Phenylene-Bridged Binuclear Organolanthanide Complexes as


Apr 1, 2009 - These binuclear organolanthanide complexes efficiently catalyze the intramolecular hydroamination/cyclization of aminoalkenes, ...
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Organometallics 2009, 28, 2423–2440

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Phenylene-Bridged Binuclear Organolanthanide Complexes as Catalysts for Intramolecular and Intermolecular Hydroamination Holming F. Yuen and Tobin J. Marks* Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208-3113 ReceiVed January 2, 2009

The phenylene-bridged bimetallic organolanthanide complexes p-bis{Cp′′Ln[N(SiHMe2)2]2}phenylene (p-Ln2; Ln ) Y, La, Cp′′ ) tetramethylcyclopentadienyl) and m-bis{Cp′′La[N(SiHMe2)2]2}phenylene (m-La2) are investigated for possible Ln · · · Ln cooperative effects in hydroamination processes. These binuclear organolanthanide complexes efficiently catalyze the intramolecular hydroamination/cyclization of aminoalkenes, aminoalkynes, aminoallenes, and conjugated aminodienes with turnover frequencies as high as 10 h-1 at 60 °C, as well as intermolecular hydroaminations with turnover frequencies as high as 0.4 h-1 at 90 °C. Substrates include those having both compressed and extended junctures between the C-C unsaturation and the -NH2 group, as well as those with multiple -NH2 groups or places of C-C unsaturation. Reactivity trends appear to be dominated by nonbonded repulsive interactions, resulting in catalytic activities following the general trend m-Ln2 < p-Ln2. Organolanthanide amide complexes are also shown to catalyze the prototropic isomerization of alkynes. Introduction The catalytic addition of an N-H bond across carbon-carbon unsaturation is a highly desirable, atom-economical transformation for the synthesis of organonitrogen molecules.1 While catalyst development for hydroamination processes now spans much of the periodic table,2-7 many catalysts suffer from short lifetimes, modest selectivity or scope, and sluggish turnover rates. In contrast, it has been demonstrated that organolanthanides are highly efficient catalysts for inter- and intramolecular hydroamination of various C-C unsaturations, including * To whom correspondence should be addressed. E-mail: [email protected] (1) For recent hydroamination reviews, see: (a) Mu¨ller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M. Chem. ReV. 2008, 108, 3795–3892. (b) Brunet, J.-J.; Chu, N.-C.; Rodriguez-Zubiri, M. Eur. J. Inorg. Chem. 2007, 4711–4722. (c) Severin, R.; Doye, S. Chem. Soc. ReV. 2007, 36, 1407– 1420. (d) Lee, A. V.; Schafer, L. L. Eur. J. Inorg. Chem. 2007, 2243– 2255. (e) Widenhoefer, R. A.; Han, X Eur. J. Org. Chem. 2006, 4555– 4563. (f) Odom, A. L. Dalton Trans. 2005, 225–233. (g) Hultzsch, K. C. AdV. Synth. Catal. 2005, 347, 367–391. (h) Hultzsch, K. C.; Gribkov, D. V.; Hampel, F. J. Organomet. Chem. 2005, 690, 4441–4452. (i) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673–686. (j) Doye, S. Synlett 2004, 1653–1672. (k) Roesky, P. W.; Mueller, T. E. Angew. Chem., Int. Ed. 2003, 42, 2708–2710. (l) Pohlki, F.; Doye, S. Chem. Soc. ReV. 2003, 32, 104– 114. (m) Bytschkov, I.; Doye, S. Eur. J. Org. Chem. 2003, 935–946. (n) Seayad, J.; Tillack, A.; Hartung, C. G.; Beller, M. AdV. Synth. Catal. 2002, 344, 795–813. (2) For examples of alkali-metal and alkaline-earth-metal catalysts, see: (a) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Hitchcock, P. B.; KociokKo¨hn, G.; Procopiou, P. A. Inorg. Chem. 2008, 47, 7366–7376. (b) Datta, S.; Gamer, M. T.; Roesky, P. W. Organometallics 2008, 27, 1207–1213. (c) Ogata, T.; Ujihara, A.; Tsuchida, S.; Shimizu, T.; Kaneshige, A.; Tomioka, K. Tetrahedron Lett. 2007, 48, 6648–6650. (d) Datta, S.; Roesky, P. W.; Blechert, S. Organometallics 2007, 26, 4392–4394. (e) HorrilloMartı´nez, P.; Hultzsch, K. C.; Gil, A.; Branchadell, V Eur. J. Org. Chem. 2007, 3311–3325. (f) Crimmin, M. R.; Casely, I., J.; Hill, M. S. J. Am. Chem. Soc. 2005, 127, 2042–2043. (g) Ates, A.; Quinet, C. Eur. J. Org. Chem. 2003, 1623–1626. (h) Ref 1m and references therein. (3) For examples of Brønsted acid catalysts, see: (a) Marcsekova´, K.; Doye, S. Synthesis 2007, 145–154. (b) Rosenfeld, D. C.; Shekhar, S.; Takemiya, A.; Utsunomiya, M.; Hartwig, J. F. Org. Lett. 2006, 8, 4179– 4182. (c) Li, Z.; Zhang, J.; Brouwer, C.; Yang, C.-G.; Reich, N. W.; He, C. Org. Lett. 2006, 8, 4175–4178. (d) Anderson, L. L.; Arnold, J.; Bergman, R. G. J. Am. Chem. Soc. 2005, 127, 14542–14543. (e) Schlummer, B.; Hartwig, J. F. Org. Lett. 2002, 4, 1471–1474.

alkenes, alkynes, allenes, and conjugated dienes, as well as coupling hydroamination to C-C bond-formation and polymerization processes.8-11 Lanthanides are nontoxic and relatively abundant in nature and have distinctive features that promote unique reactivity patterns. Owing to high electrophilicity and kinetic lability, organolanthanide centers exhibit two principal reactivity modes: σ-bond metathesis (eq 1) and insertion of C-C unsaturation (e.g., eq 2). Lanthanides exist predominantly in the 3+ oxidation state, generally suppressing side reactions such as oxidativeaddition/reductive-elimination cycles. Furthermore, the lanthanide 4f orbitals have minimal covalent interactions with the ligand sphere,12 and therefore catalytic activities/selectivities are governed primarily by the steric environment around the metal center. Catalyst tunability is thus readily achieved by varying (4) For examples of early-transition-metal catalysts, see: (a) Majumder, S.; Odom, A. L. Organometallics 2008, 27, 1174–1177. (b) Kim, H.; Livinghouse, T.; Lee, P. H. Tetrahedron 2008, 64, 2525–2529. (c) Smolensky, E.; Kapon, M.; Eisen, M. S. Organometallics 2007, 26, 4510– 4527. (d) Gott, A. L.; Clarke, A. J.; Clarkson, G. J.; Scott, P. Organometallics 2007, 26, 1729–1737. (e) Wood, M. C.; Leitch, D. C.; Yeung, C. S.; Kozak, J. A.; Schafer, L. L. Angew. Chem., Int. Ed. 2007, 46, 354–358. (f) Buil, M. L.; Esteruelas, M. A.; Lo´pez, A. M.; Mateo, A. C.; On˜ate, E. Organometallics 2007, 26, 554–565. (g) Lee, A. V.; Schafer, L. L. Organometallics 2006, 25, 5249–5254. (h) Watson, D. A.; Chiu, M.; Bergman, R. G. Organometallics 2006, 25, 4731–4733. (i) Esteruelas, M. A.; Lo´pez, A. M.; Mateo, A. C.; On˜ate, E. Organometallics 2006, 25, 1448– 1460. (j) Kim, H.; Lee, P. H.; Livinghouse, T. Chem. Commun. 2005, 5205– 5207. (k) Hoover, J. M.; Peterson, J. R.; Pikul, J. H.; Johnson, A. R. Organometallics 2004, 23, 4614–4620. (l) Gribkov, D. V.; Hultzsch, K. C. Angew. Chem., Int. Ed. 2004, 43, 5542–5546. (m) Anderson, L. L.; Arnold, J.; Bergman, R. G. Org. Lett. 2004, 6, 2519–2522. (n) Lorber, C.; Choukroun, R.; Vendier, L. Organometallics 2004, 23, 1845–1850. (o) Ackermann, L.; Bergman, R. G.; Loy, R. N. J. Am. Chem. Soc. 2003, 125, 11956–11963. (5) For examples of actinide catalysts, see: (a) Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 6149–6167. (b) Stubbert, B. D.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 4253–4271. (c) Stubbert, B. D.; Stern, C. L.; Marks, T. J. Organometallics 2003, 22, 4836–4838. (d) Straub, T.; Haskel, A.; Neyroud, T. G.; Kapon, M.; Botoshansky, M.; Eisen, M. S. Organometallics 2001, 20, 5017–5035. (e) Haskel, A.; Straub, T.; Eisen, M. S. Organometallics 1996, 15, 3773–3775. (f) Marks, T. J. Science 1982, 217, 989–997.

