Charge-Transfer Complex Promoted C-N Bond Activation for Ni-Catalyzed Carbonylation

Article abstract of DOI:10.1021/acs.orglett.7b01488

A new strategy was developed for activation of C-N bond via formation of an amine-I₂ charge-transfer complex, which facilitates the inert C-N bond activation via oxidative addition with Ni(0). This strategy has been successfully applied in the Ni-catalyzed carbonylation of benzylamines via direct insertion of CO into the C-N bond, which provided a straightforward and rapid approach to arylacetamides in the presence of catalytic amounts of I₂ and Ni catalyst. Mechanistic studies suggested that a benzyl radical generated via the oxidative addition was involved in the present reaction.

Full text of DOI:10.1021/acs.orglett.7b01488

Letter
pubs.acs.org/OrgLett
Charge-Transfer Complex Promoted C−N Bond Activation for Ni-
Catalyzed Carbonylation
Hui Yu,†,§ Bao Gao,‡ Bin Hu,*,† and Hanmin Huang*,†,‡
†State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, P. R. China
‡Hefei National Laboratory for Physical Sciences at the Microscale, and Department of Chemistry, University of Science and
Technology of China, Hefei 230026, P. R. China
§University of Chinese Academy of Sciences, Beijing 100049, P. R. China
S
* Supporting Information
ABSTRACT: A new strategy was developed for activation of
C−N bond via formation of an amine−I2 charge-transfer
complex, which facilitates the inert C−N bond activation via
oxidative addition with Ni(0). This strategy has been
successfully applied in the Ni-catalyzed carbonylation of
benzylamines via direct insertion of CO into the C−N bond, which provided a straightforward and rapid approach to
arylacetamides in the presence of catalytic amounts of I2 and Ni catalyst. Mechanistic studies suggested that a benzyl radical
generated via the oxidative addition was involved in the present reaction.
Scheme 1. Transition-Metal-Catalyzed C−N Bond Activation
he C−N bond is one of the most ample chemical bonds and
Texists in myriad organic molecules and naturally occurring
biomacromolecules.1 The ability to directly insert small
molecules or unsaturated chemical bonds into the C−N bond
is highly desirable for the late-stage derivatization of complex
molecules.2 In this context, the direct insertion of CO into the
C−N bond of amines via C−N bond activation has been proven
to be one of the most direct and promising pathways to access a
range of valuable amides.3 However, the full potential of this
strategy has not yet been realized, and most of those reactions are
only feasible for energetically favored three- or four-membered-
ring nitric heterocycles, active allylamines, or propargylamines,4
in large part due to the formidable challenge of activation of
simple C−N bonds.
As for the activation of the C−N bond of tertiary amines, the
most promising method with transition metals would be
oxidative addition of an amine to a low-valent metal center.5
However, the direct activation of simple C−N bonds via
oxidative addition is still a great challenge exacerbated by their
high bond dissociation energy and high δ-donation ability of the
nitrogen atom.6 One promising strategy to facilitate the oxidative
addition is improving the redox potential of the amines to
enhance the π-back bonding by decreasing the electron-density
of the nitrogen atom. In this context, several catalytic approaches
have been developed to cleave the C−N of benzylamines by
conversion of the simple amines to quaternary ammonium salts
(Scheme 1, top). However, only the resultant carbon moiety was
used as a coupling partner for the subsequent C−C bond
formation process in these reactions; thus, the cleaved amine
moiety was discarded, leading to wasteful byproduct generation
and low atom-efficiency.7 Therefore, the development of efficient
C−N bond activation strategies that are capable of utilization of
both of the carbon and nitrogen nucleophiles generated by the
C−N bond cleavage with high atom economy is highly desirable.
