The conversion of 1,1′-dibromoferrocene to 1,2-dibromoferrocene: The ferrocene-chemist's dream reaction

Article abstract of DOI:10.1016/j.inoche.2007.09.020

The conversion of 1,1′-dibromoferrocene to a range of 2-substituted bromoferrocenes is described. The products are useful in the preparation of a wide number of 1,2-disubstituted ferrocenes. This includes the preparation of 1,2-dibromoferrocene and 2-(diisopropylphosphino)bromoferrocene in what is a clean and simple synthesis. The synthesis may be carried out on a large scale and for the first time produces multigram quantities of these important precursor compounds. In addition the synthetic procedure may be modified to produce a new family of ferrocene-based tris-phosphine ligands such as bis(2-diphenylphosphinoferrocenyl)phenylphosphine.

Full text of DOI:10.1016/j.inoche.2007.09.020

Available online at www.sciencedirect.com
Inorganic Chemistry Communications 11 (2008) 15–19
www.elsevier.com/locate/inoche
The conversion of 1,10-dibromoferrocene to 1,2-dibromoferrocene:
The ferrocene-chemist’s dream reaction
*
Ian R. Butler
School of Chemistry, University of Wales, Bangor, Gwynedd LL57 2UW, United Kingdom
Received 12 July 2007; accepted 22 September 2007
Available online 1 October 2007
Abstract
The conversion of 1,10-dibromoferrocene to a range of 2-substituted bromoferrocenes is described. The products are useful in the
preparation of a wide number of 1,2-disubstituted ferrocenes. This includes the preparation of 1,2-dibromoferrocene and
2-(diisopropylphosphino)bromoferrocene in what is a clean and simple synthesis. The synthesis may be carried out on a large scale
and for the first time produces multigram quantities of these important precursor compounds. In addition the synthetic procedure
may be modified to produce
ferrocenyl)phenylphosphine.
a new family of ferrocene-based tris-phosphine ligands such as bis(2-diphenylphosphino-
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Lithiation; Bromoferrocene; Phosphine cycopentadienyl; Iron
The substitution chemistry of ferrocene has been exten-
sively explored since its discovery in 1951 and there are
now a myriad of applications which use ferrocene as a
key component. These range from its use in ligand chemis-
try [1], sensor chemistry [2], electrochemistry [3], medicine
[4] to the many applications in material science [5]. It is
now possible to prepare a range of ferrocene compounds
essentially at will using what is very facile chemistry.
The research in our group has been aimed at the synthe-
sis of key ferrocene compounds using very simple synthetic
methodology. For example we reported the effective large
scale synthesis of 1,10-diiodoferrocene [6], the synthesis of
a range of phosphine ligands [7], oligopyridines [8], all of
which have been subsumed into the large research portfo-
lios of the ever expanding number of ferrocene research
chemists.
disubstituted ferrocenes is more difficult normally requiring
the initial introduction of a directing group which facili-
tates substitution into the 2-position. More often than
not the directing group is necessarily retained, although
in some recent work particularly that of Kagan and
coworkers [9], it is possible to replace the directing group
to produce a range of asymmetrically substituted ferro-
cenes. It has been a universal goal of ferrocene chemistry
to obtain 1,2-disubstituted ferrocenes in a simple and effec-
tive manner for many years. We have long recognised this
fact and have sought to obtain 2-bromo-substituted ferro-
cenes, which would be ideal synthons, in a straightforward
manner. The reason this compound is key is because it
unlocks the potential to obtain both chiral and achiral fer-
rocenes such as the 1,2-diamines, diphosphines, diacids,
dialdehydes, dithiols, and dihydroxy compounds – in short,
a myriad of important compounds could be obtained rap-
idly. Although we have already reported its preparation
[10] the synthesis we used, which makes use in part of
Kagan’s methodology, is a little long-winded. We therefore
have continued our impetus to prepare this and related
strategic compounds efficiently. This paper reports a new
definitive synthesis.
The rapid synthesis of 1,10-disubstituted ferrocene is rel-
atively very easy now including the preparation of hetero-
disubstituted ferrocene compounds. The synthesis of 1,2-
*
Tel.: +44 01573 420256; fax: +44 01248 370528.
E-mail address: i.r.butler@bangor.ac.uk
1387-7003/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.09.020

