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Epoxidation of cyclohexene, methylcyclohexene and cis-cyclooctene by molecular oxygen using ruthenium(III) aquo ion as catalyst: a kinetic study
265
Journal of Molecular Catalysis, 62 (1990) 265-276
EPOXIDATION OF CYCLOHEXENE, ME THYLCYCLOIIEXENE AND cis-CYCLOOCTENE BY MOLECULAR OXYGEN USING RUTHENIUM(111) AQUO ION AS CATALYST: A EINETIC STUDY M. M. TAQUI
KHAN*,
A. PRAKASH
RAO, S. D. BHAlT
AND
R. R. MERCHANT
Discipline of Coordinution Chemistry and Homogeneous Catalysis, Centml Salt and Marine Chemicals Research Institute, Bhavnagar 364 002 Undiu) (Received January 4,199O; accepted April 27,199O)
Kinetics of the epoxidation of cyclohexene, methylcyclohexene and by molecular oxygen catalysed by RdIII) ion, cis -cyclooctene [RuCLJH~O)~]+ at pH 2.0 in a mixed water-dioxane solvent medium are reported. The reactions were studied manometrically as a function of catalyst, substrate and dioxygen concentrations. For all the substrates studied, the rate of oxidation is first order with respect to catalyst and substrate concentrations and one-half order with respect to dioxygen concentration. The oxidations yielded epoxide as the only product in all cases; these were identified and analysed by GLC technique. Based on the kinetics, a mechanism involving homolytic cleavage of the G-G (peroxo) bond with concerted transfer of the oxygen atom to the substrate was proposed. The presence of methyl group on the cyclohexene ring (reaction 2) and an increase in the number of carbon atoms in the ring for cyclooctene (reaction 3) cause a decrease in the rates of epoxidation. The rate of epoxide formation and the corresponding yields for reactions (11, (2 1 and (3) decreases in the order: cyclohexene > methylcyclohexene > cyclooctene.
Iutroduction The catalytic oxidation of organic substances in the liquid phase constitutes one of the important types of industrial chemical reactions [l-4]. The study of their mechanisms is important for the creation of new selective processes for the synthesis of a desired product 1561. The selective epoxidation of olefins constitutes one such reaction 17-121. In the options available to date for oxygenations catalyzed by transition metal complexes of Groups 3d, 4d and 5d, the oxidants are restricted to oxygen atom transfer agents such as H202, iodosylbenzene 113-201 and peroxyacids 121,223. Catalytic oxygen atom transfer reactions that are performed directly with molecular oxygen as the oxidant have been little studied 123-251. The *Author to whom correspondence should be addressed. 0304-5102/90/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
266
successful utilization of Ru(III)-EDTA as catalyst in a number of oxygen atom transfer reactions 126-301 prompted us to try the simple [RuCl,(H,O),l+ ion, stable and dominant in the pH range 2.0-2.5 [311, as the catalyst for the epoxidation of olefins in mixed solvent systems. In the present paper we report the epoxidation of cyclohexene, methylcyclohexene and cis -cyclooctene by molecular oxygen using [ RuClz( Hz0 I41+ as the catalyst at pH 2.00 in HzO-dioxane medium at 35°C (p = 0.1
MWNO,)). Experimental Materials and methods
A stock solution of ruthenium(II1) (0.01 M) was prepared in 1 M hydrochloric acid by dissolving the appropriate amount of ruthenium trichloride trihydrate (RuC13.3Hz0) obtained from Johnson Matthey Inc. The solution thus prepared was allowed to stand at room temperature for 3-4 days, allowing the monomeric chloro complexes to reach equilibrium. The ruthenium content in this solution (0.01 M) was estimated by the known spectrophotometric method. The values were also checked by comparison with those of a standard ruthenium(II1) solution (AAS) supplied by Aldrich Chemical Co. All the chemicals used for testing and standardisation studies were of analytical grade, and the solutions were prepared with doubledistilled water. The substrates, namely 1-methylcyclohexene and cyclooctene of analytical grade (A.R. 1, were obtained from BDH and purified prior to use by the published procedure 1321. Cyclohexene was prepared by the known method [333 and freed from peroxides by washing with acidified ferrous sulphate, dried over calcium chloride and distilled under inert atmosphere. Double-distilled A.R. grade 1,Cdioxane was used as a cosolvent to prepare fresh solutions of cyclohexene, methylcyclohexene, and cyclooctene for each experimental run. The ionic strength of all the solutions was maintained at 0.1 M using A.R. grade KN03 as a supporting electrolyte. All pH measurements were made with a Digisun Model Digital pH meter readable to 0.01 pH. The absorption spectra were recorded on a Beckman Model DU-7 high speed UV-Vis spectrophotometer. In the kinetic measurements, the apparatus consisted of a manometer, burette and two double-walled glass cells containing a reference and experimental reaction mixture, respectively 1341. Stability measurements The stability constants for the formation of metal-ole6n
complex were determined spectrophotometrically at 35 “C by the reported method [351. The stoichiometry of these complexes was determined by spectrophotometric titration. Increasing amounts of olefin, with varying concentrations, were added to flasks containing a fixed concentration of the metal ion solution. The absorbances at equilibrium - at the wavelength of maximum absorption of
267
metal-olefin complex - of reaction mixtures were recorded in order to estimate the stoichiometric ratio and also the stability constants of various metal-olefln complexes. Instrumentation The substrates and their corresponding epoxides were analysed on a Shimadzu gas chromatograph GC-9A assembled with a stainless-steel column containing 10% Carbowax 20M on 90-100 mesh Anakrom. A thermal conductivity detector at 200 “C using helium as carrier gas was used for product analysis. Synthetic standards of the oxidised products of the substrates were used to identify the reaction products under identical experimental conditions at a column temperature of 175 “C. Area normalisation was employed to calculate the percentage yields of the epoxide formed. Infrared spectra were recorded on a Shimadzu IR-435 instrument for the characterisation of the products formed. The proton NMR spectra were recorded on a JEOL FXlOO NMR spectrometer with tetramethylsilane as the standard. The electrochemical