Ruthenacyclopentanes as Intermediates in the Regio- and

Article pubs.acs.org/Organometallics

Ruthenacyclopentanes as Intermediates in the Regio- and Stereoselective Linear Codimerization of N‑Vinylamides with Electron-Deficient Alkenes Hiroko Fukuzawa, Nozomi Aoyagi, Ruriko Sato, Yasutaka Kataoka, and Yasuyuki Ura* Department of Chemistry, Biology, and Environmental Science, Faculty of Science, Nara Women’s University, Kitauoyanishi-machi, Nara 630-8506, Japan S Supporting Information *

ABSTRACT: Tricarbonylruthenacyclopentanes were successfully synthesized by in situ reduction of RuCl3 with Zn-Cu/ CO in the presence of N-vinylacetamides and electrondeficient alkenes such as ethyl acrylate, dimethyl fumarate, and dimethyl maleate. X-ray crystallography of the ruthenacyclopentanes revealed that the three carbonyl ligands occupied coordination sites facially, and the ruthenacycle moiety was stabilized by coordination with the acetamido oxygen atom. When the ruthenacyclopentanes in toluene or DMA were heated to 130 °C for 24 h, linear codimers of the alkenes were formed in moderate to good yield. Both the ruthenacyclopentane and the RuCl3/Zn-Cu/alcohol system showed catalytic activity for the linear codimerization of alkenes, affording the codimers in good yield. The results of stoichiometric and catalytic reactions revealed that the codimerization can proceed via ruthenacyclopentane intermediates.

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β-H elimination and reductive elimination to afford a codimer, (2) the metal-hydride mechanism, in which two different alkenes are inserted in sequence into an M−H and an M− C(sp3) bond and subsequent β-H elimination affords a codimer, and (3) the C−H activation mechanism, which involves oxidative addition of the C(sp2)−H bond of the alkene, followed by insertion of another alkene into the M−H bond and reductive elimination to afford a codimer. However, a conclusive distinction among these three mechanisms is difficult when complexes without a hydride ligand are used as catalysts, because in most cases, the reaction intermediates formed in situ are elusive. Indeed, although there have been many investigations of the above catalytic reactions in the literature, few have described the isolation and unambiguous characterization of the key intermediate complexes. Hirano, Komiya, Bennett, et al.99 reported a ruthenacyclopentane formed via the oxidative cyclization of two methyl acrylates using a zerovalent ruthenium complex, Ru(η6-naphthalene)(η4cod) (cod = 1,5-cyclooctadiene), as an intermediate for the homodimerization of methyl acrylate. They also obtained ruthenacycles via the oxidative cyclization of conjugated dienes as intermediates for homodimerization of the dienes.100 Ogoshi et al. reported a dimeric nickelacycle complex obtained by the reaction of an ene-enone with Ni(cod)2 and PBu3 via intramolecular oxidative cyclization. Although the nickelacycle intermediate is only a model, it has been proposed for the

INTRODUCTION Codimerization of alkenes is a promising, atom-efficient synthetic method that affords highly substituted alkenes with various functionalities from simple alkenes.1−16 Thus far, Co,17−27 Ni,28−31,17,32−48 Ru,49−75 Rh,49,50,76,77 Pd,78−89 other transition-metal90−95 complexes, and Lewis or Brønsted acids96−98 have been used as catalysts for this reaction. As shown in Scheme 1, possible mechanisms for the transitionmetal-catalyzed codimerization fall into one of three categories: (1) the metallacycle mechanism, which involves the formation of a metallacyclopentane through oxidative cyclization of two different alkenes with a transition-metal complex, followed by Scheme 1. Possible Mechanisms for the Linear Codimerization of Alkenes

Received: July 19, 2017

© XXXX American Chemical Society

A

DOI: 10.1021/acs.organomet.7b00545 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics similar Ni(cod)2/PCy3-catalyzed codimerization of enones with aromatic or aliphatic alkenes.46 During our investigation of ruthenium catalyst systems for the regio- and stereoselective linear codimerization of Nvinylamides 1 with electron-deficient alkenes 2,63 we successfully synthesized air-stable and thermally stable divalent tricarbonylruthenacyclopentanes via intermolecular oxidative cyclization of two different alkenes under a CO atmosphere and characterized them. In this paper, the synthesis and structures of the ruthenacyclopentanes, as well as their stoichiometric reactivity and catalytic activity, are described. Although the metal-hydride mechanism which we proposed in our previous report63 is still possible, the metallacycle mechanism is more plausible for the above codimerization according to the present results.

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RESULTS AND DISCUSSION Formation of Ruthenacyclopentanes. Under a CO atmosphere, the reaction of RuCl3·nH2O with N-vinylacetamide (1a) or N-methyl-N-vinylacetamide (1b) (10 equiv) and ethyl acrylate (2a) (10 equiv) in the presence of Zn-Cu (6 equiv) and EtOH (6 equiv) in DMA (N,N-dimethylacetamide) at 100 °C afforded ruthenacyclopentanes 3aa,ba in moderate to good yield (Scheme 2). As 3aa,ba were air- and moisture-stable, they

Figure 1. Molecular structure of 3ba with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity, except for those on C1 and C4. The ethoxycarbonyl group was treated as disordered. Selected bond distances (Å): Ru1−C1 = 2.118(3), Ru1−C4 = 2.200(3), Ru1− C5 = 1.940(3), Ru1−C6 = 1.979(3), Ru1−C7 = 1.870(3), Ru1−O1 = 2.110(2), C1−C2 = 1.521(4), C2−C3 = 1.524(4), C3−C4 = 1.534(4), C5−O4 = 1.133(4), C6−O5 = 1.126(4), C7−O6 = 1.139(4).

