Iron-Catalyzed Radical Asymmetric Aminoazidation and Diazidation of Styrenes

Article abstract of DOI:10.1002/anie.202017175

Asymmetric aminoazidation and diazidation of alkenes are straightforward strategies to build value-added chiral nitrogen-containing compounds from feedstock chemicals. They provide direct access to chiral organoazides and complement enantioselective diamination. Despite the advances in non-asymmetric reactions, asymmetric aminoazidation or diazidation based on acyclic systems has not been previously reported. Here we describe the iron-catalyzed intermolecular asymmetric aminoazidation and diazidation of styrenes. The method is practically useful and requires relatively low loading of catalyst and chiral ligand. With mild reaction conditions, the reaction can be completed on a 20 mmol scale. Studies of the mechanism suggest that the reaction proceeds via a radical pathway and involves stereocontrol of an acyclic free radical which probably takes place through a group transfer mechanism.

Full text of DOI:10.1002/anie.202017175

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How to cite:
International Edition: doi.org/10.1002/anie.202017175
German Edition: doi.org/10.1002/ange.202017175
Radical Group Transfer
Iron-Catalyzed Radical Asymmetric Aminoazidation and Diazidation
of Styrenes
Daqi Lv, Qiao Sun, Huan Zhou, Liang Ge, Yanjie Qu, Taian Li, Xiaoxu Ma, Yajun Li, and
Hongli Bao*
In memory of Professor Kilian MuÇiz
Abstract: Asymmetric aminoazidation and diazidation of
alkenes are straightforward strategies to build value-added
chiral nitrogen-containing compounds from feedstock chem-
icals. They provide direct access to chiral organoazides and
complement enantioselective diamination. Despite the advan-
ces in non-asymmetric reactions, asymmetric aminoazidation
or diazidation based on acyclic systems has not been previ-
ously reported. Here we describe the iron-catalyzed intermo-
lecular asymmetric aminoazidation and diazidation of styr-
enes. The method is practically useful and requires relatively
low loading of catalyst and chiral ligand. With mild reaction
conditions, the reaction can be completed on a 20 mmol scale.
Studies of the mechanism suggest that the reaction proceeds via
a radical pathway and involves stereocontrol of an acyclic free
radical which probably takes place through a group transfer
mechanism.
Introduction
Enantioenriched, vicinal diamines are found in many
natural products, drugs, and other biologically active mole-
cules, and are also privileged chiral ligands widely used in
asymmetric synthesis and catalysis.[1] While C C bond
À
formation involving reductive coupling of aldimines[2] and/
or imines[3] is effective for the synthesis of chiral diamines, the
catalytic asymmetric diamination of alkenes or dienes is the
Scheme 1. State of the art in catalytic asymmetric diamination and
aminoazidation, and this work.
most straightforward and convenient approach to production
of these structures (Scheme 1). Shi,[1d,4] Du,[5] Gong,[6] MuÇiz[7]
and Denmark[8] have established powerful palladium-, ary-
liodine- and organoselenium-catalyzed intermolecular diami-
nation of alkenes or dienes, respectively. Michael,[9] Chem-
ler,[10] Zeng[11] and Liu[12] developed palladium or copper
catalyzed inter/intramolecular diamination with one external
amination reagent and the other derived from alkene
substrates, producing useful chiral indolines and pyrrolidines.
Aminoazidation and diazidation of alkenes are also
promising strategies because they not only provide methods
complementary to diamination, but also have additional
potential due to the rich chemistry of the azido group.
Organoazides are familiar as versatile reactants in numerous
reactions including the Curtius rearrangement,[13] Regitz
diazo transfer,[14] Staudinger ligation,[15] Aza-Wittig reac-
tion,[16] Boyer rearrangement,[17] Huisgen “click” reaction[18]
and many other reactions.[15a,19] The azide group is an
important synthon and is also found in many drugs and
bioactive molecules.[15b,20] With the significance of organic
azides and diamines, there is continuing interest in the
development of aminoazidation[21] and diazidation[22] of
alkenes and many novel methods have been developed.