16
I.R. Butler / Inorganic Chemistry Communications 11 (2008) 15–19
Lithioferrocenes are an important class of compounds in
Whilst the outcome was not as hoped for it was encour-
aging to see that the basic principle actually worked, in that
some 1,2-disubstituted product had actually formed. In the
second experiment the reaction mixture was allowed to
warm to ꢀ30 °C to facilitate the equilibration of the iso-
mers, and the temperature was held at ꢀ30 °C for 3 h
before the reaction mixture was quenched. In this case
essentially only one product was crystallised from the solu-
tion, that of the 1,2-isomer, 6a. The characteristic 31P
NMR resonance was observed at ꢀ20.11 ppm which is
slightly shifted from the positions of monophosphine
substituted resonances (ꢀ16.5 to ꢀ18.0 ppm, typically).
Encouraged, the experiment was repeated with chlorodi-
iso propylphosphine as the quenching reagent and again
only one product crystallised, which was the 1,2-isomer,
6b, which was isolated as a crystalline semi-waxy solid.
Again the phosphorus resonance of this compound (ca.
ꢀ2.9 ppm) is shifted by approximately 2 ppm in compari-
son with compounds where the cylclopentadienyl ring is
mono substituted with a di-isopropylphosphine group.
This compound is easily transformed into the known com-
pound 7 [18] following a further lithiation and quenching
strategy, Scheme 3.
In the previous work we had prepared the triphosphine
ligand 8 [19], and it was anticipated that it should be pos-
sible to obtain the related isomeric ligand 9 also using this
new synthetic strategy thus the ortho-lithiation reaction
was again carried out where the product solution was
quenched with 0.5 equiv. of dichlorophenylphosphine
(Fig. 1).
The only difference in the nature of the product tris-
phosphine is the complication which arises from the chiral
nature of the substituents which should lead to the forma-
tion of two isomeric products. Following column chroma-
tography of the reaction mixture two isomeric products
were indeed obtained. In the ferrocene region of the proton
NMR one isomer exhibited seven resonances whereas the
other exhibited 14 resonances, thus these may be tenta-
tively assigned to the meso isomers and (RR)- and (SS)-
mixtures respectively, Scheme 4.
their own right, the most common of which monolithiofer-
rocene and 1,10-dithioferrocene have been prepared and
structurally characterised [11]. They allow the synthesis of
a broad range of substituted ferrocenes by simply using
an electrophillic quench. A few other lithioferrocenes have
also been reported the most notable being decalithioferro-
cene [12] and 1,3,10,30-tetralithioferrocene as an amine
adduct reported by Mulvey and coworkers [13]. These
compounds however do have some problems when they
are quenched, as incomplete substitution is commonly
encountered [14]. In our previous research the ortho-lithia-
tion of 1,10-dibromoferrocene was reported [15]: this
opened up the possibility of the synthesis of a new range
of ferrocenylphosphines, Scheme 1b. Prior to that we had
also reported the preparation of a range of 10-(diary-
lphosphino)bromoferrocenes, Scheme 1a [16].
The simple idea behind this current research was to com-
bine these two pieces of research into one; to take 1,10-dib-
romoferrocene, 1, which is readily available on the
multigram scale [17] and this monolithiate by lithium–hal-
ogen exchange, as was done in the previous research, and
then to isomerise the product according to the principle
that the ortho-lithiated isomer should be favoured in an
equilibration according to the second arm of our research.
To test this idea 10-lithio-bromoferrocene was prepared in
THF at low temperature. As is normal this compound pre-
cipitated from solution. 2,2,6,6-Tetramethylpiperidine was
then added to generate the ortho-lithiating agent in situ.
The solution was then allowed to warm slowly to the point
when all the precipitate had redissolved. The solution was
then recooled and the lithiated species were quenched with
chlorodiphenylphosphine. The product mixture was then
worked up as normal. A mixture of two isomeric products
3a and 6a was formed in the ratio 60:40 (Scheme 2).
PR2
Li
ClPR2
Fe
Fe
Br
Br
Br
3
a
2
Fe
PiPr2
PiPr2
Br
PiPr2
L
D
A
Br
n-BuLi
Br
Br
PR2
Fe
Fe
ClPR2
Li
ClPiPr2
Fe
Fe
1
Br
Br
6b
7
b
5
4
Scheme 3.
Scheme 1.
Ph
P
Ph2P
Ph2P
Ph
P
Br
Li
Br
Br
PPh2
-70oC to -50oC
+
Fe
Fe
Fe
Fe
Fe
Fe
Fe
PPh2
Ph2P
PPh2
3a
Scheme 2.
6a
8
9
Fig. 1. Isomeric ferrocenyltriphosphines.

