Catalysis, cation electrophilic

Adjustable coordination properties Ionic liquids have the potential to be polar yet weaMy coordinating toward transition metal complexes they may enhance reaction rates involving cationic electrophilic intermediates When they contain coordinating anions (e.g., Cl ) they may stabilize anionic transition metal intermediates (e.g., Pd Heck couphng catalysis)... 

The fact that ionic liquids with weakly coordinating anions can combine, in a unique manner, relatively high polarity with low nucleophilicity allows biphasic catalysis with highly electrophilic, cationic Ni-complexes to be carried out for the first time [26]. 

Why are transition metals well suited for catalysis of this process Certainly the electrophilicity of cationic metal centers is important, as is the relative weakness of transition-metal-carbon bonds. However, similar electrophilicities and bond strengths could be found among main-group cations as well. A key to the effectiveness of Ti catalysts is the presence of two metal-based acceptor orbitals. In effect, two such orbitals are needed to choreograph the reversal of net charge flow at the two alkene carbons as the intermediate alkene complex moves through the transition state toward the final product. 

In earlier days it was fairly common to suggest that sulfenium ions, RS+, were involved as intermediates in a number of these substitutions, particularly those in which sulfenyl halides RSX reacted with very weak nucleophiles, or those where electrophilic catalysis of the substitution was observed (Parker and Kharasch, 1959). However, it has since become evident (Owsley and Helmkamp, 1967 Helmkamp et al., 1968 Capozzi et al., 1975) that sulfenium ions are almost impossible to generate as intermediates. For example, Capozzi et al., (1975) showed that although treatment of a sulfenyl chloride RSC1 with the powerful Lewis acid antimony pentafluoride led to the complete conversion of the sulfenyl chloride to a cation, what was formed was, not the sulfenium ion RS+, but rather the cation [59] in reaction (172). These results, and others... 

Progress was made by the discovery of electrophilic catalysis by acyl cations in carbonyl reactions (91ZOR1588). This catalysis type consists in the conversion of aldehydes or ketones into highly active acyloxycarbo-cations 51 by the addition of acyl cations regardless of the origin of the latter. In contrast to related hydroxy- and alkoxycarbocations (R R —OR ... 

This is a further example of a carbonyl-electrophile complex, and equivalent to the conjugate acid, so that the subsequent nucleophilic addition reaction parallels that in hemiacetal formation. Loss of the leaving group occurs first in an SNl-like process with the cation stabilized by the neighbouring oxygen an SN2-like process would be inhibited sterically. It is also possible to rationalize why base catalysis does not work. Base would simply remove a proton from the hydroxyl to initiate hemiacetal decomposition back to the aldehyde - what is needed is to transform the hydroxyl into a leaving group (see Section 6.1.4), hence the requirement for protonation. 

Electrophilic trifluoromethylation is still of minor importance in synthetic applications. The limited efficiency and the cost of the reagents able to transfer a CF3 cation are important obstacles for the development of this approach. However, CF3-S" -type reagents can react with activated enolates under Lewis acid catalysis. A recent and promising result shows that, when the reaction is performed under UV irradiation, yields significantly increase. This can lead to synthetic applications, as exemplified by the recent preparation of 7-CF3 steroids (Figure 2.37). ... 

This does not occur in the case of catalyst and reactants here described. With Bronsted-type catalysis, the reaction between the benzoyl cation, Ph-C" =0, and the hydroxy group in phenol is quicker than the electrophilic substitution in the ring. This hypothesis has been also confirmed by running the reaction between anisole and benzoic acid in this case the prevailing products were (4-methoxy)phenylmethanone (the product of para-C-benzoylation) and methylbenzoate (obtained by esterification between anisole and benzoic acid, with the co-production of phenol), with minor amounts of phenylbenzoate, phenol, 2-methylphenol and 4-methylphenol. Therefore, when the 0 atom is not available for the esterification due to the presence of the substituent, the direct C-acylation becomes the more favored reaction. 

