Rh(II)-Catalyzed Chemoselective Oxidative Amination and Nucleophilic Trapping of gem-Dimethyl Alkynyl-Tethered Sulfamates

Article abstract of DOI:10.1021/acs.orglett.7b03425

A Rh(II)-catalyzed chemoselective oxidative amination and nucleophilic trapping of gem-dimethyl sulfamates has been presented. For 2,2-dimethyl-4-arylbut-3-yn-1-yl sulfamates, the reactions underwent a metallonitrene-initiated alkyne oxidation along with nucleophilic trapping of H₂O upon oxidation, giving aroyl group containing heterocycles. For 2,2-dimethyl-4-arylpent-3-yn-1-yl sulfamates, the α-iminometal carbene intermediate was trapped by aryl group migration, delivering a styryl group containing heterocycles.

Full text of DOI:10.1021/acs.orglett.7b03425

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rh(II)-Catalyzed Chemoselective Oxidative Amination and
Nucleophilic Trapping of gem-Dimethyl Alkynyl-Tethered
Sulfamates
Dong Pan, Yin Wei, and Min Shi*
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, University of Chinese Academy of
Sciences, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
*
S Supporting Information
ABSTRACT: A Rh(II)-catalyzed chemoselective oxidative amination
and nucleophilic trapping of gem-dimethyl sulfamates has been presented.
For 2,2-dimethyl-4-arylbut-3-yn-1-yl sulfamates, the reactions underwent
a metallonitrene-initiated alkyne oxidation along with nucleophilic
trapping of H O upon oxidation, giving aroyl group containing
2
heterocycles. For 2,2-dimethyl-4-arylpent-3-yn-1-yl sulfamates, the α-
iminometal carbene intermediate was trapped by aryl group migration,
delivering a styryl group containing heterocycles.
atalytically generated metallocarbenes, primarily obtained
by metal-catalyzed decomposition of α-diazocarbonyl
Scheme 1. Previous Work and This Work
C
compounds, have been extensively investigated over the past
decades and have demonstrated broad applications in modern
organic synthetic chemistry for strategic C−C, C−O, and C−N
bond formation through a variety of well-defined reaction
1
mechanisms. Since nitrogen atom is prevalent in biologically
and pharmaceutically relevant molecules, much attention has
been paid to the development of methodologies for the
development of synthetic methods for its incorporation into
2
organicmolecules inrecentyears. The metallonitrenechemistry
would broaden the strategic methods to build structurally
3
complex nitrogen-containing molecules. With the pioneering
4
works by Du Bois, practical conditions involving iodine(III)
oxidantsforthegenerationofmetallonitrenesnowmakecatalytic
5
6
C−H amination and alkene aziridination general methods for
the preparation of nitrogen-containing molecules. While cyclo-
propenation of alkynes is a well-established method for the
7
preparation of cyclopropenes, the addition of nitrenesto alkynyl
8
derivatives has rarely been explored. It has been known that α-
imino gold carbene intermediates could be generated via
addition of iminopyridium ylides onto gold-activated alkynes
and followed by elimination of the neutral pyridines. Moreover,
9
selective oxidative amination and cyclization cascade of 1-
arylethynyl)cycloalkyl)-methyl sulfamates (Scheme 1b). For
(
Blakey and co-workers recently developed a new method to
cyclopropyl or cyclobutyl moiety containing alkynyl sulfamates,
the reactions underwent a metallonitrene initiated alkyne
oxidation along with cyclopropyl ring expansion or alkoxyl
moiety migration to give cyclobutane-fused or methylenecyclo-
generate α-iminometal carbenes through metallonitrene/alkyne
1
0
metathesis. They showed that α-iminometal carbenes could be
generated via Rh-catalyzed metallonitrene-initiated alkyne
oxidation, and the α-iminometal carbenes could be terminated
in an array of reactions, such as oxygen-ylide formation, [2,3]-
Wittig rearrangement, aromatic substitution, and cyclopropana-
tion as well as intermolecular trapping by a variety of allyl ethers
11
butane containing heterocycles.
