Insertion of carbenoids into X-H bonds catalyzed by the cyclobutadiene rhodium complexes

Article abstract of DOI:10.1016/j.jorganchem.2017.08.010

The cyclobutadiene rhodium complex [(C₄Et₄)RhCl]₂ (generated from [(C₄Et₄)Rh(p-xylene)]PF₆ and BnNEt₃Cl) catalyzes the reaction of methyl α-diazophenylacetate with R_nX

Full text of DOI:10.1016/j.jorganchem.2017.08.010

Accepted Manuscript
Insertion of carbenoids into X-H bonds catalyzed by the cyclobutadiene rhodium
complexes
Natalia L. Loskutova, Nikita V. Shvydkiy, Yulia V. Nelyubina, Dmitry S. Perekalin
PII:
S0022-328X(17)30499-0
10.1016/j.jorganchem.2017.08.010
JOM 20063
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 18 July 2017
Revised Date: 16 August 2017
Accepted Date: 17 August 2017
Please cite this article as: N.L. Loskutova, N.V. Shvydkiy, Y.V. Nelyubina, D.S. Perekalin, Insertion
of carbenoids into X-H bonds catalyzed by the cyclobutadiene rhodium complexes, Journal of
Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.08.010.
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ACCEPTED MANUSCRIPT
Insertion of carbenoids into X-H bonds catalyzed by the cyclobutadiene rhodium complexes
Natalia L. Loskutova, Nikita V. Shvydkiy, Yulia V. Nelyubina, Dmitry S. Perekalin*
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilova,
119991, Moscow, Russia. E-mail: dsp@ineos.ac.ru
Dedicated to Academician I. P. Beletskaya in recognition of her fundamental contribution to the
development of organometallic chemistry and catalysis
Graphical Abstract
Graphical Abstract Synopsis
Cyclobutadiene rhodium complexes catalyze the formation of carbenoid species from diazo acetates and
subsequent insertion of carbenoids into X-H bonds (X = B, N, O, Si). The isolated intermediate complex
unexpectedly has a binuclear structure with a bridging carbene ligand.
Abstract
The cyclobutadiene rhodium complex [(C4Et4)RhCl]2 (generated from [(C4Et4)Rh(p-xylene)]PF6 and
BnNEt3Cl) catalyzes reaction of methyl α-diazophenylacetate with RnX-H compounds to give the
insertion products methyl 2-RnX-2-phenylacetates in 70−90% yields (RnX-H = methanol, tert-butylamine,
2,6-diisopropylaniline, morpholine, diallylamine, triethylsilane, triethylamine-borane). The
stoichiometric reaction of [(C4Et4)RhI]2 with methyl α-diazophenylacetate gives the intermediate
complex (C4Et4)2Rh2(µ-I)2(µ-1-carboxymethyl-1-phenylmethylene), which has a binuclear structure with
bridging iodide and carbene ligands. This result indicates that catalytically active carbene species may
have more complex structures than the commonly assumed LnM=CR2.
Keywords: diazo compound, carbene, rhodium, cyclobutadiene, catalysis, pi-complex
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1. Introduction
Diazo compounds are widely used as a source of carbenoids in organic synthesis [1]. They
typically undergo insertion [2], cyclopropanation [3], or cyclization [4] reactions, which lead to the
formation of the new carbon-carbon bonds. The insertion of carbenoids into X–H bonds (X =
heteroatom) has been less studied, although it is a useful method for the formation of the carbon-
heteroatom bonds [5] (Scheme 1). In particular, insertion into X–H bonds has been successfully used in
the synthesis of bioactive molecules and modifications of natural compounds, where mild conditions
and functional group tolerance are required [6]. For example, a key step in the total synthesis of
Maoecrystal V was an intramolecular Rh-catalyzed insertion of carbenoid into O-H bond [7]. Insertion of
carbenoids into N-H bonds of indole rings was used for selective labelling of tryptophan residues in
proteins [8,9].
