Catalytic Performance of a Phosphapalladacycle Bearing a Biphenyl

Organometallics 2009, 28, 6281–6287 DOI: 10.1021/om9006483

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Catalytic Performance of a Phosphapalladacycle Bearing a Biphenyl Moiety, Possessing an sp3 C-Pd Bond, toward the Heck Coupling Reaction Shirin Nadri,† Mohammad Joshaghani,*,†,‡ and Ezzat Rafiee†,‡ †

Faculty of Chemistry, Razi University, Kermanshah, 67149, Iran, and ‡Kermanshah Oil Refining Company, Kermanshah, Iran Received July 22, 2009

A new palladacycle, in which the palladium is bonded both to the phosphorus and to an sp3carbon, has been used for the Heck reaction of mono- and disubstituted olefins with aryl bromides. The S-shaped kinetic curve in the reaction between R-methylstyrene and bromobenzene in the presence of amine as base shows that the reactions probably follow a conventional Pd(0)/Pd(II) cycle rather than a Pd(II)/Pd(IV) cycle.

Introduction Palladacycles have become attractive organometallic catalysts in organic synthesis, due to their remarkable catalytic potential, easy synthetic accessibility, thermal stability, and relative low toxicity. A wide variety of novel palladacycles derived from the cyclopalladation of phosphines, phosphites, amines, imines, oximes, or thioethers have been successfully used in C-C coupling reactions.1-10 Palladacycles with PC coordination (e.g., phosphapalladacycles) (Figure 1) were found to be the most active catalysts for different types of cross-coupling reactions. Herrmann and co-worker have introduced the dimeric cyclometalated complex trans-di(μ-acetato)bis[o-(di-o-tolylphosphino)benzyl]dipalladium(II) (1) as an efficient catalyst for cross-coupling reactions with aryl bromides and electron-deficient aryl chlorides.1,2 A notable feature of this *Corresponding author. Tel/Fax: þ98 831-4274559. E-mail: [email protected]. (1) Herrmann, W. A.; Br€ ossmer, C.; Reisinger, C.-P.; Riermeier, T. H.; € Ofele, K.; Beller, M. Chem.;Eur. J. 1997, 3, 1357–1364. (2) Herrmann, W. A.; B€ ohm, V. P. W.; Reisinger, C.-P. J. Organomet. Chem. 1999, 576, 23–41. (3) Bedford, R. B.; Limmert, M. E.; Hazelwood, S. L. Chem. Commun. 2002, 2610–2611. (4) Bedford, R. B.; Hazelwood, S. L.; Albisson, D. A. Organometallics 2002, 21, 2599–2600. (5) Ohff, M.; Ohff, A.; Milstein, D. Chem. Commun. 1999, 357–358. (6) Rocaboy, C.; Gladysz, J. A. Tetrahedron 2002, 58, 4007–4014. (7) Munoz, M. P.; Martin-Matute, B.; Fernandez-Rivas, C.; Cardenas, D. J.; Echavarren, A. M. Adv. Synth. Catal. 2001, 343, 338–342. (8) Gruber, A. S.; Zim, D.; Ebeling, G.; Monteiro, A. L.; Dupont, J. Org. Lett. 2000, 2, 1287–1290. (9) Iyer, S.; Ramesh, C. Tetrahedron Lett. 2000, 41, 8981–8984. (10) Beletskaya, I. P.; Kashin, A. N.; Karlstedt, N. B.; Mitin, A. V.; Cheprakov, A. V.; Kazankov, G. M. J. Organomet. Chem. 2001, 622, 89– 96. (11) Rosol, M.; Moyano, A. J. Organomet. Chem. 2005, 690, 2291– 2296. (12) Shaw, B. L.; Perera, S. D.; Staley, E. A. Chem. Commun. 1998, 1361–1362. (13) Lee, H. M.; Zeng, J. Y.; Hu, C.-H.; Lee, M.-T. Inorg. Chem. 2004, 43, 6822–6829. r 2009 American Chemical Society

Figure 1. Palladacycles with PC coordination.