The charge-transfer complex (CT complex) is an association
of two or more molecules, or of different parts of one large
molecule, in which a fraction of electronic charge is transferred
between the molecular entities.8 Because of electron-transfer, the
original chemical bonds experience polarization, thus improving
the redox potential and weakening the chemical bonds. Iodine is
one kind of good electron acceptor for its shortage of one
electron to a full octet, which has been widely used for the
formation of CT complexes with amines.9 It would be reasonable
to believe that iodine is capable of promoting the oxidative
addition of amine with low-valent metal via formation of an
Received: May 16, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01488
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
amine−iodine CT complex. Once the C−N bond is cleaved by a
transition metal, the CO would be inserted into the C−N bond
to furnish a catalytic carbonylation reaction under appropriate
conditions (Scheme 1, bottom). Herein, we report the successful
realization of this concept in a Ni-catalyzed carbonylation that
allows the introduction of CO into the C−N bond of
benzylamines via charge-transfer complex promoted C−N
bond activation. The reaction can be carried out in the presence
of a catalytic amount of iodine and NiCl2, which constitutes a
straightforward and 100% atom-efficient approach to amides.
To identify whether the CT complex could be formed between
the I2 and the tertiary amine 1, the corresponding control
experiments were conducted and monitored by NMR spectra.
The 1H NMR spectra of a solution of tribenzylamine in CDCl3
showed a cleanly resolved single peak at 3.55 ppm for the CH2N
protons, with no significant alteration between 25 and 50 °C.
However, when 1 equiv of I2 was added to this solution, a new
signal appeared at 4.07 ppm for the CH2N protons, and the signal
at 3.55 ppm for the free CH2N protons nearly disappeared with
time going on. Furthermore, the same signal at 4.07 ppm was also
observed when a catalytic amount of I2 was introduced into the
solution of tribenzylamine in CDCl3. Control experiments ruled
out that the benzyl iodide was formed in this reaction mixture
(see the Supporting Information). These results can be
interpreted by considering the fact that the CT complex of
amine−I2 was indeed formed.9i In contrast, no new signal was
observed when the I2 was introduced into the solution of N,N-
dibenzyl-2-phenylacetamide in CDCl3, which suggested that the
amide−I2 complex was not formed due to the lower electron-
donating ability of the amide−nitrogen atom.
Table 1. Screening of Reaction Conditions
entry
[Ni]
ligand
solvent
yield (%)
1
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
NiCl2
BINAP
Xantphos
DPPF
DPEphos
L1
anisole
anisole
anisole
anisole
anisole
anisole
anisole
anisole
anisole
anisole
PhCF3
toluene
dioxane
DMF
34
33
2
3
7
4
22
5
72
6
L1
78 (73)
71
7
NiBr2
L1
8
NiI2
L1
65
9
Ni(acac)2
Ni(PPh3)4
NiCl2
L1
68
10
11
12
13
14
15
L1
75
L1
70
NiCl2
L1
34
NiCl2
L1
54
NiCl2
L1
<5
NiCl2
L1
NMP
<5
b
16
NiCl2
L1
anisole
anisole
anisole
77 (73)
b,
c
d
17
18
NiCl2
L1
82 (77)
NiCl2
51
a
Reaction conditions: 1a (0.5 mmol), [Ni] (0.025 mmol), I2 (0.1
mmol), ligand (0.03 mmol), L1 = 2-(diphenylphosphino)benzoic acid
(0.06 mmol), solvent (2 mL), CO (20 atm), 140 °C, 18 h. Yields were
determined by GC using n-tetradecane as an internal standard, and
isolated yields are given within parentheses. CO (10 atm). I2 (0.075
mmol). 4% phenyl 2-phenylacetate was isolated as a byproduct.
