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Rate enhancement in palladium catalyzed Heck reactions by Lewis acid promoters
Catalysis Communications 8 (2007) 183–186 www.elsevier.com/locate/catcom
Rate enhancement in palladium catalyzed Heck reactions by Lewis acid promoters Abhishek Sud, Raj Madhukar Deshpande, Raghunath Vitthal Chaudhari
*
Homogeneous Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India Received 4 March 2006; received in revised form 16 May 2006; accepted 16 May 2006 Available online 7 June 2006
Abstract Significant enhancement in the rate of Heck reactions has been achieved when catalytic amounts of Lewis acid (like FeCl3) promoters in the presence of trace amounts of water (>2000 ppm) are used along with Pd complex catalysts. Experiments with different aryl halides, bases as well as olefins have been carried out to demonstrate the role of Lewis acid promoters. The increase in the reaction rates is due to the weakening of Ar–X bond by the Lewis acid in the manner similar to Friedel–Crafts reaction. 2006 Elsevier B.V. All rights reserved. Keywords: Palladium; Lewis acid; Heck reaction; Promoters
1. Introduction The palladium catalysed Mizoroki–Heck Reaction [1,2] involving the coupling of alkenes with aryl halides is one of the most powerful tools in synthetic organic chemistry for C–C bond formation, with wide ranging applications in chemical and pharmaceutical manufacturing industries [3–6]. Novel palladacycle complexes have demonstrated the feasibility of activation of less reactive substrates, and also achieve very high turnover numbers of the order of millions [7–10]. The use of promoters like quaternary ammonium salts to enhance the activity of the catalysts is well known [11–15]. These quaternary ammonium salts need to be employed in large quantities (20 mol% of the substrate) as the primary mode for rate enhancement is via phase transfer action. These salts are also costly and hence pose another challenge in making Heck reactions economically viable. The presence of nickel co-catalysts jointly with alkali metals has resulted in activation of the relatively inert chloroarenes using palladium catalysts. The enhancement is due to the conversion of small quanti*
Corresponding author. Tel.: +91 20 25902620; fax: +91 20 25893260. E-mail addresses: [email protected] (A. Sud), [email protected] (R.M. Deshpande), [email protected] (R.V. Chaudhari). 1566-7367/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.05.043
ties of the chloroarene to the iodo derivative which is more active [16]. Most of the present literature deals with the study of iodo or bromoarenes. The role of Lewis acids to assist the polarization of the alkane–halogen bond in Friedel–Crafts chemistry is wellknown [17]. Since, the Heck reaction also involves similar activation of the aryl–halogen bond, the presence of Lewis acids is likely to affect the rates of reaction [18–21]. AlCl3 and ZnCl2 have been used previously in Heck reactions but with no significant improvement in rate [22,23]. In this report, we demonstrate a substantial rate enhancement in the activity of palladium complex catalysts due to the presence of catalytic quantity of some of the Lewis acids such as FeCl3 in the presence of moisture (2000 ppm). 2. Experimental 2.1. Apparatus and chemicals The palladium acetate and tri(o-tolyl)phosphine were purchased from Aldrich Chemical Co. All the other chemicals were purchased from local commercial sources and were used as received, without any pre-treatment or purification. The reactions were carried out in a 50 ml 2-necked round bottom flask fitted with a condenser immersed in an
184
A. Sud et al. / Catalysis Communications 8 (2007) 183–186
oil bath held at desired temperature. The stirring was by means of a magnetic stirrer. The analysis of the contents of the reaction mixture was done using Agilent 6850A chromatograph, with HP-1 capillary column of 30 m length and FID. GC–MS was done using Agilent 6850N GC with 5973N mass selective detector. IR was carried out on Bio Rad FTS 175C instrument. Product identification was carried out using GC–MS analysis.
O Base, 1a, Lewis acid,
3. Results and discussion In order to demonstrate the promoting effect of Lewis acids, the experiments were conducted on vinylation of bromobenzene with n-butyl acrylate in the presence of sodium acetate base and palladium complex (1) as the catalyst precursor. The typical results showing the effect of different Lewis acid promoters are presented in Fig. 1. While, the promoting effect on catalytic activity was observed for most of the promoters used, FeCl3 clearly showed the most significant effect. The ratio of Fe:Pd was also found to be crucial. Under anhydrous conditions (<50 ppm moisture) the reaction with and without FeCl3 gave same results. This shows that it is probably the hydrated FeCl3 species that is important in promoting the coupling reaction. The highest R
R P
X Pd
Pd X P R
R
1. X = OAc
R= o-MeC6H4
Scheme 1.
