Ruthenium catalyzed oxidation of nitrogen-containing heteroaromatic compounds

L.I. Simhdi (Editor),Dwxygen Activation and Homogeneous Catalytic Oxidation 0 1991 Elsevier Science PublishersB.V., Amsterdam

Ruthenim compounds

cata1yzed

oxidation

of

ni trogen-containing

129

heteroaromat ic

T.Hara and M.Horii Department of Chemistry, Faculty of Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama, Japan 240 Abstract Nitrogen-containing heteroaromatic compounds such as quinoline, isoquinoline and acridine were oxidized to corresponding aromatic o-dicarboxylic acid remaining pyridinic structure in the product molecules by hypochlorite ion in the presence of ruthenium catalyst at ambient temperatures in alkaline aqueous solution. Quinoline was oxidized to pyridine-2,3-dicarboxylic acid quantitatively, in the case that the OHconcentration in aqueous solution was kept in the particular range. Acridine was oxidized using acetonitrile-aqueous solution system. Pyridine-2,3,5,6tetracarboxylic acid (yield=82%) was formed when relatively higher OHinitial concentration (1.1M) and 30°C were used as reaction conditions. On the contrary, quinoline-2,3-dicarboxylic acid (yield=23%) was formed as a reaction intermediate in addition to pyridinetetracarboxylic acid (yield=55%) when lower OH- initial concentration (0.55M) and lower temperature of 10°C were used. 1.INTRODUCTION Pyridine-2,3-dicarboxylic acid (quinolinic acid) and pyridine-3,4dicarboxylic acid (cinchomeronic acid) are important intermediate reagents for making agrochemicals, pharmaseuticals and specialized surface coatings [ 1 1 . Pyridine-2,3,5,6-tetracarboxylic acid and quinoline-2,3-dicarboxylic acid (acridinic acid) are considered to be interesting monomers for engineering plastics, being related with pyromellitic acid and naphthalene2,3-dicarboxylic acid, respectively [2]. Ruthenium tetraoxide is known as a strong oxidant to cleave aromatic rings at ambient temperature [3]. The combination of hypochlorite ion with a catalytic amount of RuCl, or RuO, has has been used for the oxidative cleavage of naphthalene to phthalic acid in two phase reaction system of CCl,/H,O or CHC13/H20 [4]. In the previous paper [5], we reported that quinoline was oxidized to quinolinic acid rapidly and quantitatively by hypochlorite ion in the presence of ruthenium catalyst, only in the case that the concentration of

130

hydroxide ion in aqueous solution was kept in the particular range and that such organic chlorinated solvent as carbon tetrachloride or chloroform was not used for the reaction system. In the present study, new reaction system composed of acetonitrile and water was employed for the oxidation of quinoline, isoquinoline and acridine. Such nitrogen-containing heteroaromatic compounds were selectively oxidized to corresponding pyridinecarboxylic acids in the reaction conditions mentioned above. The new solvent system of CH,CN/H,O is particularly convenient for the substrate being oxidized whose solubility is limited in alkaline aqueous solution. 2.EXPERIMENTAL The oxidation reaction was carried out by adding an acetonitrile solution of a substrate being oxidized (1.55~1O-*mol)or by adding a substrate directly into vigorously stirred alkaline solution, containing ruthenium trichloride ([Sub] ,/Ru = 200-4000) and hypochlorite ion ([ClO-],/[Sub] , = 20-40). The initial concentaration of hydroxide ion in aqueous hypochlorite solution was carefully fixed in advance. Two-phase solvent systems as CCl+/H,O and CHCl,/H,O were used in the experiments to understand the role of the solvent system in the oxidation reaction. Pyridinecarboxylic acids formed during the reaction were quantitatively analyzed at adequate intervals by high performance liquid chromatography (HPLC: Shimazu LC-7A) using an anion-exchange resin as a stationary phase (Zorbax SAX) which was maintained at 40°C. A mobile phase composed of a buffer (pH=5.0, citric acid/Na,HFQ,) and acetonitrile (buffer/CH,CN = 80/20 by volume) was used as a flow rate of 1.2 ml/min. Benzene-l,Z,Ctricarboxylic acid (trimellitic acid) was used for the external standard [5]. The yields of some particular pyridine-o-dicarboxylic acids as well as oxalic acid were also determined by isolation as the copper(I1) salts [5]. The concentrations of hydroxide ion and hypochlorite ion were carefully followed during the oxidation by the conventional methods [5]. 3.RESuLTS AND DISCUSSION Quinoline was oxidized to quinolinic acid rapidly and quantitatively by ruthenium catalyst in conjunction with hypochlorite ion in aqueous solution only in the case that the concentration of hydroxide ion was maintained ar the particular range (for example, the initial OH- concentration was arranged at 0.55-1.1 M) and that the use of such organic chlorinated solvent as carbon tetrachloride o r chloroform was not employed in the oxidation reaction [S]. The yield of quinolinic acid was reached to 84% at 2h of the reaction time when the initial OH- concentration was fixed at 1.1 M. The concentration of hydroxide ion in the aqueous solution was found to play a substantial role on the selectivity of the reaction. The major