10.1021/om9000023 CCC: $40.75  2009 American Chemical Society Publication on Web 04/01/2009

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Yuen and Marks

Scheme 1. General Pathway for Lanthanide-Catalyzed Intramolecular Hydroamination/Cyclization of Aminoalkenes and Aminoalkynes

the metal ionic radius (La3+ at 1.160 Å to Lu3+ at 0.977 Å)13 and ancillary ligands. Utilizing the control accessible via these lanthanide properties, a large number of heterocyclic structures, (6) For examples of late-transition-metal catalysts, see: (a) Lavallo, V.; Frey, G. D.; Donnadieu, B.; Soleilhavoup, M.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5224–5228. (b) Dochnahl, M.; Lo¨hnwitz, K.; Pissarek, J.-W.; Roesky, P. W.; Blechert, S. Dalton Trans. 2008, 2844–2848. (c) Bender, C. F.; Hudson, W. B.; Widenhoefer, R. A. Organometallics 2008, 27, 2356–2358. (d) Shaffer, A. R.; Schmidt, J. A. R. Organometallics 2008, 27, 1259–1266. (e) Liu, Z.; Hartwig, J. F. J. Am. Chem. Soc. 2008, 130, 1570–1571. (f) Prior, A. M.; Robinson, R. S. Tetrahedron Lett. 2008, 49, 411–414. (g) Hamilton, G. L.; Kang, E. J.; Mba, M.; Toste, F. D. Science 2007, 317, 496–499. (h) Burling, S.; Field, L. D.; Messerle, B. A.; Rumble, S. L. Organometallics 2007, 26, 4335–4343. (i) Munro-Leighton, C.; Delp, S. A.; Blue, E. D.; Gunnoe, T. B. Organometallics 2007, 26, 1483–1493. (j) Fadini, L.; Togni, A. HelV. Chim. Acta 2007, 90, 411–424. (k) Chang, S.; Lee, M.; Jung, D. Y.; Yoo, E. J.; Cho, S. H.; Han, S. K. J. Am. Chem. Soc. 2006, 128, 12366–12367. (l) Sun, J.; Kozmin, S. A. Angew. Chem., Int. Ed. 2006, 45, 4991–4993. (m) Komeyama, K.; Morimoto, T.; Takaki, K. Angew. Chem., Int. Ed. 2006, 45, 2938–2941. (n) Li, X.; Chianese, A. R.; Vogel, T.; Crabtree, R. H. Org. Lett. 2005, 7, 5437–5440. (o) Klein, D. P.; Ellern, A.; Angelici, R. J. Organometallics 2004, 23, 5662–5670. (7) For examples of p-block metal catalysts, see: (a) Cheng, X.; Xia, Y.; Wei, H.; Xu, B.; Zhang, C.; Li, Y.; Qian, G.; Zhang, X.; Li, K.; Li, W. Eur. J. Org. Chem. 2008, 1929–1936. (b) Wei, H.; Qian, G.; Xia, Y.; Li, K.; Li, Y.; Li, W. Eur. J. Org. Chem. 2007, 4471–4474. (c) Huang, J.-M.; Wong, C.-M.; Xu, F.-X.; Loh, T-.P. Tetrahedron Lett. 2007, 3375–3377. (d) Qin, H.; Yamagiwa, N.; Matsunaga, S.; Shibasaki, M Chem. Asian J. 2007, 2, 150–154. (e) Qin, H.; Yamagiwa, N.; Matsunaga, S.; Shibasaki, M. J. Am. Chem. Soc. 2006, 128, 1611–1614. (8) (a) Yu, X.; Marks, T. J. Organometallics 2007, 26, 365–376. (b) Seyam, A. M.; Stubbert, B. D.; Jensen, T. R.; O’Donnell, J. J., III; Stern, C. L.; Marks, T. J. Inorg. Chim. Acta 2004, 357, 4029–4035. (c) Ryu, J.S.; Marks, T. J.; McDonald, F. E. J. Org. Chem. 2004, 69, 1038–1052. (d) Hong, S.; Kawaoka, A. M.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 15878–15892. (e) Hong, S.; Tian, S.; Metz, M. V.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 14768–14783. (f) Ryu, J.-S.; Li, G. Y.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 12584–12605. (g) Tian, S.; Arredondo, V. M.; Stern, C. L.; Marks, T. J Organometallics 1999, 18, 2568–2570. (h) Arredondo, V. A.; McDonald, F. E.; Marks, T. J. Organometallics 1999, 18, 1949–1960. (i) Arredondo, V. M.; Tian, S.; McDonald, F. E.; Marks, T. J. J. Am. Chem. Soc. 1999, 121, 3633–3639. (j) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1757–1771. (k) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 9295–9306. (l) Giardello, M. A.; Conticello, V. P.; Brard, L.; Gagne´, M. R.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241– 2254. (m) Giardello, M. A.; Conticello, V. P.; Brard, L.; Sabat, M.; Rhengold, A. L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10212–2240. (n) Li, Y. W.; Fu, P. F.; Marks, T. J. Organometallics 1994, 13, 439–440. (o) Gagne´, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275–294.

including diastereomerically pure natural products, have been synthesized via regioselective organolanthanide-catalyzed hydroamination (HA) and tandem C-N/C-C bond formation processes (Scheme 1).1i The success thus far in structurally tailoring organolanthanide catalysts for activity/selectivity optimization has prompted a continuing effort to develop strategies for manipulating reactivity and exploring new hydroamination pathways.

Enzymes catalyze processes with exceptional selectivities, owing to their ability to preorganize substrates in favorable conformational and concentration conditions, often by utilizing multiple proximate metal centers.14 Similarly, multinuclear metal complexes offer unique and more selective catalytic transformations by facilitating cooperative effects between active sites.15 For example, binuclear organo group 4 catalysts (Chart 1) exhibit significant nuclearity effects in olefin polymerization processes.15a-f Binuclear binding/activation of macromonomers by these catalysts is implicated in increased branch formation during ethylene homopolymerization (Figure 1a). Likewise, such binuclear interactions are proposed to promote increased (9) For additional examples of group 3 and lanthanide catalyzed hydroamination, see: (a) Rasta¨tter, M.; Zulys, A.; Roesky, P. W. Chem. Eur. J. 2007, 13, 3606–3616. (b) Riegert, D.; Collin, J.; Daran, J.-C.; Fillebeen, T.; Schulz, E.; Lyubov, D.; Fukin, G.; Trifonov, A. Eur. J. Inorg. Chem. 2007, 8, 1159–1168. (c) Bambirra, S.; Tsurugi, H.; van Leusen, D.; Hessen, B. Dalton Trans. 2006, 1157–1161. (d) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F. J. Am. Chem. Soc. 2006, 128, 3748–3759. (e) Panda, T. K.; Zulys, A.; Gamer, M. T.; Roesky, P. W. Organometallics 2005, 24, 2197–2202. (f) Kim, J. Y.; Livinghouse, T. Org. Lett. 2005, 7, 4391–4393. (g) Molander, G. A.; Hasegawa, H. Heterocycles 2004, 64, 467–474. (h) Lauterwasser, F.; Hayes, P. G.; Brase, S.; Piers, W. E.; Schafer, L. L. Organometallics 2004, 23, 2234–2237. (i) O’Shaughnessy, P. N.; Gillespie, K. M.; Knight, P. D.; Munslow, I. J.; Scott, P. Dalton Trans. 2004, 2251– 2256. (j) Haar, C. M.; Stern, C. L.; Marks, T. J. Organometallics 1996, 15, 1765–1784.

