Orthometalated ruthenium-complex-catalysed reductive carbonylation of nitroaromatics

OF

MOLECULAR CATALYSIS Journal of Molecular Catalysis 9 1 ( 1994) 19-30

Orthometalated ruthenium-complex-catalysed reductive carbonylation of nitroaromatics Deb K. Mukherjee, Biman K. Palit, Chitta R. Saha* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India

(Received August 9, 1993; accepted January 24, 1994)

Abstract Orthometalated ruthenium( II) complexes of the type [ RuL( CO),Cl] 2 (LH = 2_phenylpyridine, 1-phenyl pyrazole, azobenzene and benzo( h) quinoline) behaved as efficient catalysts for the reductive N-carbonylation of nitroaromatics in mild coordinating solvents containing alcohol and a basic cocatalyst under high pressure, high temperature conditions. The products were mainly the corresponding N-phenylcarbamates and aniline with a small amount of azobenzene detected in some isolated cases. The effects of various reaction parameters such as Pco, temperature, concentration of alcohol and cocatalysts on the nature and yields of products have been studied. A tentative reaction mechanism has been proposed on the basis of experimental findings. Keywords: Carbonylation; Orthometallated complexes;

Nitroaromatics;

Reduction;

Ruthenium

1. Introduction In view of the industrial importance of the products [ 1,2], there has been growing interest for the catalytic synthesis of phenylisocyanates and carbamates by reductive and oxidative carbonylation of nitroaromatics [ 3-61 and anilines [ 7,8] respectively. Special attention has been paid to the synthesis of N-phenylcarbamates from nitroaromatics due to their higher stabilities under the stringent reaction conditions. The importance of the products is indicated by numerous patents reported in the literature [9-l 31. Generally d,-metal complexes of Ru(0) [5,14-191, Fe(O) [20-221, Rh(1) [ 131, Pd(I1) [23-251 and Pt(I1) [26,27] with m-acid ligands are found to be active for the carbonylation reactions. The products are generally phenylisocyanates, diphenylurea, formanilides, anilines etc. and their distribution depends on the nature of the catalytic systems and the reaction conditions. Some *Corresponding

author; fax. ( + 9 1) 32222303.

0304-5102/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSD10304-5 102( 94)00024-P

20

D.K. Mukherjee / Journal of Molecular Caralysis 91 (1994) 19-30

palladium( II) complexes with N-donor ligands are efficient for the carbonylation of mononitroarenes [ 281 while the corresponding platinum( II) ones are effective for dinitroaromatics [ 291. Reaction mechanisms have been studied in a few cases and detailed mechanisms have been reported with the complexes of ruthenium(O), specially the carbonyl clusters [ 5,30-321. The orthometalated complexes of palladium(I1) are useful for the carbonylation of coordinated ligands [ 331. They are also important homogeneous catalysts for the hydrogenation of various organic functional groups including -NO2 [ 34-361. The investigation in our laboratory showed the complexes to be completely inactive towards the reductive carbonylation of nitroaromatics as they decompose to Pd” under the stringent experimental conditions. The corresponding ruthenium( II) complexes are more stable thermally and act as better catalysts for the hydrogenation of -NO* and other functional groups under relatively drastic reaction conditions [ 371. Hence they are thought to be active for the reactions: ArNO, + 3C0 --3 ArNCO + 2C02 ArNO* + 3C0 + ROH -+ ArNHCOOR

(1) + 2C0,

(2)

The present paper reports the results of such an investigation.