131

product in the oxidation of quinoline was changed to oxalic acid o r carbon dioxide depending on the OH- concentration [5] . The spectrophotometric study indicated that the active catalytic species for selective oxidation into quinolinic acid may be [Ru(VII)O,] - (perruthenate ion) which shows its absorption maximum at 385 nm. In the higher OH- concentration as 3.3-4.5 M in the aqueous solution, the active catalytic species was changed into [RU(VI)O,]~ (ruthenate ion), which was responsible for high yield of oxalic acid as a major product in the oxidation of quinoline. In the lower OHconcentration (below 0.20-0.24 M), Ru(VIII)O, may act as an active species and quinolinic acid formed during the reaction as well as quinoline remained in the reaction solution were oxidized into carbon dioxide 151. Table 1 Comparison of pseudo-first order rate constants in the presence and absence of CC1, as an organic phase First-order rate constant (h ' 1 kH z o kcc14+Hzo (h-') kH zo/kcc1 4 + H 2 0

Amount of CC1, added 0 ml 50 ml

Initial concentration of [OH-] 0.53M I 0.70M 4.12 8.87~

3.52 7.45~

46.5

47.2

When the oxidation of quinoline was carried out using two-phase reaction system employing carbon tetrachloride o r chloroform as an organic phase, the rate of oxidation was drastically decreased, as shown in Table 1 . The rate of oxidation has a little negative dependence on the concentration of hydroxide ion in aqueous solution. In both initial concentrations of [OH ] of 0.53 and 0.70 M, the ratio of rate constant (kH20/kCC14+H20) was obtained as almost same value of 47. On the contrary, the oxidation of quinoline proceeded more rapidly when acetonitrile was added to the alkaline aqueous solution (Figure 1 ) . In case that 25 ml of acetonitrile was added to 300 ml aqueous solution containing C10- and OH-, the effect of the addition appeared substantially and the reaction time could be shortened to 2/'3 compared with the case that no acetonitrile was added. The result shows that the reaction system of CH,CN/H,O is very convenient f o r a substrate being oxidized whose solubility is limited in aqueous alkaline solution. Isoquinoline was oxidized by ruthenium catalyst in the same reaction condition. In contrast with the case of quinoline, isoquinoline produced

132

phthalic acid as a minor product in addition to cinchomeronic acid (Figure 2). In the oxidation of quinoline, the formation of phthalic acid was no: observed in any reaction condition. The effect of the addition of acetonitrile appeared more substantially in the oxidation of isoquinoline than that of quinoline. The yield of cinchomeronic acid reached up to 44% at 1 h of reaction time. The ratio of the formation rate of cinchomeronic acid to that of phthalic acid in the initial stage of the reaction was obtained in the presence and absence of acetonitrile as follows: CH,CN 20 ml CH,CN 0 ml k c i n/kp,t h a 2.87 3.85 The result shows that the addition of acetonitrile enhances not only the rate of oxidation but the reaction selectivity into cinchomeronic acid. Acridine was oxidized using CH,CN/H,O reaction system. The oxidation was carried out by adding 50 ml acetonitrile solution of acridine (15.5 mmol) into vigorouly stirred 300 ml aqueous solution containing ruthenium catalyst, hypochlorite ion and hydroxide ion. The oxidation products were isolated by the formation of copper(I1) salts [5]. The copper salts was decomposed to form carboxylic acids by the treatment with heated aqueous ammonia. The carboxylic acids isolated were characterized after esterification as pyridine-2,3,5,6-tetracarboxylic acid and acridinic acid (Figure 3 ) . Pyridinetetracarboxylic acid was formed at the yield of 82% at 3 h of

loot

I

0

I

20 Reaction

I

60

40 time

(min.)

