Gold-Catalyzed Dehydrazinative C(sp)–S Coupling Reactions of Arylsulfonyl Hydrazides with Ethynylbenziodoxolones for Accessing Alkynyl Sulfones

Article abstract of DOI:10.1002/ejoc.201700524

A gold(III)-catalyzed dehydrazinative coupling reaction between arylsulfonyl hydrazides and ethynylbenziodoxolone reagents was realized for the synthesis of alkynyl sulfones. The scope and versatility of the reaction were demonstrated by the efficient synthesis of 23 derivatives with diverse structural features.

Full text of DOI:10.1002/ejoc.201700524

A Journal of
Accepted Article
Title: Gold-Catalyzed Dehydrazinative C(sp)-Sulphur Coupling
Reactions of Arylsulfonyl Hydrazides with TIPS-EBX
Authors: Nitin T. Patil and Popat S. Shinde
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700524
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10.1002/ejoc.201700524
European Journal of Organic Chemistry
COMMUNICATION
Gold-Catalyzed
Dehydrazinative
Arylsulfonyl
C(sp)-Sulphur
Hydrazides
Coupling
Reactions
of
with
Ethynylbenziodoxolones (EBXs) for Accessing Alkynyl Sulfones
Popat S. Shinde,[ab] and Nitin T. Patil*[ab]
Abstract: A gold(III)-catalyzed dehydrazinative coupling reaction
between arylsulfonyl hydrazides and ethynylbenziodoxolones
(EBXs) reagents has been realized for the synthesis of alkynyl
sulfones. The scope and versatility of the reaction has been
demonstrated by efficient synthesis of 23 derivatives with diverse
structural features.
ethynylbenziodoxolones (EBXs) and arylsulfonyl hydrazides
which can be easily prepared in one step from the
corresponding broadly available sulfonyl chloride (Scheme 1, eq
4).[10] Although sulfonyl hydrazide has emerged as a stable and
readily available sulfonyl anion equivalent in the metal-catalyzed
reactions;[11] there exists no report on their reaction with alkynyl
cation equivalents.
Introduction
Aryl sulfones are very important class of compounds in
medicinal chemistry and are found in numerous biologically
important compounds and drugs.[ 1 ] As a subset of these
molecules, arylalkynyl sulfones represent an important class of
synthon because of their versatile reactivity.[2] Driven by their
exceptional reactivity and it’s importance in biology, medicines,
and materials science, considerable effort has been directed
toward the development of efficient methods for their synthesis.
For instance, the group of Stang[3] and Moran[4] reported the
preparation of alkynyl sulfones by the reaction of iodonium salts
and sodium sulfinate (Scheme 1, eq 1). Recently, Kuhakarn et al.
reported coupling of arylacetylenic acids and arylacetylenes with
sodium sulfinates under iodine/TBHP conditions to access
arylalkynyl sulfones (Scheme 1, eq 2). [5]
Over the last few years, 1-[(triisopropylsilyl)-ethynyl]-1,2-
benziodoxol-3(1H)-one (TIPS-EBX) reagent has emerged as a
powerful alkynylating agent.[6] The use of this reagent was first
recognized by Waser and co-workers in the metal catalyzed
alkynylation of electron-rich heteroaromatics such as indoles,
pyrroles, thiophenes and anilines.[7] Inspired by this pioneering
work on the identification of TIPS-EBX as alkynyl cation
equivalent, other researchers including Li, Loh, Huang and
Glorius also used this reagent for various organic
transformations. [8] Chen and Waser recently reported one-pot
process for the preparation of arylalkynyl sulfones through the
reaction of ethynylbenziodoxolone (EBX) reagents, DABSO
[DABCO-bis(sulfur dioxide)] with either organomagnesium
reagents or aryl iodides under palladium catalysis (Scheme 1,
eq 3). [9] Herein, we report the synthesis of alkynyl sulfones via
gold(III)-catalyzed dehydrazinative coupling reaction between
Scheme 1. Synthesis of alkynylsulfones: previous and present work.
Results and Discussion
Our initial investigation commenced with the reaction of 4-
methylbenzenesulfonohydrazide (1a) and TIPS-EBX (2a) in the
presence of various gold catalysts (5 mol %) in CH2Cl2 (Table 1).
Preliminary screening with Ph3PAuCl catalyst resulted in the
formation of desired coupled product 3a; albeit, in 21% yield
(entry 1). The use of cationic gold complex such as Ph3PAuOTf
(generated in situ by mixing equimolar ratios of Ph3PAuCl and
AgOTf) did not improve the yield either (entry 2). To our delight,
3a was obtained in 46% yield when AuCl was used as catalyst
[a]
[b]
Popat S. Shinde, Nitin T. Patil
Division of Organic Chemistry
CSIR - National Chemical Laboratory
Dr. Homi Bhabha Road, Pune - 411 008, India
E-mail: n.patil@ncl.res.in.
