Counterion, reaction with

Displacement of a tertiary amine from a quaternary (eq. lb) iavolves the attack of a nucleophile on the a-carbon of a quaternary and usually competes with the Hoffman elimination (173). The counterion greatiy iafluences the course of this reaction. For example, the reaction of propyltrimethylammonium ion with hydroxide ion yields 19% methanol and 81% propylene, whereas the reaction with phenoxide ion yields 65% methoxybenzene and 15% propylene (174). 

A novel route to 2-fluoropyridines involved the base-induced decomposition of substituted N-fluoropyridinium salts. Abstraction of the 2-H produces a singlet carbene (11) that removes F from a counterion. This is in contrast to the reaction with C nucleophiles, which are fluorinated, and is attributed to the high stability of C—F compared to O—F and N—F (89JOC1726). 

The reaction is initiated by attack of a nucleophile (Ye), usually the counterion associated with the Cu ion, at one of the CN groups of the phthalonitrile, which is activated by its coordination with the Cu ion. Subsequently a cycliza-tion reaction to an isoindoline derivative takes place. These steps are three times repeated by a series of similar reactions finally resulting in a cyclization to a CuPc ring intermediate, whose formation is facilitated by the coordinating role of the copper ion. With a copper (II) salt as reactant it is suggested that Y is eliminated from the intermediate, e.g. the Cl ion in the case of CuCl2. The monochloro de-rivate of CuPc is than formed by an electrophilic attack of Cl on the CuPc initially formed. 

Experimental studies of the oxidative cleavage of cinnamic acid by acidic permanganate [35] resulted in secondary kinetic isotope effects, kn/kp, of 0.77 (a) and 0.75 (P), while another paper from the same group on the same reaction with quaternary ammonium permanganates [36] reported very different isotope effects of 1.0 (a) and 0.91 - 0.94 (P) depending on the counterion. Different mechanisms were discussed in the literature [37, 38] to explain the variety of experimental results available, but the mechanistic issues were unresolved. The reported activation energy for the oxidation of... 

It was previously mentioned was that a large number of minor copolymers of PET have been developed over the past 50 years, with the intent of modifying textile fiber properties and processability [2, 3], Of broader interest is that some of these textile modifications, such as PET copolymers with metal salts of 5-sulfoisophthalic acid (SIPA), have their own rich chemistries when the extent of polymer modification is increased beyond textile levels. An example of such a modification is that changing the counterions associated with SIPA can significantly effect the kinetics of polyester transesterification reactions (the... 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

When trifluoroacetaldehyde ethyl hemiacetal [F3CCH(OH)OEt] is treated with enamines in hexane at room temperature, it provides a source of the aldehyde under mild conditions. Subsequent reaction with the enamine can be used to prepare -hydroxy-/ -trifluoromethyl ketones, F3CCH(OH)CH2COR. The enamine plays successive roles as base, ammonium counterion, and then carbon nucleophile as the sequence proceeds. 

Ionic polymerizations are characterized by a wide variety of modes of initiation and termination. Unlike radical polymerization, termination in ionic polymerization never involves the bimolecular reaction between two propagating polymer chains of like charge. Termination of a propagating chain occurs by its reaction with the counterion, solvent, or other species present in the reaction system. 

The major approach to extending the lifetime of propagating species involves reversible conversion of the active centers to dormant species such as covalent esters or halides by using initiation systems with Lewis acids that supply an appropriate nucleophilic counterion. The equilibrium betweem dormant covalent species and active ion pairs and free ions is driven further toward the dormant species by the common ion effect—by adding a salt that supplies the same counterion as supplied by the Lewis acid. Free ions are absent in most systems most of the species present are dormant covalent species with much smaller amounts of active ion pairs. Further, the components of the reaction system are chosen so that there is a dynamic fast equilibrium between active and dormant species, as the rates of deactivation and activation are faster than the propagation and transfer rates. The overall result is a slower but more controlled reaction with the important features of living polymerization (Sec. 3-15). 

Syndioselective polymerizations of propene are somewhat less regioselective than the isoselective reactions, with the typical highly syndiotactic polymer showing a few percent of the monomer units in head-to-head placement [Doi, 1979a,b Doi et al., 1984a,b, 1985 Zambelli et al., 1974, 1987]. The mode of insertion is secondary, contrary to what is expected for a carbanion propagating center. Apparently, steric requirements imposed by the counterion derived from the initiator force propagation to proceed by secondary insertion. 

An alternate and equally concise approach to the same compound starts with the reaction of aziridino-benzophenone (12-1) with methyl iodide. The outcome of this reaction can be rationalized by assuming the initial formation of quaternary salt (12-2). An attack on the strained ring by the iodide counterion will open the ring to afford the A-iodoethyl derivative (12-3). This then affords medazepam (11-4) on reaction with HMTA [14]. 

---