Pyrazolin-5-ylidene Palladium(II) Complexes: Synthesis

Article pubs.acs.org/Organometallics

Pyrazolin-5-ylidene Palladium(II) Complexes: Synthesis, Characterization, and Application in the Direct Arylation of Pentafluorobenzene Jan C. Bernhammer and Han Vinh Huynh* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore S Supporting Information *

ABSTRACT: Ten palladium(II) complexes bearing a pyrazolin-5-ylidene ligand have been synthesized by oxidative addition and silver carbene transfer pathways. The weakly bound acetonitrile ligand in the initially obtained trans-[PdBr2(MeCN)(Pyry)] complex (6, Pyry = 1-phenyl-2,3-dimethylpyrazolin-5-ylidene) could be replaced by other donor ligands, and additional NHC ligands were introduced either by silver carbene transfer reactions or via reaction with in situ generated free carbenes. Using our previously reported 13C NMR-based electronic parameter, the pyrazolin-5-ylidene ligand is estimated to be among the most strongly donating ligands on our scale so far. The complexes obtained were employed as catalysts for the direct arylation of pentafluorobenzene with moderate to good yields under optimized conditions.

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INTRODUCTION Two decades after their first isolation in free form,1 Nheterocyclic carbenes (NHCs) not only play a prominent role as ligands in transition metal catalysis2 but also find application as organocatalysts,3 in drug development,4 and in material sciences.5 The widespread use of NHCs can be attributed to their superior properties when compared to the phosphine ligand, which they are increasingly replacing.6 NHCs are strongly σ-donating ligands with easily tunable steric and electronic properties, and in their metal-bound forms they exhibit low sensitivity toward air and moisture. While initial research efforts were mainly focused on imidazolin-2-ylidene and imidazolidin-2-ylidene complexes, recent investigations go beyond these classical NHCs. Among the heterocyclic frameworks that have been used for the synthesis of NHCs are triazoles,7 benzimidazoles,8 and N,O-9 and N,S-heterocycles.10 Of special interest are nonclassical NHCs, which possess only one or no nitrogen atom stabilizing the carbene center.11 Since the σ-donor strength of NHCs depends mainly on the energy of their HOMO, the removal of electronegative substituents from the ylidene carbon leads to more strongly donating ligands.12,13 NHCs derived from heterocycles such as pyridine,14 Bertrand’s cyclic alkyl amino carbenes,15 or mesoionic imidazolin-4-ylidenes16 have been shown to be exceptionally strong σ-donors. However, the coordination chemistry of these compounds has not been explored as extensively as in the case of imidazolin-2-ylidene and imidazolidin-2-ylidene, and despite an increase in recent years, there are fewer examples of catalytic applications. Pyrazolin-5-ylidenes (Pyry), as well as pyrazolin-3-ylidenes, are strongly σ-donating NHCs, although less powerful than pyrazolin-4-ylidenes,17 in which no nitrogen atom is adjacent to © 2012 American Chemical Society

the carbene center. Early examples of pyrazole-derived NHC complexes of iron and rhodium were reported by the groups of Raubenheimer and Herrmann as early as 1996.18 Since then, a few pyrazolin-3-ylidene complexes of chromium,19 ruthenium,20 rhodium,21,18b palladium,13,21,19c platinum,19c and gold19c have been prepared. However, there is no general route to access complexes of pyrazole-derived NHCs, and the existing methodologies suffer from severe limitations. Besides deprotonation of the pyrazolium precursor salts by metalbound alkoxide ligands, either oxidative addition approaches or metal-templated ligand synthesis on chromium and further transmetalation steps have been employed. Only in one case20 has the feasibility of transfer reactions using an in situ generated silver complex been demonstrated. In order to provide easy access to a diverse selection of pyrazolin-5-ylidene Pd(II) complexes, we explored both oxidative addition and silver carbene transfer protocols in detail. The mono-NHC complex trans-[PdBr2(Pyry)(MeCN)] (6, Pyry = 1-phenyl-2,3-dimethylpyrazolin-5-ylidene) proved to be especially versatile, providing an excellent starting material to introduce other donor ligands, as well as enable us to synthesize the first examples of hetero-bis(NHC) complexes incorporating pyrazole-derived NHCs. To gain some insight into the catalytic properties of these new complexes, their activity in the direct arylation of pentafluorobenzene has been explored. Received: May 27, 2012 Published: June 29, 2012 5121

dx.doi.org/10.1021/om300464b | Organometallics 2012, 31, 5121−5130

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the 19F NMR spectrum. The trans-arrangement of the phosphine ligands could be unambiguously assigned on the basis of the 31P NMR spectrum, which revealed a single resonance at 22.9 ppm. In the 1H NMR spectrum, an upfield shift from 2.67 ppm in the ligand precursor to 2.05 ppm in the ligand was observed for the CH3 group on the pyrazole ring, while the NCH3 group experienced a slightly smaller shift from 3.77 ppm to 3.34 ppm. Remarkably, H-4 was only negligibly influenced by the oxidative addition, with an upfield shift of only 0.04 ppm being observed when compared to the ligand precursor. 13 C NMR spectroscopy revealed the Ccarbene resonance at 165.6 ppm, which is close to the 161.1 ppm chemical shift reported by Herrmann et al. for a structurally closely related complex.21a The pyrazolium salt 3 proved to be acidic enough to react directly with silver(I) oxide24 in the presence of a bromide source. Attempts to isolate the resulting silver NHC complex were futile due to the low stability of this species. However, its formation could be conclusively proven by ESI spectroscopy. Transmetalation to [PdBr2(MeCN)2] and [PdBr2(iPr2-bimy)]2 (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene) occurred readily and in good yields. Both complexes were obtained as yellow solids. While the hetero-bis(NHC) complex trans-[PdBr2(iPr2-bimy)(Pyry)] (7) is readily soluble in common polar organic solvents, with the exception of ethereal solvents, the acetonitrile adduct trans[PdBr2(MeCN)(Pyry)] (6) shows poor solubility in noncoordinating solvents. This can be attributed to a tendency to form highly insoluble aggregates after dissociation of the weakly bound acetonitrile ligand. In the 1H NMR spectra of complexes 6 and 7, the disappearance of the doublet at 7.91 ppm caused by the acidic H-5 in the ligand precursor 3 shows the success of the complexation. The proton adjacent to Ccarbene experiences a slight upfield shift of 0.2−0.3 ppm and shows a singlet instead of a doublet after complexation. Small upfield shifts can also be observed for the methyl groups, similar to the changes observed upon formation of 5. In the 13C NMR spectrum, the Ccarbene resonance is observed at 150.0 ppm for complex 6. Since acetonitrile is a poorly donating and weakly bound ligand, a low electron density at palladium is to be expected, which in turn would give rise to a comparatively far upfield resonance for Ccarbene. For the heterobis(NHC) complex 7, two Ccarbene resonances are observed. The resonance at 182.8 ppm was assigned to iPr2-bimy, while the resonance at 175.4 ppm was assigned to the pyrazolin-5ylidene ligand, based on comparison with structurally similar complexes. In previous works, we have proposed the use of the i Pr2-bimy Ccarbene chemical shift in trans-[PdBr2(iPr2-bimy)L] complexes as an efficient probe to characterize the donor strength of the ligand L.7e,13,23 The observed value of 182.8 ppm for complex 7 makes the pyrazolin-5-ylidene ligand the most strongly donating ligand to be characterized by this method. This is in line with the fact that only one electronegative substituent is present in the α-position to Ccarbene, giving rise to an exceptionally strongly donating NHC ligand. Complex 6 proved to be a versatile intermediate for the synthesis of other complexes. Since the acetonitrile ligand is only weakly bound, it can easily be replaced by other ligands such as pyridine or triphenylphosphine, as well as by a second NHC ligand (Scheme 4). The reaction of 6 with one equivalent of pyridine gave the pyridine adduct trans-[PdBr2(Py)(Pyry)] (8) in quantitative

