Intramolecular Aminoalkene Hydroamination Catalyzed by

Oct 13, 2010 - Both complexes, as well as L1MgiPr (L1 = 4,6-di-tert-butyl-2-[bis((3-(dimethylamino)propyl)amino)methyl]phenoxyl), were shown to be com...
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Organometallics 2010, 29, 5871–5877 DOI: 10.1021/om100675c

5871

Intramolecular Aminoalkene Hydroamination Catalyzed by Magnesium Complexes Containing Multidentate Phenoxyamine Ligands Xiaoming Zhang, Thomas J. Emge, and Kai C. Hultzsch* Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854-8087, United States Received July 11, 2010

The magnesium complexes L2MgiPr (L2 = 4-tert-butyl-6-(triphenylsilyl)-2-[bis((3-(dimethylamino)propyl)amino)methyl]phenoxyl) and L3MgiPr (L3 = 4-tert-butyl-6-(triphenylsilyl)-2-[benzyl((3(dimethylamino)propyl)amino)methyl]phenoxyl) supported by potentially tetradentate and tridentate triphenylsilyl-substituted phenoxyamine ligands have been prepared and fully characterized. The X-ray crystallographic analysis of L2MgiPr confirmed a monomeric structure in which only one of the amine side arms is bound to the four-coordinate magnesium atom. The free and coordinated side arms in L2MgiPr undergo an exchange process at 25 °C in solution, while the phenoxydiamine complex L3MgiPr, on the other hand, shows no sign of fluxionality. Both complexes, as well as L1MgiPr (L1 = 4,6-di-tert-butyl-2-[bis((3-(dimethylamino)propyl)amino)methyl]phenoxyl), were shown to be competent catalysts in the cyclization of aminoalkenes. L2MgiPr exhibited the best catalytic activity, and both triphenylsilyl-substituted complexes display zero-order rate dependence on substrate concentration and first-order rate dependence on catalyst concentration, whereas the sterically less hindered complex L1MgiPr exhibits second-order rate dependence on substrate concentration. No Schlenk-type ligand redistributions were observed, and the catalytically active magnesium species was stable after prolonged heating to 120 °C, according to an NMR spectroscopic study.

Introduction The catalyzed addition of amines to unsaturated carboncarbon bonds, the so-called hydroamination, offers a wastefree, highly atom efficient, and green pathway to produce (1) (a) Doye, S. In Science of Synthesis; Enders, D., Ed.; Thieme: Stuttgart, Germany, 2009; Vol. 40a, p 241. (b) M€ uller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M. Chem. Rev. 2008, 108, 3795. (c) Brunet, J. J.; Neibecker, D. In Catalytic Heterofunctionalization from Hydroamination to Hydrozirconation; Togni, A., Gr€utzmacher, H., Eds.; Wiley-VCH: Weinheim, Germany, 2001; p 91. (d) M€uller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675. (2) An account of group 3 metal- and lanthanide-based catalysts is provided in: (a) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673. Selected recent examples: (b) Stanlake, L. J. E.; Schafer, L. L. Organometallics 2009, 28, 3990. (c) Yuen, H. F.; Marks, T. J. Organometallics 2009, 28, 2423. (d) Lu, E.; Gan, W.; Chen, Y. Organometallics 2009, 28, 2318. (e) Ge, S.; Meetsma, A.; Hessen, B. Organometallics 2008, 27, 5339. (f ) Yu, X.; Marks, T. J. Organometallics 2007, 26, 365. (g) Kim, J. Y.; Livinghouse, T. Org. Lett. 2005, 7, 4391. (h) Kim, J. Y.; Livinghouse, T. Org. Lett. 2005, 7, 1737. (i) Hultzsch, K. C.; Hampel, F.; Wagner, T. Organometallics 2004, 23, 2601. ( j) Gagne, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275. (k) Roesky, P. W.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 4705. (3) Reviews on group 4 metal-based catalysts: (a) Severin, R.; Doye, S. Chem. Soc. Rev. 2007, 36, 1407. (b) Lee, A. V.; Schafer, L. L. Eur. J. Inorg. Chem. 2007, 2245. (c) Odom, A. L. Dalton Trans. 2005, 225. (d) Doye, S. Synlett 2004, 1653. (e) Bytschkov, I.; Doye, S. Eur. J. Org. Chem. 2003, 935. (f ) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003, 32, 104. Selected recent examples: (g) Reznichenko, A. L.; Hultzsch, K. C. Organometallics 2010, 29, 24. (h) Bexrud, J. A.; Schafer, L. L. Dalton Trans. 2010, 361. (i) Manna, K.; Ellern, A.; Sadow, A. D. Chem. Commun. 2010, 46, 339. ( j) Wood, M. C.; Leitch, D. C.; Yeung, C. S.; Kozak, J. A.; Schafer, L. L. Angew. Chem., Int. Ed. 2007, 46, 354. (k) Gott, A. L.; Clarke, A. J.; Clarkson, G. J.; Scott, P. Organometallics 2007, 26, 1729. (l) Watson, D. A.; Chiu, M.; Bergman, R. G. Organometallics 2006, 25, 4731. (m) Bexrud, J. A.; Beard, J. D.; Leitch, D. C.; Schafer, L. L. Org. Lett. 2005, 7, 1959. r 2010 American Chemical Society

