An Improved Protocol for the Synthesis of [(Î·4-C4R4)Co(Î·5-C5H5

Article pubs.acs.org/Organometallics

An Improved Protocol for the Synthesis of [(η4-C4R4)Co(η5-C5H5)] Complexes Guillaume Bertrand,†,‡ Ludovic Tortech,†,‡ Denis Fichou,†,‡ Max Malacria,† Corinne Aubert,*,† and Vincent Gandon*,§ †

UPMC, IPCM, UMR CNRS 7201, 4 place Jussieu, 75005 Paris, France CEA-Saclay, Organic Nanostructures and Semiconductors Group, SPCSI/IRAMIS, F-91191 Gif-sur-Yvette, France § Université Paris-Sud 11, ICMMO, UMR CNRS 8182, F-91405 Orsay, France ‡

S Supporting Information *

ABSTRACT: The reaction of bulky alkynes C2R2 with (η5-C5H5)Co(CO)(dimethyl fumarate) under microwave irradiation provides complexes of the type [(η4-C4R4)Co(η5-C5H5)] in good to excellent yields. This protocol represents a significant improvement over those reported previously. In particular, the formation of insertion products such as cyclopentadienones or cyclohexadienes can be avoided. In addition, because of the exceptional stability of (η5-C5H5)Co(CO)(dimethyl fumarate), the reactions can be carried out in crude solvents. The easy access to [(η4-C4R4)Co(η5-C5H5)] complexes stimulated a study of their reactivity, notably under cross-coupling conditions.

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insertion of ethene when using CpCo(C2H4)2.9 One way to prevent insertion of the precatalyst’s ligands is to use derivatives of CpCo(PPh3)2; however, the product must be separated from large quantities of PPh3.10 Finally, insertion of a third alkyne unit is also possible in some cases, giving rise to [2 + 2 + 2] cycloadducts.11 While the latter selectivity problem can be circumvented by using diarylalkynes or alkynes bearing bulky substituents, it remains a challenge to avoid the formation of the other types of insertion products. Recently, we have synthesized a cobalt complex of formula (η5-C5H5)Co(CO)(dimethyl fumarate).12 This species proved to be a highly efficient catalyst for various types of cycloaddition reactions.13 It is also much easier to handle than CpCo(CO)2 or CpCo(C2H4)2 because it is air- and moisture-stable and does not even require the use of distilled or degassed solvents. During our preliminary screening, we noticed that the reaction of this complex under microwave irradiation in the presence of diphenylacetylene provided the CpCoCb complex 1 in 83% yield, contaminated by 9% of the Pauson−Khand type product 2 (Scheme 2). Of particular

INTRODUCTION Sandwich cobalt complexes of the type [(η4-C4R4)Co(η5-C5H5)] (also referred to as CpCoCb) are useful frameworks and building blocks for many scientific disciplines. For instance, they are constituents of ligands used in palladiumcatalyzed asymmetric transformations,1 of electronic devices,2 of molecular gears,3 and of complex molecular assemblies.4 CpCoCb's also give rise to polymers with interesting properties.5 These complexes are usually air-stable and easy to purify using common chromatography techniques. They are typically synthesized by formal [2 + 2] cycloaddition of two alkynes mediated by CpCoL2, L being a labile ligand.6 In the majority of cases, the commercially available complex CpCo(CO)2 is employed. Alternatively, CpCo(C2H4)2 or other active [CpCo] sources can be used. The mechanism of CpCoCb formation involves the substitution of L by the two alkyne units, oxidative coupling to give a cobaltacyclopentadiene, and reductive elimination (Scheme 1).7 The insertion of L into the coordinatively unsaturated cobaltacyclopentadiene often leads to undesired side products, precluding high yields. For instance, cyclopentadienones are likely to be formed when using CpCo(CO)2 (Pauson−Khand type reaction).6,8 Likewise, cyclohexadienes can be isolated after © 2011 American Chemical Society