OnBu
+
OnBu
NMP, Temp.
0.35 Conc, Kmol.m-3
SnCl2
Without LA
0.2 0.15 0.1 0.05 0 0
50
100 time, min
150
Fig. 1. Effect of Lewis acids on activity of Heck reactions. Reaction conditions: bromobenzene – 0.399 kmol/m3, n-butyl acrylate – 0.602 kmol/ m3, NaOAc – 0.6 kmol/m3, catalyst (1) – 8.51 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, temperature – 150 C, and LA – 5.93 · 105 kmol/m3.
enhancement in TOF (234%) was observed at Fe:Pd ratio of 0.7:1. The enhancement in the rate by a Lewis acid was in the order of the activity of the respective Lewis acid in Friedel–Craft reactions [17]. No enhancement in the rates was observed when AlCl3 was used. This may be due to the fact that AlCl3 does not form the hydrated species and instead reacts with water to form Al(OH)3, which may not promote the reaction. Promoting effect of FeCl3 was further established by carrying out experiments with different substrates and bases. Experiments were also carried out with different palladium catalyst precursors and different temperatures. The results are presented in Fig. 2 and Table 1. In all these experiments high conversions were achieved with more than 95% mass balance and a high selectivity towards the vinylated product. This was ensured by measuring concentration-time profiles for a few selected cases. The role of Lewis acids is probably to assist in the polarization of the arene–halogen bond similar to the Friedel– Craft reactions. It is also possible that FeCl3 interacts with the olefin and draws away the electron density from it thus making it more reactive. It is known that electron withdrawing groups on the olefin increase the rate of Heck reaction. Thus an enhancement in the activity is observed on addition of FeCl3 with different olefins, for a constant aryl bromide. As seen in the mechanism (Scheme 3), the oxidative addition of the aryl halide to the palladacycle is the
O O n Bu
HBr
SnCl4
0.25
Br +
+
FeCl3
0.3
2.2. General procedure for Heck reaction In a typical experiment 10 mmol bromobenzene, 15 mmol n-butyl acrylate, 15 mmol sodium acetate, 2 · 103 mmol palladium complex catalyst (1) shown in Scheme 1 [7], and 1.4 · 103 mmol FeCl3 were taken in a 50 ml 2-necked RBF. This was then immersed in an oil bath pre-heated to the required temperature. The reaction was then started by switching the stirrer on. Samples were withdrawn at regular intervals and analyzed by GC for conversion of bromobenzene and cinnamate formation (Scheme 2).
O
Br
NaOAc, Cat
O n Bu
FeCl3 , NM P, 150C
+ NaBr 2
O
Scheme 2.
+ HOAc
A. Sud et al. / Catalysis Communications 8 (2007) 183–186 R
R P
OAc
N
H
O
R=o-MePh
R
3
1
120
C
N Pd
OAc
P R
C
Pd
Pd
Pd(OAc) 2 /TPP
(Ph2N)2PdCl2
(Phen)PdCl 2
4
5
2
TOF, min-1
100 no FeCl3
80
with FeCl3 60 40 20 0
1
2
3 Catalyst
4
5
Fig. 2. Influence of FeCl3 on the activity for different catalyst precursors. Reaction conditions: bromobenzene – 0.399 kmol/m3, n-butyl acrylate – 0.602 kmol/m3, NaOAc – 0.6 kmol/m3, catalyst – 8.51 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, temperature – 150 C, and FeCl3 – 5.93 · 105 kmol/m3.
185
rate determining step [24]. Presence of the Lewis acid can provide a pre-activated halide wherein the C–X bond can be easily cleaved by the Pd to undergo oxidative addition. As per the mechanism, the addition of R–X to the species 1a is the slowest step. The presence of FeCl3 can promote the oxidative addition of R–Br to species 1a. In case of organic bases like tri n-butyl amine, marginal enhancement in the activity was observed. This could be due to the complexation of the Lewis acid by the free amine base, thus preventing its role in weakening the C–X bond of the aryl halide [25]. As seen in Table 1, FeCl3 was also found to improve the rate of reaction for vinylation of bromobenzene with different olefins. The role of FeCl3 in influencing the rate of the Heck reaction was assessed for different halides. The results are presented in Fig. 3. The enhancement in the rate was observed for chloro-, bromo- as well as iodo-substituted arenes. In case of bromobenzene the rate was found to increase by almost three times in the presence of FeCl3. The initial rates were enhanced by almost two times in case of 4 0 -chloroacetophenone but the overall reaction did not
Table 1 Screening of olefins and bases No.