RESULTS AND DISCUSSION Ligand Precursor Synthesis. For oxidative addition reactions, suitable precursor salts bearing a halide or another leaving group in the 5-position had to be synthesized. 5Chloropyrazole 2a could be obtained by microwave heating of pyrazolone 1 in neat trichlorophosphine, and 5-trifluoromethansulfonylpyrazole 2b was obtained according to a reported procedure22 from the same starting material (Scheme 1). Scheme 1. Preparation of Pyrazolium Salts

Subsequent alkylation of the pyrazoles 2a and 2b using Meerwein’s salt furnished the desired pyrazolium salts 4a and 4b. Only moderate yields of alkylated materials could be obtained, which can be understood in light of the electrondeficient nature of the functionalized pyrazoles, leading to a reduced nucleophilicity.23 In contrast, the alkylation of the more electron-rich 3-methyl-1-phenyl-1H-pyrazole (3) under identical conditions gave 4c in quantitative yield. All pyrazolium salts are colorless, slightly hygroscopic solids. The solubility in hydrocarbons and ethers is low, while they are soluble in chlorinated and polar organic solvents. Synthesis of Palladium Complexes. Pd(II) complexes were obtained by two different approaches. Oxidative addition to [Pd(PPh3)4] as a suitable Pd(0) source in refluxing dichloromethane gave the cationic complex trans-[PdCl(PPh3)2(Pyry)]BF4/OTf (5) in moderate yield, if the 5triflatopyrazolium salt 4b was used as ligand precursor and tetrabutylammonium chloride (TBACl) was present as a source of chloride (Scheme 2). In the absence of TBACl, the same Scheme 2. Oxidative Addition to Tetrakis(triphenylphosphine)palladium(0)

complex 5 was obtained in small amounts, with the chlorido ligand presumably being provided by the solvent. In contrast, the 5-chloropyrazolium salt 4a failed to undergo oxidative addition under identical conditions, which reflects the lower reactivity of the C−Cl bond. Complex 5 was obtained as an off-white, microcrystalline solid with good solubility in chlorinated solvents and low solubility in hydrocarbons and ethereal solvents. Elemental analysis revealed that two-thirds of the counteranion was BF4−, while one-third was OTf−. Both anions were also observed in 5122


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Scheme 3. Silver Transfer Reaction to Palladium

Scheme 4. Ligand Exchange Reactions of Complex 6

trans-[PdBr2(IPr)(Pyry)] (11), trans-[PdBr2(SIPr)(Pyry)] (12), and trans-[PdBr2(IMes)(Pyry)] (13) in excellent yields. Complexes 7 and 10−13 are the first reported examples of hetero-bis(NHC) complexes bearing a pyrazolin-5-ylidene ligand. All of the complexes 8−14 are yellow solids, which are insoluble in hydrocarbons, ethereal solvents, and water, while being soluble in chlorinated solvents and polar organic solvents. It is noteworthy that 9 and 14, the only two complexes found to be adopting a cis-arrangement (vide inf ra), showed a markedly lower solubility than trans-configured complexes. The 1H NMR shifts of 8−10 and 14 show only little variation when compared to 6. All resonances assignable to the pyrazolin-5-ylidene ligand fall within ±0.1 ppm of their respective signals in the spectrum of 6. Complexes 11, 12, and 13 exhibit a somewhat different spectral pattern, with the pyrazolin-5-ylidene signals being shifted upfield by 0.2−0.3 ppm. In these three complexes, a pronounced broadening of signals due to the isopropyl or methyl moieties on the aryl rings can be observed, indicating a hindered rotation caused by steric crowding. In the 13C NMR spectra, a clear trend emerges (Table 1). A downfield shift of the Ccarbene resonance is observed when the weakly donating acetonitrile coligand is replaced by more strongly donating ligands. This phenomenon is well documented26 and has been exploited in our group to provide precise information about the donor strength of the respective

yield after 30 min at ambient temperature. In a similar fashion, the triphenylphosphine complex cis-[PdBr2(PPh3)(Pyry)] (9) was accessible. The replacement of acetonitrile with various NHC ligands was explored as well, and a selection of structurally diverse hetero-bis(NHC) complexes with various benzimidazole-, imidazole-, and imidazoline-based NHC ligands was prepared. Silver carbene transfer methodology gave good results in the case of benzhydrylbenzimidazolium bromide (Bh2-bimy·HBr) and salt 4c, giving rise to complexes trans-[PdBr2(Bh2-bimy)(Pyry)] (10) and cis-[PdBr2(Pyry)2] (14) in excellent yields. However, attempts to synthesize the homo-bis(NHC) complex 14 in a single step by reacting [PdBr2(MeCN)2] with two equivalents of the silver NHC intermediate were unsuccessful, and only small amounts of the ligand precursor were reisolated. The failure to detect any soluble palladium species points to the formation of highly insoluble species, which could not be separated from the inorganic byproducts of the silver carbene transfer reaction. The silver carbene transfer approach failed in the case of imidazolium and imidazolinium bromides. Instead of the expected complexes 11−13, only the known silver complexes of IPr, SIPr, and IMes could be isolated.25 Apparently, the comparatively high stability of these complexes prevented the transmetalation reaction from occurring. To circumvent these problems, the free carbenes were generated in situ from their precursor salts by deprotonation with potassium tert-butoxide and subsequently reacted with 6 to give the desired products 5123


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Table 1. Ccarbene Chemical Shifts [ppm] in Complexes 5−14

a

complex

Pyry

5 6 7 8 9 10 11 12 13 14

165.6 150.0 175.4 153.0 166.5 174.3 173.2 173.4 172.6 181.8a

NHC

182.8 (iPr2-bimy)

189.7 (Bh2-bimy) 179.4 (IPr) 207.6 (SIPr) 176.6 (IMes)

