Ferrocenes containing a pendant propargylic chain obtained via addition of propargyl alcohol to μ-vinyliminium ligands in diiron complexes

Article abstract of DOI:10.1021/om200904s

The diiron bridging vinyliminium complexes [Fe₂{μ- η¹:η³-C=N(Me)₂C(R′)=C(R″)} (μ-CO)(CO)(Cp)₂][SO₃CF₃] (R′ = H, R″ = SiMe₃, 3a; R′ = H, R″ = Tol =4-MeC ₆H₄, 3b; R′ = Me, R″ = Me, 3c; R′ = SPh, R″ = Me, 3d; R′ = H, R″ = Fc = [Fe(C₅H ₄)(Cp)], 6e) react with propargyl alcohol (HC≡CCH ₂OH), in refluxing toluene, affording the polysubstituted ferrocenes as mixtures of two isomeric forms: [1-NMe₂-2-R′-3-R″-5- CH₂OCH₂C≡CH-Fc] (R′ = H, R″ = SiMe ₃, 6a; R′ = H, R″ = Tol, 6b; R′ = Me, R″ = Me, 6c; R′ = SPh, R″ = Me, 6d, R′ = H, R″ = Fc, 6e) and [1-NMe₂-2-R′-3-R″-4-CH₂OCH₂C≡ CH-Fc] (R′ = H, R″ = SiMe₃, 7a; R′ = H, R″ = Tol, 7b; R′ = Me, R″ = Me, 7c; R′ = SPh, R″ = Me, 7d) in overall yields of about 55-65%. Formation of the functionalized cyclopentadienyl in the ferrocene products takes place through the assembly of two propargyl units with the bridging vinyliminium ligand: one alkynol is incorporated by a [3 + 2] cycloaddition with the bridging C₃ ligand; a second alkynol unit gives rise to a pendant chain through -OH substitution. Investigations show that the substitution step is catalyzed by the parent diiron complex itself or by a mononuclear iron fragment (likely the Fp⁺ complex). The pendant propargyl chain has been exploited to connect the ferrocene to other molecular fragments: in particular, the reaction of 6a with 4-biphenyl azide, by copper-catalyzed azide-alkyne cycloaddition (CuAAC), leads to the formation of the triazole-functionalized ferrocene [1-NMe ₂-2-CH₂OCH₂-N₃(C₆H ₄Ph)C₂H-4-SiMe₃-Fc] (12). Moreover, 6a reacts with Co₂(CO)₈, affording the complex [Co ₂{μ-η²-HC≡CR}(CO)₆] (13), (HC≡CR = 6a), where the alkyne adopts a η² coordination to a dicobalt hexacarbonyl fragment. The molecular structure of 7a has been determined by X-ray diffraction studies.

Full text of DOI:10.1021/om200904s

Article
pubs.acs.org/Organometallics
Ferrocenes Containing a Pendant Propargylic Chain Obtained via
Addition of Propargyl Alcohol to μ-Vinyliminium Ligands in Diiron
Complexes†
Luigi Busetto,* Rita Mazzoni, Mauro Salmi, Stefano Zacchini, and Valerio Zanotti*
̀
Dipartimento di Chimica Fisica e Inorganica, Universita di Bologna, Viale Risorgimento 4, I-40136 Bologna, Italy
S
* Supporting Information
ABSTRACT: The diiron bridging vinyliminium complexes
[Fe2{μ-η1:η3-CN(Me)2C(R′)C(R″)}(μ-CO)(CO)-
(Cp)2][SO3CF3] (R′ = H, R″ = SiMe3, 3a; R′ = H, R″ = Tol
=4-MeC6H4, 3b; R′ = Me, R″ = Me, 3c; R′ = SPh, R″ = Me, 3d;
R′ = H, R″ = Fc = [Fe(C5H4)(Cp)], 6e) react with propargyl
alcohol (HCCCH2OH), in refluxing toluene, affording the
polysubstituted ferrocenes as mixtures of two isomeric forms:
[1-NMe2-2-R′-3-R″-5-CH2OCH2CCH-Fc] (R′ = H, R″ = SiMe3, 6a; R′ = H, R″ = Tol, 6b; R′ = Me, R″ = Me, 6c; R′ = SPh, R″
= Me, 6d, R′ = H, R″ = Fc, 6e) and [1-NMe2-2-R′-3-R″-4-CH2OCH2CCH-Fc] (R′ = H, R″ = SiMe3, 7a; R′ = H, R″ = Tol, 7b;
R′ = Me, R″ = Me, 7c; R′ = SPh, R″ = Me, 7d) in overall yields of about 55−65%. Formation of the functionalized
cyclopentadienyl in the ferrocene products takes place through the assembly of two propargyl units with the bridging
vinyliminium ligand: one alkynol is incorporated by a [3 + 2] cycloaddition with the bridging C3 ligand; a second alkynol unit
gives rise to a pendant chain through −OH substitution. Investigations show that the substitution step is catalyzed by the parent
diiron complex itself or by a mononuclear iron fragment (likely the Fp+ complex). The pendant propargyl chain has been
exploited to connect the ferrocene to other molecular fragments: in particular, the reaction of 6a with 4-biphenyl azide, by
copper-catalyzed azide−alkyne cycloaddition (CuAAC), leads to the formation of the triazole-functionalized ferrocene [1-NMe2-
2-CH2OCH2-N3(C6H4Ph)C2H-4-SiMe3-Fc] (12). Moreover, 6a reacts with Co2(CO)8, affording the complex [Co2{μ-η2-HC
CR}(CO)6] (13), (HCCR = 6a), where the alkyne adopts a η2 coordination to a dicobalt hexacarbonyl fragment. The
molecular structure of 7a has been determined by X-ray diffraction studies.
