HUA Yu-Peng PANG Jing-Lin GAO Yn-Peng WEI Xue-Hong②

a (Ordos Institute of Technology, Ordos 017000, Inner Mongolia, China)

b (Scientific Instrument Center, Shanxi University, Taiyuan 030006, China)

ABSTRACT The sequential reaction of the N,N,O-tridentate ligand HOC(CH2)5CH2N(Me)CH2 CH2NMe2 (LH) in diethyl ether with AlEtCl2 and MeLi afforded a lithium-aluminum complex containing a polymeric structure of one-dimensional chain [{-LiOC(CH2)5CH2N(Me)CH2CH2NMe2}AlMe2CH3-}n] (1), which was characterized by 1H, 13C NMR spectra, elemental analyses and single-crystal X-ray diffraction analysis. Complex 1 crystallizes in triclinic, space group P with a = 10.526(3), b = 10.949(3), c = 17.799(4) ?, β = 76.211(11)°, V = 1915.3(8) ?3, Dc = 1.014 g·cm–3, F(000) = 648 and μ = 0.104 mm–1. In addition, the heterobimetallic complex 1 was used to catalyze the Meerwein-Ponndorf-Verley (MPV) reaction and exhibited excellent catalytic activities in good yield (up to 97%).

Keywords: heterobimetallic complex, synthesis, characterization, catalytic reaction;

1 INTRODUCTION

The heterobimetallic complexes were widely reported because they often exhibit a special synergy that makes them more reactive than their homometallic precursors. These heterobimetallic formulations as metalate (ate) reagents are surveyed focusing mainly on containing an alkali metal paired with another metal (alkali metal, magnesium, calcium, zinc, aluminum, gallium, etc)[1-6].

The first alkali metal zincate [Na(ZnEt3)] was announced by Wanklyn in 1959[7,8]. In recent decades, various types of heterobimetallic complexes were continually reported to selectively functionalize neighbouring meta and para positions of aromatic molecules. For example, Mulvey and co-workers reported a range of lithium aluminates and alkali amide magnesites which had been successfully utilized in metal-hydrogen exchange reactions of challenging weakly acidic aromatic substrates[9-17]. Mongin′s group provided a series of bimetallic metalate complexes combining lithium with a variety of secondary metals[18-26]. Wheatley contributed a number of lithium-aluminum complexes[27-30], of whichiBu3Al(TMP)Li is designed for regionally and chemically selective direct generation of functionalized aromatic com- pounds[30].

Recently, Blair has reported four heterobimetallic (Na/Mg) complexes and revealed the contrasting heterobimetallic deprotonation with homometallic induced ethene elimination reactivity[31]. A series of lithium aluminates and lithium alkylmagnesiates have been synthesized by our group[32], such as [{-LiOC(CH2)5CH2N(Me)CH2CH2NMe2}Al(Me)(nBu)CH3-}n] (a), [{-LiOC(CH2)5CH2N(Me)CH2CH2NMe2}Al(nBu)2CH3-}n] (b), [{nBu2Mg{LiOC(CH2)5CH2N(Me)CH2CH2NMe2}}2] (c) and [{nBuMgOLi[LiOC(CH2)5CH2N(Me)CH2CH2NMe2]2}2] (d). Each of complexes a～d was utilized as catalyst in Meerwein-Ponndorf-Verley (MPV) reduction[33-37]applica- tions of selected carbonyl compound and isopropanol. Hopefully, complex c showed the best catalytic activity of its homometallic magnesium precursor. At the same time, it is worth noting that the catalytic activity of complex a is significantly higher than that of b with similar structure. It would be due to the steric hindrance effect of normal-butyl group for b, which added the steric hindrance of metal center and decreased the attack probability to benzaldehyde. As part of our ongoing studies in the influence of initiators’ structures on the catalytic activities and the synergistic effect of heterobimetallic complexes, we report here the synthesis and characterization of a lithium-aluminum complex [{-LiOC-

Complex 1 was characterized by NMR spectra and single- crystal X-ray diffraction techniques and was used as catalysts in MPV reactions.

