Nucleophilic counteranion

MeBr is a strong poison only with Et2 All coinitiator. Since Et2 All forms the least nucleophilic counterion, Et2AHXe, it is expected to produce a relatively free carbenium ion, facilitating bromonium ion formation by interaction with MeBr solvent. With more nucleophilic counteranions, like Me3 AlXe or Et2 AlXf (X = Cl, Br), bromonium ion formation is more difficult and poisoning is modest. Evidently, the less stable bromonium ions form only with weakly nucleophilic counterions. MeCl is the weakest poison or may be inert, since chloronium ions are highly unstable. 

Fluoropyridinium salts, in most cases, are freely soluble in polar solvents such as acetonitrile, but insoluble or sparingly soluble in nonpolar organic solvents.38 39 The solubilities of 1-fluoro-pyridinium triflate, tetrafluoroborate, hexafluoroantimonate and perchlorate in dichloro-methane are 0.63, 0.05, 0.09 and 0.035 mg cm 3, respectively, and of 1-fluoropyridinium triflate in tetrahydrofuran is 1.71 mg cm-3. The salts were dissolved in water and decomposed very slowly at room temperature. When a solution of 1-fluoropyridinium triflate in water was allowed to stand for 14 days at room temperature it decomposed completely to give pyridin-2-ol in 66% yield. 1-Fluoropyridinium salts with non-nucleophilic counteranions are generally stable crystals with high melting points. 

The common ion effect does not influence the kinetics of collapse of the ion pairs to dormant covalent species since it is a unimolecular reaction. In this case, however, deactivation of the ion pair can be increased by using less stable and more nucleophilic counteranions. The nucleophi-licity of both pure halides and complex anions with halide ligands increases in the order FOther examples of polymerizations which are well behaved because the equilibrium is favorable due to nucleophilic counteranions include Hl/I2 initiated polymerizations of vinyl ethers and polymerizations of isobutene and styrene using acetate-based initiators in the presence of BC13. 

Lewis acids, should therefore be used in carbocationic polymerizations to prevent /3-proton abstraction. Hexafluoroantimonate is probably the least basic and least nucleophilic counteranion, whereas the conjugate bases of oxy acids are more basic. Anion basicity, which correlates reciprocally with the acid strength, decreases in the order HOCIO3 = HO-S02CF3 > H0S02C1 > HI > HBr > S02(0H)2 > HC1 > H0(0)CCH3. As discussed in Section B.l, triflate, and perchlorate anions are the least basic and least nucleophilic of the oxy-acid anions. 

Controlled/Living Polymerizations with Nucleophilic Counteranions... 

Initiating Systems with Nucleophilic Counteranions The first generation of initiating systems for vinyl ether controlled/living polymerizations consists of a protonic acid (HB, initiator) with a Lewis acid (MtX , activator), [63,69,70]. The reported examples ofHB and MtX ... 

J. Initiating Systems with Nucleophilic Counteranions For isobutene, this group refers almost exclusively to the BCh-based initiating systems without external additives. As listed in Table 3.A, combinations of BCl3 with tertiary esters, ethers (methoxides), or alcohols induce controlled/living polymerization of isobutene in CH3C1 or other solvents at temperatures below -30° C. Scheme 10 illustrates the proposed pathway for the polymerization initiated with the cumyl acetate (12)/BC13 system [35] ... 

It is proposed that in the presence of McsAlOH , a less nucleophilic counteranion than Me3AlCl , the methylation of t-Bu to form neopentane is slower than the t-butylation of the Cg olefin yielding the first generation products. 

The thermolysis of iodonium salts possessing a nucleophilic counteranion can be realised either in solution or in the molten state under rather drastic conditions. It affords the arylated derivative of the anion together with an aryl iodide. 

Several inorganic anions have been arylated by diaryliodonium salts, possessing a non-nucleophilic counteranion. A number of examples are reported in table 5.2. 

B externally added Lewis base X weakly nucleophile counteranions... 

Scheme 3 Stabilization of the carbocationic intermediate by nucleophilic interaction with a suitably nucleophilic counteranion or an externally added Lewis base...

Nucleophilic counteranion with a weak Lewis acid 530... 

This method is for generating carbocation from a dormant carbon-halogen (often chloride) bond, activated with a weak Lewis acid, such as iodine and zinc halides (Scheme 3). Suitably nucleophilic counteranions capture ionic spedes fast and effectively to convert them into the dormant counterparts. This method is suitahle for polymerization of active monomers induding VEs and p-alkoxystyrenes. 

Control of polymerization can be difficult with a nucleophilic counteranion if a strong Lewis add is used. An active catalyst would produce more free ionic spedes, some of which cause side reactions. The addition of an appropriate additive such as a Lewis base and a quaternary ammonium salt into a reaction with a strong Lewis acid shifts the equilibrium to induce living polymerization. 

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