After the investigation on these cycloalkyl sulfamates, we then
simultaneously attempted to examine the corresponding gem-
dimethyl sulfamates such as 2,2-dimethyl-4-arylbut-3-yn-1-yl
(Scheme 1a). With these termination methods of carbene/
alkyne cascade alkyl migration process and on the basis of our
ongoing investigation on the chemical transformations of
strained small rings, we reported a Rh(II)-catalyzed chemo-
Received: November 3, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03425
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
sulfamates and 2,2-dimethyl-5-arylpent-3-yn-1-yl sulfamates in
this transformation. However, we observed totally different
results from our previous cycloalkyl sulfamates and Blakey’s
work. When R = Ar, the α-iminometal carbene intermediate was
trapped by water and then oxidized by iodine(III) oxidants to
give benzoyl group containing heterocycles. When R = ArCH2,
the α-iminometal carbene intermediate was trapped by aryl
migration rather than β-H migration to give a styryl group
With the optimal conditions in hand, we next investigated the
generality of this reaction using a variety of substrates 1b−1l
(Scheme 2). Regardless of whether electron-donating or
a,b
Scheme 2. Substrate Scope of 1
12
containing heterocycles (Scheme 1c). Herein, we wish to
report these new findings.
First, we utilized the sulfamic ester 1a for the initial
examination using [Rh (esp) ] (2.5 mol %) as the catalyst and
2
2
PhI(OPiv) (2.3 equiv) as the oxidant with 4.6 equiv of CaO as a
2
base at room temperature in dichloromethane (DCM) under
argon atmosphere. We were pleased to find that the desired
product 2a was given in 66% NMR yield within 24 h (Table 1,
a
Table 1. Optimization of the Reaction Conditions
a
Unless otherwise specified, all reactions were carried out using 1 (0.2
yield
mmol), Phl(OPiv) (0.46 mmol), Rh (esp) (2.5 mol %), H O (1.0
2
2
2
2
b
b
entry cat. (mol %)
oxidant
additive
solvent
(%)
mmol), and CaO (0.92 mmol) in DCM (4.0 mL), rt, 24 h. Isolated
c
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
Rh (esp)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhIO
PhI(OAc)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
PhI(OPiv)2
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
THF
66
yields are provided. No water was added.
2
Rh (Oct)4
2
<5
<5
<5
<5
<5
<5
48
68
73
80
87 (85)
77
Rh (OAc)4
2
electron-withdrawing groups were introduced on the aromatic
R group, the reactions proceeded smoothly, affording the desired
products 2b−2l in moderate to good yields. The electron-
donating group could be a methyl group at the para- or meta-
position as well as phenyl group or methoxyl group, affording the
corresponding products 2b, 2c, 2d, and 2f in 76, 79, 86, and 80%
yields, respectively. Naphthyl moiety was also compatible, giving
2e in 83% yield. The halogen substituents were also tolerated,
furnishing the desired products 2g, 2h, and 2i in good yields at
room temperature. Substrate 1j with a strongly electron-
withdrawing trifluoromethyl group and substrate 1k having an
ester moiety could also provide the desired products 2j and 2k in
86 and 84% yields. Furthermore, substrate 1k having a thiophene
moietygavethecorrespondingproduct2kin56%yieldunderthe
standard conditions. With substrate 1m bearing a nitryl group,
the reaction gave a complex mixture. For substrates 1n and 1o,
none of the desired products were obtained. The structure of 2a
was further confirmed by X-ray diffraction, and the ORTEP
drawing is shown in the Supporting Information.
Rh (OPiv)4
2
Rh (TFA)4
2
Rh (esp)2
2
Rh (esp)2
2
c
Rh (esp)2
2
4 Å MS
O2
d
Rh (esp)2
2
e
f
0
1
2
3
4
5
6
Rh (esp)2
2
H O (1.0)
2
Rh (esp)2
2
H O (2.0)
2
g
h
g
g
g
i
Rh (esp)2
2
H O (5.0)
2
Rh (esp)2
2
H O (10)
2
Rh (esp)2
2
H O (5.0)
2
<5
<5
63
Rh (esp)2
2
H O (5.0)
2
CH CN
3
Rh (esp)2
2
H O (5.0)
2
toluene
a
Unless otherwise specified, all reactions were carried out using 1a
0.2 mmol), oxidant (0.46 mmol), catalyst (2.5 mol %), CaO (0.92
mmol), and additive in solvent (4.0 mL), rt, 24 h. The yield was
(
b
1
determined by H NMR spectroscopic data using mesitylene as an
c
d
e
internal standard. 50 mg of 4 Å MS was added. 1.0 atm O . 1.0
2
f
g
equiv of H O was added. 2.0 equiv of H O was added. 5.0 equiv of
H O was added. 10 equiv of H O was added. Isolated yield.
