CONCENTRATION EFFECTS IN THE CATALYTIC ACTION OF METALLOPORPHYRINS

IN NONAQUEOUS MEDIA

A. B. Solov'eva, E. A. Lukashova, A. I. Ivanova, and S. A. Vol'fson

UDC 541.127:542.943.7:547. 313:541.49:546.712: 547.979.733

Association of metalloporphyrins (MP), highly efficient catalysts of nonchain oxidation of unsaturated hydrocarbons [i, 2] in nonaqueous media, is usually studied by NMR and paramagnetic NMR relaxation methods [3-7], but these methods are not sensitive with the concentrations of MP <10 -3 M usually used in catalysis. The property of manganese porphyrinates (MnP) of catalyzing oxidation of cholesterol* in soft conditions permitted detecting the association of MnP in nonaqueous media in concentrations much lower than the limit of sensitivity of the NMR method and using a kinetic method for studying the state of MP in solution ina wide range of concentrations7 The purpose of the present study was to examine more closely the concepts of association of MP in nonaqueous media and to study the effect of the polarity of the solvent, structure of the MP, and presence of a substrate on this process. EXPERIMENTAL MnP were prepared by the acetate method from metal-free porphyrin bases [8]. Porphyrin complexes of Mn were used: meso-substituted, tetraphenylporphyrinate (TPP)MnOAc, tetra-paminophenylporhyrinate (TAPP)MnOAc, tetra-p-methoxyphenylporphyrinate (TMPP)MnOAc, tetra-ppyridylporphyrinate (TPiP)MnOAc, and Mn porphyrinate with substituents only in the pyrrole rings: hemato-MnOAc. Cholesterol, Koch-light, mp 147-150~ was used as the subs%rate. NaBH4 was used as the reducing agent. The solvents were purified by the standard methods. Oxidation of cholesterol was conducted in ethanol and in mixtures of ethanol with CHCI 3 or C6H 6 with air with intense stirring% and thermostatting in the I0-40~ region, the concentrations of the reactants varied within the limits of: [MnP] = 5.10-s-6-I0 -~ M, [cholesterol] = 5"10-3-2.5"10 -2 M, [NaBH 4] = 10-2-7.5"10 -2 M. The MnP were added to a solution of cholesterol, the mixture was stirred until completely dissolved, NaBH~ was added, and counting of the reaction time was begun. The kinetics of the process were followed by absorption of 02 [9] and by TLC. RESULTS AND DISCUSSION It was shown in [I0, ii] that the rate of aerobic oxidation of olefins in the presence of the catalytic system MnP-NaBH 4 is proportional to the concentrations of the reactants in the solution and the effective rate constant is not a function of the starting concentrations of catalyst (p) and substrate (m). However, subsequent studies, whose results are reported in the present article showed that there is a region of values of p and m in which this condi ~ %ion is not satisfied. It follows from Fig. 1 (curves 1-5) that the effective rate constant of oxidation of cholesterol kef f is almost constant only in the range of concentrations which exceed the value of p'(m), which are a function of the type of MnP. The dependences of log kef f on the concentration of cholesterol m (Fig. I, curves 6, 7) have a similar shape: the characteristic concentrations m' (when m > m', kef f is not dependent on m) are also a function of m' = m'(p). The kinetics of oxidation of cholesterol were studied

*Oxidation of A5-steroids, cholesterol in particular, in the presence of Mn and NaBH 4 porphyrinates results in the formation of a natural product: the corresponding 5~-steroid alcohol. %The reaction takes place without diffusion hindrances: a change in the stirring rate did not affect the effective rate of the reaction. N. N. Semenov Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated form Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1242-1247, June, 1989. Original article submitted February 8, 1988.

0568-5230/89/3806-1129512.50

9 1989 Plenum Publishing Corporation

!129

L9 keff

f.0

!

