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Orthometalated palladium(II) complex-catalysed reduction of nitroalkanes and nitriles
Journal of Molecular Catalysis, 49 (1989)
271 - 283
271
ORTHOMETALATED PALLADIUM(I1) COMPLEX-CATALYSED REDUCTION OF NITROALKANES AND NITRILES ASISH BOSE and C. R. SAHA* Department
of Chemistry, Indian Institute of Technology,
(Received February 11,1988;
Kharagpur-721302
(India)
accepted April 28, 1988)
Dihydrogen reduction of nitroalkanes and aromatic nitriles to the corresponding amines was efficiently achieved at (4.0 - 12.0) X lo3 kN rnp2 in the temperature range 65 - 80 “c using a series of di- and mononuclear orthopalladated complexes of the type trans-Pd, L2X2 and trans-PdLL’Cl [L = 2-( 2-pyridylcarbonyl)phenyl-C’ ,N( pcph), 2-( 2-picolyl)phenyl-C1 ,N(piph); (N,Ndimethylbenzylamine)-C2 ,N(dmba); L’ = pyridine, triphenylphosphine; X = OAc, Cl] as catalysts in DMF medium. The existence of metal-arene u-bonds in the complexes was found essential for their catalytic activities. Six-membered orthopalladated complexes were generally superior catalysts to the five-membered ones, and the binuclear benzoylpyridine complex was the best in the series. The complexes were the catalyst precursors, and the actual catalytic species was always found to be the corresponding mononuclear palladium hydride-DMF adduct. Kinetic studies indicate first-order rate dependence on catalyst concentration and hydrogen pressure, and zero-order on substrate concentration. A tentative mechanism for this reduction process has been suggested on the basis of kinetic data and other experimental observations.
Introduction Very few reports have thus far been published on the academically and industrially important problem of reduction of nitroalkanes to the corresponding amines. Knifton has reported the homogeneous catalytic reduction of nitroalkane to the corresponding amine in presence of ruthenium(I1) complexes [l] and to the corresponding oximes by carbon monoxide in the presence of copper(I1) salts solubilized in alkyl polyamines [2]. Aliphatic amines have been catalytically produced by ammonolysis of alcohols [ 31, dihydrogen reduction of nitriles [4] and by reduction of nitroalkanes by hydrogen transfer agents [5]. The nature and yield of products depend on *Author to whom correspondence 0304-5102/89/$3.50
should be addressed. 0 Elsevier Sequoia/Printed in The Netherlands
272
both the type of catalyst used and the experimental conditions. The reduction mechanism in many cases could not be ascertained due to irreproducible data, catalyst decomposition and stringent reaction conditions [ 3,6]. Orthopalladated complexes with azobenzene, in spite of having excellent catalytic activities towards the reduction of nitroaromatics, alkenes and alkynes, were completely inert towards the reduction of nitroalkanes and underwent decomposition at higher temperatures [ ‘71. Similar complexes with greater thermal stabilities are expected to be catalytically active towards the reduction of nitroalkanes; the present paper describes the superior catalytic activities of some five- and six-membered orthopalladated complexes in reducing the nitroalkanes and nitriles almost completely to the corrresponding amines.
Experimental Analytical grade reagents, distilled solvents and pure dry hydrogen gas were used throughout the investigation. Dimethylformamide (DMF) was purified by storing over CaH, under nitrogen for 24 h, followed by distillation under reduced pressure. The complexes used in the present investigation, i.e., Pd2L2X2, Pd(LH)2C12 and PdLL’Cl (where L = pcph, piph, dmba; L’ = pyridine (py), triphenylphosphine (PPhs); X = OAc, Cl), were prepared according to literature methods [8] and purified by recrystallization wherever possible. Their purities were checked by physicochemical means before their use as catalysts. The metals were estimated gravimetrically, and C and N contents were estimated by semi-microanalytical methods. Vibrational, electronic and PMR spectra were taken on Perkin-Elmer 237 B, Cary-17 D and Varian 390, 90 MHz instruments respectively. Gas chromatographic analysis was carried out on a Varian-3700 instrument using SE-30, Carbowax 20M and 15% FFAP column. Molecular weights were determined in a Knauer Dampfdruck osmometer. The kinetic studies were carried out in a series of stainless steel autoclaves fitted with glass liners containing 10 ml aliquots of a solution of catalyst and substrate. The reactions were quenched at different times and immediately analysed . Isolation of PdL(H)DMF
(L = pcph, piph)
A solution of the parent dinuclear complex PdzLzXz (0.3 mmol) was prepared in 100 ml of dry deaerated DMF under nitrogen in a 250 ml glass reactor. Nitrogen was pumped out and the reactor was filled with hydrogen. The solution was stirred continuously under 1 atm hydrogen. The yellow solution changed color gradually to deep brown, and stirring was continued until there was no further deepening of colour. After -1 h, the hydrogen supply was cut off and the solution was concentrated to about 20 ml under vacuum. Addition of a large amount of dry ether at this stage precipitated a
273 deep brown compound, which was filtered and washed with dry ether under hydrogen and dried in vacuum. Further concentration of the solution to -5 ml initiated decomposition of the complex to Pd’. Reduction procedure In a typical experiment, the DMF solution (10 ml) containing the catalyst (5.5 X 10e2 mmol) and the substrate (2.5 mmol) was introduced into a glass-lined stainless steel autoclave (100 ml) equipped with a magnetic stirrer and immersed in a thermostat. The reactor was first evacuated, flushed thrice with pure dry hydrogen, and then allowed to attain the temperature of the thermostatted silicone oil bath. It was then subjected to the desired pressure of hydrogen, which was maintained constant throughout the reduction period. No reduction occurred under 1 atm hydrogen at the experimental temperature, and the extent of reduction occurring during the attainment of the desired pressure was found to be negligible. Reaction time was measured from the moment the reactor attained the desired temperature and pressure. The reaction was quenched after the desired time, and the gaseous amine was passed through standard H2S04 solution for estimation. The components present in DMF solution were normally estimated by GLC. The liquid and solid components were separated by fractional distillation and column chromatography respectively. The products were identified by b.p., m.p., IR and PMR spectra wherever possible. The catalyst solution did not show any sign of decomposition at higher pressure (up to 27.5 X lo3 kN mW2examined) but started slow decomposition if the solution temperature rose above 120 “C.
