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Organometallics 2009, 28, 5968–5981 DOI: 10.1021/om9006888
The Mechanism of the Catalytic Functionalization of Haloalkanes by Carbene Insertion: An Experimental and Theoretical Study § Juan Urbano,‡ Ataualpa A. C. Braga,† Feliu Maseras,*,† Eleuterio Alvarez, ,‡ ,‡ M. Mar Dı´ az-Requejo,* and Pedro J. Perez* ‡
Laboratorio de Cat alisis Homog enea, Departamento de Quı´mica y Ciencia de los Materiales, Unidad Asociada al CSIC, Campus de El Carmen s/n, Universidad de Huelva, 21007-Huelva, Spain, †Institute of Chemical Research of Catalonia (ICIQ), 43007 Tarragona, Catalonia, Spain, and §Instituto de Investigaciones Quı´micas, CSIC-Universidad de Sevilla, Avenida Am erico Vespucio 49, 41092 Sevilla, Spain Received August 4, 2009
Carbon-halogen (C-X) bonds (X=Cl, Br) can be easily functionalized with ethyl diazoacetate (N2CHCO2Et) in the presence of silver-based catalysts containing the TpxAg core (Tpx = hydrotrispyrazolylborate ligand). Polyhalomethanes are converted into products derived from the formal insertion of the carbene CHCO2Et units into the C-X bond. In the case of monohaloalkanes (C4-C6), cleavage of the C-X bond is observed, with formation of XCH2CO2Et and the corresponding olefin. Experimental evidence and theoretical calculations have led to the proposal of a novel mechanism to account for these transformations, in which the metal participates along the pathway in all the reaction steps. Among the experimental data, the first example of a metal-induced, asymmetric functionalization of a C-Cl bond by carbene insertion is included (ee=14 ( 2%).
Introduction The activation of carbon-chlorine (C-Cl) bonds is a transformation of interest since they constitute part of the skeleton of non-environmental friendly substances such as polyvinyl chloride (PVC) and hydrochlorofluorocarbons (HCFCs).1 Several heterogeneous systems have been described for the functionalization of C-Cl bonds.2 In the homogeneous phase, most efforts have been directed to aryl halides, a common reagent in many metal-mediated C-N bond formation reactions.3 This is in contrast with chloroalkanes, where the C-Cl bond resembles those existing in HCFC and PVC, among others. Although the activation of this unit with transition metal complexes has been achieved, very often through an oxidative addition step,4 the subsequent *Corresponding authors. E-mail:
[email protected]; mmdiaz@dqcm. uhu.es;
[email protected]. (1) (a) Wiersma, A.; van de Sant, E. J. A. X.; Makkee, M.; van Bekkum, H.; Moulijn, J. A. Stud. Surf. Sci. Catal. 1996, 101, 369. (b) Manzer, L. E.; Rao, V. N. M. Adv. Catal. 1993, 39, 329. (2) See for example: (a) Park, K. H.; Jung, I. G.; Chung, Y. K.; Han, J. W. Adv. Synth. Catal. 2007, 349, 411. (b) York, S. C.; Cox, D. F. J. Phys. Chem. B 2003, 107, 5182. (c) Zhou, G.; Gellman, A. J. J. Catal. 2002, 194, 233. (3) (a) Buchwald, S. L.; Mauger, C.; Mignani, G.; Scholz, U. Adv. Synth. Catal. 2006, 348, 23. (b) Ley, S. V.; Thomas, A. W. Angew. Chem., Int. Ed. 2003, 42, 5400. (4) (a) Vetter, A. J.; Rieth, R. D.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2009, ASAP doi: 10.1021/ja9002316. (b) Lobana, T. S.; Isobe, K.; Kitayama, H.; Nishioka, T.; Kinoshita, I. Angew. Chem., Int. Ed. 2004, 43, 213. (c) Aballay, A.; Godoy, F.; Buono-Core, G. E.; Klahn, A. H.; Oelckers, B.; Garland, M. T.; Mu~noz, J. C. J. Organomet. Chem. 2003, 688, 168. (d) Dorta, R.; Shimon, L. J. W.; Rozenberg, H.; Milstein, D. Eur. J. Inorg. Chem. 2002, 1827. (e) St€ohr, F.; Sturmayr, D.; Kickelbick, G.; Schubert, U. Eur. J. Inorg. Chem. 2002, 2305. (f) Haarman, H. F.; Ernsting, J. M.; Kranenburg, M.; Kooijman, H.; Veldman, N.; Spek, A. L.; van Leeuwen, P. W. N. M.; Vrieze, K. Organometallics 1997, 16, 887. pubs.acs.org/Organometallics
Published on Web 09/11/2009