In spite of the advances in related non-asymmetric
reactions,[21a–k,23] enantioselective aminoazidation and diazi-
[*] D. Lv, Dr. Q. Sun, Dr. H. Zhou, L. Ge, Y. Qu, T. Li, X. Ma, Dr. Y. Li,
Prof. H. Bao
Key Laboratory of Coal to Ethylene Glycol and Its Related Technology,
State Key Laboratory of Structural Chemistry, Center for Excellence in
Molecular Synthesis, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences
155 Yangqiao Road West, Fuzhou, Fujian 350002 (P. R. China)
E-mail: hlbao@fjirsm.ac.cn
Prof. H. Bao
University of Chinese Academy of Sciences
Beijing 100049 (P. R. China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202017175.
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Table 1: Reaction optimization using styrene as a substrate.[a]
dation are underdeveloped. There is only one example of
a successful enantioselective aminoazidation, an intra/ inter-
molecular aminoazidation leading to structurally diverse
substituted piperidines (Scheme 1c).[24] There is a need for
asymmetric aminoazidation/diazidation reactions but those
based on the intermolecular mode on an acyclic system
remain a challenge. Previous studies on azidation reactions
showed that the metal-catalyzed radical azidation often
involves an intermolecular group transfer mechanism.[25]
Our studies on the mechanism of iron-catalyzed carboazida-
tion of alkenes also have suggested an azido group transfer
pathway.[26] In general, covalent, dative, ionic and hydrogen
bonds between a chiral metal catalyst and a radical reaction
partner in a metal-catalyzed intermolecular group transfer
reaction are usually absent. Consequently, interactions be-
tween these two species tend to be inefficient. Stereocontrol
of untethered radicals in a group transfer radical reaction is
therefore highly challenging and successful examples of such
reactions are uncommon (vide infra).[27] The enantioselective
Kharasch addition reaction reported by Ready et al.[27a] is
probably the first successful example of intermolecular
asymmetric radical group transfer reaction dealing with
Entry
Ligand
Solvent
Yield [%][b]
er[c]
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
L4
L5
Et2O
Et2O
Et2O
Et2O
Et2O
DME
THF
PhMe
CH3CN
DCE
DCM
CHCl3
CCl4
8
29
0
55
4
48
trace
16
5
25
46
96
66
90
14
98
96
64:36
80.5:19.5
–
91.5:8.5
91:9
89:11
–
89:11
65.5:34.5
82:18
85.5:14.5
87:13
88:12
91:9
90.5:9.5
91:9
93:7
9
10
11
12
13
14[d]
15[e]
16[d,f]
17[d,g]
untethered radicals. Zhang et al. recently reported the elegant
[27d]
À
enantioselective intermolecular radical C H amination,
and we[27c] and Liu[27e] independently developed asymmetric
carboazidation of styrenes and acrylamides. Despite these
recent breakthroughs, asymmetric aminoazidation and dia-
zidation remain important unsolved problems. Herein, we
report our results of the iron-catalyzed radical asymmetric
aminoazidation and diazidation of styrenes.
CHCl3
CHCl3
CHCl3
CHCl3
[a] The reaction was performed with styrene (0.1 mmol), NFSI
(0.25 mmol), TMSN3 (0.25 mmol), Fe(OTf)2 (5 mol%), ligand
(7.5 mol%), solvent (2 mL), 608C, 5 h. [b] Yield of isolated product.
[c] Determined by HPLC analysis on a chiral stationary phase. [d] rt, 48 h.
[e] 108C, 48 h. [f] Fe(OTf)2 (1 mol%), L4 (1.2 mol%). [g] Fe(OTf)2
(1 mol%), L5 (1.2 mol%).