Phenylene-Bridged Organolanthanide Complexes

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Chart 1. Examples of Binuclear Organo Group 4 Precatalysts

Chart 2. Mono- and Bis(tetramethylcyclopentadienyl)-Substituted Ligands

Chart 3. Phenylene-Bridged Half-Lanthanocenes

comonomer enchainment in ethylene + R-olefin copolymerizations (e.g., Figure 1b) and enhanced styrene homopolymerization activity (Figure 1c). These observations suggest the intriguing

Figure 1. Proposed interactions with binuclear organo group 4 catalysts resulting in (a) increased polyethylene branch formation, (b) increased comonomer enchainment, and (c) enhanced styrene polymerization reactivity. P, P′ ) polymeryl fragment. Adapted from ref 15f.

possibility of joining catalytically active lanthanide centers via multinuclear ligand scaffolds. Thus far, no binuclear lanthanide catalysts have been devised to investigate possible cooperative effects involving transformations of the type in eqs 1 and 2. Furthermore, multinuclear catalysis with d0 elements has only been explored for olefin polymerization processes. Previously, we briefly communicated the synthesis and characterization of a new class of phenylene-bridged binuclear tetramethylcyclopentadienyl (Cp′′) organolanthanide complexes and initial catalytic intramolecular hydroamination studies.16 The phenylene fragment is employed as a bridging element to modulate the distance between lanthanide active centers via

ortho, meta, or para substitution (Chart 2), and such phenylenebridged bis-Cp′′H reagents provide straightforward access to binuclear organolanthanide complexes (Chart 3). Herein we report full synthetic and characterization studies, encompassing ligand reagents 1-4 and organolanthanide complexes 5-9, along with an additional p-phenylene-bridged binuclear lantha(10) For computational studies of lanthanide-catalyzed hydroamination, see: (a) Tobisch, S. Chem. Eur. J. 2007, 13, 9127–9136. (b) Motta, A.; Fragala`, I. L.; Marks, T. J. Organometallics 2006, 25, 5533–5539. (c) Tobisch, S. Chem. Eur. J. 2006, 12, 2520–2531. (d) Tobisch, S. Chem. Eur. J. 2005, 11, 6372–6385. (e) Tobisch, S. J. Am. Chem. Soc. 2005, 127, 11979–11988. (f) Motta, A.; Lanza, G.; Fragala`, I. L.; Marks, T. J. Organometallics 2004, 23, 4097–4104. (11) For organolanthanide-mediated synthesis of amine-capped polyolefins, see: (a) Amin, S. B.; Seo, S.; Marks, T. J. Organometallics 2008, 27, 2411–2420. (b) Amin, S. B.; Marks, T. J. Angew. Chem., Int. Ed. 2008, 47, 2006–2025. (12) Aspinall, H. C. Chemistry of the f-Block Elements; Phillips, D., O’Brien, P., Roberts, S., Eds.; Gordon and Breach: Amsterdam, 2001; Advanced Chemistry Texts, Vol. 5. (13) Shannon, R. D. Acta Crystallogr. 1976, A32, 751–767. (14) See, for example: (a) Collman, J. P.; Dey, A.; Decreau, R. A.; Yang, Y.; Hosseini, A.; Solomon, E. I.; Eberspacher, T. A. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9892–9896. (b) Ivanov, I.; Trainer, J. A.; McCammon, J. A. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 1465–1470. (c) BauerSiebenlist, B.; Dechert, S.; Meyer, F. Chem. Eur. J. 2005, 11, 5343–5352. (d) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. ReV. 2004, 104, 561–588. (e) Krishnan, R.; Voo, J. K.; Riordan, C. G.; Zahkarov, L.; Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 4422–4423. (f) Bruice, T. C. Acc. Chem. Res. 2002, 35, 139–148. (g) Bruice, T. C.; Benkovic, S. J. Biochemistry 2000, 39, 6267–6274. (h) O’Brien, D. P.; Entress, R. M. N.; Matthew, A. C.; O’Brien, S. W.; Hopkinson, A.; Williams, D. H. J. Am. Chem. Soc. 1999, 121, 5259–5265. (i) Carazo-Salas, R. E.; Guarguaglini, G.; Gruss, O. J.; Segref, A.; Karsenti, E.; Mattaj, L. W. Nature 1999, 400, 178–181. (j) Menger, F. M. Acc. Chem. Res. 1993, 26, 206– 212. (k) Page, M. I. In The Chemistry of Enzyme Action; Page, M. I., Ed.; Elsevier: New York, 1984; pp 1-54.

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num derivative. The remarkable kinetic stability of the binuclear organolanthanide complexes in contrast to that of mononuclear monocyclopentadienyl Ln3+ complexes is discussed. Next, a full discussion of binuclear organolanthanide-catalyzed intramolecular hydroamination/cyclization of aminoalkenes, aminoalkynes, aminoallenes, and conjugated aminodienes is presented. We compare/contrast the results with those for mononuclear organolanthanide catalysts and extend the discussion to include intermolecular hydroamination processes and a variety of substrates having both extended and compressed junctures between the -NH2 group and the C-C unsaturation, as well as bifunctional H2N-R-NH2 substrates. Finally, we describe a new organolanthanide-catalyzed alkyne isomerization process.

Experimental Section Materials and Methods. All manipulations of air-sensitive materials were carried out with rigorous exclusion of oxygen and moisture in flame- or oven-dried Schlenk-type glassware on a dualmanifold Schlenk line, or interfaced to a high-vacuum line (10-6 Torr), or in a nitrogen-filled Vacuum Atmospheres glovebox with a high-capacity recirculator ( 2σ) R Indices (all data) largest peak, hole in diff map (e/Å3)

C44H96La2N6Si10 1267.99 colorless, block 293(2) 0.710 73 triclinic P1j 10.8376(15) 11.1389(15) 15.445(2) 84.802(2) 83.862(2) 61.352(2) 1625.2(4) 1 1.296 1.513 656 1.33-28.55 88.5 14 170 (7320, 0.1086) 310 0.971 R1 ) 0.0549, wR2 ) 0.1174 R1 ) 0.0914, wR2 ) 0.1307 1.800, -0.661

classes that might uniquely interact with bimetallic catalyst arrays. This section begins with an account of ligand synthetic approaches, followed by the preparation of binuclear organolanthanide complexes and the crystallographic characterization of a para-bridged binuclear lanthanum derivative. Next is reported the remarkable resistance to ligand redistribution of the binuclear lanthanide compounds in contrast to the case for analogous mononuclear monocyclopentadienyl Ln3+ complexes. The efficacy of the binuclear compounds as intramolecular hydroamination/cyclization catalysts is examined in full for aminoalkenes, aminoalkynes, aminoallenes, and conjugated aminodienes, followed by intermolecular hydroaminations. The binuclear catalysts are next probed for unusual reactivity patterns via hydroamination/cyclization of long-chain substrates, reactions involving bifunctional substrates, and intermolecular hydroamination of short-chain substrates. Finally, a heretofore unreported lanthanide-mediated alkyne isomerization process is described. Precatalyst Synthesis and Characterization. The p-phenylene-bridged ligand 118 and m-phenylene-bridged ligand 2 (Chart 2) were synthesized from the respective dibromobenzene via stepwise lithiation/cyclopentenone addition reactions. An analogous ligand for mononuclear complexes, 4,19 was synthesized similarly from bromobenzene (Scheme 2). This synthetic strategy was unsuccessful for the ortho-substituted 3, owing to established rapid decomposition of (o-bromophenyl)lithium to highly unstable benzyne species.28 Temperatures sufficiently low to suppress benzyne formation also impede reaction of the aryllithium reagent with 2,3,4,5-tetramethylcyclopent-2-en-1one. Furthermore, the reaction of o-dilithiobenzene29 with excess 2,3,4,5-tetramethylcyclopent-2-en-1-one yields only the monosubstituted product, phenyltetramethylcyclopentadiene (identified by NMR and MS). Small amounts of 3 were identified in the product mixture of a Pd(PPh3)4-catalyzed Heck coupling reaction between 1,2-dibromobenzene and 2 equiv of 2,3,4,5tetramethylcyclopent-2-en-1-one but could not be isolated in sufficient purity for subsequent reactions. (28) Chen, L. S.; Chen, G. J.; Tamborski, C. J. Organomet. Chem. 1980, 193, 283–292. (29) Winkler, H. J. S.; Wittig, G. J. Org. Chem. 1963, 28, 1733–1740.

Phenylene-Bridged Organolanthanide Complexes

Organometallics, Vol. 28, No. 8, 2009 2431 Scheme 2. Ligand Synthetic Routes

Scheme 3. Syntheses of Phenylene-Bridged Half-Lanthanocenes

The preparation of phenylene-bridged binuclear organolanthanide complexes via protodeaminative and protodealkylative methods utilizing homoleptic Ln[N(SiMe3)2]320 amides, related Ln[CH(SiMe3)2]330 alkyls, and Ln[N(SiHMe2)2]3(THF)x31 silylamides showed little promise. Thus, no reaction occurs between the ligand reagent 1 or 2 and 2 equiv of Ln[N(SiMe3)2]3, Ln[CH(SiMe3)2]3, or Lu[N(SiHMe2)2]3(THF). Earlier lanthanide Ln[N(SiHMe2)2]3(THF)x (Ln ) Y, La) complexes undergo protodeamination with 1, although only very slowly over weeks in refluxing toluene, to produce the desired complexes 5 and 6 in very low yields along with unidentified side products. However, utilizing less encumbered solvent-free Ln[N(SiHMe2)2]316 tetramethyldisilylamido reagents effect efficient protodeamination to yield the desired phenylene-bridged halflanthanocenes in good yield (Scheme 3). The solvent-free Ln[N(SiHMe2)2]3 reagents are readily generated in hydrocarbon media by transamination of the corresponding Ln[N(SiMe3)2]3 amide with HN(SiHMe2)2 (eq 3). X-ray diffraction analysis of La[N(SiHMe2)2]3 reveals an unusual C1-symmetric dimeric structure (Figure 2).16 Furthermore, La[N(SiHMe2)2]3 exhibits pronounced Si-H β-agostic interactions with the La3+ center. Each of the terminal silylamide ligands is tilted to allow one close La · · · H-Si contact with minimum La · · · Si and La · · · H distances of 3.191(2) and 2.56(6) Å, respectively. The IR spectrum of La[N(SiHMe2)2]3 supports the presence of both noninteracting (νSi-H 2092, 2060 cm-1) and interacting (νSi-H 2023, 1920 cm-1) 29Si-H units.32 Furthermore, the smaller Si-H coupling in La[N(SiHMe2)2]3 (1JSiH ) 154 Hz) vs HN(SiHMe2)2 (1JSiH ) 170 Hz)31 in the 1H NMR spectrum also supports the presence of Si-H β-agostic interactions.33 The binuclear lanthanum complex 6 is readily obtained in 66% yield after 2 days from the room-temperature reaction of 1 with 2.0 equiv of La[N(SiHMe2)2]3 (eq 4). 1H NMR in situ experiments (30) Hitchcock, P. B.; Lappert, M. F.; Smith, R. G.; Bartlett, R. A.; Power, P. P. J. Chem. Soc., Chem. Commun. 1988, 15, 1007–1009. (31) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.; Herdtweck, E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847–858.

