Cyclometalation of Anthyridine-Based Ligands with Dirhodium

Article pubs.acs.org/Organometallics

Cyclometalation of Anthyridine-Based Ligands with Dirhodium Acetates: Structure and Catalytic Activity Da-wei Huang, Ying-Hao Lo, Yi-Hung Liu, Shie-Ming Peng, and Shiuh-Tzung Liu* Department of Chemistry, National Taiwan University, Taipei, Taiwan 106 S Supporting Information *

ABSTRACT: Coordination of 2,8-Ar2-5-phenylanthyridines (Ar = 2-thienyl, 1a; Ar = 2-ClC6H4−, 1b) with dirhodium tetraacetate yielded the cyclometalated complexes [Rh2(OAc)3(metalated-1a)] (3a) and [Rh2(OAc)3(metalated-1b)] (3b), respectively. Under acidic conditions, cleavage of the Rh−C bond in 3a,b took place to give the corresponding coordination complexes 4a,b. Treatment of 3a,b with PPh3 led to the phosphine-cyclometalated species [Rh2(OAc)2{P,C-(C6H4)PPh2}(metalated-1a)] (5a) and [Rh2(OAc)2{P,C-(C6H4)PPh2}(metalated-1b)] (5b), respectively. These new dirhodium complexes have been structurally characterized by NMR spectroscopy, and some representative compounds were also analyzed by X-ray methods. The use of these newly prepared dirhodium complexes as catalysts for the allylic oxidation of cyclohexenes was investigated.

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INTRODUCTION Cyclometalated dirhodium acetato complexes constitute a class of interesting organometallic molecules in both bioinorganic1 and catalytic processes.2 These complexes are generally prepared via the ortho metalation of an aromatic ring from the donor system (Scheme 1). It is generally accepted that the axial coordination position of the dinuclear cage is the active site for C−H activation or catalysis.2b,3 However, metalation takes place at the equatorial position in all of the reported dirhodium acetato species (Scheme 1),4−7 presumably due to the ring strain of the resulting metallacycles (five- vs four-membered ring). In this report, we demonstrate the cyclometalation of dirhodium acetato complexes with anthyridines 1a,b occurring at the axial position through the ligand design. Furthermore, the catalytic activity of these complexes is investigated.

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(2) with methyl aryl ketones proceeded smoothly to give the desired compounds in good yields, and these compounds were characterized by NMR spectroscopy. With the anthyridine ligands in hands, we first examined the coordination of 1a,b toward dirhodium(II) tetraacetate. Thermal reaction of 1a with an equimolar amount of [Rh2(CH3COO)4] in chloroform provided the rhodium complex 3a exclusively (Scheme 2): i.e., no other metal complex was formed. The diamagnetic dirhodium(II) complex 3a shows nine sets of signals for thienyl and anthyridinyl protons (Table 1), revealing that the C2 symmetry of the ligand 1a is no longer maintained in the complex. The 1H NMR signal corresponding to the C-1 position of the thienyl ring disappears, and the 13C NMR signal corresponding to the C-1 carbon appears as a doublet with JRh−C = 29.4 Hz, clearly indicating the metalation taking place at that position. The two sets of signals from the methyl groups of acetate ligands in 3a indicate the dissymmetry introduced by the ligand 1a. In order to unambiguously characterize this compound, the crystal structure of 3a was determined. The molecular structure of 3a is depicted in Figure 1, whereas selected bonding parameters are summarized in Table 2. The two rhodium atoms

RESULTS AND DISCUSSION

Dirhodium Complexes 3a and 4a. In order to achieve cyclometalation at the axial position with the dirhodium acetato complex, the ligands must have an accessible C−H group near that position. In an earlier finding, we learned that C−H activation does not occur in the complexation of 2,7-diaryl-1,8naphthyridine with Rh2(OAc)4, presumably due to steric hindrance. Thus, we thought that ligands 1a,b are possible candidates for this investigation. The nitrogen donors in the anthyridine rings of 1a,b are capable of binding with Rh2(OAc)4 at equatorial positions as a bridging ligand,8 whereas the ortho hydrogens of the side-arm aryl rings are readily available for cyclometalation. Ligands 1a,b were prepared by a doubleFriedländer reaction according to the literature procedure (eq 1).9 Reaction of 2,6-diamino-3,5-pyridinedicarboxaldehyde © XXXX American Chemical Society

Received: June 3, 2013

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dx.doi.org/10.1021/om400508a | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. C−H Activation with Dirhodium Acetate Complexes

Scheme 2

Table 1. 1H NMR Shifts (ppm) of Ligand 1a and the Related Complexes 3a and 4a

compd 1a 3a 4a

1

1

H NMR shifts of thienyl rings

H NMR shifts of anthyridine

7.86 (d), 7.57 (d), 7.16 (m) 8.29 (d, H-2), 7.63(d, H-8), 7.62 (t, H-9), 7.49 (d, H-10), 7.32 (d, H-3) 8.28 (d, H-1), 8.19 (d, H-10), 8.17 (d, H-3), 7.87 (d, H-8), 7.63 (t, H-2), 7.35 (t, H-9)