1,0

0

I

I 5 9--....,__ 1

1o [~]oiO~, M 5

f

T

J

$ [cholesterol ] 9I0 -2

Fig. i. Dependence of log kef f in ethanol on the concentration of MnP: I) TPiPMnOAc; 2) TMPPMnOAc; 3) TAPPMnOAc; 4) TPPMnOAc; 5) hemato-MnOAc ([NaBH4] = 5"10 -2 M, [cholesterol] = 5"10 -3 M); on the concentration of cholesterol: 6) TPiPMnOAc; 7) TMPPMnOAc (MP = 5"i0 -s M, [NaBH4] = 5. 10 -2 M). in the regions of m > m'(p) and p > p'(m) in [i0, ii]. concentrations of p < p'(m) and m < m'(p).'*

The values of kef f increase with

The results obtained suggest that the starting reactants form associates of molecules of the catalyst and substrate and also associates of a mixed type from the MnP and cholesterol molecules. Molecules of MP can associate due to covalent metal-metal bonds or through an axial ligand which plays the role of a bridge. In aqueous solutions of electrolytes, water-soluble MP, for example, Fe-protoporphyrin, form ~-oxodimers. Association due to an intermolecular ~-~* interaction, formation of hydrogen and Van der Waals bonds between side substitutents, etc., is also possible [12]. Sterols also easily form self-associates and associates with phenols, alcohols, amines, and polyenes [13]. An examination of the sequence of equilibrium relations for the concentrations of MnP, cholesterol, and associates (MnP) 2 ~ 2 M n P

(i)

(MnP) --cholesterol) ~ MnP k_2 (cholesterol)n ~

n

~- cholesterol

cholesterol

*In the range of concentrations of MnP of 10-s-10 -2 M, 36, 5=-cholestanediol is the only product of oxidation of cholesterol in the presence of reduced MnP and FeP.

1130

(2) (3 )

Ln ~ eff

s

F

q.O

7,o

[9 k eff 1.5

2.0

I •0 0.5 0

I~ r

0

-I -O,b"

m

Zr

Fig. 2

.I

Fig. 3

Fig. 2. Dependence of log kef f of oxidation of cholesterol in the presence of hemato-MnOAc (i) and TMPPMnOAc (2) in mixtures of ethanol with chloroform and benzene on the Kirkwood coordinate ([MnP] = 5" i0 -s, [cholesterol] = 5"10 -3 , [NaBH,] = 5-10 -2 M). Fig. 3. Dependence of kef f of oxidation of cholesterol in the presence of hemato-MnOAc (5"i0 -s M) on the temperature in Arrhenius coordinates with different concentrations of ethanol in mixtures with CHCI3, %: i) i00; 2) 90; 3) 60; 4) 50; 5) i0; 6) 40 ([NaBH~] = 5-10 -2 , [cholesterol] = 5"10 -3 M). results in the conclusion that kef f is not a function of p and m when p > p' and m > m', since in these cases equilibria (1)-(3) are shifted toward the formation of associates.* The rate constants of the elementary stages of nonchain oxidation of olefins in the presence of TMPPMnCI-NaBH4 found in [I0, ii] can thus be assigned to transformations involving associates. The associates dissociate with a decrease in the concentration of MnP and cholesterol, and the reactivity of the reactants increases, resulting in an increase in the effective rate constant (Fig. i). It is possible to hypothesize that aggregation of MnP and sterols in nonaqueous media primarily takes place due to polarization, hydrophobic, and other reactions. The cyclopentahydrophenanthrene ring plays the determining role in such processes in the cholesterol molecule [13]. Little is known about the factors which determine noncovalent association of MP [5, 14]. The structure of the associates formed first is probably a function of the structure of the side substituents in the porphyrin ring. In all probability, molecules of hemato-Mn form stronger associates than molecules of meso-substituted MnP, since it has not been possible to reach the concentration regions of p < p' where the value of kef f is a function of p for hemato-Mn in oxidation of cholesterol in ethanol, while the characteristic value of p' for TMPPMn is 2"10 -~ M, i.e., for hemato-Mn, in contrast to TMPPMn, equilibria (i) and (2) are shifted toward formations of associates in the entire range of concentrations of p. Dissociation of dimers of hemato-Mn could only be observed when a nonpolar solvent was added to the ethanol. For noncovalent aggregation of MP, the degree of aggregation is symbatic to an increase in the dielectric constant of the solution [3, 5]. Our experiments showed that in the case of hemato-MnOAc and TMPPMnOAc, on addition of a nonpolar component (benzene, chloroform) to the starting polar solvent (ethanol), kef f increases with an increase in the so-called Kirkwood coordinate [15] (Sm - I~(2E m + i), where Em is the dielectric constant of the reaction mixture, due to a decrease in the dielectric *We will hypothesize that in agreement with [5], the molecules of MP only form dimers in the experimental conditions; the observed independence of kef f from m when m > m' can indicate that the number of molecules in the associates formed by molecules of cholesterol is limited.

