Cycloaddition of propargylic alcohols and α-oxo ketene dithioacetals: Synthesis of functionalized cyclopentadienes and further application in a diels-alder reaction

Article abstract of DOI:10.1002/anie.201403014

Cyclopentadienes are valuable intermediates in organic synthesis and also ubiquitous as the Cp ligands in organometallic chemistry. As part of ongoing efforts to develop novel organic reactions that employ functionalized alkynes, a [3+2] cycloaddition of propargylic alcohols and ketene dithioacetals has been developed, which leads to fully substituted 2,5-dialkylthio cyclopentadienes in good to excellent yields. In an unusual dethiolating Diels-Alder reaction, the cyclopentadienes were further reacted with maleimides to afford a family of novel fluorescent polycyclic compounds.

Full text of DOI:10.1002/anie.201403014

Angewandte
Chemie
DOI: 10.1002/anie.201403014
[3+2] Cycloaddition
[3+2] Cycloaddition of Propargylic Alcohols and a-Oxo Ketene
Dithioacetals: Synthesis of Functionalized Cyclopentadienes and
Further Application in a Diels–Alder Reaction**
Zhongxue Fang, Jianquan Liu, Qun Liu, and Xihe Bi*
Abstract: Cyclopentadienes are valuable intermediates in
organic synthesis and also ubiquitous as the Cp ligands in
organometallic chemistry. As part of ongoing efforts to develop
novel organic reactions that employ functionalized alkynes,
a [3+2] cycloaddition of propargylic alcohols and ketene
dithioacetals has been developed, which leads to fully sub-
stituted 2,5-dialkylthio cyclopentadienes in good to excellent
yields. In an unusual dethiolating Diels–Alder reaction, the
cyclopentadienes were further reacted with maleimides to
afford a family of novel fluorescent polycyclic compounds.
S
ince the first report by Kelber in 1910,[1] a-oxo ketene
Figure 1. a-Oxo ketene dithioacetals as polarized alkenes in [2+x]
cyclization reactions.
dithioacetals have been shown to be versatile intermediates in
organic synthesis because of the characteristic bis(alkylthio)
groups, which are strongly electron-donating moieties and
easily cleaved.[2] a-Oxo ketene dithioacetals and their
extended conjugated derivatives are important 1,3-,[3] 1,5-,[4]
and 1,7-electrophilic synthons[5] and have been extensively
investigated in cyclization reactions. As a-oxo ketene dithioa-
cetals are polar functionalized alkenes, they can be used as
a two-carbon fragment in cyclization reactions. However,
such synthetic applications are rare, and all of the reports are
limited to the synthesis of heterocycles by classic alkylthio
displacement reactions (Figure 1).[6] Applications to the syn-
thesis of carbocycles have remained elusive. As part of our
ongoing efforts to develop novel organic reactions that
employ functionalized alkynes,[2,7] we herein report a novel
[3+2] cycloaddition of propargylic alcohols and a-oxo ketene
dithioacetals, which affords a new class of fully substituted
2,5-dialkylthio cyclopentadienes (Figure 1).[8] To the best of
our knowledge, this is the first example in which a-oxo ketene
dithioacetals were used as the polarized alkene component
for the synthesis of carbocycles. Cyclopentadienes are val-
uable intermediates in organic synthesis and also ubiquitous
as the Cp ligands in organometallic chemistry.[9] Despite
a number of available synthetic strategies, the development of
new efficient methods,[10–14] particularly those starting from
easily available starting materials to afford densely function-
alized cyclopentadienes, is still of great value.
Initially, the reaction conditions were optimized using
a-oxo ketene dithioacetal 1a and propargylic alcohol 2a. As
shown in Table 1, different catalysts and solvents were
screened. First, metal-based Lewis acids were investigated;
FeCl3, TiCl4, and SnCl4 afforded a trace amount of the desired
cyclopentadiene 3a in 1,4-dioxane, whereas CuCl2·H2O was
ineffective (entries 1–4). To our delight, the nonmetal Lewis
acid BF3·Et2O efficiently catalyzed the cycloaddition of 1a
and 2a, affording 3a in very good yield (86%; entry 5). The
strong protonic acid CF3SO3H was also an effective catalyst of
this reaction, but provided the desired product in a slightly
Table 1: Optimization of the reaction conditions.
