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Synthesis and Catalytic Applications of a Triptycene-Based Monophosphine Ligand for Palladium-Mediated Organic Transformations Franco King-Chi Leung,†,‡ Fumitaka Ishiwari,† Yoshiaki Shoji,† Tsuyoshi Nishikawa,§ Ryohei Takeda,§ Yuuya Nagata,§ Michinori Suginome,§ Yasuhiro Uozumi,‡,∥ Yoichi M. A. Yamada,*,‡ and Takanori Fukushima*,† †

Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ‡ RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako 351-0198, Japan § Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Institute for Molecular Science and The Graduate School for Advanced Studies, Myodaiji, Okazaki 444-8787, Japan S Supporting Information *

ABSTRACT: 1-Methoxy-8-(diphenylphosphino)triptycene (1), featuring high structural rigidity and steric bulkiness around the phosphine functionality, was synthesized as a new chiral monophosphine ligand for transition metal-catalyzed reactions. In the presence of 5−10 mol ppm (i.e., 0.0005−0.001 mol %) Pd(OAc)2 and 1 (2 equiv for Pd), Suzuki−Miyaura cross-coupling reactions of aryl bromides and arylboronic acids proceeded effectively under mild atmospheric conditions to give the corresponding biaryl compounds in a high yield. The single-crystal X-ray analysis of a Pd(II) complex of 1 revealed its coordination structure, in which two homochiral molecules form a dimer, suggesting that triptycene could provide a chiral environment for asymmetric organic transformations. In fact, optically active 1 obtained by optical resolution showed good enantioselectivity in the palladium-catalyzed asymmetric hydrosilylation of styrene, which represents, for the first time, the asymmetric catalytic activity of triptycene-based monophosphine ligands.

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INTRODUCTION

To date, several 1,8-substituted triptycene-based bisphosphine ligands have been developed, so as to take advantage of the particular bite angle and geometry of chelation.8 In this context, 1,8-substituted triptycene-based monophosphine ligands should also be interesting because sterically bulky and rigid monodentate ligands generally show a high catalytic activity for various transition metal-catalyzed reactions.10 Furthermore, when different functional groups are incorporated into these positions, the resultant substituted triptycene should become chiral (Figure 1c). However, to the best of our knowledge, only one previous report has described the synthesis of a triptycene-based chiral monophosphine ligand, and the catalytic activity of its transition metal complex has not been investigated.9 Here, we report the synthesis of 1-methoxy8-(diphenylphosphino)triptycene (1, Figure 1c, Scheme 1) as a prototype of a triptycene-based chiral monophosphine ligand.

Triptycene is a propeller-shaped molecule composed of three 120°-oriented phenylene blades (Figure 1a).1 Along with this high molecular symmetry, triptycene provides a large free volume around the aromatic skeleton. These structural features of triptycene have inspired chemists to develop organic,2−4 polymeric,5 and supramolecular6,7 functional materials, which have mainly focused on the use of the free volume in terms of host−guest chemistry. For this purpose, a large number of triptycene derivatives with functional groups at the metapositions of the bridgehead carbons (e.g., 2-, 3-, 6-, 7-, 14-, and 15positions) have been synthesized. Meanwhile, regiospecific functionalization of triptycene at the three ortho positions (i.e., 1-, 8-, 13- or 4-, 5-, 16-positions) of the bridgehead carbon can force the attached functional groups to assemble in proximity to each other at a distance of approximately 4.5 Å (Figure 1b). Although this type of triptycene is relatively rare, its stereochemistry would be useful in the design of ligands for transition metal catalysis.8,9 © 2017 American Chemical Society

Received: February 20, 2017 Accepted: April 28, 2017 Published: May 9, 2017 1930

DOI: 10.1021/acsomega.7b00200 ACS Omega 2017, 2, 1930−1937

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reacted with trifluoromethanesulfonic anhydride (4.0 equiv) in 1,2-dichloroethane at 60 °C in the presence of pyridine (8.0 equiv) to give 1,8-bis(trifluoromethanesulfonyloxy)triptycene (3) in 75% yield. Monophosphinylated triptycene 4 was obtained as a racemic mixture in 90% yield through the reaction of 3 with diphenylphosphine oxide (2.0 equiv) in the presence of Pd(OAc)2 (5.0 mol %) and 1,4-bis(diphenylphosphono)butane (5.0 mol %) in dimethyl sulfoxide (DMSO) at 100 °C. Upon treatment with NaOH, the triflate group of 4 was converted into a hydroxy group, which was subjected to methylation with MeI (2.9 equiv) in the presence of K2CO3 (2.9 equiv) to give 6 in 89% yield (two steps). The phosphine oxide group of 6 was reduced by HSiCl3 in the presence of triethylamine in toluene at 120 °C, to allow the isolation of 1 in 90% yield. All compounds were unambiguously characterized by 1H and 13C NMR spectroscopies, infrared (IR) spectroscopy, and high-resolution atmospheric pressure chemical ionization time-of-flight (APCI-TOF) mass spectrometry (Figures S5−S16). Many attempts to achieve the optical resolution of 1 using chiral high-performance liquid chromatography (HPLC) under various conditions were not successful. Instead, we found that chiral HPLC (CHIRAL ART Amylose-SA column, CHCl3/ hexane = 1/2 v/v) of 6 (the precursor of 1) gave rise to wellseparated fractions from which enantiomers of 6 (Figure 2a) Figure 1. (a) Perspective and (b) top view of triptycene and its numbering. (c) Structures of chiral 1,8,13-substituted triptycenes and (d) their geometrical models.

