This work has been digitalized and published in 2013 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution4.0 International License.

Dieses Werk wurde im Jahr 2013 vom Verlag Zeitschrift für Naturforschungin Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung derWissenschaften e.V. digitalisiert und unter folgender Lizenz veröffentlicht:Creative Commons Namensnennung 4.0 Lizenz.

ALKYLATION OF AROMATIC COMPOUNDS WITH PHOSPHORUS ESTERS 1 3 3 9

Analyse der Verbindungen: Die Einlagerungsverbin-dungen wurden vorsichtig mit HN03 aufgeschlossen. Das im Fall der Wolframverbindungen dabei ausfal-lende W03 wurde alkalisch gelöst. Wolfram und Mo-lybdän wurden als Oxinat, Titan als TiOa und Schwe-fel als BaS04 bestimmt. Die Bestimmung der Alkali-metalle erfolgte flammenphotometrisch. Kalium-wolf ramdisulfid: Präparat I: Kalium 8,3%,

Wolfram 66,9%, Schwefel 23,7%, Summe: 98,9%. 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 53,6%, Sdiwefel 35,5%, Summe: 99,9%. Zusam-mensetzung: K0!49MoS1?98 .

1 Auszug aus der Dissertation E. BAYER. Tübingen 1970. 2 W . RÜDORFF, Chimia [Zürich] 1 9 , 4 8 9 [ 1 9 6 5 ] , 3 H. M. SICK, Dissertation Tübingen 1959. 4 C . STEIN. J . P O U L E N A R D , L . B O N N E T A I N U. J . G O L E , C . R .

hebd. Seances Acad. Sei. 260, 4503 [1965].

Kalium-titandisulfid: Kalium 21,0%, Titan 33, 8%, Schwefel 44,2%, Summe: 99,0%. Zusammen-setzung: Ko^TiSj,^ .

Reaktionsprodukt von WS* mit Li-naphthalid (Uber-schuß) : Lithium 9,8%, Wolfram 66,3%, Schwefel 23,5%, Summe: 99,6%. Verhältnis 1 W : 2 Lili95S.

Reaktionsprodukt WS2 mit Na-naphthalid (Überschuß) : Natrium 26,9%, Wolfram 53,1%, Schwefel 18,8%, Summe: 98,8%. Verhältnis: 1 W : 2 Na2,0S.

Wir danken der Deutschen Forschungsgemeinschaft und dem Fonds der Chemie für die Unterstützung die-ser Arbeit.

5 T . E. HOVEN-ESCH U. J. SMID, J. Amer . chem. Soc . 87, 669 [ 1 9 6 5 ] .

6 W . BILTZ, P. EHRLICH U. M . MEISEL, Z. anorg. allg. Chem. 2 3 4 , 9 7 [ 1 9 3 4 ] ,

Friedel-Crafts Alkylation of Aromatic Compounds with Phosphorus Estersla b

G . S O S N O V S K Y a n d M . W . S H E N D E

Department of Chemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201

(Z. Naturforsch. 27 b, 1339—1348 11972] ; received June 26/August 22, 1972)

F r i e d e l - C r a f t s , 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.

F r i e d e l - C r a f t s alkylations of organic com-pounds with alkyl halides, alkenes, alcohols and a variety of alkylating agents are relatively old reac-tions on the chemist's list of synthetic techniques. The application of alkyl esters of organic and mine-ral acids to the F r i e d e l - C r a f t s synthesis 2 was

Requests for reprints should be sent to Prof. G. SOSNOVSKY, Department of Chemistry, University of Wisconsin-Mil-waukee, Milwaukee, Wisconsin 53201, U.S.A.

an important advance in the alkylation reaction. However, the alkylation reaction with alkyl esters of inorganic acids has not been investigated fundamen-tally or exploited practically to the same extent as the acid-catalyzed alkylations with either olefins or alkyl halides. The reason for this neglect might be the easy availability of alkyl halides and other alky-lating agents as compared to alkyl esters of in-organic acids as starting materials.

1340 G. SOSNOWSKY AND M. W. SHENDE

The literature contains a survey of alkylation reactions with alkyl esters of inorganic acids such as alkyl sulfates2, sulfites2, chlorosulfites2, sulfonates2, chlorosulfonates2, p-toluene sulfonates2, alkyl o-silicates2, alkyl carbonates2, alkyl borates2, and hypochlorites 2. However, the analogous reactions in-volving the use of phosphorus esters, such as tri-alkyl phosphites (1) , dialkyl phosphites (2) , and tri alkyl phosphates (3) , have received relatively little attention. Thus, n-'butyl phosphate3 was used to alkylate benzene in the presence of boron fluoride, and triethyl phosphate 4' 5 triisopropyl phos-phate 4 and tributyl phosphate4 were used for the alkylation of benzene in the presence of aluminum chloride. These experiments were limited to benzene as the substrate, and the reaction products were not critically examined. Definitive studies in this area have become possible only since the advent of gas chromatography.

Recently, a systematic attempt has been made to investigate the use of phosphorus esters in alkyla-tions6a 'b . At present a variety of phosphate and phosphite esters are commercially available at a low cost. These esters are liquids of low volatility and, therefore, make very suitable starting materials. The objectives of the present work were to study the scope and mechanism of alkylation reactions of various aromatic hydrocarbons with trialkyl phos-phites (1) , dialkyl phosphites (2) , and trialkyl phosphates (3) .

RCK. R n R O — P : p n > p - ° H R 0 / RO

1 2 R = C H 3 , j-C3H- R = C H 3 , COH, .

i-C3H7

Results and Discussion

The conditions under which phosphorus esters are employed in alkylations are different from those used in alkylations with alkyl halides 2. One reason is that the inorganic acid, formed as a by-product, is non-volatile and cannot escape from the reaction mixture in the manner of hydrogen halides. The resultant increase in the acid concentration in the reaction medium as the reaction proceeds probably causes secondary reactions, thus diminishing the yield of the primary products.

At an early stage in this investigation it was found that the reactions of trialkyl phosphites, di-

alkyl phosphites and trialkyl phosphates with aro-matic substrates in the presence of aluminum chlo-ride are extremely exothermic. Therefore, it was necessary to add these esters to the mixture of the aromatic substrate — aluminum chloride at 5° ± 2 ° , followed by stirring the reaction mixture at room temperature.