2. Experimental The solvents were dried and freshly distilled before use. Nitrobenzene was distilled at reduced pressure under dinitrogen atmosphere. Carbon monoxide (99.5%) purchased from IOL, Bombay, and RuCl, . 3H20 from Arora Matthey Ltd., India, were used as received. The complexes [ RuL( CO),Cl] 2 (LH = 2-phenylpyridine, Hphpy; 1-phenylpyrazole, Hphpz; azobenzene, Azb; and benzo(h)quinoline, Hbzqn) were prepared and purified according to literature methods [ 38,391. The GC analyses were carried out in Varian MPC3700 gas chromatograph equipped with 15% FFAP in 17% OV-210 S.S. columns supported on Chromosorb W. Programming was done from 100 to 240°C at the rate of lO”C/min. IR and UV-Vis spectra were recorded on Perkin-Elmer 883 and Shimadzu UV-Vis-NIR 3 100 spectrophotometers respectively. Molecular weights were measured in Dampfdruck vapour pressure osmometer. 2.1. High pressure catalytic run The high pressure autoclave was first evacuated and then flushed thrice with nitrogen. The DMF or DMSO solution (5 ml) of the catalyst containing substrate (0.5 ml), cosolvent (2.0 ml) and cocatalyst (30 mg) was introduced into the autoclave under dinitrogen. The reactor was then heated to the required temperature with a thermoregulated oil bath and magnetic stirring was applied. The reaction mixture was quickly subjected to the desired pressure of pure carbon monoxide which was kept constant during the run. At the end, the autoclave was rapidly cooled in an ice-salt bath and blown off. The products were analysed after subsequent work up by gas chromatograph using authentic samples for comparison.

21

D.K. Mukherjee / Journal of Molecular Catalysis 91 (1994) IS30 1 Analytical and IR” data of the complexes isolated from DMSO solution Compoundlmo1.wt.b

Anal. Found (calcd) (J) C

1

[Ru(phpy)(CO)*(CI)PhNO*l 458

2

[Ru(phpy)(CO)(PhNO>)(COOCHj)(DMSO)] 537

3

[Ru(phpy)(CO)(PhNO)(COOCH~)(DMSO)] 519

H

v(cm-‘) GO

N

XOOCH,

NOJNO

48.20

2.98

5.92

2040,

1540,

(48.51)

(2.76)

(5.96)

1980

I343

2020

48.26

3.77

5.10

(48.62)

(4.08)

(5.15)

50.18

4.25

5.41

(50.09)

(4.17)

(5.31)

1620

1543,

DMSO -

1062

1307 I949

1622

1336

1058

‘KBr pellets. “DMF solution.

2.2. Isolation of [Ru(phpy)(CO),Cl *PhNOJ Dry, deaerated DMSO ( 10.0 ml) containing [ Ru( phpy) (CO),Cl] 2 (0.38 g) was stirred for 2 h under N, at 70°C and filtered. The yellow filtrate was mixed with pure and dry PhNOp (0.5 ml), stirred for 30 min and then evaporated to 1.5 ml under reduced pressure at the same temperature. The concentrated solution produced a yellow precipitate of [ Ru( phpy) (CO)&1 . PhNO,] on keeping at 0°C for 24 h under closed conditions. This was washed with dry acetone and vacuum dried. No such adduct could be isolated in DMF medium (Table 1) . 2.3. Isolation of [Ru(phpy)(CO)(S)(COOCH,)(DMSO]

S = PHNOJPHNO:

Sodium methoxide (0.55 mmol) was added to the DMSO solution of [ Ru(phpy) (CO),Cl], (0.04 mmol) containing either PhN02 (4.8 mmol) or PhNO (4.4 mmol) and the mixture was stirred for 12 h under N,. The filtered solution after solvent evaporation under reduced pressure at 70°C was kept at 0°C for 48 h, when the yellow precipitate of the desired compound separated in each case. These were washed with cold DMSO ( - 10°C) and dry acetone successively, and dried under vacuum (Table 1) .

3. Results and discussion The present series of catalysts, [ RuL( CO),Cl] 2 (HL = 2-phenylpyridine, Hphpy; benzo(h)quinoline, Hbzqn; 1-phenylpyrazole, Hphpz; and azobenzene, Hazb) are highly active for the dihydrogen reduction of -NO*, -C=C-, >C=O, >C=C<, -C=N etc. in DMF and DMSO under normal or high pressure conditions [ 371, but found inefficient for the carbonylation of nitroaromatics to isocyanates or diphenylurea under various experimental conditions. In absence of any cocatalysts or cosolvents, the bulk of the nitrobenzene remained unchanged and GC analysis of the final product mixture indicated the presence of a small