measurements were made with PAR electrochemical instruments equipped with a high precision X-Y recorder (PAR RE 0089). Cyclic voltammetry measurements were carried out with the help of a PAR Model 175 universal programmer. The three-electrode measurements were carried out with a PAR 303 SMDE assembly provided with a glassy carbon working electrode, a platinum wire auxiliary electrode, and a Ag/AgCl reference electrode. Kinetic/rate measurements The rate constants of the oxidation reaction were derived by following the kinetics of oxygen absorption by the manometric technique. The rate of absorption of oxygen was measured using a glass manometric apparatus provided with leak-proof Springham stopcocks. High-vacuum silicone grease was applied to stopcocks to keep the system airtight. The temperature of the reaction mixture was maintained constant by circulating water at the desired temperature (fO.l “C) through the jacketted glass cells. The system was evacuated and filled with oxygen several times to ensure that a complete oxygen atmosphere prevails in the reaction cell. Molecular oxygen was presaturated with water vapour by streaming through a wash-bottle containing an electrolyte concentration identical to that of the reaction solution. The substrates were dissolved in sufficient quantities of dioxane to maintain a 1:l water-dioxane ratio in the reaction mixture. In order to minimize the effect of solvent vapour, a blank was run simultaneously under the same conditions of temperature, pressure and volume. The absorption of dioxygen was measured manometrically by noting the change in the levels of the indicator solution in the measuring burette at suitable time intervals. In order to obtain the desired partial pressure of molecular oxygen, mixtures of dioxygen and nitrogen varying in composition, as well as pure dioxygen, were employed. The solubility of molecular oxygen
263
in solution at 1.0 atm was determined from physical data on the solubility of molecular oxygen. The data at other partial pressures were computed from the solubility data at 1.0 atm pressure of molecular oxygen.
R43SUltS
Solution chemistry studies Studies on the aqueous solution chemistry of ruthenium(II1) ion over a wide range of acidic pH were carried out by means of spectrophotometric and electrochemical techniques. The distribution of various chloro complexes of ruthenium(II1) chloride in aqueous solution in the pH range 0.4-2.0 has been reported earlier 1313. The various ruthenium(II1) chloro species existing in solution in the said pH range were identified as [RuCL(H,O)J, [RuCl,(H,O),l, [RuCl~(H~O)~l+ and ~RuC~(H~O)~I~+.With the help of the equilibrium data available [311, it is possible to calculate the concentration of a particular rutheniumUI1) species present in solution at a particular pH. Thus, in the present studies carried out at pH 2.0, [RuC12(H20),l+ is the predominant species (~90%) present in solution and was thus assumed as the catalytic species involved in the oxidation of cyclohexene, methylcyclohexene and cyclooctene at pH 2.00.
Kinetic studies The epoxidation of cyclohexene, 1-methylcyclohexene and cyclooctene by molecular oxygen was first studied with respect to the variation of concentration of [RuCI~(H~O)~I catalyst keeping the concentration of the other
[Ru hl] x 103, M
Fig. 1. Plot of observed rates (k,,) uerszm[Ru”‘l indicating the effect of catalyst concentration on the rates of cyclohexene oxidation at 35 “C; [C,H,,l = 1.0 x 10m2M, pH = 2.00, Pop = 1.0 atm.
269
TABLE 1 Effect of catalyst concentration on the rates of oxidation of cyclohexene, methyl cyclohexene and cyclooctene catalysed by Ru(II1) ion
CM)
[Ru”‘] x 10’
Reaction (1) k*. x 106 (Mm%‘)
Reaction (2) kOb.x 10s (Mmin-‘)
Reaction (3) k,, x 10s (M mm-‘)
0.58 1.15 1.50 1.75 2.00 2.30
1.02 1.61 2.25 2.72 3.01 3.32
0.56 0.98 1.28 1.47 1.67 1.80
0.21 0.29 0.42 0.56 0.63 0.67
Conditions: [olefinl = 1.0 x lo-‘M,
PO, = 1.0 atm, pH = 2.00, p = 0.1 M KCl, temp. = 35 “C.
parameters such as substrate and O2 concentration constant. The rate of the reaction in each case was found to be first order with respect to Ru(II1) ion concentration. Figure 1 shows one such plot of observed rate (kobs) versus [Ru(III)] for the epoxidation of cyclohexene in reaction (1). Kinetic data for the dependence of the epoxidation rate on catalyst concentration are given in Table 1 for each reaction studied. Experiments were also carried out at different molar concentrations of cyclohexene, 1-methylcyclohexene and cyclooctene in order to observe the dependence of rate with respect to substrate concentration. In the reactions
[Cyclohexene]
x 102,
M
Rig. 2. Plot of observed rate (k,) versus cyclohexene concentration which exhibits an effect of the substrate on the rate of oxidation at pH 2.00, [Rum] = 1.0 x 10e3M, Pal = 1.0 atm, temp. = 35 “C.
270 TABLE 2 Variation of rates of oxidation with respect to the concentrations of cyclohexene, lmethylcyclohexene and cyclooctene as catalysed by RuUII) ion in Reactions (1 ), (2) and (3) [olefin]” X 10’ (M)
Reaction (1) kobsx 106 (M min-‘)
Reaction ( 2 ) k,, x 10s (M mir-‘1
Reaction (3) k,, x lo6 (M min-‘)
1.50 2.50 4.00 5.00 6.00 7.00
1.00 1.51 2.52 3.01 3.50 4.03
0.53 0.34 1.36 1.71 1.94 2.22
0.20 0.32 0.53 0.62 0.73 0.64
Conditions: [Rum] = 1.0 x 10e3 M, Paz= 1.0 atm, pH = 2.00, temp. = 35 “C. “Cyclohexene/methyl cyclohexene/cyclooctene.
studied, the rate of epoxidation increases linearly with increasing concentrations of substrate, with a first order dependence in the range 1.5 x 10e2 M to 7.5 x 10e2 M. One such plot of a first order rate dependence for reaction (1) is shown in Fig. 2. The kinetic data obtained for all such reactions are listed in Table 2. The rate dependence with respect to the variation in dioxygen concentration, at constant concentrations of other reactants, shows a one-half order dependence on dioxygen concentration for reactions (1 J-(3). For all the reactions, the plots of observed rate uers’susO2 concentration pass through the origin at au angle of about 45”. One such plot of rate uersus [O,l for reaction (1) is shown in Fig. 3. The dependences with respect to dioxygen concentra-
3.0 I 2,5.’ E I 2,o w-
[o,]x103,M Fig. 3. Plot of observed rates of cyclohexene oxidation with respect to variation of dioxygen concentration at pH 2.00, [Rum] = 1.0 X 1O-3 M, [cyclohexene] = 1.0 x lo-‘M, temp. = 35 “C.
