A novel route to a bromo-cyano-substituted azulene and its exploitation in the construction of an acetylenic scaffold

Article abstract of DOI:10.1002/ejoc.200601052

A novel route to functionalized azulenes is devised from a dihydroazulene precursor. Thus, bromination of 1,1-dicyano-2-phenyl-1,8a-dihydroazulene followed by heating in the presence of bromide ions provides an efficient way to generate 3-bromo-1-cyano-2-

Full text of DOI:10.1002/ejoc.200601052

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200601052
A Novel Route to a Bromo-Cyano-Substituted Azulene and Its Exploitation in
the Construction of an Acetylenic Scaffold
Michael Åxman Petersen,[a] Kristine Kilså,[a] Anders Kadziola,[a] and
Mogens Brøndsted Nielsen*[a]
Keywords: Acetylenic scaffolding / Alkynes / Azulenes / Chromophores / Cross-coupling
A novel route to functionalized azulenes is devised from a
dihydroazulene precursor. Thus, bromination of 1,1-dicyano-
2-phenyl-1,8a-dihydroazulene followed by heating in the
presence of bromide ions provides an efficient way to gener-
ate 3-bromo-1-cyano-2-phenylazulene. Formation of this
somewhat unexpected product was confirmed by X-ray crys-
tallographic analysis. It undergoes a palladium-catalyzed
cross-coupling reaction with trimethylsilylacetylene, afford-
ing a new azulene building block for acetylenic scaffolding.
Oxidative homo-coupling hereof provides an azulene dimer,
for which the optical and electrochemical properties are com-
pared to the other azulenes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
shown that halide-functionalized azulenes undergo the
Sonogashira cross-coupling reaction[6] with alkynes and
that the resulting ethynylazulenes are efficient building
blocks for acetylenic scaffolding[7] as well as for metathe-
sis.[8]
Introduction
Derivatives of azulene (1) are interesting building blocks
for advanced materials with electronic and photonic appli-
cations.[1] The azulene system has a remarkable polarizabil-
ity and a tendency to form a stabilized tropylium cation as
well as a cyclopentadienyl anion, which may be enhanced
by suitable functionalization by donor and acceptor
groups.[2] Cyano-substituted azulenes have proven as good
electron acceptors that may be employed in organic metals.
Thus, Hafner and co-workers[3] showed that the electron
donor tetrathiafulvalene forms a charge-transfer complex
with 2,4,6,8-tetracyanoazulene (2). This compound was
prepared from 2-cyanoazulene (3) by nucleophilic substitu-
tion reactions in the seven-membered ring, followed by hy-
drolysis and dehydrogenation steps. In contrast, the five-
membered ring of azulene is reactive towards electrophiles,
and mostly so at the 1- and 3-positions.[4] For example, re-
action of cyanogen bromide with azulene (1) in the presence
of stannic chloride has provided 1-cyanoazulene (4).[5]
However, with a ten-fold excess of the cyanogen bromide–
stannic chloride complex, 1,3-dibromoazulene and 1-
bromo-3-cyanoazulene (5) were obtained.[5] Interestingly, it
was observed that compound 4 was inert to an excess of
cyanogen bromide and stannic chloride and thus not able
to act as a precursor for 5 under these conditions. Cyano-
azulene derivatives such as 5 containing a reactive bromide
substituent are potential precursors for larger conjugated
electron-accepting scaffolds. Thus, it has recently been
We became interested to exploit the possibility for dihy-
droazulenes, such as 6, to act as precursors for cyano-
bromo-substituted azulenes. Dihydroazulenes have at-
tracted attention as photoswitches as they undergo, after
light irradiation, a 10-electron retro-electrocyclization to the
isomeric vinylheptafulvene compounds, which, in turn, un-
dergo a thermal cyclization back to the dihydroazulene
forms.[9,10] Gierisch and Daub[9] showed that dihydroazul-
enes could in fact also be converted into the corresponding
1-cyanoazulenes, albeit in low yields. We report here a new
efficient synthetic procedure for obtaining a 3-bromo-1-cy-
ano-substituted azulene from a dihydroazulene precursor
and its further reactivity towards alkynes, providing new
electron acceptors and chromophores.