Results and Discussion
Our study began with screening of the conditions of the
Fe-catalyzed asymmetric aminoazidation of styrene (1a)
using N-fluorobenzenesulfonimide (NFSI) as the N-radical
precursor, trimethylsilyl azide (TMSN3) as the N3 source and
a ligand. An initial trial with 5 mol% Fe(OTf)2 and 7.5 mol%
of a ligand (L1) in Et2O at 608C for 5 h afforded the desired
aminoazidation product (2) in 8% yield with a 64:36 enan-
tiomeric ratio (er) (Table 1, entry 1). The ligand (L1) contains
a large planar aromatic system that could support interactions
with p electrons, and two phenyl groups that may protect the
radicals from non-selective azidation. Bulkier ligands (L2–
L5) were designed. Ligand L2, with four phenyl groups was
found to deliver the product in 29% yield and 80.5:19.5 er
(entry 2). Ligand L4 which has a t-Bu group at the para-
position of each of the phenyl groups in L2 was found to
increase the yield of the product (2) to 55% and the er to
91.5:8.5 (entry 4). With a more sterically-hindered ligand
(L5), the yield fell to 4% and the er slightly decreased to 91:9
(entry 5). Several Box ligands (L6–L10) were also examined,
and these reactions gave lower yields and enantioselectivities
(see Table S1† in Supplementary Information—SI). A series
of metal catalysts including various iron compounds were
tested, and Fe(OTf)2 was found to be the best catalyst (see SI,
Table S2†). Solvent effects were investigated (entries 6–13),
and it was found that use of CHCl3 rather than Et2O
significantly improved the yield to 96% albeit with a de-
creased er of 87:13 (entry 12). Upon reducing the temperature
from 608C to room temperature (rt) and extending the
reaction time to 48 h, the product was obtained in a yield of
90% and with 91:9 er (entry 14). Further lowering the
temperature failed to improve the reaction yield (entry 15).
By reducing the catalyst and ligand loading to 1 mol% and
1.2 mol%, respectively (entry 16), the yield could be further
increased to 98% without affecting the enantioselectivity.
Using the conditions in entry 16 and replacing L4 with L5
afforded the product in 96% yield with 93:7 er (entry 17).
With the optimized conditions in hand, we explored the
scope of the aminoazidation reaction and obtained the results
shown in Scheme 2. Styrenes with one or more substituents on
the phenyl groups were examined and were found to provide
the corresponding products (3–32) with good yields and er.
Most of the reactions were performed with L4 or L5, and it
was found that L4 afforded products with better enantiose-
lectivity in most cases with the exception of reactions forming
products 5, 16 and 32.
Electron-donating groups such as methoxyl, methyl,
isopropyl or n-octyl, and electron-withdrawing groups, in-
cluding F, Cl, Br, CF3, or CHF2 are all tolerated. When vinyl
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Scheme 2. Scope of iron-catalyzed intermolecular asymmetric aminoazidation and diazidation of styrenes.[a,b] [a] The aminoazidation reactions
were performed with styrene (0.1 mmol), NFSI (2.5 equiv), TMSN3 (2.5 equiv), Fe(OTf)2 (1 mol%), L4/L5 (1.2 mol%), CHCl3 (2 mL), rt, 48 h. The
diazidation reaction was performed with alkene (0.2 mmol), alkyl perester 36 (3.5 equiv),TMSN3 (3.5 equiv), Fe(OTf)2 (5 mol%), L4 (6 mol%),
CHCl3 (2 mL), rt, 72 h. [b] Yields of isolated products. [c] Olefin (0.1 mmol), NFSI (5.0 equiv), TMSN3 (5.0 equiv), Fe(OTf)2 (2 mol%), L4
(2.4 mol%), CHCl3 (4 mL), rt, 48 h. [d] Toluene as solvent.
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and allyl/alkynyl groups are present in the same molecule the
of styrene afforded the corresponding product (50) in
aminoazidation occurs selectively at the vinyl group, afford-
ing products 25 and 26. 2-Vinylbenzo[b]thiophene was also
compatible with the reaction and afforded the desired
product (33) in 90% yield and 88:12 er. By doubling the
amount of reagents and the catalytic loading, 1,3-divinylben-
zene and 1-bromo-3,5-divinylbenzene underwent double
reactions affording the products 34 and 35, respectively, with
moderate diastereoselective ratios and high er. The absolute
stereochemistry of products 8, 17 and 24 has been confirmed
as R by single crystal X-ray crystallography. It should be noted
that the results of this reaction when performed on a scale of
up to 20 mmol with 0.75 mol% of iron catalyst remained
excellent.
