КИНЕТИКА И КАТАЛИЗ, 2013, том 54, № 3, с. 328-335
UDC 541.128.12:547.539.4:546.98-386
INSIGHT INTO PdCl2(bipy) COMPLEX AS AN EFFICIENT CATALYST FOR HECK REACTION AND KINETIC INVESTIGATIONS IN HOMOGENEOUS MEDIUM © 2013 S. V. Jagtap12, *, R. M. Deshpande13
1Chemical Engineering Division, National Chemical Laboratory, Maharashtra, Pune, India
2B.G. College, Sangvi, Pune 3Dow Chemicals International Pvt. Ltd., Yerawda, Pune *E-mail: sangeetajagtap@rediffmail.com Received 24.05.2012
PdCl2(bipy) complex (bipy = 2,2'-bipyrydine) efficiently catalyzes the vinylation of aryl halides. The activity of this catalyst for the Heck reaction was demonstrated for a variety of aryl halides and olefins in the presence of different organic and inorganic bases. The catalyst is stable under the reaction conditions and no degradation was observed. The kinetics of the Heck coupling of styrene with iodobenzene using the PdCl2(bipy) complex with potassium acetate as a base was studied over a temperature range of393—413 K in 2-nitro-2-methyl-1-propanol medium. An empirical rate model has been proposed to fit the observed data and is found to be in good agreement with experimental results. The activation energy of the reaction was found to be 98.70 kJ/mol.
DOI: 10.7868/S0453881113030052
The Heck reaction [1, 2]
Ar4 /CK
^CK2
CK Olefin
+ Ar—X
Aryl halide 150°C Base
PdCl2(bipy) Ar4 y,CK + ее
NMi' Y.f \A BHX
CK Ar
Product
(bipy = 2,2'-bipyrydine, NMP = 2-nitro-2-methyl-1-propanol)
is an important class of C—C coupling reactions [3, 4], as it is tolerant to many functional groups [5]. The use of Pd—phosphine complexes for the Heck reaction has been reported in a number of studies [6]. These catalysts are known to suffer decomposition in the course of the reaction involving the palladacycle catalysts, which are reportedly more stable than the monoden-tate phosphine complexes [7]. Hence, the development of stable phosphine-free catalysts resistent to the reaction temperatures encountered in the Heck reaction would be an important achievement. Compared to the phosphine modified Pd catalysts, the Pd complexes incorporating nitrogen-containing ligands although equally active have received less attention [8—11]. There are still fewreports on the kinetics of Heck reactions using Pd complexes with nitrogen containing ligands [12, 13].
Cabri et al. [8] have reported that Pd complexes of bidentate nitrogen ligand such as 1,10-phenanthroline derivatives are active for Heck reactions in N,N-dim-ethylformamide (DMF) to give a 94% yield of product. Surprisingly, the same authors report very little or no activity for Heck reaction using the Pd complexes of 2,2'-bioxazoline or of 2,2'-bipyridine ligands. Tsai et al. [14] have used heterogenized palladium bipyridyl
complex anchored on MCM-41 as an efficient and recyclable heterogeneous catalyst for Heck reactions with a Turn Over Number (TON) up to 106. n-Butyl acrylate on reaction with p-bromoacetophenone was reported to give a 98% conversion in 16 h, but for iodobenzene 96 h are needed to achieve the same conversion level. There is however no study on the Heck reaction in a homogeneous medium using this Pd complex catalyst. The Heck reaction using iminophosphine-pal-ladium(0) complexes in NMP [15], has been reported by Scrivanti et al. This catalyst is active only in an inert atmosphere and is easily poisoned in the presence of oxygen.
The kinetics of Heck reaction using Pd complexes has been reported in a few studies [16—21]. In general it has been observed that the catalyst activity has a positive dependence on olefin, aryl halide and the base. All these studies have been conducted with PdCl2 or Pd-phosphine complexes. For the phosphine-free complexes only few studies on the kinetics exist. Rosner et al. [12] have investigated the kinetics of olefina-tion ofp-bromobenzaldehyde with butyl acrylate using a dimeric CN-palladacycle complex underthe dry conditions. They observed a first order dependence on olefin concentration and a zero order dependence on aryl halide (p-bromobenzaldehyde) concentration. They also proposed an empirical rate model to predict the rate observed for the dimeric palladacycle complex with chelating nitrogen ligands [18]. Consorti and coworkers [13] reported a first order dependence with respect to Pd catalyst and a fractional order with respect to iodobenzene and methyl acrylate for the coupling of
aryl halides with butyl acrylate and methyl acrylate using the CN-palladacycle. Van Strijdonck et al. [22] reported a first order kinetics on styrene and a zero order on iodobenzene concentration for Pd(dba) catalyst (dba = dibenzylideneacetone). An empirical rate equation was derived to fit the experimental data.