1131

Ln

Ln

Z

8O

I00 0

a

qo

6O

.30 ZO IZ" I0 8

-f

80

2O 1

T

I

]

I

I

I

~Z' kcal/mole Fig. 4. a) Dependence of the preexponential function of the effective rate constant of oxidation of cholesterol in the presence of hemato-MnOAc in A on the concentration of ethanol: i) with T > 293 K; 2) with T < 293 K; b) compensation dependence of in A= on Q2 in the low-temperature region ([cholesterol] = 5"10 -3 , [NaBH ] = 5-10 -2 , [hemato-MnOAc] = 5.10 -s M). constant of the medium (Fig9 2), attaining the maximum value with the following compositions of the solvent: in ethanol-C6H 6 mixture: 65% C6H6; in ethanol-CHCl 3 mixture: 75% CHC!3 with (em - l)/(e m + i) = 0.43. The decrease in kef f with a further increase in the concentration of the nonpolar component of the solvent with (em - l)/(2em + i) = 0.41 (Fig. 2) is basically due to a decrease in t h e solubility of NaBH 4 in the system (NaBH~ is insoluble in pure CHCI 3 and benzene and the process virtually does not take place). The molar quantities of NaBH~ in the systems studied are two-three and one-two orders of magnitude higher than the concentrations of MnP and cholesterol, respectively. NaBH~ apparently does not dissolve completely even in polar solvents (it is possible to assume the presence of heterogeneous inclusions of submicron crystals of NaBH4 in the reaction mixture). However, the observed dependences cannot be explained by a decrease in the amount of dissolved NaBH4 alone, even with consideration of the hypothesis on the proportionality of kef f and (gm - l)/(2em § i), although the range of the changes in the value of kef f decreases by one order of magnitude when these factors are quantitatively considered9 The effect of thetemperature on the value of kef f in the 283-313 K range (Fig. 3) was studied to reveal the basic factors which determine the extreme character of the dependence analyzed The breaks in the observed dependences of in k' on I/T are usually due to a 9 elf change in the limiting stage of the reaction. The observed activation energy Qz in the hightemperature region changes within the limits of 4-7 kcal/mole, which probably reflects the diffusion character of the stages of formation of an intermediate complex in a mixture of solvents with a concentration of CHCI 3 > 10%; in oxidation of cholesterol in alcohol, diffusion=controlled kinetics are observed in the entire temperature range (Fig. 3; curves I, 2)~ The increase in the rate constant in the diffusion region on addition of CHCI 3 to the system is due to an increase in the preexponential function of the entropic factor (Fig. 4a, curve i). In the low-temperature region where the rate of the process is limited by chemical conversion (Q= = 10-60 kcal/mole), both the preexponential function and the activation energy, which are correlated by a compensation dependence (Fig. 4b), change. The increase in the preexponential function in both temperature regions is probably due to perturbation of the compactness of the solvate shells of the olefin molecule, the environment of the reaction site, and "loosening" of the structure of the solution on addition of chloroform [16]. CONCLUSIONS i. The dependence of the effective rate constant of oxidation of cholesterol in the presence of manganese porphyrinates and sodium borohydride on the concentration of the complex of manganese and cholesterol was found. 2. The results obtained were explained within the framework of the hypothesis on formation of solvated associates (dimers) of the manganese porphyrinate-cholesterol, manganese porphyrinate, and cholesterol-cholesterol type. 1132

LITERATURE CITED I. 2. 3. 4. 5. 6. 7. 8. 9. 10. II. 12. 13. 14. 15. 16.

