Using the Wittig reaction to produce alkenylcarbaboranes

Article abstract of DOI:10.1039/b921122e

This communication reports the first use of the Wittig reaction to produce alkenylcarbaboranes. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b921122e

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Using the Wittig reaction to produce alkenylcarbaboranesw
˜
Antonio Sousa-Pedrares,ab Clara Vinasa and Francesc Teixidor*a
Received (in Cambridge, UK) 8th October 2009, Accepted 12th February 2010
First published as an Advance Article on the web 8th March 2010
DOI: 10.1039/b921122e
This communication reports the first use of the Wittig reaction
to produce alkenylcarbaboranes.
carbaborane cluster upon the reaction of the appropriate
alkyne, that already incorporates the alkene function, with
decaborane(14);6 or in E2 reactions.7 Recently, a method that
starts from the carboryne complex has been used for the
synthesis of cyclic alkenylcarbaboranes.8 Therefore methods
that could start either from C mono-substituted or from
pristine carbaborane leading to the alkenylcarbaborane could
be very advantageous.
o-Carbaboranes are cluster compounds with ten boron and
two adjacent carbon atoms in an icosahedral arrangement.
Different methods for incorporating this unit into various
substrates have been developed and their properties studied.1
Therefore, it is very useful to develop efficient methodologies
that lead to the formation of carbaborane derivatives
functionalized either through C or B that can be used for
applications. Although carbaborane moieties have been
successfully attached to a wide range of residues, the
connection of o-carbaborane as a reagent to aromatic or
conjugated systems has turned out to be particularly difficult.2
However, some molecules containing a carbaborane moiety
linked to an organic fragment through a conjugated spacer
exhibit nonlinear optical behaviour and have attracted great
interest due to their applications.3–5 The conjugated linker is
very important as it provides electronic coupling between the
bridged units, thus permitting intercomponent processes.
Several alkenyl linkers between the carbaborane unit and the
donor group have been investigated.4,5 The Wittig reaction
allows the preparation of an alkene by the reaction of an
aldehyde or ketone with a triphenyl phosphonium ylide. The
phosphonium ylide is prepared in situ by deprotonation of the
corresponding phosphonium halide, which is in turn prepared
by the reaction of triphenylphosphine with an alkyl halide.
Despite the possibilities the Wittig reaction offers in organic
chemistry, it has never been used to generate an alkenyl
group bonded to the o-carbaborane. The strong electron-
withdrawing character of the carboranyl group could have
prevented this reaction. With this communication we provide
first evidence that this is not the case and that the Wittig
reaction is a real option to produce (C)-carbaborane alkenyl
moities. The preparation of the anthracene derivative 1 is
reported (Scheme 1). As an additional measure of the
feasibility of this reaction it has been applied on the bis-ylide
The recent publication of a single-step high-yield preparation
of C-formyl-o-carbaborane9 has encouraged us to investigate
the use of this aldehyde in the Wittig reaction to produce the
anthracene derivatives 1 and 2. In the case of compound 1,
the phosphonium salt 9-(triphenylphospho-niumchloride)-
methylanthracene was prepared from 9-(chloromethyl)-
anthracene.10 Starting from the phosphonium chloride
precursor, the target compound was prepared using the Wittig
methodology as shown in the reaction Scheme 2.
Following this one-pot synthesis the desired product 1 was
isolated. The yield of this reaction is quite high, around 70%,
although it was not optimized. However, the yield refers to the
combined yield for the E and Z diastereoisomers. This mixture
of isomers was isolated from the reaction mixture by column
1
chromatography and studied by H NMR. The signal of the
proton of the spacer closer to the anthracene moiety is very
informative. This signal appears for both isomers between
6 and 6.5 ppm as a doublet. Analysis of the coupling
3
constants, JHH, between the protons of the spacer of both
forms enables the distinction of both isomers. These constants
were 15.9 Hz and 12.3 Hz which were assigned respectively to
the E and Z forms. Comparison between the integrals of these
doublets gives the E : Z ratio, which was 60 : 40. The mixture of
E and Z isomers was separated by successive preparative TLC
using hexane as eluent. After separation, the E isomer was
crystallized from
a THF–hexane solution and studied
by X-ray crystallography11 that revealed a very interesting
supramolecular helical structure (see Fig. 1).