Recorded in DMSO-d6. Figure 1. Molecular structure of 5 showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Pd1−C1 1.987(3), Pd1−P1 2.333(1), Pd1−P2 2.33(1), Pd1−Cl1 2.346(1); C1−Pd1−P1 90.2(1), C1−Pd1−P2 94.8(1), P1− Pd1−Cl1 86.80(4), P2−Pd1−Cl1 88.23(4), C1−Pd1−Cl(1) 176.9(1), P1−Pd1−P2 174.97(4).

coligands.7e,13,23 The chemical shift of 150.0 ppm in 6 changes to 154.1 ppm for the pyridine adduct 8 and to 166.5 ppm for the triphenylphosphine complex 9. For the bis(NHC) complexes, two Ccarbene resonances are observed. The resonance assigned to the pyrazolin-5-ylidene Ccarbene is observed in the narrow range of 172.6−174.3 ppm. However, even here the relationship between coligand donor strength and chemical shift can still be clearly observed. In order of increasing donor strength,13 IMes < IPr < SIPr < Bh2-bimy < iPr2-bimy, a downfield shift of the carbene resonance is found. Finally, Ccarbene in 14 resonates at 181.8 ppm. Even though a direct comparison is not possible due to a different coordination geometry and a different solvent being used, this still reflects the exceptionally high donating abilities of the pyrazolin-5ylidene ligand. The second Ccarbene present in 10−13 shows a higher variance in chemical shift. The imidazolin-2-ylidene Ccarbene in 11 and 13 is observed at 179.4 and 176.6 ppm, respectively, while the imidazolidin-2-ylidene Ccarbene in 12 resonates at 207.6 ppm. For the benzimidazolin-2-ylidene Ccarbene in 10, a resonance occurs at 189.7 ppm, which is comparable to the value found for complex 7. Crystal Structures. Single crystals suitable for X-ray diffraction analysis were obtained from slow evaporation of concentrated dichloromethane/hexane or dichloromethane/ acetonitrile solutions for 5, 6, 7, 11, 12, and 14 or by slow diffusion of diethyl ether into concentrated dichloromethane solutions for 8 and 9. All complexes under scrutiny were found to adopt a trans-arrangement, with the notable exception of the mixed NHC−phosphine complex 9 and the homo-bis(NHC) complex 14, which prefer a cis-arrangement. This can be attributed to the pronounced transphobia effect in mixed NHC−phosphine complexes27 and the relatively small steric demand of the pyrazolin-5-ylidene ligand, respectively. In complex 5, the two phosphine ligands are located trans to each other. The steric bulk of the phosphine ligands leads to a slight distortion of the ideal square-planar arrangement, with P1−Pd1−C1 angles deviating from 90°. The Pd−C and Pd−Cl bond lengths in 5 are close to the values observed by Herrmann et al. for a structurally related complex.21a The solvent adducts 6 and 8 adopt a less distorted squareplanar arrangement, with the solvent occupying the transposition with respect to the Pyry ligand. The Pd−C bond lengths of 1.950(2) and 1.962(2) Å, respectively, are somewhat shorter when compared to those of 5. This reflects the higher Lewis acidity of the palladium center in the solvent adducts. In the pyridine adduct 8, the planes defined by the pyridine and the NHC ring system are slightly twisted by 6.5(2)° with

respect to each other. Further bond distances and angles of these remarkably similar complexes are given in Table 2. The hetero-bis(NHC) complexes 7, 11, and 12 adopt a trans-arrangement of the NHC ligands. Due to a stronger transinfluence exerted by the additional NHC ligand, a longer Pd− CPyry bond distance ranging between 1.989(5) Å in 11 and 2.030(5) Å in 7 is observed in these complexes. The bond distance between palladium and the second Ccarbene shows less variation and ranges from 2.031(5) Å in 7 to 2.049(4) Å in 12. The planes defined by the NHC ligands are twisted with respect to each other. In 11 and 12, which incorporate the more bulky IPr or SIPr ligand, the torsion angle between the planes was found to be 30.9(6)° and 43.2(4)°, respectively. In contrast, the less bulky iPr2-bimy ligand in 7 induces a twist of only 17.8(5)°. A detailed summary of bond distances and angles is given in Table 3. A cis-arrangement was observed for complexes 9 and 14. The Pd−C bonds in these complexes are very similar with a distance of 1.977(5) Å in the case of 9 and an average bond distance of 1.979 Å in the homo-bis(NHC) complex 14. Despite the bulky phosphine ligand, the torsion angle between the plane defined by the bromido ligands and the NHC plane was found to be a nearly ideal, 87.6(5)°. In contrast, the two NHC ligands in 14 are tilted by 52.3(4)° and 57.0(4)° and arranged in an antiorientation in order to avoid unfavorable steric interactions. Selected bond distances and angles are given in Table 4. Catalysis. Previously, we have shown that the direct arylation of pentafluorobenzene can be catalyzed by heterobis(NHC) complexes.7e Since this type of reaction provides biaryls without the byproducts associated with classical crosscoupling reactions, interest in it has increased recently.28 We were interested to explore the catalytic activity with our newly developed class of pyrazolin-5-ylidene complexes. In our previous work, the coupling of the substrates was conducted at 120 °C with 0.5 mol % precatalyst in the presence of K2CO3 as base and in 0.2 mL of DMA as solvent. Since the formation of catalytically inactive colloidal palladium species is the main pathway for catalyst inactivation, we diluted the reaction mixture by using a larger amount of solvent (2 mL) when compared to the initial conditions. This resulted in a significant increase in yield (Table 5, entries 2 and 3), and all 5124


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Figure 2. Molecular structures of 6 and 8 showing 50% probability ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity.

accelerated, while at lower temperatures a marked drop of reaction rates occurs. A screening of different bases gave no improved yields (entries 5−9). While inorganic bases other than potassium carbonate gave moderate yields, the use of organic bases lead to the loss of catalytic activity. This can be rationalized by the crucial role played by the carbonate anion in the proposed mechanism.29 Solvents other than DMA lead to rapid formation of palladium black, and consequently, no or very little product could be isolated from the reaction mixture (entries 10−12). Having ascertained that the initial conditions were indeed serendipitously chosen, we went on to explore the substrate scope. Electron-rich bromoarenes reacted smoothly and in good yields if the donating group was located in meta- or paraposition. Ortho-substituents were not tolerated, however, and no product was obtained for these substrates. Electronwithdrawing substituents on the bromoarene had an adverse effect on the reactivity, resulting in moderate to poor yields of isolated product. It is noteworthy that in the presence of excess pentafluorobenzene, no doubly arylated product was obtained for para-bromochlorobenzene. Obviously, the arylation of chloroarenes is not feasible using our catalyst. However, 1,3dibromobenzene gave doubly arylated product exclusively. When heteroaromatic bromoarenes were used as substrate, no product could be isolated.

