Alkyl Complexes of Lanthanum and Samarium Supported by a Chiral

ARTICLE pubs.acs.org/Organometallics

Alkyl Complexes of Lanthanum and Samarium Supported by a Chiral Binaphthylamido Ligand. Syntheses, Structures, and Catalytic Activity in Hydroamination of Amino-1,3-dienes Isabelle Aillaud,† Clarisse Olier,† Yulia Chapurina,†,§ Jacqueline Collin,†,‡ Emmanuelle Schulz,†,‡ R
egis Guillot,‡ J
er^ome Hannedouche,*,†,‡ and Alexander Trifonov*,§ †

Equipe de Catalyse Mol
eculaire, Universit
e Paris-Sud, ICMMO, UMR 8182, Orsay, F-91405, France CNRS, Orsay, F-91405, France § G. A. Razuvaev Institute of Organometallic Chemistry of Russian Academy of Sciences, Tropinia 49, 603600 Nizhny Novgorod, GSP-445, Russia ‡

bS Supporting Information ABSTRACT: The reactions of (R)-(þ)-2,20 -bis(cyclopentylamino)-1,10 -binaphthyldiamine with [Li(THF)n]3[LnMe6] (Ln = La, Sm) in THF at 0 °C depending on the metal ion size result in the formation of trinuclear [(R)-C20H12(NC5H9)2]Sm[(μ-Me)2Li(THF)2(μ-Me)Li(THF)] (1) or tetranuclear [(R)-C20H12(NC5H9)2]La{(μ-Me)2Li(THF)[(μMe)Li(THF)]2} (2) complexes with methyl ligands μ-bridging lanthanide and lithium atoms. The structures of complexes 1 and 2 were established by single-crystal X-ray diffraction and solution NMR studies. These new complexes were evaluated as catalysts to promote the enantioselective hydroamination of amino-1,3dienes.

’ INTRODUCTION Lanthanides possess a unique set of properties that makes their derivatives promising objects for investigation in both stoichiometric and catalytic reactions. The combination of large ionic radii,1 Lewis acidity, and the presence of unoccupied 5d and 6s orbitals provides a pronounced tendency of the lanthanide compounds to coordinate. A substantial variation of the ionic radius in the series of lanthanide metals (from 0.885 Å for Sc to 1.160 Å for La)1b gives a key possibility for tuning the geometric parameters of the metal coordination sphere and optimizing the selectivity by choosing the central atom with appropriate radius according to the specific features of the catalytic reaction. The fact that reactivity of lanthanide compounds is mainly determined by electrophilicity and large ion size emphasizes the importance of design of new ancillary ligand sets that provide a steric saturation of the metal center and kinetic stability of the metal complexes. Alkyl derivatives of lanthanides attract special attention as extremely active species that exhibit unique reactivity2 and high potential in the catalysis of a wide range of conversions of unsaturated substrates.3 Marks and co-workers were the first to report the intramolecular hydroamination of alkenes (including an enantioselective version) catalyzed by metallocene-type lanthanide alkyl complexes.4 However, the epimerization of chiral C1-symmetric catalysts in the presence of protic amine molecules occurred, thus lowering the enantioselectivity. Employment of chelating non-cyclopentadienyl chiral r 2011 American Chemical Society

ligand systems bound to the metal center by two covalent M
X (X = N, O) bonds could allow overcoming this limitation. Moreover, multidentate ancillary ligand systems provide enhanced means of design of the steric environment around the metal center and control of the reactivity. That is why in past decade the trend toward the search for new catalysts based on non-cyclopentadienyl ligands was observed.5 Recently we described the synthesis of new chiral binaphthylamido alkyl ate and neutral yttrium and ytterbium complexes that proved to be very efficient catalysts for enantioselective intramolecular hydroamination of aminopentenes or aminohexene at room temperature with enantiomeric excesses of up to 83%.6 At this stage the challenge was to elaborate new highly efficient catalysts able to promote hydroamination of sterically demanding substrates and to obtain more sophisticated scalemic nitrogen heterocycles. Formerly Marks and co-workers demonstrated that the reaction rate in the series of organolanthanides is very sensitive to steric demands around the metal center, increasing with larger Ln3þ ionic radius and more open supporting ligation,3a,b,7 that is why we focused on the synthesis of chiral alkyl complexes of larger lanthanides (La and Sm). Here we report on the synthesis of new ate alkyl complexes of lanthanum and samarium coordinated by a Received: March 25, 2011 Published: June 02, 2011 3378

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Scheme 1. Synthesis of Samarium and Lanthanum Alkyl Ate Complexes Supported by Chiral Binaphthylamido Ligands

chiral binaphthylamido ligand and their catalytic activity in enantioselective hydroamination of amino-1,3-dienes.

