Cyclobutadiene Rhodium Complexes as Catalysts for the Synthesis of Amides from Electron-rich Arenes, Tosyl Azide and CO

Article abstract of DOI:10.1002/cctc.202100344

Cyclobutadiene rhodium complexes [(C₄R₄)RhX]₂ (R=Et, Ph; X=I, Cl) at 0.5 mol % loading catalyze the formation of aromatic amides from various indoles, tosyl azide, and carbon monoxide (1 atm). The reaction gives good yield

Full text of DOI:10.1002/cctc.202100344

The European Society Journal for Catalysis
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Accepted Article
Title: Cyclobutadiene rhodium complexes as catalysts for the synthesis
of amides from electron-rich arenes, tosyl azide and CO
Authors: Nikita Shvydkiy, Tatiana N. Petrushina, and Dmitry S.
Perekalin
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ChemCatChem
Cyclobutadiene rhodium complexes as catalysts for the synthesis of amides
from electron-rich arenes, tosyl azide and CO
Dr. Nikita V. Shvydkiy,*a Tatiana N. Petrushina,a,b Prof. Dr. Dmitry S. Perekalin*a
a A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 Vavilova, Moscow, 119991, Russian Federation. E-mail: nikich-shv@mail.ru, dsp@ineos.ac.ru
b Department of Chemistry, Lomonosov Moscow State University, GSP-1, Leninskie Gory, Moscow,
119991, Russian Federation.
Dedicated to Professor Elena S. Shubina in recognition of her outstanding studies of organometallic
compounds by IR spectroscopy and on the occasion of her anniversary.
Abstract
Cyclobutadiene rhodium complexes [(C4R4)RhX]2 (R=Et, Ph; X = I, Cl) at 0.5 mol% loading catalyze the
formation of aromatic amides from various indoles, tosyl azide, and carbon monoxide (1 atm). The
reaction gives good yields of the target products (typically 60-85%) and tolerates both donor and
moderately acceptor substituents in the indoles (alkyl, OMe, Hal, COOMe, but not CN). The DFT
calculations at the PBE/3ζ level revealed a new possible mechanism for the reaction of TsN3 with CO via
cycloaddition and formation of RhCON3 metallacycle, which is more favorable than the commonly
assumed formation of nitrene intermediate.
Introduction
Diene rhodium complexes are readily available[1] from RhCl3 and are widely used as catalysts for various
organic transformations.[2–4] However, in most cases the labile diene ligands are replaced at the first step
of the catalytic cycle to give some other active species. Recently, we have developed a general approach
to the rhodium complexes with cyclobutadiene ligands,[5,6] which bind metals much stronger than dienes
(Scheme 1).[7,8] Rhodium cyclobutadiene complexes proved to be effective catalysts for the insertion of
diazoesters into E−H bonds,[9] as well as for reductive amination of aldehydes and ketones in the
presence of CO.[6,10] In both reactions the cyclobutadiene ligand remains coordinated with rhodium
throughout the catalytic cycle and stabilizes the active species. Interestingly, the catalytic activity of
cyclobutadiene complexes has not been explored before these works.
Recently, Bao, Gu, Xia et al. have reported a new unusual method for the synthesis of aryl amides from
electron-rich arenes, tosyl azide, and CO.[11–13] The described reaction does not require the presence of
any directing groups in arene and various functional groups. However, its efficiency is undermined by
the use of the large amounts of the diene rhodium catalyst [(cod)RhCl]2 (10 mol % based on Rh).
Therefore, we decided to find out if it is possible to improve the reaction efficiency by using the more
robust cyclobutadiene rhodium complexes as catalysts.
1
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Scheme 1. Hypothesis for investigation of cyclobutadiene complexes as catalysts.
Results and discussion
Catalytic studies
We started our investigation with the model reaction of N-methylindole with tosyl azide in MeCN in the
presence of 1 atm of CO at 80 °C (Scheme 2, Table 1). The readily available[6] complex
[(C4Et4)Rh(xylene)]PF6 (Rh1) with strongly-bound cyclobutadiene and loosely-bound xylene ligand indeed
promoted this reaction at 1 mol% catalyst loading giving the desired amide 1a in 10% yield (entry 1).