271 TABLE 3 Effect of molecular oxygen on the rates of oxidation of cyclohexene, 1-methylcyclohexene and cyclooctene catalyeed by Ru(II1) ion at 35 “C [031 x lo3 (M)
Reaction (1) k*, x 106 (Mmh-‘)
Reaction (2) k&, x 106 (M min-‘1
Reaction (3) kobsx 106 (M mill_‘)
0.62 0.83 1.24 1.84 2.30
0.70 0.93 1.11 1.52 2.11
0.40 0.53 0.64 0.85 1.15
0.15 0.19 0.23 0.32 0.44
Conditions: [Ru”‘] = 1.0 x lo-’ M, [olefinl = 1.0 x lo-’ M, pH = 2.00, p = 0.1 M, temp = 35 “C.
tion for reactions (l)-(3) are given in Table 5. Detailed kinetic results for all these reactions are presented in Table 3. Equilibrium studies The equilibrium constants involved in the epoxidation of cyclohexene, I-methylcyclohexene and cyclooctene as calculated spectrophotometrically and kinetically are represented by the following equations, and the corresponding values listed in Table 4. + S x,l,s? [Ru”‘Cl,(H,O),(S)I+ K [RurvC1,(H~O)~(S)l,O~~[Ru”~C~,(H,O),(S)I+ + O2 s [RumCl,(H,O),I+
TABLE 4 Equilibrium data for the various complexes formed during epoxidation of cyclohexene, methyl cyclohexene and cyclooctene catalyaed by Ru(II1) ion Equilibrium quotient
K, (mol-‘1
[Ru”‘(C,H,,)I [Ru”‘l[C,H,,l
4.2 x 10’ a
[Ru”(C,H,,)(O;)I [Ru”‘G,H,,)1[031
1.0 x 103 b
[Ru”‘W&&H,)1 [Ru”‘l [C&&H31
2.5 x 10’ a
[Ru”(C,H,CH,)(O,)I [Ru”‘(C,H&H3)1[0,1
9.4 x 102 b
[Ru”‘W,H,,)I [Ru”‘1[C,H,,1
0.7 x 10’ a
[Ru’“(C,H,,)(O,)I [Ru”‘(C,H,,)1[0,1
3.5 x 102 b
“Calculated spectrophotometrically. bCalculated kinetically.
272 TABLE! 5 Dependence on [O,], rate and percentage yield of epoxide formation in reactions cl), (2) and (3) catalyeed by Ru(II1) ion at 35 “C System
Order w.r.t.
K (min-‘)
Percentage yield
Reaction (1) Reaction (2) Reaction (3)
0.62 0.49 0.41
2.0 x 10-z 9.5 x 10-3 4.2 x 1O-3
40 28 14
where S = substrate; Ki, K4 (S = cyclohexene); cyclohexene); K3, KS (S = cyclooctene).
K2,
K5
(S =methyl-
Stoichimnetric studies and product analysis Oxygen uptake measurements and the results of product analysis indicate the following stoichiometric equations for ruthenium(II1) ioncatalysed epoxidation of cyclohexene (reaction ( 1) >, methylcyclohexene (reaction (2)) and cyclooctene (reaction (3)) by molecular oxygen. CsHlo+ P& + &.HioG C&H&H3 + 40, + C6Hg(OXHS C~HM+ 4% +
C,H,,G
(1) (2) (3)
From the stoichiometry of reactions (11, (2) and (31, it is evident that the molecularity of dioxygen is 0.5, which means that for every mole of substrate involved, one-half mole of dioxygen is required for the formation of the epoxide. Reactions (11, (2) and (3) are thus examples of a homodioxygenase system, in which both the oxygen atoms of dioxygen are eventually transferred to two substrate molecules to give two molecules of product at the end of the catalytic cycle. The oxidised products were extracted with ether from the solution containing ruthenium(II1) catalyst and subjected to GLC analysis which indicated neat and exclusive epoxidation of the olefins cyclohexene, cyclooctene and methyl cyclohexene to their respective epoxides. The IR spectrum of the products exhibited three bands at 1250, 900 and 800cm-’ which represent the characteristic peaks of epoxide. The proton NMR spectra of the oxidised products show a peak at 2.9ppm, suggesting formation of the epoxide. The quantitative yields of the epoxide formed in all the reactions are given in Table 5. Discussion
Based on the kinetic results, stoichiometry and product analysis, a general and common mechanism is proposed for ruthenium(II1) ioncatalysed epoxidation of cyclohexene, 1-methylcyclohexene and cyclooctene by molecular oxygen as depicted in Scheme 1, which involves homolytic
273
o-o 2[C12(H20)3
R:‘!j(]++
O2 K2,
[c12(H20)2
R:Y’
‘R:(H20)2C12]2+ ‘r/q
‘1
o/c\c‘I
2
;RZI_
t-
[CI~(H~O)~R
H20 ;&C:
+ [R:II(H20)LC12]
2+
Scheme 1. A mechanism proposed for the aerobic oxidation of oleiins (reactions (l)-(3)) catalysed by dichIorotetraaquorutheniumUI1) ion in the pH range 2.00-2.50.
