Organolanthanide-catalyzed imine hydrogenation. Scope, selectivity, mechanistic observations, and unusual byproducts

Article abstract of DOI:10.1021/ja963775+

In this paper we report the Cp'₂LnMe₂SiCp''₂Ln-catalyzed (Cp' = η⁵-Me₅C₅; Cp'' = η⁵-Me₄C₅) hydrogenation of acyclic imines to yield the corresponding amines. At 190 psi of H₂, the observed turnover frequencies (h^-1) (100:1 substrate:catalyst ratio, Cp'₂Ln, temperature (°C)) are (1) (N-benzylidene(methyl)amine, Ln = La, 50) 0.03; (Ln = Sm, 90) 1.0; (Ln = Sm + PhSiH₃, 90) 2.2; (Ln = Lu, 90) 0.60; (2) (N-benzylideneaniline, Ln = Sm, 90) 0.10; (3) (N-benzylidene(trimethylsilyl)amine, Ln = Sm, 90) 0.40; (4) (N-(α-methylbenzylidene)(methyl)amine, Ln = Sm, 90) 0.20; (5) (N-(α-methylbenzylidene)(benzyl)amine, Ln = Sm, 90) 0.70. The stoichiometric reaction of N-benzylidene(methyl)amine with Cp'₂SmCH(SiMe₃)₂ or (Cp'₂SmH)₂ yields an orthometalated Cp'₂Sm-substrate complex which undergoes either hydrogenolysis/hydrogenation or competing C=N insertion of a second substrate molecule to yield a Cp'₂Sm-imine-amido complex with a seven-membered chelate ring. The stoichiometric reaction of 2-methyl-1-pyrroline with Cp'₂SmCH(SiMe₃)₂ or (Cp'₂SmH)₂ yields a Cp'₂Sm-imine-amido complex in which two substrate molecules have been coupled to form a six-membered chelate ring (characterized by X-ray diffraction). The stoichiometric reaction of N-benzylidene(trimethylsilyl)amine with (Cp'₂SmH)₂ yields a desilylated Cp'₂Sm-imine-amido complex with a four-membered Sm(NSiMe₃)(CPh)N=CHPh chelate ring (characterized by X-ray diffraction). Additional heating of this product under H₂ yields S₆-symmetric (Cp'₂SmCN)₆, which contains an unusual chairlike 18-membered (SmCN)₆ ring (characterized by X-ray diffraction).

Full text of DOI:10.1021/ja963775+

J. Am. Chem. Soc. 1997, 119, 3745-3755
3745
Organolanthanide-Catalyzed Imine Hydrogenation. Scope,
Selectivity, Mechanistic Observations, and Unusual Byproducts
Yasushi Obora, Tetsuo Ohta, Charlotte L. Stern, and Tobin J. Marks*
Contribution from the Department of Chemistry, Northwestern UniVersity,
EVanston, Illinois 60208-3113
ReceiVed October 30, 1996X
Abstract: In this paper we report the Cp′2Ln/Me2SiCp′′2Ln-catalyzed (Cp′ ) η5-Me5C5; Cp′′ ) η5-Me4C5)
hydrogenation of acyclic imines to yield the corresponding amines. At 190 psi of H2, the observed turnover frequencies
(h-1) (100:1 substrate:catalyst ratio, Cp′2Ln, temperature (°C)) are (1) (N-benzylidene(methyl)amine, Ln ) La, 50)
0.03; (Ln ) Sm, 90) 1.0; (Ln ) Sm + PhSiH3, 90) 2.2; (Ln ) Lu, 90) 0.60; (2) (N-benzylideneaniline, Ln ) Sm,
90) 0.10; (3) (N-benzylidene(trimethylsilyl)amine, Ln ) Sm, 90) 0.40; (4) (N-(R-methylbenzylidene)(methyl)amine,
Ln ) Sm, 90) 0.20; (5) (N-(R-methylbenzylidene)(benzyl)amine, Ln ) Sm, 90) 0.70. The stoichiometric reaction
of N-benzylidene(methyl)amine with Cp′2SmCH(SiMe3)2 or (Cp′2SmH)2 yields an orthometalated Cp′2Sm-substrate
complex which undergoes either hydrogenolysis/hydrogenation or competing CdN insertion of a second substrate
molecule to yield a Cp′2Sm-imine-amido complex with a seven-membered chelate ring. The stoichiometric reaction
of 2-methyl-1-pyrroline with Cp′2SmCH(SiMe3)2 or (Cp′2SmH)2 yields a Cp′2Sm-imine-amido complex in which
two substrate molecules have been coupled to form a six-membered chelate ring (characterized by X-ray diffraction).
The stoichiometric reaction of N-benzylidene(trimethylsilyl)amine with (Cp′2SmH)2 yields a desilylated Cp′2Sm-
imine-amido complex with a four-membered Sm(NSiMe3)(CPh)NdCHPh chelate ring (characterized by X-ray
diffraction). Additional heating of this product under H2 yields S6-symmetric (Cp′2SmCN)6, which contains an unusual
chairlike 18-membered (SmCN)6 ring (characterized by X-ray diffraction).
In contrast to the catalytic hydrogenation of olefins, far less
is known about the equally exothermic hydrogenation of imines
(e.g., eqs 1 and 2),1,2 although the latter offers routes to a diverse
and metal-ligand bond enthalpy considerations4-6 suggest, a
priori, that the comparatively greater strength of early transition
metal, d0,f-metal-heteroatom bonds might seriously impede the
product-releasing hydrogenolysis step in many plausible ho-
mogeneous catalytic cycles. Hypothetical Scheme 1 illustrates
this point for group 4 (only M ) Zr thermochemical data are
available), 4f, and 5f complexes,4-6 where it can be seen that
the M-N bond-forming step (i) is rather exothermic while the
product-releasing M-N hydrogenolysis step (ii) is generally less
so.
A striking example where early transition metal complexes
are effective imine hydrogenation catalysts is the work of
Buchwald and co-workers.7 Here several classes of acyclic and
cyclic imines are hydrogenated in moderate to high enantiomeric
excess using a chiral ethylene-1,2-bis(tetrahydroindenyl)tita-
H2CdCH2 + H2 f H3CsCH3
∆H ) -32.6 kcal/mol
(1)
H2CdNH + H2 f H3CsNH2
∆H ) -32.0 kcal/mol
(2)
variety of amines, both achiral and chiral, natural and unnatural.3
To date, the great majority of imine hydrogenation studies have
employed late transition metal catalysts.1,3 This is not surprising,
(4) The accuracy of these estimates is limited to some degree by the
small current D(M-N) data base. Nevertheless, the ∆Hcalcd sums for Scheme
1, steps i and ii, are in reasonable agreement with the estimated ∆H for
imine hydrogenation. Metal-ligand bond enthalpies are from the follow-
ing: (a) Giardello, M. A.; King, W. A.; Nolan S. P.; Porchia, M.; Sishta,
C.; Marks, T. J. In Energetics of Organometallic Species; Martinho Simoes,
J. A., Ed.; Kluwer: Dordrecht, The Netherlands, 1992; pp 35-54. (b) Nolan,
S. P.; Stern, D.; Hedden, D.; Marks, T. J. ACS Symp. Ser. 1990, 428, 159-
174. (c) Nolan, S. P.; Stern, D.; Marks, T. J. J. Am. Chem. Soc. 1989, 111,
7844-7853. (d) Schock, L. E.; Marks, T. J. J. Am. Chem. Soc. 1988, 110,
7701-7715. (e) Bruno, J. W.; Marks, T. J.; Morss, L. R. J. Am. Chem.
Soc. 1983, 105, 6824-6832.
(5) Organic fragment bond enthalpies are from the following: (a) Giller,
D.; Kanabus-Kaminska, J. M.; Maccoll, A. J. Mol. Struct. 1988, 163, 125-
131. (b) McMillan, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982,
33, 493-532 and references therein. (c) Benson, S. W. Thermochemical
Kinetics, 2nd ed.; John Wiley and Sons: New York, 1976; Appendix Tables
A.10, A.11, and A.22. (d) Benson, S. W. J. Chem. Educ. 1965, 42, 502-
518.
X Abstract published in AdVance ACS Abstracts, April 1, 1997.
(1) (a) Bartok, M. Stereochemistry of Heterogeneous Metal Catalysis;
Wiley: Chichester, 1985, pp 290-293. (b) Rylander, P. N. Hydrogenation
Methods; Academic Press: New York, 1985, Chapter 7. (c) Tennant, G. In
ComprehensiVe Organic Chemistry; Sutherland, O., Barton, D., Ollis, W.
D., Eds.; Pergamon: Oxford, 1979; Vol. 2, pp 440-443.
(2) Heats of hydrogenation were computed using the AM1 Hamiltonian
in the MOPAC molecular orbital package.
(3) For some recent examples of the homogeneous hydrogenation of
imines and related substrates, see: (a) Hegedus, L. S. In ComprehensiVe
Organometallic Chemistry II; Hegedus, L. S., Abel, E. W., Stone, F. G.
