Energetics and Mechanism of Organolanthanide-Mediated

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Organometallics 2004, 23, 4097-4104

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Energetics and Mechanism of Organolanthanide-Mediated Aminoalkene Hydroamination/Cyclization. A Density Functional Theory Analysis Alessandro Motta, Giuseppe Lanza, and Ignazio L. Fragala`* Dipartimento di Scienze Chimiche, Universita` di Catania, and INSTM, UdR Catania, Viale A. Doria 6, 95125 Catania, Italy

Tobin J. Marks* Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113 Received May 12, 2004

This contribution focuses on organolanthanide-mediated hydroamination processes and analyzes the hydroamination/cyclization of a prototypical aminoalkene, NH2(CH2)3CHdCH2, catalyzed by Cp2LaCH(TMS)2, using density functional theory. The reaction is found to occur in two steps, namely, cyclization to form La-C and C-N bonds, and subsequent La-C protonolysis. Calculations have been carried out for (i) insertion of the olefinic moiety into the La-N bond via a four-center transition state and (ii) protonolysis by a second substrate molecule. The cyclized amine then dissociates, thus restoring the active catalyst. DFT energy profiles have been determined for the turnover-limiting insertion of the 1-amidopent-4-ene CdC double bond into the La-NH- bond. DFT calculations of geometries and the stabilities of reactants, intermediates, and products have been analyzed. The picture that emerges involves concerted, rate-limiting, slightly endothermic insertion of the alkene fragment into the La-N(amido) bond via a highly organized, seven-membered chairlike cyclic transition state (∆Hqcalcd ) 11.3 kcal/mol, ∆Sqcalcd ) -14.6 cal/mol K). The resulting cyclopentylmethyl complex then undergoes exothermic protonolysis to yield an amine-amido complex, the resting state of the catalyst. Thermodynamic and kinetic estimates are in excellent agreement with experimental data. Introduction The use of electrophilic early transition metal and f-element complexes to effect synthetically useful organic transformations is rapidly becoming an important interfacial boundary between traditional organometallic and synthetic organic chemistry.1,2 Carbon-nitrogen bond-forming processes are of fundamental importance (1) For some leading review references, see: (a) Tsuji, J. Transition Metal Reagents and Catalysts. Innovations in Organic Synthesis; Wiley: New York, 2000. (b) Kobayashi, S., Ed. Lanthanides: Chemistry and Use in Organic Synthesis; Springer-Verlag: Berlin, 1999. (c) Imamoto, T. Lanthanides in Organic Synthesis; Academic Press: New York, 1994. (d) Hegedus, L. S., Ed. Comprehensive Organometallic Chemistry II. Transition Metal Reagents in Organic Synthesis; Pergamon: Oxford, 1995; Vol. 12. (e) Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic Molecules; University Science Books: Mill Valley, CA, 1994. (f) Molander, G. A. In The Chemistry of the Metal-Carbon Bond; Hartley, F. R., Ed.; John Wiley Ltd: London, 1989; Vol. 5, Chapter 8. (g) McMurry, J. E. Chem. Rev. 1989, 89, 15231524. (h) Buchwald, S. L.; Nielsen, R. B. Chem. Rev. 1988, 88, 10471058. (i) Collman, J. P.; Hegedus, L. S.; Norton J. R.; Finke R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; Part III. (j) Reez, M. T. Organo-lanthanum Reagents in Organic Synthesis; Spinger-Verlag: Berlin, 1986. (k) Kagan, H. B. In Fundamental and Technological Aspects of Organo-f-Elements Chemistry; Marks, T. J., Fragala`, I. L., Eds.; Reidel: Dordrecht, The Netherlands, 1985; Chapter 2. (2) (a) Bytschkov, I.; Doye, S. Eur. J. Org. Chem. 2003, 6, 935-946. (b) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003, 32 (2), 104-114. (c) Eayad, J.; Tillack, A.; Hartung, C. G.; Beller, M. Adv. Synth. Catal. 2002, 344 (8), 795-813.