22

D.K. Mukherjee /Journal

of Molecular Catalysis 91 (I 994) 19-30

amount of aniline (2-3%). This was probably formed due to presence of moisture in DMF or DMSO contaminated from atmosphere or during subsequent work up procedure. Carbonylation of nitrobenzene in completely dry DMF at 150-180°C in the P,, [ RuL( CO) zC1. DMF] could be isolated from the final mixture. The solvent adduct was also isolated from the DMF solution of the catalyst by evaporation of the solvent under reduced pressure [ 371. Literature survey indicates that in many cases the presence of cocatalyst was essential for the catalytic carbonylation of nitrobenzene to various carbonylated products [ . addition of alcohol the catalytic mixture was necessary for the of It however, observed with present series catalysts that the of some cosolvents as ROH = CH,, or HZ0 essential for the between PhNO* CO to proceed high pressure, temperature conditions, since there hardly any in absence The of sufficient amounts of in the medium resulted in complete conversion of nitroaromatics the corresponding amines, the addiof alcohol the dry medium produced aniline’( major), (minor) (O-10%). Highest of carbamate, 20%, was with optimum and alcohol (Table 2). carried out several reactions in presence of alcohol and various to out their effects the nature yield of The carried out both acidic basic cocatalysts in and absence alcohol in reaction medium. catalyst system with any the cocatalysts did favour the between PhNO* CO in alcohol-free medium. in presence of alcohol the addition of cocatalysts such as NEt,Cl, SnCl,, and p-toluenesulphonic acid increased only the yield of aniline at the cost of carbamate. The carbonylation of nitroaromatics in pure ROH (R = CH3, C,H,) medium in absence of any cocatalyst always led to the formation of corresponding aniline as the sole product. The required hydrogens surely come from alcohol. Acid cocatalysts were then replaced by the basic ones, MOH (M = K, Na), Et,N, py, RONa (R = Ph, CH,, C,H,), and the runs were conducted under comparable experimental conditions. The yield of carbamates increased in all cases except in the case of MOH where the reverse effect was observed. The reduction of PhNO, to PhNH, by CO with M,(CO) ,,/NaOH/ROH (M=Fe, Ru) system in presence of phase transfer catalyst is well known [

[ RuL( CO) *Cl] 2 exhibits several IR peaks in the vco region (2020, 1970, 1945 and 1930 cm-‘) due to the presence of several isomers in it [ 371. In DMF or DMSO solution, all change to the most stable and structurally similar cisdicarbonyl form as is evident from the presence of only two vco peaks in the region (2040 and 1978 cm-‘). The dissociation of the complex in DMF or DMSO according to Eq. 3 is supported by the isolation of [Ru(phpy)(CO),Cl.DMFl and [W&v)(CO) $1. PhN02] and the molecular weight measurement of the species in solution [ 371.

D. K. Mukherjee /Journal of Molecular Catalysis 91 (I 994) 19-30 Table 2 Carbonylation” Expt. No.

of nitrobenzene

in presence of different cocatalysts

Cocatalyst [O.l M]

Aniline

Carbamate

(2 ml)

(%)

(%)

CH,OH

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

C>H,OH

FeCl, NEt&l SnCl, p-toluene sulphonic acid KOH PY Et,N CH,ONa LiCl FeCl, p-toluene sulphonic acid KOH cu-picoline PY Et,NH Et,N NaOEt NaOMe NaOPh

and cosolvents

Cosolvent

Hz0 _

23

2.0 98.0 75.0 80.0 1.0 Trace 1.0 5.0

_ _

CHIOH CH,OH CH,OH

50.0 2.0 I.0 1.0 82.0 96.0 97.0

2.0 14.0 3.0 2.0

CH,OH CHsOH CHzOH CH,OH CH,OH CIHsOH CH,OH PhOH

90.0 70.0 65.0 53.0 45.0 20.0 18.0 55.0

10.0 28.0 30.0 46.0 55.0 77.0 80.0 20.0

“Cat.= [RuCl(CO),(phpy)]z=8.66X IO-’ M; [PhNO,] =0.9 M. Medium=DMSO; Pco=7.1 X lo-’ kN m-*; T= 150°C; r=6 h; total volume= 10 ml. The conversion (%) is approximately the sum of the columns headed aniline and carbamate. From serial No. 13 to 22, conversion is about 100% with small amounts of azobenzene and/or tarry material ( < 1%) in the products.