indicate that the reaction proceeds in quantitative yield and in >95% purity, with the yield-limiting step being crystallization of the extremely lipophilic oily product. Complex 6exhibits Si-H β-agostic interactions with the La3+ center, as evidenced by the low νSi-H IR frequencies (1005, 1874 cm-1) and the diminished 29Si-H coupling (1JSiH ) 144 Hz) in the 1H NMR spectrum. Although Y[N(SiHMe2)2]3 and Lu[N(SiHMe2)2]3 were

Figure 2. Crystal structure of La[N(SiHMe2)2]3. All hydrogen atoms, except those directly attached to Si, are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond distances (Å) and angles (deg): La1-N1, 2.381(5); La1-N2, 2.386(4); La1-N5, 2.695(4); La1-N6, 2.670(4); La2-N3, 2.375(5); La2-N4, 2.366(5); La2-N5, 2.629(5); La2-N6, 2.622(5); La1 · · · Si2, 3.234(2); La1 · · · Si3, 3.191(2); La1 · · · Si10, 3.306(2); La1 · · · Si11, 3.430(2); La2 · · · Si5, 3.335(2); La2 · · · Si8, 3.295(2); La2 · · · Si9, 3.295(2); La2 · · · Si12, 3.336(2); La1 · · · H2, 2.56(6); La1 · · · H3, 2.61(6); La1 · · · H10, 2.62(6); La1 · · · H11, 2.94(8); La2 · · · H5, 2.85(8); La2 · · · H8, 2.76(8); La2 · · · H9, 2.61(6); La2 · · · H12, 2.62(6);La1-N5-La2,95.4(1);La1-N6-La2,96.1(1);N1-La1-N2, 97.2(2);N1-La1-N5,114.4(2);N1-La1-N6,138.5(2);N2-La1-N5, 126.2(2); N2-La1-N6, 105.9(2); N5-La1-N6, 78.6(1). From ref 16.

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Scheme 4. Generation of Bidentate Amido-Amine Ligated Species

not scrupulously purified, reaction of each with ligand reagent 1 affords binuclear complexes 5 and 7, respectively. Pronounced Si-H β-agostic interactions are also evident from the diminished 29 Si-H coupling in both Y complex 5 (1JSiH ) 151 Hz) and Lu complex 7 (1JSiH ) 149 Hz). Reaction of 2.0 equiv of La[N(SiHMe2)2]3 with the m-phenylene-bridged ligand reagent 2 yields binuclear complex 8, which similarly displays evidence for Si-H β-agostic interactions (1JSiH ) 144 Hz).

the Nd product mixture is instead reacted with the binuclear lanthanum complex 6 in pentane, ligand exchange occurs, affording the binuclear lanthanum complex 10 (Scheme 4). Complex 10 was characterized by single-crystal X-ray diffraction (Figure 3, Table 1). The structural analysis reveals that the molecule is located on an inversion center and that the bridging phenylene unit is twisted from the plane of the cyclopentadienyl rings by 44.1°, presumably reflecting steric repulsion between the phenylene ring hydrogen atoms and the cyclopentadienyl methyl substituents. As expected, the dative La-N(amine) bond (La1-N2 ) 2.593(3) Å) is significantly longer than the La-N(amide) bonds (La1-N1 ) 2.365(4) Å, La1-N3 ) 2.439(4) Å). Interestingly, while each terminal silylamide ligand in the precursor La[N(SiHMe2)2]3 forms one close Si-H β-agostic contact with the electrophilic La3+ center (Figure 4a),

During a transamination reaction as outlined in eq 3 to prepare Nd[N(SiHMe2)2]3, a small amount of air was inadvertently introduced into the reaction vessel. The result (see below) is consistent with the generation of a bidentate amido-amine ligand, presumably via a ligand rearrangement similar to that described previously for a Ta amido complex (eq 5).34 Reaction of the resulting Nd product mixture with p-phenylene-bridged ligand 1 does not yield the desired binuclear Nd complex. When

(32) Weidlein, J.; Mu¨ller, U.; Dehnicke, K. Vibration Frequencies; Thieme: Stuttgart, New York, 1986; Vol. 1. (33) (a) Hieringer, W.; Eppinger, J.; Anwander, R.; Herrmann, W. A. J. Am. Chem. Soc. 2000, 122, 11983–11994. (b) McGrady, G. S.; Downs, A. J.; Haaland, A.; Sherer, W.; McKean, D. C. J. Chem. Soc., Chem. Commun. 1997, 1547–1548. (c) Barnhart, D. M.; Clark, D. L.; Gordon, J. C.; Huffman, J. C.; Watkin, J. G.; Zwick, B. D J. Am. Chem. Soc. 1993, 115, 8461–8462. (d) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem. 1988, 36, 1–124. (e) McKean, D. C.; McQuillan, G. P.; Torto, I.; Morrison, A. R. J. Mol. Struct. 1986, 141, 457–464. (f) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395–408.

Figure 3. Crystal structure of binuclear lanthanum complex 10. All hydrogen atoms, except those directly attached to Si, are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond distances (Å) and angles (deg): La1-N1, 2.365(4); La1-N2, 2.593(3); La1-N3, 2.439(4); La1 · · · Si1, 3.331(1); La1 · · · H1, 2.77(5); La1 · · · Si2, 3.294(1); La1 · · · H2 2.74(5); La1-C4, 2.783(4); La1-C5, 2.796(4); La1-C6, 2.846(4); La1-C7, 2.828(5); La1-C8, 2.801(5); La1-N1-Si1, 109.5(2); La1-N1-Si2, 108.0(2); Si1-N1-Si2, 142.1(2); La1-N2-Si3, 134.9(1);La1-N2-Si5,95.0(1);Si3-N2-Si5,130.0(2);La1-N3-Si4, 124.6(2);La1-N3-Si5,101.7(1);Si4-N3-Si5,133.2(2);N2-Si5-N3, 100.5(1).

Phenylene-Bridged Organolanthanide Complexes

Organometallics, Vol. 28, No. 8, 2009 2433 Chart 4

the monodentate silylamide ligand in 10 forms close contacts with both Si-H units (La1 · · · Si1 ) 3.331(1) Å, La1 · · · H1 ) 2.77(5) Å, La1 · · · Si2 ) 3.294(1) Å, La1 · · · H2 ) 2.74(5) Å). Such a symmetric Si-H β-diagostic interaction (Figure 4b) has previously only been observed in ansa-lanthanocenes.33a,35 Neither Si-H unit of the bidentate amido-amine ligand forms close contacts with the La3+ center. The protodeaminative reaction of ligand 4 with La[N(SiHMe2)2]3 in hydrocarbon solvents proceeds to completion to produce the mononuclear half-sandwich complex 9 in approximately 2 days at 25 °C (eq 6), which is approximately the same length of time required for eq 4 in the synthesis of binuclear La complex 6. The reaction between the sterically less bulky pentamethylcyclopentadiene and La[N(SiHMe2)2]3 proceeds to completion to afford one-ring Cp′La[N(SiHMe2)2]2 in less than 1 h. However, within minutes of the complete Cp′La[N(SiHMe2)2]2 generation, disproportionation is observed, and after a few more hours, ligand redistribution is complete (eq 7). Although 9 could not be isolated in complete purity despite exhaustive attempts via numerous approaches, crude 9 was observed to fully disproportionate in solution at room temperature only over the course of more than 2 weeks (eq 6). In marked contrast, binuclear complex 6 shows no signs of ligand redistribution, even at 90 °C in solution, as judged by in situ 1H NMR spectroscopy. Reaction of 6 with 1.0 equiv of 1

Figure 4. (a) Mono-agostic coordination mode and (b) symmetric diagostic coordination mode of silylamide moieties.

at 90 °C over the course of 1 week slowly produced a multitude of products (NMR assay); however, these products could not be separated sufficiently for identification.

