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Organometallics 2009, 28, 394–396
Nitroso Compounds Serve as Precursors to Late-Metal η2(N,O)-Hydroxylamido Complexes Ryan L. Holland and Joseph M. O’Connor* Department of Chemistry and Biochemistry 0358, UniVersity of California, San Diego, La Jolla, California 92093-0358 ReceiVed NoVember 19, 2008 Summary: The metallacyclobutene complex (η5-C5H5)(PPh3)Co{κ2(C1,C3)-C(SO2Ph)dC(SiMe3)CH(CO2Et) undergoes a regioand stereoselectiVe reaction with both nitrosobenzene and 2-methyl-2-nitrosopropane to giVe ring-expanded metallacycles with an η2(N,O)-hydroxylamido ligand. These transformations represent a new synthetic route toward late-metal η2(N,O)hydroxylamido complexes. N,N-Dialkylhydroxylamido complexes, I and II, are intriguing intermediates that may prove useful for the formation of highly functionalized organic compounds (Chart 1).1 Both monodentate and bidentate coordination to transition metals have been observed, with early metals more often adopting η2 bonding (I)2,3 and late metals typically adopting κ1 bonding (II).4-7 Of particular interest for potential synthetic applications are the less oxophilic late-metal hydroxylamido complexes. The most common synthetic routes toward late-metal hydroxylamido complexes employ preformed nitroxyl5 or hydroxylamine6 reagents. The development of late-metal hydroxylamido chemistry would be facilitated by new methods for the preparation of η2(N,O)N,N-dialkylhydroxylamido complexes which contain two different alkyl groups bound to nitrogen. Here we report the first syntheses of late-metal η2(N,O)-N,N-dialkylhydroxylamido com-
* To whom correspondence should be addressed. E-mail: jmoconnor@ ucsd.edu. (1) Complexes with the connectivity shown for I and II are also referred to in the literature as “hydroxylaminato”, “hydroxylamino”, “nitroxide”, and “nitroxyl” complexes. The last two terms typically are used for paramagnetic complexes. Nitroxyl also refers to HNdO. (2) For leading references to early-metal κ1-hydroxylamido complexes: (a) Kraft, B. M.; Huang, K.-W.; Cole, A. P.; Waymouth, R. M. HelV. Chim. Acta 2006, 89, 1589. (b) Rehder, D.; Jaitner, P. J. Organomet. Chem. 1987, 329, 337. (3) For leading references to early-metal η2-hydroxylamido complexes: (a) Smee, J. J.; Epps, J. A.; Teissedre, G.; Maes, M.; Harding, N.; Yang, L.; Baruah, B.; Miller, S. M.; Anderson, O. P.; Willsky, G. R.; Crans, D. C. Inorg. Chem. 2007, 46, 9827. (b) Dove, A. P.; Xie, X.; Waymouth, R. M. Chem. Commun. 2005, 2152. (c) Wieghardt, K.; Quilitzsch, U.; Nuber, B.; Weiss, J. Angew. Chem., Int. Ed. 1978, 17, 351. (4) Late-metal κ1-nitroxyl complexes often display interesting magnetic properties: (a) Sessoli, R. Angew. Chem., Int. Ed. 2008, 47, 5508, and references therein. (b) Ishii, N.; Okamura, Y.; Chiba, S.; Nogami, T.; Ishida, T. J. Am. Chem. Soc. 2008, 130, 24. (5) For leading references to late-metal η2-hydroxylamido complexes prepared from nitroxyl radicals: (a) Mindiola, D. J.; Waterman, R.; Jenkins, D. M.; Hillhouse, G. L. Inorg. Chim. Acta 2003, 345, 299. (b) Laugier, J.; Latour, J. M.; Caneschi, A.; Rey, P. Inorg. Chem. 1991, 30, 4474. (6) For leading references to late-metal η2-hydroxylamidos prepared from hydroxylamines: (a) Vogel, S.; Huttner, G.; Zsolnai, L.; Emmerich, C. Z. Naturforsch. 1993, 48b, 353. (b) Middleton, A. R.; Thornback, J. R.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1980, 174. (7) For a late-metal η2-hydroxylamido complex prepared from a metal nitrosyl complex: Kura, S.; Kuwata, S.; Ikariya, T. Angew. Chem., Int. Ed. 2005, 44, 6406.