nitrogen-containing compounds, such as amines, enamines, and imines, which are valuable and industrially important bulk chemicals, specialty chemicals, and pharmaceuticals.1 Research efforts have focused primarily on the development of transition-metal-based catalyst systems (early transition metals of group 3-5, including the lanthanides2,3 and actinides,4 and late transition metals of groups 8-125) in the last two decades, while the potential of main-group-metalbased catalysts has attracted less attention. For example, alkali-metal-based hydroamination catalysts have a long history going back more than 50 years,6 but more detailed studies have only begun to emerge in recent years.7 It is not too astounding that alkaline-earth-metal complexes are active (4) A review on actinide-based catalysts: (a) Andrea, T.; Eisen, M. S. Chem. Soc. Rev. 2008, 37, 550. Selected recent examples: (b) Broderick, E. M.; Gutzwiller, N. P.; Diaconescu, P. L. Organometallics 2010, 29, 3242. (c) Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 6149. (d) Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 4253. (5) Reviews on late-transition-metal-based catalysts: (a) Brunet, J. J.; Chu, N. C.; Rodriguez-Zubiri, M. Eur. J. Inorg. Chem. 2007, 4711. (b) Widenhoefer, R. A.; Han, X. Eur. J. Org. Chem. 2006, 4555. (c) Hartwig, J. F. Pure Appl. Chem. 2004, 76, 507. (d) Beller, M.; Breindl, C.; Eichberger, M.; Hartung, C. G.; Seayad, J.; Thiel, O. R.; Tillack, A.; Trauthwein, H. Synlett 2002, 1579. Selected recent examples: (e) Shen, X.; Buchwald, S. L. Angew. Chem., Int. Ed. 2010, 49, 564. (f ) Zhang, Z.; Lee, S. D.; Widenhoefer, R. A. J. Am. Chem. Soc. 2009, 131, 5372. (g) Bender, C. F.; Hudson, W. B.; Widenhoefer, R. A. Organometallics 2008, 27, 2356. (h) Zhou, J.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 12220. (i) Liu, Z.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 1570. ( j) Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2005, 127, 1070. (k) Pissarek, J.-W.; Schlesiger, D.; Roesky, P. W.; Blechert, S. Adv. Synth. Catal. 2009, 351, 2081. (l) Dochnahl, M.; L€ohnwitz, K.; Pissarek, J.-W.; Biyikal, M.; Schulz, S. R.; Sch€on, S.; Meyer, N.; Roesky, P. W.; Blechert, S. Chem. Eur. J. 2007, 13, 6654. Published on Web 10/13/2010

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catalysts for hydroamination reactions,8-11 due to the similarity between the chemistry of alkaline-earth metals and that of the rare-earth metals.12 However, alkaline-earth complexes are prone to facile ligand redistribution reactions and such Schlenk equilibria may limit their applications in asymmetric hydroamination. Hill and co-workers reported recently that β-diketiminate8a-c and bis(imidazolin-2-ylidene-1-yl)borate8e alkaline-earth-metal amide complexes exhibit catalytic activity comparable to that of rare-earth-metal-based catalysts in hydroamination. However, a facile Schlenk-type ligand redistribution reaction was observed under the catalytic conditions, resulting in catalyst deactivation and formation of the homoleptic bis(β-diketiminato) and diamido species.8a,b,13 Similar Schlenk equilibria have hampered attempts to perform asymmetric hydroamination reactions14 using a chiral bisoxazolinato calcium10a and a diamidobinaphthyl magnesium complex11 as catalyst. Thus, the lack of alkaline-earthmetal complexes that can resist ligand redistribution reactions under the conditions of hydroamination catalysis seems to be a significant obstacle for the development of efficient chiral alkaline-earth-metal-based hydroamination catalysts. (6) (a) Seayad, J.; Tillack, A.; Hartung, C. G.; Beller, M. Adv. Synth. Catal. 2002, 344, 795. (b) Closson, R. D.; Napolitano, J. P.; Ecke, G. G.; Kolka, A. J. Org. Chem. 1957, 22, 646. (c) Howk, B. W.; Little, E. L.; Scott, S. L.; Whitman, G. M. J. Am. Chem. Soc. 1954, 76, 1899. (d) Wegler, R.; Pieper, G. Chem. Ber. 1950, 83, 1. (7) (a) Quinet, C.; Jourdain, P.; Hermans, C.; Ates, A.; Lucas, I.; Marko, I. E. Tetrahedron 2008, 64, 1077. (b) Ogata, T.; Ujihara, A.; Tsuchida, S.; Shimizu, T.; Kaneshige, A.; Tomioka, K. Tetrahedron Lett. 2007, 48, 6648. (c) Horrillo-Martínez, P.; Hultzsch, K. C.; Gil, A.; Branchadell, V. Eur. J. Org. Chem. 2007, 3311. (d) Horrillo Martínez, P.; Hultzsch, K. C.; Hampel, F. Chem. Commun. 2006, 2221. (e) Khedkar, V.; Tillack, A.; Benisch, C.; Melder, J.-P.; Beller, M. J. Mol. Catal. A: Chem. 2005, 241, 175. (f ) Ates, A.; Quinet, C. Eur. J. Org. Chem. 2003, 1623. (g) Tzalis, D.; Koradin, C.; Knochel, P. Tetrahedron Lett. 1999, 40, 6193. (h) Beller, M.; Breindl, C.; Riermeier, T. H.; Eichberger, M.; Trauthwein, H. Angew. Chem., Int. Ed. 1998, 37, 3389. (8) (a) Crimmin, M. R.; Arrowsmith, M.; Barrett, A. G. M.; Casely, I. J.; Hill, M. S.; Procopiou, P. A. J. Am. Chem. Soc. 2009, 131, 9670. (b) Crimmin, M. R.; Casely, I. J.; Hill, M. S. J. Am. Chem. Soc. 2005, 127, 2042. (c) Barrett, A. G. M.; Brinkmann, C.; Crimmin, M. R.; Hill, M. S.; Hunt, P.; Procopiou, P. A. J. Am. Chem. Soc. 2009, 131, 12906. (d) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; Kociok-K€ohn, G.; Procopiou, P. A. Inorg. Chem. 2008, 47, 7366. (e) Arrowsmith, M.; Hill, M. S.; KociokK€ ohn, G. Organometallics 2009, 28, 1730. (f ) Arrowsmith, M.; Heath, A.; Hill, M. S.; Hitchcock, P. B.; Kociok-K€ohn, G. Organometallics 2009, 28, 3550. (9) (a) Datta, S.; Gamer, M. T.; Roesky, P. W. Organometallics 2007, 26, 4392. (b) Datta, S.; Gamer, M. T.; Roesky, P. W. Organometallics 2008, 27, 1207. (10) (a) Buch, F.; Harder, S. Z. Naturforsch. 2008, 63b, 169. (b) Gauvin, R. M.; Buch, F.; Delevoye, L.; Harder, S. Chem. Eur. J. 2009, 15, 4382. (11) Horrillo-Martı´ nez, P.; Hultzsch, K. C. Tetrahedron Lett. 2009, 50, 2054. (12) (a) Harder, S. Angew. Chem., Int. Ed. 2004, 43, 2714. (b) Buch, F.; Brettar, J.; Harder, S. Angew. Chem., Int. Ed. 2006, 45, 2741. (13) Avent, A. G.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B. Dalton Trans. 2005, 278. (14) For reviews on asymmetric hydroamination see: (a) Reznichenko, A. L.; Hultzsch, K. C. In Chiral Amine Synthesis: Methods, Developments and Applications; Nugent, T., Ed.; Wiley-VCH: Weinheim, Germany, 2010; p 341. (b) Zi, G. Dalton Trans. 2009, 9101. (c) Chemler, S. R. Org. Biomol. Chem. 2009, 7, 3009. (d) Aillaud, I.; Collin, J.; Hannedouche, J.; Schulz, E. Dalton Trans. 2007, 5105. (e) Hultzsch, K. C. Adv. Synth. Catal. 2005, 347, 367. (f ) Hultzsch, K. C. Org. Biomol. Chem. 2005, 3, 1819. (g) Hultzsch, K. C.; Gribkov, D. V.; Hampel, F. J. Organomet. Chem. 2005, 690, 4441. (h) Roesky, P. W.; M€ uller, T. E. Angew. Chem., Int. Ed. 2003, 42, 2708. (15) (a) Reznichenko, A. L.; Hampel, F.; Hultzsch, K. C. Chem. Eur. J. 2009, 15, 12819. (b) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F. J. Am. Chem. Soc. 2006, 128, 3748. (c) Gribkov, D. V.; Hultzsch, K. C. Chem. Commun. 2004, 730. (d) Gribkov, D. V.; Hampel, F.; Hultzsch, K. C. Eur. J. Inorg. Chem. 2004, 4091. (e) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F. Chem. Eur. J. 2003, 9, 4796. (f ) Reznichenko, A. L.; Nguyen, H. N.; Hultzsch, K. C. Angew. Chem., Int. Ed. 2010, 49, DOI: 10.1002/ anie.201004570.