I.R. Butler / Inorganic Chemistry Communications 11 (2008) 15–19
17
Br
Br
Br
Ph
P
Ph
P
H
Fe
Br
Br
Fe
Fe
Fe
Fe
LiTMPip
HTMPip
H
LiTMPip
Br
HTMPip
10a(R,R)
10b(S,S)
,
Br
Br
Br
Ph
P
Ph
P
Li
Fe
Fe
Br
Br
Li
HTMPip
Fe
Fe
Fe
Fe
LiTMPip
HTMPip
Br
LiTMPip
10d(S,R)
10c(R,S)
H
Fe
Scheme 4.
H
A fraction from the meso isomers was converted to the
tris-phosphine 9 to verify the synthetic protocol. This was
achieved simply by treating a dry sample of compound
10c/d with a slight excess of n-butyllithium in THF and
quenching with chlorodiphenylphosphine. The spectral
data of the product, 9c, d, are in accord with the formula-
tion (31P: d = ꢀ23.97 (d, J = 64.5 Hz) (2P), ꢀ30.93 (t,
J = 64.5 Hz) (1P)). These ligands represent the first in what
should be an interesting ligand family, Scheme 5.
Scheme 7.
this idea was abandoned on purely practical grounds at
least for the present time. This will be reinvestigated at a
later date where a solubilising agent (other tertiary amine)
or simply larger solvent to reagent ratios may be required.
Indeed it may well be that no ‘‘catalyst’’ is required under
the proper experimental regime.
With all the evidence now in hand it was decided to
attempt the preparation of 1,2-dibromoferrocene. The lith-
ium salt was prepared once again in situ and the reaction
mixture was quenched with dibromotetrafluoroethane to
give the 1,2-dibromoferrocene product as expected, which
was isolated from the solution as either a yellow powder
or as dark brown nuggets in over 85% yield, Scheme 6.
The product was spectroscopically identical to a pure sam-
ple from our laboratory.
Basic experimental procedures for these preparations
are given elsewhere [20]. The isomerisation reaction should
in theory be catalytic in terms of the tetramethylpiperidine
added according to Scheme 7, therefore an attempt was
made to carry out the reaction under catalytic conditions
by only adding 0.1 mol equiv. of N,N0-2,2,6,6-tetramethyl-
piperidine to the reaction solution when it was run on a
20 mmol scale. However on warming to ꢀ30 °C most of
the precipitated lithium salt had not dissolved, therefore
Overall the original objective has worked effectively
where we now have access to multigram quantities of 1,2-
dibromoferrocene. It is also possible to obtain 2-
(bromo)iodoferrocene, Scheme 8, cleanly by following the
same experimental method, quenching with diiodotetraflu-
oroethane [21]. The proton NMR of this compound is
definitive and with all three protons on the substituted ring
clearly delineated, it is directly comparable with the ferro-
cene part of the spectrum of 10,2-dibromo(iodoferrocene)
of which we have authentic samples. This compound
should be useful in the desymmetrisation of the ferrocenes
as iodoferrocenes are more readily coupled particularly
with alkynes in comparison with bromoferrocenes [22]. It
should be noted that we have carried out a similar chemis-
try with 1,10-diiodoferrocene and in general the results are
similar although it has been known for a considerable per-
iod that 1,10-diiodoferrocenes are more prone to the forma-
tion of ferrocene dimers by coupling of the lithio species
with unreacted iodoferrocene when 1,10-diiodoferrocene is
mono-lithiated with n-butyllithium. 1,10-Diiodoferrocene
however may be ortho-lithiated in the same manner as
1,10-dibromoferrocene. The results of these investigations
will be communicated in future work [23].
Ph
P
Ph
P
2.2 n-BuLi
2.2ClPPh2
Br
PPh2
Br
Ph2P
Fe
Fe
Fe
Fe
In addition we have already carried out some dilithiation
and quenching reactions to obtain, in one step, a number of
symmetrically 1,2-disubstituted products. Here a broad
range of substituted products should be easily obtained
such as dihydroxy- dithio-, bis-phosphines etc. The only res-
ervation in this case is where the quench is not clean, for
example initial attempts to obtain 1,2-diaminoferrocene
quenching 1,2-dilithioferrocene generated in situ with O-
benzylhydroxylamine gave only traces of the desired prod-
uct; therefore, the method of Arnold [24] starting with
meso mix
9c,d
Scheme 5.
Br
Br i) n-BuLi(-70oC)
Br
DBTFE
Li
Br
Fe
ii) TMP(-30oC)
Fe
Fe
Br
Scheme 6.

