Nucleophiles counterions

Alkylation, acylation, etc. at the heteroatom lead to onium salts. In small systems these are difficult to isolate, and very weakly nucleophilic counterions must be used, such as... 

Dale improved this procedure by conducting the cyclooligomerization in the presence of certain templating cations paired with non-nucleophilic counterions (e.g.,... 

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

The nucleophilic addition of Grignard reagents to a-epoxy ketones 44 proceeds with remarkably high diastereoselectivity70. The chelation-controlled reaction products are obtained in ratios >99 1 when tetrahydrofuran or tetrahydrofuran/hexamethylphosphoric triamide is used as reaction solvent. The increased diastereoselectivity in the presence of hexamethylphos-phoric triamide is unusual as it is known from addition reactions to a-alkoxy aldehydes that co-solvents with chelating ability compete with the substrate for the nucleophile counterion, thus reducing the proportion of the chelation-controlled reaction product (vide infra). 

Thus, in the systems under consideration, MeX may form haionium ions with growing carbenium ions. Since the stability of haionium ions depends on the polarizability of ttie halogen38 —I > —Br > —Cl, Mel should form the most stable haionium ions, le., have most pronounced poisoning effect, followed by MeBr and MeCl. Indeed, Mel may even compete for the carbocation with highly nucleophilic counterions. 

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. 

Several trends emerge in these data (1) The reductive elimination of bromine is 6-13kJmol more facile than reductive elimination of chlorine in similar structures, which is consistent with weaker chalcogen-bromine bonds relative to chalcogen-chlorine bonds.(2) The reductive elimination of chlorine is accelerated by the presence of a chloride counterion as opposed to a less nucleophilic counterion such as hexafluorophosphate. (3) The rate of reductive elimination is accelerated by the presence of a more polar solvent (acetonitrile) relative to tetrachloroethane, which is consistent with development of charge in the rate-determining step. These observations suggest mechanisms for oxidative... 

Lewis acids were also screened for the ROP of lactones [65]. The polymerization takes place according to a cationic mechanism provided that the counterion is not too nucleophilic. Conversely, when Lewis acids with a nucleophilic counterion are used, several examples are reported where the polymerization takes place according to the usual coordination-insertion mechanism (Fig. 12). This coordination-insertion mechanism was indeed reported for the ROP initiated by ZnCl2 [66], TiCU, and AICI3 [67]. 

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). 

In addition to the choice of Lewis acid, added common ion salt, and temperature, the fast equilibrium between active and dormant species can be fostered by including additional nucleophiles (separate from the nucleophilic counterion) in the reaction system and by variations in solvent polarity. Nucleophiles act by further driving of the dynamic equilibrium toward the covalent species and/or decreasing the reactivity of ion pairs. Nucleophilic counterions and added nucleophiles work best in nonpolar solvents such as toluene and hexane. Their action in polar solvents is weaker because the polar solvents interact with the nucleophiles and nucleophilic counterions, as well as the ion pairs. Polar solvents such as methylene... 

The decrease in the activation energy difference in the series EtAlCl2 to PF5 might be due to differences in the nucleophilicity of the counterions formed by the various coinitiators. The less nucleophilic counterion forms a looser ion pair with the cationic chain end, which in turn reduces the steric barrier to propagation without affecting isomerization. This is reflected in a decrease in the activation energy difference between the two processes and results in a decrease in the change of I with temperature. 

The transient nature of carbocations arises from their extreme reactivity with nucleophiles. The use of low-nucleophilicity counterions, particularly tetrafluorobo-rates (B I, ), enabled Meerwein in the 1940s to prepare a series of oxonium and carboxonium ion salts, such as R30+BF4 and HC(OR)2+BF4, respectively.13 These Meerwein salts are effective alkylating agents, and they transfer alkyl cations in SN2-type reactions. However, simple alkyl cation salts (R 1 BF4 ) were not obtained in Meerwein s studies. The first acetyl tetrafluoroborate—that is, acetylium tetrafluor-oborate—was obtained by Seel14 in 1943 by reacting acetyl fluoride with boron trifluoride at low temperature [Eq. (3.1)]. 

Acids must be fairly strong for this job and must have a non-nucleophilic counterion to avoid... 

