Catalyzed Oxidative Cleavage of Pyrrolidine Ring in l-Proline by

Ind. Eng. Chem. Res. 2009, 48, 10381–10386

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Osmium(VIII) Catalyzed Oxidative Cleavage of Pyrrolidine Ring in L-Proline by Hexacyanoferrate(III) in Alkaline Media K. Sharanabasamma, Mahantesh A. Angadi, Manjalee S. Salunke, and Suresh M. Tuwar* Department of Chemistry, Karnatak Science College, Dharwad-580 001, Karnataka, India

Kinetics of oxidation of L-proline by hexacyanoferrate(III) catalyzed by osmium(VIII) in alkaline medium was studied at 30 °C. A mechanism involving free radical path was proposed. Rate law of the reaction was derived as rate )

kK1K2[HCF][L-proline][OH-][Os(VIII)] 1 + K1[OH-] + K1K2[L-proline][OH-]

The rate constant k and equilibrium constants K1 and K2 were evaluated as 3.79 × 103 dm3 mol-1 s-1, 0.52 dm3 mol-1, and 3.78 × 103 dm3 mol-1, respectively. The main reactive species of the catalyst appears to be OsO5(OH)3-. The oxidative product of L-proline was identified as L-glutamic acid which is different from the earlier reports. The literature reveals that 4-amino butyric acid, 4-amino butaraldehyde and keto acids were the oxidative products. Activation parameters were evaluated as Ea ) 33.5 ( 2 kJ mol-1, ∆Hq ) 31.0 ( 0.2 kJ mol-1, ∆Sq ) -24 ( 1.5 J K-1 mol-1, ∆Gq ) 38.0 ( 2 kJ mol-1. 1. Introduction The chemistry of hexacyanoferrate(III)(HCF) in alkaline medium is well understood,1-8 in particular its oxidative capacity of oxidation of inorganic and organic compounds. It may be due to its unequivocal stability, solubility, single equivalent change, and its moderate reduction potential, [Fe(CN)6]3-/[Fe(CN)6]4- of +0.45 V. Thus, this potent oxidant, HCF is used mainly as electron abstracting or proton abstracting agent and also in free radical generation during the oxidation of numerous organic compounds. Osmium compounds are highly stable in +8 oxidation state. The OsO4 (Os(VIII) undergoes reduction up to +2 via +4 and +6 oxidation states in acid medium. However, in alkaline medium it undergoes reduction9 up to +6 oxidation state only. Together the unique nature and the moderate reduction potential,10 Os(VIII)/Os(VI) of +0.85 V in acid medium and +0.30 V in alkaline medium act as a catalyst with a large number of oxidants in the oxidation of both organic and inorganic substrates.11 The literature survey reveals that the catalytic role12 of osmium(VIII) is well established. Nevertheless, Os(VIII) has been used as catalyst until today to understand the reaction paths during oxidation of various organic substrates, particularly with HCF, wherein HCF regenerates Os(VI)/Os(IV) to Os(VIII). 13 L-proline is a nonessential amino acid and it is an important constituent of collagen. It is also an ingredient in energy drinks. A recent report13 reveals that L-proline is considered to be the world’s smallest natural enzyme and is used in catalyzing the aldol condensation of the acetone to various aldehydes with high stereospecificity. It is an imino acid and also a unique proteogenic R-imino acid where the R-imino group is secondary with pyrrolidine ring. Some of the reports of oxidation of L-proline claim that the ring cleavage takes place between N and C of

by retaining the -NH2 group with the main moiety without liberating ammonia, and they have proposed14 a mechanism for * To whom correspondence should be addressed. E-mail: sm.tuwar@ gmail.com. Tel.: +919449796557.

