Fine-tuning the reactivity and stability by systematic ligand variations in CpCoI complexes as catalysts for [2+2+2] cycloaddition reactions

Article abstract of DOI:10.1002/chem.201202946

CpCo^I-olefin-phosphite and CpCo^I-bisphosphite complexes were systematically prepared and their reactivity in [2+2+2] cycloaddition reactions compared with highly active [CpCo(H₂C- CHSiMe₃)₂] (1). Whereas 1 is an excellent precursor for the synthesis of [CpCo(olefin)(phosphite)] complexes (2 a-f), [CpCo(phosphite)₂] complexes (3 a-e) were prepared photochemically from [CpCo(cod)]. The complexes were evaluated in the cyclotrimerization reaction of diynes with nitriles yielding pyridines. For [CpCo(olefin) (phosphite)], as well as some of the [CpCo(phosphite)₂] complexes, reaction temperatures as low as 50 °C were sufficient to perform the cycloaddition reaction. A direct comparison showed that the order of reactivity for the complex ligands was olefin₂>olefin/ phosphite>phosphites₂. The complexes with mixed ligands favorably combine reactivity and stability. Calculations on the ligand dissociation from [CpCo(olefin)(phosphite)] proved that the phosphite is dissociating before the olefin. [CpCo(H₂C-CHSiMe₃){P(OPh)₃}] (2 a) was investigated for the co-cyclization of diynes and nitriles and found to be an efficient catalyst at rather mild temperatures. Directed symbiosis: By fusing two different catalysts, new CpCo^I complexes that contain the combined properties assigned to both original complexes were developed (see scheme). These unique catalysts were rather stable yet possess high activity at relatively mild conditions in the [2+2+2] cycloaddition reaction for the synthesis of pyridine derivatives. Copyright

Full text of DOI:10.1002/chem.201202946

DOI: 10.1002/chem.201202946
Fine-Tuning the Reactivity and Stability by Systematic Ligand Variations in
CpCoI Complexes as Catalysts for [2+2+2] Cycloaddition Reactions
Indre Thiel, Haijun Jiao, Anke Spannenberg, and Marko Hapke*[a]
Abstract: CpCoI-olefin-phosphite and
CpCoI-bisphosphite complexes were
systematically prepared and their re-
nitriles yielding pyridines. For [CpCo-
(olefin)(phosphite)], as well as some of
olefin/phosphite>phosphites2.
complexes with mixed ligands faACHTUNGTRENNUNG
ACHTUNGTRENNUNG
The
vor-
G
the [CpCo(phosphite)2] complexes, re-
action temperatures as low as 508C
were sufficient to perform the cycload-
dition reaction. A direct comparison
showed that the order of reactivity for
the complex ligands was olefin2 >
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
[CpCo(H2C=CHSiMe3)2] (1). Whereas
1 is an excellent precursor for the syn-
thesis of [CpCo
N
ACHTUNGTRENNUNG
complexes (2a–f), [CpCo(phosphite)2]
complexes (3a–e) were prepared pho-
tochemically from [CpCoACTHNUGRTENUNG(cod)]. The
complexes were evaluated in the cyclo-
trimerization reaction of diynes with
gated for the co-cyclization of diynes
and nitriles and found to be an efficient
catalyst at rather mild temperatures.
Keywords: cobalt · cycloaddition ·
cyclopentadienyl ligands · olefin li-
gands · P ligands
Introduction
cation of [CpCo(CO)(dialkyl fumarate)] complexes, which
can be prepared from [CpCo(CO)2] and are air-stable
solids.[4] Nevertheless, all the regularly applied catalysts,
except the Jonas catalyst, need either fairly high reaction
temperatures or/and irradiation to become catalytically
active.