Results and discussion A series of orthometalated and non-orthometalated complexes were used as catalysts in the present investigation. All the former complexes were active for these reductions, while the latters were found to be inactive in a variety of solvents under different experimental conditions (pressures up to 15 X lo3 kN mW2and temperature up to 120 “C). The yellow DMF solutions of non-orthometalated complexes underwent slow decomposition to Pd” on long standing under hydrogen without turning brown at any stage, which is in contrast to the behavior of the binuclear solutions. The high catalytic activities of the orthopalladated complexes are probably due to greater r-acidic character of the corresponding ligands. The ligands are normally r-acidic (except NJVdimethylbenzylamine) due to the presence of the pyridine ring, and their a-acidity is greatly enhanced in orthometalated complexes due to the formation of the metal-arene u-bond. The ligand N,Ndimethylbenzylamine is n-acidic only in its orthometalated complexes. The catalytic activities of the complexes in both the mono- and binuclear series, PdLL’Cl and Pd2L2(OAc), (L = pcph, piph, dmba; L’ = pyridine, triphenylphosphine) respectively run almost parallel to the s-acidic
274
character of the corresponding ligands. The highest activity of Pdz(pcph)2(OAc), may be due to the highest n-acidic character of the ligand, resulting from the a-electron delocalization in the chelate ring. The molecular weights of the dinuclear species Pd2LzX2 determined in DMF/DMSO medium indicated their extensive dissociation to the corresponding monomeric species. However, such dissociation did not occur in noncoordinating solvents such as benzene or chloroform, as was evidenced from their molecular weight determinations (Table 1). The electronic spectra of PdzL2Xz in the solid state and in benzene medium are almost identical, and differ from those in DMF solution (Table 1). The above experimental results and the bridge splitting behaviour of these dinuclear complexes by pyridine, triphenylphosphine and acetylacetone [8] suggest the following type of equilibria in the present reaction medium: Pdz Lz (OAc],
DMF ti
2PdL(OAc)DMF
(1)
The dissociation appears to be nearly complete in the dilute solution (-lOA M) used in the present investigation. The isolation of the DMF adduct was not possible, as the solution always produced crystals of the corresponding binuclear parent complex on solvent evaporation. The visible electronic spectra of highly concentrated DMF solutions also indicated the presence of some monomeric species. The dissociation of L’ from PdLL’Cl (L = pcph, piph; L’ = pyridine, triphenylphosphine) in DMF solution in the absence of hydrogen was not indicated, either from their molecular weight measurements or from their spectral data (Table 1). TABLE 1 Molecular weight and electronic spectral data of the complexes Complexes
Molecular wt. in benzene exp. (theor.)
Molecular wt. in DMF exp. (theor.)
Ain benzene (nm)
Ain DMF (nm)
hmC in activated solution (nm)
[WPcPh)(OAc)lz
700.5 (716.6)
492.6a (420.4)b
210,335
265,320
265,430, 490
W(PiPh)(OAc)lz
659.2 (666.8)
489.5 (406.4)b
265,345 420
265,310
270,400, 495
[Pd(dmba)C1]2
550.6 (551.8)
420.6 (348.9)b
275,330, 350
270,300 320,345
270,400, 485
538.2 (543.8)
542 (543.8)
265,405
260,405
582.5 (585.9)
592 (585.9)
290,335, 470,540
290,335, 465,530
tPd(Dpcph)2Clz
1
[Pd(pcph)(PPh3)C1]
aO.OO1 M DMF solution was used. bAssuming complete dissociation. Y%e solution was activated by stirring under hydrogen for 30 min.
295,335, 435,490
275
The yellow solution of the complex, PdzLzXz and PdLL’Cl in DMF and DMSO, changed to deep brown on stirring under normal pressure of hydrogen at ambient temperature within a few minutes. The chloro-bridged species required shorter periods than the acetate-bridged ones for this conversion, while the orthopalladated mononuclear complexes required the longest time. The brown solutions thus obtained were active towards these reduction processes under high pressure conditions, and their efficiencies were equal to those of the original compounds. The yellow solutions of the complexes turn brown under high pressure hydrogen treatment, and the brown solutions, obtained by both high or normal pressure hydrogen treatment, have identical electronic spectra provided they are derived from the same parent compound. The visible electronic spectra of all the brown solutions obtained from different sources are comparable (Table l), indicating the formation of similar brown catalytic species in all cases. The catalyst solution left at the end of high pressure reduction did not show the presence of any suspended particles when examined by strong light or filtered through kieselguhr under nitrogen atmosphere. The solution could be reused for at least four cycles without any loss of catalytic activity. Solutions of these compounds in non-coordinating solvents such as CHzClz, CHCls, CCL, C6H6, C6H,CH3 etc. remained inert toward both hydrogen activation and the catalytic reduction process. Yellow solutions of the active complexes in alcohol (methanol, ethanol, n-propanol) turned deep brown very quickly in the presence of hydrogen, with subsequent slow decomposition to Pd” even at 0 “C. The pH and conductance of the initial solution decreased and increased respectively during hydrogenation. In the case of PdzLzClz and PdLL’Cl, the liberation of free HCI during this process was confirmed by the evolution of CO* from NaHCOa. In the case of acetate-bridged complexes, the decrease in initial rate on addition of CHaCOOH, HCOOH and HCl suggests the liberation of free acid during hydrogen activation. Isolation of the active species from the deep brown solution was very difficult. The solution is very sensitive to air and moisture, and underwent slow decomposition at high concentration under vacuum. The cause of this decomposition may be due to the inability of the hydride complex (eqns. 2, 3) to form the corresponding dimer with a hydride bridge. Under similar conditions, the species (C) (eqns. 2, 3) produced the corresponding dimer with a chloride (or acetate) bridge without difficulty. DMF Pdz L2X2
e
2PdLXDMF 2
PdL(H)DMF
(2)
((7 Hz, -HCl
PdL, L'Cl -\-
PdLIL’(H) DME:PdLl(H)DMF + L’
L = pcph, piph, dmba; L1 = pcph, piph; L’ = py, PPh3 ; X = OAc, Cl
(3)
276
However, the brown solutions obtained from either the mono- or binuclear complexes showed PMR signals at -20 - 22 r. Addition of large amounts of dry ether to the deep brown solutions originating from [PdL(OAc)], (L = pcph, piph) produced a deep brown precipitate of PdL(H)DMF. These compounds were stable under nitrogen or hydrogen atmosphere and decomposed to Pd” in the presence of air, very slowly in the solid state and quickly in solution. Comparison of their IR spectra with the corresponding parent dinuclear complexes indicated the absence of peaks due to acetate group and coordinated DMF in both of them (Table 2). A new peak at -1960 cm-’ (v(Pd-H) [9] in their spectra and the PMR signal at -20 - 22 T of their solutions in DMSOd6 support the presence of Pd-H in them. Their visible electronic spectra in DMF are almost identical to those of the corresponding brown solution formed during hydrogen activation. The brown compounds are catalytically active in DMF medium, but their catalytic activities could be neither determined quantitatively nor compared with those of their parent compounds, as some decomposition always occurred during their dissolution in DMF. However, attempts to isolate such brown compounds from [Pd(dmba)C112 or PdLPPhsCl (L = pcph, piph) did not meet with success. The brown compound obtained from the activated solution of PdLPPhsCl on addition of dry ether gave inconsistent analytical data. The physicochemical properties of the compound indicated the presence of PPhs and a Pd-H bond. The presence of PPhs was detected in the clear filtrate also. The presence of an equilibrium in eqn. (2) was suggested from the fact that on addition of a very small amount of NH4C1 to the brown solution derived from [Pd(dmba)Cl],, it reverts to the yellow solution from which the original compound could be recovered.