its oxidation as decarboxylation. Nonetheless, the present investigation is undertaken to know whether the reported product results or not. The results in the present study are found to be substantially different and very prominently establish a mechanism for its oxidation. Hence, the present investigation was undertaken in alkaline medium to setup the oxidative paths of L-proline with a unique oxidant and a catalyst, namely, HCF and osmium(VIII), respectively. 2. Experimental Details 2.1. Chemicals and Solutions. Reagent grade chemicals and double-distilled water (from alkaline KMnO4 in all-glass apparatus) were used. An aqueous solution of HCF was prepared by dissolving K3[Fe(CN)6] (BDH) in water and was standardized15 iodometrically. L-proline, a colorless crystalline compound (E-Merck) was used without further purification for the preparation of aqueous stock solution. The stock solution of osmium(VIII) was obtained by dissolving osmic acid (OsO4) (JohnsonMatthey) in 0.5 mol dm-3 sodium hydroxide solution, and its concentration was ascertained16 against standard ceric ammonium sulfate solution in acid medium. Aqueous solutions of K4[Fe(CN)6] and L-glutamic acid were used to study the effect of products on rate of reaction. Aqueous solution of NaOH and NaCl were used to maintain the [OH-] and ionic strength, respectively. In the present investigation, the effect of concentration of [HCF] was studied from 5.0 × 10-5 to 5.0 × 10-4 and [L-proline] was studied between 4.0 × 10-4 to 4.0 × 10-3. 2.2. Instruments Used. For kinetic measurements, Hitachi U-3010 spectrophotometer (Tokyo, Japan) was used. 2.3. Kinetic Studies. The reaction was initiated by mixing K3[Fe(CN)6] solution to L-proline which also contained required amounts of Os(VIII), NaOH, and NaCl. The reaction at 30 ( 0.1O C was studied at least 10 times under pseudo-first-order condition where [L-proline] > [HCF]. Progress of the reaction was followed spectrophotometrically by measuring the decrease in the absorbance of HCF at its λmax 420 nm in a 1 cm cell placed in the thermostatted cell compartment of the spectro-

10.1021/ie901049p CCC: $40.75  2009 American Chemical Society Published on Web 10/01/2009

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photometer. The spectrophotometer was attached to a desktop computer having the required program to evaluate the kinetic data. The obedience to Beer’s law of the absorption of HCF at 420 nm in the concentration range of 6.0 × 10-5 to 6.0 × 10-4 mol dm-3 under the reaction conditions had been tested earlier (ε ) 1060 ( 1.5% dm3 mol-1 cm-1). The pseudo-first-order rate constants kobs were calculated from the slopes of log [HCF] versus time plots, which were linear up to 80% completion of the reaction in most of the variations of concentrations of oxidant, reductant, and catalyst. Order with respect to each reactant is determined from the slopes of plots of log kobs versus log(concn) except in [HCF]. The results were reproducible within (4%. The moderately higher concentration of NaOH was used to maintain the OH- concentration in the reaction. Hence, the effect of dissolved CO2 on the rate was examined by carrying out the kinetics in the presence of CO2 and N2. It was found that there was no variation of rate constants which indicates that dissolved CO2 had no effect on rate of reaction.

Table 1. Effect of [HCF] and [L-Proline] on Oxidation of L-Proline by Hexacyanoferrate(III) Catalyzed by Osmium(VIII) in Alkaline Medium at 30 °Ca

3. Results

Table 2. Effect of [Os(VIII)] and [OH-] on Oxidation of L-Proline by Hexacyano Ferrate(III) Catalyzed by Osmium(VIII) in Alkaline Medium at 30 °Ca

3.1. Stoichiometry and Product Analysis. Different sets of [reactants] and [catalyst] in 0.10 mol dm-3 NaOH at a constant ionic strength of 0.50 mol dm-3 were kept for over 24 h at 30O C. When [HCF] > [L-proline], the [HCF] was analyzed by measuring its absorbance at 420 nm. It was also estimated iodometrically.15 The main oxidative product of L-proline was identified as L-glutamic acid by its spot test in which the intense blue color was obtained by adding ninhydrin.17 It supports the earlier work.18 It is also estimated quantitatively as ninhydrin derivative by spectrophotometric methods.19 It is found that L-proline was oxidized to L-glutamic acid. Other plausible products like glutamic semialdehyde and R-keto acid were not found. Further, the L-glutamic acid was separated from reaction mixture by ether extraction on acidification and subjected to IR scanning. It was observed that the stretching frequencies of -NH2, -COOH, and carbonyl appeared to be 3430, 3060, and 1684 cm-1, respectively, and C-N vibration frequencies at 1124 cm-1 were also observed. This clearly indicates that the oxidative product of L-proline was found to be L-glutamic acid which is formed by reacting with 4 moles of [Fe(CN)6]3- as shown in eq 1.

