Zirconocene bis(perfluorooctanesulfonate) s-catalyzed highly efficient synthesis of 1,3,5-triaryl benzene via cyclotrimerization of ketones

Article abstract of DOI:10.1080/00397911.2010.532277

Air-stable zirconocene bis(perfluoroctanesulfonate) s [Cp ₂Zr]OSO₂C₈F₁₇) ₂] (1) with high Lewis acidity and high thermal stability was prepared by the reaction between Cp₂ZrCl₂ a

Full text of DOI:10.1080/00397911.2010.532277

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Zirconocene
Bis(perfluorooctanesulfonate)s-Catalyzed
Highly Efficient Synthesis of 1,3,5-
Triaryl Benzene via Cyclotrimerization of
Ketones
Guo Ping Zhang a b , Ren Hua Qiu a , Xin Hua Xu a c , Hai Hui Zhou a
Ya Fei Kuang a & Si Hai Chen a
,
a College of Chemistry and Chemical Engineering, Hunan University ,
Changsha , China
b Shenzhen Institutes of Advanced Technology, Chinese Academy of
Sciences , Shenzhen , China
c National Key Laboratory of Elemeno-organic Chemistry, NanKai
University , TianJin , China
Accepted author version posted online: 30 Aug 2011.Published
online: 11 Nov 2011.
To cite this article: Guo Ping Zhang , Ren Hua Qiu , Xin Hua Xu , Hai Hui Zhou , Ya Fei Kuang
& Si Hai Chen (2012) Zirconocene Bis(perfluorooctanesulfonate)s-Catalyzed Highly Efficient
Synthesis of 1,3,5-Triaryl Benzene via Cyclotrimerization of Ketones, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:6, 858-864, DOI:
10.1080/00397911.2010.532277
To link to this article: http://dx.doi.org/10.1080/00397911.2010.532277
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Synthetic Communications1, 42: 858–864, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.532277
ZIRCONOCENE BIS(PERFLUOROOCTANESULFONATE)
S-CATALYZED HIGHLY EFFICIENT SYNTHESIS OF
1,3,5-TRIARYL BENZENE VIA CYCLOTRIMERIZATION
OF KETONES
Guo Ping Zhang,1,2 Ren Hua Qiu,1 Xin Hua Xu,1,3 Hai Hui
Zhou,1 Ya Fei Kuang,1 and Si Hai Chen1
1College of Chemistry and Chemical Engineering, Hunan University,
Changsha, China
2Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences,
Shenzhen, China
3National Key Laboratory of Elemeno-organic Chemistry, NanKai
University, TianJin, China
GRAPHICAL ABSTRACT
Abstract Air-stable zirconocene bis(perfluoroctanesulfonate)s [Cp2Zr]OSO2C8F17)2] (1)
with high Lewis acidity and high thermal stability was prepared by the reaction between
Cp2ZrCl2 and AgOSO2C8F17. The as-prepared complex 1 exhibits good activity and selec-
tivity in the cyclotrimerization of various ketones to the desired 1,3,5-triaylbenzenes.
Furthermore, in the cyclic experiments, the complex 1 shows little loss of activity after
10 cycles. The results afford a general and efficient method for the cyclotrimerization of
ketones using Cp2Zr(OSO2C8F17) as catalyst.
2
Keywords Cyclotrimerization; Lewis acid; perfluorooctanesulfonate; triarylbenzene;
zirconocene
INTRODUCTION
The cyclotrimerization reaction is a powerful strategy for the construction of
polyaromatic compounds, such as 1,3,5-trisubstitued benzene moiety.[1] Generally,
in the process of cyclotrimerization, the reactions of aldolization and dehydration
were completed in the presence of Lewis acids in situ to form a central benzene
moiety. As far as we know, the Lewis acids used in the cyclotrimerization reaction
Received January 21, 2010.
Address correspondence to Xin Hua Xu or Ya Fei Kuang, College of Chemistry and Chemical
Engineering, Hunan University, Changsha 410082, China. E-mail: xhx1581@yahoo.com.cn; yafeik@
163.com
858