A number of variables were examined in order to determine their effect on the reaction. It was found that the combined yield of mono- and diethyl benzenes was very dependent on the ratio of the ester to aluminum chloride. At least a 1:1 molar ratio of alkyl group in the alkylating ester to alu-minum chloride is required for optimum yields (Table I) . In practice, a slight excess of aluminium chloride was used in order to compensate for losses during handling. Thus, the molar ratios of alumi-

Table I. Ethylation of Benzene a with One Mole Triethyl Phosphate. Effect of Variation in the molar Ratio of Alumi-

num Chloride to Triethyl phosphate.

Products13

AlCl3 / (C 2 H 5 0)3P0 Monoethyl Diethyl mole ratio Benzene Benzene

Yield % Yield %

0.5 11 0.9C

1 16 2.6C

1.5 18 1.4C

3.5 26 44 D

4 26 41D

;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-lected or positively identified by glpc. It shows the same reten-

tion time as /n-diethyl benzene d m-Diethyl benzene.

num chloride to alkylating ester were 2.5 to 1 and 3.5 to 1 in the case of diester and triester, respec-tively.

Ferric chloride, cupric chloride, stannic chloride, zinc chloride, sulfuric acid (98%) and phosphoric acid (85%) were examined as catalysts, but with the exception of ferric chloride, no detectable reac-tion was observed, even at elevated temperatures and on prolonged reaction times, as evidenced by glpc analysis. Although a reaction was observed in the presence of ferric chloride, as evidenced by the evolution of hydrogen chloride, it was not possible to separate the organic phase from the inorganic by-products. A number of homologs of benzene were alkylated with triethyl phosphate and triisopropyl phosphite. The results (Table II) indicate that cumene and ethyl benzene undergo dealkylation to

R O \ R O — P = Ö : R O ^ 3

R = C 2 H 5

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 Excess Substrate

Aromatic Alkylating Substrate Ester (Mole) (Mole)

Monoalkylated % Isomer Distribu- Dialkylated Products tion of Mono- Products (Yield % ) alkylated Products (Yield % )

% Isomer Distribution of Dialkylated Products

Toluene3 ( C 2 H 5 0 ) 3 P 0 Monoethyl toluenes 7 73 20 (1.5) (0.1) (57) Benzene c ( i -C 3 H 7 0) 3 P Cumene (0.75) (0.05) (60) Toluene a ( i -C 3 H 7 0) 3 P Cymenes tracee 71 29 (0.75) (0.05) (44)

Cumene ( i -C 3 H 7 0) 3 P Diisopropyl benzenes 0 65 35 (0.75) (0.05) (114) Ethyl ( i -C 3 H 7 0) 3 P h benzene c

(0.75) (0.05)

Diethyl toluene l-Methyl-3,5-diethyl (19) benzene on ly b

Diisopropyl benezenes o d + p = 32 (12) " m = 68 Diisopropyl l-Methyl-3,5-diisopropyl toluene benzene (traces of (10) 1,2,4-isomer f) Triisopropyl benzene 1,3,5-Triiso-propyl (27) benzene only

a Reaction time 4.5 hr. b Compound identified by NMR. c Reaction time 3.5 hr. d Contains trace quantities of o-isomer. c Trace quantities of o-isomer formed but could not be separated by glpc. f IR spectrum indicates 1,2,4-substitution. S Sub-strate shows dealkylation to benzene; 6.17 g benzene isolated by fractionation. h Extensive dealkylation of the substrate; among products identified on column (A) are benzene, cumene, m-diisopropyl benzene, p-diisopropyl benzene, and on column (B) m-diethyl benzene, p-diethyl benzene. In addition products corresponding to four major peaks were not identified. § Mole ratio of aluminum chloride to alkylating ester, see p. 1340. Reaction temperature, see p. 1346.

benzene. In the isopropylation of ethyl benzene a number of aromatic hydrocarbons, such as benzene, cumene, m-diethyl benzene, p-diethyl benzene, m-diisopropyl benzene, p-diisopropyl benzene, and substantial quantities of dialkylated products are formed.

These results are in agreement with the already known fact that, depending on the alkylating agent, the nature and amount of catalyst, and the reaction conditions, dealkylations, intramolecular and inter-molecular isomerizations (disproportionation or transalkylation) occur in F r i e d e l - C r a f t s type alkylations of alkyl aromatics7a ' 8. These reactions also occur when alkyl aromatics are treated with a L e w i s acid, e. g.. metal halide7a. In these reac-tions 7a' 9, the methyl group is much more resistant to migration than higher alkyl groups. Toluene did not dealkylate under these conditions.

The dealkylations observed with cumene and ethyl benzene, as well as subsequent disproportiona-tion with ethyl benzene yielding a number of aro-matic hydrocarbons is due to the use of aluminum chloride in amounts far in excess of a catalytic quantity7a. Variation in the molar ratio of alkyl aromatic to alkylating ester was examined in order to find conditions which minimize the formation of higher alkylated products. The use of a large excess of aromatic hydrocarbon has been recommended 7b

in order to reduce polysubstitution. In the ethylation of toluene with triethyl phosphate, the yield of di-ethyl toluene could be reduced to trace quantities by increasing the toluene to triethyl phosphate molar ratio to 45 : 1, or to 15 moles : 1 mole of substrate to alkyl group in the ester. The overall yield of monoethyl toluenes decreased beyond a 30 : 1 molar ratio of toluene to triethyl phosphate, presumably

Table III. Ethylation of Toluene with Triethyl Phosphate in the Presence of Aluminum Chloride. Effect of Variation in Molar Ratio of Substrate to Alkylating Ester

PhMe/(C2H 50) 3P0 Monoethyl % Isomer Distribution Diethyl Toluene % Isomer Distribution Mole Ratio Toluenes Yrield % of Monoethyl Toluenes Yield % of Diethyl Toluenes

0 m V 15: l a 57 7 73 20 19 l-Methyl-3,5-diethyl

benzene only b

30: l c 83 4 80 16 traces'1 —

45: l e 74 6 77 18 tracesd —

§ Mole ratio of aluminum chloride to alkylating ester, see p. 1340. Reaction temperature, see p. 1346. a Reaction time 4.5 hr. t> Compound identified by NMR. c Reaction time 3 hr. d Product could not be collected over glpc and analyzed.

