Catalytic Diazosulfonylation of Enynals toward Diazoindenes via Oxidative Radical-Triggered 5-exo-trig Carbocyclizations

Article abstract of DOI:10.1021/acs.orglett.6b00655

Catalytic diazosulfonylation of enynals with arylsulfonyl hydrazides has been established by using tert-butyl hydroperoxide (TBHP) as the oxidant with tetrabutylammonium iodide (TBAI) under a convenient system. The reaction occurred through oxidative radical-triggered 5-exo-trig carbocyclization cascading to afford sulfonylated diazoindenes regioselectively. The new diazosulfonylation reaction features a broad substrate scope, readily accessible starting materials, and a simple one-pot process.

Full text of DOI:10.1021/acs.orglett.6b00655

Letter
pubs.acs.org/OrgLett
Catalytic Diazosulfonylation of Enynals toward Diazoindenes via
Oxidative Radical-Triggered 5-exo-trig Carbocyclizations
Wen-Juan Hao,†,∥ Yan Du,†,∥ Dan Wang,† Bo Jiang,*,†,‡ Qian Gao,† Shu-Jiang Tu,*,† and Guigen Li*,‡,§
†School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou 221116, P. R. China
‡Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States
§Institute of Chemistry & BioMedical Sciences, Collaborative Innovation Center of Chemistry for Life Sciences, Nanjing University,
Nanjing 210093, P. R. China
S
* Supporting Information
ABSTRACT: Catalytic diazosulfonylation of enynals with arylsulfonyl
hydrazides has been established by using tert-butyl hydroperoxide (TBHP)
as the oxidant with tetrabutylammonium iodide (TBAI) under a
convenient system. The reaction occurred through oxidative radical-
triggered 5-exo-trig carbocyclization cascading to afford sulfonylated
diazoindenes regioselectively. The new diazosulfonylation reaction features
a broad substrate scope, readily accessible starting materials, and a simple one-pot process.
VI).11 Although significant advances have been made in the past
iazo compounds have been widely utilized in organic
Dsynthesis and appear as ubiquitous structural motifs that several decades, a few crucial issues still remain in regard to the
exist in many natural products and therapeutic agents.1 For
limited substrate scopes, the use of expensive and toxic transition
metals, and the thermal lability of pregenerated diazo precursors
and their potential safety concerns. Therefore, the exploration of
an efficient access toward new diazo compounds associated with
these challenges would be among the most attractive but
intractable tasks. Radical-based cascades offer a strategic platform
for the construction of densely functionalized structures in
convergent manners through orchestrated multiple C−C/C−X
bond formations.12 Over the years, alkynes act as good radical
acceptors and allow radical-triggered addition across a triple bond
to access their functionalization with high compatibility.13
Recently, sulfonylation of alkynes via sulfonyl radicals generated
in situ from sulfonohydrazides has attracted special attention,
because it can provide an effective pathway to sulfones with
significant synthetic potential under mild conditions.14 Very
recently, our groups have also reported the addition of
arylsulfonyl radicals to 1,5-enynes as a way to trigger a
sulfonylation/5-exo-dig/6-endo-trig bicyclization sequence.15
Considering the direct conversion of sulfonylhydrazones into
diazo compounds10 and our ongoing project on radical-based
methodologies,16 we envisioned that introducing an sulfonylhy-
drazone group onto the ortho position of the aromatic ring of an
aryl alkyne would be able to control the regioselectivity of the
initial radical addition to the CC conjugate system,13 enabling
C−S and C−C bond forming events across alkynes to assemble
densely functionalized carbocycles. Herein, we would like to
report the successful realization of this analysis with the