Table 2. Bond Distances [Å] and Angles [deg] in Solvent Adduct Complexes 6 and 8 bond parameter

6

8·CH2Cl2

Pd1−C1 Pd1−N3 Pd1−Br1 Pd1−Br2 C1−Pd1−Br1 C1−Pd1−Br2 Br1−Pd1−N3 Br2−Pd1−N3 C1−Pd1−N3 Br1−Pd1−Br2

1.950(2) 2.083(3) 2.4289(3) 2.4279(3) 89.08(7) 88.88(7) 92.18(6) 89.86(6) 178.50(9) 177.80(1)

1.962(2) 2.104(4) 2.4376(4) 2.4294(4) 89.12(6) 88.65(6) 92.39(5) 89.91(5) 177.88(8) 176.83(1)

Table 3. Bond Distances [Å] and Angles [deg] in Heterobis(NHC) Complexes Complexes 7, 11, and 12 bond parameter

7

11

12 (cocrystal)

Pd1−C1 Pd1−CPyry Pd1−Br1 Pd1−Br2 C1−Pd1−Br1 C1−Pd1−Br2 Br1−Pd1−CPyry Br2−Pd1−CPyry C1−Pd1−CPyry Br1−Pd1−Br2

2.031(5) 2.030(5) 2.4325(7) 2.4339(7) 91.2(1) 89.4(1) 88.3(1) 91.2(1) 179.0(2) 174.55(2)

2.040(4) 1.989(5) 2.4343(9) 2.4430(9) 95.5(1) 89.2(1) 87.2(2) 88.2(2) 1175.4(2) 174.85(3)

2.049(4) 2.007(4) 2.4267(5) 2.4289(4) 88.59(9) 96.68(9) 88.3(1) 86.54(9) 173.8(1) 174.66(2)

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CONCLUSION We have developed an easy and high-yielding synthetic pathway to structurally diverse pyrazolin-5-ylidene palladium(II) complexes. The exceptionally high donor strength of the pyrazolin5-ylidene ligand has been confirmed using 13C NMR spectroscopy. A series of pyrazolin-5-ylidene palladium(II) complexes has been synthesized and fully characterized, including the first examples of hetero-bis(NHC) complexes featuring this ligand. These complexes have been shown to be catalytically active in the direct arylation of pentafluorobenzene, with hetero-bis(NHC) complexes or mixed NHC−phosphine complexes giving the best results. Further fine-tuning of the catalytic activity by incorporating other suitable NHC ligands or by replacing the bromido ligands seems promising, and research directed toward such complexes is underway in our laboratory.

further reactions were carried out under these improved conditions. An initial screening revealed that all complexes 5− 14 were catalytically active, while in the absence of catalyst no product was formed. The diversity of the catalysts under scrutiny makes it difficult to discern clear-cut structure−activity relationships for the complexes, but certain trends emerge. The best yields were obtained for the hetero-bis(NHC) complexes 7, 11, and 12 and the mixed phosphine−NHC complex 9. It seems that two different strongly donating ligands favor the reaction, especially if the sterical bulk of one of these ligands falls within a certain window of reactivity. The somewhat less bulky IMes ligand in complex 13 and the extremely bulky benzhydryl-substituted NHC in 10 do not seem to fulfill this second criterion and, hence, perform less well. We then attempted to optimize the reaction conditions further using the best-performing catalyst, 12. The variation of temperature (Table 6, entries 1−4 and 13) leads to decreased yields. At higher temperatures, catalyst decomposition seems

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EXPERIMENTAL SECTION

General Considerations. All reactions were carried out without precautions to exclude air and moisture, unless stated otherwise.

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Figure 3. Molecular structures of 7, 11, and 12 showing 50% probability ellipsoids. Hydrogen atoms (with the exception of those at C4/5 in IPr and SIPr) and solvent molecules are omitted for clarity. 12 was obtained as a cocrystal with [PdBr2(SIPr)]2. Only the molecule of interest is shown here; the full structure is available from the Supporting Information.

Table 4. Bond Distances [Å] and Angles [deg] in cisComplexes 9 and 14 bond parameter

9·CH2Cl2

14·CH2Cl2

Pd1−C1 Pd1−P1/C12 Pd1−Br1 Pd1−Br2 C1−Pd1−P1/C12 C1−Pd1−Br1 Br1−Pd1−Br2 Br2−Pd1−P1/C12 C1−Pd1−Br2 P1/C12−Pd1−Br1

1.977(5) 2.255(2) 2.4971(8) 2.4878(6) 92.0(2) 88.0(2) 91.06(2) 89.91(4) 171.1(2) 173.47(4)

1.978(4) 1.980(4) 2.4935(6) 2.5191(5) 89.3(2) 87.9(1) 82.23(2) 90.7(1) 173.8(1) 176.6(1)

signals relative to tetramethylsilane (1H, 13C) or externally to CF3CO2H (19F) or 85% H3PO4 (31P). ESI mass spectra were measured using a Finnigan LCQ spectrometer (ESI). Elemental analyses were performed on an Elementar Vario Micro Cube

Table 5. Catalyst Screening for the Direct Arylation of Pentafluorobenzenea

Figure 4. Molecular structures of 9 and 14 showing 50% probability ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity.

entry

catalyst

yield (%)b

1 2c 3 4 5 6 7 8 9 10 11 12

5 5 6 7 8 9 10 11 12 13 14

0 39 57 45 68 55 71 44 69 80 54 42

a

Reaction conditions: 0.5 mol % precatalyst, K2CO3 (1.1 equiv), pentafluorobenzene (1.1 equiv), 4-bromotoluene (0.6 mmol), DMA (2 mL), 120 °C, 24 h. bIsolated yields, average of two runs. c200 μL of DMA, 0.3 mmol of 4-bromotoluene.

Solvents were used as received or dried using standard procedures. NMR spectra were recorded on a Bruker AV 300 spectrometer. The chemical shifts (δ) were internally referenced to the residual solvent 5126


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Table 6. Optimization of Reactions Conditionsa entry

catalyst

solvent

base

T (°C)