Zhang et al. Chart 1

Our group has previously studied biphenolate and binaphtholate rare-earth-metal complexes that were found to be competent and configurational stable catalysts for asymmetric hydroamination reactions.15 In an analogous approach, we came to the conclusion that monoanionic multidentate phenolate ligands may furnish alkaline-earth-metal complexes with a well-defined coordination environment. Several phenoxyamine alkaline-earth-metal complexes have been studied recently as initiators for the polymerization of cyclic esters and lactide.16 In particular, the four-coordinate monomeric phenoxytriamine17,18 complex L1MgiPr (1) (Chart 1) reported by Gibson and co-workers19 appealed to us as a prospective candidate for our catalytic study. Herein we describe the synthesis and structural characterization of monomeric phenoxyamine magnesium complexes related to complex 1 and discuss their effectiveness in catalytic hydroamination/cyclization reactions of aminoalkenes as well as their resistance toward ligand redistribution reactions. (16) (a) Ejfler, J.; Krauzy-Dziedzic, K.; Szafert, S.; Jerzykiewicz, L. B.; Sobota, L. B. Eur. J. Inorg. Chem. 2010, 3602. (b) Sarazin, Y.; Poirier, V.; Roisnel, T.; Carpentier, J.-F. Eur. J. Inorg. Chem. 2010, 3423. (c) Poirier, V.; Roisnel, T.; Carpentier, J.-F.; Sarazin, Y. Dalton Trans. 2009, 9820. (d) Hung, W.-C.; Lin, C.-C. Inorg. Chem. 2009, 48, 728. (e) Zheng, Z.; Zhao, G.; Fablet, R.; Bouyahyi, M.; Thomas, C. M.; Roisnel, T.; Casagrande, O., Jr.; Carpentier, J.-F. New J. Chem. 2008, 32, 2279. (f ) Darensbourg, D. J.; Choi, W.; Richers, C. P. Macromolecules 2007, 40, 3521. (g) Davidson, M. G.; O'Hara, C. T.; Jones, M. D.; Keir, C. G.; Mahon, M. F.; Kociok-K€ohn, G. Inorg. Chem. 2007, 46, 7686. (h) Sarazin, Y.; Howard, R. H.; Hughes, D. L.; Humphrey, S. M.; Bochmann, M. Dalton Trans. 2006, 340. (i) Davidson, M. G.; Jones, M. D.; Meng, D.; O'Hara, C. T. Main Group Chem. 2006, 5, 3. ( j) Ejfler, J.; Kobyzka, M.; Jerzykiewicz, L. B.; Sobota, P. Dalton Trans. 2005, 2047. (17) For an overview on the chemistry of complexes consisting of phenoxytriamine and related monoanionic tetradentate ligands see: Chomitz, W. A.; Arnold, J. Chem. Eur. J. 2009, 15, 2020. (18) For various complexes using tetradentate phenoxyamine ligands see for example: (a) Acosta-Ramı´ rez, A.; Douglas, A. F.; Yu, I.; Patrick, B. O.; Diaconescu, P. L.; Mehrkhodavandi, P. Inorg. Chem. 2010, 49, 5444. (b) Labourdette, G.; Lee, D. J.; Patrick, B. O.; Ezhova, M. B.; Mehrkhodavandi, P. Organometallics 2009, 28, 1309. (c) Douglas, A. F.; Patrick, B. O.; Mehrkhodavandi, P. Angew. Chem., Int. Ed. 2008, 47, 2290. (d) Ang, J. C.; Mulyana, Y.; Ritchie, C.; Clerac, R.; Boskovic, C. Aust. J. Chem. 2009, 62, 1124. (e) Marinescu, S. C.; Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2007, 26, 1178. (f ) Chomitz, W. A.; Minasian, S. G.; Sutton, A. D.; Arnold, J. Inorg. Chem. 2007, 46, 7199. (g) Binda, P. I.; Delbridge, E. E. Dalton Trans. 2007, 4685. (h) Groysman, S.; Sergeeva, E.; Goldberg, I.; Kol, M. Eur. J. Inorg. Chem. 2006, 2739. (i) Westmoreland, I.; Arnold, J. Dalton Trans. 2006, 4155. ( j) Groysman, S.; Sergeeva, E.; Goldberg, I.; Kol, M. Inorg. Chem. 2005, 44, 8188. (k) Zhang, D.; Hillmyer, M. A.; Tolman, W. B. Macromolecules 2004, 37, 8198. (l) Inoue, Y.; Matyjaszewski, K. Macromolecules 2004, 37, 4014. (m) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young, V. G.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125, 11350. (n) Inoue, Y.; Matyjaszewski, K. Macromolecules 2003, 36, 7432. (o) Skinner, M. E. G.; Tyrrell, B. R.; Ward, B. D.; Mountford, P. J. Organomet. Chem. 2002, 647, 145. (p) Bylikin, S. Y.; Robson, D. A.; Male, N. A. H.; Rees, L. H.; Mountford, P.; Schr€oder, M. Dalton Trans. 2001, 170. (q) Robson, D. A.; Bylikin, S. Y.; Cantuel, M.; Male, N. A. H.; Rees, L. H.; Mountford, P.; Schr€oder, M. Dalton Trans. 2001, 157. (19) Cox, A. R. F.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.; Yeldon, D. Dalton Trans. 2006, 5014.