Scheme 1. Synthesis of 1a

Figure 2. (a) Chiral HPLC traces of racemic 6, 6CD(−) and 6CD(+) (CHIRAL ART Amylose-SA column, YMC CO., LTD.; diameter: 4.6 mm; length: 250 mm; eluent: CHCl3/hexane = 1/2, v/v; flow rate: 0.5 mL/min; detection: UV absorption at 250 nm). (b) CD and UV spectra of 6CD(−) (50 μM) and 6CD(+) (50 μM) in CH2Cl2 at 25 °C.

a

Reagents and conditions: (a) Tf2O, pyridine, 1,2-dichloroethane, 60 °C, 75%; (b) diphenylphosphine oxide, Pd(OAc) 2 , 1,4-bis(diphenylphosphino)butane, diisopropylethylamine, DMSO, 100 °C, 90%; (c) NaOH, 1,4-dioxane, MeOH, 25 °C, 94%; (d) MeI, K2CO3, acetone, 25 °C, 95%; and (e) HSiCl3, triethylamine, toluene, 120 °C, 90%.

were isolated. The circular dichroism (CD) spectra of the enantiomers were complete mirror images (Figure 2b); hereafter, each enantiomer of 6 is expressed as 6CD(+) or 6CD(−) based on the sign of the CD signal at 300 nm. As in the case of racemic 6, reduction of optically active 6, for example, 6CD(+), with HSiCl3 gave optically active 1CD(+), which displayed clear CD peaks (Figure S1). Pd-Catalyzed Suzuki−Miyaura Cross-Coupling Using Racemic 1. With racemic 1, we examined the catalytic activity of the triptycene-based monophosphine ligand for Pd-catalyzed Suzuki−Miyaura cross-coupling (Table 1). As a typical example, when p-bromotoluene (7a, 1.0 mmol) was reacted with phenylboronic acid (8a, 1.25 mmol) in 2-methoxyethanol for 2 h at 60 °C under aerobic conditions in the presence of 1.0 mol % Pd(OAc)2, 2.0 mol % 1, and K3PO4 (2.0 mmol), 4methyldiphenyl (9a) was isolated in 99% yield (entry 1, Table 1). We investigated the effects of the solvent (Table S1), base (Table S3), and Pd source (Table S2) on the cross-coupling of

Using Pd-catalyzed Suzuki−Miyaura cross-coupling and asymmetric hydrosilylation as model reactions, we demonstrate for the first time the catalytic activity of a monophosphine consisting of a sterically bulky and structurally rigid triptycene scaffold.

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RESULTS AND DISCUSSION Synthesis of 1-Methoxy-8-(diphenylphosphino)triptycene (1). In the light of the structures of axially chiral monodentate phosphine (MOP) ligands11 and a simple geometrical model (Figure 1d), we designed monophosphine ligand 1 (Figure 1c), which was successfully synthesized from 1,8-dihydroxytriptycene (2)13 using procedures similar to those for MOP ligands11,12 (Scheme 1). Thus, compound 2 was 1931


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Table 1. Pd(OAc)2/1-Catalyzed Suzuki−Miyaura Cross-Coupling of p-Bromotoluene 7a with Phenylboronic Acid 8a under Various Reaction Conditionsa

entry

Pd (mol ppm)b

1 (mol ppm)b

temperature (°C)

time (h)

yield (%)c

TON

TOF (h−1)

1 2 3 4d 5e 6f

10 000 10 5 10 10 10

20 000 20 10 0 0 0

60 100 100 100 100 100

2 24 24 24 24 24

99 99 54 15 35 70

99 99 000 108 000 15 000 35 000 70 000

50 4125 4500 625 1458 2917

a

Reaction conditions: 1 (2 equiv for Pd), 4-bromotoluene (1.0 mmol), phenylboronic acid (1.25 mmol), K3PO4 (2.0 mmol), 2-methoxyethanol (6 mL), 100 °C, 24 h. b1 mol ppm equals 0.0001 mol %. cIsolated yield. dIn the absence of 1. eIn the presence of 20 mol ppm PPh3. fIn the presence of 20 mol ppm (2-biphenylyl)diphenylphosphine.