[RuL(CO),Cl]

2

s

PhN02 [RuL(CO)z .CI.S] wRuL(C0)2C1.PhNOz] (A) (B)

(3)

S = DMF/DMSO The present series of complexes have comparable structures in solution but differ appreciably in their catalytic activities (Table 3) in the following order: [ Ru( phpy) (CO),Cl] 2 (I) 2 [Wbzqn)(CO)2C112 (11) > [Ru(phpz)(CO)2C112 (111)> [Ru(azb)(COLCl12 (IV). Detail catalytic investigations were carried out with (I) due to its highest activity. The isolation of [ Ru( phpy ) (CO) &l( PhN02) ] reveals the initial formation of metal substrate adduct in each case. The catalytic activity of the present series of structurally similar complexes seems to depend mainly on the extent of rr electron delocalization in the orthometalated ring and the bulkiness of the ring substituents. The stability of the intermediate

D.K. Mukherjee / Journal of Molecular Catalysis 91 (1994) 19-30

24

__ 60

180 240 120 TIME IN MIN

300

360

Fig. 1. Carbonylation of PhNOz under optimum reaction conditions. Catalyst = [R&1( CO),phpy] 2 = 8.66 x IO-’ M; solvent=DMSO; CH,OH= 2.0 ml; CH,ONa=0.03 g; Pco=7.1 X 10’ kN m-‘; T= 150°C. (0) Aniline, ( W) nitrobenzene, (0) N-phenylmethylcarbamate.

Table 3 Catalytic efficiency” of the complexes Catalysts

1

[RuCI(CO)zphpyl, (I)

2

[RuCKCOMbzqn)12 (II)

3

[RuCI(CO)zphpzl, (III)

4

[RuC1(CO)zazb]* (TV)

in different solvents.

Nature of produc&

Products (%) in different solvent Methanol’

Ethanold

DMF

DMSW

o-Xylene’

(a) aniline (b) azobenzene (c) carbamate

80.0

40.0 60.0

20.0

15.0

75.0 5.0 10.0

30.0 5.0 10.0

(a) aniline (b) azobenzene (c) carbamate

80.0 10.0 10.0

75.0 10.0 5.0

40.0 60.0

(a) aniline (b)azobenzene (c) carbamate

40.0 10.0 10.0

40.0

50.0

5.0

(a) aniline (b) azobenzene (c) carbamate

10.0 10.0

10.0 5.0

5.0

‘[Cat] =8.66X lo-’ M; [C,H,NOJ =0.978 M; Pco=9.OX 10’ kN m-*; NaOMe = NaOEt = 0.1 I M. bathe conversion ( %) is the summation of (a + b + c) in each case. ‘NaOMe was added. dNaOEt was added. eCH,OH (2.0 ml) was added.

80.0 30.0 70.0

40.0 5.0 5.0 20.0

20.0

50.0 _ 25.0

15.0 5.0 10.0

15.0 _ 10.0

10.0 -

T= 150°C; V,=5.0

_

ml; t=6

h;