Transition-metal-catalyzed [2+2+2] cycloadditions have ma-
tured into a versatile methodology for constructing new
carbo- and heterocyclic structures including enantio- and
stereoselectivities.[1] The application in the assembly of com-
plex structures such as natural products highlights [2+2+2]
cycloadditions as an important tool for strategic reactions.[2]
Next to a large number of transition-metal complexes
known to catalyze these reactions, group 9 complexes, in
particular CpCoI complexes, still represent the predominant-
Our group focused on the preparation of new and reactive
CpCoI-olefin complexes for [2+2+2] cycloadditions. We re-
cently reported the synthesis and the catalytic activity of
[CpCo(H2C=CHSiMe3)2] (1), which was demonstrated pre-
sumably to be the most active precatalyst for Co-catalyzed
cycloadditions for the preparation of pyridines and also a
great precursor for the synthesis of a variety of [CpCo-
ly used catalysts.[1u,2] Besides [CpCo
[CpCo(CO)2] with s-donating ligands, p-donating olefin lig-
ands have also been established for these precatalysts as
demonstrated by the Jonas catalyst [CpCo(H2C=CH2)2] and
[CpCo(cod)] (cod=1,5-cyclooctadiene). Recently, catalysts
ACHTUNGTERN(NUNG PPh3)2] and
G
(olefin)2] complexes.[5,6] However, the thermal instability of
ACHTUNGTRENNUNG
E
1, which degrades rapidly above À308C, is a critical draw-
back making the handling of the catalyst somewhat tricky.
This degradation forms stable tetranuclear CpCo-cluster
complexes, which are inactive in cycloadditions under ambi-
ent conditions.[7] To improve the stability of precatalyst 1
without interfering too strongly with its capability to rather
easily generate the catalytically active species, we set out to
prepare CpCoI complexes with one olefin ligand substituted
by a ligand that would be able to provide sufficient stability
and reactivity. Apart from the fact that a large number of
phosphites with different electronic and steric properties are
commercially available, we chose phosphites for our goals
for additional other reasons: Phosphites should coordinate
less strongly since they are more electron-deficient s-donors
and also allow a weaker p-backbonding compared with the
corresponding phosphine analogues.[8] On the other hand
they should bind tighter to the metal than olefins.
ACHTUNGTRENNUNG
containing two different donor ligands were reported. For
example, Butenschçn et al., reported a catalyst with an eth-
ylene ligand and
a Cp-tethered phosphine ligand for
[2+2+2] cycloadditions in aqueous medium,[3] as well as
Aubert and Gandon who described the synthesis and appli-
[a] I. Thiel, Dr. H. Jiao, Dr. A. Spannenberg, Dr. M. Hapke
Leibniz-Institut fꢀr Katalyse e.V.
an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a
18059 Rostock (Germany)
Fax : (+49)381-1281-51213
E-mail: marko.hapke@catalysis.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202946.
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Chem. Eur. J. 2013, 19, 2548 – 2554

FULL PAPER
Results and Discussion
Following the observation of the ease of ligand displacement
in 1, we investigated the reaction of 1 with one equivalent
of P
ceeded very smoothly and the mixed [CpCo(H2C=
CHSiMe3){P(OPh)3}] complex (2a) could be obtained in ex-
cellent yield (84%). To our surprise the corresponding bis
phosphite [CpCo{P(OPh)3}2] complex (3a) could not be ob-
tained in pure form even with an excess of P(OPh)3 or pro-
ACHTUNGTRENNUNG(OPh)3 to substitute one olefin ligand. This reaction pro-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG-
G
Scheme 2. New photochemical route for the synthesis of the CpCoI-bis-
A
ACHTUNGTRNE(NUNG cod)] (the yields are that of the
AHCTUNGTRENNUNG
longed reaction times.[9] As it turned out, this reaction was
highly suitable to systematically prepare a large array of
mixed [CpCo(H2C=CHSiMe3){P(OR)3}] complexes for the
first time, allowing a broad variety of phosphites with differ-
ligand; this opens the possibility for the coordination of new
potentially stronger coordinating phosphite ligands. To our
delight, we noted that this substitution proceeded smoothly
and led to quantitative conversion after 24 h at room tem-
perature (Scheme 2). Generally, these new complexes can
then be isolated in very high to excellent yields by a short
filtration over aluminum oxide, as shown for complexes 3b
and 3c with electron-rich phosphites and complexes 3d and
3e with electron-deficient phosphites.[14,15] These complexes
are stable and can easily be handled under inert condi-
tions.[15]
ent electronic propACHTUNGTRENNUNGerACHUTNTGERNNUGties (Scheme 1). The yields are high to
very high and the products can be easily isolated. Com-
plexes 2a–f proved to be stable at room-temperature under
inert conditions, depicting the first successful step towards
stable and active precatalysts for [2+2+2] cycloadditions.