Article

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Scheme 1. Synthesis of Triphenylsilyl-Substituted Aminophenolate Magnesium Complexes

Results and Discussion Synthesis and Characterization of Monomeric Magnesium Complexes. On the basis of our previous experience with diolate rare-earth-metal complexes,15 we envisioned to increase the steric bulk of the substituent ortho to the phenol oxygen in order to reduce the risk of ligand redistribution via bridging phenolate species and thereby improve catalytic activity. As the noncoordinated second amino group may also impact catalytic performance, we planned to substitute one of the 3-(dimethylamino)propyl side arms by a noncoordinating benzyl group. The triphenylsilyl-substituted ligands L2H and L3H were obtained through reductive amination of a triphenylsilyl-substituted salicylaldehyde and the corresponding amines in good yields using NaHB(AcO)3 as the reducing agent in dichloroethane (Scheme 1). The magnesium complexes 2 and 3 were obtained as white crystalline solids in 73% and 81% yields via salt metathesis of the lithiated phenol ligands with iPrMgCl in toluene at room temperature (Scheme 1) following a procedure analogous to the preparation of 1.19 The complexes are stable for several hours at 60 °C in solution. Similar to complex 1, the 1H NMR spectrum of complex 2 exhibits quite broad proton resonance signals in the aliphatic region in the temperature range of 25-80 °C due to an apparent exchange process between the coordinated and free side arm. At lower temperature, decoalescence of the signals was observed at ca. 0 °C and separate signal sets for the two amine side arms were observed at -40 °C in toluene-d8.20 Complex 3, on the other hand, lacks an exchangeable side arm and its 1H NMR spectrum is sharp at room temperature in C6D6. All protons of the propylene side chain as well as the two N-methyl groups are diastereotopic, indicating that the side arm in 3 remains bound to magnesium on the NMR time scale under these conditions. The characteristic magnesium-methine proton septet resonates at 0.17 ppm for 2 at -40 °C (in toluene-d8) and 0.27 ppm for 3 at 25 °C (in C6D6) in their 1H NMR spectra, while the corresponding magnesium-carbon resonance was observed at 9.9 ppm for 2 (in THF-d8)20b and 8.0 ppm for 3 (in toluene-d8) in their respective 13C NMR spectra. The X-ray crystallographic analysis of L2MgiPr (2) revealed further details of its molecular structure in the solid state (Figure 1). The central magnesium ion is coordinated by one (20) (a) Assignments of signals are based on 1H-1H COSY spectra (see the Supporting Information). (b) The 13C NMR spectra of 2 were broad in the range of -60 to þ60 °C in toluene-d8, and an interpretable spectrum could only be obtained in THF-d8.

Figure 1. ORTEP diagram of the molecular structure of L2MgiPr (2). Thermal ellipsoids are shown at the 50% probability level, and hydrogen atoms as well as one position of the disordered tert-butyl group are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Mg1-O1 = 1.9187(15), Mg1-N2 = 2.1465(19), Mg1-C41 = 2.159(2), Mg1-N1 = 2.1691(18); O1-Mg1-N2 = 101.61(7), O1-Mg1-C41 = 121.48(8), N2Mg1-C41 = 110.27(8), O1-Mg1-N1 = 89.34(6), N2-Mg1N1 = 96.61(7), C41-Mg1-N1 = 131.72(8), Mg1-O1-C2 = 131.24(13).