D.K. Mukherjee / Journal of Molecular Catalysis 91 (1994) 19-30

25

[ RuL( CO) ,Cl . RNO,] is the resultant of the same two factors with the former increasing and the latter decreasing the stability. Possibly the activity order of the complexes and the stability order of the corresponding intermediates run parallel. The structurally similar N,N-donor complexes, RuL’( CO) ,Cl, (L’ = dipy, o-phen) were found to be inactive in the present investigation. It appears that strong interaction of the ligand r-orbital with Ru-drr orbitals in these orthometalated complexes results in greater r-electron delocalization in the chelate ring and makes these complexes highly efficient for the present carbonylation reactions. Mild coordinating solvents are found to be more efficient and most of the catalytic runs were taken in DMSO, though DMF is comparably effective. The former solutions are insensitive to moist air while the latter suffer the disadvantage of turning green and becoming inactive on aerial contact [ 371. Very slow reaction occurs in strong coordinating solvents like CH,CN or non-polar solvents like C6H6, C,H,CH3, p-(CH3)&H4 etc. In alcohol, the carbonylation product was solely the corresponding aniline. The non-polar solvents probably cannot break the dinuclear complex to the corresponding monomer to any appreciable extent, while in strong coordinating medium, the corresponding solvent adducts do not allow the formation of a metal-substrate complex. The reaction rates, therefore, are expected to be very slow in these media. The coordinating solvents, sufficiently strong to form the solvent adduct by chlorobridge cleavage and sufficiently labile to favour metal-substrate complex formation, appear to be most suitable for the present investigation (Table 3). The effective cocatalysts are all basic in nature and vary greatly in their efficiencies. The best one in RONa (R=CH3, C,H,) followed by E&N, Et,NH, pyridine and a-picoline. The activity order is consistent with their corresponding basicities. The alkoxides are generally more effective than phenoxide. Carbonylation reaction of substituted aromatic nitrocompounds was investigated under optimum experimental conditions using (I) as catalyst. For the nitrobenzene derivatives, moderate yields (3040%) of the corresponding carbamate is obtained while the dinitroarenes produce still lower percentages of carbamates ( < 20%)) the major product being the corresponding anilines (Table 4). Several runs were taken in completely dry, alcohol-free medium with or without any cocatalyst to see whether phenylisocyanate, if any, was formed at the intermediate stage. Phenylisocyanate was never detected at any intermediate stage of any of the runs. When phenylisocyanate was used with or without nitrobenzene, the final product mixture contained mainly the unconverted phenylisocyanate and some high boiling unidentified liquid. Such high boiling liquids could not be detected at any stage of the catalytic runs. Hence nitrobenzene does not produce phenylisocyanate at any stage of the present catalytic conditions. Monitoring the IR and NMR spectra of the solutions during catalytic run did not show the formation of any intermediate metal-hydride species. To be sure, carbonylations of nitrobenzene were conducted in DMSO containing either 1-hexene or phenylacetylene under optimum reaction conditions. The alkene or the alkyne remained unchanged while the conversion of nitrobenzene to carbamate occurred to the extent of about 80%. Dihydrogen reduction of the substrate mixture under comparable conditions, however, led to the complete reduction of nitrobenzene, 1-hexene and phenylacetylene to aniline, hexane and phenylethane respectively. The formation of any metal hydride species under carbonylation conditions should reduce alkene or alkyne to the corresponding saturated products. Several runs were taken in DMSO using PhNO and PhNH2 as substrates and

26

D.K. Mukherjee / Journal of Molecular Catalysis 91 (1994) 19-30

Table 4 Reductive N-carbonylation”

of various nitroarenes

Substrate

Product

Yield (%)

2 3 4

p-CIC,H,NO, P-CH,C,H,NQ o-C1C,H,N02 p-MeOC,H,NO,

5

],3-C,H,(NO,)z

6

2.4.CH,C,H3(

1

NWz

Methyl 4-chlorophenyl carbamate Methyl 4-methyl phenyl carbamate Methyl 2-chloro phenyl carbamate Methyl 4-methoxy phenyl carbamate Dimethyl 1,3-phenylene bis carbamate Dimethyl 4-methyl- 1,3-phenylene biscarbamate

42.0 40.0 32.0 48.0 24.0 20.0

“[nitroarene] =0.6 M; [RuC1(CO),phpylz=8.66X 10-j M; medium=DMSO; Pco=9.1 X lo3 kN m-‘; T= 150°C; CH,OH = 2 ml; CH,ONa= 0.03 g. bIdentified as per the literature [ 26,281 and all data determined by gas chromatography based on nitroarenes charged.