Scheme 1
INTRODUCTION
■
Transition-metal-mediated cycloaddition reactions are emerg-
ing as powerful synthetic tools for the construction of
molecular architectures through the assembly of readily
accessible building blocks. Alkynes are commonly involved in
these reactions, such as in copper-catalyzed azide−alkyne
cycloaddition (CuAAC),1 which corresponds well to the “near-
perfect” bond-forming reactions condition required for a rapid
combination of molecules according to the synthetic concepts
developed by Sharpless.2 Indeed, the number of transition-
metal-promoted cycloadditions involving alkynes is much
broader, and many of these reactions, although far from
achieving the status of “click reactions”, have been developed in
useful synthetic protocols: for example, for the construction of
heterocycles.3
polysubstituted ciclopentadienyl ligand and directly affords
the ferrocene product 2.5 The substituents present on the
parent C3 and C2 fragments are retained in the resulting
functionalized Cp ring of the ferrocene product. Conversely,
the dinuclear complex undergoes fragmentation, with elimi-
nation of one iron fragment that formally corresponds to
[Fe(H)(CO)2Cp].
The second example (Scheme 2) shows a similar cyclo-
addition involving a cationic vinyliminium ligand in place of a
vinylalkylidene.6 Once more, the reaction affords, as the major
Conversely, there are few examples of cycloaddition reactions
involving alkynes and hydrocarbyl fragments acting as bridging
ligands, in spite of the fact that there is a large variety of
dinuclear complexes with unsaturated bridging ligands
potentially able to undergo cycloadditions.4
We have recently reported on [3 + 2] cycloadditions of
alkynes with bridging C3 ligands in diiron complexes.5,6 One
example, shown in Scheme 1, illustrates the [3 + 2]
cycloaddition of a bridging vinylaminoalkylidene ligand with
PhCCPh. The reaction results in the formation of a
Special Issue: F. GordonA. Stone Commemorative Issue
Received: September 28, 2011
Published: November 22, 2011
© 2011 American Chemical Society
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Scheme 2
Scheme 3
product, a ferrocene containing a polysubstituted Cp ring,
which results from the assembly of the bridging C3 ligand with
an alkyne. However, in this case, an additional product is
formed resulting from a [3 + 2 + 1] cycloaddition where a CO
is incorporated, affording the cyclohexadienyloxo derivative 5.
The incorporation of CO makes the cycloaddition more similar
previous findings (see Scheme 2) except for the CO
incorporation, which is not observed: therefore, cyclohexadie-
none complexes are not formed in this case. Another unique
feature is the incorporation of a second propargyl unit which
originates the pendant chain on the ferrocenyl products. This
point will be further discussed in detail, since we will first
describe the cycloaddition reaction. The [3 + 2] cycloaddition
is not regiospecific: a mixture of two isomers (compounds 6
and 7) are generally obtained, corresponding to the two
possible modes in which the propargyl alcohol can be
incorporated into the five-membered Cp ring. Usually, the
regioisomer of type 6 is largely predominant and, in the case of
6d, it is the only product formed. Conversely, the reaction with
3e is less selective and 6e and 7e are obtained in comparable
amounts. The different regioselectivity observed can be
associated with the nature of the substituents R′ and R″ on
the bridging C3 frame; however, it is not apparent whether the
selectivity depends on steric arguments or is the consequence
of a more complex balance of steric and electronic effects.
Separation of the isomeric mixture by column chromatog-
raphy (on alumina) was only partially achieved for 6a−7a, 6b−
7b, and 6c−7c, respectively. Nevertheless, assignments of the
NMR resonances to each isomer was possible by NOE
experiments and by consideration of the fact that one isomer
is largely prevalent. For example, complexes of type 6 where the
propargyl substituent and the amino group are in adjacent
positions exhibit NOE effects between the NMe2 group and the
protons of the −CH2OCH2CCH substituent, whereas no
significant effect can be observed between the R″ substituent
and the propargyl ether.
to the classic Dotz benzannulation, which typically involves
̈
vinylcarbenes (α,β-unsaturated carbene ligands), alkynes, and
CO.7
Despite the different natures of the parent complexes, a
common feature of the two reactions is the direct access to
ferrocenes in which only one cyclopentadienyl ring contains
different functionalities. However, the cycloaddition of bridging
vinylalkylidene ligands with alkynes (Scheme 1) has a general
character, whereas those shown in Scheme 2, published in the
form of a preliminary communication,6 are restricted to a
limited number of propargyls and diiron vinyliminium
complexes.
Therefore, since a variety of diiron μ-vinyliminium complexes
bearing different functional groups and substituents are readily
available by simple synthetic procedures,8 we decided to extend
our studies and specifically investigate the reactions of
vinyliminium complexes with the propargyl alcohol, with the
aim of increasing the library of polysubstituted Cp ferrocenes
also containing hydroxymethyl functions.
RESULTS AND DISCUSSION
■
The diiron bridging vinyliminium complexes 3a−e react with
propargyl alcohol (HCCCH2OH), in refluxing toluene,
affording the corresponding polysubstituted ferrocenes as
mixtures of two isomeric forms: 6a−e and 7a−c,e in overall
yields of about 55−65% (Scheme 3).
Compounds 6a−e and 7a−c,e have been characterized by
NMR spectroscopy and elemental analysis. The molecular
structure of 7e has been determined by X-ray diffraction (see
below).
The reaction leads to the [3 + 2] cycloaddition of the
bridging C3 ligand and the alkyne to form a cyclopentadienyl
ligand, consequently affording functionalized ferrocenes. In
Scheme 3, the carbon atoms of the bridging C3 ligand and those
of functionalized Cp have been numbered in order to make
clearer which part of the cyclopentadienyl derives from the
bridging vinyliminium (C1, C2, and C3) and which originates
from the alkyne (C4 and C5). The result is consistent with
Among the products shown in Scheme 3 the isomers 6e and
7e deserve more attention for their biferrocene character
(Chart 1). As recently reported, the [3 + 2] cycloaddition of
alkynes with bridging C3 ligands containing a ferrocenyl
provides a general synthetic route to biferrocene complexes
with unusual substitution patterns.9 This approach also applies
to the reaction of 3e with propargyl alcohol. Indeed, common
synthetic methods make it possible to obtain 1′,1‴-
disubstituted biferrocene compounds or similar centrosym-
metric compounds which are, by far, the most investigated
biferrocenes.10 Conversely, 6e and 7e are disubstituted-1,1″-
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Chart 1
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 7a
Fe(1)−
2.012(3)−2.111(3), av C−C
1.414(4)−1.427(3), av
a
b
c
a
b
c
Cp(1)
2.049(7)
Cp(1)
1.424(7)
Fe(1)−
2.016(4)−2.034(4), av C−C
1.413(4)−1.434(4), av
Cp(2)
2.028(9)
Cp(2)
1.426(9)
Fe(2)−
2.031(3)−2.059(3), av C−C
1.412(6)−1.439(4), av
Cp(3)
2.041(7)
Cp(3)
1.423(10)
Fe(2)−
2.034(3)−2.041(3), av C−C
1.395(5)−1.423(5), av
d
d
Cp(4)
2.037(7)
1.467(4)
Cp(4)
1.406(11)
C(3)−
C(1)−
1.397(3)
1.444(4)
1.434(3)
1.453(4)
C(14)
N(1)
N(1)−
1.455(4)
1.497(4)
1.403(3)
N(1)−
C(21)
C(22)
C(4)−
C(23)−
biferrocenes with substitution patterns normally unavailable by
usual substitution methods or by coupling of ferrocenyl units.
