Decarbonylative C-C bond-forming reactions of saccharins by nickel catalysis: Homocoupling and cycloaddition

Article abstract of DOI:10.1002/chem.201406227

Decarbonylation of saccharins by nickel catalysis enables two kinds of C-C bond-forming reactions; homocoupling of saccharins to form biaryls and cycloaddition with alkynes to form benzosultams. The former represents the first reported nickel-catalyzed decarbonylative C-C homocoupling reaction, whereas the latter constitutes a powerful method to pharmaceutically relevant benzosultams. The reactions proceed with good functional-group tolerance and excellent regioselectivity.

Full text of DOI:10.1002/chem.201406227

DOI: 10.1002/chem.201406227
Communication
&
Decarbonylation
Decarbonylative CÀC Bond-Forming Reactions of Saccharins by
Nickel Catalysis: Homocoupling and Cycloaddition
[a]
[a]
[c]
[a, b]
Pengbing Mi, Peiqiu Liao, Tao Tu, and Xihe Bi*
Abstract: Decarbonylation of saccharins by nickel catalysis
enables two kinds of CÀC bond-forming reactions; homo-
coupling of saccharins to form biaryls and cycloaddition
with alkynes to form benzosultams. The former represents
the first reported nickel-catalyzed decarbonylative CÀC
homocoupling reaction, whereas the latter constitutes
a powerful method to pharmaceutically relevant benzosul-
tams. The reactions proceed with good functional-group
tolerance and excellent regioselectivity.
Scheme 1. Transition metal-catalyzed decarbonylative CÀC bond-forming re-
Carbon–carbon bond formation is an intensely studied objec-
tive in organic chemistry. The development of transition metal
actions.
(
TM)-catalyzed processes has seen the concept undergo tre-
bonds, which has been utilized in decarbonylative CÀC bond-
[
1]
mendous progress in the last few decades. A number of ac-
claimed techniques for CÀC coupling reactions , including the
Ullmann, Kumada, Negishi, Suzuki, and Stille reactions, have
forming reactions such as cross-coupling and cycloaddition re-
[
5,7,9]
actions.
However, nickel-catalyzed decarbonylative CÀC ho-
mocoupling reactions remain unknown. Saccharins are easily
[
2]
[10]
been reported. Rapid progress in the field of CÀH bond acti-
available compounds with a range of synthetic applications,
[11]
[12]
[13]
vation has particularly revealed many powerful methods for
including in amination,
ring-expansion,
isomerization,
[
3]
[14]
such developments. Nevertheless, the search for new meth-
ods for CÀC bond formation remains of great demand to or-
ganic chemists. Carbonyl compounds, such as aldehydes, ke-
tones, carboxylic acids and their derivatives, are among the
most abundant chemicals and the ubiquitous nature of car-
bonyl compounds makes TM-catalyzed decarbonylation strat-
egies a versatile mode of reactivity, especially in CÀC bond
and carbonylation reactions. Herein, we report the first in-
vestigation of nickel-catalyzed decarbonylation of saccharins,
which leads to the development of two CÀC bond-forming re-
actions; a mechanistically novel homocoupling to form biaryls
and an efficient cycloaddition with alkynes to form benzosul-
tams (Scheme 1b).
The initial discovery of the decarbonylative homocoupling of
saccharins stemmed from the preparation of ortho-deuterium-
[
4]
forming reactions. Four modes of CÀC bond formation fol-
lowing the in situ generation of CO-coordinated organometal-
lic species (CÀM(CO)ÀZ; Scheme 1a) are known to date: Cross-
[15]
labeled sulfonamides during our studies in alkyne chemistry.
Recently, the research group of Kurahashi and Matsubara de-
scribed a series of nickel-catalyzed decarbonylative cycloaddi-
[
5]
[6]
coupling, homocoupling, addition to unsaturated hydrocar-
[
7]
[8]
[7]
bons, and the extrusion of CO from ketones. Reports related
to decarbonylative homocoupling of carbonyls have been few
tion reactions of carbonyls with unsaturated hydrocarbons.