Scheme 2. Formation of Ruthenacyclopentanes 3aa,ba

Scheme 3. Formation of Ruthenacyclopentanes 3bb,bb′,bb″

could be purified by open column chromatography using silica gel. Complex 3ba was recrystallized from CHCl3/hexane, and the single crystals were analyzed by X-ray crystallography (Figure 1). The three C−C bonds (C1−C2, C2−C3, and C3− C4) in the ruthenacycle had lengths typical of a single bond. The three carbonyl ligands occupied coordination sites facially, and the ruthenacycle moiety was stabilized by coordination with the acetamido oxygen atom. The N-methylacetamido and the ethoxycarbonyl groups were located trans to one another. The bond distances of Ru1−C5 and Ru1−C6 were longer than that of Ru1−C7 by ca. 0.07−0.11 Å, because of the trans influence of the strongly σ donating alkyl ligands. The C−O bond distances of the three carbonyl ligands were nearly identical. In a similar manner, ruthenacyclopentane 3bb was synthesized as the major product using 1b and dimethyl fumarate (trans-2b), along with the formation of diastereomers 3bb′,bb″, in a total yield of 93% (Scheme 3). X-ray crystallography of 3bb,bb′ revealed that the methoxycarbonyl

groups at C3 and C4 were cis and trans in 3bb and trans and trans in 3bb′ with respect to the N-methylacetamido group, respectively (Figures 2 and 3). It is likely that ruthenium hydride species were generated in situ and isomerized trans-2b to cis-2b, leading to the formation of 3bb′. Although no single crystals were obtained for 3bb″, the coupling constants between the methylene protons on C2 and the methyne proton on C3 in 3bb″ (5.1 and 13.8 Hz) were consistent with those in 3bb′ (4.5 and 13.5 Hz) and different from those in 3bb (1.5 and 9.0 Hz), indicating that the methoxycarbonyl group at C3 in 3bb″ is trans to the Nmethylacetamido group. The relatively large coupling constant between the methyne proton on C3 and that on C4 in 3bb″ (axial and axial, 9.6 Hz) in comparison to those in 3bb (equatorial and equatorial, 4.2 Hz) and 3bb′ (axial and B


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Organometallics

electron-deficient alkene instead of trans-2b, 3bb′ was obtained selectively in moderate yield (Scheme 3). As an alternative synthetic method, ruthenacyclopentane 3ba was obtained using [RuCl2(CO)3]2 as the starting complex instead of RuCl3·nH2O, in the absence of CO (Scheme 4). This reaction proceeded much more quickly in comparison to that outlined in Scheme 2, probably because no excess CO was present in the reaction system. Scheme 4. Alternative Synthesis of 3ba

Figure 2. Molecular structure of 3bb with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity, except for those on C1, C3, and C4. Selected bond distances (Å): Ru1−C1 = 2.116(5), Ru1−C4 = 2.174(5), Ru1−C5 = 1.951(6), Ru1−C6 = 1.977(5), Ru1−C7 = 1.882(6), Ru1−O1 = 2.113(4), C1−C2 = 1.531(8), C2−C3 = 1.526(7), C3−C4 = 1.555(7).

Formation of Linear Codimers of Alkenes from Ruthenacyclopentanes. The reactivity of the synthesized ruthenacyclopentanes was then investigated (Table 1). Initially, 3aa was heated in toluene under an argon atmosphere at 100 or 130 °C (bath temperature) for 24 h (entries 1 and 2). Although 3aa was converted in both cases (52% and 100%), 4aa and 5aa (linear codimers of 1a and 2a) were not detected at all. On the Table 1. Formation of Linear Codimers 4 and 5 from Ruthenacyclopentanes 3a

entry complex

Figure 3. Molecular structure of 3bb′ with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity, except for those on C1, C3, and C4. Selected bond distances (Å): Ru1−C1 = 2.118(3), Ru1−C4 = 2.191(3), Ru1−C5 = 1.950(3), Ru1−C6 = 1.994(4), Ru1−C7 = 1.875(4), Ru1−O1 = 2.109(2), C1−C2 = 1.539(4), C2−C3 = 1.521(4), C3−C4 = 1.534(4), C5−O6 = 1.127(4), C6−O7 = 1.121(4), C7−O8 = 1.134(4).

product

conversn of 3 (%)b

total yield of 4 and 5 (%)b,c

1 2

3aa

4aa/5aa

100 °C 130 °C

52 100

0 0

3 4d 5

3ba

4ba/5ba

100 °C 130 °C 130 °C, CO (1 atm) 130 °C, CO (20 atm) 130 °C

0 100 100

0 65 (95:5) 65 (95:5)

26

4

100

72 (67:33)

130 °C 130 °C

100 100

37 (100:0) 36 (100:0)

6 7e 8 9

equatorial, 8.1 Hz) also supported the proposed stereochemistry of 3bb″. In addition, a correlation between the acetyl protons and the methyne proton on C3 was observed in the NOESY spectrum of 3bb″, also supporting the stereochemistry (Figure S11 in the Supporting Information). In contrast, when dimethyl maleate (cis-2b) was used as an

conditions

3bb 3bb′

4bb/5bb 4bb/5bb

a Reaction conditions unless specified otherwise: ruthenacyclopentane 3 (0.20 mmol), toluene (2.0 mL), bath temperature, 24 h, under Ar. b Determined by 1H NMR. cThe ratios of 4 to 5 are shown in parentheses. d20 h. eDMA was used as a solvent.