7a and 8a with 1.0 mol % PdCl2, 2.0 mol % 1, and K3PO4 (2.0 mmol). The cross-coupling reaction in 2-methoxyethanol or a mixture of tetrahydrofuran (THF)/H2O (1/1 in vol) led to the highest yield of 9a (99%, entries 4 and 13, Table S1), whereas that in methanol or isopropyl alcohol resulted in 9a in less than 10% yield. Among the various bases examined, the use of K3PO4, in combination with 2-methoxyethanol, gave the best result (entry 1, Table S3). For the Pd(II) source, PdCl2 and Pd(OAc)2 allowed the quantitative conversion of 7a and 8a into 9a (entries 1 and 4, Table S2), whereas the use of PdCl2(PPh3)2 or Pd2(dibenzylideneacetone)3 under identical conditions lowered the yield of 9a (entries 2 and 3, Table S2). Based on the above experimental results, we set 2methoxyethanol, K3PO4, and Pd(OAc)2 as the standard combination for Pd-catalyzed Suzuki−Miyaura cross-coupling with 1. Remarkably, the use of only 0.001 mol % (i.e., 10 mol ppm) Pd(OAc)2 and 1 (2 equiv for Pd) could achieve the quantitative formation of 9a from 7a and 8a (entry 2, Table 1), which indicates that the turnover number (TON) of the catalytic system is as high as 99 000. An even higher TON (108 000) was obtained when the cross-coupling was performed with a 5 mol ppm Pd loading, though the yield of 9a decreased to 54% (entry 3, Table 1). In sharp contrast, when the cross-coupling reaction was conducted using Pd(OAc)2 alone (10 mol ppm of Pd loading), 9a was obtained in only 15% yield (entry 4, Table 1). In the presence of PPh3 (2 equiv for Pd), the yield of 9a was increased to some extent (35%; entry 5, Table 1). When a sterically bulky phosphine ligand such as (2-biphenylyl)diphenylphosphine was used (2 equiv for Pd) in place of PPh3, the yield of 9a was greatly improved (70%; entry 6, Table 1), yet it was lower than that achieved in the cross-coupling reaction with Pd(OAc)2/1. Therefore, we consider that a synergetic effect, arising from the steric bulkiness around the phosphine group and the rigidity of the triptycene scaffold, would lead to the excellent catalytic activity of 1.10 Substrate Scope of Pd(OAc)2/1 Catalyst for Suzuki− Miyaura Cross-Coupling. Table 2 shows the substrate scope of the catalytic system of Pd(OAc)2/1 for the Suzuki−Miyaura cross-coupling of various aryl bromides (7b−7j, 1.0 mmol) and phenylboronic acid (8a, 1.25 mmol) with Pd(OAc)2 (10 mol ppm), 1 (20 mol ppm), and K3PO4 (2.0 mmol) in 2-

Table 2. Scope of Aryl Bromides 7b−7j for Pd/1-Catalyzed Suzuki−Miyaura Cross-Coupling with Phenylboronic Acid 8aa

entry

R1

product

yield (%)b

1 2 3 4 5c 6 7 8c 9

p-NO2 (7b) p-CF3 (7c) p-COMe (7d) p-CHO (7e) m-NH2 (7f) p-OMe (7g) m-OMe (7h) o-OMe (7i) o-Me (7j)

9b 9c 9d 9e 9f 9g 9h 9i 9j

99 99 99 99 80 85 80 85 93

a

Reaction conditions: Pd(OAc)2 (10 mol ppm), 1 (20 mol ppm), aryl bromide (1.0 mmol), phenylboronic acid (1.25 mmol), K3PO4 (2.0 mmol), 2-methoxyethanol (6 mL), 100 °C, 24 h. bIsolated yield. c Pd(OAc)2 (50 mol ppm) and 1 (100 mol ppm).

methoxyethanol under aerobic conditions. Biaryl products 9b− 9e were formed quantitatively (99% yield) from aryl bromides 7b−7e with an electron-withdrawing substituent such as NO2, CF3, COMe, or CHO (entries 1−4). On the other hand, the efficiency of the cross-coupling between 8a and aryl bromides 7f−7h containing an electron-donating substituent (NH2, pOMe, m-OMe, or o-OMe) was slightly decreased, giving rise to biaryls 9f−9h in 80−85% yield (entries 5−7). Notably, when 7i and 7j bearing relatively sterically demanding o-OMe and oCH3 substituents, respectively, were subjected to Pd(OAc)2/1catalyzed cross-coupling, the corresponding products were obtained in good yields (9i: 85% and 9j: 93%, entries 8 and 9). Meanwhile, the cross-coupling of p-chlorotoluene with 8a did not proceed under the reaction conditions, resulting in complete recovery of the starting materials. 1932


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As summarized in Table 3, we further examined Pd(OAc)2/ 1-catalyzed Suzuki−Miyaura cross-coupling using aryl bromides Table 3. Scope of Arylboronic Acids 8b−8d for Pd/1Catalyzed Suzuki−Miyaura Cross-Coupling with Aryl Bromides 7b, 7d, and 7ga

entry

R1

R2

product

yield (%)b

1 2 3 4 5 6c 7c 8c,d 9c,d

p-NO2 (7b) p-COMe (7d) p-NO2 (7b) p-COMe (7d) p-NO2 (7b) p-COMe (7d) p-OMe (7g) p-OMe (7g) p-OMe (7g)

p-CF3 (8b) p-CF3 (8b) m,p-(OMe)2 (8c) m,p-(OMe)2 (8c) o-Me (8d) o-Me (8d) p-CF3 (8b) m,p-(OMe)2 (8c) o-Me (8d)

9k 9l 9m 9n 9o 9p 9q 9r 9s

99 86 99 90 99 99 99 97 86

Figure 3. Molecular structure of dimeric complex 10 (50% probability ellipsoids). Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond distances (Å) and angles (deg): P(2)−O(2) = 4.495(4), P(1)−O(1) = 4.424(4), P(2)−Pd(2) = 2.2493(12), P(1)− Pd(1) = 2.2456(12), Pd(1)−O(1) = 3.446(4), Pd(2)−O(2) = 4.580(4), P(2)−Pd(2)−Br(3) = 93.6(4), and P(1)−Pd(1)−Br(3) = 93.6(4).