in organic chemistry, and hydroamination (catalytic N-H bond addition to an unsatured carbon-carbon multiple bond) represents both a challenging and highly desirable, atom-efficient transformation3-5 for the synthesis of nitrogen-containing molecules. Furthermore, hydroamination, mediated by organolanthanides, offers an attractive component of multistep routes to heterocyclic systems bearing key frameworks present in naturally occurring alkaloids.6 A large database has recently emerged on catalytic hydroamination of internal olefins activated by neighboring functional groups.7 (3) For general references see: (a) March, J. Adv. Org. Chem., 4th ed.; J. Wiley & Sons: New York, 1992; pp 768-770, and references therein. (b) Brunet, J.-J.; Neibecker, D.; Niedercorn, F. J. Mol. Catal. 1989, 49, 235-259. (c) Collman, J. P.; Hegedus. L. S.; Norton, J. R.; Finke. R. G. Principles and Application of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; Chapters 7.1-17.1. (d) Trost, B. M.; Verhoeven, T. R. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Vol. 8, pp 892-895, and references therein. (e) Gibson, M. S. In The Chemistry of the Amino Group; Patai, S., Ed.; Interscience: New York, 1968; pp 61-65. (4) For leading reviews of amine addition to olefins and alkynes, see: (a) Hegedus, L. S. Angew. Chem., Int. Ed. Engl. 1988, 27, 11131126. (b) Gase, M. B.; Latties, A.; Perie, J. J. Tetrahedron 1983, 39, 703-731. (c) Backvall, J.-E. Acc. Chem. Res. 1983, 16, 335-342. (d) Ja¨ger, V.; Viehe, H. G. Houben-Weyl, Methoden der Organischen Chemie; Thieme Verlag: Stuttgart, Germany, 1977; Vol. 5/2a, pp 713724. (e) Suminov, S. I.; Kost, A. N. Russ. Chem. Rev. 1969, 38, 884899. (f) Checulaeva, I. A.; Kondrat’eva, L. V. Russ. Chem. Rev. 1965, 34, 669-680.

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Thus, Marks et al. have implemented C1- and C2symmetric organolanthanide catalysts in the enantioselective and diastereoselective hydroamination/cyclization of aminoalkenes and aminodienes.8 Togni et al.9 developed an enantioselective version of a known Ircatalyzed hydroamination reaction,10 while Hartwig et al. investigated the Pd-catalyzed hydroamination of 1,3dienes by anilines.7d Olefin hydroamination, which in a formal sense consists of the addition of a N-H bond across a CdC bond, is a transformation of seemingly fundamental simplicity and atom economy (eq 1) and would appear to offer an attractive route to numerous classes of organonitrogen molecules.

This chemistry is facilitated by unique features of organolanthanide complexes:11-15 (i) highly electrophilic, (5) (a) Funke, F.; Steinbrenner, U.; Boehling, R.; Rudolf, P.; Breuer, K.; Hibst, H. Method and catalysts for producing isopropylamine. PCT Int. Appl. 2003. (b) Steinbrenner, U.; Funke, F.; Boehling, R. Method and device for producing ethylamines and butylamines. PCT Int. Appl. 2003. (c) Boehling, R.; Steinbrenner, U.; Funke, F.; Dier, R. Method for producing amines by means of olefin hydroamination in the presence of unsaturated nitrogen compounds. PCT Int. Appl. 2003. (d) Funke, F.; Steinbrenner, U.; Boehling, R. Method for producing dialkylethylamines from dialkylamines and ethylene in the presence of a sodium dialkylamide hydroamination catalyst. PCT Int. Appl. 2003. (e) Boehling, R.; Steinbrenner, U.; Funke, F. Method for the production of n-butyraldehyde and 1-butanol from 1,3-butadienecontaining hydrocarbon streams via hydroamination, isomerization and hydrolysis. PCT Int. Appl. 2003. (6) (a) Arredondo, V. M.; Tian, S.; McDonald, F. E.; Marks, T. J. J. Am. Chem. Soc. 1999, 121, 3633-3639. (b) Ryu, J.-S.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 12584-12605. (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, 1587815892. (e) Ryu, J.-S.; Marks, T. J.; McDonald, F. E. J. Org. Chem. 2004, 69, 1038-1052. (7) (a) Beller, M.; Breindl, C.; Eichberger, M.; Hartung, C. G.; Seayad, J.; Thiel, O. R.; Tillack, A.; Trauthwein, H. Synlett 2002, 10, 1579-1594, and references therein. (b) Pawlas, J.; Nakao, Y.; Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 3669-3679. (c) Hong, S.; Marks, T. J. J. Am. Chem. Soc. 2002, 124, 7886. (d) Lo¨ber, O.; Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 43664367. (e) Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.; Yoshifuji, M. Angew. Chem., Int. Ed. 2001, 40, 4501-4503. (f) Nettekoven, U.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 1166-1167. (g) Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 95469547. (h) Beller, M.; Thiel, O. R.; Trauthwein, H.; Hartung, C. G. Chem. Eur. J. 2000, 6, 2513-2522. (i) Bozel, J. J.; Hegedus, L. S. J. Org. Chem. 1981, 46, 2561-2563. (l) Kawatsura, M.; Hartwig, J. F. Organometallics 2001, 20, 1960-1964. (m) Fadini, L.; Togni, A. Chem. Commun. 2003, 30-31. (8) (a) Giardello, M. A.; Conticello, V. P.; Brard, L.; Sabat, M.; Rheingold, A. L.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10212-10240. (b) Giardello, M. A.; Conticello, V. P.; Brard, L.; Gagne´ M. R.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241-10254. (c) Gagne´ M. R.; Brard, L.; Conticello, V. P.; Giardello, M. A.; Stern, C. L.; Marks, T. J. Organometallics 1992, 11, 2003-2005. (d) Douglass, M. R.; Ogasawara, M.; Hong, S.; Metz, M. V.; Marks, T. J. Organometallics 2002, 21, 283-292. (e) Hong, S.; Tian, S.; Metz, M. V.; Marks, T. J. J. Am. Chem. Soc., 2003, 125, 14768-14783. (f) Hong, S.; Marks, T. J. Accts. Chem. Res., published online Jun. 29, 2004 http://dx.doi.org/ 10.1021/ar040051r. (9) (a) Dorta, R.; Egli, P.; Zu¨rcher, F.; Togni, A. J. Am. Chem. Soc. 1997, 119, 10857-10858. (b) Bieler, N.; Egli, P.; Dorta, R.; Togni, A.; Eyer, M. (Lonza A.G.) EP 0 909 762A2, 1999 [Chem. Abstr. 1999, 130, 305622]. (10) (a) Casalnuovo, A. L.; Calabrese, J. C.; Milsein, D. J. Am. Chem. Soc. 1988, 110, 6738-6744. (b) Aizenberg, M.; Milstein, D.; Tulip, T. H. Organometallics 1996, 15, 4093-4095. (11) Hydrogenation: (a) Obora, Y.; Otha, T.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 3745-3755. (b) Roesky, P. W.; Denninger, U.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 4486-4492. (c) Haar, C M.; Sern, C. L.; Marks, T. J. Organometallics 1996, 15, 1765-1784.