C


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Organometallics other hand, when 3ba was heated in toluene at 100 °C, no conversion occurred (entry 3); however, heating at 130 °C (bath temperature) afforded linear codimers in good yield with a high formation ratio of 4ba to 5ba (95:5) and high stereoselectivity of 4ba (trans:cis ≥ 30:1) (entry 4). The reaction was also performed under CO (1 or 20 atm). Although the same result was obtained under 1 atm of CO (entry 5), the reaction was distinctly suppressed under 20 atm of CO (entry 6). When DMA was used as a solvent instead of toluene, not only 4ba but also isomerized codimer 5ba were obtained in a total yield of 72% with a ratio of 67:33 (the stereoselectivities of both 4ba and 5ba were trans:cis ≥ 30:1) (entry 7). In the case of tricarbonylruthenacyclopentenes reported by Itami and Yoshida et al., cyclopentenones were obtained via CO insertion into a Ru−C bond and subsequent reductive elimination.101,102 In contrast, no ruthenium complexes formed via CO insertion, nor were any cyclopentanones observed from our tricarbonylruthenacyclopentane 3ba under the aforementioned reaction conditions. Ruthenacyclopentanes 3bb and 3bb′ afforded the corresponding linear codimer 4bb selectively (entries 8 and 9). Although both 3bb and 3bb′ were consumed completely, the yields of 4bb were low (trans:cis ≥ 30:1 in both cases). Catalytic Activity of Ruthenacyclopentane and a RuCl3/Zn-Cu/Alcohol System. The catalytic activity of 3ba and the RuCl3/Zn-Cu/alcohol system61 toward the linear codimerization was then examined. The latter system is the same combination as that used for the synthesis of 3 (Schemes 2 and 3). Under an argon atmosphere, N-methyl-N-vinylacetamide 1b and electron-deficient alkenes 2 were allowed to react in the presence of either 3ba (catalyst A) or RuCl3/ZnCu/alcohol (catalyst B) (Table 2). When the codimerization of 1b with 2a using catalyst A was performed in toluene at 100 °C, no codimers were formed, as was expected from the reactivity of 3ba shown in entry 3 of Table 1. Upon an increase in the reaction temperature to 130 °C (bath temperature), codimers 4ba and 5ba were obtained in a total yield of 69% (4ba:5ba = 87:13) (entry 1, Table 2). Although more sluggish, the reaction also proceeded when DMA was used instead of toluene (entry 2). A relatively high reaction temperature seems to be required to afford 5ba (entry 4 vs 7 in Table 1 and entry 1 vs 2 in Table 2). Although the bath temperature is 130 °C in entry 4, Table 1, and in entry 1, Table 2, the real temperature of the reaction mixture would be ca. 110 °C (boiling point of toluene). On the other hand, the use of DMA as a solvent was essential for catalyst B to generate the catalytically active species. Unlike the case for catalyst A, catalyst B facilitated the codimerization even at 100 °C to afford 4ba selectively (entry 3), probably due to the absence of CO in catalyst B. Again, the ratio of 4ba to 5ba decreased by increasing the reaction temperature (entry 4). The codimerization of 1b with trans-2b was efficiently catalyzed by catalyst A in toluene to afford 4bb exclusively, although a longer reaction time was required than for the above codimerizations (entry 5). In this case, the reaction using catalyst B was also sluggish at 100 °C (entry 6), and increasing the temperature to 170 °C (bath temperature) improved the yield of 4bb (entry 7). The results of these stoichiometric and catalytic reactions revealed that the present codimerization proceeds via ruthenacyclopentane intermediates. To clarify whether the isomerization of 4 to 5 proceeds under catalytic conditions, isolated 4ba was treated with 5 mol % of 3bb′ in DMA at 130 °C for 24 h. A 93% amount of 4ba was recovered, along with the formation of 4% 4bb, which was derived from 3bb′. This result, in combination with the results

Table 2. Catalytic Activity of Ruthenacyclopentane 3ba and a RuCl3/Zn-Cu/ROH Systema

conversn (%)b

entry

product

catalyst

conditions

1

2

total yield of 4 and 5 (%)b,c

1

4ba/5ba

A

toluene, 130 °C, 5 h DMA, 130 °C, 24 h DMA, 100 °C, 5 h, R4 = Et DMA, 130 °C, 3 h, R4 = Et

76

100

69 (87:13)

60

82

53 (81:19)

87

100

51 (100:0)

85

100

72 (74:26)

96

100

78 (100:0)

36

27

6 (100:0)

99

NDd

42 (100:0)

2

A

3

B

4

B

5

4bb/5bb

A

6

B

7

B

toluene, 130 °C, 15 h DMA, 100 °C, 5 h, R4 = Me DMA, 170 °C, 5 h, R4 = Me

a Reaction conditions: 1 (0.55 mmol), 2 (0.50 mmol), solvent (1.5 mL), bath temperature, under Ar. Catalyst A: 3ba (0.025 mmol). Catalyst B: RuCl3·nH2O (0.025 mmol), Zn-Cu (0.25 mmol), R4OH (0.15 mmol). bDetermined by 1H NMR. cThe ratios of 4 to 5 are shown in parentheses. dNot determined.

implied by entry 2 in Table 2, indicated that 5ba is formed not from 4ba via isomerization but from the corresponding ruthenacyclopentane intermediate directly in the catalytic codimerization. Time Course of the Stoichiometric and Catalytic Reactions. Time-dependent changes in the yields of products in the stoichiometric and catalytic reactions were monitored by 1 H NMR spectroscopy (Figure 4). The stoichiometric conversion of 3ba in toluene at 130 °C (bath temperature) proceeded without an induction period (Figure 4a). On the other hand, induction periods were observed in the catalytic codimerization of 1b with 2a and of 1b with trans-2b when 5 mol % of 3ba was used as a catalyst in toluene at 130 °C (bath temperature) (Figure 4b,c). Interestingly, a longer reaction time (ca. 13 h) was required for the stoichiometric conversion of 3ba to 4ba and 5ba than for the catalytic codimerization of 1b with 2a to afford 4ba and 5ba (ca. 4 h), although [3ba]0 for the stoichiometric conversion is higher (Figure 4a, 0.10 M) than that for the catalytic codimerization (Figure 4b, 0.017 M). These results indicate that the active species in the catalytic reactions have higher reactivity than 3ba and are presumably ruthenium species having two or fewer CO ligands (see below for further discussion). Proposed Reaction Mechanism. Taking into account the obtained experimental results, a reaction mechanism for the D


