Communication pubs.acs.org/Organometallics
Skeletal Modification of Benzothiophene Mediated by Iron Carbonyls: Insertion of Terminal Alkynes with Migration of Amino and Alkoxy Groups Kyohei Kobayashi,† Masakazu Hirotsu,*,† and Isamu Kinoshita†,‡ †
Graduate School of Science and ‡The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *
ABSTRACT: A thiolate-bridged diiron carbonyl complex derived from benzothiophene, [Fe2(μ-SC6H4CHCH)(CO)6], reacted with terminal alkynes HCCR (R = SiMe3, Ph, isobutyl) under photoirradiation conditions to afford diiron complexes with a 2,4pentadienoyl moiety, [Fe2{μ-SC6H4(CH)3C(R)CO}(CO)5], via alkyne and CO insertion. In a similar reaction with N,Ndimethylpropargylamine, a diiron complex with a pentadienyl moiety, [Fe2{μ-SC6H4(CH)3C(NMe2)CH2}(CO)5], was obtained as an alkyne insertion product without CO insertion. This reaction involves 1,2-migration of a dimethylamino group. The corresponding reactions with alkyl propargyl ethers also produced diiron complexes containing pentadienyl moieties with an alkoxycarbonyl group via alkoxy migration with CO insertion. The migration process via C−N or C−O bond cleavage could be related to the coordination ability of N or O in the propargyl compounds.
T
In this communication, we report the insertion of terminal alkynes into the ring-opened benzothiophene in [Fe2(μSC6H4CHCH)(CO)6] (1), which is derived from benzothiophene and [Fe3(CO)12].17 Various functional groups were introduced on the extended carbon chain of the insertion products. This process involves insertion of CO and/or migration of coordinating functional groups. A toluene solution of 1 and (trimethylsilyl)acetylene was irradiated with a high-pressure mercury lamp to give a dark redorange solution. In the 1H NMR spectrum (C6D6), the two alkenyl signals of 1 (4.77, 8.55 ppm) decreased, and three CH signals appeared upfield at 1.61, 2.38, and 3.34 ppm, which suggested that the terminal alkyne HCCSiMe3 was inserted into the Fe−C(alkenyl) bond in 1 to form a 1,4-substituted butadienyl structure. Recrystallization from n-hexane at −30 °C gave red-orange crystals of [Fe2{μ-SC6H4(CH)3C(SiMe3)CO}(CO)5] (2) in 48% yield (Scheme 1). A single-crystal X-ray analysis confirmed that 2 is a diiron carbonyl complex of a bridging thiolate ligand containing a 2,5substituted 2,4-pentadienoyl group (Figure 1a). In addition to the insertion of the alkyne, CO is inserted between Fe2 and C10 to form an acyl group: the CO stretching frequency was observed at 1771 cm−1. The 2,4-pentadienoyl group is bound to two iron centers in a η1:η4 or η2:η4 coordination mode because of the relatively short Fe1···C8 distance (2.597(2) Å). The 13C NMR spectrum of 2 (C6D6, 20 °C) showed three signals in the
hiophene-containing organic compounds such as oligothiophenes, polythiophenes, and fused thiophenes have emerged as useful building blocks for electronic, optoelectronic, sensing, and molecular switching materials.1−5 Synthesis and functionalization of oligothiophenes have been extensively investigated because of the utilization of the π-conjugated systems.2−4 Extension of the π conjugation and modification of the thiophene ring are important issues in the area of thiophene-based materials. On the other hand, alkynes are useful starting materials for the preparation of π-conjugated organic compounds.6,7 Late-transition-metal complexes catalyze addition reactions of thioesters and sulfides to alkynes via C−S bond cleavage.8,9 However, only a few studies have been reported on metal-catalyzed reactions of thiophenes with alkynes.9,10 The C−S bond cleavage of thiophene, benzothiophene, dibenzothiophene, and their ring-substituted derivatives has been well studied using low-valent transition-metal complexes.11 Metal-inserted complexes of thiophenes are potential precursors for functional organic and organometallic materials.12−16 For example, Chen and Angelici reported that Cp*Ir(2,5-dimethylthiophene) reacts with alkynes to produce bicyclocarbene complexes.12 Diiron complexes derived from functionalized dibenzothiophenes act as electrocatalysts for proton reduction.16 Research on the reactivity of the cyclometalated thiophene ring is of fundamental importance in creating new functionalities and developing metal-mediated reactions. © 2013 American Chemical Society
