A rapid and convenient method for the formation of (diene)Fe(CO)3 complexes

Article abstract of DOI:10.1016/S0022-328X(98)00827-4

The formation of (diene)Fe(CO)₃ complexes has been achieved in the absence of solvent by the action of mild heat on a mixture of the diene and a preformed mixture of Fe₂(CO)₉ and silica gel. The yield of complex obtained compares well with literature examples especially for polar substrates. The first complexation of Danishefsky's diene, 1-methoxy-3-trimethylsilyloxybuta-1,3-diene, to Fe(CO)₃ has been achieved using this method. The speed of the reaction (typically complete after 2 h at 85°C) and the ease of work-up make this solid-state procedure the method of choice for the preparation of complexes of many polar dienes and heterodienes.

Full text of DOI:10.1016/S0022-328X(98)00827-4

Journal of Organometallic Chemistry 568 (1998) 287–290
A rapid and convenient method for the formation of (diene)Fe(CO)3
complexes
Gordon F. Docherty, Graham R. Knox 1, Peter L. Pauson *
Department of Pure and Applied Chemistry, Uni6ersity of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
Received 15 June 1998
Abstract
The formation of (diene)Fe(CO)3 complexes has been achieved in the absence of solvent by the action of mild heat on a mixture
of the diene and a preformed mixture of Fe2(CO)9 and silica gel. The yield of complex obtained compares well with literature
examples especially for polar substrates. The first complexation of Danishefsky’s diene, 1-methoxy-3-trimethylsilyloxybuta-1,3-di-
ene, to Fe(CO)3 has been achieved using this method. The speed of the reaction (typically complete after 2 h at 85°C) and the ease
of work-up make this solid-state procedure the method of choice for the preparation of complexes of many polar dienes and
heterodienes. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Iron; Diene; Solid state synthesis
1. Introduction
versible hydride abstraction to give cationic dienyl
complexes. The attached tricarbonyliron group controls
reactions at the diene ligand and can provide a high
degree of regio- as well as stereo- and even enantiose-
lectivity–yet is easily removed by photolysis or oxida-
tion when the desired organic moiety has been built up.
Typical examples of such transformations have been
reviewed in textbooks and journal articles e.g. [5–7].
The original method for the preparation of the diene-
iron complexes requires elevated temperatures (typically
100–150°C) and fairly long reaction times–conditions
under which pentacarbonyliron also catalyses double
bond migration; sensitive substrates may be destroyed
and pressure vessels must be used for the more volatile
dienes. Hence there is continuing interest in the replace-
ment of pentacarbonyliron by more reactive starting
materials which allow diene complexes to be formed at,
or below, room temperature. Nonacarbonyldiiron, in
spite of its insolubility, meets this requirement in most
cases, e.g. [8]. Its main drawback is the utilisation of
only one of the iron atoms:
For 25 years after its preparation by Reihlen et al. in
1930 [1], butadienetricarbonyliron remained in the
chemical literature as a curiosity. Its reinvestigation
only became attractive after the discovery of the metal-
locenes and the work of Dewar [2] and of Chatt and
Duncanson [3] which provided the theoretical basis for
formulating such substances as y-complexes. Repetition
of Reihlen’s synthesis from pentacarbonyliron and bu-
tadiene and extension of this method to the synthesis of
tricarbonylcyclohexadieneiron [4] opened the door lead-
ing to a vast number of diene-iron compounds and
further to the whole field of alkene-transition metal
complexes.
Among such compounds the diene-iron complexes
have a special place as useful intermediates in organic
synthesis. Their stability allows them to smoothly un-
dergo Friedel–Crafts type substitution as well as re-
* Corresponding author. Tel.: +44 141 5482800; fax: +44 141
5484822; e-mail: m.d.spicer@strath.ac.uk
1 Also corresponding author.
Fe2(CO)9+diene“(diene)Fe(CO)3+Fe(CO)5+CO
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00827-4