[a] Department of Chemistry, University of Copenhagen,
Universitetsparken 5, 2100 Copenhagen Ø, Denmark
E-mail: mbn@kiku.dk
Results and Discussion
Our synthesis starts from a 2-phenyl-substituted deriva-
tive of 6, namely compound 7 (Scheme 1) that was readily
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2007, 1415–1418
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1415

M. Å. Petersen, K. Kilså, A. Kadziola, M. B. Nielsen
SHORT COMMUNICATION
Scheme 1.
prepared according to a general literature protocol.[11] We
reckoned from calculations (vide infra) that bromination of
this compound should give preferably the 7,8-dibromo
compound 8. Indeed, treating 7 with bromine gave 8 in
quantitative yield.[12] Single crystals of 8 were grown from
CH2Cl2/pentane and subjected to an X-ray crystallographic
analysis, which confirmed the proposed structure (Fig-
ure 1). When a 0.05  solution of 8 in CH2Cl2 was left over-
night at 40 °C, it was converted to a mixture of the two
azulenes 9 and 10[13] formed in a ratio of 4:7 and in a total
yield of 61%. In contrast, heating simply the dihy-
droazulene 7 overnight at 40 °C in CH2Cl2 caused no azu-
lene formation. Heating 8 in the presence of one equivalent
of bromide (Bu4NBr) gave solely compound 9 in a yield of
79%.[14] Yet, running the reaction under more dilute condi-
tions (0.009 ) and in the absence of Bu4NBr produced so-
lely the cyanoazulene 10 in a yield of 72%. While exploi-
tation of 1-cyanoazulene (4) as a precursor for 1-bromo-3-
cyanoazulene (5) was previously discarded,[5] we find that
treating azulene 10 with a small excess of Br2 in CH2Cl2
resulted in clean conversion to the bromide 9. These obser-
vations suggest that at least two mechanisms can be respon-
sible for the formation of 9 from the precursor 8: i) initial
elimination of either HBr or HCN, followed by nucleophilic
attack of bromide at C3 with expulsion of either cyanide or
bromide (SN2Ј), followed by a final elimination reaction, or
ii) initial formation of 10 after two elimination reactions,
followed by electrophilic substitution by attack of either Br2
or BrCN. The structures of both 9 and 10 were confirmed
by X-ray crystallography (Figure 1).[15]
Figure 1. X-ray crystal structures of 8, 9, and 10.
analysis was performed to secure that a real minimum had
been obtained. Then single point energy calculations were
performed at the B3LYP/6-311+G(2d,p) level. The relative
energies (at 0 K) of the ions are depicted in Figure 2. As
The formation of azulenes 9 and 10 from dibromide 8
can also be promoted by light irradiation. Thus, photolysis
at 350 nm of a dilute sample of 8 (0.009 ) in dry CH2Cl2
(1.5 mL; without stabilizator) for 1 hour showed complete
1
conversion to the cyanoazulene 10 (as judged from an H
NMR spectroscopic investigation of the reaction mixture
after evaporation of the solvent), while photolysis for
1 hour of 8 (0.009 ) in the presence of Bu4NBr (0.05 )
instead provided the bromide 9 (together with a minor un-
identified by-product).
The first step of the bromination of dihydroazulene 7 was
subjected to a computational study employing the Gaussian
03 program package[16] in order to compare stabilities of
possible cations. The isomeric ions were optimized at the
Figure 2. Relative cation energies calculated at the B3LYP/6-
semiempirical PM3 level, and on each structure a frequency 311+G(2d,p) level on PM3-optimized structures.
