Iridium-Catalyzed Unreactive C(sp3)-H Amination with 2,2,2-Trichloroethoxycarbonyl Azide

Article abstract of DOI:10.1021/acs.orglett.8b02738

An additive-assisted iridium-catalyzed directed C(sp³)-H amination with 2,2,2-trichloroethoxycarbonyl azide as an amino source is reported. Both carboxylate anions and the corresponding cations in the additives are crucial to achieve satisfactory efficiency. Sodium acetate or n-pentanoic acid can promote the amination of various primary C(sp³)-H bonds adjacent to secondary, tertiary, and quaternary carbons in ketoximes or N-aromatic heterocycles, respectively, providing a practical route to versatile β-amino ketoxime and N-heteroaryl ethanamine derivatives. The amination products can be treated as isocyanate analogues and can be converted to other useful amino functionalities. An iridacyclic compound was isolated and identified as a plausible intermediate.

Full text of DOI:10.1021/acs.orglett.8b02738

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Iridium-Catalyzed Unreactive C(sp3)−H Amination with 2,2,2-
Trichloroethoxycarbonyl Azide
Tao Zhang,† Xuejiao Hu,† Xunqing Dong,† Guigen Li,†,‡ and Hongjian Lu*,†
†Institute of Chemistry and BioMedical Sciences, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing, 210093, China
‡Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States
S
* Supporting Information
ABSTRACT: An additive-assisted iridium-catalyzed directed C(sp3)−H amination with 2,2,2-trichloroethoxycarbonyl azide as
an amino source is reported. Both carboxylate anions and the corresponding cations in the additives are crucial to achieve
satisfactory efficiency. Sodium acetate or n-pentanoic acid can promote the amination of various primary C(sp3)−H bonds
adjacent to secondary, tertiary, and quaternary carbons in ketoximes or N-aromatic heterocycles, respectively, providing a
practical route to versatile β-amino ketoxime and N-heteroaryl ethanamine derivatives. The amination products can be treated
as isocyanate analogues and can be converted to other useful amino functionalities. An iridacyclic compound was isolated and
identified as a plausible intermediate.
ransition-metal catalyzed direct amination of alkanes via
TC−H activation is receiving attention because it favors
amination of primary C(sp3)−H bonds1 and is thus an
excellent complement to C−H insertion chemistry in which
amination of secondary and tertiary C(sp3)−H bonds is
preferred.2 However, the high bond dissociation energy of
primary C(sp3)−H bonds, the absence of filled high-energy
orbitals or empty low-energy orbitals,3 and the coordination
ability of newly formed amino groups4 all contribute to the low
efficiency of this transformation. To overcome the inherently
low reactivity and poor selectivity in intermolecular C(sp3)−H
amination reactions, the use of a coordinated directing
Figure 1. Metal-catalyzed unactivated C(sp3)−H amination of
ketoximes and N-aromatic heterocycles
group5−8 at the appropriate site relative to the targeted C−H
bond is critical. Ketoxime derived from ketone,6 and a pyridine
moiety,7 which are commonly found in natural products, have
been evaluated for use as representative directing groups
(DGs). With the ketoxime unit as a DG, Che’s group6a
reported pioneering work on Pd-catalysis with amides under
oxidative conditions. Chang et al.6b−d developed the first Ir-
catalysis with TsN3 and BocN3, and the Li group6e reported
Rh-catalysis with 3-substituted 1,4,2-dioxazol-5-ones (Figure
1A). With pyridine as a DG, Li’s7b and Loh’s7c groups reported
Rh-catalyzed amination with anthranils or amidobenzo-
dioxolones, respectively, as amination reagents. In both these
cases, C−H bonds with adjacent tertiary or quaternary carbons
could be aminated (Figure 1B). In addition to the limited
catalytic systems that have been developed for C(sp3)−H
amination,5−8 substrates with different directing groups must
use different catalysts or amino sources due to the diverse
coordination ability and basicity of the DGs. You’s Rh catalytic
system is effective for both ketoximes and pyridines when
NsNH2 is used as an amino source under oxidative conditions,
in which only C(sp3)−H bonds with neighboring quaternary
carbons7a are effective through tuning the coordination ability
of directing groups and facilitating the formation of five-
membered metallacyclic intermediates by a Thorpe−Ingold
Received: August 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02738
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
effect. Herein, we report a method of Ir-catalyzed intermo-
lecular C(sp3)−H amination with 2,2,2-trichloroethoxy-
carbonyl azide (TrocN3)9,10 (Figure 1C). Using different
additives to match the coordinative ability of directing groups
with the metal catalyst and the basicity of the reaction system,
this method is applicable to both ketoximes and N-aromatic
heterocycles with a broad scope of C(sp3)−H bonds. The
NHTroc group introduced to the products can be directly
transformed to other useful amino functionalities.