1342 G. SOSNOWSKY AND M. W. SHENDE

Table IV. Alkylation of Aromatic Compounds with Phosphorus Esters in the Presence of Aluminum Chloride. Effect of Varia-tion in Reaction Time

Aromatic Alkylating Reaction Monoalkylated % Isomer Distri- Dialkylated % Isomer Distribution Substrate Ester Time Products bution of Mono- Products of Dialkylated Products (Mole) (Mole) hr (Yield % ) alkylated Products (Yield % )

o m p Cumene (i-C3H70)3P 1 Diisopropyl 0 65 35 Triisopropyl 1,3,5-Triisopropyl (0.75) (0.05) benzenes

(114)a benzene (27)

benzene only

Cumene (i-C3H70)3P 3.5 Diisopropyl 0 65 35 Triisopropyl 1,3.5-Triisopropyl (0.75) (0.05) benzenes

(114)a benzene (27)

benzene only

Cumene (*-C3H70)3P 24 Diisopropyl 0 65 35 Triisopropyl 1,3,5,-Triisopropyl (0.75) (0.05) benzenes benezene benzene only (0.75) (0.05)

(94) (52) benzene only

Tol uene ( i -C 3H 70) 2P(0)H 0.5 Cymenes trace'3 75 25 Diisopropyl l-Methyl-3,5-(2.0) (0.05) (67) toluene (trace0) diisopropyl benezene Toluene (i-C3H70)3P 4.5 Cvmencs trace13 71 29 Diisopropyl l-Methyl-3,5-diisoprop\ (0.75) (0.05) (43) toluene

(10) benzene (traces of l,2,4-isomerd)

Toluene (C 2H 50) 2P(0)H 0.5 Monoethyl 27 56 17 trace e

(2.0) (0.05) toluenes (61) Toluene (C 2 H 5 0) 3 P0 3 Monoethyl 6 77 18 trace e

(1.5) (0.033) toluenes (74) Toluene (CH 30) 2P(0)H 0.5 Xylenes 51 21 28 trace e

(2.0) (0.05) (56) Toluene (CH 3 0) 2 P(0)H 24 Xvlenes 26 62 12 trace e

(2.0) (0.05) (57)

5 Mole ratio of aluminum chloride to alkylating ester, see p. 1340. Reaction temperature, see p. 1346. a Substrate shows de-alkylation to benzene, hence, more than theoretical amount of product formed. b The o-isomer, formed in trace quantities, could not be separated by glpc. c Yield could not be calculated. d The IR spectrum indicates 1,2,4-substitution. e Yield could not be calculated. The product could not be collected or identified by glpc.

because of additional losses during the isolation procedure (Table III and Experimental pp. 1346.

Finally, the reaction time was examined in order to determine its effect on the orientation and yield of the monoalkylated and dialkylated products (Table IV ) . Short reaction times could not be exa-mined conveniently since the exothermic reaction precluded a rapid addition of the alkylating ester to the substrate — aluminum chloride mixture. The al-kylations reported so far in Tables I, II, III, and IV used an excess of the aromatic substrate as the reactant and solvent, and were, therefore, particu-larly sensitive to isomerizations 10. The orientation of products in these reactions indicates the forma-tion of considerable amounts of the m-dialkyl ben-zenes and the 1,3,5-isomer. Due to isomerizing con-ditions, considerable disproportionation and trans-alkylation also occur to give thermodynamically controlled equilibrium mixtures of xylenes, diethyl benzenes, monoethyl toluenes, cymenes and diiso-propvl benzenes. In the literature are found nume-rous examples of the formation of such equilibrium mixtures containing high proportions of meta-iso-

mers in the aluminum chloride catalyzed isomeriza-tions of dialkyl benzenes 1 1 - 1 3 a - d and in the alumi-num chloride catalyzed alkylations of toluene with ethylene14 and propylene14 , and of cumene with propylene 14. The evidence of whether the high m-isomer contents are due to direct substitution in-volving a reaction of low selectivity 1 5 a _ d or to a concurrent or consecutive secondary isomerization of the initially formed o- and p-dialkyl benzenes is not conclusive 7 c ' 1 0 ' 1 6 ' 1 7 .

In order to eliminate side reactions such as de-alkylation and to minimize polysubstitution, we found that a solvent system composed of nitromethane and dichloromethane is advantageous for the alky-lation reactions with trialkyl phosphites, dialkyl phosphites and trialkyl phosphates. Nitromethane is used to dissolve the aluminum chloride, and di-chloromethane is used as the solvent for the sub-strate and the alkylating ester, thus giving homo-geneous conditions. The molar ratios of aromatic substrate to aluminum chloride to alkylating ester were the same as those used in the previous experi-ments, viz., 45 : 3.5 : 1 and 30 : 2.5 : 1 in the case

ALKYLATION OF AROMATIC COMPOUNDS WITH PHOSPHORUS ESTERS 1343

Table V. Alkylation of Aromatic Compounds with Phosphorus Esters a in Dichloromethane in the Presence of Aluminum Chloride —Nitromethane. Effect of Variation in Reaction Time

% Isomer Aromatic Substrate Alkylating Ester Reaction Monoalkvlated Products Distribution of Mono-(Mole) (Mole) Time hr (Yield % ) alkylated Products15

Toluene ( / -C3H70)3P 0.5 Cymenes o m 43 25

P 32

(1.125) (0.025) (52) Toluene ( i -C3H70)3P '•> Cymenes c 43 26 31 (1.125) (0.025) (69) Toluene ( i -C3H70)3P 24 Cymenes 43 26 31 (1.125) (0.025) (83) Ethyl benezene ( i -C3H70)3P 0.5 Ethyl isopropvl benzenes'1 Not resolved (0.75) (0.016) (46) Ethyl benzene (t-C3H70)3P 24 Ethvl isopropvl benzenes'1 Not resolved (0.75) (0.016) (87)" Ethvl benzene (C 2 H 5 0) 2 P(0)H 4 Diethyl benezenes 42 26 32 (0.75) (0.025) (17) Ethyl benzene (C 2 H 5 0) 2 P(0)H 24 Diethvl benzenes 38 29 33 (0.75) (0.025) (50) Cumene ( i -C3H70)3P 18 Diisopropvl benzenes o + p = 62 (1.125) (0.025) (84) m = 38 Toluene (C 2 H 5 0) 3 P0 20 Monoethyl toluenes 51 30 19 (0.562) (0.0125) (34) Toluene (C 2 H 5 0) 2 P(0)H 20 Monoethyl toluenes 54 28 18 (0.75) (0.025) (38)