1416 Eur. J. Org. Chem. 2007, 1415–1418
www.eurjoc.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Bromo-Cyano-Substituted Azulene
SHORT COMMUNICATION
expected from simple resonance formulas, an attack at C8 green in the solid state, which is also the case in solution
is the most favorable. Electrophilic attack of Br+ at for ex- for the dimer 12, while the monomer 11 exhibits a bright
ample the 2- or 3-positions requires more than blue color in solution. Cyclic voltammetry investigations in
20 kcalmol–1 relative to attack at C8. For the subsequent CH2Cl2 (0.1  Bu4NPF6) show irreversible reductions at
attack of bromide, we find that the kinetic product of ad- –1.23 (9), –1.38 (10), –1.80 (11), and –1.81 (12) V vs. Fc+/
dition (assumed so from a proximity effect), namely the 7,8- Fc. Taking into account the electron-accepting nature of
dibromide 8 (7S,8S,8aR stereoisomer), is also the thermo- acetylenic scaffolds,[21] it is somewhat surprising that com-
dynamic product as the 2,8-, 3a,8-, and 5,8-dibromides are pounds 11 and 12 are the poorest acceptors in the series.
more energetic by +17.9, +25.3, and +7.1 kcalmol–1,
respectively. All in all, the calculations substantiate the ex-
clusive formation of the dibromide 8 from bromination of
7.
The bromide 9 was subjected to a Sonogashira cross-
coupling reaction[6] with trimethylsilylacetylene employing
the catalyst system of Hundertmark et al.[17] (Scheme 2).
The product 11[18] was desilylated with K2CO3 in MeOH/
THF, and the terminal alkyne intermediate was then sub-
jected to an oxidative Hay homo-coupling reaction[19] to
give the azulene dimer 12.[20] Single crystals of 12 were
grown from toluene/CHCl3, and the crystal structure is
shown in Figure 3. Angles of 22° and 38° are observed be-
Figure 4. UV/Vis absorption spectra in CH2Cl2.
tween the phenyl and azulene rings.
In conclusion, we have developed a new efficient synthe-
sis of a 3-bromo-functionalized azulene. This compound is
readily incorporated into new redox-active acetylenic chro-
mophores. We are currently investigating the possibility for
controlling the light-induced conversion of 8 into azulenes
via a ring-opened intermediate.
Supporting Information (see also the footnote on the first page of
this article): A Table containing a summary of general crystallo-
graphic data.
CCDC-626060 (for 8), -626062 (for 9), -626061 (for 10), -626063
(for 12) contain the detailled supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
datarequest/cif.
Scheme 2.
Acknowledgments
We thank the Danish Research Agency (grant #2111-04-0018) for
financial support.
[1] a) G. Iftime, P. G. Lacroix, K. Nakatani, A. C. Razus, Tetrahe-
dron Lett. 1998, 39, 6853–6856; b) S. Ito, S. Kikuchi, N. Mor-
ita, T. Asao, J. Org. Chem. 1999, 64, 5815–5821; c) P. G. Lac-
roix, I. Malfant, G. Iftime, A. C. Razus, K. Nakatani, J. A.
Delaire, Chem. Eur. J. 2000, 6, 2599–2608; d) S. Ito, H. Inabe,
N. Morita, K. Ohta, T. Kitamura, K. Imafuku, J. Am. Chem.
Soc. 2003, 125, 1669–1680; e) F. Wang, Y.-H. Lai, M. Y. Han,
Macromolecules 2004, 37, 3222–3230.
[2] H. Möllerstedt, M. C. Piqueras, R. Crespo, H. Ottosson, J.
Am. Chem. Soc. 2004, 126, 13938–13939.
Figure 3. Crystal structure of 12.
[3] S. Schmitt, M. Baumgarten, J. Simon, K. Hafner, Angew.
Chem. Int. Ed. 1998, 37, 1077–1081.