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Organometallics

tanes, β-hydrogen elimination from the ruthenacycles can proceed smoothly (ΔG⧧ values were estimated to be only 12.1 and 5.6 kcal mol−1, respectively).103 On the other hand, dissociation of the CO ligand trans to the acetyl oxygen and subsequent β-hydrogen elimination and reductive elimination afford 5ba, which is also a trans isomer. In this case, a βhydrogen (Hc) cis to the ethoxycarbonyl group is eliminated. As mentioned above, a ruthenacyclopentane(s) having two or fewer CO ligands can be the active species for the formation of 4ba and 5ba in the catalytic codimerization using 3ba. An explanation for the induction period observed in the catalytic codimerization (Figure 4b,c) is as follows. After the formation of “Ru(CO)3” species and codimer 4ba from 3ba via β-hydrogen elimination and reductive elimination, oxidative cyclization involving “Ru(CO)3”, 1b, and 2a (or trans-2b) can be very sluggish in the catalytic reactions. Thereafter, gradual loss of CO ligand(s) from “Ru(CO)3” occurs to afford “Ru(CO)n” (n ≤ 2), from which oxidative cyclization, βhydrogen elimination, and reductive elimination proceed rapidly: i.e., “Ru(CO)n” is the catalytically active species. In such cases the induction periods can be observed. Because in the absence of CO ligands (i.e., under the reaction conditions of catalyst B in Table 2) catalytic reaction proceeded rapidly (entry 2 vs entries 3 and 4 in Table 2), the above explanation would be consistent with these results, although the possibility of the involvement of other mechanisms still cannot be excluded (see below). Deuterium-Labeling Experiment. To clarify whether ruthenium hydride species are formed from the ruthenacycles in situ, the codimerization of 1b with deuterium-labeled dimethyl fumarate (trans-2b-d2) was examined in the presence of 5 mol % of 3ba in toluene at 130 °C (bath temperature) for 15 h (Scheme 6). As a result, significant deuterium scrambling

Figure 4. Time-dependent changes in the yields of products in (a) the stoichiometric conversion of 3ba ([3ba]0 = 0.10 M), (b) the catalytic codimerization of 1b with 2a using 3ba as a catalyst ([1b]0 = 0.37 M, [2a]0 = 0.33 M, [3ba]0 = 0.017 M), and (c) the catalytic codimerization of 1b with trans-2b using 3ba as a catalyst ([1b]0 = 0.37 M, [trans-2b]0 = 0.33 M, [3ba]0 = 0.017 M), in toluene at 130 °C (bath temperature), respectively.

formation of 4ba and 5ba starting from RuCl3 under CO is proposed, as depicted in Scheme 5. First, ruthenium is reduced Scheme 5. Proposed Mechanism for the Formation of 4ba and 5ba

Scheme 6. Catalytic Codimerization using trans-2b-d2

was observed in the codimer 4bb-d2, similar to that observed previously.63 This result indicates that rapid H/D exchange occurred by in situ formed ruthenium hydride species prior to the formation of the codimer. Related to this, the formation of 3bb′ using trans-2b as shown in Scheme 3, in which the isomerization of trans-2b to cis-2b occurred prior to the oxidative cyclization, was probably caused by the in situ formed ruthenium hydride species as well. Although these results indicate that the metal-hydride and C−H activation mechanisms are still possibly operative, the other results support that the present catalytic codimerization can proceed via the metallacycle mechanism.

from the trivalent state to the zerovalent state by Zn-Cu/ROH. Electron-deficient alkenes such as 2a usually coordinate to Ru(0) more strongly than electron-rich alkenes due to stronger π back-donation. However, N-vinylacetamides such as 1b can coordinate in a bidentate manner and thus the coordination ability of these alkenes would balance well. Thus, the Ru(0) intermediate which is coordinated by 1b and 2a would be formed, giving ruthenacycle 3ba after oxidative cyclization and further coordination of CO. If dissociation of the acetyl oxygen proceeds from 3ba, subsequent β-hydrogen elimination and reductive elimination would afford 4ba. The β-elimination of the hydrogen (Hb) cis to the N-methylacetamido group can explain the formation of trans-4ba. According to the DFT calculations for Cp and benzene-coordinated ruthenacyclopen-

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CONCLUSION Ruthenacyclopentanes, which are usually elusive, could be synthesized and isolated as air- and moisture-stable complexes by the reaction of RuCl3·nH2O with Zn-Cu, N-vinylacetamides, and electron-deficient alkenes under a CO atmosphere. Heating the ruthenacyclopentanes having an N-methylacetamido group E