18
I.R. Butler / Inorganic Chemistry Communications 11 (2008) 15–19
Br
I
Br
I
I
Br
diiodotetrafluoroethane
I
+
Fe
Li
Fe
Fe
Fe
12
11a,b
Scheme 8.
[3] (a) For recent references see: F. Wang, Y. Xu, L. Gao, B.-X. Ye, J.
Chin. Chem. Soc. 54 (2007) 205;
1,2-dibromoferrocene or 2-aminobromoferrocene is recom-
mended for such a preparation (clearly there are safety
implications here and a cautious approach is advised).
These reactions are being carried out in collaboration with
others as there is clearly much to explore. Our immediate
goal will be to make the isomerisation asymmetric and here
we invite a collaborative approach from interested parties.
To summarise, a new simple and effective synthesis of 1,2-
disubstituted ferrocene compounds has been established
which opens a door to the preparation of a broad range
of ferrocenes both new and old.
(b) M. Zamora, S. Hererro, J. Losada, I. Cusdrado, C.M. Casado, B.
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Meijboom, J.R. Moss, P.J. Smith, H. Su, Dalton Trans. (2003) 3046;
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Acknowledgements
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I.R.B gratefully acknowledges the help of Denis J. Wil-
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University of Wales, Bangor, in the producing the mass
spectral data and NMR spectra.
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Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.inoche.2007.
09.020.
(b) L.J. Hobson, I.R. Butler, Synthetic Met. 71 (1995) 2199;
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reagents the first of these is the reagent of choice however its
commercial availability is limited.
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[20] Pertinent experimental data is given as Supplementary material.
[21] In this case diiodotetrafluoroethane is the quenching reagent of choice
although iodine is commonly used as a quenching reagent in similar
reactions. Where a bulk preparation of 1,10-diiodoferrocene (>50 g) is
required the method using tin precursors [6] leads to clean products
on economic grounds however where small sample are required
(ꢁ5 g) quenching 1,10-dilithioferrocene with diiodotetrafluoroethane
is the reagent of choice and indeed we have successfully used this
method in our undergraduate laboratories for more than 10 years. An
alternative synthesis via the boronic acid [22c] is also available for
iodoferrocene which can itself serve as a direct precursor for the
ortho-lithiation reaction. The ortho-lithation of di-iodoferrocene has
also been carried out as an undergraduate class experiment at the
University of Wales to prove its ease of synthesis.
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[14] The synthesis of the tetra metallated ferrocene compound is very easy
however the quenching reaction of the polylithio species with
electrophiles, which would be expected to be clean, invariably gives
a mixture of products with varying degrees of substitution on the
ferrocene cyclopentadienyl rings. We have also observed this
difficultly first hand in attempts to quench deca-lithioferrocene which
has been prepared from deca-mercury substituted ferrocene
precursors.
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[23] The ortho-lithiation of 1,10-diiodoferrocene has been studied in
undergraduate laboratory experiments at the University of Wales
and has been shown to be equally as useful as that of the lithiation of
dibromoferrocene.
[24] A. Shafir, M.P. Power, J. Arnold, Organometallics 19 (2000) 3978.