Chart 1. η2(N,O)- and K1(O)-Hydroxylamido Coordination
Scheme 1. Mechanistic Speculation for the Conversion of 1 and Maleic Anhydride to 210
plexes via a formal insertion of nitroso compounds into a metal-carbon bond.8,9 We recently reported the reaction of the cobaltacyclobutene complex (η5-C5H5)(PPh3)Co{κ2(C1,C3)-C(SO2Ph)dC(SiMe3)CH(CO2Et)} (1) with maleic anhydride to give metallacyclohexene products (2, Scheme 1).10 The observed selectivity for reaction at C(3) in preference to C(1) of the metallacycle contrasts with the reactions of 1 with carbon monoxide, ethyl diazoacetate, and isonitriles, all of which undergo coupling at the Co-C(1) bond to form vinylketene, vinylketenimine, and 1,3-diene products.11 In principle, all four reactions of 1 could proceed via the vinylcarbene intermediate 3 (Scheme 1).12 Reaction of 3 with CO, isocyanides, and carbenes would involve substrate coordination to cobalt followed by coupling to the carbene ligand. In the case of maleic anhydride, a [4 + 2] (8) Nitroso insertions into early-transition-metal-carbon bonds have been observed: (a) Erker, G.; Humphrey, M. G. J. Organomet. Chem. 1989, 378, 163. (b) Doxsee, K. M.; Juliette, J. J. J.; Weakley, T. J. R.; Zientara, K. Inorg. Chim. Acta 1994, 222, 305. (c) Nakamoto, M.; Tilley, T. D. Organometallics 2001, 20, 5515. (d) Cummings, S. A.; Radford, R.; Erker, G.; Kehr, G.; Fro¨hlich, R. Organometallics 2006, 25, 839. (9) The reactions of nitroso compounds with aromatic iridacycles give addition products without ring expansion: Bleeke, J. R. Acc. Chem. Res. 2007, 40, 1035. Bleeke, J. R.; Behm, R.; Xie, Y.-F.; Chiang, M. Y.; Robinson, K. D.; Beatty, A. M. Organometallics 1997, 16, 606. Bleeke, J. R.; Blanchard, J. M. B.; Donnay, E. Organometallics 2001, 20, 324. Bleeke, J. R.; Hinkle, P. V.; Rath, N. P. Organometallics 2001, 20, 1939. (10) Complex 2 was formed as both endo and exo products: Holland, R. L.; Bunker, K. D.; Chen, C. H.; DiPasquale, A. G.; Rheingold, A. L.; Baldridge, K. K.; O’Connor, J. M. J. Am. Chem. Soc. 2008, 130, 10093.
10.1021/om801105x CCC: $40.75 2009 American Chemical Society Publication on Web 12/30/2008
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Scheme 2. Formation of η2(N,O)-N,N-Dialkylhydroxylamido Complexes from 1 and Nitroso Reagents
cycloaddition reaction involving 3 or oxametallacycle 4 would lead directly to metallacyclohexene 2. In this respect, 3 and 4 may be viewed as “metalladiene” participants in a Diels-Alder reaction with activated alkenes. This speculation has now led us to examine the reactions of 1 with the classic heterodienophiles nitrosobenzene and 2-methyl-2-nitrosopropane. In both cases the nitroso compounds undergo regioselective coupling to C(3) of the metallacycle and formation of ring-expanded metallacycles which contain an η2(N,O)-N,N-dialkylhydroxylamido ligand. A toluene solution (25 mL) of metallacycle 1 (245 mg, 0.35 mmol) and excess 2-methyl-2-nitrosopropane dimer (0.66 mmol) was heated under a nitrogen atmosphere at 70 °C for 4 h, followed by chromatography on silica gel (35% ethyl acetate/ hexanes), to provide 5-trans as a dark burgundy powder in 83% yield (Scheme 2). Analytically pure 5-trans was obtained by recrystallization from chloroform/hexanes. In the 1H NMR (CDCl3) spectrum of 5-trans, singlets were observed at δ 4.93 (C5H5) and 4.08 (CHCO2Et). Both resonances were significantly downfield of the corresponding signals for 1 (δ 4.20 (C5H5), 1.43 (d, JPH ) 7.0 Hz, CHCO2Et)) and appeared at chemical shift values very similar to those observed for metallacyclohexene 2 (δ 5.02 (C5H5), 4.05 (d, JHH ) 4.2 Hz, CHCO2Et)). In the 13C{1H} NMR spectrum (CDCl3) of 5-trans, the carbon bearing the ester substituent was observed at δ 80.6 (CHCO2Et), which is substantially downfield of the δ 49.6 (CHCO2Et) resonance observed for the corresponding carbon in 2. In the