1 2 3 4 5 6 7
Olefin
Base
Methyl acrylate Ethyl acrylate n-Butyl acrylate Styrene n-Butyl acrylate n-Butyl acrylate n-Butyl acrylate
TOF, min1
Product
NaOAc NaOAc NaOAc NaOAc NaHCO3 Na2CO3 KOAc
Methyl cinnamate Ethyl cinnamate n-Butyl cinnamate Stilbenea n-Butyl cinnamate n-Butyl cinnamate n-Butyl cinnamate
No FeCl3
With FeCl3
57 37 23 92 111 35 113
113 143 104 148 136 51 251
Reaction conditions: bromobenzene – 0.399 kmol/m3, olefin – 0.602 kmol/m3, base – 0.6 kmol/m3, catalyst (1) – 8.51 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, and temperature – 150 C, FeCl3 – 5.93 · 105 kmol/m3. a (trans + cis) Stilbene. FeCl3
baseHBr 1 base
Ar------Br------FeCl3 Br
Br 5
P
Pd
H
P
C
Pd
Ar
C 2 R
Ar R
R
Br R P
Pd
4
+ 3 P
C
Ar
Pd
Ar
+ Br-
C
Scheme 3. Mechanism of the Heck reaction catalyzed by palladacycle catalysts.
Ar-Br
186
A. Sud et al. / Catalysis Communications 8 (2007) 183–186
4. Conclusions
1000 900 800 TOF, min-1
700 600
no FeCl3 With FeCl3
500 400 300 200 100 IB # 4' -B AP 3' * -B AP 4' -B AP 4' -B AP 4' ** -B AP $
BB
2' -B AP 4' -C AP 4Ba n IB #*
0
We have reported here a new and very effective promoter for the Heck reactions which can be a cheap alternative to the quarternary ammonium salt promoters. A remarkable enhancement in the rate of palladium catalyzed Heck reactions has been achieved by introduction of catalytic amounts of Lewis acids, like FeCl3 when used in the presence of moisture (>2000 ppm). This effect has been demonstrated for a variety of bases, alkenes, aryl halides and palladium catalysts. Although the exact nature of the interaction of the Lewis acids with the substrate needs further elucidation, this strategy can nevertheless be applied to enhance the activities for Heck reactions. The same methodology may also be applicable for improving the yields of less active substrates.
ArX Fig. 3. Effect of addition of FeCl3 on the TOF of various aryl halides. Reaction conditions: ArX – 0.399 kmol/m3, BA – 0.602 kmol/m3, NaOAc – 0.6 kmol/m3, catalyst (1) – 8.51 · 105 kmol/m3, FeCl3 – 5.93 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, and temperature – 150 C. * Reaction at 140 C, ** catalyst (1) – 4.26 · 105 kmol/m3, $ catalyst (1) – 2.13 · 105 kmol/m3. # Reaction conditions – 0.198 kmol/m3, BA – 0.300 kmol/m3, NaOAc – 0.301 kmol/ m3, catalyst (1) – 8.51 · 105 kmol/m3, FeCl3 – 5.93 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, and temperature – 150 C. BAP – bromo acetophenone, CAP – chloro acetophenone, Ban – bromoanisole, IB – iodobenzene, BB – bromobenzene, and BA – butyl acrylate.