As mentioned above, the molecular structure of 7e has been
determined by X-ray diffraction (Figure 1 and Table 1) and can
C(23)
O(1)
O(1)−
C(24)−
C(24)
C(25)
C(25)−
1.167(5)
C(26)
a
b
sum of angles at Cp(1)
sum of angles at Cp(3)
C(1)−N(1)−C(21)
C(21)−N(1)−C(22)
C(23)−O(1)−C(24)
C(24)−C(25)−C(26)
539.9(4) sum of angles at Cp(2)
539.9(7) sum of angles at Cp(4)
115.5(2) C(1)−N(1)−C(22)
114.0(3) C(4)−C(23)−O(1)
112.1(2) O(1)−C(24)−C(25)
179.3(4)
539.9(9)
540.2(7)
115.4(3)
113.6(2)
109.8(3)
c
d
a
Cp(1) is defined by atoms C(1), C(2), C(3), C(4), and C(5).
Cp(2) is defined by atoms C(6), C(7), C(8), C(9), and C(10).
Cp(3) is defined by atoms C(11), C(12), C(13), C(14), and C(15).
b
c
d
Cp(4) is defined by atoms C(16), C(17), C(18), C(19), and C(20).
the products 6 and 7: one is a constituent of the five-membered
ring, while the second propargyl unit gives rise to the
CH2OCH2CHCCCH pendant chain. We do not have direct
evidence for the reaction mechanism; however, a reasonable
possibility is a two-step process with the ferrocene intermediate
I (Scheme 4).
Figure 1. ORTEP diagram of 7e. All the hydrogens have been omitted
An alternative explanation based on the coupling of two
HCCCH2OH molecules to form the dipropargyl ether
which, in turn, undergoes [3 + 2] cycloaddition with 3 has to be
excluded in that 3a was found unreactive upon treatment with
dipropargyl ether. On the other hand, any attempt to isolate the
intermediate I was unsuccessful, including the use of an
equimolar amount of propargyl alcohol. Even in this case the
reaction afforded the ferrocenes 6 and 7, although in lower
yields. Therefore, the addition of the second propargyl unit
must be very fast under the reaction conditions, which is
unexpected, in that etherification of alcohols is not a prompt
process unless it is catalyzed. The formation of the propargyl
ether pendant substituent might occur by OH desplacement
either at the propargyl alcohol or at the ferrocenylmethanol.
Both possibilities are well documented in the literature.
Propargylic substitution is generally promoted by transition-
metal complexes; for example, the use of Co2(CO)9 (Nicholas
reaction)11 provides an effective tool for the substitution of
propargyl alcohols with a variety of nucleophiles, including
alcohols. More recently, ruthenium-catalyzed propargylic
substitution has emerged as a most effective approach for the
transformation of alkynols into valuable products. Examples
include the thiolate-bridged diruthenium complexes developed
by Nishibayashi et al.12 and the mononuclear ruthenium
complexes developed by Gimeno and co-workers13 and the
Rennes group.14 For these ruthenium catalysts the role of
allenylidene intermediates has been proposed. In addition,
Lewis acids, such as BiCl3,15 InBr3,16 CuBr2,17 and FeCl3,18
behave as catalysts in the propargylic substitution.
for clarity. Thermal ellipsoids are drawn at 30% probability level.
be described as a 1,3,5-trisubstituted ferrocene, where the 1-
substituent is a ferrocenyl group. The two Cp rings Cp(1)
(defined by atoms C(1), C(2), C(3), C(4), and C(5)) and
Cp(2) (C(6), C(7), C(8), C(9), and C(10)) bonded to Fe(1),
as well as Cp(3) (C(11), C(12), C(13), C(14), and C(15))
and Cp(4) (C(16), C(17), C(18), C(19), and C(20)) bonded
to Fe(2), are almost parallel (angles between the least-squares
mean planes of the five-membered rings are 3.8° and 1.3°,
respectively). The C−C-bonded Cp(1) and Cp(3) rings are
almost coplanar (mean deviation from the least-squares plane
0.0799 Å) and the inter-Cp contact (C(3)−C(14) = 1.467(4)
Å) is essentially a single bond.
The Fe−C interactions with the two substituted C5 rings
(average Fe(1)−Cp(1) = 2.049(7) Å and Fe(2)−Cp(3) =
2.041(7) Å) are sensibly longer than those with the
unsubstuted Cp rings (average Fe(1)−Cp(2) = 2.028(9) Å
and Fe(2)−Cp(4) = 2.037(7) Å) and increase in the order Fe−
Cp(unsubstituted) < Fe−Cp(monosubstituted) < Fe−Cp-
(trisubstituted).
The N(1) atom is essentially sp3 hybridized, displaying a
strong pyramidalization (sum of angles 344.9(5)°), whereas
C(24)−C(25)−C(26) (179.3(4)°) is nearly perfectly linear, in
agreement with the acetylenic nature of the C(25)−C(26)
triple bond (1.167(5) Å).