CYCLOTRIMERIZATION OF KETONES
859
included proton acids (H2SO4, HCl, p-MeC6H4SO3H, HOOCCH2SO3H) and metallic
halides (InCl3, TiCl4, SnCl4, BeCl2, SiCl4, Cp2ZrCl2).[2–6] However, because of the
harsh reaction conditions, most of these procedures have some disadvantages such
as poor yield and poor regioselectivity. Moreover, most of these metallic halides are
highly sensitive to air and not recyclable. Therefore, developing an air-stable, highly
efficient and recyclable catalyst for the cyclotrimerization of ketones is desirable.
Recently, the cationic group of four metallocene compounds and metal triflate
have attracted increasing attention,[7,8] initially obtained by treatment of Cp2MCl2
with AgOTf and later from Cp2MMe2 and TfOH.[9,10] The complexes were success-
fully employed as catalysts for various carbon–carbon bond-forming reactions.[11]
Unfortunately, these metallocene bis(triflate)s compounds are not stable in open
air and suffer from facile hydrolysis.[12] Accordingly, improvement of the hygro-
scopic character of the cationic metallocene derivatives is in highly demand for prac-
tical utilization. Recently, Otera et al. found that organotin perfluorooctanesulfonate
was air stable and water tolerant, in sharp contrast to the corresponding organotin
triflates, which are highly hygroscopic.[13] With this in mind, we prepared novel
2
=
metallocene complexes: Cp2M(OSO2C8F17)2, [M Zr, Ti], [fCpZr(OH2)3g2(m -
OH)2][OSO2C6F5]4, and [fCpHf(OH2)3g2(m2-OH)2][OSO2C8F17]4.[14] In this article,
we report the preparation and the physiochemical properties of Cp2Zr(OSO2C8F17)2
as well as its application in the catalytic synthesis of 1,3,5-triaylbenzenes.
RESULTS AND DISCUSSION
Zirconocene bis(perfluorooctanesulfonate) (1) was prepared conveniently by the
reaction between zirconocene dichloride (Cp2ZrCl2) and silver perfluorooctanesulfo-
nate (AgOSO2C8F17) in tetrahydrofuran (THF).[14–17] Notably, the complex remained
as a dry crystal or powder and suffered no color change after being kept in open air for
1 year. The Lewis acidity of the metal complexes can be estimated by the binding
energies (4E values) of Lewis acid metal ions with Oꢀ2 on the basis of electron spin
ꢁ
resonance (ESR) spectra. The 4E values of the zirconium complex ðOꢀ2ꢁ 1Þ exhibits
relatively large value (Zr4þ: gzz ¼ 2.0331, 4E ¼ 0.91 eV), which falls between those
of Sc(OTf)3 (gzz ¼ 2.0304, 4E ¼ 1.00 eV) and Y(OTf)3 (gzz ¼ 2.0349, 4E ¼ 0.85 eV).
Therefore, the Lewis acidity of 1 was found to fall between those of Sc(OTf)3 and
Y(OTf)3.[16,18,19] The thermogravimetry–differential scanning calorimetry (TG-DSC)
curves of complex 1 are shown in Fig. 1. It can be seen that the TG curve shows three
stages of weight loss. The endothermic step below 220 ꢂC can be assigned to the
removal of water molecules. The material is stable up to about 300 ꢂC, after which
two overlapping weight losses of exothermic nature appear, plausibly a result of the
oxidation of organic entities. We observed the removal of pentafluorooctanesulfuryl
ligands at 400 ꢂC, and then compounds of zirconium fluorides were left behind.
To investigate the catalytic activity of the complex Cp2Zr(OSO2C8F17)2 (1), the
cyclotrimerization of acetophenones were employed to prepare 1,3,5-triaylbenzenes.
The experiments were carried out in refluxing toluene in the presence of 5 mol% of
Cp2Zr(OSO2C8F17)2 to give the desired 1,3,5-triaylbenzenes with excellent yields
and good stereoselectivity (Scheme 1).
All the results show that the reaction proceeded well for a variety of acetophe-
nones in Table 1. However, the properties of substituting groups present in the