2
2
h
i
The further examination of substrates 3 containing an
arylmethyl moiety was also conducted. At first, we conducted
the reaction of 3a under the same conditions as that of 1, and we
were pleasedto findthat the aryl migrationcouldtakeplace via α-
iminometal carbene, giving the styryl group containing
2
2
entry 1). This result inspired us to further explore the better
conditions for this reaction, and the results are summarized in
Table 1. First, we screened various rhodium catalysts such as
t
[
Rh (Oct) ], [Rh (OAc) ], [Rh (O C Bu) ], and [Rh (TFA) ]
heterocyclic product 4a in good yield rather than the H O-
2
4
2
4
2
2
4
2
4
2
(
Table 1, entries 1−5). [Rh (esp) ] was found to be the best one
trapping product or H migration product. The structure of 4f was
further confirmed by X-ray diffraction. This result indicates the
special reaction property of α-iminometal carbene derived from
sulfamates with alkyne. We found that using 1.3 equiv of
2
2
t
for this cascade reaction. Then, PhI(O C Bu) was identified as
the more efficient oxidant than PhI(OAc) and PhIO (Table 1,
entries6and7).Withtheadditionof4ÅMS(50mgfor0.2mmol
of 1a), 2a could be obtained in 48% yield (Table 1, entry 8). The
2
2
2
t
PhI(O C Bu) as the oxidant, 2.5 mol % of [Rh (esp) ] as the
2
2
2
2
yield of 2a was 68% under O atmosphere (Table 1, entry 9).
catalyst, and 2.6 equiv of CaO as a base and carrying out the
reaction at room temperature in DCM for 2 h gave the desired
product 4a in 87% isolated yield. The substrate scope was also
explored. As summarized in Scheme 3, the reactions proceeded
smoothly with various substrates 3b−3l, allowing the facile
synthesis of product 4 in moderate to good yields. The substrates
2
Next, different equiv of H O was used as the additive for this
2
reaction, and we identified that adding 5.0 equiv of H O afforded
2
2
b in 87% NMR yield and 85% isolated yield (Table 1, entries
1
0−13). The examination of solvent effects revealed that DCM
was the best choice (Table 1, entries 14−16).
B
DOI: 10.1021/acs.orglett.7b03425
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a,b
Scheme 3. Substrate Scope of 3
Scheme 5. Proposed Reaction Mechanism
a
theformationofiminoiodinaneintermediateA,whichundergoes
exchange with dirhodium catalyst to generate the metallonitrene
intermediate B. Then intramolecular metallonitrene/alkyne
addition cascades afford intermediate C or its α-iminometal
carbene D. The Thorpe−Ingold effect of gem-dimethyl groups
facilitated the formation of 6-exo α-iminometal carbene
intermediate D and did not favor 7-endo cyclization, thereby
Unless otherwise specified, all reactions were carried out using 3 (0.2
mmol), Phl(OPiv) (0.26 mmol), Rh (esp) (2.5 mol %), and CaO
2
2
2
b
(0.52 mmol) in DCM (4.0 mL), rt, 2 h. Isolated yields are provided.
