Synthesis and characterisation of Dewar benzene-ferrocene conjugates

Article abstract of DOI:10.1039/b904488b

The first Dewar benzene-ferrocene conjugates were synthesised by the reaction of tetraalkylcyclobutadiene-AlCl₃ complexes with 3-ferrocenylpropynoates. Owing to extensive delocalisation, these conjugates feature unusually high thermal and chemi

Full text of DOI:10.1039/b904488b

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/dalton | Dalton Transactions
Synthesis and characterisation of Dewar benzene–ferrocene conjugates†
a
b
c
b
a,d
ˇ
ˇ
Steˇpa´nka Jankova´, Ivana C´ısarˇova´, Filip Uhl´ık, Petr Stepnicˇka* and Martin Kotora*
Received 6th February 2009, Accepted 13th March 2009
First published as an Advance Article on the web 25th March 2009
DOI: 10.1039/b904488b
The first Dewar benzene–ferrocene conjugates were syn-
thesised by the reaction of tetraalkylcyclobutadiene–AlCl3
complexes with 3-ferrocenylpropynoates. Owing to extensive
delocalisation, these conjugates feature unusually high ther-
mal and chemical stability, rearranging to their corresponding
phenylferrocenes (benzenes) under forcing microwave irradi-
ation conditions.
Scheme 1 Synthesis of DB–ferrocene conjugates 3a and 3b.
As valence isomers to benzenes, Dewar benzenes (DBs) have
attracted considerable synthetic and theoretical interest.1 The
first DBs were prepared by a combination of conventional
reactions and/or the UV irradiation of suitable precursors.2 Later
on, the discovery of the remarkable trimerisation of 2-butyne
to hexamethylbicyclo[2.2.0]hexa-2,5-diene (or hexamethyl(DB)),
promoted by AlCl3,3 opened a simple route to various DB
derivatives.4–7 Careful consideration of suitable substituents was
shown to markedly increase DBs’ stability, which in turn allowed
for the synthesis of oligophenylene macrocycles,8 ladderanes, and
high DB-content polymers,9 as well as for the utilisation of the DB
moiety as a supramolecular protecting group.10 Our recent study
on phenyl-substituted DBs indicated that the rate of the DB-to-
benzene rearrangement is controlled by electronic effects of the
substituents at the DB scaffold.11
by X-ray crystallography (Fig. 1).‡ The X-ray structure of 3a
not only corroborated with the anticipated DB structure but
also revealed that the substituted cyclopentadienyl ring (C1–5),
its adjacent double bond (C11–C12), and the carbonyl group
˚
(C17–O1) are coplanar within 0.097(2) A, which is likely to be due
to conjugation (cf. the torsion angles C1–C11–C12–C17 = 2.0(3)◦,
and C5–C1–C11–C12 = 4.7(3)◦). The observed geometry was very
well reproduced by DFT calculations (see ESI†), including the
overall molecular conformation (e.g., the aforementioned torsion
angles were predicted to be 6 and 11◦, respectively).
Since the phenyl and cyclopentadienyl moieties are in many
regards similar (e.g., iso-p-electronic), we set out to prepare
hitherto unknown DB–ferrocene conjugates. The required fer-
rocene synthon, ethyl 3-ferrocenylpropynoate (1), was prepared by
lithiation of ethynylferrocene with n-BuLi followed by the reaction
with ethyl chloroformate. This compound reacted smoothly with
tetramethyl- (2a) or tetraethyl-cyclobutadiene–AlCl3 complex (2b)
under standard conditions (-15 ◦C, 2 h)11 to afford the correspond-
ing ferrocenyl-substituted DBs 3a and 3b in 31 and 17% isolated
yields, respectively (Scheme 1).
The compounds 3a and 3b were characterised by conventional
spectral methods and the crystal structure of 3a was established
Fig. 1 PLATON plot of 3a at the 30% probability level. Selected distances
◦
˚
(A) and angles ( ): Fe–Cg1 1.6440(9), Fe–Cg2 1.652(1), C1–C11 1.454(2),
C11–C12 1.351(2), C12–C13 1.532(3), C13–C14 1.523(3), C13–C16
1.574(3), C14–C15 1.342(3), C15–C16 1.528(3), C16–C11 1.540(2),
C12–C17 1.461(2), C17–O1 1.212(2), C17–O2 1.346(2); Cg1–Fe–Cg2
179.13(5), the range of in-ring angles for the C4-rings: 85.5(1)–94.8(1),
O1–C17–O2 123.2(2). Cg1 and Cg2 are the centroids of the rings C(1–5)
and C(6–10) respectively (also see ESI†).