The success of the Wittig methodology in preparing both
isomers of compound 1 led us to attempt the synthesis of the
isomers of the disubstituted anthracene 2. Starting from the
of anthracene in
a single step, producing the sought
disubstitution leading to 2. There are indeed ways to produce
alkenyl groups connected to the carbaborane moiety. Most of
them, however, rely on the typical reaction of assembling the
a I.C.M.B.A.C (CSIC), Campus U.A.B., 08193 Bellaterra, Spain.
E-mail: teixidor@icmab.es; Fax: +34 93 5805729;
Tel: +34 93 5805729
b On leave from Universidad de Santiago de Compostela, Spain
w Electronic supplementary information (ESI) available: Synthesis
and characterization of E-1, Z-1 and 2 and crystal structure and
packing of compound E-1 and E,E-2. CCDC 746811 & 746812. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b921122e
Scheme 1 Carbaborane derivatives obtained using the Wittig reaction.
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Scheme 2 Synthetic route to compound 1.
Fig. 2 Molecular structure of E,E-2 with 30% thermal ellipsoid
probability.
interaction in the crystal packing of this compound is a
p-stacking interaction among the anthracene groups. Thus,
the anthracene groups stack along the crystallographic b axis,
as shown in Fig. 1. If we take a close look at one of those
chains, we see that the ethylene-carboranyl side groups on the
anthracene moieties spiral along the anthracene row, forming
a
helix. This helical structure is responsible for the
non-centrosymmetry of the crystal packing.
Compound E,E-2 crystallizes in the centric space group
P21/c. The molecular structure of this compound is shown
in Fig. 2. The complete descriptions of the molecular
structures of compounds E-1 and E,E-2, along with a full
discussion of their crystal packing, can be found in the ESI.w
In conclusion, we have shown that the Wittig reaction is an
efficient and simple method for preparing alkenylcarbaborane
derivatives. We have also shown that some of the final
compounds are potentially interesting for technological
applications. Compound E-1 is a good example of the
structurally interesting materials that can be prepared using
this methodology.
Fig. 1 Crystal packing of E-1 along the crystallographic b axis.
phosphonium salt 9,10-bis(triphenylphosphoniumchloride)-
di-methylanthracene,9 obtained from 9,10-bis(chloromethyl)-
anthracene,12 the reaction was done in a similar way as for
compound 1. After purification, the reaction yielded the
mixture of all possible isomers of compound 2. All attempts
to separate the mixture were unsuccessful, although single
crystals of the major E,E isomer could be obtained from a
THF–hexane solution of the isomer mixture.13 These studies
show, however, that the Wittig reaction can be successfully
used to prepare the isomers of the disubstituted compound 2.
The Wittig methodology presents some advantages
compared to the methods already described in the literature
(vide supra). The major advantage is the fact that it forms the
double bond in one position with no ambiguity, unlike
elimination reactions which produce mixtures of alkene
regioisomers. Another advantage is the possibility of forming
all possible isomers of the double bond, while the other
available methods only produce the trans isomer. Although
this may seem like an experimental drawback, it enables the
possibility of obtaining the elusive Z isomers. However, the
Wittig reaction has been extensively studied and there are
known variations of the reaction that favours the formation of
only one of the isomeric forms.14 More research is on the way
to show how these known modifications can affect the ratio of
the alkenylcarbaborane isomers.
We acknowledge the financial support from Xunta de
Galicia (Parga Pondal contract); Generalitat Catalunya
2005/SGR/00709.
Notes and references
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Commun., 1996, 2033; (d) C. Frixa, M. F. Mahon, A. S. Thompson
and M. D. Threadgill, Tetrahedron Lett., 2002, 43, 1557.
