Indenyl rhodium complexes. Synthesis and catalytic activity in reductive amination using carbon monoxide as a reducing agent

Article abstract of DOI:10.1016/j.jorganchem.2017.11.003

Indenyl-ligated rhodium(III) catalyst [(η⁵-indenyl)RhX₂]_n (1) is reported for the synthesis of alkylated amines via catalytic reductive amination using carbon monoxide as a reducing agent. Water as a solvent was found to b

Full text of DOI:10.1016/j.jorganchem.2017.11.003

Accepted Manuscript
Indenyl rhodium complexes. Synthesis and catalytic activity in reductive amination
using carbon monoxide as a reducing agent
Sofiya A. Runikhina, Mikhail A. Arsenov, Vladimir B. Kharitonov, Elizaveta R.
Sovdagarova, Olga Chusova, Yulia V. Nelyubina, Gleb L. Denisov, Dmitry L. Usanov,
Denis Chusov, Dmitry A. Loginov
PII:
S0022-328X(17)30635-6
10.1016/j.jorganchem.2017.11.003
JOM 20168
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 15 August 2017
Revised Date: 2 November 2017
Accepted Date: 2 November 2017
Please cite this article as: S.A. Runikhina, M.A. Arsenov, V.B. Kharitonov, E.R. Sovdagarova, O.
Chusova, Y.V. Nelyubina, G.L. Denisov, D.L. Usanov, D. Chusov, D.A. Loginov, Indenyl rhodium
complexes. Synthesis and catalytic activity in reductive amination using carbon monoxide as a reducing
agent, Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.11.003.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
Indenyl rhodium complexes. Synthesis and catalytic activity in reductive amination using
carbon monoxide as a reducing agent
a
a
a
a
Sofiya A. Runikhina, Mikhail A. Arsenov, Vladimir B. Kharitonov, Elizaveta R. Sovdagarova,
b
a
a
c
a,b
Olga Chusova, Yulia V. Nelyubina, Gleb L. Denisov, Dmitry L. Usanov, Denis Chusov,*
a
Dmitry A. Loginov*
a
A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
Vavilova st. 28, Moscow 119991, Russian Federation.
b
Faculty of Science, RUDN University, 6 Miklukho-Maklaya St., Moscow 117198, Russian
Federation
c
c/a: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street,
Cambridge, MA 02138, United States
E-mail: Chusov@ineos.ac.ru or dloginov@ineos.ac.ru
th
Dedicated to 85 anniversary of Prof. Irina P. Beletskaya in recognition of her great contribution
to the development of catalysis with metal complexes.
5
Abstract: Indenyl-ligated rhodium(III) catalyst [(η -indenyl)RhX ] (1) is reported for the
2
n
synthesis of alkylated amines via catalytic reductive amination using carbon monoxide as a
reducing agent. Water as a solvent was found to be the best media for the reaction. Complex 1
was synthesized via a high-yielding procedure based on the reaction of bis(ethylene) derivative
5
(
η -indenyl)Rh(C H ) with iodine.
2
4 2
Introduction
N-alkylated amines are important synthetic intermediates for pharmaceuticals and fine
chemicals and considerable efforts have been focused on the development of general and
efficient methodologies for the generation of alkylated amines[1]. Some of those include
traditional coupling of amines with halogenated molecules, reductive amination of
aldehydes or ketones[2–8], as well as hydroamination[9–17] and N-alkylation of
alcohols[18–20]. Recently, we described an atom- and step-economical approach to the
synthesis of amines without an external hydrogen source[21,22]. This methodology uses
carbon monoxide[23–28] as a reducing agent and tolerates various functional groups,
including aromatic and aliphatic nitriles[29], esters[30], phenols[31] and
cyclopropanes[32], such moieties as Cbz- [31], trifluoroacetamido-[31], N-benzyl-[33],
O-benzyl-[22], aromatic fluoro-[29], chloro-[22], bromo-[33] and even aromatic nitro-
[
31]. Experimental data consistently demonstrated that the presence of water leads to
decreased catalytic activity. On the contrary, herein we describe new indenyl rhodium
complexes with highest catalytic activity observed in water, which constitutes a desirable
feature in the context of economic and ecological impacts.
Results