Diazidation of alkenes is a powerful strategy with which to
incorporate two azido groups. Asymmetric diazidation how-
ever has not been reported. We explored the diazidation of
styrenes with this catalytic system and found the chiral iron
catalyst is suitable for disubstituted and trisubstituted styr-
enes and produces the otherwise inaccessible chiral diazida-
tion products. With trisubstituted styrenes, products 37–49
with up to 97:3 er were obtained. Products 51 and 52, which
contain two asymmetric centers were produced from unsym-
metric styrenes, each isomer with a moderate er. Diazide
compounds 53–56 were delivered from symmetrical styrenes.
The chiral products delivered er as high as 97:3. Diazidation
moderate yield but with low er.
To demonstrate the synthetic utility of the reaction,
different transformations of the aminoazidation product (8)
were performed (Scheme 3). Click reactions of 8 with
terminal[21d,28] or internal alkynes,[29] a benzyne precursor,[30]
or acetylacetone[31] afforded the corresponding triazoles (57–
62) with good yields and without loss of enantioselectivity.
The chiral azide (8) was reduced in 79% yield to the
corresponding chiral primary amine (63) by refluxing the
chlorophenyl compound (8) with indium powder in MeOH.[32]
Compound 8 could be further reduced with P(OMe)3 to
afford a chiral phosphoramide (64) in 81% yield.[21f] Besides
azide transformations, compound 8 undergoes mono-desul-
fonylation upon treatment with n-Bu4NF[33] to afford a prod-
uct (65) in a quantitative yield. Further reduction of 65 with
indium gave the monoprotected 1,2-diamine (66) in 62%
yield.[32] One-pot propargylation/intramolecular [3+2] cyclo-
addition of compound 66 afforded a novel triazolopyrazine
derivative (67).[34]
To gain insight into the radical nature and mechanism of
the reaction, control experiments were performed. In radical
trap experiments, the reaction was significantly inhibited by
the addition of TEMPO (2,2,6,6-tetramethylpiperidine-1-
oxyl), and the desired product (8) was formed in only 24%
yield (Scheme 4a). An aminooxygenated product (68) was
Scheme 3. Synthetic applications of the aminoazidation products. Reaction conditions: a) 8 (0.2 mmol), aryl alkyne (2.0 equiv), CuSO4·5H2O
(20 mol%), Na-ascorbate (0.4 equiv), t-BuOH/H2O/DCM (2:1:0.2), rt, 24 h; b) 8 (0.2 mmol), but-3-yn-2-one (1.2 equiv), Cu(OAc)2 (10 mol%), 2-
aminophenol (5 mol%), DCM/H2O (1:1), rt, 18 h; c) 8 (0.2 mmol), DMAD (1.1 equiv), t-BuOH/H2O/DCM (2:1:0.25), 708C, 18 h; d) 8 (0.2 mmol),
2-(trimethylsilyl)phenyl trifluoromethanesulfonate (2.0 equiv), CsF (2.0 equiv), CH3CN, rt, 4 h; e) 8 (0.2 mmol), acetylacetone (2.0 equiv), K2CO3
(0.2 equiv), DMF, 408C, 17 h; f) 8 (0.2 mmol), In powder (1.5 equiv), NH4Cl (1.5 equiv), MeOH, reflux, 5 h; g) 8 (0.2 mmol), P(OMe)3 (1.5 equiv),
PhMe, 808C, 12 h; h) 8 (0.5 mmol), TBAF (1.1 equiv, 1 M in THF), THF, 708C, 1.5 h; i) 8 (0.5 mmol), propargyl bromide (1.1 equiv), K2CO3
(2.0 equiv), CH3CN, 608C, 4 h. DMAD=dimethyl acetylenedicarboxylate.
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Scheme 4. Mechanistic studies.