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Organometallics

Ruthenacyclopentane 3ba. To a mixture of RuCl3·nH2O (0.654 g, 2.5 mmol) and Zn-Cu couple (0.981 g, 15 mmol) were added DMA (50 mL), 1b (2.58 mL, 25 mmol), 2a (2.72 mL, 25 mmol), and absolute EtOH (0.88 mL, 15 mmol) under a CO atmosphere. The reaction mixture was stirred at 100 °C for 24 h. After removal of the solvent and other volatile compounds, the residual sticky oil was treated with ethyl acetate and then filtered. The filtrate was concentrated to dry, and the residue was purified by silica gel column chromatography (eluent hexane/ethyl acetate 3/1) in air to give ruthenacyclopentane 3ba in 61% isolated yield (0.587 g, 1.53 mmol) as a white solid along with codimer 4ba (0.242 g, 1.2 mmol, 49% isolated yield based on RuCl3). Ruthenacyclopentane 3ba was air- and moisture-stable. Mp: 86−89 °C dec. IR (KBr): 2091, 2027, 1997, 1670, 1588, 1175, 1148 cm−1. 1H NMR (300 MHz, CDCl3): δ 4.43 (s, 1H, NCHCH2), 4.14 (dq, 2JHH = 10.8 Hz and 3JHH = 7.2 Hz, 1H, OCHHCH3), 4.01 (dq, 2JHH = 10.8 Hz and 3JHH = 7.2 Hz, 1H, OCHHCH3), 3.06 (s, 3H, NCH3), 2.72 (d, 3JHH = 8.1 Hz, 1H, EtO2CH), 2.30−2.25 (m, 1H, NCHCH), 2.02 (s, 3H, NCOCH3), 2.02−1.90 (m, 2H, NCHCH and EtO2CHCHH), 1.21 (t, 3JHH = 7.2 Hz, 3H, OCH2CH3), 0.94−0.80 (m, 1H, EtO2CHCHH). 13C{1H} NMR (CDCl3, 75 MHz): δ 198.9 (CO), 191.0 (CO), 190.4 (C O), 183.4 (CO 2 Et), 178.0 (NCOCH 3 ), 73.9 (CHN), 58.6 (OCH2CH3), 41.4 (CHCO2Et), 41.0 (NCHCH2), 36.6 (NCH3), 25.8 (EtO2CHCH2), 19.9 (NCOCH3), 14.4 (OCH2CH3). Anal. Calcd for C13H17NO6Ru: C, 40.62; H, 4.46; N, 3.64. Found: C, 40.75; H, 4.47; N, 3.53. Ruthenacyclopentanes 3bb,bb′,bb″. To a mixture of RuCl3·nH2O (52.3 mg, 0.2 mmol) and Zn-Cu couple (78.5 mg, 1.2 mmol) were added DMA (4.0 mL), 1b (0.21 mL, 2.0 mmol), trans-2b (288 mg, 2.0 mmol), and dry MeOH (48.7 μL, 1.2 mmol) under a CO atmosphere. The reaction mixture was stirred at 100 °C for 20 h. After removal of the solvent and other volatile compounds, the residual sticky oil was treated with MeOH and then filtered. The filtrate was concentrated to dry, and the residue was purified by silica gel column chromatography (eluent hexane/ethyl acetate 1/1) in air to give ruthenacyclopentanes 3bb,bb′,bb″. These ruthenacyclopentanes were air- and moisturestable. Ruthenacyclopentane 3bb. White solid (51.6 mg, 0.12 mmol, 60% isolated yield). Mp: 116−121 °C dec. IR (KBr): 2093, 2013, 1996, 1713, 1679, 1597, 1200, 1175, 1157 cm−1. 1H NMR (CDCl3, 300 MHz): δ 4.38 (dd, 3JHH = 1.7 and 3.9 Hz, 1H, NCHCH2), 3.66 (s, 3H, CO2CH3), 3.57 (s, 3H, CO2CH3), 3.45 (ddd, 3JHH = 1.5, 4.2, and 9.0 Hz, 1H, CH2CHCO2Me), 3.12 (s, 3H, NCH3), 3.10 (d, 3JHH = 4.2 Hz, 1H, RuCHCO2Me), 3.01 (dt, 2JHH = 14.7 Hz and 3JHH = 1.7 Hz, 1H, NCHCHH), 2.23 (ddd, 2JHH = 14.7 Hz and 3JHH = 4.2 and 9.0 Hz, 1H, NCHCHH), 1.95 (s, 3H, NCOCH3). 13C{1H} NMR (CDCl3, 75 MHz): δ 198.3 (CO), 190.20 (CO), 190.17 (CO), 182.9 (CO2Me), 178.5 (NCOCH3), 177.4 (CO2Me), 72.0 (CHN), 51.6 (CO2CH3), 50.5 (CO2CH3), 45.7 (CH2CHCO2Me), 44.49 (RuCHCO2Me), 44.46 (CH2), 36.9 (NCH3), 20.1 (NCOCH3). Anal. Calcd for C14H17NO8Ru: C, 39.26; H, 4.00; N, 3.27. Found: C, 39.10; H, 4.13; N, 3.01. Ruthenacyclopentane 3bb′. White solid (13.2 mg, 0.031 mmol, 16% isolated yield). Mp: 151−152 °C dec. IR (KBr): 2097, 2015, 1987, 1728, 1681, 1589, 1181 cm−1. 1H NMR (CDCl3, 300 MHz): δ 4.34 (br s, 1H, NCHCH2), 3.63 (s, 3H, CO2CH3), 3.59 (s, 3H, CO2CH3), 3.13 (s, 3H, NCH3), 3.10 (d, 3JHH = 8.1 Hz, 1H, RuCHCO2Me), 2.67 (ddd, 2JHH = 13.8 Hz and 3JHH = 2.4 and 4.5 Hz, 1H, NCHCHH), 2.45 (dt, 3JHH = 3.6 Hz and 2,3JHH = 13.6 Hz, 1H, NCHCHH), 2.16 (ddd, 3JHH = 4.5, 8.1, and 13.2 Hz, 1H, CH2CHCO2Me), 2.07 (s, 3H, NCOCH3). 13C{1H} NMR (CDCl3, 75 MHz): δ 198.1 (CO), 190.0 (CO), 189.9 (CO), 181.4 (CO2Me), 178.7 (NCOCH3), 176.2 (CO2Me), 67.8 (CHN), 51.5 (CO2CH3), 50.2 (CO2CH3), 43.9 (CH2CHCO2Me), 41.9 (CH2), 41.1 (RuCHCO2Me), 36.9 (NCH3), 19.9 (NCOCH3). Anal. Calcd for C14H17NO8Ru: C, 39.26; H, 4.00; N, 3.27. Found: C, 39.57; H, 3.95; N, 3.13. Ruthenacyclopentane 3bb″. White solid (14.4 mg, 0.034 mmol, 17% isolated yield). Mp: 72−75 °C dec. IR (KBr): 2084, 2021, 1975, 1726, 1695, 1594 cm−1. 1H NMR (CDCl3, 300 MHz): δ 4.38 (t, 3JHH