’ RESULTS AND DISCUSSION Synthesis and Characterization of New Chiral Ate Amido Alkyl Lanthanide Complexes. Recently we reported on the

synthesis of a new family of neutral and ate rare earth alkyl complexes supported by chiral binaphthylamido ligands.6 The alkane elimination reactions of (R)-(þ)-2,20 -bis(cyclopentylamino)-1,10 -binaphthyldiamine (binamH2) with Ln(CH2SiMe3)3(THF)2 (Ln = Y,8a Yb8b) or with in situ-generated species [LiLn][LnMe4] (Ln = Y, Yb)6 were used as a synthetic approach. The ate alkyl derivatives [(R)-C20H12(NC5H9)2]Ln[(μ-Me)2Li(L)2(μ-Me)Li(L)] (Ln = Y, Yb; L = THF, Et2O, TMEDA) turned out to possess enhanced thermal stability (at room temperature in inert atmosphere) compared to that of related neutral alkyl compounds [(R)-C20H12(NC5H9)2]LnCH2SiMe3(L) (Ln = Y, Yb; L = THF, DME), demonstrating similar enantioselectivity in intramolecular hydroamination reactions with lower catalytic activity for the neutral alkyl compounds. The accessibility of neutral alkyl compounds is limited to the derivatives of small lanthanide metals for which tris(alkyl) derivatives Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Y, Tb, Er, Yb, Lu)8 are isolable. Since catalytic activity of organolanthanides is known to increase with an increase of ionic radius of the metal atom,3a,b,7 we focused on development of synthetic methods that allow the preparation of stable alkyl derivatives of larger lanthanides coordinated by chiral binaphthylamido ligands. The anionic alkyl complexes [LiLn][LnR4] (R = CH2SiMe3, CMe3; Ln = Y, Sm, Er, Tb, Yb, Lu; L = THF, Et2O, TMEDA) and [LiLn]3[LnMe6] (Ln = Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; L = TMEDA, DME) were first described in the late 1970s and attracted our attention as promising precursors for the preparation of chiral alkyl ate complexes.8b,9 Anionic complexes [Li(THF)n]3[LnMe6] were generated in situ by reacting anhydrous rare earth chlorides LnCl3 (Ln = La, Sm) with six equivalents of MeLi in THF at 0 °C. Addition of an equimolar amount of (R)-(þ)-2,20 -bis(cyclopentylamino)-1,10 -binaphthyldiamine L1H2 to the resulting solutions occurred with methane evolution (Scheme 1). Evaporation of THF under vacuum, extraction of the solid residue with toluene, and subsequent recrystallization of the product from THF
hexane mixtures

allowed obtaining complexes 1 and 2 (Scheme 1) in 77% and 69% yields, respectively. New complexes were isolated as orange (1) and dark yellow (2) air- and moisture-sensitive crystalline solids that are soluble in THF, Et2O, and toluene and poorly soluble in hexane. When kept in C6D6 solution at ambient temperature, both complexes decompose during one day but can be kept at
20 °C for several weeks without decomposition. It should be noted that depending on the metal ionic radius these reactions afford compounds differing in the number of MeLi fragments included in the molecule and consequently in the coordination number of the central atom. The samarium (ionic radius of six-coordinate Sm3þ is 1.098 Å)1a derivative 1 has a structure analogous to that previously found for the yttrium (1.04 Å)6 complex (see 3 in Scheme 1). Both samarium and yttrium compounds6 contain two MeLi fragments, while in the case of larger lanthanum (1.172 Å)1a three MeLi units are present in the coordination sphere. The 1H and 13C NMR spectra of diamagnetic complex 2 in C6D6 at ambient temperature demonstrated the expected sets of signals corresponding to the binaphthyldiamido ligand and coordinated THF molecules. In the 1H NMR spectrum of complex 2 four methyl ligands bound to the lanthanum atom give rise to a sole, broadened singlet (
0.71 ppm) in contrast to two singlets (1:2 integral intensities ratio) in the spectrum of the yttrium analogue 3, which appear due to the presence of two different types of μ-bridging methyl ligands bound to the yttrium and lithium atoms. In the 13C{1H} NMR spectrum one singlet at 14.1 ppm corresponds to these groups. Surprisingly in the 7Li NMR spectrum of 2 despite the presence in the molecule of three lithium atoms in different coordination environments a sole, broad singlet appears (4.27 ppm), while in the 7Li NMR spectrum of the yttrium analogue 3 two signals were observed. The addition of a 10-fold molar excess of THF into a solution of 2 in C6D6 at ambient temperature resulted in narrowing the signals in the 1H NMR spectrum and their better resolution; nevertheless the signal attributed to μbridging methyl ligands still appears as a singlet. The variabletemperature 1H NMR study of complex 2 revealed highly fluxional behavior in C7D8 solution. Below 260 K the broad singlet corresponding to the μ-bridging methyl protons starts to decoalesce, splitting at 255 K into two signals with an integral intensities ratio of 10:2. At 215 K these protons are present as six sharp singlets in the region from
1.49 to
0.30 ppm. 3379

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Figure 1. ORTEP diagram (30% probability thermal ellipsoids) of 1 showing the atom-numbering scheme. Hydrogen atoms are omitted for clarity. Selected bond distances [Å] and angles [deg] for one of the two crystallographically independent molecules: N(1)
Sm 2.4558(12), N(2)
Sm 2.3411(12), C(50)
Sm 2.5879(14), C(60)
Sm 2.5893(15), C(70)
Sm 2.5703(17), C(1)
Sm 2.7756(13), C(2)
Sm 2.8560(13), C(3)
Sm 2.9533(15), C(4)
Sm 2.8649(14), C(50)
Li(2) 2.186(3), C(60)
Li(1) 2.161(4), C(70)
Li(1) 2.214(4), Li(1)
O(80) 1.933(4), Li(2)
O(40) 1.916(3), Li(2)
N(1) 2.053(3), Li(1)
O(90) 1.937(4), N(1)
Sm
N(2) 114.20(4), C(60)
Sm
C(70) 87.03(6), C(50)
Sm
C(70) 80.76(5).