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kinetically labile f-element centers which are compatible with a variety of nondissociable ancillary ligands, (ii) a generally single, thermodynamically stable oxidation state (Ln3+), thus avoiding the complications of competing oxidative addition/reductive elimination processes, (iii) large coordination numbers (8-12) for which the metal ions are in most cases coordinatively unsaturated, (iv) 4fn orbitals shielded by filled 5s2 5p6 orbitals, which renders the chemistry of lanthanides ionic and governed more by electrostatic and steric factors than by orbitalfilling interactions. Organolanthanide complexes of the type Cp′2LnR (Cp′ ) η5-Me5C5; R ) H, CH(TMS)2; Ln ) La, Nd, Sm, Y, Lu) have been shown to be highly reactive with respect to hydroamination/cyclization transformations involving aminoalkenes, aminoalkynes, aminoallenes, and aminodienes,6,8,14b,16-19 and in all cases, the formation of the catalytically-active lanthanide amido species is found to occur via rapid, quantitative proton transfer from the substrate to the hydrocarbyl ligand16,20 (eq 2; Scheme 1, step i):

Cp′2LnCH(TMS)2 + H2NR f Cp′2LnNHR + CH2(TMS)2 (2) Furthermore, experimental kinetic and mechanistic data6,8,16-19 strongly argue that catalytic hydroamination/cyclization of amino-olefins by organolanthanide complexes involves the turnover-limiting insertion of olefinic functionalities into Ln-N bonds within the framework of a bis(cyclopentadienyl)lanthanide envi(12) Hydrosilylation: (a) Molander, G. A.; Corrette, C. P. Organometallics 1998, 17, 5504-5512. (b) Schumann, H.; Keitsch, M. R.; Winterfeld, J.; Muhle, S.; Molander, G. A. J. Organomet. Chem. 1998, 559, 181-190. (c) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 7157-7168. (d) Molander, G. A.; Julius, M. J. Org. Chem. 1992, 57, 6347-6351. (e) Sakakura, T.; Lautenschlager, H.; Tanaka, M. J. Chem. Soc., Chem. Commun. 1991, 40-41. (13) Hydroboration: (a) Molander, G. A.; Pfeiffer, D. Org. Lett. 2001, 3, 361-363. (b) Bijpost, E. A.; Duchateau, R.; Teuben, J. H. J. Mol. Catal. 1995, 95, 121-128. (c) Harrison, K. N.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 9220-9221. (14) Hydrophosphination: (a) Douglass, M. R.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 1824-1825. (b) Douglass, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 2001, 123, 10221-10238. (c) Kawaoka, A. M.; Douglass, M. R.; Marks, T. J. Organometallics 2003, 22, 40304032. (15) For recent organolanthanide reviews, see: (a) Aspinall, H. C. Chem. Rev. 2002, 102, 1807-1850. (b) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem. Rev. 2002, 102, 1851-1896. (c) Arndt, S.; Okuda, J. Chem. Rev. 2002, 102, 1953-1976. (d) Molander, G. A.; Romero, A. C. Chem. Rev. 2002, 102, 2161-2186. (e) Shibasaki, M.; Yoshikawa, N. Chem. Rev. 2002, 102, 2187-2210. (f) Inanaga, J.; Furuno, H.; Hayano, T. Chem. Rev. 2002, 102, 2211-2226. (g) Molander, G. A. Chemtracts: Org. Chem. 1998, 18, 237-263. (h) Edelmann, F. T. Top. Curr. Chem. 1996, 179, 247-276. (i) Edelmann, F. T. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 4, Chapter 2. (j) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. Rev. 1995, 95, 865-986. (k) Schaverien, C. J. Adv. Organomet. Chem. 1994, 36, 283-362. (l) Evans, W. J. Adv. Organomet. Chem. 1985, 24, 131-177. (m) Marks, T. J.; Ernst, R. D. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Chapter 21. (16) Gagne´, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275-294. (17) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 9295-9306. (18) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1757-1771. (19) (a) Hong, S.; Kawaoka, A. M.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 15878-15892. (b) Ryu, J. S.; Li, G. Y.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 12584. (20) (a) Piers, W. E.; Shapiro P. J.; Bunel, E. E.; Bercaw, J. E. Synlett 1990, 74-84. (b) Doherty, N. M.; Bercaw, J. E. J. Am. Chem. Soc. 1985, 107, 2670-2682. (c) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Berkaw, J. E. J. Am. Chem. Soc. 1987, 109, 203-219.