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13c

12 12 12 12 12 12 12 12 12 12 12 12 12

DMA DMA DMA DMA DMA DMA DMA DMA DMA DMSO toluene 1,4-dioxane DMA

K2CO3 K2CO3 K2CO3 K2CO3 Cs2CO3 KHCO3 KOH NEt3 DBU K2CO3 K2CO3 K2CO3 K2CO3

80 100 130 140 120 120 120 120 120 120 120 120 120

36 51 65 58 61 42 49 0 1 0 2 0 3

layers were successively washed with aqueous sodium hydroxide solution (2 N, 2 × 40 mL) and water (40 mL), dried over sodium sulfate, and filtered, and the solvent was removed in vacuo. The product was obtained as a colorless liquid (4.49 g, 23.3 mmol, 81%). 1 H NMR (300 MHz, CDCl3): δ 7.57−7.51 (m, 2 H, Ar−H), 7.50− 7.43 (m, 2 H, Ar−H), 7.41−7.35 (m, 1 H, Ar−H), 6.19 (s, 1 H, CH), 2.32 (s, 3 H, CH3). The analytical data were in accordance with reported values.31 3-Methyl-1-phenyl-1H-pyrazol-5-yltrifluoromethanesulfonate (2b). Pyrazolone 1 (600 mg, 3.44 mmol, 1.00 equiv) was dissolved in anhydrous dichloromethane (40 mL). N-Phenylbis(trifluoromethanesulfoneimide) (1.47 g, 4.13 mmol, 1.20 equiv) and triethylamine (1.04 g, 1.40 mL, 10.3 mmol, 3.00 equiv) were added, and the mixture was heated to reflux for 3 h. After cooling to ambient temperature, it was diluted by addition of dichloromethane (20 mL), washed with water (60 mL), dried over sodium sulfate, filtered, and concentrated in vacuo. The residue was purified by column chromatography (silica gel, ethyl acetate/hexane, 1:3) to give the product as a pale yellow liquid (1.04 g, 3.40 mmol, 99%). 1H NMR (300 MHz, CDCl3): δ 7.54−7.33 (m, 5 H, Ar−H), 6.14 (s, 1 H, CH), 2.34 (s, 3 H, CH3). The analytical data were in accordance with reported values.22 5-Chloro-2,3-dimethyl-1-phenylpyrazolium Tetrafluoroborate (4a). 2a (384 mg, 2.00 mmol, 1.00 equiv) was dissolved in anhydrous dichloromethane (6 mL). Trimethyloxonium tetrafluoroborate (473 mg, 3.20 mmol, 1.6 equiv) was added, and the mixture was heated to reflux for 20 h under a dry nitrogen atmosphere. After cooling to ambient temperature, the solvent was removed in vacuo. The residue was washed with diethyl ether (3 mL) and dried in vacuo. The product was obtained as a white solid (350 mg, 1.19 mmol, 59%). 1 H NMR (300 MHz, DMSO-d6): δ 7.87−7.72 (m, 5 H, Ar−H), 7.29 (s, 1 H, CH), 3.68 (s, 3 H, NCH3), 2.58 (s, 3 H, CH3). 13C{1H} NMR (75 MHz, DMSO-d6): δ 149.0 (CN), 135.5 (Cl−C−N), 133.0,

a

Reaction conditions: 0.5 mol % precatalyst, base (1.1 equiv), pentafluorobenzene (1.1 equiv), 4-bromotoluene (0.6 mmol), solvent (2 mL), 24 h. bIsolated yields, average of two runs. c1.5 h, microwave heating. elemental analyzer at the Department of Chemistry, National University of Singapore. IPr·HBr,30a−c SIPr·HBr,30d IMes·HBr,30a−c Bh2-bimy·HBr,30e and [PdBr2(iPr2-bimy)]230f were prepared according to known procedures. 5-Chloro-3-methyl-1-phenyl-1H-pyrazole (2a). Pyrazolone 1 (5.00 g, 28.7 mmol, 1.00 equiv) and phosphorus trichloride (5.91 g, 43.1 mmol, 1.50 equiv) were thoroughly mixed and heated to 160 °C under microwave irradiation for 2 h. After having cooled to ambient temperature, ice water (30 mL) was added slowly. The aqueous phase was extracted with diethyl ether (2 × 45 mL). The combined organic

Table 7. Substrate Scope for the Direct Arylation of Pentafluorobenzenea

a Reaction conditions: 0.5 mol % precatalyst, K2CO3 (1.1 equiv), pentafluorobenzene (1.1 equiv), 4-bromotoluene (0.6 mmol), DMA (2 mL), 120 °C, 24 h. bIsolated yields, average of two runs. cK2CO3 (2.2 equiv), pentafluorobenzene (2.2 equiv). dMonocoupled product. eDoubly coupled product.