aDepartment of Organic and Nuclear Chemistry, Faculty of Science,
Charles University, Hlavova 2030, 128 43, Prague, Czech Republic. E-mail:
kotora@natur.cuni.cz; Fax: +420 221 951 326
bDepartment of Inorganic Chemistry, Faculty of Science, Charles Univer-
sity, Hlavova 2030, 128 43, Prague, Czech Republic. E-mail: stepnic@
natur.cuni.cz; Fax: +420 221 951 253
cDepartment of Physical and Macromolecular Chemistry, Faculty of Science,
Charles University, Hlavova 2030, 128 43, Prague, Czech Republic
The DBs 3a and 3b demonstrated remarkably high stability.
They did not observably rearrange to their corresponding ben-
zenes upon heating to their melting points or to 150 ◦C in DMSO
for 30 minutes, whilst heating to 180 ◦C in the same solvent
resulted in their decomposition. Finally, the rearrangement was
achieved when THF solutions of 3a and 3b were heated in a
microwave reactor (180 ◦C, 6 h). The respective benzenes 4a
dInstitute of Organic Chemistry and Biochemistry, Academy of Sciences of
the Czech Republic, Flemingovo n. 2, 166 10, Prague, Czech Republic
† Electronic supplementary information (ESI) available: Preparative proce-
dures and analytical data for all compounds, electrochemical data, details
of the DFT calculations, selected molecular orbitals, and crystallographic
data for 3a, 4a, and 6. CCDC reference numbers 718552–718554. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b904488b
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3137–3139 | 3137
©

View Article Online
and 4b were isolated in good yields (80 and 76%; Scheme 2)
and identified on the basis of their spectral data (note that
these compounds were previously prepared from tetramethyl-
or tetraethylzirconacyclopentadienes and 1 in the presence of
CuCl12). The crystal structure of 4a was established by X-ray
diffraction analysis (Fig. 2).‡
Scheme 3 Reaction of propynylferrocene with 1a.
Scheme 2 MW-induced rearrangement of 3 to 4.
Fig. 3 HOMO orbitals for 3a and 4a (0.05 a.u. isosurface plots).
electronic stabilisation, the conjugation reduces steric strain in
the bicyclic framework. The double bonds, as well as the central
C–C bond within the bicyclic unit in 3a, fall among the longest
for the structurally characterised DBs (for a comparison of the
structural data, see ESI†).13 Besides, compound 3a shows the
=
least pronounced alternation of the C C and C–C bond lengths.
The extent of conjugation is nicely reflected in the IR spectra,
displaying the nC O bands for 3a shifted by 30 cm-1 to lower energies
=
compared to 4a (1687 and 1718 cm-1, respectively).
The conjugated nature of 3, 4, and 6 was also established by
voltammetric techniques (Table 1). The compounds uniformly
underwent one-electron reversible oxidation attributable to the
ferrocene/ferrocenium couple (note that the HOMOs are not
exclusively ferrocene-based but contributions still came mainly
from the iron orbitals; Fig. 3). The redox potentials were all close
to that of ferrocene itself14 butclearly reflectedelectronic properties
of the substituents (E◦¢: R4(CO2Et) > R5).
Fig. 2 PLATON plot of 4a at the 30% probability level. Selected distances
(A) and angles ( ): Fe–Cg1 1.652(1), Fe–Cg2 1.647(1), C1–C11 1.497(3),
◦
˚
C–C bond lengths within the benzene ring: 1.395(4)–1.412(4), C12–C17
1.493(4), C17–O1 1.209(3), C17–O2 1.343(3); Cg1–Fe–Cg2 176.86(6),
in-ring angles for the ring C(11–16): 118.6(2)–121.7(2), O1–C17–O2
123.2(2). Cg1 and Cg2 denote the centroids of the rings C(1–5) and
C(6–10), respectively (for further data, see ESI†).
Extensive conjugation in 3 and its partial disruption in 4
together with the steric hindrance imposed by the ferrocenyl group
and the substituents in ortho-positions of the phenyl ring (CO2R >
Me) are likely to account for the unusually high thermal stability
of the DB–ferrocene conjugates compared to other DBs. Our
results confirm that the substituents at the DB unit play a vital
role in the stabilisation of the DB scaffold. The reason, for which
no ferrocenylpentamethyl–DB could be isolated, remains unclear.