b
c
d
The above positive results intrigued us to investigate the
carbonylation reaction of benzylamines with CO via the CT
complex enabled C−N bond activation according to the
proposed reaction pathway (Scheme 1). N,N-Diisopropylbenzyl-
amine (1a) was chosen as a model substrate to identify a suitable
metal complex and reaction conditions. Various metal
complexes, including palladium and nickel, were tested as
catalysts. After extensive experimentation, Ni(cod)2 proved to be
a promising catalyst for promoting the desired reaction. The
desired amide 2a was obtained in 34% yield when BINAP was
utilized as a ligand under 20 atm of CO at 140 °C in the presence
of a catalytic amount of I2.10 Further evaluation of the phosphine
ligand (Table 1, entries 1−5) demonstrated that the 2-
(diphenylphosphino)benzoic acid (L1) stood out as the best
ligand to deliver the desired amide in 72% yield (Table 1, entry
5). Furthermore, the impact of the nickel catalyst precursors was
carried out with L1 as a ligand, revealing that the inexpensive and
commercially available NiCl2 was the most effective catalytic
precursor for this reaction to give the desired product 2a in 73%
isolated yield (Table 1, entry 6). Other simple nickel salts or
complexes such as NiBr2, NiI2, Ni(acac)2, and Ni(PPh3)4 were
also effective for this transformation to give the desired product
2a in moderate to good yields (Table 1, entries 7−10). Screening
of some representative solvents revealed that the most efficient
catalysis was furnished in anisole (Table 1, entry 6). To our
delight, the reaction could be completed under 10 atm of CO to
yield the corresponding product in 73% isolated yield (Table 1,
entry 16). When the catalyst loading of I2 was further decreased
to 15 mol %, the desired product 2a was still obtained in 77%
isolated yield under 10 atm of CO. Control experiments
demonstrated that no reaction occurred when the reaction was
conducted in the absence of either I2 or Ni catalyst (see the SI),
indicating the critical importance of the I2 and Ni catalyst for
activation of the C−N bond. However, the target product 2a was
obtained in 52% yield in the absence of ligand (Table 1, entry
18), which revealed that the ligand is not essential for the present
catalytic system.
Having established the optimized reaction conditions, the
substrates with different substituents on the nitrogen atom were
investigated under the standard conditions. As summarized in
Table 2, tertiary benzylamines containing different groups on the
nitrogen atom (1a−h) gave the desired product in distinctly
a
Table 2. Substrate Scope of Benzylamines
b
yield (%)
entry substrate
R
conditions A conditions B conditions C
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
i-Pr
H
77
<5
<5
<5
19
59
68
8
73
<5
<5
14
35
73
65
14
CH3
Et
38
67
73
n-Pr
Bn
1g
1h
Cy
Ph
73
a
Conditions A: 1 (0.5 mmol), CO (10 atm), NiCl2 (0.025 mmol), I2
(0.075 mmol), L1 (0.06 mmol), anisole (2 mL), 140 °C, 18 h.
Conditions B: 1 (0.5 mmol), CO (20 atm), NiCl2 (0.025 mmol), I2
(0.1 mmol), L1 (0.06 mmol), anisole (2 mL), 140 °C, 18 h.
Conditions C: 1 (0.5 mmol), CO (20 atm), NiCl2 (0.05 mmol), I2
b
(0.1 mmol), L1 (0.12 mmol), anisole (2 mL), 140 °C, 48 h. Isolated
yield.
B
DOI: 10.1021/acs.orglett.7b01488
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Letter
a
different yields. For the benzylamines with isopropyl (i-Pr),
benzyl (Bn), and cyclohexyl (Cy) as substituents, the carbon-
ylation adduct 2 could be obtained in good yields even under
relatively mild conditions (Table 2, entries 1, 6, and 7). When
other nitrogen groups, such as NH2, N(CH3)2, NEt2, N(n-Pr)2,
and NPh2, were installed in the benzyl skeleton, the conversion
was dramatically decreased under conditions A or B. To our
delight, the desired adducts 2d, 2e, and 2h could be obtained in
good yields by increasing the catalyst loading and prolonging the
reaction time (Table 2, entries 4, 5, and 8). The relatively lower
activity of these substrates might be attributed to either the
difficult cleavage of the corresponding C−NR2 bonds or the
reluctance for undergoing the reductive elimination.