We were able to obtain crystals of 3a and 3e from a satu-
rated pentane solution at 48C suitable for X-ray structure
analysis.[16] Figure 1 shows the molecular structures of the
CpCoI-bisphosphite complexes. The structural features of
the complexes with respect to their substitution at the phos-
phites are very similar, which is displayed, for example, by
À
the Co P bond distances (3a: 2.0389(5) and 2.0486(4) ꢃ;
3e: 2.0435(6) and 2.0428(6) ꢃ) and the P1-Co-P2 angles
(3a: 95.78(2)8; 3e: 94.62(2)8).
Scheme 1. New mixed CpCoI-olefin-phosphite complexes obtained
through a substitution reaction from 1 (the yields are that of isolated
product).
Having a large array of new potential precatalysts (2a–f
and 3a–e) in our hands which showed high stability at room
temperature, we tested their catalytic performance in the
[2+2+2] reaction between 1,6-heptadiyne (4) and benzoni-
trile, giving pyridine 8 (Figure 2).[17] We chose 508C as the
reaction temperature to test the activity at rather mild con-
ditions and were pleased to detect a very high catalytic ac-
tivity towards the synthesis of pyridines.
Figure 2 (top) shows that most of complexes 2a–f provide
complete conversions after short reaction times, except for
2 f, which reaches conversions between 60 and 70% only
after t>20 h. It can therefore be concluded that the nature
of the phosphite ligand did not play a large role in providing
high reactivity, but rather in stabilizing the complex by coor-
dinating more strongly to the cobalt center than the olefin
ligands in 1. The catalytic performance of 3a–e was investi-
gated in the same catalytic screening setting as 2a–f
(Figure 2, bottom). The outcome showed that in most cases
the phosphite ligands coordinate stronger to the cobalt
center, resulting in a lower reactivity compared with 2a–f.
However, the most active complex 3c shows almost full con-
version of the substrate already after 3 h.
To obtain a complete series of the complexes 1, 2, and 3,
as well as to clarify the influence of the phosphite ligand, we
wanted to prepare the respective [CpCo{P(OR)3}2] com-
plexes, and in addition, investigate their catalytic properties.
Due to the fact, that only very few [CpCo{P(OR)3}2] com-
plexes are known[10,11] and to our best knowledge none of
them have been tested in catalytic [2+2+2] cycloadditions,
we initially turned our attention to the published synthesis
of [CpCo{PACHTUNGTRENNUNG(OPh)3}2] (3a). Werner et al. were the first to de-
scribe the synthesis of 3a,[10] followed by a modified proce-
dure by McKinney et al.[11] Unfortunately, further experi-
mental work proved none of these conditions suitable as a
general approach for the synthesis of [CpCo{P(OR)3}2] com-
plexes.[12] Therefore, a new, general and efficient method for
preparing [CpCo{P(OR)3}2] complexes with different elec-
tronic properties was sought. We took advantage from our
experience with photocatalytic reactions and succeeded in
developing a photochemical substitution reaction as a route
to various [CpCo{P(OR)3}2] complexes (Scheme 2). In con-
trast to 1, [CpCo
terial for these substitution reactions.[13]
[CpCo(cod)] can be easily activated at low and ambient
temperature through irradiation by releasing the COD
ACHTUNGTRENNUNG(cod)] proved to be a suitable starting ma-
Comparing the reactivity of the complexes one can see
that the electronic properties of the phosphite ligands do
play a large role. Whereas electron-rich phosphites such as
A
ACHTUNGTRENNUNG
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2549

M. Hapke et al.
Figure 1. ORTEP drawings of the molecular structure of 3a (top) and 3e
(bottom). Ellipsoids are drawn at the 30% level. Hydrogen atoms are
omitted for clarity.