of the O-O bond with concerted transfer of each oxygen atom to the substrate. In the proposed mechanism (Scheme 11, the pre-equilibrium step (K,) involves the formation of Ru( III)-olefin complex, two moles of which combine with one mole of dioxygen forming a p-peroxo-bridged complex as defined by step K2. In the rate-determining step (k 1, overlap of a filled n-orbital of the peroxo oxygen atom with the empty n* orbital of olefin causes homolytic cleavage of the O-O bond with the formation of an oxetane intermediate (Scheme l), which cleaves to give the product and the catalyst. The epoxidation is stereospecific in that cis-cyclooctene gives the ci.s-epoxide, supporting the oxetane intermediate. cleavage
Evaluation of rate parameters
Based on kinetic observations and the mechanism proposed, the following common rate expression is derived for ruthenium(II1) ion-catalysed epoxidation of cyclohexene, methylcyclohexene and cyclooctene by dioxygen in 1:l water-dioxane solvent: (4)
The above rate equation is subjected to steady state and equilibrium conditions, where the concentration of the catalyst is expressed in terms of its total concentration [RumIT by taking into consideration the amount of ruthenium(II1) present in the intermediate complexes.
-d[O,l= dt
IzK,K,~Sl[Ru”*lT[0211/2 1+ KJSI + K,K,[Sl10,11~
(5)
For the evaluation of rate constant (k) and equilibrium constants (K, and K2), the rate expression (5) is subsequently rearranged to give the following
274
rate law: [Rum],
1
1
1
-d[Oddt =kxI&[sl[0211~ + ~qo,l~n + i
(6)
With the help of the above rate law (eqn. (6)), the values of equilibrium constants K1 and & and the value of the rate cons~t K were determined. Values of equilibrium constants for reactions (1), (2) and (3) are given in Tables 4 and 5, respectively. From the relative values of rate constants for the slow step (K) in the reactions catalysed by Ru(II1) ion, it is obvious that the rate of transfer of both the oxygen atoms to two coordinated olefin molecules by the homolytic dteavage of the peroxo bond is two times faster for reaction (1) as compared to reaction (Z), about five times faster than reaction (3). Reactions (l), (2) and (3) represent a model homodioxygenase system in which both dioxygen atoms are simultaneously incorporated into two substrate molecules, giving rise to two product molecules. The epoxidation of cyclohexene by Ru(II1) ion is much faster as compared to that of l-methylcyclohexene and cyclooctene. This is attributed to the fact that the presence of a methyl substituent in the ring makes it bulkier, causing a steric hindrance during the orientation of the reactant molecules into proper position while complexing with the metal ion. This is reflected in the values of the equilibrium constants (Table 4) wherein l-methylcyclohexene forms a relatively less stable complex with Athena as compared to cyclohexene. The co~s~n~~ dioxygen complex with the former is subsequently less stable than the latter. The difference in equilibrium constants for cyclohexene and cyclooctene can be attributed to the ring size of the olefina. The steric blocking of one side of the double bond by the aliphatic ring in the case of cyclooctene makes the
Fig. 4. A plot of [Ru(III)l&ate reaction (1) kqn. (6)).
uersw [SJ-’ for the resolution of ‘k,’ and ‘K, con&anta in
275
mixed ligand complex less stable. The rate constants (Table 5) also indicate the fact that reaction (1)is faster than reaction (21, which in turn is faster than reaction (31, a fact also reflected in the percentage yield of epoxide formed (Table 5) in reactions Cl),(2) and (3).
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
L. F. Fieser and M. Fieser, Reagents for Organic Syntheses, Wiley, New York, 1967. W. A. Waters, Mechanisms of Oxidation of Organic Compounds, Methuen, London, 1964. T. A. Turney, Oxidation Mechanisms, Buttenvorths, London, 1965. N. M. Emanuel. E. T. Denisov and Z. K. Maizus, Chain Oxidation Reactions of Hydrocarbons in the Liquid Phase, Izd. Nauka, Moscow, 1965. N. M. Emanuel and D. Gal, Oxidation of Ethylbenzene -A Model Reaction, Izd. Nauka, Moscow, 1984. Reich and S. S. Stivala, Autoxidution of Hydrocarbons and Polyokfins, Marcel Dekker, New York, 1969. E. Guihnet and B. Meunier, J. Mol. Catal., 23 (1984) 115. B. Meunier, M. E. Decarvalho and A. Robert, J. Mol. Catal., 41 (1987) 