2-Aminopyrrolyl Dilithium Compounds: Synthesis, Structural Diversity

Aug 12, 2013 - Lithium complex of 2-amino-functionalized benzoylpyrrole: Synthesis, structure, and catalytic activity for the cyclotrimerization of is...
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2‑Aminopyrrolyl Dilithium Compounds: Synthesis, Structural Diversity, and Catalytic Activity for Amidation of Aldehydes with Amines Zhiqiang Guo,† Qiao Liu,‡ Xuehong Wei,*,†,‡ Yongbin Zhang,† Hongbo Tong,† Jianbin Chao,† Jianping Guo,§ and Diansheng Liu*,† †

Institute of Applied Chemistry, Shanxi University, Taiyuan, 030006, People’s Republic of China School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006, People’s Republic of China § Solid Wastes Resource Utilization and Energy Saving Building Materials State Key Laboratory, Beijing, 100041, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Five dilithium compounds containing bidentate dianionic pyrrolyl ligands, [{2-(CH3NCH2)C4H3N}Li2(TMEDA)3] (1), {[μ-η5-2-[CH3CH2NCH2]C4H3N]Li2(TMEDA)}2 (2), {[μη 5 :η 1 -2-[(CH 3 ) 3 CNCH 2 ]C 4 H 3 N]Li 2 (TMEDA)} 2 (3), {[η 5 -2[(CH3)2CHNCH2]C4H3N]Li2 (TMEDA)}2 (4), and {[η5-2[(CH2)5CHNCH2]C4H3N]Li2(TMEDA)}2 (5), were synthesized, and their structural features were provided. Compounds 1−5 were proved to be a series of efficient catalysts for amidation reactions of aldehydes with amines in good to excellent yields under mild conditions.

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INTRODUCTION Over the past few years, substituted pyrrolyl as a supporting ligand with various metals has attracted increasing attention by virtue of its strong metal−ligand bonds and exceptional tunable steric and electronic features required for compensating coordinative unsaturation of metal centers.1 Especially, they hold more flexible coordination modes ranging from N-η1 (σ) to -η5 (π). It was demonstrated that donor-substituted pyrrolyl ligands can bind to the metal atoms in either a η1 or a η5 fashion,2 but only a few examples exist in which the pyrrolyl units bind to metal atoms in both ways simultaneously.3 Furthermore, the coordination mode (η1 or η5) of the pyrrolyl ligands may show a dramatic influence on the catalytic activity in many cases. For example, Hou and co-workers reported that rare earth metal aminobenzyl complexes with η5-coordinated pyrrolide showed a high catalytic activity and stereocontrol for the polymerization of styrene; however, η1-coordinated pyrrolyl rare earth metal aminobenzyl complexes had no catalytic activity.4 In addition, Cui’s group reported that bidentate pyrrole-imino lanthanide complexes with mixed η5/η1 coordination modes exhibited moderate to high catalytic activities toward polymerizations of lactide or isoprene; however, the polymerization of isoprene is less controlled.5 Usually, these pyrrolyl metal complexes are synthesized based on the deprotonation of iminopyrrolyl ligand precursors with nBuLi, LDA, or NaH. Then the mixture is reacted with the © 2013 American Chemical Society

corresponding transition or rare-earth metal salts. The intermediate iminopyrrolyl alkali-metal complexes are generally prepared and employed in situ.6 Hence, they have rarely been isolated from reaction solution and are poorly characterized in the solid state, let alone the characterization of their structural features. Structure elucidation is a crucial part of the understanding of reactivities and reaction mechanisms and for selective and specific applications in organolithium chemistry.7 Recently, we reported the synthesis, characterization, and application of lithium compounds containing substituted pyrrolyl ligands for the cyclotrimerization of isocyanate to corresponding isocyanurate.8 Unlike monolithium reagents, organo-dilithium reagents provide access to interesting and useful compounds that are not available by other means in terms of reactivity and synthetic application.9 In our continuing interest in developing alkali metal compounds containing substituted pyrrolyl ligands as catalysts, here we report the synthesis and structural features of five dilithium compounds containing bidentate dianionic pyrrolyl ligands with η1, η5, or both bonding modes and their catalytic activity for amidation of aldehydes with amines. Received: July 4, 2013 Published: August 12, 2013 4677