Figure 2. Screening experiments of co-cyclotrimerization reactions with
CpCoI-olefin-phosphite complexes (2a–f; top) and CpCoI-bisphosphite
complexes (3a–e; bottom) as catalysts (the yields were determined by
PACHTUNGTRENNUNG(OiPr)3 and PACHTUNGTRENNUNG(OEt)3 (3c and 3b) lead to easily activated
complexes, which reach quantitative conversions to the pyri-
dine derivative, more electron-deficient phosphites such as
GC analysis):[17] Complexes 2a–f (top): =[CpCo(H2C=CHSiMe3){P-
&
P(OPh)3 (3a) only reach conversions up to 20% after 24 h.
~
!
E
=[CpCo(H2C=CHSiMe3){P
G
=
[CpCo(H2C=CHSiMe3){P(OC6H4-4-CF3)3}]
The introduction of electron-withdrawing groups as in the
case of P(OC6H4-4-CF3)3 even leads to a complete loss of ac-
tivity of the respective complex 3e. Ligand PACHTUNGTRNEG(UN OCH2CF3)3 is
an exception being rather electron-deficient but reaching
conversions up to 33% in 24 h under these conditions (3d).
Due to the now possible and unprecedented access to
three complex types with either only olefin or phosphite and
also the mixed ligand combination, it was possible to direct-
^
CHSiMe3){P
G
=[CpCo(H2C=CHSiMe3){PACHTNUGTRENNNUG
*
=[CpCo(H2C=CHSiMe3)
ACHTUNGTRENNUNG
&
[CpCo{P
(OiPr)3}2] (3c); =[CpCo{P
CF3)3}2] (3e).
U
ACHTUNGTRENNUNG
~
*
N
ACTHGNUNERTN(UNG OCH2CF3)3}2] (3d); =[CpCo{P(OC6H4-4-
tures as low as 08C. Complex 3a requires 1008C for comple-
tion of the cycloaddition reaction and a much longer reac-
tion time (24 h). However, the reaction temperature here is
still lower or at least in the same range than for many other
CpCo complexes used as catalysts. This comparison explicit-
ly shows the stabilizing influence of the phosphite ligands. It
was also possible to perform the cycloaddition reaction with
2a and 3a in non-dried and non-degassed toluene without
any loss in the product yield.[18] The combination with one
olefin ligand therefore leads to a unique complex that offers
higher thermal stability compared with the purely olefin-li-
ly compare the performance of these catalysts. The com
ACHTUNGTNERpNUNG ar-
ACHTUNGTRENNUNGaACHTUNGTRENNUNGtive reactivity profile of complexes 1, 2a, and 3a for the
[2+2+2] cycloaddition in Figure 2 at different temperatures
and within different time frames is displayed in Figure 3.[17]
Whereas complex 1 is most active, accomplishing the re-
ACHTUNGTRENNUNGaction at 08C in a very short time (within minutes), the
chart shows very clearly that the stable complex 2a still
maintains a significantly high reactivity as precatalyst, lead-
ing to completion of the pyridine formation already at 508C
within an hour and also showing some reactivity at tempera-
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Fine-Tuning Reactivity and Stability in CpCoI Complexes
FULL PAPER
Figure 3. Temperature-dependent yields of pyridine (8) determined for
[CpCo(H2C=CHSiMe3)2] (1, dark grey), [CpCo(H2C=CHSiMe3){P-
A
ACHTUNGTRNE(NUNG OPh)3}2] (3a, light grey), respective-
gated 1, but still provides significant reactivity for cycloaddi-
tion reactions.
To fortify our experimental results we were interested to
further evaluate the influence of the phosphite ligand for
the reactivity and stability features of these complexes by
using DFT calculations. We performed DFT calculations on
the formation of the complexes as well as the ligand dissoci-
ation, being the initial step of the catalytic cycle (Figure 4).
In our calculations we used the real-size complexes and lig-
ACHTUNGTRENNUNGands and the computational details are given in the Support-
ing Information.