[*] Z. Fang, J. Liu, Prof. Q. Liu, Prof. X. Bi
Department of Chemistry, Northeast Normal University
Changchun 130024 (China)
E-mail: bixh507@nenu.edu.cn
Homepage: http://www.bigroup.com.cn/
Entry
Catalyst
Solvent
t [h]
Yield[a] [%]
1
2
3
4
5
6
7
8
9
FeCl3
TiCl4
SnCl4
CuCl2·H2O
BF3·Et2O
CF3SO3H
BF3·Et2O
BF3·Et2O
BF3·Et2O
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
toluene
3.0
3.0
3.0
1.0
1.5
1.0
1.0
1.0
1.0
trace
trace
trace
0
86
65
70
75
43
Prof. X. Bi
State Key Laboratory of Elemento-Organic Chemistry
Nankai University
Tianjin 300071 (China)
[**] This work was supported by the NNSFC (21172029, 21202016,
21372038, 31101010), the Ministry of Education of the People’s
Republic of China (NCET-13-0714), and the Jilin Provincial Research
Foundation for Basic Research (20140519008JH).
CH3CN
MeOH
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403014.
[a] Yields of isolated products.
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lower yield (65%; entry 6). Next, the influence of the solvent
was studied using BF3·Et2O as the catalyst; non-polar
(toluene), polar (CH3CN), and protic (MeOH) solvents
could be used, but the products were obtained in lower
yields (entries 7–9). Therefore, the reaction conditions that
are described in Table 1, entry 5 were found to be the most
suitable for this transformation and selected for further
investigations.
With the optimized reaction conditions in hand (Table 1,
entry 5), the substrate scope of the cyclopentadiene synthesis
was investigated (Scheme 1). First, the use of various a-oxo
ketene dithioacetals with different R1 and R2 substituents was
investigated in the reaction with 2a. Diverse functional
groups, such as amino, alkoxy, aryl, and styryl groups, as the
R1 group were tolerated under these reaction conditions, and
the corresponding cyclopentadienes were obtained in good to
high yields (3b–3e, 64–84%). The length of the alkylthio
moiety of the a-oxo ketene dithioacetal significantly affected
the outcome of the reaction. For example, the methylthio-
substituted substrate afforded cyclopentadiene 3 f in 92%
yield, whereas the bulky n-butylthio-substituted substrate was
converted into product 3g in only 68% yield. Next, we
systematically varied the R3, R4, and R5 substituents of the
propargylic alcohol derivative and subjected these derivatives
to the reaction with 1a. To our delight, diverse tertiary
propargylic alcohols could be used in this cycloaddition
reaction, affording the corresponding cyclopentadienes in
good to excellent yields (3h-3z; 62–94%). Various functional
groups, such as electron-donating and -withdrawing (hetero)-
aryl, fused aryl, alkyl, and ether groups, could be installed at
the 1- and/or 3-position of the cyclopentadiene skeleton.
Notably, a substrate possessing a terminal alkyne unit (R5 =
H) also participated in the reaction with 1a, the correspond-
ing cyclopentadiene 3z with an unoccupied 3-position was
isolated in 79% yield. Although cyclopentadienes 3 were
thoroughly characterized by NMR spectroscopy and high-
resolution mass spectrometry (HRMS), their structures were
confirmed by X-ray diffraction (XRD) of compound 3q.
Moreover, biscyclopentadienes, such as 3aa and 3ab, could be
prepared in one step from the corresponding bispropargylic
alcohols.
