Rhodium(II)-Catalyzed Synthesis of N-Aryl-N′-tosyldiazenes from Primary Aromatic Amines Using (Tosylimino)aryliodinane: A Potent Stable Surrogate for Diazonium Salts

Article abstract of DOI:10.1002/ejoc.201601627

A single-step synthesis of N-aryl-N′-tosyldiazenes from primary aromatic amines was developed. A wide range of aromatic amines provided the corresponding products in high yields by N–N bond formation with Rh^II–nitrene intermediates, which were generated in situ from dirhodium(II) complex catalysts and (tosylimino)-2,4,6-trimethylphenyliodinane, followed by oxidation. The synthesized N-aryl-N′-tosyldiazenes were successfully demonstrated to serve as potent and stable surrogates for diazonium salts in the two-step deaminative transformation of primary aromatic amines.

Full text of DOI:10.1002/ejoc.201601627

A Journal of
Accepted Article
Title: Rhodium(II)-Catalyzed Synthesis of N-Aryl-N'-Tosyldiazenes
from Primary Aromatic Amines Using (Tosylimino)aryliodinane: A
Potent Stable Surrogate for Diazonium Salts
Authors: Motoki Ito, Arisa Tanaka, Kazuhiro Higuchi, and Shigeo
Sugiyama
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601627
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10.1002/ejoc.201601627
European Journal of Organic Chemistry
COMMUNICATION
Rhodium(II)-Catalyzed Synthesis of N-Aryl-N’-Tosyldiazenes from
Primary Aromatic Amines Using (Tosylimino)aryliodinane: A
Potent Stable Surrogate for Diazonium Salts
Motoki Ito,* Arisa Tanaka, Kazuhiro Higuchi, and Shigeo Sugiyama*[a]
Abstract: The first example of the single-step synthesis of N-aryl-N’-
tosyldiazenes from primary aromatic amines is described. A wide
range of aromatic amines provided the corresponding products in high
yields via an N−N bond formation with Rh(II)-nitrene intermediates,
generated in situ from dirhodium(II) complex catalysts and
(tosylimino)-2,4,6-trimethylphenyliodinane (TsN=IMes), followed by
oxidation. The synthesized N-aryl-N’-tosyldiazene was successfully
demonstrated to serve as a potent stable surrogate for diazonium
salts in two-step deaminative transformations of primary aromatic
amine.
with arylboronic acids or acrylates proceeded via the intermediacy
of diazenes 1 generated by the oxidation of hydrazines (Scheme
1, c and d).[5,6] Furthermore, in 2016, Fagnoni and Protti reported
the visible-light-induced arylation of 1 (R’ = Me) without the use of
a photosensitizer or photocatalyst in the presence of an excess
amount of aromatic compounds (Scheme 1, e).[7]
Aryl diazonium salts represent unrivaled intermediates in a wide
range of aromatic C−N bond transformations including the
Sandmeyer,
Gomberg-Bachmann,
and
Balz-Schiemann
reactions since their discovery in the late 19th century.[1a,c,f]
Despite their instability and explosibility, the synthetic importance
of diazonium salts has further increased during the past four
decades with the development of transition metal-catalyzed C−N
bond transformations,[1b,d,e] pioneered by Kikukawa and
Matsuda.[2] Consequently, it is not surprising that a great deal of
effort has been devoted to use diazonium salts safely and
remarkable progress has recently been achieved by exploiting
one-pot diazotization/coupling protocols or flow reactor, as well as
the development of storable salts (tetrafluoroborate,
hexafluorophosphate, and tosylate).[1e] However, there still
remains a need for the development of a surrogate for diazonium
salts in terms of safety, handling, and accessibility from primary
aromatic amines.[3]
N-Aryl-N’-sulfonyldiazenes 1, known as stable azo
compounds, have been paid little attention as substrates for C−N