A typical catalyst for the reactions of diazo compounds is rhodium(II) acetate Rh2(OAc)4 or its
derivatives [2a]. However, it has been recently found that the diene rhodium(I) complexes [(diene)RhCl]2
can also promote such process (see examples of catalytic cyclopropanation of alkenes [10], insertion
into N-H [11], Si-H bonds [12], and B-H bonds [13]).
Two years ago, we have developed a general approach to the cyclobutadiene rhodium(I)
complexes [14]. It has been expected that these complexes would have more robust catalytic activity
than their diene congeners, because cyclobutadiene ligand provides better stabilization for the active
metal center. Indeed, we have found that the cyclobutadiene complex [(C4Et4)Rh(p-xylene)]PF6 (1) is one
of the best catalyst for the selective reductive amination of carbonyl compounds in the presence of the
carbon monoxide [15]. Herein we report the application of the cyclobutadiene complexes as a catalysts
for insertion of carbenoids into X–H bonds.
Scheme 1. Insertion of carbenoids into X-H bonds (X = heteroatom; MLn = metal catalyst).
2. Results and discussions
At the outset of this investigation, we examined the catalytic activity of the cyclobutadiene
complexes in a model reaction of methyl α-diazophenylacetate (3) with tert-butylamine (Scheme 2). It
was found that the sandwich complex 1 is completely inactive in this process, apparently because it has
no vacant sites for coordination of the reactants. Addition of halide anions to the complex 1 leads to
replacement of the labile p-xylene ligand and formation of the more active half-sandwich species
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[(C4Et4)RhHal]2 (2a-c) [14]. It was found that the rate of this reaction increases in the order I < Br ≈ Cl. At
the same time, the catalytic activity of 2a−c in the model reaction of α-diazophenylacetate increases in
the opposite order: Cl ≈ Br < I (Table 1). For further investigation, we selected the chloride complex 2a,
which we generated in situ from 1.
Scheme 2. Generation of catalysts 2a−c and the model reaction of α-diazophenylacetate (3).
Table 1. The catalytic activity of 2a−c in the model reaction,
depending on the time of the catalyst generation.
Catalyst Time of the catalyst
loading generation (1 + Hal−) product 4a after 30 min
NMR yielda of the
Cl−
Br−
I−
2 mol-%
1 mol-%
5 minb
87%
23%
75%
17%
17%
44%
20 hoursc
a
1
All conversions were determined by H NMR. Catalysts 2a-c were generated in situ from precatalyst 1
b
and halide salts BnNEt3Cl, Bu4NBr, and NEt4I, respectively. The reaction was performed with 1 (2 mol-
t
%), Hal− source (2 mol-%), 3 (0.057 mmol), and BuNH2 (0.113 mmol) in 0.5 ml of CDCl3 at room
temperature. c The reaction was performed with 1 (1 mol-%), Hal− source (1 mol-%), 3 (0.111 mmol), and
tBuNH2 (0.222 mmol) in 0.5 ml of CDCl3 at room temperature.
Next, we evaluated the diversity of amines that can react with α-diazophenylacetate 3 in the
presence of our catalyst (Scheme 3). The insertion of carbenoid into tert-butyl amine proceeded
smoothly to give the product 4a in 83% isolated yield. Similar reaction with hindered and less
nucleophilic 2,6-diisopropylaniline still led to the formation of the amine 4b in excellent 95% yield. The
reactions with secondary amines morpholine and diallylamine gave products 4c and 4d in 78% and 93%
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yields. Note, that no cyclopropanation was observed in the case of diallylamine. Depending on the steric
and electronic effects of the amines, the insertion reactions typically required 1−3 mol-% catalyst
loadings for complete conversion of the diazoacetate 3.
Substrates with other X-H bonds also reacted with the diazo compound 3. For example,
methanol produced the ether 4e (91%), while triethylsilane gave the silane 4f (73%). A carbon-boron
bond was formed by reaction of 3 with triethylamine-borane, which produced the stable adduct 4g in
91% yield. The first example of such transformation has been reported only several years ago [16], in
particular with the use of the related diene rhodium complexes [(diene)RhCl]2 as catalysts [13]. This
method is an attractive way to prepare functionalized organoboranes from the commercially available
amine-borane adducts.