A., Wilkinson, G., Eds.; Elsevier: Oxford, 1995; Vol. 12, pp 26-27 and
references therein. (b) Noyori, R. Asymmetric Catalysis in Organic Synthesis;
Wiley: New York, 1994; pp 82-85 and references therein. (c) Ball, G. E.;
Cullen, W. R.; Fryzuk, M. D.; Henderson, W. J.; James, B. R.; MacFarlane,
K. S. Inorg. Chem. 1994, 33, 1464-1468. (d) Bolm, C. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 232-234. (e) Takaya, H.; Ohta, T.; Noyori, R. In
Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH Publishers: New York,
1993; Chapter 1 and references therein. (f) Burk, M. J.; Feaster, J. E. J.
Am. Chem. Soc. 1992, 114, 626-627. (g) Becalski, A. G.; Cullen, W. R.;
Fryzuk, M. D.; James, B. R.; Kang, G.-J.; Rettig, S. J. Inorg. Chem. 1991,
30, 5002-5008. (h) Spindler, F.; Pugin, B.; Blaser, H.-U. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 558-559.
(6) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of
Organic Compounds, 2nd ed.; Chapman and Hall: London, 1986; Appendix
Tables 1 and 3.
(7) (a) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem. Soc. 1994,
116, 11703-11714. (b) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem.
Soc. 1994, 116, 8952-8965. (c) Willoughby, C. A.; Buchwald, S. L. J.
Am. Chem. Soc. 1992, 114, 7562-7564.
S0002-7863(96)03775-4 CCC: $14.00 © 1997 American Chemical Society

3746 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
Obora et al.
Scheme 1. Thermodynamics of Catalytic Imine
Hydrogenation
imine hydrogenation, using well-defined precatalysts and focus-
ing on scope, selectivity, mechanistic observations, and the
informative nature of several unusual organolanthanide byprod-
ucts. Subsequent to our initial communication on this subject15
and while this paper was being completed, the stoichiometric,
aryl C-H activating reaction of (Cp′2SmH)2 (Cp′ ) η5-Me5C5)
with 2-phenyl-1-pyrroline and several acyclic benzylideneamines
was communicated by Buchwald, Burns, et al.16
Experimental Section
General Considerations. All manipulations of air-sensitive materi-
als were performed with the rigorous exclusion of oxygen and moisture
in flame-dried Schlenk-type glassware on a dual manifold Schlenk line,
interfaced to a high-vacuum (10-6 Torr) line, or in a nitrogen-filled
Vacuum Atmospheres glovebox with a high-capacity recirculator (<1
ppm O2). Argon (Matheson, prepurified) and dihydrogen (Linde) were
purified by passage through a MnO/SiO217 oxygen-removal column and
a Davison 4A molecular sieve column. Ether solvents (tetrahydrofuran,
ethyl ether) were predried over KOH and distilled under nitrogen from
sodium-benzophenone ketyl. Hydrocarbon solvents (toluene, pentane)
were distilled under nitrogen from Na/K alloy. All solvents for vacuum
line manipulations or catalytic experiments were stored in Vacuo over
Na/K alloy in resealable bulbs. Deuterated solvents were obtained from
Cambridge Isotope Laboratories (all 99 atom % D) and were degassed
and dried over Na/K alloy. Anhydrous lanthanide trichlorides were
prepared from the corresponding sesquioxides Ln2O3 (Cerac, Ln ) La,
Sm, Lu) and ammonium chloride.18 The complexes Cp′2LaCH(TMS)2,9f
Cp′2SmCH(TMS)2,9f Cp′2LuCH(TMS)2,9f and Me2SiCp′′2SmCH(TMS)2
(Cp′′ ) η5-Me4Cp)9e were synthesized according to published proce-
dures. The solid lithium reagent LiCH(SiMe3)2 was prepared according
to literature methods.19 Pentamethylcyclopentadiene was synthesized
according to a procedure developed in this laboratory.20 The imine
substrates N-benzylidene(trimethylsilyl)amine (2c),21 N-(R-methyl-
benzylidene)(methyl)amine (2d),22 and N-(R-methylbenzylidene)-
(benzyl)amine (2e)23 were synthesized via modifications of literature
methods. N-Benzylidene(methyl)amine (2a) and 2-methyl-1-pyrroline
(4) were purchased from Aldrich and were distilled from BaO and
LiAlH4 and twice from freshly activated 4A molecular sieves. N-
Benzylideneaniline (2b) was purchased from Aldrich and vacuum
sublimed twice. Substrates 2c, 3a, and 3b were distilled from BaO
twice.
nium(IV) binaphthalenediolate complex activated with n-BuLi
+ PhSiH3. Required pressures are rather high (80-2000 psi),
turnover frequencies are modest (Nt ≈ 0.5-4.0 h-1 at 65 °C),
and added silane is required to stabilize the active species. The
rate law (ν ≈ k[Ti]1[H2]1[imine]0), lack of isotopic evidence
for significant reversibility of imine insertion, and k(H2)/k(D2)
) 1.5 are in accord with Scheme 1, where step i is operationally
irreversible under catalytic conditions and step ii not unexpect-
edly (vide supra) turnover-limiting. It was proposed that the
active species is a d1 Ti(III) complex (as opposed to the d0
Ti(IV) of the precatalyst).7
Recent research in several laboratories has shown that
lanthanocene complexes are highly active catalysts for olefin/
acetylene transformations such as hydrogenation,8 oligomer-
ization/polymerization,9 hydrosilylation,9c,10 hydrophosphina-
tion,11 hydroboration,12 and ring-opening Ziegler polymerization.13
With reference to organonitrogen transformations, these catalysts
also mediate a variety of alkene and alkyne intramolecular and
intermolecular hydroamination processes.14 These character-
istics, the general predictability/immutability of most lanthanide
formal oxidation states, and thermochemical data4-6 arguing that
step ii of Scheme 1 is less endothermic for lanthanides suggest
that organolanthanides might be competent and instructive imine
hydrogenation catalysts. We report here a catalytic, synthetic,
and molecular structure study of organolanthanide-mediated
Analytical Measurements. NMR spectra were recorded on a Varian
Gemini 300 (FT, 300 MHz, 1H), VXR-300 (FT, 300 MHz, 1H), or XL-
(8) (a) Haar, C. M.; Stern, C. L.; Marks, T. J. Organometallics 1996,
15, 1765-1784. (b) Giardello, M. A.; Conticello, V. P.; Brard, L.; Gagne,
M. R.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241-10254. (c)
Molander, G. A.; Hoberg, J. O. J. Am. Chem. Soc. 1992, 114, 3123-3125.
(d) Molander, G. A.; Hoberg, J. O. J. Org. Chem. 1992, 57, 3266-3268.
(e) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J.
Am. Chem. Soc. 1985, 107, 8111-8118.
(9) (a) Mitchell, J. P.; Hajela, S.; Brookhart, S. K.; Hardcastle, K. I.;
Henling, L.M.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 1045-1053.
(b) Ihara, E.; Nodono, M.; Yasuda, H.; Kanehisa, N.; Kai, Y. Macromol.
Chem. Phys. 1996, 197, 1909-1917. (c) Fu, P.-F.; Marks, T. J. J. Am.
Chem. Soc. 1995, 117, 10747-10748. (d) Heeres, H. J.; Renkema, J.; Booij,
M.; Meetsma, A.; Teuben, J. H. Organometallics 1988, 7, 2495-2502. (e)
den Haan, K. H.; de Boer, J. L.; Teuben, J. H.; Spek, A. L.; Kajic-Prodic,
B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726-1733. (f) Jeske,
G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am.
Chem. Soc. 1985, 107, 8103-8110. (g) Jeske, G.; Lauke, H.; Mauermann,
H.; Swepton, P. N.; Schumann, H.; Marks, T.J. J. Am. Chem. Soc. 1985,
107, 8091-8103. (h) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985,
18, 51-55.
(10) (a) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc.
1995, 117, 7157-7168. (b) Molander, G. A.; Nichols, P. J. J. Am. Chem.
Soc. 1995, 117, 4415-4416. (c) Molander, G. A.; Julius, M. J. Org. Chem.
1992, 57, 6347-6351. (d) Sakakura, T.; Lautenschlager, H.; Tanaka, M. J.
Chem. Soc., Chem. Commun. 1991, 40-41.
(11) Giardello, M. A.; King, W. A.; Nolan, S. P.; Porchia, M.; Sishta,
C.; Marks, T. J. In ref 4a, pp 35-51.
(12) (a) Harrison, K. N.; Marks, T. J. J. Am. Chem. Soc. 1992, 114,
9220-9221. (b) Bijpost, E. A.; Duchateau, R.; Teuden, J. H. J. Mol. Catal.
1995, 95, 121-128.
1
400 (FT, 400 MHz, H) instrument. 1H and 13C chemical shifts are
(14) (a) Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 9295-9306.
(b) Li, Y.; Marks, T. J. Organometallics 1996, 15, 3770-3772. (c) Li, Y.;
Marks, T. J. J. Am. Chem. Soc. 1996, 118, 707-708. (d) Li, Y.; Fu, P.-F.;
Marks, T. J. Organometallics 1994, 13, 439-440. (e) Giardello, M. A.;
Conticello, V. P.; Brard, L.; Gagne, M. R.; Marks, T. J. J. Am. Chem. Soc.