2 EXPERIMENTAL

2. 1 Instruments and reagents

All experiments were carried out under purified dry nitrogen atmosphere by using standard Schlenk techniques. Solvents were dried and freshly distilled under nitrogen. The carbonyl compounds were distilled, sublimed, or recrystalli- zed before use. MeLi (1.6 M solution in diethyl ether), AlMe3(2.0 M solution in hexane) and AlEtCl2(1.0 M solution in heptane) were used as purchased from Alfa Aesar.1H NMR and13C NMR spectra of the complexes were recorded in THF-D8with a Bruker AVANCE III HD 600 instrument, using TMS as an internal standard. Melting point was determined on a STUART SMP10 melting point apparatus and uncorrected. Elemental analyses were conducted on a Vario EL-III instrument.

2. 2 Synthesis of complex 1, [{-LiOC(CH2)5CH2N(Me)- CH2CH2NMe2}AlMe2CH3-}n]

Method 1 The ligand HOC(CH2)5CH2N(Me)CH2CH2NMe2(LH) was prepared with reference to the literature[38]. To a stirred solution of LH (0.23 g, 1.1 mmol) in diethyl ether (10 mL), AlMe3(0.55 mL, 1.1 mmol) was added dropwise at 0 °C. The solution was slowly warmed to room temperature and stirred for an additional 2 h. Then MeLi (0.69 mL, 1.1 mmol) was added to the reaction mixture at 0 °C. The mixture was allowed to warm to room temperature and stirred for 2 h. The reaction mixture was filtered and crystallized from tetrahydrofuran-hexane at –5oC to give colourless crystals of complex 1. Yield: 0.24 g (77%).

Method 2 AlEtCl2(2.1 mL, 2.1 mmol) was added dropwise at 0oC to a solution of LH (0.44 g, 2.1 mmol) in diethyl ether (10 mL). The solution was then warmed to room temperature and stirred for 2 h. Then MeLi (2.6 mL, 4.2 mmol) was added to the reaction mixture at 0 °C. The mixture was allowed to warm to room temperature and stirred for 2 h. Colorless crystals were obtained from tetrahydrofuran-hexane at –5oC. Yield: 0.54 g (89%), m. p.: 176～179 °C.1H NMR (THF-D8, δ/ppm): –1.30 (s, 9H, AlCH3Li and AlC2H6), 0.98～1.02 (m, 1H, C5H10), 1.09～1.14 (m, 2H, C5H10), 1.39～1.44 (m, 5H, C5H10and NC2H6), 1.54 (s, 2H, NC2H6), 1.69～1.80 (m, 1H, NC2H6), 1.86～1.95 (m, 1H, NC2H4N), 2.04～2.15 (m, 6H, C5H10, NC2H6and C5H10CH2), 2.17～2.23 (m, 4H, NCH3and NC2H4N), 2.46～2.56 (m, 3H, NC2H4N and C5H10CH2).13C NMR (THF-D8, δ/ppm): –5.41 (AlCH3Li and AlC2H6), 24.08 (C5H10), 24.21 (C5H10), 26.00 (C5H10), 37.84 (NC2H6), 40.99 (NC2H6), 46.54 (NCH3), 57.52 (NC2H4N), 57.84 (NC2H4N), 64.66 (C5H10CH2), 72.11 (C5H10CO-).7Li NMR (THF-D8, δ/ppm): 0.73.27Al NMR (THF-D8, δ/ppm): 149.55. Anal. Calcd. (%) for C15H33AlLiN2O: C, 61.83; H, 11.42; N, 9.61. Found (%): C, 61.74; H, 11.75; N, 9.93.

2. 3 General procedure employed for the MPV reaction

The carbonyl compound (2.0 mmol) and complex 1 (0.1 mmol) were added to a 50 mL Schlenk flask, followed by the addition of isopropanol (0.19 mL, 2.4 mmol). The reaction mixture was then refluxed for 6 hours, and the yield was determined by1H NMR spectroscopic study based on the integration in the methylene and the CHO region of the benzyl group.

2. 4 Structure determination

The crystal of complex 1 was coated in oil and then directly mounted on the diffractometer under a stream of cold nitrogen gas. Single-crystal X-ray diffraction data of complex 1 were collected on a Bruker Smart Apex CCD diffractometer using monochromated MoKα radiation (λ = 0.71073 ?). A total of N reflections were collected by using an ω scan mode. A total of 6698 reflections were collected with 8994 unique ones (Rint= 0.0937), of which 5066 observed reflections with I > 2σ(I) were used in the succeeding refinements. Corrections were applied for Lorentz and polarization effects as well as absorption using multi-scans (SADABS)[39]. The molecular structure was solved by direct methods and refined on F2by full-matrix least-squares technique (SHELX-97)[40]. All non-hydrogen atoms were refined with anisotropic dis- placement parameters, whereas the hydrogen atoms were constrained to parent sites, using a riding mode (SHE- LXTL)[41]. The final R = 0.0650, wR = 0.1424 (w = 1/[σ2(Fo2) + (0.0231P)2+ 5.3400P], where P = (Fo2+ 2Fc2)/3), S = 0.988, (Δ/σ)max= 0.000, (Δρ)max= 0.363 and (Δρ)min= –0.346 e/?3. The selected bond lengths and bond angles for complex 1 are given in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