bearing electron-withdrawing or electron-donating group on the
aromatic ring underwent the cascade cyclization efficiently to
deliver the desired products 4b−4h in yields ranging from 83 to
10
exclusively affording the six-membered ring products. For
substrate 1a (R = Ph), intermediate D′ undergoes nucleophilic
8
9%. Of note, for substrates 3c and 3d with a methyl substituent
at the meta-position or ortho-position, the corresponding
products 4c and 4d were formed in 83 and 85% yields. Substrates
attack of H O to form intermediate E′, which furnishes the
2
desiredproduct2auponoxidationwithPhI(OPiv) .However, in
2
3
i and 3k having heteroaromatic moiety were tolerable in this
the case of 3a (R = PhCH ), the corresponding intermediate D″
2
14
reaction, affording the desired products 4i and 4k in 71 and 77%
yields, respectively. In addition, for substrate 3l, in which methyl
group was replaced by ethyl group, the reaction could also take
place, affording 4l in 54% yield. However, when both of the two
methyl groups were replaced by phenyl groups, such as substrate
undergoes the aryl group migration through intermediate E″.
The elimination of Rh catalyst from E″ gives product 4a.
In summary, we have disclosed a novel chemoselective
oxidative amination and nucleophilic trapping of gem-dimethyl
sulfamates in the presence of Rh(II) catalyst. For 2,2-dimethyl-4-
arylbut-3-yn-1-yl sulfamates, the reactions underwent a metal-
lonitrene-initiated alkyne oxidation along with nucleophilic
3m, none of the desired product was obtained.
1
8
To identify the source of O in product 2, we conducted the O
1
8
labeling experiment with the addition of 5.0 equiv of H O into
attacking of H O to give aroyl group containing heterocycles in
2
2
the reaction mixture and found that the obtained product
incorporated 51% O content, suggesting that oxygen atom in
moderate to good yields depending on the substituents under
mild conditions. For 2,2-dimethyl-4-arylpent-3-yn-1-yl sulfa-
mates, the α-iminometal carbene intermediate was trapped by
aryl migration, affording styryl group containing heterocycles in
good yields. Further investigations on expanding the scope and
applications of this method for the synthesis of diversified
nitrogen-containing heterocyclic products are ongoing in our
laboratory.
18
product 2 was derived from H O. For the aryl migration, alkyne
2
bearing a benzyl group could be transformed to allene under the
basic condition. To exclude the possible allenic intermediate in
the pathway, we synthesized the allenic compound 5 and found
that the corresponding cycloaddition product 6 was formed in
7
5% under the standard conditions rather than the aryl migration
product 4, rendering unlikely the involvement of allenic
intermediate (Scheme 4).
On the basis of the previous reports and the control
experiments mentioned above, a plausible reaction mechanism
has been outlined in Scheme 5 to account for the different
reaction outcomes. The oxidation of substrate 1a or 3a initiates
13
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03425.
Experimental procedure and characterization data for all
compounds (PDF)
Scheme 4. Mechanistic Experiments
Accession Codes
CCDC 1493128 and 1525668 contain 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: mshi@mail.sioc.ac.cn.
C
DOI: 10.1021/acs.orglett.7b03425
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
(f)Nowlan, D. T., III;Singleton, D. A. J. Am. Chem. Soc. 2005, 127, 6190.
g) Lou, Y.; Horikawa, M.; Kloster, R. A.; Hawryluk, N. A.; Corey, E. J. J.
Am. Chem. Soc. 2004, 126, 8916. (h) Perez, P. J.; Brookhart, M.;
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Doyle, M. P.; Mueller, P.; Ene, D. J. Am. Chem. Soc. 1992, 114, 2755.
(
Yin Wei: 0000-0003-0484-9231
Min Shi: 0000-0003-0016-5211
Notes
(
(
8) Yeom, H.-S.; Shin, S. Acc. Chem. Res. 2014, 47, 966.
The authors declare no competing financial interest.
9)(a)Ling,H.-B.;Chen, Z.-S.;Yang, F.;Xu,B.;Gao, J.-M.;Ji, K. J.Org.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National Basic
Research Program of China ((973)-2015CB856603), the
Strategic Priority Research Program of the Chinese Academy
of Sciences (Grant No. XDB20000000), and the National
Natural Science Foundation of China (20472096, 21372241,
1572052, 20672127, 21421091, 21372250, 21121062,
1302203, 20732008, and 21772037).
2
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DOI: 10.1021/acs.orglett.7b03425
Org. Lett. XXXX, XXX, XXX−XXX