TABLE
2
Analytical and IR data of the complexes Compound
v(Pd-H) (cm-‘)
1670
1580,
-
-
-
1425 1585,
-
-
-
1650
1965
-
-
1655
1965
-
-
1714s
-
N
Pd
v(C=O) (Benzoyl pyridine) (cm-‘)
47.21 (47.36)
3.70 (3.91)
29.40
50.15 (50.38)
$619)
(31.91)
[Pd(pcph)HDMF]
49.42 (49.66)
7.50 (7.72)
29.29 (29.36)
1665
[Pd(piph)HDMF]
51.30 (51.66) -
8.1 (8.03)
30.4 (30.54) -
[Pd(pcph)(OAc)]s [Pd(piph)(OAc)Js
DMF aVoir [ll].
Found (Caicd) (I)
Bridging V(C=O)(DMF) acetate (cm-i) (cm-’ )
C
(29.69) 31.75
1420
277 TABLE 3 Time period of nitroethane reduction using different metal complexesa Catalyst
Reduction time (h)
Yield of main product (W)
(1) W(pcphh(OAc)2
5.15 5.40 5.90 6.10 5.30 5.70
ethylamine ethylamine ethylamine ethylamine ethylamine ethylamine
(2) (3) (4) (5) (6)
Pdz(piphh(CAc)2 W(dmbah(OAch Pdz(dmba)KG Pd(pcph)pya Pd(piph)pyCl
(92) (90) (85) (86) (90) (90)
aMedium = DMF, substrate = nitroethane, [catalyst ] = 5.5 X 10e4 mol 1-l) [substrate] = 0.25 mol l-r, pH, = 11.5 X lo3 kN mm2, temp. = 75 “C, total volume = 10 ml.
The activities of various catalysts towards the reduction of nitroalkanes are presented in Table 3. Among these orthopalladated complexes, the binuclear ones are generally more active than the mononuclear ones, and [Pd(pcph)(OAc)], is the most efficient catalyst in the series. Detailed investigations were made using [Pd(pcph)(OAc)12 due to its highest catalytic activity. The corresponding chloro-bridged complex could not be used, due to its insolubility in DMF. The nitroalkanes used in the present investigation are almost completely reduced to the corresponding amines at the final stage (Table 4). Lower nitroalkanes are generally reduced at faster rates than the higher ones. The reduction rates decreased with increasing branching at the o-carbon atom. Greater steric hindrance experienced TABLE 4 Reduction time and yields of products* Substrate
Time period (h)
Products (%)
(1) (2) (3) (4) (5 ) (6) (7 ) (8 ) (9)
5.0 5.15 5.15 6.0 5.7 5.4 8.0 6.0 7.5
methylamine (93) ethylamine (92) 1-aminopropane (92) 2-aminopropane (90) 1-aminoheptane (88) 2-phenylethylamine (90) aminocyclohexane (7 3 ) diphenylmethanol(95) dibenzylamine (90) and benzylamine (10)
nitromethane nitroethane 1-nitropropane 2-nitropropane 1-nitroheptane 2-phenylnitroethane nitrocyclohexane benzophenone benzonitrile
*Medium = DMF, catalyst = Pd2(pcph),(OAch, [Pdz(pcph)2(OAc)p] = 5.5 x 10e4 mol l-l, [substrate] (except benzophenone and benzonitrile) = 0.25 mol l-l, [benzophenone] = 0.2 mol l-r, [benzonitrile] = 0.32 mol l-l, pi, = 11.5 x lo3 kN m-‘, temp. = 75 “C, total volume = 10 ml.
278
1
3
2 Reaction
4 time
5
6
7
(hr)
Fig. 1. Reduction of 1-nitropropane with trens-Pdz(pcph)2(OAch in DMF at 75 “C under 11.5 x lo3 kN rn-’ Hz, in DMF medium (---) and in the presence of acetic acid ); [Pd2(pcph)2(OAc)2] = 5.5 x 10e4 mall-‘; [1-nitropropane] = 0.25 mol (1O-2 M) (1-l ; 0 = 1-nitropropane; m = 1-aminopropane; A = IV-propylhydroxylamine.
by highly branched or higher nitroalkanes during their coordination to the metal atom centre may be the cause of their lower rates of reduction. During the reduction of nitroalkanes, corresponding hydroxylamines were always detected at various intermediate stages. The concentration of the hydroxylamines increased with the progress of reduction, and a small amount remained in the system even after the complete disappearance of nitroalkane (Fig. 1). The reduction rate of alkylhydroxylamine was comparable to that of the corresponding nitroalkane under similar experimental conditions. The reduction period of nitroalkanes increased with increasing strength and concentration of the acid added (CH,COOH, HCOOH, HCI). This may be due to a lower concentration of the active species, as per Scheme 2. The concentration of hydroxylamine in acid medium decreased at various intermediate stages (Fig. l), indicating the formation of a metal-oxygen bond during the reduction. If the reduction of hydroxylamine required metalnitrogen bond formation, the proportion of hydroxylamine in acidic medium would increase at various intermediate stages. In the presence of base (NaOH, 10e3 M), the rate of nitroalkane reduction increased with an increasing proportion of corresponding hydroxylamine at intermediate stages (Fig. 2). This is possible provided the base increases the reduction rate of nitroalkane without affecting the rate for hydroxylamine. The base may increase the concentration of active species as per Scheme 2, or it may produce RCHNOz from RCH,NOz. Such formation of RCHN02 has been reported to be essential for the catalytic reduction of the nitroalkane [ 11. In the present case,
279
25
1
2 Reaction
L
3 tim
5
(hr)
Fig. 2. Reduction of 1-nitropropane in ;he presen’ce of base (NaOH) with tians-Pdz~~JIJI);~~_~. at 75 “C tinder 11.5 x 10 kN me2 of Hz; [Pdz(pcph)2(OAc)2] = 5.5 x , [1-nitropropane] = 0.25 mol 1-l; [NaOH] = 0.005 mol 1-l; @ = l-nitropropane; IJ = l-aminopropane; * = N-propylhydroxylamine.
RCH,NO, appears to be reduced as such without the intermediate formation of the corresponding nitroalkyl anion, at least in the absence of base. Though alkylhydroxylamine could be reduced under conditions comparable to those for nitroalkane, all attempts to reduce alkyloximes were unsuccessful under different experimental conditions. The rates of dihydrogen reduction of alkylhydroxylamine to the corresponding amine using these catalysts do not undergo any appreciable change in presence of acid or base (10m3 M). This supports the interaction of a metal atom centre with the oxygen atom of hydroxylamine during its reduction. Such a metal-hydroxylamine intermediate could not, however, be isolated. The complexes are active towards the reduction of benzonitrile and benzophenone, but inert towards acetonitrile. High pressure and high ternperature conditions are necessary for the reduction of both benzophenone and benzonitrile, the formel’ producing only the corresponding alcohol while the latter produced both benzylamine (10%) and dibenzylamine (90%) (Table 4). Benzonitrile reduction was accompanied by liberation of ammonia, and the initial addition of benzylamine increased the proportion of dibenzylamine in the final product mixture. The reduction probably occurs as per Scheme 1 [ 41. The reduction rates of all these substrates increased in the presence of small concentrations of acetonitrile in the reaction medium (up to 0.3 M), but decreased if the concentration of acetonitrile was increased further. A very long reduction time is required in pure acetonitrile. Both the mild proton acceptor property of DMF and its ability to coordinate to the metal atom centre in a z-type manner [lo] favours the stabilization of (C) (eqns. 2,3).