proceed beyond 30% conversions in both the cases – with or without FeCl3. Good rate enhancements were observed for the vinylation of 3 0 -bromoacetophenone (100%) and 4 0 bromoacetophenone (70%) in presence of FeCl3. The rates with 2 0 -bromoacetophenone were low as this is an intrinsically inactive system for the Heck vinylation reactions. The role of FeCl3 is not so evident in this case. In case of iodobenzene, the enhancement in the rate was more pronounced at lower temperature (140 C) than that at higher temperature (150 C). In case of vinylation of 4-bromoacetophenone, a very active substrate for these reactions [7], the presence of FeCl3 (Fe::Pd 0.7:1, Pd-8.51 · 105 kmol/m3) resulted in only a minor enhancement in the rate (6%). However, at lower catalyst concentrations (4.26 · 105 and 2.13 · 105 kmol/m3) the rate enhancement of 20% and 72%, respectively, was observed. In case of 4 0 -bromo-nitrobenzene and 4 0 -bromoaniline no enhancement in the rates of the reactions with FeCl3was observed. This is because nitro- and amino- groups can coordinate to the Lewis acid via lone pair of electrons on the nitrogen. This is consistent with the Friedel–Craft chemistry, where the alkylation of the compounds containing these two groups gives very poor results [17].
Acknowledgement Abhishek Sud is grateful to CSIR, New Delhi, for the grant for research fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
[18] [19] [20] [21] [22] [23] [24] [25]
T. Mizoroki, K. Mori, A. Ozaki, Bull. Chem. Soc. Jap. 44 (1971) 581. R.F. Heck, J.P. Nolley, J. Org. Chem. 37 (1972) 2320. R.F. Heck, J.B. Melpolder, J. Org. Chem. 41 (1976) 265. A. Spencer, J. Organomet. Chem. 247 (1983) 117. A.J. Chalk, S.A. Magennis, J. Org. Chem. 41 (1976) 273. Y. Tamaru, Y. Yamada, Z. Yoshida, J. Org. Chem. 43 (1978) 3396. A.A. Herrmann, C. Brossmer, C. Reisinger, T.H. Riermeier, K. Ofele, M. Beller, Chem. Eur. J. 3 (1997) 1357. I.P. Beletskaya, A.N. Kashin, N.B. Karlstedt, A.V. Mitin, A.V. Cheprakov, G.M. Kazankov, J. Organomet. Chem. 622 (2002) 89. A.S. Gruber, D. Zim, G. Ebeling, A.L. Monteiro, J. Dupont, Org. Lett. 2 (2000) 1287. C.S. Consorti, G. Ebeling, F.R. Flores, F. Rominger, J. Dupont, Adv. Synth. Catal. 346 (2004) 617. T. Jeffery, J. Chem. Soc. Chem. Commun. (1984) 1287. T. Jeffery, Tetrahedron Lett. 26 (1985) 2667. T. Jeffery, Tetrahedron Lett. 35 (1994) 3051. T. Jeffery, Tetrahedron Lett. 35 (1994) 4103. T. Jeffery, Tetrahedron 52 (1996) 10113. J.J. Bozell, C.E. Vogt, J. Am. Chem. Soc. 110 (1988) 2655. M.B. Smith, J. March, March’s Advanced Organic Chemistry Reactions Mechanisms and Structures, Springer Ltd., Berlin, 2001, pp. 707–711. B. Shaw, New J. Chem. (1998) 77. A. Jutand, C. Amatore, Acc. Chem. Res. 33 (2000) 314. V.P.W. Bohm, W.A. Herrmann, Chem. Eur. J. 2 (2001) 4191. G.T. Crisp, Chem. Soc. Rev. 27 (1998) 427. S. Iyer, C. Ramesh, Tetrahedron Lett. 41 (2000) 8981. S. Iyer, G.M. Kulkarni, C. Ramesh, Tetrahedron Lett. 60 (2004) 2163. A. Sundermann, O. Uzan, J.M.L. Martin, Chem. Eur. J. 7 (2001) 1703. G.A. Olah, Friedel–Crafts and Related Reactions, Interscience Publishers, John Wiley and Sons, Ltd., London, 1963, p. 583.