Treatment of the complex Rh1 with [Et3NBn]Cl led to the displacement of xylene ligand and the
formation of the neutral chloride complex [(C4Et4)RhCl]2 (Rh2) in almost quantitative yield.[5] This dimeric
complex is a close analog of the classical catalyst [(cod)RhCl]2. Complex Rh2 has 16 valence electrons
and vacant coordination sites on rhodium and therefore can be a more active catalyst. Indeed, the
chloride complex Rh2 (at 0.5 mol% loading, that is 1 mol% based on rhodium) was notably more active
than the xylene complex Rh1 and gave the product 1a in 33% yield (entry 2). Screening of the solvents
showed that the yield of 1a can be increased up to 70% in nitromethane (entries 4−9). Noteworthy, the
addition of water retarded the process (entry 4) presumably because of hydrolysis of the intermediately
formed TsNCO (see below). Therefore, it was necessary to use dry distilled solvents to achieve good
yields. The use of the iodide complex[9] [(C4Et4)RhI]2 (Rh3) instead of the chloride [(C4Et4)RhCl]2 (Rh2)
increased the yield of the product up to 80% in 1,2-dichloroethane (entry 10), while in nitromethane the
yield dropped to 42% (entry 11). The decrease of the catalyst loading from 0.5 to 0.25 mol% led to the
notably lower yield of 59% (entry 13).
We also tested the more bulky tetraphenyl-cyclobutadiene complexes [(C4Ph4)Rh(xylene)]PF6 (Rh4) and
[(C4Ph4)RhCl]2 (Rh5) as catalysts and found that the latter have similar activity to Rh3 under the standard
conditions (entry 16 vs 10). Interestingly, under the increased pressure of CO (40 bar) even very low
amounts of the catalyst Rh5 (0.05 mol%) gave the product 1a in good yield 62% (entry 16). Finally, we
tested the cyclooctadiene complex [(cod)RhCl]2 as the catalyst under our optimized conditions and
unexpectedly found very similar activity (entry 18). Apparently, the strongly coordinating MeCN solvent
that has been used previously[11] diminished the catalytic activity of this complex.
2
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Scheme 2. Optimization of catalyst and conditions for the model catalytic reaction.
№
Catalyst
Loading,
mol%
1
0.5
0.5
Solvent
Yield of
1a, %
10
1
2
3
4
[(C4Et4)Rh(xylene)]PF6 (Rh1)
[(C4Et4)RhCl]2 (Rh2)
[(C4Et4)RhCl]2 (Rh2)
MeCN
MeCN
MeCN
MeCN
+5 eq. H2O
DCM
Acetone
DMF
DCE
MeNO2
DCE
MeNO2
THF
33
40a
0
[(C4Et4)RhCl]2 (Rh2)
0.5
5
6
7
8
[(C4Et4)RhCl]2 (Rh2)
[(C4Et4)RhCl]2 (Rh2)
[(C4Et4)RhCl]2 (Rh2)
[(C4Et4)RhCl]2 (Rh2)
[(C4Et4)RhCl]2 (Rh2)
[(C4Et4)RhI]2 (Rh3)
[(C4Et4)RhI]2 (Rh3)
[(C4Et4)RhI]2 (Rh3)
[(C4Et4)RhI]2 (Rh3)
[(C4Ph4)Rh(xylene)]PF6 (Rh4)
[(C4Ph4)RhCl]2 (Rh5)
[(C4Ph4)RhCl]2 (Rh5)
[(C4Ph4)RhCl]2 (Rh5)
[(cod)RhCl]2 (Rh6)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.25
1
19b
15
33
38
70
80
42
24
9
10
11
12
13
14
15
16
17
18
DCE
59
traces
24
MeCN
MeCN
DCE
DCE
DCE
0.5
0.5
0.05
0.5
80
62 c
79
Typical conditions: N-methylindole (0.5 mmol), TsN3 (0.6 mmol), CO (balloon), catalyst (1 mol% based on
Rh), solvent (2 mL), 80 °C, 24 h. The yields were determined by NMR spectroscopy using 1,1,2,2-
tetrachloroethane as the internal standard. a) Two equivalents of tosyl azide were used. b) The
temperature of the reaction was 55 °C. c) The pressure of CO was 40 bar.
