Article pubs.acs.org/Organometallics
Pd/Josiphos-Catalyzed Enantioselective α-Arylation of Silyl Ketene Acetals and Mechanistic Studies on Transmetalation and Enantioselection Kenya Kobayashi, Yasunori Yamamoto,* and Norio Miyaura Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan S Supporting Information *
ABSTRACT: Palladium/(4-MeO-3,5-MePh)2PF-Pcy2) catalyzed the enantioselective arylation of silyl ketene acetals with iodoarenes in the presence of TlOAc to promote transmetalation of silyl ketene acetals. The highest enantioselectivities giving α-arylesters up to 91% ee were achieved when (E)-1-methoxy-1-(trimethylsiloxy)propene (E/Z = 88/12) was used for the silyl ketene acetal. The effect on enantioselection of a chiral ligand is discussed on the basis of the X-ray structure of the palladium/ (4-MeO-3,5-MePh)2PF-Pcy2) complex and results of DFT computational studies on mechanistic aspects of the catalytic cycle.
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Shiina.10 More recently, MacMillan and Gaunt have independently reported the copper-catalyzed enantioselective α-arylation of carbonyls with iodonium salts.11 Asymmetric α-arylation of carbonyl compounds that can form tertiary centers with high enantioselectivity was a longstanding problem due to enolization. Herein, we report the palladium/ Josiphos complex-catalyzed enantioselective coupling of iodoarenes with silyl ketene acetals for the synthesis of α-arylesters (Scheme 1).
INTRODUCTION Metal-catalyzed α-arylation of carbonyl compounds is one of the useful C−C bond-forming reactions.1 Palladium-catalyzed intermolecular α-arylation of ketones with aryl halides was independently discovered by Miura, 2a Buchwald,2b and Hartwig2c in 1997. The asymmetric arylation of enolates provides an effective method for constructing quaternary stereocenters.3 Unfortunately, these methods cannot be applied to the asymmetric synthesis of common tertiary stereocenters, because of the tendency of α-aryl ketones to enolize under the basic conditions. Thus, the use of preformed enolates, such as silyl ketene acetals, can be the key for achieving highly enantioselective construction of tertiary-α-arylated carbonyl compounds with prevention of racemization in mild reaction conditions. A well-established method for the synthesis of α-arylsubstituted carboxylic acid esters is palladium-catalyzed reaction of silyl ketene acetals with aryl halides or triflates.4,5 The coupling of aryl halides with ester enolates provides easy access to α-aryl esters,6 including important intermediates in the syntheses of nonsteroidal anti-inflammatory drugs (NSAIDs), such as Ibuprofen, Ketoprofen, Flurbiprofen, and Naproxen.7 Recently, some effective methods for obtaining such chiral α-aryl carboxylic esters have been reported. Fu and co-workers reported nickel-catalyzed cross-coupling of α-halocarbonyls with aryl-organometallic reagents to obtain α-aryl carbonyl compounds in high enantioselectivity.8 We have reported an asymmetric γ-selective cross-coupling reaction of potassium allyltrifluoroborates with bromoarenes catalyzed by a palladium/ CyPF-t-Bu complex.9 Moreover, optically active carboxylic esters are produced by the kinetic resolution of racemic α-aryl carboxylic acids using chiral acyl-transfer catalysts reported by © 2011 American Chemical Society
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RESULTS AND DISCUSSION
Optimization of Reaction Conditions. The yields and enantioselectivities were highly sensitive to phosphine ligands employed for palladium acetate in the reaction between methyl 4-iodobenzoate and (E)-1-methoxy-1-(trimethylsiloxy)propene (E/Z = 88/12). Among phosphine ligands screened for optimization, (R)-(S)-(4-MeO-3,5-MePh)2PF-Pcy2 (4e) was found to be the best ligand to achieve 91% ee with a quantitative yield (entry 1 in Table 1), whereas reaction using other derivatives in the Josiphos series (4), Walphos series (5), (S)-BINAP, and (R,R)-DIOP resulted in poor yields and low enantioselectivities (entries 2−9). The reaction proceeded smoothly in toluene but was very slow in other solvents, such as dioxane, DMF, and 1,2-dichloroethane (entries 10−12). Iodoarene was the best substrate to achieve high yield and ee compared to the corresponding bromoarene or ArOTf (entries 1, 13, 14).12 Although TlOAc was used to accelerate the reaction, other Received: October 7, 2011 Published: November 2, 2011 6323
dx.doi.org/10.1021/om200945q | Organometallics 2011, 30, 6323−6327
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Table 2. Effect of Substitutions on Silyl Ketene Acetalsa
Scheme 1
Table 1. Optimizationa
entry
1
R1 =
R2 =
E/Z
3
1 2 3 4 5 6 7 8 9 10 11 12 13
1a 1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l
Me Me Me Me Me Me Me Me Et n-Pr i-Pr Cy i-Bu
Me Me Et n-Pr i-Pr Cy t-Bu Ph Me Me Me Me Me
88/12 51/49 83/17 n.d. 88/12 83/17 >99/1 91/9 92/8 91/9 97/3 n.d. 94/6
3aa 3aa 3ba 3ca 3da 3ea 3fa 3ga 3ha 3ia 3ja 3ka 3la
yield/%
ee/%
99 45 95 99 96 62 45 28 95 85 74 43 92
91 73 89 90 60 71 61d 0 64 60 55 50 91
b
c
a
All reactions were carried out at 70 °C for 6 h in the presence of Pd(OAc)2 (5 mol %), (R)-(S)-(4-MeO-3,5-MePh)2PF-Pcy2 (4a, 5.5 mol %), methyl 4-iodobenzoate (0.25 mmol), 1 (1.5 equiv), TlOAc (2 equiv), and toluene (2.5 mL). bIsolated yield. cEnantiomer excess was determined by a chiral stationary column. dEnantiomer excess was determined after hydrolysis.