Scheme 5. Proposed mechanism.
isolated from this reaction in 14% yield. This suggests the
existence of a benzyl radical, generated from the addition of
a disulfonimidyl radical to the alkene. The radical inhibitor
BHT (2,6-di-tert-butyl-4-methylphenol) was found to com-
afford the final product regenerating the active catalyst
species A.[26,27c]
pletely inhibit the aminoazidation reaction (Scheme 4b). The Conclusion
addition of 1,1-diphenylethylene severely suppressed the
reaction, the yield of the desired product (8) decreasing to
40% (Scheme 4c). A radical clock experiment with (2-
phenylcyclopropyl)styrene (69) was conducted and the ring-
opened product (70) was formed in 32% yield with no
aminoazidation product (71) (Scheme 4d). These results are
consistent and suggest that the reaction proceeds via a radical
pathway and that a benzyl radical may be an intermediate.
To identify the catalytically active species in this reaction,
two complexes, L2-FeIIOTf2·THF and L2-FeIIOTf2·CH3CN
were synthesized and their structures were established by X-
ray crystallography (Scheme 4e). The complexes catalyze the
model reaction in terms of both the yield and enantioselec-
tivity. This suggests that an L-FeIIOTf2 complex may serve as
the catalytically active species. Based on the above studies of
the mechanism, a catalytic cycle is proposed in Scheme 5 for
the iron-catalyzed asymmetric aminoazidation of styrenes.
Coordination of the iron catalyst with the ligand affords an
iron(II) complex (A), which undergoes ligand exchange with
TMSN3 to afford an iron(II) azide complex (B). A single
electron transfer (SET) process between NFSI and B affords
an iron(III) azide species (C) and a bis-sulfonylamidyl radical
(D). Alternatively, the SET process could proceed prior to the
ligand exchange with TMSN3. Complex A participates in
a SET with NFSI to afford an oxidized iron species (E) and
a radical (D) and E reacts with TMSN3 to generate the
iron(III) azide species (C). Addition of the bis-sulfonylamidyl
radical (D) to the styrene gives a benzyl radical (F), which can
participate in a group transfer mechanism with species C to
We are reporting the first iron-catalyzed radical asym-
metric aminoazidation and diazidation of styrenes. The
reaction proceeds under mild reaction conditions, with low
catalyst loading and broad styrene scope. A range of chiral
aminative organoazides and diazides have been synthesized,
and can be readily further transformed into various useful
nitrogen-containing compounds. Studies on the mechanism of
the reaction suggest the involvement of a benzyl radical, and
a radical pathway has been proposed for the transformation.
Acknowledgements
We thank Professor Daqiang Yuan from our institute for X-
ray crystallography analysis. This work was supported by the
National Key R&D Program of China (Grant No.
2017YFA0700103), the NSFC (Grant Nos. 21871258,
21922112, and 22001251), the Strategic Priority Research
Program of the Chinese Academy of Sciences (Grant No.
XDB20000000), Scientific Research Foundation of Fujian
Normal University in China and Innovative Research Teams
Program II of Fujian Normal University in China
(IRTL1703).
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1793; e) C. E. Hendrick, K. J. Bitting, S. Cho, Q. Wang, J. Am.
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The authors declare no conflict of interest.
Keywords: aminoazidation · asymmetric catalysis ·
iron catalysis · radical group transfer
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Manuscript received: December 26, 2020
Accepted manuscript online: March 22, 2021
Version of record online: && &&
,
&&&&
Angew. Chem. Int. Ed. 2021, 60, 2 – 8
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Radical Group Transfer
D. Lv, Q. Sun, H. Zhou, L. Ge, Y. Qu, T. Li,
X. Ma, Y. Li, H. Bao*
&&&& — &&&&
Iron-Catalyzed Radical Asymmetric
Aminoazidation and Diazidation of
Styrenes
Biologically accessible hydroxylation in
nature often proceeds through a radical
rebound process. However, such a radical
rebound usually leads to non-stereose-
lectivity. Rendering the radical rebound
enantioselective is quite challenging.
Herein, the first radical asymmetric ami-
noazidation and diazidation on acyclic
systems via radical rebound is reported.
&&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 8
These are not the final page numbers!