phosphapalladacycle is that it possesses an sp3 carbonpalladium bond rather than a sp2 carbon-palladium bond, which makes it different from other palladacycles. On the other hand, prompted by Buchwald’s pioneering discovery that introduced biphenyl-based phosphines as a new class of electron-rich and sterically crowded ligands (Figure 2), many useful applications of these phosphines in organic chemistry are established, such as amination,14,15 Suzuki16,17 and Negishi18 coupling processes, and the R-arylation of carbonyl-containing compounds.19 Not only have these ligands been shown to stabilize metal centers by an arene-Pd interaction,20-24 they also can easily be (14) Wolfe, J. P.; Buchwald, S. L. Angew. Chem., Int. Ed. 1999, 38, 2413–2416. (15) Wolfe, J. P.; Tomori, H.; Sadighi, J. P.; Yin, J. J.; Buchwald, S. L. J. Org. Chem. 2002, 65, 1158–1174. (16) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am. Chem. Soc. 2005, 127, 4685–4696. (17) Billingsley, K. L.; Anderson, K. W.; Buchwald, S. L. Angew. Chem., Int. Ed. 2006, 45, 3484–3488. (18) Milne, J. E.; Buchwald, S. L. J. Am. Chem. Soc. 2004, 126, 13028–13032. (19) Moradi, W. A.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 7996–8002. (20) Yin, J.; Rainka, M. P.; Zhang, X.-X.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 1162–1163. (21) Walker, S. D.; Barder, T. E.; Martinelli, J. R.; Buchwald, S. L. Angew. Chem., Int. Ed. 2004, 43, 1871–1876. (22) Barder, T. E. J. Am. Chem. Soc. 2006, 128, 898–904. (23) Barder, T. E.; Buchwald, S. L. J. Am. Chem. Soc. 2007, 129, 12003–12010. (24) Barder, T. E.; Biscoe, M. R.; Buchwald, S. L. Organometallics 2007, 26, 2183–2192. Published on Web 10/15/2009

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Figure 2. Selected examples of biphenyl-based phosphines. Scheme 1. Synthesis of Phosphapalladacycle 9

modified to increase the reactivity of the catalyst. With the above idea, we found that (2-diphenylphosphino-20 -methylbiphenyl) (8) is efficacious at promoting the Suzuki coupling reaction.25 We became interested in designing a “mixed catalyst” combining Herrmann’s framework with a sterically demanding biphenyl moiety in the hope of constructing a new and more efficient catalyst. The outcome of this construction was phosphapalladacycle 9, in which palladium is bonded both to the biphenyl-based phosphine phosphorus and to an sp3-carbon.26 Recently, we checked the efficacy of this phosphapalladacycle as a catalyst in Suzuki coupling reactions of aryl halides as well as bromoarylphosphines and bromoarylphosphine oxides.26 One of the most important tools for the formation of the carbon-carbon bond is the palladium-catalyzed coupling of olefins with aryl halides, known as the Heck reaction.27 Having demonstrated that biphenyl-based phosphine 8 as ligand provided an excellent condition for the Heck coupling reaction of monosubstituted28 and 1,1-disubstituted olefins29 with various aryl bromides, we report here the scope of phosphapalladacycle 9 in Heck coupling reactions.

Experimental Section General Remarks. All reactions were performed under an atmosphere of dry nitrogen. All chemicals purchased from Fluka and/or Merck were used without further purification. The biphenyl-based phosphine 8 and phosphapalladacycle 9 were prepared according to our previous work.25,26 1 H (200 MHz), 13C (100 MHz), and 31P (81 MHz) NMR spectra were recorded on a Bruker Avance spectrometer. Elemental analysis was performed using CHN Herause rapid model. Gas chromatography was performed on a Varian CP-3800 (column: CP-Sil 8 CB fused silica capillary column). Thin-layer chromatography on precoated silica gel fluorescent 254 nm (0.2 mm) on aluminum plates was used for monitoring the reactions. The cross-coupling products were characterized by their 1H NMR spectra or GC analysis. (25) Joshaghani, M.; Daryanavard, M.; Rafiee, E.; Xiao, J.; Baillie, C. Tetrahedron Lett. 2007, 48, 2025–2027. (26) Joshaghani, M.; Daryanavard, M.; Rafiee, E.; Nadri, Sh. J. Organomet. Chem. 2008, 693, 3135–3140. (27) Heck, R. F.; Nolley, J. P. J. Org. Chem. 1972, 37, 2320–2322. (28) Nadri, Sh.; Joshaghani, M.; Rafiee, E. Appl. Catal. A: Gen. 2009, 362, 163–168. (29) Nadri, Sh.; Joshaghani, M.; Rafiee, E. Tetrahedron Lett. 2009, 50, 5470–5473.