860
G. P. ZHANG ET AL.
Figure 1. The TG-DSC curves of complex Cp2Zr(OSO2C8F17)2.
Scheme 1. Synthesis of 1,3,5-triarylbenzene from ketones catalyzed by 1.
phenyl influence the reaction speed. The acetophenones with electron-donating
groups in the reaction exhibited higher reactivity than those with electron-
withdrawing groups. It can be deduced that the electron-donating groups enhanced
=
the density of superconjugated p-electrons in the phenyl plane and C O bond, lead-
ing to stronger interaction of carbonyl group of acetophenones with the zirconium
atom of the complex of 1.
Because of the air stability and thermal stability of complex 1, the catalyst can
be recycled in this reaction. The cyclic experiment was carried out by cyclotrimeriza-
tion of acetonphone to 1,3,5-triphenylbenzene on a larger scale (10 mmol). After
the reaction was finished, methanol was added, and the yellow solid product was
Table 1. Yields of 1,3,5-triarylbenzene synthesized from acetophenones catalyzed by 1 in refluxing
toluene
Entry
Substrate
R
Product
3a
Time =h
Yield =%a
Select. (3/4)=%b
1
2
3
4
5
6
2a
2b
2c
2d
2e
2f
H
Cl
4a
4b
4c
4d
4e
4f
3
6
98
89
85
95
93
78
99=1
92=8
90=10
95=5
94=6
89=11
3b
3c
3d
3e
3f
Br
6
CH3
OCH3
NO2
3.5
3
24
aIsolated yield of 3.
bSelectivity is determined by 1H NMR.

CYCLOTRIMERIZATION OF KETONES
861
Figure 2. Yield of 1,3,5-triphenyl benzene and recovered catalyst 1 in the catalyst recycle procedure.
precipitated immediately. The superior solubility of the catalyst in methanol can be
up to 1222 gLꢁ1. Therefore, the catalyst can be recovered easily from the methanol
solvent by the usual workup and used directly for the next catalysis cycle in the same
reaction. The yield of the recovered catalyst decreased gradually down to 78%
isolated yield in the tenth run relative to the first run, while the yield of 1,3,5-
tripenylbenzene was still 85% (Fig. 2).
CONCLUSION
In conclusion, we have demonstrated a versatile method for the cyclotrimeriza-
tion of ketones using zirconocene bis(perfluorooctanesulfonate) as catalyst. The
approach has merits such as high selectivity and yield, operational simplicity, mild
reaction conditions, and catalyst recyclability. It is reasonably expected that this
method will be useful in organic synthesis.
EXPERIMENTAL
All chemicals were purchased from Aldrich and used as received unless other-
wise noted. The catalyst preparation reaction was carried out under nitrogen atmos-
phere with freshly distilled solvent. Tetrahydrofuran (THF) and n-hexane were
distilled from sodium=benzophenone. Melting points were measured on a RY-1
melting-point apparatus (Tianjing, China) and were uncorrected. NMR spectra
13
(1H NMR 400 M, 19F NMR 376 M, C NMR 100 M) were recorded at 25 ꢂC with
CDCl3 as solvent (unless otherwise noted) on an Inova-400 M (USA) instrument
calibrated with tetramethysilane (TMS) as an internal reference. Mass (MS) spectra
were determined on a HP5989A spectrometer. Elemental analyses were performed
on a Vario EL III (Germany). The thermal stability of the samples was investigated
with a thermal gravimetric analyzer (TGA, a Netzsch STA 449C) under a nitrogen
atmosphere.