The most peculiar and unprecedented feature of the
reactions shown in Scheme 3 is that two propargyl units are
incorporated in the functionalized cyclopentadienyl ligand of
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Scheme 4
On the other hand, ferrocenylmethylation of propargyl
alcohol, via formation of the stabilized ferrocenylmethylidene
cation, might also explain the formation of the propargyl ether
pendant chain. Desplacement of OH in ferrocenyl alcohols also
requires a suitable catalytic system, usually provided by
Brønsted acids, Lewis acids, or transition-metal compounds.19
On the basis of these considerations, it is realistic to assume the
role of the parent diiron complexes in promoting the second
step in the reaction sequence shown in Scheme 4. In order to
prove this possibility, we have investigated the reaction of
hydroxymethylferrocene, [Fc-CH2OH] (8), with propargyl
alcohol in the presence of catalytic amounts (0.1 equiv) of
the vinyliminium complex 3a (Scheme 5).
primary or secondary and tertiary propargyls can behave quite
differently with respect to propargylic substitution.
We have also investigated the reaction of propargyl alcohol
with the diiron vinylalkylidene complex 10 (Scheme 6).
Scheme 6
Scheme 5
Also in this case a [3 + 2] cycloaddition between the bridging
vinylalkylidene ligand and the propargyl alcohol takes place,
leading to the formation of the functionalized ferrocene 11.
Complex 11 contains a hydroxymethyl substituent which does
not undergo addition of a further alkynol unit. Therefore, only
vinyliminium−diiron complexes (compounds of type 3), and
not neutral vinylalkylidene complexes, are able to promote OH
substitution.
Under these conditions, etherification takes place, yielding 9,
whereas no reaction occurs under the same conditions in the
absence of the diiron complex 3a. Nevertheless, it is not
obvious how 3a can catalyze the reaction. Some structural
similarities with the Nishibayashi diruthenium catalysts should
be envisaged, but this is certainly not enough to hypothesize a
similar propargylic activation. Iron-bound propargyl units can
be transformed into vinylidene and allenylidene complexes,20
but iron is apparently less effective than ruthenium in the
activation of propargyl alcohol. Moreover, dinuclear complexes
exhibit some distinct features. To the best of our knowledge,
none of the numerous diiron complexes containing unsaturated
C3 ligands, including bridging allenylidene complexes, can
properly account for the possible involvement of 3a in catalytic
propargylic substitution.21 As an example, no propargylic
susbstitution is observed in the reaction of alkynols and
MeOH with Fe3(CO)12, in spite of the formation of bridging
allenylidene intermediates; the result is a rather complex
rearrangement of the bridging ligand with incorporation of a
CO.22
At this point it should be evident what is reported above in
Scheme 2: the reaction of 3a with the diphenyl propargylic
alcohol affords the ferrocene complexes 4 as the major product,
which, in spite of the presence of a hydroxymethyl substituent
and of the diiron species 3a (expected to act as catalyst), does
not undergo OH substitution. In this case, the absence of
reactivity should be associated with the different nature of the
alkynol: HCCCPh2OH in place of HCCCH2OH. Indeed,
Considerations on the possible catalytic role of our diiron
complexes should also include the observation that, in some
cases, the catalytic properties attributed to polynuclear
complexes are actually due to mononuclear species generated
by fragmentation of the parent complexes. This possibility is
particularly realistic in our case, in that vinyliminium complexes
3 undoubtedly undergo fragmentation in order to form the
observed ferrocene products. The fragment released formally
correspond to [Fe(CO)2Cp]+ (usually indicated as Fp+), which
is known to behave as a Lewis acid and can coordinate alkynes
by η2 coordination.23 Therefore, Fp+ might activate the
propargyl alcohol and catalyze propargylic substitution. Never-
theless, the ferrocenylmethylation reaction cannot be excluded.
In any case the catalytic role of Fp+ is clearly demonstrated by
running the same reaction shown in Scheme 5, except for the
fact that [Fp][SO3CF3] was used in the place of the dinuclear
complex 3a. The results were quite similar, and compound 9
was obtained in comparable yields, indicating that Fp+ might be
also the effective catalyst in the etherification step of the
reaction, leading to the ferrocene complexes 6 and 7. This
hypothesis should also explain the absence of catalytic activity
in the case of the vinyl alkylidene complex 10. The
fragmentation of 10, occurring in the reaction shown in
Scheme 6, is expected to eliminate the neutral mononuclear
iron fragment HFp, which does not have the acidic character of
Fp+.
Ferrocenes Containing Pendant Propargylic Substitu-
ents and Their Reactions. So far our attention has been
almost exclusively directed to the reaction sequence, [3 + 2]
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Scheme 7
cycloaddition and propargylic substitution, in order to provide a
reasonable picture of transformations that occur in the
synthesis of the ferrocene isomers 6 and 7 and, in particular,
to account for the OH substitution step. However, the structure
of these functionalized ferrocenes deserve some comment.
Some peculiar features of the substitution pattern in the
biferrocene complexes 6e and 7e have already been outlined.
The ferrocene products 6 and 7 display, as a characteristic
feature, one cyclopentadienyl ring containing different
functionalities. Since ferrocenes with a single polysubstituted
Cp ligand are difficult to obtain by common synthetic
methods,24 the cycloadditions described above might provide
a valuble synthetic approach, in that the “one pot” synthesis
would largely compensate the synthetic effort for the
construction of bridging C3 ligands containing appropriate
functionalities. Moreover, ferrocenes provided with a pendant
propargylic chain and heteroatom-containing functionalities
have potential applications as organometallic ligands or
functional building blocks for the construction of more complex
molecular architectures. This point is of particular importance
in view of the increasing interest in the use of ferrocene
derivatives as selective and sensitive multichannel chemical
probes with applications spanning from biomolecules to
nanomaterials and sensors.25 The pendant propargylic chain
might be used to connect the ferrocenyl unit to a variety of
molecular fragments via covalent or coordination bonds. In
order to investigate these possibilities, we have studied a classic
copper-catalyzed azide−alkyne cycloaddition. In particular, we
have investigated the reaction of 6a with 4-biphenyl azide and
have observed the formation of the expected triazole derivative
12 in high yield (Scheme 7). It should be remarked that, in
spite of careful alumina column chromatography, samples of 6a
used in these experiments contained trace amounts of the
isomeric form 7a. These were too limited to allow the
formation of the corresponding products of type 12 (or 13; see
later) in detectable amounts.