IR spectrum (thin film) of 5-trans, a strong ν(CdO) stretch was observed at 1748 cm-1, which is at much higher frequency than that observed for 2 (1638 cm-1). Thus, the spectroscopic data for 5-trans suggested that nitroso coupling to C(3) had occurred, but the data were inconsistent with the anticipated structure III (Scheme 2). A single-crystal X-ray diffraction study on 5-trans confirmed that the ester oxygen was not chelating and revealed a much different type of bicyclic structure, with κ1(C)-η2(N,O) coordination to cobalt (Figure 1, Table S1 (Supporting Information)).13 The relative stereochemistry at the cobalt, nitrogen, and carbon stereocenters is SCoSNRC,RCoRNSC, with O(1) and the ester substituent C(13) located on opposite faces of the five-membered azametallacycle defined by (Co, C(1), C(2), C(3), N). The trans relationship for the ester substituent and O(1) is reflected in a C(13)-C(3)-N(1)-O(1) dihedral angle of 172.95(0.12)°. The fold angle between the Co-N(1)-O(1) and C(1)-Co-N(1) planes is 87.37(5)°, and the largest deviations of ring atoms from the C(1)-Co-N(1) plane are 0.150(2) Å for C(2), 0.410(2) for C(3), and -1.289(1) for O(1). The quaternary carbon of the tert-butyl substituent, C(16), is displaced only 0.207(2) Å from the C(1)-Co-N(1) plane. To our knowledge, 5-trans is the first cobalt(III)-η2(N,O)hydroxylamido complex and the only example of an η2(N,O)hydroxylamido group which is incorporated into a [3.1.0] fusedring framework. The constraints of this bicyclic structure cause significant perturbations on the geometry at N(1). The
Figure 1. Solid-state structures of 5-trans (left) and 7-cis (right). For clarity, H(3) is the only hydrogen atom shown.
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Scheme 3. Mechanistic Speculation for the Formation of 5-trans from 1 and Nitroso Compounds
Co-N(1)-O angle is constrained to 68.4(1)° by the threemembered ring, the Co-N(1)-C(3) angle is constrained to 111.51(10)° by the five-membered ring, and the Co-N(1)-C(16) angle is opened up to 127.6(1)°. The corresponding Co-N-C angles in the cobalt(I) complex (triphos)Co{η2(N,O)-Me2NO} (6; triphos ) 1,1,1-tris(diphenylphosphinomethyl)ethane)6a are 121.5(5) and 122.7(4)°. The Co-N and N-O bond distances in 6 are similar to the 1.917(1) and 1.388(2) Å distances in 5-trans; however, the 1.908(1) Å Co-O(1) bond distance in 5-trans is significantly longer than the 1.854(4) Å distance observed in 6. For comparison the Co-N and N-O bond distances in the κ1(N)-nitroso complex (η5-C5H5)(PPh3)Co{κ1(N)N(dO)CH3} are 1.7822(16) and 1.2846(19) Å, respectively.14 The nature of the nitroso reagent has a dramatic effect on the diastereoselectivity observed for the reactions of 1. Heating a toluene solution (25 mL) of 1 (0.37 mmol) and excess nitrosobenzene (1.27 mmol) at 70 °C leads to the formation and isolation of an 8:1 mixture of 7-cis and 7-trans, in 71% combined yield. The structure of the minor isomer, 7-trans, was tentatively assigned on the basis of the 1H NMR spectral data, by comparison to those for 5-trans. Specifically, in the 1H NMR spectrum (CDCl3) of the minor isomer, singlets are observed at δ 4.78 (C5H5) and 3.90 (CHCO2Et), which are chemical shift values similar to those observed for 5-trans (δ 4.93 (5H) and 4.08 (1H)). In contrast, the major isomer, 7-cis, exhibits resonances in the 1H NMR spectrum (CDCl3) at δ 4.48 (C5H5) and 5.23 (CHCO2Et). Attempts to separate the two isomers by chromatography and recrystallization failed, and extended heating at 70 °C led to eventual decomposition of the compounds without evidence for a change in the 8:1 cis:trans ratio. An analytically pure sample of the mixture was obtained by (11) (a) O‘Connor, J. M.; Ji, H.; Iranpour, M.