microcrystalline powders in nearly quantitative yield. Solutions of the cations 2a-c in aromatic solvents are stable at room temperature. The stibonium ion 2d decomposes in solution slowly, yielding unidentified products. 

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. 

Crum Brown and Fraser (6) in 1868 attributed curariform activity to the onium ion. Even at that early date, they suggested therapeutic possibilities of synthetic quaternary salts which could be prepared of constant purity and effectiveness. Boehm (7, 8) first showed that the active alkaloids of curare preparations are quaternary ammonium compounds. Since these discoveries, a great many quaternary compounds have been investigated, and even the simplest quaternary ammonium salts, as well as sul-fonium, phosphonium, arsonium, and stibonium salts, exhibit curare activity (9). Quaternary ammonium compounds widely varied in character have been found to exhibit curare activity. Hunt and Renshaw and collaborators (10, 11) have published a series of papers on onium compounds and their effects on the autonomic nervous system. In addition to curare action, they were shown to exhibit muscarinic and nicotinic actions in varying degrees. 

Meitens A, Lammeitama K, Arvanaghi M, Olah GA (1983) Onium ions. 26. Aminodiazonium ions preparation, H, C, and N NMR structural studies, and electrophilic amination of aromtics. J Am Chem Soc 105 5657-5660... 

Preparation of Organo Hectorites. Hectorite SHCa-1 was obtained from the Qay Mineral Society Repository at the University of Missouri, Columbia. The mineral was purified by sedimentation and subsequent treatment with NaHSO solution to remove carbonates. A 1.0 wt. % aqueous suspension of sodium hectorite (100 mL, 0.73 meq) was mixed with a 0.073 M solution of the chloride or bromide salt of the desired onium ion (20 mL, 1.46 mmol) dissolved in either water or ethanol. The products were then washed free of excess onium salt with ethanol as determined by a bromide or chloride specific electrode. The clay was then resuspended in de-ionized water, centrifuged and air dried at room temperature. 

Scheme 31 Preparation of onium ions from solvent-stabilized silylium ions [87-91]...

Preparation of Carbocations, Onium Ions, and Their Salts. SbFs is a preferred medium for the preparation of carbocations and onium ions. In fact, the first observation of stable carbocations was achieved in this medium. - By dissolving /-butyl fluoride in an excess of SbFs, the /-butyl cation was obtained (eq 26). Subsequently, many alkyl cations have been obtained in SbFs (either neat or diluted with SO2, SO2CIF, or SO2F2). The 2-norbomyl cation, one of the most controversial ions in the history of physical organic chemistry, was prepared from exo-2-fluoronorbomane in SbFs/S02 (or SO2CIF) solution (eq 27). Bridgehead cations such as 1-adamantyl, 1-trishomobarrelyl, and 1-trishomobullvalyl cations have also been similarly prepared with the use of SbFs. ... 

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