entry
X=
solvents
ligands
yield/%b
ee/%c
1 2 3 4 5 6 7 8 9 10 11 12 13 14
I I I I I I I I I I I I Br OTf
toluene toluene toluene toluene toluene toluene toluene toluene toluene dioxane DMF DCE toluene toluene
4a 4b 4c 4d 4e 4f 5 (S)-BINAP (R,R)-DIOP 4a 4a 4a 4a 4a
99 86 n.r. n.r. 52 90 67 97 83 trace trace 36 41 45
91 83
9−13 in Table 2). As an exception, 91% ee was achieved with the R = i-Bu group for an unknown reason (entry 13). Thus, methyl propionate is the most suitable precursor of the silyl ketene acetal as the coupling partner with iodoarene. Scope and Limitation. Under the conditions optimized for methyl 4-iodobenzoate, the representative iodoarene possessing an electron-donating or electron-withdrawing substituent resulted in 71−91% ee with good yields (Table 3). This asymmetric α-arylation reaction is available in the synthesis of the methyl esters of chiral anti-inflammatory drugs, such as Naproxen, Cicloprofen, Ketoprofen, Ibuprofen, and Flurbiprofen (entries 12−21). Although S-isomers of the Profen family have pharmacological activity, the corresponding R-isomers are not effective.13 S-isomers of α-arylated propionates can be obtained by using (S)-(R)-(4-MeO-3,5-MePh)2PF-Pcy2 as a ligand (entries 13, 17, 19, and 21, Table 3). Catalytic Cycle. We propose a catalytic cycle that starts from oxidative addition of iodoarene to palladium(0). Binding of the acetate anion facilitates transmetalation of enolate from silicon to palladium via an acetate-bridged structure. Subsequent reductive elimination provides the arylation product. Musco proposed that the roles of TlOAc are conversion of Pd-I into Pd-OAc and activation of the Si−O bond for transmetalation from Si to Pd.5a,14 To clarify the role of TlOAc, we isolated two intermediates, Ar-Pd-I and Ar-Pd-OAc, and reacted them with silyl ketene acetals (Table 4). Ar-Pd-I did not react with silyl ketene acetals in the absence of acetate anion (entries 1−4, Table 4). On the other hand, Ar-Pd-I smoothly reacted with the silyl ketene acetal in the presence of TlOAc (entry 4). Formation of Tl-I is a strong driving force of transmetalation. Ar-Pd-OAc provided the arylation product in the absence of TlOAc (entry 5). Thus, Ar-Pd-OAc was found to be the reactive species for transmetalation with silyl ketene acetals in the catalytic cycle (eq 1).15
39 28 0 2 19 19 62 24 88 37
a
All reactions were carried out at 70 °C for 6 h in the presence of Pd(OAc)2 (5 mol %), chiral ligand (5.5 mol %), methyl 4-iodobenzoate (0.25 mmol), 1 (1.5 equiv), TlOAc (2 equiv), and solvent (2.5 mL). bIsolated yield. c Enantiomer excess was determined by a chiral stationary column.