Despite DFT calculations indicating that such molecule could be
stable (see ESI†), its rearrangement or decomposition can well
reflect an instability under the reaction conditions (e.g., in the
presence of AlCl3).
In the structure of 4a, the phenyl ring is rotated by as much as
42.4(1)◦ from the plane of its bonding cyclopentadienyl ring and
subtends a dihedral angle 66.8(3)◦ with the carboxyl plane. Such
geometry, apparently reflecting steric demands of the substituents,
is also reproduced by the DFT calculations (cf. the torsion
angles C5–C1–C11–C12: exp. 132.0(3)◦, calc. 131◦; and C11–C12–
C17–O1: exp. 117.5(3)◦, calc. 116◦).
The following experiments have shown that the structure of
the ferrocene precursor has a pronounced impact on the course
of the reaction. An attempted synthesis of DB from 1b and
propynylferrocene (5) gave a complex reaction mixture, from
which only (pentamethylphenyl)ferrocene (6) could be isolated
(15% yield; Scheme 3). This compound was characterised by
spectral methods and by X-ray crystallography.‡
The differences in reactivity can be assigned to differences
in electronic properties and the geometry in the DB–benzene
pair. DFT calculations revealed that the conjugation in 3a is
considerably more extensive than in 4a (Fig. 3 and ESI†). The
delocalisation in esters 3 is clearly aided by the combination of
the electron donating and withdrawing groups (ferrocenyl and
CO2R), forming a sort of a “push–pull” system. In addition to
Table 1 Summary of the electrochemical dataa
DB
E◦¢/V
Benzene
E◦¢/V
3a
3b
—
+0.08
+0.09
—
4a
4b
6
+0.05
+0.08
-0.02
a In CH2Cl2 at a Pt-electrode. The potentials are given relative to the
ferrocene/ferrocenium reference. See ESI for details.†
3138 | Dalton Trans., 2009, 3137–3139
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
In summary, we prepared the first DB–ferrocene conjugates
that proved to be highly thermally stable. This particular combi-
nation of unique stability and conjugation nature makes these
compounds attractive for the preparation of new conjugated
organometallic molecules and materials.15
discussion on this issue, see: L. Sementsov, J. Chem. Educ., 1966, 43,
151.
2 (a) E. E. van Tamelen and S. P. Pappas, J. Am. Chem. Soc., 1962, 84,
3789; (b) E. E. van Tamelen and S. P. Pappas, J. Am. Chem. Soc., 1963,
85, 3297; (c) E. E. van Tamelen, S. P. Pappas and K. L. Kirk, J. Am.
Chem. Soc., 1971, 93, 6092.
3 (a) W. Schaefer, Angew. Chem., 1966, 78, 716; (b) W. Schaefer, R.
Criegee, R. Askani and H. Gruener, Angew. Chem., 1967, 79, 54.
4 (a) J. B. Koster, G. J. Timmermans and H. van Bekkum, Synthesis,
1971, 139; (b) F. van Rantwijk, G. J. Timmermans and H. van Bekkum,
Recl. Trav. Chim. Pays-Bas, 1976, 95, 39.
5 (a) J. H. Dopper, B. Greijdanus and H. Wynberg, Tetrahedron Lett.,
1975, 4297; (b) J. H. Dopper, B. Greijdanus and H. Wynberg, J. Am.
Chem. Soc., 1975, 97, 216.
Acknowledgements
This work was financially supported by the Ministry of Ed-
ucation, Youth and Sports of the Czech Republic (projects
MSM0021620857, LC06070, and Z40550506).
6 (a) P. B. J. Driessen and H. Hogeveen, J. Organomet. Chem., 1978, 156,
265; (b) P. B. J. Driessen and H. Hogeveen, J. Am. Chem. Soc., 1978,
100, 1193.
Notes and references
7 (a) R. Gleiter and F. Ohlbach, J. Chem. Soc., Chem. Commun., 1994,
2049; (b) R. Gleiter, F. Ohlbach, T. Oeser and H. Irngartinger, Liebigs
Ann., 1996, 785.
8 (a) M. Ohkita, K. Ando, K. Yamamoto, T. Suzuki and T. Tsuji, Chem.
Commun., 2000, 83; (b) M. Ohkita, K. Ando, T. Suzuki and T. Tsuji,
J. Org. Chem., 2000, 65, 4385; (c) M. Ohkita, K. Ando and T. Tsuji,
Chem. Commun., 2001, 2570.