Scheme 3. Substrate scope of X-H insertion reactions. Isolated yields are given.
The reaction times are indicated, the catalyst loadings are put in parentheses.
It should be mentioned that reactions of the diazoacetate 3 with alkenes and alkynes in the
presence of the rhodium catalyst 2a were slow and non-selective. Fortunately, the unsaturated bonds
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do not interfere with the insertion of carbenoids into X-H bonds (e.g. product 4d). At the same time,
thiols completely inhibited the catalysis and no reaction was observed when 4-aminobenzenethiol was
used as X-H reagent.
We also evaluated similar insertion reactions of ethyl diazoacetate, but found that they
proceeded too fast and non-selective. This can be exemplified by the reaction of ethyl diazoacetate with
tert-butylamine in the presence of the generated catalyst 2a, which produced the products of single and
double insertions as well as the product of homocoupling of the diazo compound (Scheme 4). Attempts
to improve the selectivity by the slow addition of the diazo compound or lowering the reaction
temperature were not sufficiently successful. It should be noted, that some known catalysts provide the
sole product of mono insertion without undesired dimerization of ethyl diazoacetate [17].
Et
Et
Et
Et
tBu
tBu
+
Rh
PF6
NH
CO2Et
N
CO2Et
CO2Et
+
N2
+
H
CO2Et
2 mol-%
H
CO2Et
CO2Et
BnNEt3Cl
tBu NH2
+
CHCl3, 20 С
H
Scheme 4. Non-selective reaction of ethyl diazoacetate with tert-butylamine.
In order to get some insight into the nature of the carbenoid rhodium species we have carried
out the reaction of the iodide complex 2c with the stoichiometric amount of diazoacetate 3 (Scheme 5).
1
Surprisingly, the bright-green complex 5 was formed as a sole organometallic product according to H
NMR. This green color was also typically observed in the catalytic reactions, which suggested the
intermediate formation of 5. Furthermore, the test reaction of 5 with tert-butylamine in 10 min gave the
expected insertion product 4a and the starting iodide complex 2c in almost quantitative yields.
Complex 5 was somewhat unstable in air and was isolated by the column chromatography under
1
argon and subsequent crystallization in 72% yield. The H NMR spectra of 5 indicated the hindered
rotation of the phenyl substituent at room temperature. At −30 °C it was so slow that the protons of the
phenyl ring appeared as four separated resolved multiplets (see supporting information). At the same
1
time, the rotation of the cyclobutadiene ligands remained fast on H NMR time scale even at −30 °C.
However, the signals of CH2 groups of the substituted cyclobutadiene appeared as two doublets of
quadruplets indicating that the corresponding protons are diastereotopic. The 13C NMR of 5 shows
characteristic doublet at 94.74 (JRh-C = 9.5 Hz), which corresponds to the cyclobutadiene ligand, but no
clear signal of the carbene atom. The latter is not surprising as this signal is presumably broad, because
of the dynamic behavior of the complex and the spin coupling interaction with two rhodium atoms.
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Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
N2
Rh
Rh
Ph
CO2Me
CHCl3
NEt4I
Ph
CO2Me
Et
+
Rh
PF6
Rh
I
I
I
CH2Cl2
2
Et
1
2c
Et
Et
5
Scheme 5. Synthesis of the reaction intermediate 5 from the precatalyst 1.
Finally, the unusual binuclear structure of the complex 5 was revealed by the X-ray diffraction
analysis (Figure 1). The cyclobutadiene ligands are non-symmetrically coordinated to the rhodium atoms
because of the different trans-effects of iodide and carbene ligands. In particular, the Rh−Ccyclobutadiene
distances in 5 vary in 2.11−2.17 Å range. For comparison, these distances in more symmetrical sandwich
cation 1 are in 2.10−2.12 Å range [14]. The distance between two rhodium atoms of 2.6280(3) Å is
consistent with the presence of a single covalent bond, which is required by 18 electron rule.