additives, such as LiOAc4c and ZnF2,4g,h were less effective than TlOAc. The use of an E/Z mixture of silyl ketene acetals (E/Z = 51/49) resulted in a lower yield and lower enantioselectivity (45%, 73% ee) than those in the case of an E-rich substrate (entries 1 and 2, Table 2). The enantioselectivity of the product was dependent on the bulkiness of the ester moiety of the substrate, and the best results were obtained with the methyl ester as the substrate (entries 1−8 in Table 2). As the bulkiness of the ester substituent increases, the enantioselectivity tends to decrease. Substrate generality was then investigated under the optimized reaction conditions. Acid derivatives other than propionic acid were also examined. The acid substituent attached to the α-carbon should be small to achieve higher enantioselectivity (entries 1, 6324
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Table 3. Scope and Limitationa
entry
iodoarene
3
yield/%b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
4-MeO2CC6H4I 4-MeC6H4I 4-PhC6H4I 4-BnOC6H4I 4-MeOC6H4I 3-MeOC6H4I 2-MeOC6H4I 2-MeC6H4I 1-naphthyl-I 2-naphthyl-I (3,5-CH2O2)C6H3I 2-iodo-6-methoxynaphthalene 2-iodo-6-methoxynaphthalene 2-iodo-9H-fluorene 2-iodo-9H-fluorene 3-(PhCO)C6H4I 3-(PhCO)C6H4I 4-(i-Bu)C6H4I 4-(i-Bu)C6H4I 3-F-4-Ph-C6H3I 3-F-4-Ph-C6H3I
3aa 3ab 3ac 3ad 3ae 3af 3ag 3ah 3ai 3aj 3ak 3al 3al 3am 3am 3an 3an 3ao 3ao 3ap 3ap
99 76 98 86 99 99 76 76 73 93 83 79 98 89 98 99 89 85 80 85 84
ee/%c 91 83 90 90 87 85 71 83 77 81 86 91 89 91 89 85 84 83 85 89 84
(−) (R) (−) (−) (R) (−) (R) (−) (R) (R) (−) (R) (S)d (−) (+)d (R) (S)d (R) (S)d (R) (S)d
Figure 1. X-ray structure of ((R)-(S)-(4-MeO-3,5-MePh)2PF-Pcy2)PdCl2.
are given in Table 5. The molecular structure has a square-planar coordination geometry. The sum of the angles around Pd is 360°, the significant bond angles being ∠P(1)−Pd−P(2) (96.30°), ∠Cl(1)−Pd−Cl(2) (89.47°), ∠P(1)−Pd−Cl(1) (90.81°), and ∠P(2)−Pd−Cl(2) (83.42°). Structure of ((R)-(S)-(4-MeO-3,5-MePh) 2PF-Pcy2)Pd(C6H4CO2Me)(I). Since Josiphos ligands have unsymmetrical structures, it is possible that the ((R)-(S)-(4-MeO-3,5-MePh)2PF-Pcy2)Pd(C6H4CO2Me)(I) complex has two stereoisomers. In the cis isomer, the C6H4CO2Me group on Pd is located in the cis position of the (4-MeO-3,5-Me2C6H2)2P group on the Josiphos ligand, and in the trans isomer, it is trans.16 To determine the ratio of cis/trans, oxidative addition of Ar-I to Pd(PPh3)4 in the presence of Josiphos in toluene-d8 was monitored by 31P NMR spectra. After heating the mixture at 50 °C for 6 h, ((R)-(S)-(4MeO-3,5-MePh)2PF-Pcy2)Pd(C6H4CO2Me)(I) was formed with the ratio of cis/trans = 10/1 (eq 2). Thus, the cis complex is the responsible species for catalytic reaction and enantioselection.
a All reactions were carried out at 70 °C for 6 h in the presence of Pd(OAc)2 (5 mol %), (R)-(S)-(4-MeO-3,5-MePh)2PF-Pcy2 (4a, 5.5 mol %), iodoarene (0.25 mmol), 1 (1.5 equiv), TlOAc (2 equiv), and toluene (2.5 mL). bIsolated yield. cEnantiomer excess was determined by a chiral stationary column. The letters in the parentheses indicate the absolute configurations of the chiral center of products. d(S)-(R)(4-MeO-3,5-MePh)2PF-Pcy2 was used as a ligand.