Catalytic Hydroamination Studies. In this section are presented the results of intra- and intermolecular hydroamination experiments for a range of C-C unsaturations utilizing the aforementioned binuclear organolanthanide complexes as precatalysts. In addition, substrates that might preferentially form preorganized structures with bimetallic catalysts vs the mononuclear analogues are examined (Chart 4). Bifunctional diamine and diene/diyne substrates might simultaneously bind both ends

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Table 2. Aminoalkene and Aminoalkyne Hydroamination/Cyclization with Binuclear Ln Catalysts

a Yields >95% determined by 1H NMR spectroscopy. of Ln centers). c Reference 8o. d Reference 8k.

b

Turnover frequencies measured in C6D6 with 3-5 mol % precatalyst (normalized to number

to proximate catalyst active sites, facilitating multiple hydroaminations. Short-chain substrates which would disfavor simple intramolecular hydroamination/cyclization (due to ring strain) and instead react with a second substrate molecule bound at the adjacent Ln active site were also examined. (a) Aminoalkene and Aminoalkyne Substrates. Binuclear complexes 5, 6, and 8 as well as La[N(SiHMe2)2]3 and Cp′La[N(SiHMe2)2]2 exhibit significant activity for aminoalkene and aminoalkyne intramolecular hydroamination/cyclization (Table 2). All conversions proceed smoothly with zero-order dependence on substrate concentration, and in situ 1H NMR spectroscopy reveals that the Ln3+ centers remain bound to their respective ligands throughout catalytic turnover. Conversions of 11, 13, 15, and 17 proceed cleanly as with mononuclear lanthanocene catalysts5b,8k,n to yield exocyclic products. The binuclear La3+ complex 6 mediates these conversions with slightly increased turnover frequencies versus binuclear Y3+ complex 5 (Table 2, entry 2 vs entry 1, for example). For aminoalkenes 11 and 15, the homoleptic La[N(SiHMe2)2]3 complex, which lacks cyclopentadienyl ligand steric crowding, effects rapid conversions (3.0 h-1; Table 2, entry 5), although with lower activity than the corresponding lanthanocene8o (95 h-1; Table 2, entry 7). An exception is aminoalkyne 17, where the homoleptic complex La[N(SiHMe2)2]3 exhibits the lowest efficiency while mononuclear lanthanocenes are most efficient. Although single-ring Cp′La[N(SiHMe2)2]2 disproportionates soon after generation (eq 6), careful monitoring of the catalyst formation and subsequent ligand redistribution processes by in situ 1H NMR spectroscopy allows identification of conditions (34) Chen, S.-J.; Zhang, X.-H.; Yu, X.; Qiu, H.; Yap, G. P. A.; Guzei, I. A.; Lin, Z.; Wu, Y.-D.; Xue, Z.-L. J. Am. Chem. Soc. 2007, 129, 14408– 14421. (35) Eppinger, J.; Spiegler, M.; Hieringer, W.; Herrmann, W. A.; Anwander, R. J. Am. Chem. Soc. 2000, 122, 3080–3096.

under which the amounts of La[N(SiHMe2)2]3 and Cp′2La[N(SiHMe2)2] present in solution are below the 1H NMR detection limits. At this point, the essentially pure Cp’La[N(SiHMe2)2]2 can be intercepted by addition of substrate, such as aminoalkene 11 (Table 2, entry 6). No disproportionation is observed by 1H NMR spectroscopy throughout catalytic turnover, and moderate HA/cyclization activity is displayed (0.7 h-1). (b) Aminoallene and Conjugated Aminodiene Substrates. Significant activity is also observed for aminoallene and conjugated aminodiene hydroamination/cyclization processes (Table 3). All conversions proceed with zero-order dependence on substrate concentration. Reactions of aminoallene 19 (Table 3, entries 1 and 2) parallel trends observed with Cp′2Ln catalysts8h (Table 3, entries 3 and 4), exclusively generating the trans-pyrrolidine product with a slight Z isomer preference. However, the strong E isomer preference for conjugated aminodiene conversions exhibited by Cp′2LaCH(TMS)28d (Table 3, entries 9, 14, and 23) is not observed with the present binuclear catalysts (Table 3, entries 5-8, 10-13, 16-18, and 20-22). Interestingly, meta-bridged binuclear complex 8 shows a moderate Z isomer preference. As with the aminoalkene and aminoalkyne cyclizations, the present catalysts exhibit decreased activities versus mononuclear lanthanocenes.8d,h (c) Intermolecular Hydroamination. Significant activity is also observed for intermolecular catalytic hydroamination of alkenes and alkynes with n-propylamine (Table 4). The reactions with styrene employed a [Ln]/[amine]/[styrene] ratio of 1/10/ 100 (normalized to the number of Ln3+ centers) to minimize catalytic inhibition by amine and to shorten reaction times. Similarly, reactions with 1-phenylpropyne 29 employed a [Ln]/ [amine]/[alkyne] ratio of 1/20/100 (normalized to the number of Ln3+ centers). Reactions with para-bridged binuclear catalyst 6 yield the same anti-Markovnikov product 28 and imine

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Organometallics, Vol. 28, No. 8, 2009 2435

Table 3. Aminoallene and Conjugated Aminodiene Hydroamination/Cyclization

a Yields >95% determined by 1H NMR spectroscopy. of Ln centers). c Reference 8h. d Reference 8d.

b

Turnover frequencies measured in C6D6 with 3-5 mol % precatalyst (normalized to number

Table 4. Intermolecular Hydroamination of Alkenes and Alkynes

a Yields >95% determined by 1H NMR spectroscopy. b Turnover frequencies measured in C6D6. Reaction conditions (normalized to number of Ln centers): [Ln]/[amine]/[styrene] ) 1/10/100 employed for pseudo-zero-order kinetic plot; [Ln]/[amine]/[phenylpropyne] ) 1/20/100 employed; [Ln]/ [amine]/[methylenecyclopropane] ) 1/20/20 employed; [Ln] ) 0.05 mM. c Small quantities of polystyrene are observed by 1H NMR spectroscopy along with the hydroamination product. d Polystyrene is observed to form. e Reference 8f. f Dimers and trimers of the alkene are observed to form.

products 30 and 32 (Table 4, entries 2, 6, and 9, respectively) as mononuclear lanthanocenes8f (Table 4, entries 4, 7, and 12, respectively), although with somewhat decreased activities. For styrene reactions, complex 6 was observed by 1H NMR spectroscopy to also produce small quantities of polystyrene at 90 °C. No intermolecular hydroamination is observed with metabridged binuclear complex 8 as the precatalyst, even at 120 °C. For styrene reactions, complex 8 only sluggishly produces polystyrene at 90 °C and above, as judged by 1H NMR spectroscopy. For reactions with methylenecyclopropane 31 at 120 °C, complex 8 slowly generates 31-derived dimers and trimers, as judged by 1H NMR spectroscopy and GC-MS analysis (Table 4). (d) Longer Chain Unsaturated Amine Substrates. To date, mononuclear organolanthanide catalysts have not shown competence for intramolecular HA/cyclization processes that would produce heterocycles having greater than seven members. Thus,

a series of long-chain substrates (Chart 4) was examined with the multinuclear catalysts. When alkene substrates 33 and 34 and alkyne substrates 36-38 are each exposed to 5 mol % (normalized to number of Ln3+ centers) of binuclear La3+ precatalyst 6, rapid transamination/catalyst activation is observed by 1H NMR at 25 °C with release of HN(SiHMe2)2. However, no other detectable reactions are observed with any of these substrates up to 120 °C over a period of 4 weeks. Exocyclic hydroamination/cyclization of these substrates is most likely to produce eight-membered heterocycles, while substrate 35 would generate a nine-membered ring (Chart 4). Again, rapid transamination/catalyst activation is observed at 25 °C with 35 for binuclear complexes 6 and 8, as well as with La[N(SiHMe2)2]3 and (CGC)SmN(TMS)2. However, no other reaction is observed for any of the catalysts up to 120 °C over a period of 2 weeks. (e) Bifunctional Substrates. The reactivity of the binuclear catalysts with bifunctional substrates (Chart 4) was also