functionalization and extraction of the modified substrate is still rare. Alternative methods are based on the use of a strong base, in processes lacking regio- or stereochemical control.5 Very recently, Yorimitsu, Oshima. and co-workers have reported a cobalt-based system for the regioselective dehydrohalogenation with Grignard’s reagents.6 However, the use of an efficient procedure that would result in the removal of the halide from the hydrocarbon chain with high atomic economy is still of interest and remains to be developed. Half a century ago, seminal work by Urry et al.7 described the photochemical reaction of diazomethane or methyl diazoacetate with polyhalomethanes (Scheme 1a), leading to products derived from the insertion of the :CHR units into one or more C-X bonds (X = Cl, Br). Reaction of ethyl diazoacetate with monohaloalkanes also proceeded under irradiation conditions, although when a β-hydrogen atom (with respect to the halogen) is present, the reaction affords a mixture of the haloacetate and an olefin (Scheme 1b).8 In the absence of that hydrogen, the reaction proceeds through insertion into the C-X bond. The main drawback of these transformations is that they occur through the intermediacy of photolytically generated free carbenes, and therefore no control of the selectivity can be exerted. (5) (a) Bartsch, R. A.; Za0 vada, J. Chem. Rev. 1980, 80, 453. (b) Ma, Y.; Ramirez, A.; Singh, K. J.; Keresztes, I.; Collum, D. B. J. Am. Chem. Soc. 2006, 128, 15399. (6) Kobayashi, T.; Ohmiya, H.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc. 2008, 130, 11276. (7) (a) Urry, W. H.; Eiszner, J. R. J. Am. Chem. Soc. 1951, 73, 2977. (b) Urry, W. H.; Eiszner, J. R. J. Am. Chem. Soc. 1952, 74, 5822. (c) Urry, W. H.; Wilt, J. W. J. Am. Chem. Soc. 1954, 76, 2594. (d) Urry, W. H.; Eiszner, J. R.; Wilt, J. W. J. Am. Chem. Soc. 1957, 79, 918. (8) (a) Marchand, A. P.; Brockway, N. M. Chem. Rev. 1974, 74, 441. (b) Marchand, A. P.; Brockway, N. M. J. Am. Chem. Soc. 1970, 92, 5801. r 2009 American Chemical Society
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Scheme 1. Photochemical Functionalization of C-X (X=halogen) with Diazo Compounds
Figure 1. Trispyrazolylborate ligands (Tpx) relevant to this work. Scheme 2. First Examples of Metal-Catalyzed Carbene Insertion into C-Cl Bonds
Late transition metal complexes are known to promote diazo compound decomposition and subsequent transfer of the carbene moiety to unsaturated or saturated substrates.9 In the early 1990s, Pirrung and co-workers studied the behavior of Rh2(OAc)4 as the catalyst for dipolar cycloaddition reactions using cyclic diazo compounds. During their studies they observed10 that the reaction of 2-diazo-1,3-cyclohexanedione with furan could be performed in several solvents such as THF, benzene, or fluorobenzene; however, in the case of methylene or ethylene chloride as the solvent no reaction occurred due to the formation of another product that was later characterized11 as the formal HCl abstraction product (Scheme 2a). To explain the formation of this compound, the authors invoked the rhodium-catalyzed formation of a chloronium ylide that undergoes loss of chlorine-containing moieties. Further work by M€ uller and co-workers12 with the same system led to the observation of different compounds in which a CH2Cl group migrates from the chloronium to the vicinal carbonyl O-atom (Scheme 2b). Migration of ArCH2 was also observed13 by Lee and co-workers with a related system using aryl halides ArCH2Cl as the substrate in the reaction with 2-diazo-1,3-cyclohexanedione. A few years ago Dias, Lovely, et al. reported14 the first example based on this methodology of a metal-catalyzed functionalization of a C-X bond (X = Cl, Br) using simple ethyl diazoacetate (EDA) as the carbene source. They observed a similar behavior to that previously reported under irradiation: polyhalomethanes underwent formal insertion of the carbene group, whereas in the case of halocyclohexane, ethyl haloacetate and cyclohexene were obtained (Scheme 3). (9) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods for Organic Synthesis with Diazo Compounds; John Wiley & Sons: New York 1998. (10) Pirrung, M. C.; Zhang, J.; McPhail, A. T. J. Org. Chem. 1991, 56, 6269. (11) Pirrung, M. C.; Zhang, J.; Lackey, J.; Sternbach, D. D.; Brown, F. J. Org. Chem. 1995, 60, 2112. (12) M€ uller, P.; Allenbach, Y. F.; Bernardinelli, G. Helv. Chim. Acta 2003, 86, 3164. (13) Lee, Y. R.; Cho, B. S.; Kwon, H. J. Tetrahedron 2003, 59, 9333. (14) Dias, H. V. R.; Browning, R. G.; Polach, S. A.; Diyabalange, H. V. K.; Lovely, C. J. J. Am. Chem. Soc. 2003, 125, 9270.
