Polystyrene-bound tetrafluorophenylbis(trifiyl)methane as an organic-solvent-swellable and strong Bronsted acid catalyst

Article abstract of DOI:10.1002/1521-3773(20011105)40:21<4077::AID-ANIE4077>3.0.CO;2-1

Several advantages over inorganic solid acids such as zeolites and perfluororesinsulfonic acids such as Nafion are offered by the new reusable polystyrene-bound catalyst 1: a broader range of applications, improved yields, improved selectivity, and milder reaction conditions. Tf = F₃CSO₂.

Full text of DOI:10.1002/1521-3773(20011105)40:21<4077::AID-ANIE4077>3.0.CO;2-1

COMMUNICATIONS
CF3SO2Na (1.3 equiv)
Polystyrene-Bound
10 mol% Bu4NI
Tetrafluorophenylbis(triflyl)methane as an
Organic-Solvent-Swellable and Strong
Brùnsted Acid Catalyst**
C6F5CH2Br
C6F5CH2Tf
EtCN, reflux
4
5 (91% yield)
(1)
1. tBuLi (1 equiv), Et2O, –78 °C, 0.5 h
2 (95% yield)
Kazuaki Ishihara, Aiko Hasegawa, and
Hisashi Yamamoto*
2. Tf2O (0.5 equiv), –78 °C
3. 4 M HCl
RT, 1 h
Perfluororesinsulfonic acids such as Nafion are highly
acidic solid catalysts, and they are superior to conventional
resinsulfonic acids such as Dowex-50, Amberlite IR-112, and
Permutit-Q with regard to catalytic activity, thermal stability,
and chemical resistance.[1, 2] However, like inorganic solid
acids such as zeolites, they are not effectively swollen by most
aprotic organic solvents.[3] A resin-bound superacidic Brùnst-
ed acid that is effectively swollen by such solvents may offer
several advantages over Nafion, including a broader range of
applications, improved yields, improved selectivity, and mild-
er reaction conditions.
50% yield. When tBuLi (2 equiv) and triflic anhydride
(1 equiv) were used with 5, 4-tert-butyl-2,3,5,6-tetrafluoro-
phenylbis(triflyl)methane (6a) was obtained instead of 2 as a
major product.
To explore the generality and scope of the nucleophilic para
substitution of 2, the reaction was examined with several
alkyllithium reagents (Table 1). We confirmed that compound
6 was always produced as a single isomer by para substitution
Table 1. Regiospecific nucleophilic para substitution of 2.
The trifluoromethanesulfonyl (triflyl, Tf) group is one of
the strongest neutral electron-withdrawing groups.[4] In par-
ticular, it greatly increases the acidity of a-hydrogen atoms.[5±7]
For example, phenylbis(triflyl)methane (1, pKa ˆ 7.83 in
MeCN)[6] is a strong nonoxidizing acid. The steric and
electronic effects of the aromatic ring in arylbis(triflyl)-
methanes are expected to greatly influence their Brùnsted
acidity and their catalytic activity and selectivity for organic
reactions. We report here on concise syntheses of the new
strong organic Brùnsted acids pentafluorophenylbis(triflyl)-
methane (2) and polystyrene-bound tetrafluorophenylbis(trif-
lyl)methane (3) and their catalytic applications in organic
synthesis.
RLi[a]
Conditions
6
Yield [%]
tBuLi
BuLi
PhLi
788C, 1 h
788C, 1 h
788C !RT, 1 d
6a
6b
6c
87
> 95
> 95
[a] RLi (3 equiv) was added.
by 1H, 13C, 19F, F-F COSY, and NOE difference NMR
spectroscopy, and by single-crystal X-ray analysis on 6c[10]
(Figure 1). The C13 H13 bond in 6c is almost coplanar with
F
F
F
F
Tf
Tf
F
H
H
Tf
Tf
3
2
F
F
F
F
The synthetic strategy for 2 involves the preparation of
2,3,4,5,6-pentafluorobenzyltriflone (5) by nucleophilic substi-
tution of 2,3,4,5,6-pentafluorobenzyl bromide (4) with sodium
trifluoromethanesulfinate and subsequent reaction of 5 with
triflic anhydride [Eq. (1)].[8, 9] The first step was accomplished
in 91% yield by a slight modification of the method described
by Hendrickson et al.[4] The second step was achieved by
successive addition of tBuLi (1 equiv) and triflic anhydride
(0.5 equiv) to a solution of 5. Thus, 2 was obtained in 95%
yield based on triflic anhydride, and 5 was recovered in ca.