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Scheme 1. Synthetic Routes to Compounds 1−5

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C4H3N), 6.67 (s, 2H, C4H3N), 4.42 (s, 4H, CH2NCH3), 3.09 (s, 6H, CH2NCH3), 2.22(s, 12H, CH2), 1.90 (s, 36H, N(CH3)2). 13C NMR (C6D6): 136.39 (C4H3N), 105.73 (C4H3N), 102.20 (C4H3N), 100.73 (C4H3N), 60.12 (CH2NCH3), 57.79 (TMEDA), 50.20 (CH2NCH3), 45.73 (TMEDA). 7Li NMR (C6D6): −1.47, −2.26. Anal. Calcd for C30H64Li4N10: C, 60.80; H, 10.88; N, 23.63. Found: C, 60.49; H, 11.13; N, 23.34. {[2-(CH3CH2NCH2)C4H3N]Li2(TMEDA)}2 (2). The synthesis of compound 2 was carried out following procedures similar to those used for the preparation of 1. The white solid was recrystallized from a saturated diethyl ether and hexane solution to yield colorless crystals of 2 (0.583 g, 77%). Mp: 73 °C (dec). 1H NMR (C6D6): 6.95 (s, 2H, C4H3N), 6.64 (s, 2H, C4H3N), 6.37 (s, 2H, C4H3N), 4.60 (s, 4H, CH2NC2H5), 3.52 (s, 4H, CH2CH3), 2.00 (s, 24H, N(CH3)2), 1.83 (s, 8H, CH2) 1.65 (s, 6H, CH2CH3). 13C NMR (C6D6): 146.76 (C4H3N), 123.38 (C 4 H 3 N), 107.43 (C 4 H 3 N), 100.34 (C 4 H 3 N), 57.43 (CH 2 NC 2 H 5 ), 55.77 (TMEDA), 52.28 (CH 2 CH 3 ), 44.35 (TMEDA), 19.96 (CH2CH3). 7Li NMR (C6D6): 2.48, −3.99. Anal. Calcd for C26H52Li4N8: C, 61.90; H, 10.39; N, 22.21. Found: C, 61.73; H, 10.21; N, 22.08. [{μ-η5:η1-2-[(CH3)3CNCH2]C4H3N}Li2(TMEDA)]2 (3). The synthesis of compound 3 was carried out following procedures similar to those used for the preparation of 1. The white solid was recrystallized from a saturated hexane solution to yield colorless crystals of 3 (0.563 g, 67%). Mp: 71 °C (dec). 1H NMR (C6D6): 6.92 (s, 2H, C4H3N), 6.75 (s, 2H, C4H3N), 6.48 (s, 2H, C4H3N), 4.56 (s, 4H, CH2NBut), 1.90 (s,

EXPERIMENTAL SECTION

General Remarks. Unless otherwise noted, all syntheses and manipulations of air-sensitive materials were performed under a purified nitrogen atmosphere using standard Schlenk techniques. Tetrahydrofuran and diethyl ether were distilled from sodiumbenzophenone under nitrogen. Hexane and toluene were dried using sodium potassium alloy and distilled under nitrogen prior to use. All aldehydes and amines were sublimed, recrystallized, or distilled before use. 1H NMR (300 MHz), 13C NMR (75.5 MHz), and 7Li NMR (116.6 MHz) spectra of the compounds were recorded on a Bruker DRX 300 instrument in C6D6 at 298 K and referenced internally to the residual solvent resonances (1H, 13C) or externally (7Li). Elemental analyses were performed on a Vario EL-III instrument. Melting points were determined on a Stuart SMP10 melting point apparatus and uncorrected. All aminopyrrolyl ligands were synthesized according to the literature procedure.10 Preparation. [{2-(CH3NCH2)C4H3N}Li2(TMEDA)3] (1). A nBuLi hexane solution (6.0 mmol, 2.2 M) was added dropwise to a hexane (20 mL) solution of C4H3NH(CH2NHCH3)-2 (0.330 g, 3.0 mmol) at 0 °C under nitrogen. After the reaction mixture was stirred at room temperature for 2 h, TMEDA (0.679 mL, 4.5 mmol) was stepwise added, and the mixture was then stirred for another 6 h. The white solid was isolated by filtration and recrystallized from a saturated hexane solution to yield colorless crystals of 1 (0.413 g, 53%). Mp: 84 °C (dec). 1H NMR (C6D6): 7.13 (s, 2H, C4H3N), 6.84 (s, 2H, 4678

