Highly Efficient and 1,2-Regioselective Method for the Oligomerization

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Highly Efficient and 1,2-Regioselective Method for the Oligomerization of 1‑Hexene Promoted by Zirconium Precatalysts with [OSSO]-Type Bis(phenolate) Ligands Norio Nakata,* Kazuaki Nakamura, and Akihiko Ishii* Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan

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S Supporting Information *

ABSTRACT: A mixture of zirconium complex 7, which carries a phenyl-substituted [OSSO]-type bis(phenolate) ligand, and dried modified methylaluminoxane (dMMAO) catalyzes the 1,2-regioselective oligomerization of 1-hexene at relatively low catalyst loadings (0.0056 mol %) to produce the corresponding vinylidene-terminated dimer, 5methyleneundecane (74−80%), and trimer, 7-butyl-5-methylenetridecane (8−11%). The observed turnover frequencies (TOFs) are relatively high (up to 11 100 h−1). When a mixture of 2,6-dimethylphenylsubstituted precatalyst 8 and dMMAO was used, the oligomerization of 1-hexene proceeded effectively to afford predominantly the dimer (87− 91%) together with a small amount of the trimer (8−11%) at remarkably high TOFs (up to 6640 h−1).

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highest TOF (1945 h−1), and the conversion and dimerization selectivity reached 100 and 94%, respectively.10 Nonmetallocene complexes of zirconium (3) and hafnium (4) supported by 1,4-dithiabutane-bridged [OSSO]-type bis(phenolate) ligands, which were developed by Okuda et al., can act as precatalysts in this reaction.11 Upon activation with B(C6F5)3, precatalysts 3 and 4 selectively produce vinylidene-terminated oligo(1-hexene)s (93−95%), albeit at lower TOFs (196 h−1 for 3; 3.8 h−1 for 4) compared to those of the 1/MAO system. Very recently, Abu-Omar et al. presented that B(C6F5)3activated complexes of zirconium (5) and hafnium (6) that bear [ONNO]-type amine bis(phenolate) ligands catalyze the oligomerization of 1-hexene to produce exclusively vinylideneterminated dimers in up to 97% yield with relatively high TOFs (430 h−1 for 5; 25 h−1 for 6) at 85 °C.12 However, these catalyst systems suffer from low activity even at high temperature, and their use for the industrial production of renewable fuels is not feasible. Recently, our group reported several zirconium complexes that contain an [OSSO]-type bis(phenolate) ligand based on a trans-1,2-cyclooctanediyl core. These complexes serve as precise polymerization catalysts for α-olefins and yield excellent isotactic poly(α-olefin)s.13,14 Motivated by these results and the desire to further explore reactions catalyzed by [OSSO]-type complexes, we would like to report herein the high performance of dried-modified methylaluminoxaneactivated (dMMAO)15 zirconium precatalysts 7 and 8, which

INTRODUCTION Over the past two decades, metal-catalyzed α-olefin oligomerization has emerged as an important process to generate detergents, synthetic fuels, plasticizers, and lubricants.1,2 In this context, regioselectivity is an important aspect that influences the composition of the resulting oligomers. For example, the dimerization of 1-hexene via a metal−hydride intermediate can produce 10 constitutional and geometric isomers that include vinylidenes and vinylenes when 1,2-insertion or 2,1-insertion followed by β-H elimination is combined.3 Of particular interest is the regioselective oligomerization of 1-hexene, as the corresponding vinylidene-terminated dimers and trimers such as 5-methyleneundecane and 7-butyl-5-methylenetridecane serve as suitable precursors for renewable C8−C22 platforms for jet and diesel fuels.4 Although extensive research efforts have been devoted to developing new catalyst systems that improve the 2,1-regioselectivity in the oligomerization of 1hexene for the production of vinylene-terminated oligo(1hexene)s,5 only very few catalyst systems have been reported that insert 1-hexene 1,2-regioselectively to produce vinylideneterminated dimers as the major product.6,7 In their pioneering study, Slaugh and Schoenthal discovered that the (η5C5H5)2ZrCl2(1)/Me3Al system with an Al/Zr ratio of 4:1 realizes the dimerization of 1-hexene under complete 1,2regioselectivity at turnover frequencies (TOFs) of 189−960 h−1 to afford 5-methyleneundecane in up to 95% yield.8 Some substituted zirconocenes and ansa-type complexes also oligomerize 1-hexene in the presence of a minimal excess of MAO to yield 5-methyleneundecane as the main product.4d,e,9 Among these, 1,3-siloxo-bridged zirconocene 2 showed the © XXXX American Chemical Society