B. Meunier, Bull. Soc. Chim. France, Nos. 7-8, 578 (1986). M. R. Tarasevich and K. A. Radyushkina, Catalysis and Electrocatalysis with Metalloporyphyrins [in Russian], Nauka, Moscow (1982), p. 168. D. Dolfin, Porphyrins, Vol. 4, Academic Press, New York (1979), p. 490. E. B. Fleisher, Acc. Chem. Res., ~, 105 (1970). R. V. Snygger and G.-N. La Mar, J. Am. Chem. Soc., 99, 7178 (1977). G. P. Fulton and G.-N. La Mar, J. Am. Chem. Soc., 98, 2119 (1976). A. N. Kitaigorodskii, A. G. Stepanov, A. B. Solov'eva, and E. A. Lukashova, Khim. Fiz., 6, No. 8, ii13 (1987). K. M. Smith, Porphyrins and Metalloporphyrins, Elsevier, Amsterdam (1975). V. F. Tsepalov, Zavod. Lab., No. I, i01 (1964). A. B. Solov'eva, A. I. Samokhvalova, E. I. Karakozova, et al., Kinet. Katalo, 2__5, No. 5, 1080 (1984). E. A. Lukashova, A. B. Solov'eva, G. Ya. Mailnovskii, et al., Kinet. Katal., 26, No. i, 56 (1985). R. J. Abraham, Tetrahedron, 29, 553 (1973). D. V. loffe, Usp. Khim., ~, No. 2, 333 (1986). G.-N. La Mar, J. Am. Chem. Soc., 96, 7354 (1974). So G. Entelis and R. P. Tiger, Kinetics of Reactions in the Liquid Phase [in Russian], Khimiya, Moscow (1973). S. F. Timashev, Synergetics-86 [in Russian], Shtiintsa, Kishinev (1986), p. 48.

ESTERIFICATION OF PHTHALIC ANHYDRIDE BY ALCOHOLS IN THE PRESENCE OF TITANIUM BIS(8-DIKETONATES) UDC 541.128:542.951o3: 547.584:547.2661

E. A. Khrustaleva, Yu. G. Yatluk, and A. L. Suvorov

Alkyl orthotitanates have been widely studied as catalysts for esterification reactions used, in particular, in the industrial manufacture of phthalic anhydride-based plasticizers [1-5]. The use of titanium chelates for this purpose is known [6-8], but there is practically no information on the kinetics in relation to various factors and on the reaction mechanism. Previously we investigated the esterification of phthalic anhydride by 2-ethylhexanol in a dioctyl phthalate medium using a wide range of titanium chelates as catalysts [9]. The present paper is devoted to a study of the kinetics of reaction of phthalic anhydride with excess 2-ethylhexanol and normal CG-C 9 alcohols in a medium of the corresponding ester in the presence of titanium bis(B-diketonates). EXPERIMENTAL Phthalic anhydride of analytical purity and distilled 2-ethylhexanol and normal C6-C 9 alcohols were used. Bis(B-diketonato)dialkyoxytitaniums were synthesized from butyl orthotitanate and the corresponding chelating agents according to [i0], and the hydrolysis products w e r e synthesized according to [ii]. The esterification was carried out at 185~ either in excess alcohol or in a medium of the resulting ester with a stoichiometric reagent ratio. The concentrations of phthalic anhydride and the catalyst were 0~2 and 4.35~i0 -4 mole/liter, respectively. The reaction and the calculation of the rate constants were carried out similarly to [9].

Institute of Chemistry, Ural Branch, Academy of Sciences of the USSR, Sverdlovsko Translated from Izvestiya Akademii.Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1247-1250, June, 1989. Original article submitted May 4, 1987. 0568-5230/89/3806-1133512.50

9 1989 Plenum Publishing corporation

1133