Received: July 22, 2011 Published: December 15, 2011 126

dx.doi.org/10.1021/om200662g | Organometallics 2012, 31, 126−132

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Scheme 1. Key Steps in the Formation of CpCoCb and Other Insertion Products

Table 2. Cobalt-Mediated Dimerization of Symmetrical Diphenylalkynes

Scheme 2. Cobalt-Mediated Cyclodimerization of Diphenylacetylene

Table 1. Solvent Screening

entry

solvent

T (°C)

1 2 3 4 5 6 7 8 9 10

xylenes THF EtOH THF/EtOH (14/1) MeCN pyridine DMF DMSO CH2Cl2 Et2O

175 150 150 150 150 170 200 200 130 100

yield (%)a 88 99 98 99 51 90 82

(59) (66) (65) (66) (34) (60) (55) 0b 81 (54) 27 (18)

a

Isolated yields relative to the alkyne (limiting reagent); yields based on cobalt are given in parentheses. bDegradation.

Isolated yields relative to the alkyne (limiting reagent); yields based on cobalt are given in parentheses. bThe best yield reported so far in the literature is given in brackets. cDecomposition. dNo reaction.

interest, under the same conditions, 1 and 2 were isolated in 52% and 40% yields, respectively, when using CpCo(CO)2.6h On the other hand, CpCo(C2H4)2 led to the insertion product 3b.14

Thus, by keeping the incorporation of CO to a minimum extent, and because dimethyl fumarate does not insert, CpCo(CO)(dimethyl fumarate) appeared to be an expedient

a

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in high yields when using alkynes bearing phenyls para- or meta-substituted by methoxy, amino, halo, and alkyl groups (entries 1−6).17,18 Although carboxylic acid and azide substituents led to decomposition (entries 7 and 10), an ester, and even a pinacol boronic ester, could be used successfully (entries 8 and 9). It is also worthy of note that the experimental procedure used herein proved to be compatible with thienyl-substituted alkynes (Chart 1). Again, in contrast with a previously described protocol, no Pauson−Khand product was formed and the yield was significantly improved.6h The exceptional stability of CpCoCbs was exploited for the formation of the tetracid derivative 14 (Scheme 3, which could not be formed directly (see Table 2, entry 7). In spite of a prolonged time in a refluxing basic medium, the desired product was isolated quantitatively by saponification of 10.17 Likewise, the reduction of 10 by LiAlH4 furnished 15 quantitatively. The straightforward preparation of the tetrabromide 7 and of the tetraiodide 8 (see Table 1, entries 4 and 5) prompted us to carry out cross-coupling reactions (Scheme 4). Gratifyingly, the tetra-Heck, the tetra-Suzuki,15 and the tetra-Sonogashira coupling reactions proved successful, giving rise to the new polyconjugated complexes 16−18.19 Interestingly, the synthesis of 17 is also possible from the borylated derivative 11 and phenyl iodide, with a better yield of 99% (Scheme 5).20 The case of unsymmetrical diphenylalkynes was also investigated (Table 3). As before, all substituents were tolerated, except CO2H, which led to decomposition products (entry 6). Although the yields were moderate with borylated groups, they were excellent with other substituents, including NO2 (entry 2) and SiMe3 (entry 8). However, in line with previous reports,6 a low regioselectivity was observed and the mixtures could not be separated. Unsymmetrical diarylalkynes bearing pyridyl, thienyl, and ferrocenyl groups were also tested (Table 4). This time, better regioselectivities could be obtained. In particular, with phenyl and 2-thienylcarbaldehyde groups at the starting alkyne (entry 3), a 10/1 ratio was observed by 1H NMR. Only one regioisomer of 32 was isolated when using ferrocenyl-2thienylcarbaldehyde acetylene as the starting material, albeit

choice for the selective construction of CpCoCb's. We describe herein an experimental procedure which allows the rapid and efficient synthesis of such complexes. The ease by which CpCoCb's are made available by this strategy also prompted us to study their reactivity.