280
PhCN --%
PhCN
PhCHzNHz -
PhCH+H PhC=NH 1
PhCH2TH
CH2Ph -
H2
PhCH2S:
-NH3
CHPh-
H2
PhCH2rH
PhCHNH2
Scheme 1.
However, its stronger n-acceptor capacity may disfavour metal-nitroalkane x-interaction. Acetonitrile may form species similar to (C) and, being a weak x-acceptor ligand, may favour metal-nitroalkane n-interaction under comparable conditions. This increases the concentration of [ metal-nitroalkane] intermediate, with a consequent increase in reduction rate. This is only possible when acetonitrile is present in very small concentrations. Increased acetonitrile concentration may lower the concentration of species such as (C), due to the former’s stronger u-coordination to the metal atom centre. This a-coordination then prevents substrate coordination to the metal, with a consequent decrease in the reduction rate. Addition of strong s-acid ligand L’ (L’ = PPhs or py) to the reaction medium drastically reduces the reduction rate, due to the formation of stable complexes of the type PdLL’X (X = anion) [8].
Reaction kinetics and mechanism The kinetic studies were carried out using four autoclaves joined in series. The pressure in the autoclaves was maintained constant during the reaction by connecting them to the hydrogen cylinder. The rate was determined by analysis of the product composition at different time intervals, and the initial reaction rate was measured by graphical extrapolation of the rate curve to zero time. The initial rate was found to be first-order dependent on catalyst concentration (Fig. 3) and hydrogen pressure (Fig. 4) and independent of substrate concentration. The following mechanism (Scheme 2), consistent with the kinetic data was suggested on the basis of experimental evidence. According to Scheme 2, Rate = ki[B] [H2] Let [Cat], = total catalyst concentration. The spectral studies of different solutions indicated almost complete absence of original dimer (G) and the corresponding monomer (H) in the hydrogen activated solution. Hence,
281
I 2
L
6
8
10
[Cat] xlOL (mol lit-')
Fig. 3. Rate dependence on Pdz(pcph)z(OAch concentration 11.5 X lo3 kN m-* of Hz. [1-nitropropane] = 0.25 mall-’ (0).
in DMF at 75 “C under
[p,,] X10s3(kNmw2)
Fig. 4. Rate dependence on hydrogen pressure in DMF at 75 “C. [Pdz(pcphh(OAc)2] 5.5 x 10m4 mol 1-l; [1-nitropropane] = 0.25 mol 1-l (a). [CatIT
=
[Al
+
WI
+
[Cl
+
PI + [El
= [BIPMFI + KI [RN021
[B] + k’[B] +
= [B]{[DMF]
(1+ k’)K1[RN02]
+
JWI [RNHOHI + K, WI [RNH,I [RN%1 [RN021 + &K[RNHOH]
K,[RN021
+ K1K’[RNH2])
=
282
‘Pd’ ,‘;
P;“)
X
N’
=
(;;PdI;MF
_g
‘c (HI
(G)
C
/H
‘Pd
C + RNOZ
N’
’
‘Pd
/H
kl, Hz
N /(B;C$NR
DMF
-H20 1 Slow)
(A)
c
H ‘Pd’ / \
N
+
H2
L
(“Pd/
H Loi
\N’
ON-R
(Cl H
K2 DMF
‘Pdl N’
NH2R
,ti_
‘NHR
(D)
C
H
RNHOH
(El K3 DMF Ji RNH2 C, (
Pd
N’
N'
,H
(A) C
‘DMF
= pcph,
pibh,
dmbajX=OAc,Cl
N
(A) Scheme 2.
where k,/k2
=
pd\DMF
= k’, l/K,K,
PI&
= K, 1/K,K3
+ k,WJOzl + K,
= K’
[RNHOH]
+ Kg[RNH2])
K1 WW
where [DMF] = K,;
LB1= Rate = =
(l+
k’)K, = k6, KK, = K,,
&
KI [Cat1dRN021 K, + k, [ RN02]
+ K, [ RNHOH]
kl&[Ca%[RN021 K, + kg [ RN02]
klKSWdRN021
If K, < k6 [RN02],
+ K, [ RNH,]
W21
+ K, [ RNHOH]
K5 + k6[.RN02]
Rate =
K,K’=
W21 when
+ KS [RNH2] [RNHOH]
then
klK1[WdRN021 [%I = KICatlTW~l k6NW1
-
0,
[RNH,]
__f
0
283
References 1 J. F. Knifton, J. Org. Chem., 40 (1975) 519. 2 J. F. Knifton, J. Org. Chem., 38 (1973) 3296. 3 Hung. Pat. 34 431 (1985) to J. Antal, E. Csaszar, L. Horvath, J. Jesztl, G. Kineses, L. Kisgergely, J. Medve, B. Szeiler and I. Vida, see Chem. A bstr., 104 (1986) 226705m; E. Ger. Pat. 213 432 (1984) to R. Kioss, K. Becker, H. D. Berrouschot, H. J. Biiinger, K. Klein and R. Merk, see Chem. Aktr., 102 (1985) 184710k; Jap. Pat. 60 45 550 (85 45 550) (1986) to Nitto Chem. Ind. Co. Ltd., see Chem. Abstr., 103 (1985) 104539u. 4 N. L. Holy, J. Org. Chem., 44 (1979) 239; N. L. Holy, J. Chem. Sot., Chem. Commun., (1978) 1074. 5 S. Ram and E. Ehrenkaufer, Tetrahedron Lett., 25 (1984) 3415. 6 J. Kwiatek, Catal. Rev., 1 (1967) 37. 7 P. K. Santra and C. R. Saha, J. Mol. Catal., 39 (1987) 279. 8 Y. Fuchita, K. Hiraki and Y. Kage, Bull. Chem. Sot. Jpn., 55 (1982) 955; K. Hiiaki, Y. Fuchita and K. Takechi, Znorg. Chem., 20 (1981).4316; J. M. Thomson and R. F. Heck, J. Org. Chem., 40 (1975) 2667; A. C. Cope and E. C. Friedrich, J. Am. Chem. Sot., 90 (1968) 909; S. Otsuka, A. Nakamura, T. Kano and K. Tani, J. Am. Chem. sot., 93 (1971) 4301. 9 J. V. Kingston and G. R. ScoiIary, Chem. Commun., (1969) 455; E. H. Brooks and F. Glocking, J. Chem. Sot., A, (1966) 1241. 10 J. A. Davies and F. R. Hartley, Chem. Rev., 81 (1981) 79. 11 R. L. Hones, J. Mol. Spectra, 11 (1963)411.