resulted in the formation of linear codimers of the alkenes. The ruthenacyclopentanes were also found to operate as catalysts for linear codimerization. Although the metal-hydride and C− H activation mechanisms are still possible because the ruthenium hydride species which cause isomerization and H/ D exchange of alkenes appeared to be generated in situ under the conditions of synthesis of the ruthenacycles and the catalytic reactions, the aforementioned experimental facts established that the present codimerization can proceed via the metallacycle mechanism. The formation ratio of the codimers was influenced by the reaction temperature: i.e., the isomeric codimer 5ba was formed at higher temperature. Codimer 5ba must be formed via a pathway independent from that for 4ba, because no isomerization from 4ba to 5ba was observed under the catalytic reaction conditions. The catalytic codimerization proceeded at lower temperature when a RuCl3/ Zn-Cu/ROH system was used in the absence of CO. These results would be useful for further enhancing the catalytic activity in the codimerization of alkenes via metallacyclopentanes.

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EXPERIMENTAL SECTION

Materials and Methods. All manipulations were performed under an argon atmosphere using standard Schlenk techniques unless otherwise noted. RuCl3·nH2O and [RuCl2(CO)3]2 were obtained from Furuya Metal and Strem, respectively. Dry DMA was purchased from Wako Chemical. Toluene was also purchased from Wako Chemical and was distilled from sodium and benzophenone before use. Zn-Cu104 and trans-2b-d2105 were prepared as described in the literature. Flash column chromatography was performed using silica gel SILICYCLE SiliaFlash F60 (40−63 μm, 230−400 mesh). Physical and Analytical Measurements. NMR spectra were recorded on either a JEOL AL-400 (400 MHz (1H), 100 MHz (13C)) or a Bruker AV-300N (300 MHz (1H), 75 MHz (13C)) spectrometer. Chemical shift values (δ) are expressed relative to SiMe4. IR measurements (ATR) were carried out using a JASCO FT/IR-6100 spectrometer. Elemental analysis was obtained using a J-SCIENCE LAB JM-10 analyzer. High-resolution mass spectra were recorded on a JEOL JMS-T100LC spectrometer (ESI-TOF MS) with positive ionization mode. Melting points were measured on a Yanagimoto micro melting point apparatus. Synthesis of Ruthenacyclopentanes 3. Ruthenacyclopentane 3aa. To a mixture of RuCl3·nH2O (0.105 g, 0.4 mmol) and Zn-Cu couple (0.157 g, 2.4 mmol) were added DMA (5.0 mL), 1a (0.340 g, 4.0 mmol), 2a (0.44 mL, 4.0 mmol), and absolute EtOH (0.14 mL, 2.4 mmol) under a CO atmosphere. The reaction mixture was stirred at 100 °C for 20 h. After removal of the solvent and other volatile compounds, the residual sticky oil was treated with MeOH and then filtered. The filtrate was concentrated to dry, and the residue was purified by silica gel column chromatography (eluent hexane/ethyl acetate 3/2) in air to give ruthenacyclopentane 3aa in 78% isolated yield (57.5 mg, 0.155 mmol) as a white solid. Ruthenacyclopentane 3aa was air- and moisture-stable. Mp: 133−135 °C dec. IR (KBr): 3261, 2088, 2020, 1995, 1979, 1658, 1608, 1548, 1178, 1154 cm−1. 1H NMR (CDCl3, 300 MHz): δ 6.65 (brs, 1H, NH), 4.60 (br s, 1H, NCHCH2), 4.13 (dq, 2JHH = 10.8 Hz and 3JHH = 7.2 Hz, 1H, OCHHCH3), 4.01 (dq, 2JHH = 10.8 Hz and 3JHH = 7.2 Hz, 1H, OCHHCH3), 2.81 (d, 3JHH = 7.5 Hz, 1H, EtO2CH), 2.08 (s, 3H, NCOCH3), 2.04−1.99 (m, 3H, NCHCH2 and EtO2CHCHH), 1.24 (t, 3 JHH = 7.2 Hz, 3H, OCH2CH3), 1.19−1.02 (m, 1H, EtO2CHCHH). 13 C{1H} NMR (CDCl3, 75 MHz): δ 198.8 (CO), 190.8 (CO), 190.3 (CO), 183.6 (CO2Et), 179.9 (NCOCH3), 64.2 (CHN), 58.7 (OCH 2 CH 3 ), 45.0 (NCHCH 2 ), 41.3 (CHCO 2 Et), 26.2 (EtO2CHCH2), 19.1 (NCOCH3), 14.4 (OCH2CH3). Anal. Calcd for C12H15NO6Ru: C, 38.92; H, 4.08; N, 3.78. Found: C, 38.96; H, 3.96; N, 3.68. F