Figure 2. ORTEP diagram (30% probability thermal ellipsoids) of 2 showing the atom-numbering scheme. Hydrogen atoms are omitted for clarity. Selected bond distances [Å] and angles [deg] for one of the two crystallographically independent molecules: C(50)
La 2.832(11), La
C(55) 2.716(10), C(60)
La 2.704(10), C(70)
La 2.704(8), C(1)
La 2.996(7), C(2)
La 3.189(9), C(3)
La 3.173(8), C(4)
La 3.079(9), N(1)
La 2.626(7), N(2)
La 2.677(8), C(50)
Li(2) 2.23(2), C(60)
Li(1) 2.21(2), C(70)
Li(1) 2.18(2), Li(1)
O(80) 1.946(19), Li(2)
O(40) 1.993(19), Li(2)
N(1) 2.017(19), Li(3)
N(2) 2.068(19), C(55)
Li(3) 2.12(2), Li(3)
O(50) 1.968(19), N(1)
La
N(2) 103.9(2), C(50)
La
C(60) 105.4(4), C(60)
La
C(70) 83.3(3), C(55)
La
C(70) 100.1(4), C(50)
La
C(55) 173.3(3).

Crystals suitable for single-crystal X-ray diffraction studies were obtained by slow condensation of hexane into the THF solutions at
20 °C. The molecular structures of 1 and 2 are depicted in Figures 1 and 2; the crystal and structural refinement data are listed in Table 3. Complexes 1 and 2 crystallize in the monoclinic space group P21 with four molecules in the unit cell. Both compounds crystallize with two crystallographically independent molecules in the asymmetric unit. Since both crystallographically independent molecules have similar geometric parameters, herein we discuss only one of them. Complex 2 crystallizes as a solvate containing a half of a THF molecule per unit, while crystals of complex 1 do not contain molecules of solvent. The single-crystal X-ray structure determinations revealed that 1 and 2 are monomeric heterobimetallic complexes. As it was mentioned above, both compounds in addition to a lanthanide atom contain lithium atoms: two in complex 1 and three in complex 2. The coordination sphere of the samarium atom in 1 is set up by the two nitrogen atoms of the chelating binaphthylamido ligand and the three carbon atoms of the μ2bridging methyl groups, thus resulting in a formal coordination number 5. In 2 the formal coordination number of La is six since one additional methyl ligand is present in the coordination sphere. In 1 the coordination environment of the samarium atom can be described as a distorted trigonal bipyramid, while for lanthanum it adopts the geometry of a distorted square pyramid. In complex 1 one of the lithium atoms is bound with two carbon atoms of the methyl fragments and coordinated by two THF molecules. The second lithium atom is coordinated by one μ2bridging methyl group, the nitrogen atom of the binaphthylamido ligand, and one THF molecule. The short contacts between the Sm atom and the carbon atoms in ipso- and ortho-positions to the amido groups of the naphthalene fragments have been

observed in complex 1 (2.7756(13), 2.8560(13), 2.9533(15), 2.8649(14) Å). The Sm
C bond lengths in 1 (C(50)
Sm 2.5879(14), C(60)
Sm 2.5893(15), C(70)
Sm 2.5703(17) Å) are comparable to the related distances in complexes [{Me3SiC5H4}2Sm(μ-Me)]2 (2.556(4), 2.590(4) Å) and [{(Me3Si)2C5H3}2Sm(μ-Me)]2 (2.578(5), 2.580(5) Å)10a but shorter than the μ-bridging bonds in heterobimetallic complex (C5Me5)2Sm[(μ-Me)A1Me2(μ-Me)]2Sm(C5Me5)2 (2.644(21), 2 0.639(22), 2.689(21), 2.688(21) Å).10b The Sm
N bond lengths in 1 are 2.4558(12) and 2.3411(12) Å. The average Sm
C bond length in 1 (2.5825 Å) is close to the value reported for the yttrium analogue 3 (2.540 Å)6 when the difference in ionic radii is taken into consideration. In complex 2 the coordination environment of the lanthanum atom is set up by four methyl groups μ-bridging La and Li atoms and two nitrogen atoms of the binaphthylamido ligand. Complex 2 also contains three Li atoms. One Li atom is coordinated by two methyl fragments and one THF molecule, while two others are bound with one methyl group, one nitrogen atom of the binaphthylamido ligand, and one THF molecule. A short contact (2.996(7) Å) between the lanthanum atom and the ipso-carbon atom of one of the naphthalene fragments was found in 2. The La
C(methyl) bond lengths in 2 fall into the region 2.704(10)
2.832(11) Å, and the average La
C(methyl) bond length (2.739 Å) is close to the related value reported for the sevencoordinate lanthanum methyl ate complex (Me3Si)3C5H2La[(μMe)2AlMe2]2 (2.749 Å)11 but considerably shorter than that in seven-coordinate La(AlMe4)3 (2.811 Å).12 The La
N bond distances in 2 are 2.626(7) and 2.677(8) Å. Rare Earth-Catalyzed Hydroamination of Amino-1,3dienes. Rare earth-catalyzed enantioselective intramolecular hydroamination of aminoolefins has been a focus of intense 3380

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Table 1. Asymmetric Hydroamination of Amino-1,3-diene 4 Catalyzed by the (R)-LH2/LnCl3/MeLi Precatalyst Systema

entry

metal b

LH2/Ln/MeLi molar ratio

t (h)

conv (%)

E/Z

ee(E) (%)

ee(Z) (%)