At first we computed the stepwise exchange reaction of
one olefin ligand in 1 by one phosphite ligand at a time to
form 2a and 3a, respectively. PACHTUNRGTNEUNG(OPh)3 has three isomers;
Figure 4. Calculations on the formation of the new complexes and free
energies of ligand dissociation from 1, 2a, and 3a at BP86/TZVP level.
which is in agreement with our experimental findings. For
complex 2a, both the olefin and phosphite ligand can disso-
ciate. Here the selectivity depends solely on the dissociation
free-energies. It is found that the phosphite dissociation of
complex 2a leading to the formation of complex A needs
lower free energy than the olefin dissociation with the for-
mation of complex B (3.81 vs. 7.88 kcalmolÀ1). This indicates
that complex 2a dissociates selectively into complex A, sub-
sequently leading to the active catalyst. This also explains
why the reactivity of the mixed complexes did not largely
depend on the type of phosphite ligand, as Figure 2 (left)
showed, since in each case the phosphite is dissociating first,
leading to the same intermediate. The computed first ligand
dissociation free energies reveal that the stability of the
complexes increases in the following order: 1 (À2.20 kcal
molÀ1)<2a (3.81 kcalmolÀ1)<3a (10.41 kcalmolÀ1). This
stability order correlates well with our experimental obser-
vations as discussed above.
Since complex 2a proved to be one of the most active but
at the same time rather stable CpCoI catalysts, we expanded
the substrate scope for cycloadditions with 2a as precatalyst
to different diynes and various nitriles (Table 1).[19] Pyridines
possessing annellated five- and six-membered rings were iso-
lated in moderate to excellent yields (Table 1, entries 1–6).
The reactions of 1,7-octadiyne (5) with PhCN, iPrCN, and
tBuCN (Table 1, entries 4, 5, and 6) generally gave lower
yields compared with the corresponding reactions with 1,6-
heptadiyne (4) (Table 1, entries 1, 2, and 3). However, the
the isomer with C1 symmetry is more stable than the C3 as
well as Cs isomer by 1.34 and 0.94 kcalmolÀ1, respectively.
Therefore, the C1 isomer was used for our energetic compar-
ison. It was found that the formation of 2a is highly favor-
ACHTUNGTRENNUNGable, as indicated by the computed negative free energy
(À6.01 kcalmolÀ1). For the formation of 3a, however, the
negative free energy (À2.54 kcalmolÀ1) still indicates a fa-
vorable process, but to a lesser extent when compared with
the formation of 2a.
The stability of complexes 1, 2a, and 3a towards ligand
dissociation has also been computed. It is found that the 16-
electron complex [CpCo(H2C=CHSiMe3)] (A) has a triplet
ground state, which is more stable than the corresponding
singlet state by 11.84 kcalmolÀ1; the computed dissociation
free energy is À2.20 kcalmolÀ1, revealing the instability of
complex 1 at ambient conditions. This reflects the experi-
mentally discovered instability of complex 1 above À308C.
For complex 3a, the monophosphite complex [CpCo{P-
ACHTUNGTRENNUNG(OPh)3}] (B) has a triplet ground state as well, which is
16.4 kcalmolÀ1 more stable than the singlet state. The com-
puted dissociation free energy of complex 3a is 10.41 kcal
molÀ1, indicating its high stability towards dissociation,
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M. Hapke et al.
Table 1. Investigation of 2a as catalyst for cycloaddition reactions be-
tween diynes and nitriles.
Scheme 3. Investigation of 2a as catalyst for an intermolecular cycloaddi-
tion reaction between 3-hexyne (16) and benzonitrile (isolated product
yield).
Entry
Nitrile
Diyne
Product[a]
Yield
[%][b]
1
2
3
4
5
6
7
PhCN
iPrCN
tBuCN
PhCN
iPrCN
tBuCN
PhCN
4
4
4
5
5
5
6
98[c], 97[d]
96
49
67
46
35
0
Scheme 4. Application of 2a as catalyst for the intramolecular cycloaddi-
tion reaction of triynes 18 and 19, leading to carbocyclic products (isolat-
ed product yield).
lower compared with reactions with other cobalt catalysts
under more extreme conditions.[4]
Conclusion
We have described new and efficient synthetic routes to a
large number of novel CpCoI-complexes containing either
olefin-phosphite ligand combinations ([CpCo(H2C=CHSi-
Me3){P(OR)3}]) or two phosphite ligands ([CpCo{P(OR)3}2])
by selective substitution from [CpCo(H2C=CHSiMe3)2] (1)
8
PhCN
7
42[e]
or by photochemical exchange reactions from [CpCo
respectively. The screening of the catalytic activity in
[2+2+2] cycloadditions showed that [CpCo{P(OEt)3}2] (3b)
and [CpCo{P(OiPr)3}2] (3c) are the most reactive bisphos-
phite complexes at a reaction temperature as mild as 508C.