a

Reaction conditions: Pd(OAc)2 (10 mol ppm), 1 (20 mol ppm), aryl bromide (1.0 mmol), arylboronic acid (1.25 mmol), K3PO4 (2.0 mmol), 2-methoxyethanol (6 mL), 100 °C, 24 h. bIsolated yield. cIn the presence of n-Bu4NBr (2.0 mmol). dPd(OAc)2 (50 mol ppm) and 1 (100 mol ppm).

phosphorous atoms of 1 bind to Pd with interatomic Pd−P distances of 2.2493(12) and 2.2456(12) Å (Figure 3).16 Notably, although 1 exists as a racemic mixture in the unit cell (space group = P21/c, Z = 4), each molecule of PdBr2/1 is composed of homochiral 1. This structural feature suggests that 1 could provide a chiral environment around the coordination sphere of Pd for asymmetric organic transformations. For reference, we tested the Suzuki−Miyaura cross-coupling of 7a and 8a using complex 10 (10 mol ppm) as a catalyst under otherwise identical conditions to those in Table 1. As shown in Scheme S1, the coupling reaction took place very efficiently to give 9a in 95% yield. Given that the 1:1 complex of PdBr2 and 1 (10) exhibits an excellent catalytic activity, a 1:1 complex of other Pd source and 1 would be the active species in the catalytic system for the Suzuki−Miyaura cross-coupling. Pd-Catalyzed Asymmetric Reactions Using Optically Active 1CD(+). Because of the limited variation in the chiral MOP ligands in comparison with the large variation in bidentate phosphine ligands, 1 seemed to be promising as a new candidate for the chiral MOP ligand. First, we conducted the Suzuki−Miyaura cross-coupling of 1-bromo-2-methylnaphthalene (7k) or 1-bromo-2-methoxynaphthalene (7l) with 2methylphenylboronic acid (8d) in the presence of optically active 1CD(+) (Scheme 2).17 Although these reactions showed high or moderate yields (9t: 86% and 9u: 63%), no enantiomeric excesses (ee) of these products were observed (Figures S2 and S3). Next, we conducted a Pd-catalyzed hydrosilylation reaction11,18 by using 1CD(+) as a chiral ligand (Scheme 3). Styrene was reacted with HSiCl3 in the presence of [PdCl(π-allyl)]2 (0.25 mol %) and 1CD(+) (0.60 mol %) in toluene at 25 °C to give 11 in 52% isolated yield. The Tamao oxidation of 11 using H2O2, KF, and KHCO3 afforded an optically active 1phenylethanol (12), whose ee value was determined to be 58% with R configuration (Figure S4). Although the enantioselectivity was moderate in this reaction, this result

(7b, 7d, and 7g) and substituted phenylboronic acids (8b−8d). Under conditions identical to those described in Table 2, the cross-coupling of electron-deficient aryl bromides (7b and 7d) with 8b bearing an electron-withdrawing CF3 group gave the corresponding biaryls in high yields (86−99%, entries 1 and 2). A similar result was obtained when electron-rich arylboronic acid 8c was used in place of 8b (entries 3 and 4). As represented by entries 5 and 6, the catalytic system of Pd(OAc)2/1 worked very efficiently for cross-coupling even with sterically demanding o-methylphenylboronic acid (8d). Meanwhile, for electron-rich aryl bromides such as 7g, to achieve a satisfactory yield of biaryl products, the reaction should be conducted with a larger amount of the Pd source, for example, 50 mol ppm of Pd(OAc)2 (entry 7) or in the presence of tetra-n-butylammonium bromide (entries 8 and 9), which is a promising additive for improving Pd-catalyzed reactions.14 Single-Crystal X-ray Structural Analysis of a Pd Complex with Racemic 1. To gain insight into the structure of the catalytic species, we attempted to crystallize a Pd/1 complex directly from a mixture of Pd(OAc)2 and racemic 1. Although this attempt was not successful, a dark orange-colored block-shaped single crystal suitable for X-ray crystallography was obtained when a 1:1 mixture of Pd(OAc)2 and 1 was treated with NaBr and recrystallized from a mixture of CH2Cl2 and diethyl ether (see the Supporting Information).15 The Xray diffraction analysis of the single crystal revealed the formation of a dimer of PdBr2/1 (10), where Pd atoms are bridged by μ-Br atoms (Figure 3). The dimer formation is in contrast to the case of a previously reported mononuclear PdCl2 complex of chiral 1-p-tolylthio-8-(diphenylphosphino)triptycene,9 where the PdCl2(II) is simply ligated with the P atom of the triptycene and a THF molecule without forming such a dimeric structure. In the crystal structure of 10, the 1933


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phosphine and alkoxy groups, might lead to new highly efficient catalytic systems to give a particular stereoselectivity.

