Article pubs.acs.org/Organometallics
Syntheses of RuHCl(CO)(PAr3)3 and RuH2(CO)(PAr3)3 Containing Various Triarylphosphines and Their Use for Arylation of Sterically Congested Aromatic C−H Bonds Yohei Ogiwara,† Masashi Miyake,† Takuya Kochi,† and Fumitoshi Kakiuchi*,†,‡ †
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan ‡ JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: A series of ruthenium complexes, RuHCl(CO)(PAr3)3 and RuH2(CO)(PAr3)3, containing various triarylphosphines were synthesized. Screening of these complexes as catalysts for direct arylation of sterically congested ortho C−H bonds of aromatic ketones improved the yields of the arylation products.
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INTRODUCTION Phosphine-containing transition-metal complexes are often used as catalysts for a variety of organic transformations. In many cases, the choice of an appropriate phosphine is critical to achieve the desired reactivity, and tuning of the steric and electronic properties of phosphine ligands is considered as an important process in the development of catalytic reactions. Ruthenium hydrido carbonyl complexes, RuHCl(CO)(PPh3)3 (1a) and RuH2(CO)(PPh3)3 (2a), have been used as catalysts for a variety of transformations.1−7 For example, our group reported a direct C−H arylation of aromatic ketones with arylboronates using dihydrido complex 2a as a catalyst.8,9 A combination of 1a with CsF was also found to catalyze the C−H arylation, albeit with lower efficiency.8b However, there have not been many reported syntheses of analogous complexes containing other triarylphosphines, and most of the studies on RuHCl(CO)(PAr3)3 (1)10 and RuH2(CO)(PAr3)3 (2)11 have been limited to PPh3 complexes.12,13 We envisioned that the fine tuning of the triarylphosphine ligands of complexes 1 and 2 may lead to an improvement in catalytic efficiency in the direct C−H arylation of some challenging substrates. Direct C−H arylation of arenes is one of the most intensely investigated reactions in the past decade, and many catalyst systems have been developed for this reaction.14 However, it has been difficult to achieve direct arylation reaction at sterically congested aromatic C−H bonds whose two adjacent positions are substituted.15 Particularly, there have been no reports on high-yielding chelation-assisted regioselective arylation at sterically congested ortho C−H bonds of aromatic compounds possessing meta substituents. Here we describe the syntheses of ruthenium complexes 1 and 2 containing various triarylphosphines. Chelation-assisted direct arylation of sterically congested C−H bonds of aromatic ketones was also achieved using the new series of ruthenium complexes. © XXXX American Chemical Society
RESULTS AND DISCUSSION Synthesis of RuHCl(CO)(PAr3)3 (1) Containing Various Triarylphosphines. First, we examined the synthesis of hydrido chloro complexes 1. There have been several examples reported of RuHCl(CO)(PAr3)3-type complexes (Ar ≠ Ph), and most of them contain aryl groups bearing polar substituents such as CO2H and SO3Na.10 These complexes were prepared either by phosphine exchange reaction of 1a with the corresponding phosphine10c,d or by following the previously reported synthetic procedure of 1a by Robinson and coworkers,16 in which ruthenium(III) chloride was reacted with PPh3 and aqueous formaldehyde in 2-methoxyethanol.10a,b Initially, we examined the synthesis of 1 using 2-methoxyethanol as a solvent, but the corresponding complex could not be obtained. Then the screening of the reaction conditions was conducted, and it was found that the use of methanol or ethanol as