3.2. Reaction Orders. Reaction order and order with respect to each reactant was determined by varying the concentrations of oxidant, reductant, catalyst and alkali in turn, while keeping the others constant. 3.3. Effect of Concentration Hexacyanoferrate(III). The order in [HCF] in the concentration range, 5.0 × 10-5-5.0 × 10-4 mol dm-3 was found to be unity as the plot of log [HCF] versus time was linear with non- variation in slopes for different [HCF] (Table 1). 3.4. Effect of Concentration L-Proline. The order in Lproline in the concentration range of 1.0 × 10-3 to 1.0 × 10-2 mol dm-3 was found to be less than unity (0.4) (Table 1). 3.5. Effect of Concentration Osmium(VIII). Effect of varying [Os(VIII)] in the range of 1.0 × 10-7-4.0 × 10-6 mol

[Fe(CN)6]3- × 104

[L-prol] × 103

kobs × 103

kcalb × 103

0.5 mol dm-3 0.8 mol dm-3 1.0 mol dm-3 2.0 mol dm-3 3.0 mol dm-3 4.0 mol dm-3 5.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3

4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 4.0 mol dm-3 1.0 mol dm-3 2.0 mol dm-3 3.0 mol dm-3 4.0 mol dm-3 5.0 mol dm-3 7.0 mol dm-3 10.0 mol dm-3

1.44 s-1 1.44 s-1 1.43 s-1 1.44 s-1 1.45 s-1 1.44 s-1 1.45 s-1 0.52 s-1 0.92 s-1 1.22 s-1 1.43 s-1 1.65 s-1 1.97 s-1 2.50 s-1

1.44 s-1 1.44 s-1 1.44 s-1 1.44 s-1 1.44 s-1 1.44 s-1 1.44 s-1 0.57 s-1 1.00 s-1 1.32 s-1 1.55 s-1 1.71 s-1 2.08 s-1 2.56 s-1

a [OH-] ) 0.1, [Os(VIII)] ) 1.0 × 10-6, I ) 0.5 /mol dm-3. b kobs were recalculated using k ) 3.79 × 103 dm3 mol-1 s-1, K1 ) 0.52 dm3 mol-1, and K2 ) 3.78 × 103 in the rate eq 4.

[Os(VIII)] × 106

[OH-]

kobs × 103

kcalb × 103

1.0 mol dm-3 1.0 mol dm-3 1.0 mol dm-3 1.0 mol dm-3 1.0 mol dm-3 1.0 mol dm-3 0.1 mol dm-3 0.2 mol dm-3 0.3 mol dm-3 0.4 mol dm-3 0.5 mol dm-3 0.6 mol dm-3 0.8 mol dm-3 1.0 mol dm-3 2.0 mol dm-3 4.0 mol dm-3

0.050 mol dm-3 0.075 mol dm-3 0.10 mol dm-3 0.20 mol dm-3 0.30 mol dm-3 0.40 mol dm-3 0.10 mol dm-3 0.10 mol dm-3 0.10 mol dm-3 0.10 mol dm-3 0.10 mol dm-3 0.10 mol dm-3 0.10 mol dm-3 0.10 mol dm-3 0.10 mol dm-3 0.10 mol dm-3

0.92 s-1 1.36 s-1 1.44 s-1 2.04 s-1 2.55 s-1 2.84 s-1 0.17 s-1 0.30 s-1 0.42 s-1 0.60 s-1 0.77 s-1 0.84 s-1 1.22 s-1 1.44 s-1 2.80 s-1 6.20 s-1