bond transformation until recently,[4−8] while it has been reported
that 1 can act as precursors of aryl radicals or cations similar to
diazonium salts.[4] For example, diazenes 1 are known to provide
protodeazosulfonylation and iododeazosulfonylation products
upon heating in DMF and treatment with potassium iodide in the
presence of 18-crown-6, respectively (Scheme 1, a and b).[4f]
Recently, the potent synthetic utility of diazenes 1 has been
demonstrated by the work of Lu and Liu, in which the
palladium(II)-catalyzed coupling of N-aryl-N’-sulfonylhydrazines
Scheme 1. C−N Bond transformations of N-aryl-N'-sulfonyldiazenes 1
On the other hand, the synthesis of diazenes 1 starting from
primary aromatic amines generally requires multiple steps
including a diazotization.[9] Hence, despite their growing synthetic
importance, the lack of methods for the single-step synthesis of
diazenes 1 has precluded their utilization as a surrogate for
diazonium salts. In this context, we herein report the efficient
synthesis of N-aryl-N’-tosyldiazenes 1 (R’ = p-Tol) with a
combinational use of dirhodium(II) complex catalysts and
(tosylimino)aryliodinanes. This represents the first example of a
single-step synthesis of 1 from primary aromatic amines 2.
Electrophilic Rh(II)-nitrene species generated from
dirhodium(II) complexes and (N-arylsulfonylimino)aryliodinanes
are presumed to be intermediates in the inter- and intramolecular
C−H amination of both aliphatic and aromatic compounds as well
as the aziridination of alkenes.[10] In contrast, X−H (X =
heteroatom) insertion reactions of metal-nitrene have remained
unexplored.[11] However, based on the analogy between the
reactivity of nitrenes and carbenes,[12] we envisioned that the
reaction between primary aromatic amines 2 and a Rh(II)-nitrene
should provide N-aryl-N’-sulfonyldiazenes 1 via the N−H insertion
of nitrene followed by oxidation (Scheme 2).[13,14]
[a]
Dr. M. Ito, A. Tanaka, Dr. K. Higuchi, Prof. Dr. S. Sugiyama
Meiji Pharmaceutical University
2-522-1 Noshio Kiyose, Tokyo 204-8588, Japan
E-mail: mito@my-pharm.ac.jp, sugiyama@my-pharm.ac.jp
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Table 1. Synthesis of diazene 1a from p-toluidine (2a) catalyzed by
dirhodium(II)-complexes.[a]
Scheme 2. Strategy for a single-step synthesis of N-aryl-N'-sulfonyldiazenes 1
via N−H insertion of Rh(II)-nitrene intermediates.
On the basis of the above anticipation, we initially examined
the reaction of p-toluidine (2a) with
(tosylimino)phenyliodinane (TsN=IPh) in CH2Cl2 using 2 mol% of
Rh2(esp)2 (esp α,α,α,α-tetramethyl-1,3-
2
equiv of
=
Entry
1
Conditions
Yield[b]
74%
benzenedipropanoate).[15] The reaction proceeded smoothly to
completion at 0 °C in less than 1 h, affording N-(p-tolyl)-N’-
tosyldiazene (1a) in 74% yield (Table 1, entry 1, determined by 1H
NMR). To our surprise, 1a could be obtained even in the absence
of catalyst albeit in low yield (entry 2).[16] The use of Rh2(OAc)4 in
place of Rh2(esp)2 provided 1a in only 45% yield (entry 3). An
increase in the reaction temperature or the concentration of the
substrate was accompanied by a decrease in the product yield
(entries 4 and 5). The reaction in the absence of 4Å MS also
resulted in a slight drop in the product yield (entry 6). Solvents
such as Et2O, MeCN, toluene, and benzotrifluoride were less
effective than CH2Cl2 (entries 7−10). Finally, switching the nitrene
standard
2
without Rh2(esp)2
23%
3
Rh2(OAc)4 instead of Rh2 (esp)2
rt instead of 0 °C
45%
4
62%
5
0.1 M instead of 0.025 M
without 4Å MS
58%
6
59%
7
Et2O instead of CH2Cl2
MeCN instead of CH2Cl2
toluene instead of CH2Cl2