Figure 1. ORTEP plot of 6c. Ellipsoids are drawn at 26% probability.
Selected distances [Š] and torsion angles [8]: C13-H13 0.993(4), H13 ´´´ F2
2.2516; C9-C10-C13-H13 6.2(3), C9-C10-C13-S1 116.7(4).
the tetrafluorobenzene ring, and the torsion angle C9-C10-
C13-H13 is 6.28. Furthermore, 6 may prefer an asymmetrical
conformation in CDCl3 at room temperature, according to
NMR spectroscopic data. These experimental results can be
explained by conformational relaxation to avoid steric
repulsion between the triflyl groups and the ortho-fluoro
substituents of the tetrafluorobenzene moiety in 6.
To understand the para selectivity of the nucleophilic
substitution of 2, we recorded the 13C NMR spectra of 2 and
its lithium salt in CD3OD, and compared the chemical shifts of
the reaction sites. The 13C NMR signals of C4 were observed
at d ˆ 143.0.[11] This means that conjugation between the
anionic carbon center and the pentafluorobenzene ring is lost.
In addition, the next lowest unoccupied molecular orbital
(NLUMO) coefficients[12] and charges on the pentafluoro-
phenyl carbon atoms of the lithium salt were estimated by
theoretical calculations (RHF/STO-3G) (Figure 2).[13] The
[*] Prof. Dr. H. Yamamoto, Assoc. Prof. Dr. K. Ishihara, A. Hasegawa
Graduate School of Engineering, Nagoya University
CREST, Japan Science and Technology Corporation (JST)
Furo-cho, Chikusa, Nagoya 464-8603 (Japan)
Fax : (‡81)52-789-3222
E-mail: yamamoto@cc.nagoya-u.ac.jp
[**] The authors thank Mr. Shoichi Kondo for the single-crystal X-ray
analysis. Sodium triflate was generously donated by Central Glass Co.,
Ltd., Japan. The authors also acknowledge Dr. Yuko Wasada and Dr.
Manabu Kubota for their helpful discussions on the theoretical
calculations.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
Angew. Chem. Int. Ed. 2001, 40, No. 21
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4021-4077 $ 17.50+.50/0
4077

COMMUNICATIONS
Li+ –
Li+ –
CTf2
The catalytic activities of Brùnsted acids 2 and 3 were first
tested in the acylation of alcohols with carboxylic anhydrides,
which is known to be catalyzed by Lewis acids (Table 3).[18] As
expected, 3 exhibited an extremely high catalytic activity for
CTf2
F
F
F
F
F
F
2
6
0.223
0.049
0.187
0.073
0.210
0.145
2
6
0.235
0.146
3
5
F
F
3
5
4
4
F
F
0.402
0.196
Figure 2. Frontier electron densities (left) and atomic charges (right) of the
NLUMO at C2 ± C5 in the lithium salt of 2.
Table 3. Acetylation of ( )-menthol with carboxylic anhydrides (1.5 equiv).
Entry Catalyst [mol% Tf2CH units] (RCO)2O[a] t [h] Yield [%]
1
2
3
4
5
6
3 (0.1)
2 (3)
6c (3)
3 (3)
Ac2O
Bz2O
Bz2O
Bz2O
Bz2O
Bz2O
< 1
< 1
< 1
4
17
17
> 99
> 99
> 99
71
> 99
0
frontier electron density of NLUMO and the positive charge
at the C4 position are much greater than those at the C3 and
C5 positions. Therefore, interception of the electron donation
from the anionic carbon atom to the aromatic ring and the
steric hindrance of triflyl groups on the ortho position cause
higher reactivity at the para position.[14]
3 (6)
Nafion SAC-13[b]
[a] Ac ˆ acetyl, Bz ˆ benzoyl. [b] Nafion SAC-13 (60 gmol 1) was used.