The bridging carbene ligand is symmetrically coordinated to both rhodium atoms with 2.124(3)
Å average Rh−C25 distance. Interestingly, the carbene center has no significant conjugation with the
phenyl ring, which is indicated by the rather long C25-C26 bond of 1.481(5) Å (compare with 1.41−1.43 Å
in the mononuclear carbene rhodium complexes [18]). The carbene atom C25 is well hidden in the
molecular environment (Figure 2), so it seems that ‘opening of the bridge’ with the cleavage of one of
the Rh−C25 bond is required for its reaction with the X-H bonds. In order to access such possibility we
carried out the DFT calculations (Scheme 6). It was found that formation of the opened structure 6 from
5 requires only 14.3 kcal mol−1 and therefore is quite feasible at room temperature.
Figure 1. The crystal structure of the complex 5 in 50% thermal ellipsoids. The disordered part of one of
ethyl groups and all hydrogen atoms are omitted for clarity. Selected interatomic distances (Å): Rh1—
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Rh2 2.6280(3), Rh1—I1 2.7173(3), Rh1—I2 2.9081(3), Rh1—C25 2.122(3), Rh2—I1 2.7438(3), Rh2—I2
2.8493(3), Rh2—C25 2.126(3), C25—C26 1.482(4), C25—C32 1.507(4).
Figure 2. The crystal structure of the complex 5 with atoms represented as spheres with van der Waals
radii. The carbene atom is in the center, shown in yellow (in black in printed version).
Et
Et
Et
Et
Et
Et
Et
Et
Rh
Rh
Rh
Ph
Ph
CO2Me
Et
I
I
I
subsequent
reaction
I
CO2Me
Rh
Et
Et
Et
Et
Et
H XR3
Et
Et
5
6
E = 14.3 kcal mol-1
5
relative to
Scheme 6. Proposed opening of the bridged structure of the intermediate 5
according to DFT calculations.
3. Conclusions
To conclude, the cyclobutadiene rhodium complexes [(C4Et4)RhHal]2 catalyze decomposition of
diazoacetates and subsequent insertion of carbenoids into various X-H bonds with formation of the new
C−B, C−N, C−O, and C−Si bonds. The intermediate of this reaction has an unusual binuclear structure
with a bridging carbene ligand. Catalytically active carbenoid complexes of transition metals are typically
assumed to be monomeric [18,19]. Our results show that it may not always be the case.
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4. Experimental section
4.1. General
All reactions were carried out under argon in anhydrous solvents, while the products were isolated in
air, unless stated otherwise. Chloroform was deacidified over calcium hydride, degassed by bubbling of
argon for 15 minutes, and stored over molecular sieves (5 Å) in the dark. Anhydrous 1,2-dichloroethane
was obtained by distillation under argon from calcium hydride. Column chromatography was performed
with standard Macherey-Nagel silica gel 60 (0.04–0.063 mm particle size). The progress of the catalytic
reactions were monitored by NMR or TLC using PE/ether = 10/1 as an eluent and starting methyl α-
diazophenylacetate (3) as a reference compound (Rf = 0.3). 1H, 13С, 11B NMR spectra were recorded on a
Bruker Avance 400 spectrometer at 20°C in CDCl3. Data were reported as chemical shifts in ppm relative
1
to the residual signals of the solvent CDCl3 (7.26 ppm for H, 77.2 ppm for 13C). The abbreviations used
for explaining the multiplicities were as follows: s = singlet, d = doublet, t = triplet, q = quartet, m =
multiplet, br = broad. Mass spectra were measured on high-resolution time-of-flight Bruker micrOTOF
instrument using electrospray ionization (ESI-MS) [20]. Measurements were performed in positive ion
mode, interface capillary voltage at 4.5 kV, effective scan range at m/z 100 – 3000, external calibration
ESI-L Low Concentration Tuning Mix, Agilent Technologies, direct syringe injection at flow rate of 3
μL/min, nitrogen as dry gas at 4 L/min, interface temperature at 180°C. The spectra were processed
using Bruker Data Analysis 4.0 software package. Methyl α-diazophenylacetate (3) [21] and complex
-
[(C4Et4)Rh(p-xylene)]+PF6 (1) [14] were synthesized according to the published procedures. All other
reagents were purchased from Acros or Aldrich and used as received.