185. J. C. Dobson, W. K. Seok and T. J. Meyer, Znorg. Chem., 25 (1986) 1513. I. Tabushi, M. Kodera and M. J. Yokoyama, J. Am. Chem. Sot., 107 (1985) 4466. G. Balavoine, C. Eskenasi, F. Meunier and H. Riviere, Tetrahedron L&t., 25 (1984) 3187. R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidation of Organic Compounds, Academic Press, New York, 1981. P. A. Ganeshpure and S. Satish, J. Chem. Sot., Chem. Commun., (1988) 981. T. G. Traylor, T. Nakano, B. E. Dunlap, P. S. Traylor and D. Dolphin, J. Am. Chem. Sot., 106 (1986) 2782. J. P. Collman, T. Kodadek and J. I. Brauman, J. Am. Chem. Sot., 108 (1986) 2588. C. M. Che, W. H. Leung and C. K. Poon, J. Chem. Sot., Chem. Commun., (1987) 173. C. M. Che, W. K. Cheng, W. H. Leung and T. C. W. Mak, J. Chem. Sot., Chem. Commun., (1987) 418. Be&a de Pooter and B. Meunier, J. Chem. Sot. Perkin Trans. 2, (1985) 1735. (a) A. Robert and B. Meunier, New J. C&m., 12 (1988) 885; (b) E. G. Saurrel, K. Srinivasan and J. K. Kochi, J. Am. Chem. Sot., 107 (1985) 7607. (a) Wah-Hun Wong, Drazen Ostovic and T. C. Bruice, J. Am. Chem. Sot., 109 (1987) 3428; (b) F. P. Geungerich and T. L. MacDonald, Acts. Chem. Res., 17 (1984) 9. (a) K. Srinivasan, P. Michaud and J. K. Kochi, J. Am. Chem. Sot., 108 (1986) 2309; (b) J. T. Groves and Y. Watanabe, J. Am. Chem. Sot., 106 (1986) 507. (a) H. J. Ledon, P. Durbut and F. Varescon, J. Am. Chem. Sbc., 103 (1981) 3601; (b) D. Mansuy, J. F. Bartoli and M. Momenteau, Tetrahedron L&t., (1982) 2781. J. T: Groves and K. H. Ahn, Znorg. Chem. 26 (1987) 3831. J. T. Groves and R. Quinn, J. Am. Chem. Sot., 107 (1985) 5790. J. P. Colhnan, C. E. Barnes, P. J. Brothers, T. J. Collins, T. Ozawa, J. C. Gallucci and J. A. lbers, J. Am. Chem. Sot., loS(1984) 5151. (a) M. M. Taqui Khan, Oxid. Commun., 9 (1986) 105; (b) M. M. Taqui Khan, Proc. Zndian Natl. Sci. Acad., 524 (1986) 818. M. M. Taqui Khan and A. Prakash Rao, J. Mol. Catal., 39 (1987) 331. M. M. Taqui Khan, A. Prakash Rao and R. S. Shukla, J. Mol. Catal., 50 (1989) 45. M. M. Taqui Khan, R. S. Shukla and A. Prakash Rao, Zrwrg. Chem., 28 (1989) 452. (a) M. M. Taqui Khan, H. C. Bajaj, R. S. Shukla and S. A. Mirsa, J. Mol. Catal., 46 (1988) 51; (b) M. M. Taqui Khan, S. A. Mirsa and H. C. Bajaj, J. Mol. Catal., 37 (1986) 253; (c) M. M. Taqui Khan, S. A. Mirsa and H. C. Baj& J. Mol. Catal., 42 (1987) 323.
276 31 M. M. Taqui Khan, G. Ramachandraiah and A. Prakaah Rao, Znmg. Chem. 25 (1986) 665. 32 D. D. Penin, W. L. F. Armarego and D. R. Perrin, Purifiation of Lahatory Chemicals, 2nd edn., Pergamon, New York, 1981. 33 A. I. Vogel, Textbook of Pmctical Organic Chemistry, 4th edn. Elbs, Longman, London, 1978, p. 332. 34 M. M. Taqui Khan and A. Prakash Rao, J. Mol. Catal., 35 (1936) 237. 35 K. L. Brown, Zmrg. Ckim. Acta, 37(1979) W13.
Journal of Molecular Catalysis, 62 (1990) 265-276
EPOXIDATION OF CYCLOHEXENE, ME THYLCYCLOIIEXENE AND cis-CYCLOOCTENE BY MOLECULAR OXYGEN USING RUTHENIUM(111) AQUO ION AS CATALYST: A EINETIC STUDY M. M. TAQUI
KHAN*,
A. PRAKASH
RAO, S. D. BHAlT
AND
R. R. MERCHANT
Discipline of Coordinution Chemistry and Homogeneous Catalysis, Centml Salt and Marine Chemicals Research Institute, Bhavnagar 364 002 Undiu) (Received January 4,199O; accepted April 27,199O)
Kinetics of the epoxidation of cyclohexene, methylcyclohexene and by molecular oxygen catalysed by RdIII) ion, cis -cyclooctene [RuCLJH~O)~]+ at pH 2.0 in a mixed water-dioxane solvent medium are reported. The reactions were studied manometrically as a function of catalyst, substrate and dioxygen concentrations. For all the substrates studied, the rate of oxidation is first order with respect to catalyst and substrate concentrations and one-half order with respect to dioxygen concentration. The oxidations yielded epoxide as the only product in all cases; these were identified and analysed by GLC technique. Based on the kinetics, a mechanism involving homolytic cleavage of the G-G (peroxo) bond with concerted transfer of the oxygen atom to the substrate was proposed. The presence of methyl group on the cyclohexene ring (reaction 2) and an increase in the number of carbon atoms in the ring for cyclooctene (reaction 3) cause a decrease in the rates of epoxidation. The rate of epoxide formation and the corresponding yields for reactions (11, (2 1 and (3) decreases in the order: cyclohexene > methylcyclohexene > cyclooctene.