§ Ratios of aluminum chloride, see p. 1342. Reaction temperature, see p. 1346. a No reaction obtained between ethyl benzene and trimethyl phosphite, and toluene and trimethyl phosphite and dimethyl hydrogen phosphite, respectively, b Only trace quantities of dialkylated products were formed in these reactions. c The isomer distribution of cymenes was not resolved by glpc. The IR spectroscopic examination for variation in isomer distribution indicated no change from the isomer distribu tions of cymenes formed in experiments with reaction times of 0.5 and 24 hr, respectively. d Ethyl isopropyl benzenes were

isomerically similar by IR spectroscopy.

of triesters and diesters, respectively. The following weight ratios were maintained with different reactants and their solvents: alkyl aromatic to di-chloromethane, 1 : 1 ; aluminum chloride to nitro-methane, 1 : 3.5; and the alkylating ester to di-chloromethane, 1 : 3.5.

The use of this solvent system prevented the de-alkylation of alkyl aromatics to benzene, and the formation of dialkylated products was reduced to a minimum. The scope of the alkylation reaction using phosphorus esters could thus be extended to a number of alkyl benzenes (Table V ) . In the di-chloromethane — nitromethane solvent system, the isopropyl esters are the most reactive, ethyl esters have intermediate reactivity and the methyl esters fail to react. This result is in agreement with the observation by S C H M E R L I N G 18 that in the aluminum chloride — nitromethane system the secondary alkyl chlorides are more reactive than the primary iso-mers. The isopropylation of toluene with triisopro-pyl phosphite using aluminum chloride — nitro-methane and dichloromethane yields cymenes con-taining a higher percentage of the m-isomer 19 than

is produced in alkylations with isopropyl bromide 16

and propylene16 in nitromethane solvent in the presence of aluminum chloride.

The effect of reaction time in the alkylation reactions with these esters with aluminum chloride — nitromethane and dichloromethane is shown in Table V. Enhanced yields of alkylated products were obtained with increased reaction time. The variation in reaction time had no effect on the iso-meric composition of the products formed in ethylation and isopropylation reactions. The simi-larity in the isomer distributions of these dialkyl benzenes was determined by glpc and by infrared spectroscopy using the out-of-plane carbon — hydro-gen deformation absorption bands at 13.0 — 13.6/ / for the o, at 1 2 . 3 - 1 3 . 3 / * and 1 4 . 0 - 14.8 ju for the m, and at 11.6 — 12.5 JLI for the p-isomers. The mixture of isomers of dialkylated benzenes formed in each experiment was collected over glpc and was found on infrared analysis to be spectroscopicallv identical with a mixture of isomers formed in a similar experiment with a different reaction time. Subjecting a mixture of known composition of o-,

1344 G. SOSNOWSKY AND M. W. SHENDE

m- and p-diethyl benzenes to simulated reaction conditions over a period of 4 hrs did not change the composition of the mixture as regards percentages of o-, m- and p-isomers.

These results indicate that the aluminum chlo-ride — nitromethane — dichloromethane solvent sys-tem provides practically non-isomerizing reaction conditions. Additional information on the activity of these esters was obtained from studies of com-petitive isopropylations of toluene — benzene and re-lated alkyl benzenes — benzene in the nitromethane — dichloromethane solvent system. The molar ratio of combined aromatic substrate to aluminum chloride to triisopropyl phosphite was 45 : 3.5 : 1; thus a constant excess of the combined aromatic substrate was maintained. The relative reactivities A;a,.omaticf b̂enzene were calculated 20 from the amounts of iso-

propylated aromatic and cumene formed. As only trace quantities of dialkylated products were formed in these competitive alkylations, they did not affect the calculated &aromatjc/A:benzene results seriously 16.

The method of competitive rate determination can be used to obtain relative reactivities only if the observed relative rates are dependent on the aro-matic substrates 1 0 '1 6 . In order to establish whether actual competition exists between benzene and the investigated alkyl benzenes, the relative reactivities in toluene — benzene mixtures were determined by changing the concentration of toluene — benzene mixtures. The results in Table VI show that the re-lative rate remains constant and that a first order dependence on the aromatic substrate concentration is indicated10 '16 . The relative reactivities of a number of alkyl benzenes over that of benzene, as obtained by the method of competitive isopropyla-tion, are summarized in Table VII.

The isopropylation with triisopropyl phosphite shows low substrate selectivity between the two aro-matic compounds and low positional selectivity be-tween the m and p positions of toluene. This result is plausible considering the value of 3.39 for the relative reactivity of toluene to benzene, and the formation of 26% m-cymene and 31% p-cymene in the isopropylation of toluene. The average value of 3.39 is somewhat higher than the values of kyjk^ for the isopropylation reactions involving different alkylating agents 1 0 '1 5 d '1 6 - 20. The values of selecti-vity factor, Sf1 5 d ' 21, and partial rate factors for o (of) , m (m f ) , and p (pf) positions were calculated from the orientation data obtained in the isopropy-

lation of toluene with triisopropyl phosphite, for comparison with similar values reported previously for other isopropylations (Table VIII) . From a

Table VI. Competitive Isopropylations a of Toluene —Benzene in Dichloromethane with Triisopropyl Phosphite and Alumi-num Chloride —Nitromethane. First Order Dependence of

the Isopropylation in Aromatics §.