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24H, N(CH3)2), 1.73(s, 8H, CH2), 1.55 (s, 18H, CH2NBut). 13C NMR (C6D6): 138.86 (C4H3N), 115.64 (C4H3N), 99.70 (C4H3N), 93.87 (C 4 H 3 N), 48.35 (TMEDA), 43.40 (C(CH 3 ) 3 ), 39.07 (CH2NBut), 37.39 (TMEDA), 24.17(C(CH3)3). 7Li NMR (C6D6): 0.56, −4.55. 1H NMR (d8-THF): 6.49 (s, 2H, C4H3N), 5.94 (s, 2H, C4H3N), 5.64 (s, 2H, C4H3N), 4.15 (s, 4H, CH2NBut), 2.27 (s, 8H, CH2), 2.11 (s, 24H, N(CH3)2), 1.08 (s, 18H, CH2NBut). 13C NMR (d8-THF): 145.23 (C4H3N), 123.99 (C4H3N), 108.28 (C4H3N), 101.39 (C4H3N), 58.51 (TMEDA), 52.14 (C(CH3) 3), 48.68 (CH2NBut), 46.14 (TMEDA), 32.79 (C(CH3)3). Anal. Calcd for C30H60Li4N8: C, 64.27; H, 10.79; N, 19.99. Found: C, 64.01; H, 10.83; N, 19.73. [{η5-2-[(CH3)2CHNCH2]C4H3N}Li2(TMEDA)]2 (4). The synthesis of compound 4 was carried out following procedures similar to those used for the preparation of 1. The white solid was recrystallized from a saturated diethyl ether and hexane solution to yield colorless crystals of 4 (0.567 g, 71%). Mp: 93 °C (dec). 1H NMR (C6D6): 6.99 (s, 2H, C4H3N), 6.71 (s, 2H, C4H3N), 6.39 (s, 2H, C4H3N), 4.63 (s, 4H, CH2N iPr), 3.28 (s, 2H, CH(CH3)2), 1.96 (s, 24H, N(CH3)2), 1.70 (s, 8H, CH2) 1.32−1.44 (d, 6H, CH2N iPr). 13C NMR (C6D6): 147.92 (C4H3N), 124.27 (C4H3N), 108.34 (C4H3N), 101.58 (C4H3N), 56.83 (TMEDA), 55.75 (CH2N iPr), 55.29 (CH(CH3)2), 45.85 (TMEDA), 28.59 (CH(CH3)2). 7Li NMR (C6D6): −1.47, −8.69. Anal. Calcd for C28H56Li4N8: C, 63.15; H, 10.60; N, 21.04. Found: C, 63.19; H, 10.30; N, 21.19. {[η5-2-[(CH2)5CHNCH2]C4H3N]Li2(TMEDA)}2 (5). The synthesis of compound 5 was carried out following procedures similar to those used for the preparation of 1. The white solid was recrystallized from a saturated diethyl ether solution to yield colorless crystals of 5 (0.597 g, 65%). Mp: 77 °C (dec). 1H NMR (C6D6): 7.22 (s, 2H, C4H3N), 6.83 (s, 2H, C4H3N), 6.70 (s, 2H, C4H3N), 4.03 (s, 4H, CH2NCy), 2.50 (m, 2H, CH), 1.88 (s, 24H, N(CH3)2), 1.65(s, 8H, CH2), 1.60 (d, 4H, Cy), 1.54 (d, 4H, Cy), 1.19 (d, 4H, Cy), 1.04 (d, 4H, Cy), 0.79 (d, 4H, Cy). 13C NMR (C6D6): 135.93 (C4H3N), 125.84 (C4H3N), 105.04 (C4H3N), 104.24 (C4H3N), 54.05 (TMEDA), 45.46 (TMEDA),42.64 (CH2NCy), 42.00 (Cy), 30.92 (Cy), 23.50 (Cy), 22.66 (Cy). 7Li NMR (C6D6): 6.04, 5.83. Anal. Calcd for C34H64Li4N8: C, 66.65; H, 10.53; N, 18.29. Found: C, 66.27; H, 10.27; N, 18.47. Typical Procedure for Amidation Reaction. A 30 mL Schlenk flask was charged with the ether solution of dilithium compound (10.00 mL, 0.1 mmol). Aniline was added (0.09 mL, 1.00 mmol); after stirring for 0.5 h, benzaldehyde was then added (0.31 mL, 3.00 mmol). The resulting mixture was stirred at room temperature for 3 h. The reaction was monitored by TLC until complete consumption of amine. All the volatiles were removed under vacuum. Purification of the crude product by column chromatography (ethyl acetate/petroleum ether, 1:5) afforded amides. All the amides were identified by spectral comparison with literature data. X-ray Crystallography. Single-crystal X-ray diffraction data of the compounds were collected on a Bruker Smart Apex CCD diffractometer using monochromated Mo Kα radiation, λ = 0.71073 Å. A total of N reflections were collected by using the ω scan mode. Corrections were applied for Lorentz and polarization effects as well as absorption using multiscans (SADABS).11 Each structure was solved by the direct method and refined on F2 by full matrix least-squares (SHELX97)12 using all unique data. Then the remaining nonhydrogen atoms were obtained from the successive difference Fourier map. All non-hydrogen atoms were refined with anisotropic displacement parameters, whereas the hydrogen atoms were constrained to parent sites, using a riding mode (SHELXTL).13 Details of the modeling of disorder in the crystals can be found in their CIF files.