Scheme 1. Substrate Scope of N-Aromatic Heterocycles 1
Our investigation started with 2-ethylpyridine (1a) as the
substrate, TrocN3 (2a) as the amino source, and [IrCp*Cl2]2
as the catalyst and was based on the following considerations.
First, pyridine-directed catalytic C−H amination reactions
would be the most efficient and direct method to produce 2-
(2-aminoethyl)pyridine derivatives, a type of motif that is
important in medical and bioactive molecules.11 However, the
amination of C−H bonds adjacent to secondary carbons has
not been reported (Figure 1B).7 Second, amines containing a
strong electron-withdrawing sulfonyl group represent perhaps
the most widely used amine sources in C−H amination
reactions.1 TrocN3, containing a strongly electron-withdrawing
2,2,2-trichloroethoxycarbonyl group,12 which can easily be
deprotected or transformed,13 may be a suitable amino source
for development of a general catalytic C(sp3)−H amination
reaction. Third, since it has been reported that iridium can
catalyze the C(sp3)−H amination of ketoximes6b−d and shows
higher reactivity than rhodium in certain C(sp2)−H amination
reactions,14 it is necessary to investigate the substrate scope of
Ir-catalyzed C(sp3)−H aminations.
a
Conditions A: 1 (0.3 mmol), 2 (0.2 mmol), [IrCp*Cl2]2 (5 mol %),
AgNTf2 (20 mol %), n-C4H9CO2H (50 mol %), DCE, 100 °C, 24 h;
isolated yield.
DGs. The C−H bonds adjacent to secondary carbons were
aminated efficiently (3aa−da, 3ce), implying the uniqueness of
this catalytic system among the known methods.7 The
amination of C−H bonds adjacent to tertiary and quaternary
carbons provided the desired products 3ea−3ha in moderate
to good yields. Benzylic C(sp3)−H amination of 8-methyl-
quinolines was observed with high functional group tolerance
(3ia−3la). The desired monoamination of an isoquinoline 1m
was achieved selectively producing 3ma, but a reasonable yield
of the diamination product 3ma′ was also produced when
excess TrocN3 was used. Such double C−H aminations are
challenging due to the formation of an intramolecular
hydrogen bond in other catalytic systems.4
Our investigation was focused on the additives (Table 1, see
the details in Table S1 in Supporting Information (SI)).15 The
a
Table 1. Selected Results of Pyridine 1a
Then, the C(sp3)−H amination using ketoxime 4a as the
substrate and TrocN3 as the amino source was tested. The
amination product 5aa was achieved, albeit in moderate yield,
using Chang’s conditions.6d The results from optimization of
the reactions are shown in Table 2 (see the details in Table S2
a
Table 2. Selected Results of Ketoxime 4a
a
1
Yield was calculated based on crude H NMR using CH2Br2 as the
b
c
d
e
standard. 2-NAA = 2-naphthylacetic acid. 80 °C. 100 °C. Isolated
yield. 120 °C.
f
reaction failed to proceed without additives, and little pyridine
3aa was formed under basic conditions (Table1, entries 1−6).
After a screening of different acids and temperatures, 3aa was
obtained in 67% yield under 100 °C with n-pentanoic acid as
the additive (Table 1, entries 7−11). TrocN3 and its analogue
2e have proven to the best of all the different azidoformates
(Table 1, entries 12−15).
The scope of N-aromatic heterocycles was then explored,
giving the results shown in Scheme 1. In addition to pyridine,
quinoline, isoquinoline, and even pyrazine could be used as
a
1
Yield was calculated based on crude H NMR using CH2Br2 as the
b
c
d
e
standard. 80 °C. 40 °C. 20 °C; isolated yield. 60 °C.
B
DOI: 10.1021/acs.orglett.8b02738
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in SI). Among the different alkali metal acetates, NaOAc was
shown to be the best, giving 5aa in 99% yield, even at room
temperature (Table 2, entries 1−5). In screening the different
azidoformates, TrocN3 and its analogue 2e gave the best yields
(Table 2, entries 6−10).