18 1.5

81 78

52/48 60/40

21 38

5 21

1.5

95

44/56

17

12

95

58/42

23


8

1 2

Y (3) Sm (1)c

3

La (2)c

L1H2

4

Sm

L1H2

1/1/5

5

Sm

L2H2

1/1/5

1.8

99

83/17

53

8

6

Sm

L2H2

1/1/6

2

89

78/22

52


2

7

Sm

L3H2

1/1/5

1

99

81/19

55

12

8

La

L2H2

1/1/6

0.25

93

66/34

50

5

L2H2 (THF) L1H2 (THF)

1/0/4 1/0/4

0.33 1

99 95

77/23 61/39

52 33

0 11

9 10 a

ligand L1H2 L1H2

25

Reactions were performed with 10 mol % catalyst at rt. b Isolated complex YL1MeLi (ether/TMEDA). c Isolated complex

Table 2. Asymmetric Hydroamination of Amino-1,3-diene 6 Catalyzed by the (R)-LH2/LnCl3/MeLi Precatalyst Systema

entry

a

metal b

ligand

t (h)

LH2/Ln/MeLi molar ratio

conv

E/Z

ee(E) %

ee(Z) (%)

32 24

82 73

70/30 68/32

56 35

11 27

1 2

Y (3) Sm (1)c

3

La (2)c

L1H2

24

88

80/20

23

33

4

Y

L2H2

1/1/5

18

90

90/10

55

48

5

Sm

L2H2

1/1/5

23

99

90/10

39

46

6

La

L2H2

1/1/6

23

93

92/8

33

34

7

L1H2 (THF)

1/0/5

20

70

67/33

48

1

8

L2H2 (THF)

1/0/5

95

83/17

64

15

L1H2 L1H2

2.5

Reactions were performed with 10 mol % catalyst. b Isolated complex YL1MeLi (ether/TMEDA). c Isolated complex.

interest during the past decade,5,13 but only a few studies have been devoted to the cyclization of amino-1,3-dienes, although this reaction gives access to synthetically useful heterocycles.14 Marks reported the sole example of lanthanocene-catalyzed cyclization of amino-1,3-dienes and an asymmetric version with up to 71% ee (measured on the hydrogenated products).15 We have recently found that chiral lithium amide salts prepared by in situ combination of an N-substituted (R)-(þ)-1,10 -binaphthyl2,20 -diamine ligand and a commercial methyllithium solution are efficient for the catalysis of hydroamination/cyclization of primary amino-1,3-dienes, affording the expected heterocycles with diastereomeric ratios (E:Z) of up to 92:8 and enantiomeric excesses of up to 72% for diastereomer E.16 Rare earth amido alkyl ate complexes, isolated or in situ prepared, are now investigated for the catalysis of the same reactions, and the results are compared with those given by lithium amide salts (Tables 1 and 2). Isolated yttrium complex 3 coordinated by N-cyclopentyl binaphthylamine ligand (R)-L1H2 was found to be a highly active and enantioselective catalyst for the cyclization of aminoolefins in pyrrolidines and piperidines with up to 75% ee.6 We

thus examined first its efficiency for the hydroamination reaction of (4E,6)-heptadien-1-amine 4. The reaction performed with 10 mol % of isolated yttrium complex 3 at room temperature afforded a mixture of diastereomers E and Z in equal amounts and with low enantiomeric excesses (Table 1, entry 1). Isolated samarium complex 1 proved to be by far more active and led to the pyrrolidine 5 with a similar diastereomeric ratio and slightly higher enantiomeric excesses (entry 2). The lanthanum analogue complex 2 coordinated by the same chiral ligand furnished low selectivities (entry 3) with, however, a similar activity to 1. For the hydroamination of aminoolefins catalyzed by yttrium complex 3 we found similar results whether the catalyst was prepared in situ or recrystallyzed as observed for other binaphthylamido rare earth catalysts.6 Conversely the samarium catalyst prepared in situ from ligand (R)-L1H2, SmCl3, and MeLi in a 1/1/5 ratio was less active and afforded enantiomeric excesses of 5E and 5Z with different values compared to those given by complex 1 (entries 4 and 2). In an effort to improve the selectivities for the samarium-catalyzed reactions, we prepared complexes with N-benzyl-substituted binaphthylamine ligand (R)-L2H2 by in situ combination with five or six equivalents of methyllithium and 3381

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Table 3. Crystallographic Data and Structure Refinement Details for 1 and 2 1

2

formula

C45H54Li2N2O3Sm

C96H116La2Li6N4O7

Mr cryst size, mm3

835.14 0.31  0.30  0.28

1757.39 0.31  0.125  0.03

cryst syst

monoclinic

monoclinic

space group

P21

P21

a, Å

16.6559(4)

19.1789(13)

b, Å

13.5871(3)

10.6715(6)

c, Å

18.4721(4)

22.0313(14)

R, deg

90

90

β, deg γ, deg

90.9210(10) 90

96.469(4) 90

cell volume, Å3

4179.80(16)

4480.4(5)

Z

4, 2

2

T, K

100(1)

100(1)

F000

1720

1816

μ, mm
1

1.444

0.995

2θ range, deg

1.63
45.33

0.93
30.58

reflns collected reflns unique

138 035 60 991

101 214 26 335

Rint

0.0234

0.0588

GOF

1.033

1.264

reflns obsd (I > 2σ(I))

54 060

24 407

No. of params

955

1001

Flack param

0.006(3)

0.11(2)

wR2 (all data)

0.0776

0.2427

R value (I > 2σ(I)) largest diff peak and

0.0332
1.093; 1.276

0.0980
0.855; 0.914

hole (e Å
3)

Figure 3. Structures of ligands LH2 for hydroamination reactions.