Following our original idea, we chose the catalyst Rh3 for further studies and investigated the scope of
suitable arenes for this reaction (Scheme 3). N-methyl-indoles with substituents in various positions of
aromatic ring smoothly reacted with TsN3 and CO to give the desired amides 1a−e in 76−88% yields. The
yields were somewhat lower (45-71%) in the case of derivatives of non-methylated indoles (1f-1i),
probably because of some side reactions, such as an electrophilic attack of TsNCO intermediate at the
nitrogen atom. Interestingly, N-acetyl-indole did not give the corresponding amide product (1m)
because the electron acceptor acetyl group reduced the nucleophilicity of arene. Otherwise, the nature
of the substituents did not significantly affect the outcome of the reaction: the reaction tolerated both
donor (alkyl, OMe) and acceptor groups (halogens, COOMe). Mild steric hindrance in 2-phenyl-indole
did not prevent the formation of the product 1g. Other electron-rich arenes such as pyrrole and
guaiazulene also reacted with TsN3 and CO giving the corresponding amides 1j and 1k, albeit in lower
3
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yields because of the lower stability of these arenes themselves. On the other hand, less nucleophilic
arenes, such as 1,4-dimetoxybenzene, N,N-dimethylaniline, and ferrocene did not produce the expected
products 1n−p.
Scheme 3. The substrate scope of catalytic amidation of heteroarenes with CO and tosyl azide. Reaction
conditions: arene (0.5 mmol), TsN3 (0.6 mmol), CO (1 atm), [(C4Et4)RhI]2 (0.0025 mmol), 1,2-
dichloroethane (2 mL), 80 °C, 24 h. Isolated yields are given. a) 2 mol% of the catalyst were used.
Attempts to use benzyl azide, tert-butyldimethylsilyl azide or the combination of p-nitrotoluene and
CO[14] as a source of nitrene instead of tosyl azide were unsuccessful. Similarly, the use of ethyl formate
as a source of CO was also ineffective. To sum up, the reaction scope, the yields of products, and the
limitations of our catalytic system are very similar to those observed previously with the cyclooctadiene
rhodium catalyst [(cod)RhCl]2.[11] Noteworthy, this low loading (0.5 mol%) makes the cyclobutadiene
4
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rhodium complex more active than palladium catalysts,[13,15] which are often used in various
carbonylation reactions.
Detailed analysis of the substrate scope (Scheme 3) revealed that the outcome of this catalytic reaction
is determined by the nucleophilic properties of the arene and correlates with Mayr’s nucleophilicity
scale.[16] In particular, the nucleophilicity parameter (N) for N-methyl-indole is 5.8, for the free indole N =
5.6, for methyl- and methoxy-substituted indoles N > 6.0, and for halogen-substituted indoles N ≈ 4.5.[17]
All these arenes reacted with TsN3 and CO to give the corresponding amides 1a−i. At the same time, no
reaction was observed for the arenes with lower nucleophilicity, such as cyano-indole (N = 2.8), N-
acetyl-indole (N = 1.7 for the related N-Boc-indole), or 1,4-dimethoxybenzene (N = 2.5 for the related
1,3-isomer). This approach also explains that benzyl azide did not react because the intermediate
isocyanate PhCH2NCO is significantly less electrophilic than TsNCO (E = −7.7 for TsNCO vs E = −15.4 for
PhNCO).[18] Based on Mayr’s nucleophilicity scale, it can be predicted that other nucleophiles with
parameter N > 4 (such as silyl ethers of enols, enamines, etc.) can also react with TsN3 and CO to give the
corresponding amides.
The synthesized amides 1a-1i were moderately soluble in acetone and ethyl acetate and poorly soluble
in other common organic solvents (Et2O, CH2Cl2, etc.). This low solubility significantly complicated their
isolation because it required large amounts of solvents for column chromatography. Therefore, we
developed an alternative and more efficient purification procedure, which avoids chromatography. First,
the excess of tosyl azide and admixture of tosylamide were removed by washing with Et2O, and then the
remaining product was dissolved in acetone and filtered through the short pad of silica gel to remove
the residual catalyst. The obtained samples were spectroscopically pure and did not require additional
purification in most cases.
