Oxidation-induced C-H amination leads to a new avenue to build C-N bonds

Article abstract of DOI:10.1039/c7cc04955b

In this work, we develop an oxidation-induced C-H functionalization strategy, which not only leads to a new avenue to build C-N bonds, but also leads to different site-selectivity compared with "classic directing-groups". The high selectivity of our new catalytic system originates from the heterogeneous electron-density distribution of the radical cation species which are induced by single electron transfer between the aromatics and oxidant-Cu(ii) species.

Full text of DOI:10.1039/c7cc04955b

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Oxidation-induced C-H amination leads new avenue
to build C-N bonds
Cite this: DOI: 10.1039/x0xx00000x
Hong Yi,a Zilu Tang,a Changliang Bian,a Hong Chen,a Xiaotian Qi,c Xiaoyu Yue,c Yu
Lan,c JyhꢀFu Leed and Aiwen Lei *a,b
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this work, we developed an oxidation-induced C-H selective CꢀH activation, drawbacks still remain. For example,
functionalization strategy, which leads not only a new avenue installation and elimination of directingꢀgroup increased at least two
to build C-N bonds, but also leads different site-selectivity synthetic processes, which limit its application in synthetic industry.
compared with “classic-directing-groups”. Using readily Therefore, it is a great demand to develop new and efficient
available CuBr as the catalyst precursor and (NH4)2S2O8 as strategies which are able to functionalize specific CꢀH bond.
the oxidant, a series of aromatics and azoles could be coupled
in selective-manner up to 92% yield. The high selectivity of
our new catalytic system originated from the heterogeneous
electron-density distribution of the radical cation species
which were induced by single electron transfer between
aromatics and oxidant-Cu(II) species.
During the past several decades, transition metal catalyzed CꢀH
activation/functionalization has brought a revolution in the field of
chemical transformations.1 Due to the broad existence of CꢀH bonds
in organic molecules, it is undoubtedly an optimal pathway to
accomplish the conversion of CꢀH bond to CꢀC/CꢀX bond directly.
However, how to differentiate diverse CꢀH bonds that are present in
complex organic molecules is a great challenge. In another word, the
Scheme 1. Three CꢀH activation/functionalization strategies. (A) Directingꢀ
siteꢀselectivity in CꢀH activation is the crucial problem to be solved
group strategy for CꢀH activation: CꢀM species serve as the key intermediate.
in this field.
(B) Radical Csp3ꢀH activation: the cation species serve as the key
Employing the nobleꢀtransition metals such as ruthenium2,
intermediate. (C) Oxidationꢀinduced Csp2ꢀH activation: the radical cation
rhodium3, palladium4 and platinum5, diverse kinds of directingꢀ
species serve as the key intermediate.
groups including alkenyl, pyridine, amide, cyan, hydroxyl etc. have
been developed to distinguish different CꢀH bonds in complicated
structures6. Applying this directingꢀgroup strategy, explosively
achievements have been made during the past several decades for the
transformation of CꢀH bonds to CꢀC/CꢀX bonds.7 In addition, siteꢀ
selective CꢀH activation/functionalization induced by directingꢀ
group has shown great application value in the synthesis of complex
products.8 From the aspect of mechanism, CꢀM (Pd, Rh, Ru etc)
species are always believed to be formed, serving as the essential
intermediate (Scheme 1A), and the reductive elimination usually
serves as the pivotal step for the final bond formation in these
reactions.9 Despite the great success of directingꢀgroup induced siteꢀ
Along with the development of transition metal catalyzed CꢀH
activation reactions, more and more firstꢀrow transition metal (such
as Mn10, Cu11, Co12, Fe13 etc) salts have been employed as the
catalyst. Different from the nobleꢀtransition metal catalyzed
reactions which usually underwent twoꢀelectron transfer process,
singleꢀelectron transfer (SET) process was involved mostly in the
firstꢀrow transition metal catalyzed reactions.14 In addition, most of
these reactions employed Csp3ꢀH bonds which were usually adjacent
to a heteroatom (Scheme 1B). In this model, the CꢀH bond was
activated via electron transfer process, in which radical species
(including radical ions and free radical) played the crucial role in the
reactions. This radical model provides an alternative route for CꢀH
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DOI: 10.1039/C7CC04955B
bond functionalization, especially for inert Csp3ꢀH activation15. We
envisioned if an aromatic CꢀH could transfer one electron to an
oxidant, forming a radical cation, which could be attacked by a
nucleophile (Scheme 1C). The electron distribution of this radical
cations in the aromatic ring might be varied, which would furnish
unique selectivity for aromatic CꢀH functionalization. Photochemical
and electrochemical oxidation has been proved to be the efficient
routes for generating the radical cation species.16 Herein, employing
cheap copper as catalyst, we developed a new method which
produced the radical cation species through chemical oxidation and
realized the construction of aryl CꢀN bond in a selective manner.