2436 Organometallics, Vol. 28, No. 8, 2009

examined. When a 1/1 mixture of ethylenediamine 39 and hexadiene 41 is reacted with 5 mol % (per Ln3+ center) of binuclear precatalyst 6, rapid transamination/catalyst activation is observed; however, no other reaction is observed up to 120 °C over a period of 4 weeks. The same results are obtained with a 1/1 diamine 40/diene 41 mixture and with a 1/1 40/heptadiyne 42 mixture. At 90 °C and above, slow catalytic oligomerization of 42 is observed, as judged by 1H NMR spectroscopy and GC-MS analysis. (f) Shorter Chain Unsaturated Amine Substrates. For substrates too short for intramolecular hydroamination/cyclization, multinuclear catalysts might facilitate intermolecular hydroamination processes, as observed in certain aminoalkyne hydroaminations.8j The substrates surveyed (Chart 4) included primary amines with C-C unsaturation at the 2- (43-45) and 3-positions (46-49). Exposing allylamine 43 to 5 mol % (per Ln3+ center) of binuclear precatalyst 6 or 8 results in rapid transamination/catalyst activation; however, no other reaction is observed at 120 °C for periods up to 2 weeks for either catalyst. Similarly, transamination/catalyst activation of precatalyst 6 proceeds at 25 °C with aminoalkynes 45, 48, and 49, but no other reaction at 120 °C over a period of 4 weeks. Furthermore, negligible hydroamination is observed at 120 °C in the presence of terminal alkynes 44 and 46, although alkyne polymerization occurs, as judged by 1H NMR spectroscopy. Interestingly, 3-pentynylamine 47 undergoes HA/cyclization to 2-methyl-1-pyrroline in the presence of binuclear complexes 5, 6, and 8, as well as La[N(SiHMe2)2]3, La[N(TMS)2]3, Lu[N(TMS)2]3, (CGC)SmN(TMS)2, and (CGC′)LuN(TMS)2(THF) (eq 8). Simple exocyclic cyclization of 47 should produce a four-membered heterocycle (see Scheme 1), not the fivemembered heterocycle obtained. It is hypothesized that 47 initially undergoes isomerization to 4-pentynylamine via Ln3+mediated C-H activation (eq 9), and the terminal alkyne product then undergoes exocyclic HA/cyclization to yield the observed 2-methyl-1-pyrroline.

Alkyne Isomerization. Examining additional substrates indicates that the process of eq 9 is rather general. As monitored by 1H NMR, both 3-hexyne (50) and 2-pentyne (51) undergo clean prototropic isomerization in the presence of Ln3+ complexes. For the reaction of 50, at equilibrium the major species observed in solution is 2-hexyne with some 3-hexyne and 2,3hexadiene and traces of 1,2-hexadiene (eq 10). For 51, at equilibrium the major species observed in solution is 2-pentyne with traces of 2,3-pentadiene, 1,2-pentadiene, and 1-pentyne (eq 11). As anticipated from the preceding results, in the presence of Ln3+ complexes aminoallene 52 undergoes isomerization to 4-pentynylamine and then undergoes intramolecular hydroamination/cyclization (eq 12). Furthermore, aminoallene 53 undergoes isomerization to 6-octynylamine in the presence of complex 6 at 60 °C. Subsequent heating of the 6-octynylamine at 120 °C in the presence of 6 affords the exocyclic intramolecular, seven-membered hydroamination/cyclization product (eq 13).

Yuen and Marks

Discussion This section begins with a discussion of catalyst synthesis, followed by the synthetic and crystallographic characterization aspects of binuclear complex 10. The remarkable stability exhibited by the binuclear half-sandwich organolanthanide compounds is then discussed. Binuclear complex catalyzed intramolecular hydroamination reactions of aminoalkenes are then presented in terms of reactivity trends, followed by reactivity trend analyses for intramolecular hydroamination/ cyclization of aminoalkynes, aminoallenes, and conjugated aminodienes and then for intermolecular hydroamination. Using long-chain substrates to further probe binuclear lanthanide catalyst reactivity is discussed, followed by studies of bifunctional and short-chain substrates. Finally, lanthanide-promoted prototropic isomerization reactions of alkynes are presented. Precatalyst Synthesis and Characterization. While pphenylene-bridged ligand 1 and m-phenylene-bridged ligand 2 can be prepared from the respective dibromobenzenes via stepwise lithiations/cyclopentenone additions (Scheme 2), the ortho-substituted analogue proved to be problematic owing to known, rapid decomposition of (o-bromophenyl)lithium to highly unstable benzyne species. While there are reports28,36 that generating o-bromophenyllithium below -110 °C slows benzyne formation sufficiently to allow reactions with other reagents, reaction between (o-bromophenyl)lithium and 2,3,4,5tetramethylcyclopent-2-en-1-one is exceedingly sluggish at these temperatures. Synthesis of 1 or 2 via the respective dilithiobenzene rather than via a stepwise procedure depresses the yield ∼10×.37 Therefore, the results with o-dilithiobenzene are understandable. Furthermore, that monosubstituted PhCp′′H is the sole o-dilithiobenzene + 2,3,4,5-tetramethylcyclopent-2-en1-one reaction product suggests that steric encumbrance disfavors further addition. Unlike salt elimination reactions, protodeaminative Ln-NR2 bond-forming reactions avoid coordinating solvents and undesired “ate” byproducts.8g,38 While the Ln[N(SiMe3)2]3 amides do (36) (a) Quintard, D.; Keller, M.; Breit, B. Synthesis 2004, 905–908. (b) Saito, M.; Nitta, M.; Yoshioka, M Organometallics 2001, 20, 749–753. (c) Omote, M.; Kominato, A.; Sugawara, M.; Sato, K.; Ando, A.; Kumadaki, I. Tetrahedron Lett. 1999, 40, 5583–5585. (d) Kyushin, S.; Shinnai, T.; Kubota, T.; Matsumoto, H. Organometallics 1997, 16, 3800–3804. (37) Yuen, H. F.; Marks, T. J. Unpublished results. (38) (a) Berthet, J. C.; Ephritikhine, M. Coord. Chem. ReV. 1998, 178180, 83–116. (b) Kuhlman, R. Coord. Chem. ReV. 1997, 167, 205–232. (c) Bradley, D. C.; Chisholm, M. H. Acc. Chem. Res. 1976, 9, 273–280. (39) (a) Fraser, R. R.; Monsour, T. S.; Savard, S. J. Org. Chem. 1985, 50, 3232–3234. (b) Eppinger, J.; Herdtweck, E.; Anwander, R. Polyhedron 1998, 17, 1195–1201.

Phenylene-Bridged Organolanthanide Complexes

Organometallics, Vol. 28, No. 8, 2009 2437 Chart 5

not undergo appreciable reaction with ligand reagents 1 and 2 to yield the desired binuclear complexes, protodeaminative approaches employing less sterically encumbered tetramethyldisilylamido Ln[N(SiHMe2)2]3 reagents proved efficient and clean (Scheme 3). This probably reflects a combination of diminished steric bulk allowing greater ease of approach and Cp ligand proton transfer, as well as stabilization afforded by Si-H β-agostic interactions in the resulting complexes (Figure 4). Ln[N(SiHMe2)2]3 reagents are readily generated by transamination of the corresponding Ln[N(TMS)2]3 amide with HN(SiHMe2)2 (eq 3), as anticipated from pKa differences between the parent disilylamines (25.8, HN(TMS)2;39a 22.6, HN(SiHMe2)239b). During the attempted transamination between Nd[N(TMS)2]3 and HN(SiHMe2)2, a small amount of inadvertently introduced air induced a ligand rearrangement of -N(SiHMe2)2 moieties to yield a bidentate amido-amine species. It is tentatively hypothesized that this occurs via a process similar to that described by Xue and co-workers34 for Ta-amido complexes (eq 5). Since the resulting Nd product mixture does not cleanly yield a binuclear Nd3+ complex on reaction with ligand reagent 1, attempts were made to access a binuclear Nd3+ complex via transmetalation with binuclear La3+ complex 6. However, instead of transmetalation, ligand exchange occurs to deliver the bidentate amido-amine ligand to La3+ centers, yielding complex 10 (Scheme 4). This transformation is understandable, since the bidentate amido-amine ligand provides additional steric and electronic La3+ saturation in complex 10 vs 6. The crystal structure of complex 10 (Figure 3) reveals that the molecule is located on an inversion center, with Ln3+ centers oriented anti to each other and having essentially coplanar Cp rings. The bridging phenylene unit is twisted from this plane by 44.1°, which compares well with values for the tungsten analogue p-[Cp′′W(CO)3Me]2phenylene (39°)40a and cobalt analogue p-[Cp′′Co(Et2C2B3H5)]2phenylene (47°)40b (Chart 5). The structures of p-[Cp′′W(CO)3Me]2phenylene and p-[Cp′′Co(Et2C2B3H5)]2phenylene also contain anti-oriented metal centers with essentially coplanar Cp rings. The La-C distances in 10 range from 2.783(4) to 2.846(4) Å and are slightly elongated vs ansa-lanthanocene (Me2SiCp′′2)LaN(SiHMe2)2 (2.738(2)-2.786(7) Å)35 but are comparable to the (40) (a) Southard, G. E.; Curtis, M. D. Synthesis 2002, 1177–1184. (b) Davis, J. H., Jr.; Sinn, E.; Grimes, R. N. J. Am. Chem. Soc. 1989, 111, 4784–4790.