However, the mixed CpCoI-olefin-phosphite complexes ex-
hibited much higher catalytic activities but were also stable
compounds at room temperature. This highly beneficial
combination of stability and high catalytic activity was
proven by comparison of the reaction temperatures for
[CpCo(H2C=CHSiMe3)2] (1), [CpCo(H2C=CHSiMe3){P-
ACHTUNGTRENNUNG(cod)],
[a] General procedure: Diyne (1 mmol), nitrile (5 mmol), and
[CpCo(H2C=CHSiMe3){P(OPh)3}] (2a, 5 mol%) were dissolved in dry
THF (3 mL) and stirred at 508C for 3 h. [b] Yield of the isolated product.
[c] GC yield. [d] [CpCo(H2C=CHSiMe3){PACHTNUTRGNE(NUG OPh)3}] (2a, 1 mol%).
[e] Yield after 6 h.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
very bulky tBuCN gave lower yields in both cases (Table 1,
entries 3 and 6). The conversion of internal diynes needs
longer reaction times and results in lower yields, probably
due to steric hindrance, as was demonstrated with diyne 7
(Table 1, entry 8). The use of 1,8-nonadiyne (6) resulted in
the intermolecular [2+2+2] cycloaddition between two ni-
triles and two diynes and the formation of a macrocyclic bis-
pyridine instead of 14 (Table 1, entry 7).[20]
We also investigated the intermolecular cycloaddition re-
action between 3-hexyne (16) and benzonitrile, yielding the
respective pyridine 17 in very good yield (Scheme 3). In ad-
dition, the intramolecular cycloaddition reaction was dem-
onstrated exemplarily for terminal and internal triynes (18
and 19), yielding the benzene derivatives (Scheme 4). In the
latter reaction moderate yields are achieved at a reaction
temperature of 1008C, which in the case of 20 are slightly
ACHUNTGRENUN(G OPh)3}] (2a) and [CpCo{PAHCTUNGTERN(NUGN OPh)3}2] (3a). Computational
results showed that in the mixed complexes [CpCo(H2C=
CHSiMe3){P(OR)3}], the dissociation of the phosphite
ligand is preferred compared to the olefin. Also, the reactiv-
ity and stability differences between [CpCo(H2C=
CHSiMe3)2] (1) and [CpCo{P(OR)3}2] can be demonstrated.
The synthetic scope in pyridine synthesis from diynes and
nitriles was investigated for 2a in intramolecular as well as
intermolecular reactions, demonstrating the high utility of
this new precatalyst. The concept of using two different lig-
AHCTUNGTRENNUNG
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Fine-Tuning Reactivity and Stability in CpCoI Complexes
FULL PAPER
[CpCo(L2)2] proved to be very useful for the development
of new, reactive and stable CpCoI-precatalysts.
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[9] A similar observation was described by Brookhart et al., who were
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remove the second trimethylvinylsilane ligand of [(Me5C5)Co(H2C=
Experimental Section
General procedure for the synthesis of [CpCo(H2C=CHSiMe3){P(OR)3}]:
To a solution of [CpCo(H2C=CHSiMe3)2] (1) in diethyl ether a solution
of the respective phosphite (1 equiv) in THF (2 mL) was slowly added at
À788C. The solution was stirred at À788C for 2 h before it is slowly
warmed to room temperature. The solution was filtered over a small
amount of neutral Al2O3 (Brockman Type I), which was washed with
THF (10 mL). After removal of the solvent under reduced pressure the
resulting red-black oil was dried under vacuum.
General procedure for the synthesis of [CpCo{P(OR)3}2]: A solution of
[CpCoACHTUNGTRENNUNG(cod)] (232 mg, 1 mmol) and the respective phosphite (2 mmol) in
THF (6 mL) was irradiated with light (two 460 W lambs) in a thermostat-
ed Schlenk-type vessel under argon for 24 h. The solution was filtered
over a small amount of neutral Al2O3 (Brockman Type I), which was
washed with diethyl ether (10 mL). After removal of the solvent under
reduced pressure the resulting deep red oil/solid was dried under
vacuum.