Table 3. Substrate Scope of Benzylamines
On the basis of the results described above, a variety of
benzylamines with two isopropyl groups attached on the N atom
were subjected to the nickel-catalyzed C−N bond carbonylation
reactions to investigate its substrate scope and generality (Table
3). A series of substituents regardless of electron-donating or
-withdrawing properties on the phenyl ring of the tertiary
benzylamines were well tolerated, providing the desired adducts
in moderate to good yields (50−77% yields). Typical functional
groups, such as halides, ether, ester, ketone, cyano, and
sulfonylamide, were compatible with this method (Table 3,
entries 2−16), giving ample opportunities for further elaboration
by transition-metal-catalyzed coupling or other reactions. In
addition to phenyl-substituted amines, naphthyl-substituted
amines were also compatible with this reaction, generating the
corresponding amides (2x and 2y) in good yields (Table 3,
entries 17 and 18). To our great delight, reaction of a heteroaryl-
substituted amine such as N-isopropyl-N-(thiophene-2-
ylmethyl)propan-2-amine also proceeded smoothly to provide
the carbonylation product in moderate yield under modified
reaction conditions (Table 3, entry 19). However, only a lower
yield was obtained for the α-substituted benzylamine (Table 3,
entry 20). Unfortunately, no desired reaction occurred for the α-
disubstituted benzylamines. Furthermore, allylamines and
propargylamines were found to be incompatible with these
reaction conditions (see the SI). The structures of 2w and 2z
were confirmed by single-crystal X-ray diffraction.11
To gain further insight into the mechanism, the following
control experiments were carried out (Scheme 2). When radical
scavenger TEMPO was introduced into the standard reaction,
the desired product could not be obtained. In addition, when 1,1-
diphenylethylene was introduced into this carbonylation system,
the benzylic adduct 3 was obtained in 40% yield and almost no
desired carbonylation product was detected, which suggested
that a free-radical process was most likely involved in this
carbonylation reaction. Competition reaction between 1f and 1k
produced four kinds of amides, which indicated that an outer-
sphere electron-transfer pathway was involved in the present
reaction for transferring the benzyl radical to the desired product
(see the SI).
While a precise reaction mechanism is not yet clear at the
present stage, the most plausible mechanism in line with our
experimental results and previous reports on Ni-catalyzed
carbonylation reactions10 is illustrated in Figure 1. Initially, the
amine−I2 charge-transfer complex A was most likely formed,
which underwent oxidative addition with an active Ni(0) center
to form the radical-containing Ni(I) complex B via a radical
pathway. Then acylnickel(I) species C was generated by CO
migratory insertion along with the release of I2 due to the lower
electron density of the amide moiety. Subsequently, oxidative
addition of the benzylic radical to the active Ni(I) center of
a
Conditions A: 1 (0.5 mmol), CO (10 atm), NiCl2 (0.025 mmol), I2
(0.075 mmol), L1 (0.06 mmol), anisole (2 mL), 140 °C, 18 h.
Conditions B: 1 (0.5 mmol), CO (20 atm), NiCl2 (0.025 mmol), I2
(0.1 mmol), L1 (0.06 mmol), anisole (2 mL), 140 °C, 18 h.
