Acyl halides and related compounds

As described in detail in CHEC-II(1996) 1996CHEC-II(6)1 N-acylation of 1,2-diazines is often used to make the nucleus more susceptible for nucleophilic attack (see Section 8.01.5.4.4). Intramolecular reactions on nitrogen 

Alkyl pyrazole-l-carboxylates 146, which were readily prepared from an alkyl chloroformate and pyrazole 1, are useful alkoxycarbonylating agents towards Grignard reagents for the synthesis of carboxylic esters 147 or towards 

Reaction of 3(5)-aminopyrazole with chloroacetyl chloride gives a mixture of 3-amino-l-(chloro-acetyl)pyrazole and 5-amino-l-(chloroacetyl)pyrazole, both of which rearrange in the solid state to 

Azoles containing a free NH group react comparatively readily with acyl halides. yV-Acyl-pyrazoles, -imidazoles, etc. can be prepared by reaction sequences of either type (75) — (76) or type (79)— (80) or (81). Such reactions have been carried out with benzoyl halides, sulfonyl halides, isocyanates, isothiocyanates and chloroformates. Reactions occur under Schotten-Baumann conditions or in inert solvents. 

When two isomeric products could result, only the thermodynamically stable one is usually obtained because the acylation reactions are reversible and the products interconvert readily. Thus, benzotriazole forms 1-acyl derivatives (115) which preserve the Kekule resonance of the benzene ring and are therefore more stable than the isomeric 2-acyl derivatives. Acylation of pyrazoles also usually gives the more stable isomer as the sole product (66AHC(6)347). The imidazole-catalyzed hydrolysis of esters can be classified as an electrophilic attack on the multiply bonded imidazole nitrogen. 

3-Triazoles are acylated with acyl halides, usually initially at the 1-position, but the acyl group may migrate to the 2-position on heating or on treatment with base. Thus, acetylation with acetyl chloride often gives 1-acetyl derivatives, which rearrange to the 2-isomers above 120°C 74AHC(I6)33 . 

Reactivity of Five-membered Rings with Two or More Heteroatoms 

Whether tetrazoles are acylated in the 1- or 2-position depends on the 5-substituent. 2-Acyltetrazoles are unstable (see Section 3.4.3.12.4) (77AHC(2l)323) 1-alkylsulfonyltriazoles are also unstable (see Section 3.4.1.2.3). 

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Acyl halides and related compounds and Michael-type reactions... 

Dimethyl sulfoxide decomposes violently when in contact with a wide range of acyl halides and related compounds, such as cyanuric, acetyl, benzoyl, benzenesulfonyl, thionyl, and phosphoryl chlorides and PC13- DMS should be used with caution as a solvent in exploratory reactions... 

Alkyl halides and related compounds compounds without a free NH group Alkyl halides and related compounds compounds with a free NH group Acyl halides and related compounds... 

The radical reduction of acyl halides and related compounds, such as acyl chal-cogenides, to aldehydes may find significance for primary, vinyl, and aromatic acyl radicals whose decarbonylation rates are significantly slower than those of the corresponding secondary and tertiary radicals [6], In practice, however, this method is restricted to substrates which have serious incompatibilities with more traditional methods. 

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