1.01 s-1 1.33 s-1 1.55 s-1 2.18 s-1 2.51 s-1 2.84 s-1 0.16 s-1 0.37 s-1 0.47 s-1 0.63 s-1 0.79 s-1 0.95 s-1 1.25 s-1 1.55 s-1 3.17 s-1 6.34 s-1

a [Fe (CN)6]3- ) 4.0 × 10-4, [L-proline] ) 4.0 × 10-3, I ) 0.5 /mol dm-3. b kobs were recalculated using k ) 3.79 × 103 dm3 mol-1 s-1, K1 ) 0.52 dm3 mol-1, and K2 ) 3.78 × 103 in the rate eq 4.

dm-3 in the reaction was studied under the constant concentrations of oxidant and reductant at 4.0 × 10-4 and 4.0 × 10-3 mol dm-3, respectively, when [OH-] ) 0.1 mol dm-3 (Table 2). The order in [Os(VIII)] was found to be unity as calculated from log-log plots. 3.6. Effect of Initially Added Products. The effect of initially added products [Fe(CN)6]4- and L-glutamic acid were studied in the concentration range of 1.0 × 10-4-1.0 × 10-3 mol dm-3 at constant ionic strength and all other conditions being constant. It is found that both of the products did not alter the rate of reaction. 3.7. Effect of Concentration of Alkali. At a fixed ionic strength of 0.5 mol dm-3 and other conditions remaining constant, [OH-] was varied from 0.05 to 0.50 mol dm-3. It was noticed that as [OH-] increased the rate of reaction was also increased (Table 2). The plot of log kobs versus log [OH-] was linear and the order was determined as 0.5. Its increasing effect on rate is due to the variation of concentration of hydroxide species of Os(VIII) at different [OH-]. The various forms of hydroxide complexes in alkaline medium such as [OsO3(OH)3]-, [OsO4(OH)2]2-, and [OsO5(OH)]3- are in equilibrium with each other and can be shown in eq 2 and eq 3.

Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009 KI

[OsO3(OH)3]- + OH- y\z [OsO4(OH)2]2- + H2O KII

[OsO4(OH)2]2- + OH- y\z [OsO5(OH)]3- + H2O

(2)

(3)

The total concentration of Osmium(VIII), [Os(VIII)]T, is the sum of different Os(VIII) species concentrations, [OsO3(OH)3]-, [OsO4(OH)2]2-, and [OsO5(OH)]3-; the complexes having the equilibrium constants KI and KII as 24 and 6.4 dm3 mol-1, respectively,20 are used to calculate the concentrations of such species (Table 3). The results of such calculations are utilized to draw Figure 1, and it is seen that, of the concentrations of different species, the variation of only [OsO5(OH)]3- with [OH-] shows any parallelism with the variation of kobs with alkali. 3.8. Effect of Ionic Strength and Dielectric Constant. The ionic strength of the reaction medium was varied between 0.1 and 0.9 mol dm-3 with sodium chloride at constant concentrations of alkali, oxidant, reductant, and catalyst at 30 °C. The apparent first-order rate constants showed a nearly 10-fold increase (Table 4). A plot of log kobs versus I was linear with a unit positive slope (Figure 2). The dielectric constant of the medium D was varied by varying the tert-butyl alcohol/H2O percentage (v/v). Since the D for various percentage compositions was not available in literature, they were computed by using their D in pure state. In the reaction, as D decreases the kobs values were increased (Table 4). Earlier the reactions between tert-butyl alcohol and oxidant /catalyst were studied. It was observed that there was neither a reaction between solvent and catalyst nor with the oxidant. The graph of log kobs versus 1/D was found to be linear with positive slope (Figure 2). 3.9. Polymerization Study. Since the [Fe(CN)6]3- is 1 equiv oxidant and oxidation of organic compounds by such an oxidant was expected to intervene the free radical generated by L-proline. Therefore, the intervention of free radical during the reaction was studied by an initial addition of monomer, acrylonitrile, followed by dilution with methanol, resulting in a copious precipitation. This indicates that the reaction was routed through a free radical path. It is also ascertained by the decrease in rate with initial addition of monomer. 3.10. Effect of Temperature. The kinetics were also studied at 30, 35, 40, and 45 °C at constant concentrations of all reactants and other conditions being constant. The kobs at various temperatures are calculated and tabulated (Table 5). From the Arrhenius plot of log kobs vs 1/T, the Ea was calculated as 33.5 ( 2 kJ mol-1 and other activation parameters were calculated as ∆Hq ) 31.0 ( 0.2 kJ mol-1, ∆Sq ) -24 ( 1.5 J K-1 mol-1, ∆Gq ) 38.0 ( 2 kJ mol-1 and log A ) 3.0. 4. Discussion The variation of concentration of Os(VIII) with alkali as shown in Figure 1 indicates that [OsO5(OH)]3- is the reactive species; its concentration was varied linearly with [OH-]. The concentrations of the other two species, [OsO4(OH)2]2- and [OsO3(OH)3]- are either decreased or increased drastically with various [OH-] and are not varied parallel to the variation of kobs for different [OH-]. Hence, they are not considered as reactive species. The formation of [OsO5(OH)]3- as in eq 3 is important in this study as reported earlier.20 Each fractional order in [OH-] and [L-proline] is an implicit fact to support the expectation of the pre-equilibrium before rate determining step, and copious precipitate during the polymerization study strongly supports the free radical interven-