CF3C6H5 instead of CH2Cl2
TsN=IMes instead of TsN=IPh
10%
precursor
from
TsN=IPh
to
(tosylimino)-2,4,6-
8
42%
trimethylphenyliodinane (TsN=IMes) dramatically improved the
product yield,[17] providing 1a in 86% yield (determined by 1H
NMR) and in 66% yield after column chromatography on silica gel
(entry 11).[18,19]
9
48%
10
11
56%
86% (66%)[c]
With optimized conditions in hand, we then investigated the
applicability of the reaction to a range of aromatic amines 2 (Table
2). Aniline (2b) provided N-phenyl-N’-tosyldiazene (1b) in 68%
yield (entry 1).[20] High product yields were consistently observed
with amines bearing tert-butyl, chloro,[21] trifluoromethyl,
alkoxycarbonyl, methoxy,[18] or trifluoromethoxy groups at the
para position on the benzene ring (71–99% yield, entries 2−8).
Again, the advantage of the use of TsN=IMes over TsN=IPh was
demonstrated with 4-chloroaniline (2d), 4-trifluoromethylaniline
(2e), or methyl 4-aminobenzoate (2f) (TsN=IMes; 78−99%,
TsN=IPh; 30−49%, see entries 3−5), although the reason for
these results is not clear at present. It is also notable that no trace
of the aziridination product was observed with allyl ester 2g (entry
6). Acid-sensitive functional groups like tert-butyldimethylsilyl and
trityl ethers remained untouched during the reaction (entries 9 and
10). No detectable benzylic C−H amination or oxidative removal
of the p-methoxybenzyl ether was observed under the reaction
conditions (entry 11). Furthermore, the present protocol was
found to be applicable to amino acid and steroid derivatives 2m
and 2n providing diazenes 1m and 1n in 70% and 79%,
respectively (entries 12 and 13).
[a] Reactions were carried out as follows, unless otherwise noticed:
iminoiodinane (0.2 mmol) was added to a mixture of 1a (0.1 mmol), Rh(II)
catalyst (0.002 mmol, 2 mol%) and 4Å MS (powder, 40 mg) in the indicated
solvent (4 mL) at the indicated temperature under Ar atmosphere. [b] Yields
were determined by 1H NMR using 1,1,2,2-tetrachloroethane as an internal
standard (average of two trials). [c] Yield in parenthesis refers to isolated
yield. Ts = tosyl.
We next examined ortho-substituted amines as substrates
(Table 3). However, unfortunately, sterically hindered ortho-
substituted amines 2o−s were found to be less effective
substrates under the catalysis by Rh2(esp)2 (<30% yield). To our
delight, the yield of N-(2-bromophenyl)-N’-tosyldiazene (1o) was
[15b]
greatly improved by switching the catalyst to Rh2(HNCOCF3)4
with less bulky amidate ligands (entry 1).[22] High yields were
maintained with electron-withdrawing methoxycarbonyl or
sterically bulky phenyl groups at the ortho position on the benzene
ring (entries 2 and 3). In contrast, the introduction of electron-
donating methyl and methoxy groups markedly diminished the
yields (entries 4 and 5). In addition, Rh2(HNCOCF3)4 was found
to be well suited for the transformation of a heteroaromatic amine,
namely 3-aminopyridine (2t), providing diazene 1t as the sole
product in 70% yield (entry 6).[13]
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COMMUNICATION
Table 2. Synthesis of diazenes 1 catalyzed by Rh2(esp)2.[a]
Table 3. Synthesis of ortho-substituted- and heteroaromatic diazenes 1
catalyzed by Rh2(HNCOCF3)4.[a]
Entry
Diazene 1
Yield[b]
1
2
3
4
5
6
7
8
R = H (1b)
t-Bu (1c)
68%
Entry
Diazene 1
Yield[b]
83%
Cl (1d)
99% (49%)[c]
88% (30%)[c]
86%[d] (31%)[c]
85%
1
2
3
4
5
R = Br (1o)
CO2Me (1p)
79%
79%
67%
ND
CF3 (1e)
CO2Me (1f)
CO2Allyl (1g)
OMe (1h)
OCF3 (1i)
Ph (1q)
Me (1r)
73%
OMe (1s)
20%
71%
6
70%
[a] All reactions were performed on a 0.1 mmol scale. [b] Isolated yield. ND
= not detected.
