Synthesis and Reactivity of a 1,8-Bis(methylium)naphthalenediyl Dication

Article abstract of DOI:10.1002/anie.200353011

A voracious appetite for electrons is displayed by the 1,8-bis(methylium)naphthalenediyl dication (see picture). Cooperative effects arising from the proximity of the methylium centers intensify the electron affinity of this molecule and make it very sens

Full text of DOI:10.1002/anie.200353011

Angewandte
Chemie
Communications
The enforced proximity of the methylium
centers in 1,8-bis(bis(p-methoxyphenyl)methylium)-
naphthalenediyl dication makes them cooperate and ultimately
magnifies the electron affinity of this molecule. Like the carnivorous
plant Dionaea traps flies, this dication readily captures electrons and
then holds them in a newly formed carbon–carbon bond. For more
information on this chemistry, see the following Communication by
F. P. Gabbaï and H. Wang.
Angew. Chem. Int. Ed. 2004, 43, 183
DOI: 10.1002/anie.200353011
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
183

Communications
Arene Electrochemistry
Synthesis and Reactivity of a 1,8-
Bis(methylium)naphthalenediyl Dication**
Huadong Wang and François P. Gabbaï*
Triarylmethylium cations constitute one of the best studied
[
1,2]
classes of stable carbocations.
In addition to being used as
[
3]
dyes, such species have become ubiquitous in the domain of
olefin polymerization catalysis where they serve as activa-
tors. Recent advances in the chemistry of these compounds
focus on their incorporation in bifunctional derivatives.
Among these, a series of dicationic systems in which two
triarylmethylium cations are linked by a biphenyl (as in A )
or a binaphthyl backbone have been pre-
[
4]
2+
Scheme 1. Synthesis of 2 and 2. Ar=p-MeO(C
H ). a) Ar CO in THF
6 4 2
at À788C; b) aq[HBF ]/(CF CO) O; c) Li[BEt H] in THF; d) aq[HBF ]/
4
3
2
3
4
[
5]
CF CO H. TMEDA=N,N,N’,N’-tetramethylethylenediamine.
3 2
2
+
pared and have been shown to display
[
6,7]
unique electrochromic properties.
Struc-
attached to the aromatic core of the p-methoxyphenyl groups
1
3
tural studies indicate that the methylium
centers of these biphenyl- or binaphthyl-
based systems are separated by only
.5–3.7 Š. Owing to inherent coulombic
repulsions, it can be assumed that the
physical separation between the methylium
indicates restricted rotation of the latter. The C resonance of
the carbocationic centers appears at d = 191.8 ppm, which is
comparable to that observed for other carbocations such as
[
9]
3
bis(4-methoxyphenyl)phenylmethylium.
[
10]
A single-crystal X-ray analysis revealed the existence of
a sterically congested structure (Figure 1, top). The tight
2
+
centers governs the stability and reactivity of such derivatives.
Hence, control over this separation could serve to fine-tune
the electrophilic and Lewis acidic properties of such bifunc-
tional molecules. Applying this paradigm to the synthesis of
highly electrophilic and Lewis acidic bidentate molecules, we
are nowtargeting bis(methylium) dications that feature short
intercationic separation. Herein, we report on the synthesis,
characterization, and reactivity of 1,8-bis(bis(p-methoxyphe-
geometrical constraints present in the structure of 2 induce
distortions of the naphthalenediyl skeleton. Especially note-
worthy are the C9-C8-C02 and C9-C1-C01 angles (125.36(16)
and 126.60(15)8, respectively) which are larger than the ideal
value of 1208. As indicated by the sum of the bond angles at
C01 (ꢀ(C-C01-C) = 359.6) and C02 (ꢀ(C-C02-C) = 359.8), each meth-
ylium center adopts a trigonal-planar arrangement. The
trigonal coordination planes of the C01 and C02 centers
form relatively large dihedral angles with the plane containing
the naphthalene backbone (59.7 and 59.68, respectively),
indicating that conjugation of the methylium center empty pz
orbital with the p-system of the naphthalene backbone can
only be modest. By contrast, the p-methoxyphenyl groups
strongly stabilize the methylium centers, as indicated by the
small dihedral angles that they form with the trigonal
coordination planes of the C01 and C02 centers (18.1–
28.58). This mesomeric stabilization is also reflected by the
short C01ÀC11, C01ÀC21, C02ÀC31, and C02ÀC41 bonds.
