Friedel-Crafts Alkylation of Aromatic Compounds with Phosphorus Esters

• Published in: Zeitschrift für Naturforschung. B, A journal of chemical sciences Vol. 27; no. 11; pp. 1339 - 1348
• Main Authors: Sosnovsky, G., Shende, M. W.
• Format: Journal Article
• Language: English
• Published: Verlag der Zeitschrift für Naturforschung 01.11.1972
• ISSN: 0932-0776; 1865-7117
• DOI: 10.1515/znb-1972-1112

The alkylations of aromatic compounds with trialkyl phosphites (1), dialkyl phosphites (2), and trialkyl phosphates (3) in the presence of aluminum chloride were studied involving several raction variables, such as time, ratio of reactants, nature of catalyst and solvent, and combinations thereof. Extensive disproportionation and isomerization were observed in the reaction with monosubstituted alkylbenzenes under heterogeneous reaction conditions obtained by the use of an excess of aromatic substrates. A combination of aluminum chloride—nitromethane complex and dichloromethane as solvent was used to eliminate these undesirable effects and to give homogeneous and practically non-isomerizing conditions. The scope of the reaction was studied with a number of aromatic substrates, and their relative reactivities were compared to that of benzene in competitive isopropylations with triisopropyl phosphite. The relative rates and isomer distributions showed low substrate and low positional selectivities and poor agreement with Brown’s selectivity relationship. The substrate selectivity was somewhat higher and the positional selectivities were somewhat lower than those obtained in competitive isopropylation reactions with other isopropylating agents. The selectivity factor, Sf, and partial rate factors were calculated. An electrophilic alkylation mechanism is proposed on the basis of (1) the relative rates of isopropylation, (2) the isomer distribution of dialkylated aromatics, and (3) the necessity of a strong Lewis acid in these reactions.