introduction of an arylsulfonyl radical for the flexible synthesis
of unprecedented sulfonylated diazoindenes in a highly selective
and functional-group-compatible fashion (Scheme 2). In this
example, the kinamycins (e.g., A−D, F, and J) are endowed with a
diazofluorene unit which is responsible for potent biological
properties including antibiotic and antitumor activities.2 There
have been numerous examples in which diazo compounds served
as versatile building blocks due to their accessibility and attractive
functionalization potential via dediazoniation.3 Among them,
transition-metal-catalyzed reactions of diazo compounds have
taken a dominant position, enabling powerful access to the
formations of carbon−carbon and carbon−heteroatom bonds via
additions and insertions of highly reactive metal carbene
intermediates.4 The significant biological interests and synthetic
profiles of diazo compounds have triggered the development of
novel methods to incorporate diazo functionality into more
challenging structures.5 So far, the vast majority of well-
established routes for accessing diazo compounds have been
represented by the following: diazotization of activated
methylene compounds (Scheme 1, Route I)6 and primary amines
(Route II),7 alkaline cleavage of N-alkyl-N-nitrosoureas (Route
III),8 dehydrogenation of hydrazones (Route IV),9 cleavage by
treating sulfonylhydrazones with bases (Route V),10 and
structural modifications of an existing diazo compound (Route
Scheme 1. Synthetic Routes to Diazo Compounds
Received: March 7, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00655
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Scheme 2. Exploration of New Reactivity of Enynal
Scheme 3. Substrate Scope for Synthesis of 3
reaction, arylsulfonyl hydrazides were found to play dual roles as
both diazo source and arylsulfonyl radical precursor.
2-(Phenylethynyl)benzaldehyde 1a was initially selected as the
benchmark substrate to study the radical addition−cyclization
reaction with p-toluenesulfonyl hydrazide 2a. This model
reaction was first performed by using different oxidants, such as
tert-butyl hydroperoxide (TBHP), tert-butyl peroxybenzoate
(TBPB), and di-tert-butyl peroxide (DTBP) in the presence of
tetrabutylammonium iodide (TBAI) in an acetonitrile solution.
The use of 70% TBHP in aqueous solution gave the
unprecedented diazoindene 3a, albeit with a low yield of 21%.
An increased yield was obtained when TBPB was employed as the
oxidant. The reaction did not give product 3a by using the DTBP
oxidant. Delightedly, the anhydrous TBHP showed the best
performance (41%), indicating that water inhibited this reaction.
We thus utilized the anhydrous system for further optimization.
Replacing TBAI with KI or CuI resulted in relatively lower yields
of3a,whereasiodinecanhardlyfacilitatethereactionspeed. Next,
different dry solvents, such as DMSO, EtOH, and toluene, were
examined. We observed diazoindene 3a in 33% yield together
with some unreacted sulfonylhydrazones (entry 8). Other
solvents including EtOH and toluene were found to be less
effective (entries 9−10). Then, 4 Å molecular sieves (MS) were
added into the reaction system, resulting in a higher yield of 46%
(entry 11). Eventually, the combination of acetonitrile with
DMSO allowed us to isolate 3a in 53% yield, as shown in Table 1,
entry 12 (see Supporting Information (SI)).
a
Reaction conditions: enynals (1, 0.5 mmol), sulfonyl hydrazide (2,
1.5 mmol), TBAI (0.1 mmol), TBHP (5.5 M in nonane, 2.0 mmol),
CH3CN (3.0 mL), DMSO (0.1 mL), 70 °C, 2.5 h. Isolated yields
based on 1.
b
With the established optimal conditions, we turned to evaluate
the generality of this metal-free radical cyclization with respect to
a variety of enynals 1 and sulfonyl hydrazides 2 (Scheme 3).
