Aminotroponiminate–Zinc Complex‐Functionalized Mesoporous Materials: Efficient and Recyclable Intramolecular Hydroamination Catalysts (2009)

Recycling Day: A heterogeneous aminotroponiminate–Zn complex-functionalized mesoporous material prepared by grafting and post-synthetic modification, is used as a recyclable efficient catalyst for the intramolecular hydroamination reaction of non-activated alkenes. Hydroamination is carried out through five cycles without the loss of catalytic activity. The formation of nitrogen heterocycles is of considerable interest in the production of fine chemicals, pharmaceuticals, and synthetic natural products. To avoid lengthy synthetic methods, there has been a wealth of research toward these products through intramolecular hydroamination using organo-Rh,1 -Pt,2, and -Zr3 complexes, as well as organolanthanides and group three metal complexes4 as catalysts; such catalysts generally help in generating moderate to excellent yields with good reaction times (7–48 h) for a broad range of reagent functionalities. These materials, however, are generally air sensitive, costly to produce, and lack recyclability necessary for industrial scale syntheses. Zinc-based zeolites have been very effective in the cyclization of 6-aminohex-1-yne, but they have not proven useful with more bulky non-activated alkenes or alkynes, nor have they been shown recyclable.5 Herein, we report the synthesis and characterization of a zinc-aminotroponiminate (ATI) complex immobilized on mesoporous silica for use as a recyclable catalyst in the intramolecular hydroamination of a non-activated alkene, which to our knowledge, is the first of this kind on functionalized mesoporous silica. Recent studies performed by Roesky and Blechert have shown homogenous zinc aminotroponiminates (ATIs) and zinc amintroponates (ATOs) to be very effective intramolecular hydroamination catalysts for non-activated alkenes and alkynes with a broad range of functional group compatibility.6, 7 Furthermore, these compounds may easily be modified to alter the steric/electronic nature of the catalyst, are relatively stable in air and moisture at various pH levels, and utilize a nontoxic metal. Owing to its bidentate anionic nature, the ATI ligand forms very stable chelate complexes with a variety of metals.8 Such chelating properties make it an excellent material for heterogeneous catalysis to aid in the prevention of metal (active site) leaching over multiple uses. Synthesis of the ATI ligand was accomplished in a multistep process on highly ordered mesoporous silica SBA-15 with a surface area of 727 m2 g−1and pore diameter of 85 Å by grafting and post-synthesis modification (Scheme 1). Briefly, 3-aminopropyltrimethoxysilane in isopropanol was grafted to serve as the amine source toward imine formation of the ATI complex. Excess silanols were then capped using hexamethyldisilazane to produce capped aminopropyl-functionalized SBA-15 (CAPI) and to reduce possible catalyst/cocatalyst quenching. After capping, the formation of immobilized N-propyl-2-(propylamino)troponimine (DPRT) and [N-propyl-2-(propylamino)troponiminato]methylzinc (ZnDPRT) were carried out as described previously for a solution phase synthesis6b with a slightly modified procedure, which produced yellow mesoporous powders. Synthesis of DPRT and ZnDPRT. The formation of DPRT was established by FTIR, UV/Vis spectroscopy, thermogravimetric analysis (TGA), 13C and 29Si cross-polarization magic angle spinning (CP-MAS) solid state NMR spectroscopy, and elemental analysis (see the Supporting Information). The formation of DPRT is indicated in FTIR spectra by the appearance of absorption bands at 3378 and 3256 cm−1 (free and H-bonded NH stretches), a shift of the band at 1633 cm−1 (NH2 deformation) to 1625 cm−1, and concomitant formation of a peak at 1515 cm−1 (CN stretching). Comparing the TGA profiles of CAPI and DPRT, there is an additional weight loss of 3.18 wt. % in the temperature range corresponding to silanol condensation and/or organoamine loss (100–600 °C). In fact, the TGA profile of DPRT shows a steep weight loss (5.72 wt. %) over the range of 200–350 °C, which is in good agreement with the previously reported results of a [{η-8-1,4-(Me3Si)2C8H6}Y-{(iPr)2ATI}(THF)] complex and other analogues.8a, 9 Addition of 2 M ZnMe2 to DPRT formed the final active catalyst, ZnDPRT. Elemental analysis showed the immobilization of zinc and adsorption of some zinc unassociated with the ligand (see the Supporting Information). N2 sorption studies of ZnDPRT showed a type IV adsorption