2
+
nyl)methylium) naphthalenediyl dication (2 ), which fea-
tures two methylium centers separated by 3.076(2) Š.
[
8]
The reaction of 1,8-dilithionaphthalene·TMEDA with
two molar equivalents of 4,4-dimethoxybenzophenone
affords the corresponding diol, namely 1,8-bis[bis(p-methoxy-
phenyl)hydroxymethyl]naphthalene (1) (Scheme 1). Upon
treatment with a mixture of aq [HBF ] and (CF CO) O, this
4
3
2
diol undergoes a double dehydroxylation reaction to afford
2
+
1
the corresponding dication, 2 (Scheme 1). The H NMR
2
+
spectrum of 2 features the expected signals for a symmetri-
cally peri-substituted naphthalene derivative: the signals for
the hydrogen atoms at the 2- and 7-positions are shifted
downfield by 1.05 ppm with respect to the analogous signals
of 1. The appearance of four distinct signals for the hydrogen
Finally, as a result of this unique molecular structure, the
vicinal methylium centers are separated by 3.076(2) Š, which
[
5–7]
is the shortest such distance so far reported.
2
+
The structure of 2 was computationally optimized using
DFT methods (B3LYP, 6-31 + G* for the methylium carbon
centers, 6-31G for all other atoms). The fully optimized
geometry approaches that observed for the dication in [2 ]
BF ] . Most importantly, examination of the DFT orbitals
[
11]
[*] H. Wang, Prof. Dr. F. P. Gabbaï
2
+
Chemistry Department
À
Texas A&M University 3255 TAMU
College Station, TX 77843-3255 (USA)
Fax: (+1)979-845-4719
[
4
2
reveals that the methylium p orbitals largely contribute to the
z
LUMO and are oriented toward one another in a trans-
E-mail: gabbai@mail.chem.tamu.edu
2
+
annular fashion (Figure 1, bottom). Thus, 2 is both structur-
ally and electronically similar to 1,8-bis(diphenylboryl)naph-
thalene, which features an interboron separation of
3.002(2) Š and whose LUMO bears strong contributions
[
**] This work was supported by the Welch Foundation (A-1423) and the
National Science Foundation (CHE-9807975, purchase of the X-ray
diffractometers; CHE-0077917 purchase of NMR instrumentation).
We thank Lisa Perez for her help with the calculations.
[
12]
from the converging 2p boron orbitals.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
z
1
84
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200353011
Angew. Chem. Int. Ed. 2004, 43, 184 –187

Angewandte
Chemie
2
+
Figure 2. Cyclic voltammogram of 2 in CH Cl with a Pt working elec-
2
2
À1
trode; scan rate: 100 mVs , 0.1m NBu PF .
4
6
2
+
centers in 2 ; it also reflects the increased electrophilicity of
2
+
2+
2
. We also note that the oxidation of 2 into 2 necessitates a
less cathodic potential (0.68 V) than that observed for its
biphenyl analogue (oxidation at 1.01 V).
It is well known that triarylmethylium cations add
[
14]
hydrides to afford the neutral triaryl methane derivatives.
2
+
In the present case, however, treatment of 2 with LiHBEt3,
KH, or (p-Me NC H ) CH leads to reductive coupling and
2
6
4 3
formation of 2, which is accompanied by gas evolution
Scheme 1). Reaction with LiDBEt does not lead to any
(
3
deuterium incorporation, ruling out a mechanism in which the
hydride would attack one of the aromatic rings. Hence, we
2
+
2+
Figure 1. Top: ORTEP plot of the molecular structure of 2 in [2
]
À
[
BF4 ] (MeCN) with thermal ellipsoids set at the 50% probability
2
2
level. Hydrogen atoms are omitted for clarity. Selected bond lengths
Š] and bond angles [8]: C01-C02 3.076(2), C01-C11 1.418(2), C01-C21
.440(2), C01 C1 1.483(2), C02-C41 1.430(2), C02-C31 1.432(2), C02-
[
1
C8 1.482(2); C11-C01-C21 124.06(16), C11-C01-C1 117.59(15), C21-
C01-C1 118.00(15), C41-C02-C31 123.65(15), C41-C02-C8 117.48(15),
C31-C02-C8 118.64(15), C2-C1-C01 113.78(16), C9-C1-C01 126.60(15),
C7-C8-C02 114.71(16), C9-C8-C02 125.36(16). Bottom: DFT orbital pic-
2
+
ture showing the LUMO in 2
.