Scheme 2. Asymmetric Suzuki−Miyaura Cross-Coupling Using Optically Active 1CD(+)a,b

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EXPERIMENTAL SECTION Materials. Unless otherwise noted, all commercial reagents were purchased from commercial suppliers (Sigma-Aldrich and Tokyo Chemical Industry) and used without purifications. Anhydrous CH2Cl2 and toluene were dried by passing them through an activated alumina column and a Q-5 column (Nikko Hansen). 1,8-Dihydroxytriptycene13 was prepared according to previously reported procedures and unambiguously characterized by nuclear magnetic resonance (NMR) spectroscopy and APCI-TOF mass spectrometry. General. Column chromatography was carried out using Wakogel silica C-300 (particle size: 45−75 μm). Recycling preparative chiral HPLC was carried out on an LC-9210 NEXT recycling preparative HPLC system (Japan Analytical Industry), equipped with CHIRAL ART Amylose-SA column, YMC Co., Ltd. (diameter: 20 mm; length: 250 mm) and a multiwavelength detector (MD-2010Plus). Analytical chiral HPLC was carried out on a JASCO HSS-1500 system, equipped with CHIRAL ART Amylose-SA column, YMC Co., Ltd. (diameter: 4.6 mm; length: 250 mm) or CHIRALCEL OD-H column, DAICEL Co., Ltd. (diameter: 4.6 mm; length: 250 mm). Melting points (mp) and decomposition points (dp) were recorded on a Yanaco MP-500D melting-point apparatus. IR spectra were recorded at 25 °C on a JASCO FT/IR-660Plus Fourier transform IR spectrometer. CD spectra were recorded on a JASCO J-820 spectropolarimeter. NMR spectroscopy measurements were taken at 25 °C on a Bruker AVANCE III HD500 spectrometer (1H: 500.0 MHz, 13C: 125.0 MHz, 19F: 470.5 MHz, and 31P: 202.0 MHz) or on a Bruker AVANCE400 spectrometer (1H: 400.0 MHz and 13C: 100.0 MHz) or on a Varian 400-MR (1H: 400.0 MHz and 13C: 100.0 MHz). Chemical shifts (δ) are expressed relative to the resonances of the residual nondeuterated solvent for 1H (CDCl3: 1H (δ) = 7.26 ppm and DMSO-d6: 1H (δ) = 2.50 ppm), 13C (CDCl3: 13 C (δ) = 78.0 ppm and DMSO-d6: 13C (δ) = 39.5 ppm), external CF3CO2H in CDCl3 for 19F (19F (δ) = −78.5 ppm), and external H3PO4 in CDCl3 for 31P (31P (δ) = 0.0 ppm). Absolute values of the coupling constants are given in hertz, regardless of their sign. Multiplicities are abbreviated as singlet (s), doublet (d), triplet (t), quartet (q), and multiplet (m). APCI-TOF mass spectrometry measurements were taken on a Bruker micrOTOF II mass spectrometer equipped with an atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI) probe. Specific optical rotations of optically active compounds were measured on a JASCO P-2200 digital polarimeter in a 5 cm quartz cuvette at 25 °C. 1,8-Bis-(trifluoromethanesulfonyloxy)triptycene (3). At 25 °C, triflic anhydride (3.0 mL, 18 mmol) was added to a 1,2dichloroethane solution (50 mL) of a mixture of 1,8dihydroxytriptycene 2 (1.30 g, 4.55 mmol) and pyridine (3.00 mL, 37.3 mmol). The resultant mixture was stirred at 60 °C for 18 h, allowed to cool to 25 °C, poured into water (100 mL), and extracted with CHCl3 (100 mL, three times). The organic extract was washed successively with water and brine, dried over anhydrous MgSO4, and evaporated to dryness under reduced pressure. The residue was subjected to column chromatography on SiO2 (hexane/CHCl3; v/v = 3/1) to allow the isolation of 3 as a white powder (1.88 g, 3.42 mmol) in 75% yield: mp: 191 °C. FT-IR (KBr) v (cm−1): 3078, 1613, 1579, 1465, 1429, 1251, 1212, 1135, 1098, 996, 958, 892, 862,

a

Isolated yield. bThe ee values were determined by HPLC (CHIRALCEL OD-H with n-hexane, see the Supporting Information).

Scheme 3. Asymmetric Hydrosilylation Reaction Using Optically Active 1CD(+)a,b

a

Isolated yield. b The ee value was determined by HPLC (CHIRALCEL OD-H with n-hexane/2-PrOH = 95/5, see the Supporting Information).18

clearly indicated that 1CD(+) could serve as a chiral monophosphine ligand.

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CONCLUSIONS In conclusion, inspired by a geometrical model (Figure 1d), we designed a triptycene-based monophosphine ligand, 1-methoxy-8-(diphenylphosphino)triptycene (1), for the development of a new catalytic system for transition metal-catalyzed organic transformation. Surprisingly, there has been no previous report on the catalytic application of a triptycene of this type. Compound 1 was obtained in a manner similar to the synthesis of well-known MOP.11,12 Using Pd-catalyzed Suzuki− Miyaura cross-coupling, we demonstrated the catalytic activity of 1 complexed with Pd(II). The catalytic system worked efficiently for cross-coupling reactions between a wide variety of aryl bromides and arylboronic acids and gave the corresponding biaryls in good to excellent yields (80−99%), even when the Pd loading was as low as 0.001 mol %. The successful chiral HPLC separation of the precursor of 1 allowed the synthesis of optically active 1. Although this chiral ligand was unable to induce chirality in Suzuki−Miyaura cross-coupling, it was applicable to a Pd-catalyzed asymmetric hydrosilylation reaction. The ee of the product was moderate (58% ee), but this was surprising considering the rather simple stereochemistry of 1, which may be described as a triangle with three dif ferent poles at the apexes (Figure 1d). These results show the potential of this triptycene-based monophosphine ligand with structural rigidity and steric bulkiness around the phosphine group. Thus, proper modifications of this prototype, in terms of the triptycene skeleton itself and substituents of 1934