Inspired by this work, we envisaged that treatment of the sac-
charins with stoichiometric amounts of nickel(0) catalyst and
subsequent protonation with CF SO H could give sulfon-
[
4,6]
0
and restricted to aldehydes and acyl chlorides. Ni is capable
of undergoing oxidative addition to a variety of CÀheteroatom
3
3
amides. Surprisingly, the anticipated reaction failed to yield sul-
fonamide 2a’; instead, the decarbonylative homocoupling
product 2a, a biphenyl-2,2’-disulfonamide, was isolated in 31%
yield accompanied with 58% recovered saccharin (Table 1,
entry 1). This reaction is quite unique, as it represents an un-
precedented decarbonylative homocoupling mode for CÀC
bond formation. Consequently, detailed studies to elucidate
the effect of the additive on the reaction outcome were carried
out. Firstly, the amount of CF SO H was found to significantly
[
a] P. Mi, Dr. P. Liao, Prof. X. Bi
Department of Chemistry, Northeast Normal University
Changchun 130024 (P. R. China)
E-mail: bixh507@nenu.edu.cn
Homepage: http://www.bigroup.com.cn/
[b] Prof. X. Bi
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (P. R. China)
[
c] Prof. T. Tu
Department of Chemistry, Fudan University
3
3
affect the distribution ratio of product 2a and recovered sub-
strate 1a. For our three initial experiments, an exceptional
result that afforded 94% yield of 2a was realized from the
2
00433 Shanghai (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406227.
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Table 1. Screening of reaction conditions.
Entry Substrate Additive Amount Product
Recovered starting
material [%]
[
a]
[equiv]
yield [%]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1ab
CF
CF
CF
CF
3
3
3
3
SO
SO
SO
3
3
3
H
H
H
2.0
5.0
10.0
2a, 31
2a, 70
2a, 94
2a, 67
2a, 42
2a, trace
2a, 0
2ab, 0
58
18
0
19
51
90
95
100
COOH 10.0
[
b]
HCl
BF
NaBH
CF
10.0
10.0
10.0
10.0
3
·Et
2
O
4
3
SO
3
H
2
[a] Yields of isolated compound; [b] HCl in Et O (5m).
Scheme 2. Substrate scope for the decarbonylative homocoupling of saccha-
rins. cod=1,5-cyclooctadiene.
complete conversion of 1a upon treatment with 10 equiva-
lents of CF SO H (Table 1, entries 2 and 3). However, replace-
3
3
ment of CF SO H with the slightly weaker CF COOH led to an
3
3
3
incomplete reaction with 67% yield of 2a (Table 1, entry 4).
The proton-promoted coupling was verified, because HCl in
A cross-coupling reaction between saccharins 1a and 1h
was performed by mixing the same equivalents of reactants in
the reaction. A mixture consisting of homo-coupling products
2a, 2h, and a cross-coupling product 2ah, was obtained in
86% total yield [Eq. (1)]. This result is mechanistically impor-
tant, as it implies that the decarbonylative CÀC homocoupling
proceeded through an intermolecular process.
Et O could promote this reaction and result in a fair yield of
2
2
a, which excluded the necessity of acid anions (Table 1,
entry 5). Furthermore, BF ·Et O (a Lewis acid) and NaBH (a hy-
3
2
4
dride donor) proved to be totally inactive for the decarbonyla-
tion as substrate 1a was recovered almost completely (Table 1,
entries 6 and 7). With the conditions listed in entry 3, the reac-
tivity of phthalimide 1ab, a compound similar to saccharin 1a
in structure, was examined. Surprisingly, substrate 1ab was
completely recovered, without coupled product 2ab (Table 1,
entry 8). The remarkable difference in reactivity between 1a
and 1ab may be due to the strong electronegativity and steric
[
16]
effect of the SO group in saccharins.
2
From a synthetic point of view, we have found a new reac-
[
17]
tion toward the synthesis of biaryls. Although biphenyl-2,2’-
disulfonamides, such as 2a, are key structural motifs in radia-
tion-sensitive pigments, known synthetic methods have been
limited to the copper-catalyzed Ullmann coupling of ortho-io-
To capture the organonickel intermediates with other com-
ponents and utilize this nickel-catalyzed decarbonylative meth-
odology, we examined the cycloaddition of saccharins with al-
kynes, a potential pathway to pharmaceutically important ben-
[
18]
dosulfonamides, with low yields and efficiency. This limita-
tion inspired us to investigate the reaction scope for this new
decarbonylative homocoupling reaction under nickel catalysis
[19]
(Scheme 2). The N-aryl saccharins 1b–h were readily applied to
zosultams. Although a number of synthetic protocols toward
benzosultams have been reported, most of these methods in-
volve intramolecular cyclizations using elaborately designed
the nickel-promoted reactions, affording the corresponding
coupling products 2b–h in excellent yields. A diverse range of
aromatic substituents, including electron-rich and -poor aryl,
heteroaryl, and fused aryl groups, were well tolerated. The
structure of 2c was unambiguously confirmed by single-crystal
X-ray diffraction analysis. The inability of the N-methyl reactant
[
20]
substrates and multistep routes.