Received: June 14, 2018

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DOI: 10.1021/acs.organomet.8b00411 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

integration ratio of vinylidene peaks suggested an oligomeric distribution of 78% of dimer, 9% of trimer, and 13% of other oligomers. The GC-MS analysis of the obtained oligomers revealed the following distribution: 76% of dimer, 11% of trimer, 6% of tetramer, 4% of pentamer, and 3% of hexamer (Figure 1). This oligomeric distribution was in relatively good agreement with that calculated based on the 13C NMR data. When the oligomerization temperature was increased to 40 °C, the TOF and conversion values increased to 7560 h−1 and 42%, respectively (Table 1, run 2). The neat oligomerization of 1-hexene with 7/dMMAO was successful at 25 or 40 °C, and the dimer was obtained as the main product (74−78%) under complete 1,2-regioselectivity, and increased TOF (7510 h−1 at 25 °C; 8580 h−1 at 40 °C) and conversion values (42% at 25 °C; 48% at 40 °C) were observed (Table 1, runs 3 and 4). However, a full conversion of 1-hexene was not achieved, not even upon increasing the catalyst loading (0.056 mol %) and/ or extending the reaction time (24 h) (Table 1, run 5). Thus, it seems that the reaction does not reach completion due to the deactivation of the Zr−H-active species during the oligomerization.3 To investigate the effect of different loadings of the cocatalyst, we subsequently screened varying amounts of dMMAO in the oligomerization of 1-hexene (Table 1, runs 6− 8). Reducing the amount of dMMAO to 100, 150, and 200 equiv led to the formation of vinylidene-terminated oligomers that contain high contents of dimers (74−80%) under significant improvement of the conversion and TOF. Especially noteworthy is the use of 150 equiv of dMMAO, which afforded the highest TOF (11 100 h−1) in this study (Table 1, run 7). The neat oligomerization of 1-hexene (3.0 g, 35.6 mmol) using precatalyst 8 (0.002 mmol, 0.0056 mol %) and different loadings of dMMAO (150−300 equiv) at 25 °C for 1 h also efficiently furnished vinylidene-terminated oligomers in 11− 20% conversion (Table 1, runs 9−12). In contrast to the aforementioned 7/dMMAO system, reducing the amount of dMMAO affects the TOF of this oligomerization. Thus, when precatalyst 8 was activated with 250 equiv of dMMAO, the oligomerization of 1-hexene showed the highest TOF (3,530 h−1) for the 8/dMMAO system (Table 1, run 11). Despite the sterically more congested environment at the zirconium center of 8, the corresponding TOFs (1,950−3,530 h−1) are still higher than those of the previously reported precatalysts 1− 6.8−12 Similar results were observed for oligomerizations carried out at 40 °C, which produced oligo(1-hexene)s in excellent 1,2-regioselectivity with slightly increased TOFs (3,700 h−1) (Table 1, run 13). The 13C NMR and GC-MS analyses revealed that the thus obtained oligomers possess three different chain lengths (cf. SI), i.e., there is a strong preference for the formation of the dimer (85−91%) relative to the trimer and tetramer. Interestingly, the oligomerization of 1hexene can be carried out even with a low catalyst loading (0.0019 mol %) under retention of a remarkably high TOF (6,640 h−1) (Table 1, run 14). The resulting oligomers exhibit an exceptional selectivity toward vinylidene termini (>99%) and show a dimer distribution (89%) similar to that in run 12, indicating that the catalyst loading does not influence the regioselectivity and the formation ratio of the dimer. The resulting dimer-enriched oligo(1-hexene)s represent a promising potential precursor for the alternative jet diesel fuel 5-methylundecane. Hydrogenation of the oligo(1-hexene)s obtained from run 14 in Table 1 (dimer, 89%; trimer, 8%; tetramer, 3%) with a catalytic amount of 5% Pd/C (1 mol %) in MeOH under an atmosphere of H2 led to the quantitative

Chart 1. Previously Reported Precatalysts for the 1,2Regioselective Oligomerization of 1-Hexene

possess [OSSO]-type ancillary ligands with ortho, paradiphenylphenol or ortho-{2,6-dimethylphenyl (Dmp)}-paramethylphenol substituents, respectively, in the 1,2-regioselective oligomerization of 1-hexene using relatively low catalyst loadings.16

