Intermediates in the Paal-Knorr Synthesis of Furans
摘要:
New experimental evidence for the mechanism of the Paal-Knorr reaction involving the acid-catalyzed cyclization of a 1,4-diketone to form a furan is reported. In aqueous or alcoholic solutions containing hydrochloric acid and in chloroform containing boron trifluoride-etherate d,l- and meso-3,4-diethyl-2,5-hexanediones (2r and 2m) cyclize at unequal rates; the stereochemical configuration of the unchanged done is preserved during the reaction. This disagrees with the commonly accepted mechanism involving the ring closure of the rapidly formed monoenol (11b) followed by loss of water. A pathway involving the rapid protonation of one of the carbonyls followed by the electrophilic attack on the protonated carbonyl by the enol being formed at the other carbonyl group (10c) is proposed to account for the difference in reaction rates between the diastereomers of 3,4-disubstituted 2,5-hexanediones (1-3). The following results also seem to support the intermediacy of 10c. The presence of two isopropyl groups in 3,4-diisopropyl-2,5-hexanedione (3) considerably reduces the rate of cyclization, The catalytic constants k(H)(+) for the cyclization of 2r and 2m are larger than the constants for enolization of methyl ketones. The diastereomers of 2,3-dimethyl-and 2,3-diethyl-1,4-diphenyl-1,4-butanediones (4 and 5), which could enolize only toward the center of the molecule, also react at different rates. The d,l and meso dideuterio analogs (d(2)-4r and d(2)-4m) exhibit a primary isotope effect during cyclization. The order of cyclization of 1,4-diphenyl-1,4-butanedione (6) and its analogs (7-9) reveals that the presence of electron-donating groups facilitate the reaction.
The reaction of carbanions with 2-substituted-2-nitropropanes
作者:Glen A. Russell、Boguslaw Mudryk、Francisco Ros、Mikolaj Jawdosiuk
DOI:10.1016/0040-4020(82)80123-3
日期:1982.1
The reaction of mono-enolate anions with O2NCMe2X where X = Cl, NO2, p-MePhSO2 yield coupling (RCOCH(R′)(CMe2NO2) and enolate dimerization products (RCOCH(R′)CH(R′)COR) by free radical chain mechanisms involving bimolecular substitution or electrontransferreactions between the enolate anion and the intermediate nitro alkane radical anion (XCMe2NO2−).
单烯酸根阴离子与X = Cl,NO 2,p -MePhSO 2的O 2 NCMe 2 X反应产生偶联(RCOCH(R')(CMe 2 NO 2)和烯醇二聚产物(RCOCH(R')CH (R')COR)通过涉及烯醇阴离子和中间体硝基烷烃自由基阴离子之间的双分子取代或电子转移反应的自由基链机构(XCMe 2 NO 2 - )。
Electron transfer processes. 31. Coupling of enolates of phenones with 2-chloro-2-nitropropane
作者:Glen A. Russell、Boguslaw Mudryk、Mikolaj Jawdosiuk、Zbigniew Wrobel
DOI:10.1021/jo00349a012
日期:1982.5
Alkylation and oxidative dimerization of enolate anions by radical chain processes
作者:Glen A. Russell、Boguslaw Mudryk、Mikolaj Jawdosiuk