The choice of a rather unusual azide (4-biphenyl azide)27 was
aimed at obtaining adjacent triazole and biphenyl rings, which
might produce interesting properties that go beyond the scope
of this report.
Compound 12 was characterized by NMR spectroscopy and
elemental analysis. In particular, the NMR spectrum shows a
single set of resonances, thus indicating that the reaction leads
to the formation of a single product. Indeed, the cycloaddition
of the azide is regioselective, affording the 1,4-disubstituted
1,2,3-triazole product in excellent yields and near-perfect
regioselectivity.
The presence of a pendant propargylic chain in complexes 6
and 7 should be also exploited to connect the ferrocene unit to
metal fragments through η2 coordination of the alkyne unit. We
have investigated the reaction of 3a with Co2(CO)8 in order to
obtain the corresponding dicobalt hexacarbonyl complex.
Substitution of two carbonyl groups in Co2(CO)8 by acetylenic
molecules has been known for a long time28 and can generate
stable alkyne complexes and also opens possible applications
and useful protocols in organic synthesis.29
Compound 13 was characterized by NMR spectroscopy and
elemental analysis. Spectroscopic data are consistent with
dicobalt hexacarbonyl alkyne complexes, including those
obtained by transition-metal propargyl compounds.30 In
particular, in the 1H NMR spectra, the resonance of the alkyne
proton is shifted downfield in comparison with the signal for
the parent compound 6a (6a shows a triplet at 2.46 ppm, while
for 13 it is shifted to 6.05 ppm). Likewise, in the 13C NMR
spectrum, the resonances due to the carbon atoms of the alkyne
(at 91.6 and 70.3 ppm for the quaternary carbon and the CH
carbon, respectively) are consistent with typical values for
alkyne complexes with dicobalt hexacarbonyls.
CONCLUSIONS
■
The assembly of propagyl alcohol with bridging vinyliminium
ligands in diiron complexes led to to the formation of ferrocene
containing a pendant propargyl chain. In comparison to
previous results concerning the analogous reaction with tertiary
propargyl alcohols, the reactions with HCCCH2OH exhibit
some distinct and unprecedented features. In particular, two
The synthesis follows a literature procedure that takes
advantage of the in situ reduction of Cu(II) by sodium
ascorbate, to give Cu(I), which catalyzes the [3 + 2]
cycloaddition between the terminal alkyne and the azide.26
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Scheme 8
Synthesis of 6a and 7a. Compound 3a (400 mg, 0.883 mmol)
was dissolved in toluene (30 mL); then HCCCH2OH (0.5 mL, 8.9
mmol) was added. The resulting mixture was stirred at reflux overnight
and then was cooled to room temperature and filtered on a Celite pad.
Solvent removal and chromatography of the residue on an alumina
column with petroleum ether (bp 40−60 °C) as eluent first gave a
yellow fraction, corresponding to nonsubstituted ferrocene (FeCp2).
Two orange fractions, not fully separated, corresponding to a mixture
of the isomers 6a and 7a were collected by using a mixture of
petroleum ether and diethyl ether (1/1, v/v) as eluent. Yield: 201 mg,
62%. Anal. Calcd for C19H27FeNOSi: C, 61.78; H, 7.37. Found: C,
61.64; H, 7.46.
propargyl units are involved in the assembly with the bridging
C3 ligand: one alkynol unit undergoes [3 + 2] cycloaddition
with the vinyliminium ligand, whereas a second alkynol gives
rise to the pendant propargyl chain by −OH substitution. We
have evidenced that this second step is catalyzed: the parent
diiron complex itself or a mononuclear fragment (likely the Fp+
complex) is able to promote the formation of propargyl
ferrocenylmethyl ethers. These results are interesting for the
presence on the polysubstituted Cp of a propargyl pendant
chain functionality whose reactivity could open routes to new
ferrocenes with unusual substitution patterns. We have
described a couple of reactions in which the pendant alkynyl
has been connected by formation of a covalent bond in a
copper-catalyzed azide−alkyne cycloaddition (CuAAC) and, in
another example, by η2 coordination to a dicobalt carbonyl
fragment. Finally, having found iron complexes which can
produce catalytic activation of alcohols will prompt us to
further investigate the subject, which is certainly of general
interest; so far, it has been almost completely dominated by the
more expensive and toxic ruthenium derivatives.
1
Data for 6a are as follows. H NMR (CDCl3): δ 4.73, 4.31 (d, 2H,
2JHH = 10.6 Hz, C5-CH2O−); 4.23, 4.16 (dd, 2H, 2JHH = 15.8 Hz, 4JHH
4
= 2.4 Hz, −CH2OCH2−); 4.11 (s, 5H, Cp); 3.98 (d, 1H, JHH = 1.4
Hz, C4H); 3.81 (d, 1H, 4JHH = 1.4 Hz, C2H); 2.66 (s, 6H, NMe2); 2.46
4
(t, 1H, JHH = 2.4 Hz, −OCH2CCH); 0.19 (s, 9H, SiMe3). 13C
NMR (CDCl3): δ 117.3 (C1); 80.2 (CCH); 77.5 (C5); 74.6 (C
CH); 73.0 (C4); 68.9 (Cp); 67.3 (C5-CH2O−); 66.7 (C3); 60.9 (C2);
57.3 (−OCH2−CCH); 45.0 (NMe2); 0.3 (SiMe3).
1
Data for 7a are as follows. H NMR (CDCl3): δ 4.49, 4.20 (d, 2H,
2JHH = 10.6 Hz, C4-CH2O−); 4.11, 4.04 (dd, 2H, 2JHH = 15.8 Hz, 4JHH
4
= 2.4 Hz, −CH2OCH2−); 4.19 (s, 5H, Cp); 4.02 (d, 1H, JHH = 1.6
EXPERIMENTAL SECTION
Hz, C5H); 3.67 (d, 1H, 4JHH = 1.6 Hz, C2H); 2.59 (s, 6H, NMe2); 2.42
■
4
(t, 1H, JHH = 2.40 Hz, −OCH2CCH); 0.2 (s, 9H, SiMe3).