1994, 116, 10241-10254. (f) Gagne, M. R.; Stern, C. L.; Marks, T. J. J.
Am. Chem. Soc. 1992, 114, 275-294. (g) Gagne, M. R.; Brard, L.;
Conticello, V. P.; Giardello, M. A.; Stern, C. L.; Marks, T. J. Organome-
tallics 1992, 11, 2003-2005. (h) Gagne, M. R.; Nolan, S. P.; Marks, T. J.
Organometallics 1990, 9, 1716-1718.
(15) Obora, Y.; Ohta, T.; Stern, C. L.; Marks, T. J. Abstracts of Papers,
211th National Meeting of the American Chemical Society, New Orleans,
LA, March 1996; American Chemical Society: Washington, DC, 1996;
INOR 129.
(16) Radu, N. S.; Buchwald, S. L.; Scott, B.; Burns, C. J. Organometallics
1996, 15, 3913-3915.
(17) (a) Moeseler, R.; Horvath, B.; Lindenau, D.; Horvath, E. G.; Krauss,
H. L. Z. Naturforsch. 1976, 31B, 892-893. (b) McIlwrick, C. R.; Phillips,
C. S. G. J. Chem. Phys. E 1973, 6, 1208-1210. (c) He, M.-Y.; Xiong, G.;
Toscano, P. J.; Burwell, R. L., Jr.; Marks, T. J. J. Am. Chem. Soc. 1985,
107, 641-652.
(18) Meyer, G. Inorg. Synth. 1989, 25, 146-150.
(19) Cowley, A. H.; Kemp, R. A. Synth. React. Inorg. Met.-Org. Chem.
1981, 11, 591-595.
(20) Fendrick, C. M.; Schertz, L. D.; Mintz, E. A.; Marks, T. J. Inorg.
Synth. 1992, 29, 193-197.
(21) Hart, D. J.; Kanai, K.; Thomas, D. G.; Yang, T.-K. J. Org. Chem.
1983, 48, 289-294.
(13) (a) Jia, L.; Yang, X.; Seyam, A. M.; Albert, I. D. L.; Fu, P.-F.;
Yang, S.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 7900-7913. (b) Yang,
X.; Seyam, A. M.; Fu, P.-F.; Marks, T. J. Macromolecules 1994, 27, 4625-
4626.
(22) Evans, D. A.; Domeier, L. A. Organic Syntheses; Wiley: New York,
1988; Vol. VI, 818-820.
(23) In the Supplementary Material of ref 7c.

Organolanthanide-Catalyzed Imine Hydrogenation
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3747
referenced to internal solvent resonances and reported relative to TMS.
NMR experiments on air-sensitive samples were conducted in either
Teflon valve-sealed tubes (J. Young) or in screw-capped tubes fitted
with septa (Wilmad). IR spectra were recorded on a Mattson FT-IR
spectrometer. MS studies were performed on a VG70-250 SE
instrument with 70 eV electron impact ionization. We thank Dr. Doris
Hung for assistance. Elemental analyses were performed by Oneida
Research Services, Inc., Whitesboro, NY.
2 days, the solvent was removed in Vacuo, and the product 6 was
obtained by recrystallization from pentane (-78 °C) as an orange solid.
Yield: 51 mg (50%). 1H NMR (toluene-d8): δ -18.5 (br s, 1H), -15.9
(br s, 1H), -11.8 (br s, 1H), -9.0 (br s, 1H), -3.60 (br s, 1H), -2.90
(br s, 1H), 1.18 (s, 15H), 1.30 (s, 15H), 1.35-1.45 (m, 3H), 1.5-1.7
(m, 1H), 3.52 (ddd, 1H, J ) 6, 9, 12 Hz), 4.92 (ddd, 1H, J ) 6, 11, 11
Hz), 5.6 (s, 3H), 5.65 (d, 1H, J ) 15 Hz), 10.9 (br, 1H). MS (EI, 70
eV, relative intensity): m/e 588 (M+(152Sm), 2), 583 (M+(147Sm), 1),
573 (M+(152Sm) - CH3, 8), 571 (M+(150Sm) - CH3, 4), 568(M+-
Catalytic Hydrogenation Reactions. A typical procedure is
described for the high-pressure hydrogenation of 2a using Cp′2SmCH-
(TMS)2 (1b) as the precatalyst. The quartz Griffin-Warden vessel
(60 mL) and 500 mL dihydrogen reservoir described previously24 were
employed in these experiments. In the glovebox, a solution of Cp′2-
SmCH(TMS)2 (1b; 58 mg, 0.10 mmol) in toluene (2.0 mL) and 2a
(1.18 g, 10.0 mmol) were placed in the dry (flamed under vacuum)
quartz vessel. The closed vessel was then removed from the glovebox
and connected to the high-vacuum line, and the solution was degassed
by three freeze-pump-thaw cycles. To this mixture was then added
pressurized Matheson UHP H2 (190 psi), and the reaction mixture was
rapidly stirred at 90 °C. The progress of the reaction was monitored
by the drop in H2 pressure, and runs were typically terminated (by
venting) after 96 h or when the pressure no longer decreased. Analysis
(
147Sm) - CH3, 5), 513 (10), 512 (4), 511 (12), 509 (7), 508 (8), 507
(8), 506 (6), 455 (12), 454 (6), 453 (13), 451 (6), 450 (9), 449 (8), 448
(7), 441 (19), 439 (27), 438 (100), 437 (10), 436 (48), 435 (71), 434
(66), 433 (72), 430 (15), 424 (16), 423 (15), 422 (18), 421 (14), 419
(12), 418 (16), 417 (14), 416 (10), 413 (7), 412 (8), 410 (7), 399 (11),
397 (13). HRMS (EI, 70 eV): m/e calcd for C29H45N2147Sm (M+
CH3) 568.2732, found 568.2712.
-
Preparation of Cp′2Sm(C17H21NSi) (7). For an NMR scale
experiment, a 5 mm NMR tube with a Teflon valve (previously flamed
in Vacuo) was charged in the glovebox with Cp′2SmCH(TMS)2 (29
mg, 0.050 mmol), N-benzylidene(trimethylsilyl)amine (2c; 71 mg, 0.40
mmol), and C6D6 (1 mL). The tube was then pressurized with H2 (ca.
20 psi). Subsequent concentration of this mixture to 0.3 mL and
standing at room temperature overnight afforded yellow crystals of 7.
For a preparative scale synthesis, a 20 mL J. Young valve equipped
storage tube was loaded in a glovebox with Cp′2SmCH(TMS)2 (100
mg, 0.17 mmol), N-benzylidene(trimethylsilyl)amine (71 mg, 0.40
mmol), and toluene (2.0 mL). The mixture was then placed under a
H2 atmosphere. After 2 days, solvent was removed in Vacuo, and the
product was obtained by recrystallization from pentane (-78 °C) as a
yellow solid. Yield: 55 mg (44%). 1H NMR (C6D6): δ -9.90 (br s,
2H, ortho of Ph(1)), -3.40 (br s, 9H, Si(CH3)3), 1.3 (s, 15H, C5(CH3)5),
1.50 (s, 15H, C5(CH3)5), 2.60 (br s, 1H), 3.60 (t, 2H, meta of Ph(1)),
4.60 (t, 1H, para of Ph(1)), 8.00 (t, 1H, para of Ph(2)), 8.40 (t, 2H,
meta of Ph(2)), 11.5 (br s, 2H, ortho of Ph(2)), and 13.15 (s, 1H); MS
(EI, 70 eV, relative intensity): m/e 633 (M+(152Sm) - (CH3)3Si, 5),
631 (M+(150Sm) - (CH3)3Si, 6), 628 (M+(147Sm) - (CH3)3Si, 4), 500
(6), 499 (34), 498 (90), 497 (39), 496 (100), 495 (17), 494 (52), 493
(71), 492 (64), 491 (62), 489 (6), 488 (17), 363 (24), 362 (5), 361
(28), 359 (12), 358 (18), 357 (16), 356 (17). Anal. Calcd for C37H51N2-
SiSm: C, 63.28; H, 7.32; N, 3.99. Found: C, 62.54; H, 7.15; N, 3.58.
Synthesis of [Cp′2SmCN]6 (8). A mixture of Cp′2SmCH(TMS)2
(29 mg, 0.05 mmol) and N-benzylidene(trimethylsilyl)amine (2c; 53
mg, 0.30 mmol) in C6D6 (1.0 mL) was heated at 90 °C for 1 week
under H2 (ca. 20 psi of pressure) in a 5 mm NMR tube with a Teflon
valve. During heating, orange crystals were observed to grow. The
mother liquor was next removed by decantation, the crystals were dried
in Vacuo, and degassed Paratone N (Exxon) was poured over the crystals
for diffraction analysis (see below). A preparative scale synthesis was
carried out in a manner similar to that of the NMR reaction with Cp′2-
SmCH(TMS)2 (100 mg, 0.17 mmol) and N-benzylidene(trimethylsilyl)-
amine (140 mg, 0.80 mmol) in C6D6 (2 mL) under 20 psi of H2 at 90
°C. Solid 8 did not crystallize during heating; however, analytically
pure product (10-15 mg) could be obtained by crystallization from
toluene (-78 °C). 1H NMR (C6D6): δ 1.68 (s, 60H), 1.82 (s, 60H).