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CH), 3.43 (s, 3 H, NCH3), 2.29 (s, 3 H, CH3), 2.01 (s, 3 H, CH3). 13 C{1H} NMR (75 MHz, CDCl3/CD3CN): δ 150.0 (Ccarbene), 144.5 (CN), 137.0, 131.1, 130.0, 129.9, 129.8 (Ar−C), 115.9 (CN), 34.3 (NCH3), 12.3 (CH3), 2.1 (CH3). Anal. Calcd for C13H15Br2N3Pd: C, 32.56; H, 3.15; N, 8.76. Found: C, 32.98; H, 3.32; N, 8.45. MS (ESI): m/z 397 [M − Br]+. trans-Dibromo(2,3-dimethyl-1-phenyl-1H-pyrazolin-5ylidene)(1,3-diisopropylbenzimidazolin-2-ylidene)palladium(II) (7). 4c (52 mg, 0.20 mmol, 1.00 equiv), tetrabutylammonium bromide (64 mg, 0.20 mmol, 1.00 equiv), and silver(I) oxide (23 mg, 0.10 mmol, 0.50 equiv) were dissolved in dichloromethane (25 mL) and stirred shielded from light for 15 h at ambient temperature. [PdBr2(iPr2-bimy)]2 (94 mg, 0.10 mmol, 0.50 equiv) was added, and the solution was stirred at ambient temperature for another 2 h shielded from light. The resulting suspension was filtered over a short plug of Celite and concentrated in vacuo. The solid residue was washed with diethyl ether (5 mL) and dried in vacuo. The product was obtained as a yellow solid (103 mg, 0.160 mmol, 80%). 1H NMR (300 MHz, CDCl3): δ 7.90−7.80 (m, 2 H, Ar−H), 7.68−7.60 (m, 3 H, Ar− H), 7.50−7.40 (m, 2 H, Ar−H), 7.14−7.06 (m, 2 H, Ar−H), 6.51 (s, 1 H, 4-H), 5.72 (sept, 3JH−H = 7.0 Hz, 2 H, CH(CH3)2), 3.49 (s, 3 H, NCH3), 2.31 (s, 3 H, CH3), 1.57 (d, 3JH−H = 7.0 Hz, 12 H, CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 182.8 (NCN), 175.4 (NCCH), 144.0 (CN), 138.6, 134.2, 131.0, 130.5, 129.8, 122.1, 116.1, 113.0 (Ar−C), 53.9 (CH(CH3)2), 34.1 (NCH3), 21.6 (CH(CH3)2), 12.7 (CH3). Anal. Calcd for C24H30Br2N4Pd: C, 44.99; H, 4.72; N, 8.74. Found: C, 44.96; H, 4.73; N, 8.84. MS (ESI): m/z 559 [M − Br]+. trans-Dibromo(2,3-dimethyl-1-phenylpyrazolin-5-ylidene)(pyridine)palladium(II) (8). Pyridine (7.9 mg, 8.0 μL, 0.10 mmol, 1.00 equiv) and 6 (50 mg, 0.10 mmol, 1.00 equiv) were dissolved in dichloromethane (5 mL). The solution was stirred for 30 min at ambient temperature. Then, the solvent was evaporated in vacuo. The product was obtained as a yellow solid (52 mg, 0.10 mmol, >99%). 1H NMR (300 MHz, CDCl3): δ 8.80−8.75 (m, 2 H, Ar−H), 7.84−7.77 (m, 2 H, Ar−H), 7.65−7.57 (m, 4 H, Ar−H), 7.22−7.14 (m, 2 H, Ar− H), 6.50 (s, 1 H, CH), 3.48 (s, 3 H, NCH3), 2.31 (s, 3 H, CH3). 13 C{1H} NMR (75 MHz, CDCl3): δ 154.1 (Ccarbene), 153.0 (Ar−C), 144.4 (CN), 137.9, 137.7, 131.3, 130.4, 130.1, 124.8 (Ar−C), 116.9 (CH), 34.5 (NCH3), 12.9 (CH3). Anal. Calcd for C16H17Br2N3Pd·CH2Cl2: C, 33.89; H, 3.18; N, 6.97. Found: C, 33.94; H, 3.16; N, 6.94. MS (ESI): m/z 436 [M − Br]+. cis-Dibromo(2,3-dimethyl-1-phenylpyrazolin-5-ylidene)(triphenylphosphine)palladium(II) (9). Triphenylphosphine (26 mg, 0.10 mmol, 1.00 equiv) and 6 (50 mg, 0.10 mmol, 1.00 equiv) were dissolved in dichloromethane (5 mL). The solution was stirred for 30 min at ambient temperature. Then, the solvent was evaporated in vacuo. The residue was washed with diethyl ether (5 mL) and dried. The product was obtained as a yellow solid (71 mg, 0.10 mmol, >99%). 1H NMR (300 MHz, CD2Cl2): δ 7.77−7.41 (m, 8 H, Ar−H), 7.35−7.24 (m, 12 H, Ar−H), 6.39 (s, 1 H, CH), 3.31 (s, 3 H, NCH3), 2.27 (s, 3 H, CH3). 13C{1H} NMR (75 MHz, CD2Cl2): δ 166.5 (Ccarbene), 145.0 (CN), 136.0 (Ar−C), 135.6 (d, JC−P = 11 Hz, Ar− H), 131.4, 130.7 (Ar−C), 130.5 (d, JC−P = 3 Hz, Ar−H), 130.3, 129.7 (Ar−C), 127.7 (d, JC−P = 11 Hz, Ar−C), 115.8 (CH), 34.4 (NCH3), 11.9 (CH3). 31P NMR (158 MHz, CDCl3): δ 27.3. Anal. Calcd for C29H27Br2N2Pd: C, 49.71; H, 3.88; N, 4.00. Found: C, 50.01; H, 3.64; N, 4.06. MS (ESI): m/z 619 [M − Br]+. trans-Dibromo(2,3-dimethyl-1-phenyl-1H-pyrazolin-5ylidene)(1,3-dibenzhydrylbenzimidazolin-2-ylidene)palladium(II) (10). 1,3-Dibenzhydrylbenzimidazolium bromide (Bh2bimy·HBr) (106 mg, 0.20 mmol, 1.00 equiv) and silver(I) oxide (23 mg, 0.10 mmol, 0.50 equiv) were dissolved in dichloromethane (10 mL) and stirred shielded from light for 15 h at ambient temperature. 6 (96 mg, 0.20 mmol, 1.00 equiv) was added, and the solution was stirred at ambient temperature for another 1 h shielded from light. The resulting suspension was filtered over a short plug of Celite and concentrated in vacuo. The solid residue was washed with diethyl ether (5 mL) and dried in vacuo. The product was obtained as a yellow solid (179 mg, 0.200 mmol, >99%). 1H NMR (300 MHz, CDCl3): δ 8.46−

130.6, 130.2, 129.1 (Ar−C), 107.8 (CH), 35.5 (NCH3), 11.9 (CH3). 19 F NMR (170 MHz, DMSO-d6): δ −77.23, −77.18 (BF4). Anal. Calcd for C11H12BClF4N2: C, 44.86; H, 4.11; N, 9.51. Found: C, 44.57; H, 4.13; N, 9.52. MS (ESI): m/z 207 [M − BF4]+. 5-Trifluoromethanesulfonyl-2,3-dimethyl-1-phenylpyrazolium Tetrafluoroborate (4b). 2b (3.34 g, 10.9 mmol, 1.00 equiv) was dissolved in anhydrous dichloromethane (50 mL). Trimethyloxonium tetrafluoroborate (1.94 g, 13.1 mmol, 1.20 equiv) was added, and the mixture was heated to reflux for 10 h under a dry nitrogen atmosphere. After cooling to ambient temperature, the solvent was removed in vacuo. The residue was dissolved in THF and precipitated by addition of diethyl ether, collected by filtration, and dried in vacuo. The product was obtained as an off-white solid (3.36 g, 8.23 mmol, 76%). 1H NMR (300 MHz, CDCl3): δ 7.82−7.62 (m, 5 H, Ar−H), 6.67 (s, 1 H, CH), 3.77 (s, 3 H, NCH3), 2.67 (s, 3 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 151.6 (CN), 145.4 (O−C−N), 134.2, 131.5, 129.8, 128.8 (Ar−C), 98.9 (CH), 35.7 (NCH3), 13.4 (CH3); CF3 not observed. 19F NMR (170 MHz, CDCl3): δ −77.83 (BF4), −77.78 (BF4), −4.38 (OTf). Anal. Calcd for C12H12BF7N2O3S: C, 35.32; H, 2.96; N, 6.86. Found: C, 35.09; H, 2.61; N, 6.78. MS (ESI): m/z 321 [M − BF4]+. 2,3-Dimethyl-1-phenyl-1H-pyrazolium Tetrafluoroborate (4c). 3 (1.00 g, 6.32 mmol, 1.00 equiv) was dissolved in anhydrous dichloromethane (15 mL). Trimethyloxonium tetrafluoroborate (1.12 g, 7.59 mmol, 1.20 equiv) was added, and the mixture was heated to reflux for 15 h under a dry nitrogen atmosphere. After cooling to ambient temperature, the solvent was removed in vacuo and the residue was washed with diethyl ether (5 mL). The product was obtained as a white solid (1.64 g, 6.31 mmol, >99%). 1H NMR (300 MHz, CDCl3): δ 7.91 (d, 3JH−H = 3.0 Hz, 1 H, 5-H), 7.71−7.51 (m, 5 H, Ar−H), 6.69 (d, 3JH−H = 2.9 Hz, 1 H, 4-H), 3.75 (s, 3 H, NCH3), 2.56 (s, 3 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 149.9 (C-3), 137.2 (C-5), 133.3, 133.2, 131.2, 128.5 (Ar−C), 109.4 (C-4), 35.2 (NCH3), 13.0 (CH3). 19F NMR (170 MHz, CDCl3): δ −76.92 (BF4), −76.87 (BF4). Anal. Calcd for C11H13BF4N2: C, 50.81; H, 5.04; N, 10.77. Found: C, 50.43; H, 4.76; N, 10.62. MS (ESI): m/z 173 [M − BF4]+. trans-Chloro(2,3-dimethyl-1-phenylpyrazolin-5-ylidene)bis(triphenylphosphine)palladium(II) Tetrafluoroborate/triflate (5). Tetrakis(triphenylphosphine)palladium (1.16 g, 1.00 mmol, 1.00 equiv) was dissolved in anhydrous dichloromethane (15 mL), and 2b (429 mg, 1.05 mmol, 1.05 equiv) and tetrabutylammonium chloride (292 mg, 1.05 mmol, 1.05 equiv) were added. The mixture was heated to reflux for 10 h under a dry nitrogen atmosphere. After cooling to ambient temperature, the solvent was evaporated in vacuo. The residue was washed with a diethyl ether/THF mixture (1:1, 5 mL) and dried in vacuo. After recrystallization from CHCl3/hexane, the product was obtained as a pale yellow solid (477 mg, 0.515 mmol, 52%). 1H NMR (300 MHz, CDCl3): δ 7.80−7.29 (m, 35 H, 2 × PPh3, Ph), 6.63 (s, 1 H, CH), 3.35 (s, 3 H, NCH3), 2.05 (s, 3 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 165.6 (Ccarbene), 148.7 (CN), 136.3 (Ar−C), 135.0 (t, JC−P = 6.3 Hz, Ar−C), 132.1, 131.3, 130.4, 130.0, 129.7 (Ar−C), 129.3 (t, JC−P = 5.3 Hz, Ar−C), 127.3 (CH), 36.2 (NCH3), 12.7 (CH3). 19F NMR (170 MHz, CDCl3): δ −77.06 (BF4), −77.00 (BF4), −2.22 (OTf). 31P NMR (158 MHz, CDCl3): δ 22.9. Anal. Calcd for C47H42ClN2P2Pd(BF4)2/3OTf1/3: C, 60.08; H, 4.47; N, 2.96. Found: C, 59.50; H, 4.83; N, 3.73. MS (ESI): m/z 837 [M − BF4]+. trans-Dibromo(2,3-dimethyl-1-phenylpyrazolin-5-ylidene)(acetonitrile)palladium(II) (6). 4c (650 mg, 2.50 mmol, 1.00 equiv), silver(I) oxide (290 mg, 1.25 mmol, 0.50 equiv), and tetrabutylammonium bromide (806 mg, 2.50 mmol, 1.00 equiv) were suspended in dichloromethane (60 mL) and stirred at ambient temperature for 15 h shielded from light. PdBr2 (660 mg, 2.50 mmol, 1.00 equiv) was dissolved in acetonitrile (60 mL) and heated to 50 °C for 20 min. The resulting palladium complex solution was slowly added to the reaction mixture. After stirring for an additional 2 h, the mixture was filtered over a short plug of Celite, and the solvent was concentrated to 10 mL. The precipitate was collected and dried under reduced pressure. The product was obtained as a yellow solid (937 mg, 1.95 mmol, 78%). 1H NMR (300 MHz, CDCl3): δ 7.72−7.78 (m, 5 H, Ar−H), 6.40 (s, 1 H, 5128