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 1586. (b) O’Connor, J. M.; Ji, H.-L.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 9846. (c) O‘Connor, J. M.; Fong, B. S.; Ji, H.-L.; Hiibner, K.; Rheingold, A. L. J. Am. Chem. Soc. 1995, 117, 8029. (d) O’Connor, J. M.; Chen, M.-C.; Frohn, M.; Rheingold, A. L.; Guzei, I. A. Organometallics 1997, 16, 5589. (e) O‘Connor, J. M.; Chen, M.-C.; Rheingold, A. L. Tetrahedron Lett. 1997, 38, 5241. (12) A model structure for intermediate 4, (η5-C5H5)Co{κ2(C,O)C(SO2Me)C(SiH3)CH(CO2Me)}, has been studied using quantum-mechanical methods, and the electrostatic potential map indicates relatively high charge density at the CH ring carbon.10 (13) The reactions of nitroso compounds with early-transition-metal metallacyclobutenes also leads to insertion into the metal-carbon (sp3) bond, but the ring-expanded product does not engage in metal-nitrogen bonding.8b (14) O’Connor, J. M.; Bunker, K. D. Organometallics 2003, 22, 5268.
recrystallization from chloroform/hexanes, and an X-ray diffraction study was carried out on a crystal of 7-cis (Figure 1, Table S1). The structural data for 7-cis revealed a bicyclic structure with κ1(C)-η2(N,O) coordination to cobalt, as was observed for 5-trans, but with the ester substituent and O(1) on the same face of the five-membered azametallacycle ring. The cis relationship of the ester substituent and O(1) leads to a C(13)-C(3)-N(1)-O(1) dihedral angle of -62.80(0.23)°. The ipso carbon of the phenyl substituent, C(16), is displaced only -0.354(2) Å from the C(1)-Co-N(1) plane. The relative stereochemistry at the cobalt, nitrogen, and carbon stereocenters is SCoSNSC,RCoRNRC). With the exception of a 6.8° smaller Co-N(1)-C(16) angle in 7-cis and the difference in stereochemistry at C(3), the bond distance and angle data for 7-cis are very similar to those observed for 5-trans (Table S1). In the case of 1, the available data do not allow us to distinguish between a traditional metallacycle insertion mechanism, involving intermediates such as IV and V, from a [4 + 2] cycloaddition reaction involving the nitroso compound and intermediate 4 (Scheme 3).15 Regardless of the mechanism, it is becoming clear that, for the late-metal metallacyclobutene 1, one-atom addends (CO, carbenes, and isocyanides)11 exhibit selective coupling at the Co-C(sp2) bond, whereas two-atom cycloaddends (alkenes,10 nitroso compounds, and possibly alkynes11c) exhibit selective coupling at the Co-C(sp3) bond. Efforts are underway to elucidate the factors that control the diastereochemistry observed for 5-trans and 7-cis and to extend this novel reaction chemistry to acyclic late-metal alkyl and vinyl complexes.
Acknowledgment. Financial support by the National Science Foundation (Grant No. CHE-0518707 and instrumentation Grant Nos. CHE-9709183, CHE-0116662, and CHE-0741968) is gratefully acknowledged. Supporting Information Available: Text, figures, and tables giving synthesis details, characterization data, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. OM801105X (15) We are also unable to rule out associative mechanisms. Attempts at kinetic studies were unsuccessful due to the reaction of the nitroso compounds with triphenylphosphine.