9
PG = TBS (1j)
Tr (1k)
81%
10
11
78%
85%
PMB (1l)
As a result, no signs of the formation of hydrazine 3a were
detected by NMR spectroscopy of the crude reaction mixture,
while 1a was obtained in 28% yield. This result should not be
inconsistent with a mechanism involving a formal N−H insertion,
but neither it supports such a pathway.[24] Hence, at this point,
both reaction pathways are equally possible. On the other hand,
the reaction of N-allylaniline (4) provided the N−H insertion
product 5 in 57% yield under the same conditions [Eq. (2)]. This
result indirectly suggests the formation of hydrazine 3 from
primary amine 2.
12[e]
70%
79%
13
[a] All reactions were performed on a 0.1 mmol scale. [b] Isolated yield. [c]
Yields in parentheses refer to yields obtained when TsN=IPh was used
instead of TsN=IMes. [d] 1.0 mmol scale. [e] 4 mol% of Rh2(esp)2 was used.
TBS = tert-butyldimethylsilyl, Tr = trityl, PMB = p-methoxybenzyl, Boc = tert-
butoxycarbonyl.
A plausible reaction mechanism for this transformation is
illustrated in Scheme 3. The nucleophilic addition of aromatic
amine 2 to Rh(II)-nitrene would generate zwitterionic intermediate
I. The formation of diazene 1 from I can be rationalized with two
pathways. Path A involves the formation of a formal N−H insertion
product, namely N-aryl-N’-tosylhydrazine 3, via a proton transfer
from I, followed by oxidation with the iminoiodinane to afford
diazene 1. Alternatively, trapping of I by the iminoiodinane can
generate intermediate II possessing an N−I(III) bond, which, upon
elimination of iodoarene and p-toluenesulfonamide, provides
diazene 1 without the intermediacy of hydrazine 3 (path B).[23] To
gain an insight into the reaction mechanism, the reaction of 2a
with 1 equiv of TsN=IMes was performed [Eq. (1)].
Scheme 3. Plausible reaction mechanism.
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10.1002/ejoc.201601627
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COMMUNICATION
In summary, we have developed a new and efficient method
for the synthesis of N-aryl-N’-tosyldiazenes with a combinational
use of dirhodium(II) complex catalysts and iminoiodinane. This
represents the first example of the synthesis of N-aryl-N’-
tosyldiazenes in a single step starting from primary aromatic
amines through an N−N bond formation with Rh(II)-nitrene
followed by oxidation. The obtained N-aryl-N’-tosyldiazene was
successfully demonstrated to serve as a potent stable surrogate
for diazonium salts in a two-step deaminative transformation of
aromatic amines. Further studies aimed to expand the utility of
diazenes are currently in progress as well as the applications of
this method to the synthesis of bioactive compounds.
Acknowledgements
We thank T. Koseki of the Analytical Center of Meiji
Pharmaceutical University for mass spectral measurements.