7.99 (d), 7.71 (d) 8.46 (d, H-4), 8.14 (d, H-5), 7.92 (d, H-7), 7.78 (d, H-6) 8.43 (d, H-4), 8.14 (d, H-5), 8.26 (d, H-6), 8.09 (d, H-7)

the normal range for this type of compound. The lengths of Rh−O trans to nitrogen donors are slightly longer than those trans to oxygen donors (bridging acetates). It is noteworthy that there is no coordinating ligand on the other axial position of the dirhodium core. The distance between Rh(2) and S(2) is 4.336 Å, indicating that there is no coordination of the sulfur donor toward the metal center. It has been demonstrated that the cyclometalation of dirhodium complexes with thienylphosphines is a reversible process, as evidenced by the H/D exchange study.5d Treatment

are bridged by the three acetate groups and two nitrogen donors of the anthyridine fragment in 1a. One of the thienyl groups in 1a occupies the axial site trans to the Rh−Rh bond with a Rh(1)−C(7) distance of 2.064(6) Å. The C(7)−Rh(1)− Rh(2) angle is 170.2(2)°, consistent with the metalation at the axial position of the dirhodium system. The Rh−Rh bond distance of 2.4399(6) Å for 3a is similar to those found in the equatorial-metalated dirhodium complexes with a rhodium− rhodium single bond,5d indicating no significant influence of this axial ligand. The Rh−N and Rh−O distances are all within B


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Figure 1. ORTEP plot of the crystal structure of 3a (30% probability ellipsoids).

Figure 2. ORTEP plot of complex 3b (30% probability ellipsoids).

Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for 3a

Table 3. Selected Bond Distances (Å) and Bond Angles (deg) for 3b

Rh(1)−N(1) Rh(1)−C(7) Rh(1)−O(1) Rh(1)−O(5) Rh(2)−O(4) C(7)−Rh(1)−Rh(2) N(2)−Rh(2)−O(4) O(6)−Rh(2)−O(2) N(1)−Rh(1)−O(1) N(1)−Rh(1)−O(5)

2.011(5) 2.064(6) 2.045(4) 2.035(4) 2.058(4) 170.23(18) 175.34(18) 174.20(16) 91.45(18) 89.44(17)

Rh(2)−N(2) Rh(1)−Rh(2) Rh(1)−O(3) Rh(2)−O(2) Rh(2)−O(6) N(1)−Rh(1)−O(3) O(5)−Rh(1)−O(1) O(2)−C(1)−O(1) N(2)−Rh(2)−O(2) N(2)−Rh(2)−O(6)

2.008(5) 2.4399(6) 2.074(4) 2.051(4) 2.035(4) 175.84(18) 175.97(16) 127.3(6) 91.69(17) 90.20(18)

Rh(1)−N(1) Rh(1)−C(7) Rh(1)−O(1) Rh(1)−O(5) Rh(2)−O(4) C(7)−Rh(1)−Rh(2) N(2)−Rh(2)−O(4) O(6)−Rh(2)−O(2) N(1)−Rh(1)−O(1) N(1)−Rh(1)−O(5)

of 3a with ammonium hexafluorophosphate readily caused the cleavage of the Rh−C bond to yield 4a, which was identified by spectroscopic characterization. Partial 1H NMR data are summarized in Table 1. The shift at δ 8.28 comes from the C-1 proton, which was confirmed by a deuterium-labeling experiment. This cleavage of the Rh−C bond is quite similar to those in dirhodium complexes cyclometalated at equatorial positions.5 Dirhodium Complexes 3b and 4b. The coordination chemistry of ligand 1b toward dirhodium(II) tetraacetate is quite similar to that of 1a. Substitution of Rh2(OAc)4 with an equimolar amount of 1b gave 3b as the exclusive product, which was isolated in 86% yield. Recrystallization from a CHCl3/hexane solution gave 3b as a dark red crystalline solid. NMR experiments and an X-ray crystal structure determination support the structure assignment for complex 3b. The 13C NMR signal corresponding to the ortho position of the “ClC6H4” group appears to be a doublet at δ 174.4 with JRh−C = 30 Hz, clearly suggesting bonding between the rhodium and carbon centers. 1H NMR data are also consistent with the proposed structure (Experimental Section). The structure of dirhodium complex 3b is depicted in Figure 2, while selected bond distances and angles are presented in Table 3. It consists of one dirhodium unit with a Rh−Rh bond length of 2.4363(6) Å. The four equatorial sites are occupied by anthyridinyl nitrogen donors, acting as a N∼N bridging ligand, and three bridging acetate groups. One of the axial positions of the

1.991(5) 2.023(5) 2.046(4) 2.039(4) 2.048(4) 171.4(2) 173.79(16) 174.05(15) 91.58(17) 91.42(18)