MS (EI, 70 eV, relative intensity): m/e 315 ([Cp′ 152SmCN]+, 4), 313
([Cp′ 150 SmCN]+, 6), 298 (8), 296 (11), 287 (5), 285 (5), 267 (8), 265
(5), 240 (22), 193 (33), 150 (76), 149 (72), 135 (100). IR (Nujol
mull): 2980-2850 s, 2360 m, 2332 m, 1458 s, 1376 s, 1243 w, 1150
w, 1088 m, 936 m, 840 w, 722 m cm-1. Anal. Calcd for C126H180N6-
Sm6: C, 56.44; H, 6.77; N, 3.13. Found: C, 56.57; H, 7.38; N, 3.00.
1
of the rather concentrated reaction mixture by H NMR was straight-
forward and indicated only the presence of 2a (8%) and of the
hydrogenation product, N-benzylmethylamine (92%). All hydrogena-
tion products were identified by comparison to literature 1H NMR
spectra.25 NMR scale reactions were carried out in NMR tubes
equipped with Teflon J. Young valves. Sample charging was similar
to that described above, and H2 pressures were on the order of 20 psi.
Sample tubes were regularly shaken during hydrogenation studies.
NMR Study of the Reaction of Cp′2SmCH(TMS)2 with N-
Benzylidene(methyl)amine (2a). Synthesis of 4a and 4b. A 5 mm
NMR tube equipped with a Teflon valve was charged in the glovebox
with Cp′2SmCH(TMS)2 (20 mg, 0.050 mmol), 2a (21 mg, 0.18 mmol),
and C6D6 (1.0 mL). The tube was then sealed, and the reaction was
monitored by NMR at room temperature over a period of 136 h. After
this time, the tube was pressurized with H2 (ca. 20 psi). After 22 h at
room temperature, the tube was then heated at 90 °C for 120 h. From
the NMR spectra, two new samarium complexes (4a, 4b) could be
detected. Formation of complexes 4a and 4b was further confirmed
by GC/MS analysis of the organic products formed upon D2O quenching
of this reaction mixture (5a, 5b).
Data for 4a. 1H NMR (C6D6): δ -7.30 (s, 3H, Me), 1.30 (s, 30H,
C5(CH3)5), 6.45 (br d, 1H), 6.80 (br s, 1H), 7.20 (t, 1H), 8.00 (t, 1H),
8.60 (d, 1H). D2O-quenched organic product (5a) 1H NMR (C6D6): δ
3.40 (s, 3H), 7.0-7.9 (5H). MS (EI, 70 eV, relative intensity): m/e
120 (M+, 100), 91 (53), 77 (22), 65 (27).
Data for 4b. 1H NMR (C6D6): δ -13.3 (s, 3H, Me), -10.6 (s,
3H, Me), 1.20 (s, 30H, C5(CH3)5), 6.80 (d, 1H), 7.00 (t, 2H), 7.0-7.2
(2H, m), 7.60 (d,2H), 7.90 (d, 1H), 8.40 (br s, 2H), 11.8 (very br s,
1H). D2O-quenched organic product (5b) 1H NMR (C6D6): δ 2.40 (s,
3H), 3.20 (s, 3H), 3.40 (s, 1H), 7.0-7.9 (m, 9H). MS (EI, 70 eV,
relative intensity): m/e 224 (M+ - CH3, 60), 120 (14), 105 (100), 91
(16), 77 (40), 51 (12).
Synthesis of Cp′2Sm(C10H18N2) (6). For an NMR scale experiment,
a 5 mm NMR tube with a Teflon valve (previously flamed in Vacuo)
was charged in the glovebox with Cp′2SmCH(TMS)2 (29 mg, 0.050
mmol), 2-methyl-1-pyrroline (2f; 12 mg, 0.15 mmol), and toluene-d8
(1.0 mL). The tube was then pressurized with H2 (ca. 20 psi). After
12 h, the title compound 7 was detected in 88% yield. For a preparative
scale synthesis, a 20 mL J. Young valve equipped storage tube was
loaded in the glovebox with Cp′2SmCH(TMS)2 (100 mg, 0.17 mmol),
2-methyl-1-pyrroline (42 mg, 0.50 mmol), and toluene (2 mL). After
Synthesis of Cp′2La(C13H12N) (10). For an NMR scale experiment,
a 5 mm NMR tube with a Teflon valve (previously flamed in Vacuo)
was charged with Cp′2LaCH(TMS)2 (17 mg, 0.030 mmol), N-benz-
ylidene(trimethylsilyl)amine (2c; 5 mg, 0.03 mmol), and C6D6 (1.0 mL).
After 1 day, no reaction was detectable. H2 (20 psi) was then added
to this mixture. Immediately, the lemon-yellow color changed to
orange. The Cp2′LaCH(TMS)2 was no longer visible in the 1H NMR,
and a new complex (10) was detected in ∼60% yield. For a preparative
scale synthesis, a 20 mL J. Young valve equipped storage tube was
loaded in the glovebox with Cp2′LaCH(TMS)2 (100 mg, 0.18 mmol),
2c (33 mg, 0.18 mmol), and toluene (2.0 mL). The reaction mixture
was then placed under a H2 atmosphere. After 1 week, the solvent
(24) Eisen, M. S.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 10358-
10368.
(25) (a) 3a: Bernatis, P.; Laurie, J. C. V.; Dubois, M. R. Organometallics
1990, 9, 1607-1617. (b) 3b: Hayashi, T.; Abe, F.; Sakakura, T.; Tanaka,
M. J. Mol. Catal. 1990, 58, 165-170. (c) 3c: Narula, S. P.; Kapur, N.
Inorg. Chim. Acta 1983, 73, 183-187. (d) 3d: Whitesides, G. M.; Lewis,
D. W. J. Am. Chem. Soc. 1971, 93, 5914-5916. (e) 3e: Bakos, J.; Orosz,
A.; Heil, B.; Laghmari, M.; Lhoste, P.; Sinou, D. J. Chem. Soc., Chem.
Commun. 1991, 1684-1685. (f) 3f: Gagne, M. R.; Nolan, S. P.; Marks, T.
J. Organometallics 1990, 9, 1716-1718.

3748 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
Obora et al.
Table 1. Summary of Crystal Structure Data for Complexes 6, 7, and 8a
complex
formula
crystal system
space group
a, Å
b, Å
c, Å
6
7
8
SmC30H48N2
monoclinic
P21/n (no. 14)
10.517(2)
17.243(2)
15.227(3)
93.90(2)
2755
SmC37H52N2Si
monoclinic
P21/n (no. 14)
14.064(4)
14.668(2)
16.801(6)
92.44(2)
3462
Sm6C126H180N6
hexagonal
R3(h) (no. 148)
31.310(7)
10.873(2)
â (deg)
V, Å3
9231(6)
18
1.486
0.35 × 0.30 × 0.08
yellow, platey
28.75
Zh
4
4
d(calcd), g/cm3
crystal size, mm
color, habit
1.413
1.349
0.48 × 0.31 × 0.08
orange, platey
21.58
0.22 × 0.13 × 0.06
yellow, platey
17.60
µ, cm-1
scan type
ω-θ
ω-θ
ω-θ
transmission factors range
2θ range, deg
intensities (unique, Ri)
intensities > 3.00σ(I)
number of parameters
R
0.54-0.84
2.0-46
4150 (3982, 0.026)
3244
299
0.027
0.71-0.88
2.0-47.9
5887 (5675, 0.087)
2959
371
0.039
0.66-0.87
2.0-46
5889 (2854, 0.071)
2235
207
0.038
Rw
0.035
0.037
0.047
GOF
1.65
0.01
1.51
0.00
1.86
0.01
max shift/error in final cycle
a Diffractometer, Enraf-Nonius, CAD4; temperature for data collection, -120 °C; radiation, graphite monochromator, Mo KR, λ ) 0.710 69.
was removed under high vacuum, and pentane (2 mL) was added. The
product 10 was obtained by recrystallization from pentane at -78 °C
as a yellow solid. Yield: 34 mg (33%). 1H NMR (C6D6): δ 1.80 (s,
30H), 4.25 (s, 2H), 6.25 (br s, 2H), 6.90 (t, 2H, J ) 7.0 Hz), 7.0-7.2
(m, 6H). MS (EI, 70 eV, relative intensity): m/e 591 (M+, 8) 457
(24), 456 (100), 425 (19), 410 (20), 409 (98), 408 (18), 282 (12), 273
(46). HRMS (EI, 70 eV): m/e calcd for C33H42NLa 591.2381, found
591.2414. Anal. Calcd for C33H42NLa: C, 67.00; H, 7.16; N, 2.37.
Found: C, 66.42; H, 7.26; N, 2.42.
showed no significant variations. Intensity data were all corrected for
absorption, anomalous dispersion and Lorentz and polarization effects.26
The space group choice for each complex was unambiguously
determined. Unit cell and data collection parameters are summarized
in Table 1.