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821, 792, 752, 733, 712, 666, 648, 629, 608, 571, 517, 499. 1H NMR (500 MHz, CDCl3): δ (ppm) 7.50−7.49 (m, 1H), 7.43− 7.41 (m, 3H), 7.11−7.07 (m, 4H), 6.98 (d, J = 10.5 Hz, 2H), 6.15 (s, 1H), 5.58 (s, 1H). 13C NMR (125 MHz, CDCl3): δ (ppm) 148.7, 144.3, 144.1, 141.6, 136.0, 127.5, 126.2, 126.1, 125.2, 124.0, 123.7, 122.5, 120.0, 118.8, 117.4, 114.9, 53.7, 42.4. 19 F NMR (470 MHz, CDCl3): δ (ppm) −73.2. High-resolution ACPI-TOF mass: calcd for C22H12O6F6S2 [M]+: m/z = 549.9974; found, 549.9953. 1H and 13C NMR spectra of 3 are shown in Figures S7 and S8, respectively. 8-Trifluoromethanesulfonyltriptycene-1-diphenylphosphine Oxide (4). Under argon at 25 °C, N,N-diisopropylethylamine (0.63 mL, 3.6 mmol) was added to a DMSO solution (4.0 mL) of a mixture of 3 (0.50 g, 0.91 mmol), diphenylphosphine oxide (0.38 g, 1.88 mmol), Pd(OAc)2 (10.2 mg, 0.045 mmol), and 1,4-bis(diphenylphosphino)butane (19.4 mg, 0.045 mmol). The resultant mixture was stirred at 100 °C for 12 h, allowed to cool to 25 °C, poured into water (100 mL), and extracted with ethyl acetate (100 mL, three times). The organic extract was washed successively with water and brine, dried over anhydrous MgSO4, and evaporated to dryness under reduced pressure. The residue was subjected to column chromatography on SiO2 (ethyl acetate/CH2Cl2; v/v = 9/1) to allow the isolation of a racemic mixture of 4 as a white powder (0.49 g, 0.81 mmol) in 90% yield: mp: 206 °C. FT-IR (KBr) v (cm−1): 3419, 3073, 2959, 2932, 2834, 2345, 2216, 1941, 1701, 1613, 1580, 1480, 1465, 1428, 1322, 1206, 1135, 1098, 1067, 996, 958, 892, 862, 822, 791, 752, 722, 708, 696, 679, 666, 647, 629, 608, 570, 543, 517, 499, 458. 1H NMR (500 MHz, CDCl3): δ (ppm) 7.71−7.67 (dd, J = 7.5, 4.5 Hz, 4H), 7.56−7.54 (m, 3H), 7.47−7.44 (m, 4H), 7.34−7.32 (dd, J = 7.5, 2.5 Hz, 2H), 7.02−6.95 (m, 3H), 6.86−6.85 (m, 3H), 6.57 (d, J = 7.5 Hz, 1H), 6.52 (s, 1H), 5.51 (s, 1H). 13C NMR (125 MHz, CDCl3): δ (ppm) 149.1, 148.3, 148.2, 146.9, 146.8, 144.8, 144.3, 142.0, 136.3, 133.1, 132.7, 132.3, 132.2, 132.1, 132.0, 131.9, 131.8, 131.7, 129.4, 129.3, 129.2, 128.8, 128.7, 128.6, 128.5, 128.4, 127.5, 127.4, 126.9, 125.6, 125.5, 125.2, 125.1, 125.0, 123.4, 123.3, 119.9, 118.7, 117.3, 54.0, 46.0. 19F NMR (470 MHz, CDCl3): δ (ppm) −73.3. 31P NMR (202 MHz, CDCl3): δ (ppm) 29.2. High-resolution ACPI-TOF mass: calcd for C33H22O4F3PS [M]+: m/z = 602.0934; found: 602.0922. 1H and 13C NMR spectra of 4 are shown in Figures S9 and S10, respectively. 8-Hydroxytriptycene-1-diphenylphosphine Oxide (5). An aqueous solution (10 mL) of NaOH (1.20 g, 30 mmol) was dropwise added to a MeOH/1,4-dioxane solution (18 mL; v/v = 1/2) of 4 (0.40 g, 0.77 mmol), and the mixture was stirred at 25 °C for 12 h. After the addition of an aqueous solution of HCl (1 N, 20 mL) to the reaction mixture, a white precipitate formed was collected by filtration, washed with water (100 mL), and dried under reduced pressure. The residue was subjected to column chromatography on SiO2 (CH2Cl2) to allow the isolation of a racemic mixture of 5 as a white powder (0.34 g, 0.72 mmol) in 94% yield: mp: 313 °C. FT-IR (KBr) v (cm−1): 3071, 1592, 1474, 1455, 1437, 1418, 1238, 1169, 1142, 1116, 930, 832, 790, 725, 706, 644, 611, 565, 542, 454. 1H NMR (500 MHz, CDCl3): δ (ppm) 8.87 (s, 1H), 7.78 (dd, J = 7.5, 4.5 Hz, 1H), 7.67−7.65 (m, 3H), 7.64−7.49 (m, 6H), 7.40 (d, J = 8.0 Hz, 1H), 7.00 (dd, J = 7.5, 7.5 Hz, 1H), 6.95 (d, J = 7.0 Hz, 1H), 6.87−6.83 (m, 3H), 6.70 (d, J = 7.5 Hz, 1H), 6.41 (dd, J = 7.5, 4.5 Hz, 1H), 6.41 (s, 1H), 6.39 (d, J = 7.5 Hz, 1H), 5.45 (s, 1H). 13C NMR (125 MHz, CDCl3): δ (ppm) 152.5, 150.9, 150.8, 148.3, 148.2, 146.6, 144.7, 142.6, 132.5, 132.5,