Iutroduction The catalytic oxidation of organic substances in the liquid phase constitutes one of the important types of industrial chemical reactions [l-4]. The study of their mechanisms is important for the creation of new selective processes for the synthesis of a desired product 1561. The selective epoxidation of olefins constitutes one such reaction 17-121. In the options available to date for oxygenations catalyzed by transition metal complexes of Groups 3d, 4d and 5d, the oxidants are restricted to oxygen atom transfer agents such as H202, iodosylbenzene 113-201 and peroxyacids 121,223. Catalytic oxygen atom transfer reactions that are performed directly with molecular oxygen as the oxidant have been little studied 123-251. The *Author to whom correspondence should be addressed. 0304-5102/90/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
266
successful utilization of Ru(III)-EDTA as catalyst in a number of oxygen atom transfer reactions 126-301 prompted us to try the simple [RuCl,(H,O),l+ ion, stable and dominant in the pH range 2.0-2.5 [311, as the catalyst for the epoxidation of olefins in mixed solvent systems. In the present paper we report the epoxidation of cyclohexene, methylcyclohexene and cis -cyclooctene by molecular oxygen using [ RuClz( Hz0 I41+ as the catalyst at pH 2.00 in HzO-dioxane medium at 35°C (p = 0.1
MWNO,)). Experimental Materials and methods
A stock solution of ruthenium(II1) (0.01 M) was prepared in 1 M hydrochloric acid by dissolving the appropriate amount of ruthenium trichloride trihydrate (RuC13.3Hz0) obtained from Johnson Matthey Inc. The solution thus prepared was allowed to stand at room temperature for 3-4 days, allowing the monomeric chloro complexes to reach equilibrium. The ruthenium content in this solution (0.01 M) was estimated by the known spectrophotometric method. The values were also checked by comparison with those of a standard ruthenium(II1) solution (AAS) supplied by Aldrich Chemical Co. All the chemicals used for testing and standardisation studies were of analytical grade, and the solutions were prepared with doubledistilled water. The substrates, namely 1-methylcyclohexene and cyclooctene of analytical grade (A.R. 1, were obtained from BDH and purified prior to use by the published procedure 1321. Cyclohexene was prepared by the known method [333 and freed from peroxides by washing with acidified ferrous sulphate, dried over calcium chloride and distilled under inert atmosphere. Double-distilled A.R. grade 1,Cdioxane was used as a cosolvent to prepare fresh solutions of cyclohexene, methylcyclohexene, and cyclooctene for each experimental run. The ionic strength of all the solutions was maintained at 0.1 M using A.R. grade KN03 as a supporting electrolyte. All pH measurements were made with a Digisun Model Digital pH meter readable to 0.01 pH. The absorption spectra were recorded on a Beckman Model DU-7 high speed UV-Vis spectrophotometer. In the kinetic measurements, the apparatus consisted of a manometer, burette and two double-walled glass cells containing a reference and experimental reaction mixture, respectively 1341. Stability measurements The stability constants for the formation of metal-ole6n
complex were determined spectrophotometrically at 35 “C by the reported method [351. The stoichiometry of these complexes was determined by spectrophotometric titration. Increasing amounts of olefin, with varying concentrations, were added to flasks containing a fixed concentration of the metal ion solution. The absorbances at equilibrium - at the wavelength of maximum absorption of
267
metal-olefin complex - of reaction mixtures were recorded in order to estimate the stoichiometric ratio and also the stability constants of various metal-olefln complexes. Instrumentation The substrates and their corresponding epoxides were analysed on a Shimadzu gas chromatograph GC-9A assembled with a stainless-steel column containing 10% Carbowax 20M on 90-100 mesh Anakrom. A thermal conductivity detector at 200 “C using helium as carrier gas was used for product analysis. Synthetic standards of the oxidised products of the substrates were used to identify the reaction products under identical experimental conditions at a column temperature of 175 “C. Area normalisation was employed to calculate the percentage yields of the epoxide formed. Infrared spectra were recorded on a Shimadzu IR-435 instrument for the characterisation of the products formed. The proton NMR spectra were recorded on a JEOL FXlOO NMR spectrometer with tetramethylsilane as the standard. The electrochemical measurements were made with PAR electrochemical instruments equipped with a high precision X-Y recorder (PAR RE 0089). Cyclic voltammetry measurements were carried out with the help of a PAR Model 175 universal programmer. The three-electrode measurements were carried out with a PAR 303 SMDE assembly provided with a glassy carbon working electrode, a platinum wire auxiliary electrode, and a Ag/AgCl reference electrode. Kinetic/rate measurements The rate constants of the oxidation reaction were derived by following the kinetics of oxygen absorption by the manometric technique. The rate of absorption of oxygen was measured using a glass manometric apparatus provided with leak-proof Springham stopcocks. High-vacuum silicone grease was applied to stopcocks to keep the system airtight. The temperature of the reaction mixture was maintained constant by circulating water at the desired temperature (fO.l “C) through the jacketted glass cells. The system was evacuated and filled with oxygen several times to ensure that a complete oxygen atmosphere prevails in the reaction cell. Molecular oxygen was presaturated with water vapour by streaming through a wash-bottle containing an electrolyte concentration identical to that of the reaction solution. The substrates were dissolved in sufficient quantities of dioxane to maintain a 1:l water-dioxane ratio in the reaction mixture. In order to minimize the effect of solvent vapour, a blank was run simultaneously under the same conditions of temperature, pressure and volume. The absorption of dioxygen was measured manometrically by noting the change in the levels of the indicator solution in the measuring burette at suitable time intervals. In order to obtain the desired partial pressure of molecular oxygen, mixtures of dioxygen and nitrogen varying in composition, as well as pure dioxygen, were employed. The solubility of molecular oxygen
263
in solution at 1.0 atm was determined from physical data on the solubility of molecular oxygen. The data at other partial pressures were computed from the solubility data at 1.0 atm pressure of molecular oxygen.
R43SUltS
Solution chemistry studies Studies on the aqueous solution chemistry of ruthenium(II1) ion over a wide range of acidic pH were carried out by means of spectrophotometric and electrochemical techniques. The distribution of various chloro complexes of ruthenium(II1) chloride in aqueous solution in the pH range 0.4-2.0 has been reported earlier 1313. The various ruthenium(II1) chloro species existing in solution in the said pH range were identified as [RuCL(H,O)J, [RuCl,(H,O),l, [RuCl~(H~O)~l+ and ~RuC~(H~O)~I~+.With the help of the equilibrium data available [311, it is possible to calculate the concentration of a particular rutheniumUI1) species present in solution at a particular pH. Thus, in the present studies carried out at pH 2.0, [RuC12(H20),l+ is the predominant species (~90%) present in solution and was thus assumed as the catalytic species involved in the oxidation of cyclohexene, methylcyclohexene and cyclooctene at pH 2.00.
Kinetic studies The epoxidation of cyclohexene, 1-methylcyclohexene and cyclooctene by molecular oxygen was first studied with respect to the variation of concentration of [RuCI~(H~O)~I catalyst keeping the concentration of the other
[Ru hl] x 103, M
Fig. 1. Plot of observed rates (k,,) uerszm[Ru”‘l indicating the effect of catalyst concentration on the rates of cyclohexene oxidation at 35 “C; [C,H,,l = 1.0 x 10m2M, pH = 2.00, Pop = 1.0 atm.