Sulfur migration has emerged as a valuable process for
organic chemists; 1,2-sulfur migration has been widely inves-
tigated and applied in organic synthesis.[15] However, other
types of sulfur migration, such as a 1,4-sulfur shift, are rarely
described. Recently, Wang and co-workers reported an
unprecedented 1,4-shift of the sulfanyl group in the transi-
tion-metal-catalyzed reaction of allenyl sulfides.[16] We envi-
sioned that a 1,4-alkylthio shift may be involved in the
formation of cyclopentadienes 3. To confirm this hypothesis,
control experiments were carried out. First, a competition
experiment was performed by adding two equivalents of
benzyl mercaptan (BnSH) to the reaction of 1a with 2a under
the standard reaction conditions [BF3·Et2O (30 mol%), 1,4-
dioxane, 608C; Eq. (1)]. Cyclopentadiene 3a was obtained in
75% yield, whereas the benzylthio-substituted cyclopenta-
diene 3a’ was not observed by HRMS analysis of the reaction
mixture, indicating that the alkylthio shift is an intramolecular
process. Moreover, to test the specificity of the alkylthio shift,
propargylic alcohol 2z, which contains a terminal alkyne
group, was subjected to the reaction with 1a, because 2z
would facilitate both 1,4- and 1,3-alkylthio shifts. However,
only the 1,4-alkylthio shift occurred, affording 3z as the sole
product, without forming 3z’ through a 1,3-alkylthio shift
[Eq. (2)]. This result confirmed that the 1,4-alkylthio shift
occurred as a specific process.
Scheme 1. Substrate scope of the cyclopentadiene formation.
[a] CF3SO3H (30 mol%).
2
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Based on these results and related precedents,[2,17] a plau-
sible mechanism for the cyclopentadiene synthesis was
proposed (Scheme 2). The polarity of the hydroxy group in
To elucidate the reaction mechanism for the formation of
4a, the overall transformation was conducted in a stepwise
fashion (Scheme 3). In the absence of maleimide, deacety-
Scheme 3. Investigation of the reaction mechanism.
lated product 5a was obtained in nearly quantitative yield
under the acid-catalyzed conditions. Further treatment of 5a
with CF3SO3H at 1008C for two hours did not result in any
reaction. Similarly, no reaction was observed when a mixture
of 5a and maleimide was heated at 1008C in the absence of
CF3SO3H for two hours. However, when the CF3SO3H
catalyst was added to the reaction mixture of 5a and
maleimide, the Diels–Alder reaction smoothly proceeded
and afforded 4a in 87% yield. Apparently, CF3SO3H played
a dual role in the cycloaddition of cyclopentadiene with
maleimide: 1) It promotes the deacetylation of the cyclo-
pentadiene and 2) enhances the ability of maleimide to act as
the dienophile, probably through protonation of the nitrogen
and/or oxygen atom.[21] Therefore, the mechanism for the
Diels–Alder reaction of cyclopentadienes with maleimides
may involve sequential deacetylation, Diels–Alder cyclo-
addition with participation of the aryl ring at the 3-position,
and final dethiolation, which leads to aromatization.
Next, we investigated the substrate scope of this unusual
Diels–Alder reaction (Scheme 4). Diverse cyclopentadienes 3
underwent this cascade reaction with maleimides to afford the
corresponding polycyclic products in good to excellent yields
(4b–4m, 60–92%; Scheme 4). Variations in the substituents
and their position on the aryl ring at the 1-position of
cyclopentadienes 3 did not affect the efficiency of this
reaction (4b–4e). Although the structures of products 4
were evident from the NMR and HRMS data, additional
confirmation was obtained by single-crystal XRD analysis of
product 4e. The efficiency of this transformation was not
significantly affected by the nature of the aryl group at the
3-position of cyclopentadienes 3. Thus, substrates with
electron-rich or electron-deficient substituents afforded the
corresponding polycyclic products in comparable yields (4 f
and 4g; Scheme 4). Moreover, a cyclopentadiene with a spiro
center underwent facile cycloaddition with maleimide to give
the desired product in a good yield (4h, 60%). Furthermore,
the reactions of N-protected maleimides, such as N-methyl or
N-phenyl derivatives, with 3a smoothly produced the corre-
sponding products, 4i and 4j, in 90 and 79% yield, respec-
tively. Remarkably, cyclopentadienes 3 with an unsymmetric
aryl group at the 3-position reacted with excellent regiose-
lectivity, and the reactive site was the sterically hindered ortho
position (4k, 4l, and 4m; Scheme 4).
