Efficient Amination of Activated and Non-Activated C(sp3)?H Bonds with a Simple Iron–Phenanthroline Catalyst

Article abstract of DOI:10.1002/anie.202013687

A readily available catalyst consisting of iron dichloride in combination with 1,10-phenanthroline catalyzes the ring-closing C?H amination of N-benzoyloxyurea to form imidazolidin-2-ones in high yields. The C?H amination reaction is very general and applicable to benzylic, allylic, propargylic, and completely non-activated aliphatic C(sp³)?H bonds, and it also works for C(sp²)?H bonds. The surprisingly simple method can be performed under open flask conditions.

Full text of DOI:10.1002/anie.202013687

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Accepted Article
Title: Efficient Amination of Activated and Non-Activated C(sp3)-H
Bonds with Simple Iron-Phenanthroline Catalyst
Authors: Lucie Jarrige, Zijun Zhou, Marcel Hemming, and Eric Meggers
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10.1002/anie.202013687
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COMMUNICATION
Efficient Amination of Activated and Non-Activated C(sp3)-H
Bonds with Simple Iron−Phenanthroline Catalyst
Lucie Jarrige, Zijun Zhou, Marcel Hemming and Eric Meggers*
Abstract: A readily available catalyst consisting of iron dichloride in
combination with 1,10-phenanthroline catalyzes the ring-closing C-H
amination of N-benzoyloxyurea to form imidazolidin-2-ones in high
yields. The C-H amination reaction is very general and applicable to
benzylic, allylic, propargylic, and completely non-activated aliphatic
C(sp3)-H bonds, and it also works for C(sp2)-H bonds. The surprisingly
simple method can be performed under open flask conditions.
Aspects of sustainability are playing an increasingly important role
for the development of new synthetic methods. While it is now
generally considered possible to prepare almost any stable
molecule of interest with existing methods, current and future
challenges shift towards the development of new reactions that
make use of benign and abundant reagents and catalysts. For
example, the direct conversion of C-H bonds into C-heteroatom
bonds has been recognized as a powerful toolbox for designing
streamlined and more efficient synthetic routes. Much progress
has been made towards C-H functionalization over the past
decades.[1] However, most reported methods require an activation
of the C-H bond such as a neighboring -system or heteroatom in
order to overcome the low reactivity of aliphatic C-H bonds.[2]
Furthermore, most reported methods rely on catalysis with noble
metals such as Rh, Pd, Ir and Ru. Considering economic,
environmental as well as health aspects, earth-abundant first row
transition metals are more desirable, and especially iron as a non-
toxic metal with a high abundance in the Earth’s crust.[3-5]
However, often used in combination with ligands which need to
be synthesized in a multistep fashion, the advantage of this
catalysis is often diminished. Thus, despite the enormous
progress made over the past decades regarding the development
of direct C-H functionalization methodology, issues of
practicability and sustainability in combination with the
functionalization of aliphatic, non-activated C(sp3)-H bonds
remain an important problem (Figure 1a).
Herein, we report a direct C-H amination of benzylic, allylic,
propargylic, and completely non-activated aliphatic C(sp3)-H
bonds using a simple iron salt in combination with a readily
available ligand under mild conditions without the need for inert
atmosphere and dry solvents (Figure 1b). The method is also
applicable to C(sp2)-H bonds. Specifically, iron dichloride in
combination with 1,10-phenanthroline catalyzes the smooth
conversion of N-benzoyloxyurea to imidazolidin-2-ones which are
important motifs in bioactive compounds[6] (Figure 1c) and
precursors of valuable vicinal diamines.
Figure 1. Challenges for C-H functionalization and this work.