862
G. P. ZHANG ET AL.
Preparation of Cp2Zr(OSO2C8F17)2 (1)
A solution of AgOSO2C8F17 (1.21 g, 2.0 mmol) in THF (10 mL) was added to a
solution of Cp2ZrCl2 (292 mg, 1.0 mmol) and THF (20 mL). After the mixture was
stirred at room temperature for an hour, it was filtered. Dry n-hexane (40 mL) was
laid over the colorless filtrate, and after keeping it in the refrigerator for 24 h, the col-
1
orless crystal was collected (0.794 g, 65%). MP 133–136 ꢂC. H NMR (400 MHz,
CD3CN) d 1.93 to 1.95 (t, 4H, THF), 2.37 (s, n H, H2O), 3.78 (s, 4H, THF), 6.65
(s, 10H, Cp). 19 F NMR (376 MHz CD3CN) d ꢁ79.96 to ꢁ79.00 (t, 3F, CF3–),
ꢁ112.79 (s, 2 F, –CF2–), ꢁ118.66 (s, 2 F, –CF2–), ꢁ119.62 to ꢁ119.81 [d, 6F,
–(CF2)3–], ꢁ120.65 (s, 2 F, –CF2–), ꢁ124.03 to ꢁ124.13 (m, 2F, –CF2–). Elemental
analysis calculated (%) for C26H10F34O6S2Zr (as no hydrate and no THF): C,
25.60; H, 0.83; found: C, 25.67; H, 0.82 (after pumping for a week at room tempera-
ture or heating at 80 ꢂC in the vacuum for 30 min, the catalyst used in this article).
ESR Detection of Oꢀ2ꢁ1 Complex[16]
A quartz ESR tube (4.5 mm i.d.) containing an oxygen-saturated solution
of dimeric 1-benzyl-1,4-dihydronicotinamide (BNA)2 (1.0 ꢃ 10ꢁ2 M) and
1
(1.0 ꢃ 10ꢁ3 M) in MeCN was irradiated in the cavity of the ESR spectrometer with
the focused light of a 1000 W high-pressure Hg lamp through an aqueous filter.
Dimeric (BNA)2, which was used as an electron donor to reduce oxygen, was pre-
pared according to the literature. The ESR spectra of ðOꢀ2ꢁ 1Þ complex in frozen
MeCN were measured at 143 K with a Jeol X-band apparatus under nonsaturating
microwave power conditions. The g values were calibrated precisely with a Mn2þ
marker, which was used as a reference.
Typical Procedure of Synthesis of 1,3,5-Triphenyl Benzene in
Toluene (Representative)
Acetophenone (2a) (1.0 mmol, 120 mg), 1 (0.05 mmol, 65 mg), and toluene
(3 mL) were added to a 25-mL, round-bottom flask. The mixture was stirred for
3 h at 110 ꢂC, poured into methanol (20 mL), stirred, and filtered. Crude product
of 3a was obtained. Recrystallization of the crude product was completed in ethanol
and methylene dichloride (v=v ¼ 1:1) to obtain 3a (100 mg, mp 170–171 ꢂC, yield
98%). All the analytical data for the compounds (3a–3f) are listed.
Selected Data
1
1,3,5-Triphenyl benzene (3a).[20] Mp 170–171 ꢂC; H NMR d: 7.79 (s, 3H),
7.70 (d, 6H, J ¼ 8.4 Hz), 7.49 (t, 6H, J ¼ 7.6 Hz), 7.40 (t, 3H, J ¼ 7.2 Hz); 13C NMR d:
142.1, 140.9, 136.2, 129.0, 127.5, 127.4, 125.0; MS (m=z): 306 (Mþ). Elemental analy-
sis calculated (%) for C24H18: C, 94.08; H, 5.92. Found: C, 94.12; H, 5.91.
1,3,5-Tri(4-chlorophenyl) benzene (3b).[20] Mp 243–245 ꢂC; 1H NMR d
7.72 (s, 3H), 7.61 (d, 6H, J ¼ 8.0 Hz), 7.45 (d, 6H, J ¼ 8.0 Hz); 13C NMR d 141.7,
139.1, 134.0, 129.3, 128.8, 125.1; MS (m=z): 408 (Mþ). Elemental analysis calculated
(%) for C24H15Cl3: C, 70.35; H, 3.69. Found: C, 70.40; H, 3.70.