271 - 283
271
ORTHOMETALATED PALLADIUM(I1) COMPLEX-CATALYSED REDUCTION OF NITROALKANES AND NITRILES ASISH BOSE and C. R. SAHA* Department
of Chemistry, Indian Institute of Technology,
(Received February 11,1988;
Kharagpur-721302
(India)
accepted April 28, 1988)
Dihydrogen reduction of nitroalkanes and aromatic nitriles to the corresponding amines was efficiently achieved at (4.0 - 12.0) X lo3 kN rnp2 in the temperature range 65 - 80 “c using a series of di- and mononuclear orthopalladated complexes of the type trans-Pd, L2X2 and trans-PdLL’Cl [L = 2-( 2-pyridylcarbonyl)phenyl-C’ ,N( pcph), 2-( 2-picolyl)phenyl-C1 ,N(piph); (N,Ndimethylbenzylamine)-C2 ,N(dmba); L’ = pyridine, triphenylphosphine; X = OAc, Cl] as catalysts in DMF medium. The existence of metal-arene u-bonds in the complexes was found essential for their catalytic activities. Six-membered orthopalladated complexes were generally superior catalysts to the five-membered ones, and the binuclear benzoylpyridine complex was the best in the series. The complexes were the catalyst precursors, and the actual catalytic species was always found to be the corresponding mononuclear palladium hydride-DMF adduct. Kinetic studies indicate first-order rate dependence on catalyst concentration and hydrogen pressure, and zero-order on substrate concentration. A tentative mechanism for this reduction process has been suggested on the basis of kinetic data and other experimental observations.
Introduction Very few reports have thus far been published on the academically and industrially important problem of reduction of nitroalkanes to the corresponding amines. Knifton has reported the homogeneous catalytic reduction of nitroalkane to the corresponding amine in presence of ruthenium(I1) complexes [l] and to the corresponding oximes by carbon monoxide in the presence of copper(I1) salts solubilized in alkyl polyamines [2]. Aliphatic amines have been catalytically produced by ammonolysis of alcohols [ 31, dihydrogen reduction of nitriles [4] and by reduction of nitroalkanes by hydrogen transfer agents [5]. The nature and yield of products depend on *Author to whom correspondence 0304-5102/89/$3.50
should be addressed. 0 Elsevier Sequoia/Printed in The Netherlands
272
both the type of catalyst used and the experimental conditions. The reduction mechanism in many cases could not be ascertained due to irreproducible data, catalyst decomposition and stringent reaction conditions [ 3,6]. Orthopalladated complexes with azobenzene, in spite of having excellent catalytic activities towards the reduction of nitroaromatics, alkenes and alkynes, were completely inert towards the reduction of nitroalkanes and underwent decomposition at higher temperatures [ ‘71. Similar complexes with greater thermal stabilities are expected to be catalytically active towards the reduction of nitroalkanes; the present paper describes the superior catalytic activities of some five- and six-membered orthopalladated complexes in reducing the nitroalkanes and nitriles almost completely to the corrresponding amines.
Experimental Analytical grade reagents, distilled solvents and pure dry hydrogen gas were used throughout the investigation. Dimethylformamide (DMF) was purified by storing over CaH, under nitrogen for 24 h, followed by distillation under reduced pressure. The complexes used in the present investigation, i.e., Pd2L2X2, Pd(LH)2C12 and PdLL’Cl (where L = pcph, piph, dmba; L’ = pyridine (py), triphenylphosphine (PPhs); X = OAc, Cl), were prepared according to literature methods [8] and purified by recrystallization wherever possible. Their purities were checked by physicochemical means before their use as catalysts. The metals were estimated gravimetrically, and C and N contents were estimated by semi-microanalytical methods. Vibrational, electronic and PMR spectra were taken on Perkin-Elmer 237 B, Cary-17 D and Varian 390, 90 MHz instruments respectively. Gas chromatographic analysis was carried out on a Varian-3700 instrument using SE-30, Carbowax 20M and 15% FFAP column. Molecular weights were determined in a Knauer Dampfdruck osmometer. The kinetic studies were carried out in a series of stainless steel autoclaves fitted with glass liners containing 10 ml aliquots of a solution of catalyst and substrate. The reactions were quenched at different times and immediately analysed . Isolation of PdL(H)DMF
(L = pcph, piph)
A solution of the parent dinuclear complex PdzLzXz (0.3 mmol) was prepared in 100 ml of dry deaerated DMF under nitrogen in a 250 ml glass reactor. Nitrogen was pumped out and the reactor was filled with hydrogen. The solution was stirred continuously under 1 atm hydrogen. The yellow solution changed color gradually to deep brown, and stirring was continued until there was no further deepening of colour. After -1 h, the hydrogen supply was cut off and the solution was concentrated to about 20 ml under vacuum. Addition of a large amount of dry ether at this stage precipitated a
273 deep brown compound, which was filtered and washed with dry ether under hydrogen and dried in vacuum. Further concentration of the solution to -5 ml initiated decomposition of the complex to Pd’. Reduction procedure In a typical experiment, the DMF solution (10 ml) containing the catalyst (5.5 X 10e2 mmol) and the substrate (2.5 mmol) was introduced into a glass-lined stainless steel autoclave (100 ml) equipped with a magnetic stirrer and immersed in a thermostat. The reactor was first evacuated, flushed thrice with pure dry hydrogen, and then allowed to attain the temperature of the thermostatted silicone oil bath. It was then subjected to the desired pressure of hydrogen, which was maintained constant throughout the reduction period. No reduction occurred under 1 atm hydrogen at the experimental temperature, and the extent of reduction occurring during the attainment of the desired pressure was found to be negligible. Reaction time was measured from the moment the reactor attained the desired temperature and pressure. The reaction was quenched after the desired time, and the gaseous amine was passed through standard H2S04 solution for estimation. The components present in DMF solution were normally estimated by GLC. The liquid and solid components were separated by fractional distillation and column chromatography respectively. The products were identified by b.p., m.p., IR and PMR spectra wherever possible. The catalyst solution did not show any sign of decomposition at higher pressure (up to 27.5 X lo3 kN mW2examined) but started slow decomposition if the solution temperature rose above 120 “C.