2+
Scheme 2. Proposed pathways for the reactions of 2 with hydrides
and of 2 with protons.
2
+
The cyclic voltammogram of 2 shows a two-electron
propose that this reaction proceeds through formation of
[2·H] , which undergoes rapid deprotonation (Scheme 2). By
+
+
reduction wave at À0.17 V (vs Fc/Fc ), corresponding to the
formation of acenaphthylene 2 (Figure 2). As a confirmation,
analogy with the reactivity of 1,8-diborylnaphathlene hydride
[
15]
+
the same voltammogram is obtained when starting from pure
(vide infra). The reduction of 2 does not meet the
electrochemical criteria of reversibility, and reoxidation
necessitates a more positive potential of 0.68 V (vs Fc/Fc ).
sponges, it is tempting to suggest that intermediate [2·H]
+
2
+
[16]
2
possesses a CÀHÀC 3c–2e bridge ([2·H] isomer a). How-
ever, we have not been able to confirm this formulation, and
+
+
an unsymmetrical structure should also be considered ([2·H]
In comparison to 2,2’-bis(bis(p-methoxyphenyl)methylium)-
isomer b) (Scheme 2). In the presence of a substoichiometric
2
+
[6a,13]
2+
biphenyl (A ), whose reduction occurs at À0.28 V,
amount of LiHBEt , only mixtures of 2 and 2 are observed.
3
2
+
+
reduction of 2 appears remarkably facile as it is shifted by
more than 0.11 V toward cathodic potentials. In turn, this
difference provides a measure of the cooperative effects that
result from the proximity of the neighboring methylium
This suggests that the proposed intermediate [2·H] is labile
and can revert to the dication 2 by extrusion of a hydride.
2
+
[
10]
We have determined the crystal structure of 2. Remark-
ably, the bond of 1.670(3) Š linking the former methylium
Angew. Chem. Int. Ed. 2004, 43, 184 –187
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
185

Communications
3
3
carbon centers constitutes one of the longest C(sp )ÀC(sp )
quenched by adding 5% aq NH Cl (20 mL). The organic phase was
4
[
17,18]
extracted with ether, dried with MgSO , and concentrated. Column
bonds so far reported.
Because of this lengthening, this
4
chromatography on silica gel (n-hexane/ethyl acetate 8:1) afforded 2
bond can be anticipated to be inherently weak and therefore
possibly reactive. As a matter of fact, 2 undergoes a slow
oxidation reaction when treated with aq [HBF ]/CF CO H
leading to the formation of 2 , which could be isolated in 55–
0% yield (Scheme 2). While Brønsted acids are known to act
(30 mg, 74%) as a light yellowsolid. Single crystals of 2 suitable for X-
ray structural analysis were obtained by slow evaporation of the
1
4
3
2
solvent from a solution of 2 in acetonitrile. H NMR ([D ]acetone,
6
2
+
5
00 MHz): d = 3.67 (s, 12H, CH ), 6.50 (d, J = 8.5 Hz, 8H, C H ), 6.84
3
6
4
7
(d, J = 8.8 Hz, 8H, C H ), 7.23 (d, J = 7.1 Hz, 2H, H ), 7.60 (t, J =
6 4 Naph
[
19]
13
as oxidants in the formation of carbenium ions, formation
of two methylium centers by oxidative protonolysis of a
7.3 Hz, 2H, HNaph), 7.82 ppm (d, J = 8.3 Hz, 2H, HNaph); C NMR
[D ]acetone, 125.9 MHz): d = 54.6, 73.4, 112.0, 112.5, 123.4, 128.3,
(
6
3
3
[20]
131.8, 136.9, 137.8, 150.8, 157.6 ppm; Elemental analysis calcd (%) for
C H O : C 83.02, H 5.92; found: C 82.59, H 5.90.