However, step-economi-
[21]
cal, metal-catalyzed cycloaddition approaches to benzosul-
[22]
tams are comparatively less well-studied. Screening of reac-
tion conditions for the cycloaddition of saccharin 1a with
alkyne 3a was conducted and the identified optimal condi-
tions afforded the desired benzosultam 4a in 85% yield in the
1
i in this coupling reaction demonstrated the significant influ-
ence of N-substituents.
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presence of 50 mol% of nickel(0) catalyst [Eq. (2)]. This consti-
tuted a straightforward route to benzosultams.
cal alkynes, we were pleased to note that the regioisomers
4s–u were generated regioselectively in good yields. The re-
giochemistry was established by NOE experiments.
Next, attention was focused on the intramolecular decarbon-
ylative cycloaddition. Under the standard conditions, the reac-
tion of 5a smoothly proceeded to afford a novel tricyclic prod-
uct 6a in 78% yield, with a 1:1 ratio of Z and E stereoisomers
(
Scheme 4). The tricyclic structure was unequivocally resolved
The substrate scope for saccharins was next investigated
(Scheme 3) Saccharins containing electron-rich or -poor N-aryl
groups reacted efficiently with alkynes 3, affording the corre-
sponding products (4b–g) in good to excellent yields (70–
93%). Representative functional groups such as ether, chloride,
trifluoromethyl, and ester, were well tolerated. In addition to
Scheme 4. Intramolecular decarbonylative cyclization.
by X-ray crystallographic analysis. The length of the alkyl chain
between the saccharin and alkyne moieties significantly affect-
ed the reaction outcome, as the reaction of 5b resulted in an
unidentified mixture.
Although the precise mechanism of the reaction remains un-
clear, we proposed the following reaction pathways based on
[
7,9,23]
related precedents (Scheme 5).
First, oxidative addition of
0
1
a and Ni catalyst takes place to afford the nickelacycle A,
which can be converted into intermediate B by the extrusion
of CO. In the presence of large excess of TfOH, the decarbon-
ylative homocoupling reaction takes place giving biaryl 2a, in-
Scheme 3. Substrate scope for the decarbonylative cycloaddition of saccha-
rins with alkynes.
the aryl groups, N-alkyl groups that also underwent reaction
included benzyl (4h), methyl (4i), n-butyl (4j, 4l), and notably,
2,2-dimethoxyethyl (4k), a masked aldehyde group capable of
undergoing further synthetic derivatization. It is also notable
that the 3-thienyl substituent was also amenable to the reac-
tion conditions, affording N-thienyl benzosultam 4m in 87%
yield. The scope of internal alkynes was also examined for their
reaction with N-(p-tolyl)saccharin 1b. Pleasingly, all selected
symmetrical aryl and alkyl alkynes reacted smoothly with 1b
to afford the corresponding benzosultams 4n–u in good
yields. The structures of both 4m and 4o were unambiguously
confirmed by X-ray diffraction analysis. Regarding unsymmetri-
Scheme 5. Proposed mechanism.
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Communication
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[
26]
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[
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is formed and subsequently forms intermediate H.
After
reductive elimination the tricyclic product 6a is released along
0
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In conclusion, our studies on the decarbonylative reactions
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[
[
Keywords: cycloaddition · decarbonylation · homocoupling ·
nickel · saccharins
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Published online on && &&, 0000
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Decarbonylation of saccharins by
&
Decarbonylation
nickel catalysis has led to the discovery
of two novel CÀC bond-forming reac-
tions; homocoupling to form biaryls,
and cycloaddition with alkynes to give
benzosultams. The former represents
the first nickel-catalyzed decarbonyla-
tive CÀC homocoupling reaction,
whereas the latter constitutes a powerful
method to pharmaceutically relevant
benzosultams with good functional-
group tolerance and excellent regiose-
lectivity.
P. Mi, P. Liao, T. Tu, X. Bi*
& – &&
&
Decarbonylative CÀC Bond-Forming
Reactions of Saccharins by Nickel
Catalysis: Homocoupling and
Cycloaddition
&
&
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!