General procedure for the catalytic trial: 1,6-Heptadiyne (114 mL,
1 mmol), benzonitrile (515 mL, 5 mmol) and dodecahydrotriphenylene
(DHT, 36 mg, 0.15 mmol) as internal standard were dissolved in THF
(2 mL). A solution of the respective complex (5 mol% regarding the
diyne) in THF (1 mL) was added and the reaction mixture stirred at
508C. At given times small aliquots of the reaction mixture (0.1 mL)
were taken for GC analysis.
Substrate screening: Diyne (1 mmol), nitrile (5 mmol) and [CpCo(H2C=
CHSiMe3){PACHTUNGTRENNUNG(OPh)3}] (2a, 26 mg, 5 mol%) were dissolved in dry THF
(3 mL). The solution was stirred at 508C for 3 h before the solvent was
removed under reduced pressure. Column chromatography (n-hexane/
ethyl acetate 6:1, v/v) afforded the respective pyridines as white solids or
clear oils. The compounds were identified by NMR spectroscopy and MS
and comparison with reported data.
CHSiMe3){PACHTUNRGTENNUG(OMe)3}] to yield [(Me5C5)Co{PCAHTUNGTERN(NUGN OMe)3}2]. Substitution
reactions with N-donor ligands such as pyridine and 2,2’-bipyridine
were also reported: c) C. P. Lenges, P. S. White, W. J. Marshall, M.
Brookhart, Organometallics 2000, 19, 1247.
[10] V. Harder, J. Mꢀller, H. Werner, Helv. Chim. Acta 1971, 54, 1.
[11] R. J. McKinney, Inorg. Chem. 1982, 21, 2051.
[12] Essentially, cobaltocene was heated at reflux with the phosphite at
high temperatures over long reaction times. We investigated various
other routes for the synthesis of 3a under reducing conditions in the
Acknowledgements
presence of PACHTNUGRTENUNG(OPh)3 but only the application of NaK (sodium/potas-
sium alloy) was found effective, albeit yielding 3a in a lower yield
than the original procedure. Interestingly, all experiments involving
the reduction of cobaltocene with pure alkali metals (Li, Na, or K)
led to no reaction at all, not even a decomposition of the phosphite
ligands was observed.
We thank Prof. Uwe Rosenthal for his enduring support and helpful dis-
cussions and comments. We thank the DFG (HA3511/3–1) for financial
support and the Fonds der Chemischen Industrie for a Kekulꢄ-Fellowship
for I. Thiel.
[13] We also investigated the direct synthesis of complex 2a from [CpCo-
A
ACHTUGNTERN(NUNG OPh)3 (1 mmol), and H2C=CHSiMe3 (10 mmol)
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duced (see the Supporting Information).
[14] Experiments using phosphites with large aryl- (e.g., tris-(2,4-tert-bu-
tylphenyl)phosphite) or large alkyl groups (e.g., tris-(dimethyl-tert-
ACHTUNGTREN(NUNG cod)]/2a/3a was pro-
butylsilyl)phosphite) gave no reaction at all, which could be at
AHCTUNGTREGUNuNN ted to the steric bulkiness of these phosphites.
ACHTUNGTRENNUNGtrib-
[15] Whereas complex 3a and 3e are solids, complexes 3b–d are red oils.
[16] CCDC-879909 (3a) and 879908 (3e) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[17] Figures 2 and 3 are displayed in color in the Supporting Informa-
tion.
[18] For more details and comparison with the reaction in anhydrous sol-
vents see the Supporting Information.
[19] The reaction of diynes with nitriles was investigated with catalyst
systems of different metals. Selected original work: a) Y. Yamamoto,
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[20] GC-MS and HRMS (ESI) analysis showed signals ([M+H+] found:
m/z 447.2794) corresponding to a macrocyclic bispyridine yielded by
the cycloaddition of two molecules of 1,8-nonadiyne and two mole-
cules of benzonitrile ([M+H+] calcd: m/z 447.2795).
Received: August 17, 2012
Published online: December 18, 2012
2554
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ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2548 – 2554