C(sp )À(sp ) bond remains unprecedented. Moreover, we
note that acid-induced CÀC bond activations typically
4
0
34
4
[
21]
necessitate the use of superacidic media. This oxidation
Received: October 3, 2003 [Z53011]
reaction likely proceeds by protonation of 2, which under
acidic conditions extrudes a hydride to afford 2 . Once again,
the intermediacy of [2·H] can be invoked (Scheme 2).
Theoretical investigations are currently underway to support
this proposal.
In summary, we report the first stable compound featuring
two methylium cation centers directly connected by a peri-
substituted naphthalene backbone. The enforced proximity of
2
+
Keywords: arenes · carbocations · cooperative effects ·
+
.
electrochemistry · Lewis acids
[1] H. H. Freedman in Carbonium Ions, Vol. IV (Eds.: G. A. Olah, P.
von R. Schleyer), Wiley-Interscience, NewYork, 1973, chap. 28.
[2] C. Herse, D. Bas, F. C. Krebs, T. Bürgi, J. Weber, T. Wesolowski,
B. W. Laursen, J. Lacour, Angew. Chem. 2003, 115, 3270 – 3274;
Angew. Chem. Int. Ed. Engl. 2003, 42, 3162 – 3166.
2
+
the methylium centers in 2 intensifies the electron affinity of
this unusual molecule. In the presence of hydrides, this
dication undergoes reductive coupling which also leads to the
formation of 2. The newly formed CÀC bond of the latter is
[
[
3] D. F. Duxbury, Chem. Rev. 1993, 93, 381 – 433.
4] For examples, see: a) E. Y.-X. Chen, T. J. Marks, Chem. Rev.
2000, 100, 1391 – 1434; b) W. E. Piers, G. J. Irvine, V. C. Williams,
Eur. J. Inorg. Chem. 2000, 2131 – 2142.
long and undergoes an oxidative protonolysis reaction which
2
+
affords 2 .
[
5] J. Nishida, T. Suzuki, T. Tsuji, J. Syn. Org. Chem. Jpn. 2002, 60,
40 – 51, and references therein.
[
6] a) T. Suzuki, J. Nishida, T. Tsuji, Angew. Chem. 1997, 109, 1387 –
1389; Angew. Chem. Int. Ed. Engl. 1997, 36, 1329 – 1331; b) T.
Suzuki, J. Nishida, T. Tsuji, Chem. Commun. 1998, 2193 – 2194.
Experimental Section
1
: A solution of 4,4-dimethoxylbenzophenone (1.12 g, 4.52 mmol) in
THF (15 mL) was added to a solution of 1,8-dilithionaphthalene
[7] a) J. Nishida, T. Suzuki, M. Ohkita, T. Tsuji, Angew. Chem. 2001,
113, 3351 – 3354; Angew. Chem. Int. Ed. 2001, 40, 3251 – 3254;
b) H. Higuchi, E. Ohta, H. Kawai, K. Fujiwara, T. Tsuji, T.
Suzuki, J. Org. Chem. 2003, 68, 6605 – 6610.
(
0.50 g, 1.95 mmol) in THF (5 mL) at À788C. The reaction mixture
was stirred for 2 h at À788C and for another 12 h at room temper-
ature. After addition of 5% aq NH Cl (20 mL) the organic phase was
4
extracted with ether, dried with MgSO , and concentrated. Column
[8] W. Neugebauer, T. Clark, P. von R. Schleyer, Chem. Ber. 1983,
116, 3283 – 3292.