a solvent provided the corresponding complexes 1b−g (Table 1). For example, complex 1b possessing P(4MeC6H4)3 ligands was obtained using either ethanol or methanol in good yields (entries 1 and 2). The reaction using P(3-MeC6H4)3 and methanol afforded complex 1c in 78% yield (entry 3), but the corresponding complex was not detected in the case of P(2-MeC6H4)3 (entry 4). The reaction of triarylphosphines bearing methoxy, ethyl, and fluoro groups at the para positions of the aryl groups gave complexes 1d−f in good yields (entries 5−7), but the corresponding complex bearing P(4-F3CC6H4)3 could not be obtained using this method (entry 8). The reactions using triarylphosphines possessing meta substituents were also examined. The use of P(3-MeOC6H4)3 provided complex 1g in 59% yield (entry 9), Special Issue: Hydrocarbon Chemistry: Activation and Beyond Received: July 4, 2016
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DOI: 10.1021/acs.organomet.6b00540 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Synthesis of RuHCl(CO)(PAr3)3 (1)a
P(4-MeC6H4)3 in refluxing ethanol gave the desired dihydrido complex 2b in 73% yield (Table 2, entry 1). The same reaction Table 2. Synthesis of RuH2(CO)(PAr3)3 (2)a
entry
PAr3
ROH
1
yield (%)
1 2 3 4 5 6 7 8 9c 10 11c
P(4-MeC6H4)3 P(4-MeC6H4)3 P(3-MeC6H4)3 P(2-MeC6H4)3 P(4-MeOC6H4)3 P(4-EtC6H4)3 P(4-FC6H4)3 P(4-F3CC6H4)3 P(3-MeOC6H4)3 P(3,5-Me2C6H3)3 P(4-MeO-3,5-Me2C6H2)3
EtOH MeOH MeOH MeOH or EtOH MeOH MeOH EtOH MeOH or EtOH MeOH EtOH MeOH
1b 1b 1c
83 73 78 ndb 60 81 79 ndb 59 90 65
1d 1e 1f 1g 1h 1i
a Reaction conditions unless specified otherwise: RuCl3·3H2O, PAr3 (6 equiv), HCHO (aq), ROH, reflux, 1 h. bNot detected. cPerformed for 3 h.
entry
1
PAr3
1 2 3 4 5 6 7 8
1b 1b 1b 1c 1d 1e 1f 1h
P(4-MeC6H4)3 P(4-MeC6H4)3 P(4-MeC6H4)3 P(3-MeC6H4)3 P(4-MeOC6H4)3 P(4-EtC6H4)3 P(4-FC6H4)3 P(3,5-Me2C6H3)3
ROH EtOH PrOH MeOH EtOH MeOH MeOH EtOH MeOH or EtOH
i
time (h) 7 6 5 12 5 12 5 13
2 2b 2b 2b 2c 2d 2e 2f
yield (%) 73 72 ndb 62 80 39 94 ndb
a Reaction conditions: RuHCl(CO)(PAr3)3, PAr3 (3.3 equiv), KOH (5 equiv), ROH, reflux. bNot detected.
and complexes containing phosphines with 3,5-dimethylphenyl and 3,5-dimethyl-4-methoxyphenyl groups (1h,i) were similarly prepared in 90% and 65% yields, respectively (entries 10 and 11). Triphenylphosphine complex 1a and complexes 1b−i prepared here showed similar characteristic signals on 1H and 31 P NMR and IR spectra. A doublet of triplets corresponding to the hydride was found in each 1H NMR spectrum in the region between −7.09 and −6.48 ppm (J = 104.2−107.1, 23.2−27.1 Hz). Two signals were observed in 31P NMR spectra, and one of them corresponding to two phosphines appeared at 35.7− 40.1 ppm, while the other was observed at 10.0−14.7 ppm. The IR spectra of complexes 1 showed a CO absorption band at 1921−1933 cm−1. Structural determination by single-crystal Xray diffraction analysis was difficult mainly because of the significant disorder of the positions of the CO and Cl ligands. Synthesis of RuH2(CO)(PAr3)3 (2) Containing Various Triarylphosphines. The synthesis of RuH2(CO)(PAr3)3 (2) was then investigated. Syntheses of RuH2(CO)(PAr3)3-type complexes (Ar ≠ Ph) have been reported so far using P(4FC6H4)3,11a PPh2(4-MeC6H4),11b P{3-(NaO3S)C6H4}3,11c PPh2{P(3-NaO3S)C6H4},11d P(4-F3CC6H4)3,11e and P{3,5(F3C)2C6H3}3.11e In particular, RuH2(CO){P(4-FC6H4)3}3 (2f) prepared by Mitsudo and co-workers has been used as an effective catalyst for their [2 + 2] cross-addition of norbornene derivatives and dimethyl acetylenedicarboxylate,7a and recently, for catalytic annulation between 1-naphthylsilanes and internal alkynes