Rate enhancement in palladium catalyzed Heck reactions by Lewis acid promoters Abhishek Sud, Raj Madhukar Deshpande, Raghunath Vitthal Chaudhari
*
Homogeneous Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India Received 4 March 2006; received in revised form 16 May 2006; accepted 16 May 2006 Available online 7 June 2006
Abstract Significant enhancement in the rate of Heck reactions has been achieved when catalytic amounts of Lewis acid (like FeCl3) promoters in the presence of trace amounts of water (>2000 ppm) are used along with Pd complex catalysts. Experiments with different aryl halides, bases as well as olefins have been carried out to demonstrate the role of Lewis acid promoters. The increase in the reaction rates is due to the weakening of Ar–X bond by the Lewis acid in the manner similar to Friedel–Crafts reaction. 2006 Elsevier B.V. All rights reserved. Keywords: Palladium; Lewis acid; Heck reaction; Promoters
1. Introduction The palladium catalysed Mizoroki–Heck Reaction [1,2] involving the coupling of alkenes with aryl halides is one of the most powerful tools in synthetic organic chemistry for C–C bond formation, with wide ranging applications in chemical and pharmaceutical manufacturing industries [3–6]. Novel palladacycle complexes have demonstrated the feasibility of activation of less reactive substrates, and also achieve very high turnover numbers of the order of millions [7–10]. The use of promoters like quaternary ammonium salts to enhance the activity of the catalysts is well known [11–15]. These quaternary ammonium salts need to be employed in large quantities (20 mol% of the substrate) as the primary mode for rate enhancement is via phase transfer action. These salts are also costly and hence pose another challenge in making Heck reactions economically viable. The presence of nickel co-catalysts jointly with alkali metals has resulted in activation of the relatively inert chloroarenes using palladium catalysts. The enhancement is due to the conversion of small quanti*
Corresponding author. Tel.: +91 20 25902620; fax: +91 20 25893260. E-mail addresses: [email protected] (A. Sud), [email protected] (R.M. Deshpande), [email protected] (R.V. Chaudhari). 1566-7367/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.05.043
ties of the chloroarene to the iodo derivative which is more active [16]. Most of the present literature deals with the study of iodo or bromoarenes. The role of Lewis acids to assist the polarization of the alkane–halogen bond in Friedel–Crafts chemistry is wellknown [17]. Since, the Heck reaction also involves similar activation of the aryl–halogen bond, the presence of Lewis acids is likely to affect the rates of reaction [18–21]. AlCl3 and ZnCl2 have been used previously in Heck reactions but with no significant improvement in rate [22,23]. In this report, we demonstrate a substantial rate enhancement in the activity of palladium complex catalysts due to the presence of catalytic quantity of some of the Lewis acids such as FeCl3 in the presence of moisture (2000 ppm). 2. Experimental 2.1. Apparatus and chemicals The palladium acetate and tri(o-tolyl)phosphine were purchased from Aldrich Chemical Co. All the other chemicals were purchased from local commercial sources and were used as received, without any pre-treatment or purification. The reactions were carried out in a 50 ml 2-necked round bottom flask fitted with a condenser immersed in an
184
A. Sud et al. / Catalysis Communications 8 (2007) 183–186
oil bath held at desired temperature. The stirring was by means of a magnetic stirrer. The analysis of the contents of the reaction mixture was done using Agilent 6850A chromatograph, with HP-1 capillary column of 30 m length and FID. GC–MS was done using Agilent 6850N GC with 5973N mass selective detector. IR was carried out on Bio Rad FTS 175C instrument. Product identification was carried out using GC–MS analysis.
O Base, 1a, Lewis acid,
3. Results and discussion In order to demonstrate the promoting effect of Lewis acids, the experiments were conducted on vinylation of bromobenzene with n-butyl acrylate in the presence of sodium acetate base and palladium complex (1) as the catalyst precursor. The typical results showing the effect of different Lewis acid promoters are presented in Fig. 1. While, the promoting effect on catalytic activity was observed for most of the promoters used, FeCl3 clearly showed the most significant effect. The ratio of Fe:Pd was also found to be crucial. Under anhydrous conditions (<50 ppm moisture) the reaction with and without FeCl3 gave same results. This shows that it is probably the hydrated FeCl3 species that is important in promoting the coupling reaction. The highest R
R P
X Pd
Pd X P R
R
1. X = OAc
R= o-MeC6H4
Scheme 1.
OnBu
+
OnBu
NMP, Temp.