ACCEPTED MANUSCRIPT
We became very interested in the catalytic properties of indenyl rhodium complexes.
We anticipated these complexes to be quite effective in catalysis due to their dual nature:
5
3
η - η isomerization (Scheme 1) allows formation of an additional coordination site[34–
7]. Importantly, ligand elimination reaction does not require extra stabilization or solvate
restructuring, which leads to lowered energy barrier to generate potentially active species.
3
5
3
Scheme 1. Reversible η -to-η isomerization of (indenyl)metal complexes.
Good leaving-group properties of halide anions naturally rendered them to be best
candidates as auxiliary ligands. For example, (cyclopentadienyl)rhodiumhalides
[
(C R )RhX ] (X = Cl, Br, I) are known as effective catalysts for numerous organic
5 5 2 2
transformations, viz. oxidative coupling of benzoic acids with internal alkynes[38–40], as
well as carbenoid C–H functionalization of benzenes and indoles[41–44]. Similar
complexes with polymethylated indenyl ligands are also well known[45,46]. In contrast,
the chemistry of unsubstituted (indenyl)rhodium complexes is almost unexplored. Welch
5
and coworkers[47] attempted to synthesize chloride complex [(
η
-indenyl)RhCl ] by the
2
n
reaction of RhCl ·xH O with indene; however, the structure of the isolated product is of
3
2
questionable reliability because its reaction with Tl[Tl(η-7,8-C B H )] did not give the
2
9
11
5
expected rhodacarborane 3-(η -indenyl)-3,1,2-RhC B H . Besides nido-derivatives, the
2 9 11
only one closo-bis(dicarbollide) complex 3-(C B H )-8-{(C H )Rh(C H )}-3,1,2-
2
9
11
9
6
9
7
RhC B H was isolated in low yield from this reaction.
2
9
10
5
We found that halide complexes [(η -indenyl)RhX ] (1: X = I; 2: X = Br; 3: X = Cl)
2
n
5
can be successfully synthesized by reactions of halogens with bis(ethylene) derivative (η -
indenyl)Rh(C H ) (Scheme 2). The latter was prepared without purification from the Cramer
2
4 2
complex [(C H ) RhCl] and indenyllithium in overall yield of 70-80% after two steps. Notably,
2
4 2
2
5
the use of cyclooctadiene derivative (η -indenyl)Rh(cod) instead of bis(ethylene) complex leads
to an unidentified mixture of products, formation of which is probably caused by partial
5
5
elimination of cyclooctadiene. One of the possible products is [(η -indenyl)RhX ][(η -
3
indenyl)Rh(cod)X], that is partially confirmed by the presence of signals from the coordinated
1
cyclooctadiene in H NMR spectrum. Similar phenomena were observed earlier for reactions of
CpIr(cod) with halogens[48].
Scheme 2. Synthesis of halide complexes 1–3.
Complexes 1–3 are almost insoluble in non-coordinating solvents (e.g., dichloromethane,
acetone, acetonitrile), which rendered difficult detection of NMR spectra. Presumably, these
5
complexes have polymeric structure with bridging halides [(η -indenyl)RhX(µ-X)] . The
n
5
presence of (η -indenyl)Rh framework in their structure was indirectly confirmed by the reaction
of 1 with CpTl in acetonitrile, which selectively furnished mixed-ligand sandwich complex [(η -
5

ACCEPTED MANUSCRIPT
indenyl)RhCp]PF (4, Scheme 3). The use of acetonitrile as a solvent was found to be necessary;
6
less polar media, such as tetrahydrofuran, gave unidentified mixture of products as a result of
incomplete abstraction of iodide-anions.
Scheme 3. Reaction of 1 with CpTl.
The structure of 4-BPh was determined by X-ray diffraction. Cation
4 has the
4
expected sandwich structure, containing both Cp and indenyl ligands (Fig. 1). Only few
substituted derivatives of this type were structurally characterized earlier [49][50]. The
planes of cyclopentadienyl and indenyl ligands are approximately parallel; the dihedral
5
Cp/indenyl angle equals to 5.30°. The indenyl ligand is η -coordinated to the rhodium
atom without significant slippage; the slip parameter (defined as difference in the average
distances from metal to the bridgehead carbons and from metal to the adjacent carbon
atoms [51–54]) equals to 0.069 Å. The distance from the rhodium atom to the plane of C5
ring of the indenyl ligand Rh···C (1.822 Å) is longer than Rh···Cp distance (1.798 Å),
5
suggesting weaker bonding of rhodium with the indenyl ligand than with Cp. According
to trans-effect, weakening of one bond leads to strengthening of the other bonds located
in the trans-position. Indeed, Rh···Cp distance in
corresponding distances in rhodocenium cation [RhCp ] (av. 1.808 Å)[55]. In contrast,
Rh···Cp distance is greatest (av. 1.824 Å) in complex [CpRhCp*] with strongly bonded
4 is somewhat shorter than the
+
2
+
Cp*-ligand [55].
Figure 1. Structure of cation
are omitted for clarity. Selected bond lengths (Å): Rh1
4
. Ellipsoids are shown at the 50% level. Hydrogen atoms
C1 2.2242(19), Rh1 C2
−
−
2
2
.1576(19), Rh1
.1685(19), Rh1
−
C3 2.1635(19), Rh1
−
C4 2.1737(19), Rh1
−
C5 2.2445(18), Rh1
−
C10
−C11 2.1700(18), Rh1
−
С12 2.1555(19), Rh1
−
С13 2.1708(19), Rh1
−С14