Toluene Cymenes Cumene Observed Relative Rates Benzene Yield % Yield % Relative According to Mole Rates First Order Ratio (ki/kn) Dependence in

Aromatics

1 : : 1 53.5 18.15 2.95 (2.99) c(L) = 2.99 1 : : l b 73.4 24.1 3.04

(2.99) c(L)

1 : : 4 28.2 34.4 0.82 (0.82 ( j ) - 3.28 2 : : 3 57.5 23.4 2.46 (2.46) (4) = 3.69 3 : : 2 75.4 14.0 5.36 (5.36) (J) = 3.57 4 : : 1 79.6 5.8 13.7 (13.7) (1) =3 3.44

Average = 3.39

§ Ratios of aluminum chloride, see p. 1342. Reaction tempera-ture, see p. 1346. a Reaction time 2.25 hrs. *> Reaction time 3 hrs. e The value 2.99 is an average of two values obtained

with 1:1 molar ratio of toluene to benzene.

Table VII. Competitive Isopropylations a of Alkyl Benzenes — Benzeneb in Dichloromethane with Triisopropyl Phosphite

and Aluminum Chloride — Nitromethane

Aromatic Substrate

Benzene 1.00 Toluene 3.39 c

Ethyl benzene 2.50d

?i-Propyl benzene 2.87 p-Cymene 2.54 Chlorobenzene 0.44

8 Ratios of aluminum chloride, see p. 1342. Reaction tempera-ture, see p. 1346. a Reaction time 2.25 hrs. b A 1:1 molar ratio of alkyl benzene:benzene used. c Average value from Table VI. d Average of two values obtained with 1:1 molar

ratio of ethyl benzene : benzene.

survey of a number of substitution reactions, B R O W N

and S T O C K 2 1 selected the limiting value of the ratio (log p f ) / ( l og m {) to be 4.04 + 0.55 for adherence to the selectivity relationship. The average value of 3.39 for kr/kft (Table VI) and the values of par-tial rate factors (Table VIII) indicate that the iso-propylation reaction with triisopropyl phosphite has a somewhat higher substrate selectivity and some-what lower positional selectivities as compared to isopropylation reactions with other isopropylating agents. The reaction does not show adherence to the selectivity relationship.

An electrophilic substitution mechanism for the isopropylation of toluene by triisopropyl phosphite is supported by the following facts: (1) the pre-

ALKYLATION OF AROMATIC COMPOUNDS WITH PHOSPHORUS ESTERS 1 3 4 5

Table VIII. Selectivity Factors and Partial Rate Factors for F r i e d e l - C r a f t s Isopropylations of Toluene with Isopropyl Bromide, Propylene and Triisopropyl Phosphite.

Alkylating Catalyst Agent

Temp. % Isomer Distri- Partial Rate Selectivity log pj [°C] bution of Cymenes Factors Factor log »if

Reference

Of rri{ p{ Isopropyl Gallium bromide 25 26.2 26.6 47.2 1.45 1.47 5.20 0.548 4.28 15a

bromide Isopropyl

bromide Gallium bromide 25 26.2 26.6 47.2 1.52 1.41 5.05 0.554 4.72 24 a

Propylene B F 3 - E t 2 0 or 5 37.5 29.8 32.7 0.342 Propylene A1C13—CH3N02 2.37 1.80 4.27b 2.46 25

Propylene B F 3 — E t 2 0 or 65 37.6 27.5 34.9 0.404 Propylene A 1 C 1 3 - C H 3 N 0 2

Triisopropyl A 1 C 1 3 - C H 3 N 0 2 , 5 ± 2C 43.1 26.1 30.8 4.38 2.65 6.26d 0.373 e 1.88 present phosphite solvent CH2C12 work

a Partial rate factors calculated from relative rate data &T/A:B = 1.82 and isomer distribution of cymenes of ref. ,5a. b Calcu-lated from isomer distributions of cymenes of ref. 28 by interpolation of data at 5 ° and 65® to 40°. c Addition of alkylating ester to substrate + A1C13—CH3N02 at this temp, followed by stirring the crude reaction mixture at room temp. d Partial

rate factors calculated from relative rate data &T/&B=3.39 with the use of the following eqs.: Of——? x r e a t * v e r a t e x ^

m x relative rate x 3 m f = 100 log (2 x % p/% m).

and pt = % p x relative rate x 6

100

100 e Selectivity factor calculated with the use of the eq. Sf =

dominant o,p-orientation with toluene; (2) the re-latively large amount of m-isomer indicating that the attacking species possesses high activity 15b and, therefore, low selectivity; (3) the necessity of a strong L e w i s acid in the reaction and the ability of the reaction to proceed at low temperatures; (4) the relative rates (^toiueneMWene) being in the same range as the values reported for those reac-tions, e. g., alkylations, entailing electrophiles of comparatively high activity22. The dialkyl phos-phites (2) show little or none of the nucleophilic reactivity of the trialkyl esters (1) 23. Furthermore, they are only weakly acidic. It is now well established that they exist almost exclusively in the phosphonate form 23 (4 ) .

ROv. R O x y O > P - O H > Pf

R O ' R O x x H Phosphite form Phosphonate form

2 4

Complex formation with a strong L e w i s acid is possible in (1) at the phosphorus and oxygen atoms of the alkoxy group, and in (3) and (4) at the oxygen atoms of the alkoxy and the phosphoryl groups. Complexation of the catalyst at the oxygen of the alkoxy group is suggestive in all three cases, as (1) the oxygen is the negative end of the phos-phorus — oxygen dipole, (2) complexing at the oxy-gen atom enhances the electrophilic character of the

alkyl group and (3) it justifies the need for a 1 : 1 molar ratio of the alkyl group in the ester to alu-minum chloride for optimum yields in the alkyla-tion reactions. For a general reaction of the type

(RO) 3P + 3 ArH + A1C13 ->• 3 RAr + A1P03 + 3 HCl

the following reaction scheme is in overall agree-ment with our observations.

