Rhodium(III)-catalyzed hydroamination of aromatic terminal alkynes with anilines

Article abstract of DOI:10.1021/om201134j

The dinuclear Rh(III) species [Cp*RhCl₂]₂ catalyzes the hydroamination reaction between an aromatic terminal alkyne (ArCCH) and an aniline (Ar′NH₂), in the presence of a salt additive, to afford the ketimine Ar′N=C(Me)(Ar). A reaction pathway has been proposed on the basis of experimental and computational studies.

Full text of DOI:10.1021/om201134j

Article
pubs.acs.org/Organometallics
Rhodium(III)-Catalyzed Hydroamination of Aromatic Terminal
Alkynes with Anilines
Elumalai Kumaran and Weng Kee Leong*
Division of Chemistry and Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
S
* Supporting Information
ABSTRACT: The dinuclear Rh(III) species [Cp*RhCl2]2
catalyzes the hydroamination reaction between an aromatic
terminal alkyne (ArCCH) and an aniline (Ar′NH2), in the
presence of a salt additive, to afford the ketimine Ar′N
C(Me)(Ar). A reaction pathway has been proposed on the
basis of experimental and computational studies.
is the relatively simple, easily accessible, and air-stable dinuclear
rhodium complex [Cp*RhCl2]2 (1).
INTRODUCTION
■
Amines and imines are important intermediates in the synthesis
of natural products, pharmacological agents, and N-hetero-
cycles and in a number of industries including fine chemicals,
agrochemicals, and dyes.1 Practical synthetic routes to imines
include the condensation of ketones with primary amines. This
method is, however, not suitable for aromatic ketimines because
of the poor reactivity of aryl ketones, the propensity toward
aldol-type side reactions with methyl ketones,2 and the poor
(40−55%) yields.3 The direct addition of amine N−H bonds to
unsaturated substrates such as alkynes and alkenes provides a
simple, efficient, and atom-economic route to the synthetically
useful ketimines and enamines. It is also superior to the other
available methods such as imination of ketones2,3 or the
aminomercuration/demercuration of alkynes.4 Catalytic hydro-
amination of alkynes has also been successfully utilized as a key step
in a variety of total syntheses of naturally occurring molecules.5
A number of metal complexes are known to catalyze the inter-
molecular hydroamination of terminal alkynes,6 and the regio-
selective hydroamination of alkynes to form aromatic ketimines
has been achieved with ruthenium6e and gold catalysts.6j Rhodium
complexes have also been reported to catalyze hydroamination
reactions, including the inter- and intramolecular hydroamination
of alkenes7 and the intramolecular cyclization of aminoalkynes.8
The first reported rhodium-catalyzed intermolecular alkyne hydro-
amination by Beller also appears to be the only one to-date which
afforded Markovnikov products;9 a closely related example was the
Markovnikov addition of hydrazine reported by Messerle.10 Other
reports on rhodium-catalyzed intermolecular alkyne hydroamina-
tion all led to the anti-Markovnikov product.11 In particular, we
noticed that all the rhodium catalysts reported have involved a
Rh(I) species. In addition, the sole example of Markovnikov
addition did not work well with phenylacetylene, affording only a
10% yield of product, because of competing oligomerization.9
Here, we wish to report our investigations into the first
successful example of a Rh(III) species acting as a catalyst
precursor, for the efficient synthesis of aromatic ketimines
based on the regioselective Markovnikov hydroamination of
terminal aromatic alkynes with anilines. The catalyst precursor
RESULTS AND DISCUSSION
■
Complex 1 catalyzes the reaction between aniline (2a) and phenyl-
acetylene (3a), in the presence of an additive, to form the ketimine
4a, via a Markovnikov alkyne hydromamination (Scheme 1).
Scheme 1
An optimization study showed that a number of additives
(AgOTf, AgPF6, AgSbF6, LiPF6, NaPF6, HBF4, and NH4PF6)
were active, although Bu4NPF6 was not (Table 1; more
Table 1. Effects of Variations in Additive and Solvent for
1-Catalyzed Hydroamination of Phenylacetylene with Aniline
S/no.
solvent
1 (mol %) additive (mol %) T (°C) yield (%)
1
toluene
toluene
toluene
toluene
toluene
THF
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.2
NH4PF6 (2.0)
NH4PF6 (3.0)
NH4PF6 (5.0)
Bu4NPF6(3.0)
AgOTf (3.0)
NH4PF6 (3.0)
NH4PF6 (3.0)
NH4PF6 (3.0)
NH4PF6 (1.5)
NH4PF6 (0.6)
NH4PF6 (3.0)
80
80
80
80
80
80
30
110
80
80
80
69
89
85
2
3
4
5
81
6
7
toluene
toluene
toluene
toluene
toluene
8
83
87
53
9
10
11
variations in the SI); as a control, use of the additive NH4PF6
alone did not catalyze the reaction. This points to a cationic
Received: November 15, 2011
Published: January 30, 2012
© 2012 American Chemical Society
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Article
complex as the active catalyst. Our subsequent studies utilized a
catalyst loading of 0.5 mol % and a 1.5 mol % loading of
NH4PF6; a higher catalyst loading did not improve the yield
much, although it did accelerate the reaction rate. Strongly
coordinating solvents such as THF, methanol, and 2-propanol
were detrimental to the reaction; higher (>80 °C) temperatures
gave slightly lower yields, and the reaction failed to proceed at
ambient temperature.