CYCLOTRIMERIZATION OF KETONES
863
1,3,5-Tri(4-bromophenyl) benzene (3c).[20] Mp 263–264 ꢂC; 1H NMR d
7.69 (s, 3H), 7.45 (d, 6H, J ¼ 8.4 Hz), 7.38 (d, 6H, J ¼ 8.4 Hz); 13C NMR d 137.8,
135.9, 132.3,129.4, 125.5, 122.1; MS (m=z): 542 (Mþ). Elemental analysis calculated
(%) for C24H15Br3: C, 53.08; H, 2.78. Found: C, 59.02; H, 2.80.
1
1,3,5-Tri(4-methylphenyl) benzene (3d).[20] Mp 176–177 ꢂC; H NMR d
7.61 (s, 3H), 7.27–7.33 (m, 6H), 7.55–7.59 (m, 6H), 2.44 (s, 9H); 13C NMR d
142.1, 138.5, 137.6, 129.9, 127.1, 124.5, 21.6; MS (m=z): 348 (Mþ). Elemental analy-
sis calculated (%) for C27H24: C, 93.06; H, 6.94. Found: C, 93.12; H, 6.95.
1,3,5-Tri(4-methoxylphenyl) benzene (3e).[20] Mp 142–143 ꢂC; 1H NMR d
7.78 (s, 3H), 7.59 (d, 6H, J ¼ 7.7 Hz), 7.29 (d, 6H, J ¼ 7.7 Hz), 3.83 (s, 9H); 13C NMR
d 160.8, 137.7, 128.8, 128.5, 125.3,114.3, 56.7; MS (m=z): 396 (Mþ). Elemental analy-
sis calculated (%) for C27H24O3: C, 81.79; H, 6.10. Found: C, 81.78; H, 6.12.
1,3,5-Tri(4-nitrophenyl) benzene (3f).[20] Mp 152–154 ꢂC; 1H NMR d
8.23–8.25 (d, 6H, J ¼ 8.3 Hz), 7.71 (d, 6H, J ¼ 8.3 Hz); 7.63 (s, 3H), 13C NMR d
147.4, 142.8, 137.7, 128.4, 125.2, 124.5; MS (m=z): 441 (Mþ). Elemental analysis
calculated (%) for C24H15N3O6: C, 65.31; H, 3.43; found: C, 65.34; H, 3.44.
Catalyst Recycling Procedure
Acetophenone (2a) (10 mmol, 1200 mg) and a catalytic amount of catalyst of 1
(0.5 mmol, 650 mg) were added to a 250-mL, round-bottom flask. After the catalyst
was partially dissolved in acetophenone, toluene (30 mL) was added as reaction
solvent. The mixture was stirred for 3 h at 110ꢂC and then poured into methanol
(200 mL), stirred, and filtered. Crude product of 3a was obtained. Recrystallization of
the crude product was completed in ethanol and methylene dichloride (v=v ¼ 1:1) to
obtain 3a (1000mg, mp 170–171 ꢂC, yield 98%). The filtrate of methanol solvent was
evaporated. Thus, the crude cyclic catalyst was obtained (98% yield, 635 mg), which
can be used for the next reaction immediately. For 1H NMR spectroscopy, it was recrys-
tallized in THF=hexane (v=v ¼ 2:5), and pure catalyst was obtained. 1H NMR (400 M,
CD3CN) d 1.92 to 1.94 (t, 4H, THF), 2.37 (s, n H, H2O), 3.77 (s, 4H, THF), 6.66 (s, 10H,
Cp). [Cp2Zr(OSO2C8F17]2 was obtained by heating at 80ꢂC in a vacuum for 30 min.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China
(Project 20372020) and Hunan Province National Science Foundation (11JJ5009).
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