Subsequent computations were carried out on a SGI Indy computer.
The structure of complex 6 was solved by direct methods (SHELXS-
86).27 The non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were fixed in idealized positions. The maximum peak on the
final difference Fourier map (0.90 e- Å-3) was located in the vicinity
of the Sm positions. Neutral atom scattering factors were taken from
Cromer and Waber.28 Anomalous dispersion effects were included in
Synthesis of Cp′2Sm(C13H12N) (11). For an NMR scale experiment,
a 5 mm NMR tube with a Teflon valve (previously flamed in Vacuo)
was charged in the glovebox with Cp′2SmCH(TMS)2 (35 mg, 0.060
mmol), N-benzylidene(trimethylsilyl)amine (2c; 21 mg, 0.12 mmol),
and C6D6 (1.0 mL). After 3 h, no reaction was detectable. H2 (ca. 20
psi) was then added to this mixture. Immediately the color changed
to dark red, and then to orange. The 1H NMR revealed that the
Cp2′SmCH(TMS)2 was consumed, and a new complex (11) formed in
quantitative yield. For a preparative scale synthesis, a 20 mL J. Young
valve equipped storage tube was loaded in the glovebox with Cp′2-
SmCH(TMS)2 (100 mg, 0.17 mmol), 2c (72 mg, 0.40 mmol), and
toluene (2.0 mL). The tube was then placed under a H2 atmosphere.
After 1 week, the solvent was removed in Vacuo and pentane (2.0 mL)
added. The product 11 was obtained by recrystallization from pentane
at -78 °C as a red solid. Yield: 51 mg (49%). 1H NMR (C6D6): δ
-32.0 (br s, 1H), -2.20 (br s, 1H), -0.42 (s, 30H), 2.75 (br d, 2H, J
) 7.4 Hz, ortho of Ph), 2.95 (br s, 2H), 5.05 (t, 1H, J ) 8.3 Hz), 5.30
(t, 2H, J ) 7.4 Hz, meta of Ph), 5.98 (t, 1H, J ) 7.4 Hz, para of Ph),
7.00 (m, 1H), and 11.0 (br s, 1H). MS (EI, 70 eV, relative intensity):
m/e 604 (M+(152Sm), 11), 602 (M+(150Sm), 5), 599 (M+(147Sm), 6),
472 (20), 471 (45), 470 (24), 469 (51), 468 (9), 467 (30), 466 (38),
465 (36), 464 (29), 463 (3), 462 (4), 461 (6), 441 (8), 440 (37), 439
(9), 438 (44), 437 (3), 436 (18), 435 (27), 434 (24), 425 (20), 424
(88), 423 (25), 422 (100), 421 (8), 420 (44), 419 (67), 418 (59), 417
(61), 415 (3), 414 (12). HRMS (EI, 70 eV): m/e calcd for C33H42-
N147Sm 599.2466, found 599.2456. Anal. Calcd for C33H42NSm: C,
65.72; H, 7.02; N, 2.32. Found: C, 65.77; H, 6.97; N, 2.28.
Fcalc;
29 the values for ∆f′ and ∆f′′ were those of Cromer.30 The structure
of 7 was solved with SHELX-86 and expanded using Fourier
techniques.31 The non-hydrogen atoms were refined, and hydrogen
atoms were fixed in idealized positions. The maximum peak on the
final difference Fourier map (0.95 e- Å-3) was located in the vicinity
of the Sm position. Neutral atom scattering factors were taken from
Cromer and Waber.28 Anomalous dispersion effects were included in
29
Fcalc
,
and the values for ∆f′ and ∆f′′ were those of Creagh and
McAuley.32 The values for the mass attenuation coefficients were those
of Creagh and Hubbel.33 The structure of 8 was solved using a
Patterson map (SHELXS-86).27 The non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were fixed in idealized positions.
The space group for 8 is not unambiguously determined. The
assignment is based on successful solution and refinement of the
proposed model and statistical analysis of the intensity distribution.
The disordered C-N bonds were refined using group isotropic thermal
parameters with N1(CN1)-C02(CN2) refined to an occupancy of 0.494
(26) Cromer, D. T.; Waber, J. T. International Tables for X-Ray
Crystallography; The Kynoch Press: Birmingham, England, 1974; Vol.
IV, pp 149-150.
(27) Sheldrick, G. M. In Crystallographic Computation; Sheldrick, G.
M., Kruger, C., Goddard, R., Eds.; Oxford University Press: Oxford, 1985;
pp 175-189.
(28) Reference 26, Vol. IV, Table 2.2A.
(29) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781.
(30) Reference 26, Vol. IV, Table 2.3.1.
X-ray Crystallographic Studies of Complexes 6, 7, and 8.
Suitable crystals of 6, 7, and 8 for diffraction studies were grown by
slow cooling of a pentane/toluene solution of each complex at -25
°C. In each case, the solvent was decanted and Paratone N (Exxon,
dried and degassed at 120 °C, 10-6 Torr, for 24 h) was poured over
the crystals in the glovebox. The crystals were mounted on glass fibers
and tranferred to the cold stream (-120 °C) of the Enraf-Nonius CAD4
diffractometer. Final cell dimensions were obtained by least-squares
fits to the automatically centered settings for 25 reflections. Three
reference reflections monitored during data collection for each crystal
(31) DIRDIF92: Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman,
W. P.; Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The
DIRDIF program system, Technical Report of the Crystallography Labora-
tory, University of Nijmegen, The Netherlands, 1992.
(32) Creagh, D. C.; McAuley, W. J. In International Tables for
Crystallography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers:
Boston, 1992; Vol. C, Table 4.2.6.8, pp 219-222.
(33) Creagh, D. C.; Hubbel, J. H. In International Tables for Crystal-
lography; Wilson, A. J. C., Ed.; Kluwer Academic Publishers: Boston,
1992; Vol. C, Table 4.2.4.3, pp 200-206.

Organolanthanide-Catalyzed Imine Hydrogenation
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3749
and N2(CN2)-C01(CN1) refined to an occupancy of 0.506. The
maximum peak in the final difference Fourier map (1.42 e- Å-3) was
located in the vicinity of the Sm position. Neutral atom scattering
Table 2. Hydrogenation of Imines Catalyzed by Organolanthanide
Complexesa
factors were taken from Cromer and Waber.28 Anomalous dispersion
29
effects were included in Fcalc
,
and the values for ∆f′ and ∆f′′ were
those of Cromer.30
All calculations were performed using the TEXSAN34 crystal-
lographic software package of Molecular Structure Corp.
Results
This section begins with a discussion of organolanthanide +
imine catalytic hydrogenation phenomenology, focusing on the
influence of imine structure and substituents, organolanthanide
metal and ancillary ligation, added silanes, and reaction condi-
tions on the hydrogenation process. In several cases, informa-
tive organolanthanide + substrate reaction products were
detected in the hydrogenation studies and were then pursued in
stoichiometric syntheses, followed by further spectroscopic and
diffractometric characterization. The resulting picture both
expands the scope and reveals some of the thermodynamic sinks
prevalent in this type of catalytic chemistry.
Organolanthanide-Catalyzed Imine Hydrogenation. Ho-
mogeneous imine hydrogenation experiments using a variety
of substrates, precatalysts (Cp′2LnCH(TMS)2, I; Me2SiCp′′2-
LnCH(TMS)2, II), and reaction conditions (typically substrate:
a Solution of substrate (4-5 mol/L) and catalyst (∼100:1) in toluene
stirred under H2 pressure. b Conversion determined by 1H NMR
analysis. c Reaction carried out in the presence of PhSiH3 (PhSiH3:
catalyst ) 3.6). d Cp′2SmR ) (η5-Me5C5)2SmCH(SiMe3)2; Cp′2LaR )
(η5-Me5C5)2LaCH(SiMe3)2; Cp′2LuR ) (η5-Me5C5)2LuCH(SiMe3)2;
SiSmR ) Me2Si(η5-Me4C5)SmCH(SiMe3)2. e Reaction carried out in
the presence of PhSiH3 (PhSiH3:catalyst ) 10). f Hydrosilylation
product obtained in 10% yield. g Reaction carried out in the presence
of PhSiH3 (PhSiH3:catalyst ) 30).
catalyst ) 100:1) were carried out in toluene solution with
rigorous exclusion of oxygen and moisture.8a,b,24 Both NMR
scale (20 psi of UHP H2) and preparative scale (190 psi of UHP
H2) reactions were performed (see the Experimental Section for
details). Since relatively concentrated solutions were employed,
NMR spectroscopy of the reaction mixtures provided an
effective and convenient analytical technique. Results are
compiled in Table 2. It was found that acyclic imines undergo
far more rapid hydrogenation than cyclic imines, with the latter
giving trace (entries 14 and 15) to negligible yields of
hydrogenation products. The acyclic imines undergo regiospe-
cific hydrogenation to form known products25 (identified by 1H
NMR) at modest turnover frequencies. Although few direct
comparisons are available, the 2e f 3e (entry 13) reduction
with Nt ≈ 0.70 h-1 at 90 °C, 190 psi of H2, compares favorably
with the titanocene-mediated reduction (Nt ≈ 2.0 h-1 at 65 °C,
2000 psi of H2).7b Using the equipment described previously24
and under conditions which were approximately zero-order in
H2 pressure, the rate of H2 uptake in a high-pressure organo-
samarium-catalyzed reduction of N-benzylidene(methyl)amine
(Table 2, entry 1) was monitored as a function of time. At 90
°C, the uptake is approximately linear (Figure 1) to J85%
conversion. These results are consistent with a catalytic rate
law which is initially zero-order in imine, although deviations
are evident at high conversion and may reflect some catalyst
deactivation (vide infra). Not surprisingly, hydrogenation rates
are greater at higher temperatures (entries 1, 3, and 4) and at
higher H2 pressures (comparisons to 20 psi NMR experiments).
