Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
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Synthesis of and Catalytic Linear Cross-Dimerizations by an Electron-Deficient Cyclic Diene Complex of Ruthenium(0) Masafumi Hirano,* Yukino Tanaka, and Nobuyuki Komine Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
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S Supporting Information *
ABSTRACT: A ruthenium(0) complex [Ru(η6-naphthalene)(η4-Phdbcot)] (1a: Ph-dbcot = 5-phenyldibenzo[a,e]cyclooctatetraene) is newly prepared in 53% yield. The IR experiment of [Ru(η4-2,3dimethylbutadiene)(η4-Ph-dbcot)(CO)] (2a) shows this formal zerovalent ruthenium complex to be very electron-deficient, even though the Phdbcot ligand does not have any particular electron-withdrawing functional group. Complex 1a shows high catalytic activity toward cross-dimerization between conjugated diene with methyl acrylate with high regioselectivity.
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toward cross-dimerization than corresponding cod and dbcot complexes. The Ph-dbcot ligand was originally reported by Grützmacher and co-workers, which was readily prepared from dibenzosuberenone.8 By a modified method of Bennett and co-workers,9 treatment of [Ru(acac)3] with Ph-dbcot and Zn in refluxing wet thf for 7 h produced a 1/1 mixture of diastereomers of new [Ru(acac)2(η4-Ph-dbcot)] (3a) in 61% total yield (Scheme 1).
atalytic linear cross-dimerization is a powerful method to construct complex molecules from simple substrates with perfect atom- and step-economy.1,2 We have reported crossdimerization of conjugated compounds with substituted alkenes catalyzed by Ru(0) complexes having a cyclic diene ligand.1d In these catalyses, employment of a tertiary phosphine ligand shut down the catalysis,3 and [Ru(η6naphthalene)(η4-cod)] (1c) (cod: 1,5-cyclooctadiene) was normally used as the catalyst. In the screening of the catalysts, we found [Ru(η6-naphthalene)(η4-dbcot)] (1b) (dbcot: dibenzo[a,e]cyclooctatetraene4) showing catalytic activity in reactions of conjugated dienes with methyl acrylate (Chart 1).5
Scheme 1. Syntheses of [Ru(η6-naphthalene)(η4-Ph-dbcot)] (1a)
Chart 1
One of the diastereomers 3a can be separated by recrystallization from cold acetone, and the molecular structure of rac-Δ-(S)-3a was revealed (Figure 1). Reduction of 3a with sodium naphthalene in thf followed by recrystallization from cold thf/hexane yielded [Ru(η6-naphthalene)(η4-Ph-dbcot)] (1a) in 53% yield as red powders. 1a was fully characterized by 1 H, 13C{1H}, correlation spectroscopy (COSY), and heteronuclear multiple quantum coherence (HMQC) analysis.10 We evaluated the catalytic properties of 1a by crossdimerizations of conjugated dienes with methyl acrylate. Treatment of 2,3-dimethylbutadiene (4a) with methyl acrylate
We are interested in this fact because the dbcot ligand was used in the past for catalyst poisoning for homogeneous catalysis such as hydrogenation catalyzed by Wilkinson’s catalyst and [RuCl2(PPh3)3].6,7 In the line of the exploration of the cyclic diene ligands, we came across the Ph-dbcot ligand (Ph-dbcot: 5-phenyldibenzo[a,e]cyclooctatetraene). In this work we report preparation of a new Ru(0) complex, [Ru(η6-naphthalene)(η4-Ph-dbcot)] (1a), which shows much more efficient catalytic activity © XXXX American Chemical Society
Received: September 3, 2018
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DOI: 10.1021/acs.organomet.8b00645 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
Table 1. Catalytic Cross-Dimerizations of Conjugated Dienes 4 with Methyl Acrylate (5)a
Figure 1. Molecular structure of rac-Δ-(S)-[Ru(acac)2(η4-Ph-dbcot)] (3a) by X-ray analysis. R1 (wR2) = 0.0376 (0.1064).
