Alkylation-acylation processes

In this chapter, only special examples of stoichiometric acylation will be commented. For example, reactions showing extraordinary level of regiose-lectivity promoted by proximity or metal template effects are described. Moreover, examples of efficient use of carboxylic acids and esters as acylating agents under soft experimental conditions in combination with ecocompatible solvents are stressed as new and practicable synthetic methods. Studies on the highly efficient multistep s)mthesis of polyfunctional compounds via bis-acylation and alkylation-acylation processes are commented upon, and some mechanistic details are also shown. 

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

Das zur Diskussion stehende 7t, n-angeregte Keton geht eine a-Spal-tung (Norrish Type I Process n>) zum entsprechenden 1,5-, 1,6- oder 1,7-Alkyl/Acyl-Biradikal ein, das... 

Friedel-Crafts (FC) alkylation, acylation, and sulfonylation reactions are important C-C or C-S bond forming reactions in organic chemistry [60-64], Since the seminal works of Charles Friedel and James Mason Crafts published in 1877 in which they report the use of A1C13 for alkylation reactions [65], the search for more active catalysts, especially for acylation reactions, continues. Due to increasing environmental concerns, the need for green catalysts and processes for the FC reaction has gained significant importance. Bi(III) salts have shown to be efficient and recoverable catalysts with applicability in this area [13]. 

Enamines are the stable products of a similar reaction between secondary amines (such as pyrrolidine or morpholine) and aldehydes and ketones.218 These vinylamines are reactive reagents of value in synthesis they function as specific enol equivalents of carbonyl compounds, readily undergoing alkylation and acylation processes (e.g. Section 5.9.2, p. 632). 

Aryl alkyl ketones are readily prepared by the Friedel-Crafts acylation process (see Section 6.11.1, p. 1006) and their Clemmensen reduction constitutes a more efficient procedure for the preparation of monoalkylbenzenes than the alternative direct Friedel-Crafts alkylation reaction (see below). Alternatively aldehydes and ketones may be reduced to the corresponding hydrocarbon by the Wolff-Kishner method which involves heating the corresponding hydrazone or semicarbazone with potassium hydroxide or with sodium ethoxide solution. 

Insertion of CO into RCo(CO)4 to give acyl complexes is facile (see Mechanisms of Reaction of Organometallic Complexes). This reaction proceeds at 1 atm of CO at ambient temperature. The alkyl-acyl equilibrium lies far towards the acyl complex. The activation energy for the process has been calculated by MO methods to be about 85 kJmol (see Molecular Orbital Theory). Thus, RCo(CO)4 complexes can only be obtained under conditions of low CO pressure, which in turn opens the way for CO dissociative decomposition. Cobalt acyl complexes can be derivatized in several ways to form various products (Scheme 5). 

DC of imines and munchnones 145, in situ generated by treatment of 5(4//)-oxazolones with chlorotrimethylsilane, allowed a diastereoselective multicomponent synthesis of highly substituted imidazolines 146, containing a four-point diversity and two stereocenters. The process is applicable to aryl, alkyl, acyl, and heterocyclic substitutions and only the trans distereomers (with respect to and R ) of 146 were observed in almost all cases <03SI433>. 

In contrast, a significant change in rate is observed with solvent variation when R in (RC02)2 is a secondary alkyl group as seen from Table 87. The solvent effects are consistent with a contribution from canonical structure (V) in the transition state. However, the magnitude of the solvent effect is difficult to assess in terms of structure (V), since the normal peroxide decomposition is in competition with an inversion reaction to give an alkyl acyl carbonate. The inversion process will be... 

Subsequent loss of carbon dioxide from the alkyl acyl carbonate may occur. It was estimated, in the decomposition of Ira 5-4-I-butylcyclohexanecarbonyl peroxide in carbon tetrachloride, that two-thirds of the reaction occurs via the inversion process and one-third by the homolytic process It is suspected that inversion may be major decomposition route for other secondary aliphatic diacyl peroxides as well as for some bridgehead peroxides . Confirmation that the inversion process does contribute to the decomposition of i-butyryl peroxide is given . Further evidence for the inversion process is found in the volumes of activation for the decomposition of i-butyryl peroxide in isooctane at 50° and ram-4-r-butylcyclohexanecarbonyl peroxide in -butane at 40 °C. The AF values are —5.1 and —4.1 cm. mole , respectively. These values may be compared to the positive values of A F for benzoyl peroxide (Table 77) where there is no inversion. While the transition states for homolytic decomposition and inversion for secondary and tertiary diacyl peroxides are both polar, it is felt that the transition state for inversion is more polar . The extent of contribution of structure (V) to the transition state in the homolytic decomposition must be held with considerable reservation. In general much of the reported data for the decomposition of secondary and tertiary alkyl diacyl peroxides should be viewed with some scepticism unless efforts were made to assess the importance of the inversion process. One clue that may be used to evaluate the importance of this process is the yield of ester, which is a product of this reaction. 

In the alkylation section, it was stated that the immobilized ionic liquids can combine the advantage of green media with solid support materials, which may enable the wide apphcation of precious ionic hquids by the reduchon of usage and also realize the sustainability of the chemical reaction process. The supported ionic hquids (SlLs) catalysts commonly employ the supports such as the macroporous polymer, metal oxide (SiO, AI2O3, etc.), zeohte, clay, and achve carbon, and after the immobilization of the ionic hquids, the ionic hquids still maintain their special solvent effect. Presently, the immobilized ionic hquids have been applied extensively to the alkylation, acylation, hydroformylation, oxidation, esterihcation, hydrolyza-tion, hydrogenahon, and other unit reactions this part of the chapter only discusses the application of immobilized ionic hquids to the acylahon. 

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