4.2. Methyl 2-(tert-butylamino)-2-phenylacetate (4a).
A 10 ml Schlenk flask was charged with complex 1 (1 mol-%) and BnNEt3Cl (1 mol-%) and 0.5 ml of 1,2-
dichloroethane was added. The solution was stirred for 10 minutes, then the diazo compound (67 mg,
0.38 mmol) was added, followed by tert-butylamine (48 ml, 0.45 mmol). Additional 2.5 ml of 1,2-
dichloroethane was added and the mixture was heated at 50ᵒC for 2 hours. After the completion of the
reaction, the solvent was evaporated under reduced pressure and the residue was purified by column
chromatography using dichloromethane (with 1% of triethylamine) as an eluent to give analytically pure
product 4a (69 mg, 83%). This compound was previously synthesized using ruthenium [22] and iridium
catalysts [23]. 1H NMR (400 MHz, CDCl3): δ = 1.10 (s, 9H, tert-butyl), 2.07 (br, 1H, NH), 3.67 (s, 3H, ester),
4.48 (s, 1H, CH), 7.30 (m, 1H, Ph), 7.34 (m, 2H, Ph), 7.40 (d, J = 7.16 Hz, 2H, Ph).
4.3. Methyl 2-(2,6-diisopropylphenylamino)-2-phenylacetate (4b).
A 10 ml Schlenk flask was charged with a complex 1 (2.0 mg, 2 mol-%) and BnNEt3Cl (0.9 mg, 2 mol-%)
and 0.5 ml of CHCl3 was added. The solution was stirred for 10 minutes, then the diazo compound (36
mg, 0.20 mmol) was added, followed by 2,6-diisopropylaniline (42 µl, 39.4 mg, 0.22 mmol). Additional
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2.5 ml of CHCl3 was added and the mixture was heated at 50ᵒC for 3 hours. After the completion of the
reaction, the solvent was evaporated under reduced pressure and the residue was purified by column
chromatography using petroleum ether – diethyl ether mixture (20:1) as eluent to give analytically pure
product 4b (62 mg, 95%). The ethyl ester analogue of 4b was synthesized previously using copper
1
catalysts [24]. H NMR (400 MHz, CDCl3): δ = 1.09 (d, J = 6.8 Hz, 6H, iso-propyl), 1.17 (d, J = 6.8 Hz, 6H,
iso-propyl), 3.17 (septet, J = 6.8 Hz, 2H, iso-propyl), 3.69 (s, 3H, ester), 4.32 (d, J = 9.9 Hz, 1H, NH), 4.68
13
(d, J = 9.9 Hz, 1H, CH), 7.06 (m, 3H, C6H3), 7.33-7.38 (m, 5H, Ph). C NMR (100 MHz, CDCl3): δ = 24.30,
24.55, 27.92, 52.63, 67.09, 123.89, 127.21, 128.34, 128.91, 138.76, 141.48, 142.19, 174.00. ESI-MS:
calculated for C21H28NO2 (M+H) 326.2115, recorded 326.2105.
4.4. Methyl 2-morpholino-2-phenylacetate (4c).
A 10 ml Schlenk flask was charged with a complex 1 (2.0 mg, 2 mol-%) and BnNEt3Cl (0.9 mg, 2 mol-%)
and 0.5 ml of CHCl3 was added. The solution was stirred for 10 minutes, then the diazo compound (36
mg, 0.20 mmol) was added, followed by morpholine (34 µl, 34.2 mg, 0.39 mmol). Additional 2.5 ml of
CHCl3 was added and the mixture was heated at 50ᵒC for 4 hours. After the completion of the reaction,
the solvent was evaporated under reduced pressure and the residue was purified by column
chromatography using petroleum ether – ethyl acetate mixture (5:1) as eluent to give product 4c (37
mg, 78%). The compound 4c was previously synthesized previously by the methods that do not involve
diazo compounds [25, 26, 27]. 1H NMR (400 MHz, CDCl3): δ = 2.46 (d, 2.13 Hz, 4H, -(CH2)2O), 3.69 (m, 3H,
ester), 3.74 (m, 4H, –(CH2)2N-), 3.99 (s, 1H, CH), 7.34 (m, 3H, Ph), 7.44 (m, 2H, Ph).