Rh(2)−N(2) Rh(1)−Rh(2) Rh(1)−O(3) Rh(2)−O(2) Rh(2)−O(6) N(1)−Rh(1)−O(3) O(5)−Rh(1)−O(1) O(2)−C(1)−O(1) N(2)−Rh(2)−O(2) N(2)−Rh(2)−O(6)

2.001(4) 2.4363(6) 2.056(4) 2.034(4) 2.030(4) 177.04(16) 174.17(15) 126.1(5) 92.94(17) 90.08(17)

dirhodium unit is occupied by the carbon donor with an Rh(1)− C(7) distance of 2.023(5) Å. The Rh(1)−Rh(2)−C(7) angle, 171.4(2)°, deviates from linearity most likely due to the strain of the resulting cyclometalated ring. Other bond distances and angles are in the normal ranges. Under acidic conditions, cleavage of the Rh−C bond in complex 3b took place to give 4b, which is similar to the case for 3a. However, isolation of a pure form of 4b was not possible, due to its rapid conversion back to 3b. Structural assignment to 4b was achieved by 1H NMR spectroscopy. Substitution of PPh3 with 3. Reaction of 3a with 1 equiv of triphenylphosphine in chloroform at room temperature yielded the phosphine-substituted complex 5a exclusively (eq 2).

The formation of 5a is corroborated by its NMR spectrum and crystallography. Its 31P NMR spectrum shows a doublet at δ 30.2 with JRh−P = 164 Hz, indicating a single isomeric form of the C


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product. The 13C NMR spectrum of 5a exhibits two doublets at δ 191.3 (JRh−C = 37.5 Hz) and 134.2 (JRh−C = 23.5 Hz), which are assigned to the carbons of Cthienyl−Rh and Cphenyl−Rh, respectively. The absence of a phosphorus−carbon coupling constant (JP−C) for the carbon shift of Cthienyl−Rh clearly suggusts that the coodination of phosphorus occurs on the other rhodium metal center, which is further confirmed by the crystallographic analysis. Concerning the regioslectivity of substitution, it appears that the substitution of triphenylphosphine takes place on the rhodium center away from that with the metalated aryl group, presumably due to steric hindrance. Single crystals of complex 5a suitable for X-ray analysis were obtained from a CH2Cl2/hexane solution. The crystallographic analysis confirms the structure of 5a (Figure 3). Important

Rh(2)−O(2) bond (2.121(3) Å, trans to the phosphorus donor), which is due to the trans influence of M−C versus M−P bonds. Other structural parameters are within the normal range. Similarly, complex 3b reacted with triphenylphosphine to give 5b exclusively. The structure of this complex has been characterized by NMR spectroscopy and elemental analysis (Experimental Section). The 31P NMR spectrum of 5b shows a doublet at δ 34.6 with JRh−P = 163 Hz, which is comparable to that of 5a. The carbon shifts of CCl‑Ph−Rh and Cphenyl−Rh in the 13C NMR of 5b appear at δ 177.4 (JRh−C = 26.9 Hz) and 163.3 (JRh−C = 23.5 Hz) ppm, respectively. All these data, similar to those of 5a, are characteristic for the cyclometalated species.

Catalytic Allylic Oxidation. Dirhodium complexes have been reported to have catalytic activity in the allylic oxidation of olefins with peroxide under mild conditions.10,11 However, the ligands have significant influence on the activity. In this context, Doyle and co-workers demonstrated dirhodium tetracaprolactamate [Rh2(Cap)4] to be an excellent catalyst for allylic oxidation.11 Thus, with these dirhodium complexes in hand, their catalytic activities toward the oxidation of cyclohexenes were investigated. We would particularly like to examine the axial ligand effect on the catalysis, since the axial position in complexes 3−5 is capped with a carbon donor. We first examined the oxidation of 1-phenylcyclohexene with TBHP as a model reaction for surveying reaction parameters such as catalysts, bases, oxidants, and solvents (eq 3). The

Figure 3. Molecular structure of 5a (30% probability ellipsoids).

bond distances and angles are given in Table 4. In the structure the two rhodium atoms are bridged by two acetate ligands, by a Table 4. Selected Bond Distances (Å) and Bond Angles (deg) for 5a Rh(1)−N(1) Rh(1)−C(7) Rh(1)−O(1) Rh(1)−O(3) Rh(1)−C(32) C(7)−Rh(1)−Rh(2) N(1)−Rh(1)−O(3) O(1)−Rh(1)−O(3) C(32)−Rh(1)−O(1) C(32)−Rh(1)−N(1)

2.005(4) 2.000(4) 2.210(3) 2.070(3) 2.017(4) 165.63(13) 175.49(13) 90.43(12) 178.71(15) 89.91(16)

Rh(1)−Rh(2) Rh(2)−N(2) Rh(2)−O(2) Rh(2)−O(4) Rh(2)−P(1) N(2)−Rh(2)−O(4) P(1)−Rh(2)−O(2) P(1)−Rh(2)−Rh(1) P(1)−Rh(2)−N(2) C(32)−Rh(1)−C(7)

