a Family of Silyl-Substituted PdâNHC Complexes - ACS Publications

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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[Pd(4‑R3Si-IPr)(allyl)Cl], a Family of Silyl-Substituted Pd−NHC Complexes: Catalytic Systems for the Buchwald−Hartwig Amination Norihisa Fukaya,*,†,‡ Tomoteru Mizusaki,§,∥ Kouhei Hatakeyama,†,‡ Yuto Seo,†,‡ Yuuya Inaba,†,§ Kazuhiro Matsumoto,† Vladimir Ya. Lee,⊥ Yukio Takagi,∥ Junpei Kuwabara,§ Takaki Kanbara,§ Yoong-Kee Choe,† and Jun-Chul Choi*,†,§

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†

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Department of Chemistry, Faculty of Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan § Tsukuba Research Center for Energy Materials Science (TREMS), Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ∥ N.E. CHEMCAT Corporation, 678 Ipponmatsu, Numazu, Shizuoka 410-0314, Japan ⊥ Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan S Supporting Information *

ABSTRACT: A family of Pd−IPr complexes (1Pd−6Pd) featuring electropositive and bulky R3Si substituents at the 4-position of the NHC ring was prepared and fully characterized. The catalytic performance of 1Pd (R3Si = SiMe3) and 3Pd (R3Si = Si(allyl)Me2) in the Buchwald−Hartwig amination was excellent, notably outperforming those of other previously reported catalytic systems by requiring shorter reaction times, lower catalyst loadings, and milder reaction conditions. Furthermore, a systematic evaluation of both the electronic and steric influences of the 4-R3Si-IPr ligand on the overall catalytic performance of 1Pd−6Pd revealed that electronic rather than steric factors play a dominant role.

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INTRODUCTION

changeably in favor of the oxidative addition and the reductive elimination processes. The rates of the two key steps of the overall amination catalytic cycle, namely oxidative addition and reductive elimination, are mostly dependent on the electronic and steric factors of ligand L. As very strong σ-donors manifesting a remarkably shielding steric pattern, N-heterocyclic carbenes (NHC) have emerged as a reasonable alternative and very prominent alternative of tertiary phosphines, in many aspects outperforming them as ligands of choice.4 It is well-established that the very strong σ-electron-donating ability of the NHC ligands stabilizes the low-coordinated catalytically active LPd0 species, thus accelerating the oxidative addition step,5,6 whereas the high steric crowding caused by the bulky NHC ligands facilitates the reductive elimination of the product in the last step of the catalytic cycle.6,7 Recently, César and Lavigne et al. reported their rational synthetic strategy for novel highly effective Buchwald−Hartwig amination Pd−NHC precatalysts, based on a simple

Making bonds to carbon is the ultimate goal of organic chemistry. Among a variety of such bonds, the C−N bond (i.e., amine formation) is of paramount importance. The Pdcatalyzed C−N cross-coupling reaction of amines and aryl halides, thus forming arylamines that are ubiquitous in natural products and pharmaceuticals, is one of the numerous synthetic procedures available for the making of carbon− nitrogen bonds. This benchmark case is called the Buchwald− Hartwig amination reaction, named after the milestone reports of Buchwald and Hartwig published first in 1994,1 and is an indispensable tool in the organic chemists’ synthetic arsenal.2,3 Within its commonly recognized catalytic cycle, the palladium precatalyst first forms LPd0, which is the true catalytically active species. This then undergoes an oxidative addition of an aryl halide, followed by an amine coordination at Pd and a subsequent deprotonation of the resulting intermediate by a base. The catalytic cycle is finally completed with a reductive elimination, thereby forming the final product, arylamine, and regenerating the LPd0 intermediate. Notably, the amination rate-determining step is strongly case-sensitive, being inter© XXXX American Chemical Society

Received: October 17, 2018

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DOI: 10.1021/acs.organomet.8b00757 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

derivatives [4-(R3Si)-IPr] 1−6 under mild reaction conditions. Notably, except for [4-(Me3Si)-IPr] 1, which has been previously isolated by Bertrand12 and Robinson,11 the isolation and full characterization of the obtained NHC derivatives have not been previously reported before. Synthesis of the Pd Complexes. As a Pd precatalyst for the Buchwald−Hartwig amination, we chose a mononuclear πallyl palladium complex [(NHC)Pd(allyl)Cl] due to its straightforward synthesis and high thermal stability. 13 Typically, such complexes can be prepared via a simple “free carbene” synthetic protocol developed by Nolan and coworkers,14 which includes the addition of 2 equiv of NHC ligand to readily available commercial [Pd(allyl)Cl]2. The activation of such a precatalyst resulted in the generation of transient [(NHC)Pd0], which is a true catalytic species, thereby promoting a wide range of cross-coupling reactions (including Buchwald−Hartwig amination) using low catalyst loadings and mild reaction conditions.13 Therefore, herein we applied Nolan’s procedure to prepare our Pd precatalyst complexes by reacting commercial [Pd(allyl)Cl]2 with 4-(R3Si)-IPr 1−6 at room temperature. When performed in THF, the reaction was typically completed within 1 h, forming a series of the corresponding NHC−palladium complexes {[4-(R3Si)-IPr]Pd(allyl)Cl} 1Pd−6Pd, in moderate to good yields (Scheme 2).

modification of a 1,3-bis(2,6-diisopropylphenyl)-2H-imidazol2-ylidene (IPr) ligand at the 4- or 4,5-positions of the NHC backbone using Me2N-substituents.8 Notably, the remarkable effect of the amino groups enhanced the overall electrondonating ability of the IPr ligand, thereby improving the catalytic activity of the Pd−NHC precatalysts. As a result, the amination of p-chloroanisole with morpholine catalyzed by {Pd[4,5-(Me2N)2-IPr](3-chloropyridine)Cl2} proceeded very smoothly, reaching full conversion in just 2 h. Such a remarkable catalytic performance is quite comparable to those of the best Pd−NHC complexes known for arylamination, as well as superior phosphine−Pd systems.8 Furthermore, {Pd[4,5-(Me2N)2-IPr](3-chloropyridine)Cl2} can also very effectively catalyze amination of aryl tosylates, which is hardly achieved with other catalytic systems.9 Moreover, by further increasing the steric bulk of the NHC backbone, the same authors recently developed novel Pd−IPr precatalyst featuring a i-Pr2N-substituent at the 4-position of the NHC cycle, thereby achieving an excellent catalytic performance in the arylation of even bulkier primary amines.10 Being inspired by the above-mentioned results on the modification of the NHC backbone of Pd−IPr precatalysts with electron-donating dialkylamino groups significantly improving their catalytic activity,8−10 we decided to explore the catalytic potential of precatalysts having not π- (like NR2) but σ-electron-donating group at the 4-position of the NHC ligand. In this regard, the highly electropositive silyl-group R3Si was the substituent of choice, as such a σ-donating group was anticipated to remarkably enhance the electron-donating ability of the [4-(R3Si)-IPr] ligand and accordingly improve the overall catalytic performance of the new palladium complexes featuring such NHC ligands.