Finally, transformations of the obtained diazene 1 were
examined under both catalytic and transition metal-free conditions
(Scheme 4). The palladium(II)-catalyzed coupling of diazene 1n,
derived from 3-aminoestrone (2n), with 4-methoxyphenylboronic
acid[5a] or phenylacetylene[5b] provided biaryl 6a and diarylethyne
6b in 69% and 88% yields, respectively. 3-Iodo-3-deoxy-estrone
(6c) was obtained by treatment of 1n with KI in the presence of
18-crown-6.[4f] These reactions constitute a novel two-step
deaminative transformation of primary aromatic amines 2 using
diazenes 1 as key intermediates. Thus, the exceptional utility of 1
as a surrogate for diazonium salts has been successfully
demonstrated.
Keywords: amines • azo compounds • rhodium • nitrene •
aromatic substitution
[1]
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[3]
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[5]
a) J.-B. Liu, H. Yan, H.-X. Chen, Y. Luo, J. Weng, G. Lu, Chem. Commun.
2013, 49, 5268−5270; b) J.-B. Liu, F.-J. Chen, E. Liu, J.-H. Li, G. Qiu,
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Jiang, J. Weng, G. Lu, Org. Biomol. Chem. 2013, 11, 8014−8017; e) J.-
B. Liu, H.-P. Zhou, Y-Y. Peng, Tetrahedron Lett. 2014, 55, 2872−2875;
f) H.-P. Zhou, J.-B. Liu, J.-J. Yuan, Y.-Y. Peng, RSC Adv. 2014, 4,
25576−25579.
[6]
The first example of palladium-catalyzed coupling of diazenes 1 was
reported by Kamigata and co-workers. a) N. Kamigata, A. Satoh, T.
Kondoh, M. Kameyama, Bull. Chem. Soc. Jpn. 1988, 61, 3575−3580; b)
N. Kamigata, A. Satoh, M. Yoshida, M. Kameyama, Bull. Chem. Soc. Jpn.
1989, 62, 605−607; c) N. Kamigata, M. Satoh, M. Yoshida, Y. Nakamura,
Scheme 4. Transformations of diazene 1n.
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COMMUNICATION
K. Kikuchi, I. Ikemoto, Phosphorus Sulfur Silicon Relat. Elem. 1992, 69,
[17] TsN=IMes can be readily prepared by condensation of TsNH2 with
commercially available 2-(diacetoxyiodo)mesitylene, [MesI(OAc)2]. See
Supporting Information for details of the preparation. The consideration
about the beneficial effect of the mesityl group on the product yield is also
described in Supporting Information.
129−136.
[7]
[8]
S. Crespi, S. Protti, M. Fagnoni, J. Org. Chem. 2016, 81, 9612−9619.
For other synthetic applications of N-aryl-N’-sulfonyldiazenes, see a) I.
Sapountzis, P. Knochel, Angew. Chem., Int. Ed. 2004, 43, 897−900; b)
P. Sinha, C. C. Kofink, P. Knochel, Org. Lett. 2006, 8, 3741−3744; c) Q.
Zhang, L.-G. Meng, K. Wang, L. Wang, Org. Lett. 2015, 17, 872−875.
Diazenes 1 are generally synthesized by the treatment of diazonium salts
with sodium sulfinate. An alternative method involves the sulfonylation
and oxidation of arylhydrazines (Ar-NHNH2), which are prepared by the
reduction of diazonium salts. See Ref [5b] and references therein.
[18] It was found that electron-rich diazenes, such as 1a and 1h, underwent
partial decomposition during column chromatography on silica gel
(neutral).
[9]
[19] Unfortunately, in situ generation of iminoiodinane from corresponding
(diacetoxyiodo)arene and p-toluenesulfonamide led to the undesired
oxidation of starting amines. See also Ref [16]
[10] For reviews on metal-catalyzed nitrene insertions, see: a) P. Müller, C.
Fruit, Chem. Rev. 2003, 103, 2905−2919; b) G. Dequirez, V. Pons, P.
Dauban, Angew. Chem., Int. Ed. 2012, 51, 7384−7395.
[20] No traces of the C(sp2)−H amination product was observed. As the
reviewer pointed out, nitrenes rarely undergo C(sp2)−H insertion. a) J.
