Acyl cations reactions

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

The electrophile in a Friedel-Crafts acylation reaction is an acyl cation (also referred to as an acylium ion) Acyl cations are stabilized by resonance The acyl cation derived from propanoyl chloride is represented by the two resonance forms... 

The electrophilic site of an acyl cation is its acyl carbon An electrostatic poten tial map of the acyl cation from propanoyl chloride (Figure 12 8) illustrates nicely the concentration of positive charge at the acyl carbon as shown by the blue color The mechanism of the reaction between this cation and benzene is analogous to that of other electrophilic reagents (Figure 12 9)... 

An acyl cation is an intermediate m Friedel-Crafts acylation reactions... 

Friedel-Crafts acylation involves electrophilic attack by acyl cation (CHsCO ) on the ring, and the ring s electronic character should indicate its susceptibility to attack. Compare electrostatic potential maps of ferrocene and acetylferrocene. Which molecule contains the most electron-rich ring Which acylation reaction should be faster Does an acetyl substituent enhance or diminish ring reactivity What should be the major product when ferrocene is combined with one equivalent of acetic anhydride ... 

Figure 16.9 Mechanism of the Friedel-Crafts acylation reaction. The electrophile is a resonance-stabilized acyl cation, whose electrostatic potential map indicates that carbon is the most positive atom (blue).

This intermediate expels a dtacylglycerol as leaving group in a nucleophilic acyl substitution reaction, giving an acyl enzyme. The dtacylglycerol is protonated by the histidine cation. 

Block or graft copolymem can be obtained by cationic polymerization of THF with macromolecular initiators. The recommended groups for the initiation are the dioxolenium cation, the acyl cation and the super acid ester, each of which can be introduced into the backbone polymer by reaction with silver salts of strong acids. Introduction of the dioxolenium group into polystyrene was carried out by the following route32 ... 

The reaction between carbonium ions and carbon monoxide affording oxocarbonium ions (acyl cations) is a key step in the well-known Koch reaction for making carboxylic acids from alkenes, carbon monoxide and water ... 

Alkenes can be acylated with an acyl halide and a Lewis acid catalyst in what is essentially a Friedel-Crafts reaction at an aliphatic carbon. ° The product can arise by two paths. The initial attack is by the acyl cation RCO (or by the acyl halide free or complexed see 11-14) at the double bond to give a carbocation ... 

Exactly the same considerations apply to the esterification of hindered acids (182) in the reverse direction. It will be noticed that this mechanism requires protonation on the less favoured (cf. p. 240) hydroxyl oxygen atom (185) to allow the formation of the acyl carbocationic intermediate (184). Apart from a number of R3C types, a very well known example is 2,4,6-trimethylbenzoic (mesitoic) acid (186), which will not esterify under ordinary acid-catalysis conditions—and nor will its esters (187) hydrolyse. Dissolving acid or ester in cone. H2S04 and pouring this solution into told alcohol or water, respectively, is. found to effect essentially quantitative esterification or hydrolysis as required the reaction proceeds via the acyl cation (188) ... 

We have, however, already seen an alternative acid hydrolysis pathway (Aac1, p. 242) in which a water molecule is not involved in the slow, rate-limiting step. In addition, this step is one in which considerable +ve charge is developed at the reaction centre as the protonated ester (35a) is converted into the acyl cation intermediate (36a) a necessary requirement for a reaction with a large -ve (-3-25) p value ... 

Acyl-transfer reactions are some of the most important conversions in organic chemistry and biochemistry. Recent work has shown that adjacent cationic groups can also activate amides in acyl-transfer reactions. Friedel-Crafts acylations are known to proceed well with carboxylic acids, acid chlorides (and other halides), and acid anhydrides, but there are virtually no examples of acylations with simple amides.19 During studies related to unsaturated amides, we observed a cyclization reaction that is essentially an intramolecular acyl-transfer reaction involving an amide (eq 15). The indanone product is formed by a cyclization involving the dicationic species (40). To examine this further, the related amides 41 and 42 were studied in superacid promoted conversions (eqs 16-17). It was found that amide 42 leads to the indanone product while 41... 

The reaction of acylsilanes with acid chlorides in the presence of A1C13 leads to furans (Table 9.41) [45]. In these reactions an acyl cation initiates the addition with ensuing silyl migration yielding an intermediate vinyl cation. Attack of the carbonyl oxygen followed by proton loss affords the observed products (Scheme 9.16). An analogous reaction with nitrosyl fluoroborate provides a route to oxazoles (Table 9.42) [65]. The nitrosyl cation serves as the electrophile in this application. 

Allosteric behavior of the heterotropic variety is seen in the interaction between polymer and detergent or polymer and polymer (Shirahama, 1974 Arai et al., 1973 Tsuchida and Osada, 1973). Shinkai et al. (1977b) observed a sigmoid profile of rate constant vs. concentration of cationic detergents in the acyl transfer reaction from p-nitrophenyl acetate (PNPA) to copolymers (7). 

This section describes the nucleophilic reactions—acyl transfer reactions mostly—promoted by micelles and polysoaps. The nucleophiles are imidazoles, oxyanions and thiols, the same catalytic groups found ubiquitously in the enzyme active site. These nucleophiles are remarkably activated in the anionic form in the presence of cationic micelles and cationic polysoaps. These results are explained by the concept of the hydrophobic ion pair (Kunitake et al.,... 

A similar bait and switch approach has been exploited for acyl-transfer reactions (Janda et al., 1990b, 1991c). The design of hapten [10] incorporates both a transition state mimic and the cationic pyridinium moiety, designed to induce the presence of a potential general acid/base or nucleophilic amino acid residue in the combining site, able to assist in catalysis of the hydrolysis of substrate [11] (Appendix entry 2.6). 

Acid chlorides, RCOCl, undergo ready attack by weaker nucleophiles, e.g. H2O, ROH. The question then arises whether, with the better potential leaving group Cl , the reactions of acid chlorides could proceed either via a single step 8 2 type pathway (cf. p. 78) involving a T.S., in which attack by and loss of Q are essentially synchronous or via an 8 1 type pathway (cf. p. 79) in which the slow step is RCOCl RCO Cl , followed by fast attack by Y on the acyl cation, RCO . In fact, most reactions of acid chlorides probably proceed via the now familiar tetrahedral intermediate pathway, though there may be some exceptions. 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

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 ... 

Formylation may be carried out by use of CO, HCl, and AICI3 (the Gattermann-Koch reaction) it is doubtful whether HCOCl is ever formed, the most likely electrophile being the acyl cation,... 

Preparation of amides Ammonia, 1° and 2° amines react with carboxylic acids to produce, respectively, 1°, 2° and 3° amides, through a nucleophilic acyl substitution reaction. The reaction of ammonia and a carhoxylic acid initially forms a carhoxylate anion and an ammonium cation. Normally the... 

Santry and McClelland (1983a) also generated the tetrahedral intermediate of an 0,S-acyl transfer reaction from an orthothiolester precursor [83]. At PH < 3 the cation [84] could be detected by its strong absorption at X = 350 nm and it was concluded from a kinetic analysis that this was in equilibrium with the hemiorthothiolester [85]. 

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