The transition metal catalyzed insertion of nitrenes into C-H
bonds is an attractive method for the amination of C(sp3)-H
bonds.[7] We became interested in exploring the use of simple iron
salts for such reactions and commenced our study by
investigating the ring-closing C-H amination of the N-
benzoyloxyurea 1a[8,9] to the cyclic urea 2, a reaction which we
recently disclosed with a ruthenium complex as catalyst.[10-12]
Encouragingly, we found that FeBr2 (5 mol%) in CH2Cl2 at room
temperature afforded the imidazolidin-2-one, although only in a
low yield of just 8% (Table 1, entry 1) (see Tables S1-6 of the
Supporting Information for more details). Slightly higher yields of
11% and 15% were obtained with Fe(OTf)2 and FeCl2·H2O,
respectively (entries 2 and 3). We selected FeCl2·H2O as the iron
salt of choice and next optimized the reaction conditions and
found that 1,2-dichloroethane as the solvent at 50 °C resulted in
only slightly higher yields of 18% (entry 4). However, when we
increased the catalyst loading from 5 to 10 mol%, the yield
increased to 69% (entry 5). Almost quantitative yield of the cyclic
urea 2 was obtained when 1,10-phenanthroline (phen, 10 mol%)
and Na2CO3 were added to the reaction (entry 6). Other
co-ligands such as terpyridine (entry 7) or neocuproine (entry 8)
are inferior. Conveniently, we found that the reaction can be
performed in many solvents such as methanol, THF, acetone and
ethyl acetate and we selected MeCN as our standard solvent for
the remaining study (entry 9). Also notable for practical purposes,
the reaction is not sensitive to air and can be performed under
open flask conditions (entry 10).
[*]
Dr. L. Jarrige, Z. Zhou, M. Hemming, Prof. Dr. E. Meggers
Fachbereich Chemie, Philipps-Universität Marburg
Hans-Meerwein-Straße 4, 35043 Marburg (Germany)
E-mail: meggers@chemie.uni-marburg.de
Supporting information for this article can be found under: xxx
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Table 1. Initial experiments and optimization of reaction conditions.[a]
available. As to be expected, a benzylic position is strongly
favored over a non-activated methylene group (44:44' = 3.8:1),
but only slightly favored over an aliphatic tertiary C-H bond (45:45'
=
1.1:1). The formation of
a 5-membered heterocyclic
imidazolidin-2-one is generally favored over a 6-membered
heterocyclic pyrimidin-2(1H)-one but the ratio depends of the
degree of activation of the C-H bonds. For example, the 6-
membered ring formation by amination of a benzylic C-H bond
can compete with the 5-membered ring formation by activation of
a non-activated methylene group (46:46' = 1:1). An electron-
donating methoxy group in the phenyl moiety renders the benzylic
C-H bond more electron-rich and shifts the ratio towards the 6-
membered ring formation (47:47'). This electronic effect is
expected for a reaction that proceeds through an electrophilic iron
nitrenoid intermediate (see Supporting information for more
details).[7] Other examples for 5- vs. 6-membered ring formation
are shown for 48:48' and the citronellol derivatives 49:49'. If the
formation of a 5-membered ring is completely blocked, the 6-
membered cyclic urea can form efficiently as the main product as
shown for compound 29.
Finally, we tested this method for late-stage functionalization by
converting primary amines into their N-benzoyloxyurea in a single
step, followed by iron-catalyzed C-H amination. The resulting C-
H amination products 50-53 (52-95% yield) are shown in Figure
2d, which were obtained in overall two steps from 2-aminoindane,
N-tosyl tryptamine, and n-dodecylamine. Interestingly, the methyl
ester of phenylalanine was converted into the cyclic urea 53 with
high diastereoselectivity (> 20:1 dr).
Entry Catalyst
Loading Ligand[b] Base
[mol%]
Solvent[c] T [°C] Yield
[%][d]
1
2
3
4
5
6
7
8
9
FeBr2
5
5
5
5
none
none
none
none
none
phen
tpy
none
CH2Cl2
CH2Cl2
CH2Cl2
DCE
r.t.
r.t.
r.t.
50
50
50
50
50
50
50
50
50
8
Fe(OTf)2
none
11
15
18
69
99
82
50
99
99
40
46
·
FeCl2 4H2O
none
·
FeCl2 4H2O
none
·
FeCl2 4H2O 10
none
DCE
·
FeCl2 4H2O 10
Na2CO3
Na2CO3
DCE
·
FeCl2 4H2O 10
DCE
·
FeCl2 4H2O 10
neocup Na2CO3
DCE
·
FeCl2 4H2O 10
phen
phen
phen
phen
Na2CO3
Na2CO3
Na2CO3
Na2CO3
MeCN
MeCN
MeCN
MeCN
·
10[e] FeCl2 4H2O 10
·
11[e,f] FeCl3 6H2O 10
12[e,f] Fe(OTf)3
10
[a] All reactions were performed on 0.1 mmol scale of 1a in 1 mL of solvent
(0.1M) under nitrogen atmosphere for 14 h unless noted otherwise. [b] phen =
1,10-phenanthroline, tpy = 2,6-bis(2-pyridyl)pyridine, neocup = neocuproine.