132.4, 132.3, 132.2, 132.1, 132.0, 130.9, 129.1, 128.9, 128.7, 128.6, 127.9, 127.8, 127.7, 127.6, 126.3, 125.7, 124.3, 124.2, 124.1, 123.5, 116.2, 116.1, 54.0, 46.2, 46.1. 31P NMR (202 MHz, CDCl3): δ (ppm) 32.9. High-resolution ACPI-TOF mass: calcd for C32H23O2P [M]+: m/z = 470.1430; found: 470.1428. 1H and 13C NMR spectra of 5 are shown in Figures S11 and S12, respectively. 8-Methoxytriptycene-1-diphenylphosphine Oxide (6). Methyl iodide (0.1 mL, 1.2 mmol) was added to an acetone solution (20 mL) of 5 (0.20 g, 0.43 mmol) and K2CO3 (0.24 g, 1.2 mmol) and stirred at 25 °C for 3 h. The reaction mixture was poured into water (100 mL) and extracted with ethyl acetate (100 mL, three times). The organic extract was washed successively with water and brine, dried over anhydrous MgSO4, and evaporated to dryness under reduced pressure. The residue was subjected to column chromatography on SiO2 (CH2Cl2) to allow the isolation of 6 as a white powder (0.20 g, 0.41 mmol) in 95% yield: mp: 224 °C. FT-IR (KBr) v (cm−1): 3402, 3057, 3019, 2958, 2932, 2834, 2215, 1904, 1826, 1707, 1590, 1481, 1458, 1436, 1415, 1323, 1267, 1190, 1159, 1145, 1118, 1097, 1071, 1026, 997, 967, 927, 874, 831, 786, 752, 722, 705, 696, 647, 608, 591, 566, 544, 475. 1H NMR (500 MHz, CDCl3): δ (ppm) 7.65−7.57 (m, 4H), 7.51 (dd, J = 7.5, 3.0 Hz, 3H), 7.42−7.37 (m, 4H), 7.29 (d, J = 7.0 Hz, 1H), 6.98 (d, J = 7.0 Hz, 1H), 6.95−6.82 (m, 6H), 6.56 (s, 1H), 6.41 (d, J = 8.0 Hz, 1H), 5.43 (s, 1H), 3.41 (s, 3H). 13C NMR (125 MHz, CDCl3): δ (ppm) 154.7, 150.2, 150.1, 147.8, 147.7, 147.5, 145.5, 144.2, 134.0, 133.1, 133.0, 132.4, 132.3, 132.2, 132.1, 131.8, 131.7, 131.6, 131.4, 129.2, 129.1, 128.5, 128.4, 128.3, 128.2, 127.5, 127.4, 127.3, 126.7, 126.3, 125.2, 124.9, 124.7, 124.5, 124.4, 123.3, 116.0, 109.4, 108.0, 81.2, 55.3, 54.4, 44.8, 24.9. 31P NMR (202 MHz, CDCl3): δ (ppm) 30.1. Highresolution ACPI-TOF mass: calcd for C33H25O2P [M + H]+: m/z = 485.1665; found: 485.1651. 1H and 13C NMR spectra of 6 are shown in Figures S13 and S14, respectively. Optically active 6CD(+) and 6CD(−), which were obtained by the optical resolution using chiral HPLC (CHIRAL ART Amylose-SA column, CHCl3/hexane = 1/2 v/v), showed specific optical rotations ([α]25 D ) of 24.8 and −25.0, respectively, in CHCl3 (c = 0.04 g/dL). 8-Methoxytriptycene-1-diphenylphosphine (1). Under argon at 25 °C, HSiCl3 (0.16 mL, 1.6 mmol) was added dropwise to a toluene solution (15 mL) of a mixture of 6 (0.15 g, 0.31 mmol) and triethylamine (0.87 mL, 6.2 mmol), and the resultant mixture was stirred at 120 °C for 18 h and allowed to cool to 25 °C. An aqueous solution (50 mL) of NaHCO3 (4.2 g, 50 mmol) was added slowly to the reaction mixture and extracted with diethyl ether (100 mL, three times). The organic extract was washed successively with water and brine, dried over anhydrous MgSO4, and evaporated to dryness under reduced pressure. The residue was subjected to column chromatography on SiO2 (hexane/CH2Cl2; v/v = 3/1) to enable the isolation of 1 as a white powder (0.13 g, 0.28 mmol) in 90% yield: mp: 216 °C. FT-IR (KBr) v (cm−1): 3446, 3051, 3001, 2952, 2928, 1589, 1480, 1457, 1432, 1416, 1324, 1267, 1193, 1160, 1097, 1071, 1025, 964, 909, 786, 767, 746, 729, 694, 679, 646, 591, 539, 500, 466. 1H NMR (500 MHz, CDCl3): δ (ppm) 7.35−7.20 (m, 12H), 7.00 (dd, J = 7.0, 7.0 Hz, 2H), 6.94−6.85 (m, 4H), 6.49−6.41 (m, 3H), 5.41 (s, 1H), 5.28 (s, 1H), 3.51 (s, 3H). 13C NMR (125 MHz, CDCl3): δ (ppm) 154.7, 149.9, 149.6, 147.7, 146.0, 145.8, 144.6, 136.7, 136.6, 134.3, 134.1, 134.0, 133.9, 132.3, 131.8, 131.7, 128.9, 128.6, 128.5, 128.4, 128.3, 128.2, 125.0, 124.9, 124.3, 124.1, 1935