range in sterically encumbered lanthanocene (Ph4Cp)2LaN(SiHMe2)2 (2.744(4)-3.019(5) Å).41 The monodentate silylamide ligand in 10 exhibits La1-N1 ) 2.365(4) Å, which is comparable to that in (Ph4Cp)2LaN(SiHMe2)2 (2.361(5) Å),41 precursor La[N(SiHMe2)2]3 (2.366(5)-2.386(4) Å),16 and eightcoordinate Cp′2La(NHMe)(H2NMe) (2.32(1) Å).8o In contrast, the bidentate amido-amine ligand La-N(amide) bond distance of 2.439(4) Å in 10 is more comparable to those in (Me2SiCp′′2)LaN(SiHMe2)2 (2.449(3) Å)35 and nine-coordinate (Cp′2La)2(µ-phenazinato) (2.452(2) Å)42 (Chart 5). The dative La-N(amine) bond distance of 2.593(3) Å in 10 is slightly shorter than in Cp′2La(NHMe)(H2NMe) (2.70(1) Å).8o The symmetric Si-H β-diagostic interaction seen in 10 has previously only been observed in ansa-Cp′′2 and ansa-Ind2 lanthanocenes. The close La · · · Si contacts, 3.331(1) and 3.294(1) Å, and the close La · · · H contacts, 2.77(5) and 2.74(5) Å, are parameters comparable to the symmetric Si-H β-diagostic interactions of (Me2SiCp′′2)LaN(SiHMe2)2 (La · · · Si, 3.2460(9)/ 3.2440(9) Å; La · · · H, 2.70(3)/2.66(4) Å)35 and the Si-H β-monoagostic interactions in La[N(SiHMe2)2]3 (La · · · Si, 3.191(2)-3.335(2) Å; La · · · H, 2.56(6)-2.85(8) Å). In comparison to ubiquitous Cp2Ln-L complexes, monocyclopentadienyl complexes possess greatly diminished electronic and steric saturation. As a consequence, the synthesis of halflanthanocenes has proven problematic, commonly plagued by Lewis base complexation, “ate” formation, and ligand redistribution owing to high Cp and L ligand lability.43 As discussed above, protodeaminative synthetic routes can avoid the first two problems. However, even for isolated pure samples, halflanthanocenes often undergo ligand redistributions, even in hydrocarbon solutions.43 It is therefore remarkable that binuclear complex 6 exhibits no evidence of ligand redistribution even on heating to 90 °C in solution over periods of 3 weeks. While the markedly faster rates of both the protodeaminative synthesis and subsequent disproportionation of Cp′La[N(SiHMe2)2]2 vs phenyl-substituted analogue 9 illustrate that the increased steric and electronic saturation provided by the phenyl substituent contributes to the kinetic stability of these complexes, that complex 9 does undergo disproportionation in solution suggests (41) Klimpel, M. G.; Go¨rlitzer, H. W.; Tafipolsky, M.; Spiegler, M.; Scherer, W.; Anwander, R. J. Organomet. Chem. 2002, 647, 236–244. (42) Scholz, J.; Scholz, A.; Weimann, R.; Janiak, C.; Schumann, H. Angew. Chem., Int. Ed. 1994, 33, 1171–1174. (43) Arndt, S.; Okuda, J. Chem. ReV 2002, 102, 1953–1976, and references therein.

2438 Organometallics, Vol. 28, No. 8, 2009

that phenyl substitution is not the sole factor. It is likely that the steric bulk of the adjacent metal-ligand fragment in the binuclear complexes further stabilizes them with respect to ligand redistribution processes. Aminoalkene Intramolecular Hydroamination. In general, aminoalkene intramolecular HA/cyclization rates are largely governed by steric and strain energy factors.1 That mononuclear lanthanocenes are already highly efficient in catalyzing these processes8o raised the question of how catalysts having adjacent Ln centers might affect these transformations. Judging from the depressed HA/cyclization rate of substrate 11, the steric bulk of the adjacent Ln-containing fragment appears to dominate the rates. Under identical reaction conditions, the Nt ordering is La[N(SiHMe2)2]3 > Cp′La[N(SiHMe2)2]2 > 6 (Table 2, entries 5, 6, and 2, respectively), where the least hindered La[N(SiHMe2)2]3 is the most active. While Cp′La[N(SiHMe2)2]2 undergoes ligand redistribution in solution, this process appears by 1H NMR to be inhibited by excess substrate 11. The steric bulk of substrate/product amide and amine binding (e.g., A) likely creates sufficient hindrance around the Ln center to suppress intermolecular redistribution processes. The smaller Y3+ ion is more sterically congested than La3+, consistent with the diminished activity of binuclear Y3+ 5 (Table 2, entry 1) vs binuclear La3+ 6 (Table 2, entry 2) in HA/cyclization. Phenylcontaining substrates 13 and 15 were used to probe the possible role of Ln3+ · · · Ph interactions in modifying regiochemistry or other reaction characteristics. No obvious effects are observed, with turnover frequencies again following steric encumbrance trends. The steric hindrance imposed by the N-methyl group in substrate 13 during the turnover-limiting olefin insertion (Scheme 1) depresses Nt vs primary amine 11 (Table 2, entry 9 vs entry 3), and the bulkier N-benzyl group in 15 depresses Nt further (Table 2, entry 12).

Intramolecular Aminoalkyne Hydroamination. Interestingly, for aminoalkyne substrate 17, the most sterically open La[N(SiHMe2)2]3 precatalyst (Table 2, entry 20) exhibits far lower HA/cyclization activity than the more sterically hindered binuclear catalysts (Table 2, entries 17-19). However, within the binuclear series the opposite trend is obtained, where less open/more sterically hindered 5 and 8 display lower Nt values than the more open complex 6. While sterics and strain are major factors determining HA rate trends for aminoalkenes,1 it was previously observed for mononuclear catalysts that, for sterically less demanding CtC substrates, electronic demands supersede stericssmore open catalysts exhibit decreased activities.8k,10b The same trend is observed here, with the highly open La[N(SiHMe2)2]3 precatalyst exhibiting significantly lower activity than the less open binuclear complexes. Since the binuclear catalysts are doubtless sterically more crowded (see discussion above), and the large excess of amines in solution likely weakens stabilizing cooperative interactions, nonbonded interactions dominate reaction rate trends. Intramolecular Aminoallene and Conjugated Aminodiene Hydroamination. In general, aminoallene and conjugated aminodiene substrates adhere to the same reaction rate trends as aminoalkenes, with the more sterically hindered binuclear

Yuen and Marks

Figure 5. Possible stabilizing interactions with an adjacent metal in (a) intermolecular hydroamination (R ) weakly basic C-H or aryl group) and (b) intramolecular hydroamination of long-chain substrates. (c) Intermolecular hydroamination between two short aminoalkene/aminoalkyne substrates.

Figure 6. Plausible transition states for (a) phenyl-directed antiMarkovnikov styrene insertion, (b) phenyl-directed anti-Markovnikov alkyne insertion, and (c) phenyl-directed cyclopropane ringopening. Adapted from ref 8f.

catalysts displaying somewhat lower activities than the mononuclear analogues. For reactions of aminoallene 19, neither the electronic nor steric presence of an adjacent metal significantly affects product E/Z ratio (Table 3). In contrast, dramatic differences in conjugated aminodiene hydroamination product E/Z selectivities are observed for the binuclear catalysts vs mononuclear Cp′2LaCH(TMS)2. Thus, the binuclear catalysts favor Z isomer products, with para-bridged Y3+ complex 5 and meta-bridged La3+ complex 8 exhibiting stronger Z isomer preferences than para-bridged La3+ complex 6. This new selectivity may reflect favorable binding/orientation of the diene to the second Ln center (e.g., B).

Intermolecular Hydroamination. Intermolecular hydroaminations were studied for possible Ln · · · Ln effects on C-N bond formation (Figure 5a), and minimal effects are observed, with the steric encumbrance of the adjacent Ln fragment presumably outweighing other stabilizing interactions. Lanthanocene catalysts are known to effect anti-Markovnikov intermolecular hydroamination of vinylarene and 1-phenylalkyne substrates, with Ln3+ · · · Ph interactions proposed to stabilize anti-Markovnikov insertive transition states (Figure 6a,b). Similar Ln3+ · · · Ph interactions also play a key role in directing cyclopropane ring-opening regiochemistry (Figure 6c).8f The adjacent Ln3+ centers of the para-bridged binuclear catalysts are likely too distant to cooperatively bind these phenyl substituents, while the increased steric encumbrance of the meta-bridged system suppresses intermolecular hydroamination.