1
To determine the acid strengths of 2 and 6c, the H NMR
the acetylation of ( )-menthol with acetic anhydride: the
reaction was complete within 1 h in the presence of 3
(0.1 mol% Tf2CH units, entry 1). Benzoic anhydride is much
less reactive than acetic anhydride.[18] Nevertheless, the
benzoylation reaction proceeded quantitatively in the pres-
ence of 3 (6 mol% Tf2CH units, entry 5). Moreover, 2 and 6c
were more active than 3 for the benzoylation (entries 2 and 3
vs entry 4). Surprisingly, Nafion-H/silica nanocomposite
(Nafion SAC-13[19]), which has a 104-fold higher surface area
than Nafion-H beads (Nafion NR50),[19] was inert in the
benzoylation (entry 6).
signals of their acidic protons were compared, and their pKa
1
values in glacial acetic acid were measured by the H NMR
1
method of Schantl et al. (Table 2).[15] Their H NMR signals
were observed at a lower field in the order 1, 2 ꢀ 6c, CHTf3. If
the pKa values shown in Table 2 are valid, 2 is a remarkably
strong acid (superacid). In addition, 6c is still of equal acidity
to concentrated sulfuric acid, although the para substitution of
2 by phenyllithium lowered the Brùnsted acidity.
Table 2. 1H NMR signals and pKa for RnCHTf3 (n ˆ 0, 1; R ˆ H, aryl).
n
CH2Tf2
1
H2SO4
6c
2
CHTf3
To demonstrate the usefulness and effectiveness of 3, its
catalytic activities in several important synthetic reactions
were compared with those of Nafion SAC-13 (Equations (3) ±
(7) and Table 4). Catalyst 3 was highly active and reusable for
all of the reactions examined: direct esterification [Eq. (3)],[1]
Friedel ± Crafts acylation [Eq. (4)],[1] acetalization [Eq. (5)],[1]
the Mukaiyama aldol reaction [Eq. (6)], and Sakurai ± Ho-
somi allylation [Eq. (7)]. The catalytic activity of 3 in the
reactions examined was superior to that of Nafion, which may
be the strongest Brùnsted acid among the known solid acids.
The polymer catalyst 3 was quantitatively recovered by
simple filtration and could be reused.[20] For example, 3
d1H[a]
4.98
13
5.97
12.5
±
6.31
7.5
6.21
1.5
7.21
±
pKa in AcOH[b]
7.5 (7.0)[c]
[a] 1H NMR chemical shift for the acidic proton of RnCHTf3 in CDCl3.
[b] See ref. [15]. [c] Literature data.[15]
n
Our goal was to develop a highly acidic heterogeneous
catalyst that can be effectively swollen by organic solvents.
Polystyrene, 1 or 2% cross-linked with divinylbenzene
(DVB), is fully swollen by aprotic solvents such as aromatic
solvents, chlorinated hydrocarbons, dioxane, and THF.[16] We
envisioned that the nucleophilic para substitution of 2 with an
alkyllithium could be adapted to prepare polystyrene-bound
tetrafluorophenylbis(triflyl)methane 3 [Eq. (2)]. Three equiv-
alents of the lithium salt of 2 were added to lithiated
catalyst
Ph
Ph
(3)
CO2H
CO2Me
OMe
MeOH, 27 °C, 29 h
OMe
catalyst
1. BuLi (3 equiv)
+
(4)
(5)
Ac2O
benzene, 60 °C, 3 h
2. Removal of excess BuLi
and solvents
MeNO2, 50 °C, 2 h
(1.2 equiv)
Ac
3
(2)
Br
3. Lithium salt of 2 (3 equiv)
catalyst
O
MeO OMe
benzene/THF = 1:1
+
HC(OMe)3
(1.2 equiv)
0 °C
RT (0.5 h),
7
toluene
0 °C, 1 h
Ph
Ph
then 70 °C (6 h)
(1 equiv)
1. catalyst
toluene
polystyrene beads,[16a] prepared from commercially available
poly(4-bromostyrene) (7; 2.71 mmolBrg resin, 2% DVB
cross-linker, 200 ± 400 mesh[17]) in situ, and the reaction
mixture was heated at 708C. After treatment with 4m HCl,
3 was obtained as a brownish resin. The loading of 2 on the
resin was estimated to be 1.01 mmol of Tf2CHC6F4 units per
gram of resin, based on fluorine content (elemental analysis).