Jiao, K. Murakami, K. Itami; ACS Catal. 2016, 6, 610−633; b) R. Singh,
K. Nagesh, M. Parameshwar, ACS Catal. 2016, 6, 6520−6524.
[21] With 4-chloroaniline (2d), it was observed that lowering the catalyst
loading from 2 mol% to 1 mol% led to a decrease in the product yield (2
mol%: 99%, 1 mol%: 69%).
[11] There is an example of rhodium(II)-catalyzed B−H insertion of nitrene. S.
Postle, D. W. Stephan, Dalton Trans. 2015, 44, 4436−4439.
[12] For a review on X−H insertion of metal-carbenoids, see: D. Gillingham,
N. Fei, Chem. Soc. Rev. 2013, 42, 4918−4931.
[13] It is reported that the reactions of Rh(II)-nitrene with tertiary amines or
nitrogen-containing heteroaromatics provide zwitterionic aminimides by
N−N bond formation. a) J. Li, J. S. Cisar, C.-Y. Zhou, B. Vera, H. Williams,
A. D. Rodríguez, B. F. Cravatt, D. Romo, Nature Chem. 2013, 5,
510−517; b) S. L. Jain, V. B. Sharma, B. Sain, Tetrahedron Lett. 2003,
44, 4385−4387.
[22] In the reactions of para-substituted amines, Rh2(HNCOCF3)4 (2 mol%)
provided similar levels of product yields as those obtained with Rh2(esp)2
(73% yield for 1c, 95% yield for 1f, see Table 1, entries 2 and 5).
[23] It is reported that the zwitterionic intermediate generated from primary
aromatic amines and Rh(II)-carbene intermediates can be trapped by a
variety of electrophiles. Y. Zhu, C. Zhai, L. Yang, W. Hu, Eur. J. Org.
Chem. 2011, 1113−1124 and references therein.
[14] Lacour and co-workers reported that the reaction of Rh(II)-nitrene with
the Tröger base, a chiral bicyclic aminal, provides formal C−N insertion
products via an N−N bond formation followed by the 1,2-rearrangement
of the aminal carbon. S. A. Pujari, L. Guénée, J. Lacour, Org. Lett. 2013,
15, 3930−3933.
[24] It was confirmed that reaction of hydrazine 3a with 1 equiv of TsN=IMes
afforded diazene 1a in virtually quantitative yield in the absence of the
dirhodium(II) complex catalyst.
[15] a) C. G. Espino, K. W. Fiori, M. Kim, J. Du Bois, J. Am. Chem. Soc. 2004,
126, 15378−15379; b) K. W. Fiori, J. Du Bois, J. Am. Chem. Soc. 2007,
129, 562−568.
[16] It is reported that the reactions of primary aromatic amines with iodine(III)
reagents afford aromatic azo compounds via oxidative dimerization. K.
Monir, M. Ghosh, S. Mishra, A. Majee, A. Hajra, Eur. J. Org. Chem. 2014,
1096−1102.
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Dr. Motoki Ito,* Arisa Tanaka, Dr.
Kazuhiro Higuchi, Prof. Dr. Shigeo
Sugiyama*
Page No. – Page No.
Rhodium(II)-Catalyzed Synthesis of
N-Aryl-N’-Tosyldiazenes from Primary
Aromatic Amines Using (Tosylimino)
aryliodinane: A Potent Stable
N-Aryl-N‘-tosyldiazenes 1 have been obtained in a single step from aromatic
amines 2 via an N−N bond formation with Rh(II)-nitrene, generated in situ from an
iminoiodinane and a dirhodium(II) complex catalyst, followed by oxidation. The
synthesized diazene 1 was successfully demonstrated to serve as a potent stable
surrogate for diazonium salts in two-step deaminative transformations of primary
aromatic amine.
Surrogate for Diazonium Salts
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