We report here the results of our investigation into the Heck reaction catalyzedby the PdCl2(bipy) complex. Contrary to the reports mentioned earlier, the PdCl2(bipy) complex as catalyst was very efficient for Heck reaction in polar solvents. This catalyst is stable and does not degrade to metallic Pd as most of the other catalysts reportedly do, permitting the reactions to occur at higher temperatures. This catalyst was applied for the coupling of a variety of aryl halides and olefins in the presence of different organic and inorganic bases in different solvents. Further, in order to understand the kinetics and mechanism of this reaction, the effect of the process parameters on the catalytic activity of the PdCl2(bipy) complex was also investigated for the Heck coupling of styrene with iodobenzene in a temperature range of 393—413 K. An empirical rate model has been proposed which was found to fit the experimental data.
EXPERIMENTAL
PdCl2, 2,2'-bipyridine, the olefins and aryl halides used were procured from "Aldrich"(USA) and used
TON — Amount of product(mole) (2)
Amount of catalyst(mole)
Procedure for Preparation of Catalyst
In a typical preparation, 0.234 g (1.33 mmol) 2,2'-bipyridine was added into a 50 mL round bottom flask containing 10 ml methanol, 0.266 g (1.33 mmol) PdCl2. The mixture was stirred at room temperature for 6 h. An orange yellow colored PdCl2(bipy) complex precipitated, which was filtered and washed with methanol and then dried under vacuum. Practical yield of the complex was 0.440 g (88%). Characterization of the PdCl2(bipy) complex was carried out by IR, NMR and the elemental analysis and was found to match with the reported values [23].
General Procedure for Screening Experiments
In a 25 mL two necked round bottom flask 2.2 mmol of olefin, 2 mmol of aryl halide, 2.0 mmol of base, and 0.002 mmol of PdCl2(bipy)catalyst were added. TBAB (1% ofbase) was added as a phase transfer agent every time in case of inorganic bases. Then
without any further treatment. The solvents were obtained from "Ms SDs Chemicals"(India). Bases and tetrabutyl ammonium bromide (TBAB) used were of analytical grade, purchased from "Loba Chemi-cals"(India).
A portion of the reaction mixture was periodically taken out from the round bottom flask and was analyzed for its contents by using a capillary gas chromatograph Agilent 6850 series (India) with flame ion-ization detector, on HP1 column with film thickness 0.25 ^m (initial temperature 80°C, final temperature 250°C). Formation of stilbene was confirmed on GC by comparison with authentic standards. The identification of products of screening reaction was done using a GC—MS Agilent 6890N series equipped with 5973N mass selective detector. Besides the confirmation from GC—MS the identification was also confirmed with authentic samples of the products — cis-and trans-stilbenes. No other products except these two were observed in the GC analysis. 1,1-Diphenyl ethylene can form as an additional product in the reaction, however it was not observed in this case because its concentration was very low (below the detectable limit).
The conversion of the aryl halide (ArX) and TON were calculated by using the following relations:
(1)
N-methyl-2-pyrrolidinone was added to make the volume to 10 mL. The reaction was carried out for 1 h at 150°C.
General Procedure for Kinetic Studies
In a 25 mL two necked round bottom flask styrene, iodobenzene, potassium acetate, TBAB (1% of base), and catalyst PdCl2(bipy) were added as per the requisite concentration. N-methyl-2-pyrrolidinone was then added to make volume to 10 mL. The reaction was carried out for 2 h at desired temperature in the range 120—140°C (393-413 K).
The kinetic experiments were carried out for a short duration, such that the conversion of liquid phase reactant was less than 20-25% to ensure differential conditions. It was generally observed that in this low conversion range, the rates of Heck reaction were constant. The experiments were found to be reproducible within an error of 2-4%. In each reaction, initial, intermediate and final samples were analyzed in order to check the material balance. Following this procedure, the effect of catalyst, styrene, iodobenzene and
_ . Initial amount of ArX(mole)-Final amount of ArX(mole) ...
Conversion —-1---1-- x 100,
Initial amount of ArX(mole)
Table 1. Effect of various bases, olefins and aryl halides on activity of PdCl2(bipy) catalyst in Heck reaction in homogeneous medium
Entry Olefin Ar-X Base Conversion, % LON Trans-/cis-stilbenes
1 Styrene Iodobenzene TBA + TBAI 68 552 87/13
2 » » Et3N 66 543 87/13
3 » » KOAc + TBAB 59 482 88/12
4 » » TBA 56 455 84/16
5 » » NaOAc + TBAB 33 268 88/12
6 » » Piperidine 27 224 90/10
7 » » NaHCO3 + TBAB 23 190 89/11
8 » » Morpholine 20 161 86/14
9 » » MgOAc + TBAB 13 98 88/12
10 » 4-Bromo-acetophenone KOAc + TBAB 89 733 95/5
11 » 4-Bromo-1-iodobenzene » 70 573 88/12
12 » 4-Iodoanisole » 58 477 86/14
13 » 4-Bromotoluene » 54 445 92/8
14 » 2-Bromotoluene » 45 370 80/20
15 Methylacrylate Iodobenzene » 100 887 100/0
16 Allyl alcohol » » 92 754 100/0
17 n-Butylacrylate » » 84 706 100/0
18 4-Vinylanlsole » » 75 594 87/13
19 4-Methylstyrene » » 72 508 8
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