General Data. All reactions were routinely carried out under a
nitrogen atmosphere, using standard Schlenk techniques. Solvents
were distilled immediately before use under nitrogen from appropriate
drying agents. Chromatography separations were carried out on
columns of deactivated alumina (4% w/w water). Glassware was oven-
dried before use. Infrared spectra were recorded at 298 K on a Perkin-
Elmer Spectrum 2000 FT-IR spectrophotometer, and elemental
analyses were performed on a ThermoQuest Flash 1112 Series EA
Instrument. All NMR measurements were performed on a Varian
Mercury Plus 400 instrument. Unless otherwise stated, NMR spectra
were recorded at 298 K. Chemical shifts for 1H and 13C were
referenced to internal TMS. Spectra were fully assigned via DEPT
Isomer ratio 6a/7a: 10/1.
Synthesis of Complexes 6b−e and 7b,c,e. Complexes 6b−e
and 7b,c,e were prepared following the same procedure above-
described by reacting 3b−e, respectively, with HCCCH2OH.
6b and 7b: overall yield 55%. Anal. Calcd for C23H25FeNO: C,
71.33; H, 6.51. Found: C, 71.44; H, 6.46.
Data for 6b are as follows. 1H NMR (CDCl3): δ 7.49−7.06 (m, 4H,
2
C6H4Me); 4.83, 4.76 (d, 2H, JHH = 11.0 Hz, C5-CH2O−); 4.65 (d,
4
2
4
1H, JHH = 1.4 Hz, C4H); 4.33, 4.26 (dd, 2H, JHH = 16.1 Hz, JHH
=
2.2 Hz, −CH2OCH2−); 4.21 (s, 5H, Cp); 4.12 (d, 1H, 4JHH = 1.4 Hz,
C2H); 2.79 (s, 6H, NMe2); 2.53 (t, 1H, JHH = 2.2 Hz, −OCH2C
4
CH); 2.42 (s, 3H, C6H4Me). 13C NMR (CDCl3): δ 135.7−125.7
(C6H4Me); 115.0 (C1); 84.0 (C3); 80.3 (CCH); 75.0 (C5); 74.7
(CCH); 71.2 (C4); 70.4 (Cp); 67.6 (C5-CH2O−); 58.3 (C2); 57.1
(−OCH2CCH); 45.2 (NMe2); 21.1 (C6H4Me).
1
experiments and H,13C correlation measured through gs-HSQC and
gs-HMBC experiments. NOE measurements were recorded using the
DPFGSE-NOE sequence. For the isomers 6 and 7, not fully separated
by chromatography, yields have been determined by integration of the
NMR signals. NMR resonances are indicated according to the
numbering shown in Scheme 8.
All the reagents were commercial products (Aldrich) of the highest
purity available and used as received. [Fe2(CO)4(Cp)2] was purchased
from Strem and used as received. Compounds 3a−c,31 3d,32 3e,33 and
1034 were prepared by published methods.
Data for 7b are as follows. 1H NMR (CDCl3): δ 7.49−7.06 (m, 4H,
C6H4Me); 4.51 (d, 1H, 4JHH = 1.5 Hz, C5H); 4.49, 4.45 (d, 2H, 2JHH
=
11.0 Hz, C4−CH2O−); 4.24 (d, 1H, 4JHH = 1.5 Hz, C2H); 4.09 (s, 5H,
Cp); 4.16−4.05 (m, 2H, −CH2OCH2−); 2.68 (s, 6H, NMe2); 2.55 (t,
1H, 4JHH = 2.2 Hz, -OCH2CCH); 2.40 (s, 3H, C6H4Me). 13C NMR
(CDCl3): δ 114.8 (C1); 80.5 (CCH); 74.5 (CCH); 69.2 (Cp);
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Organometallics
Article
67.1 (C4-CH2O); 66.0 (C5); 56.9 (-OCH2CCH); 55.8 (C2); 42.4
in 0.5 mL of water). The heterogeneous mixture was stirred overnight;
then, it was diluted with water (50 mL) and cooled in ice, and the
yellow precipitate was collected by filtration. After the precipitate was
washed with cold water (three 25 mL portions), it was dried under
vacuum to afford a yellow powder. Yield: 255 mg, 91%. Anal. Calcd for
(NMe2); 21.4 (C6H4Me).
Isomer ratio 6b/7b: 6/1.
6c and 7c: yield 60%. Anal. Calcd for C18H23FeNO: C, 66.47; H,
7.13. Found: C, 66.44; H, 7.22.
1
1
C31H36FeN4OSi: C, 65.95; H, 6.43. Found: C, 66.02; H, 6.56. H
Data for 6c are as follows. H NMR (CDCl3): δ 4.66, 4.12 (d, 2H,
2
2JHH = 11.0 Hz, C5-CH2O−); 4.15 (dd, 2H, 2JHH = 15.8 Hz, 4JHH = 2.4
NMR (CDCl3): δ 8.01−7.07 (m, 10H, Ph); 4.81, 4.38 (d, 2H, JHH
=
2
10.8 Hz, C5-CH2O−); 4.79, 4.72 (d, 2H, JH4H = 15.6 Hz,
−CH2OCH2−); 4.12 (s, 5H, Cp); 4.01 (d, 1H, JHH = 1.2 Hz,
C4H); 3.84 (d, 1H, 4JHH = 1.24 Hz, C2H); 2.69 (s, 6H, NMe2); 0.2 (s,
9H, SiMe3). 13C NMR (CDCl3): δ 146.2 (−CH2CCHtriazole); 141.8,
139.4, 138.9, 130.4−119.5 (Carom); 121.0 (−CH2CCHtriazole); 116.8
(C1); 77.4 (C5); 73.2 (C4); 68.8 (Cp); 68.4 (C5-CH2O−); 66.0 (C3);
63.5 (−OCH2CCH); 61.2 (C2); 44.8 (NMe2); −0.4 (SiMe3).