The new azulene derivatives are strong chromophores as
is evident from the UV/Vis absorption spectra shown in
Figure 4. The azulene monomers 9 and 10 exhibit a bright
violet and blue-violet color, respectively, in solution. They
are only weakly fluorescent with structured emissions at
433 nm (φfl = 0.5 ‰) for 9 and 431 nm (φfl = 0.4 ‰) for 10
in CH2Cl2 (λexc = 322 nm). Compounds 11 and 12 are both
[4] H. Spiesecke, W. G. Schneider, Tetrahedron Lett. 1961, 2, 468–
472.
[5] A. G. Anderson Jr, D. J. Gale, R. N. McDonald, R. G. An-
derson, R. C. Rhodes, J. Org. Chem. 1964, 29, 1373–1377.
[6] K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett.
1975, 16, 4467–4470.
[7] a) S. Ito, H. Inabe, T. Okujima, N. Morita, M. Watanabe, N.
Harada, K. Imafuku, J. Org. Chem. 2001, 66, 7090–7101; b)
Eur. J. Org. Chem. 2007, 1415–1418
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1417

M. Å. Petersen, K. Kilså, A. Kadziola, M. B. Nielsen
SHORT COMMUNICATION
M. Lamberto, C. Pagba, P. Piotrowiak, E. Galoppini, Tetrahe-
dron Lett. 2005, 46, 4895–4899; c) K. K. H. Fabian, A. H. M.
Elwahy, K. Hafner, Eur. J. Org. Chem. 2006, 791–802.
A. Mikus, V. Sashuk, M. Kedziorek, C. Samojłowicz, S. Os-
˛
trowski, K. Grela, Synlett 2005, 1142–1146.
S. Gierisch, J. Daub, Chem. Ber. 1989, 122, 69–75.
L. Gobbi, P. Seiler, F. Diederich, Angew. Chem. Int. Ed. 1999,
38, 674–678.
L. Gobbi, P. Seiler, F. Diederich, V. Gramlich, C. Boudon, J.-
P. Gisselbrecht, M. Gross, Helv. Chim. Acta 2001, 84, 743–777.
Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M.
Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Aus-
tin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Mo-
rokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G.
Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O.
Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B.
Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cios-
lowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Ko-
maromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Po-
ple, Gaussian, Inc., Pittsburgh PA, 2003.
[8]
[9]
[10]
[11]
[12] Dihydroazulene
7 (125 mg, 0.49 mmol) was dissolved in
CH2Cl2 (4 mL) at –78 °C under N2. Then a solution of Br2 in
CH2Cl2 (0.39 , 1.25 mL) was slowly added (2 min). Stirring
for 1 h at –78 °C followed by evaporation in vacuo yielded 8
(205 mg, 100%) as a yellow-brown foam. An analytical sample
was recrystallized from CH2Cl2 and pentane. Melting range
[17] T. Hundertmark, A. F. Littke, S. L. Buchwald, G. C. Fu, Org.
Lett. 2000, 2, 1729–1731.
170–178 °C, compound turns dark-violet above 95 °C. Rf
=
[18] Compound 9 (200 mg, 0.65 mmol) was dissolved (with sonic-
ation) in dry THF (1.5 mL), dry toluene (1.5 mL), and NH-
(iPr)2 (0.2 mL) under argon. Then [Pd(PhCN)2Cl2] (33 mg,
0.09 mmol), CuI (20 mg, 0.1 mmol), and P(tBu)3 in hexane
(10%, 0.25 mL, 0.08 mmol) were added. Trimethylacetylene
(0.4 mL, 2.7 mmol) was added, and the reaction mixture was
allowed to stir at room temp. for 3 h. The resulting dark blue
solution was diluted with CH2Cl2 (25 mL) and washed with
water (2ϫ50 mL), dried (MgSO4), and the solvents evaporated
in vacuo. Purification by flash column chromatography
(EtOAc/heptane, 2:8, v/v, Rf = 0.4) gave 11 as a green solid
(150 mg, 79%). M.p. 112–113.5 °C. 1H NMR (300 MHz,
0.36 in EtOAc/heptane, 3:7 v/v, 2D-TLC showed breakdown of
main product; decomposition product observed at Rf = 0.075.