Received: July 25, 2013 Published: September 6, 2013 5030
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complexes, in which the vinylketene ligand can be described by resonance structures.18 A π-conjugated system of the pentadienoyl moiety in 2−4 is shown later. In a photoreaction of 1 with N,N-dimethylpropargylamine, alkyne insertion proceeded, but CO insertion was not detected in the IR spectrum. Three 1H NMR signals at 1.02, 1.77, and 3.42 ppm (C6D6) were assigned to the 1,4-substituted butadienyl structure. The 1H signals for CH2 appeared upfield at 0.14 and 1.22 ppm with a relatively small geminal coupling constant (2JHH = 5.5 Hz). We isolated the thiolate-bridged diiron complex [Fe2{μ-SC6H4(CH)3C(NMe2)CH2}(CO)5] (5) as red-orange crystals in 60% yield (Scheme 1). A crystal structure analysis of 5 revealed that the inserted alkyne undergoes 1,2-migration of the dimethylamino group to form a coordinated pentadienyl group with terminal CH2 (Figure 1b). In 5, Fe−C distances are in the range 2.06−2.18 Å except for Fe1−C8 (2.3750(17) Å) and Fe2−C8 (2.3221(17) Å); therefore, the coordination mode of the pentadienyl moiety is described as η2:η4. The formation of 5 implies that the CO insertion found for 2−4 is prevented by the coordination of the amino group of the inserted alkyne. Furthermore, the migration of NMe2 via C−N bond cleavage could be related to the coordination ability of N as well as polarization of the C−N bond. The migration via C− C bond cleavage was not observed for the reaction of HCCCH2iPr, which gives the CO insertion product 4 as described above. 1,2-Migration of functional groups in alkynes was reported for metal-catalyzed reactions such as cycloisomerization of alkynyl ketones and alkynyl imines and rearrangement of propargyl acetates.7,19 However, 1,2-migration of amino groups in propargylamines has not been reported. To elucidate factors in the NMe2 migration leading to 5, methyl propargyl ether, which has weak coordination ability of the ethereal O atom, was used for the reaction of 1. The 1H NMR spectrum of the reaction mixture suggested the formation of two isomeric diiron complexes, 6 and 7, in a 2:3 ratio. The products were obtained as a 2:1 mixture of 6 and 7 in 37% yield after chromatographic purification. The elemental analytical result was consistent with the formula of the insertion product (1·HCCCH2OMe). The 1H resonances for CH2 were observed at 0.50 and 2.42 ppm for 6 and 0.21 and 2.40 ppm for 7 (C6D6). The small geminal coupling constants (2JHH = 2.4 Hz for 6 and 0.7 Hz for 7) suggested the presence of a terminal methylene group formed by migration of the methoxy group. Complexes 6 and 7 were successfully isolated (vide infra) and proved to be diiron complexes with unsaturated ester moieties, as shown in Scheme 2. When a mixture of 6 and 7 was heated in acetonitrile at 60 °C, 6 remained intact, but 7 decomposed. Complex 6 ([Fe2{μSC6H4(CH)3C(COOMe)CH2}(CO)5]) was isolated from the reaction solution (12% yield) and structurally characterized by X-ray crystallography (Figure 2a). The structure of 6 is similar
Scheme 1. Reactions of 1 with Terminal Alkynes
Figure 1. ORTEP drawings of (a) 2 and (b) 5 with thermal ellipsoids at the 50% probability level. Hydrogen atoms on the coordinated carbon atoms are shown, and other hydrogen atoms are omitted for clarity.