Scheme 3. Silver-Catalyzed Functionalization of C-X (X=halogen) with Diazo Compounds
In spite of the low number of contributions regarding the reaction of haloalkanes and diazocompounds, this is an extremely interesting transformation, which could find a potential application for the degradation of polyvinyl chloride, PVC. Equation 1 shows a hypothetical decomposition of such material using the methodology of carbene transfer from ethyl diazoacetate in the presence of a metal-based catalyst. The PVC polymer chain would undergo a formal dehydrochlorination process in such a way that an unsaturated polyolefin and ethyl chloroacetate would be the final products. The former could be reused in different ways, whereas the latter, ClCH2CO2Et, is a raw material employed in chemical industry in several processes. However, the design of the appropriate catalysts requires the knowledge of the mechanism that governs this transformation. During the past decade, our group has been involved in the use of group 11 metal-based catalysts for the functionalization of hydrocarbons, either saturated or
unsaturated, with ethyl diazoacetate.15 We have been particularly interested in the conversion of simple hydrocarbons with EDA, in a reaction in which the :CHCO2Et group (from EDA) formally inserts into the C-H bond of the alkane.16-18 Among the catalysts developed for such transformation, the [TpBr3Ag]2 (1) complex was found18 to promote high conversions (15) (a) Dı´ az-Requejo, M. M.; Perez, P. J. Chem. Rev. 2008, 108, 3379. (b) Díaz-Requejo, M. M.; Perez, P. J. J. Organomet. Chem. 2005, 690, 5441. (c) Díaz-Requejo, M. M.; Perez, P. J. J. Organomet. Chem. 2001, 617, 110. (16) (a) Dı´ az-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Trofimenko, S.; Perez, P. J. J. Am. Chem. Soc. 2002, 124, 896. (b) Caballero, A.; Díaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Trofimenko, S.; Perez, P. J. J. Am. Chem. Soc. 2003, 125, 1446. (c) Caballero, A.; Díaz-Requejo, M. M.; Belderrain, T. R.; Nicasio, M. C.; Trofimenko, S.; Perez, P. J. Organometallics 2003, 22, 4145. (17) (a) Fructos, M. R.; Belderrain, T. R.; de Fremont, P.; Scott, N. M.; Nolan, S. P.; Dı´ az-Requejo, M. M.; Perez, P. J. Angew. Chem., Int. Ed. 2005, 44, 5284. (b) Fructos, M. R.; de Fremont, P.; Scott, N. M.; Nolan, S. P.; Díaz-Requejo, M. M.; Perez, P. J. Organometallics 2006, 25, 2237. (18) Urbano, J.; Belderraı´ n, T. R.; Nicasio, M. C.; Trofimenko, S.; Dı´ az-Requejo, M. M.; Perez, P. J. Organometallics 2005, 24, 1528.
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Scheme 4. Functionalization of Polyhalomethanes with EDA Using [TpBr3Ag]2 as the Catalyst Precursora
a
Diethyl fumarate and maleate account for the remaining initial EDA.
when employed with several plain alkanes (pentane, hexane, 2-methylbutane, 2,3-dimethylbutane, among others), using the alkane as the reaction solvent. Given the similarity of this complex to the Dias-Lovely catalyst14 (complex Tp(CF3)2 Ag(thf); see Figure 1 for Tpx ligands), we decided to investigate the behavior of our [TpBr3Ag]2 catalyst in the presence of carbon-halogen bonds. In this contribution we present the results obtained with these silver complexes as catalysts for the above transformations, including the first example of an asymmetric insertion of a carbene group into the C-Cl bond. Experimental and theoretical data have allowed the proposal of a novel reaction mechanism that will serve as the basis for the design of useful catalysts for these processes.