–78 °C, 7 h
OSiMe3
OH O
+
(6)
(7)
PhCHO
1
2. 1 M HCl/THF
Ph
(1.2 equiv)
Ph
Ph
(1 equiv)
1. PhCHO (1 equiv)
–40 °C, 1 h
catalyst
OH
SiMe3
(1.5 equiv)
2. 1 M HCl/THF
CH2Cl2
RT, 0.5 h
Ph
4078
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4021-4078 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 21

COMMUNICATIONS
Table 4. Comparison of catalytic activities of 3 and Nafion SAC-13.
CCDC-165028 (6c). Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
1
Reaction
3 [gmol
Yield
[%]
Nafion SAC-13
Yield
[%]
1
(mol% Tf2CH)]
[gmol
]
[11] In contrast, the 13C NMR signal (d ˆ 104.7) for C4 of BnLi was
observed at lower field than that of toluene (d ˆ 126.1). G. Vanermen,
S. Toppet, M. Van Beylen, P. Geerlings, J. Chem. Soc. Perkin Trans. 2
1986, 707.
[12] The frontier electron density was calculated to be twice the square of
the coefficient of the NLUMO. Analysis of the orbitals reveals that
the LUMO mainly participates in the unoccupied 2s orbital of the
lithium atom.
Eq. (3)
Eq. (4)
Eq. (4)
Eq. (5)
Eq. (6)
Eq. (7)
10 (1)
30 (3)
10 (1)
5 (0.5)
30 (3)
30 (3)
94
> 99
54
> 99
> 99
89
10
±
10
5
30
30
39
±
25
16
> 99
2
[13] Geometry optimizations were performed with Gaussian98 (Revisio-
nA.5), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,
K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R.
Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V.
Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-
Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M.
Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh,
PA, 1998.
(0.1 mol% Tf2CH units) was reused for acetalization [Eq. (5)]
more than ten times, and no loss of activity was observed for
the recovered catalyst.[21] This means that the turnover
number (TON) is greater than 10000 and the turnover
1
frequency (TOF) exceeds 1000 h .
In conclusion, a practical method for preparing a novel
carbon Brùnsted superacid 2 was developed and a resin-
bound strong Brùnsted acid 3 was obtained by para substi-
tution of the lithium salt of 2 with lithiated polystyrenes. To
the best of our knowledge, this is the first example of a highly
acidic heterogeneous Brùnsted acid catalyst that is effectively
swollen by nonpolar organic solvents, and its catalytic activity
is superior to that of Nafion SAC-13. Such superacids could
make a major contribution to green chemistry.
[14] The nucleophilic substitution reaction of pentafluorobenzenes C6F5X
with weakly deactivating, nonactivating, and activating X (H, Me,
CH(C6F5)2, halogens, CF3, etc.) is para-selective, while the reaction of
compounds with strongly deactivating X (O , NH2, etc.) is meta-
selective. L. S. Kobrina, Fluorine Chem. Rev. 1974, 7, 1.
Received: June 7, 2001 [Z17241]
[15] B. M. Rode, A. Engelbrecht, J. Z. Schantl, J. Prakt. Chem. (Leipzig)
1973, 253, 17.
Â
[16] a) M. J. Farrall, J. M. Frechet, J. Org. Chem. 1976, 41, 3877; b) R.