carbonyl region. These signals were attributed to the acyl group, Fe(CO)3, and Fe(CO)2, in which the carbonyl ligands on each iron center exchange rapidly. A similar fluxional behavior of the Fe(CO)3 site was reported for 1 and C,Sbridged diiron complexes.16,17 The corresponding reactions using ethynylbenzene and 4methyl-1-pentyne produced analogous diiron complexes with an acyl group, [Fe2{μ-SC6H4(CH)3C(Ph)CO}(CO)5] (3) and [Fe2{μ-SC6H4(CH)3C(iBu)CO}(CO)5] (4), in 39% and 43% yields, respectively. The crystal structures of 3 and 4 are similar to that of 2 (Supporting Information). The shortest Fe−C bond was observed for Fe2−C(acyl): 2, 1.938(2); 3, 1.920(2); 4, 1.927(2) Å. Other Fe−C distances are in the range 2.10− 2.24 Å except for Fe1···C8 (2.502.60 Å). The short Fe− C(acyl) bond was reported for η4-vinylketene carbonyl iron(0)
Scheme 2. Reaction of 1 with Methyl Propargyl Ether
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Figure 3. Dianionic thiolate ligands in (a) 2−4, (b) 5 and 6, and (c) 7 obtained by the reactions of 1 and terminal alkynes.
Scheme 3. Possible Mechanism for the Formation of 2−6
Figure 2. ORTEP drawings of (a) 6 and (b) 7 with thermal ellipsoids at the 50% probability level. Hydrogen atoms on the coordinated carbon atoms are shown, and other hydrogen atoms are omitted for clarity.
to that of 5 except that the dimethylamino group in 5 is replaced by the methoxycarbonyl group. This implies that methoxy migration and CO insertion proceeded in the photoreaction. Complex 7 ([Fe2{μ-SC6H4(CH)2C(CH2)CH(COOMe)}(CO)5]) was obtained as a major product by using a 3-fold excess of methyl propargyl ether in the photoreaction (13% yield). The crystal structure of 7 is shown in Figure 2b. Complex 7 also has a methoxycarbonyl group, which suggests that the reaction forming 7 involves similar methoxy migration and CO insertion. However, the coordinated pentadienyl skeleton is branched; therefore, the initial C−C bond formation takes place at the substituted carbon atom of the terminal alkyne as described below. The Fe2−C9 bond (1.9605(17) Å) is shorter than other Fe−C distances (2.102.15 Å except for Fe−C8) in 7. In acyl complexes 2−4 the thiolate S atom bridges two Fe atoms asymmetrically: Fe1−S1, 2.24−2.26 Å; Fe2−S1, 2.34− 2.36 Å (Table S3, Supporting Information). On the other hand, migration products 5−7 have symmetrically bridged structures: the Fe−S distances are in the range 2.24−2.28 Å. The binding character of the central C8 atom is also different between acyl and migration compounds. In 2−4, the Fe1−C8 distance is 0.26−0.39 Å larger than Fe2−C8, while in 5−7, Fe1−C8 is 0.05 Å larger and 0.16 and 0.13 Å smaller than Fe2−C8, respectively. The C−C bond lengths for the C7−C11 skeleton indicate that negative charge is delocalized over the conjugated chain, as shown in Figure 3; however, the distribution is varied by the electron-withdrawing acyl group in 2−4. The bridging character of the dianionic ligands slightly affects the Fe−Fe distances: 2.662.69 Å for 2−4 and 2.732.78 Å for 5−7. A possible mechanism for the formation of 2−6 is shown in Scheme 3. Complex 1 undergoes photochemical elimination of