1H NMR (300 MHz, CDCl3, 25 °C, 7.26 ppm): δ = 7.76 (m, 2
H, Ar), 7.48 (m, 3 H, Ar), 6.98 (s, 1 H, CH), 6.27 [dd, 3J(H,H)
= 2.2 Hz, 3J(H,H) = 7.5 Hz, 1 H, CH], 6.09 [dd, 3J(H,H) =
7.5 Hz, 3J(H,H) = 12.2 Hz, 1 H, CH], 5.91 [dd, 3J(H,H) =
5.6 Hz, 3J(H,H) = 12.0 Hz, 1 H, CH], 5.32 (m, 1 H, CH), 5.05
(m, 1 H, CH), 4.66 (br. s, 1 H, CH) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C, 77.0 ppm): δ = 144.5, 139.6, 133.9, 130.3, 130.0,
129.3, 128.5, 126.4, 125.7, 121.0, 114.6, 111.7, 53.1, 51.5, 49.0,
44.6 ppm. UV/Vis (CH2Cl2): λmax (ε) = 348 nm (26700).
[13] Compound 10 was previously prepared in another manner: T.
Nozoe, K. Takase, S. Fukuda, Bull. Chem. Soc. Jpn. 1971, 44,
2210–2213.
3
CDCl3, 25 °C, 7.26 ppm): δ = 8.71 [d, J(H,H) = 9.7 Hz, 1 H,
CH=], 8.63 [d, 3J(H,H) = 9.7 Hz, 1 H, CH=], 8.15 (m, 2 H,
3
Ar), 7.85 [t, J(H,H) = 9.8 Hz, 1 H, CH=], 7.64–7.49 (m, 5 H,
[14] Dibromide 8 (208 mg, 0.50 mmol) was dissolved in dry, unsta-
bilized CH2Cl2 (10 mL) under N2 atmosphere, whereupon
NBu4Br (171 mg, 0.53 mmol) was added. After stirring at
40 °C for 16 h (caution: HCN evolution expected), the dark-
purple reaction mixture was washed with water (2ϫ30 mL),
dried (MgSO4), and the solvents evaporated in vacuo. Purifica-
tion by flash column chromatography (EtOAc/heptane, 3:7, v/
Ar, CH=), 0.30 (s, 9 H, CH3) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C, 77.0 ppm): δ = 153.6, 144.4, 144.1, 140.1, 138.1,
136.4, 133.3, 129.9, 129.6, 129.0, 129.0, 128.5, 117.2, 109.8,
102.3, 99.3, 95.6, 0.0 ppm. HR-ES-TOF-MS (pos. mode): m/z
= 348.1193 [M·Na+]; calcd. for C22H19NNaSi: 348.1184. UV/
Vis (CH2Cl2): λmax (ε) = 273 (22000), 289 (22000), 328 (47000),
337 (51000), 372 (8000), 390 (br. sh, 7000), 601 (500) nm.
v, Rf = 0.31) yielded 3-bromo-1-cyano-2-phenylazulene 9 [19] A. S. Hay, J. Org. Chem. 1962, 27, 3320–3321.