Results and discussion A series of orthometalated and non-orthometalated complexes were used as catalysts in the present investigation. All the former complexes were active for these reductions, while the latters were found to be inactive in a variety of solvents under different experimental conditions (pressures up to 15 X lo3 kN mW2and temperature up to 120 “C). The yellow DMF solutions of non-orthometalated complexes underwent slow decomposition to Pd” on long standing under hydrogen without turning brown at any stage, which is in contrast to the behavior of the binuclear solutions. The high catalytic activities of the orthopalladated complexes are probably due to greater r-acidic character of the corresponding ligands. The ligands are normally r-acidic (except NJVdimethylbenzylamine) due to the presence of the pyridine ring, and their a-acidity is greatly enhanced in orthometalated complexes due to the formation of the metal-arene u-bond. The ligand N,Ndimethylbenzylamine is n-acidic only in its orthometalated complexes. The catalytic activities of the complexes in both the mono- and binuclear series, PdLL’Cl and Pd2L2(OAc), (L = pcph, piph, dmba; L’ = pyridine, triphenylphosphine) respectively run almost parallel to the s-acidic
274
character of the corresponding ligands. The highest activity of Pdz(pcph)2(OAc), may be due to the highest n-acidic character of the ligand, resulting from the a-electron delocalization in the chelate ring. The molecular weights of the dinuclear species Pd2LzX2 determined in DMF/DMSO medium indicated their extensive dissociation to the corresponding monomeric species. However, such dissociation did not occur in noncoordinating solvents such as benzene or chloroform, as was evidenced from their molecular weight determinations (Table 1). The electronic spectra of PdzL2Xz in the solid state and in benzene medium are almost identical, and differ from those in DMF solution (Table 1). The above experimental results and the bridge splitting behaviour of these dinuclear complexes by pyridine, triphenylphosphine and acetylacetone [8] suggest the following type of equilibria in the present reaction medium: Pdz Lz (OAc],
DMF ti
2PdL(OAc)DMF
(1)
The dissociation appears to be nearly complete in the dilute solution (-lOA M) used in the present investigation. The isolation of the DMF adduct was not possible, as the solution always produced crystals of the corresponding binuclear parent complex on solvent evaporation. The visible electronic spectra of highly concentrated DMF solutions also indicated the presence of some monomeric species. The dissociation of L’ from PdLL’Cl (L = pcph, piph; L’ = pyridine, triphenylphosphine) in DMF solution in the absence of hydrogen was not indicated, either from their molecular weight measurements or from their spectral data (Table 1). TABLE 1 Molecular weight and electronic spectral data of the complexes Complexes
Molecular wt. in benzene exp. (theor.)
Molecular wt. in DMF exp. (theor.)
Ain benzene (nm)
Ain DMF (nm)
hmC in activated solution (nm)
[WPcPh)(OAc)lz
700.5 (716.6)
492.6a (420.4)b
210,335
265,320
265,430, 490
W(PiPh)(OAc)lz
659.2 (666.8)
489.5 (406.4)b
265,345 420
265,310
270,400, 495
[Pd(dmba)C1]2
550.6 (551.8)
420.6 (348.9)b
275,330, 350
270,300 320,345
270,400, 485
538.2 (543.8)
542 (543.8)
265,405
260,405
582.5 (585.9)
592 (585.9)
290,335, 470,540
290,335, 465,530
tPd(Dpcph)2Clz
1
[Pd(pcph)(PPh3)C1]
aO.OO1 M DMF solution was used. bAssuming complete dissociation. Y%e solution was activated by stirring under hydrogen for 30 min.
295,335, 435,490
275
The yellow solution of the complex, PdzLzXz and PdLL’Cl in DMF and DMSO, changed to deep brown on stirring under normal pressure of hydrogen at ambient temperature within a few minutes. The chloro-bridged species required shorter periods than the acetate-bridged ones for this conversion, while the orthopalladated mononuclear complexes required the longest time. The brown solutions thus obtained were active towards these reduction processes under high pressure conditions, and their efficiencies were equal to those of the original compounds. The yellow solutions of the complexes turn brown under high pressure hydrogen treatment, and the brown solutions, obtained by both high or normal pressure hydrogen treatment, have identical electronic spectra provided they are derived from the same parent compound. The visible electronic spectra of all the brown solutions obtained from different sources are comparable (Table l), indicating the formation of similar brown catalytic species in all cases. The catalyst solution left at the end of high pressure reduction did not show the presence of any suspended particles when examined by strong light or filtered through kieselguhr under nitrogen atmosphere. The solution could be reused for at least four cycles without any loss of catalytic activity. Solutions of these compounds in non-coordinating solvents such as CHzClz, CHCls, CCL, C6H6, C6H,CH3 etc. remained inert toward both hydrogen activation and the catalytic reduction process. Yellow solutions of the active complexes in alcohol (methanol, ethanol, n-propanol) turned deep brown very quickly in the presence of hydrogen, with subsequent slow decomposition to Pd” even at 0 “C. The pH and conductance of the initial solution decreased and increased respectively during hydrogenation. In the case of PdzLzClz and PdLL’Cl, the liberation of free HCI during this process was confirmed by the evolution of CO* from NaHCOa. In the case of acetate-bridged complexes, the decrease in initial rate on addition of CHaCOOH, HCOOH and HCl suggests the liberation of free acid during hydrogen activation. Isolation of the active species from the deep brown solution was very difficult. The solution is very sensitive to air and moisture, and underwent slow decomposition at high concentration under vacuum. The cause of this decomposition may be due to the inability of the hydride complex (eqns. 2, 3) to form the corresponding dimer with a hydride bridge. Under similar conditions, the species (C) (eqns. 2, 3) produced the corresponding dimer with a chloride (or acetate) bridge without difficulty. DMF Pdz L2X2
e
2PdLXDMF 2
PdL(H)DMF
(2)
((7 Hz, -HCl
PdL, L'Cl -\-
PdLIL’(H) DME:PdLl(H)DMF + L’
L = pcph, piph, dmba; L1 = pcph, piph; L’ = py, PPh3 ; X = OAc, Cl
(3)
276
However, the brown solutions obtained from either the mono- or binuclear complexes showed PMR signals at -20 - 22 r. Addition of large amounts of dry ether to the deep brown solutions originating from [PdL(OAc)], (L = pcph, piph) produced a deep brown precipitate of PdL(H)DMF. These compounds were stable under nitrogen or hydrogen atmosphere and decomposed to Pd” in the presence of air, very slowly in the solid state and quickly in solution. Comparison of their IR spectra with the corresponding parent dinuclear complexes indicated the absence of peaks due to acetate group and coordinated DMF in both of them (Table 2). A new peak at -1960 cm-’ (v(Pd-H) [9] in their spectra and the PMR signal at -20 - 22 T of their solutions in DMSOd6 support the presence of Pd-H in them. Their visible electronic spectra in DMF are almost identical to those of the corresponding brown solution formed during hydrogen activation. The brown compounds are catalytically active in DMF medium, but their catalytic activities could be neither determined quantitatively nor compared with those of their parent compounds, as some decomposition always occurred during their dissolution in DMF. However, attempts to isolate such brown compounds from [Pd(dmba)C112 or PdLPPhsCl (L = pcph, piph) did not meet with success. The brown compound obtained from the activated solution of PdLPPhsCl on addition of dry ether gave inconsistent analytical data. The physicochemical properties of the compound indicated the presence of PPhs and a Pd-H bond. The presence of PPhs was detected in the clear filtrate also. The presence of an equilibrium in eqn. (2) was suggested from the fact that on addition of a very small amount of NH4C1 to the brown solution derived from [Pd(dmba)Cl],, it reverts to the yellow solution from which the original compound could be recovered.