(CH2)5)-2 with 2 equiv of nBuLi in the presence of TMEDA in hexane, respectively. As shown in Scheme 1, an equal molar ratio of the substituted pyrrole and TMEDA was employed, except for the synthesis of compound 1 (the substituted pyrrole and TMEDA in a molar ratio of 1:1.5). Each of 1, 2, 3, 4, and 5 was easily purified by crystallization from hexane, diethyl ether, or a mixture of hexane and diethyl ether and was characterized by satisfactory C, H, and N microanalysis, 1H, 13C{1H}, and 7Li NMR spectra in C6D6 at ambient temperature, and singlecrystal X-ray structural data. X-ray Single-Crystal Structures of 1−5. Crystals of 1 were obtained from a saturated hexane solution at −5 °C. As shown in Figure 1, each of the lithium atoms is four-

Figure 1. ORTEP diagram of compound 1. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Li1−N1 2.104(7), Li1−N4 2.112(7), Li2−N1 2.026(7), Li2−N4 2.025(7), Li2−N2 2.193(7), Li3−N3 2.042(7), Li3−N2 2.042(7), Li3−N4 2.190(7), Li4−N3 2.060(7), Li4−N2 2.139(7), N1−C4 1.367(5), N1−C1 1.374(5), N2−C6 1.451(5), N2−C5 1.474(5), C4−C5 1.504(6), C10−C11 1.500(5); N1−Li1−N4 101.2(3), N1−Li2−N4 107.1(3), N1−Li2−N2 97.0(3), N4−Li2−N2 98.7(3), N3−Li3−N2 107.4(3), N3−Li3−N4 97.4(3), N2−Li3−N4 98.3(3), N3−Li4−N2 103.2(3), N9−Li4−N10 82.3(2), Li2−N1−Li1 75.4(3), Li3−N2−Li4 73.1(3), Li3−N2−Li2 64.3(3), Li3−N3−Li4 74.8(3), Li2−N4−Li1 75.2(3), Li2−N4−Li3 64.6(3).