Scheme 2. Synthesis of {[4-(R3Si)-IPr]Pd(allyl)Cl} Complexes 1Pd−6Pd

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RESULTS AND DISCUSSION Synthesis of the NHC Ligands. A series of novel NHC ligands, containing a bulky silyl group at the 4-position of the five-membered ring heterocycle, was readily available by a straightforward procedure, which included an initial lithiation of the commercially available precursor IPr with n-BuLi in hexane, thus forming the corresponding lithium salt, followed by its reaction with silyl chloride in THF to afford the corresponding 4-silyl-substituted NHC ligands (Scheme 1). The synthesis of lithium derivative Li−IPr was performed according to a straightforward gram-order-scale protocol reported by Robinson,11 which allowed for its quantitative isolation and full characterization. Coupling of Li−IPr with silyl chloride R3SiCl resulted in a high-yield formation of NHC

The isolation of complexes 1Pd−6Pd was also rather straightforward: a standard workup procedure and simple recrystallization from dry hexane were employed to afford the analytically pure samples. The structures of complexes 1Pd− 6Pd were undoubtedly established by means of spectroscopic and analytical methods, and two of them, 1Pd and 2Pd, were crystallographically characterized (Figures 1 and 2, respectively). As is diagnostic for such Pd(allyl) complexes, a distorted square-planar coordination at the palladium center was observed in both 1Pd and 2Pd. The allyl group was trihapto-coordinated to the metal center, and the imidazolidene ring of the NHC ligand was practically planar with a rather negligible folding (N1−C6−C7−N2 torsion angle: −0.40° for 1Pd and 1.28° for 2Pd). The bond lengths and angles in both complexes were unexceptional. More specifically, the bond between the Pd atom and the NHC ligand, Pd1−C1, which had a length of 2.040(2) Å for 1Pd and 2.0552(16) Å for 2Pd, was quite typical to the one reported for other [(NHC)Pd(allyl)Cl] complexes, which is usually in the narrow range of 2.00−2.04 Å (e.g., 2.040(11) Å for

Scheme 1. Synthesis of Li−IPr Salt and Its Subsequent Silylation Forming Target [4-(R3Si)-IPr] Ligands 1−6

B


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Organometallics

rings (on the sterically unhindered site of the IPr ligand) was still able to rotate freely about the Cipso−N bond. Such an NMR distinction of 1Pd in different solvents might be ascribed to the potential complexation of THF, which breaks the molecular symmetry. Catalytic Performance of NHC−Pd Complexes 1Pd− 6Pd in the Buchwald−Hartwig Amination. Next, we tested the catalytic performance of newly synthesized Pd− NHC complexes 1Pd−6Pd in the Buchwald−Hartwig amination. In this respect, we reacted chlorobenzene with din-butylamine and each of the synthesized complexes in 1,2dimethoxyethane (DME) at 70 °C and obtained the desired product, N,N-di-n-butylaniline (Scheme 3, Table 1). Two of Figure 1. Crystal structure of complex 1Pd, {[4-(Me3Si)-IPr]Pd(allyl)Cl}. ORTEP view, thermal ellipsoids are given at 30% probability, H atoms are omitted. The central atom of the allyl ligand is evenly disordered between the two sites C3 and C4 (50%:50% occupancy ratio), and only one of them (C3) is shown here. Selected bond lengths (Å): Pd1−C1 = 2.040(2), Pd1−Cl1 = 2.3605(8), C1−N1 = 1.360(3), C1−N2 = 1.359(3), N1−C6 = 1.412(3), N2−C7 = 1.388(3), C6−C7 = 1.360(3), C6−Si1 = 1.899(2). Selected bond angles (°) C1−N1−C6 = 112.41(16), N1− C6−C7 = 104.00(17), C6−C7−N2 = 108.64(18), C7−N2−C1 = 110.74(16), N2−C1−N1 = 104.20(17).

Scheme 3. Catalytic Performance of Pd−NHC Complexes 1Pd−6Pd

Table 1. Comparison of the Catalytic Performances of Precatalysts 1Pd−6Pd and parent Pd−IPra

a

Figure 2. Crystal structure of complex 2Pd, {[4-((EtO)3Si)IPr]Pd(allyl)Cl}. ORTEP view, thermal ellipsoids are given at 30% probability, H atoms are omitted. The central atom of the allyl ligand is evenly disordered between the two sites C3 and C4 (50%:50% occupancy ratio), and only one of them (C3) is shown here. Selected bond lengths (Å): Pd1−C1 = 2.0552(16), Pd1−Cl1 = 2.3628(4), C1−N1 = 1.358(2), C1−N2 = 1.3624(19), N1−C6 = 1.413(2), N2− C7 = 1.388(2), C6−C7 = 1.358(2), C6−Si1 = 1.8606(17). Selected bond angles (°) C1−N1−C6 = 112.19(12), N1−C6−C7 = 104.45(14), C6−C7−N2 = 108.22(14), C7−N2−C1 = 110.98(13), N2−C1−N1 = 104.12(13).

entry

precatalyst

yield (%, by GC)

1 2 3 4 5 6 7

1Pd 2Pd 3Pd 4Pd 5Pd 6Pd Pd−IPr

64 28 80 34 12 15 31

Yields are given after 30 min.

the tested complexes, namely, 1Pd and 3Pd, showed the highest catalytic activity giving the target product in 64% for {[4-(Me 3 Si)-IPr]Pd(allyl)Cl} 1Pd and 80% for {[4(Me2(allyl)Si)-IPr]Pd(allyl)Cl} 3Pd (Table 1, entries 1 and 3, respectively). Importantly, this remarkable catalytic performance greatly exceeds that of the commercially available parent complex Pd−IPr, which yields the product in 31% under the same catalytic conditions. In an effort to shed light on the origin of the difference in the catalytic activity of palladium catalysts 1Pd−6Pd, we evaluated the effect of their both electronic and steric factors on the overall catalytic process. Initially, we estimated of the importance of electronic factors by using the Tolman electronic parameter (TEP) as a commonly accepted probe for the net electron donor/electron acceptor properties of a ligand.15 In this respect, we synthesized rhodium complexes {[4-(R3Si)-IPr]Rh(CO)2Cl} 1Rh−6Rh and employed their carbonyl stretching vibrations in the TEP calculations. All rhodium complexes were readily available by direct complexation of 4-(R3Si)-IPr ligands with commercial [Rh(CO)2Cl]2 in THF (Scheme 4). Notably, the TEPs of complexes 1Rh− 6Rh, which were calculated by applying the general formula TEP(cm−1) = 0.8001νCOav/Rh + 420.0,16 were well-correlated with the order of observed catalytic activity of 1Pd−6Pd. More specifically, the two lowest TEP values, 2040.1 and 2037.8 cm−1, were obtained for 1Rh and 3Rh, respectively (Table 2), and were notably smaller than those of the other complexes

[(IPr)Pd(allyl)Cl]14a and 2.0415(14) Å for [(4,5-Me2-IPr)Pd(allyl)Cl]).14b Interestingly, the resonances of the olefinic protons of the NHC ligands in all complexes 1Pd−6Pd exhibited a solventdependent behavior, being diagnostically lower-field shifted in THF-d8 than in benzene-d6. For example, in 1Pd the shift difference (i.e., Δ[δ(THF-d8) − δ(benzene-d6)]) was as large as 0.6 ppm. In addition, the 1Pd aromatic carbons of the IPrligand in THF-d8 were all nonequivalent, thus causing the maximum number of resonances to total 12, which testifies to the lack of molecular symmetry in THF (see Figure S60). By contrast, in benzene the number of aromatic carbon resonances in 1Pd was reduced due to the higher symmetry of the molecule. It is likely that in a polar solvent such as THF the rotation of both flanking aromatic rings in 1Pd was blocked whereas in a nonpolar solvent like benzene one of the aromatic C