Those substituents on the phenyl ring of both enynals 1 and
sulfonyl hydrazides 2 were proven not to hamper this catalysis,
and a wide range of arylsulfonylated diazoindenes 3b−3x can be
generated in acceptable yields and a functional-group-compatible
fashion using sulfonyl hydrazides as the diazo source. For
instance, all the reactions of methyl- methoxy-, or chloro-
substituted substrates 2 with 1a can lead to the formation of
desired products 3b−3d in 46%−59% yields. With a p-
methoxyphenyl (PMP) group (1b) or a 1-naphthyl (1-Np)
substituent (1d) on the alkynyl moiety, functional groups
including methyl, tert-butyl, chloride, and bromide at different
positions of the aromatic ring (Ar2) directly bound to sulfonyl
hydrazides were well tolerated under this system, providing the
corresponding sulfonated diazoindenes 3e−3k and 3m−3q with
yields ranging from 43% to 64%. Among them, a sterically
demanding o-chlorophenyl analogue (2i) proved to be more
reluctant to undergo the reaction, in which 3p was obtained in
43% yield, whereas strongly electron-withdrawing aromatic rings
(nitro, 2j) would be accommodated, confirming the reaction
efficiency, as 3q was generated in 61% yield. Next, the electronic
nature of substituents on the arylalkynyl (Ar1) moiety was
systematically investigated, and it was found that the reaction
proceeded readily with various functional groups attached by the
arylalkynyl moiety of enynals 1. The variant of substituents on the
arylalkynylmoiety,includingmethyl,methoxy, chloro, andfluoro,
would be compatible with the present catalytic oxidation system.
Generally, electron-donating groups on the arylalkynyl moiety
afforded much better yields than those with electron-withdrawing
a
Table 1. Optimization Conditions for Forming 3a
b
entry
cat. (mol %)
oxidant
solvent
CH3CN
yield (%)
c
1
TBAI (20)
TBAI (20)
TBAI (20)
TBAI (20)
KI (20)
TBHP
21
2
TBPB
DTBP
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
32
3
N.D.
41
d
4
TBHP
d
5
TBHP
17
d
6
I2 (10)
TBHP
N.D.
27
d
7
CuI (5)
TBHP
d
8
TBAI (20)
TBAI (20)
TBAI (20)
TBAI (20)
TBAI (20)
TBHP
33
d
9
TBHP
EtOH
trace
7
d
10
11
12
TBHP
toulene
d
e
TBHP
CH3CN
CH3CN/DMSO
46
d
f
e
TBHP
53
a
Reaction conditions: 1 (0.5 mmol), 2a (1.5 mmol), oxidant (2.0
b
c
mmol), solvent (3.0 mL), 2.5 h. Isolated yields based on 1. TBHP
(70% in water). TBHP (5.5 M in nonane). The use of 4 Å MS (200
mg). VMeCN/VDMSO = 30:1. N.D. = no detected.
d
e
f
B
DOI: 10.1021/acs.orglett.6b00655
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Letter
ones (3e vs 3s, 3f vs 3r), which indicates that the reactivity of the
reaction may be dominated by the electronic nature of the
substituents. In addition, we conducted fluorinated enynals 1 to
expand their synthetic and phamaceutical utility. As anticipated,
fluorinated enynals 1 proved to be adaptable substrates in this
transformation, allowing sulfonyl radical-triggered addition−
cyclization to access the corresponding sulfonated diazoindenes
3t−3x, albeit with relatively lower yields as compared with those
without the fluoro group, which might be ascribed to a strong
inductive effect of fluorine functionality. The fluoro substituent
located at the 4- or 5-position of the phenyl ring of enynals 1
enabled the reaction to occur smoothly. The resulting densely
functionalized diazoindenes 3 were fully determined by NMR
spectroscopy and HRMS. In the two cases of 3a, its structure was
unambiguously confirmed by X-ray diffraction analysis (Figure
1).
Scheme 5. Control Experiments
Scheme 6. Proposed Mechanism for Forming 3
Figure 1. ORTEP drawing of 3a.
Scheme 4. Applications of Diazoindenes 3f
NMR, were isolated (Scheme 5d). Considering these exper-
imental observations, we reasoned that arylsulfonyl radicals were
in situ generated from arylsulfonyl hydrazides 2 by oxidation
triggered addition−cyclization, which allowed the cleavage of N−
S of sulfonohydrazones via a radical process. Obviously, the
sulfonylation occurred prior to the carbocyclization step, which
was followed by diazotization.