(121 mg, 79%), as a purple solid. M.p. 191–194 °C. Crystals
[20] Compound 11 (72 mg, 0.22 mmol) was dissolved in MeOH
(3 mL) and THF (1.5 mL). Then K2CO3 (160 mg, 1.15 mmol)
was added, and the reaction mixture was stirred for 1/2 h at
room temp. and then diluted with CH2Cl2. The organic phase
was washed with water, dried (MgSO4), and the solvents evapo-
rated in vacuo. The dark-blue solid was dissolved in CH2Cl2
(15 mL), whereupon Hay catalyst [TMEDA (40 mg,
0.34 mmol) and CuCl (40 mg, 0.40 mmol) in CH2Cl2 (1 mL)]
was added. After stirring for 31/2 h at room temp., the reaction
mixture was washed with water, dried (MgSO4), and the sol-
vents evaporated in vacuo. Purification by flash column
chromatography (CH2Cl2, Rf = 0.44) followed by two pro-
cedures of precipitation, by dissolving in hot CHCl3 followed
by addition of heptane, yielded the dimer 12 (30 mg, 27%) as
a green solid. M.p. 315–317 °C (dec.). Crystals for X-ray crys-
tallography were grown from toluene/CHCl3. 1H NMR
(300 MHz, CDCl3, 25 °C, 7.26 ppm): δ = 8.79 [d, 3J(H,H) =
1
for X-ray crystallography were grown from CDCl3. H NMR
(300 MHz, 25 °C, CDCl3, 7.26 ppm): δ = 8.65 [d, 3J(H,H) =
3
10.0 Hz, 1 H, CH=], 8.62 [d, J(H,H) = 10.8 Hz, 1 H, CH=],
7.93–7.82 (m, 3 H, Ar, CH=), 7.68–7.50 (m, Ar, 5 H, CH=)
ppm. 13C NMR (75 MHz, 25 °C, CDCl3, 77.0 ppm): δ = 150.7,
143.5, 140.3, 139.2, 138.2, 136.5, 133.0, 130.2, 129.4, 128.6,
128.5, 128.2, 116.7, 104.4, 96.5 ppm; ES-TOF-MS (neg. mode):
m/z = 308 [H·M–], 339 [MeOH·M–]. UV/Vis (CH2Cl2): λmax (ε)
= 269 (13000), 321 (46500), 359 (7500), 373 (7500), 588 (500)
nm; fluorescence (CH2Cl2, λexc = 322 nm): λem = 414 (sh), 433
(tail, φ = 0.5 ‰) nm.
[15] Compound 9 crystallizes in the acentric space group Cc but
does so as a racemic twin with a ratio of 52:48. Moreover, the
azulene ring displays disorder, so that the Br and CN substitu-
ents are interchanged in 17% of the unit cells. The azulene ring
and the phenyl ring form an angle of 41°. Crystals of com-
pound 10 contain two molecules in the asymmetric unit. One
of the molecules is all planar (shown in Figure 1), induced by
the crystal packing, and disordered. Here 38% of the molecules
are rotated 180° with the phenyl ring overlapping the seven-
membered ring of the azulene, and the five-membered rings of
the azulene partially overlapping. The other molecule is well
ordered and displays an angle of 33° between the phenyl ring
and the azulene ring.
3
10.0 Hz, 2 H, CH=], 8.66 [d, J(H,H) = 10.0 Hz, 2 H, CH=],
3
8.13 [d, 3J(H,H) = 7.6 Hz, 4 H, Ar], 7.90 [t, J(H,H) = 9.4 Hz,
2 H, CH=], 7.69–7.52 (m, 10 H, CH=, Ar) ppm. 13C NMR
(75 MHz, CDCl3, 50 °C, 77.0 ppm): δ = 140.5, 138.4, 136.7,
129.8, 129.7, 129.4, 128.9 ppm, (missing signals due to poor
solubility). MALDI-TOF-MS (neg. mode): m/z = 504 [M–],
1008 [M·M–]. UV/Vis (CH3Cl): λmax (ε) = 278 (37500), 325
(71500), 345 (66500), 420 (16000), 454 (14000), 583 (1300) nm.
[21] J.-P. Gisselbrecht, N. N. P. Moonen, C. Boudon, M. B. Nielsen,
F. Diederich, M. Gross, Eur. J. Org. Chem. 2004, 2959–2972.
Received: December 4, 2006
[16] Gaussian 03, Revision B.03, M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A.
Montgomery Jr, T. Vreven, K. N. Kudin, J. C. Burant, J. M.
Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M.
Published Online: February 9, 2007
1418
www.eurjoc.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2007, 1415–1418