Results and Discussion Catalytic Functionalization of Polyhalomethanes and Monohaloalkanes with [TpBr3Ag]2 as the Catalyst Precursor. A first series of experiments was carried out with [TpBr3Ag]2 as the catalyst in the reaction of ethyl diazoacetate and several polyhalomethanes as the substrate (and solvent). A 1:20 [catalyst]:[EDA] ratio was employed in all cases (considering TpxAg monomeric units). As shown in Scheme 4, chloro and bromo derivatives were converted into the corresponding products derived from the formal insertion of the :CHCO2Et unit into the C-X bond. In no case did the iodo analogues verify any reaction, all the initial EDA being converted into a mixture of diethyl fumarate and maleate after extended reaction times. Some decomposition was observed, and therefore the formation of the olefins could be due to the presence of some naked silver in the mixture. We believe that the iodo-containing substrates induce a certain blocking of the silver center, avoiding further catalytic reaction. In the case of Cl- and Br-containing substrates, the conversions observed are quite similar to those reported14 with Tp(CF3)2Ag(thf), as it could be expected, since both catalysts have also been reported to induce a similar activity in the C-H bond functionalization by carbene insertion from EDA.18,19 Competition experiments (Scheme 5) have also been carried out to determine the relative reactivity of C-Cl and C-Br bonds. Thus, when (19) (a) Dias, H. V. R.; Browning, R. G.; Richey, S. A.; Lovely, C. J. Organometallics 2005, 24, 5784. (b) Dias, H. V. R.; Browning, R. G.; Richey, S. A.; Lovely, C. J. Organometallics 2004, 23, 1200. (c) Dias, H. V. R.; Lovely, C. J. Chem. Rev. 2008, 108, 3233.
Urbano et al. Scheme 5. Competition Experiments with Polyhalomethanes
equimolar mixtures of dichloro- and dibromomethane were reacted with EDA, a 7-fold excess of the product derived from the bromo derivative was observed. This trend is in accord with the well-known order of bond dissociation energies (BDE) for C-Cl and C-Br bonds, the latter being ca. 16 kcal/mol lower than the former. In another competition experiment with dichloromethane and chloroform, only the product formed from the latter was obtained. Once it was demonstrated that the TpBr3-containing silver complex was also capable of inducing the functionalization of C-X bonds by this methodology, we explored this feature toward monohaloalkanes. Chloro- and bromobutane or -hexane were employed as the substrates, again with a 1:20 ratio of [catalyst]:[EDA]. In all cases, mixtures of several products were obtained. For example, the use of 1-chlorohexane as the substrate afforded a mixture of ethyl chloroacetate, 1-hexene, and several compounds derived from the insertion of the carbene unit into the C-H bonds of 1-chlorohexane (Scheme 6). This is a notable variance with the results reported by Dias, Lovely, et al., which did not mention the functionalization of C-H bond with haloalkanes as the substrates: only the formation of the ethyl haloacetate was mentioned,14 in spite of the reported catalytic capabilities of their Tp(CF3)2-containing complex for this transformation.19 Back to our system, the four haloalkanes studied displayed the same behavior using [TpBr3Ag]2 as the catalyst (Scheme 6), the respective amounts of products derived from C-X cleavage and the C-H functionalization being nearly similar. It could seem that the bromoalkanes are less reactive than the chloroalkanes, but these results just reflect the relative reactivity of each haloalkane compared with EDA dimerization. Intermolecular competition experiments provided intractable mixtures of compounds, so we decided to evaluate the intramolecular competition using 1-chloro-4-bromobutane as the substrate (Scheme 7). Products derived from the cleavage of both the C-Cl and the C-Br bonds were observed, the ratio of products indicating that the C-Br bond is more prone to be activated, in good accord with its lower value of BDE. Chloro- and bromocyclohexane have also been studied in this transformation (Scheme 7). They converted mainly into a mixture of ethyl haloacetate and cyclohexene, the bromo derivative being more active toward the functionalization. Small amounts of the products derived from C-H functionalization were also detected (