3 RESULTS AND DISCUSSION

3. 1 Description of the structure

The synthesis of complex 1 by ligand LH presented here is shown in Scheme 1, which has two ways. Method 1: Reaction of LH with trimethyl aluminum in diethyl ether results in the facile evolution of methane gas and the formation of colorless solution. Treatment of the solution with equivalent amount of methyl lithium obtained the colorless crystal complex 1 in 77% yield.

Method 2: The sequential reaction of LH with equivalent amount of ethylaluminum dichloride and two equivalents of methyl lithium in diethyl ether afforded complex 1 in 89% yield. The yield of this method is obviously better than that of method 1. It is worth noting that the adding sequence of methyl lithium and aluminum reagent is tried to be changed and the expected product is not available.

Scheme 1. Synthesis of lithium-aluminum complex 1

Complex 1 crystallizes in triclinic, space group P. The ORTEP drawing of complex 1 indicated that the crystal structure is of one-dimensional polymerization (Fig. a). The fragment of two monomers results from a weak intermolecular bond of lithium atom by Al-bonded methyl-C atom (mean Li(2)–C(15) = 2.428 ?) in the structure unit of 1 (Fig. b), which is similar to that of complexes a and b[32]. The four-coordinated aluminum atom is surrounded by the oxygen atoms from the ligand and three methyl-C atoms in an approximately tetrahedral geometry in the structure of monomer. The charge on the negative aluminate fragment is offset by that on the metal lithium centre. The geometry around the lithium (Li(1)) atom is three-coordinated with the oxygen (O(1)) atom from ligand and two nitrogen (N(1), N(2)) atoms. The bond distance of Li(2)–C(15) is significantly longer than that in complexes a (2.373 ?) and b (2.337 ?). The Al(1)–C(15) (2.015 ?) is slightly longer than Al(1)–C(13) (1.992 ?) and Al(1)–C(14) (1.995 ?), respectively. Meanwhile, Al(1)–C(15) is slightly longer than that (2.002 and 2.010 ?, respectively) in complexes a and b. Compared with the structures of complexes a and b, there is a greater spatial advantage around aluminum atoms in complex 1.

Fig. 1. (a) Polymeric chain structure of 1, (b) Structure unit of 1 (at 25% probability level). Hydrogen atoms are omitted for clarity

3. 2 Complex 1 as catalyst for the MPV reaction

According to the previous work[32,42,43], the study of the MPV reaction was carried out using selected carbonyl compounds and dry isopropanol catalyzed by the heterobime- tallic complex 1. The results are shown in Table 2. Firstly, complex 1 at 5 mol% loading under solvent-free in 6 hours afford 94% yield of benzaldehyde to benzyl alcohol (Table 2, entry 1). Then the electronic effects were examined by employing 4-subsitituted benzaldehyde. 4-Chlorobenzalde- hyde of electron-withdrawing group on the phenyl ring gave an obviously better yield than 4-anisaldehyde (97%, 87% yield, Table 2, entries 2 and 3). In addition, citronellal and some α,β-unsaturated aldehydes were also reduced to the corresponding alcohols under our catalytic system in good yield (Table 2, entries 4～6). Due to the conjugative effect, the yield of citronellal was significantly higher than that of cinnamaldehyde and citral. Finally, low active acetophenone and cyclohexanone were also reduced to corresponding alcohols in 78% and 85% yields, respectively (Table 2, entries 7 and 8). The chirality of the product of MPV reaction was further tested. Each of 1-phenylethanol products catalyzed complexes 1～3 was proved to be a racemic mixture, respectively.

Table 2. Complex 1 Catalyzed MPV Reactions of Selected Carbonyl Compoundsa

4 CONCLUSION

In summary, a new heterobimetallic complex 1 was synthesized, characterized and used for catalytic study of MPV reaction. 1 exhibited good catalytic activity for a range of aldehydes and ketones, which could be rapidly and selectively transferred to the corresponding alcohols about 78～97% yield.