Nadri et al. General Procedure for Heck Coupling of Acrylate Derivatives with Aryl Bromides. A reaction tube was charged with bromobenzene (4 mmol), n-butyl methacrylate (5 mmol), Na2CO3 (6 mmol), and a solution of diethyleneglycol di-n-butyl ether (GC internal standard) under a dry nitrogen atmosphere. A solution of phosphapalladacycle 9 (0.1 mol % in 5 mL of DMAc) was added through a rubber septum, and the resulting mixture was heated at 130 °C for 1 h. The reaction mixture was then cooled to room temperature. After extraction with CH2Cl2 (3  20 mL), the combined organic layer was dried over MgSO4. The solvent was evaporated, and the crude product was analyzed by GC. General Procedure for Heck Coupling of Styrene Derivatives with Aryl Bromides. A reaction tube was charged with bromobenzene (4 mmol), R-methylstyrene (5 mmol), and Na2CO3 (6 mmol) under a dry nitrogen atmosphere. A solution of phosphapalladacycle 9 (0.1 mol % in 5 mL of DMAc) was added through a rubber septum, and the resulting mixture was heated at 130 °C for 1 h. The reaction mixture was then cooled to room temperature. After extraction with CH2Cl2 (3  20 mL), the combined organic layer was dried over MgSO4. The solvent was evaporated, and the crude product was characterized by 1H NMR spectroscopy.

Result and Discussion Optimization of the Solvent and Base. Starting with the well-defined phosphapalladacycle 9, we tested its performance as a catalyst for Heck coupling reactions using bromobenzene and R-methylstyrene as the olefin. It is well known that both solvent and base have an important effect in Heck coupling reactions. Thus, initial experiments were performed to discover the optimum solvent and base. Selected results are listed in Table 1. Investigations into the most suitable solvent for this reaction showed that reaction rates were significantly enhanced by using polar solvents, with N,N-dimethylacetamide (DMAc) being the solvent of choice. The reaction rates were also strongly dependent on the base employed, and a remarkable increase in activity was observed with inorganic bases such as Na2CO3, whereas in the case of organic bases (e.g., triethyllamine) the yield was considerably reduced. In the arylation of 1,1-disubstituted olefins such as R-methylstyrene and n-butyl methacrylate, there are two possibilities for β-hydride elimination: (a) elimination of benzylic hydrogen leads to the internal double-bond product 10, the stereochemistry of which can be E or Z, and (b) elimination of methyl hydrogen leads to the terminal product 11, which is susceptible to further arylation to give doublearylated product 12 (Scheme 2).30 We observed that like Herrmann’s palladacycle system, the nature of base significantly influences the regioselectivity of the double bond.30,31 Beller found that in Heck arylation of these olefins, catalyzed by Herrmann’s palladacycle, inorganic bases such as sodium acetate gave a mixture of the two regioisomers with the terminal olefin as the major product, whereas the internal olefin was favored by organic bases such as Bu3N or diisopropylethylamine (DIPEA) and proposed that a direct proton abstraction by the amine takes place toward the most acidic benzylic protons.30 In our system, inorganic bases such as sodium carbonate favor the formation of terminal olefins (Table 1, entry 3), while (30) Beller, M.; Riermeier, T. H. Eur. J. Inorg. Chem. 1998, 29–35. (31) Beller, M.; Riermeier, T. H. Tetrahedron. Lett. 1996, 37, 6535– 6538.

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Table 1. Optimizations of Solvent and Basea

Entry 1 2 3 4 5 6 7

Base

Solvent

Conversionb (%)

Terminalb (%)

Internalb (%)

Double arylatedb (%)

TONc

Na2CO3 Na2CO3 Na2CO3 K2CO3 NaHCO3 NaOAc NEt3

DMF toluene DMAc DMAc DMAc DMAc DMAc

88 76 97 94 87 90 65

60 53 52 50 45 50 10

24 20 40 38 34 30 50

4 3 5 6 8 10 5

880 760 970 940 870 900 650

Reaction conditions: bromobenzene (4 mmol), R-methylstyrene (5 mmol), base (6 mmol), phosphapalladacycle 9 (0.1 mol %), and solvent (5 mL), 130 °C, 1 h. b Determinated by NMR. c TON = mol product/mol catalyst. a