4.5. Methyl 2-(diallylamino)-2-phenylacetate (4d).
A 10 ml Schlenk flask was charged with a complex 1 (3.1 mg, 3 mol %) and BnNEt3Cl (1.4 mg, 3 mol %)
and 0.5 ml of CHCl3 was added. The solution was stirred for 10 minutes, then the diazo compound (36
mg, 0.20 mmol) was added, followed by diallylamine (49 µl, 38.6 mg, 0.40 mmol). Additional 2.5 ml of
CHCl3 was added and the mixture was heated at 50ᵒC for 4 hours. After the completion of the reaction,
the solvent was evaporated under reduced pressure and the residue was purified by column
chromatography using petroleum ether – diethyl ether mixture (7:1) as eluent to give analytically pure
product 4d (46 mg, 93%). The compound 4d was synthesized previously by the methods that do not
1
involve diazo compounds [25, 28]. H NMR (400 MHz, CDCl3): δ = 3.24 (br, 4H, 2CH2), 3.73 (s, 3H,
COOMe), 4.64 (s, 1H, CH), 5.20 (m, 4H, 2=CH2), 5.84 (m, 2H, 2=CH), 7.30-7.40 (m, 5H, Ph).
4.6. Methyl 2-methoxy-2-phenylacetate (4e).
A 10 ml Schlenk flask was charged with a complex 1 (2.0 mg, 2 mol-%) and BnNEt3Cl (0.9 mg, 2 mol-%)
and 0.5 ml of CHCl3 was added. The solution was stirred for 10 minutes, then the diazo compound (36
mg, 0.20 mmol) was added, followed by methanol (40 µl, 31.7 mg, 0.99 mmol). Additional 2.5 ml of
CHCl3 was added and the mixture was heated at 50ᵒC for 2 hours. After the completion of the reaction,
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the solvent was evaporated under reduced pressure and the residue was purified by column
chromatography using petroleum ether – diethyl ether mixture (10:1) with 1% of triethylamine as an
eluent to give analytically pure product 4e (33 mg, 91%). The compound 4e was previously synthesized
using rhodium [29], iron [30], and copper catalysts [31]. 1H NMR (400 MHz, CDCl3): δ = 3.39 (s, 3H, OMe),
3.71 (s, 3H, COOMe), 4.77 (s, 1H, CH), 7.34-7.42 (m, 5H, Ph).
4.7. Methyl 2-phenyl-2-(triethylsilyl)acetate (4f).
A 10 ml Schlenk flask was charged with a complex 1 (2.0 mg, 2 mol-%) and BnNEt3Cl (0.9 mg, 2 mol-%)
and 0.5 ml of CHCl3 was added. The solution was stirred for 10 minutes, then the diazo compound (36
mg, 0.20 mmol) was added, followed by triethylsilane (64 µl, 46.6 mg, 0.40 mmol). Additional 2.5 ml of
CHCl3 was added and the mixture was heated at 50ᵒC for 3 hours. After the completion of the reaction,
the solvent was evaporated under reduced pressure and the residue was purified by column
chromatography using petroleum ether – ethyl acetate (3:1) with 1% of triethylamine as an eluent to
give product 4f (39 mg, 73%). The compound 4f was previously synthesized using rhodium [32, 33],
1
iridium [34], and copper catalysts [35]. H NMR (400 MHz, CDCl3): δ = 0.57 (m, 6H, 3CH2), 0.90 (m, 9H,
3CH3), 3.52 (s, 1H, CH), 3.67 (s, 3H, ester), 7.17 (m, 1H, Ph), 7.28 (m, 2H, Ph), 7.37 (m, 2H, Ph).