ACCEPTED MANUSCRIPT
C2 1.441(3), C1 С5 1.443(3), C2 С3 1.428(3), С3−С4 1.417(3), С4−С5
2
1
.1750(19), C1
.435(3).
−
−
−
We examined indenyl complexes 1−4 and cyclopentadienyl derivative [CpRhI ] as
2 n
catalysts for reductive amination of aldehydes and ketones using carbon monoxide as a
reducing agent. The reaction of p-tolylaldehyde with p-anisidine was chosen as a model
process. Traditionally, tetrahydrofuran (THF) behaved as the best solvent for CO-
mediated reductive amination catalyzed by rhodium derivatives. We tested different
rhodium species in THF (Table 1) and only trace amount of the product was detected
when cyclopentadienyl complexes were used (Table 1, entries 4, 5). On the contrary,
halide complexes with indenyl ligands did show catalytic activity. This data corroborates
5
3
the hypothesis that the possibility of indenyl ligands to isomerize from η to η
coordination might be useful for catalysis. No significant effect of the halide nature in
rhodium species was observed (Table 1, entries 1 3).
−
Table 1. Reductive amination of p-tolylaldehyde with p-anisidine catalyzed by
a
rhodium complexes in THF
b
b
Entry
catalyst
[(η -indenyl)RhI ] , 1
Loading catalyst, mol %
Yield of 6
Yield of
Schiff base
81%
5
1
2
3
4
a5
3
1%
1%
1%
1%
1%
8%
2
n
5
[(η -indenyl)RhBr ] , 2
11%
10%
trace
trace
78%
68%
80%
93%
2
n
5
[(η -indenyl)RhCl ] , 3
2
n
5
[(η -indenyl)RhCp]PF , 4
6
[CpRhI ] , 5
2
n
1.4 – 81.2 mg (0.26 – 0.66 mmol) anisidine, 20 - 52µL (0.17 – 0.44 mmol) tolylaldehyde, 1 mol % of the catalyst,
b
1
20 – 400 µL of THF. Yields were determined by GC.
Solvent screening expectedly demonstrated inferior results in reactions conducted in
acetonitrile, methanol, toluene or under solvent-free conditions (Table 2, entries 1−4 vs. 5).
Isopropanol and ethanol showed higher activity (Table 2, entries 6−7 vs. 5). The same trend was
found in the case of rhodium cyclobutadienyl catalysts[31]. Whereas WGSR reductive amination
traditionally requires water to be mixed with another organic solvent (usually, an alcohol) [56–
5
8], to our surprise the best results were observed when pure water was used as the reaction
medium (Table 2, entry 8 vs. 5).
a
Table 2. Investigation of the solvent influence

ACCEPTED MANUSCRIPT
b
Entry
Solvent
MeCN
solvent-free
MeOH
toluene
THF
Yield of 6
3%
1
2
3
4
5
6
7
8
5%
7%
7%
8%
15%
55%
99%
i-PrOH
EtOH
H O
2
a
4
0.6 mg (0.33 mmol) anisidine, 26µL (0.22 mmol) tolylaldehyde, 200 µL of solvent
b
Yields were determined by GC.
With these results in hand, we decided to re-test the activity of all rhodium complexes in
water (Table 3). The influence of the halogen ligand in indenyl rhodium complexes became
much more significant. Iodide complex 1 was found to be the most active catalyst (Table 3, entry
1
); excellent yield of amine 6 could be achieved even when the catalyst loading was decreased
twofold (Table 3, entry 2). Bromide complex 2 gave product 6 with slightly lower yield of 87%
vs. 99%, Table 3, entry 3 vs. 1). Chloride complex 3 led to average yield of 58% (Table 3, entry
). High catalytic activity was observed for cyclopentadienyl derivative 5 (Table 3, entry 5),
which implied that a different reaction mechanism could be taking place. Interestingly, complex
containing two strongly bonded ligands (both indenyl and cyclopentadienyl) proved to be
(
4
4
inactive even in water (Table 3, entry 6).
Table 3. Reductive amination of p-tolylaldehyde with p-anisidine catalyzed by
a
rhodium complexes in water
b
entry
catalyst
[(η -indenyl)RhI ] , 1
catalyst loading, mol %
yield of 6
yield of the
Schiff base
1%
5
1
2
3
4
5
1%
0.5 %
1%
1%
1%
99%
90%
87%
58%
93%
trace
2
n
5
[(η -indenyl)RhI ] , 1
9%
9%
35%
5%
2
n
5
[(η -indenyl)RhBr ] , 2
2
n
5
[(η -indenyl)RhCl ] , 3
2
n
[CpRhI ] , 5
[(η -indenyl)RhCp]PF , 4
2
n
5
6
4
1%
92%
6
a
0.6 – 81.2 mg (0.33 – 0.66 mmol) anisidine, 26 - 52µL (0.22 – 0.44 mmol) tolylaldehyde, 0.5 – 1mol % of
b
catalyst, 200 – 400 µL of water. Yields were determined by GC.
When the best catalyst was found, we proceeded to screening of other parameters of the
system (see supporting information). Temperature studies showed that in the range of 120−160
°
1
°
C product yields of 92−99% can indeed be achieved; however, decreasing temperature from
20 to 100 °C leads to lower reaction rate. When the temperature was increased to 140 or 160
C, smaller amounts of bis-alkylated tertiary amine (4-methoxy-N,N-bis(4-methylbenzyl)aniline,
see supporting information) were detected. The influence of the carbon monoxide pressure was
also investigated. When the pressure was decreased from 30 to 10 bar the yield of the product
dropped to 27%. This might be explained by the necessity for carbon monoxide to be in the