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Organometallics

SambVca 2 web tool, based on the energy minimum structures of complexes 1Pd−6Pd. Geometry optimization was performed at the DFT M06-L level of theory using LANL8(f) basis set for palladium and 6-31G(d) basis set for all other atoms. The resulting steric map pictures of the complexes 1Pd−6Pd are shown in Figure 3. Analysis of these steric maps clearly revealed that the NHC ligands in 1Pd−6Pd formed very similar catalytic pockets, in which the silyl-substituted part was remarkably more sterically congested (see bottom-right quadrants in the steric maps). This, in turn, suggests that the approach of the aryl halide substrate at the oxidative addition step of the catalytic cycle would be more likely to proceed preferentially from the less hindered site. Meanwhile, the reductive elimination might be further promoted by the presence of these most highly congested quadrants, which are caused by the presence of voluminous R3Si-substituents. However, the percent buried volume descriptor (%Vbur) values of the complexes do not indicate any substantial difference in the steric bulkiness of the NHC ligands, as they are within the narrow range of 41.0−41.6 (except for 4Pd, which has the bulkiest substituent, giving a value of 42.2), which is very close to the value of the unsubstituted parent Pd−IPr complex (40.9). Therefore, the analysis of the steric maps and buried volumes implies the minor role of the steric factors in the overall catalytic activity of the Pd complexes. Although it is still premature to draw a definite conclusion as to which is the rate-determining step, it is likely that in our particular case the oxidative addition (rather than the reductive elimination) plays a decisive role in the complete catalytic process. Having established that 3Pd manifests the best catalytic activity among all synthesized Pd−NHC complexes, we then studied its performance in different solvents. As a result, we found that the model coupling reaction of p-chloroanisole and the pharmaceutically relevant morpholine tolerates a variety of different solvents when catalyzed by 3Pd in the presence of tBuOK base (Scheme 5, Figure 4). THF and 1,4-dioxane gave the best results in this case, producing the coupling product nearly quantitatively (99 and 96%, respectively) in only 10 min. Toluene, which is commonly used in the Buchwald− Hartwig aminations, also allowed the product in nearly quantitative yield (96%), although the reaction proceeded more slowly, requiring 30 min to reach the highest yield plateau. Although lower that the rest, the yield obtained with DME was still relatively high (89%), but the reaction rate was lower. Next, we explored the scope and limitations of applicability of the precatalyst with the best catalytic activity, namely, 3Pd. The coupling reaction of aryl chlorides (with both electrondonating and -withdrawing substituents R) and amines (both primary and secondary) in THF catalyzed by 3Pd was found to tolerate a wide range of aromatic substrates and amine reagents, thereby leading to quantitative formation of the cross-coupling products, arylamines, in just 5 min at low catalyst loading (Table 3). Among these, only the carbazole reaction failed to give the desired product, presumably due to metalation of the proton of NH group of the starting carbazole by the base (t-BuOK) and inhibition of the catalytic process.20 The reaction of chlorobenzene with n-butylamine produced a mixture of phenyl-n-butylamine (27%) and diphenyl-n-butylamine (11%). In the case of the reaction of chlorobenzene with

Scheme 4. Synthesis of {[4-(R3Si)-IPr]Rh(CO)2Cl} Complexes 1Rh−6Rh

Table 2. Carbonyl Stretching Frequencies of {[4-(R3Si)IPr]Rh(CO)2Cl} 1Rh−6Rh and {[IPr]Rh(CO)2Cl}a 1Rh 2Rh 3Rh 4Rh 5Rh 6Rh Rh-IPr

νCORh

νCOav/Rh

TEP

2070.2, 1979.6 2071.2, 1996.9 2062.5, 1981.5 2078.9, 1983.4 2075.0, 1986.3 2070.2, 1990.2 2071.2, 1987.3

2024.9 2034.1 2022.0 2031.2 2030.7 2030.2 2029.2

2040.1(−3.5) 2047.4(+3.9) 2037.8(−5.8) 2045.1(+1.5) 2044.7(+1.2) 2044.4(+0.8) 2043.6(0.0)

Stretching vibrations νCORh, νCOav/Rh, and TEP are given in cm−1.

a

(i.e., 2Rh and 4Rh-6Rh, the range of 2044.4−2047.4 cm−1) and for the parent complex Rh−IPr (2043.6 cm−1). Moreover, the lowest TEP value of 2037.8 cm−1 for 3Rh was also smaller than those of the structurally similar Rh complexes with IMesligands, featuring one or two Me2N-substituents at the 4- or 4,5- positions of the NHC backbone (2048.6 and 2046.6 cm−1, respectively).8 The electronic impact of silyl-substitution is even more pronounced when comparing the TEP values of unsubstituted and monosubstituted Rh-NHC complexes: −5.8 (TEP Rh‑IPr − TEP 3Rh ) versus −2.2 (TEP Rh‑IMes − TEPRh‑(4‑Me2N‑IMes)). Such a decrease in the CO stretching frequencies of 1Rh and 3Rh clearly reflects their stronger Rh−CO π-back-donation and accordingly weaker C−O bonding, which were caused by the markedly stronger electron-donation from the NHC ligand to the Rh metal center. This, in turn, implies that from [4(R3Si)-IPr] NHC ligands 1−6, 4-(Me3Si)-IPr 1 and 4[Me2(allyl)Si]-IPr 3 were the strongest σ-donors. Accordingly, 1Pd and 3Pd were the most effective precatalysts for the C−N coupling reaction. This goes to show that the electronic effects of the NHC ligands play a crucial role in the catalytic performance of the respective Pd complexes. Next, we then turned our attention to the role of steric factors on the catalytic activity of 1Pd−6Pd. Although there are various approaches on this topic, some of them would be inadequate in our case. For example, estimating the Tolman cone angle17 is a very popular approach, but this single-value descriptor can be reliably applied to mostly phosphines. Other general steric descriptors, including the recently introduced buried volume parameter,18 also do not provide a clear picture of the catalytic pocket of a particular ligand. Therefore, for our catalytic pocket estimations, we employed one of the most recently developed approaches in the field, namely, analysis of the topographic steric maps.19 This was done using the D


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Organometallics

Figure 3. Steric maps of Pd complexes 1Pd−6Pd and parent Pd−IPr.

Scheme 5. Solvent Scope of the Buchwald−Hartwig Amination Catalyzed by 3Pd Precatalyst

aniline, only diphenylamine was obtained in trace amounts (2%), and no formation of triphenylamine was observed. Compared to other Pd−NHC complexes capable of catalysis of the C−N cross-coupling reactions, complexes 1Pd and 3Pd exhibited an excellent catalytic performance. In order to further examine their catalytic activity, we applied 1Pd and 3Pd with 2.0 mol % catalyst loading, using the test reaction depicted on Scheme 5. The reaction employed t-BuOK as a base (1.5 mmol) and DME as a solvent, and was performed at room temperature for 30 min. As a result, both 1Pd and 3Pd

Figure 4. Solvent effects on the catalytic performance of precatalyst 3Pd in the reaction of p-chloroanisole and morpholine (yields after 90 min are given in parentheses).

