Acyl cations reactions with weak nucleophiles

Related classes of gitonic superelectrophiles are the previously mentioned protoacetyl dications and activated acyl cationic electrophiles. The acyl cations themselves have been extensively studied by theoretical and experimental methods,22 as they are intermediates in many Friedel-Crafts reactions. Several types of acyl cations have been directly observed by spectroscopic methods and even were characterized by X-ray crystal structure analysis. Acyl cations are relative weak electrophiles as they are effectively stabilized by resonance. They are capable of reacting with aromatics such as benzene and activated arenes, but do not generally react with weaker nucleophiles such as deactivated arenes or saturated alkanes. 

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. 

Like other superelectrophiles, protonation of the acyl cations leads to decrease of neighboring group participation and a lowering of the LUMO energy level, which facilitates reactions with even weak nucleophiles. 

Ammonium salts with two different alkyl chains were prepared directly via subsequent alkylations of dimethylamine with primary bromides and crystallization. Commercial hexadecyl-methylamine can be conveniently applied in the same way in order to convey functionality to cationic synkinons. A recent example describes subsequent alkylations with a small functional and a long-chain primary bromide (Scheme 2.4). A-acylated / -phenylenediamine was also alkylated at the second nitrogen atom which had two different alkyl chains, with or without extra functionality . After deacylation, this head group can be diazotized or coupled oxidatively with various heterocycles in water (Scheme 2.4). Photoactive and coloured membrane surfaces are thus obtained. Phenylene-diamine, pyridine and in particular A-methyl-4,4-bipyridinium chloride are relatively weak nucleophiles. Substitution of bromides is slow and the more reactive iodides can rarely be obtained commercially, but the selection of nitromethanes as solvent for bromide substitution is of great help as well as the addition of sodium iodide to enforce a Finkelstein reaction or a combination of both. 

The simplest example of a functional micelle is (49), previously demonstrated to be more effective than its trimethylammonium analogue in both esterolysis and bimolecular elimination reactions. It has now been demonstrated that micelles of (49) are more effective catalysts for the hydrolysis of p-nitrobenzoyl phosphate dianion at high pH than non-functional surfactants. " 2,4-Dinitrochloro- and fluoro-benzene react with micelles of (49) at high pH 10" times faster than with hydroxide ion at a comparable external pH. The initial product is (50) and this in turn is hydrolysed in micelles 2.6 x 10 times faster than is 2,4-dinitrophenyl 2-(trimethylammonium)ethyl ether in water at pH 12. Acyl transfer between p-nitrophenyl acetate and (49) gives an intermediate whose hydrolysis is not micelle catalysed. In contrast to the rate acceleration observed in that case, hydrolysis of p-nitrophenyl acetate is inhibited by micelles of (51) since the phenoxide nucleophile is weak and at the reaction pH its micelles are zwitterionic, not cationic. Synthesis of functional choline-type micelles is facilitated by the use of sulphonate (52), which is reactive towards thiophenoxide in aqueous micelles, but its water-insoluble trifluoromethanesulphonate reacts with a range of anions under phase-transfer conditions. " ... 

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