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G.F. Docherty et al. / Journal of Organometallic Chemistry 568 (1998) 287–290
Even better results are achieved [9] by using the diene
to displace one of the less firmly held heterodienes, e.g.
PhCHꢀCHCHꢀNPh or PhCHꢀCHCOMe:
Among the more polar dienes used, several of the
yields are at least comparable to those reported using
other techniques. In the best case, the complex of
(E,E)-hexa-2,4-dien-1-ol, the yield (77%) is significantly
greater than any reported values. Notably, the method
also succeeds well for heterodiene complexes.
(PhCHꢀCHCOMe)Fe(CO)3+diene
“(diene)Fe(CO)3+PhCHꢀCHCOMe
Hexa-2,4-dienal was selected as a convenient example
to compare the efficacy of silica with other adsorbents
and also to study different iron carbonyl to silica ratios.
When ground without ‘inert’ adsorbent, Fe2(CO)9 re-
acted at 85°C in 2 h to give the complex in a 49% yield.
In contrast to SiO2 (73%), all other adsorbents tried
lowered this value (Al2O3, 28%; Florisil, 37%; K2CO3,
37%; MgSO4, 45%). The yield quoted for silica was
achieved with an apparently optimal 5:1 ratio of
SiO2:Fe2(CO)9. Both an increase to 12.5:1 or a decrease
to 2.5:1 lowered the yield (to 66 and 59%, respectively).
In summary, we believe that for many polar dienes
and heterodienes, the use of the essentially low toxicity,
non-volatile, Fe2(CO)9–SiO2 mixtures, their speed of
reaction at modest temperatures and the ease of work-
up will make this solid-state procedure the method of
choice and especially when suitable ultrasound and
photochemical equipment (needed for other methods) is
not readily available.
Alternative methods rely on activation of pentacar-
bonyliron by either ultrasound [10] or amine-N-oxides
[11] to remove one CO group. The same procedures
have also been found to be effective in reactions of
(carbene)Cr(CO)5 [12] and of (alkyne)Co2(CO)6 [13].
Smit and co-workers [14] introduced a dry-state adsorp-
tion technique for reaction of alkenes with the alkyne–
cobalt complexes and demonstrated that the reactions
were greatly accelerated and proceeded cleanly and in
high yield. This approach was also found useful in
displacement
of
CO
from
carbenepentacar-
bonylchromium [12] and we now describe its use as a
rapid and convenient method for reactions of nonacar-
bonyldiiron with dienes.
Various adsorbents including alumina and magne-
sium carbonate have been used, but silica has proved
most consistently successful. The earliest examples in-
volved
intramolecular
Khand
reactions
of
(enyne)hexacarbonyldicobalt; solutions of these com-
plexes were added to the adsorbent and the solvent was
then removed on a rotary evaporator. Gentle heating
then sufficed for complete reaction and the product
could be purified by adding the reaction mixture to the
top of a prepacked chromatography column of the
same adsorbent. Extensions to intermolecular reactions
were possible [14] by adding the pure alkenes (if liquid)
to the adsorbed alkyne metal complex.
3. Experimental
3.1. General procedure for diene complexation
The diene (0.5 g) is added to a carefully ground
mixture of nonacarbonyldiiron (1.1 equivalents) and
silica (230–400 mesh; 5 g per g of Fe2(CO)9). The flask
is shaken to obtain an even dispersion and then heated
under nitrogen under the conditions specified in the
table. After cooling, the mixture is added to the top of
a prepacked column of dry silica gel; the products are
separated by gradient elution using hexane/ether. If the
silica is dried overnight at 140°C the yield of diene
complex is diminished, but this was necessary to permit
isolation of the unstable complex of 1-methoxy-3-
trimethylsilyloxybuta-1,3-diene.
2. Discussion
To find whether a related dry-state method could be
devised for the synthesis of diene–iron complexes we
chose the binuclear carbonyl as the most appropriate
precursor. Its insolubility prevents its deposition from
solution, but by careful and thorough grinding together
of a mixture of this carbonyl with the adsorbent in a
mortar we seem to have achieved sufficiently close
contact to enhance the reactivity of the carbonyl. Addi-
tion of dienes as pure liquids or, in the case of solids, as
solutions in easily removable volatile solvents then
yielded diene–iron complexes on heating. Useful yields
were however only obtained from highly polar dienes;
the results are summarised in Table 1 [15–17]. In most
cases the reactions were much faster than by other
techniques, being complete after 2 h at 85°C. They were
very clean and the chromatographic method described
above provided pure products easily and quickly.
The following new spectroscopic data have been ob-
tained: (E,E)tricarbonyl(hexa-2,4-dienal)iron lC(C6D6)
19.1, 55.5, 60.6, 81.7, 89.7, 195.3, 210.5; tricarbonyl
(1-trimethylsilyloxybuta-1,3-diene)iron
wmax(hexane)
1992, 2059 cm−1; lH(C6D6) 0.01(9H, s), 0.85(2H, m),
1.58(1H, broad s), 5.4–6.0(2H, m); tricarbonyl(1-
acetoxybuta-1,3-diene)iron wmax(hexane) 1999, 2060
cm−1; lH(C6D6) 1.49(1H, broad s), 1.80(3H, s), 1.95–
2.06(1H, m), 4.17(1H, d), 5.03–5.20(2H, m); tricar-
bonyl(1-methoxy-3-trimethylsilyloxybuta-1,3-diene)iron
wmax(C6D6) 1985, 2069 cm−1; lH(C6D6) 0.01(9H,s),
1.25(1H, d), 1.56(1H, d), 2.86(1H, m), 3.56(3H, broad
s), 4.74(1H, broad s); tricarbonyl(hexa-2,4-dien-1-yl ac-

G.F. Docherty et al. / Journal of Organometallic Chemistry 568 (1998) 287–290
289
Table 1
Reactions of dienes with Fe2(CO)9/SiO2
etate)iron wmax(hexane) 1750, 1984, 2049 cm−1
lH(C6D6) 1.15–1.24(2H, m), 1.57(3H, d), 1.74(3H, s),
4.54(2H, d), 5.4–5.6(2H, m).
;
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Houben-Weyl, Thieme, 1986.
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Acknowledgements
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The authors thank the EPSRC for a grant (to GFD).
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