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RESULTS AND DISCUSSION We started our investigation by looking for experimental conditions that would improve the selectivity for the CpCoCb products. The 83% yield in 1 shown in Scheme 2 could be slightly increased to 88% by using a slight excess of CpCo(CO)(dimethyl fumarate) (Co/alkyne ratio of 1.5/2 instead of 1/2; see Table 1, entry 1). Keeping this Co/alkyne ratio, excellent yields of 99% and 98% were obtained in THF and EtOH, respectively (entries 2 and 3). Using a 14/1 mixture of THF and EtOH, compound 1 was also isolated in 99% yield.15 However, the rise of the temperature within the microwave vial is much faster in this polar mixture than in THF alone (20 s vs 180 s), allowing us to maintain the microwave power between 60 and 80 W instead of 150 W (entry 4). In the other solvents, the yields were lower (entries 5−10). Using the THF/EtOH medium, the cyclodimerization of symmetrical diarylalkynes was investigated next (Table 2). Electron-donating and -withdrawing groups could be employed, provided no substituents are present at the ortho position (entries 11 and 12).16 The expected complexes were isolated Chart 1. CpCoCb Derived from Thienylalkynesa

a

Isolated yields relative to the alkyne (limiting reagent); yields based on cobalt are given in parentheses.

Scheme 3. Saponification and Reduction of 10

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Scheme 4. Cross-Coupling Reactions Involving 7 and 8

Scheme 5. Cross-Coupling Reactions between 11 and Phenyl Iodide

in moderate yield (entry 5).21 Unfortunately, regioselectivity remained an issue in the other cases. Finally, the case of diynes was briefly studied (Scheme 6). When they bear aryl or silyl groups at the alkyne termini, these substrates are also prone to formal CpCo-mediated [2 + 2] and [2 + 2 + 1] cycloadditions.22 We found that the selectivity between [2 + 2] and [2 + 2 + 1] cycloadditions was critically dependent on the nature of the tether. While the three-carbon linker led virtually exclusively to the Pauson−Khand type complex in moderate yield,23 the desired CpCoCb became the major component of the mixture with a four-carbon tether. With a five-carbon tether, the CpCoCb was isolated in 93% yield, accompanied by 4% of the Pauson−Khand product.

Clearly the ring strain in the 4/5 bicyclic product 33 is detrimental to its formation and favors the competitive [2 + 2 + 1] cycloaddition process.

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CONCLUSION An expedient method for the synthesis of CpCoCb's has been devised. It is highly chemoselective in the sense that the formation of insertion products can be minimized. The desired products can be obtained in good to excellent yields, which will favor their use as building blocks, notably in materials science. The experimental procedure is very convenient to carry out in crude solvents without taking any specific precautions. We are currently using this original synthetic pathway to synthesize 129


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Table 3. Cobalt-Mediated Dimerization of Unsymmetrical Diphenylalkynes

a c

Isolated yields relative to the alkyne (limiting reagent); yields based on cobalt are given in parentheses. bDetermined by 1H NMR, unattributed. Decomposition.

Scheme 6. Cyclodimerization of Diynesa

Table 4. Cobalt-Mediated Dimerization of Unsymmetrical Diarylalkynes

a

Isolated yields relative to the alkyne (limiting reagent); yields based on cobalt are given in parentheses.

novel CpCoCb derivatives bearing π-conjugated peripheral arms on the Cb moiety in view of applications in the field of organic devices such as field-effect transistors, photovoltaic solar cells, and magnetic heterostructures.24 In particular we have recently shown that thin solid films of CpCoCb's substituted by oligothiophenes of various lengths are efficient electron-donating materials (i.e., p-type semiconductors), leading to high-performance organic solar cells.25

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EXPERIMENTAL SECTION

General Procedure for the [2 + 2] Cycloadditions. In a 2−5 mL Biotage microwave vial, 2 equiv of diarylacetylene and 1.5 equiv of (η5-C5H5)Co(CO)(dimethyl fumarate) were dissolved in 4.6 mL of THF and 0.4 mL of EtOH. The mixture was heated at 150 °C for

a

Isolated yields relative to the alkyne (limiting reagent); yields based on cobalt are given in parentheses. bDetermined by 1H NMR, unattributed. 130