Phenylene-Bridged Organolanthanide Complexes

Longer Chain, Bifunctional, and Shorter Chain Substrates. Cyclization reactions yielding rings of eight or more members44 are classically disfavored, owing to entropic and ring strain factors,45 and present a significant, general synthetic challenge. Long-chain aminoalkene and aminoalkyne substrates were examined with the present bimetallic catalysts to probe possible stabilizing/preorganizing effects (e.g., Figure 5b). C-H agostic interactions that are central in binuclear group 4 polymerization catalysis15f are insufficient here to effect HA of substrates 33 and 36. Furthermore, a secondary amine placed within the chain and sufficiently close to the C-C unsaturation to prevent intramolecular HA/cyclization with the secondary amine does not accelerate reaction (e.g., substrates 34 and 35). It is conceivable that the secondary amine also undergoes proton transfer, forming a chelating diamide ligand that suppresses catalytic activity. Alternative polar groups appended beyond the C-C unsaturation that are unlikely to form such chelates were also investigated; however, results with substrates 37 and 38 suggest they also do not provide sufficient binding/stabilization to effect HA. This may reflect the large excess of amines in solution competing for Ln coordination, weakening any stabilizing C-H agostic or appended polar group interactions. Bifunctional substrates such as 39-42 might potentially utilize adjacent Ln centers simultaneously. The lack of response of ethylenediamine 39 may again reflect chelating diamide ligand formation. While the rigid p-phenylenediamine linkage of 40 should disfavor chelation and favorably orient the -NH2 groups to geometrically match the p-phenylene-bridged catalysts, negligible HA of either diene 41 or diyne 42 with 40 is observed. Intramolecular hydroamination proceeds readily to form heterocycles containing as few as five members.1i Smaller ring precursors do not undergo cyclization, owing to the excessive ring strain in the insertive transition state and product. With two such short substrate molecules held in close proximity, intermolecular hydroamination may be facilitated1i (Figure 5c); however, negligible intermolecular turnover is observed for substrates 43-49. For terminal alkyne substrates 44 and 46, a known alkyne oligomerization process46 is found to be more rapid than intermolecular HA, following trends previously observed with mononuclear lanthanocene catalysts, where terminal alkyne oligomerization is more rapid than intermolecular hydroamination.8f (44) (a) Chattopadhyay, S. K.; Neogi, K.; Singha, S. K.; Dey, R. Synlett 2008, 1137–1140. (b) Lu, S.-M.; Alper, H. J. Am. Chem. Soc. 2008, 130, 6451–6455. (c) Springer, J.; Jansen, T. P.; Ingemann, S.; Hiemstra, H.; van Maarseveen, J. H. Eur. J. Org. Chem. 2008, 36 ]?>, 1–367. (d) Metay, E.; Leonel, E.; Nedelec, J.-Y. Synth. Commun. 2008, 38, 889–904. (e) Tsuji, H.; Yamagata, K.; Itoh, Y.; Endo, K.; Nakamura, M.; Nakamura, E. Angew. Chem., Int. Ed. 2007, 46, 8060–8062. (f) Block, E.; Glass, R. S.; Dikarev, E. V.; Gruhn, N. E.; Jin, J.; Li, B.; Lorance, E.; Zakai, U. I.; Zhang, S.-Z. Heteroat. Chem. 2007, 18, 509–515. (g) Chattopadhyay, S. K.; Karmakar, S.; Biswas, T.; Majumdar, K. C.; Rahaman, H.; Roy, B. Tetrahedron 2007, 63, 3919–3952. (h) Klapars, A.; Parris, S.; Anderson, K. W.; Buchwald, S. L. J. Am. Chem. Soc. 2004, 126, 3529–3533. (i) Molander, G. A. Acc. Chem. Res. 1998, 31, 603–609. (j) Shaw, R. W.; Gallagher, T. J. Chem. Soc. Perkin Trans. 1 1994, 3549–3555. (k) Begley, M. J.; Crombie, L.; Haigh, D.; Jones, R. C. F.; Osborne, S.; Webster, R. A. B. J. Chem. Soc., Perkin Trans. 1 1993, 2027–2046. (l) Evans, P. A.; Holmes, A. B. Tetrahedron 1991, 47, 9131–9166. (45) Illuminati, G.; Mandolini, L. Acc. Chem. Res. 1981, 14, 95–102. (46) (a) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1993, 12, 2618–2633. (b) Heeres, H. J.; Nijhoff, J.; Teuben, J. H. Organometallics 1993, 12, 2609–2617. (c) Heeres, H. J.; Teuben, J. H. Organometallics 1991, 10, 1980–1986. (d) Heeres, H. J.; Meetsma, A.; Teuben, J. H. Organometallics 1990, 9, 1508–1510. (e) Heeres, H. J.; Meetsma, A.; Teuben, J. H.; Rogers, R. D. Organometallics 1989, 8, 2637–2646. (f) Den Haan, K. H.; Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053– 2060.

Organometallics, Vol. 28, No. 8, 2009 2439 Table 5. Formation Enthalpies of Pentyne Isomers at 298 K

a

compd

∆fH° (kcal/mol)

2-pentyne 2,3-pentadiene 1,2-pentadiene 1-pentyne

30.79 ( 0.50a 31.77 ( 0.17b 33.58 ( 0.16b 34.47 ( 0.50a

Reference 48a. b Reference 48b.

Prototropic Alkyne Isomerization. Alkali-metal bases, including amides, are known to promote efficient alkyne prototropic isomerization.47 However, to the best of our knowledge, there have been no previous reports of lanthanidemediated alkyne isomerization. As seen from the equilibria of eqs 10 and 11, 2-alkynes are the most stable isomers, although terminal alkyne isomers also form, in agreement with documented thermodynamics (Table 5).48 The kinetic accessibility of terminal alkynes allows 4-pentynylamine formation from substrate 47, which then rapidly undergoes intramolecular HA/ cyclization. This rapid hydroamination removes the terminal alkyne species, displacing eq 9 to the right until all of 47 is consumed. As expected, intermediate aminoallene 52 also undergoes isomerization and again the terminal alkyne is intercepted by intramolecular hydroamination (eq 12). Substrates 46, 48, and 49, which are H-, Ph-, and SiMe3-substituted versions of 47, respectively, cannot undergo isomerization of the CtC unit to the 4-position, explaining why cyclization is not observed for these substrates. For aminoallene 53, prototropic isomerization proceeds more rapidly than subsequent intramolecular hydroamination (eq 13), as expected, considering that cyclohydroaminations to form seven-membered rings are sluggish.1i

Conclusions A series of bimetallic organolanthanide complexes has been synthesized and characterized. These complexes are competent catalysts for intramolecular aminoalkene, aminoalkyne, aminoallene, and conjugated aminodiene hydroamination/ cyclization. Nonbonded repulsive considerations appear to be overriding factors influencing reactivity trends in these catalytic reactions. The steric contribution of the adjacent metal-containing fragment also affords remarkable kinetic stability to these binuclear complexes against ligand redistribution, although it also depresses catalytic activity vs their mononuclear analogues. This trend persists with regard to catalytic intermolecular reactions, where the p-phenylenebridged catalysts exhibit reduced activity vs mononuclear catalysts, and the more sterically crowded m-phenylenebridged catalysts exhibit negligible activity. Long-chain, bifunctional, and short-chain substrates were examined for possible cooperative effects; however, insignificant activity enhancements and only modest selectivity effects are observed. The sea of Lewis basic amines that immerses the catalysts likely weakens cooperative binding. Organolanthanide amide complexes were discovered to promote (47) (a) Trofimov, B. A. Curr. Org. Chem. 2002, 6, 1121–1162. (b) Abrams, S. R.; Shaw, A. C. J. Org. Chem. 1987, 52, 1835–1839. (c) Abrams, S. R.; Nucciarone, D. D.; Steck, W. F. Can. J. Chem. 1983, 61, 1073– 1076. (d) Brown, C. A.; Yamashita, A. J. Am. Chem. Soc. 1975, 97, 891– 892. (e) Ben-Efraim, D. A. Tetrahedron 1973, 29, 4111–4125. (f) Carr, M. D.; Gan, L. H.; Reid, I. J. Chem. Soc., Perkin Trans. 2 1973, 672–676. (g) Carr, M. D.; Gan, L. H.; Reid, I. J. Chem. Soc., Perkin Trans. 2 1973, 668–672. (48) (a) Wagman, D. D.; Kilpatrick, J. E.; Pitzer, K. S.; Rossini, F. D. J. Res. Natl. Bur. Stand. 1945, 35, 467–496. (b) Fraser, F. M.; Prosen, E. J. J. Res. Natl. Bur. Stand. 1955, 54, 143–148.

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catalytic alkyne isomerization, enabling tandem isomerization/hydroamination processes, of which several were demonstrated.

Acknowledgment. We thank the NSF for funding this research (Grant No. CHE-0809589), Mr. C. J. Weiss for catalytic cross-coupling synthetic studies of ligand 3, Dr. B. D. Stubbert for generously providing substrates 46-49,

Yuen and Marks

Ms. S. L. Wegener for preparing substrate 45, and Ms. C. L. Stern for collecting single-crystal X-ray diffraction data. Supporting Information Available: A CIF file giving crystallographic data for complex 10. This material is available free of charge via the Internet at http://pubs.acs.org. OM9000023