269
TABLE 1 Effect of catalyst concentration on the rates of oxidation of cyclohexene, methyl cyclohexene and cyclooctene catalysed by Ru(II1) ion
CM)
[Ru”‘] x 10’
Reaction (1) k*. x 106 (Mm%‘)
Reaction (2) kOb.x 10s (Mmin-‘)
Reaction (3) k,, x 10s (M mm-‘)
0.58 1.15 1.50 1.75 2.00 2.30
1.02 1.61 2.25 2.72 3.01 3.32
0.56 0.98 1.28 1.47 1.67 1.80
0.21 0.29 0.42 0.56 0.63 0.67
Conditions: [olefinl = 1.0 x lo-‘M,
PO, = 1.0 atm, pH = 2.00, p = 0.1 M KCl, temp. = 35 “C.
parameters such as substrate and O2 concentration constant. The rate of the reaction in each case was found to be first order with respect to Ru(II1) ion concentration. Figure 1 shows one such plot of observed rate (kobs) versus [Ru(III)] for the epoxidation of cyclohexene in reaction (1). Kinetic data for the dependence of the epoxidation rate on catalyst concentration are given in Table 1 for each reaction studied. Experiments were also carried out at different molar concentrations of cyclohexene, 1-methylcyclohexene and cyclooctene in order to observe the dependence of rate with respect to substrate concentration. In the reactions
[Cyclohexene]
x 102,
M
Rig. 2. Plot of observed rate (k,) versus cyclohexene concentration which exhibits an effect of the substrate on the rate of oxidation at pH 2.00, [Rum] = 1.0 x 10e3M, Pal = 1.0 atm, temp. = 35 “C.
270 TABLE 2 Variation of rates of oxidation with respect to the concentrations of cyclohexene, lmethylcyclohexene and cyclooctene as catalysed by RuUII) ion in Reactions (1 ), (2) and (3) [olefin]” X 10’ (M)
Reaction (1) kobsx 106 (M min-‘)
Reaction ( 2 ) k,, x 10s (M mir-‘1
Reaction (3) k,, x lo6 (M min-‘)
1.50 2.50 4.00 5.00 6.00 7.00
1.00 1.51 2.52 3.01 3.50 4.03
0.53 0.34 1.36 1.71 1.94 2.22
0.20 0.32 0.53 0.62 0.73 0.64
Conditions: [Rum] = 1.0 x 10e3 M, Paz= 1.0 atm, pH = 2.00, temp. = 35 “C. “Cyclohexene/methyl cyclohexene/cyclooctene.
studied, the rate of epoxidation increases linearly with increasing concentrations of substrate, with a first order dependence in the range 1.5 x 10e2 M to 7.5 x 10e2 M. One such plot of a first order rate dependence for reaction (1) is shown in Fig. 2. The kinetic data obtained for all such reactions are listed in Table 2. The rate dependence with respect to the variation in dioxygen concentration, at constant concentrations of other reactants, shows a one-half order dependence on dioxygen concentration for reactions (1 J-(3). For all the reactions, the plots of observed rate uers’susO2 concentration pass through the origin at au angle of about 45”. One such plot of rate uersus [O,l for reaction (1) is shown in Fig. 3. The dependences with respect to dioxygen concentra-
3.0 I 2,5.’ E I 2,o w-
[o,]x103,M Fig. 3. Plot of observed rates of cyclohexene oxidation with respect to variation of dioxygen concentration at pH 2.00, [Rum] = 1.0 X 1O-3 M, [cyclohexene] = 1.0 x lo-‘M, temp. = 35 “C.
271 TABLE 3 Effect of molecular oxygen on the rates of oxidation of cyclohexene, 1-methylcyclohexene and cyclooctene catalyeed by Ru(II1) ion at 35 “C [031 x lo3 (M)
Reaction (1) k*, x 106 (Mmh-‘)
Reaction (2) k&, x 106 (M min-‘1
Reaction (3) kobsx 106 (M mill_‘)
0.62 0.83 1.24 1.84 2.30
0.70 0.93 1.11 1.52 2.11
0.40 0.53 0.64 0.85 1.15
0.15 0.19 0.23 0.32 0.44
Conditions: [Ru”‘] = 1.0 x lo-’ M, [olefinl = 1.0 x lo-’ M, pH = 2.00, p = 0.1 M, temp = 35 “C.
tion for reactions (l)-(3) are given in Table 5. Detailed kinetic results for all these reactions are presented in Table 3. Equilibrium studies The equilibrium constants involved in the epoxidation of cyclohexene, I-methylcyclohexene and cyclooctene as calculated spectrophotometrically and kinetically are represented by the following equations, and the corresponding values listed in Table 4. + S x,l,s? [Ru”‘Cl,(H,O),(S)I+ K [RurvC1,(H~O)~(S)l,O~~[Ru”~C~,(H,O),(S)I+ + O2 s [RumCl,(H,O),I+
TABLE 4 Equilibrium data for the various complexes formed during epoxidation of cyclohexene, methyl cyclohexene and cyclooctene catalyaed by Ru(II1) ion Equilibrium quotient
K, (mol-‘1
[Ru”‘(C,H,,)I [Ru”‘l[C,H,,l
4.2 x 10’ a
[Ru”(C,H,,)(O;)I [Ru”‘G,H,,)1[031
1.0 x 103 b
[Ru”‘W&&H,)1 [Ru”‘l [C&&H31
2.5 x 10’ a
[Ru”(C,H,CH,)(O,)I [Ru”‘(C,H&H3)1[0,1
9.4 x 102 b
[Ru”‘W,H,,)I [Ru”‘1[C,H,,1
0.7 x 10’ a
[Ru’“(C,H,,)(O,)I [Ru”‘(C,H,,)1[0,1
3.5 x 102 b
“Calculated spectrophotometrically. bCalculated kinetically.
272 TABLE! 5 Dependence on [O,], rate and percentage yield of epoxide formation in reactions cl), (2) and (3) catalyeed by Ru(II1) ion at 35 “C System
Order w.r.t.