used them without isolation for the cyclization of 4. Higher diastereoselectivities than with catalysts prepared from (R)-L1H2 were observed with five equivalents of MeLi as well as good enantiomeric excess (53%) for 5E (entry 5). Increasing the amount of MeLi from five to six equivalents during the preparation phase of the catalyst furnished no improvement in the stereoselectivities (entry 5 vs entry 6). Changing the ligand to (R)-(þ)-2,20 -bis(2-naphthyl)-1,10 -binaphthyldiamine, L3H2, afforded a more active catalyst than the one with ligand (R)-L2H2, with similar results in terms of diastereo- and enantioselectivities (entry 5 vs entry 7). An analogous lanthanum complex in situ prepared from (R)-L2H2 led extremely rapidly to pyrrolidine 5, albeit with no better diastereo- or enantioselectivities (entry 8). The cyclization of 4 catalyzed by the lithium salt (R)-L2Li2 has been previously studied.16 Catalyst that was prepared in THF with a 1/4 (R)-L2H2/MeLi ratio afforded 52% ee for 5E, while

5Z was racemic (entry 9). The analogous in situ-prepared samarium catalyst afforded the same enantiomeric excesses for 5E and 5Z as lithium amide (R)-L2Li2 but proved to be less active (entry 9 vs 5). Comparison of lanthanum and lithium catalysts prepared from (R)-L2H2 did not show marked differences in enantioselectivities or rate of reactions (entry 9 vs 8). The reaction of cyclization of 4 catalyzed by the dilithium salt prepared from ligand (R)-L1H2 was faster than cyclizations catalyzed by either isolated complex Y(3), Sm(1), or La(2) (entry 10 vs entries 1
3). The stereodifferentiation values for this cyclization were similar to the ones observed with complex Sm(1) except for the ee value of minor diastereomer Z (entry 10 vs 2). Transformation of (5E,7)-octadien-1-amine 6 in piperidine 7 was next studied (Table 2). All reactions were realized at 50 °C due to lower reaction rates for the formation of piperidines than for pyrrolidines. Comparison of reactions catalyzed by isolated complexes 1
3 coordinated by ligand (R)-L1H2 gave the highest enantiomeric excess for isomer 7E (56%) with yttrium complex 3, while the highest ee for 7Z (33%) was obtained with the lanthanum complex 2 (entries 1
3). Interestingly these values are higher than those observed with lithium salt (R)-L1Li2 prepared in THF (entry 7). With yttrium, samarium, and lanthanum catalysts in situ prepared from (R)-L2H2, LnCl3, and MeLi, the reaction rates and diastereomeric ratios were similar, while enantiomeric excesses varied with the metal to only a small extent (33
55% for 7E, 34
48% for 7Z) (entries 4
6). The chiral amide (R)-L2Li2 prepared in THF yielded 64% ee for 7E and 15% ee for 7Z (entry 8). Enantiomeric excesses of piperidine 7Z were thus higher with catalysts prepared from rare earth chlorides and (R)-L2H2 than with (R)-L2Li2, indicating the rare earth plays a role in these reactions. All new prepared complexes thus proved efficient in promoting the asymmetric hydroamination of both aminodienes, with the chiral lithium salts demonstrating in all cases higher reactivities than their rare earth counterparts. Contrary to our findings for the hydroamination/cyclization of simple aminoolefins, binaphthyl ligands bearing benzyl-type groups afforded higher enantioselectivities than the sterically more hindered cyclopentyl moiety. This trend was already observed in lithium-based catalysis.16 The easily in situ-prepared samarium- and lanthanum-based catalysts led to the expected substituted pyrrolidine and piperidine with enantioselectivities matching those obtained with corresponding lithium salts, except for the preparation of 7Z, which could be isolated with higher ee values. In general, the use of rare earth amido alkyl catalysts isolated or in situ prepared did not allow an improvement of enantioselectivity in comparison with chiral amide lithium salts for the cyclization of amino1,3-dienes. Moreover, contrary to previous observations concerning yttrium complexes involved in aminoolefin cyclization,6 different values were obtained in terms of activity and selectivity whether the samarium catalyst was in situ prepared or not. This probably accounts for the existence of different species in equilibrium, in solution, possessing obviously different reactivity. Nevertheless, in these reactions a lithium species could be one of the active species either with in situ-prepared catalysts or with isolated complexes since crystals of the latter contain MeLi.

’ CONCLUSION The reactions of (R)-(þ)-2,20 -bis(cyclopentylamino)-1,10 -binaphthyldiamine with [Li(THF)n]3[LnMe6] (Ln = La, Sm) in 3382

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Organometallics THF enable obtaining isolable alkyl derivatives supported by a chiral ligand that are generally hard to access for lanthanide metals having large ionic radii. Stabilization of methyl derivatives in the form of ate complexes allows handling these highly reactive species at room temperature and facilitates their application for catalytic reactions. Depending on the metal ion size trinuclear [(R)-C20H12(NC5H9)2]Sm[(μ-Me)2Li(THF)2(μ-Me)Li(THF)] or tetranuclear [(R)-C20H12(NC5H9)2]La{(μ-Me)2Li(THF)[(μ-Me)Li(THF)]2} complexes with methyl ligands μbridging lanthanide and lithium atoms were isolated and structurally characterized. The propensity of these new alkyl amido ate complexes to promote the hydroamination/cyclization of amino1,3-dienes was compared to analogous yttrium or lithium complexes. As it is now well-established,17 higher reaction rates were obtained by using rare earth precatalysts with increasing ionic radii, a tendency markedly pronounced for the synthesis of pyrrolidine 5. Nevertheless, and for both hydroamination tests, the highest reactivities were recorded by using the chiral lithium salts as catalysts. Some evidence for the implication of a rare earth ate-catalyzed path has also been detected here, which suggests that these bimetallic complexes should be investigated as catalysts for other asymmetric intramolecular hydroamination reactions.