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tion generated from L-proline in its oxidation by alkaline HCF catalyzed by Os(VIII). First-order in oxidant and catalyst can also be accommodated in the mechanism as shown in Scheme 1. Hence, the scheme is written in accordance with the above facts and the consideration of active species of osmium(VIII) in alkali as [OsO5(OH)]3- in the first equilibrium step. L-Proline has two donor atoms. namely, N from imino moiety and O from the carboxylic group having a lone pair of electrons. It is a known21 fact that N is a small potent atom and can donate a pair of electrons to the central metal ion of Os(VIII) to form an adduct. The presence of two -CH2 groups on either side of the N atom favors the positive charge on the N atom and makes it easy to form the complex. Thus, formation of a complex between Os(VIII) and an O atom of carboxylic group can be ruled out. The adduct formed in this way with N might be very reactive and undergoes oxidation easily by HCF. This is evidenced by the fact that in the absence of Os(VIII), the reaction between L-proline and HCF was not observed. Therefore, the intermediate as shown in the second step of Scheme 1 reacts with HCF in the rate-determining step to give a free radical generated from L-proline. This justifies the unit order each in oxidant and catalyst. Nevertheless, one can predict that the Os(VIII) could oxidize the substrate by yielding Os(VI) and it is reoxidation to Os(VIII) by HCF. If that is the case the order in substrate would be unity, and no free radical intervention during oxidation could be expected. However, in the present investigation the free-radical test was positive and order with respect to substrate is a fraction. Hence, such a possibility is ruled out. The rate law for the mechanism as in Scheme 1 could be derived as kobs )

kK1K2[L-proline][Os(VIII)][OH-] 1 + K1[OH-] + K1K2[OH-][L-proline]

(4)

Equation 4 is rearranged in to eq 5. 1 1 + ) kobs kK1K2[L-proline][Os(VIII)][OH-] 1 1 + kK2[L-proline][Os(VIII)] k[Os(VIII)]

(5)

The mechanism as in Scheme 1 and rate law eq 4 are verified in the form of eq 5 by plotting the graphs of 1/kobs versus 1/[OH-] and 1/[L-proline] which should be linear (Figure 3). From the slopes and intercepts of such plots, the values of k, K1, and K2 are calculated as 3.79 × 103 dm3 mol-1 s-1, 0.52 dm3 mol-1 (literature value: 6.4 dm3 mol-1), and 3.78 × 103 dm3 mol-1, respectively. The K1 found in this study is in close agreement with the reported value.20 This justifies the formation of [OsO5(OH)]3-. Using these values, rate constants under different experimental conditions were calculated. It is found that such calculated rate constants agree with the experimental values (Tables 1-3). An increase in rate with increasing ionic strength is in the expected direction as a negatively charged complex, resulted by interacting Os(VIII) species with L-proline, reacts with another negatively charged [Fe(CN)6]3- in the rate determining step. Ion pairing9,22 between K+ and [Fe(CN)6]3- is understood. Thus, the ion pairing between Na+ and [Fe(CN)6]3- may also be expected as NaOH and NaCl were used to maintain required concentrations of alkali and ionic strength, respectively. Therefore, the unit positive slope in the graph of log kobs versus I