0.35 Conc, Kmol.m-3
SnCl2
Without LA
0.2 0.15 0.1 0.05 0 0
50
100 time, min
150
Fig. 1. Effect of Lewis acids on activity of Heck reactions. Reaction conditions: bromobenzene – 0.399 kmol/m3, n-butyl acrylate – 0.602 kmol/ m3, NaOAc – 0.6 kmol/m3, catalyst (1) – 8.51 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, temperature – 150 C, and LA – 5.93 · 105 kmol/m3.
enhancement in TOF (234%) was observed at Fe:Pd ratio of 0.7:1. The enhancement in the rate by a Lewis acid was in the order of the activity of the respective Lewis acid in Friedel–Craft reactions [17]. No enhancement in the rates was observed when AlCl3 was used. This may be due to the fact that AlCl3 does not form the hydrated species and instead reacts with water to form Al(OH)3, which may not promote the reaction. Promoting effect of FeCl3 was further established by carrying out experiments with different substrates and bases. Experiments were also carried out with different palladium catalyst precursors and different temperatures. The results are presented in Fig. 2 and Table 1. In all these experiments high conversions were achieved with more than 95% mass balance and a high selectivity towards the vinylated product. This was ensured by measuring concentration-time profiles for a few selected cases. The role of Lewis acids is probably to assist in the polarization of the arene–halogen bond similar to the Friedel– Craft reactions. It is also possible that FeCl3 interacts with the olefin and draws away the electron density from it thus making it more reactive. It is known that electron withdrawing groups on the olefin increase the rate of Heck reaction. Thus an enhancement in the activity is observed on addition of FeCl3 with different olefins, for a constant aryl bromide. As seen in the mechanism (Scheme 3), the oxidative addition of the aryl halide to the palladacycle is the
O O n Bu
HBr
SnCl4
0.25
Br +
+
FeCl3
0.3
2.2. General procedure for Heck reaction In a typical experiment 10 mmol bromobenzene, 15 mmol n-butyl acrylate, 15 mmol sodium acetate, 2 · 103 mmol palladium complex catalyst (1) shown in Scheme 1 [7], and 1.4 · 103 mmol FeCl3 were taken in a 50 ml 2-necked RBF. This was then immersed in an oil bath pre-heated to the required temperature. The reaction was then started by switching the stirrer on. Samples were withdrawn at regular intervals and analyzed by GC for conversion of bromobenzene and cinnamate formation (Scheme 2).
O
Br
NaOAc, Cat
O n Bu
FeCl3 , NM P, 150C
+ NaBr 2
O
Scheme 2.
+ HOAc
A. Sud et al. / Catalysis Communications 8 (2007) 183–186 R
R P
OAc
N
H
O
R=o-MePh
R
3
1
120
C
N Pd
OAc
P R
C
Pd
Pd
Pd(OAc) 2 /TPP
(Ph2N)2PdCl2
(Phen)PdCl 2
4
5
2
TOF, min-1
100 no FeCl3
80
with FeCl3 60 40 20 0
1
2
3 Catalyst
4
5
Fig. 2. Influence of FeCl3 on the activity for different catalyst precursors. Reaction conditions: bromobenzene – 0.399 kmol/m3, n-butyl acrylate – 0.602 kmol/m3, NaOAc – 0.6 kmol/m3, catalyst – 8.51 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, temperature – 150 C, and FeCl3 – 5.93 · 105 kmol/m3.
185
rate determining step [24]. Presence of the Lewis acid can provide a pre-activated halide wherein the C–X bond can be easily cleaved by the Pd to undergo oxidative addition. As per the mechanism, the addition of R–X to the species 1a is the slowest step. The presence of FeCl3 can promote the oxidative addition of R–Br to species 1a. In case of organic bases like tri n-butyl amine, marginal enhancement in the activity was observed. This could be due to the complexation of the Lewis acid by the free amine base, thus preventing its role in weakening the C–X bond of the aryl halide [25]. As seen in Table 1, FeCl3 was also found to improve the rate of reaction for vinylation of bromobenzene with different olefins. The role of FeCl3 in influencing the rate of the Heck reaction was assessed for different halides. The results are presented in Fig. 3. The enhancement in the rate was observed for chloro-, bromo- as well as iodo-substituted arenes. In case of bromobenzene the rate was found to increase by almost three times in the presence of FeCl3. The initial rates were enhanced by almost two times in case of 4 0 -chloroacetophenone but the overall reaction did not
Table 1 Screening of olefins and bases No.