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123.3, 116.1, 108.2, 55.5, 54.5, 44.6, 44.5. 31P NMR (202 MHz, CDCl3): δ (ppm) −14.7. High-resolution ACPI-TOF mass: calcd for C33H25OP [M + H]+: m/z = 469.1716; found: 469.1728. 1H and 13C NMR spectra of 1 are shown in Figures S5 and S6, respectively. Optically active 1CD(+) that was prepared from 6CD(+) showed a specific optical rotation ([α]25 D) of 77.9 in CHCl3 (c = 0.04 g/dL). [PdBr2-1]2 (10). At 25 °C, NaBr (100 mg, 0.97 mmol) was added to a 2-methoxyethanol solution (4 mL) of a mixture of racemic 1 (22 mg, 0.047 mmol) and Pd(OAc)2 (10.6 mg, 0.047 mmol), and the mixture was stirred at 25 °C for 1 h and then poured into water (10 mL). The resultant brown precipitate was collected by filtration, washed with water (20 mL), and dried under reduced pressure. The residue was recrystallized from CH2Cl2 under diethyl ether vapor to give [PdBr2-1]2 10 as dark orange single crystals (68 mg, 0.0465 mmol) in 99% yield: mp: 252 °C. FT-IR (KBr) v (cm−1): 3447, 3056, 2956, 2931, 2832, 2356, 1590, 1481, 1457, 1435, 1416, 1314, 1267, 1187, 1159, 1141, 1094, 1074, 998, 966, 924, 871, 828, 786, 750, 730, 691, 646, 607, 591, 543, 514, 475. 1H NMR (500 MHz, DMSO-d6): δ (ppm) 7.82 (dd, J = 8.0, 4.0 Hz, 2H), 7.64 (d, J = 7.0 Hz, 1H), 7.59−7.58 (m, 1H), 7.51−7.47 (m, 3H), 7.41 (d, J = 7.0 Hz, 3H), 7.20−7.09 (m, 5H), 7.04−6.89 (m, 4H), 6.77 (dd, J = 10.0, 10.0 Hz, 1H), 6.58 (d, J = 8.0 Hz, 1H), 5.74 (s, 1H), 3.32 (s, 3H). 13C NMR (125 MHz, DMSO-d6): δ (ppm) 154.8, 148.9, 148.8, 147.8, 147.6, 147.5, 146.2, 144.5, 137.4, 137.3, 134.9, 134.8, 132.2, 131.5, 131.0, 128.7, 128.6, 128.2, 127.8, 127.6, 127.2, 126.9, 125.7, 125.2, 124.9, 123.8, 116.3, 108.5, 55.1, 53.5, 45.9, 45.8. 31P NMR (202 MHz, DMSO-d6): δ (ppm) 25.5. High-resolution ESI-TOF mass: calcd for C66H50Pd2O2P2Br4 [M − Br]+: m/z = 1388.8901; found: 1388.8916. 1H and 13C NMR spectra of 10 are shown in Figures S15 and S16, respectively. General Experimental Procedures for Pd/1-Catalyzed Suzuki−Miyaura Cross-Coupling. A 2-methoxyethanol solution (6 mL) of a mixture of aryl bromides 7 (1.0 mmol), arylboronic acid 8 (1.25 mmol), Pd(OAc)2 (0.001 mol %, 10 mol ppm), 1 (0.002 mol %, 20 mol ppm), and K3PO4 (2.0 mmol) was added to a reaction tube. The reaction mixture was stirred at 100 °C for 24 h under air, allowed to cool to 25 °C, poured into water (25 mL), and extracted with ethyl acetate (30 mL, three times). The organic extract was washed successively with water and brine, dried over anhydrous MgSO4, and evaporated to dryness under reduced pressure. The residue was subjected to column chromatography, which enabled the isolation of the desired product 9.

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Takanori Fukushima: 0000-0001-5586-9238 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas “π-Figuration” (no. 26102008 for T.F., no. 15H00994 for Y.N.) and “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT). F.K.C.L. kindly acknowledges the International Program Associate (IPA) of RIKEN. We thank K. Osakada and D. Takeuchi (Tokyo Institute of Technology) for their support with the specific optical rotation measurement. We thank Suzukakedai Materials Analysis Division, Technical Department, Tokyo Institute of Technology, for their support with the NMR measurement.

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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/acsomega.7b00200. Tables S1−S3, Scheme S1, and Figures S1−S60 (PDF) Crystallographic data for 10 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.M.A.Y.). *E-mail: [email protected] (T.F.). ORCID

Yoshiaki Shoji: 0000-0001-8437-1965 Yuuya Nagata: 0000-0001-5926-5845 Michinori Suginome: 0000-0003-3023-2219 1936


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