’ EXPERIMENTAL SECTION A. General Procedures. All manipulations were carried out under an argon atmosphere by using standard Schlenk or glovebox techniques. THF was distilled from sodium benzophenone ketyl, degassed by freeze
pump
thaw method, and stored over activated 4 Å molecular sieves. d6-Benzene and diethyl ether were dried with sodium benzophenone ketyl, transferred under vacuum, and stored over activated 4 Å molecular sieves. MeLi (1.6 M in ether) was purchased from Acros Organics and used as received. (R)-(þ)-1,10 -Binaphthyl-2,20 -diamine (BinamH2) was purchased from Sigma-Aldrich and used without any further purification. Ligands (R)-L1H2,18 (R)-L2H2,19 and (R)-L3H213g were prepared according to reported procedures. Substrates 4 and 6 were prepared as reported15a and dried overnight on 4 Å molecular sieves with a few drops of d6-benzene prior to use. Yttrium complex 3 was prepared according to ref 6. Lanthanide metal analyses were carried out by complexometric titration. The C, H, N elemental analyses were made in the microanalytical laboratory of the G. A. Razuvaev Institute of Organometallic Chemistry. Bruker AM250, Bruker AV300 and AV360, and DRX400 NMR spectrometers, operating at 250, 300, 360, and 400 MHz, respectively, were used for recording the 1H NMR spectra. Chemical shifts were referenced internally according to the residual solvent resonances. B. Preparation of the Precatalytic Species. [(R)-C20H12(NC5H9)2]Sm[(μ-Me)2Li(THF)2(μ-Me)Li(THF)] (1). MeLi (1.46 mL, 1.6 M in Et2O, 2.34 mmol) was added to a suspension of SmCl3 (0.100 g, 0.39 mmol) in THF (5 mL) at 0 °C under vigorous stirring. The reaction mixture was stirred at 0 °C for 10 min. When the precipitate of SmCl3 disappeared, a solution of binamH2 (0.163 g, 0.39 mmol) in THF (2 mL) was added. Gas evolution was observed, and the orange solution was stirred at 0 °C for 10 min and allowed to warm to room temperature. The solvent was evaporated under vacuum, and the solid residue was extracted with toluene (2  5 mL). Filtration of the solution, evaporation of toluene by vacuum condensation, and recrystallization of the product by slow hexane condensation into the THF solution at
20 °C allowed isolation of complex 1 as an orange crystalline solid (0.25 g, 77%). IR (Nujol, KBr, cm
1): 1929 s, 1818 s, 1602 w, 1587 w, 1533 m, 1334 s, 1302 s, 1284 s, 1073 m, 1047 m, 853 s, 774 s, 676 m, 620 m, 514 s, 470 w. 1H NMR (300 MHz, C6D6, 303 K):
0.30 (br s, 6 H, CH3 Sm(μ-

ARTICLE

CH3)2Li), 0.31 (br s, 3 H, CH3 Sm(μ-CH3)Li), 0.76 (br s, 6 H, CH2, C5H9), 0.92 (br s, 6 H, CH2, C5H9), 1.48 (s, 14 H, CH2, β-CH2 THF and CH2, C5H9), 1.71 (s, 2 H, CH2, C5H9), 3.85 (s, 12 H, CH2, R-CH2 THF), 4.12 (m, 2 H, CH, C5H9), 6.86
8.21 (m, 12 H, CH, Ar) ppm. 13 C{1H} NMR (360 MHz, C6D6, 303 K): 9.0 (s, Sm(μ-CH3)2Li(THF)2), 11.7 (s, Sm(μ-CH3)Li(THF)), 22.0 (s, CH2, C5H9), 22.8 (s, CH2, C5H9), 25.0 (s, CH2, C5H9), 25.4 (s, CH2, β-THF), 30.0 (br s, CH2, C5H9), 57.5 (br s, CH, C5H9), 68.5 (s, CH2, β-THF), 116.5 (s, Ar), 118.0 (s, Ar), 120.2 (s, Ar), 122.9 (s, Ar), 125.9 (s, Ar), 129.1 (s, Ar), 129.7 (s, Ar), 131.1 (s, Ar) ppm. 7Li NMR (155.4 MHz, C6D6, 303 K): 9.52 (s), 12.91 (s) ppm. Anal. Calcd for C45H63Li2N2O3Sm (844.18): C 64.02, H 7.52, Sm 17.81. Found: C 64.12, H 7.47, Sm 17.93. [(R)-C20H12(NC5H9)2]La[{(μ-Me)Li(THF)}2(μ-Me)2Li(THF)] (2). MeLi (1.52 mL, 1.6 M in Et2O, 2.44 mmol) was added to a suspension of LaCl3 (0.10 g, 0.41 mmol) in THF (5 mL) at 0 °C under rigorous stirring. The reaction mixture was stirred at 0 °C for 10 min. When the precipitate of LaCl3 disappeared, a solution of binamH2 (0.17 g, 0.41 mmol) in THF (2 mL) was added. When the gas evolution was finished, the orange solution was stirred at 0 °C for 10 min and allowed to warm to room temperature. The solvent was evaporated under vacuum, and the solid residue was extracted with toluene (2  5 mL). Filtration of the solution, evaporation of toluene by vacuum condensation, and recrystallization of the product by slow hexane condensation into the THF solution at
20 °C allowed isolation of complex 2 as a dark yellow, crystalline solid (0.25 g, 69%). IR (Nujol, KBr, cm
1): 1608 s, 1589 s, 1535 w, 1292 m, 1153 m, 1128 w, 1095 w, 1066 w, 1033 s, 846 w, 807 s, 771 w, 741 s, 620 w, 585 w, 532 w. 1H NMR (300 MHz, C7D8, 293 K):
0.71 (br s, 12 H, CH3, La(μ-CH3)Li), 1.26 (s, 18 H, CH2, C5H9 and β-CH2, THF), 1.73 (s, 8 H, CH2, C5H9), 2.58 (br s, 4 H, CH2, C5H9), 3.37 (m, 14 H, R-CH2 THF), 4.46 (br s, 2 H, CH, C5H9), 6.81
7.00 (m, 6 H, CH, Ar), 7.50
7.81 (m, 6H, CH, Ar) ppm. 13C{1H} NMR (400 MHz, C7D8, 303 K): 14.1 (br s, CH3, La(μ-CH3)Li), 24.4 (s, CH2, C5H9), 24.6 (s, CH2, C5H9), 25.1 (s, CH2 β-THF), 35.5 (s, CH2, C5H9), 36.7 (s, CH2, C5H9), 58.9 (s, CH, C5H9), 67.7 (s, CH2 R-THF), 113.1 (s, Ar), 116.2 (s, Ar), 117.7 (s, Ar), 124.8 (s, Ar), 125.2 (s, Ar), 125.6 (s, Ar), 127.0 (s, Ar), 128.1 (s, Ar), 128.9 (s, Ar), 138.3 (s, Ar), 155.8 (s, Ar) ppm. 7Li NMR (155.4 MHz, C6D6, 293 K): 4.34 (s) ppm. Anal. Calcd for C48H70Li3N2O3.5La (890.74): C 64.72, H 7.91, La 15.59. Found: C 64.79, H 7.81, La 16.01.

