ALKYLATION OF AROMATIC COMPOUNDS WITH PHOSPHORUS ESTERS 1339 Analyse der Verbindungen: Die Einlagerungsverbin- Kalium-titandisulfid: Kalium 21,0%, Titan 33, 8%, dungen wurden vorsichtig mit HN03 aufgeschlossen. Schwefel 44,2%, Summe: 99,0%. Zusammen- Das im Fall der Wolframverbindungen dabei ausfal- setzung: Ko^TiSj,^ . lende W03 wurde alkalisch gelöst. Wolfram und Mo- Reaktionsprodukt von WS* mit Li-naphthalid (Uber- lybdän wurden als Oxinat, Titan als TiOa und Schwe- schuß) : Lithium 9,8%, Wolfram 66,3%, Schwefel fel als BaS04 bestimmt. Die Bestimmung der Alkali- 23,5%, Summe: 99,6%. Verhältnis 1 W : 2 Lili95S. metalle erfolgte flammenphotometrisch. Reaktionsprodukt WS2 mit Na-naphthalid (Überschuß) : Kalium-wolf ramdisulfid: Präparat I: Kalium 8,3%, Natrium 26,9%, Wolfram 53,1%, Schwefel 18,8%, Wolfram 66,9%, Schwefel 23,7%, Summe: 98,9%. Summe: 98,8%. Verhältnis: 1 W : 2 Na2,0S. Zusammensetzung: K0.59WS2)0 . Präparat II: Kalium 8,1%, Wolfram 67,1%, Schwe- fel 23,4%, Summe: 98,6%. Zusammensetzung: K0.57WS2,O . Kalium-molybdändisulfid: Kalium 10,86%, Molybdän Wir danken der Deutschen Forschungsgemeinschaft 53,6%, Sdiwefel 35,5%, Summe: 99,9%. Zusam- und dem Fonds der Chemie für die Unterstützung die- mensetzung: K0!49MoS1?98 . ser Arbeit. 1 Auszug aus der Dissertation E. BAYER. Tübingen 1970. 5 T. E. HOVEN-ESCH U. J. SMID, J. Amer. chem. Soc. 87, 669 2 W. RÜDORFF, Chimia [Zürich] 19,489 [1965], [1965]. 3 H. M. SICK, Dissertation Tübingen 1959. 6 W. BILTZ, P. EHRLICH U. M. MEISEL, Z. anorg. allg. Chem. 4 C. STEIN. J. POULENARD, L. BONNETAIN U. J. GOLE, C. R. 234,97 [1934], hebd. Seances Acad. Sei. 260, 4503 [1965]. Friedel-Crafts Alkylation of Aromatic Compounds with Phosphorus Estersla b G. SOSNOVSKY and M. W. SHENDE Department of Chemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201 (Z. Naturforsch. 27 b, 1339—1348 11972] ; received June 26/August 22, 1972) Friedel-Crafts, Electrophilic Alkylation, Phosphorus Esters, Alkylbenzenes 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 mono- substituted alkylbenzenes under heterogeneous reaction conditions obtained by the use of an excess of aromatic substrates. A combination of aluminum chloride —nitromethane complex and dichloro- methane 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 B r o w n's selectivity relation- ship. 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. Friedel-Crafts alkylations of organic com- an important advance in the alkylation reaction. pounds with alkyl halides, alkenes, alcohols and a However, the alkylation reaction with alkyl esters of variety of alkylating agents are relatively old reac- inorganic acids has not been investigated fundamen- tions on the chemist's list of synthetic techniques. tally or exploited practically to the same extent as The application of alkyl esters of organic and mine- the acid-catalyzed alkylations with either olefins or ral acids to the Friedel-Crafts synthesis 2 was alkyl halides. The reason for this neglect might be the easy availability of alkyl halides and other alky- Requests for reprints should be sent to Prof. G. SOSNOVSKY, lating agents as compared to alkyl esters of in- Department of Chemistry, University of Wisconsin-Mil- waukee, Milwaukee, Wisconsin 53201, U.S.A. organic acids as starting materials. Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschung This work has been digitalized and published in 2013 by Verlag Zeitschrift in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der für Naturforschung in cooperation with the Max Planck Society for the Wissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht: Advancement of Science under a Creative Commons Attribution Creative Commons Namensnennung 4.0 Lizenz. 4.0 International License. 