Article

Organometallics 3

JHH = 6.6 Hz, 1H, CHHCOOCH3), 2.16 (s, 3H, NCOCH3). 1H NMR for minor rotamer of 4bb (CDCl3, 300 MHz): δ 7.44 (d, 3JHH = 14.4 Hz, 1H, NCHCH), 4.88 (dd, 3JHH = 14.4 and 9.6 Hz, 1H, NCHCH), 3.64 (s, 3H, OCH3), 3.61 (s, 3H, OCH3), 3.57−3.46 (m, 1H, CHCOOCH3), 3.02 (s, 3H, NCH3), 2.80 (dd, 2JHH = 16.3 Hz and 3 JHH = 8.6 Hz, 1H, CHHCOOCH3), 2.51 (dd, 2JHH = 16.3 Hz and 3 JHH = 5.9 Hz, 1H, CHHCOOCH3), 2.14 (s, 3H, NCOCH3). 13C{1H} NMR for major rotamer of 4bb (CDCl3, 100 MHz): δ 173.6 (NCOCH3), 171.8 (COOCH3), 169.3 (COOCH3), 132.3 (NCHCH), 106.3 (NCHCH), 52.5 (COOCH 3 ), 52.0 (COOCH 3 ), 43.1 (CHCOOCH 3 ), 37.2 (CH 2 COOCH 3 ), 29.4 (NCH 3 ), 22.0 (COCH3). 13C{1H} NMR for minor rotamer of 4bb (CDCl3, 100 MHz): δ 173.9 (NCOCH3), 171.8 (COOCH3), 169.2 (COOCH3), 130.1 (NCHCH), 106.5 (NCHCH), 52.4 (COOCH 3 ), 52.0 (COOCH3), 43.0 (CHCOOCH3), 37.3 (CH2COOCH3), 29.8 (NCH 3 ), 22.7 (COCH 3 ). HRMS (ESI): m/z calcd for C11H17NO5Na [M + Na]+ 266.1004, found 266.0991. Catalytic Linear Codimerization by 3ba. The following procedure using toluene as a solvent is representative. To a toluene solution (1.5 mL) of 3ba (9.6 mg, 0.025 mmol) were added 1b (57 μL, 0.55 mmol) and 2a (54 μL, 0.50 mmol) under an argon atmosphere, and the mixture was heated at 130 °C (bath temperature) for 5 h. After the mixture was cooled to room temperature, pbis(trifluoromethyl)benzene (77 μL, 0.50 mmol) was added as an internal standard. A portion of the reaction mixture (0.2 mL) was taken and was mixed with 0.5 mL of CDCl3. The total yield of the codimers and the formation ratio were determined by 1H NMR analysis. Catalytic Linear Codimerization by a RuCl3/Zn-Cu/ROH System. The following procedure for the formation of 4ba is representative. To a mixture of RuCl3·nH2O (6.5 mg, 0.025 mmol) and Zn-Cu couple (16.3 mg, 0.25 mmol) were added DMA (1.5 mL), 1b (57 μL, 0.55 mmol), 2a (54 μL, 0.50 mmol), and absolute EtOH (8.8 μL, 0.15 mmol) under an argon atmosphere. The reaction mixture was stirred at 100 °C for 5 h. After the mixture was cooled to room temperature, dibenzyl ether (90 μL, 0.47 mmol) was added as an internal standard. A portion of the reaction mixture (0.2 mL) was taken and was mixed with 0.5 mL of CDCl3. The total yield of the codimers and the formation ratio were determined by 1H NMR analysis. Catalytic Linear Codimerization of 1b with trans-2b-d2 by 3ba. The reaction was performed in the same manner as described above. The isolated codimer 4bb-d2 (89 mg, 0.36 mmol, 73% yield) was analyzed by 1H NMR, and the deuterium incorporation ratios were determined by comparison of the integral values of the corresponding signals. Crystallographic Study of 3ba,bb,bb′. Single crystals suitable for X-ray diffraction measurements obtained from CHCl3/hexane for 3ba (colorless block, by vapor diffusion) and 3bb′ (colorless block), and CH2Cl2/hexane for 3bb (colorless block) were mounted using a cryoloop. The diffraction data were collected with a Rigaku Saturn CCD detector (Mo Kα, λ = 0.71069 Å). Crystal data are given in Table S1 in the Supporting Information. The structures were solved by direct methods using SHELXS-97 and refined by least-squares on F2 using SHELXL-2013.106−108 Non-hydrogen atoms were anisotropically refined except for disordered atoms, which were isotropically refined. Refinements were continued until all shifts were smaller than one-tenth of the standard deviations of the parameters involved. Atomic scattering factors and anomalous dispersion terms were taken from ref 109.