[c] DCE = 1,2-dichloroethane. [d] All yields refer to the isolated product after
column chromatography. [e] Performed under air. [f] Full conversion.
With optimized reaction conditions in hands, we explored the
scope of this new iron catalysis employing a broad variety of N-
benzoyloxyurea as depicted in Figure 2 (see Figure S1 of the
Supporting Information for a variation of the carboxylate leaving
group). We started with the C-H amination of benzylic C(sp3)-H
bonds (Figure 2a, imidazolidin-2-ones 2-23) and found that the
method tolerates well a large variety of N-substituents at the urea,
including different alkyl groups (2-6), a trimethylsilylmethyl (7),
and propargyl group (8). Even an unprotected N-H group provides
the C-H amination product (9) in high yields of 92%. Both electron
donating and accepting substituents in the para-position of the
phenyl moiety were also successfully converted (products 10-16),
as well as sterically more demanding meta- and ortho-methyl
groups (17 and 18), 1-naphthyl (19) and 2-naphthyl groups (20),
heteroaromatic thiophene (21) and even a coordinating pyridine
moiety (22). For the majority of the reactions, the isolated yields
were above 90%, including the amination of a tertiary benzylic C-
H bond (30). The desymmetrization of a N-benzoyloxyurea
derived from 1,3-diphenyl-2-propanamine providing the 4,5-
difunctionalized 2-imidazolidinones 23 with two adjacent
stereocenters as the trans diastereomer is a notable exception
with only a low yield of 32% for the cyclic urea. Beyond benzylic
C-H bonds, this amination method is also applicable to allylic (24-
26, 77-81% yields) and propargylic C-H bonds (27 and 28, 84 and
91% yields).
It is also noteworthy, that the Fe/phen-catalyzed C-H amination
is easily scalable. For example, through C(sp3)-H activation of a
non-activated C-H bond, imidazolidine-2-one 42 was obtained in
94% yield on a 1 mmol scale.
Our new method is clearly distinguished from reported iron
catalyzed C-H aminations which most frequently employ organic
azides as substrates or reagents, require hypervalent iodine
oxidants, or need elaborate ligands for iron coordination, or a
combination thereof.[13,14] For example, with respect to azide
substrates, Betley[14f] and Che[14n] recently reported ring-closing
C(sp3)-H aminations of organic azides to pyrrolidines using an
iron-dipyrrinato complex and iron porphyrin complex with axially
coordinated N-heterocyclic carbene ligands, respectively. De
Bruin and van der Vlugt used an iron catalyst with a redox-active
pyridine-aminophenol ligand for the same conversion.[14m] Plietker
reported an intramolecular C(sp3)-H amination of alkylaryl azides
using a nucleophilic iron carbonyl nitroso complex.[14l] Beyond
azides, sulfamate esters have been used as substrates for
intramolecular C(sp3)-H aminations. For example, White[14e]
reported an iron-phthalocyanine catalyzed intramolecular C-H
amination of sulfonate esters and Che[14g] used an interesting
quinquepyridine iron catalyst. But both studies require PhI(OAc)2
as oxidant. In contrast, N-benzoyloxyurea have not been reported
for iron-catalyzed C-H aminations. They are easily synthesized
from primary and secondary amines in two steps and, as
demonstrated in this study, undergo C-H aminations under mild
“open-flask” reaction conditions using iron dichloride together with
1,10-phenanthroline as a simple and readily available catalyst.
While most reported C(sp3)-H amination reactions only provide
satisfactory results for aminations of C(sp3)-H bonds activated by
arenes, alkenes, alkynes, ether, or amines, our method is
applicable to both activated and non-activated C-H bonds.
We next turned our attention to C(sp3)-H bonds that are not
activated by a neighboring -system (Figure 2b). Such non
activated C(sp3)-H bonds are notoriously challenging due to their
increased C-H bond strength. To our delight, the amination of
secondary (32-37) and tertiary (38-42) aliphatic C(sp3)-H bonds
occurred efficiently in good to high yields of 68-95% with just two
exceptions (36 in 36% yield and 43 in 16% yield).
Site competition experiments shown in Figure 2c reveal that
product mixtures are obtained when more than one C-H group is
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Figure 2. Substrate scope. Standard reaction conditions: Substrate (0.20 mmol), FeCl2·4H2O (0.02 mmol), 1,10-phenanthroline (0.04 mmol), Na2CO3 (0.60 mmol),
MeCN (2 mL, 0.1M), 50 °C, 14 h. All yields refer to the isolated product after column chromatography. [a] Performed on 1 mmol scale.