Figure 1 Effect of Addition of Acetonitrile on Yield of Quinolinic Acid in Ruthenium Catalyzed Oxidation of Quinoline Reaction conditions: Quinoline=l5.5 mmol, [Sub/Ru]=200, NaClOaq 300 ml [OH-],=O.SS M, [ClO~/Sub],=30.0, 30°C

133

CHSCN

0

1 &action time

2

3

(h)

Figure 2 Ruthenium Catalyzed Oxidation of Isoquinoline in the Presence and Absence of Acetonitrile Solvent Reaction conditions: Isoquinoline=15.5 mmol, [Sub/Ru]=200, NaClOaq 300 ml, [OH-]0=0.63 M, [ClO-/Sub],=30.0, 30°C reaction time when relatively higher initial OH- concentration (1.1 M) and reaction temperature of 30°C were employed for the oxidation (Figure 4 ) . On the other hand, acridinic acid was formed at the yield of 23% in addition to pyridinetetracarboxylic acid (yield=55%) when lower initial OH- concentration (0.55 M) and lower temperature of 10°C were used (Figure 5). Acridinic acid is the reaction intermediate to form pyridinetetracarboxylic acid in the ruthenium catalyzed oxidation of acridine. Relative reactivity of each aromatic ring for the oxidative cleavage by perruthenate ion was estimated in the oxidation of quinoline, isoquinoline and naphthalene at the same reaction condition using CH,CN/H,O system, as shown in Table 2. In case of quinoline, only benzene ring was subjected to be cleaved, whereas for isoquinoline both rings could be cleaved by the same oxidant. Relative reactivity of each benzene ring included in quinoline, isoquinoline and naphthalene was estimated as 1.00 : 0 . 6 4 : 0.022/2=0.011, respectively.

134

.

..

..'

'

(b) .

?l?S

h

L

2

( 11) tetramethyl-2,3,5,6-pyridine te tracarboxylate

.

. ! I

-. c

Figure 3 NMR Chart of Methyl Esters of Two Kinds of Carboxylic Acid Formed in Ruthenium Catalyzed Oxidation of Acridine

135

0 pyridinetetracarboxyllc acid acrldlnic acid @

oxalic acid

Figure 4 Formation of Pyridinetetracarboxylic Acid in Ruthenium Catalyzed Oxidation of Acridine Reaction conditions: Acridine=15.5 mmol, [Sub/Ru]=200, NaClOaq 300 ml, CH,CN added = 501111,[OH-]o = l . 1 M, [ClO-/Sub],=30.0, 30°C

h

50 -

s.

0 wridinetetracarboxyllc acid 0 acridinic acid

Ooxalic acid

u

a .-a

--

d

>

0

'1

Reaction Time ( h )

2

Figure 5 Formation of Acridinic Acid in Ruthenium Catalyzed Oxidation of Acridine Reaction conditions: Acridine=15.5 mmol, [Sub/Ru]=200, NaClOaq 300 ml, CH,CN added = 50 ml, [OH-],=O.55 M, [ClO-/Sub],=30.0, 10°C

136

Table 2 Rate constants in ruthenium catalyzed oxidation of various two-ring aromatics Substrate

Product

Quinolinic acid Isoquinoline Cinchomeronic - acid Phthalic acid -Naphthalene Phthalic acid

CH,CN added (ml)

Quinoline

20 ~~

0

20 0

20 0

20

rate constant (h '1 2.30 1.83 1.50 0.27 0.39 0.094 0.053 ~~~

relative rate constant

~

1 0.78 0.64 0.11 0.17 0.040 0.022

Rate constant was measured by formation rate of each product by HPLC. Reaction conditions: Substrate=l.55 mmol, [Sub/Ru],=200, NaClOaq=300 ml, [OH-],=0.63 M, [ClO-/Sub],=30.0, 30°C 4.CONCLUSION (1) Nitrogen-containing heteroaromatic compounds are selectively oxidized to corresponding aromatic o-dicarboxylic acid remaining pyridinic structure in the product molecule by the ruthenium catalyzed oxidation. (2) The new solvent system composed of acetonitrile and alkaline aqueous solution is found to be very efficient, particularly f o r a substrate whose solubility is limited in alkaline aqueous solution. (3) Quinoline is oxidized to quinolinic acid up to 842 (isolation yield). (4) Isoquinoline is oxidized to cinchomeronic acid (44%) and phthalic acid (13%). (5) Acridine is oxidized to pyridine-2,3,5,6-tetracarboxylic acid up to 82%. Acridinic acid is the intermediate product to form pyridinetetracarboxylic acid. Maximum yield of acridinic acid is obtained as 23% in addition to the formation of pyridinetetracarboxylic acid (yield=55%) in the lower OH- concentration and temperature. (6) Relative reactivity of each benzene ring of quinoline, isoquinoline and naphthalene to be cleaved by perruthenate ion is estimated as 1.00 : 0.64 : 0.011, respectively. REFERENCES 1 P.A.O'Sullivan, C&EN, May 7 (1984) p19. 2 M.Komatsu, Petrotech, 1 1 (1988) 773. 3 R.A.Sheldon and J.K.Kochi, Metal-Catalyzed Oxidations of Organic Compounds, Academic Press, New York, 1981. 4 D.C.Ayres, J.Chem.Soc. Perkin trans. I (1975) 707. 5 T.Hara and T.Matsumura, The Role of Oxygen in Chemistry and Biochemistry, Elsevier, Amsterdam, 1988 p347.