3 D. M. Murphy, D. M. P. Mingos, J. L. Hagitt, H. R. Powell,
S. A. Westcott, T. B. Marder, N. J. Taylor and D. R. Kanis,
J. Mater. Chem., 1993, 3, 139.
4 (a) M. Lamrani, R. Hamasaki, M. Mitsuishi, T. Miyashita and
Y. Yamamoto, Chem. Commun., 2000, 1595–1596; (b) R. Hamasaki,
M. Ito, M. Lamrani, M. Mitsuishi, T. Miyashita and Y. Yamamoto,
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T. Miyashita and Y. Yamamoto, J. Mater. Chem., 2002, 12, 2701.
6 (a) L. I. Zakharkin, K. M. Uyzbaev, A. V. Kazantsev and
E. Gafurov, Seriya Khim., 1985, 9, 2134; (b) J. F. Valliant,
P. Schaffer, K. A. Stephenson and J. F. Britten, J. Org. Chem.,
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R. G. Fairchild, Tetrahedron Lett., 1990, 31(16), 2247;
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N. N. Ivanova, Zhurnal Obshchei Khimii, 1973, 43(11), 2389;
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The Wittig reaction can thus provide an efficient method to
prepare compounds with o-carbaborane moieties linked to an
aromatic organic residue through an ethylene spacer. As
mentioned before, these materials are very promising as
technology oriented materials, especially in the field of
NLO materials. One limitation is that, in the solid state, a
non-centrosymmetric arrangement of the molecules is an
essential prerequisite, if the molecular NLO response is to
contribute to an observable bulk nonlinearity.3 One of
the molecules prepared using the Wittig reaction, E-1,
fulfills this condition. Compound E-1 crystallizes in the
non-centrosymmetric space group Pn. The main secondary
7 (a) H. Nemoto, J. Cai and Y. Yamamoto, Tetrahedron Lett., 1996,
37(4), 539; (b) H. Nemoto, J. Cai, H. Nakamura, M. Fujiwara and
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Y. Yamamoto, J. Organomet. Chem., 1999, 581, 170;
(c) G. V. Grintselev-Knyazev, M. Y. Antipin, S. P. Knyazev,
V. N. Kirinb and E. A. Chernyshev, Acta Crystallogr., Sect. E:
Struct. Rep. Online, 2001, 58, o16; (d) M. Cheung, H. Chan and
Z. Xie, Dalton Trans., 2005, 2375; (e) J. C. Clark, F. R. Fronczek
and M. G. H. Vicente, Tetrahedron Lett., 2005, 46, 2365.
8 Z. Qiu and Z. Xie, Angew. Chem., Int. Ed., 2008, 47(35), 6572.
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8053.
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J. Organomet. Chem., 2006, 691, 1389.
11 Crystal data for E-1: C28H28B10; M = 346.46; Monoclinic, space
group Pn; a = 13.7778(3), b = 14.5476(3), c = 19.4191(4) A;
b = 97.0820(10)1; V = 3862.55(14) A3; T = 100(2) K; Z = 8;
Dc = 1.192 g.cmÀ3; m(Mo-Ka) = 0.059 mmÀ1; Reflections
collected/unique = 36376/7889 [Rint = 0.0650]; Data/parameters =
7889/1009; R1 = 0.0589 and wR2 = 0.1242 for 5405 reflections
with I 4 2s(I); R1=0.0911 and wR2=0.1386 for all 7889 unique
reflections. CCDC 746812.
12 M. W. Miller, R. W. Amidon and P. O. Tawney, J. Am. Chem.
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13 Crystal data for E,E-2: C22H34B20; M = 514.69; Monoclinic, space
group P21/c; a = 6.7914(2), b = 21.7400(7), c = 9.7036(3) A;
b = 104.800(2)1; V = 1385.16(7) A3; T = 100(2) K; Z = 2; Dc =
1.234 g.cmÀ3; m(Mo-Ka) = 0.059 mmÀ1; Reflections collected/
unique
= 21103/2724 [Rint = 0.0329]; Data/parameters =
2724/190; R1 = 0.1086 and wR2 = 0.3846 for 2267 reflections
with I 4 2s(I); R1 = 0.1216 and wR2 = 0.4033 for all 2724 unique
reflections. CCDC 746811.
14 B. A. Maryanoff and A. B. Reitz, Chem. Rev., 1989, 89, 863–927.
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3000 | Chem. Commun., 2010, 46, 2998–3000