(top) and those that were converted to the amine by NaBH4
reduction and isolated as such (bottom). Both electron-donating
and -withdrawing substituents on the alkyne and the aniline are
tolerated. However, the reaction failed with aliphatic amines or
alkynes and with internal alkynes.
Mechanistic Considerations. Isotopic labeling experi-
ments employing (a) d7-aniline and 4-methoxyphenyl acetylene
and (b) 4-methoxyaniline and PhCCD afforded 4c with two
and 4f with one of the CH3 protons being deuterated (Scheme 2).
The substrate scope has also been studied; this is summarized in
Table 2, for both imines that were sufficiently stable for isolation
Scheme 2
Table 2. Substrate Scope Study for Isolated Imine (Top) and
Imine Converted to, and Isolated As, the Amine (Bottom)
a
entry
R1
R2
yield (%)
1
2
H
H
H
H
H
4a (89)
4b (90)
4c (92)
4d (84)
4e (91)
4f (93)
4g (89)
4h (91)
4i (92)
4j (93)
4-CH3
4-OCH3
4-Cl
3
4
5
4-CH3
H
These results clearly indicate that all three H atoms in the CH3
group are derived solely from the terminal alkyne CH and the
aniline NH2.
6
4-OCH3
4-OCH3
4-OCH3
4-OCH3
4-OCH3
H
7
3-CH3
4-CH3
4-Cl
8
There are a number of possibilities for the reaction pathway.
One involves an insertion into the N−H bond,8c,11a,b but this is
known mainly for the group 4 metals and lanthanides12 and
would also involve a Rh(V) species, which is likely to lose HCl
(and hence loss of deuterium in the labeling experiments).
Complex 1 does not react with alkyne, NH4PF6, or both, even
upon heating. It does, however, react immediately at room
temperature with PhNH2 to form Cp*RhCl2(NH2Ph) (6),
which is also an active catalyst for the reaction. It is therefore
reasonable to assume that the initial reaction involved the
formation of 6 followed by loss of a chloride to the cationic
species [Cp*RhCl(NH2Ph)]+ (A). A plausible reaction path-
way for the catalytic cycle is that depicted in Figure 1.
We have also examined computationally the energetics
involved, using DFT at the B3LYP/LANL2DZ level of theory,
for the reaction between PhCCH and PhNH2; the computed
free energies (ΔGo) are also given in Figure 1. The first step,
involving binding of the alkyne to A to afford B, has a slightly
positive ΔGo. The computed structure of B is interesting, as it
appears to be quite similar to that proposed by Messerle for a
similar cationic alkyne complex,8e but the binding of the alkyne
is so asymmetric that it is essentially an alkene-like carbonium
ion; the ipso and α carbons carry positive Mulliken charges.
That the binding of the alkyne is endergonic is consistent with
the suppression of the reaction in coordinating solvents; we
have computed a ΔGo of −32 kJ mol−1 for the binding of
2-propanol to A.
9
10
4-OCH3
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
H
2-CH3
2-OCH3
3-CH3
3-OCH3
3-Cl
5a (66)
5b (77)
5c (77)
5d (61)
5e (65)
5f (71)
5g (81)
5h (76)
5e (55)
5j (79)
5k (78)
5l (73)
5m (81)
5n (76)
5o (74)
5p (80)
5q (67)
5r (83)
5s (78)
5t (82)
H
H
H
H
H
3-F
H
4-Br
H
4-F
H
4-CH2OH
4-tBu
H
H
2-CH3, 4-OCH3
C10H8
H
H
6-OCH3-1-naphthyl
4-OCH3
4-OCH3
3-OCH3
3,5-(OCH3)2
4-OCH3
4-Br
2-OCH3
3-OCH3
H
H
H
H
The second step (step B → C) involves attack of a second
aniline molecule on the bound alkyne; the isomeric form of C
in which the amino group is trans to the Rh lies 67 kJ mol−1
higher. The possibility of direct migration of the Rh-bound
aniline to the alkyne has a computed ΔGo of −16 kJ mol−1, but
4-Cl
a
Yields reported are isolated yields.
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Article
Figure 1. Proposed reaction pathway for the catalytic reaction involving PhCCH and PhNH2. Gibbs free energies (in kJ mol−1) for the various steps
in the cycle are also given.
binding of the aniline to the metal center would be expected to
lower its nucleophilicity.13 Consistent with this is our
observation that although 6 is also an active catalyst for the
reaction, it does not give any ketimine when reacted with
alkyne and NH4PF6 without the addition of aniline.
alkyne, followed by an intramolecular 1,3-H shift to form an
Rh-bound iminium, before its release from the catalyst.