TABLE
2
Analytical and IR data of the complexes Compound
v(Pd-H) (cm-‘)
1670
1580,
-
-
-
1425 1585,
-
-
-
1650
1965
-
-
1655
1965
-
-
1714s
-
N
Pd
v(C=O) (Benzoyl pyridine) (cm-‘)
47.21 (47.36)
3.70 (3.91)
29.40
50.15 (50.38)
$619)
(31.91)
[Pd(pcph)HDMF]
49.42 (49.66)
7.50 (7.72)
29.29 (29.36)
1665
[Pd(piph)HDMF]
51.30 (51.66) -
8.1 (8.03)
30.4 (30.54) -
[Pd(pcph)(OAc)]s [Pd(piph)(OAc)Js
DMF aVoir [ll].
Found (Caicd) (I)
Bridging V(C=O)(DMF) acetate (cm-i) (cm-’ )
C
(29.69) 31.75
1420
277 TABLE 3 Time period of nitroethane reduction using different metal complexesa Catalyst
Reduction time (h)
Yield of main product (W)
(1) W(pcphh(OAc)2
5.15 5.40 5.90 6.10 5.30 5.70
ethylamine ethylamine ethylamine ethylamine ethylamine ethylamine
(2) (3) (4) (5) (6)
Pdz(piphh(CAc)2 W(dmbah(OAch Pdz(dmba)KG Pd(pcph)pya Pd(piph)pyCl
(92) (90) (85) (86) (90) (90)
aMedium = DMF, substrate = nitroethane, [catalyst ] = 5.5 X 10e4 mol 1-l) [substrate] = 0.25 mol l-r, pH, = 11.5 X lo3 kN mm2, temp. = 75 “C, total volume = 10 ml.
The activities of various catalysts towards the reduction of nitroalkanes are presented in Table 3. Among these orthopalladated complexes, the binuclear ones are generally more active than the mononuclear ones, and [Pd(pcph)(OAc)], is the most efficient catalyst in the series. Detailed investigations were made using [Pd(pcph)(OAc)12 due to its highest catalytic activity. The corresponding chloro-bridged complex could not be used, due to its insolubility in DMF. The nitroalkanes used in the present investigation are almost completely reduced to the corresponding amines at the final stage (Table 4). Lower nitroalkanes are generally reduced at faster rates than the higher ones. The reduction rates decreased with increasing branching at the o-carbon atom. Greater steric hindrance experienced TABLE 4 Reduction time and yields of products* Substrate
Time period (h)
Products (%)
(1) (2) (3) (4) (5 ) (6) (7 ) (8 ) (9)
5.0 5.15 5.15 6.0 5.7 5.4 8.0 6.0 7.5
methylamine (93) ethylamine (92) 1-aminopropane (92) 2-aminopropane (90) 1-aminoheptane (88) 2-phenylethylamine (90) aminocyclohexane (7 3 ) diphenylmethanol(95) dibenzylamine (90) and benzylamine (10)
nitromethane nitroethane 1-nitropropane 2-nitropropane 1-nitroheptane 2-phenylnitroethane nitrocyclohexane benzophenone benzonitrile
*Medium = DMF, catalyst = Pd2(pcph),(OAch, [Pdz(pcph)2(OAc)p] = 5.5 x 10e4 mol l-l, [substrate] (except benzophenone and benzonitrile) = 0.25 mol l-l, [benzophenone] = 0.2 mol l-r, [benzonitrile] = 0.32 mol l-l, pi, = 11.5 x lo3 kN m-‘, temp. = 75 “C, total volume = 10 ml.
278
1
3
2 Reaction
4 time
5
6
7
(hr)
Fig. 1. Reduction of 1-nitropropane with trens-Pdz(pcph)2(OAch in DMF at 75 “C under 11.5 x lo3 kN rn-’ Hz, in DMF medium (---) and in the presence of acetic acid ); [Pd2(pcph)2(OAc)2] = 5.5 x 10e4 mall-‘; [1-nitropropane] = 0.25 mol (1O-2 M) (1-l ; 0 = 1-nitropropane; m = 1-aminopropane; A = IV-propylhydroxylamine.
by highly branched or higher nitroalkanes during their coordination to the metal atom centre may be the cause of their lower rates of reduction. During the reduction of nitroalkanes, corresponding hydroxylamines were always detected at various intermediate stages. The concentration of the hydroxylamines increased with the progress of reduction, and a small amount remained in the system even after the complete disappearance of nitroalkane (Fig. 1). The reduction rate of alkylhydroxylamine was comparable to that of the corresponding nitroalkane under similar experimental conditions. The reduction period of nitroalkanes increased with increasing strength and concentration of the acid added (CH,COOH, HCOOH, HCI). This may be due to a lower concentration of the active species, as per Scheme 2. The concentration of hydroxylamine in acid medium decreased at various intermediate stages (Fig. l), indicating the formation of a metal-oxygen bond during the reduction. If the reduction of hydroxylamine required metalnitrogen bond formation, the proportion of hydroxylamine in acidic medium would increase at various intermediate stages. In the presence of base (NaOH, 10e3 M), the rate of nitroalkane reduction increased with an increasing proportion of corresponding hydroxylamine at intermediate stages (Fig. 2). This is possible provided the base increases the reduction rate of nitroalkane without affecting the rate for hydroxylamine. The base may increase the concentration of active species as per Scheme 2, or it may produce RCHNOz from RCH,NOz. Such formation of RCHN02 has been reported to be essential for the catalytic reduction of the nitroalkane [ 11. In the present case,
279
25
1
2 Reaction
L
3 tim
5
(hr)
Fig. 2. Reduction of 1-nitropropane in ;he presen’ce of base (NaOH) with tians-Pdz~~JIJI);~~_~. at 75 “C tinder 11.5 x 10 kN me2 of Hz; [Pdz(pcph)2(OAc)2] = 5.5 x , [1-nitropropane] = 0.25 mol 1-l; [NaOH] = 0.005 mol 1-l; @ = l-nitropropane; IJ = l-aminopropane; * = N-propylhydroxylamine.
RCH,NO, appears to be reduced as such without the intermediate formation of the corresponding nitroalkyl anion, at least in the absence of base. Though alkylhydroxylamine could be reduced under conditions comparable to those for nitroalkane, all attempts to reduce alkyloximes were unsuccessful under different experimental conditions. The rates of dihydrogen reduction of alkylhydroxylamine to the corresponding amine using these catalysts do not undergo any appreciable change in presence of acid or base (10m3 M). This supports the interaction of a metal atom centre with the oxygen atom of hydroxylamine during its reduction. Such a metal-hydroxylamine intermediate could not, however, be isolated. The complexes are active towards the reduction of benzonitrile and benzophenone, but inert towards acetonitrile. High pressure and high ternperature conditions are necessary for the reduction of both benzophenone and benzonitrile, the formel’ producing only the corresponding alcohol while the latter produced both benzylamine (10%) and dibenzylamine (90%) (Table 4). Benzonitrile reduction was accompanied by liberation of ammonia, and the initial addition of benzylamine increased the proportion of dibenzylamine in the final product mixture. The reduction probably occurs as per Scheme 1 [ 41. The reduction rates of all these substrates increased in the presence of small concentrations of acetonitrile in the reaction medium (up to 0.3 M), but decreased if the concentration of acetonitrile was increased further. A very long reduction time is required in pure acetonitrile. Both the mild proton acceptor property of DMF and its ability to coordinate to the metal atom centre in a z-type manner [lo] favours the stabilization of (C) (eqns. 2,3).