isotherm with delayed desorption branch, which is evidence of a slight agglomeration/bottlenecking upon the ZnMe2 addition. The BET surface area of the catalyst was 499 m2 g−1 and its pore size distribution showed bimodal Barrett–Joyner–Halenda (BJH) pore diameters of 63 and 37 Å. The latter may be a result of the formation of bottleneck pores upon immobilization of ZnMe2 species into the pore channels of the material. To explore the binding of ZnMe2 to the surface, X-ray photoelectron spectroscopy (XPS) was employed. Figure 1 shows the binding interactions in calcined SBA, CAPI, and DPRT. In all three samples, the attachment of Zn to the oxygen atoms was indicated by the binding energies of 1022.38 (ZnSBA), 1022.72 (ZnCAPI), and 1022.09 eV (ZnDPRT), respectively. Pure ZnO is known to have a binding energy of 1021.10-1021.80 eV.10, 11 The higher binding energies indicate a lower electronegativity of oxygen, as would be the case with surface silanols in mesoporous silica. X-ray photoemission (XPS) spectra showing Zn 2p peaks for a) ZnSBA, b) ZnCAPI, and c) ZnDPRT. During the synthesis of ZnDPRT, the varying concentrations of surface silanols and primary amines and the addition of the ATI ligand led to ZnO and ZnN bindings with distinguishable energies. Although the exact nature of Zn2+ binding can not be easily determined, the presence of new bonds unique to ZnDPRT was shown by the peaks on the XPS spectra of N 1s for ZnCAPI and ZnDPRT. ZnCAPI exhibited two distinct N 1s species at 400.73 and 396.88 eV, which likely correspond to free amine NH and ZnN bonds (Figure 2), and are similar to analogous materials previously reported.11 Upon formation of DPRT and subsequent Zn2+ addition, a marked shift was detected in the peak of the free amine (400.88 eV). In addition, the formation of two different ZnN bonds was evident due to the presence of both the free amine and the ATI ligand (Figure 2 B), which is comparable to other N-containing chelates.12 X-ray photoemission (XPS) spectra showing N 1s peaks for samples a) ZnCAPI and b) ZnDPRT. The material’s catalytic property was tested in intramolecular hydroamination (Scheme 2). (2,2-Diphenyl-4-pentyl)-(4-nitrobenzyl)amine (1, Scheme 2) was used as the target compound in the catalytic/recyclability trials of ZnDPRT. The catalyst was found to exhibit good efficiency over many runs without loss of activity after repeated use in the formation of the corresponding N-containing heterocycle, 2-methyl-1-(4-nitrobenzyl)-4,4-diphenyl-pyrrolidine (2, Scheme 2, Table 1). It can be seen that ZnDPRT performs competitively with homogeneous zinc ATO/ATI complexes used in the cyclization of compounds analogous to 1. Additionally, the catalyst exhibited only slight reduction in catalytic efficiency after 130 days of storage. As in previous studies, [HNMe2Ph][B(C6F5)4] was used as a cocatalyst.6, 7 Other homogeneous catalysts have lower percentage conversions and/or longer reaction time without recyclability. Intramolecular hydroamination reaction catalyzed by ZnDPRT-functionalized catalyst. Catalyst t [h] T [°C] Conversion [%][a] ZnDPRT[a,b] 1st Run 8 110 74 2nd Run 8 110 83 3rd Run 8 110 86 4th Run 8 110 86 5th Run 8 110 86 6th Run 8 110 83 ZnDPRT[a,b,c] 8 110 70 [(iPr)2(ATI)ZnMe][b,d] 1.5 80 >99 [{(iPrAT)ZnMe}2][b,e] 13 120 92 [{(iPrAT)ZnEt}2][b,e] 13 120 >99 [HNMe2Ph][B(C6F5)4][f] 24 130 83 [PtCl2(CH2=CH2)]2 : PPh3[g] 40 120 72 [Rh(cod)2]BF4[h] 7 70 62-72 Elemental analysis of C, H, N, and F content in ZnDPRT used in a single run and over six runs showed non-negligible adsorption of the cocatalyst. However, recyclability tests performed without the addition of a cocatalyst showed significantly lowered efficiency, which indicates any possible physi-/chemisorbed cocatalyst loses its catalytic activity. Therefore, recyclability trials were carried out with a fresh equivalent of cocatalyst. The catalyst was fully recyclable for five trials without loss of activity or apparent metal leaching; furthermore, there were also increases in catalytic efficiency when the catalyst was recycled, which is unlike many other heterogeneous organometallic catalysts.14 Control studies performed with ZnMe2 treated CAPI, which contains no ATI ligand and left in air in the same way as ZnDPRT also showed negligible catalytic activity. This result is in agreement with previous tests on ZnO and ZnO/SiO2 materials.5a Additionally, calcined SBA-15 impregnated with Zn(OTf)2 or Zn(OAc)2 exhibited only 4 % and 14 % conversion, respectively, over 28 h (see the Supporting Information). These results clearly indicate