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RESULTS AND DISCUSSION The oligomerization of 1-hexene catalyzed by dMMAOactivated zirconium complexes 7 and 8 was conducted under varying reaction conditions (Scheme 1), and the results are Scheme 1. Oligomerization of 1-Hexene Using Zirconium Precatalysts 7 and 8

summarized in Table 1. Upon activation with 250 equiv of dMMAO (0.50 mmol), phenyl-substituted precatalyst 7 (0.002 mmol, 0.0056 mol %) promoted the oligomerization of 1hexene (3.0 g, 35.6 mmol) in toluene to yield 1.00 g (conversion = 33%) of a product of low volatility after 1 h at 25 °C (Table 1, run 1). The corresponding TOF (5950 h−1) is approximately 6 times higher than that of Et3Al-activated zirconocene 1 (0.063 mol %, conversion = 58.6%, 40 °C), which was reported by Slaugh and Schoenthal (TOF < 960 h−1).8 The microstructure of the produced oligomers was determined by 1H and 13C NMR spectroscopy. In the 1H NMR spectrum (see Figure S1 in the Supporting Information), a sharp signal due to the vinylidene-terminated dimer was observed at 4.68 ppm. Other much smaller signals appeared at 4.68 (overlapping with the aforementioned signal) and 4.73 ppm, which are consistent with those of isolated 7-butyl-5methylenetridecane.12,17 In sharp contrast, signals corresponding to vinylene-terminated oligomers were not observed, indicating that the oligomerization proceeded under excellent 1,2-regioselectivity (>99%). In the 13C{1H} NMR spectrum (see Figure S2 in the Supporting Information), the relative B

DOI: 10.1021/acs.organomet.8b00411 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Oligomerization of 1-Hexene Using Zirconium Precatalysts 7 and 8 Activated with dMMAOa

oligomer distributionf (%) −1

e

run

cat.

Al/Zr

temp (°C)

conv. (%)

TOF (h )

vinylidene selectivity (%)

dimer

trimer

others

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

7 7 7 7 7 7 7 7 8 8 8 8 8 8

250 250 250 250 250 100 150 200 150 200 250 300 250 250

25 40 25 40 25 25 25 25 25 25 25 25 40 25

33 42 42 48 77 61 62 42 14 11 20 11 21 13

5950 7560 7510 8580 57 10900 11100 8460 2570 2020 3530 1950 3700 6640

>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99

76 76 78 74 80 80 79 79 91 89 87 87 88 89

10 11 9 11 10 9 8 8 8 9 10 11 10 8

14 13 13 15 10 11 13 12 1g 2g 3g 2g 2g 3g

a

Conditions: 0.002 mmol 7 or 8; 1-hexene (3.0 g, 35.6 mmol); reaction time = 1 h. bSolvent: toluene (5 mL). cConditions: 0.02 mmol 7; 1-hexene (3.0 g, 35.6 mmol); reaction time = 24 h. dConditions: 0.002 mmol 8; 1-hexene (9.0 g, 107 mmol); reaction time = 1 h. eDetermined by 1H NMR spectroscopy in CDCl3. fDetermined by 13C NMR spectroscopy in CDCl3. gOnly the tetramer was formed.

Scheme 3. Oligomerization of 1-Octene Using Zirconium Precatalyst 8 and dMMAO

cane) are consistent with literature values.18 The TOF (2880 h−1) is comparable to that in run 12 for the oligomerization of 1-hexene and remarkably higher than those of arenesubstituted cyclopentadienyl zirconium complexes18,19 and dimeric lanthanide hydrides.20

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Figure 1. GC-MS chart for oligo(1-hexene)s in CDCl3 obtained using 7/dMMAO at 25 °C (Table 1, run 1).

CONCLUSION In summary, we have established a highly regioselective method for the oligomerization of 1-hexene employing zirconium(IV) complexes 7 and 8 supported by arylsubstituted [OSSO]-type bis(phenolate) ligands, which are based on the trans-cyclooctanediyl platform. Upon activation with dMMAO, these precatalysts actively oligomerize 1-hexene at low catalyst loadings (0.0019−0.0056 mol %) with excellent vinylidene selectivity. The TOF values can be tuned by changing the aryl substituent at the ortho position of the phenolate moiety in the [OSSO]-type ligand and the number of dMMAO equivalents employed. Notably, the highest TOF (11 100 h−1) in this study was observed for phenyl-substituted precatalyst 7. For the dominant formation of the corresponding dimer, 5-methyleneundecane, in up to 91% yield with remarkably high TOFs (1950−6640 h−1), the use of Dmpsubstituted precatalyst 8 was crucial. These studies on welldefined [OSSO]-type complexes may help to improve production processes for renewable jet fuels from bio-derived precursors.

formation of a viscous oil of saturated hydrocarbons, for which the 13C NMR analysis revealed 5-methylundecane as the major product (Scheme 2), which could be isolated in 81% yield by vacuum distillation. Scheme 2. Hydrogenation of Oligo(1-hexene)s