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Table 3. Variation of Concentration of Active Speciesa of Osmium(VIII) at Different [OH-] with Rate Constants of Oxidation of L-Proline by Hexacyanoferrate(III) Catalyzed by Osmium(VIII) at 30 °C [OH-]

[OsO3(OH)3]- × 107 -3

-3

0.05 mol dm 0.075 mol dm-3 0.10 mol dm-3 0.20 mol dm-3 0.30 mol dm-3 0.50 mol dm-3

[OsO4(OH)2]2- × 107 -3



[OsO5(OH)]3- × 107 -3


kobs × 103 -1

0.92 s 1.36 s-1 1.44 s-1 2.04 s-1 2.55 s-1 2.85 s-1

kCala × 103 1.01 s-1 1.33 s-1 1.55 s-1 2.18 s-1 2.51 s-1 2.84 s-1

a Os(VIII) in various forms at different [OH-] as per eq 2 and 3, and KI ) 24 dm3 mol-1 and KII ) 6.4 dm3 mol-1 are used to calculate the species (ref 20).

Figure 1. Variation of concentrations of different Os(VIII) species and rate constants with [OH-]: (b) [OsO3(OH)3]-, (9) [OsO4(OH)2]2-, (2) [OsO5(OH)]3-, (×) log kobs. Table 4. Effect of Ionic Strength and Dielectric Constant of the Reaction Medium on Osmium(VIII) Catalyzed Oxidation of a L-Proline by Hexacyanoferrate(III) in Aqueous Alkali at 30 °C ionic strength I

dielectric constant D I ) 0.5 mol dm-3

D ) 78.5 kobs × 10

I 0.1 0.2 0.3 0.5 0.7 0.9

mol mol mol mol mol mol

dm-3 dm-3 dm-3 dm-3 dm-3 dm-3

Figure 2. Effect of ionic strength and dielectric constant on osmium(VIII) catalyzed oxidation of L-proline by hexacyanoferrate(III) in alkaline medium at 30 °C.

0.75 s-1 0.89 s-1 1.08 s-1 1.44 s-1 1.95 s-1 2.35 s-1

3

D

kobs × 103

78.5 75.1 71.7 68.4 65.0 62.5

1.44 s-1 1.52 s-1 1.58 s-1 1.81 s-1 2.12 s-1 2.40 s-1

a [Fe(CN)6]3- ) 4.0 × 10-4, [L-proline] ) 4.0 × 10-3, [Os(VIII)] ) 1.0 × 10-6, [OH-] ) 0.1/ mol dm-3.

explains qualitatively the interaction between the species of similar charges in the rate-determining step. An increase in rate with decreasing the dielectric constant of reaction media is contradictory to the expected direction as ionic species are involved in the slow step as shown in Scheme 1. This may be due to the solvation of the activated complex at low dielectric constant media rather than that of a higher one where the reactants are more solvated.23 A negative value of ∆Sq (-24.2 ( 2 J K-1 mol-1) suggests that the two ionic species combine in rate determining step to give a single intermediate complex which is more ordered than the reactants.24,25 The smaller rate constant of the slow step of the mechanism indicates that the oxidation presumably occurs through an inner-sphere mechanism. This conclusion was supported by earlier reports.26-28 The unexpected low value of frequency factor (log A ) 3.0) clearly indicates the intervention

Figure 3. Verification of rate law eq 5 at a different [OH-] and [L-proline] on oxidation of L-proline by osmium(VIII) catalyzed hexacyanoferrete(III).

of free radical and similarly charged ions that are interacting rather than the direct interaction between un-ionized L-proline with Os(VIII)/ HCF in a slow step that fortifies the mechanism as shown in Scheme 1.