2.5102(5) 2.031(4) 2.121(3) 2.072(3) 2.1895(12) 171.08(13) 178.35(9) 88.62(3) 92.46(10) 91.91(18)

typical experimental conditions are as follows: a mixture of 1-phenylcyclohexene (0.32 mmol), K2CO3 (0.16 mmol), rhodium complex (3.2 × 10−4 mmol), and t-BuOOH (1.58 mmol) in a solvent (2 mL) was stirred at ambient temperature for 10 h. The results are summarized in Table 5. With complex 3a as the catalyst, it appears that a number of bases do assist the reactions, but sodium carbonate gives the best result (Table 5, entries 1−4). Using a solvent such as dichloromethane is important in obtaining the best results. The precatalyst 3a shows a poor activity in other organic solvents (Table 5, entries 8−10). As for the oxidant, TBHP appears to be the best reagent. It is noted that the reactions using H2O2 and K2S2O8 as the oxidants did not perform well, presumably due to the solubility in dichloromethane. Catalyst screening suggests that both complexes 3a and 5a give rise to the best catalysts for the oxidation of 1-phenylcyclohexene, indicating that the phosphine substituted at an

PPh3 metalated in one phenyl ring, and by two nitrogen donors of the anthyridinyl ligand. The Rh−Rh bond distance is 2.5102(5) Å, slightly longer than those in 3a,b, which compares well to those observed in the related complexes. The conformation of the metalated phosphine appears to be in an “envelope” form, which is normal in these types of cyclometalated dirhodium complexes. It is noted that cyclometalations of thienyl and phenyl rings occur at the same rhodium metal center. The Rh(1)−C(7) bond trans to the Rh−Rh bond, 2.000(4) Å, is quite similar to that trans to the oxygen donor, Rh(1)−C(32) = 2.017(4) Å, showing a small effect of the trans influence of the metal−metal bond on this structural parameter. The Rh(1)−O(1) bond distance (2.210(3) Å, trans to the carbon donor) is longer than the D


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Summary. The two new dirhodium(II) compounds 3a,b with metalation at the axial position have been synthesized by the reaction of [Rh2(OAc)4] with the designed ligands 1a,b. Further treatment of 3a,b with triphenylphosphine led to metalation of the equatorial phosphine, providing the corresponding dirhodium complexes 5a,b. Both 3a and 5a appear to be good catalysts in the allylic oxidation, presumably due to the influence of the axial ligand in the dirhodium complexes. Studies on the catalytic reactivity and ligand effects on these complexes are ongoing in our laboratory.

Table 5. Results of Oxidation of 1-Phenylcyclohexene with TBHPa entry

catalyst

base

oxidant

solvent

yield, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3a 3b 4a 5a Rh2(OAc)4

Na2CO3 K3PO4 DBU Et3N K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

TBHP TBHP TBHP TBHP H2O2 O2 K2S2O8 TBHP TBHP TBHP TBHP TBHP TBHP TBHP TBHP

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CHCl3 ether EtOAc CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