oxygen, one carbon, and two nitrogen atoms in a distortedtetrahedral geometry with one noncoordinating amino side arm. The Mg-O (1.9187(15) A˚), Mg-C (2.159(2) A˚), and Mg-N (2.1465(19) and 2.1691(18) A˚) bond lengths are quite comparable to those of complex 119 and other known magnesium complexes.16a-d,21 However, the angles around magnesium in 2 are slightly distorted when compared to 1. The bite angle of the phenoxyamine N,O chelate ring O1-Mg1-N1 (89.34(6)°), as well as O1-Mg1-N2 (101.61(7)°) and O1Mg1-C41 (121.48(8)°), are more acute in 2 than in complex 1 (94.23(5), 105.19(5), and 129.58(6)°, respectively),19 while the C41-Mg1-N1 angle is significantly widened (131.72(8)° for 2 compared to 114.91(6)° for 1). Also, the Mg1-O1-C2 angle is significantly widened (131.24(13)° for 2 compared to 125.94° for 1). The six-membered N,O chelate ring adopts a half-chair geometry due to steric repulsion of the triphenylsilyl group, while an energetically more favorable twisted-boat geometry is found in complex 1. In 2, Mg1, C2, C3, C25, and O1 are ideally coplanar and N1 resides 0.933 A˚ out of this plane. The dihedral angle between the planes consisting of Mg1, C2, C3, C25, O1 and Mg1, C25, N1 is 57.7°. Similar to the case for complex 1, the six-membered N,N0 -chelate ring adopts a chair conformation with N1, N2, C30, C32 being coplanar within 0.03 A˚ and Mg1 and C31 lying ca. þ0.86 and -0.71 A˚, respectively, out of the plane. Intramolecular Aminoalkene Hydroamination Catalyzed by Magnesium Complexes. With magnesium complexes 1-3 in hand, we began to study their catalytic behavior in the (21) (a) Schofield, A. D.; Barros, M. L.; Cushion, M. G.; Schwarz, A. D.; Mountford, P. Dalton Trans. 2009, 85. (b) Spielmann, J.; Bolteb, M.; Harder, S. Chem. Commun. 2009, 6934. (c) Pajerski, A. D.; Squiller, E. P.; Parvez, M.; Whittle, R. R.; Richey, H. G., Jr. Organometallics 2005, 24, 809. (d) Dove, A. P.; Gibson, V. C.; Hormnirun, P.; Marshall, E. L.; Segal, J. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 3088. (e) Henderson, K. W.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E.; Parkinson, J. A.; Sherrington, D. C. Dalton Trans. 2003, 1365.

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Table 1. Magnesium-Catalyzed Hydroamination of Primary Aminoalkenesa

entry substrate cat. (amt (mol %)) T (°C) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17c 18c 19c 20 21 22 23 24 25c 26c 27c 28 29c 30c

4a 4a 4a 4a 4a 4b 4b 4b 4b 4c 4c 4c 4d 4d 4e 4e 4f 4f 4f 4g 4g 4g 4h 4h 5a 5a 5a 5b 6 6

1 (10) 1 (5) 2 (3) 3 (10) 3 (5) 1 (10) 2 (10) 2 (5) 3 (5) 1 (10) 2 (10) 3 (10) 2 (10) 3 (10) 2 (10) 2 (20) 1 (10) 2 (10) 3 (10) 1 (10) 2 (10) 3 (10) 1 (20) 2 (20) 1 (10) 2 (10) 3 (10) 2 (10) 1 (20) 2 (15)

25 25 25 25 25 25 25 25 25 100 100 100 60 100 120 120 25 25 25 100 100 100 150 150 60 60 60 120 120 120

t (h)

conversn (%)b

1.5 4 3 1.5 5 12 6 13 22 40 18 40 18 50 50 100 2 1.5 2 7 12 24 48 48 13 2.5 8 40 20 20

99 99 99 99 98 98 97 96 81 98 98 48 95d 79d 30 87 99 99 99 99 99 98 47 69 99 99 99 93 nr nr

a Reaction conditions: 0.2 mmol of substrate, 0.6 mL of C6D6, Ar atmosphere. b Determined by 1H NMR spectroscopy, using ferrocene as internal standard. nr denotes no reaction observed. c 0.1 mmol of substrate. d dr = 1.3:1.

hydroamination/cyclization of aminoalkenes (Table 1). All three complexes are efficient in the cyclization of 2,2-diphenylpent-4-enylamine (4a) at room temperature in C6D6 at catalyst loadings of 3-10 mol % (Table 1, entries 1-5). To our surprise, the qualitative analysis of the catalytic results indicates that catalyst 2 exhibits the highest catalytic activity, followed by catalysts 3 and 1. As anticipated,22 decreasing the steric bulk of the gemdialkyl substituent in the aminopentene substrates led to lower rates of cyclization (Table 1, entries 6-12) and required increased reaction temperatures for the dimethyl-substituted aminopentene 4c. The unsubstituted aminopentene 4e required 20 mol % of 2 to reach 87% conversion in 100 h at 120 °C (Table 1, entry 16). Interestingly, cyclization of the (22) Jung, M. E.; Piizzi, G. Chem. Rev. 2005, 105, 1735.

gem-diphenyl-activated aminoalkene 4f proceeded rapidly at room temperature (Table 1, entries 17-19), despite the increased steric hindrance of the 1,2-disubstituted double bond that generally requires harsher reaction conditions.1 The cyclization of 4g requires heating to 100 °C and longer reaction times due to the decreased activating effect of the gemdimethyl substitutents (Table 1, entries 20-22). Substitution of the vinylic phenyl group in 4f by a methyl group requires even harsher reaction conditions (150 °C, 48 h) for aminoalkene 4h (Table 1, entries 23 and 24), and the reaction remains incomplete. Cyclization of the aminodialkene 4d produced the two diastereomeric products with low diastereoselectivity (Table 1, entries 13 and 14, dr = 1.3:1), apparently due to the remote β position of the prochiral center in the substrate. The cyclization of aminohexenes to form piperidines (Table 1, entries 25-28) was achieved at elevated reaction temperatures, but no cyclization of aminoheptene 6 was observed even at 120 °C (Table 1, entries 29 and 30). Reaction of the more hindered, secondary N-benzylated aminoalkene 7 required higher reaction temperatures (140 °C) to show appreciable turnover (eq 1), and no conversion was observed at lower temperatures.