ACCEPTED MANUSCRIPT
supercritical form for best performance (the critical point of CO is 35 bar, -140 °C. When 30 bar
of CO is heated to 120 °C the pressure becomes higher than critical). The reaction barely
proceeds at 5 bar of CO and further decrease of the pressure to 3 bar abolishes the catalytic
activity (see supporting information). Various types of carbonyl compounds and amines were
suitable for the reaction (Scheme 4). Both primary and secondary amines worked well. Ketones
were naturally found to be less active, however reductive amination of acetone proceeds with
excellent yield. Whereas aliphatic aldehydes often undergo self-aldol reaction in the presence of
amines, under our conditions the reaction between cyclohexane carboxaldehyde and p-anisidine
successfully furnished the corresponding product in 83% yield.
Scheme 4. Substrate scope of the reductive amination reaction.
We believe that different mechanisms might take place in different media (Scheme 5).
When the reaction is run in organic solvents, the main process might proceed via formation of a
hemiaminal or an iminium cation. Initially, the indenyl rhodium species furnishes intermediate A
followed by the hydroxyl group attacking the carbonyl moiety. Subsequent elimination of carbon
dioxide leads to the formation of hydride C and reductive elimination leads to the formation of
the product and regeneration of the catalytic species. When the reaction proceeds in water, we
hypothesize that the cycle starts with the formation of hydroxide anion and ammonium cation.
An intermolecular attack of hydroxide on the pre-catalytic species leads to intermediate D, and
intramolecular attack on the carbonyl group leads to species E. Subsequent elimination of carbon
dioxide yields hydride F, which reacts with iminium cation/Schiff base leading to the formation
of the product. We believe that the key step of both processes involves changing the
5
3
configuration of the complex from η to η .

ACCEPTED MANUSCRIPT
Scheme 5. Possible mechanisms of reductive amination in water (Path II) and water-free media
(Path I).
In conclusion, new indenyl rhodium complexes were synthesized and fully characterized.
Their application in CO-mediated reductive amination shows their high potential in redox
reactions, especially in aqueous media. We believe that such types of rhodium complexes can be
5
3
quite effective in catalysis due to their dual nature: η to η isomerization allows liberation of an
additional coordination site. The ligand elimination reaction does not require extra stabilization
or solvate restructuring, which leads to lowered energy barrier to generate potentially active
species. We corroborated the structural merits of the new indenyl complexes by demonstrating
their superiority with respect to cyclopentadienyl analogues.
Acknowledgments
This work was supported by the Russian Foundation for Basic Research (15-03-02548). D.A.L.
gratefully acknowledges support of the Russian Foundation for Basic Research (project 16-33-
6
0140 mol_а_dk). O.C. and D.C. thanks the Ministry of Education and Science of the Russian
Federation (Agreement number 02.a03.21.0008). The contribution of the Center of molecular
composition studies of INEOS RAS is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary
data
related
to
this
article
can
be
found
at
http://
dx.doi.org/10.1016/j.jorganchem.
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Graphical abstract

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•
•
New indenyl rhodium complexes were synthesized and fully characterized
Such types of rhodium complexes can be quite effective in catalysis due to their dual
nature
5
3
•
•
η to η isomerization allows liberation of an additional coordination site.
Their application in CO-mediated reductive amination shows their high potential in redox
reactions, especially in aqueous media.