CO and coordination of terminal alkynes. The coordinated alkyne is inserted into the Fe−C(alkenyl) bond to form the 1,4substituted butadienyl intermediate A. Binding and insertion of CO to A affords acyl complexes 2−4. In the case of N,Ndimethylpropargylamine, the N atom in A is bound to Fe, which induces 1,2-migration of NMe2 to form 5 via an allenelike structure. Propargyl−allenyl isomerization has been proposed for the metal-catalyzed transformation of propargyl compounds with 1,2-migration.7,19 In the formation of 6, insertion of CO and migration of OMe are required. We assumed that the coordinating interactions between Fe and the methoxy O atom in A assist the OMe migration via C−O bond cleavage. The methoxy group migrates to the carbonyl C atom to form a methoxycarbonyl group and a terminal methylene group first, and then the methoxycarbonyl group undergoes migration to form 6. This process is regarded as 1,2-migration of OMe with CO insertion. A formation mechanism for the branched isomer 7 is probably similar to that for the linear isomer 6, except that the initial C−C bond formation takes place at the substituted carbon atom to form intermediate B (Scheme 4). In the 5032
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(3) (a) Stott, T. L.; Wolf, M. O. Coord. Chem. Rev. 2003, 246, 89− 101. (b) Cremer, J.; Briehn, C. A. Chem. Mater. 2007, 19, 4155−4165. (c) Zambianchi, M.; Maria, F. D.; Cazzato, A.; Gigli, G.; Piacenza, M.; Sala, F. D.; Barbarella, G. J. Am. Chem. Soc. 2009, 131, 10892−10900. (d) Takamizu, K.; Nomura, K. J. Am. Chem. Soc. 2012, 134, 7892− 7895. (4) (a) Gendron, D.; Leclerc, M. Energy Environ. Sci. 2011, 4, 1225− 1237. (b) Powar, S.; Daeneke, T.; Ma, M. T.; Fu, D.; Duffy, N. W.; Götz, G.; Weidelener, M.; Mishra, A.; Bäuerle, P.; Spiccia, L.; Bach, U. Angew. Chem., Int. Ed. 2013, 52, 602−605. (c) Kim, B.-G.; Chung, K.; Kim, J. Chem. Eur. J. 2013, 19, 5220−5230. (5) (a) Irie, M. Chem. Rev. 2000, 100, 1685−1716. (b) Kurata, H.; Kim, S.; Fujimoto, T.; Matsumoto, K.; Kawase, T.; Kubo, T. Org. Lett. 2008, 10, 3837−3840. (6) (a) Saito, S.; Yamamoto, Y. Chem. Rev. 2000, 100, 2901−2915. (b) Liu, J.; Lam, J. W. Y.; Tang, B. Z. Chem. Rev. 2009, 109, 5799− 5867. (7) Gulevich, A. V.; Dudnik, A. S.; Chernyak, N.; Gevorgyan, V. Chem. Rev. 2013, 113, 3084−3213. (8) (a) Hua, R.; Takeda, H.; Onozawa, S.; Abe, Y.; Tanaka, M. J. Am. Chem. Soc. 2001, 123, 2899−2900. (b) Sugoh, K.; Kuniyasu, H.; Sugae, T.; Ohtaka, A.; Takai, Y.; Tanaka, A.; Machino, C.; Kambe, N.; Kurosawa, H. J. Am. Chem. Soc. 2001, 123, 5108−5109. (c) Yamashita, F.; Kuniyasu, H.; Terao, J.; Kambe, N. Org. Lett. 2008, 10, 101−104. (d) Minami, Y.; Kuniyasu, H.; Kambe, N. Org. Lett. 2008, 10, 2469− 2472. (e) Minami, Y.; Kuniyasu, H.; Sanagawa, A.; Kambe, N. Org. Lett. 2010, 12, 3744−3747. (f) Hooper, J. F.; Chaplin, A. B.; GonzálezRodríguez, C.; Thompson, A. L.; Weller, A. S.; Willis, M. C. J. Am. Chem. Soc. 2012, 134, 2906−2909. (9) Wang, L.; He, W.; Yu, Z. Chem. Soc. Rev. 2013, 42, 599−621. (10) (a) Huang, H.; Li, J.; Lescop, C.; Duan, Z. Org. Lett. 2011, 13, 