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Organometallics Table 3. Scope and Limitations of the Buchwald−Hartwig Amination Using Complex 3Pd as a Precatalyst

Figure 5. Catalytic performance of 1Pd and 3Pd. For the sake of comparison, the conversion data of the previously reported Buchwald−Hartwig amination precatalysts are given in this graph (along with our own data): PEPPSI-(Me2N)2IPr, PEPPSI-(Me2N)IPr, and PEPPSI-IPr (see ref 8).

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EXPERIMENTAL SECTION

General Procedures. All experimental manipulations involving air-sensitive compounds were performed using high-vacuum line techniques or in a nitrogen atmosphere of MBRAUN LABmaster Pro SP glovebox. All solvents were dried and degassed by MBRAUN MB SPS (solvent purification system) prior to use. NMR spectra were recorded on Bruker AVANCE III HD (1H NMR at 400.1 MHz; 13 C{1H} NMR at 100.6 MHz; 29Si NMR at 79.5 MHz; 7Li NMR at 155.5 MHz) NMR spectrometer. IR spectra were recorded on a JASCO ATRS-100-VIR spectrophotometer. High-resolution massspectra were measured on a micrOTOFII ESI-TOF mass spectrometer (Bruker Daltonics). Elemental analyses were performed using PerkinElmer 2400 II elemental analyzer. Gas chromatographic (GC) analysis was performed using Shimadzu GC-2014 instrument. Gas chromatographic-mass spectrometric (GC-MS) analysis was performed using Shimadzu QP-2010 spectrometer. All computations were performed with the Gaussian 16 suite of programs.21 Geometry optimization and frequency calculations were performed using density functional theory (DFT) with M06-L functional.22 For Pd, core electrons were represented by the HayWadt relativistic effective core potential.23 For valence electrons of Pd, the LANL8 basis set24 augmented with f-polarization function25 was used, whereas 6-31G(d) basis set was used for all other atoms. Density fitting approximation was used when we applied auto keyword in the Gaussian program to automatically generate density fitting sets.26 Synthesis of Li−IPr. n-BuLi (1.6 M hexane solution, 7.8 mmol) was added dropwise to a suspension of 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene (2.1 g, 1.5 mmol) in hexane (50 mL) at −30 °C. The mixture was then stirred at room temperature for 15 h. The precipitated Li−IPr was filtered and washed with dry hexane (10 mL × 5 times). The resulting Li−IPr was dried at room temperature under vacuum for 1 h to give the pure product as colorless solid (2.1 g, 97%). 1 H NMR (THF-d8, δ, ppm) 1.14−1.16 (m, 24H), 3.04−3.14 (m, 2H), 3.17−3.27 (m, 2H), 6.24 (s, 1H), 7.10−7.20 (m, 6H). 13C{1H} NMR (THF-d8, δ, ppm) 22.7, 23.3, 24.0, 24.4, 27.6, 27.7, 121.6, 122.0, 124.9, 125.9, 126.8, 142.1, 145.9, 146.0, 147.9, 214.3. 7Li NMR (THF-d8, δ, ppm) 0.92.

a

Yield after 60 min.

outperformed the recently developed {[4-Me2N-IPr]Pd(allyl)Cl} PEPPSI-(Me2N)IPr8 and {[4,5-(Me2N)2-IPr]Pd(allyl)Cl} PEPPSI-(Me2N)2IPr8 complexes (conversion after 30 min: 99 and 95% versus 808 and 33%),8 also catalyzing the reaction much better than the commercially available PEPPSI-IPr (14%)8 (Figure 5).

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CONCLUSIONS In this work, we developed a straightforward gram-order-scale procedure for the synthesis of a series of novel silyl-substituted NHC derivatives [4-(R3Si)-IPr] from readily available commercial starting materials. On the basis of these NHC ligands, a family of the palladium complexes with the formula {[4-(R3Si)-IPr]Pd(allyl)Cl}, 1Pd−6Pd, was prepared as potential precatalysts for the Buchwald−Hartwig amination reaction. Two of these complexes, namely, 1Pd and 3Pd, showed excellent catalytic performance, notably outperforming that of the previously reported Pd−NHC complexes in terms of the reaction time, catalyst loading, reaction temperature, and reaction scope. Investigation of the electronic and steric effects of the NHC ligands on the catalytic activity of the respective Pd precatalysts revealed that the overall catalytic performance was primarily governed by the electronic effects, whereas the role of the steric factors was less pronounced. F