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Scheme 1

Table 5. Effect of Temperature on Osmium(VIII) Catalyzed Oxidation of L-Proline by Hexacyanoferrate(III) in Aqueous Alkalia temperature (°C) 30 35 40 45

kobs × 103 1.44 1.84 2.29 3.00

s-1 s-1 s-1 s-1

a [Fe(CN)6]3- ) 4.0 × 10-4, [L-proline] ) 4.0 × 10-3, [Os(VIII)] ) 1.0 × 10-6, [OH-] ) 0.1/ mol dm-3, I ) 0.5 mol dm-3.

Appendix Derivation of Rate Law for the Scheme 1 [Os(VIII)]T is equal to the sum of concentration of -

d[Fe(CN)6]3) rate ) k[Fe(CN)6]3-[complex] dt ) kK2[Fe(CN)6]3-[L-proline]f[OsO5(OH)]3) kK1K2[Fe(CN)6]3-[L-proline]f ×

[OsO4(OH)]f2-[OH-]f (i)

5. Conclusion Oxidation of L-proline was set-up to mimic the biological path. The reaction product was found to be L-glutamic acid. However, earlier studies reveal that the products were 4-amino butyric acid,14 4-amino butaraldehyde,29 and keto acids.30 The 4-amino butaraldehyde is the most unpredictable product, as L-proline oxidizes through a pyrrolidine ring cleavage without decarboxylation. If aldehyde is formed at all, it would be glutamic semialdehyde. In the absence of Os(VIII), the reaction between L-proline and hexacyanoferrate(III) is almost imperceptible, whereas the addition of a small amount of Os(VIII) favors the spontaneity of the reaction. This might be the reactive species of adduct, which is formed by interacting L-proline with osmium(VIII). Though osmium(VIII) is used as a catalyst it did not undergo reduction to Os(VI), but it catalyzes through the formation of active adduct and regenerates in the rate determining step by reacting with HCF to give a free radical generated by L-proline. Acknowledgment Suresh M. Tuwar is grateful to the University Grants Commission, SWRO, Bangalore, for the financial assistance under the research project. Supporting Information Available: Additional data, calculations, tables, and graphs. This material is available free of charge via the Internet at http://pubs.acs.org.

[OsO4(OH)2]2-, [OsO5(OH)]3-, and [complex]. ) [OsO4(OH)2]f2- + K1[OsO4(OH)2]f2-[OH-]f + K1K2[OsO4(OH)2]f2-[L-proline]f[OH]f

) [OsO4(OH)2]f2-{1 + K1[OH-]f + K1K2[L-proline]f[OH-]f}

∴[OsO4(OH)2]f2- ) [Os(VIII)]T -

1 + K1[OH ]f + K1K2[OH-]f[L-proline]f

(ii)

Similarly, [(OH)]f- )

[OH]T 1 + K1[OsO4(OH)2]2-

(iii)

and [L-proline]f )

[L-proline]T 1 + K2[OsO5(OH)]3-

(iv)

In view of low concentration of Os(VIII) used in the present investigation, the denominators of eqs iii and iv in the parentheses are negligible. Thus, [OH]f- ) [OH]T- and [L-proline]f ) [L-proline]T and on incorporating all these and by substituting eq ii in i, eq v results.

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-


d[Fe(CN)6]3) rate ) dt kK1K2[Fe(CN)6]3-[L-proline]T[Os(VIII)]T[OH-]T 1 + K1[OH-]f + K1K2[L-proline]f[OH-]f

or kobs )

kK1K2[L-proline]T[Os(VIII)]T[OH-]T 1 + K1[OH-]f + K1K2[OH-]f[L-proline]f

(v)

For the verification of the rate law, the subscripts ‘T’ and ‘f’ are omitted and hence eq v becomes kobs )

kK1K2[L-proline][Os(VIII)][OH-] 1 + K1[OH- + K1K2[OH-][L-proline]

(vi)

Equation vi is rearranged in to eq vii: 1 1 + ) kobs kK1K2[L-proline][Os(VIII)][OH-] 1 1 + kK2[L-proline][Os(VIII)] k[Os(VIII)]

(vii)

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ReceiVed for reView June 30, 2009 ReVised manuscript receiVed August 27, 2009 Accepted August 30, 2009 IE901049P

Catalyzed Oxidative Cleavage of Pyrrolidine Ring in l-Proline by

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