coordinated by a dianionic bidentate pyrrolyl ligand and TMEDA. One of the TMEDA molecules acts as a bridged ligand, and its two nitrogen atoms are coordinated to Li2 and Li3, respectively. The bond distances of N1−Li1, N1−Li2, N3−Li3, and N3−Li4 are 2.104(7), 2.026(7), 2.042(7), and 2.060(7) Å, respectively, which are close to those in the monoanionic substituted pyrrolyl lithium compounds.7 Compound 2 was crystallized from a mixed solution of hexane and diethyl ether. As illustrated in Figure 2, the two ligand moieties are linked by two lithium atoms (Li1, Li3), where Li1 is coordinated by two pyrrolyl rings in η5 mode to form a distorted sandwich geometry. The bond distances of Li1−N1 2.195(10) Å, Li1−C1 2.213(11) Å, Li−C2 2.303(11) Å, Li1−C3 2.322(12) Å, and Li1−C4 2.247(11) Å are shorter than Li1−N3 2.280(10) Å, Li1−C8 2.240(10) Å, Li−C9 2.277(10) Å, Li1−C10 2.321(11) Å, and Li1−C11 2.322(11) Å, respectively, and the distances of Li1 to the N1C1C2C3C4 plane and the N3C8C9C10C11 plane are 1.925 and 1.961 Å, respectively. The dihedral angle between the N1C1C2C3C4 plane and the N3C8C9C10C11 plane is 17.01°.

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RESULTS AND DISCUSSION Synthesis and Characterization of Dilithium Compounds. Each of the dilithium compounds 1−5 was readily prepared in good yield from the reaction of substituted pyrrole precursor C4H3NH(CH2NHCH3)-2, C4H3NH(CH 2 NHCH 2 CH 3 )-2, C 4 H 3 NH(CH 2 NHCH(CH 3 ) 2 )-2, C4H3NH(CH2NHCH(CH3)3)-2, or C4H3NH(CH2NHCH4679

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Figure 2. ORTEP diagram of compound 2. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Li1−N1 2.195(10), Li1−C1 2.213(11), Li−C2 2.303(11), Li1−C3 2.322(12), Li1−C4 2.247(11), Li1−N3 2.280(10), Li1−C8 2.240(10), Li−C9 2.277(10), Li1−C10 2.321(11), Li1−C11 2.322(11), Li2−N2 1.967(10), Li2−N1 2.051(9), Li3−N4 1.966(10), Li3−N2 1.968(10), Li3−N3 2.312(10), Li4−N4 1.978(9), Li4−N3 2.086(9); N2−Li2−N1 84.5(4), N4−Li3−N3 76.8(3), N2−Li3−N1 69.7(3), N4−Li4−N3 82.1(3), Li2−N2−Li3 87.1(4), Li3−N4−Li4 83.2(4), N2−C5−C4 112.0(5), N4−C12−C11 110.6(4).

membered rings; the bond angles of N1−Li2−N2 and N3− Li2−N4 are 81.2(2)° and 86.6(3)°. The molecular structures of the dimeric compounds 4 and 5 are very similar and are illustrated in Figure 4 and Figure 5, respectively. The half-sandwich geometries of 4 and 5 are close to that of 3. For compound 4, the bond lengths of Li2−N2′, Li2−C1′, Li2−C2′, Li2−C3′, and Li2−C4′ are 2.187(4), 2.203(4), 2.293(4), 2.297(4), and 2.234(4) Å, respectively, and for compound 5, the bond lengths of Li2−N1, Li2−C1,

Compound 3 was crystallized from a saturated solution of hexane as a dimer (Figure 3). Li1 is coordinated by two