[ Ru( phpy) (CO) $11 2 as catalyst under optimum conditions of carbamate synthesis. The presence of carbamate could not be detected at any stage of the catalytic run using PhNH, as substrate indicating the inactivity of the present complexes towards aniline carbonylation. However, the nature and yields of the final products using PhNO as substrate are almost comparable to that of PhNO*. The higher initial rate which is most probably due to higher concentration of PhNO at the start slowed down and became comparable to that of PhNO, after some time. This suggests the preliminary reduction of PhNO,, by CO to PhNO at some faster initial steps. Addition of PhNO to the yellow DMSO solution of the catalyst at 25°C immediately changed the colour to green and then to yellowish brown. The electronic spectrum of the yellowish brown solution is different from that of the substrate or catalyst and exhibits an absorption peak at 510 nm which is also observed in the intermediate catalytic solution using PhNO, as substrate. Infrared spectroscopy of [ Ru(phpy) (CO) (WNO) (COOCH,)DMSO] and [Ru(phpy) (CO) ( PhNOI) (COOCH,)DMSO] showed among other bands, two strong absorption bands of almost equal intensity at 1949 and 1622 cm- ’ (for PhNO) and 2020 and 1620 cm- ’ (for PhNO,) respectively (Table 1). The former and latter bands in each case are due to terminal CO and metal bonded-COOCH, group respectively [ 321. Weak absorption bands at 1058 cm- ’ (for PhNO) and 1062 cm- ’ (for PhNO,) were accounted for coordinated DMSO in both cases. PhNO may be attached to the metal either by a-bonding or by $-type of bonding [ 321, but v,, peak at 1336 cm- ’ suggest o-type nitrogen coordination [ 401. [ Ru( phpy) (CO),( Cl) (PhNO;?) ] could only be isolated from DMSO solution. Evaporation of the DMF solution of the complex resulted in the isolation of the solvent adduct [ Ru( phpy ) (CO) $1. DMF] . The catalytic activities of [ Ru( phpy) (Cl) (CO) ,DMF] and [Ru(phpy) (CO),( Cl)] 2 are almost identical. The action of CO on [ Ru( phpy ) (CO) *Cl. PhNO*] under catalytic conditions converted the coordinated PhNOp to PhNH* only. The absence of carbamate in the product mixture is probably due to moisture contaminated with the substrate adduct during its isolation. However, the final product

D.K. Mukherjee /Journal of Molecular Catalysis 91 (1994) 19-30

21

mixture obtained after subjecting [ Ru( phpy ) (CO) (COOMe)S( DMSO) ] (S = PhNO,/ PhNO) to catalytic conditions (without addition of substrate) contained carbamate and aniline in the liquid and CO* in the gaseous phase. To examine the activity of the final solution, a further amount of nitrobenzene or nitrosobenzene was added to it and the catalytic run was repeated. In both cases, complete conversion of the added substrate to the products in the expected proportions was observed. The solid residue left after vacuum evaporation of the final solution mixture exhibits an IR peak at 720 cm - ’ indicating the presence of orthometalated ruthenium( II) in it. It possesses catalytic activities comparable to that of the original. TLC analysis indicates it to be a mixture of two components but we failed to isolate any pure compound out of it. With (I) as catalyst, the activity and the selectivity in carbamate formation increased with increasing PC, pressure from 2 X lo3 kN me2 to 12.2 X lo3 kN m-* at 150°C. The rate of carbamate formation is immeasurably slow below 2.0 X lo3 kN rn-’ and increased steadily up to 9.1 X lo3 kN rne2. The maximum selectivity to carbamate formation is observed at 7.07 X lo3 kN me2 with longer reaction period (Fig. 2). At lower temperatures (below 60°C)) there was hardly any reaction, while above 180°C substantial amounts of tarry products were formed. The temperature for the best selectivity of carbamate formation was 150°C. Significant conversion of nitrobenzene to carbamate was obtained in presence of sodium alkoxide or phenoxide. Sodium methoxide was the best cocatalyst in the series. The studies of the dependence of the ratio ( [ NaOMe ] /I) on the selectivity of carbamate formation show that the best ratio lies in the range 8 to 12. Methanol-sodium methoxide was the best combination followed by ethanol-sodium ethoxide pair. Phenol-sodium phenoxide was the worst producing only 20% of PhNHCOOPh. Mixed pairs, such as ROH-NaOR’, produce mixed carbamates and the major product contains R group. Under optimum reaction con-

P,X10e3

KNniz

Fig. 2. Effect of PC0 on carbonylation under optimum conditions. [RuC1(CO)z(phpy)], =8.66x IO-’ M: CH,OH = 2.0 ml; CH,ONa = 0.03 g; T= 150°C; total time = 6 h. (0) PhNO?. ( 9 ) PhNHCOOCH, ( 0) aniline