4
chromatography on silica gel (n-hexane/ethyl acetate 6:1) afforded 1
1
(
0.51 g, 43%) as a light yellowsolid. H NMR (CDCl , 500 MHz): d =
[9] E. M. Arentt, R. A. Flowers II, R. T. Ludwig, A. E. Meekhof,
S. A. Walek, J. Phys. Org. Chem. 1997, 10, 499.
3
3
2
.73 (s, 12H, CH ), 6.55 (d, J = 8.8 Hz, 8H, C H ), 6.92 (d, J = 7.3 Hz,
3
6
4
2+
À
2
H, HNaph), 6.97 (d, J = 8.8 Hz, 8H, C H ), 7.37 (t, J = 7.8 Hz, 2H,
[10] Crystal data: [2 ][BF4 ] ·(MeCN) : C H O B F N , M =
6
4
2 44 40 4 2 8 2 r
1
3
HNaph), 7.80 ppm (d, J = 8.3 Hz, 2H, HNaph); C NMR (CDCl3,
834.40, monoclinic, space group P2(1)/n, a = 7.7412(17), b =
3
1
25.9 MHz) d = 55.3, 85.1, 112.7, 113.2, 123.2, 129.0, 129.2, 131.0,
30.885(7), c = 16.494(4) Š, b = 94.675(4)8, V= 3930.5(15) Š ,
À3
1
33.5, 141.9, 142.8, 158.4 ppm; Elemental analysis calcd (%) for
Z = 4, 1calcd = 1.410 gcm , MoKa radiation (l = 0.71073 Š), T=
C H O ·0.85CHCl : C 68.70, H 5.20; found: C 68.82, H 5.10.
110(2) K, 29394 measured reflections, 9351 unique, Rint = 0.0729,
R = 0.0560, wR = 0.1397 (all data). 2: C H O , M = 578.71,
4
0
36
2
6
3
+
À
2
[
2 ][BF4 ] : A suspension of 1 (0.19 g, 0.31 mmol) in (CF CO )O
3 2
1
2
40 34
4
¯
(
5 mL) was treated with aq [HBF ] (48%, 0.20 mL, 1.5 mmol). The
triclinic, space group P1, a = 10.0914(17), b = 10.8633(19), c =
15.308(3) Š, a = 101.893(3)8, b = 98.924(3)8, g = 111.528(3)8,
4
reaction mixture was stirred for 2 h before Et O (20 mL) was added to
2
3
À3
the mixture, which resulted in precipitation of the product as a dark-
V= 1477.7(4) Š , Z = 2, 1
= 1.301 gcm , MoKa radiation
calcd
2
+
]
red solid. It was washed with small portions of Et O to afford a [2
(l = 0.71073 Š), T= 110(2) K, 12635 measured reflections,
6539 unique, Rint = 0.0424, R = 0.0633, wR = 0.1306 (all data).
2
À
2+
À
[
BF4 ]2 (0.19 g, yield 79%). Single crystals of [2 ][BF4 ]2·(MeCN)2
1
2
2
+
À
2
were obtained by vapor diffusion of diethyl ether into a solution of
The crystals (0.23 ” 0.22 ” 0.11 mm for [2 ][BF4 ] ·(MeCN) and
2
2
+
À
1
[
2
][BF4 ]2 in acetonitrile. H NMR ([D ]acetone, 500 MHz): d =
0.46 ” 0.27 ” 0.11 mm for 2) were mounted onto a glass fiber with
Apiezon grease. All the structures were solved by direct
methods, which successfully located most of the non-hydrogen
6
4
.19 (s, 12H, CH ), 6.97 (dd, J = 2.2, 9.0 Hz, 4H, C H ), 7.17 (dd,
3
6
4
J = 9.0, 2.7 Hz, 4H, C H ), 7.27 (d, J = 8.6 Hz, 4H, C H ), 7.67 (dd, J =
6
4
6
4
2
1
.2, 7.3 Hz, 2H, HNaph), 7.89 (dd, J = 2.4, 9.2 Hz, 4H, C H ), 8.03 (dt,
atoms. Subsequent refinement on F using the SHELXTL/PC
6
4
J = 0.9, 7.8 Hz, 2H, HNaph), 8.85 ppm (dd, J = 1.2, 8.3 Hz, 2H, HNaph);
package (version 5.1) allowed location of the remaining non-
hydrogen atoms. CCDC-220813 and 220814 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB21EZ, UK; fax: (+ 44)1223-
336-033; or deposit@ccdc.cam.ac.uk).
1
3
CNMR ([D ]acetone, 125.9 MHz): d = 57.7, 117.8, 118.1, 127.3,
6
1
31.4, 135.2, 135.7, 136.9, 138.7, 142.8, 144.2, 145.6, 172.9, 191.8 ppm;
Elemental analysis calcd (%) for C H O B F ·2CH CN: C 62.25, H
4
0
34
4
2
8
3
4
.98; found: C 61.71, H 4.75. UV/Vis (CH CN): lmax(log(e)) = 482
3
(5.46).