X-ray Structure of ((R)-(S)-(4-MeO-3,5-MePh)2PF-Pcy2)PdCl2. An ORTEP of ((R)-(S)-(4-MeO-3,5-MePh)2PF-Pcy2)PdCl2 is shown in Figure 1; crystal data and refinement details Table 4. Stoichiometric Reactionsa
entry
X
additives
yield/%b
1 2 3 4 5
I I I I OAc
none LiOAc NaOAc TlOAc none
trace 13 35 86 78
a
All reactions were carried out at 70 °C for 3 h in the presence of 1 (5 equiv), oxidative adduct (0.1 mmol), PPh3 (2 equiv), additives (2 eq., if used), and toluene (20 mL). bNMR yield. 6325
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to explain the enantioselectivity (13 or 16, Δ = 1.08 kcal/mol). In addition, O-bound enolates may require a vacant coordination site for tautomerization. However, the bidentate ligand Josiphos provides no coordination site for tautomerization. The enolate moiety of the O-bound Pd-enolate rotates anticlockwise so that the double bond may become close to the ipso-carbon of Pd−Ph (12 or 15). Although both Si- and Re-coordination of the substrate is preferred thermodynamically without steric interaction (11 and 14), the intermediate giving the experimentally observed R product has a low energy barrier for reaction from the Si-face (12, 27 kcal/mol). On the other hand, for the coordination of an enolate from its opposite Re-face was observed a similar low energy level (14, −0.45 kcal/mol), but the subsequent reductive elimination process does not advance for a high energy barrier (15, 44 kcal/mol). Finally, we propose the catalytic cycle in Figure 3. Thus,
Table 5. Crystallographic Data for ((R)-(S)-(4-MeO-3,5MePh)2PF-Pcy2)PdCl2 empirical formula fw cryst color, habit cryst dimens, mm cryst syst space group a, Å b, Å c, Å β, ° V , Å3 Z Dcalc, g/cm3 temperature, °C total no. of reflns measured no. of unique data Rint refln/param ratio R1 (I > 2.00σ(I)) R (all reflections) wR2 (all reflections) GOF indicator Friedel pairs Flack parameter (Friedel pairs = 5377)
C49H64Cl4FeO2P2Pd 1051.05 orange, block 0.18 × 0.17 × 0.16 monoclinic P21 (No. 4) 11.0323(5) 18.9610(7) 12.4461(5) 108.620(2) 2467.2(2) 2 1.415 −60 23 919 11 163 0.075 20.94 0.0576 0.1336 0.1380 1.107 5377 −0.03(3)
Enantioselection Mechanism. The minimum energy mode of O-bound palladium enolate was calculated on the basis of the X-ray structure of ((R)-(S)-(4-MeO-3,5-MePh) 2PF-Pcy2)PdCl2 and results of NMR analysis of ((R)-(S)-(4MeO-3,5-MePh)2PF-Pcy2)Pd(C6H5CO2Me)(I). As discussed above, Ar-Pd-OAc is an active species for transmetalation with silyl ketene acetals. It is estimated that the immediate product of transmetalation of silyl ketene acetals to Ar-Pd-OAc is an O-bound Pd-enolate.17 Thus, the mode of substrate coordination to the (R)-(S)-(4-MeO-3,5-MePh)2PF-Pcy2-phenylpalladium intermediate was calculated: that is, the conformation of the stereo-determining step was estimated by DFT computations at the B3LYP/LANL2DZ level of theory (Figure 2). 11 shows the most stable conformation estimated by DFT calculation of the O-bound enolate facing the Pd−Ph at the Si-face. The O-bound Pd-enolate with the Pd−Ph bond facing the enolate of the Re-face (14) also has a similar energy level. Although they may transform to the C-bound enolate, the energy gap between two C-bound enolates is not large enough
Figure 3. Proposed catalytic cycle.
the enantioselectivity is determined at this reductive elimination step.
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CONCLUSION
In conclusion, we have developed the Pd/(4-MeO-3,5MePh)2PF-Pcy2-catalyzed enantioselective α-arylation of silyl ketene acetals. This method provides a simple access for the synthesis of optically active α-arylesters up to 91% ee. In addition, we discovered an active species that can react
Figure 2. Stable conformations optimized at the B3LYP/LANL2DZ level of theory: ((R)-(S)-(4-MeO-3,5-MePh) 2PF-Pcy2)Pd(Ph)(O−C(OMe) CHMe). 6326
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smoothly with silyl ketene acetals. Extension to an advanced version based on the nature of the catalytic cycle and enantioselection is in progress in our laboratory.
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ASSOCIATED CONTENT S Supporting Information * Text, tables, and figures providing experimental details, spectral and/or analytical data of the products, and X-ray data. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. Tel/Fax: +81-11-7066560.
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ACKNOWLEDGMENTS This work was supported by the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank S. Oka, A. Tokumitsu, K. Owada, and A. Nagao of the Instrumental Analysis Division, Equipment Management Center, Creative Research Institution, Hokkaido University, for technical assistance in MS analyses.
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