Counter nucleophilicity

Compared with the simple iminium ions, the enhanced reactivity of acyliminium salts opens up a greatly enlarged spectrum of counter-nucleophiles, which now includes alkenes and aromatic compounds possessing no donor substituents (see Section 4.2.2.2.2). 

Since the cyanide anion is an ambident nucleophile, isonitriles R—NC may be obtained as by-products. The reaction pathway to either nitrile or isonitrile can be controlled by proper choice of the counter cation for the cyanide anion. 

When concentrated sulphuric acid alone was used as the initiator, the polymerization was found to follow a different path. It is well known that Bronsted acids can function as cationic/pseudocationic initiators in the oligomerization of olifins [174]. If the counter ion has a higher nucleophilicity as it forms cation-conjugate pairs, which collapse rapidly, polymerization will not take place. As the counter ion in the case of sulphuric acid is not very strong compared to the cation, oligomerization can take place, but may not be to a very high molecular weight. This, however, depends on the nature of the... 

Conversion of Acid Anhydrides into Amides Acetic anhydride is also commonly used to prepare iV-substituted acetamides from amines. For example, acetaminophen, a drug used in over-the-counter analgesics such as Tylenol, is prepared by reaction of p-hydroxyaniline with acetic anhydride. Note that the more nucleophilic -NH2 group reacts rather than the less nucleophilic -OH group. 

The product selectivity arises from the reaction of this intermediate with the various nucleophiles present in the reaction medium. Therefore, the structure of this intermediate and its close environment (counter-ion, solvent molecules, etc.) determines the product selectivity. It is important to note that the relevant data concern the intermediate as it is formed during the reaction and not prepared under extreme conditions where it is stable, such as those used to obtain the nmr spectra of these ions, for example (ref. 5). Since the life-time of this ionic species in the reaction medium is very short, in the 10 9 - 10-10 s range (ref. 6), there is no way to observe it directly. Consequently, the available data, useful for understanding... 

Both the mode and site of nucleophilic attack of pyran-2-ones by alkoxides is influenced by the counter ion such that a variety of products can be obtained. In particular, chelation effects play a significant role <96JCS(P1)2715>. 

At the second stage of chlorine substitution in the tetramers there is a greater statistical probability for the incoming nucleophile to attack the phosphorus adjacent to =P(C1)(NHR), viz. P4 or P8, rather than the remote phosphorus, viz. P6 (Fig. 9). However, this statistical effect is countered by the electron releasing effect of the substituent already present on P2, which tends to deactivate P2 as well as P4 and P8 towards further nucleophilic substitution. It is observed that reactive amines such as dimethylamine (94) or ethylamine (95) react with N4P4C18 and... 

In fact, the analogy between the mechanisms of heterolytic nucleophilic substitutions and electrophilic bromine additions, shown by the similarity of kinetic substituent and solvent effects (Ruasse and Motallebi, 1991), tends to support Brown s conclusion. If cationic intermediates are formed reversibly in solvolysis, analogous bromocations obtained from bromine and an ethylenic compound could also be formed reversibly. Nevertheless, return is a priori less favourable in bromination than in solvolysis because of the charge distribution in the bromocations. Return in bromination implies that the counter-ion, a bromide ion in protic solvents, attacks the bromine atom of the bromonium ion rather than a carbon atom (see [27]). Now, it is known (Galland et al, 1990) that the charge on this bromine atom is very small in bridged intermediates and obviously nil in /f-bromocarbocations [28]. 

In halogenated solvents the results indicate that return can occur, even for the uncongested stilbenes. Unfortunately, its importance, as measured by the k i/kN ratio (Fig. 10), cannot be estimated. It must be noted that Bellucci s experiments prove only that return is possible, but do not demonstrate conclusively that it occurs in bromination, since reversibility is controlled by the relative energy levels of TS and TSN which can be affected by the reaction conditions. Now, these conditions are not the same for nucleophilic substitution on bromohydrins and for bromine addition in particular, the counter-ions, Br and Br3 respectively, can alter the lifetime of the intermediate and thus control its partitioning between return and nucleophilic attack. 

The cation pool method is based on the irreversible oxidative generation of organic cations. In the first step, the cation precursor is oxidized via an electrochemical method. An organic cation thus generated is accumulated in the solution in the absence of a nucleophile that we want to introduce onto the cationic carbon. Counter anions which are normally considered to be very weak nucleophiles are used to avoid the nucleophilic attack on the cationic center. In order to avoid thermal decomposition of the cation, electrolysis should be carried out at low temperatures such as -78 °C. After electrolysis is complete, the nucleophile is then added to obtain the desired product. The use of a carbon nucleophile results the direct carbon-carbon bond formation. 

FIGURE 2.16 Coupling reagents made up of an additive linked to a charged atom bearing dialkylamino substituents and a non-nucleophilic counter ion. 

The protonation of the triplet jtjt state of 3-bromonitrobenzene is shown to be responsible for the acid-catalysed promotion of halogen exchange which follows a S y23Ar mechanism26 (equation 23). Cationic micellar effects on the nucleophilic aromatic substitution of nitroaryl ethers by bromide and hydroxide ions have also been studied27. The quantum efficiency is dependent on the chain length of the micelle. The involvement of counter ion exchanges at the surface of ionic micelles is proposed to influence the composition of the Stem-layer. 

The mechanism is illustrated in Figure 13.9. Nucleophilic attack leads to rotation and formation of the 7t-alkene complex. The left-hand structure will undergo a counter clockwise rotation and experience large steric hindrance. The clockwise rotation on the right leads to a much more favourable situation. 

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