Despite the relatively harsh conditions for some of the catalytic transformations, the spectroscopic analysis of the catalytic reaction mixtures did not reveal any signs of a potential Schlenktype ligand redistribution process and a further indicator for the integrity of the catalytic species is the well-behaved reaction kinetics. The reaction of 3 with a stoichiometric amount of 4a (3 equiv) revealed no observable catalyst decomposition even after 10 h of heating to 120 °C, and an additional portion of substrate was consumed rapidly (see NMR spectra in Figures S6-S9 in the Supporting Information). Thus, the catalytically active magnesium amide species seems to be highly thermally robust. The cyclization of 2,2-diphenylpent-4-enyl amine (4a) proceeded in the presence of 3 mol % of the triphenylsilyl-substituted catalysts 2 or 3 with a zero-order rate dependence on substrate concentration (Figure 2). As noted in our substrate screening, catalyst 2 exhibited the highest catalytic activity with an Nt value of 14.5 h-1 at 25 °C compared to 5.1 h-1 for 3. While generally it is anticipated that a larger number of donor arms should render the catalyst less electrophilic and less active, it has been reported that an ansa-yttrocene containing an ether functionality tethered to its silicon bridge exhibited up to 5-fold higher catalytic activity in the cyclization of aminoalkenes relative to the corresponding nonfunctionalized ansa-yttrocene.2k It was suggested that this effect results from a stabilization of the polar olefin-insertion transition state through an intramolecular chelation of the tethered donor group, and it is apparent that such a chelating stabilization (Figure 3) would be more efficient in the presence of two donor arms present in complex 2, opposed to the presence of only one donor arm in complex 3. The sterically less hindered complex 1 showed an apparent second-order rate dependence on catalyst concentration (see also Figure S1 in the Supporting Information). While the reaction with 1 proceeds initially faster than with complex 2,

Article

Figure 2. Time dependence of normalized substrate concentration in the hydroamination/cyclization of 2,2-diphenylpent-4enylamine (4a) ([C]0 = 0.167 mol L-1) with 1 (b), 2 () and 3 ([) ([cat.] = 0.005 mol L-1) in C6D6 at 25 °C.

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Figure 5. Dependence of catalyst concentration on observed rate for the hydroamination/cyclization of 4a ([subst]0 = 0.167 mol L-1) with catalyst 3 in C6D6 at 25 °C.

Figure 3. Proposed stabilization of the olefin-insertion transition state through an intramolecular chelation.

Figure 6. Time dependence of substrate concentration in the hydroamination/cyclization of 2,2-diphenylpent-4-enylamine (4a) ([subst]0 = 0.123 mol L-1) using 3 ([cat.] = 0.0081 mol L-1) at 30 °C (a) first run; (b) second run with a second batch of substrate 4a added; (c) third run with a third batch of substrate 4a added.

Figure 4. Time dependence of normalized substrate concentration in the hydroamination/cyclization of 4a ([subst]0 = 0.167 mol L-1) in C6D6 at 25 °C using varying concentrations of 3.

the overall reaction time to reach high conversion is substantially longer due to the significant rate dependence on substrate concentration. Second-order rate dependencies have been observed previously, for example in a sterically undemanding biphenolate yttrium catalyst,15e but the exact cause for this kinetic behavior remains unclear. Further kinetic analysis using complex 3 showed that the reaction is first-order in catalyst (Figures 4 and 5). The data acquired at

298 K provided a good fit over the 5-20 mM catalyst concentration range. In contrast to previous studies on binaphtholate rareearth-metal complexes15b and β-diketiminate calcium amide complexes,8b the phenoxyamine magnesium catalysts 2 and 3 did not show signs of substrate or product inhibition. No rate depression was observed for catalyst 3 when the substrate was added in three batches (Figure 6), and all three runs displayed comparable reaction rates (9.1, 9.8, and 9.0 h-1, respectively). Similar observations were made for catalyst 2 (Figure S2, see the Supporting Information) when the substrate was added in two batches. The kinetic data for catalysts 2 and 3 is in agreement with a mechanism analogous to rare-earth-metal-catalyzed hydroamination/cyclization reactions.1,2a,2j The zero-order rate

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dependence on substrate concentrations indicates that the insertion of the alkene in the magnesium amide bond is rate determining, followed by rapid protonolysis of the resulting magnesium-alkyl intermediate.

Conclusions In summary, phenoxyamine magnesium complexes are competent catalysts for the hydroamination/cyclization of aminoalkenes. The sterically demanding triphenylsilyl substituent effectively shields the metal center from undesired excessive amine binding and catalyst aggregation. Also, the absence of Schlenk-type ligand redistribution processes is an important requirement for the development of chiral catalysts for enantioselective hydroamination reactions.14