28

D. K. Mukherjee /Journal of Molecular Catalysis 9/ (I 994) 19-30

I

I

4

I

I

I

1

2

3

4

5

ALCOHOL

CONTENT

(ml)

Fig. 3. Effect of alcohol on distribution of components. [Cat] = 8.66 X 10m3 M; Solvent = DMSO; [PhNO,] M; T= 150°C; CH,ONa=0.03 g; Pc,= 7 1 X IO3kN m-*. (0) PhNHg (0) PhNHCOOCH,.

ditions, carbamate formation increased with increasing alcohol concentration ning and then decreased significantly with excess alcohol (Fig. 3).

= 0.9

at the begin-

3.1. Results of kinetic studies

The kinetic studies for the carbonylation of nitrobenzene with the catalyst [ Ru( phpy) (CO),Cl] 2 were carried out in glass lined stainless steel autoclaves connected in series. The runs were taken at 150°C in dry DMSO medium. All reaction parameters except that varied were kept constant during any set of catalytic runs. The initial rate in each case was determined from graphical extrapolation of the rate curve to zero time. The initial rate was found to be first order dependent on catalyst concentration in the range (2.00-9.00) X lop3 mol 1-l and second order dependent on carbon monoxide pressure in the range of (4.00-10.1) X lo3 kN rne2. The rate was, however, found to be independent of substrate concentration in the range (0. l-l .O) mol l- ‘. The experimental results show that the yield of carbamate is enhanced by basic cocatalysts while the acidic ones have the reverse effect. Carbamate formation probably requires the presence of OR- ion which is provided to different extent by different basic cocatalysts according to the following reaction ROH+B+RO-

+BH+

The acidic cocatalysts, having the opposite effect, decrease the yield of carbamate relative to neutral condition. Addition of chloride ion, such as LiCl, decreases the yield of carbamate (Table 2) as it retards the formation of carbamoyl complex. Based on these facts, the following tentative mechanism has been proposed for the reaction (Scheme 1) .

D. K. Mukherjee /Journal of Molecular Calalysis 91 (I 994) 19-30

29

(A)

+R

C\? R/l

N’I

N/ 1 ‘COOR

RU

‘CO

N/Ph

co (B)

ROH co (Cl

Scheme 1.

References [ 11 G.S. Hartley and T.F. West, Chemicals for Pest Control, Pergamon, New York, 1969, p. 174-183. [21 G.W. Parshall, Homogeneous Catalysis, Wiley, New York, 1980, p. 93, and references therein. [ 31 S. Cenini, M. Pizzotti and C. Crotti, in R. Ugo (Ed.), Aspects of Homogeneous Catalysis, Vol. 6, Reidel, Dordrecht, 1988, p. 97-198. [ 41 S. Cenini, M. Pizzotti, C. Crotti, F. Porta and G. la. Monica, J. Chem. Sot. Chem. Commun., ( 1984) 1286. [5] S. Cenini, C. Crotti, M. Pizzotti and F. Porta, J. Org. Chem., 53 ( 1988) 1243. [ 61 S.B. Halligudi, K.N. Bhatt, N.H. Khan and M.M. Taqui Khan, J. Mol. Catal., 72 ( 1992) 139. [7] S. Fukuoka, M. Chono and M. Kohno, J. Org. Chem., 49 ( 1984) 1458. [8] H. Alper, G. Vasapollo, F.W. Hartstock and M. Mlekuz. Organometallics, 6 ( 1987) 2391. [9] US Pat. 4629804 ( 1986) to J.H. Grate, D.R. Hamm and D.H. Valentine. [ 101 US Pat. 4603216 ( 1986) to J.H. Grate, D.R. Hamm and D.H. Valentine. [ 111 US Pat. 4705883 ( 1987) to J.H. Grate, D.R. Hamm and D.H. Valentine. [ 121 Eur. Pat. 0086281 Al (1983) to E. Drent and P. Vanleeuwen. [ 131 DDR Pat. 121509 (1977) to K. Schwetlick, K. Unverferth, R. Hoentsch and M. Pfeifer.

30

D. K. Mukherjee /Journal

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