Aminoalkene Hydroamination/Cyclization Scheme 1. Proposed Catalytic Cycle for Organolanthanide-Catalyzed Cyclohydroamination of Aminoalkenes

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a minimal energy reaction coordinate to compare with experimental kinetic and thermodynamic data,14,16-18 as well as to highlight the role of interactions among ligand, reactant, and product species within the lanthanide coordination sphere. Computational Details

ronment (Scheme 1, step ii), coupled with subsequent rapid Ln-C protonolysis to effect efficient catalytic N-C bond-forming processes (Scheme 1, step iii). The present study represents the first theoretical analysis of the salient mechanistic aspects associated with amino-olefin hydroamination/cyclization mediated by Cp′2LnR complexes (Cp′ ) η5-Me5C5; R ) H, CH(TMS)2; Ln ) La, Nd, Sm, Y, Lu). Here (C5H5)2LaCH(TMS)2 has been adopted as a model catalyst, while the substrate is represented by 1-aminopent-4ene (CH2dCH(CH2)3NH2). The substrate undergoes regiospecific cyclization to form a five-membered azacylic ring with the absence of regioisomers avoiding additional complexities in the present analysis associated with either regio- or stereoisomers.6,16,21 Experimental data,16 in addition, show that the reaction proceeds equally well in pentane, toluene, benzene, and related solvents, while in donor solvents such THF, catalytic rates are significantly depressed.16 It therefore transpires that noncoordinating solvents do not significantly affect the reaction. In the present study, the substrate molecule involved in the protonolytic step (step iii) following cyclization has been modeled with the simpler methylamine (CH3-NH2) molecule. There is in fact evidence (vide infra) that the effects on the protonolysis due to the nonproximate CH2dCH- unsaturation represent only minor energetic perturbations. The geometries of the intermediates as well as of the transition states have been evaluated here to define (21) (a) Roesky, P. W.; Mueller, T. E. Angew. Chem., Int. Ed. 2003, 42 (24), 2708-2710. (b) O’Shaughnessy, P. N.; Knight, P. D.; Morton, C.; Gillespie, K. M.; Scott, P. Chem. Commun. 2003, 14, 1770-1771. (c) Hong, S.; Ryu, Jae-Sang; Tian, S.; Metz, M. V.; Marks, T. J. New C2-symmetric chiral amido-lanthanide catalysts for enantioselective hydroamination. Abstracts of Papers, 24th ACS National Meeting, Boston, MA, August 18-22, 2002. (d) Gribkov, D. V.; Hultzsch, K. C. Chem. Commun. 2004, 730-731.

Calculations were performed adopting the B3LYP formalism. The effective core potential (ECP) of Hay and Wadt,22 which explicitly treats 5s and 5p electrons and a basis set contracted as [3s, 3p, 2d, 1f], was used for the lanthanum atom. The standard all-electron 6-31G** basis was used for the remaining atoms.23 Polarization functions are required in such calculations to obtain correct results. Molecular geometry optimization of stationary points used analytical gradient techniques. The transition state was searched with the synchronous, transit-guided quasi-Newton method.24 IRC calculations were performed to follow the reaction path around the transition states both in the cyclization step and in the protonolysis step. Frequency analyses were performed to obtain thermochemical information about the reaction pathways at 298 K. The force constants were determined analytically. Solvent effects were modeled using the polarized continuum (overlapping spheres) formalism (PCM) of Tomasi and co-workers.25 The PCM method models the solvent as a continuum of uniform dielectric constant, and the solute is placed into a cavity within the solvent. The cavity is constructed by placing a sphere around each solute heavy atom. Hydrogen atoms are always enclosed within the sphere of the atom to which they are bonded. For the atomic radii, the Bondi approximation was used. In this method, the effects of solvation are folded into the iterative SCF procedure. The dielectric constants of the solvents investigated are as follows: C6H6, 2.247; C6H5CH3, 2.379; heptane, 1.92. All calculations were performed using G9826 codes on IBM-SP and Origin 3000 systems.