71 50 58 14 trace 39 13 25 32 35 75 52 15 77 trace

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General Information. Nuclear magnetic resonance spectra were recorded in CDCl3 on a Bruker AVANCE 400 or Bruker AVIII 800 MHz spectrometer. Chemical shifts are given in parts per million relative to Me4Si for 1H and 13C NMR and relative to 85% H3PO4 for 31 P NMR. All reaction and manipulation steps were performed under a dry nitrogen atmosphere. Tetrahydrofuran was distilled under nitrogen from sodium benzophenone ketyl. Dichloromethane was dried over CaH2 and distilled under nitrogen. Other chemicals and solvents were of analytical grade and were used after a degassing process. 2,6-Diamino-4-phenylpyridine-3,5-dicarbaldehyde (2) was prepared according to the literature method.12 5-Phenyl-2,8-di-2-thienylanthyridine (1a). A mixture of 2,6diamino-4-phenylpyridine-3,5-dicarbaldehyde (0.1 g, 0.415 mmol) and 1-(thiophen-2-yl)ethanone (0.15 mL, 174 mg, 1.38 mmol) in ethanol (4 mL) was heated to reflux under a nitrogen atmosphere. After 30 min, a 10% NaOH aqueous solution (0.25 mL) was added. The mixture turned dark green immediately, which then slowly changed to brown upon heating for another 12 h. After removal of solvents under reduced pressure, the residue was passed through a flash column with DCM as the eluent. The filtrate was reprecipitated with DCM/diethyl ether three times. The desired product was obtained as a light yellow solid (163 mg, 93%): mp 291−291.5 °C; 1H NMR (400 MHz, CDCl3) δ 7.99 (d, J = 8 Hz, 2 H), 7.86 (d, J = 4 Hz, 2 H), 7.71 (d, J = 8 Hz, 2 H), 7.63 (m, 3 H), 7.57 (d, J = 8 Hz, 2 H), 7.46 (m, 2 H), 7.16 (m, 2 H); 13C NMR (100 MHz, CDCl3) δ 157.5, 156.3, 150.3, 145.0, 136.3, 134.1, 131.4, 130.5, 129.1, 128.8, 128.2, 119.2, 118.3. ESIHRMS m/z for [M + H]+: calcd 422.0786 (C25H16N3S2); found 422.0785. 2,8-Bis(2-chlorophenyl)-5-phenyl-1,9,10-anthyridine (1b). A mixture of 2 (0.1 g, 0.42 mmol) and 2′-chloroacetophenone (0.16 g, 1.1 mmol) in ethanol (4 mL) was heated at 60 °C for 2 h. Then, a solution of 10% KOH in EtOH (0.2 mL) was added to the above reaction mixture. The solution turned brown immediately. The resulting mixture was heated to reflux for 12 h. Upon cooling, water (20 mL) was added and the solution was extracted with CH2Cl2 (25 mL × 2). The extracts were dried, concentrated, and reprecipitated from hexane/CH2Cl2 to give 1b as a yellow solid (0.14 g, 70%): mp 294−295 °C; 1H NMR (400 MHz, CDCl3) δ 8.16 (d, J = 8.0 Hz, 2 H), 7.97 (d, J = 4.0 Hz, 2 H), 7.87 (d, J = 8.0 Hz, 2 H), 7.62 (m, 3 H), 7.47 (m, 4 H), 7.42 (m, 4 H); 13C NMR (100 MHz, CDCl3) δ 163.6, 155.96, 151.3, 138.6, 135.2, 134.0, 132.6, 132.3, 130.8, 130.5, 130.2, 129.3, 128.9, 127.3, 123.8, 119.7. Preparation of Complex 3a. A mixture of Rh2(O2CCH3)4 (52.4 mg, 0.12 mmol) and 1a (50 mg, 0.12 mmol) in chloroform (1 mL) was heated to reflux for 12 h under a nitrogen atmosphere. The mixture turned dark green immediately and then slowly changed to dark red during heating. After removal of solvents under reduced pressure, the residue was passed through a flash column with CH2Cl2 as the eluent. The filtrate was reprecipitated with CH2Cl2/hexane three times. The desired product was obtained as a dark red crystalline solid (78 mg, 82%). Further recrystallization from CHCl3/hexane provided single crystals suitable for X-ray analysis: mp 226−229 °C dec; 1H NMR (400 MHz, CDCl3) δ 8.46 (d, J = 4 Hz, 1H, H-4), 8.29 (d, J = 4 Hz, 1H, H-2), 8.14 (d, J = 4 Hz, 1H, H-5), 7.92 (d, J = 8 Hz, 1H, H-7), 7.79 (m, 1H, Ph-H), 7.78 (d, J = 8 Hz, 1H, H-6), 7.63 (d, J = 4 Hz, 1H, H-8), 7.62 (t, J = 4 Hz, 1H, H-9), 7.61 (m, 2H, Ph-H),

Reaction conditions: catalyst (3.2 × 10−4 mmol), 1-phenylcyclohexene (0.32 mmol), base (0.16 mmol), and oxidant (1.58 mmol) in solvent (1.2 mL) at ambient temperature with stirring for 10 h.

a

equatorial position does not affect the catalytic activity. A comparison of entries 11, 13, and 14 shows that the coordination of an axial ligand on the dirhodium core causes a major difference in activity: i.e., the catalytic activities of 3a and 5a are better than that of 4a. The catalytic activity of complex 3b, in which the axial ligand is a chlorophenyl group, is slightly less active than that of 3a (Table 5, entry 12). Nevertheless, the catalytic activities of these complexes are much better than that of Rh2(OAc)4 (Table 5, entry 15), showing the ligand effect on the metal complexes.11b With optimized conditions, we examined a series of cyclohexene derivatives containing various functional groups. Table 6 summarizes the results of the allylic oxidation. We were Table 6. Results of Allylic Oxidation of Cyclohexenesa

entry

substrate R

yield, %c

1 2b 3 4 5 6 7.

Ph− HCC− MeO− CH3CO− CH3C(NOH)− CH3C(NOMe)− NO2−

75 83 87 79 0 83 0

EXPERIMENTAL SECTION

Reaction conditions: 3a (3.2 × 10−4 mmol), cyclohexene (0.32 mmol), K2CO3 (0.16 mmol), and TBHP (1.58 mmol) in CH2Cl2 (1.2 mL) at ambient temperature for 10 h. bAt 35 °C. cIsolated yield.

a

pleased to find that various 1-substituted cyclohexenes do undergo allylic oxidation to give the corresponding cyclohexenone except for those with nitro and oxime groups. In terms of catalytic activity, the turnover frequency (TOF) of 3a in the production of 3-phenylcyclohexenone (Table 6, entry 1) reaches 75 mol of product (mol of catalyst)−1 h−1, which is slightly superior to that of the known catalyst [Rh2(Cap)4] (TOF 65 mol of product (mol of catalyst)−1 h−1), indicating that the axial ligand might affect the activity of the metal complex. E