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Organometallics

2Rh. 1H NMR (CDCl3, δ, ppm) 1.06 (t, J = 7.0 Hz, 9H), 1.12 (d, J = 6.8 Hz, 6H), 1.27 (d, J = 6.8 Hz, 6H), 1.38−1.44 (m, 12H), 2.75 (sept, J = 6.7 Hz, 2H), 3.02 (sept, J = 6.8 Hz, 2H), 3.59 (q, J = 7.0 Hz, 6H), 7.28−7.32 (m, 4H), 7.22 (s, 1H), 7.45−7.50 (m, 2H). 13C{1H} NMR (CDCl3, δ, ppm) 17.8, 22.9, 25.1, 25.2, 26.3, 28.4, 29.0, 58.8, 124.1, 124.2, 129.4, 129.7, 130.1, 135.0, 135.6, 136.4, 145.7, 146.5, 182.8 (d, JRh−C = 74.2 Hz, CO), 184.8 (d, JRh−C = 45.6 Hz, NCN), 184.9 (d, JRh−C = 54.2 Hz, CO). 29Si NMR (CDCl3, δ, ppm) −69.5. 3Rh. 1H NMR (CDCl3, δ, ppm) −0.6 (s, 6H), 1.13 (d, J = 6.8 Hz, 6H), 1.25 (d, J = 6.8 Hz, 6H), 1.41−1.44 (m, 12H), 1.59 (d, J = 8.0 Hz, 2H), 2.85 (sept, J = 6.7 Hz, 2H), 3.01 (sept, J = 6.8 Hz, 2H), 4.82−4.90 (m, 2H), 5.63−5.69 (m, 1H), 7.25 (s, 1H), 7.32−7.36 (m, 4H), 7.47−7.54 (m, 2H). 13C{1H} NMR (CDCl3, δ, ppm) −2.8, 22.8, 25.3, 25.4, 26.3, 28.3, 29.0, 124.1, 124.6, 130.1, 130.3, 134.1, 135.0, 135.1, 136.3, 145.7, 146.2, 182.9 (d, JRh−C = 74.0 Hz, CO), 183.3 (d, JRh−C = 46.4 Hz, NCN), 184.8 (d, JRh−C = 54.3 Hz, CO). 29 Si NMR (CDCl3, δ, ppm) −9.2. 4Rh. 1H NMR (CDCl3, δ, ppm) 0.13 (s, 6H), 1.03 (d, J = 6.8 Hz, 3H), 1.13 (d, J = 6.8 Hz, 3H), 1.39 (d, J = 6.7 Hz, 3H), 1.47 (d, J = 6.7 Hz, 3H), 2.85 (sept, J = 6.7 Hz, 2H), 3.11 (sept, J = 6.8 Hz, 2H), 7.31 (s, 1H), 7.32−7.43 (m, 4H), 7.48−7.54 (m, 2H). 13C{1H} NMR (CDCl3, δ, ppm) −1.8, 22.9, 25.2, 25.4, 26.4, 28.4, 29.0, 124.1, 124.6, 128.1, 129.7, 130.2, 130.3, 133.5, 134.1, 135.1, 135.2, 136.3, 136.9, 145.7, 146.4, 182.9 (d, JRh−C = 74.1 Hz, CO), 184.0 (d, JRh−C = 45.6 Hz, NCN), 184.8 (d, JRh−C = 54.1 Hz, CO). 29Si NMR (CDCl3, δ, ppm) −13.0. 5Rh. 1H NMR (CDCl3, δ, ppm) −0.06 (s, 6H), 0.10 (s, 9H), 1.12 (d, J = 6.8 Hz, 6H), 1.24 (d, J = 6.8 Hz, 6H), 1.40−1.44 (m, 12H), 2.85 (sept, J = 6.8 Hz, 2H), 3.02 (sept, J = 6.8 Hz, 2H), 7.14 (s, 1H), 7.31−7.34 (m, 4H), 7.46−7.54 (m, 2H). 13C{1H} NMR (CDCl3, δ, ppm) −3.0, −1.8, 22.8, 25.2, 25.7, 26.5, 28.4, 28.9, 124.0, 124.5, 130.1, 130.2, 133.2, 135.1, 136.4, 145.7, 146.2, 182.9 (d, JRh−C = 74.2 Hz, CO), 183.5 (d, JRh−C = 45.5 Hz, NCN), 184.9 (d, JRh−C = 54.6 Hz, CO). 29Si NMR (CDCl3, δ, ppm) −18.1, 26.2. 6Rh. 1H NMR (CDCl3, δ, ppm) 0.09 (s, 6H), 0.95 (s, 9H), 1.12 (d, J = 6.8 Hz, 6H), 1.27 (d, J = 6.8 Hz, 6H), 1.44 (d, J = 6.6 Hz, 12H), 2.79 (sept, J = 6.7 Hz, 2H), 3.02 (sept, J = 6.8 Hz, 2H), 7.28− 7.34 (m, 5H), 7.44−7.54 (m, 2H). 13C{1H} NMR (CDCl3, δ, ppm) −4.8, 23.0, 25.3, 26.1, 26.5, 26.9, 28.1, 28.9, 124.0, 124.7, 130.0, 130.1, 133.6, 133.7, 135.2, 136.7, 145.7, 146.4, 182.7 (d, JRh−C = 74.2 Hz, CO), 184.5 (d, JRh−C = 46.3 Hz, NCN), 184.8 (d, JRh−C = 54.0 Hz, CO). 29Si NMR (CDCl3, δ, ppm) −1.4. Typical Procedure for the Synthesis of Pd Complexes with the 4Silyl-Substituted IPr Ligand, 1Pd−6Pd. To a solution of 4-silylsubstituted IPr (1.5 mmol) in THF (2 mL) was added dropwise the THF solution of [Pd(allyl)Cl]2 (0.28 g, 0.76 mmol) at −20 °C. The mixture was then stirred at room temperature for 2 h. The solvent was removed under vacuum, and dry hexane was added to the residue. Precipitated solid was filtered off, and the solvents were evaporated. The residue was recrystallized from hexane to give Pd complexes with 4-silyl-substituted IPr ligand 1Pd−6Pd as brown crystals. 1Pd. Mp 167 °C (dec). 1H NMR (THF-d8, δ, ppm) 0.02 (s, 9H), 1.03 (d, J = 6.7 Hz, 3H), 1.14−1.24 (m, 9H), 1.27−1.39 (m, 12H), 1.55 (d, J = 12.0 Hz, 1H), 2.63 (d, J = 13.4 Hz, 1H), 2.84−2.96 (m, 1H), 2.96−3.08 (m, 2H), 3.16−3.32 (m, 2H), 3.60−3.67 (m, 1H), 4.68−4.83 (m, 1H), 7.23−7.33 (m, 4H), 7.33−7.39 (m, 1H), 7.39− 7.45 (m, 1H), 7.60 (s, 1H). 13C{1H} NMR (THF-d8, δ, ppm) 0.04, 22.8, 23.5, 24.8, 24.9, 25.4, 25.6, 25.6, 26.3, 28.7, 28.9, 29.2, 29.3, 50.1, 71.5, 114.1, 123.9, 124.4, 124.6, 124.7, 129.9, 130.2, 135.1, 135.9, 137.3, 138.5, 146.5, 146.8, 147.0, 147.3, 189.4. 29Si NMR (THF-d8, δ, ppm) −8.3. Anal. Calcd for C33H49ClN2PdSi: C, 61.57; H, 7.67. Found: C, 61.56; H, 7.64. The single crystals of 1Pd for X-ray diffraction analysis were grown from a hexane solution. Diffraction data were collected at 90 K on a Bruker APEX-II CCD X-ray diffractometer (Mo Kα radiation, λ = 0.71069 Å, 50 kV/90 mA). The structure was solved by the direct method with the SIR2002 program27 and refined by the full-matrix least-squares method with the SHELXL-97 program.28 Crystal data for 1Pd: MF = C33H49ClN2PdSi, MW = 643.70, monoclinic, P21/c, a = 10.4846(5), b = 16.0964(8), c = 19.7252(10) Å, β = 93.9940(10)°,