Synthesis of 13. Compound 6a (183 mg, 0.5 mmol) was
dissolved in CH2Cl2 (20 mL). Then, Co2(CO)8 (171 mg, 0.5
mmol) was added. The resulting solution was stirred at room
temperature for 2 h, and then it was filtered on a Celite pad. Solvent
removal afforded a dark red solid, corresponding to 6. Yield: 291 mg,
89%. Anal. Calcd for C25H27Co2FeNO7Si: C, 45.82; H, 4.15. Found:
C, 45.93; H, 4.16. IR (CH2Cl2): ν(CO) 2084 (vs), 2025 (vs) cm−1. 1H
Hz, −CH2OCH2−; 3.98 (s, 5H, Cp); 3.95 (s, 1H, C4H); 2.76 (s, 6H,
4
NMe2); 2.42 (t, 1H, JHH = 2.4 Hz, −OCH2CCH); 1.96 (s, 3H,
Me); 1.88 (s, 3H, Me). 13C NMR (CDCl3): 110.9 (C1); 81.0 (C
CH); 80.4 (C3); 80.0 (C2); 74.5 (CCH); 74.3 (CCH); 72.8 (C5);
70.6 (Cp); 67.5 (C4); 67.4 (C5-CH2O−); 56.5 (−OCH2CCH);
45.6 (NMe2); 13.4 (Me); 11.7 (Me).
1
Data for 7c are as follows. H NMR (CDCl3): δ 4.55, 4.25 (d, 2H,
2JHH = 11.0 Hz, C4-CH2O−); 4.07 (dd, 2H, 2JHH = 15.8 Hz, 4JHH = 2.4
Hz, −CH2OCH2−); 3.89 (br s, 5H + 1H, Cp and C5H); 2.56 (s, 6H,
4
NMe2); 2.45 (t, 1H, JHH = 2.4 Hz, −OCH2CCH); 2.01 (s, 3H,
Me); 1.83 (s, 3H, Me). 13C NMR (CDCl3) δ 112.4 (C1); 81.3 (C
CH); 80.3 (C3); 78.8 (C2); 75.1 (C4); 70.3 (Cp); 67.1 (C4-CH2O−);
56.9 (−OCH2CCH); 56.1 (C2); 45.1 (NMe2); 12.1 (Me); 11.4
(Me).
NMR (CDCl3): δ 6.05 (br s, 1H, HCC−); 4.80, 4.43 (d, 2H, 2JHH
=
Isomer ratio 6c/7c: 7/1.
11.2 Hz, C5-CH2O−); 4.38−4.25 (m, 2H, −CH2OCH2−); 4.10 (s,
6d: yield 55%. Anal. Calcd for C23H25FeNOS: C, 65.87; H, 6.01.
Found: C, 65.94; H, 6.12.
5H, Cp); 3.95 (d, 1H, JHH = 1.2 Hz, C4H); 3.82 (d, 1H, JHH = 1.2
Hz, C2H); 2.66 (s, 6H, NMe2); 0.2 (s, 9H, SiMe3). 13C NMR
(CDCl3): δ 200.2−199.4 (CO); 117.2 (C1); 91.6 (HCC−); 78.2
(C5); 72.6 (C4); 70.3 (HCC−); 69.0 (Cp); 68.3 (C5-CH2O−); 66.5
(C3); 64.5 (−OCH2CCH); 60.8 (C2); 45.0 (NMe2); 1.2 (SiMe3).
X-ray Crystallography. Crystal data and collection details for 7e
are reported in Table 2. The diffraction experiments were carried out
4
4
Data for 6d are as follows. 1H NMR (CDCl3): δ 7.20−6.95 (m, 5H,
2
SPh); 4.63, 4.26 (d, 2H, JHH = 10.6 Hz, C5-CH2O−); 4.35 (s, 1H,
C4H); 4.23−4.19 (m, 2H, −CH2OCH2−); 4.15 (s, 5H, Cp); 2.75 (s,
6H, NMe2); 2.47 (t, 1H, 4JHH = 2.4 Hz, −OCH2CCH); 1.94 (s, 3H,
C3Me). 13C NMR (CDCl3): δ 140.8, 128.9, 125.3, 124.6 (Carom); 114.0
(C1); 85.9 (C2); 80.1 (CCH); 77.3 (C5); 74.7 (CCH); 71.5
(Cp); 71.0 (C3); 69.5 (C4); 67.1 (C5-CH2O−); 57.4 (-OCH2C
CH); 45.9 (NMe2); 13.2 (C3Me).
Table 2. Crystal Data and Experimental Details for 7e
Isomers 6e and 7e were separated by chromatography. Crystals of
7e suitable for X-ray diffraction were obtained by slow evaporation
from a CH2Cl2 solution.
formula
C26H27Fe2NO
481.19
fw
T, K
295(2)
6e: yield 36%. Anal. Calcd for C26H27Fe2NO: C, 64.89; H, 5.66.
Found: C, 64.94; H, 5.57.
λ, Å
0.710 73
triclinic
1
cryst syst
Data for 6e are as follows. H NMR (CDCl3): δ 4.53, 4.32 (d, 2H,
2JHH = 11.0 Hz, C5-CH2O−); 4.36 (d, 1H, 4JHH = 1.5 Hz, C4H); 4.29
(m, 2H, −CH2OCH2−); 4.19 (s, 5H, Cp); 4.01 (s, 5H, Cp); 4.06−
space group
P1
̅
a, Å
7.9265(11)
9.8809(13)
15.145(2)
105.555(2)
95.967(2)
101.465(2)
1104.3(3)
2
4
3.95 (br m, 4H, C5H4); (d, 1H, JHH = 1.5 Hz, C2H); 2.71 (s, 6H,
b, Å
4
NMe2); 2.50 (t, 1H, JHH = 2.4 Hz, −OCH2CCH). 13C NMR
c, Å
(CDCl3): δ 115.1 (C1); 86.4 (C3); 74.0 (CCH); 70.0, (Cp); 69.3
(Cp); 88.1 72.0, 71.4, 69.9, 68.9, 68.0, 66.9 (C2, C4, and C5H4); 67.0
(C5-CH2O−); 57.5 (−OCH2CCH); 43.4 (NMe2).