Compound 8 (0.002 mmol, 0.0075 mol %) and dMMAO (250 equiv) were also applied to the oligomerization of 1octene under similar conditions (neat, 25 °C, 1 h), which produced predominantly vinylidene-terminated oligo(1octene)s (>99%), as evident from 1H NMR data (Scheme 3). The 13C NMR spectrum of these oligo(1-octene)s showed the formation of the dimer as the major product (91%), together with minor fractions of the trimer (7%) and tetramer (2%) (see Figure S9 in the Supporting Information). The obtained NMR data for the dimer (i.e., 7-methylenepentade-

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EXPERIMENTAL SECTION

General. All manipulations of air- and/or moisture-sensitive compounds were performed either using standard Schlenk line techniques or in a Japan E300 glovebox under an inert atmosphere of

C

DOI: 10.1021/acs.organomet.8b00411 Organometallics XXXX, XXX, XXX−XXX

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argon. Toluene, 1-hexene, and 1-octene were dried and degassed over a potassium mirror by a freeze−thaw cycle prior to use. Other chemicals were used as received. 1H (400 MHz) and 13C{1H} (101 MHz) NMR spectra were obtained with a Bruker AVANCE-400 spectrometer using CDCl3 or C6D6 as a solvent at room temperature. GC-MS charts were obtained with a Bruker SCION SQ 456-GC/MS system equipped with an InertCap Pure-WAX 30 m column. [OSSO]-type zirconium complexes (7 and 8)16 and dMMAO15 were prepared by the literature procedures. General Procedure of 1-Hexene Oligomerization. A 50 mL Schlenk flask was charged sequentially with precatalyst 7 or 8 (0.0020 mol, 0.0056 mol %), dMMAO as an activator, and toluene (5 mL) at an appropriate temperature (except neat condition). After being stirred for 5 min at the temperature, 1-hexene (3.0 g, 35.6 mmol) was added to the reaction mixture. The mixture was stirred for the desired time at the temperature. The reaction was quenched by addition of MeOH and aqueous HCl, and volatile components were removed under reduced pressure. The residue was extracted with CH2Cl2, and the organic layer was washed with water and dried over anhydrous Na2SO4. The solvent was removed in vacuo to leave oligo(1-hexene) as a colorless liquid. Hydrogenation of Oligo(1-hexene). To a 100 mL three-necked flask were added oligomer (500 mg, 2.78 mmol for double bonds) and 5% Pd/C, and then the flask was purged with dry H2 gas. To the reaction mixture was added MeOH (5 mL), and the reaction mixture was stirred vigorously for 18 h at 25 °C. The mixture was diluted with CH2Cl2 (10 mL), and the mixture was passed through a pad of Celite. Then volatile components were removed under reduced pressure to leave hydrogenated product (486 mg) as a colorless liquid. Finally, 5methylundecane (442 mg, 81% yield) was purified by distillation under reduced pressure (0.1 Pa/32 °C). 5-Methylundecane:4d 1H NMR (400 MHz, C6D6) δ 0.84 (d, J = 7 Hz, 3H), 0.87−0.91 (m, 6H), 1.04−1.12 (m, 2H), 1.26 (br s, 15H); 13C{1H} NMR (101 MHz, C6D6) δ 14.1 (CH3), 14.2 (CH3), 19.7 (CH3), 22.8 (CH2), 23.1 (CH2), 27.1 (CH2), 29.4 (CH2), 29.8 (CH2), 32.0 (CH2), 32.8 (CH), 36.8 (CH2), 37.2 (CH2). General Procedure of 1-Octene Oligomerization. A 50 mL Schlenk flask was charged sequentially with precatalyst 8 (0.002 mol, 0.0075 mol %) and dMMAO (250 equiv) as an activator at 25 °C. After being stirred for 5 min at 25 °C, 1-octene (3.0 g, 26.7 mmol) was added to the reaction mixture. The mixture was stirred for desired time at the temperature. The reaction was quenched by addition of MeOH and aqueous HCl, and volatile components were removed under reduced pressure. The residue was extracted with CH2Cl2, and the organic layer was washed with water and dried over anhydrous Na2SO4. The solvent was removed in vacuo to leave 0.647 g of oligo(1-octene) as a colorless liquid.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grants 25410035 and 17K05771 (to N.N.).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00411. NMR and GC-MS charts of the obtained oligomers

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Norio Nakata: 0000-0003-4120-7924 Akihiko Ishii: 0000-0003-4638-1294 Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.organomet.8b00411 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00411 Organometallics XXXX, XXX, XXX−XXX