Typical Procedure for the Synthesis of 4-Silyl-Substituted IPr, 1− 6. To a solution of Li−IPr (0.6 g, 1.5 mmol) in THF (6 mL) was added dropwise the corresponding chlorosilane R3SiCl (2.3 mmol) at −20 °C. The mixture was then stirred at room temperature for 1 h. The solvent was removed under vacuum, and dry hexane (3 mL) was added to the residue. Precipitated Li salt was filtered off, and the solvent was evaporated. The residue was recrystallized from hexane to give 4-silyl-substituted IPr 1−6 as pale-yellow crystals. 4-(Me3Si)-IPr (1). Yield 96%. 1H NMR (THF-d8, δ, ppm) 0.02 (s, 9H), 1.18−1.22 (m, 18H), 1.32 (d, J = 6.9 Hz, 6H), 2.70 (sept, J = 6.8 Hz, 2H), 2.85 (sept, J = 6.9 Hz, 2H), 7.27−7.30 (m, 5H), 7.36− 7.42 (m, 2H). 13C{1H} NMR (THF-d8, δ, ppm) −1.1, 21.2, 23.2, 23.8, 25.6, 28.1, 28.6, 122.6, 123.0, 128.1, 128.4, 130.8, 131.3, 138.5, 139.8, 145.7, 146.1, 222.8. 29Si NMR (THF-d8, δ, ppm) −11.6. 4-[(EtO)3Si]-IPr (2). Yield 87%. 1H NMR (THF-d8, δ, ppm) 1.09 (t, J = 7.0 Hz, 9H), 1.16−1.23 (m, 18H), 1.33 (d, J = 6.9 Hz, 6H), 2.74 (sept, J = 6.9 Hz, 2H), 2.86 (sept, J = 6.79 Hz, 2H), 3.62 (q, J = 7.0 Hz, 6H), 7.26−7.31 (m, 4H), 7.34−7.42 (m, 2H), 7.45 (s, 1H). 13 C{1H} NMR (THF-d8, δ, ppm) 17.4, 21.7, 23.0, 23.6, 24.8, 28.2, 28.4, 58.2, 122.5, 123.0, 125.1, 127.8, 128.2, 132.8, 138.3, 139.6, 145.6, 146.2, 223.3. 29Si NMR (THF-d8, δ, ppm) −65.2. 4-[(allyl)Me2Si]-IPr (3). Yield 85%. 1H NMR (THF-d8, δ, ppm) −0.04 (s, 6H), 1.19−1.23 (m, 18H), 1.33 (d, J = 6.9 Hz, 6H), 1.60 (d, J = 8.0 Hz, 2H), 2.71 (sept, J = 6.8 Hz, 2H), 2.86 (sept, J = 6.9 Hz, 2H), 4.80−4.84 (m, 2H), 5.66−5.77 (m, 1H), 7.28−7.31 (m, 4H), 7.33 (s, 1H), 7.37−7.43 (m, 2H). 13C{1H} NMR (THF-d8, δ, ppm) −3.5, 21.2, 23.2, 23.6, 23.7, 25.4, 28.1, 28.5, 113.1, 122.7, 123.0, 128.2, 128.4, 129.5, 131.3, 134.0, 138.4, 139.7, 145.7, 146.1, 222.7. 29 Si NMR (THF-d8, δ, ppm) −11.8. 4-(PhMe2Si)-IPr (4). Yield 93%. 1H NMR (THF-d8, δ, ppm) −0.21 (s, 6H), 1.04 (d, J = 6.8 Hz, 6H), 1.15 (d, J = 6.8 Hz, 6H), 1.21 (d, J = 6.9 Hz, 6H), 1.24 (d, J = 7.0 Hz, 6H), 2.89 (sept, J = 6.9 Hz, 2H), 7.22−7.24 (m, 2H), 7.28−7.34 (m, 6H), 7.37−7.44 (m, 4H). 13 C{1H} NMR (THF-d8, δ, ppm) −2.7, 20.8, 23.3, 23.7, 25.5, 28.1, 28.6, 122.6, 123.0, 127.6, 128.2, 128.4,129.0, 129.5, 132.4, 133.7, 137.3, 138.4, 139.6, 145.7, 146.1, 223.0. 29Si NMR (THF-d8, δ, ppm) −16.4. 4-(Me3SiMe2Si)-IPr (5). Yield 89%. 1H NMR (THF-d8, δ, ppm) −0.07 (s, 6H), 0.14 (s, 9H), 1.18−1.22 (m, 18H), 1.32 (d, J = 6.9 Hz, 6H), 2.72 (sept, J = 6.9 Hz, 2H), 2.87 (sept, J = 6.9 Hz, 2H), 7.21 (s, 1H), 7.27−7.30 (m, 4H), 7.37−7.42 (m, 2H). 13C{1H} NMR (THFd8, δ, ppm) −3.6, −3.0, 21.5, 23.2, 23.6, 25.1, 28.1, 28.4, 122.7, 123.0, 128.2, 128.3, 129.0, 130.3, 138.4, 139.8, 145.7, 146.0, 222.4. 29Si NMR (THF-d8, δ, ppm) −29.8, −19.0. 4-(t-BuMe2Si)-IPr (6). Yield 70%. 1H NMR (THF-d8, δ, ppm) −0.13 (s, 6H), 0.98 (s, 9H), 1.19−1.23 (m, 18H), 1.33 (d, J = 6.9 Hz, 6H), 2.73 (sept, J = 6.8 Hz, 2H), 2.88 (sept, J = 6.9 Hz, 2H), 7.27− 7.30 (m, 4H) 7.37−7.41 (m, 3H). 13C{1H} NMR (THF-d8, δ, ppm) −5.3, 17.1, 21.3, 23.2, 23.5, 25.3, 26.3, 28.1, 28.4, 122.7, 123.0, 128.2, 128.3, 128.6, 131.0, 138.4, 139.9, 145.7, 146.1, 221.6. 29Si NMR (THF-d8, δ, ppm) −3.0. Typical Procedure for the Synthesis of Rh Complexes with the 4Silyl-Substituted IPr Ligand, 1Rh−6Rh. To a solution of 4-silylsubstituted IPr (0.3 mmol) in THF (1 mL), the THF solution of [Rh(CO)2Cl]2 (0.061 g, 0.16 mmol) was added dropwise at −20 °C. The mixture was then stirred at room temperature for 1 h. The solvent was removed under vacuum, and dry toluene was added to the residue. Precipitated solid was filtered off, and the solvents were evaporated. The residue was recrystallized from toluene to give Rh complexes with 4-silyl-substituted IPr ligand 1Rh−6Rh as brown crystals. 1Rh. 1H NMR (CDCl3, δ, ppm) 0.02 (s, 9H), 1.13 (d, J = 6.9 Hz, 6H), 1.23 (d, J = 6.8 Hz, 6H), 1.40−1.43 (m, 12H), 2.85 (sept, J = 6.7 Hz, 2H), 3.01 (sept, J = 6.8 Hz, 2H), 7.22 (s, 1H), 7.32−7.34 (m, 4H), 7.47−7.54 (m, 2H). 13C{1H} NMR (CDCl3, δ, ppm) 0.0, 22.9, 25.3, 25.4, 26.3, 28.4, 29.0, 124.1, 124.6, 130.2, 133.8, 135.2, 136.4, 137.1, 145.8, 146.3, 182.8 (d, JRh−C = 45.5 Hz, NCN), 183.0 (d, JRh−C = 74.2 Hz, CO), 184.9 (d, JRh−C = 54.0 Hz, CO). 29Si NMR (CDCl3, δ, ppm) −7.7. G