Results and Discussion In this section, the geometries of the precatalyst and of the other species along the catalytic reaction coordinate are first analyzed. Next, the related energetics are evaluated in a similar manner. The entire catalytic reaction has been partitioned into the component steps, namely, catalyst generation, cyclization, and, finally, the protonolysis. These are analyzed in sequence. We begin by discussing the lanthanocene hydrocarbyl precatalyst and its protonolytic activation by the aminoalkene (22) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (23) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (b) Franel, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (24) Peng, C.; Schlegel, H. B. Combining Synchronous Transit and Quasi-Newton Methods for Finding Transition States. Isr. J. Chem. 1993, 33, 449-454. (25) (a) Miertus, S.; Tomasi, J. Chem. Phys. 1982, 65, 239. (b) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117. (c) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett. 1996, 255, 327. (d) Cances, M. T.; Mennucci, V.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (e) Barone, V.; Cossi, M.; Tomasi, J. J. Comput. Chem. 1998, 19, 404. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challcombe, M.; Peng, C. J.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. GAUSSIAN-98; Gaussian Inc.: Pittsburgh, PA, 1995.

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Figure 1. Optimized molecular structure of the model precatalyst (C5H5)2LaCH(TMS)2.

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Figure 2. Optimized molecular structures of the activated catalyst (C5H5)2LaNH(CH2)3CHdCH2.

Scheme 2. Catalyst Activation

substrate to form an amidoalkene complex, which we consider to be the active catalytic species. Catalyst Generation. Precatalyst. The geometry of the model precatalyst, (C5H5)2LaCH(TMS)2, adopts a pseudo-trigonal arrangement around the La3+ ion. The La-C(σ) bond lies outside the [Cp(centroid)]2La plane. This distortion together with the computed bond angles and bond lengths associated with the La-bonded TMS groups (La-C(1)-Si(1), La-C(1)-Si(2), La-H(1), LaH(2) in Figure 1) indicate a surprising deviation from Cs symmetry, thus suggesting agostic interaction involving the Si-C(1) bond. The evidence of agostic interaction (La-H(1) ) 2.82 Å) is in accordance with XRD data on a related Nd catalyst27 and is a direct consequence of the electron-deficient character of the metal center. Experimental data16,28 indicate that the catalyst activation process (Scheme 1, step i; eq 2) is rapid and exothermic (∆Hcalc ≈ -20 kcal/mol).28 It will not be discussed in detail since the mechanism involves a protonolysis similar to the second step of the catalytic cycle, which will be analyzed in detail below. Cyclization Step. It will be seen that coordination of the aminoalkene to the lanthanocene center activates the catalyst and precedes the olefin hydroamination/ cyclization process (Scheme 1, step ii). Activated Catalyst. Local energy minima have been located for two stable conformations of the amidoalkene complex resulting from initial precursor protonolysis (Figure 2). Both have a pseudo-trigonal geometry with the La-N vector lying out of the Cp-La-Cp plane, (27) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schuman, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091-8103. (28) Metal-ligand bond enthalpies from: (a) Nolan, S. P.; Stern, D.; Hedden, D.; Marks, T. J. ACS Symp. Ser. 1990, 428, 159-174. (b) Nolan, S. P.; Stern, D.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 7844-7853. (c) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110, 7701-7715. (d) Bruno, J. W.; Marks, T. J.; Morss, L. R. J. Am. Chem. Soc. 1983, 105, 6824-6832.

Figure 3. Optimized molecular structure of the hydroamination/cyclization transition state.

similar to the precatalyst geometry (Figure 1). Conformation B is more stable than A by 3.5 kcal/mol due to the chelating effect associated with ligand-to-metal donation from the electron-rich CdC bond. This donation finds counterpart in a sizable computed lengthening of the CdC distance (0.02 Å) relative to A. In addition, the La-N bond distance in B is slightly elongated versus that in less coordinatively saturated A. Cyclization Transition State. In the transition state, addition of the CdC double bond across the La-N bond occurs. Here the four-membered La-N(1)-C(2)C(1) metallacycle (Figure 3) exhibits an 8.1° folding angle (Table 1), which in this conformation allows the La-N(1) and the CdC bonds to be eclipsed. In this context, it is also of interest to consider the transition state geometry with respect to those of the corresponding activated catalyst and cyclization product (vide infra). There is evidence that the cyclization produces an increase in the lengths of La-N(1) and C(1)-C(2) (since the amido coordination evolves into amino coordination and the double bond evolves into a single bond) and a decrease in the N(1)-C(2) distance (due to the bond-forming process of the cyclization). Data in Table 1 show that the lengths of the La-N(1), C(1)-C(2), and N(1)-C(2) bonds in the transition state are approximately intermediate between those of reagents and products, while the La-C(1) distance remains closer to that in the product. IRC calculations of 12 points around the cyclization transition state provide quantitative details of such variations, and the data in Table S14