Santini, M. C. Griffith, M. Qi, Tetrahedron Lett. 1998, 39, 8951.
[17] Purchased from Tokyo Kasei Kogyo Co., Ltd.
[18] K. Ishihara, M. Kubota, H. Yamamoto, Synlett 1996, 265.
[1] G. A. Olah, P. S. Iyer, G. K. S. Prakash, Synthesis 1986, 513.
[2] a) S. Kobayashi, S. Nagayama, J. Org. Chem. 1996, 61, 2256; b) S.
Nagayama, S. Kobayashi, Angew. Chem. 2000, 112, 578; Angew.
Chem. Int. Ed. 2000, 39, 567, and references therein; c) S. Murata, R.
Noyari, Tetrahedron Lett. 1980, 21, 767.
Â
[19] Purchased from Aldrich. B. Török, I. Kiricsi, A. Molnar, G. A. Olah, J.
Catal. 2000, 193, 132.
[20] The aromatic polymer backbone of 3, which was prepared by
nucleophilic substitution at the active surface of 7, was inert to
electrophilic attack.
[21] Reaction conditions: 3 (0.1 mol%), benzylacetone (1 equiv), trimeth-
yl orthoformate (1.2 equiv), toluene, 08C, 1 ± 2 h. After the reaction,
the solution was decanted and the residual catalyst 3 was reused
without drying.
[3] A. Corma, Chem. Rev. 1995, 95, 559.
[4] a) J. B. Hendrickson, A. Giga, J. Wareing, J. Am. Chem. Soc. 1974, 96,
2275; b) R. Goumont, N. Faucher, G. Moutiers, M. Tordeux, C.
Wakselman, Synthesis 1997, 691; c) F. Eugene, B. Langlois, E. Laurent,
J. Fluorine Chem. 1994, 66, 301.
[5] A. R. Siedle, R. A. Newmark, L. H. Pignolet, R. D. Howells, J. Am.
Chem. Soc. 1984, 106, 1510.
[6] I. Leito, I. Kaljurand, I. A. Koppel, L. M. Yagupolskii, V. M. Vlasov, J.
Org. Chem. 1998, 63, 7868.
[7] I. A. Koppel, R. W. Taft, F. Anvia, S.-Z. Zhu, L.-Q. Hu, K.-S. Sung,
D. D. DesMarteau, L. M. Yagupolskii, Y. L. Yagupolskii, N. V.
Ignatꢁev, N. V. Kondratenko, A. Y. Volkonskii, V. M. Vlasov, R.
Notario, P.-C. Maria, J. Am. Chem. Soc. 1994, 116, 3047.
[8] To the best of our knowledge, there are only two reported methods for
the synthesis of 1:[9] the reaction of benzylmagnesium chloride with
triflyl fluoride and the photochemical reaction of phenyliodonium
bis(triflyl) methide with benzene give 1 in yields of 40 and 61%,
respectively.[9a,b] The former method requires gaseous triflyl fluoride
(b.p. 218C), which is not commercially available, as an electrophilic
triflyl source, while the latter requires a large amount of benzene as
solvent, and upon photochemical reaction with arenes bearing
electron-withdrawing groups such as fluorobenzene, no corresponding
arylbis(triflyl)methanes are formed.
[9] a) R. J. Koshar, R. A. Mitsch, J. Org. Chem. 1973, 38, 3358; b) S.-Z.
Zhu, Heteroat. Chem. 1994, 5, 9; c) S.-Z. Zhu, J. Fluorine Chem. 1993,
64, 47; d) According to Zhu,[9c] 1a can be prepared in 73% yield by the
pyrolysis of benzenediazonium bis(triflyl)methide. However, we
ˆ
obtained the O-phenylation product PhO(CF3)S(O) CHTf in 71%
yield instead of 1a by following his procedure. K. Ishihara, A.
Hasegawa, H. Yamamoto, J. Fluorine Chem. 2000, 106, 139.
[10] Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
Angew. Chem. Int. Ed. 2001, 40, No. 21
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4021-4079 $ 17.50+.50/0
4079