Reactions electrophilic displacement

In complete parallel to the discussion in Section 3.5, the conditions for forming a stable 2c->-lc complex or T-bonded compound are intimately related to those for catalyzing the corresponding electrophilic displacement reaction. Thus, the implications of 3c/2e T-bonding go far beyond the chemistry of boron compounds. 

Some of the most important electrophilic displacement reactions of lignins are listed in Table I with the reactive species indicated. In general, the complexity of the products from these reactions makes structural evaluation extremely difficult. Therefore, an approach using model compounds related to lignin appears desirable. 

Bromosilatrane (50) can be involved in electrophilic displacement reactions with silver compounds26,27 to give products 21, 37, 61 and 62 (equations 77-80). 

Studies have also been carried out in systems containing excess BF3 (17,18). The results (18) show that when the base is dimethyl ether, anisole, tetrahydrofuran, or pyridine, the exchange of BF3 is rapid and probably proceeds through an electrophilic displacement reaction in which the excess BF3 attacks the complex. These reactions all have activation energies of less than 10 kcal/mole, eliminating the possibility of a dissociation process. The data available, however, do not allow a complete evaluation of the reaction mechanism. Studies carried out on BF3-methanol complexes by 19F NMR (17) indicate displacement reactions having an activation energy of 5.3 kcal/mole. 

Recently, more accurate determinations of isomer distributions and a /3 reactivity ratios in electrophilic substitutions of benzofuran and benzothiophene have been determined using the gas chromatographic technique.77 The data are summarized in Table XVI which also includes a /3 reactivity ratios referring to electrophilic displacement reactions such as protodesilylation and to side-chain reactions related to electrophilic substitution, such as the solvolysis of 1-arylethyl-acetates239 and 1-arylethyl- -nitrobenzoates.240... 

Fig. 30. Potential surfaces (energy versus reaction coordinate) for typical gas-phase, ion-molecule reactions (a) proton-transfer reactions, (b) electrophilic displacement reactions.

The 3-substituted metal chelates of acetylacetone undergo some unusual electrophilic displacement reactions. The acetyl groups in the acetylated... 

This view is supported by the fact that if diazominobenzene is dissolved in dimethylaniline in the presence of the hydrochloride of the latter, the main product is p-dimethylamino-azobenzene, CgHjN=NCgHgN(CH3)2 this is because dimethylaniline couples in the nucleus more readily than does aniline. The reaction is an electrophilic displacement of hydrogen by the diazonium ioii ... 

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

Aromatic compounds may be chlorinated with chlorine in the presence of a catalyst such as iron, ferric chloride, or other Lewis acids. The halogenation reaction involves electrophilic displacement of the aromatic hydrogen by halogen. Introduction of a second chlorine atom into the monochloro aromatic stmcture leads to ortho and para substitution. The presence of a Lewis acid favors polarization of the chlorine molecule, thereby increasing its electrophilic character. Because the polarization does not lead to complete ionization, the reaction should be represented as shown in equation 26. 

Acyl-pyrroles, -furans and -thiophenes in general have a similar pattern of reactivity to benzenoid ketones. Acyl groups in 2,5-disubstituted derivatives are sometimes displaced during the course of electrophilic substitution reactions. iV-Alkyl-2-acylpyrroles are converted by strong anhydrous acid to A-alkyl-3-acylpyrroles. Similar treatment of N-unsubstituted 2- or 3-acyIpyrroles yields an equilibrium mixture of 2- and 3-acylpyrroles pyrrolecarbaldehydes also afford isomeric mixtures 81JOC839). The probable mechanism of these rearrangements is shown in Scheme 65. A similar mechanism has been proposed for the isomerization of acetylindoles. 

Kinetic studies have been carried out on the displacement reactions of various chloroazanaphthalenes with ethoxide ions and piperi-dine. - 2-Chloroquinoxaline is even more reactive than 2-chloro-quinazoline, thus demonstrating the powerfully electrophilic nature of the -carbon atoms in the quinoxaline nucleus. The ease of displacement of a-chlorine in the quinoxaline series is of preparative value thus, 2-alkoxy-, 2-amino-, - 2-raethylamino-, 2-dimethyl-amino-,2-benzylamino-, 2-mercapto-quinoxalines are all readily prepared from 2-chloroquinoxaline. The anions derived from substituted acetonitriles have also been used to displace chloride ion from 2-chloroquinoxaline, ... 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

In conclusion, it should be mentioned that though the great majority of aromatic electrophilic substitution reactions involve displacement of hydrogen, other atoms or groups can be involved. Thus we have already seen the displacement of S03H in the reversal of sulphonation (p. 140), of alkyl in dealkylation (p. 143), and a further, less common, displacement is that of SiR3 in protodesilylation (cf. also p. 161) ... 

The carbonium ion rearrangement is essentially an internal displacement reaction in which a carbonium ion becomes bonded to some other portion of the same molecule. Certain base-catalyzed rearrangements have been discussed in the section on carbonium ion rearrangements because they have the distinguishing characteristic of being internal electrophilic displacements by an atom bearing at least a partial positive charge. 

Alkylaryldichlorogermanes produce substituted cyclogermanes. In the case shown in reaction 9, three of the four possible configurations of the tetragermane product 29 are formed, but none is all-c/s. It is possible to carry out an electrophilic displacement of the phenyl groups to form the 1,2,3,4-tetrachloro derivative 30, which is aW-lrans, the configuration of 29 notwithstanding120. 

In Lambert s approach, the triarylstannylium ion is generated by the reaction of an electrophile with an allyltri-arylstannane. The bulky aryl groups sterically protect the tin center in the stannylium ion from attack by nucleophiles, yet the allyl ligand permits unhindered conjugate electrophilic displacement of the tin (Equation (42)).145... 

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