reported by Tokoro and Fukuzawa.3e The most convenient method for the synthesis of RuH2(CO)(PPh3)3 (2a) is the reaction of ruthenium(III) chloride with PPh3 and aqueous formaldehyde in ethanol in the presence of potassium hydroxide, reported by Robinson.16 However, the attempted synthesis of RuH2(CO){P(4-MeC6H4)3}3 (2b) using this method was not successful, because the reaction did not give 2b selectively and it was difficult to isolate 2b in a pure form. Junk and Steed reported that conversion of 1a to 2a can be achieved by treatment of 1a with potassium hydroxide in the presence of PPh3.17 Therefore, the synthesis of 2 containing various triarylphosphines from complexes 1 was examined. Treatment of chloro complex 1b with potassium hydroxide and
in 2-propanol also provided 2b in 72% yield (entry 2), but complex 2b was not obtained when methanol was used as a solvent in this case (entry 3). The reaction with 1c in ethanol also gave the corresponding complex 2c containing P(3MeC6H4)3 as a ligand (entry 4). Conversion of complexes 1d−f having para-substituted triarylphosphines also successfully afforded the corresponding complexes 2d−f in 39−94% yields (entries 5−7), but the reaction with complex 1h using methanol and ethanol as solvents failed to give the corresponding dihydrido complex. Dihydrido complexes 2b−f also showed characteristic signals similar to those of triphenylphosphine complex 2a in their 1H and 31P NMR and IR spectra. Two doublets of doublets of triplets having similar spin−spin coupling constants were observed in the 1H NMR spectra as signals corresponding to the hydrides. Two signals were also found in 31P NMR spectra at 53.2−57.6 and 41.9−46.1 ppm. CO absorption bands in the IR specta were observed at 1926−1939 cm−1. Single-crystal Xray diffraction analysis was also successfully conducted for complexes 2b,d,f to confirm that these complexes possess similar structures, and no significant differences in bond lengths and angles were observed. Regioselective Direct Arylation of Sterically Congested Ortho C−H Bonds of Aromatic Ketones. As mentioned in the Introduction, direct arylations at sterically congested aromatic C−H bonds are still rare, and there has been no report on high-yielding chelation-assisted regioselective arylation of ortho C−H bonds whose adjacent meta positions possess carbon substituents. Therefore, we examined ruthenium-catalyzed arylation of sterically congested ortho C− H bonds of aromatic ketones with arylboronates. First, C−H arylation of benzophenone derivative 3 was investigated. The most standard catalyst we have used for the C−H arylation is complex 2a, and the reaction is usually performed in pinacolone, which functions as a solvent and a hydride acceptor. When benzophenone derivative 3, whose all four meta positions possess methyl groups, was reacted with phenylboronate 4a in pinacolone at 120 °C for 24 h, a 22% GC yield of monophenylation product 5a was formed (Table 3, entry 1). Screening of the conditions using catalyst 2a revealed B
DOI: 10.1021/acs.organomet.6b00540 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 3. C−H Phenylation of Benzophenone Derivative 3 Using Triphenylphosphine Complexes 1a and 2aa
Table 4. Screening of Phosphines in Ruthenium Catalysts 1 and 2 for C−H Phenylation of Benzophenone Derivative 3
yield (%)c entry
Ru cat.
1a 2a 3a 4a 5a 6b 7b 8b 9b 10b 11b 12b 13b
2b 2c 2d 2e 2f 1b 1c 1d 1e 1f 1g 1h 1i
yield (%)b entry
Ru cat.
1 2
2a 2a
3 4
1a 1a
5
1a
6
2a
CsF (equiv)
2 2
2
solvent pinacolone acetone/ mesitylenec pinacolone acetone/ mesitylened acetone/ mesitylened acetone/ mesitylenec
temp (°C)
5a
6a
120 140
22 55