1 2 3 4 5 6 7
Olefin
Base
Methyl acrylate Ethyl acrylate n-Butyl acrylate Styrene n-Butyl acrylate n-Butyl acrylate n-Butyl acrylate
TOF, min1
Product
NaOAc NaOAc NaOAc NaOAc NaHCO3 Na2CO3 KOAc
Methyl cinnamate Ethyl cinnamate n-Butyl cinnamate Stilbenea n-Butyl cinnamate n-Butyl cinnamate n-Butyl cinnamate
No FeCl3
With FeCl3
57 37 23 92 111 35 113
113 143 104 148 136 51 251
Reaction conditions: bromobenzene – 0.399 kmol/m3, olefin – 0.602 kmol/m3, base – 0.6 kmol/m3, catalyst (1) – 8.51 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, and temperature – 150 C, FeCl3 – 5.93 · 105 kmol/m3. a (trans + cis) Stilbene. FeCl3
baseHBr 1 base
Ar------Br------FeCl3 Br
Br 5
P
Pd
H
P
C
Pd
Ar
C 2 R
Ar R
R
Br R P
Pd
4
+ 3 P
C
Ar
Pd
Ar
+ Br-
C
Scheme 3. Mechanism of the Heck reaction catalyzed by palladacycle catalysts.
Ar-Br
186
A. Sud et al. / Catalysis Communications 8 (2007) 183–186
4. Conclusions
1000 900 800 TOF, min-1
700 600
no FeCl3 With FeCl3
500 400 300 200 100 IB # 4' -B AP 3' * -B AP 4' -B AP 4' -B AP 4' ** -B AP $
BB
2' -B AP 4' -C AP 4Ba n IB #*
0
We have reported here a new and very effective promoter for the Heck reactions which can be a cheap alternative to the quarternary ammonium salt promoters. A remarkable enhancement in the rate of palladium catalyzed Heck reactions has been achieved by introduction of catalytic amounts of Lewis acids, like FeCl3 when used in the presence of moisture (>2000 ppm). This effect has been demonstrated for a variety of bases, alkenes, aryl halides and palladium catalysts. Although the exact nature of the interaction of the Lewis acids with the substrate needs further elucidation, this strategy can nevertheless be applied to enhance the activities for Heck reactions. The same methodology may also be applicable for improving the yields of less active substrates.
ArX Fig. 3. Effect of addition of FeCl3 on the TOF of various aryl halides. Reaction conditions: ArX – 0.399 kmol/m3, BA – 0.602 kmol/m3, NaOAc – 0.6 kmol/m3, catalyst (1) – 8.51 · 105 kmol/m3, FeCl3 – 5.93 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, and temperature – 150 C. * Reaction at 140 C, ** catalyst (1) – 4.26 · 105 kmol/m3, $ catalyst (1) – 2.13 · 105 kmol/m3. # Reaction conditions – 0.198 kmol/m3, BA – 0.300 kmol/m3, NaOAc – 0.301 kmol/ m3, catalyst (1) – 8.51 · 105 kmol/m3, FeCl3 – 5.93 · 105 kmol/m3, solvent – NMP (moisture content 2000 ppm), total volume – 25 ml, and temperature – 150 C. BAP – bromo acetophenone, CAP – chloro acetophenone, Ban – bromoanisole, IB – iodobenzene, BB – bromobenzene, and BA – butyl acrylate.
proceed beyond 30% conversions in both the cases – with or without FeCl3. Good rate enhancements were observed for the vinylation of 3 0 -bromoacetophenone (100%) and 4 0 bromoacetophenone (70%) in presence of FeCl3. The rates with 2 0 -bromoacetophenone were low as this is an intrinsically inactive system for the Heck vinylation reactions. The role of FeCl3 is not so evident in this case. In case of iodobenzene, the enhancement in the rate was more pronounced at lower temperature (140 C) than that at higher temperature (150 C). In case of vinylation of 4-bromoacetophenone, a very active substrate for these reactions [7], the presence of FeCl3 (Fe::Pd 0.7:1, Pd-8.51 · 105 kmol/m3) resulted in only a minor enhancement in the rate (6%). However, at lower catalyst concentrations (4.26 · 105 and 2.13 · 105 kmol/m3) the rate enhancement of 20% and 72%, respectively, was observed. In case of 4 0 -bromo-nitrobenzene and 4 0 -bromoaniline no enhancement in the rates of the reactions with FeCl3was observed. This is because nitro- and amino- groups can coordinate to the Lewis acid via lone pair of electrons on the nitrogen. This is consistent with the Friedel–Craft chemistry, where the alkylation of the compounds containing these two groups gives very poor results [17].
Acknowledgement Abhishek Sud is grateful to CSIR, New Delhi, for the grant for research fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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