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(7) (a) Fritch, J. R.; Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1979, 18, 409. (b) Ville, G.; Vollhardt, K. P. C. J. Am. Chem. Soc. 1981, 103, 5267. (c) Ville, G.; Vollhardt, K. P. C.; Winter, M. J. Organometallics 1984, 3, 1177. (d) Veiros, L. F.; Dazinger, G.; Kirchner, K.; Calhorda, M. J.; Schmid, R. Chem. Eur. J. 2004, 10, 5860. (e) Weng, C. M.; Hong, F. E. Organometallics 2011, 30, 3740. (f) Avilés, T.; Jansat, S.; Martínez, M.; Montilla, F.; Rodríguez, C. Organometallics 2011, 30, 3919. (8) (a) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E. Chem. Commun. 1971, 36. (b) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E.; Foreman, M. I. J. Chem. Soc., Perkin Trans. 1 1973, 977. (9) Geny, A.; Lebœuf, D.; Rouquié, G.; Vollhardt, K. P. C.; Malacria, M.; Gandon, V.; Aubert, C. Chem. Eur. J. 2007, 13, 5408. (10) (a) Nguyen, H. V.; Yeamine, M. R.; Amin, J.; Motevalli, M.; Richards, C. J. J. Organomet. Chem. 2008, 693, 3668. (b) O’Donohue, P.; McAdam, C. J.; Courtney, D.; Ortin, Y.; Müller-Bunz, H.; Manning, A. R.; McGlinchey, M. J.; Simpson, J. J. Organomet. Chem. 2011, 696, 1496. (c) Singh, N.; Elias, A. J. Dalton Trans. 2011, 40, 4882. (11) (a) Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984, 23, 539. (b) Agenet, N.; Gandon, V.; Buisine, O.; Slowinski, F.; Aubert, C.; Malacria, M. In Organic Reactions; RajanBabu, T. V., Ed.; Wiley: Hoboken, NJ, 2007; Vol. 68, pp 1−302. (c) Lebœuf, D.; Gandon, V.; Malacria, M. In Handbook of Cyclization Reactions; Ma, S., Ed.; WileyVCH: Weinheim, Germany, 2009; Vol. 1, pp 367−406. (d) Domínguez, G.; Pérez-Castells, J. Chem. Soc. Rev. 2011, 40, 3430. (e) Weding, N.; Hapke, M. Chem. Soc. Rev. 2011, 40, 4525. (12) Geny, A.; Agenet, N.; Iannazzo, L.; Malacria, M.; Aubert, C.; Gandon, V. Angew. Chem., Int. Ed. 2009, 48, 1810. (13) (a) Lebœuf, D.; Iannazzo, L.; Geny, A.; Malacria, M.; Vollhardt, K. P. C; Aubert, C.; Gandon, V. Chem. Eur. J. 2010, 16, 8904. (b) Garcia, P.; Evanno, Y.; George, P.; Sevrin, M.; Ricci, G.; Malacria, M.; Aubert, C.; Gandon, V. Org. Lett. 2011, 13, 2030. (c) Iannazzo, L.; Vollhardt, K. P. C; Malacria, M.; Aubert, C.; Gandon, V. Eur. J. Org. Chem. 2011, 3283. (14) This result is consistent with: Benn, R.; Cibura, K.; Hofmann, P.; Jonas, K.; Rufińska, A. Organometallics 1985, 4, 2214. (15) If the reaction is stopped after 5 min, the yield is 34%. After 15 min, it is 84%. (16) The steric hindrance could be detrimental to the oxidative coupling of the two alkyne units. (17) The structure of compounds 5, 9, and 17 was unambiguously assigned by X-ray diffraction studies. CCDC files 835250−835252 contain the supplementary crystallographic data for this paper. See the Supporting Information for details. (18) Harrison, R. M.; Brotin, T.; Noll, B. C.; Michl, J. Organometallics 1997, 16, 3401. (19) For tetra-Sonogashira cross-couplings carried out on a CpCoCb, see refs 4a and 17. For mono-Suzuki and -Negishi couplings, see ref 3 and: (a) Bergin, E.; Hughes, D. L.; Richards, C. J. Tetrahedron: Asymmetry 2010, 21, 1619. (b) Nguyen, H. V.; Butler, D. C. D.; Richards, C. J. Org. Lett. 2006, 8, 769. (c) Zheng, X.; Mulcahy, M. E.; Horinek, D.; Galeotti, F.; Magnera, T. F.; Michl, J. J. Am. Chem. Soc. 2004, 126, 4540. (20) The coupling of 11 with 7 or 8 was attempted, but it gave rise to an insoluble material which we could not analyze. (21) Although it could not be ascertained, we suppose that the major isomers are of type a. The cis selectivity would be consistent with the formation of an intermediate metallacycle bearing the bulkier substituents at the carbons α to cobalt, as previously described: (a) Wakatsuki, Y.; Nomura, O.; Kitaura, K.; Morokuma, K.; Yamazaki, H. J. Am. Chem. Soc. 1983, 105, 1907. A recent report also describes cis cycloadducts as the major species: (b) Zhang, P.; Brkic, Z.; Berg, D. J.; Mitchell, R. H.; Oliver, A. G. Organometallics 2011, 30, 5396. (22) See inter alia: (a) Aubert, C.; Gandon, V.; Han, S.; Johnson, B. M.; Malacria, M.; Schömenauer, S.; Vollhardt, K. P. C.; Whitener, G. D. Synthesis 2010, 2179. (b) Gleiter, R.; Stahr, H.; Nuber, B. Organometallics 1997, 16, 646.