1340 G. SOSNOWSKY AND M. W. SHENDE The literature contains a survey of alkylation alkyl phosphites and trialkyl phosphates with aro- reactions with alkyl esters of inorganic acids such as matic substrates in the presence of aluminum chlo- alkyl sulfates2, sulfites2, chlorosulfites2, sulfonates2, ride are extremely exothermic. Therefore, it was chlorosulfonates2, p-toluene sulfonates2, alkyl o- necessary to add these esters to the mixture of the silicates2, alkyl carbonates2, alkyl borates2, and aromatic substrate — aluminum chloride at 5° ±2°, hypochlorites 2. However, the analogous reactions in- followed by stirring the reaction mixture at room volving the use of phosphorus esters, such as tri- temperature. alkyl phosphites (1), dialkyl phosphites (2), and A number of variables were examined in order tri alkyl phosphates (3), have received relatively to determine their effect on the reaction. It was little attention. Thus, n-'butyl phosphate3 was used found that the combined yield of mono- and diethyl to alkylate benzene in the presence of boron benzenes was very dependent on the ratio of the fluoride, and triethyl phosphate 4' 5 triisopropyl phos- ester to aluminum chloride. At least a 1:1 molar phate 4 and tributyl phosphate4 were used for the ratio of alkyl group in the alkylating ester to alu- alkylation of benzene in the presence of aluminum minum chloride is required for optimum yields chloride. These experiments were limited to benzene (Table I). In practice, a slight excess of aluminium as the substrate, and the reaction products were not chloride was used in order to compensate for losses critically examined. Definitive studies in this area during handling. Thus, the molar ratios of alumi- have become possible only since the advent of gas a chromatography. Table I. Ethylation of Benzene with One Mole Triethyl Phosphate. Effect of Variation in the molar Ratio of Alumi- Recently, a systematic attempt has been made to num Chloride to Triethyl phosphate. investigate the use of phosphorus esters in alkyla- Products13 6a b tions ' . At present a variety of phosphate and AlCl3/(C2H50)3P0 Monoethyl Diethyl phosphite esters are commercially available at a mole ratio Benzene Benzene low cost. These esters are liquids of low volatility Yield % Yield % and, therefore, make very suitable starting materials. 0.5 11 0.9C The objectives of the present work were to study 1 16 2.6C C the scope and mechanism of alkylation reactions of 1.5 18 1.4 3.5 26 44 D various aromatic hydrocarbons with trialkyl phos- 4 26 41D phites (1), dialkyl phosphites (2), and trialkyl phosphates (3). ;l Benzene to triethyl phosphate molar ration 20:1. b Reaction time 2.5 hr. (Reaction temperature, see pp. 1340 and 1346). c As the product was formed in traces, it could not be col- RCK. Rn RO\ RO— P: pn>p-°H RO—P=Ö: lected or positively identified by glpc. It shows the same reten- R0/ RO RO^ tion time as /n-diethyl benzene d m-Diethyl benzene. 1 2 3 num chloride to alkylating ester were 2.5 to 1 and R = CH3, j-C3H- R = CH3, COH, . R=C2H5 i-C3H7 3.5 to 1 in the case of diester and triester, respec- tively. Results and Discussion Ferric chloride, cupric chloride, stannic chloride, zinc chloride, sulfuric acid (98%) and phosphoric The conditions under which phosphorus esters acid (85%) were examined as catalysts, but with are employed in alkylations are different from those the exception of ferric chloride, no detectable reac- used in alkylations with alkyl halides 2. One reason tion was observed, even at elevated temperatures is that the inorganic acid, formed as a by-product, and on prolonged reaction times, as evidenced by is non-volatile and cannot escape from the reaction glpc analysis. Although a reaction was observed in mixture in the manner of hydrogen halides. The the presence of ferric chloride, as evidenced by the resultant increase in the acid concentration in the evolution of hydrogen chloride, it was not possible reaction medium as the reaction proceeds probably to separate the organic phase from the inorganic causes secondary reactions, thus diminishing the by-products. A number of homologs of benzene were yield of the primary products. alkylated with triethyl phosphate and triisopropyl At an early stage in this investigation it was phosphite. The results (Table II) indicate that found that the reactions of trialkyl phosphites, di- cumene and ethyl benzene undergo dealkylation to ALKYLATION OF AROMATIC COMPOUNDS WITH PHOSPHORUS ESTERS 1341 Table II. Alkylation of Aromatic Compounds with Triethyl Phosphate and Triisopropyl Phosphite in the Presence of Alumi- num Chloride in