Arylamines are privileged structures in materials and biologyꢀ
oriented functional molecules. The high binding ability of newly
introduced amino moieties to metal centers plays an essential role in
coordination chemistry.17 Therefore, the development of efficient
and atomꢀeconomy methods for the construction of aromatic CꢀN
bonds have been recognized as one of the central issues in synthetic
chemistry community.18 To probe the feasibility of our strategy, we
chose naphthalene 1a and benzotriazole 2a as model substrates and
screened a series of copperꢀbased catalytic systems. As our
expectation, copper(II) was an initiator of the oxidation process, we
chose some common copper(II) as the catalyst precursor to test our
proposal initially. Interestingly, best result was achieved when
cuprous bromide was employed as the catalyst (Table S1, entry 6).
Screening of oxidants and solvents showed that using (NH4)2S2O8 as
oxidant and MeCN/H2O as mixture solvent could get the best yield
(Table S1, entry 8). Nevertheless, no product was detected when the
reaction was operated at room temperature (Table S1, entry 18).
Control experiments indicated that oxidant was essential to this
transformation and copper catalyst played a role as a promoter.
Conditions: 0.5 mmol aromatics, 2.0 mmol azoles, 20 mol% CuBr and 2
equiv (NH4)2S2O8 were stirred in MeCN/H2O for 12 h at 80 oC. a 24 h, b 28 h,
c Cu(OTf)2 instead of CuBr.
With the optimized conditions established, we turned our attention
to explore substrate scope. We firstly evaluated different aromatics
employing benzotriazole as coꢀpartner which usually used as
corrosion inhibitor of copper and copper alloys.19 As shown in
Scheme 2, different electronꢀrich/electronꢀneutral aromatics were
compatible to afford corresponding CꢀN coupling products in good
to excellent yields under our catalytic system. Simple πꢀsystem
aromatics including naphthalene and phenanthrene reacted well,
affording single regioisomers (Scheme 2, 3a, 3b). For other electronꢀ
rich aromatics, methoxybenzene and methoxynaphthalene
derivatives gave the corresponding C−N coupling products in
moderate to good yields (3c-3f, 3h-3j). It is worth mentioning that
halogen atoms which could be further functionalized were tolerated
(3e-3f). In particular, 1,3ꢀdimethoxybenzene (3d) and orthoꢀ
halogenated anisoles (3e, 3f) gave single regioisomers. The
heterocycles bearing electron releasing substitution were competent
substrates, 2,6ꢀdimeoxypyridine showed excellent reactivity under
our conditions, giving siteꢀselective CꢀN coupling product in 66%
yield (3k). In addition, Csp2ꢀH was functionalized selectively to
afford arylamine for mesitylene, which contained Csp3ꢀH in methyl
group (3g). The low yields of some products were mainly due to the
low conversation of the substrates
Scheme 3. General substrate scope of azoles for oxidationꢀinduced selective
CꢀH functionalization
Scheme 2. General substrate scope of aromatics for oxidationꢀinduced
selective CꢀH functionalization.
Conditions: 0.5 mmol aromatics, 2.0 mmol azoles, 20 mol% CuBr and 2
a
equiv (NH4)2S2O8 was stirred in MeCN/H2O for 12 h at 80 oC. 2 equiv
K2CO3 was added, b 24 h.
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DOI: 1C0.O10M39M/CU7CNCI0C4A95T5IOB N
It is of great significance to develop efficient methods to the to confirm this singleꢀelectron transfer process, an EPR silent species
synthesis of functionalized azoles because they are important was formed when mixing CuBr2 and 1j in MeCN for 1h (Figure S2).
structural moieties in many kinds of pharmacologically active The EXAFS and EPR results did show that the aromatic substrates
compounds and usually act as directingꢀgroups in transition metal could go through singleꢀelectron transfer with Cu(II) species.
catalyzed reactions.20 As shown in Scheme 3, methyl group were
well tolerated for benzotriazole to afford the corresponding molecule
1.2
in 71% yield (4a). In addition to benzotriazole, 1,2,3ꢀtriazol was
good reaction partner under our conditions (4a, 4b). Apart from
triazole derivatives, tetrazole was also compatible with our CꢀN
0.8
coupling strategy (4c). Notably, tetrazole was a skeleton of cilostazol
which was used as an antithrombotic drug.21 Moreover, a diverse
0.4
range of pyrazoles were employed as nucleophiles which involved in
our conversion as good reaction partners(4d-4h). Halogen groups
which can provide a further functionalized site in pyrazoles were
viable to produce the amination structures (4g, 4h). It is worth noting
0.0
8970
8980
8990
9000
9010
that our method can be used to the modification of caffeine (4i).