Table 2. Heck Reaction of Monosubstituted Olefins with Aryl Bromidesa

Entry 1 2 3 4 5d 6d b

X

R

Conversionb (%)

transb

cisb

gemb

TONc

H Me OMe Ph H Me

Ph Ph Ph Ph COOnBu COOnBu

97 95 70 83 95 90

60 60 47 53 45 43

6 5 4 5 20 22

31 30 19 25 30 25

970 950 700 830 950 900

a Reaction conditions: aryl bromide (4 mmol), olefin (5 mmol), Na2CO3 (6 mmol), phosphapalladacycle 9 (0.1 mol %), and DMAc (5 mL), 130 °C, 1 h. Determinated by NMR. c TON = mol product/mol catalyst. d Determinated by GC.

Scheme 2. Heck Arylation of 1,1-Disubstituted Olefins

in the presence of triethylamine the internal olefin is formed as the major product (Table 1, entry 7). Using a phosphinito Pd complex in DMF at 180 °C, Jensen observed that in Heck arylation of olefins in the presence of both inorganic and organic bases, the internal olefin was obtained as the major product.32 Sun and coworkers33 reported that during the arylation of 1,1-disubstituted olefins catalyzed by Co hollow nanospheres, only internal olefins were obtained, in contrast to the palladium nanoparticle-catalyzed reaction reported by Cal o et al., in which isomerized products (terminal isomer) were generated.34 Heck Reaction of Monosubstituted Olefins with Aryl Bromides. With the optimized conditions in hand, we proceeded to study the scope and limitations of the Heck coupling reactions of monosubstituted olefins using phosphapallada(32) Morales-Morales, D.; Grause, C.; Kasaoka, K.; Red on, R.; Cramer, R. E.; Jensen, C. M. Inorg. Chim. Acta 2000, 30-302, 958–963. (33) Zhou, P.; Li, Y.; Sun, P.; Zhou, L.; Bao, J. Chem. Commun. 2007, 1418–1420. (34) Cal o, V.; Nacci, A.; Monopoli, A.; Detomaso, A.; Iliade, P. Organometallics 2003, 22, 4193–4197.

cycle 9 in the reaction of a series of aryl bromides with styrene and n-butyl acrylate. Selected results are summarized in Table 2. The phosphapalladacycle 9 was an efficient catalyst for the reaction of aryl bromides with styrene. The isomer distributions of 1,1-diphenylethylene, cis-stilbene, and transstilbene are in the expected ratios, with the trans-stilbene always being formed as the main product (Table 2, entries 1-4). Interestingly, Jensen observed that the reaction of bromobenzene with styrene catalyzed by a phosphinito Pd complex produced exclusively triphenylethylene rather than the stilbenes.32 Using the phosphapalladacycle 9 with n-butyl acrylate for the synthesis of the corresponding cinnamic esters was also successful (Table 2, entries 5, 6). The time for completion of the reaction is only 1 h, less than that of Herrmann’s palladacycle 1 (48 h at similar reaction conditions).1 We used the reactions of bromobenzene with styrene and n-butyl acrylate to compare the efficacy of the palladacycle 9 with other reported palladacycle systems (Table 3). The difference in activity between various palladacycles could be partly attributed to differences in the degree of basicity of the coordinating donor heteroatoms and/or electronicsteric effects of the substituents. For instance, the presence of an electron-donating Me on the aryl groups bound to phosphorus in Herrmann’s palladacycle 1 causes the P atom be more basic than the P atom of the palladacycle 9, which has an additional phenyl group. Therefore, the oxidation state of palladium(II) in the palladacycle 1 is

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Table 3. Comparison of Heck Coupling of Bromobenzene and Different Olefins Using Palladacycle 9 and Other Palladacycles

a

trans-Isomer exclusively. b4-Bromotoluene was used.