In regard to imine substituents, both N-aryl and N-SiMe3
substitutions are deactivating with respect to hydrogenation
(entries 9 and 11 vs entry 1), as is imine C-methyl substitution
Figure 1. Time dependence of the reactor H2 pressure in the
hydrogenation of N-benzylidene(methyl)amine (2a) using Cp′2SmCH-
(SiMe3)2 as the precatalyst at 90 °C. The line represents a least-squares
fit to the initial data points. Fits of the data to first-order behavior were
less convincing.
(entry 12 vs entry 1). As in the case of titanocene-mediated
reduction, PhSiH3 has a modest accelerating effect (entry 1 vs
entry 2); however, in the case of N-benzylideneaniline (2b), a
hydrosilylation product is detected in ca. 10% yield (eq 3).35
Interestingly and in marked contrast to metal and ancillary ligand
sensitivities evident in a variety of organolanthanide-catalyzed-
(34) TEXSAN Structure Solution Package; Molecular Structure Corp.:
The Woodlands, 1985 and 1992.

3750 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
Obora et al.
Scheme 2. Reaction of N-Benzylidene(methyl)amine (2a)
with Cp′2SmCH(TMS)2
olefin transformations (including hydrogenation),8a,b,e,9f,g,10a,12a
the present reductions are rather insensitive to the identity of
the lanthanide (entry 1 vs entries 7 and 3 vs entries 6 and 4 vs
entry 5). Furthermore, in the case of entries 1 and 8, the more
“open” Sm coordination geometry actually depresses the
hydrogenation rate. The observation in several NMR scale
catalytic reactions of spectral features attributable to new
organolanthanide complexes promoted studies of stoichiometric
reactions.
Figure 2. Perspective ORTEP drawing of the molecular structure of
complex 6. All non-hydrogen atoms are represented by thermal
ellipsoids drawn to encompass 50% probability, and hydrogen atoms
are deleted for ease of viewing.
Scheme 3. Reaction of 2-Methylpyrroline (2f) with
Organosamarium Complexes
Stoichiometric Imine-Organolanthanide Chemistry. N-
Benzylidene(methyl)amine (2a). The reaction of Cp2′SmCH-
(TMS)2 with 2a in toluene-d8 (1:3.6 equiv/equiv) was monitored
1
in the absence of H2 at 25 °C by H NMR over the course of
136 h. The formation of two new (paramagnetic) organosa-
marium complexes (4a, 4b) and the concurrent production of
CH2(TMS)2 (useful as an internal NMR quantitation standard)
are observed. Complex 4a is formed initially, with the 4b:4a
ratio increasing over time and suggesting that 4b arises from
4a. Complex 4a exhibits a relatively simple 1H NMR spectrum
with 30 Cp′ (singlet), 3 Me (singlet), and 4 aryl protons
(1:1:1:1) readily identifiable. The spectrum of 4b is more
complex with 30 Cp′ (singlet) and 9 aryl protons detectable (see
the Experimental Section for data).
An ortho-metalated structure for 4a (for which there is
considerable precedent16,36) and an imine insertion related
structure for 4b are proposed (Scheme 2) on the basis of the
NMR data and D2O quenching/GC-MS analysis of the organic
products 5a and 5b. Pressurizing the 4a/4b reaction mixture
(4a:4b ≈ 1:3) to 20 psi of H2 results in no change in the mixture
composition after 22 h at room temperature. However, subse-
quent increase of the temperature to 90 °C effects slow
diminution of the 4a and 2a signals (substrate hydrogenation)
and negligle change in the 4b signal. These results suggest the
scenario shown in Scheme 2 in which metalated complex 4a
undergoes slow Sm-C(aryl) hydrogenolysis, while complex 4b
is more inert. Futher confirmation of this scenario is provided
by the observation that 2a reacts rapidly with (Cp2′SmH)2 at
25 °C in the absence of H2 to yield a ∼8:1 mixture of 4a:4b.
Attempts to crystallize and/or to separate 4a and 4b by fractional
crystallization were unsuccessful.
Reaction of 2-Methyl-1-pyrroline (2f) with Organo-
samarium Complexes. The cyclic imine 2-methyl-1-pyrroline
undergoes minimal hydrogenation in the presence of Cp2′SmCH-
(TMS)2 (Table 1), but rather stoichiometric experiments reveal
the formation of complex 6 and the evolution of CH2(TMS)2
(Scheme 3). Complex 6 can also be prepared from 2f and
(Cp2′SmH)2, and was characterized by standard analytical
1
techniques (the H NMR reveals magnetically nonequivalent
Cp′ ligands; see the Experimental Section for details) and X-ray
diffraction. Selected bond distances and bond angles are
compiled in Table 3. It can be seen (Figure 2) that the molecular
structure of 6 conforms closely to the SmN2 imine-amido motif
shown in Scheme 3, with SmsN2(amido) and SmsN1(imine)
distances of 2.242(4) and 2.523(4) Å, respectively. These
amido-imine assignments agree with the corresponding formal
CsN and CdN distances of 1.454(6) and 1.276(7) Å, respec-
tively. The Cp′CgsSmsCp′Cg angle of 132.0° and the average
SmsC(Cp′) distance of 2.775(4) Å are unexceptional for
Cp2′SmX2 compounds.9fg,37
A reasonable scenario for the formation of complex 6 invokes
CsH activation at the allylic methyl position in 2f, followed
by CdN insertion into the resulting, metallacyclic SmsC bond
(Scheme 4). The former process has precedent in Cp′2SmR-
mediated activation of allylic CsH bonds in propylene and other
(35) 1H NMR (CDCl3): δ 4.82 (s, 2H, CH2-N), 5.40 (s, 2H, SiH2), 6.83-
7.60 (m, 15H, phenyl-H). HRMS: m/e calcd for C19H19NSi(M+) 289.1287,
found 289.1298.
(36) (a) Duchateau, R.; van Wee, C. T.; Teuben, J. H. Organometallics
1996, 15, 2291-2302. (b) Deehman, B.-J.; Booij, M.; Meetsma, A.; Teuben,
J. H.; Kooijman, H.; Spek, A.L. Organometallics 1995, 14, 2306-2317.
(c) Deelman, B.-J.; Stevels, W. M.; Teuben, J. H.; Lakin, M. T.; Spek, A.
L. Organometallics 1994, 13, 3381-3391 and references therein.
(37) (a) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. ReV.
1995, 95, 865-986. (b) Evans, W. J.; Foster, S. E. J. Organomet. Chem.
1992, 433, 79-94.

Organolanthanide-Catalyzed Imine Hydrogenation
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3751
Table 3. Selected Bond Distances (Å) and Angles (deg) for Complex 6
Bond Distances
Sm-N1
Sm-N2
Sm-C1
Sm-C2
Sm-C3
Sm-C4
Sm-C5
Sm-C11
Sm-C12
Sm-C13
Sm-C14
Sm-C15
N1-C21
N1-C24
N2-C26
2.523(4)
2.242(4)
2.773(4)
2.770(5)
2.761(5)
2.778(4)
2.763(4)
2.745(5)
2.800(5)
2.812(4)
2.789(4)
2.757(4)
1.478(6)
1.276(7)
1.458(6)
N2-C29
C1-C2
1.454(6)
1.398(7)
1.396(7)
1.512(8)
1.415(7)
1.492(8)
1.407(7)
1.495(7)
1.389(7)
1.497(8)
1.512(8)
1.419(7)
1.409(7)
1.506(7)
1.408(6)
C12-C17
C13-C14
C13-C18
C14-C15
C14-C19
C15-C20
C21-C22
C22-C23
C23-C24
C24-C25
C25-C29
C26-C27
C27-C28
C28-C29
C29-C30
1.500(7)
1.417(7)
1.504(7)
1.409(6)
1.508(7)
1.498(7)
1.502(8)
1.486(8)
1.502(7)
1.538(8)
1.501(8)
1.503(8)
1.473(9)
1.547(8)
1.533(8)
C1-C5
C1-C6
C2-C3
C2-C7
C3-C4
C3-C8
C4-C5
C4-C9
C5-C10
C11-C12
C11-C15
C11-C16
C12-C13
Bond Angles
N1-Sm-N2
N1-Sm-Cg1
N1-Sm-Cg2
N2-Sm-Cg1
N2-Sm-Cg2
Cg1-Sm-Cg2
Sm-N1-C21
Sm-N1-C24
C21-N1-C24
Sm-N2-C26
78.0(1)
105.7
104.8
109.6
112.3
Sm-N2-C29
C26-N2-C29
N1-C21-C22
C21-C22-C23
C22-C23-C24
N1-C24-C23
N1-C24-C25
C23-C24-C25
C24-C25-C29
N2-C26-C27
135.4(3)
106.0(4)
107.2(4)
105.0(4)
103.2(5)
114.6(5)
125.6(5)
119.6(5)
116.6(5)
106.0(5)
C26-C27-C28
C27-C28-C29
N2-C29-C25
N2-C29-C28
N2-C29-C30
C25-C29-C28
C25-C29-C30
C28-C29-C30
103.9(5)
107.3(5)
112.5(4)
104.9(4)
111.5(5)
110.5(5)
110.2(5)
107.0(5)
132.0
125.1(3)
126.4(3)
108.4(4)
118.5(3)
Scheme 4. Plausible Reaction Mechanism for the Formation
of Complex 6
Scheme 5. Reaction of N-Benzylidene(trimethylsilyl)amine
(2c) with Cp′2SmCH(TMS)2
olefins,9fg,38 and is likely assisted by substrate precoordination
via the basic pyrroline functionality. The subsequent CdN
insertion was previously observed in Scheme 2.