Article

Organometallics V = 3320.8(3) Å3, Z = 4, Dcalcd = 1.287 g cm−3. The final R factor was 0.0312 for 6423 reflections with I0 > 2σ(I0) (Rw = 0.0821 for all data), GOF = 1.071. The X-ray crystallographic data for 1Pd have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition no. CCDC 1859969. These data can be obtained free of charge from the CCDC (www.ccdc.cam.ac.uk/data_request/cif). 2Pd. Mp 150 °C (dec). 1H NMR (THF-d8, δ, ppm) 1.03 (t, J = 7.0 Hz, 9H), 1.15 (d, J = 6.8 Hz, 4H), 1.22−1.40 (m, 20H), 1.48 (d, J = 11.8 Hz, 1H), 2.62 (d, J = 13.4 Hz, 1H), 2.68−2.79 (m, 1H), 2.94 (d, J = 5.8 Hz, 1H), 3.04−3.25 (m, 3H), 3.59−3.67 (m, 7H), 4.66−4.78 (m, 1H), 7.20−7.30 (m, 4H), 7.32−7.40 (m, 2H), 7.64 (s, 1H). 13 C{1H} NMR (THF-d8, δ, ppm) 18.0, 22.8, 23.5, 25.0, 25.1, 25.3, 25.6, 25.8, 26.5, 28.9 (2C), 29.3, 29.4, 50.4, 59.3, 71.3, 114.1, 124.0, 124.2, 124.5 (2C), 128.4, 129.7, 129.9, 136.6, 137.0, 138.6, 146.4, 146.8, 147.3, 147.7, 191.6. 29Si NMR (THF-d8, δ, ppm) −68.2. Anal. Calcd for C36H55ClN2O3PdSi: C, 58.93; H, 7.56. Found: C, 59.07; H, 7.51. The single crystals of 2Pd for X-ray diffraction analysis were grown from a hexane solution. Diffraction data were collected at 90 K on a Bruker APEX-II CCD X-ray diffractometer (Mo Kα radiation, λ = 0.71069 Å, 50 kV/90 mA). The structure was solved by the direct method with the SIR2002 program27 and refined by the full-matrix least-squares method with the SHELXL-97 program.28 Crystal data for 2Pd: MF = C36H55ClN2O3PdSi, MW = 733.78, monoclinic, P21/c, a = 18.1934(9), b = 13.2031(6), c = 16.3077(8) Å, β = 102.4490(10)°, V = 3825.2(4) Å3, Z = 4, Dcalcd = 1.274 g cm−3. The final R factor was 0.0222 for 7781 reflections with I0 > 2σ(I0) (Rw = 0.0614 for all data), GOF = 1.060. The X-ray crystallographic data for 2Pd have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition no. CCDC 1859968. These data can be obtained free of charge from the CCDC (www.ccdc.cam.ac.uk/data_request/cif). 3Pd. Mp 140 °C (dec). 1H NMR (THF-d8, δ, ppm) −0.06 (d, J = 16.0 Hz, 6H), 0.97−1.43 (m, 24H), 1.55 (d, J = 11.0 Hz, 1H), 1.63 (d, J = 7.9 Hz, 1H), 2.64 (d, J = 13.4 Hz, 1H), 2.81−3.11 (m, 4H), 3.11−3.32 (m, 2H), 3.64 (d, J = 7.2 Hz, 1H), 4.70−4.79 (m, 1H), 4.79−4.83 (m, 1H), 4.83−4.86 (m, 1H), 5.64−5.78 (m, 1H), 7.20− 7.33 (m, 4H), 7.33−7.38 (m, 1H), 7.39−7.46 (m, 1H), 7.59 (s, 1H). 13 C{1H} NMR (THF-d8, δ, ppm) −2.7, 22.8, 23.5, 24.8, 25.0, 25.2, 25.3, 25.5, 25.7, 26.5, 28.7, 28.9, 29.3 (2C), 50.3, 71.5, 114.1, 114.6, 124.0, 124.4, 124.7, 124.8, 129.9, 130.3, 134.1, 134.2, 135.3, 137.3, 138.5, 146.5, 146.8, 146.9, 147.3, 190.0. 29Si NMR (THF-d8, δ, ppm) −9.54. HRMS (ESI): m/z calcd. for C35H51N2PdSi [M − Cl]+ 633.2856. Found: 633.2852. 4Pd. Mp 185 °C (dec). 1H NMR (THF-d8, δ, ppm) 0.05 (s, 3H), 0.19 (s, 3H), 0.84−0.96 (m, 3H), 0.99−1.11 (m, 6H), 1.14−1.21 (m, 3H), 1.25−1.46 (m, 12H), 1.60 (d, J = 11.8 Hz, 1H), 2.65 (d, J = 13.4 Hz, 1H), 2.80−2.94 (m, 1H), 2.97−3.08 (m, 1H), 3.10−3.33 (m, 3H), 3.66 (d, J = 7.2 Hz, 1H), 4.71−4.85 (m, 1H), 7.23−7.33 (m, 7H), 7.34−7.44 (m, 2H), 7.45−7.51 (m, 2H), 7.73 (s, 1H). 13C{1H} NMR (THF-d8, δ, ppm) −2.0, −1.3, 22.8, 23.6, 24.7, 24.9, 25.5, 25.8, 25.9, 26.6, 28.7, 29.0, 29.3, 29.4, 50.3, 71.5, 114.2, 123.9, 124.4, 124.6, 124.9, 128.5, 130.0, 130.0, 130.2, 132.9, 134.4, 136.5, 137.2, 138.3, 138.4, 146.5, 146.9, 147.0, 147.5, 190.7. 29Si NMR (THF-d8, δ, ppm) −13.4. Anal. Calcd for C38H51ClN2PdSi: C, 64.67; H, 7.28. Found: C, 64.66; H, 7.43. 5Pd. Mp 138 °C (dec). 1H NMR (THF-d8, δ, ppm) −0.07 to −0.01 (m, 6H), 0.11 (s, 9H), 1.03 (d, J = 6.6 Hz, 3H), 1.16 (d, J = 6.6 Hz, 3H), 1.22 (d, J = 5.4 Hz, 6H), 1.25−1.41 (m, 12H), 1.50 (d, J = 12.2 Hz, 1H), 2.46 (s, 1H), 2.62 (d, J = 13.4 Hz, 1H), 2.81−2.91 (m, 1H), 2.94 (d, J = 6.0 Hz, 1H), 3.13−3.23 (m, 1H), 3.23−3.33 (m, 1H), 3.60−3.67 (m, 1H), 4.67−4.80 (m, 1H), 7.18−7.33 (m, 4H), 7.33−7.38 (m, 1H), 7.38 (s, 1H), 7.39−7.44 (m, 1H). 13C{1H} NMR (THF-d8, δ, ppm) −3.0, −2.4, −1.8, 22.8, 23.5, 24.7, 25.4, 25.6, 25.9, 26.8, 28.7, 28.9, 29.2, 29.3, 50.2, 71.5, 114.1, 124.0, 124.5, 124.5, 124.8, 129.9, 130.2, 133.8, 134.3, 137.2, 138.6, 146.5, 146.8, 146.9, 147.4, 190.4. 29Si NMR (THF-d8, δ, ppm) −26.7, −18.2. Anal. Calcd for C35H55ClN2PdSi2: C, 59.89; H, 7.90. Found: C, 60.05; H, 7.94.

