Cyclobutadiene cobalt complexes as catalysts for insertion of diazo compounds into X–H bonds

Article abstract of DOI:10.1016/j.mencom.2021.04.022

Novel cyclobutadiene cobalt complex with labile naphthalene ligand [(C₄Et₄)Co(C₁₀H₈)]PF₆ catalyzes the insertion of ethyl diazoacetate into X–H bonds giving the corresponding products in 15–75% yields

Full text of DOI:10.1016/j.mencom.2021.04.022

Mendeleev
Communications
Mendeleev Commun., 2021, 31, 350–351
Cyclobutadiene cobalt complexes as catalysts
for insertion of diazo compounds into X–H bonds
Nikita V. Shvydkiy*a and Dmitry S. Perekalina,b
a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. E-mail: nikich-shv@mail.ru
b G. V. Plekhanov Russian University of Economics, 117997 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2021.05.022
Novel cyclobutadiene cobalt complex with labile naphthalene
ligand [(C4Et4)Co(C10H8)]PF6 catalyzes the insertion of ethyl
diazoacetate into X–H bonds giving the corresponding
products in 15–75% yields. The reaction proceeds with
primary and secondary aliphatic amines, hydrosilanes and
triethylamine borane but not with anilines and alcohols.
+
Et
Et
Et
Et
[Co]
2 mol%
N2
PF6–
H
XRn
CO2Et
15–75%
RnXH
+
[Co] =
– N2
H
CO2Et
Co
H
X = N, Si, B
Keywords: cyclobutadiene, cobalt complexes, catalysis, diazo compounds, insertion, organosilicon compounds.
Cyclobutadiene complexes are well known for many transition
metals.1 However, most of the reported compounds have inert
sandwich structures2,3 and hence they have not been used as
catalysts until 2015.4 We have previously discovered a simple
method for the synthesis of the cyclobutadiene complexes of
rhodium with labile ligands, such as [(C4Et4)Rh(xylene)]PF6,
[(C4Et4)Rh(MeCN)3]PF6, and [(C4Et4)RhCl]2.4,5 These
compounds were proved to be active catalysts for insertion of
diazo compounds,6 reductive amination,7 and cycloisomerization
of unsaturated substrates.4
One of the trends in modern catalysis is the replacement of
catalysts based on expensive platinum metals with more
affordable derivatives of 3d metals.8,9 Therefore, we decided to
investigate the activity of cyclobutadiene cobalt complexes in
one of the reactions catalyzed by cyclobutadiene rhodium
complexes, namely, the insertion of diazo compounds into X–H
bonds. Noteworthy, to the best of our knowledge, cyclobutadiene
cobalt complexes are almost unexplored type of the catalysts; the
only example is the catalytic oxidation of alkanes.10
The insertion of diazo compounds into X–H bonds is an
efficient approach to the formation of C–X bonds (X = N, O, S,
Si, etc.),11,12 which is often used for the synthesis of bioactive
molecules and modifications of natural compounds.13,14 This
transformation is typically catalyzed by copper and rhodium
complexes.15 Interestingly, despite the widespread use of
rhodium catalysts, the related cobalt derivatives have rarely been
tested for this reaction.16 At the same time, cobalt compounds
have been reported to catalyze cyclopropanation of olefins with
diazo compounds.17,18
Based on the data on the catalytic activity of cyclobutadiene
rhodium complexes6 we initially tested the analogous benzene
complex of cobalt [(C4Et4)Co(C6H6)]+ as a catalyst for model
reaction between ethyl diazoacetate (EDA) and tert-butylamine.
It was found, however, that the desired insertion product was
formed only in trace amounts. The possible explanation for this
fact is that the displacement of benzene in [(C4Et4)Co(C6H6)]+
complex, which is required for the initiation of the catalytic
cycle, is very slow.19 Therefore, we switched to the related
naphthalene complex [(C4Et4)Co(C10H8)]+, which would
undergo the displacement of the arene ligand much faster due
to the transition from η6- to η4-coordination.20,21 This complex
was obtained from 3-hexyne and Co2(CO)8 following the
protocol developed earlier for the tetramethyl-substituted
analogue [(C4Me4)Co(C10H8)]+ (Scheme 1).22,23 It should be
noted that 3-hexyne is readily available and more easy to
handle than 2-butyne that was used for assembling C4Me4-
ligand. Initially, 3-hexyne formed a cyclobutadienyl complex
with aluminum chloride, which reacted with cobalt carbonyl
giving [(C4Et4)Co(CO)3]+.24 This complex was converted
in situ into the iodide (C4Et4)Co(CO)2I 1 by treatment with NaI
in the presence of mild CO-abstracting agent25 Me3NO∙2H2O.
The refluxing of 1 with AlCl3 in benzene produced the benzene
complex [(C4Et4)Co(C6H6)]+ 2 in 30% yield. The preparation
of complex 2 was not optimized and it was used in further
reactions without additional purification. At the final stage of
the synthesis, the benzene ligand in 2 was displaced by aceto-
nitrile to form the labile intermediate [(C4Et4)Co(MeCN)3]+,
which then reacted with naphthalene to give the target complex
+
Et
Et
Et
Et
Et
Et
Et
Et
PF6–
i
ii
45%
30%
Et
Co
Co
Et
OC
I
CO
2
1
+
Et
Et
Et
Et
iii
58%
PF6–
Co
3
Scheme 1 Reagents and conditions: i, AlCl3, Co2(CO)8, C6H6, reflux, then
Me3NO/NaI∙2H2O (cf. ref. 22); ii, KPF6/H2O; iii, MeCN, reflux, then
C10H8.
© 2021 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 350 –