Figure 3. ORTEP diagram of compound 3. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Li1−N1′ 2.397(6), Li1−C1′ 2.390(7), Li1−C2′ 2.384(7), Li1−C3′ 2.363(8), Li1−C4′ 2.360(7), Li1−N2 1.986(7), Li1−N1 2.144(7), Li2−N2 1.979(7), Li2−N1 2.088(6), N2−C5 1.458(4), N2−C6 1.458(4), C4−C5 1.498(5); N2−Li1−N1 79.7(2), N2−Li2−N1 81.2(2), C4− N1−Li2 96.0(3), C4−N1−Li1 95.2(3), C5−N2−C6 113.4(3), C5− N2−Li2 97.9(3), N1−C4−C5 118.8(3), N2−C5−C4 109.3(3). Symmetry elements for 3: ′ −x, y, −z+1/2. Figure 4. ORTEP diagram of compound 4. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Li2−N2′ 2.187(4), Li2−C1′ 2.203(4), Li2−C2′ 2.293(4), Li2−C3′ 2.297(4), Li2−C4′ 2.234(4), Li1−N2 2.014(4), Li1−N1 2.029(4), Li2−N1 1.923(4), N1−C5 1.454(3), N1−C6 1.458(3), N2−Li2′ 2.187(4), C4−C5 1.495(3); N2−Li1−N1 88.79(16), C5−N1−C6 110.34(18), C5−N1−Li1 98.54(16), C4−N2−Li1 103.58(17), N2−C4−C5 119.8(2), N1−C5−C4 112.38(18). Symmetry elements for 4: ′ −x +2, −y+2, −z.

nitrogen atoms in one ligand and a pyrrolyl ring of the other ligand in η5 mode to form a half-sandwich geometry. The bond lengths of Li1−N1′, Li1−C1′, Li1−C2′, Li1−C3′, and Li1−C4′ are 2.397(6), 2.390(7), 2.384(7), 2.363(8), 2.360(7) Å, respectively. The distance of Li1 to the N1′C1′C2′C3′C4′ is 2.067 Å, which is much longer than those (1.925, 1.961 Å) in compound 2. Both of the nitrogen atoms of the bidentate ligands and TMEDA bond to Li2, forming two spiral five4680

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compounds containing bidentate dianionic pyrrolyl ligands in the amidation reaction of aldehydes with amines was tested in this paper. The initial study of the amidation reaction was carried out using benzaldehyde with aniline as a model substrate catalyzed by dilithium compounds 1−5. The results are listed in Table 1. Table 1. Optimized Reaction Conditions Using Compounds 1−5 for the Amidation of Aldehydes with Aminesa

Figure 5. ORTEP diagram of compound 5. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg): Li2−N1 2.223(11), Li2−C1 2.207(12), Li2−C2 2.261(12), Li2−C3 2.286(12), Li2−C4 2.250(11), N1−Li1 1.979(10), Li1−N2 2.005(10), Li2−N2′ 1.916(11), N2−C6 1.458(6), N2−C5 1.469(6), C4−C5 1.508(7); C4−N1−Li1 104.4(4), N2−Li1−N1 88.2(4), C6−N2−C5 110.1(4), C5−N2−Li1 99.9(4), N1−C4−C5 119.9(5), N2−C5−C4 110.9(5). Symmetry elements for 3: ′ −x+1, −y+2, −z.

entry

catalyst

mol % of catalyst

solvent

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14

1 2 3 4 5 3 3 3 1 2 4 5 3 3

5% 5% 5% 5% 5% 5% 5% 5% 5% 5% 5% 5% 10% 15%

THF THF THF THF THF hexane toluene Et2O Et2O Et2O Et2O Et2O Et2O Et2O

37.4 39.6 44.7 40.3 35.9 49.4 48.3 73.7 61.3 68.8 72.6 70.7 94.9 95.2

a The molar ratio of amine and aldehyde is 3:1, and amine was first added to the catalyst solution; then aldehyde was added after 30 min. b Isolated yield based on amine.