Experimental Section General Considerations. All operations were performed under an inert atmosphere of nitrogen or argon using standard Schlenk-line or glovebox techniques. Hexanes and toluene were purified by distillation from sodium/benzophenone ketyl. 4,6-Di-tert-butyl-2-[bis((3-(dimethylamino)propyl)amino)methyl]phenol (L1H),19 L1MgiPr (1),19 and 5-(tert-butyl)-2-hydroxy-3-(triphenylsilyl)benzaldehyde23 were synthesized according to literature protocols. The aminoalkene substrates 4a,7d,15b 4b,5j,7d 4c,24a 4d,15e 4e,2j 4f,2h,15b,24b 4g,24b 5a,24b 5b,15b 6,3m and 75j were prepared as reported previously and dried by distillation twice from CaH2. The hydroamination products are known compounds and were identified by comparison to the literature NMR spectroscopic data.2h-j,3g,5j,7d,15b 1H and 13C NMR spectra were recorded on Varian (300, 400, 500 MHz) spectrometers at 25 °C unless stated otherwise. Chemical shifts are reported in ppm downfield from tetramethylsilane with the undeuterated portion of the solvent as internal standard. Elemental analyses were performed by Robertson Microlit Laboratories, Inc., Madison, NJ. Despite repeated attempts the elemental analyses of complexes 2 and 3 gave low carbon content, potentially as a result of carbide formation. Three exemplary analyses for each complex are provided. L2H. 5-tert-Butyl-2-hydroxy-3-(triphenylsilyl)benzaldehyde (2.18 g, 5.0 mmol) and bis[3-(dimethylamino)propyl]amine (1.4 g, 7.5 mmol) were mixed in 1,2-dichloroethane (30 mL) and then treated with sodium triacetoxyborohydride (1.7 g, 8.0 mmol) and AcOH (0.48 g, 8.0 mmol). The mixture was stirred at 30 °C under a nitrogen atmosphere for 2 days. The reaction mixture was quenched by addition of aqueous saturated NaHCO3 (100 mL), and the product was extracted with diethyl ether (3  50 mL). The combined organic layers were washed with water (3  100 mL) and dried over MgSO4. The solvent was evaporated under vacuum to give the product as a light yellow viscous oil which was used without further purification. Yield: 2.48 g (82%). 1H NMR (500 MHz, CDCl3): δ 7.65 (m, 6H, Si(C6H5)3), 7.36 (m, 9H, Si(C6H5)3), 7.07 (d, 4JH,H = 3.0 Hz, 2H, aryl-H), 3.80 (s, 2H, ArCH2N), 2.56 (t, 3JH,H = 7.5 Hz, 4H, N(CH2CH2CH2NMe2)2), 2.22 (t, 3JH,H = 7.5 Hz, 4H, N(CH2CH2CH2NMe2)2), 2.16 (s, 12H, N(CH3)2), 1.66 (m, 4H, N(CH2CH2CH2NMe2)2), 1.13 (s, 9H, C(CH3)3). 13C{1H} NMR (125 MHz, CDCl3): δ 161.1, 141.1, 136.6, 135.8, 134.7, 129.2, 128.3, 127.7, 120.6, 119.2 (aryl), 59.1 (ArCH2N), 57.8 (N(CH2CH2CH2NMe2)2), 51.7 (N(CH2CH2CH2NMe2)2), 45.6 (N(CH2CH2CH2N(CH3)2)2), 34.2 (C(CH3)3), 31.7 (C(CH3)3), 24.5 (N(CH2CH2CH2NMe2)2). (23) Thadani, A. N.; Huang, Y.; Rawal, V. H. Org. Lett. 2007, 9, 3873. (24) (a) Tamaru, Y.; Hojo, M.; Higashimura, H.; Yoshida, Z.-I. J. Am. Chem. Soc. 1988, 110, 3994. (b) Kondo, T.; Okada, T.; Mitsudo, T. J. Am. Chem. Soc. 2002, 124, 186.

Zhang et al. N1-Benzyl-N3,N3-dimethylpropane-1,3-diamine.25 Benzaldehyde (2.12 g, 20.0 mmol) and N,N-dimethyl-1,3-propanediamine (2.14 g, 21.0 mmol) were mixed in MeOH (40 mL) at room temperature under a nitrogen atmosphere. The mixture was stirred at room temperature overnight. The solution of the aldimine in MeOH was carefully treated with solid NaBH4 (1.20 g, 32.0 mmol). The reaction mixture was stirred for 30 min and quenched with 1.0 M NaOH (100 mL). The product was extracted with diethyl ether (3  50 mL). The ether extract was washed with water (2  100 mL) and dried over MgSO4. The solvent was evaporated to give the product as a colorless oil. Yield: 3.66 g (95%). 1H NMR (400 MHz, CDCl3): δ 7.29 (m, 4H, C6H5), 7.21 (m, 1H, C6H5), 3.77 (s, 2H, NCH2C6H5), 2.66 (t, 3 JH,H = 7.6 Hz, 2H, NCH2CH2CH2NMe2), 2.30 (t, 3JH,H = 7.6 Hz, 2H, NCH2CH2CH2NMe2), 2.20 (s, 6H, N(CH3)2), 1.67 (m, 2H, NCH2CH2CH2NMe2). 13C{1H} NMR (125 MHz, CDCl3): δ 140.8, 128.6, 128.3, 127.0 (aryl), 58.3 (NCH2C6H5), 54.3 (NCH2CH2CH2NMe2), 48.1, (CH2NMe2), 45.8, (N(CH3)2), 28.3 (NCH2CH2CH2NMe2). L3H. A solution of 5-(tert-butyl)-2-hydroxy-3-(triphenylsilyl)benzaldehyde (2.40 g, 5.5 mmol) and N1-benzyl-N3,N3-dimethylpropane-1,3-diamine (0.96 g, 5.0 mmol) in 1,2-dichloroethane (30 mL) was treated with sodium triacetoxyborohydride (1.70 g, 8.0 mmol) and AcOH (0.48 g, 8.0 mmol). The mixture was stirred at room temperature under a nitrogen atmosphere for 2 days. The reaction mixture was quenched by addition of saturated aqueous NaHCO3 (100 mL), and the product was extracted with diethyl ether (3  50 mL). The combined extracts were washed with water (2  100 mL) and dried over MgSO4. The solvent was evaporated to give the product as a slightly yellow viscous oil which can be used without further purification. Yield: 2.23 g (73%). Further purification of the ligand may be achieved by dissolving the oil in diethyl ether (100 mL) and treatment with ethereal HCl to precipitate the hydrochloride salt. The isolated salt was dissolved in water, neutralized with saturated aqueous NaHCO3 (100 mL), and extracted with diethyl ether (2  50 mL) to give the pure product. 1H NMR (500 MHz, CDCl3): δ 7.66 (m, 6H, Si(C6H5)3), 7.40 (m, 3H, Si(C6H5)3), 7.34 (m, 6H, Si(C6H5)3), 7.22 (m, 3H, CH2C6H5), 7.14 (m, 2H, CH2C6H5), 7.08 (m, 2H, aryl-H), 3.83 (s, 2H, ArCH2N), 3.59 (s, 2H, C6H5CH2N), 2.48 (t, 3JH,H = 7.5 Hz, 2H, NCH2CH2CH2NMe2), 2.14 (s, 6H, N(CH3)2), 2.13 (m, 2H, NCH2CH2CH2NMe2), 1.68 (m, 2H, NCH2CH2CH2NMe2), 1.12 (s, 9H, C(CH3)3). 13C{1H} NMR (125 MHz, CDCl3): δ 160.8, 141.3, 137.2, 136.7, 135.7, 134.8, 129.9, 129.2, 128.6, 128.5, 127.7, 127.5, 120.6, 119.5 (aryl), 58.8 (ArCH2N), 58.1 (CH2C6H5), 57.8 (NCH2CH2CH2NMe2), 51.6 (NCH2CH2CH2NMe2), 45.6 (NCH2CH2CH2N(CH3)2), 34.1 (C(CH3)3), 31.7 (C(CH3)3), 24.4 (NCH2CH2CH2NMe2). L2MgiPr (2). At -78 °C, a 2.5 M hexanes solution of n-BuLi (0.42 mL, 1.05 mmol) was added to a solution of L2H (607 mg, 1.00 mmol) in toluene (10 mL). The mixture was warmed to room temperature, and after 1 h, a 2.0 M iPrMgCl solution in THF (0.53 mL, 1.05 mmol) was added. After it was stirred for a further 2 h, the solution was filtered; the filtrate was evaporated under reduced pressure. A small amount of hexanes was added to wash the solid at -78 °C and then removed in vacuo, affording 2 as a light yellow crystalline solid. Yield: 491 mg (73%). 1H NMR (400 MHz, toluene-d8, -40 °C): δ 7.93 (m, 6H, Si(C6H5)3), 7.54 (d, 4JH,H = 2.8 Hz, 1H, aryl-H), 7.23 (m, 9H, Si(C6H5)3), 7.16 (d, 4JH,H = 2.8 Hz, 1H, aryl-H), 4.06 (d, 2JH,H = 14.0 Hz, 1H, ArCH2N), 2.80 (t, 3JH,H = 12.0 Hz, 1H, NCH2CH2CH2NMe2), 2.71 (d, 2JH,H = 14.0 Hz, 1H, ArCH2N), 2.58 (t, 3JH,H = 12.0 Hz, 1H, NCH2CH2CH2NMe2), 2.05 (s, 3H, N(CH3)2), 2.00 (t, 3JH,H = 12.0 Hz, 1H, NCH2CH2CH2NMe2), 1.87 (d, 3JH,H = 8.0 Hz, (25) Khan, N. H.; Ahmad, G.; Serajuddin, A. S. M. Pakistan J. Biol. Sci. 1968, 11; 49 [CA70:96290]. (26) (a) SMART, SAINTplus, TWINABS, XPREP programs; Bruker Analytical X-Ray, Madison, WI, 2008. (b) Sheldrick, G. M. SHELXTLBruker. Acta Crystallogr. 2008, A64, 112.