(5) catalyzed by 1a (1 mol %) at 30 °C for 4 h produced methyl (2E)-5,6-dimethylhepta-2,5-dienoate (6a) (75%) (Table 1, entry 1). Complexes 1b and 1c also catalyzed the cross-dimerization, but they were less active than 1a (entries 2 and 3) under the same conditions. With 1,3-pentadiene (4b), the C4-position in 4b was dominantly reacted with methyl acrylate and the Z-configuration of the 5-position in 6b suggests the coupling of cisoid-4b with 5 (entries 4−6). In the reactions with isoprene (4c) (entries 7−10), (E)-3-methyl-1,3pentadiene (4d) (entries 11−13), and (E)-2,3-dimethyl-1,3pentadiene (4e) (entries 14−16), 1a produced the crossdimers with high catalytic activity, where the C−C bond formation dominantly occurred at the more hindered position. Notably, an overwhelming effect of the Ph-dbcot ligand on the catalytic activity was observed for the coupling reaction using β-myrcene (4f) (entries 17−19). In our previous study, a related 1,4-diene product favored to remain attached on the Ru(0), and this species falls down to a stable intermediate.11 The tethered alkenyl group (Me2CCHCH2CH2−) in 6f/7f may further stabilize the coordination of the product 6f/7f to satisfy the 18e rule or interrupt to replace the coordinated product with substrates for 1b and 1c. However, it is reasonable to consider the Ph group in 1a promoting the dissociation of the coupling product from the Ru(0) center. A mercury poisoning test never spoiled the catalytic activity of 1a, suggesting that 1a should act as a homogeneous catalyst (entry 8). To characterize the high catalytic activity of 1a, 1a was treated with 2,3-dimethylbutadiene and MeCN, and then the solution was exposed to CO atmosphere to give [Ru(η4-2,3dimethylbutadiene)(η4-Ph-dbcot)(CO)] (2a) in 44% yield (eq 1).
a
Typical conditions: 1 (1 mol %), 4 (0.25 mmol), 5 (0.25 mmol), benzene-d6 (600 μL), 30 ̊C, 4 h, products and yields are characterized by 1H NMR and GLC/GC-MS. bMercury (1.1 mmol) added. c Isomers (4%). dIsomer (8%). eTime = 8 h. fIsomer (5%). gIsomer (2%). hTime = 7 h. iTime = 1 h. jIsomer (10%). kIsomer (15%).
(2c: 1964 cm−1). The difference in the νCO band between 2a and 2c corresponds to P(OPh)3 (1985.3 cm−1) and PMe3 (1964.1 cm−1) in [Ni(CO)3L] according to Tolman’s electronic parameters.12 Notably, the electronic nature of the cyclic diene ligand can be controlled without special electronwithdrawing functional groups. A similar comparison was
The IR spectrum of 2a in KBr shows an intense νCO band at 1985 cm −1 . This is comparable to [Ru(η 4 -1,3dimethylbutadiene)(η4-dbcot)(CO)]5 (2b: 1984 cm−1), showing the Ph-dbcot ligand to be more electron-deficient than the cod ligand in [Ru(η4-1,3-dimethylbutadiene)(η4-cod)(CO)]5 B
DOI: 10.1021/acs.organomet.8b00645 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics reported for [Mo(η4-dbcot)(CO)4] (2052 cm−1) and [Mo(η4cod)(CO)4] (2038 cm−1).13 We also evaluated the steric bulk of the Ph-dbcot ligand. The steric map and percent buried volume14 of the Ph-dbcot (49.7% Vbur), dbcot (45.7% Vbur), and cod (46.0% Vbur) in 3a, 3b, and 3c are illustrated in Figure 2 and Table 2. The Phdbcot ligand is the bulkiest ligand, and the dbcot and cod ligands have almost the same steric bulk according to this criterion.
ruled out, and the detailed analysis of these diastereomers in the reactivity failed. A 1:1-diastereomeric mixture of 8a (1 mol %) catalyzed the cross-dimerization of 4c with 5 in benzene-d6 at 30 °C for 1 h to produce 6c (65%, 2E/2Z = 89/11) and 7c (29%, 2E/2Z = 100/0) based on 1H NMR. Because this result is roughly comparable to the reaction catalyzed by 1a (entry 7 in Table 1), a (η4-isoprene)ruthenium(0) species is presumably involved in the catalysis. The mechanism for the cross-dimerization is probably the oxidative coupling mechanism as same as the cod complex 1c (see the Supporting Information).15 With this agreement, a 7membered ruthenacycle (or a η1:η3-ruthenacycle) would be formed by the oxidative coupling of a conjugated diene with methyl acrylate. Chart 2 shows two possible simplified regioisomers of the intermediate for the reaction of isoprene with methyl acrylate. Chart 2 Figure 2. Steric maps for (A) the Ph-dbcot in [Ru(acac)2(η4-Phdbcot)] (3a), (B) dbcot in [Ru(acac)2(η4-dbcot)] (3b), and (C) cod ligand in [Ru(acac)2(η4-cod)] (3c) based on the crystallographic data. The ruthenium atom is placed at the original point, and the C=C double bonds in the cyclic diene ligand are placed parallel to the y axis. The Ph group in the Ph-dbcot ligand is placed in the fourth quadrant. These are views from the +z axis. Panel D is an example of placement of the cod ligand.