Academy of Scientific and Innovative Research (AcSIR)
New Delhi – 110 025, India
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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(entry 3). Next, we focused our attention on the use of Au(III)
salts as they have proved to be efficient catalysts in related
reactions.[7g, 7i, 8m,n] Indeed, when AuCl3 was used as catalyst, 3a
was isolated in 64% yield (entry 4). Inspired by these results, we
screened various gold (III) catalysts; however, none of them was
found to be beneficial (entry 5-8). The yield of 3a was improved
when 2a was used in excess (entries 9 and 10). Use of large
excess of 2a (2 equiv) did not prove beneficial (entry 11). Hence,
we considered the results described in entry 10 are optimal.
Further solvent screening revealed that CH2Cl2 should be the
solvent of choice (entries 12-14 vs. entry 10). The fact that
reaction failed to produce 3a in the absence of gold catalyst
clearly indicates the crucial role of AuCl3 as a catalyst for
present C-S coupling reaction (entry 15).
with electron withdrawing group like -NO2 on the phenyl ring
produced the requisite product 3g; albeit, in low yield (55%).
Notably, the reaction with arylsulfonylhydrazide bearing an
amide group at p-position also proceeded smoothly to give the
alkynylated sulfone 3h in 62% yield. The reactions of arylsulfonyl
hydrazides bearing various groups (-CH3, -OCH3 and -Cl) at the
meta position gave 3i-3k in comparable yields (65−70%). When
we investigated the substrate scope of arylsulfonyl hydrazides
bearing various disubstitution pattern on the aryl ring, the
reaction faithfully produced the desired
Table 2. Substrate scope[a,b]
Table 1. Optimization of the reaction conditions[a]
Yield
2a (x
equiv)
entry
Cat. [Au]
solvent
(%)[b]
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
1
2
3
4
5
6
7
Ph3PAuCl
Ph3PAuOTf[c]
AuCl
1.1
1.1
1.1
1.1
1.1
1.1
1.1
21
26
46
64
63
58
60
AuCl3
Py·AuCl3
AuCl3·3H2O
NaAuCl4·2H2O
dichloro(2-
pyridinecarboxylato)gold
CH2Cl2
8
1.1
59
CH2Cl2
CH2Cl2
CH2Cl2
ACN
9
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
-
1.3
1.5
2.0
1.5
1.5
1.5
1.5
75
84
85
55
51
47
-
10
11
12
13
14
15
THF
toluene
CH2Cl2
aReaction conditions: 0.25 mmol 1a, x equiv 2a, 5 mol% [Au], solvent (3 mL),
30 °C, 10 h. bIsolated yields. cThe catalyst Ph3PAuOTf was generated in situ
by mixing equimolar quantities of Ph3PAuCl and AgOTf.
The optimized reaction conditions (Tabel 1, entry 10) were
applied to the set of sulfonyl hydrazides 1 by keeping TIPS-EBX
(2a) as stationary. As shown in Table 2, a variety of sulfonyl
hydrazides bearing electron donating as well as electron
withdrawing groups smoothly reacted with TIPS-EBX (2a) to
afford corresponding arylalkynylsulfones in moderate to good
yields. Benzenealkynyl sulfone 3b could be synthesized in 79%
yield starting from benzenesulfonyl hydrazide. The introduction
of 4-methoxy group at phenyl ring of sulfonyl hydrazide was also
found to be suitable for the reaction to give product 3c in 86%
yield. Halo substituents at p–position of arylsulfonylhydrazide
were also well tolerated to afford 3d, 3e and 3f in 65, 63 and
68% yields respectively. However, the arylsulfonyl hydrazides
aReaction conditions:0.25 mmol 1, 0.38 mmol 2a, 5 mol% AuCl3, CH2Cl2 (3
mL), 30 °C, 10 h. bIsolated yield. cA complex mixture of products was obtained.
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10.1002/ejoc.201700524
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COMMUNICATION
report, the proposed mechanism for gold(III)-catalyzed C-S bond
forming reactions is depicted in Scheme 4. At first, the
coordination of AuCl3 to the triple bond of TIPS-EBX (2a) would
take place to form gold−alkyne complex Ia. The iodine(III) center
could be capable of activating the alkyne triple bond more
efficiently than the gold center, as shown by Ariafard et al.,[12] to
products 3l-3o in 57-63% yields. Next, β-naphthyl sulfonyl
hydrazide was tested under the optimized reaction conditions
and in this case 3p was obtained in 68% yield. It is worth
mentioning that not only aryl sulfonyl hydrazide but also benzyl
sulfonyl hydrazide was compatible under the reaction conditions
giving 3q in 52% yield. The product 3r was obtained in 47%
yield when the reaction of phenylethenesulfonohydrazide was
performed with 2a under established reaction conditions.
generate
another
complex
Ib.