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7.49 (d, J = 8 Hz, 1H, H-10), 7.32 (d, J = 4 Hz, 1H, H-3), 7.31 (m, 2H, Ph-H), 2.30 (s, 3H), 1.64 (s, 6H); 13C NMR (200 MHz, CDCl3) δ 189.9, 189.3, 180.9 (d, J = 29.4 Hz), 176.8, 169.5, 163.8, 161.2, 159.2, 145.2, 144.0, 140.2, 137.2, 135.3, 135.2, 134.8, 132.9, 132.6, 129.9, 129.8, 129.4, 129.2, 129.1, 120.4, 119.7, 119.3, 119.0, 24.4, 23.7; ESIHRMS m/z for [M + H]+: calcd 803.9204 (C31H24N3O6S2103Rh2); found 803.9202. Preparation of Complex 3b. The procedure for the preparation of 3b is similar to that for 3a. Complex 3b is a dark red crystalline solid (78 mg, 86%): mp 221−224 °C dec; 1H NMR (400 MHz, CDCl3, 298 K) δ 9.19 (d, J = 8 Hz, 1H, H-5), 8.82 (d, J = 8 Hz, 1H, H-4), 8.68 (d, J = 8 Hz, 1H, H-12), 8.13 (d, J = 8 Hz, 1H, H-7), 8.10 (d, J = 8 Hz, 1H, H-8), 7.91 (d, J = 8 Hz, 1H, H-6), 7.73 (t, J = 8 Hz, 1H, H-3), 7.64 (m, 2H, Ph-H), 7.63 (m, 2H, Ph-H), 7.60 (t, J = 8 Hz, 1H, H-2), 7.56 (t, J = 8 Hz, 1H, H-11), 7.40 (t, J = 8 Hz, 1H, H-10), 7.39 (m, 1H, Ph-H), 7.36 (d, J = 8 Hz, 1H, H-9), 2.25 (s, 3H), 1.63 (s, 6H); 13C NMR (200 MHz, CDCl3, 298 K) δ 189.7, 188.6, 174.4 (d, J = 29.8 Hz), 174.3, 164.8, 163.6, 162.0, 144.4, 140.4, 137.5, 135.8, 134.7, 133.8, 133.7, 132.9, 132.5, 132.3, 132.0, 131.6, 130.8, 130.0, 130.0, 129.2, 128.3, 128.1, 125.6, 123.8, 122.5, 120.0, 24.3, 24.0. ESIHRMS (TOF) m/z for [M + H]+: calcd for C35H26N3O6Cl2103Rh2 859.9309; found 859.9308. Preparation of Complex 4a. A mixture of 3a (20 mg, 0.025 mmol) and NH4PF6 (32.46 mg, 0.2 mmol) in chloroform was heated to reflux for 24 h. The solvent was evaporated under reduced pressure, and the residue was extracted by DCM/water (three times) to remove the salt. After reprecipitation with ditheyl ether, the desired product was obtained as a pure pale red solid (17.5 mg, 74%): mp 239−243 °C dec; 1H NMR (400 MHz, CDCl3) δ 8.43 (d, J = 8 Hz, 1H, H-4), 8.31 (d, J = 8 Hz, 1H, H-5), 8.28 (d, J = 4 Hz, 1H, H-1), 8.26 (d, J = 8 Hz, 1H, H-6), 8.19 (d, J = 8 Hz, 1H, H-10), 8.17 (d, J = 4 Hz, 1H, H-3), 8.09 (d, J = 8 Hz, 1H, H-7), 7.87 (d, J = 4 Hz, 1H, H-8), 7.69 (m, 1H, Ph-H), 7.68 (m, 2H, Ph-H), 7.63 (t, J = 4 Hz, 1H, H-2), 7.53 (d, J = 8 Hz, 2H, H-11), 7.35 (t, J = 4 Hz, 1H, H-9), 2.38 (s, 3H), 1.81 (s, 6H); 13 C NMR (200 MHz, CDCl3, 298 K) δ 192.7, 190.7, 163.6, 