Article 3H, MgCH(CH3)2), 1.84 (t, 3JH,H = 12.0 Hz, 1H, NCH2CH2CH2NMe2), 1.79 (d, 3JH,H = 8.0 Hz, 3H, MgCH(CH3)2), 1.71, 1.56 (m, 1H, NCH2CH2CH2NMe2), 1.47 (s, 3H, N(CH3)2), 1.40 (9H, s, C(CH3)3), 1.32 (m, 1H, NCH2CH2CH2NMe2), 1.24 (s, 3H, N(CH3)2), 1.19 (m, 1H, NCH2CH2CH2NMe2), 1.06 (s, 3H, N(CH3)2), 1.01, 0.98, 0.83, 0.38 (m, 1H, NCH2CH2CH2NMe2), 0.17 (m, 1H, MgCH(CH3)2). 1H NMR (500 MHz, THF-d8, 25 °C): δ 7.61 (m, 6H, Si(C6H5)3), 7.26 (m, 9H, Si(C6H5)3), 7.04 (d, 4JH,H = 3.0 Hz, 1H, aryl-H), 7.00 (d, 4JH,H = 3.0 Hz, 1H, aryl-H), 3.74 (br s, 2H, ArCH2N), 2.68 (s, 4H, N(CH2CH2CH2NMe2)2), 2.16 (br s, 4H, N(CH2CH2CH2NMe2)2), 2.00 (br s, 12H, N(CH2CH2CH2N(CH3)2)2), 1.76 (br s, 4H, N(CH2CH2CH2NMe2)2), 1.26 (d, 3JH,H = 7.5 Hz, 6H, MgCH(CH3)2), 1.09 (s, 9H, C(CH3)3), -0.29 (m, 1H, MgCH(CH3)2). 13C{1H} NMR (500 MHz, THF-d8, 25 °C): δ 171.4, 139.8, 138.6, 137.7, 136.2, 132.0, 130.2, 129.0, 122.7, 122.4 (aryl), 62.4 (ArCH2N), 60.9 (N(CH2CH2CH2NMe2)2), 54.2 (N(CH2CH2CH2NMe2)2), 46.3 (N(CH2CH2CH2N(CH3)2)2), 35.3, (C(CH3)3), 33.2 (C(CH3)3), 27.3 (MgCH(CH3)2), 23.1 (N(CH2CH2CH2NMe2)2), 9.9 (MgCH(CH3)2). Anal. Calcd for C42H59MgN3OSi: C, 74.81; H, 8.82; N, 6.23. Found: C, 72.33, 72.56, 73.03; H, 8.41, 8.31, 8.45; N, 5.88, 5.88, 5.89. Crystallography of 2. Clear, colorless brick-shaped crystals of 2 suitable for X-ray diffraction analysis were obtained by slow evaporation of the solvent from a hexanes solution at room temperature. Data were collected on a Bruker SMART APEX diffractometer: C42H59MgN3OSi, Mr = 674.32, monoclinic, space group P21/c, a = 10.7798(7) A˚, b = 14.4804(9) A˚, c = 25.3779(15) A˚, β = 94.550(1)°, V = 3948.9(4) A˚3, Z = 4, Fcalcd = 1.134 g cm-3, F(000) = 1464, Mo KR radiation (λ = 0.710 73 A˚), μ = 0.110 mm-1, crystal dimensions 0.24  0.04  0.06 mm, T = 100(2) K, 8110 independent reflections for 1.9 e θ e 26.4°, GOF = 1.008, R1(I > 2σ(I)) = 0.0518, wR2(all data) = 0.1236, largest difference peak and hole 0.377 and -0.319 e A˚-3. X-ray data for a minor twin component (average reflection intensity ratio