A1C13

A1C13 + P (OR) 3 P ( O R ) 2 O R — * ArR + HCl + P(0R)20A1C12

A1C13

P (OR) 20A1C12 + A1C13 - > P (OR) (OR) 0A1C12

ArR + HCl + P(OR) (0A1C12)2

AICI3

P(OR) (0A1C12)2 + A1C13 P (ÖR) (0A1C12)2

ArR + HCl + P(0A1C12)3

P (0A1C12) 3 A1P03 + 2 A1C13

In the alkylations with triethyl phosphate and tri-isopropyl phosphite, the presence of the phosphate ion in the aqueous layer resulting from work-up of the reaction mixture was confirmed by qualitative analysis. In the dichloromethane — nitromethane sol-vent combination no free aluminum chloride capable of direct coordination with the alkylating ester is present since aluminum chloride forms a stable com-plex 16 with nitromethane, A1C1 3 CH 3 N0 2 . Any

1346 G. SOSNOWSKY AND M. W. SHENDE

interaction with the alkylating ester must, therefore, involve the aluminum chloride — nitromethane com-plex. Owing to the competition of complexing of aluminum chloride with nitromethane and the alky-lating ester, the alkylation reactions performed in this solvent system show a marked slowness. The aluminum chloride — nitromethane complex pre-sumably fails to polarize the carbon — oxygen bonds in the methoxy groups of trimethyl phosphite and dimethyl hydrogen phosphite, leading to the un-reactivity of these esters in the solvent system.

It is also possible to consider isopropylation with aluminum chloride — nitromethane in dichloro-methane solvent as a concerted nucleophilic dis-placement reaction by the aromatic substrate of the catalyst : nitromethane : alkylating ester complex. Such displacement mechanisms have been suggested by B R O W N and co-workers 1 5 c ' d for alkylations in-volving primary halides. As to the nature of the ef-fective alkylating species, the data of the present investigation do not allow final conclusion. The sug-gested L e w i s acid — alkylating ester complexes may split into relatively free carbonium ions, or they may remain in a partially polarized state and react. The discussion of the carbonium ion character is only justified on the basis of overall product distribution.

Experimental

Materials. The aromatic hydrocarbons were dried azeotropically and stored over anhydrous calcium chloride before use. They were of high purity according to glpc analysis. Nitromethane (spectrograde, Eastman Organic Chemicals) was distilled over anhydrous cal-cium chloride, and dichloromethane (Aldrich Chemical Co.) was distilled over calcium hydride before use. An-hydrous aluminum chloride (reagent grade, Allied Chemical and Mallinckrodt Chemical Works) was used without further purification. The phosphorus esters were best commercial grade used without purification. The following aromatic compounds were used as authentic materials for identification of products: o-and p-xylene (Eastman Kodak Co., White Label), m-xylene (spectrograde, Eastman Kodak Co., White Label), o-, m-, and p-diethyl benzene (Aldrich Chemical Co.), m- and p-diisopropyl benzene (Aldrich Chemical Co.), o-, m- and p-ethyl toluene (Aldrich Chemical Co.), 3,5-diisopropyl toluene (Alfred Bader Chemicals, Division of Aldrich Chemical Co.), 1,3,5-triisopropyl benzene (J. T. Baker Chemical Co.), and p-cvmene (terpene-free, Eastman Organic Chemicals).

Analytical Procedures. Glpc analyses were performed on a Varian Aerograph A90P3 gas Chromatograph equipped with a thermal conductivity detector. T h e

following overall conditions were maintained: injector temperature, 150°; detector temperature, 220°; bridge current, 150 ma; helium pressure, 50 psi. The fol-lowing columns were used: Column (A) — 6 ft by 0.25 in., Aluminum column, 20% Ucon 50 HB 280 on 60/80 mesh acid washed Chromosorb W ; Column (B) — 21 ft by 0.25 in., Aluminum column, 9.5% Bentone 34 + 4.5% Dow 550 on 60/80 mesh acid washed, DCMS-treated Chromosorb P.

Infrared spectra were obtained with a Perkin-Elmer 137 spectrophotometer, and nmr spectra with a Varian T-60 spectrometer on 10% (v/v) samples in carbon tetrachloride using an internal TMS standard. Mole-cular weights were determined isopiestically on a Hitachi Perkin-Elmer Model 115 Molecular Weight apparatus.

Alkylations with Phosphorus Esters in the Presence of Excess Aromatic as Reactant and Solvent (Tables I through IV). General Procedure A: The ester was ad-ded as rapidly as the exothermic reaction permitted at 5° ± 2 ° to a well stirred suspension of aluminum chloride in the aromatic hydrocarbon maintaining the reaction mixture at 5° ± 2°by external cooling. On com-pletion of the addition the reaction mixture was stir-red at room temperature. The total reaction time spe-cified for a particular reaction includes the time re-quired for addition of ester followed by stirring the crude reaction mixture for the rest of the time. The reaction mixture was then carefully poured onto crushed ice with stirring. The organic phase was sepa-rated and the aqueous phase was extracted with ether (50 ml). The ether extract was combined with the main organic phase and the ether removed by distillation. The organic phase was then washed successively with 10% sulfuric acid, 10% sodium bicarbonate solution and water until neutral to litmus, dried (Na2S04) , and concentrated by removing excess aromatic substrate by distillation under nitrogen at atmospheric pressure. The aromatic substrate, thus removed, was analyzed by glpc in order to ascertain that the reaction pro-ducts were not lost. The resultant concentrated solution containing the reaction products was then analyzed by glpc.

Alkylations with Phosphorus Esters in the Presence of Aluminum Chloride— Nitromethane and Dichloro-methane (Table V through VII). General Procedure B: To a cold (5°) solution of the aromatic hydrocarbon in dichloromethane was added a solution of aluminum chloride in nitromethane in one portion. A solution of the ester in dichloromethane was then added to the resultant solution at 5° + 2° as rapidly as the exother-mic reaction permitted maintaining the reaction mixture at this temperature by external cooling. On completion of the addition the reaction mixture was stirred at room temperature as indicated in General Procedure A. The reaction mixture was then carefully poured onto crushed ice with stirring. The organic phase was separated, washed successively with 10% sulfuric acid, 10% sodium bicarbonate solution and water until neutral to litmus, dried (Na2S04), and con-centrated by removing dichloromethane, nitromethane.

ALKYLATION OF AROMATIC COMPOUNDS WITH PHOSPHORUS ESTERS 1 3 4 7

and excess aromatic substrate by distillation under nitrogen at atmospheric pressure. Dichloromethane, nitromethane, and excess aromatic substrate, thus re-moved, were analyzed by glpc in order to ascertain that the reaction products were not lost. The resultant concentrated solution containing the reaction products was then analyzed by glpc.