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cis-Dibromobis(2,3-dimethyl-1-phenyl-1H-pyrazolin-5ylidene)palladium(II) (14). 2,3-Dimethyl-1-phenyl-1H-pyrazolium tetrafluoroborate (52 mg, 0.20 mmol, 1.00 equiv), silver(I) oxide (23 mg, 0.10 mmol, 0.50 equiv), and tetrabutylammonium bromide (64 mg, 0.10 mmol, 1.00 equiv) were suspended in dichloromethane (10 mL) and stirred at ambient temperature for 15 h shielded from light. 6 (96 mg, 0.20 mmol, 1.00 equiv) was added, and stirring was continued for 2 h. Then, the solution was filtered over a short plug of Celite and concentrated in vacuo. The residue was washed with a mixture of diethyl ether and dichloromethane (3:1, 5 mL). The product was obtained as a yellow solid (113 mg, 0.185 mmol, 93%). 1 H NMR (300 MHz, CD2Cl2): δ 7.58−7.52 (m, 6 H, Ar−H), 7.50− 7.44 (m, 4 H, Ar−H), 6.29 (s, 2 H, CH), 3.39 (s, 6 H, NCH3), 2.30 (s, 6 H, CH3). 13C{1H} NMR (75 MHz, DMSO-d6): δ 181.8 (Ccarbene), 136.7 (CN), 130.2, 129.3, 128.9, 128.3 (Ar−C), 79.2 (CH), 34.5 (NCH3), 11.4 (CH3). Anal. Calcd for C22H24Br2N4Pd·CH2Cl2: C, 39.71; H, 3.77; N, 8.05. Found: C, 41.20; H, 4.37; N, 7.68. MS (ESI): m/z 531 [M − Br]+. General Procedure for the Direct Arylation of Pentafluorobenzene. A Schlenk tube was charged with precatalyst (3.0 μmol, 0.5 mol %), base (0.66 mmol, 1.1 equiv), and aryl halide (0.60 mmol, 1.0 equiv) if it was a solid. DMA (2 mL) was added, and the vessel was evacuated and backfilled with dry nitrogen three times. Pentafluorobenzene (74 μL, 111 mg, 0.66 mmol, 1.1 equiv) and aryl halide (0.60 mmol, 1.0 equiv), if it was a liquid, were added, and the mixture was immersed in a preheated oil bath for 24 h. After cooling to ambient temperature, dichloromethane (4 mL) was added, and the resulting suspension was filtered over a short plug of Celite. The solution was concentrated under reduced pressure, and the residue was purified by column chromatography (silica gel, hexane, or hexane/diethyl ether). X-ray Diffraction Studies. X-ray data were collected with a Bruker AXS SMART APEX diffractometer, using Mo Kα radiation at 100(2) K (6, 7, 8·CH2Cl2, 9·CH2Cl2, 11, 12, 14·CH2Cl2) or 223(2) K (5), with the SMART suite of programs.32 Data were processed and corrected for Lorentz and polarization effects with SAINT33 and for absorption effect with SADABS.34 Structural solution and refinement were carried out with the SHELXTL suite of programs.35 The structure was solved by direct methods to locate the heavy atoms, followed by difference maps for the light, non-hydrogen atoms. All hydrogen atoms were put at calculated positions. All non-hydrogen atoms were generally given anisotropic displacement parameters in the final model. A summary of crystallographic data is given in Tables 2−4 and the Supporting Information.