thermodynamically stabilized by the stronger σ-donor ability of the o-tolyl groups. Heck Reaction of Disubstituted Olefins with Aryl Bromides. We also examined the catalytic activity of phosphapalladacycle 9 in Heck coupling reactions of the 1,1-disubstituted olefins (R-methylstyrene and n-butyl methacrylate) with various bromobenzenes (Table 4, entries 1-7). As seen in Table 4 conditions were found for >85% conversion of all pairs of substrates to corresponding products. It is clear that in the presence of Na2CO3 the main product is a terminal olefin. To reduce double-arylated product formation, Beller used a 5-fold excess of olefin versus aryl halide in palladacyclecatalyzed Heck reaction of 1,1-disusbstituted olefins,31 while in our system there is no need for a large amount of olefin and the double-arylated product formed as a minor product. Arylation of trans-stilbene by bromobenzene provided triphenylethylene in 65% yield (Table 4, entry 8). Reaction

of bromobenzene with triphenylethylene provided tetraphenylethylene in 25% yield after 1 h, increasing up to 40% yield after 24 h (Table 4, entries 9, 10). To gain further mechanistic information about our system, we investigated the progress of the Heck reaction during the time for the reaction of bromobenzene with R-methylstyrene catalyzed by phosphapalladacycle 9 in the presence of Na2CO3 and NEt3 (Figure 3). In agreement with Beller’s theory,31 our experiments showed that in the presence of Na2CO3 the reaction starts immediately, leading to the terminal olefin as the major isomer (Figure 3, top diagram). In contrast, in reaction with NEt3 as base an induction period was observed at the start of the reaction, which caused a sigmoidal Heck product versus time diagram (Figure 3, bottom diagram). This induction period is attributable to coordination of amine to the phosphapalladacycle 9.

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Table 4. Heck Reaction of Disubstituted Olefins with Aryl Bromidesa

Entry 1 2 3 4 5 6d 7d 8 9 10 b

X

R

R0

R00

Conversionb (%)

Terminalb (%)

Internalb (%)

Double arylatedb (%)

TONc

H Me Cl OMe Ph H Me H H H

Ph Ph Ph Ph Ph CO2nBu CO2nBu Ph Ph Ph

Me Me Me Me Me Me Me H Ph Ph

H H H H H H H Ph Ph Ph

97 92 90 75 85 95 85 65 25 40

52 51 47 51 52 50 45

40 36 38 21 27 30 26 65 25 40

5 5 5 3 6 15 15

970 920 900 750 850 950 850 650 250 400

a Reaction conditions: aryl bromide (4 mmol), olefin (5 mmol), Na2CO3 (6 mmol), phosphapalladacycle 9 (0.1 mol %), and DMAc (5 mL), 130 °C, 1 h. Determinated by NMR. c TON = mol product/mol catalyst. d Determinated by GC.

The mercury drop test is more useful for the distinction between the homogeneous and heterogeneous natures of the catalysis.36 It has been generally accepted that addition of metallic mercury to Heck reactions catalyzed by Pd nanoparticles results in the immediate poisoning of the catalyst and significantly reduces the rate of reaction, which can be regarded as evidence of a heterogeneous pathway.36 In a related evaluation, we subjected two experiments to mercury drop. Addition of a drop of mercury to the reaction mixture for the identity of the true catalyst in the presence of NEt3 as base did not affect the conversion of the reaction, which suggests that the reaction is catalyzed by a homogeneous Pd(0) complex. Probably, NEt3 playing a role of coordinating ligand thus prevents or inhibits Pd colloid formation. The addition of mercury to the reaction medium, in the presence of Na2CO3, slows the initial rate; however, no irreversible loss of catalytic activity was observed, and the reaction did reach complete conversion. In addition, no visible palladium black was observed during the reactions. These experimental results may be interpreted by a quasi-homogeneous mechanism (in the presence of inorganic base); that is, Pd complexes or colloidal particles are the active species in solution, as was already proposed earlier.37 Although a distinction between these two kinds of Pd active species is not possible on the basis of our results, their natures have been discussed in many studies. Jutand has reported that in DMF a monophosphine-Pd(0) complex, Pd0{P(o-Tol)2(o-benzyl-OAc)}(DMF), 13, is generated in situ from Herrmann’s palladacycle 1, in the absence of any reducing agents, presumably via a reductive elimination (Scheme 3).38 Herrmann has proposed the reduction of the palladacycle 1 to the anionic Pd(0) complex 15 still ligated to the benzyl moiety of the ligand (Scheme 4).39,40 (35) Shaw, B. L. New J. Chem. 1998, 22, 77–79. (36) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 198, 317–341. (37) K€ ohler, K.; Heidenreich, R. G.; Krauter, J. G. E.; Pietsch, J. Chem.;Eur. J. 2002, 8, 622–631. (38) d’Orlye, F.; Jutand, A. Tetrahedron 2005, 61, 9670–9678. (39) B€ ohm, V. P. W.; Herrmann, W. A. Chem.;Eur. J. 2001, 7, 4191– 4197. € (40) Herrmann, W. A.; Ofele, K.; Preysing, D. V.; Schneider, S. K. J. Organomet. Chem. 2003, 687, 229–248.