2
: A solution of LiBEt H in THF (1.0m, 0.5 mL, 0.5 mmol) was
3
2
+
À
added to a stirred solution of [2 ][BF4 ]2 (53 mg, 70 mmol) in THF
10 mL) at room temperature. After 30 min the reaction mixture was
[11] Computational details have been assembled in the Supporting
Information.
(
1
86
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 184 –187

Angewandte
Chemie
[
12] a) J. D. Hoefelmeyer, F. P. Gabbaꢀ, J. Am. Chem. Soc. 2000, 122,
054 – 9055; b) J. D. Hoefelmeyer, M. Schulte, M. Tschinkl, F. P.
Gabbaꢀ, Coord. Chem. Rev. 2002, 235, 93 – 103.
9
2
+
[
13] The potentials reported for A /A in ref. [6a] are: red. 0.18 V, ox.
+
1
.47 V versus SCE. Versus Fc/Fc , these potentials can be
calculated at À0.28 and 1.01 V by substraction of 0.46 V (see:
N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877 – 910.
14] T.-Y. Cheng, R. M. Bullock, J. Am. Chem. Soc. 1999, 121, 3150 –
[
[
3155.
15] a) H. E. Katz, J. Am. Chem. Soc. 1985, 107, 1420 – 1421; b) F. P.
Gabbaꢀ, Angew. Chem. 2003, 115, 2318 – 2321; Angew. Chem. Int.
Ed. 2003, 42, 2218 – 2221.
[
[
16] J. E. McMurry, T. Lectka, Acc. Chem. Res. 1992, 25, 47 – 53.
17] a) K. K. Baldridge, Y. Kasahara, K. Ogawa, J. S. Siegel, K.
Tanaka, F. Toda, J. Am. Chem. Soc. 1998, 120, 6167 – 6168, and
references therein; b) H. F. Bettinger, P. von R. Schleyer, H. F.
Schaefer, Chem. Commun. 1998, 769 – 770; c) K. Toda, Tanaka,
M. Watanabe, K. Tamura, I. Miyahara, T. Nakai, K. Hirotsu, J.
Org. Chem. 1999, 64, 3102; d) B. Kahr, D. V. Engen, K. Mislow,J.
Am. Chem. Soc. 1986, 108, 8305; e) T. Suzuki, K. Ono, H. Kawai,
T. Tsuji J. Chem. Soc. Perkin Trans. 2 2001, 1798 – 1801.
18] Long carbon–carbon bonding 4c–2e interactions are encoun-
[
2
À
tered in species such [(TCNE)2] . See a) J. J. Novoa, P.
Lafuente, R. E. Del Sesto, J. S. Miller, Angew. Chem. 2001,
1
13, 2608 – 2613; Angew. Chem. Int. Ed. 2001, 40, 2540 – 2545;
b) J.-M. Lü, S. V. Rosokha, J. K. Kochi, J. Am. Chem. Soc. 2003,
25, 12161 – 12171.
19] a) R. K. Thauer, A. R. Klein, G. C. Hartmann, Chem. Rev. 1996,
6, 3031 – 3042; b) S. Sankararaman, W. Lau, J.K. Kochi, J.
1
[
9
Chem. Soc. Chem. Commun. 1991, 396 – 398; c) K. Komatsu, H.
Akamatsu, S. Aonuma, Y. Jinbu, N. Maekawa, K. Takeuchi,
Tetrahedron 1991, 47, 6951 – 6966.
[
20] This reaction bears some analogy with the acid-induced for-
mation of a monocationic ketocarbenium species. See: T. Suzuki,
T. Nagasu, H. Kawai, K. Fujiwara, T. Tsuji, Tetrahedron Lett.
2003, 44, 6095 – 6098.
[
21] G. A. Olah, G. K. S. Prakash, J. Sommer, Superacids, Wiley-
Interscience, NewYork, 1985.
Angew. Chem. Int. Ed. 2004, 43, 184 –187
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
187