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alkene (E)-24 (Figure 3a). This experiment supports a radical
mechanism through an intermediate allylic radical. A stepwise
hydrogen abstraction followed by radical rebound instead of a
concerted nitrene insertion is also supported by an experiment
that revealed loss of stereochemical information in the course of
the C-H amination of (S)-1c to rac-30 (Figure 3b). Finally,
determination of relative initial rates for the substrate 1a and its
deuterated analogue 1a' provided a kinetic isotope effect of 1.2,
revealing that the C-H cleavage is not the rate determining step
of the overall C-H bond amination process but probably rather the
N-O cleavage leading to the nitrenoid intermediate (Figure 3c). An
intramolecular competition experiment with substrate 1d provided
a significantly higher KIE of 2.2 as determined from the product
ratio 6':6'', which is consistent with a different rate for homolytic
C-H vs. C-D bond cleavage in a stepwise radical mechanism.
In conclusion, we here reported that iron dichloride in
combination with 1,10-phenanthroline catalyzes the ring-closing
C-H amination of N-benzoyloxyurea to smoothly form
imidazolidin-2-ones in high yields. The new method combines
several attractive features: First, the catalyst is very simple and
readily available without the need for an elaborate organic ligand
that needs to be synthesized in a multistep fashion. Second, the
reaction conditions are mild and not sensitive to moisture or air
(open flask conditions). Third, the C-H amination reaction is
surprisingly general and applicable to benzylic, allylic, propargylic,
and completely non-activated aliphatic C(sp3)-H bonds, and even
works for C(sp2)-H bonds. Finally, the formed imidazolidin-2-ones
are important motifs in bioactive compounds and precursors of
valuable vicinal diamines.[16] It is likely that this new method will
become widely applied as a new tool in organic synthesis.
Acknowledgements
L.J. is grateful for a postdoctoral fellowship from the Alexander
von Humboldt Foundation.
Keywords: iron • catalysis • nitrene • C‒H amination •
imidazolidin-2-ones
Figure 3. Proposed mechanism and supporting experiments. HAT = hydrogen
atom transfer.
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A proposed mechanism is shown in Figure 3. The active catalyst,
iron(II) bound to one or two phen ligands,[15] coordinates to the N-
benzoyloxyurea and upon base-induced deprotonation of the N-
H group and subsequent release of the benzoate leaving group,
an intermediate iron nitrenoid (I) is formed. This iron nitrenoid in
its triplet state undergoes a 1,5-hydrogen atom transfer (HAT) to
generate the radical intermediate II, followed by a rapid radical
rebound to establish a new C-N bond (III). Release of the product
then regenerates the iron catalyst for a new catalytic cycle. A
number of control experiments support this mechanism (see
Supporting information for more details). Using FeCl3·6H2O or
Fe(OTf)3 as the catalyst instead of FeCl2·4H2O provided
significantly reduced yields of 40% and 46%, respectively (Table
1, entries 11 and 12), reinforcing that the active catalyst under
standard conditions contains Fe in the oxidation state +2. Allylic
C-H amination the substrate (Z)-1b provided a product mixture in
which the majority of the double bond is isomerized to the E-
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[15] [Fe(phen)3]Cl2 is unstable in organic solvents and releases a phen ligand
to from [Fe(phen)2Cl2]. See: a.) K. Madeja, W. Wilke, S. Schmidt, Z. anorg. allg.
Chem. 1966, 346, 306-315; b.) R. D. Gillard, Inorg. Chim. Acta 1979, 37, 103-
105.
[16] See Supporting Information for converting imidazolidin-2-ones 9 and 42 to
their respective vicinal diamines under acidic conditions using microwave.
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10.1002/anie.202013687
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
L. Jarrige, Z. Zhou, M. Hemming and E.
Meggers*
Page No. – Page No.
Efficient Amination of Activated and
Non-Activated C(sp3)-H Bonds with
Simple Iron−Phenanthroline Catalyst
Simple, cheap and efficient: A readily available catalytic system composed of
iron(II) chloride and 1,10-phenanthroline is widely applicable for the ring-closing C-H
amination of N-benzoyloxyurea. High yields are provided both for activated and
completely non-activated C-H bonds.
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