‡ Selected crystallographic data and structure refinement parameters.
Compound 3a: C23H26FeO2, M = 390.29, triclinic, space group P1 (no.
¯
˚
2), T = 150(2) K, a = 7.3695(2), b = 10.1814(5), c = 13.4315(6) A,
◦
3
˚
a = 89.394(2), b = 86.159(3), g = 75.385(3) , V = 972.97(7) A , Z =
2, Dcalc = 1.332 g mL-1, m(Mo Ka) = 0.788 mm-1; 14778 diffractions
of which 4282 were unique and 3653 observed according to Io > 2s(Io)
criterion (Rint = 3.2%); R(observed diffractions) = 3.43%, R(all data) =
9 M. J. Marsella, S. Estassi, L.-S. Wang and K. Yoon, Synlett, 2004, 192.
10 M. J. Marsella, M. M. Meyer and F. S. Tham, Org. Lett., 2001, 3, 3847.
-3
˚
4.48%, wR(all data) = 8.00%, residual electron density: 0.47, -0.43 e A ,
CCDC reference number 718552. Compound 4a: C23H26FeO2, M = 390.29,
monoclinic, space group P21/c (no. 14), T = 150(2) K, a = 14.9261(5),
ˇ
11 S. Jankova´, M. Dracˇ´ınsky´, I. C´ısarˇova´ and M. Kotora, Eur. J. Org.
◦
3
Chem., 2008, 47.
˚
˚
b = 7.3837(2), c = 18.0000(6) A, b = 107.659(1) , V = 1890.3(1) A ,
Z = 4, Dcalc = 1.371 g mL-1, m(Mo Ka) = 0.812 mm-1; 26499 diffractions
of which 3694 were unique and 3349 observed according to Io > 2s(Io)
criterion (Rint = 4.0%); R(observed diffractions) = 3.20%, R(all data) =
ˇ
12 L. Dufkova´, H. Matsumura, D. Necˇas, P. Steˇpnicˇka, F. Uhl´ık and M.
Kotora, Collect. Czech. Chem. Commun., 2004, 69, 351.
13 A search for non-coordinated bicyclo[2.2.0]hexa-2,5-dienes bearing one
or more carbon substituents in the Cambridge Structural Database
(version 5.30 with updates of November 2008) resulted in seven struc-
tures (refcodes: CITSII, JUJYOC, KELZIK, KELZUW, KEMBAF,
KEMBEJ, and NADXEV). For structural data, see ESI†.
-3
˚
3.75%, wR(all data) = 7.40%, residual electron density: 0.37, -0.49 e A ,
CCDC reference number 718553. Compound 6: C21H24Fe, M = 332.25,
monoclinic, space group P21/c (no. 14), T = 150(2) K, a = 20.1331(2),
◦
3
˚
˚
b = 9.1392(2), c = 8.9108(4) A, b = 102.700(1) , V = 1599.48(8) A ,
Z = 4, Dcalc = 1.380 g mL-1, m(Mo Ka) = 0.937 mm-1; 22400 diffractions
of which 3665 were unique and 3133 observed according to Io > 2s(Io)
criterion (Rint = 4.20%); R(observed diffractions) = 3.01%, R(all data) =
14 Compare the observed redox potential values with that of phenyl-
ferrocene: E◦¢ = +0.025 V (in MeCN): (a) G. L. K. Hoh, W. E.
McEwen and J. Kleinberg, J. Am. Chem. Soc., 1961, 83, 3949; (b) W. E.
Britton, R. Kashyap, M. El-Hashas, M. El-Kady and M. Herbehold,
Organometallics, 1986, 5, 1029.
-3
˚
3.92%, wR(all data) = 8.01%, residual electron density: 0.33, -0.37 e A ,
CCDC reference number 718554. For full crystallographic information,
see ESI.†
ˇ
15 (a) Ferrocenes: Ligands, Materials and Biomolecules, ed. P. Steˇpnicˇka,
Wiley, Chichester, UK, 2008; (b) Ferrocenes: Homogeneous Catalysis,
Organic Synthesis, Materials Science, ed. A. Togni and T. Hayashi,
VCH, Weinheim, Germany, 1995.
1 The description of the bridged formula for benzene is usually attributed
to J. Dewar: J. Dewar, Proc. R. Soc. Edinburgh, 1867, 6, 82. For further
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3137–3139 | 3139
©