Simultaneously,
the
dehydrazination in arylsulfonyl hydrazides might occur to
generate incipient sulfonyl anions[11] which would spontaneously
attack on C-C triple bond of intermediate Ia/Ib to generate
gold−vinylidene complex (II). The vinyl gold intermediate II, thus
formed, would undergo α/β elimination depending on the
regioselectivity of the addition followed by a 1,2-silyl shift to
provide alkynyl sulphones.[7c]
Moreover,
heteroaryl
sulfonylhydrazides
were
worked
satisfactorily under the standard reaction conditions giving 3s-3u
in yields ranging from 54-64%. On the other hand, we observed
no formation of arylalkynylsulfones 3v when sterically hindered
arylsulfonyl hydrazide like mesitylenesulfonyl hydrazide was
employed as substrate. The scalability of our approach was
examined by performing the reaction on a gram scale (5.37
mmol of 1a). Notably, the yield was comparable and product 3a
was isolated in 79% yield.
Next, we investigated the utility of various EBX analogues for
the C-S coupling reactions. Accordingly, substrate 1a was
treated with TBDMS-EBX (2b), TBDPS-EBX (2c) and Ph-EBX
(2d) under standard reaction conditions. The reaction worked
well with 2b and 2c, giving rise to the products 4a and 4b in 70
and 62% yields, respectively. However, the reaction failed to
yield the desired alkynylated sulfone product 4c when Ph-EBX
(2d) was employed as an alkynylating agent leaving the starting
materials decomposed (Scheme 2). Similarly, Me-EBX, tBu-EBX,
nPent-EBX and CF3-EBX did not give the desired sulfones when
treated with 1a under standard reaction conditions. The utility of
product 3a was shown by deprotecting TIPS group with NaF to
obtain tolyl-ethynyl sulfone 5 in 63% yield (Scheme 3).12 Be
noted that the several functionalization reaction of 5 is well
known in literature.
Scheme 4. A plausible mechanism
Conclusions
In conclusion, gold(III)-catalyzed dehydrazinative coupling
reaction between TIPS-EBX and arylsulfonyl hydrazides has
been developed to synthesize a variety of arylalkynyl sulfones.
In the presence of 5 mol% AuCl3, a range of aryl-, heteroaryl-,
and benzyl sulfonyl hydrazides smoothly underwent coupling
with TIPS-EBX to give structurally diverse arylalkynylsulfones in
moderate to good yields. Though there exists several reports on
the use of sulfonyl hydrazides as sulfonyl anion equivalent in the
metal catalyzed reactions, this work represent the first example
of the reaction of sulfonyl hydrazides with alkynyl cation
equivalent. Moreover, the current study paves the way for the
use of sulfonyl hydrazides as unique sulfur nucleophiles in gold-
catalyzed reactions.
Scheme 2. Scope with various ethynylbenziodoxolones (EBXs).
Experimental Section
Scheme 3. Deprotection of TIPS group in 3a
Representative procedure: To a screw-cap vial containing a stir bar
were added p-toluenesulfonyl hydrazide (1a, 47 mg, 0.25 mmol), TIPS-
EBX (2a, 162 mg, 0.38 mmol, 1.5 eq), AuCl3 (3.7 mg, 5 mol%) and
CH2Cl2 (3 mL). The reaction vial was fitted with a cap, evacuated and
back filled with N2 and stirred at 30 °C for 10 h. The reaction mixture was
diluted with EtOAc and washed with NaHCO3 followed by brine. The
collective organic layers were dried over Na2SO4 and the solvent was
removed under vacuo. The resulting residue was purified by flash column
The mechanistic aspect of the gold-catalyzed electrophilic
alkynylation is highly warranted. Both π-activation and oxidative
addition pathways are plausible. Since the oxidative addition
pathway requires a gold(I) species, this activation mode seems
less likely as phosphine ligated gold(I) complexes exhibited poor
activity. Accordingly, based on the Waser's[7c] and Ariafard's[13]
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10.1002/ejoc.201700524
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COMMUNICATION
chromatography on silica gel (eluent: pet. ether/EtOAc) to give
analytically pure 3a (71 mg, 84% yield).
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Acknowledgements
Generous financial support by the Board of Research in Nuclear
Science
(BRNS),
Mumbai
(Grant
No.
37(2)/14/12/2015/BRNS/10549) is gratefully acknowledged. P. S.
S. thanks CSIR for the award of Senior Research Fellowship.
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Keywords: Gold catalysis • hydrazides • coupling • sulfones •
synthetic methodology
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Alkynyl Sulfones
Popat S. Shinde, Nitin T. Patil*
Page No. – Page No.
Gold-Catalyzed
Dehydrazinative
C(sp)-Sulphur Coupling Reactions of
The synthesis of alkynyl sulfones via gold(III)-catalyzed dehydrazinative coupling
reaction between ethynylbenziodoxolones (EBXs) and arylsulfonyl hydrazides has
been described. The mild reaction conditions and the tolerance electron-
withdrawing and electron-donating substituents in arylsulfonyl hydrazides make this
process amenable for the gram scale synthesis of functionalized arylalkynyl
sulfones.
Arylsulfonyl
Ethynylbenziodoxolones (EBXs) for
Accessing Alkynyl Sulfones
Hydrazides
with
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