On the basis of our own observations and literature survey,17
a
After the successful formation of functional diazoindenes 3, we
then attempted to render the potential applications of the
resulting diazoindenes. The reaction between diazoindenes 3f
and phenylboronic acid 5 was carried out in the presence of
K2CO3 in 1,4-dioxane at 120 °C. Surprisingly, an unprecedented
hydrazine-tethered bifluorenes 6 was afforded in a 76% yield
(Scheme 4), confirmed by X-ray diffraction analysis (SI).
To gain insights into the reaction mechanism for forming
diazoindenes 3, several control experiments were conducted
(Scheme 5). The enynal 1a was subjected to reaction with 2a in
the presence of the radical scavenger TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy) or BHT (2,6-di-tert-butyl-4-meth-
ylphenol) (4.0 equiv), and sulfonohydrazones 4 was obtained in
89% yield without observation of the expected product 3a,
indicating the possibility of a radical mechanism (Scheme 5a). To
providefurthersupportforthereactionpathway,treatmentwith4
and 2a under standard conditions gave the desired product 3a in
52% yield (Scheme 5b), suggesting that the reaction involves the
in situ formation of sulfonohydrazones 4. The reaction failed to
proceed under standard conditions, and the preformed substrate
4 was recovered in this reaction system (Scheme 5c), confirming
that the oxidative cleavage of the N−S bond did not take place.
Moreover, when 4 was reacted with 2c, two products 3a and 3c as
an inseparable mixture in a 1:2.75 ratio, as analyzed by their 1H
tentative mechanism for forming diazoindenes 3 was proposed in
Scheme 6. The first step is to form sulfonohydrazones 4 derived
from enynals 1 and sulfonohydrazides 2, which captures sulfonyl
radicals generated in situ from the oxidative decomposition of
sulfonyl hydrazides mediated by TBAI and TBHP,18 to give A.
Intermediate A then undergoes 5-exo-trig cyclization and H-
abstraction to access arylsulfonyldiazene intermediate C, which
releases sulfonyl radicals by a second H-abstraction and the
cleavage of the N−S bond, leading to the formation of
diazoindenes 3. Although the conversion of arylsulfonyl
hydrazides into sulfonyl radicals by various oxidants has been
documented well,17,18 in situ generated sulfonyl radical initiated
addition−cyclization toward diazoindenes, in which sulfonyl
hydrazides play dual roles as both a diazo source and an
arylsulfonyl radical precursor, has not been reported yet.
In summary, we have discovered a novel diazosulfonylation of
enynals under catalytic oxidation conditions. The addition of
sulfonyl radicals tothe triple bond of enynals can trigger a domino
5-exo-trig carbocyclization and H-abstraction sequence, giving
access to unprecedented sulfonylated diazoindenes in multiple
C−N/C−S/C−C bond-forming processes. The present new
synthetic strategy paves the way for the collection of significant
functional sulfones for potential applications in organic and
medicinal chemistry.
C
DOI: 10.1021/acs.orglett.6b00655
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Organic Letters
Letter
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ASSOCIATED CONTENT
■
S
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.6b00655.
Experimental procedures and spectroscopic data for all
new compounds 3a−3x, 6 (PDF)
X-ray crystal data for 3a (CIF)
X-ray crystal data for 6 (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: jiangchem@jsnu.edu.cn.
*E-mail: laotu@jsnu.edu.cn.
*E-mail: guigen.li@ttu.edu.
Author Contributions
∥W.-J.H. and Y.D. contributed equally.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (Nos.
21232004, 21332005, and 21472071), PAPD of Jiangsu Higher
Education Institutions, Robert A. Welch Foundation (D-1361,
USA), NIH(R33DA031860, USA), theOutstandingYouthFund
ofJSNU(YQ2015003), NSFofJiangsuProvince (BK20151163),
NSF of JSNU (14XLR005), and NSF of Jiangsu Education
Committee (15KJB150006).
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