3469-21-4Relevant articles and documents
Hexafluoroisopropanol as solvent and promotor in the Paal-Knorr synthesis of N-substituted diaryl pyrroles
Schirmacher, Robert H.E.,R?sch, Daniel,Thomas, Franziska
, (2021/02/20)
An additive-free synthesis of challenging N-substituted aryl pyrroles from the often poorly soluble corresponding 1,4-diketones by means of the Paal-Knorr pyrrole synthesis is reported, which makes use of the unique properties of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as a solvent and reaction promotor. Our procedure offers simple execution and purification as well as easy scale-up and can be applied in the Paal-Knorr synthesis of a large number of structurally diverse pyrroles including the synthetically challenging tetra- and penta-substituted pyrroles in moderate to excellent yields. HFIP can also be used as solvent in the Paal-Knorr synthesis of furans and thiophenes; however, the solvent effect is more pronounced in synthesis of pyrroles.
Electrochemical oxidation and EPR spectroscopy of radical cations of N-substituted 2,3,4,5-Tetramethylpyrroles
Klu?k, Robert,Kubá?ek, Pavel
, p. 399 - 406 (2007/10/03)
Electrochemical oxidation of 19 N-substituted 2,3,4,5-tetramethylpyrroles has been studied in acetonitrile and dichloromethane by means of slow cyclic voltammetry and coulometry. The first oxidation consumes one electron and occurs within the potential range 0.60-0.94 V in acetonitrile and 0.78-1.17 V in dichloromethane (vs. SCE). Twelve in situ generated primary radical cations were sufficiently stable at lowered temperature in dichloromethane for EPR measurement and showed well resolved HFS. The g-values (≈2.0026) and the coupling constants of 2,5-methyls (aH≈1.5 mT), 3,4-methyls (aH≈0.35 mT), and of the pyrrole nitrogen (aN≈0.42 mT) are very proximate for all 12 radical cations. It can be concluded, with support from quantum chemical calculations, that the odd electron is localised entirely on the pyrrole ring in the a2 HOMO of the parent molecule. Despite the odd electron distribution, the stability of the radical cations depends on the particular substituent attached to the pyrrole nitrogen. Acta Chemica Scandinavica 1998.
Stereoisomer Effects on the Paal-Knorr Synthesis of Pyrroles
Szakal-Quin, Gyoengyi,Graham, Doyle G.,Millington, David S.,Maltby, David A.,McPhail, Andrew T.
, p. 621 - 624 (2007/10/02)
The neurotoxicity of n-hexane has been postulated to result from the reactivity of its γ-diketone metabolite, 2,5-hexanedione (1), with lysil amino groups of proteins to form pyrroles (Paal-Knorr synthesis).We have synthesized a series of 3,4-disubstituted γ-diketones in order to explore the relationship between rate of pyrrole formation and neurotoxicity.The γ-diketones were prepared through oxidative coupling of ketones.Yields were improved to 60-70percent with the use a Soxhlet apparatus containing PbO2 in the extraction thimble.Diketones prepared were 3,4-dimethylhexane-2,5-dione (2), 3,4-diethylhexane-2,5-dione (3), 3,4-diisopropylhexane-2,5-dione (4), and 3,4-diphenylhexane-2,5-dione (5).The reactions yielded mixtures of the d,l (a) and meso (b) diastereomers, which were separated by column chromatography, fractional distillation, or crystallization.Structures of the diastereomeric forms were established by 13C NMR techniques and, in the case of 4b, by single crystal X-ray diffraction.The relative reactivities of the d,l and meso isomers of each γ-diketone were determined with benzylamine in cyclohexane and the rate of pyrrole formation was determined by HPLC.For each pair of diastereomeric diketones the d,l reacted 4-40 times faster than the meso form.The reactivities of the γ-diketones were in the order 2 > 1 > 3 > 5 > 4 with pseudo-first-order rate constants ranging from 4*10-4 to 3*10-8 s-1 at 30 deg C.