Mendeleev Commun., 2021, 31, 350–351
[(C4Et4)Co(C10H8)]+ 3 in 58% yield (for details, see Online
Supplementary Materials).
and B–H bonds. Naphthalene complex [(C4Et4)Co(C10H8)]PF6 3
is more active than the benzene analogue 2 due to the more labile
arene ligand. At the same time, the activity of the naphthalene
cyclobutadiene complex 3 is notably lower than that of
cyclobutadiene rhodium analogues.
In contrast to benzene derivative 2, the new naphthalene
complex 3 readily promoted decomposition of ethyl diazoacetate
(EDA) inTHF solution in the presence of acetonitrile (Scheme 2).
The addition of acetonitrile was required to displace the
naphthalene ligand and to generate active solvate species
[(C4Et4)Co(MeCN)3]+.22 The use of this acetonitrile complex as
a catalyst itself led to non-reproducible results, apparently due to
its sensitivity to oxidation. Thus, the naphthalene complex acts
as an air-stable source of the catalytically active species
representing a common way of application of arene complexes
in catalysis.19
This work was financially supported by the Russian Science
Foundation (grant no. 19-73-00278). Access to the electronic
resources and databases was provided by INEOS RAS with
financial support from the Ministry of Science and Higher
Education of the Russian Federation.
Online Supplementary Materials
The reaction of EDA with tert-butylamine in the presence of
5 mol% of catalyst 3 provided product 4a in 70% yield. The
main by-products were ethene-1,2-dicarboxylate 5 as the result
of the decomposition of EDA, as well as a small amount of
ButN(CH2COOEt)2 as a product of the second insertion of EDA
into 4a. Neither lowering the temperature to –10 °C nor slow
addition of EDA to the reaction mixture did not significantly
suppress these side processes. Similar behaviour has been
observed previously for other catalytic systems.26 The yield of
the target product 4a slightly depended on the catalyst loading,
in particular, 2 mol% of the catalyst 3 produced 60% yield.
The reaction with secondary amines afforded the
corresponding products 4b−d in 50–75% yields (see Scheme 2).
At the same time, aromatic amines such as aniline did not give
insertion products. Oxygen-containing nucleophiles such as
methanol or tert-butyl alcohol gave only traces of the target
products. Reaction with silanes provided the products with new
C−Si bonds 6a−c. The product yield dropped for disubstituted
silanes (6c) in comparison with trisubstituted ones. Finally, the
reaction of EDA with triethylamine borane furnished compound
7 thus demonstrating the application of cyclobutadiene cobalt
catalyst for the formation of C−B bonds.
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2021.05.022.
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EtO2C
N2
H
NR1R2
i
+ R1R2N
H
+
H
CO2Et
CO2Et
H
CO2Et
4a–d
5
a R1 = H, R2 = But, 45%
b R1 + R2 = (CH2)2O(CH2)2, 78%
c R1 = R2 = Pri, 54%
d R1 = R2 = allyl, 52%
N2
N2
H
SiR12R2
i
a R1 = R2 = Et, 41%
b R1 = R2 = Ph, 58%
c R1 = Ph, R2 = H, 16%
1
2
+
H
R2R Si
H
CO2Et
H
H
CO2Et
6a–c
O
i
H2B
H
+ Et3N
H2B
Et3N
OEt
CO2Et
7, 46%
Scheme 2 Reagents and conditions: i, [(C4Et4)Co(C10H8)]+PF6–
(2 mol%), MeCN/THF, 0 °C, 1 h, then ~20 °C, 18 h.
3
Received: 27th January 2021; Com. 21/6435
– 351 –