5252−5255. (b) Li, J.; Huang, H.; Liang, W.; Gao, Q.; Duan, Z. Org. Lett. 2013, 15, 282−285. (11) (a) Rauchfuss, T. B. Prog. Inorg. Chem. 1991, 39, 259−329. (b) Jones, W. D.; Dong, L. J. Am. Chem. Soc. 1991, 113, 559−564. (c) Vicic, D. A.; Jones, W. D. J. Am. Chem. Soc. 1999, 121, 7606−7617. (d) Chen, J.; Angelici, R. J. Coord. Chem. Rev. 2000, 206−207, 63−99. (e) Angelici, R. J. Organometallics 2001, 20, 1259−1275. (f) Chehata, A.; Oviedo, A.; Arévalo, A.; Bernès, S.; Garcı ́a, J. J. Organometallics 2003, 22, 1585−1587. (g) Shibue, M.; Hirotsu, M.; Nishioka, T.; Kinoshita, I. Organometallics 2008, 27, 4475−4483. (12) Chen, J.; Angelici, R. J. Organometallics 1992, 11, 992−996. (13) (a) Chin, R. M.; Jones, W. D. Angew. Chem., Int. Ed. 1992, 3, 357−358. (b) Jones, W. D.; Chin, R. M. J. Am. Chem. Soc. 1992, 114, 9851−9858. (14) Iwasa, K.; Seino, H.; Mizobe, Y. J. Organomet. Chem. 2008, 693, 3197−3200. (15) Hirotsu, M.; Tsuboi, C.; Nishioka, T.; Kinoshita, I. Dalton Trans. 2011, 40, 785−787. (16) Hirotsu, M.; Santo, K.; Hashimoto, H.; Kinoshita, I. Organometallics 2012, 31, 7548−7557. (17) Ogilvy, A. E.; Draganjac, M.; Rauchfuss, T. B.; Wilson, S. R. Organometallics 1988, 7, 1171−1177. (18) (a) Mitsudo, T.; Watanabe, H.; Sasaki, T.; Takegami, Y.; Watanabe, Y.; Kafuku, K.; Nakatsu, K. Organometallics 1989, 8, 368− 378. (b) Alcock, N. W.; Richards, C. J.; Thomas, S. E. Organometallics 1991, 10, 231−238. (19) (a) Kim, J. T.; Kel’in, A. V.; Gevorgyan, V. Angew. Chem., Int. Ed. 2003, 42, 98−101. (b) Dudnik, A. S.; Sromek, A. W.; Rubina, M.; Kim, J. T.; Kel’in, A. V.; Gevorgyan, V. J. Am. Chem. Soc. 2008, 130, 1440− 1452.
Scheme 4. Plausible Intermediate for the Formation of 7
branched structure, 1,3-migration of OMe with CO insertion takes place to form 7. The reaction via B probably causes unfavorable steric interactions. Indeed, the reaction using tertbutyl propargyl ether produced the linear isomer [Fe2{μSC6H4(CH)3C(COOtBu)CH2}(CO)5] (8) as a major product (isolated yield 22%). In summary, we have demonstrated that a thiophene ring of benzothiophene is converted to thiolate and conjugated carbon chains by using iron carbonyls and terminal alkynes. Various functional units can be introduced on the conjugated chains derived from the ethynyl and alkenyl groups. The propargyl compounds showed migration of dimethylamino and alkoxy groups in the photoreactions, which could be related to the coordination ability of N or O. We are currently investigating the modification of other thiophene-containing compounds as well as the redox properties and the detailed mechanisms for the formation of the insertion products.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Text, tables, figures, and CIF files giving experimental details, NMR data for 2−8, and crystallographic data for 2−7. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author
*M.H.: tel, +81-6-6605-2519; fax, +81-6-6690-2753; e-mail,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (No. 22550064) from the Japan Society for the Promotion of Science.
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REFERENCES
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