30 min in a Biotage initiator 2.0 microwave (P = 60−75 W). The crude material was then concentrated under reduced pressure. The remaining solid was washed with 25 mL of EtOH to dissolve all organic impurities. The residue was paper-filtered using CH2Cl2. The solution was concentrated under reduced pressure to afford the desired product.

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ASSOCIATED CONTENT S Supporting Information * Text, figures, tables, and CIF files giving experimental details, characterization data, crystal data and refinement tables, and X-ray crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.A.); vincent.gandon@ u-psud.fr (V.G.).

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ACKNOWLEDGMENTS G.B. is thankful to the MERS for a Ph.D. grant. We thank Xavier Dorland for the synthesis of some compounds.

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REFERENCES

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(23) Microwave conditions have previously been applied to the synthesis of complexes structurally close to 34: Taylor, C. J.; Motevalli, M.; Richards, C. J. Organometallics 2006, 25, 2899. (24) (a) Fichou, D.; Horowitz, G.; Nishikatani, Y.; Garnier, F. Synth. Met. 1989, 28, C723. (b) Fichou, D. J. Mater. Chem. 2000, 10, 571. (c) Dennler, G.; Bereznev, S.; Fichou, D.; Holl, K.; Ilic, D.; Koeppe, R.; Krebs, M.; Labouret, A.; Lungenschmied, C.; Marchenko, A.; Meissner, D.; Mellikov, E.; Meot, J.; Meyer, A.; Meyer, T.; Neugebauer, H.; Opik, A.; Sariciftci, N. S.; Taillemite, S.; Wohrle, T. Solar Energy 2007, 81, 947. (d) Berny, S.; Tortech, L.; Véber, M.; Fichou, D. ACS Appl. Mater. Interfaces 2010, 2, 3059. (e) Berny, S.; Matzen, S.; Tortech, L.; Moussy, J.-B.; Fichou, D. Appl. Phys. Lett. 2010, 97, 253303. (25) (a) Bertrand, G.; Tortech, L.; Fichou, D.; Malacria, M.; Aubert, C.; Gandon, V. French Patent 1,152,641, 2011. (b) Bertrand, G.; Tortech, L.; Fichou, D.; Malacria, M.; Aubert, C.; Gandon, V. To be submitted for publication.

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An Improved Protocol for the Synthesis of [(Î·4-C4R4)Co(Î·5-C5H5

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