8.36 (m, 2 H, CH), 7.84−7.78 (m, 2 H, Ar−H), 7.49−7.17 (m, 23 H, Ar−H), 6.83 (s, 4 H, Ar−H), 6.53, (s, 1 H, CH), 3.39 (s, 3 H, NCH3), 2.28 (s, 3 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 189.7 (NCN), 174.3 (NCCH), 144.0 (CN), 138.5, 138.4, 134.9, 130.6, 130.3, 130.0, 129.8, 128.8, 128.2, 122.4 (Ar−C), 116.2 (CH), 114.3 (Ar−C), 68.1 (CH), 34.1 (NCH3), 12.2 (CH3). Anal. Calcd for C44H38Br2N4Pd: C, 59.44; H, 4.31; N, 6.30. Found: C, 60.28; H, 4.98; N, 6.01. MS (ESI): m/z 809 [M − Br]+. trans-Dibromo(IPr)(1-phenyl-2,3-dimethylpyrazolin-5ylidene)palladium(II) (11). IPr·HBr (70 mg, 0.15 mmol, 1.00 equiv) and potassium tert-butoxide (17 mg, 0.15 mmol, 1.00 equiv) were dissolved in anhydrous THF (15 mL) and stirred for 30 min at ambient temperature under a dry nitrogen atmosphere. Then 6 (72 mg, 0.15 mmol, 1.00 equiv) was added, and stirring was continued for another 2 h. The solvent was removed in vacuo, and the solid residue was taken up in dichloromethane (20 mL) and filtered over a short plug of Celite. The solvent was removed in vacuo. The product was obtained as a yellow solid (124 mg, 0.149 mmol, >99%). 1H NMR (300 MHz, CDCl3): δ 7.50−7.42 (m, 2 H, Ar−H), 7.39−7.22 (m, 7 H, Ar−H), 7.18−7.10 (m, 2 H, Ar−H), 6.94 (s, 2 H, HCCH), 6.06 (s, 1 H, CH), 3.25 (s, 3 H, NCH3), 3.14 (sept, 3JH−H = 6.9 Hz, 4 H, CH(CH3)2), 2.12 (s, 3 H, CH3), 1.39−1.24 (m, 12 H, CH(CH3)2), 1.01 (d, 3JH−H = 6.9 Hz, 12 H, CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 179.4 (NCN), 173.2 (NCCH), 147.3 (Ar−C), 143.8 (C N), 137.8, 136.8, 130.0, 129.6, 129.4, 129.2 (Ar−C), 124.7 (C-sp2), 124.3 (Ar−C), 116.1 (CH), 34.1 (NCH3), 29.2 (CH(CH3)2), 26.9 (CH(CH3)2), 24.0 (CH(CH3)2), 12.6 (CH3). Anal. Calcd for C38H48Br2N4Pd·CH2Cl2: C, 51.36; H, 5.53; N, 6.14. Found: C, 51.90; H, 5.82; N, 5.92. MS (ESI): m/z 747 [M − Br]+. trans-Dibromo(SIPr)(1-phenyl-2,3-dimethylpyrazolin-5ylidene)palladium(II) (12). SIPr·HBr (71 mg, 0.15 mmol, 1.00 equiv) and potassium tert-butoxide (26 mg, 0.23 mmol, 1.50 equiv) were dissolved in anhydrous THF (15 mL) and stirred for 30 min at ambient temperature under a dry nitrogen atmosphere. Then 6 (72 mg, 0.15 mmol, 1.00 equiv) was added, and stirring was continued for another 2 h. The solvent was removed in vacuo, and the solid residue was taken up in dichloromethane (20 mL) and filtered over a short plug of Celite. The solvent was removed in vacuo. The product was obtained as a dark yellow solid (107 mg, 0.129 mmol, 86%). 1H NMR (300 MHz, CDCl3): δ 7.42−7.28 (m, 5 H, Ar−H), 7.25−7.17 (m, 4 H, Ar−H), 7.11−7.03 (m, 2 H, Ar−H), 6.03 (s, 1 H, CH), 3.87 (s, 4 H, CH2), 3.55 (sept, 3JH−H = 6.6 Hz, 4 H, CH(CH3)2), 3.23 (s, 3 H, NCH3), 2.10 (s, 3 H, CH3), 1.51−1.22 (m, 12 H, CH(CH3)2), 1.16 (d, 3JH−H = 6.6 Hz, 12 H, CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 207.6 (NCN), 173.4 (NCCH), 148.3 (Ar−C), 143.7 (C N), 137.7, 137.2, 129.7, 129.4, 129.2, 129.1, 124.8 (Ar−C), 116.1 (CH), 54.4 (CH2−CH2), 34.1 (NCH3), 29.1 (CH(CH3)2), 27.4 (CH(CH3)2), 25.0 (CH(CH3)2), 12.6 (CH3). Anal. Calcd for C38H50Br2N4Pd·CH2Cl2: C, 51.25; H, 5.73; N, 6.13. Found: C, 50.62; H, 5.82; N, 6.41. MS (ESI): m/z 749 [M − Br]+. trans-Dibromo(IMes)(1-phenyl-2,3-dimethylpyrazolin-5ylidene)palladium(II) (13). IMes·HBr (58 mg, 0.15 mmol, 1.00 equiv) and potassium tert-butoxide (17 mg, 0.15 mmol, 1.00 equiv) were dissolved in anhydrous THF (15 mL) and stirred for 30 min at ambient temperature under a dry nitrogen atmosphere. Then 6 (72 mg, 0.15 mmol, 1.00 equiv) was added, and stirring was continued for another 2 h. The solvent was removed in vacuo, and the solid residue was taken up in dichloromethane (20 mL) and filtered over a short plug of Celite. The solvent was removed in vacuo. The product was obtained as a yellow solid (107 mg, 0.144 mmol, 96%). 1H NMR (300 MHz, CDCl3): δ 7.51−7.38 (m, 3 H, Ar−H), 7.35−7.27 (m, 2 H, Ar− H), 6.97−6.88 (m, 4 H, Ar−H), 6.83 (s, 2 H, HCCH), 6.08 (s, 1 H, CH), 3.27 (s, 3 H, NCH3), 2.40 (s, 6 H, CH3), 2.23 (sbr, 12 H, CH3), 2.14 (s, 3 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 176.6 (NCN), 172.6 (NCCH), 143.5 (CN), 137.9, 136.7, 129.7, 129.6, 129.6, 129.3 (Ar−C), 123.5 (C(sp2)), 123.4 (Ar−C), 115.6 (CH), 34.0 (NCH3), 21.9 (CH3), 20.3 (CH3), 12.6 (CH3). Anal. Calcd for C32H36Br2N4Pd: C, 51.74; H, 4.88; N, 7.54. Found: C, 51.62; H, 5.09; N, 7.24. MS (ESI): m/z 663 [M − Br]+.

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ASSOCIATED CONTENT

* Supporting Information S

H and 13C NMR spectra, CIF files for complexes 5, 6, 7, 8, 9, 11, 12, and 14, summary of crystallographic data, and plot of 12 and cocrystallized [PdBr2(SIPr)]2. This material is available free of charge via the Internet at http://pubs.acs.org. 1

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National University of Singapore for financial support (Grant No. R143-000-410-112 and SINGA scholarship) and the CMMAC staff of the Department of Chemistry for technical assistance.

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REFERENCES

(1) (a) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991, 113, 361. (b) Arduengo, A. J.; Goerlich, J. R.; Marshall, W. J. J. Am. Chem. Soc. 1995, 117, 11027.

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Organometallics

Article

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