Scheme 3. Transformations of Aminated Products 3 and 5
The scope of ketoximes was then explored (Scheme 2). C−
H bonds adjacent to tertiary or quaternary carbons reacted
a
Scheme 2. Substrate Scope of Ketoximes
a
b
K2CO3 (4.0 equiv), alcohol, 100 °C, 20 h. DIPEA (1.0 equiv),
c
amine (1.5 equiv), DMSO, 100 °C, 20 h. Amine (1.1 equiv).
d
e
PhMgBr (3.0 equiv), THF, −78 °C−rt, 10 h. LiOH·H2O (5.0
equiv), MeCN/H2O (1/1), rt, 3 h.
a
Conditions B: 4 (0.3 mmol), 2 (0.2 mmol), [IrCp*Cl2]2 (5 mol %),
This newly developed method has been utilized as a tool for
late-stage functionalization (Scheme 4).16 For example, the
AgNTf2 (20 mol %), NaOAc (50 mol %), DCE, 24 h; isolated yield.
b
2.0 mmol scale.
Scheme 4. Late-Stage C(sp3)−H Amination
smoothly even at room temperature, giving products 5aa−5ca
and 5ae. The amination of C−H bonds adjacent to secondary
carbons proceeded slowly, generating the desired products
5da−ea in moderate yields. Compounds 5fa and 5ga,
important precursors of 1,2-amino alcohols, were obtained in
moderate yields. Finally, the competing reactions at different
C−H bonds were investigated, and the results show that C−H
bonds adjacent to tertiary or quaternary carbon were greatly
favored as a result of the Thorpe−Ingold effect, providing the
desired products (5ha−ia) exclusively. Interestingly, the
ketoxime (5aa) was easily deprotected without affecting the
Troc functional group, producing a ketone derivative (6aa) in
95% yield.
The products formed in this reaction could be treated as
analogs of isocyanates and directly transformed to compounds
bearing other important and functional amino groups (Scheme
3). Using ethanol and isopropanol as nucleophiles, different
carbamates were formed efficiently (7a−b). When tert-amyl
alcohol was used, the symmetric urea (7c) was obtained
instead of a carbamate. When different amines containing
secondary, tertiary, or quaternary carbons were used, various
asymmetric urea compounds were obtained directly (7d−g).
The reactions of amantadine, an antiviral and an antiparkinso-
nian medication, and an amine derivative of cholesterol could
provide urea derivatives of the drug or natural product directly
(7h−i). With PhMgBr as a nucleophile, the amide 7j could be
obtained directly. Under basic conditions, 3aa was efficiently
converted to the free primary amine 7k. Ketoximes can also go
through similar transformations, forming carbamate and urea
derivatives 8a−b without affecting the ketoxime group.
ketoxime 4j, derived from oleanolic acid, was prepared and
subjected to the reaction under the optimized conditions. The
catalytic amination proceeded smoothly at the methyl group
affording monoaminated isomers in 91% yield (5ja, 71%; 5ja′,
20%), which could be separated by column chromatography.
The quinoline derivative 1n, a member of a promising new
class of antiosteoporosis compounds,17 was readily aminated,
providing 3na in 75% isolated yield as the major product. An
equal equivalent of substrate 1n was used, showing the high
efficiency of this transformation.
A series of experiments were carried out to further
understand the mechanism (Scheme 5). Significant primary
kinetic isotope effects were observed in an intermolecular
competition experiment between [D6]-1g and 1g (3ga/[D5]-
3ga = 5.3, eq 1) and an intramolecular competion reaction of
[D3]-1g ([D3]-3ga/[D2]-3ga = 6.1, eq 2), while no H/D
scrambling occurred in these two reactions. These results
C
DOI: 10.1021/acs.orglett.8b02738
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ORCID
Scheme 5. Mechanistic Studies
Guigen Li: 0000-0002-9312-412X
Hongjian Lu: 0000-0001-7132-3905
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We would like to acknowledge the financial support from the
National Natural Science Foundation of China (Nos.
21332005, 21472085, 21672100, 21871131) and the Funda-
mental Research Funds for the Central Universities (No.
020514380131).
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02738.
Experimental details and characterization data for all
new compounds (PDF)
Accession Codes
CCDC 1863905 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
■
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(a) Shin, K.; Kim, H.; Chang, S. Acc. Chem. Res. 2015, 48, 1040.
*E-mail: hongjianlu@nju.edu.cn.
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