Reaction mechanism
In order to clarify the nature of the active species we carried out the model catalytic reaction at 80 °C
with 30 mol% loading of the iodide complex Rh3 and analyzed the resulting organometallic products by
IR- and NMR-spectroscopy as well as mass spectrometry. The mass spectrum contained the signals of
the starting complex Rh3. The 1H NMR spectrum showed only the signals of the coordinated
cyclobutadiene ligand and the IR spectrum did not show the absorption bands of the possible
decomposition products [(CO)2RhI]2 or [(CO)2RhI2]− indicating that the cyclobutadiene remained intact
during the catalytic cycle. The reaction of [(C4Et4)RhI]2 (Rh3) with CO at room temperature in the
absence of other organic reagents produced an interesting product Rh7 instead of the expected 18-
electron complex (C4Et4)Rh(CO)2I (Scheme 4). The NMR spectra of Rh7 revealed ambiguous picture with
only one type of the cyclobutadiene ligand present in the 13C NMR spectrum but presumably two sets of
cyclobutadiene signals in 1H NMR spectrum. The IR spectrum of Rh7 contained a single vibration band of
CO ligand at 1855 cm−1 indicating its bridging coordination mode. The elemental analysis and the high-
resolution mass spectrum suggested that the formula of Rh7 was [(C4Et4)RhI]2(CO) (higher oligomers
were also observed in HRMS). The DFT calculations of the IR spectra for the model compound
[(C4Me4)RhI]2(CO) (IM0, see below) gave a very similar CO vibration frequency of 1869 cm−1. Also, the
compound with a similar dimeric structure [(C4Et4)RhI]2(CR2) has been previously identified as
intermediate for the rhodium catalyzed carbene insertion reactions.[9] We were not able to confirm the
structure of Rh7 by X-ray diffraction because of its high solubility and low stability. In fact, the formation
of Rh7 was reversible and CO ligand dissociated after several days to give back the starting iodide Rh3.
Similar instability has been reported for the related rhodium carbonyl complexes with borole ligands.[19]
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Scheme 4. The reaction of the complex Rh3 with carbon monoxide and the possible structure of Rh7.
The previous works have shown the formation of the aryl amides 1 from indoles, TsN3, and CO proceeds
in two steps.[11,13] The first step is the catalytic reaction of TsN3 with CO to give the tosyl isocyanate
TsNCO.[15,20] The second step is the reaction of TsNCO with indoles to give amides, which does not
require a catalyst.[18] Bao, Gu, and Xia et al. have proposed that the catalytic conversion of TsN3 into
TsNCO occurs via coordination of TsN3 with [Rh(CO)2Cl] species and the subsequent dissociation of N2
with the formation of the rhodium nitrene intermediate Cl(OC)2Rh=NTs.[11] The calculated barrier for the
formation of the rhodium nitrene intermediate Cl(OC)2Rh=NTs from [Rh(CO)2Cl] species is 29 kcal mol−1,
suggesting that the reaction can proceed with a reasonable speed at 80 °C in accordance with
experimental observations. However, according to our calculations, the formation of the starting species
[Rh(CO)2Cl] from the stable dimer [Rh(CO)2Cl]2 is unfavorable for enormous 79 kcal mol−1,[21] which is not
surprising for the highly unsaturated 14-electron species with acceptor CO ligands. The proposed nitrene
intermediate Cl(OC)2Rh=NTs has similarly low stability relative to the starting dimer [Rh(CO)2Cl]2.
Therefore, we think that the formation of TsNCO from TsN3 in the presence of [(cod)RhCl]2 or
[Rh(CO)2Cl]2 catalysts proceeds via some other mechanism, possibly via dimeric complexes or via
cycloaddition of TsN3 as described below for the cyclobutadiene rhodium complexes.