Product Yields. The yields of various mono-, di- and trialkylated aromatics were determined by glpc on column (A).

In competitive isopropylations (Tables VI and VII) the relative response ratios of the components present in the concentrated solution containing the reaction products to that of an added internal standard were determined27. Benzene was used as an internal stan-dard. One of the pure components, the isopropylated aromatic, was previously collected over glpc for use in the mixture of known composition. Authentic samples of other components were used. The yields of alkylated aromatics are based on the moles of alkylating ester used in each reaction, assuming that every alkyl group can react.

Product Identification and Isomer Distributions. The concentrated solution containing the reaction pro-ducts obtained in procedure A or B was analyzed by glpc on column (A). The peaks corresponding to mono-, di-, and tri-substituted alkyl aromatics were always well separated and products corresponding to these peaks were collected from the same column. The mono-substituted aromatic hydrocarbons were identi-field by comparison of their IR spectra with those of authentic samples.

The mixture of di-substituted aromatic hydrocar-bons, collected on column (A) was analyzed by IR spectroscopy for the aromatic substitution pattern in the 11.6 — 14.8 /u region. Characteristic absorption wavelengths (strong absorptions), associated with each isomer, were used to determine the isomers in the pro-duct mixture.

Di-substituted Absorption Isomer Range o 13.0 — 13.6 p m 12.3-13.3 p

14.0-14.8 p p 11.6 — 12.5 p

The isomeric di-substituted aromatic hydrocarbons were then separated on column (B). Each of the isomers was identified by peak enhancement on injection with an authentic sample. As the authentic samples of o-cymene, m-cymene and o-diisopropyl benzene could not be obtained, and as these products could not be separately collected over glpc for identification, the peak assignment on the gas chromatogram was done in these cases on the strength of the IR absorption pattern of the mixture of isomers collected on column (A). .

The isomeric tri-substituted aromatic hydrocarbons were well separated on column (A), and were col-lected and identified by comparison of their IR spectra with those of authentic samples, if available, or by

NMR. The following absorption wavelengths were used for the identification of the pattern of substitu-tion: Tri-substituted Isomer Absorption Range 1.2.4-trisubstituted 11.7 — 12.4/* (strong)

11.0—11.5 p (medium) 1.3.5-trisubstituted 11.0 — 12 .0« (strong)

14.1 —15.0 p (medium)

Isomerization and Recovery Studies. A known mix-ture of isomeric diethyl benzenes (4.0 g, 19.6% o, 31.0% m, and 49.4% p) was prepared. To 3.70 g of the above mixture, dissolved in ethyl benzene (79.62 g, 0.75 mole) and dichloromethane (65 ml), was added a solution of aluminum chloride (8.33 g, 0.0625 mole) in nitro-methane (27 ml) at 5° in one portion. On completion of the addition the mixture was stirred at room tem-perature for 4 hrs. After the work-up as described in General Procedure B, 128.97 g of dry organic phase was isolated. A portion of the organic phase, 35.21 g, was concentrated by removing solvent and excess ethyl benzene by distillation at atmopheric pressure under a current of nitrogen yielding 5.80 g of a solution con-taining isomeric diethyl benzenes. The analysis by glpc on column (A) indicated 3.54 g (96.0%) of di-ethyl benzenes. The mixture of isomeric diethyl ben-zenes present in the concentrated solution was col-lected from column (A). The recovered diethyl ben-zene mixture was compared to the starting material by IR spectroscopy and was found to be isomerically identical within experimental limits.

Competitive Isopropylation of Toluene —Benzene with Triisopropyl Phosphite. Competitive isopropylation of toluene — benzene mixtures was carried out by changing the concentration of toluene — benzene mix-tures from 1:1 molar ratio to 1 :4 , 2:3, 3 :2 , and 4:1. The data obtained are summarized in Table VI.

Competitive Isopropylation of Alkyl Benzenes —Ben-zene. These were carried out with a 1:1 molar ratio of alkyl benzene — benzene. The data obtained are summarized in Table VII. The analyses of the pro-ducts obtained on isopropylation of the alkyl ben-zenes are indicated for each case.

Ethyl Benzene. The molecular weight of the mix-ture of monoalkylated products corresponded to the molecular formula CX1H16. Found: 151, calcd. for CnH 1 6 : 148. The IR spectroscopic analysis of the product mixture indicated the presence of o-, m-, and p-isomers. The product mixture could not be resolved by glpc.

n-Propyl Benzene. The glpc analysis for the pro-ducts was done on column (B). The molecular weight of the mixture of monoalkylated products corresponded to the molecular formula C12H18 . Found: 165, calcd. for C12H18: 162. The IR spectroscopic examination of the product mixture failed to show clear bands due to o-, m-, and p-isomers. The product mixture could not be resolved by glpc analysis.

p-Cymene. The glpc analysis for the products was done on column (B). The product was identfied to be diisopropyl toluene, corresponding to the molecular

1 3 4 8 ALKYLATION OF AROMATIC COMPOUNDS WITH PHOSPHORUS ESTERS 1348

formula C1 3H2 0 • Molecular weight found: 176, calcd. for C 1 3 H 2 0 : 176. The IR spectroscopic examination showed a medium band at 11.2 /z and a strong band at 12.2 n (1,2,4-substitution), and a strong isopropyl doublet at 7.3 n and 7.4 /u. The position of the in-coming isopropyl group (o to methyl or isopropyl) could not be assigned.

Chlor ob enzene. The glpc analysis for the products was done on column (B ) . The molecular weight of the mixture of monoalkylated products corresponded to the molecular formula C 9 H U C 1 . Found: 151, calcd. for

1 (a) This investigation was supported by grants from the Public Health Service, U.S. Department of Health, Educa-tion and Welfare, GM 16741 and from the Graduate School of the University of Wisconsin-Milwaukee; (b) Taken in part from M. S. Thesis of M. W. SHENDE, University of Wisconsin-Milwaukee, 1971.

2 F. A. DRAHOWZAL, in: " F r i e d e l - C r a f t s and Related Reactions", Vol. II, p. 641, G. OLAH, Ed., Interscience Publishers, New York 1964.