Aminoalkene Hydroamination/Cyclization

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Table 1. DFT-Derived Bond Lengths (Å) and Bond Angles (deg) of Structures Involved in the La-NH(CH2)3CHdCH2 Cyclization Stepa activated catalyst B

cyclization transition state

cyclization product C

Bond Lengths (Å) La-Cpcentr La-N(1) La-C(1) La-C(2) N(1)-C(2) C(1)-C(2)

2.64 2.34 3.08 3.22 2.98 1.35

2.64 2.51 2.68 3.09 2.00 1.43

2.64 2.71 2.62 3.16 1.54 1.53

Bond Angles (deg) Cp-La-Cp

130.0

133.2

133.0

144.9 -8.1

150.8 -19.1

Torsion Angles Cp-La-Cp-N(1) La-N(1)-C(2)-C(1) a

142.4 -5.6

The other geometrical parameters are no longer affected.

Figure 4. Energy profile for the N(1)-C(2) bond-forming process of the cyclization step.

show that the La-N(1) and C(1)-C(2) distances increase 0.03 and 0.05 Å, respectively, while the N(1)C(2) distance decreases 0.23 Å, thus indicating that the major evolution in this step is the ring closure. The relative trend of energies associated with N(1)-C(2) bond forming is depicted in Figure 4. Cyclization Product. The cyclization product is found to assume either of two conformations differentiated by coordinative details of the N-amino atom to the lanthanide ion (Figure 5). N-Coordinated structure C is more stable by 16.4 kcal/mol than structure D and represents the initial direct insertion product. The optimized structures C and D are depicted in Figure 5. The most significant geometrical parameters of structure C are compiled in Table 1. There is a profound coordinative reorganization about the La3+ ion as the cyclization reaction coordinate is traversed. First, there is a progressive and pronounced elongation of the La-N(1) bond, reflecting the evolution from a strong amido La-N bond to a formal LarN dative amino bond. A similar progressive shortening of the N(1)-C(2) distance is observed due to the direct olefin insertion process and subsequent formation of the aminocyclopentane σ-bonded product. Energetics of the Cyclization Process. The calculated energetic profile along the cyclization reaction coordinate is shown in Figure 6. Relevant data are compiled in Table 2 and compared with related experimental literature data.14b,16-18 It can be seen that the activated catalyst has a configuration stabilized by donation to the La-N(1) bond by the electron-rich Cd C double bond, which lies almost eclipsed with the LaN(1) bond in a chairlike seven-membered cyclic transi-

Figure 5. Optimized molecular structures of the hydroamination/cyclization products. In structure C, the hydrogen atom on N(1) is obscured from view by the nitrogen atom.

tion state. This basic type of transition state has been invoked in a number of experimental studies to explain the stereochemical course of various organolanthanide-mediated hydroelementation/cyclization processes.7c,8,14,16,19 This arrangement anticipates the cyclization process, which, in turn, is clearly driven by the overlap of the nonbonded N 2p lone pair with a π* Cd C orbital. The activation barrier to insertion is computed to be 11.9 kcal/mol, in excellent agreement with the experimental value for the cyclization process of ∆Hq ) 12.7 ( 1.4 kcal/mol.16 The energy of the cyclization product lies close to that of the activated catalyst, and therefore the process is computed to be thermoneutral, in accordance with experimental bond enthalpy estimates.16,28 The calculated energetic pathway in different solvents along the cyclization reaction coordinate is reported in Table 3. The relative energies are referenced to structure B of the activated catalyst. The data show that in benzene, toluene, and heptane the cyclization reaction steps involve energies rather similar to those in a vacuum. This is consistent with experimental data showing minimal solvent effects on rate and stereochemistry, except in coordinating solvents. Protonolysis Step. Following cyclization, the protonolysis process regenerates the catalyst and releases the cycloamine product. This process has been modeled

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Figure 6. Energetic profile for the hydroamination/cyclization pathway in kcal/mol. The energies of the isolated reactants are always assumed as references for the energy scale. Table 2. Relative SCF Energies, Enthalpies, Gibbs Free Energies, and Entropies for the Entire Cyclization/Hydroamination Process of CH2dCH(CH2)3NH2 Catalyzed by the (C5H5)2LaCH(TMS)2 System

a

∆E (kcal/mol)