Scheme 2. Proposed reaction mechanism.
2a was enhanced by the interaction between BF3·Et2O and
the hydroxy group, which leads to the formation of prop-
argylic carbocation A by the loss of the OH species. Because
of the resonance stabilization between propargylic carbocat-
ion A and allenic carbocation B, less sterically hindered B is
preferably attacked by the electron-rich a-carbon atom of 1a,
affording intermediate C. The allenic carbon atom attacks the
thiolanium ion species to afford a five-membered carbocycle,
along with allylic carbocation D. Next, attack of a lone pair of
electrons of the sulfur atom to the allylic carbocation affords
À
intermediate E with the formation of a new C S bond, thus
enabling the 1,4-alkylthio shift.[18] Finally, elimination of
À
a proton and cleavage of the C S bond produce cyclo-
pentadiene 3a. Meanwhile, the catalyst is regenerated by the
elimination of one molecule of H2O, which completes the
catalytic cycle.
Interestingly, when 3a and maleimide were reacted in the
presence of CF3SO3H (30 mol%), the desired Diels–Alder
cycloaddition reaction with the cyclopentadiene ring acting as
the diene was not observed [Eq. (3)].[19] Instead, an unprece-
dented dethiolating Diels–Alder reaction with participation
of the phenyl ring in the 3-position occurred; polycyclic
compound 4a was isolated in high yield (81%).[20] Notably, 4a
represents a new family of polycyclic structures. This unusual
reactivity of 3a can be attributed to the unique substitution
pattern. Moreover, the type of dienophile also played a critical
role in this cascade reaction because the use of maleic
anhydride resulted in a mixture of unidentified products.
Naturally, we were interested in studying this unusual Diels–
Alder reaction.
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have great potential as a family of small-molecule fluoro-
phores.
In conclusion, we have developed a conceptually new
strategy for the synthesis of cyclopentadienes by the BF3·Et2O
catalyzed regiospecific [3+2] cycloaddition of propargylic
alcohols and a-oxo ketene dithioacetals. A new class of fully
substituted 2,5-dialkylthio cyclopentadienes were obtained in
good to excellent yields. A mechanistically novel 1,4-alkylthio
shift was observed during the ring-closure process. The unique
substitution pattern of these cyclopentadienes facilitated an
unusual dethiolating Diels–Alder reaction with maleimides,
and therefore afforded a family of novel fluorescent polycyc-
lic compounds.
Received: March 5, 2014
Revised: April 14, 2014
Published online: && &&, &&&&
Keywords: [3+2] cycloaddition · cyclopentadienes · Diels–
.
Alder reaction · ketene dithioacetals · propargylic alcohols
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
[3+2] Cycloaddition
Z. Fang, J. Liu, Q. Liu,
X. Bi*
&&&& — &&&&
[3+2] Cycloaddition of Propargylic
Alcohols and a-Oxo Ketene
Dithioacetals: Synthesis of
Functionalized Cyclopentadienes and
Further Application in a Diels–Alder
Reaction
2,5-Dialkylthio cyclopentadienes were
prepared by the BF3·Et2O catalyzed
regiospecific [3+2] cycloaddition of
propargylic alcohols and a-oxo ketene
dithioacetals in good to excellent yields.
Furthermore, an unusual dethiolating
Diels–Alder reaction of these cyclopenta-
dienes with maleimides was discovered,
which yielded novel fluorescent polycyclic
compounds.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!