3 J . F . M C K E N N A a n d F . J. S O W A , J . A m e r . c h e m . S o c . 5 9 , 1204 [19371.

4 N . BERMAN a n d A . L O W Y , J . A m e r . d i e m . S o c . 6 0 , 2 5 9 6 [ 1 9 3 8 ] .

5 B . V . T R O N O V a n d A . M . PETROVA, Z h . O b s h c h . K h i m . 2 3 , 1019 [1953],

6 a ) G . SOSNOVSKY, E . H . ZARET, a n d W . M E R T Z , S y n . 1 4 2 [ 1 9 7 1 ] ; b ) G . SOSNOVSKY, E . H . ZARET, a n d M . KONIECZ-NY, J. org. Chemistry 37, 2267 [1972].

7 G. OLAH, in: " F r i e d e l - C r a f t s and Related Reac-tions", Vol. I, G. OLAH, Ed., Interscience Publishers, New Y o r k 1 9 6 4 , ( a ) p p . 3 6 - 3 7 ; ( b ) p . 3 9 ; ( c ) p . 4 0 .

8 D. A. MCCAULAY, in: " F r i e d e l - C r a f t s and Related Reactions", Vol. II, p. 1061, G. OLAH, Ed., Interscience Publishers, New York 1964.

9 A. W. FRANCIS, Chem. Reviews 43, 257 [1948]. 1 0 G . A . OLAH, S . J. K U H N , a n d S . H . FLOOD, J . A m e r . d i e m .

Soc. 84, 1688 [1962]. 1 1 G . A . OLAH, M . W . MEYER, a n d N . A . OVERCHUK, J . o r g .

Chemistry 29, 2315 [1964]. 1 2 G . A . OLAH, M . W . MEYER, a n d N . A . OVERCHUK, J . o r g .

Chemistry 29, 2313 [1964]. 1 3 a ) R . H . ALLEN, L . D . Y A T S , a n d D . S . ERLEY, J . A m e r .

chem. Soc. 82, 4853 [1960]; b) R. H. ALLEN, T. ALFREY, JR., and L. D. YATS, J. Amer. chem. Soc. 81, 42 [1959]; c ) R . H . ALLEN a n d L . D . Y A T S , J . A m e r . d i e m . S o c . 8 1 , 5289 [1959] ; d) H. C. BROWN and J. JUNGK, J. Amer. chem. Soc. 77, 5579 [1955],

C 9 H U C 1 : 154. The IR spectroscopic examination in-dicated the presence of o-, m-, and p-isomers. The pro-duct mixture could not be resolved by glpc analysis.

We thank Eastman Organic Chemicals, Rochester. N .Y . ; Hooker Chemical Corp., Industrial Chemicals Division, Niagara Falls, N .Y . ; Mobil Chemical, In-dustrial Chemicals Division, Richman, Va . ; Victor Chemical Division, Stauffer Chemical Co., Dobbs Ferry, N .Y . ; and Aldrich Chemical Co., Milwaukee, Wise, for supplies of various phosphorus esters.

1 4 S . H . PATINKIN a n d B . S . FRIEDMAN, i n : " F R I E D E L -C r a f t s and Related Reactions", Vol. II, pp. 147, 149, 171, G. OLAH, Ed., Interscience Publishers, New York 1 9 6 4 .

1 5 a ) H . C . B R O W N a n d J . D . BRADY, J. A m e r . c h e m . S o c . 7 4 . 3 5 7 0 [ 1 9 5 2 ] ; b ) H . C . B R O W N a n d K . L . NELSON, J . A m e r . c h e m . S o c . 7 5 , 6 2 9 2 [ 1 9 5 3 ] ; c ) H . C . B R O W N a n d M . GRAYSON, J . A m e r . c h e m . S o c . 7 5 , 6 2 8 5 [ 1 9 5 3 ] ; d ) H . C . B R O W N a n d C . R . SMOOT, J . A m e r . c h e m . S o c . 7 8 , 6 2 5 5 [ 1 9 5 6 ] ,

1 0 G . A . O L A H , S . H . FLOOD, S . J. K U H N , M . E . MOFFATT, and N. A. OVERCHUK, J. Amer. chem. Soc. 86, 1046 [ 1 9 6 4 ] ,

1 7 R . H . ALLEN a n d L . D . YATS, J . A m e r . c h e m . S o c . 8 3 , 2 7 9 9 [ 1 9 6 1 ] .

1 8 L . SCHMERLING, I n d . E n g . C h e m . 4 0 , 2 0 7 2 [ 1 9 4 8 ] . 1 9 G . A . O L A H , N . A . OVERCHUK, a n d J. C . LAPIERRE, J . A m e r .

d i e m . S o c . 8 7 , 5 7 8 5 [ 1 9 6 5 ] . 2 0 S . U . CHOI a n d H . C . BROWN, J . A m e r . c h e m . S o c . 8 1 , 3 3 1 5

[ 1 9 5 9 ] . 2 1 L . M . STOCK a n d II . C . BROWN, J. A m e r . c h e m . S o c . 8 1 ,

3 3 2 3 [ 1 9 5 9 ] . 2 2 P . K O V A C I C a n d S. T . MORNEWECK, J. A m e r . c h e m . S o c .

8 7 , 1 5 6 6 [ 1 9 6 5 ] , 2 3 A . J . KIRBY a n d S. G . W A R R E N , " T h e O r g a n i c C h e m i s t r y

of Phosphorus", pp. 17, 18, Elsevier Publishing Co., Am-sterdam/London/New York 1967.

2 4 M . J. SCHLATTER a n d R . D . CLARK, J . A m e r . c h e m . S o c . 7 5 . 3 6 1 [ 1 9 5 3 ] .

2 5 F . E . C O N D O N , J . A m e r . c h e m . S o c . 7 1 , 3 5 4 4 [ 1 9 4 9 ] . 26 C. C. PRICE, Chem. Reviews 29, 37 [1941], 27 D. J. PASTO and C. R. JOHNSON, "Organic Structure Deter-

mination", pp. 43, 44, Prentice-Hall, Englewood Cliffs, N. J. 1 9 6 9 .