To further confirm our designed strategy, two equivalents 2,2,6,6ꢀ
tetramethlpiperidinooxy (TEMPO) was added to the model reaction
system and the reaction was totally inhibited, which implied a
radical mechanism was involved (Scheme 4A). In addition, based on
our strategy, we proposed the singleꢀelectron transfer process instead
of cleavage of CꢀH bond may be involved in the rateꢀdetermining
step. Intermolecular KIE experiment showed a 1.07 kinetic isotope
coefficient, which was in accordance with our proposal (Scheme 4B).
In addition, for the excellent biological activities such as antiꢀHIV
activity, antimicrobial activity and antiꢀinflammatory of 1, 2, 3ꢀ
triazoles,22 we synthesized two phenylꢀsubstituted 1, 2, 3ꢀtriazoles
and afforded the CꢀN adducts successfully in 43% yield using our
method, which confirmed the application potential of our CꢀN
coupling strategy (Scheme 4C).
Figure 1. XANES evidence for single electron transfer between aromatics
and Cu(II) species.
To get further insights into the selectivity, we chose some electronꢀ
neutral/electronꢀrich aromatics to test our proposal using theoretical
calculation method firstly. Density functional theory (DFT)
calculation of their radical cations of four aromatic compounds 1 (1a,
1d, 1h and 1k) were performed and the results were illustrated in
Figure 2. It was shown that cation and radical distribution of arenes
did have their selectivities.24 The calculated Mulliken atomic spin
densities (ASD) of these radical cations showed the distributions of
spin density on each carbon atom when radical cations formed. The
combination of frontier molecular orbital (FMO) analysis with
Mulliken ASD analysis revealed that the most active Cꢀcenter of 1
(1a, 1d, 1h and 1k) was C1, C4(C6), C4 and C2(C4), respectively.
The theoretical study could provide us an opportunity to realize the
selective Csp2ꢀH functionalization via radical CꢀH activation (Figure
2 and S1).
Figure 2. Lowest unoccupied molecular orbitals (βꢀLUMOs) and Mulliken
atomic spin densities (ASD) of formed radical cation species of aromatics.
Scheme 4. (A) Radical inhibition experiment. (B) Intermolecular KIE
In summary, through an in-situ generated radical cation species
whose electron density distribution changed dramatically relative to
aromatic molecules, here we reported a new method which realized
regioselective aromatic Csp2ꢀH activation amination reaction
utilizing oxidationꢀinduced CꢀH functionalization strategy. A variety
of simple aromatics were compatible in our catalytic system and
different azoles were employed as reaction partners successfully.
Spectroscopy methods and calculation results verified our protocol.
In addition, intermolecular KIE experiment and radical inhibition
experiment provided further proof of the mechanism. Moreover,
successful modification of caffeine and synthesis of 1,2,3ꢀtriazole
experiment. (C) Application in synthesis of natural product skeleton.
Furthermore, EXAFS experiments of the aromatics 1d, 1j and 1m
with CuBr2 were performed and demonstrated the feasibility of
oxidantionꢀinduced CꢀH functionalization. As shown in Figure 1, the
EXAFS spectrum of the CuBr2 solution gave a preꢀedge at 8977.0
eV (Figure 1, dark gray line). After the addition of different
aromatics (including 1d, 1h and 1k) at 80 oC, a new Cu species with
the edge energy of 8981.6 eV was observed. This edge energy is
close to reported Cu(I) sample.23 EPR spectroscopy was also utilized
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DOI: 10.1039/C7CC04955B
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This work was supported by the National Natural Science
Foundation of China (21390400, 21520102003, 21272180,
21302148), the Natural Science Foundation of Hubei Province
(2016CFB571). The Program of Introducing Talents of
Discipline to Universities of China (111 Program) is also
appreciated. The Xꢀray absorption spectroscopy studies were
carried out at the beamline 17C1 of the National Synchrotron
Radiation Research Center (NSRRC) in Taiwan. We also thank
all the team members at BL14W1 of Shanghai Synchrotron
Radiation Facilities (SSRF).
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