Conditions D: 1 (0.5 mmol), CO (30 atm), NiCl2 (0.05 mmol), I2
b
(0.1 mmol), L1 (0.12 mmol), anisole (2 mL), 150 °C, 18 h. Isolated
yield. PhCF3 as solvent.
c
intermediate C via outer-sphere electron-transfer delivered the
acylnickel(II) species D, which underwent reductive elimination
to deliver the desired amide 2 and regenerated the active Ni(0) to
complete the catalytic cycle.12
In summary, we have developed a new strategy for activation of
inert C−N bonds by using a catalytic amount of I2 for formation
of the amine−I2 charge-transfer complex. This strategy has been
C
DOI: 10.1021/acs.orglett.7b01488
Org. Lett. XXXX, XXX, XXX−XXX

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Figure 1. Plausible reaction mechanism.
successfully applied in the first nickel-catalyzed direct carbon-
ylation of benzylamines via C−N bond activation. This new
transformation is characterized by a broad substrate scope and
represents a concise, operationally simple, and useful method for
the preparation of amides that are of interest in synthetic organic
chemistry. It is anticipated that this line of research could open a
new pathway for C−N bond transformation and could widen the
category of inert bond activation.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01488.
(8) For reviews on charge-transfer complex: (a) Leopold, K. R.;
Canagaratna, M.; Phillips, J. A. Acc. Chem. Res. 1997, 30, 57. (b) Lima, C.
Detailed experimental procedures, compound character-
ization data, and NMR spectra (PDF)
Crystallographic data for compound 2w (CIF)
Crystallographic data for compound 2z (CIF)
G. S.; Lima, T. D.; Duarte, M.; Jurberg, I. D.; Paixao, M. W. ACS Catal.
̃
2016, 6, 1389.
(9) For selected examples, see: (a) Nagakura, S. J. Am. Chem. Soc. 1958,
80, 520. (b) Yada, H.; Tanaka, J.; Nagakura, S. Bull. Chem. Soc. Jpn. 1960,
33, 1660. (c) Kobinata, S.; Nagakura, S. J. Am. Chem. Soc. 1966, 88, 3905.
(d) Bruning, W. H.; Nelson, R. F.; Marcoux, L. S.; Adams, R. N. J. Phys.
Chem. 1967, 71, 3055. (e) Halpern, A. M.; Weiss, K. J. Am. Chem. Soc.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: hanmin@ustc.edu.cn.
*E-mail: hcom@licp.cas.cn.
ORCID
́
2080. (i) Pierre, J.-L.; Handel, H.; Labbe, P.; Le Goaller, R. J. Am. Chem.
Hanmin Huang: 0000-0002-0108-6542
Notes
Soc. 1980, 102, 6574. (j) Stenzel, V.; Jeske, J.; du Mont, W.-W.; Jones, P.
G. Inorg. Chem. 1995, 34, 5166. (k) Rouhollahi, A.; Shamsipur, M. Anal.
Chem. 1999, 71, 1350.
The authors declare no competing financial interest.
(10) For selected examples on Ni-catalyzed carbonylation, see:
(a) Chiusoli, G. P.; Cassar, L. Angew. Chem., Int. Ed. Engl. 1967, 6,
ACKNOWLEDGMENTS
■
124. (b) Pages
̀
, L.; Llebaria, A.; Camps, F.; Molins, E.; Miravitlles, C.;
This research was supported by the National Natural Science
Foundation of China (21672199), the CAS Interdisciplinary In-
novation Team, and the Anhui Provincial Natural Science
Foundation (1708085MB28).
Moreto, J. M. J. Am. Chem. Soc. 1992, 114, 10449. (c) Semmelhack, M.
́
F.; Brickner, S. J. J. Am. Chem. Soc. 1981, 103, 3945. (d) Bottoni, A.;
Miscione, G. P.; Novoa, J. L.; Prat-Resina, X. J. Am. Chem. Soc. 2003, 125,
10412. (e) Hoshimoto, Y.; Ohata, T.; Sasaoka, Y.; Ohashi, M.; Ogoshi,
S. J. Am. Chem. Soc. 2014, 136, 15877.
(11) CCDC 1524398 (2w) and 1524399 (2z) contain the
supplementary crystallographic data for this paper. This data can be
obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(12) The mercury-poisoning experiments suggested that the Ni-
nanoparticle is produced in the present reaction, which further proved
that Ni(0) might be involved in the catalytic cycle.
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D
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