Table 2. Percent Buried Volume (%Vbur) for Ph-dbcot in 3a, dbcot in 3b, and cod in 3c and the Percent Buried Volume of Each Quadrant ligand
%Vbur
NE
NW
SW
SE
Ph-dbcot dbcot cod
49.7 45.7 46.0
48.0 45.9 44.8
46.2 45.6 47.0
46.0 45.9 44.9
58.5 45.5 47.3
Because of the rotation or half-rotation of the Ph-dbcot ligand with respect to the ruthenacycle, the intermediate B is disfavored because of the steric repulsion between the Me and Ph groups on the Ru(II) center. The coupling reaction with methyl acrylate therefore occurs at the C1 carbon rather than C4 in isoprene (intermediate A). This is one of the possible views for the high C1 selectivity in the C−C bond formation for the coupling using 2-substituted 1,3-dienes when 1a was used as the catalyst.16 The high catalytic activity of 1a probably arises form (i) the electronic deficient nature of the “Ru(Ph-dbcot)” fragment that favors the electron-rich conjugated dienes and (ii) the steric bulk of the ligand, which probably promotes facile displacement of the coupled 1,4-diene product from the Ru(0) center. In summary, a catalyst poison for homogeneous catalysis was reborn as an efficient ligand by the Ph substitution. Both the electron-deficient nature and the steric bulk would lead to the high catalytic activity and regioselectivity. Further studies on the substituent effect on the cyclic diene ligand and the catalysis will be reported in due course.
Of particular interest in the cross-dimerization catalyzed by 1a is the regioselectivity in the C−C bond-forming reaction, which dominantly occurs at the C1-position in 2-substituted 1,3-diene such as 4c, 4d, and 4f. In the stoichiometric reaction of 1a with 4c in the presence of MeCN in thf, a 1:1 mixture of diastereomers, rac-(R i s o p r e n e ,S P h ‑ d b c o t )-8a and rac(Sisoprene,SPh‑dbcot)-8a, was quickly produced in 46% yield (eq 2), suggesting no prostereogenic control in 4c. The pNOESY correlations (the green double-headed arrows in eq 2) revealed their conformations, and the Ph group went away to the openside in the cisoid-η4-isoprene. These diastereomers are inseparable, and stoichiometric reaction of these 1:1-mixtures with methyl acrylate (5) at 30 °C for 6 h gave the regioisomers 6c (45%) and 7c (9%) with complete conversion of 8a. The interconversion between these diastereomers 8a cannot be C
DOI: 10.1021/acs.organomet.8b00645 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
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(5) Hiroi, Y.; Komine, N.; Komiya, S.; Hirano, M. Organometallics 2014, 33, 6604−6613. (6) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855−859. (7) Sharp and Singh reported the weaker bonding of the dbcot ligand to a Pt(II) center than the cod ligand in [PtCl2(cyclic diene)]: Singh, A.; Sharp, P. R. Organometallics 2006, 25, 678−683. (8) Läng, F.; Breher, F.; Stein, D.; Grützmacher, H. Organometallics 2005, 24, 2997−3007. (9) (a) Bennett, M. A.; Byrnes, M. J.; Willis, A. C. Organometallics 2003, 22, 1018−1028. (b) Bennett, M. A.; Byrnes, M. J.; Kovácik, I. J. Organomet. Chem. 2004, 689, 4463−4474. (10) Despite several trials of elemental analysis of 1a, satisfactory results were not obtained. 1a was therefore characterized by spectroscopic methods. (11) Kiyota, S.; In, S.; Komine, N.; Hirano, M. Chem. Lett. 2017, 46, 1040−1043. (12) Tolman, C. A. Chem. Rev. 1977, 77, 313−348. (13) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 621− 627. (14) (a) Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841− 861. (b) Falivene, L.; Credendino, R.; Poater, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L. Organometallics 2016, 35, 2286−2293. (15) Hirano, M.; Ueda, T.; Komine, N.; Komiya, S.; Nakamura, S.; Deguchi, H.; Kawauchi, S. J. Organomet. Chem. 2015, 797, 174−184. (16) One of the reviewers suggested this potential steric effect of the Ph group in Ph-dbcot. We thank this reviewer for this comment.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00645. Synthetic procedures and full characterizations of the products (PDF) Accession Codes
CCDC 1854235 contains 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:
[email protected]. ORCID
Masafumi Hirano: 0000-0001-7835-1044 Nobuyuki Komine: 0000-0003-1744-695X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to Dr. Sayori Kiyota for elemental analyses, APCI, and HRMS. This work was financially supported by Japan Science and Technology Agency and the Ministry of Education, Culture, Sports, Science and Technology of Japan; Japan Science and Technology Agency (JST) ACT-C (JPMJCR12Z2); and Grant-in-Aid for Scientific Research (B) 17H03051. A part of this work was supported by JSPS Grant-in-Aid for Scientific Research on Innovative Areas, “3D Active-Site Science”, 26105003.
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ABBREVIATIONS acac, acetylacetonato (C5H7O2); cod, 1,5-cyclooctadiene (C8H12); dbcot, dibenzo[a,e]cyclooctatetraene (C16H12); Phdbcot, 5-phenyldibenzo[a,e]cyclooctatetraene (C22H16); thf, tetrahydrofuran (C4H8O)
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REFERENCES
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DOI: 10.1021/acs.organomet.8b00645 Organometallics XXXX, XXX, XXX−XXX