= 2.4 Hz, 1H, NCHCH2), 3.63 (s, 3H, CO2CH3), 3.62 (s, 3H, CO2CH3), 3.32 (d, 3JHH = 9.6 Hz,RuCHCO2Me), 3.12 (s, 3H, NCH3), 2.83 (ddd, 3JHH = 5.1, 9.6, and 13.8 Hz, 1H, CH2CHCO2Me), 2.73 (ddd, 2JHH = 13.5 Hz and 3JHH = 2.4 and 5.1 Hz, 1H, NCHCHH), 2.10 (s, 3H, NCOCH3), 2.02 (dt, 3JHH = 3.0 Hz and 2,3JHH = 13.7 Hz, 1H, NCHCHH). 13C{1H} NMR (CDCl3, 75 MHz): δ 198.3 (CO), 190.2 (CO), 189.9 (CO), 180.1 (CO2Me), 179.2 (NCOCH3), 177.0 (CO2Me), 68.5 (CHN), 51.7 (CO2CH3), 50.2 (CO2CH3), 45.2 (CH2), 43.0 (CH2CHCO2Me), 36.9 (RuCHCO2Me), 36.3 (NCH3), 14.1 (NCOCH3). Anal. Calcd for C14H17NO8Ru: C, 39.26; H, 4.00; N, 3.27. Found: C, 39.44; H, 3.99; N, 3.23. Alternative Synthesis of 3ba from [RuCl2(CO)3]2. Ruthenacyclopentane 3ba was alternatively synthesized from [RuCl2(CO)3]2. To a mixture of [RuCl2(CO)3]2 (102 mg, 0.20 mmol) and Zn-Cu couple (157 mg, 2.4 mmol) were added DMA (8 mL), 1b (0.41 mL, 4.0 mmol), 2a (0.44 mL, 4.0 mmol), and absolute EtOH (0.14 mL, 2.4 mmol) under an argon atmosphere. The reaction mixture was stirred at 100 °C for 3 h. After removal of the solvent and other volatile compounds, a viscous black oil was obtained. The residue was purified by silica gel column chromatography (eluent hexane/ethyl acetate 2/ 1) in air to give 3ba in 48% isolated yield (75 mg, 0.19 mmol). Formation of Linear Codimers 4 and 5 from Ruthenacyclopentanes 3. The following procedure for 3ba using toluene as a solvent is representative. To 3ba (77 mg, 0.20 mmol) were added toluene (2.0 mL) and dimethyldiphenylsilane (43 μL, 0.20 mmol, internal standard) under an argon atmosphere. The reaction mixture was stirred at 130 °C (bath temperature). A portion of the reaction mixture (0.2 mL) was taken several times, and each sample was mixed with 0.5 mL of CDCl3. The total yield of the codimers and the formation ratio were determined by 1H NMR analysis. The NMR data for 4ba were consistent with those reported previously.63 Isolation of Codimer 5ba. A DMA solution (0.6 mL) of 3ba (116 mg, 0.30 mmol) was heated at 170 °C (bath temperature) for 16 h under an argon atmosphere. After removal of the solvent, the residue was purified by silica gel column chromatography (hexane/ethyl acetate 10/1 to 5/1) to afford 5ba (16 mg, 0.082 mmol, 27%) along with 4ba (3.1 mg, 0.016 mmol, 5%). 1H NMR for major rotamer of 5ba (CDCl3, 300 MHz): δ 6.90 (dt, 3JHH = 15.6 and 7.2 Hz, 1H, CH2CHCH), 5.86 (dt, 3JHH = 15.6 Hz and 4JHH = 1.4 Hz, 1H, CH2CHCH), 4.17 (q, 3JHH = 7.2 Hz, 2H, OCH2CH3), 3.48 (t, 3JHH = 7.2 Hz, 2H, NCH2), 2.99 (s, 3H, NCH3), 2.45 (dq, 4JHH = 1.2 Hz and 3JHH = 7.2 Hz, 2H, NCH2CH2), 2.07 (s, 3H, NCOCH3), 1.28 (t, 3 JHH = 7.2 Hz, 3H, OCH2CH3). 1H NMR for minor rotamer of 5ba (CDCl3, 300 MHz): δ 6.88 (dt, 3JHH = 15.6 and 7.2 Hz, 1H, CH2CHCH), 5.89 (dt, 3JHH = 15.6 Hz and 4JHH = 1.4 Hz, 1H, CH2CHCH), 4.19 (q, 3JHH = 7.2 Hz, 2H, OCH2CH3), 3.42 (t, 3JHH = 7.2 Hz, 2H, NCH2), 2.92 (s, 3H, NCH3), 2.47 (dq, 4JHH = 1.2 Hz and 3JHH = 7.2 Hz, 2H, NCH2CH2), 2.08 (s, 3H, NCOCH3), 1.28 (t, 3 JHH = 7.2 Hz, 3H, OCH2CH3). 13C{1H} NMR for major rotamer of 5ba (CDCl3, 75 MHz): δ 170.6 (NCOCH3), 166.3 (CO2Et), 145.3 (CH2CHCH), 123.2 (CH2CHCH), 60.3 (OCH2), 46.5 (NCH2), 36.7 (NCH3), 30.3 (NCH2CH2), 21.9 (NCOCH3), 14.2 (OCH2CH3). 13 C{1H} NMR for minor rotamer of 5ba (CDCl3, 75 MHz): δ 170.3 (NCOCH 3 ), 165.9 (CO 2 Et), 143.7 (CH 2 CHCH), 124.0 (CH2CHCH), 60.5 (OCH2), 49.4 (NCH2), 33.3 (NCH3), 31.2 (NCH2CH2), 21.3 (NCOCH3), 14.2 (OCH2CH3). HRMS (ESI): m/z calcd for C10H17NO3Na [M + Na]+ 222.1106, found 222.1098. Isolation of Codimer 4bb. A toluene solution (1.0 mL) of 3bb′ (42 mg, 0.10 mmol) was heated at 130 °C (bath temperature) for 24 h under an argon atmosphere. After the mixture was cooled to room temperature, the supernatant solution was taken and the remaining black precipitate was washed with toluene. The combined solution was evaporated under vacuum to give a blackish brown oil, which was purified by silica gel column chromatography (hexane/ethyl acetate 1/ 2) to afford 4bb (5.6 mg, 0.020 mmol, 23%) as a colorless oil. 1H NMR for major rotamer of 4bb (CDCl3, 300 MHz): δ 6.76 (d, 3JHH = 13.8 Hz, 1H, NCHCH), 4.85 (dd, 3JHH = 13.8 and 9.3 Hz, 1H, NCHCH), 3.66 (s, 3H, OCH3), 3.62 (s, 3H, OCH3), 3.57−3.46 (m, 1H, CHCOOCH3), 2.98 (s, 3H, NCH3), 2.83 (dd, 2JHH = 16.5 Hz and 3 JHH = 7.8 Hz, 1H, CHHCOOCH3), 2.50 (dd, 2JHH = 16.5 Hz and

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00545. NMR spectra for 3−5 and crystallographic information for complexes 3ba,bb,bb′ (PDF) G


Article

Organometallics Accession Codes

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CCDC 1556414 and 1556418−1556419 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.U.: [email protected]. ORCID

Yasuyuki Ura: 0000-0003-0484-1299 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Nara Women’s University Intramural Grant for Project Research. We thank Prof. Takashi Kajiwara (Nara Women’s University) for supporting our X-ray crystallography.

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REFERENCES

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