Aroyl triflates, acylation

These reactions are presumed to occur through aroyl triflate intermediates which dissociate to aiyl acylium ions. Lithium perchlorate and scandium triflate also promote acylation. ... 

In a kinetic study of the acylation of toluene, with p-xylene and the corresponding perdeutero compounds with aroyl triflates, correlation was found between the primary kinetic isotope effect and the ortho para ratio.35 Different conformations of the bent cr complexes36 for the two isomers resulted in a much higher rate of deprotonation and rearomatization for the para isomer. By appropriately selecting reaction conditions and thereby affecting the ratio of the two conformations, unusually high amounts of ortho products may be obtained.37,38... 

More recent studies reconfirmed the excellent properties of these acylating systems and showed that aroyl triflates are able to acylate aromatics without catalysts... 

The first unequivocal evidence for the effective electrophile in an acylation reaction has been presented.59 Reaction of aromatics with aroyl triflates in organic solvents needs no catalyst and allows kinetic investigation in homogeneous solution. The rate-limiting step can be either dissociation of the triflate to the acylium ion or reaction of the latter with the substrate. Kinetic measurements in the presence of base establish firmly the mtennediacy of acylium ions. 

Alkynes are readily acylated with acid chlorides under Friedel-Crafts conditions to form, in most cases, fram-p-chlorovinyl ketones through the corresponding vinyl cation intermediate [Eq. (8.16)]. The first study in 1935 reported low yields.11 Later in acylations with acyl triflates, p-keto vinyl triflates were obtained in satisfactory yields.123 When aroyl derivatives are used, the intermediate can undergo cyclization to form indenones. Chlorovinyl ketones formed from terminal alkynes may also react further losing hydrogen chloride to yield conjugated acetylenic ketones 11,13... 

Mesylates are used for Ni-catalysed reactions. Arenediazodium salts 2 are very reactive pseudohalides undergoing facile oxidative addition to Pd(0). They are more easily available than aryl iodides or triflates. Also, acyl (aroyl) halides 4 and aroyl anhydrides 5 behave as pseudohalides after decarbonylation under certain conditions. Sulfonyl chlorides 6 react with evolution of SO2. Allylic halides are reactive, but their reactions via 7t-allyl complexes are treated in Chapter 4. Based on the reactions of those pseudohalides, several benzene derivatives such as aniline, phenol, benzoic acid and benzenesulfonic acid can be used for the reaction, in addition to phenyl halides. In Scheme 3.1, reactions of benzene as a parent ring compound are summarized. Needless to say, the reactions can be extended to various aromatic compounds including heteroaromatic compounds whenever their halides and pseudohalides are available. 

The palladium-catalyzed system can be extended to the acylation of siloxycyclopropanes with aroyl chloride/carbon monoxide or aryl triflate/carbon monoxide, which gives 1,4-diketones. Contrary to the case of doubly oxygen-substituted cyclopropanes vide infra), the acylation of 1-siloxycyclopropanes is restricted to aroyl chlorides and is not applicable to aliphatic or a, -unsaturated acyl chlorides. For the reactions with aryl triflates, tetrakis(triphenylphos-phane)palladium(O) is used as catalyst, while the reactions with aroyl chlorides employ bis(triphenylphosphane)palladium(II) chloride and ( / -allyl)chloropalladium dimer/triphenyl phosphite as catalysts. In these reactions, aroylpalladium(II) species may undergo ring opening of the siloxycyclopropanes. 

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