7e: yield 24%. Anal. Calcd for C26H27Fe2NO: C, 64.89; H, 5.66.
Found: C, 64.80; H, 5.71.
α, deg
β, deg
γ, deg
V, Å3
Z
1
Data for 7e are as follows. H NMR (CDCl3): δ 4.65, 4.36 (d, 2H,
Dc, g cm−3
1.447
2JHH = 11.0 Hz, C4-CH2O−); 4.25 (m, 2H, −CH2OCH2−); 4.13 (d,
μ, mm−1
1.331
4
1H, JHH = 1.5 Hz, C2H); 4.06 (s, 5H, Cp); 3.99 (s, 5H, Cp); 3.95−
3.92 (m, 4H, C5H4 and C5H); 2.60 (s, 6H, NMe2); 2.38 (t, 1H, 4JHH
=
F(000)
500
2.4 Hz, −OCH2CCH). 13C NMR (CDCl3): δ 114.6 (C1); 74.3
(CCH); 69.2 (Cp), 68.0 (Cp); 89.0, 87.1, 73.1, 71.8, 69.5, 67.7,
66.2, 65.9 (C2, C5, and C5H4); 66.4 (C4-CH2O−); 55.4 (−OCH2C
CH); 41.9 (NMe2).
cryst size, mm
θ limits, deg
no. of rflns collected
no. of indep rflns
no. of data/restraints/params
goodness on fit on F2
R1 (I > 2σ(I))
wR2 (all data)
largest diff peak and hole, e Å−3
0.18 × 0.13 × 0.11
1.42−26.00
11 249
4289 (Rint = 0.0265)
4289/150/271
1.040
Synthesis of 11. Complex 11 was prepared by following the same
procedure described for 6a, by reacting the complex 10 with HC
CCH2OH (yield 55%). Anal. Calcd for C14H20FeOSi: C, 58.34; H,
0.0362
1
6.99. Found: C, 58.26; H, 7.06. H NMR (CDCl3): δ 4.41−4.37 (m,
0.1099
3H, −CH2OH and C5H); 4.17 (t, 4JHH = 1.40 Hz, C2H); 4.14 (s, 5H,
0.565/−0.250
Cp); 4.09 (dd, 3JHH = 2.2 Hz, 4JHH = 1.4 Hz, C4H); 1.57 (t, 1H, 3JHH
=
5.8 Hz, −CH2OH); 0.22 (s, 9H, SiMe3). 13C NMR (CDCl3): δ 90.9
(C3); 73.1 (C2); 72.9 (C1); 72.6 (C5); 70.6 (C4); 68.5 (Cp); 60.9
(−CH2OH); −0.3 (SiMe3).
on a Bruker APEX II diffractometer equipped with a CCD detector
using Mo Kα radiation. Data were corrected for Lorentz−polarization
and absorption effects (empirical absorption correction SADABS).35
Structures were solved by direct methods and refined by full-matrix
least squares based on all data using F2.36 All hydrogen atoms were
fixed at calculated positions and refined by a riding model. Similar U
restraints (standard uncertainty 0.01) were applied to all C atoms.
Synthesis of 12. Compound 6a (183 mg, 0.5 mmol) and
diphenyl azide (Ph2N3) (98 mg, 0.5 mmol) were suspended in a 1:1
mixture of water and tert-butyl alcohol (10 mL). Sodium ascorbate
(0.05 mmol, 0.5 mL of a freshly prepared 0.1 M solution in water) was
added, followed by copper(II) sulfate pentahydrate (5 mg, 0.02 mmol,
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(16) Yadav, J. S.; Subba Reddy, B. V.; Raghavendra Rao, K. V.;
Narayana Kumar, G. G. K. S. Synthesis 2007, 3205−3210.
(17) Hui, H.-h.; Zhao, Q.; Yang, M.-y.; She, D.-b.; Chen, M.; Huang,
G.-s. Synthesis 2008, 191−196.
(18) Zhan, Z.-p.; Yu, J.-l.; Liu, H.-j.; Cui, Y.-y.; Yang, R.-f.; Yang, W.-
z.; Li, J.-p. J. Org. Chem. 2006, 71, 8298−8301.
ASSOCIATED CONTENT
■
S
* Supporting Information
A CIF file giving crystal data for 7e. This material is available
free of charge via the Internet at http://pubs.acs.org.
(19) See for example: (a) Jiang, R.; Zhang, Y.; Shen, Y. C.; Zhu, X.;
Xu, X. P.; Ji, S. J. Tetrahedron Lett. 2010, 66, 473−1179. (b) Vicennati.,
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(20) (a) Venancio, A. I. F.; da Silva, M. F. C. G.; Martins, L. M. D. R.
S.; Frausto da Silva, J. J. R.; Pombeiro, A. J. L. Organometallics 2005,
24, 4654−4665. and references therein. (b) Selegue, J. P. Coord.
Chem. Rev. 2004, 248, 1543−1563. (c) Gauss, C.; Veghini, D.; Orama,
O.; Berke, H. J. Organomet. Chem. 1997, 541, 19−38.
(21) (a) Etienne, M.; Talarmin, J.; Toupet, L. Organometallics 1992,
11, 2058−2068. (b) Akita, M.; Kato, S.-i.; Terada, M.; Masaki, Y.;
Tanaka, M.; Moro-oka, Y. Organometallics 1997, 16, 2392−2412.
(22) Bertolotti, F.; Gervasio, G.; Marabello, D.; Sappa, E.; Secco, A. J.
Organomet. Chem. 2008, 693, 2673−2678.
(23) (a) Reger, D. L. Acc. Chem. Res. 1988, 21, 229−235. (b) Akita,
M.; Kakuta, S.; Sugimoto, S.; Terada, M.; Tanaka, M.; Moro-oka, Y.
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AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +39 0512093694. E-mail: luigi.busetto@unibo.it (L.B.);
valerio.zanotti@unibo.it (V.Z.).
DEDICATION
†In memory of Professor F. G. A. Stone.
■
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