161.3, 160.3, 160.1, 153.4, 142.4, 141.9, 141.3, 138.0, 137.2, 135.3, 133.8, 132.1, 131.0, 130.7, 130.6, 130.5, 130.1, 129.6, 122.3, 122.2, 122.1, 120.1, 24.2, 24.0; ESI-MS (TOF) m/z for M + : calcd for C31H24N3O6S2103Rh2 803.92; found 803.88. Preparation of Complex 4b. The procedure for the preparation of 4b is similar to that for 4a. However, we were not able to isolate this complex in a pure form and it readily converted back into 3b: 1H NMR (400 MHz, CDCl3, 298 K) δ 8.68 (m, 2 H), 8.46 (m, 3 H), 8.15 (m, 3 H), 7.97 (m, 2 H), 7.89 (m, 1 H), 7.69 (m, 1 H), 7.62 (m, 3 H), 7.45(m, 2 H), 2.09 (s, 6 H), 1.89 (s, 3 H). Preparation of Complex 5a. Triphenylphosphine (6.6 mg, 0.025 mmol) was added to a solution of 3a (20 mg, 0.025 mmol) in CHCl3 (3 mL) under a nitrogen atmosphere. The resulting mixture was stirred at room temperature for 2 h. The solution changed from red to green. The solvent was removed under reduced pressure. The residue was reprecipitated from dichlromethane/hexane/ether. The desired complex 5a was obtained as a dark green solid (21.4 mg, 85%): mp 235−239 °C dec; 1H NMR (400 MHz, CDCl3, 298 K) δ 8.20 (d, J = 8 Hz, 1 H), 8.11 (m, 2 H), 7.85 (d, J = 4 Hz, 1 H), 7.75 (d, J = 4 Hz, 1 H), 7.59 (m, 2 H), 7.54 (d, J = 8 Hz, 1H), 7.48 (d, J = 8 Hz, 1 H), 7.41 (m, 3 H), 7.34 (d, J = 8 Hz, 2 H), 7.18 (d, J = 8 Hz, 2 H), 6.78 (m, 2 H), 6.70 (m, 3 H), 6.58 (t, J = 8 Hz, 2 H), 6.50 (t, J = 8 Hz, 2 H), 6.37 (t, J = 8 Hz, 3 H), 1.89 (s, 3 H), 1.26 (s, 3 H); 13C NMR (125 MHz, CDCl3, 298 K) δ 191.3 (d, JRh−C = 37.5 Hz), 187.6, 181.3, 167.9, 163.1, 158.4, 157.6, 146.0, 144.0, 143.5, 143.3, 142.9, 142.4, 140.9, 137.7, 136.4, 136.2, 135.8, 135.5, 135.0 (JP−C = 10 Hz), 134.2 (d, JRh−C = 23.5 Hz), 133.3, 132.3, 131.9, 131.3, 130.7 (JP−C = 9 Hz), 129.9, 129.7, 129.5, 129.4, 129.1, 128.9, 128.6, 127.5 (JP−C =10 Hz), 126.6 (JP−C = 9.5 Hz), 120.5 (JP−C = 8 Hz), 120.3, 119.3, 117.8, 24.2, 23.8; 31P NMR (161 MHz, CDCl3, 298 K) δ 30.2 (d, JRh−P = 164 Hz); ESIMS (TOF) m/z for [M + H]+: calcd for C47H35N3O4PRh2S2 1005.99; found 1006.08. Preparation of Complex 5b. To a solution of complex 3b (30 mg, 0.0349 mmol) in chloroform (1 mL) was added PPh3 (9.2 mg, 0.0349 mmol). The resulting mixture was stirred at room temperature under a nitrogen atmosphere for 3 h. The solution turned from red to