As discussed in Chapter 2, interception of cation 2.31 with a nucleophilic counterion such as acetate produces the 2,5-addition product 2.32. Tetrafluoroborate is a non-nucleophilic counterion and hence the only pathway available to 2.31 is loss of a proton to give nitrofuran 2.33 directly. 

Ethers are stable molecules, which do not react with nucleophiles they must be stable because THE and Et20 are used as solvents, But we can make them react by using an acid with a nucleophilic counterion (HBr or HI, for example) and then nucleophilic attack will occur preferentially at the more susceptible carbon atom. Aryl alkyl ethers cleave only on the alkyl side. We shall explain in Chapter 23 why nucleophilic attack does not occur on a benzene ring. 

This reaction is an equilibrium and it may be necessary in making sulfonium salts from less reactive sulfides (sterically hindered ones for example) to use more powerful alkylating agents with non-nucleophilic counterions, for example, Me30+ BF4, trimethyloxonium fluoroborate (also known as Meerwein s salt). The sulfur atom captures a methyl group from 0+, but the reverse does not happen and the BF4 anion is not a nucleophile. 

When the carbocations are generated by Laser flash photolysis, the ion pair collapse with the nucleophilic counterion Cl- is so fast [136] that the decay cannot be followed with the instrumentation used for these experiments, i.e., only those carbocations which manage to escape from the [Aryl2CH + Cl ] ion pair can be observed. Consequently, all rate constants determined for the Laser photolytically produced carbocations refer to the reactions of the nonpaired entities. 

Alkyl enol ethers polymerize under these conditions, and their reactivity, as that of other strong nucleophiles, has been determined with the LASER flash method (Table 7). Because this method of carbocation generation produces the nucleophilic counterions CL, eventual polymeriza-... 

These salt effects are schematically depicted in Scheme 8. As we will discuss later more in detail (Sections Vl.B.3 and VII.E.3), mechanistically, salts may act in two different ways. In polar solvents they will suppress the free ions and considerably reduce their lifetime. This often converts bimodal MWD to monomodal MWD and provides controlled polymers. However, in polymerization of vinyl ethers initiated by strong Lewis acids such as SnCl4, where only ion pairs are present after addition of a few percent of salts or in nonpolar toluene, control is still very poor (Fig. 17B). Controlled polymers can be obtained only after addition of a more than equimolar amount of tetra-n-butylammonium halides. This implies that the salts change the weakly nucleophilic counterion SnCIs-to the more nucleophilic SnCl62 , which faster converts growing carbo-cations to covalent species. Another effect of added salts is related to... 

It should be stressed that growing species with diffused charge distributed along —N—C—0— group are relatively weak electrophiles. Thus, as discussed in Section II.B.6.C, ionic active species may exist when coupled with relatively nucleophilic counterions such as Br or I". The collapse of ion pairs into covalent alkyl halide end groups does proceed in the system, but these covalent species still react (although slower than... 

As illustrated earlier, the inherent problem in controlled cationic polymerizations is the instability of the macrocarbocations. However, living polymerizations can be realized by stabilizing the growing carbenium ions with a suitable nucleophilic counterion (Be), 9-19, or an added Lewis base (X) containing a weakly nucleu-cleophilic counteranion (Be), 9-20 [7]. That is ... 

Both methods spread the charge on the growing macrocation and render the /9-proton less likely to transfer to monomer (as in reaction 9-41). The first method is typified by initiation with HI/I2 in which the nucleophilic counterion is the Ie//2 anion. The second primarily involves combinations of a cation-generator, like a tertiary alkyl halide, with a Lewis acid, such as EtAlCl2- A number of initiator combinations of the latter type have been reported for living cationic polymerization of isobutene [S). 

Double bonds can be hydrated by treatment with water and an acid catalyst. The most common catalyst is sulfuric acid, but other acids that have relatively non-nucleophilic counterions, such as nitric or perchloric can also be used. The mechanism is electrophilic and begins with attack of the 7i-bond on an acidic proton (see p. 1005). The resulting carbocation is then attacked by negative species, such as HS04 (or similar counterion in the case of other acids), to give the initial product 32, which can be isolated in some cases, but under the conditions of the... 

---