4.8. Methyl 2-phenyl-2-(triethylamine-dihydridoboron)acetate (4g).
A 10 ml Schlenk flask was charged with a complex 1 (1.0 mg, 1 mol-%) and BnNEt3Cl (0.5 mg, 1 mol-%)
and 0.5 ml of CHCl3 was added. The solution was stirred for 10 minutes, then the diazo compound (42.2
mg, 0.24 mmol) was added, followed by borane triethylamine complex BH3·NEt3 (36 µl, 27.8 mg, 0.24
mmol). Additional 2.5 ml of CHCl3 was added and the mixture was heated at 50ᵒC for 4 hours. After the
completion of the reaction, the solvent was evaporated under reduced pressure and the residue was
purified by column chromatography using petroleum ether – ethyl acetate (3:1) with 1% of
triethylamine as an eluent to give product 4g (58 mg, 91%). The compound 4g was previously
synthesized by rhodium catalysis [13]. 1H NMR (400 MHz, CDCl3): δ = 1.06 (t, J = 7.2 Hz, 9H, 3CH3), 1.50-
2.45 (br+d, J = 99.6 Hz, 2H, BH2), 2.70 (qd, J = 7.6 Hz and J = 3.2 Hz, 6H, 3CH2), 3.13 (br t, J = 3.83 Hz, 1H,
CH), 3.60 (s, 3H, ester), 7.08 (t, J = 7.33 Hz, 1H, Ph), 7.22 (t, J = 7.62 Hz, 2H, Ph), 7.44 (d, J = 6.8 Hz, 2H,
Ph). 11B{1H} NMR (128 MHz, CDCl3): δ = −6.25 (s, 1B).
4.9. [(C4Et4)RhI]2 (2c).
Synthesis and isolation of this complex was conducted under argon atmosphere. A Schlenk flask was
charged with complex 1 (31 mg, 0.06 mmol) and NEt4I (17 mg, 0.065 mmol) and 2 ml of
dichloromethane was added. The pale yellow solution quickly became reddish, but was stirred overnight
to ensure complete conversion. The solvent was evaporated and the residue was extracted with hexane
(3×2 ml). The hexane solution was evaporated and the residue was dried to give the analytically pure
dark red complex 2c (22 mg, 93%). The complex is stable in air in the solid state for several minutes.
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1H NMR (400 MHz, CDCl3): δ = 1.12 (t, J = 7.5 Hz, 24H, CH3), 1.74 (q, J = 7.5 Hz, 16H, CH2). 13C NMR (100
MHz, CDCl3): δ = 12.35 (CH3), 20.26 (CH2), 85.64 (d, J = 12 Hz, C4). ESI-MS: calculated for C24H40I2Rh2 (M):
787.9324, recorded: 787.9329.
4.10. (C4Et4)2Rh2(µ-I)2(µ-1-carboxymethyl-1-phenylmethylene) (5).
Synthesis and isolation of this complex was conducted under argon atmosphere. To a solution of
complex [(C4Et4)RhI]2 2c (37 mg, 0.047 mmol) in 1 ml of CDCl3 methyl α-diazophenylacetate (17 mg, 0.94
mmol) was added and the reaction mixture was stirred overnight. The solvent was evaporated and the
residue was purified by column chromatography using gradient eluent (petroleum ether, then
petroleum ether – ethyl acetate (20:1), then petroleum ether – ethyl acetate (10:1)). The green fraction
was collected and evaporated. The residue was dissolved in hexane and slow evaporation of the solvent
gave the crystals, which were suitable for the X-ray analysis (32 mg, 72%). The complex is stable in air in
1
the solid state for several minutes. H NMR (600 MHz, 20 °C, CDCl3): δ = 1.05 (t, J = 7.6 Hz, 24H, CH3),
1.74 (dq, J = 15.0, 7.6 Hz, 8H, 8CH), 1.96 (dq, J = 15.0, 7.6 Hz, 8H, 8CH), 3.73 (s, 3H, COOMe), 7.07 (br s,
2H, Ph), 7.17 (br s, 1H, Ph), 7.38 (t, J = 9.6 Hz, 1H, Ph), 8.43 (br s, 1H, Ph). The integral intensity of broad
signals is somewhat lower than expected. 13C NMR (150 MHz, 20 °C, CDCl3): δ = 13.64 (CH3), 19.71 (CH2),
50.80 (COOCH3), 94.74 (d, JRh-C = 9.5, C4Et4), 128.31, 159.44, 184.32. Elemental analysis for C33H48I2O2Rh2
calcd: C 42.33, H 5.17; found: C 41.39, H 5.01. The results of elemental analysis somewhat deviated from
the calculated values presumably because of the partial decomposition of the complex in air. ESI-MS:
calculated for C33H48IO2Rh2 (M−I): 809.0804, recorded: 809.0801; calculated for C24H40I2Rh2
(M−C(Ph)COOMe): 787.9324, recorded: 787.9323.