∆H (kcal/mol)

∆G (kcal/mol)

∆S (cal/mol K)

activated catalyst conf. A activated catalyst conf. B cyclization transition statea

0.0 -3.5 8.4

cyclization product amino complex protonolysis transition state amino-amido complex final product

1.6 -17.0 -4.2 -32.3 -13.8

0.0 -3.6 ∆Hq ) 11.3 (12.7 (1.4)) 2.5 (0.0) -14.7 -5.3 -29.9 -13.0 (-13.0)

0.0 -0.4 ∆Gq ) 12.5 (13.4 (1.5)) 6.4 0.0 10.9 -15.8 -11.5

0.0 -10.9 ∆Sq ) -14.6 (-27 (5.0)) -12.9 -49.3 -54.2 -47.2 -5.2

Values in parentheses refer to experimental data.16,28

Table 3. Relative SCF Energies (kcal/mol) in the Hydroamination/Cyclization Reaction Pathway for Different Solvents toluene benzene heptane vacuum ( ) 2.379) ( ) 2.247) ( ) 1.92) activated catalyst B 0.0 cyclization TS 11.9 cyclization product 5.1 amino complex -13.5 protonolysis TS -0.7 amino-amido complex -28.8 final product -10.3 a

0.0 12.0 4.3 -12.5 1.3 -27.2 -11.3

0.0 12.0 N.C. -15.2 -1.5 -30.0 -13.9

0.0 N.C.a N.C. -12.8 0.8 -27.6 -11.0

N.C. ) not converged.

by adopting the simpler methylamine CH3-NH2 as the second substrate. Protonolysis requires proton exchange between the -NH2 group and the Ln-bonded hydrocarbyl and creates a Ln-amido complex stabilized by the interaction with the amine lone pair. The cyclized amine then moves away, regenerating the active catalyst (Figure 7). In the amine-hydrocarbyl complex (Figure 7a), the La3+ coordination of the CH3NH2 molecule induces a sizable lengthening of the La-Cp (centroid) bond (+0.03 Å), while the La-N(1) vector folds out of the Cp-La-Cp plane by 64.3° (Table 4). Both N(1) and N(2) atoms are then at comparable distances from the

Figure 7. Optimized molecular structures of the aminehydrocarbyl complex, protonolysis state, and amine-amido product.

Ln center, as expected for comparable dative bonding interactions. Protonolysis Transition State. In the transition state (Figure 7b), the proton transfer from N(2) (CH3NH2) to La-C(1) of the cyclic product is synchronized with the approach of CH3NH2 to the electrophilic Ln metal

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Figure 8. Correlation between the N(2)-H(1) distance and H(1)-C(1) distance in the protonolysis step. Table 4. DFT Bond Lengths (Å) and Bond Angles (deg) of Structures Involved in the Protonolysis Step aminecoordinated protonolysis amine-amido complex transition state complex La-Cpcentr La-N(1) La-N(2) La-C(1) N(2)-H(1) C(1)-H(1) Cp-La-Cp N(1)-La-N(2) Cp-La-Cp-N(1)

Bond Lengths 2.67 2.71 2.76 2.65 1.02 3.01

2.64 2.71 2.53 2.95 1.31 1.47

2.64 2.80 2.35 3.89 2.80 1.09

Bond Angles 133.3 128.5 130.3 111.4

131.9 98.7

Torsion Angles 64.3 64.0

62.3

center. Along the vector associated with the proton transfer in N(2)- - - H(1)- - - C(1) (Figure 7b), the N(2)H(1) distance is 0.29 Å longer than in the methylamine complex, and similarly, the evolving H(1)-C(1) length is 0.38 Å longer than in the amine-amido protonolysis product. Both elongations are comparable, as expected for a concerted bond-breaking/forming process. IRC calculations of 12 points around the protonolysis transition state are tuned well with the dynamics of the process and provide quantitative details of such variations. Thus, quantitative data (Table S15) show that the elongation of the N(2)-H(1) bond (0.54 Å) is comparable with the decrease of the C(1)-H(1) distance (0.58 Å). Moreover, there is a linear correlation (Figure 8) between changes associated with the N(2)H(1) distance and C(1)-H(1) length, and hence, there is evidence of a concerted bond-breaking/forming process. Amine-Amido Complex. The protonolysis kinetic product consists of a pseudo-tetrahedral Ln architecture in which the Cp2Ln moiety has amido coordination derived from the N(2) methylamine protonolytic agent as well as a dative contact with N(1) of the cyclic amine. The new Ln-N(2) bond is thus comparable to Ln-N(1) in the initial activated catalyst (Figure 2), while the LnN(1) distance in the amine-amido kinetic product lies close to that in the inserted amine complex (Figure 7a,c). Note that the direct amido bond is 0.45 Å shorter than that of the weaker “dative” amine contact (Table 4). Of course, regenerating the catalyst involves release of the cyclic amine final product, which is thermodynamically more stable than the aminoalkene precursor (Figure 7d).