280
PhCN --%
PhCN
PhCHzNHz -
PhCH+H PhC=NH 1
PhCH2TH
CH2Ph -
H2
PhCH2S:
-NH3
CHPh-
H2
PhCH2rH
PhCHNH2
Scheme 1.
However, its stronger n-acceptor capacity may disfavour metal-nitroalkane x-interaction. Acetonitrile may form species similar to (C) and, being a weak x-acceptor ligand, may favour metal-nitroalkane n-interaction under comparable conditions. This increases the concentration of [ metal-nitroalkane] intermediate, with a consequent increase in reduction rate. This is only possible when acetonitrile is present in very small concentrations. Increased acetonitrile concentration may lower the concentration of species such as (C), due to the former’s stronger u-coordination to the metal atom centre. This a-coordination then prevents substrate coordination to the metal, with a consequent decrease in the reduction rate. Addition of strong s-acid ligand L’ (L’ = PPhs or py) to the reaction medium drastically reduces the reduction rate, due to the formation of stable complexes of the type PdLL’X (X = anion) [8].
Reaction kinetics and mechanism The kinetic studies were carried out using four autoclaves joined in series. The pressure in the autoclaves was maintained constant during the reaction by connecting them to the hydrogen cylinder. The rate was determined by analysis of the product composition at different time intervals, and the initial reaction rate was measured by graphical extrapolation of the rate curve to zero time. The initial rate was found to be first-order dependent on catalyst concentration (Fig. 3) and hydrogen pressure (Fig. 4) and independent of substrate concentration. The following mechanism (Scheme 2), consistent with the kinetic data was suggested on the basis of experimental evidence. According to Scheme 2, Rate = ki[B] [H2] Let [Cat], = total catalyst concentration. The spectral studies of different solutions indicated almost complete absence of original dimer (G) and the corresponding monomer (H) in the hydrogen activated solution. Hence,
281
I 2
L
6
8
10
[Cat] xlOL (mol lit-')
Fig. 3. Rate dependence on Pdz(pcph)z(OAch concentration 11.5 X lo3 kN m-* of Hz. [1-nitropropane] = 0.25 mall-’ (0).
in DMF at 75 “C under
[p,,] X10s3(kNmw2)
Fig. 4. Rate dependence on hydrogen pressure in DMF at 75 “C. [Pdz(pcphh(OAc)2] 5.5 x 10m4 mol 1-l; [1-nitropropane] = 0.25 mol 1-l (a). [CatIT
=
[Al
+
WI
+
[Cl
+
PI + [El
= [BIPMFI + KI [RN021
[B] + k’[B] +
= [B]{[DMF]
(1+ k’)K1[RN02]
+
JWI [RNHOHI + K, WI [RNH,I [RN%1 [RN021 + &K[RNHOH]
K,[RN021
+ K1K’[RNH2])
=
282
‘Pd’ ,‘;
P;“)
X
N’
=
(;;PdI;MF
_g
‘c (HI
(G)
C
/H
‘Pd
C + RNOZ
N’
’
‘Pd
/H
kl, Hz
N /(B;C$NR
DMF
-H20 1 Slow)
(A)
c
H ‘Pd’ / \
N
+
H2
L
(“Pd/
H Loi
\N’
ON-R
(Cl H
K2 DMF
‘Pdl N’
NH2R
,ti_
‘NHR
(D)
C
H
RNHOH
(El K3 DMF Ji RNH2 C, (
Pd
N’
N'
,H
(A) C
‘DMF
= pcph,
pibh,
dmbajX=OAc,Cl
N
(A) Scheme 2.
where k,/k2
=
pd\DMF
= k’, l/K,K,
PI&
= K, 1/K,K3
+ k,WJOzl + K,
= K’
[RNHOH]
+ Kg[RNH2])
K1 WW
where [DMF] = K,;
LB1= Rate = =
(l+
k’)K, = k6, KK, = K,,
&
KI [Cat1dRN021 K, + k, [ RN02]
+ K, [ RNHOH]
kl&[Ca%[RN021 K, + kg [ RN02]
klKSWdRN021
If K, < k6 [RN02],
+ K, [ RNH,]
W21
+ K, [ RNHOH]
K5 + k6[.RN02]
Rate =
K,K’=
W21 when
+ KS [RNH2] [RNHOH]
then
klK1[WdRN021 [%I = KICatlTW~l k6NW1
-
0,
[RNH,]
__f
0
283
References 1 J. F. Knifton, J. Org. Chem., 40 (1975) 519. 2 J. F. Knifton, J. Org. Chem., 38 (1973) 3296. 3 Hung. Pat. 34 431 (1985) to J. Antal, E. Csaszar, L. Horvath, J. Jesztl, G. Kineses, L. Kisgergely, J. Medve, B. Szeiler and I. Vida, see Chem. A bstr., 104 (1986) 226705m; E. Ger. Pat. 213 432 (1984) to R. Kioss, K. Becker, H. D. Berrouschot, H. J. Biiinger, K. Klein and R. Merk, see Chem. Aktr., 102 (1985) 184710k; Jap. Pat. 60 45 550 (85 45 550) (1986) to Nitto Chem. Ind. Co. Ltd., see Chem. Abstr., 103 (1985) 104539u. 4 N. L. Holy, J. Org. Chem., 44 (1979) 239; N. L. Holy, J. Chem. Sot., Chem. Commun., (1978) 1074. 5 S. Ram and E. Ehrenkaufer, Tetrahedron Lett., 25 (1984) 3415. 6 J. Kwiatek, Catal. Rev., 1 (1967) 37. 7 P. K. Santra and C. R. Saha, J. Mol. Catal., 39 (1987) 279. 8 Y. Fuchita, K. Hiraki and Y. Kage, Bull. Chem. Sot. Jpn., 55 (1982) 955; K. Hiiaki, Y. Fuchita and K. Takechi, Znorg. Chem., 20 (1981).4316; J. M. Thomson and R. F. Heck, J. Org. Chem., 40 (1975) 2667; A. C. Cope and E. C. Friedrich, J. Am. Chem. Sot., 90 (1968) 909; S. Otsuka, A. Nakamura, T. Kano and K. Tani, J. Am. Chem. sot., 93 (1971) 4301. 9 J. V. Kingston and G. R. ScoiIary, Chem. Commun., (1969) 455; E. H. Brooks and F. Glocking, J. Chem. Sot., A, (1966) 1241. 10 J. A. Davies and F. R. Hartley, Chem. Rev., 81 (1981) 79. 11 R. L. Hones, J. Mol. Spectra, 11 (1963)411.