Solid-liquid solvent-free phase-transfer catalysis (PTC) is specific for anionic reactions including base-catalysed isomerisation7. Usually, a catalyst (typically a tetraalky-lammonium salt or a cationic complexing agent) is added to an equimolar mixture of an electrophile and a nucleophile, one of which serves as both a reactant and the... 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

With both the Fu and the Denmark catalysts it can be assumed that catalysis is effected by formation of a highly electrophilic silicon cation D from tetrachlorosi-lane and the nucleophilic catalyst C, i.e. by attack of the pyridine N-oxide or of the phosphoramide O-atom on silicon, followed by ionization (Scheme 13.38). The latter cation can then activate the epoxide toward nucleophilic attack by the chloride ion. Exchange of the product silane for another molecule of tetrachlorosilane completes the catalytic cycle [75],... 

Another advantage of this approach is that we can now use electrophilic substitution on the pyrrole to add the rest of the molecule. So the secondary benzylic alcohol 106 might well cyclise to 105 with Lewis acid catalysis as the cation will be reasonably stable and the reaction is intramolecular. But the Friedel-Crafts alkylation to give 107 will not succeed as the cation would be primary. 

The Pd(0)-catalyzed allylation of 96 with acrolein dimethyl acetal gives exclusively compound 104. The 7j3-allylpalladium cationic complex (4, R = OMe) is attacked only at the center bearing the substituent MeO (80SC147), thus emphasizing the importance not only of steric effects in the electrophile but also of the electronic effects in the Tsuji-Trost reaction (92T1695). Indole 96 has been also allylated with epoxide 105 under Pd(0) catalysis by Trost and Molander (81JA5969). The intermediate cationic complex is attacked at the exocyclic position, 106 being formed, as shown in Scheme 22. 

Indole (2) undergoes electrophilic substitution preferentially at the b(C3)-position whereas pyrrole (1) reacts predominantly at the a(C2)-position [15]. The positional selectivity in these five-membered ring systems is well explained by the stability of the Wheland intermediates for electrophilic substitution. The intermediate cations from 3 (for indole, 2) and a (for pyrrole, 1) are the more stabilized. Pyrrole compounds can also participate in cycloaddition (Diels-Alder) reactions under certain conditions, such as Lewis acid catalysis, heating, or high pressure [15]. However, calculations of the frontier electron population for indole and pyrrole show that the HOMO of indole exhibits high electron density at the C3 while the HOMO of pyrrole is high at the C2 position [25-28] (Scheme 3). 

Kinetic analysis of this system enables the data of Table 20 to be derived. The ratios k 3/k2 give a measure of the relative catalytic effectiveness of the cations since k2 is independent of the electrophile. The resulting order, Li+ > K+ > n-Bu4N+, is similar to that reported for the electrophile-assisted ionizations of p-methoxy-neophyl tosylate and the p-nitrobenzoate of spiro- [4,5 ] -deca-6,9-dien-8-ol in acetone (Winstein et al., 1964). The absence of these effects with the chloro compound is in accord with the observations that aromatic chloro derivatives are not usually subject to electrophilic catalysis of leaving group departure (Bemasconi, 1973). 

Electrophilic catalysis by acyl cations in reactions of ketones... 

In the course of acyloxycarbocation investigations112 it has been noted that the reactions of both aldehydes and ketones follow an unusual course or are strongly accelerated if either acylium ions RCO+ are present in the reaction mixtures or conditions to generate them in situ arise. These observations are explained by a transformation of carbonyl compounds into the highly reactive acyloxycarbocations 163 which easily react with weak nucleophiles such as vinyl ethers, vinyl esters, etc. Hence the electrophilic catalysis by acyl cations in carbonyl reactions takes place regardless of the origin of the latter. This catalysis was used in the reaction of ketones with nitriles. 

Moreover, it has been shown102 that under conditions of electrophilic catalysis by acetyl cations the addition of ketones to nitriles proceeds more rapidly than the self-condensation of ketones shown in equations 34-36. Thus, cyclohexanone reacts with both benzonitrile and acetonitrile in the presence of perchloric acid/acetic anhydride mixture to form Af-acyliminium salts 186 which are then converted to the Af-acylimines 187. The /Lacylaminoketones 9773 did not form the salts 186 in a control experiment... 

It should be noted that the alkyl aryl ketones 191 react with the mixture of perchloric acid and acetic anhydride under the same conditions (room temperature, 10-20 h) to give the unsymmetrical pyrylium salts 197114,115. As shown in Reference 112 this reaction is also an example of electrophilic catalysis by acylium ions, proceeding via the cationic intermediates 195 and 196 (equation 62). 

---