Figure 9. Thermodynamic profiles of the aminoalkene hydroamination/cyclization pathway. The energies of isolated reagents are always assumed as references for the energy scale.

Energetics of the Protonolysis Process. The calculated energetic profile along the cyclization pathway is shown in Figure 6. Relevant data are compiled in Table 2 and compared with available experimental literature data.28 There is evidence that protonolysis does not affect the overall kinetics since all energy values associated with protonolysis lie lower than those of the reagents. In fact, the methylamine coordination brings about a 17.0 kcal/mol stabilization, closely comparable to the energetic stabilization associated with the cyclic conformer of the cyclization product (Figures 5, 6), similarly due to the amine lone-pair coordination. The protonolysis transition state lies 4.2 kcal/mol lower than the energy of the reagents and, therefore, does not affect the overall kinetics despite the 12.8 kcal/mol energetic barrier relative to the amino complex (Figure 6). The kinetic product is an amine-amido complex. It is 28.8 kcal/mol more stable than the activated catalyst (structure B). The dissociation of the final aminocyclopentane product and regeneration of the methylamido complex incurs a final energetic stabilization of -13.8 kcal/mol, closely comparable to the experimental value for 1-aminopent-4-ene cyclization (∆H ≈ -13 kcal/ mol).16,28 The calculated energies, corrected for solvent effects, for the protonolysis step are reported in Table 3. Similar to the cyclization step, the solvent does not substantially affect the reaction profile. The data in Table 3 evidence only a minor stabilization (3.6 kcal/ mol) of the final product in benzene with respect to vacuum. The other energetic parameters are only marginally influenced (∼1 kcal/mol) by solvent effects. Thermodynamics of the Hydroamination/Cyclization Reaction. Thermodynamic state functions (enthalpy, Gibbs free energy, and entropy) associated with the stationary states as well as with the transition states of the hydroamination/cyclization process are compiled in Table 2 and displayed in Figure 9. It can be seen that the enthalpy profile is entirely superimposable on that of the SCF energy profile discussed in the previous section. In contrast, the Gibbs free energy (∆G) profile differs markedly due to entropic contributions (-T∆S). In fact, the cyclization step incurs a large, monotonic decline in entropy (Table 2) due to the progressive loss of degrees of freedom as the transition state is approached. The modest ∆Hq value (11.3 kcal/ mol) suggests a concerted transition state, with signifi-

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cant bond formation to compensate for the bond breaking (vide supra). The large, negative ∆Sq value (-14.6 cal/mol K) is therefore consistent with a highly organized, polar transition state in which a significant loss of internal rotational degrees of freedom occurs. Of course, this is due to the formation of the N(1)-C(2) bond, which promotes cycle formation and is in excellent agreement with experiment (Table 2).16,19 Even greater in magnitude is the decrease in entropy associated with the formation of the amino complex coupled with the protonolysis step. Now the entropic change is due to the bimolecular association to form a single complex (two particles f one). Finally, the release of products and regeneration of the catalyst involves a sizable entropic gain. This entropic contribution significantly affects the ∆G trend, with the consequence being that related values shift to higher energies (hence to less favored processes) relative to the corresponding potential energies. These results implicate an amine-amido complex as the resting state of the catalyst, in good agreement with experiment.7c,8,14,16,19 Concluding Remarks Catalyst generation and hydroamination/cyclization processes at an electrophilic biscyclopentadienyl-La site have been investigated using DFT and analytical gradient methods to understand catalytic reaction pathways

Motta et al.

and related reaction energetics. It is found that the cyclization step is essentially thermoneutral, while the subsequent concerted protonolysis step (concerted bondbreaking/forming process) is exothermic (∆H ) -13.0 kcal/mol), in complete accord with experimental data. On kinetics grounds it is therefore evident that the olefin insertion process is the turnover-limiting step, with a computed activation reaction barrier of +11.9 kcal/mol, in excellent agreement with experiment. Noncoordinating solvents such as benzene, toluene, and heptane do not significantly affect the reaction pathway. Future studies will focus on the nature of organolanthanide-mediated hydroelementation pathways as a function of C-C unsaturation, lanthanide, and heteroelement. Acknowledgment. This research was supported by the Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (MURST Rome), the Consiglio Nazionale delle Ricerche (Rome), and by the U.S. National Science Foundation (grants CHE-0078998 and CHE-0415407). Supporting Information Available: A complete list of Cartesian coordinates of all structures and data from IRC calculations presently analyzed. This material is available free of charge via the Internet at http://pubs.acs.org. OM049666I