green. After removal of solvents, the residue was chromatographed to give 5b as a dark green solid (33.8 mg, 91%): mp 233−237 °C dec; 1H NMR (400 MHz, CDCl3) δ 9.13 (d, J = 8 Hz, 1 H), 8.49 (d, J = 8 Hz, 1 H), 8.44 (d, J = 8 Hz, 1 H), 8.14 (d, J = 8 Hz, 1 H), 8.03 (d, J = 8 Hz, 1 H), 7.71 (t, J = 8 Hz, 1 H), 7.62 (m, 1 H), 7.56 (d, J = 8 Hz, 1 H), 7.54(d, J = 8 Hz, 2 H), 7.52 (m, 2 H), 7.38 (t, J = 8 Hz, 2 H), 7.24 (d, J = 8 Hz, 1 H), 7.18 (d, J = 8 Hz, 2 H), 7.09 (m, 2 H), 7.02 (m, 3 H), 6.96 (d, J = 8 Hz, 2 H), 6.83 (t, J = 8 Hz, 1 H), 6.74 (m, 3 H), 6.70 (d, J = 8 Hz, 1 H), 6.57 (t, J = 8 Hz, 2 H), 2.25 (s, 3 H), 1.85 (s, 3 H); 13C NMR (125 MHz, CDCl3, 298 K) δ 188.4, 183.6, 179.2, 177.4 (JRh−C = 27 Hz), 175.5, 167.7, 163.3 (JRh−C = 24 Hz), 159.1, 147.7, 140.9, 138.0, 136.9 (JP−C =11.6 Hz), 135.2, 133.5, 133.0, 132.9, 132.6 (d, J = 9.8 Hz), 132.5, 132.2, 132.1, 132.0 (JP−C = 9.6 Hz), 131.2 (JP−C =13 Hz), 130.9, 130.5, 130.1, 129.9, 129.8, 129.1, 129.0, 128.8, 128.5, 128.4, 128.1, 127.8, 127.7, 127.4, 127.2 (JP−C = 9.8 Hz), 126.9 (JP−C = 9.5 Hz), 125.3, 122.9, 122.4, 119.0, 23.7, 23.0; 31P NMR (161 MHz, CDCl3) δ 34.6 (d, JRh−P = 162.6 Hz); ESI-MS m/z for [M + H]+: calcd for C51H37Cl2N3O4PRh2 1062.00; found 1062.24. General Procedure for Catalytic Oxidation. A mixture of substrate (0.32 mmol), Rh complex (3.2 × 10−4mmol), K2CO3 (0.16 mmol), and TBHP (1.58 mmol) in CH2Cl2 (1.25 mL) was loaded in a reaction tube. The reaction mixture was stirred at room temperature for 10 h. The reaction mixture was poured into a saturated NaCl solution, extracted with CH2Cl2 (3 mL × 2), and dried over anhydrous MgSO4. After removal of solvents, the residue was chromatographed on silica gel. 3-Phenylcyclohexenone: 13 1H NMR (400 MHz, CDCl3) δ 7.53− 7.50 (m, 2H), 7.40−7.38 (m, 3H), 6.40 (s, 1H), 2.76 (m, 2H), 2.47 (t, J = 6.4 Hz), 2.14 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 199.9, 130.0, 128.7, 128.3, 126.1, 125.5, 125.1, 37.3, 28.1, 22.8; IR (KBr) 1663 cm−1 (νCO); ESI-HRMS m/z for [M + H]+: calcd for C12H13O 173.0961; found 173.0960. 3-Ethynylcyclohexenone: 14 1H NMR (400 MHz, CDCl3) δ 6.25 (s, 1H), 3.51 (s, 1H), 2.47 (m, 2H), 2.43 (m, 2H), 2.05 (m, 2H); 13C NMR (100 MHz, CDCl3, 298 K) δ 198.6, 142.3, 133.9, 87.2, 82.4, 37.2, 30.1, 22.4; IR (KBr) 2095, 1674 cm−1; ESI-HRMS m/z for [M + H]+: calcd for C8H9O 121.0653; found 121.0658. 3-Methoxycyclohexenone: 15 1H NMR (400 MHz, CDCl3) δ 5.36 (s, 1 H), 3.68 (s, 3 H), 2.38 (t, J = 6 Hz, 2 H), 2.32 (t, J = 6 Hz, 2 H), 1.96 (m, 2 H); 13C NMR (100 MHz, CDCl3, 298 K) δ 199.8, 176.7, 102.3, 55.6, 36.7, 28.8, 22.2; IR (KBr) 1597 cm−1 (νCO); ESI-HRMS m/z for [M + H]+: calcd for C7H11O2 127.0754; found 127.0752. 1-(3-Oxocyclohexen-1-yl)ethanone O-methyloxime: 1H NMR (400 MHz, CDCl3) δ 6.21 (s, 1 H), 3.96 (s, 3 H), 2.64 (m, 2 H), 2.40 (m, 2 H), 1.98 (m, 2 H). 1.92 (s, 3 H); IR (KBr) 1672 cm−1 (νCO); ESI-HRMS m/z for [M + Na]+: calcd for C9H13NO2Na 190.0838; found 190.0831. 3-Acetylcyclohexenone: 13 1H NMR (400 MHz, CDCl3) δ 6.57 (s, 1 H), 2.52 (m, 2 H), 2.50 (m, 2 H), 2.46 (s, 3 H), 1.98 (m, 2 H); 13 C NMR (100 MHz, CDCl3, 298 K) δ 201.3, 200.0, 154.6, 132.4, 37.8, 26.1, 23.3, 21.8; IR (KBr) 1681 cm−1 (νCO). ESI-HRMS m/z for [M + H]+: calcd for C8H11O2 139.0759; found 139.0759. Crystallography. Crystals suitable for X-ray determination were obtained for 3a·2CHCl3·0.25C6H14, 3b·CHCl3, and 5a·4CH2Cl2 by recrystallization from chloroform/hexane, chloroform, and dichloromethane, respectively. Cell parameters were determined with a Siemens SMART CCD diffractometer. The structure was solved using the SHELXS-97 program16 and refined using the SHELXL-97 program17 by full-matrix least squares on F2 values. Crystal data of these complexes are given in Table S0 (Supporting Information). Other crystallographic data are deposited as Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Tables and CIF files providing crystal data, atomic positional parameters, bond distances and angles, anisotropic thermal parameters, and calculated hydrogen atom positions for complexes 3a,b and 5a. This material is available free of charge via the Internet at http://pubs.acs.org. F


Organometallics

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Article

(14) Uyanik, M.; Fukatsu, R.; Ishihara, K. Org. Lett. 2009, 11, 3470− 3473. (15) Winkler, C. K.; Stueckler, C.; Mueller, N. J.; Pressnitz, D.; Faber, K. Eur. J. Org. Chem. 2010, 6354−6358. (16) SHELXS-97: Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, 67. (17) Sheldrick, G. M. SHELXL-97; University of Gö ttingen, Göttingen, Germany, 1997.

AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Council for financial support for their financial support (NSC100-2113-M-002-001-MY3). We also thank Miss Yu Chang Chao for her generous support for ESI mass data analysis.

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REFERENCES

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