Reaction of N-Benzylidene(trimethylsilyl)amine (2c) with
Organosamarium Complexes. In C6D6 at room temperature,
Cp′2SmCH(TMS)2 fails to react with substrate 2c (1:8) over
the course of one day. However, when the atmosphere is
replaced with 20 psi of H2, the starting materials are consumed
within one day at 25 °C, and complex 7 is formed in quantitative
yield (Scheme 5). Concurrent formation of 1 equiv each of
1
CH2(TMS)2 and trimethylsilane is also detected by H NMR.
Complex 7 was characterized by standard analytical methods;
especially noteworthy are an SiMe3 signal and magnetically
nonequivalent Cp′ resonances in the 1H NMR (see the Experi-
mental Section for full data). The crystal structure of 7 (Figure
3) is in accord with these data and the configuration of the first
product shown in Scheme 5. Selected bond distances and angles
are compiled in Table 4. A Cp′2Sm fragment of unexcep-
tional dimensions9f,g,37 (average SmsC(Cp′) ) 2.764(9) Å,
CgsSmsCg ) 133.0°) is incorporated in a three-membered
imine-silylamido chelate ring, with SmsN1(amide) and
SmsN2(imine) distances of 2.301(7) and 2.548(7) Å, respec-
tively. The Sm, N1, C24, and N2 atoms are coplanar to within
0.07 Å, with a short, formal N2dC31 bond length of 1.27(1)
Å and an unexceptional N1sSi1 distance of 1.711(7) Å. A
possible pathway for the formation of complex 7 is depicted in
Scheme 6, where desilylation proceeds via σ-bond metathesis
(having precedent in lanthanide hydrocarbyl-silane chem-
istry9c,10a,39 and in examples of lanthanide hydride cleavage of
SisO bonds)40 to yield a benzylidene-amido complex which
undergoes subsequent CsN insertion.
The reaction of imine-amido complex 7 with 20 psi of H2
at 90 °C or the stoichiometric reaction of Cp′2SmCH(TMS)2
with 2c under 20 psi of H2 at 90 °C yields orange crystals having
only two Cp′ resonances in the 1H NMR (1:1) and what appear
to be CtN stretching modes at 2360 and 2332 cm-1 in the IR.
The crystal structure of complex 8 (Figure 4) reveals a
(39) Tilley, T. D. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; John Wiley: Chichester, 1989; Chapter 24 and
references therein.
(38) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1990,
112, 2314-2324.
(40) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. Organometallics 1991,
10, 134-142.

3752 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
Obora et al.
Table 4. Selected Bond Distances (Å) and Angles (deg) for Complex 7
Bond Distances
Sm-N1
Sm-N2
Sm-Cl
2.301(7)
2.548(7)
2.80(1)
N2-C31
C1-C2
C1-C5
C1-C6
C2-C3
C2-C7
C3-C4
C3-C8
C4-C5
C4-C9
C5-C10
C11-C12
C11-C15
C11-C16
C12-C13
C12-C17
C13-C14
C13-C18
1.27(1)
1.42(1)
1.43(1)
1.49(1)
1.41(1)
1.50(1)
1.43(1)
1.50(1)
1.41(1)
1.50(1)
1.52(1)
1.42(1)
1.42(1)
1.52(1)
1.41(1)
1.49(1)
1.41(1)
1.49(1)
C14-C15
C14-C19
C15-C20
C24-C25
C25-C26
C25-C30
C26-C27
C27-C28
C28-C29
C29-C30
C31-C32
C32-C33
C32-C37
C33-C34
C34-C35
C35-C36
C36-C37
1.39(1)
1.52(1)
1.51(1)
1.53(1)
1.39(1)
1.38(1)
1.37(1)
1.38(1)
1.38(1)
1.39(1)
1.46(1)
1.37(1)
1.38(1)
1.38(1)
1.36(1)
1.39(1)
1.39(1)
Sm-C2
Sm-C3
Sm-C4
Sm-C5
Sm-C11
Sm-C12
Sm-C13
Sm-C14
Sm-C15
Si-N1
Si-C21
Si-C22
Si-C23
N1-C24
N2-C24
2.757(9)
2.744(8)
2.765(8)
2.803(9)
2.753(9)
2.759(9)
2.761(9)
2.754(8)
2.743(8)
1.711(7)
1.88(1)
1.862(9)
1.87(1)
1.44(1)
1.53(1)
Bond Angles
Sm-N1-Si
Sm-N1-C24
Si-N1-C24
Sm-N2-C24
Sm-N2-C31
C24-N2-C31
N1-Sm-N2
N1-Sm-Cg1
N1-Sm-Cg2
N2-Sm-Cg1
N1-Sm-Cg2
Cg1-Sm-Cg2
58.2(2)
112.4
111.8
112.5
103.6
133.0
134.9(4)
103.3(5)
120.1(6)
90.5(5)
152.5(6)
116.7(7)
N1-C24-N2
N1-C24-C25
N2-C24-C25
N2-C31-C32
105.7(7)
115.6(7)
110.4(6)
125.1(8)
Table 5. Selected Bond Distances (Å) and Angles (deg) for Complex 8
Bond Distances
Sm-NC1
Sm-NC2* a
Sm-C1
2.517(8)
2.532(8)
2.695(8)
2.720(8)
2.682(8)
2.688(8)
2.694(9)
2.715(8)
2.708(8)
2.719(7)
2.721(8)
2.724(8)
NC1-NC2
1.17(1)
1.41(1)
1.41(1)
1.52(1)
1.42(1)
1.51(1)
1.40(1)
1.51(1)
1.42(1)
1.51(1)
1.50(1)
1.40(1)
C11-C15
C11-C16
C12-C13
C12-C17
C13-C14
C13-C18
C14-C15
C14-C19
C15-C20
C21-C21* a
1.41(1)
1.51(1)
1.43(1)
1.49(1)
1.40(1)
1.50(1)
1.40(1)
1.54(1)
1.49(1)
1.40(1)
C1-C2
C1-C5
C1-C6
C2-C3
C2-C7
C3-C4
C3-C8
C4-C5
C4-C9
C5-C10
C11-C12
Sm-C2
Sm-C3
Sm-C4
Sm-C5
Sm-C11
Sm-C12
Sm-C13
Sm-C14
Sm-C15
Bond Angles
NC1-Sm-NC2* a
NC1-Sm-Cg1a
NC1-Sm-Cg2a
108.2(2)
100.0
103.0
NC2* a-Sm-Cg1a
NC2* a-Sm-Cg2a
Cg1a-Sm-Cg2a
100.2
104.0
138.9
Sm-CN1-CN2
170.9(7)
175.0(6)
CN1-CN2-Sm* a
a NC2* and Sm* are neighboring atoms. Cg1 and Cg2 are ring centroids.
remarkable 18-membered ring of constitution (Cp′2Smµ-CN)6.
Selected bond angles and distances are compiled in Table 5.
The SmsCtN and -CtNsSm vectors are nearly linear (see
the Experimental Section for a discussion of how the CtN
disorder was treated) with angles of 170.9° and 175.0°, and the
CgsSmsCg angle is 138.9°stypical of Cp′2SmX2
complexes.9f,g,37 The SmsSmsSm angle in 8 is 104.8°,
C(N)sSmsN(C) ) 108.2°, and the conformation of the
18-membered ring is chairlike (S6 symmetry, 8) with fold angles
of 65.19° between planes. The magnetic nonequivalence of the
1
Cp′ signals in the solution H NMR at 25 °C suggests that the
ring has low conformation mobility, presumably due to the
difficulty of the bulky Cp′ rings in moving past one another.
The average SmsCNsSm distance in 8 of 6.187 Å is slightly
shorter than that in the more coordinatively saturated [Cp′2Sm-
(CNC6H11)(µ-CN)]3, 6.29 Å (9).41 Here the nine-membered
[Sm(µ-CN)]3 unit is planar to within 0.04 Å.