C. General Procedure for NMR-Scale Asymmetric Intramolecular Hydroamination
Cyclization of Amino-1,3dienes 4 and 6 Catalyzed by Isolated Complexes Y(3), Sm(1), and La(2). In the glovebox, the amino-1,3-diene 4 or 6 (0.20 mmol) was dissolved in C6D6 (0.1 mL) and dried on 4 Å molecular sieves for two hours at room temperature. The complex Y(3), Sm(1), or La(2) (0.02 mmol) was dissolved in C6D6 (0.7 mL) and introduced into a J. Young NMR tube equipped with a Teflon valve, and the amino-1,3diene solution was then introduced. The NMR tube was maintained at room temperature for the cyclization of amino-1,3-diene 4 and heated out of the glovebox at 50 °C for amino-1,3-diene 6. The hydroamination reaction was monitored by 1H NMR. The conversion of the reaction was monitored by comparative integration of the signal relative to the olefinic protons of the substrate and the signal relative to the protons of the product. After the appropriate time, the reaction was quenched with CH2Cl2.

D. General Procedure for NMR-Scale Asymmetric Intramolecular Hydroamination
Cyclization of Amino-1,3dienes 4 and 6 with in Situ Preparation of the Catalyst. In an argon-filled Schlenk tube equipped with an O-ring tap (J. Young) a diethyl ether solution of 1.6 M MeLi was added dropwise to a mixture of LnCl3 (0.05  10
3 mol) and ligand (0.05  10
3 mol) in THF (4 mL) at room temperature. The homogeneous reaction solution was then allowed to stir 10 min at ambient temperature and concentrated in vacuo. In an argon-filled glovebox, the corresponding residue was dissolved in benzene-d6 (2 mL), and a 800 μL aliquot of the mixture was taken off by 3383

dx.doi.org/10.1021/om2002624 |Organometallics 2011, 30, 3378–3385

Organometallics a micropipet and transferred to a vial containing the substrate (0.20  10
3 mol). The reaction mixture was then introduced into a screw-tap or a J-Young NMR tube and maintained at room temperature for the cyclization of amino-1,3-diene 4 and heated out of the glovebox at 50 °C for amino-1,3-diene 6. The conversion of the reaction was monitored by comparative integration of the signal relative to the olefinic protons of the substrate and the signal relative to the protons of the product. After the appropriate time, the reaction was quenched by addition of a small amount of dichloromethane.

E. Determination of the Enantiomeric Excess Values and Configurations. The enantiomeric excess values were determined by HPLC analysis of the derivatized product using either a (S,S)-OJcolumn (iPrOH/hexane, 10/90; 0.8 mL min
1, λ 254 nm) (5a) or a AD-H column (EtOH/hexane, 1.5/98.5; 1 mL min
1, λ 210 nm, 35 °C) (7a). Typical procedure of derivatization: To a solution of the corresponding cyclized product (0.20 mmol) in CH2Cl2 (5 mL) was added dimethylaminopyridine (6.0 mg, 0.04 mmol), triethylamine (55 μL, 0.40 mmol), and 2-benzoyl chloride (55 μL, 0.36 mmol) at ambient temperature. After stirring for 2 h, a saturated aqueous solution of ammonium chloride (4 mL) was poured into the reaction mixture and the layers were separated. The aqueous layer was extracted with CH2Cl2 (3  5 mL). The combined organic layer was then washed with a saturated aqueous solution of ammonium chloride (5 mL), dried over anhydrous magnesium sulfate, filtered, and concentrated in vacuo. The crude product was purified by preparative TLC plate (silica gel) (EtOAc/cyclohexane, 30/70).