4.11. Reaction of ethyl diazo acetate with tert-butylamine.
NMR-tube with a small stirring bar was charged with solutions of tert-butylamine (15 µl, 11 mg, 0.14
mmol) in CDCl3 (200 µl), complex 1 (1.5 mg, 2 mol-%) in CDCl3 (100 µl), and BnNEt3Cl (0.7 mg, 2 mol-%) in
CDCl3 (100 µl). Upon the addition of ethyl diazo acetate (15 µl, 16.3 mg, 0.14 mmol), vigorous gas
1
evolution was observed. According to H NMR the resulting mixture contained t-BuNHCH2CO2Et 67%, t-
BuN(CH2CO2Et)2 22%, diethyl maleate 8%, diethyl fumarate 3%, and an excess of unreacted t-BuNH2.
4.12. DFT calculations.
Geometry optimizations were performed without constraints using PBE exchange-correlation functional
[36], the scalar-relativistic Hamiltonian, atomic basis sets of generally contracted Gaussian functions,
and a density-fitting technique as implemented in Priroda 6 code [37]. The all-electron double-ζ basis set
L1 augmented by one polarization function was used [38]. Frequency calculations were performed to
confirm that the structures correspond to the minima. The molecular visualization was done by
ChemCraft software (http://www.chemcraftprog.com). Cartesian coordinates of the optimized
structures are available as supplementary data.
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4.13. X-ray diffraction analysis.
Crystals of 5 (C33H48I2O2Rh2, M = 936.33) are monoclinic, space group P21/n, at 120 K: a = 11.9250(7), b =
21.6274(13), c = 13.9321(9) Å, β = 102.5260(10)°, V = 3507.7(4) Å3, Z = 4 (Z’ = 1), dcalc = 1.773 gcm–3,
µ(MoKα) = 27.27 сm-1, F(000) = 1832. Intensities of 36087 reflections were measured with a Bruker
APEX2 CCD diffractometer [λ(MoKα) = 0.71072Å, ω-scans, 2θ<54°], and 7658 independent reflections
[Rint = 0.0424] were used in a further refinement. The structure was solved by the direct method and
refined by the full-matrix least-squares technique against F2 in the anisotropic-isotropic approximation.
Positions of hydrogen atoms were calculated, and they were refined in the isotropic approximation in
the riding model. The refinement converged to wR2 = 0.0580 and GOF = 1.050 for all the independent
reflections (R1 = 0.0256 was calculated against F for 6596 observed reflections with I>2σ(I)). All
calculations were performed using SHELXTL PLUS 5.0 [39].
Appendix A. Supplementary data
NMR spectra of the complexes 2c and 5. Cartesian coordinates for calculated structures.
Appendix B. Supplementary data
CCDC 1562824 contains the supplementary crystallographic 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
Acknowledgements
We thank the Russian Foundation for Basic Research (grant 16-33-00948) and the Council of the
President of the Russian Federation (grant МК-6254.2016.3) for financial support. The X-ray diffraction
data were obtained using the equipment of the Center for Molecular Composition Studies of INEOS RAS.
Mass spectra were recorded in Department of Structural Studies of N. D. Zelinsky Institute of Organic
Chemistry RAS.
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Highlights
Cyclobutadiene rhodium complexes catalyze decomposition of diazo compounds.
Rhodium carbenoids react with X-H bonds but do not add to C=C double bonds.
In contrast to general assumption carbenoid complexes can have a binuclear structure.