In particular, one important aspect of the proposed reaction
pathway is that all the intermediates contain a Cp*RhCl-
(aniline) moiety. The implication is that Rh(III) complexes of
the general formula Cp*Rh(X)(L) should be potential catalysts
for alkyne hydroamination. Our work on this is ongoing, and
our results will be reported shortly.
The following step, C → D, is essentially an enammonium−
iminium rearrangement. This has been shown to occur via an
intramolecular 1,3-H shift and is consistent with our deuterium
labeling experiments.14 As for the case of C, there are also two
isomeric forms for D, which differ in their stereochemistry
about the CN bond. The energies shown in Figure 1 for the
last two steps are those for the cis isomer; the corresponding
values for the trans isomer are −64 and +12 kJ mol−1,
respectively, which does not allow us to determine which, or
both, is the likely identity for D. The final step is dissociation
of the iminium, presumably as the enamine, which then tauto-
merizes to the corresponding, more stable, imine product.
This final tautomerisation also involves an intramolecular
1,3-H shift,15 which is again consistent with the deuterium
labeling experiments.
EXPERIMENTAL SECTION
■
All chemicals from commercial sources were used without further
purification, and solvents were dried over the appropriate drying
agents and distilled under argon. The catalyst 1 was prepared
according to the published method.16 All reactions and manipulations
were carried out under argon by using standard Schlenk techniques.
1H and 13C{1H} NMR spectra were recorded in CDCl3 on a JEOL
ECA400 or ECA400SL spectrometer, and the chemical shifts were
referenced to the residual proton resonance. GC/MS was recorded in
EI mode on a Thermofinnigan DSQII mass spectrometer. High-
resolution mass spectra (HRMS) were recorded in ESI mode on a
Waters UPLC-Q-TOF mass spectrometer.
General Procedure for the Hydroamination of Alkynes with
Anilines. In a typical experiment, aniline (240 μL, 2.5 mmol) and a
slight excess of phenylacetylene (302 μL, 2.75 mmol) were added
using a syringe to a suspension of 1 (7.8 mg, 0.5 mol %) and NH4PF6
(6.2 mg, 1.5 mol %) in toluene (10 mL) in a Carius tube under argon
at room temperature. The mixture was degassed (three cycles of
freeze−pump−thaw) and then stirred at 80 °C for 12 h, after which
the solvent was removed under vacuum and the crude product washed
with chilled (−10 °C), dry hexane (3 × 5 mL). A similar procedure
was used for the other imines 4. Characterization data are given in the
Supporting Information.
CONCLUDING REMARKS
■
We have shown that the readily available Rh(III) complex
[Cp*RhCl2]2 is a good catalyst precursor for alkyne hydro-
amination involving aromatic terminal alkynes and anilines. A
reaction pathway involving cationic intermediates has been
proposed on the basis of experimental and computational
evidence. This involves binding of both alkyne and aniline and
nucleophilic attack by a second aniline on the coordinated
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General Procedure for the Reduction of Imine Using NaBH4.
For reactions in which the imine is unstable, the crude imine product
obtained from the hydroamination reaction was dissolved in dry
methanol. To this was added sodium borohydride (1.2 equiv), and the
mixture stirred at room temperature for 2 h. The solvent was then
evaporated, and the residue was quenched with water and then
extracted with ethyl acetate (3 × 15 mL). The combined extracts were
dried over magnesium sulfate, and then the solvent was evaporated
under reduced pressure. The crude amine product 5 was purified by
column chromatography using ethylacetate/hexane (15:85, v/v) as
eluent. Characterization data are given in the Supporting Information.
Deuterium Labeling Experiments. A sample of 4-methoxyani-
line (20 mg, 0.16 mmol) and a slight excess of phenylacetylene-d (19 μL,
0.18 mmol) were added using syringe to a suspension of 1 (1 mg, 1 mol %)
and NH4PF6 (1 mg, 3 mol %) in toluene (2 mL). The mixture was
degassed (three cycles of freeze−pump−thaw) and then stirred at 80 °C for
12 h, after which the solvent was removed under vacuum and the crude
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1
product was characterized by H NMR and HRMS.
Similarly, aniline-d7 (20 μL, 0.2 mmol) was reacted with 4-
methoxypheneylacetylene (29 μL, 0.22 mmol) in the presence of 1
(1.2 mg, 1 mol %) and NH4PF6 (1 mg, 3 mol %) in toluene at 80 °C
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1
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Preparation of Cp*RhCl2(NH2Ph) (6). Aniline (10 μL, 0.105
mmol) and 1 (30 mg, 0.048 mmol) were dissolved in dichloromethane
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1
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3
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in the SI.
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(110 μL, 1.07 mmol), 6 (5 mg, 0.012 mmol), and NH4PF6 (3 mg,
0.018 mmol) afforded the ketimine product in 85% yield.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Further experimental details and characterization for 4 and 5,
crystallographic data for 6, and optimized geometry of A−D.
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ACKNOWLEDGMENTS
■
This work was supported by Nanyang Technological University
and the Ministry of Education (Research Grant No.
T208B1111), and one of us (E.K.) thanks the University for
a Research Scholarship.
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dx.doi.org/10.1021/om201134j | Organometallics 2012, 31, 1068−1072