K (min-‘)
Percentage yield
Reaction (1) Reaction (2) Reaction (3)
0.62 0.49 0.41
2.0 x 10-z 9.5 x 10-3 4.2 x 1O-3
40 28 14
where S = substrate; Ki, K4 (S = cyclohexene); cyclohexene); K3, KS (S = cyclooctene).
K2,
K5
(S =methyl-
Stoichimnetric studies and product analysis Oxygen uptake measurements and the results of product analysis indicate the following stoichiometric equations for ruthenium(II1) ioncatalysed epoxidation of cyclohexene (reaction ( 1) >, methylcyclohexene (reaction (2)) and cyclooctene (reaction (3)) by molecular oxygen. CsHlo+ P& + &.HioG C&H&H3 + 40, + C6Hg(OXHS C~HM+ 4% +
C,H,,G
(1) (2) (3)
From the stoichiometry of reactions (11, (2) and (31, it is evident that the molecularity of dioxygen is 0.5, which means that for every mole of substrate involved, one-half mole of dioxygen is required for the formation of the epoxide. Reactions (11, (2) and (3) are thus examples of a homodioxygenase system, in which both the oxygen atoms of dioxygen are eventually transferred to two substrate molecules to give two molecules of product at the end of the catalytic cycle. The oxidised products were extracted with ether from the solution containing ruthenium(II1) catalyst and subjected to GLC analysis which indicated neat and exclusive epoxidation of the olefins cyclohexene, cyclooctene and methyl cyclohexene to their respective epoxides. The IR spectrum of the products exhibited three bands at 1250, 900 and 800cm-’ which represent the characteristic peaks of epoxide. The proton NMR spectra of the oxidised products show a peak at 2.9ppm, suggesting formation of the epoxide. The quantitative yields of the epoxide formed in all the reactions are given in Table 5. Discussion
Based on the kinetic results, stoichiometry and product analysis, a general and common mechanism is proposed for ruthenium(II1) ioncatalysed epoxidation of cyclohexene, 1-methylcyclohexene and cyclooctene by molecular oxygen as depicted in Scheme 1, which involves homolytic
273
o-o 2[C12(H20)3
R:‘!j(]++
O2 K2,
[c12(H20)2
R:Y’
‘R:(H20)2C12]2+ ‘r/q
‘1
o/c\c‘I
2
;RZI_
t-
[CI~(H~O)~R
H20 ;&C:
+ [R:II(H20)LC12]
2+
Scheme 1. A mechanism proposed for the aerobic oxidation of oleiins (reactions (l)-(3)) catalysed by dichIorotetraaquorutheniumUI1) ion in the pH range 2.00-2.50.
of the O-O bond with concerted transfer of each oxygen atom to the substrate. In the proposed mechanism (Scheme 11, the pre-equilibrium step (K,) involves the formation of Ru( III)-olefin complex, two moles of which combine with one mole of dioxygen forming a p-peroxo-bridged complex as defined by step K2. In the rate-determining step (k 1, overlap of a filled n-orbital of the peroxo oxygen atom with the empty n* orbital of olefin causes homolytic cleavage of the O-O bond with the formation of an oxetane intermediate (Scheme l), which cleaves to give the product and the catalyst. The epoxidation is stereospecific in that cis-cyclooctene gives the ci.s-epoxide, supporting the oxetane intermediate. cleavage
Evaluation of rate parameters
Based on kinetic observations and the mechanism proposed, the following common rate expression is derived for ruthenium(II1) ion-catalysed epoxidation of cyclohexene, methylcyclohexene and cyclooctene by dioxygen in 1:l water-dioxane solvent: (4)
The above rate equation is subjected to steady state and equilibrium conditions, where the concentration of the catalyst is expressed in terms of its total concentration [RumIT by taking into consideration the amount of ruthenium(II1) present in the intermediate complexes.
-d[O,l= dt
IzK,K,~Sl[Ru”*lT[0211/2 1+ KJSI + K,K,[Sl10,11~
(5)
For the evaluation of rate constant (k) and equilibrium constants (K, and K2), the rate expression (5) is subsequently rearranged to give the following
274
rate law: [Rum],
1
1
1
-d[Oddt =kxI&[sl[0211~ + ~qo,l~n + i
(6)
With the help of the above rate law (eqn. (6)), the values of equilibrium constants K1 and & and the value of the rate cons~t K were determined. Values of equilibrium constants for reactions (1), (2) and (3) are given in Tables 4 and 5, respectively. From the relative values of rate constants for the slow step (K) in the reactions catalysed by Ru(II1) ion, it is obvious that the rate of transfer of both the oxygen atoms to two coordinated olefin molecules by the homolytic dteavage of the peroxo bond is two times faster for reaction (1) as compared to reaction (Z), about five times faster than reaction (3). Reactions (l), (2) and (3) represent a model homodioxygenase system in which both dioxygen atoms are simultaneously incorporated into two substrate molecules, giving rise to two product molecules. The epoxidation of cyclohexene by Ru(II1) ion is much faster as compared to that of l-methylcyclohexene and cyclooctene. This is attributed to the fact that the presence of a methyl substituent in the ring makes it bulkier, causing a steric hindrance during the orientation of the reactant molecules into proper position while complexing with the metal ion. This is reflected in the values of the equilibrium constants (Table 4) wherein l-methylcyclohexene forms a relatively less stable complex with Athena as compared to cyclohexene. The co~s~n~~ dioxygen complex with the former is subsequently less stable than the latter. The difference in equilibrium constants for cyclohexene and cyclooctene can be attributed to the ring size of the olefina. The steric blocking of one side of the double bond by the aliphatic ring in the case of cyclooctene makes the
Fig. 4. A plot of [Ru(III)l&ate reaction (1) kqn. (6)).
uersw [SJ-’ for the resolution of ‘k,’ and ‘K, con&anta in
275
mixed ligand complex less stable. The rate constants (Table 5) also indicate the fact that reaction (1)is faster than reaction (21, which in turn is faster than reaction (31, a fact also reflected in the percentage yield of epoxide formed (Table 5) in reactions Cl),(2) and (3).
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