6Pd. Mp 193 °C (dec). 1H NMR (THF-d8, δ, ppm) −0.14 (s, 3H), −0.01 (s, 3H), 0.96 (s, 9H), 1.00−1.46 (m, 24H), 2.46 (s, 1H), 2.65 (d, J = 13.4 Hz, 1H), 2.69−2.85 (m, 2H), 3.05−3.25 (m, 3H), 3.65 (d, J = 7.3 Hz, 1H), 4.62−4.76 (m, 1H), 7.16−7.38 (m, 5H), 7.40− 7.46 (m, 1H), 7.60 (s, 1H). 13C{1H} NMR (THF-d8, δ, ppm) −4.9, −4.1, −3.4 18.5, 22.9, 23.6, 25.3, 26.0, 26.1, 26.6, 27.3 (2C), 28.6 (2C), 29.3 (2C), 50.4, 71.3, 114.1, 124.0, 124.4, 124.8, 125.0, 129.7, 130.2, 132.6, 134.8, 137.2, 138.9, 146.4, 146.8, 147.2, 147.5, 190.9. 29 Si NMR (THF-d8, δ, ppm) −1.79. HRMS (ESI): m/z calcd. for C36H55N2PdSi [M − Cl]+ 649.3177, found: 649.3155. Typical Procedure for the Buchwald−Hartwig Amination Using Complex 3Pd as a Precatalyst. THF solution of complex 3Pd (0.2 mol % relative to aryl chloride) was added to a THF (2 mL) solution of aryl chloride (1.00 mmol), amine (1.19 mmol), t-BuOK (1.20 mmol), and dodecane (0.075 g, as an internal standard for GC analysis) at 70 °C. The reaction mixture was then stirred for 5 min. The conversion of aryl chloride and yield of the coupling product were determined by GC analysis. All coupling products were identified by their GC-MS, 1H and 13C NMR spectral data.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00757. X-ray analysis data and tables of crystallographic data (including atomic positional and thermal parameters) for 1Pd and 3Pd, as well as computational details (optimized geometries and total energies for 1Pd−6Pd and Pd−IPr) (PDF) Cartesian coordinaes (XYZ) Accession Codes

CCDC 1859968−1859969 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.F.). *E-mail: [email protected] (J.-C.C.). ORCID

Norihisa Fukaya: 0000-0001-8319-803X Kazuhiro Matsumoto: 0000-0003-1580-8822 Vladimir Ya. Lee: 0000-0002-6527-5342 Junpei Kuwabara: 0000-0002-9032-5655 Takaki Kanbara: 0000-0002-6034-1582 Jun-Chul Choi: 0000-0002-7049-5032 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the JSPS KAKENHI Grant program (No. JP18K05137) from the Ministry of Education, Science, Sports, and Culture of Japan. The authors also acknowledge the helpful discussions with Hiroshi Igarashi and Yasuhiro Seki from the N.E. CHEMCAT Corporation.

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REFERENCES

(1) (a) Paul, F.; Patt, J.; Hartwig, J. F. Palladium-Catalyzed Formation of Carbon-Nitrogen Bonds. Reaction Intermediates and

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recently, Nolan et al. reported novel IPr−Pd complexes featuring halogen substituents at the 4,5-positions of the NHC backbone: (b) Izquierdo, F.; Zinser, C.; Minenkov, Y.; Cordes, D. B.; Slawin, A. M. Z.; Cavallo, L.; Nahra, F.; Cazin, C. S. J.; Nolan, S. P. Insights into the Catalytic Activity of [Pd(NHC)(cin)Cl] (NHC = IPr, IPrCl, IPrBr) Complexes in the Suzuki−Miyaura Reaction. ChemCatChem 2018, 10, 601−611. (9) Zhang, Y.; Lavigne, G.; César, V. Buchwald−Hartwig Amination of (Hetero)Aryl Tosylates Using a Well-Defined N-Heterocyclic Carbene/Palladium(II) Precatalyst. J. Org. Chem. 2015, 80, 7666− 7673. (10) Zhang, Y.; Lavigne, G.; Lugan, N.; César, V. Buttressing Effect as a Key Design Principle towards Highly Efficient Palladium/NHeterocyclic Carbene Buchwald−Hartwig Amination Catalysts. Chem. - Eur. J. 2017, 23, 13792−13801. (11) Wang, Y.; Xie, Y.; Abraham, M. Y.; Wei, P.; Schaefer, H. F.; Schleyer, P. v. R.; Robinson, G. A Viable Anionic N-Heterocyclic Dicarbene. J. Am. Chem. Soc. 2010, 132, 14370−14372. (12) Mendoza-Espinosa, D.; Donnadieu, B.; Bertrand, G. Synthesis of 4- and 4,5-Functionalized Imidazol-2-ylidenes from a Single 4,5Unsubstituted Imidazol-2-ylidene. J. Am. Chem. Soc. 2010, 132, 7264−7265. (13) Marion, N.; Nolan, S. P. Well-Defined N-Heterocyclic Carbenes−Palladium(II) Precatalysts for Cross-Coupling Reactions. Acc. Chem. Res. 2008, 41, 1440−1449. (14) (a) Viciu, M. S.; Germaneau, R. F.; Navarro-Fernandez, O.; Stevens, E. D.; Nolan, S. P. Activation and Reactivity of (NHC)Pd(allyl)Cl (NHC = N-Heterocyclic Carbene) Complexes in CrossCoupling Reactions. Organometallics 2002, 21, 5470−5472. (b) Viciu, M. S.; Navarro, O.; Germaneau, R. F.; Kelly, R. A., III; Sommer, W.; Marion, N.; Stevens, E. D.; Cavallo, L.; Nolan, S. P. Synthetic and Structural Studies of (NHC)Pd(allyl)Cl Complexes (NHC = Nheterocyclic carbene). Organometallics 2004, 23, 1629−1635. (c) Clavier, H.; Correa, A.; Cavallo, L.; Escudero-Adán, E. C.; Benet-Buchholz, J.; Slawin, A. M. Z.; Nolan, S. P. [Pd(NHC)(allyl)Cl] Complexes: Synthesis and Determination of the NHC Percent Buried Volume (%Vbur) Steric Parameter. Eur. J. Inorg. Chem. 2009, 2009, 1767−1773. (15) (a) Tolman, C. A. Electron donor-acceptor properties of phosphorus ligands. Substituent additivity. J. Am. Chem. Soc. 1970, 92, 2953−2956. (b) Tolman, C. A. The 16 and 18 electron rule in organometallic chemistry and homogeneous catalysis. Chem. Soc. Rev. 1972, 1, 337−353. Recently, MLEP (metal−ligand electronic parameter), based on the local stretching force constant of the metal−ligand bond, was proposed to predict the catalytic activity of transition metal complexes. This parameter, which can be determined for any transition metal and any ligand by either experimental or computational techniques, is considered the generalization and further development of TEP factor: (c) Cremer, D.; Kraka, E. Generalization of the Tolman electronic parameter: the metal−ligand electronic parameter and the intrinsic strength of the metal−ligand bond. Dalton Trans 2017, 46, 8323−8338. (16) Dröge, T.; Glorius, F. The Measure of All RingsNHeterocyclic Carbenes. Angew. Chem., Int. Ed. 2010, 49, 6940−6952. (17) Tolman, C. A. Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis. Chem. Rev. 1977, 77, 313−348. (18) (a) Hillier, A. C.; Sommer, W. J.; Yong, B. S.; Petersen, J. L.; Cavallo, L.; Nolan, S. P. A Combined Experimental and Theoretical Study Examining the Binding of N-Heterocyclic Carbenes (NHC) to the Cp*RuCl (Cp* = η5-C5Me5) Moiety: Insight into Stereoelectronic Differences between Unsaturated and Saturated NHC Ligands. Organometallics 2003, 22, 4322−4326. (b) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. Steric and electronic effects in the bonding of N-heterocyclic ligands to transition metals. J. Organomet. Chem. 2005, 690, 5407−5413. (c) Poater, A.; RAgone, F.; Giudice, S.; Costabile, C.; Dorta, R.; Nolan, S. P.; Cavallo, L. Thermodynamics of N-Heterocyclic Carbene Dimerization: The Balance of Sterics and Electronics. Organometallics 2008, 27, 2679− I


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