Intermediates in acylation reactions

Intermediates in acylation reactions, 47 carbinolamine as, 41 in diazonium coupling, 77 enamine-like, 85... 

Acylation of acetylenic compounds provides /ra/ j -P-chlorovinyl ketones (181). Vinyl cations were proposed to be the intermediates in these reactions. 

Acyl sulfonylhydrazides (117) are cleaved with base to give aldehydes. This is known as the McFadyen-Stevens reduction and is applicable only to aromatic aldehydes or aliphatic aldehydes with no a hydrogen. " RCON=NH (see 10-87) has been proposed as an intermediate in this reaction. " °... 

Monothiodiacylhydrazines 127, derived from the acylation of thiosemicarbazides or as intermediates in the reactions of (1) thiohydrazides with carboxylic acids and their derivatives (see Section 5.10.9.2.2(i)) or (2) hydrazides with thiocarbonyl compounds (see Section 5.10.9.2.3(i)), cyclize in the presence of an acid catalyst to give 1,3,4-thiadiazoles 128 (Equation 39, Table 4). 

L abbe and Vermeulen have proposed salts of type (105) as key intermediates in the reactions of 5-aminothiatriazole (10) with acyl chlorides (Scheme 19) <81BSB89>. The intermediate is expected to undergo cyclization to 3-imino-A -l,2,4-oxathiazoline (106) followed by reaction with a second molecule of acyl chloride to give the corresponding thiapentalene (107). The formation of the 1,2,4-thiadiazole (104) was explained as a result of an addition of V-acetylcyanamide (108), formed by extrusion of nitrogen and loss of sulfur from 5-acetamido-1,2,3,4-thiatriazole, to the key intermediate... 

Even though N-acylated amidoximes have never been isolated, their existence as intermediates in some reactions involving the formation of the oxadiazole ring cannot be excluded. Indeed, the formation of N-acyl-amidoximes is postulated in the following methods of oxadiazole synthesis. 

Another clear example of an acetylene insertion reaction was reported by Chiusoli (15). He observed that allylic halides react catalytically with nickel carbonyl in alcoholic solution, in the presence of CO and acetylene, to form esters of cis-2,5-hexadienoic acid. The intermediate in this reaction is very probably a 7r-allylnickel carbonyl halide, X, which then undergoes acetylene insertion followed by CO insertion and alcoholysis or acyl halide elimination (35). Acetylene is obviously a considerably better inserting group than CO in this reaction since with acetylene and CO, the hexadienoate is the only product, whereas, with only CO, the 3-butenoate ester is formed (15). [See Reaction 59]. 

Further indirect evidence for the incursion of tetrahedral intermediates in acyl-transfer reactions has been obtained from experiments which indicate a change in rate-determining step and hence the incursion of an intermediate (Jencks and Gilchrist, 1964, 1968 Jencks, 1969 Johnson, 1967 Schowen et al., 1966 Fedor and Bruice, 1965 Hibbert and Satchell, 1967 Chaturvedi et... 

Both the experimental and calculated equilibrium constants indicate the great thermodynamic instability of hemiorthoesters with respect to the corresponding esters and show why it is normally impossible to detect the tetrahedral intermediates in acyl-transfer reactions. On going from intermolecular to intramolecular reactions the tetrahedral intermediate becomes relatively more stable, and if the structure is more rigid (cf. [120], [121] in Table 17) or more sterically crowded (cf. [119]) the tetrahedral intermediate is more stable still. However, it is only with structues as rigid as tetrodotoxin or with the trifluoroacetate of pinacol that the hemiorthoester is more stable than the ester (see Section 1). ... 

A carbocation is strongly stabilized by an X substituent (Figure 7.1a) through a -type interaction which also involves partial delocalization of the nonbonded electron pair of X to the formally electron-deficient center. At the same time, the LUMO is elevated, reducing the reactivity of the electron-deficient center toward attack by nucleophiles. The effects of substitution are cumulative. Thus, the more X -type substituents there are, the more thermodynamically stable is the cation and the less reactive it is as a Lewis acid. As an extreme example, guanidinium ion, which may be written as [C(NH2)3]+, is stable in water. Species of the type [— ( ) ]1 are common intermediates in acyl hydrolysis reactions. Even cations stabilized by fluorine have been reported and recently studied theoretically [127]. 

Heating of acyl- or ethoxycarbonyl-methines (123) with anhydrides yields the indolizines (120), (121) and (122). The 2-diacylmethylenepyridines (124) are thought to be intermediates in these reactions. 

The 2-hydroxyglycals provide additional source material for the study of dismutation reactions, the reaction of the acyl derivatives of the hexoses climaxing in di-O-acetylkojic acid or di-O-benzoylkojic acid through loss of acetic or benzoic acid and of the O-methyl derivatives in 5-(methoxymethyl)-2-furaldehyde through loss of methanol. The formation of the 2-hydroxyglycals as intermediates in the reaction of some alkalis on sugars has been proposed by Kusin8 in an effort to explain the cationic dependence exhibited by the products. His mechanism has not, however, been established. 

Although this mechanism of rearrangement has not been demonstrated in acylation reactions of carbohydrates, it must operate in situations where formation of the intermediate is facile. This would include terminal 1,2-diols and c/s-l,2-diols. There is considerable evidence that rearrangements occur during benzoylation of carbohydrate terminal 1,2-diols.20 Most likely,... 

Exposure of 3-(tetrahydro-2-furyl)-3-trimethylsilylpropanoic acids 178 to trifluoroacetic anhydride allows intramolecular acylative ring-opening reaction to give the corresponding eight-membered lactones 180 in moderate to good yields (Scheme 46) <1999SL1757>. However, when R1 = R2 = Me, lactones 180 are not formed. The acyloxonium ion 179 is a probable intermediate in these reactions. Similarly, acids 182 afford lactones 183 (Scheme 47). 

There is continumg interest in the organometalhc chemistry of Pd. This oxidation state has often been invoked as an intermediate in the reactions of Pd species with reactive acyl or alkyl halides. Spectroscopic evidence has been obtained for this hypothesis. When phenacyl bromide reacts with dimethylpalladium(n) complexes, such as PdMe2(bpy), a complex can be isolated at 0 °C, which is shown, by H NMR and X-ray crystallography, to be PdBrMe2(CH2COAr)(bpy) (equation 2). The same complex reacts with Mel to form PdIMe3(bpy). The d Pd complexes are expected to exhibit... 

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