that the immobilized ZnDPRT catalyst is responsible for the catalytic hydroamination reaction. Furthermore, it proves that the presence of immobilized ATI ligand is crucial for the catalytic activity the Zn complex. Our study also revealed that upon recycling the catalyst, the presence of a fresh cocatalyst is important for its activity, as is the case for their homogeneous counterparts. In summary, we have synthesized the first zinc aminotroponiminate complex functionalized mesoporous silica supported heterogeneous catalyst through grafting and post-synthesis modification. The catalyst was recyclable at least five times, without losing catalytic activity. Interestingly, the recycled catalyst exhibited slightly greater activity in subsequent catalytic runs (Table 1). The long-term viability of this catalyst (showing only marginally reduced catalytic efficiency after extended storage periods of 130 days), has also been demonstrated (Table 1). This general procedure for synthesizing aminotroponiminate-functionalized mesoporous materials has potential for broader applications in other areas, including sensing and drug delivery as aminotroponiminates, and their derivatives have shown promise in catalysis,6, 7 luminescent materials, biological applications, and anticancer properties.15 Future cyclization studies will give insight into the efficiency of heterogeneous zinc aminotroponiminates with different ligand composition by varying the grafted amine and/or the 2-(alkylamino)tropone. A broader functional group compatibility study may also be performed for comparison of efficiency in homogeneous catalysts for the formation of N-containing heterocycles, as well as general trends (e.g., Thorpe–Ingold effect, functional group hydrophobicity/-philicity). Synthesis of N-propyl-2(propylamino)troponimine-zinc functionalized mesoporous catalysts SBA-15-(ZnDPRT): API was prepared by stirring 3-aminopropyltrimethoxysilane (APTMS) in solution of calcined SBA-15 dispersed in isopropanol as performed previously.16 Remaining silanol groups were then capped by stirring API (1.0 g) in toluene (200 mL) with excess hexamethyldisilazane (5 mL, 24 mmol) at room temperature for 24 h as performed previously for capped API (CAPI).17 A solution of CAPI (0.45 g) was dispersed by sonication in CH2Cl2 (200 mL) and cooled to 0 °C. Ethylated N-(propylamino)tropone was prepared by adding 1 M EtO3BF4 (1.20 mL, 1.20 mmol) in CH2Cl2 dropwise into a solution of N-(propylamino)tropone (0.261 g, 1.20 mmol) in CH2Cl2 (2.5 mL), and stirring the solution for 3 h. The ethylated N-(propylamino)tropone was added to the CAPI solution at room temperature for 16 h. The resulting yellow mesoporous powder, DPRT, was recovered by vacuum filtration and washed copiously with 95 % ethanol. The material was dried overnight at 85 °C and stored in a dessicator for later use. Synthesis of [N-propyl-2(propylamino)troponiminato]methylzinc-functionalized SBA-15 (ZnDPRT): A solution of DPRT (0.34 g) in toluene (100 mL) was prepared by sonication. Then a solution of 2 M ZnMe2 (0.33 mL, 0.66 mmol in toluene) was added dropwise to the DPRT solution and the mixture was allowed to stir at room temperature for 6 h. The resulting yellow powder was recovered by vacuum filtration and washed with 95 % ethanol (500 mL). The material was dried overnight at 85 °C and stored in a dessicator for future use. Intramolecular hydroamination reaction: In a typical cyclization experiment ZnDPRT (0.050 g) was suspended by sonication in toluene (12.5 mL) followed by addition of (2,2-diphenyl-4-pentenyl)-(4-nitrobenzyl)amine 1 (0.125 g, 0.336 mmol) and N,N-dimethylanilinium tetra(pentafluorophenyl)borate (0.050 g, 0.06 mmol). The reaction mixture was heated to 110 °C and stirred until completion of the reaction. The progress of the reaction was monitored and yields were determined by using 1H NMR spectroscopy. To test recyclability, the catalyst was recovered by vacuum filtration, washed with toluene (100 mL), and immediately redissolved in toluene (12.5 mL) by sonication followed by the addition of 1 (0.125 g, 0.336 mmol) and N,N-dimethylanilinium tetra(pentafluorophenyl)borate (0.050 g, 0.06 mmol). The reaction mixture was heated to 110 °C and stirred until completion of the reaction. Yields were monitored as above by using 1H NMR spectroscopy. We thank the US National Science Foundation (NSF), Grant No. NSF CHE-0645348, and NSF DMR-0804846 for financially supporting this work. We thank Dr. Jonathan Shu for his help with the XPS analysis. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.