Figure 3. Heck product vs reaction time diagrams: (top) Na2CO3 as base; (bottom) NEt3 as base; ( terminal olefin; 9 internal; 2 double-arylated product. Reaction conditions: bromobenzene (4 mmol), R-methylstyrene (5 mmol), base (6 mmol), phosphapalladacycle 9 (0.1 mol %), and DMAc (5 mL), 130 °C.

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Beletskaya et al. in their work on NC-palladacycles, observed that prereduction of Pd(II) with a simultaneous disassembly of a palladacycle could be effected by the olefin in a noncatalytic Heck reaction.10 The conclusion is that phosphapalladacycle 9 is an actual reservoir of a catalytically active palladium(0) species of yet unidentified structure and operates by a convetional Pd(0)/ Pd(II) rather than Pd(II)/Pd(IV) cycle. Tables 5 and 6 show that the palladacyle 9 displays different properties with respect to the conventional Scheme 3. Formation of a Pd(0) Complex by Reductive Eliminationa)

Scheme 4. Reduction of Phosphapalladacycle 1 without Rupture of the Carbon-Palladium Bond

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palladium phosphine 8 complex, which was employed as an in situ catalyst by us, previously.28,29 For instance, as mentioned before, in the presence of phosphapalladacycle 9, the nature of the base dramatically influences the product distribution of the Heck arylation of 1,1-disubstituted olefins (Table 1, entries 4, 7). While using the phosphine 8 as a ligand, under the conventional reaction conditions (0.025 mol % Pd(OAc)2/phosphine 8 (1:2)), in the reaction between n-butyl methacrylate and bromobenzene, in the presence of both organic and inorganic bases (NEt3 and K2CO3, respectively), the same regioselectivities were observed and a terminal olefin was obtained, albeit with lower yield than in the case of NEt3 (Table 5, entries 5, 7). Consequently, in contrast with Heck arylation of 1,1-disubstituted olefins catalyzed by phosphapalladacycle 9, in the conventional palladium phosphine 8 system (e.g., in situ) the nature of the base has no effect on the regioselectivity of the reaction. Furthermore, Tables 5 and 6 show the superiority of phosphapalladacycle 9 in terms of conversion for Heck arylation of both mono- and disubstituted olefins with respect to conventional in situ catalyst produced from Pd(OAc)2 and phosphine 8, while both regio- and stereoselectivity were remarkably better for phosphine 8. For example, in the case of the reaction between 4-bromobiphenyl and R-methylstyrene, using Pd(OAc)2/phosphine 8, exclusively the terminal product together with a low total yield (15%) was obtained, while with phosphapalladacycle 9 excellent

Table 5. Comparison of Heck Coupling of 1,1-Disubstituted Olefins Using Phosphine 8a and Palladacycle 9b

a

Reaction conditions: from ref 29. b Reaction conditions: from Table 2. cNEt3 was used as base.

Table 6. Comparison of Heck Coupling of Monosubstituted Olefins Using Phosphine 8 a and Palladacycle 9b

a

Reaction conditions: from ref 28. b Reaction conditions: from Table 1.

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total yield (85%) was achieved, but the selectivity toward the terminal product was significantly lower and the reaction gave a mixture of the three coupling products (Table 5, entry 4). The catalytic activity differences observed with two different catalytic systems suggest that conventional in situ catalyst (Pd(OAc)2/phosphine 8) does not follow the formation of phosphapalladacycle and Pd(II) is reduced to the catalytically active Pd(0) in situ, through the oxidation of phosphine 8. In summary, the phosphapalladacycle 9 is an efficient catalyst for the olefinic coupling of aryl bromides with both

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mono- and the disubstituted olefins. The S-shaped kinetic curve in the reaction between R-methylstyrene and bromobenzene in the presence of amine as base shows that the catalytic cycle in the reactions under study is probably a conventional Pd(0)/Pd(II). In other words, the palladacycle 9 might be a catalyst precursor to active palladium(0) complexes.

Acknowledgment. We thank the Razi University Research Council and Kermanshah Oil Refining Company for support of this work.