Scheme 7 presents one possible pathway for the conversion
of complex 7 to complex 8. Mechanistically, the least obvious
step is that which creates a cyano functionality, which would
Figure 3. Perspective ORTEP drawing of the molecular structure of
complex 7. All non-hydrogen atoms are represented by thermal
ellipsoids drawn to encompass 50% probability, and hydrogen atoms
are deleted for ease of viewing.
appear invariably to require a C(â)sPh bond scission. Scheme
7 suggests a TMSNdCPh extrusion process from the presum-
(41) Evans, W. J.; Drummond, D. K. Organometallics 1988, 7, 797-
802.

Organolanthanide-Catalyzed Imine Hydrogenation
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3753
Scheme 7. Possible Pathway for the Formation of Complex
8 from Complex 7
involving organolanthanide hydrocarbyls have considerable
precedent.9h,13 In the present case, it is possible that phenyl
Figure 4. Perspective ORTEP drawing of the molecular structure of
complex 8: (A) top view, (B) side view. All non-hydrogen atoms are
represented by thermal ellipsoids drawn to encompass 50% probability,
and hydrogen atoms are deleted for ease of viewing. (A) is viewed
along the S6 axis, and (B) is viewed perpendicular to the S6 axis.
transfer is assisted through coordination of the imine and/or aryl
π-electron system by the electrophilic lanthanide center (e.g.,
eq 4). Following â-Ph transfer, Cp′2SmCN formation could
occur via either intramolecular or intermolecular protonolytic
C6H6 elimination, followed by oligomerization (Scheme 7).
Reaction of N-Benzylideneaniline (2b) with Organolan-
thanides. Although the Cp′2LnCH(TMS)2 complexes where
Ln ) La and Sm do not undergo reaction with 2b under inert
atmosphere, reaction under H2 yields insertion products 10 and
Scheme 6. Plausible Pathway for the Formation of Complex
7
11 (eq 5), which were characterized by standard spectroscopic
ably strained metallacycle of complex 7 (the microscopic reverse
insertion process was portrayed in Schemes 2 and 4), followed
by a â-Ph transfer to Sm. Alkyl transfer processes of this type
and analytical techniques (see the Experimental Section for
details). The 1H NMR spectra of these complexes are straight-
forward; however, they exhibit slight (10) to very large

3754 J. Am. Chem. Soc., Vol. 119, No. 16, 1997
Obora et al.
Scheme 8. Pathways for Organolanthanide-Catalyzed Imine Hydrogenation
(paramagnetic 11) displacements of one ortho phenyl proton
resonance which are suggestive of Ln interactions with either
the aryl π-system or an ortho C-H bond. Such structures have
ample precedent in organosamarium complexes of styrene
(12),42 stilbene (13),42 and azobenzene (14).43 Indeed, such
lanthanide catalysts exhibit low to negligible activity for
hydrogenation of cyclic imines. In regard to mechanism, the
plausible scenarios of Schemes 1, and 8,44,45 a variant of
pathways previously identified for organolanthanide-mediated
olefin hydrogenation,8a,b,e appear reasonable. The present
relative insensitivity of rates to lanthanide identity is consistent
with the catalytic cycle being restricted to a single lanthanide
oxidation state; however, the exact ordering has not been seen
in lanthanide-olefin chemistry before.46 Thus, the trends
contrast with olefin hydrogenation phenomenology where, for
sterically encumbered substrates, rates generally increase almost
monotonically with increasing lanthanide ionic radius or more
open (Me2SiCp′′2Ln, Me2SiCp′′(C5H3R*)Ln) coordination spheres
(olefin insertion is turnover-limiting), and for R-olefins, where
rates increase almost monotonically with decreasing lanthanide
ionic radius (metal hydrocarbyl hydrogenolysis is turnover-
limiting).8e In the present case, it seems likely (as for Ti16)
that the insertion step is rapid and irreversible (supported by
the approximate zero-order dependence of the 2a hydrogenation
rate on [2a]), while hydrogenolysis is turnover-limiting. The
exothermicity of the insertion step is consistent with a more
reactant-like transition state, which might be less sensitive to
steric constraints. Interesting and apparently analogous trends
in lanthanide ion sensitivity are noted for organolanthanide-
mediated aminoalkene and aminoalkyne cyclative hydroami-
nation (eqs 7 and 8, X ) NH, NR).14 Thus, turnover-limiting,
interactions may precede C-H activation processes such as that
leading to metalation of 2-phenyl-1-pyrroline (15; eq 6).16
Discussion
The present results demonstrate that organolanthanides are
competent for the catalytic hydrogenation of acyclic organo-
lanthanides.44 Rates are modest and roughly comparable to
those mediated by a chiral titanocene.7 However, the organo-
(42) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1990,
112, 219-223.
(43) (a) Evans, W. J.; Drummond, D. K.; Bott, S. G.; Atwood, J. L.
Organometallics 1986, 5, 2389-2391. (b) Evans, W. J.; Drummond, D.
K.; Chamberlain, L. R.; Doedens, R. J.; Bott, S. G.; Zhang, H.; Atwood, J.
L. J. Am. Chem. Soc. 1988, 110, 4983-4994.
(44) Some stages of organoscandium-mediated nitrile hydrogenation
appears to be similar: Bercaw, J. E.; Davies, D. L.; Wolczanski, P. T.
Organometallics 1986, 5, 443-450.
approximately thermoneutral olefin insertion (eq 7) is ap-
preciably accelerated by larger or more open lanthanide ions,
while turnover-limiting, exothermic alkyne insertion (eq 8) is
decelerated.
The present results include one example (Table 2, entry 2)
where small amounts of PhSiH3 accelerate the imine hydrogena-
tion process (in another example, competing hydrosilylation is
observed; see below). It seems unlikely here that the role of
(45) In principle, the initial insertion step could also proceed by the
opposite regiochemistry to yield LnCH2NH2-type products. This reaction
is actually estimated to be slightly more exothermic (∼3 kcal/mol) than
that in the schemes due to the greater product N-H bond enthalpy vs C-H.
In principle, such a structure might be further stabilized by amine
coordination36 as shown below. Nevertheless, the NMR spectra of complexes
10 and 11 and the structures of complexes 6 and 7 are inconsistent with
this regiochemistry, and an Ln-C-bonded product might be expected to
give rise, in the presence of PhSiH3, to conventional C-Si-bonded
hydrosilylation products.9c,10a
(46) For a discussion of Ln3+ redox characteristics, see: Morss, L. R.
Chem. ReV. 1976, 76, 827-842.

Organolanthanide-Catalyzed Imine Hydrogenation
J. Am. Chem. Soc., Vol. 119, No. 16, 1997 3755
Scheme 9. Possible Role of Silanes as Promoters in
Organolanthanide-Catalyzed Imine Hydrogenation
Catalytic organolanthanide chemistry almost invariably in-
volves sequences of insertions of unsaturated species into
metal-ligand σ-bonds coupled to/in competition with, four-
center σ-bond metathesis processes, which transpose σ-bonded
groups within the lanthanide coordination sphere.8-14 The
present study provides examples of this interplay, with exo-
thermic, catalyst-deactivating4-6 CdN insertion (eq 9) intercept-
ing products of σ-bond metathetical CsH activation (Scheme
8, i, iii) and CdN extrusion (Scheme 8, ii). In the case of cyclic
imines, CsH activation pathways are apparently more rapid
than CsN insertion (Schemes 3 and 4; eq 6), and low
hydrogenolytic reactivity of the products renders imine hydro-
genation at best a minor pathway. The formation of such
byproducts is an obvious complication in effecting organolan-
thanide-catalyzed imine hydrogenations for large numbers of
turnovers. Whether similar species may be of comparable
importance in the group 4 catalytic chemistry is presently
unclear.
silane can be to stabilize the catalyst oxidation state. One
possible role of silane may be deamidation of the lanthanide
center (Scheme 947). Protonolytic step ii has precedent in
lanthanide10 and group 4 hydrocarbyl38 as well as possibly in
group 4 amide-silane chemistry.48 Hydrogenolytic step iii has
precedent in organolanthanide silyl chemistry.38,49 Interestingly
and in analogy to the hydrocarbyl chemistry,10a transposition
in the opposite sense in step ii (N f Si, H f Ln) would effect
hydrosilylation of the imine.47
Acknowledgment. We are grateful to the NSF for support
of this research under Grants CHE9104112 and CHE9618589.
Supporting Information Available: Tables of positional
and anisotropic thermal parameters and full tables of bond
distances, angles, and dihedral angles for 6, 7, and 8 (111 pages).
See any current masthead page for ordering and Internet access
instructions.
(47) Bond enthalpies from refs 4-6 and the following: King, W. A.;
Marks, T. J. Inorg. Chim. Acta 1995, 229, 343-354.
(48) (a) Liu, H. Q.; Harrod, J. F. Organometallics 1992, 11, 822-827.
(b) He, J.; Liu, H. Q.; Harrod, J. F.; Hynes, R. Organometallics 1994, 13,
336-343.
(49) Radu, N.; Tilley, T. D. J. Am. Chem. Soc. 1995, 117, 5863-5864
and references therein.
JA963775+