ARTICLE

respectively obtained from derivatization of enantiomerically pure (R)-2-((Z)-prop-1-enyl)pyrrolidine 5 and (R)-2-((Z)-prop-1-enyl)piperidine 7 with 2-benzoyl chloride as previously described.16 Enantiomerically pure (R)-2-((Z)-prop-1-enyl)pyrrolidine 5, racemic (()(Z)-5, enantiomerically pure (R)-2-((Z)-prop-1-enyl)piperidine 7, and racemic (()-(Z)-7 were respectively synthezised as previously reported.16 G. X-ray Crystallography. X-ray diffraction data for 1 and 2 were collected by using a Kappa X8 APPEX II Bruker diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073 Å). Crystals were mounted on a CryoLoop (Hampton Research) with Paratone-N (Hampton Research) as cryoprotectant and then flash-frozen in a nitrogen-gas stream at 100 K. The temperature of the crystal was maintained at the selected value (100 K) by means of a 700 series Cryostream cooling device to within an accuracy of (1 K. The data were corrected for Lorentz, polarization, and absorption effects. The structures were solved by direct methods using SHELXS-9720 and refined against F2 by full-matrix least-squares techniques using SHELXL-9721 with anisotropic displacement parameters for all non-hydrogen atoms. Hydrogen atoms were located on a difference Fourier map and introduced into the calculations as a riding model with isotropic thermal parameters. All calculations were performed by using the Crystal Structure crystallographic software package WINGX.22 The absolute configuration was determined by refining the Flack23 parameter using a large of Friedel’s pairs. The crystal data collection and refinement parameters are given in Table 1. CCDC 817813 and 817814 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_ request/cif.

’ ASSOCIATED CONTENT (E)-1-N-Benzoyl-2-(prop-1-enyl)pyrrolidine, 5a: [R]25D
28.8 (c 1.65 in CHCl3). IR ν (neat, cm
1): 3583, 3469, 3058, 3027, 2966, 2880, 1626, 1576, 1446, 1411, 1349, 1259, 1075, 1028, 962. 1H NMR (300 MHz; CDCl3) (mixture of two rotamers): 7.53
7.35 (10H, m, aryl CH), 5.69 (1H, m, H0 20 ), 5.50 (1H, m, H0 10 ), 5.26
5.12 (2H, m, H10 and H20 ), 4.77 (1H, m, H0 2), 4.26 (1H, m, H2), 3.84
3.38 (4H, m, H5 and H0 5), 2.54
2.09 (8H, m, H3, H4 and H0 3, H0 4), 1.72 (3H, d, J 5.4, CH0 3), 1.54 (3H, d, J 5.4, CH3). 13C NMR (62.5 MHz; CDCl3): 170.7, 137.8, 137.7, 131.2, 130.9, 129.9, 129.5, 128.3, 128.1, 127.4, 127.0, 126.6, 125.9, 61.2, 58.5, 50.1, 46.1, 32.9, 31.6, 25.0, 22.1, 17.9, 17.6. Found 216.1387 [M þ H]þ; C14H18NO requires m/z 216.1383. (E)-1-N-Benzoyl-2-(prop-1-enyl)piperidine, 7a: [R]25D
33.7 (c 2.15 in CHCl3). IR ν (neat, cm
1): 3583, 3434, 3025, 2937, 2858, 1716, 1628, 1576, 1445, 1424, 1370, 1270, 1074, 1023, 969. 1H NMR (400 MHz; CDCl3;
20 °C) (mixture of two rotamers): 7.38
7.29 (10H, m, aryl CH), 5.87
5.41 (5H, m, H0 2, H10 , H20 and H0 10 , H0 20 ), 4.58 (1H, d, J 12.8, H6b), 4.36 (1H, m, H2), 3.49 (1H, d, J 12.8, H0 6b), 3.12 (1H, dd, J 13.2 and 12.8, H0 6a), 2.91 (1H, dd, J 13.2 and 12.8, H6a), 1.81
1.55 (18H, m, H3, H4, H5, H0 3, H0 4, H0 5 and 2  CH3). 13C NMR (100 MHz, CDCl3,
20 °C): 171.2, 170.7, 136.3, 129.5, 129.4, 129.3, 128.7, 128.5, 128.3, 127.7, 127.4, 126.8, 126.6, 126.2, 126.1, 126.0, 56.0, 49.7, 43.9, 37.7, 30.4, 29.2, 26.3, 25.6, 19.6, 18.2. Found 230.1541 [M þ H]þ; C15H20NO requires m/z 230.1539.

F. Retention Time of Derivatized Products 5a and 7a. Retention times were compared to racemic standard samples prepared by hydroamination reaction with lithium salt prepared from racemic ligand (()-H2L2 followed by a derivatization reaction as previously described.16 E/Z configurations and absolute configuration were assigned from comparison of the elution order of authentic samples of enantiomerically pure (R)-(Z)-5a and (R)-(Z)-7a, which were

bS

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*(J.H.) Tel: þ33 1 69 15 47 40. Fax: þ33 1 69 15 46 80. E-mail: [email protected]. (A.T.) Tel: þ7 8314 63 35 32. Fax: þ7 8312 12 74 97. E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been supported by the Russian Foundation for Basic Research. MENSR and ANR are acknowledged for financial support and CNRS for giving financial support through a GDRE grant (“Homogeneous Catalysis for Sustainable Development”) and a doctoral grant (together with the Conseil G
en
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