We calculated a possible mechanism for the model reaction of [(C4Me4)RhI]2 catalyst with TsN3 and CO
at PBE/3ζ level (Figure 1).[22] The initial addition of CO is favorable for 16.1 kcal mol−1 and leads the
dimeric complex with a bridging carbonyl ligand IM0. The structure of IM0 is similar to that proposed for
the experimentally obtained complex Rh7. Addition of the second CO molecule leads to the cleavage of
dimeric structure and gives the unsaturated 16-electron monomeric species [(C4Me4)Rh(CO)I] (IM1),
which have similar stability to IM0. Subsequent coordination of tosyl azide requires ca. 7 kcal mol−1
because of the very small enthalpy gain and the unfavorable entropy contribution. Note that the
rhodium atom can coordinate either with the substituted nitrogen atom of TsN3 (IM2a) or with the
terminal one (IM2b) and these intermediates have similar stability. The azide complex IM2a can then
split off a nitrogen molecule via the transition state TS1a to form the nitrene complex IM3a, but the
activation barrier for this process is rather high ΔG‡ = 30.2 kcal mol−1. Subsequent addition of CO to the
coordinated nitrene proceeds much easier via TS2a (ΔG‡ = 19.9 kcal mol−1 from IM3a) to give loosely-
bound isocyanate complex [(C4Me4)Rh(CO)(TsNCO)l], which dissociates into the catalytically active
species IM1 and the product TsNCO. The overall formation of TsNCO and N2 from TsN3 and CO is
energetically favorable for 64.6 kcal mol−1.
6
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Figure 1. Calculated standard Gibbs free energies (at PBE/3ζ level) for the possible mechanisms of the
conversion of TsN3 and CO into TsNCO and N2 in the presence of the cyclobutadiene rhodium
complexes. Energies are given in kcal mol−1 relative to the presumed active species [(C4Me4)RhI(CO)]
(IM1).
The mechanisms involving coordinated nitrenes are often assumed in the literature. Interestingly, we
found another possible pathway for the transformation of TsN3 into TsNCO, which is more energetically
favorable (Figure 1). In this case the key step is the intramolecular nucleophilic attack of azide on CO
ligand in the intermediate IM2b, which proceeds via TS1b with activation barrier G‡ = 21.7 kcal mol−1
and gives the metallacycle intermediate IM3b. Despite looking exotic, similar metallacycles have been
isolated previously from the reaction of the carbonyl molybdenum complexes with organic azides[23] and
have been shown to act as the intermediates for the formation of isocyanates.[24] In our case, however,
7
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the metallacycle IM3b is rather unstable and its retro-cycloaddition has a low barrier (ΔG‡ = 3.6 kcal
mol−1 from IM3b). As a result, IM3b directly gives the product TsNCO as well as the labile nitrogen
complex [(C4Me4)Rh(N)2I], which easily exchanges N2 for CO to regenerate the catalytically active species
IM1. Similar mechanism has been recently proposed and calculated for the palladium-catalyzed
reactions.[25,26] The important conclusion from this pathway is that reactions of TsN3 in the presence of
the transition metal catalysts may proceed without the intermediate formation of the commonly
assumed nitrene complexes. This hypothesis also provides a possible explains for the absence of by-
products of the direct nitrene insertion into C−H bond of indoles in our case. The overall proposed
mechanism for the amidation of indoles is given on Scheme 5.
Scheme 5. The proposed mechanism for the overall reaction of N-methyl-indole with TsN3 and CO in the
presence of the catalyst [(C4Et4)RhI]2 (Rh3).
8
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Conclusions
We have shown that cyclobutadiene rhodium complexes are the active catalysts for the reaction of
donor arenes with tosyl azide and CO. The range of substrates for this class of transformations is
determined by the nucleophilic and electrophilic properties of the reagents and can be predicted by
Mayr's nucleophilicity scales. According to DFT calculations the reaction is likely proceed via
cycloaddition mechanism which does not include the formation of nitrene intermediates.
Acknowledgments
The main synthetic work was supported by the Russian Science Foundation (grant # 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.
Keywords: Carbonyl ligands, Cyclobutadiene, Density functional calculations, Homogeneous catalysis,
Rhodium.
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Table of contents
Catalysts of a new type: Readily available cyclobutadiene rhodium complexes catalyze the synthesis of
amides directly from donor arenes, CO, and tosyl azide. The scope of substrates for such
transformations can be rationalized in terms of nucleophilic and electrophilic properties of reagents.
10
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