Pivaloylation of N-Methylpyrrole. Formation of a Novel 3,4-Diacylation Product

Article abstract of DOI:10.1021/jo00387a018

The pivaloylation of N-methylpyrrole (1) with pivaloyl chloride and tin(IV) chloride in benzene or 1,2-dichloroethane at 20 deg C gives mixtures of mono- and dipivaloylated pyrroles.N-Methyl-2-pivaloylpyrrole (2) is formed in only trace amounts and the 3-isomer 3 is the predominant monosubstituted product.Substantial amounts of N-methyl-2,4-dipivaloylpyrrole (4) and smaller amounts of 3,4-isomer 5 are also formed.Facile Lewis acid mediated rearrangements of 2 into 3 and of 4 into 5 were demonstrated.The 3,4-disubstituted isomer 5 is the thermodynamically most stable product and all compounds in this study were converted into 5 on treatment with tin(IV) chloride and excess pivaloyl chloride.The pivaloylation of 1 is markedly different from its acetylation.Acetylation takes place to give 2- and 3-acetyl derivatives, and diacetylation gives the 2,5- and/or 2,4-diacetyl derivatives.Fully proton-coupled 13C NMR spectra were obtained for all the acylated pyrroles in this study and complete assignment of chemical shifts and coupling constants was made.