Li2−C2, Li2−C3, and Li2−C4 are 2.223(11), 2.207(12), 2.261(12), 2.286(12), and 2.250(11) Å, respectively. The distances of lithium to the pyrrolyl ring plane are 1.910(4) and 1.831(11) Å in 4 and 5, respectively, which are much shorter than those in 2 or 3. [The dimeric compound names (3, 4, and 5) discussed here are still used, but their solution states should be monomeric; the 13CNMR data are consistent with the monomeric solution structure.] Catalytic Activities of Compounds 1, 2, 3, 4, and 5 for Amidation of Aldehydes with Amines. In recent years, the direct amidation of aldehydes with amines to produce amides has garnered significant attention because of easy availability of starting materials. The various reaction systems have been reported, including metal-free conditions,14 metal-based catalysts,15 nontransition metals,16 and a number of rare earth systems. In view of substantial waste generation, reduced toxicity profile, and requiring either stoichiometric additives or long reaction times, the alkali metals and rare-earth metals are recognized as an attractive choice for this desirable transformation. For example, Marks has carried out this transformation by simple, commercially available rare-earth amido complexes.17 Shen’s group has reported a number of rare-earth guanidinate,18 amidinate,19 and heterobimetallic complexes20 for this transformation. Wang21 and Schafer22 also showed that rare-earth metal amido and amidate complexes are effective catalysts, respectively. However, to the best of our knowledge, only a few papers about alkali metal complexes for this transformation have been published. In those studies, the oxidative mixture of potassium iodide/TBHP (tert-butyl hydroperoxide) and sodium hydride was reported for this transformation in succession.23 In the meantime, Ishihara also reported the use of lithium diisopropylamide in a Cannizzarotype reaction.16 Thus, to further explore the application of alkali metal compounds, the catalytic behavior of the five dilithium

We were pleased to find that in the presence of 5% mol 1−5 the amidation can proceed smoothly at room temperature in THF to afford corresponding amide in moderate yields. It can be seen that the steric effects of ligand is minimal (Table 1, entries 1−5), although compound 3 gave the highest yield under the same reaction conditions. Among the different solvents used for optimization (Table 1, entries 3, 6−8), the best results was obtained when diethyl ether was employed as solvent in the presence of 5% mol of compound 3. This indicates that solvent effect plays a key role in this reaction system. Hence, the catalytic behavior of compound 1, 2, 4, or 5 for this reaction in Et2O was also investigated for comparison (Table 1, entries 3, 9−12), and the results show that compound 3 still gave the highest yield. It is worth noting that the amidation reaction can afford 94.9% isolated yield when the loading of catalyst 3 is increased to 10% (Table 1, entry 13). The yield increases slightly when catalyst loading increases to 15% (Table 1, entry 14). Using the optimized reaction conditions and preferred catalyst identified, the generality and scope of the amidation reaction catalyzed by compound 3 were investigated, as summarized in Table 2. A variety of aldehydes and amines can be smoothly transformed to the corresponding amides in good to excellent isolated yields at room temperature in 3 h, revealing the practicability of the present method. The aromatic amines with an electron-withdrawing group at the p-position on the phenyl ring give high yields relative to the amines with an electron-donating group, but there is no explicit trend in the yields. The strongly withdrawing nitro substitutent dramatically reduces reactivity (Table 2, entries 7). The steric hindrance of 4681

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consistent with the previous reports. Attempts to use alkyl aldehydes or alkyl amines as substrates, such as C6H11CHO, (CH 3) 2 CHCHO, C2 H5 CHO, C3 H 7CHO, (CH 3 )3 CNH 2 , C 4 H 8 NH, and C 5 H 10 NH in catalytic amidation were unsuccessful due to side reactions. Based on the above experimental results and related literature, a mechanism similar to those proposed for the lanthanide amide catalysts is suggested.17−19,20b,22

Table 2. Amidation of Aldehydes with Amines Catalyzed by Compound 3a

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CONCLUSION In summary, a series of dilithium compounds containing bidentate dianionic pyrrolyl ligands were prepared from the reaction of corresponding aminopyrrolyl ligand precursors with two equivalents of nBuLi in the presence of TMEDA. The resulting corresponding compounds 1−5 were obtained in moderate to good yields. The X-ray diffraction analysis indicated that the compounds exhibited structural diversity with different substituents, i.e., with the pyrrolyl ring coordinated to lithium metal in η1 mode, η5 mode, or both ways simultaneously. These dilithium compounds are efficient catalysts for the amidation reactions of aldehydes with amines under mild reaction conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Copies of the 1H NMR and 13C NMR spectra for amidation products (S1), crystal data details of data collection and refinements (S2), and crystallographic data for compounds 1− 5 in CIF format (S3) are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel: 008615536566161. Fax: 0086-351-7011688. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (No. 20572065) and the Natural Science Foundation of Shanxi Province (Nos. 2008011021, 2011021011-1) is gratefully acknowledged.

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REFERENCES

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