Rhodium Thiavinyl Carbenes from 1,2,3-Thiadiazoles Enable Modular Synthesis of Multisubstituted Thiophenes

Article abstract of DOI:10.1021/acs.orglett.6b00541

The rhodium-catalyzed transannulation reaction between 1,2,3-thiadiazoles and alkynes, proceeding via intermediacy of the previously unknown Rh thiavinyl carbene, toward a highly efficient and regioselective synthesis of up to fully substituted thiophenes is described.

Full text of DOI:10.1021/acs.orglett.6b00541

Letter
pubs.acs.org/OrgLett
Rhodium Thiavinyl Carbenes from 1,2,3-Thiadiazoles Enable Modular
Synthesis of Multisubstituted Thiophenes
Daria Kurandina and Vladimir Gevorgyan*
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061, United States
S
* Supporting Information
ABSTRACT: The rhodium-catalyzed transannulation reaction between
1,2,3-thiadiazoles and alkynes, proceeding via intermediacy of the previously
unknown Rh thiavinyl carbene, toward a highly efficient and regioselective
synthesis of up to fully substituted thiophenes is described.
Scheme 2. Reported Syntheses of Thiophenes from 1,2,3-
Thiadiazoles
α-Carbonyl-derived metal carbenes are versatile synthetic
intermediates employed in a wide variety of organic trans-
formations,1 including heterocyclization reactions. Among these,
Rh oxavinyl carbene A, originated from diazocarbonyl
compound,2 a stable open-chain tautomer of 1,2,3-oxadiazole,3
was efficiently utilized by Davies4a,b and Padwa4c−e in annulation
with alkynes toward furans (Scheme 1, eq 1). More recently, it
has been reported that N-tosyldiazoimine, a minor open-chain
tautomer of N-tosyl-1,2,3-triazole,5 can be trapped by transition
metals to form azavinyl carbene B. The latter has received
increased attention as a highly useful precursor for a variety of
organic transformations,6−8 including cycloaddition with alkynes
to form a pyrrole ring, as shown by Murakami7b (M = Ni) and
our group7c (M = Rh) (eq 2). Herein, we report the first efficient
generation of Rh thiavinyl carbene C from 1,2,3-thiadiazole 1
that can undergo regioselective annulation reaction with alkynes
to produce thiophenes 3 in good to excellent yields (eq 3).
A 1,2,3-thiadiazole core is a great source of highly reactive
intermediates used in the formation of a variety of acyclic and
cyclic sulfur-containing compounds,9 including thiophenes.
However, the reported methods for synthesis of thiophenes
from 1,2,3-thiadiazoles suffer a limited scope of reactive alkynes
and 1,2,3-thiadiazoles, harsh reaction conditions, lack of
Table 1. Optimization of Transannulation Reaction of 1,2,3-
a
Thiadiazoles with Alkynes
b
ligand
(5 mol %)
t
time
(h)
yield (%) (ratio
no. cat. (2 mol %)
(°C)
3a:3a′)
1
2
3
4
5
6
7
8
Rh2(Oct)4
130
130
130
130
60
15
12
12
0.1
7
1 (1:1)
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
5 (only 3a)
c
Josiphos
DPPF
DPPF
DPPF
DPPF
DPPF
70 (10:1)
98 (19.0:1)
100 (43.6:1)
81 (7.4:1)
Scheme 1. Transition-Metal-Catalyzed Transannulation
Reactions of Azoles
d
Rh(PPh3)3Cl
130
130
130
12
3
d
RhCl3·H2O
Rh2(Oct)4
100 (16.7:1)
1 (0.4:1)
15
a
The reactions were run on 0.1 mmol in PhCl (1 M) under inert
b
atmosphere. The yield and ratio were determined by GC−MS with
pentadecane as internal standard. Isolated yield (ratio was determined
by NMR). 4 mol % of catalyst has been used.
c
d
regioselectivity, and low efficiency (Scheme 2). Hence, α-
diazothione 1′,10 in the presence of stoichiometric amounts of
strong base, decomposes into alkynethiolate (eq 4).11 The latter,
upon [3 + 2] cycloaddition with activated alkynes, leads to the
Received: February 25, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00541
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Organic Letters
Letter
a
Table 2. Synthesis of Thiophenes via Rh-Catalyzed Transannulation of 1,2,3-Thiadiazoles with Alkynes
a
Reactions conditions: 1 (0.25 mmol), 2 (0.5 mmol, 2 equiv), [Rh(COD)Cl]2 (0.005 mmol, 2 mol %), DPPF (0.0125 mmol, 5 mol %),
b
c
d
e
chlorobenzene (1 M), inert atmosphere. Isolated yield. Reactions were performed on 0.5 mmol scale. Ratio of regioisomers ≥20:1. Ratio of
regioisomers 11:1.
corresponding trisubstituted thiophenes.12 Alternatively, upon
Scheme 3. Plausible Pathways for Rh-Catalyzed
heating with over stoichiometric amounts of Fe2(CO)9, 1,2,3-
thiadiazoles are believed to release dinitrogen to produce thiirene
species, which are captured by Fe2(CO)9 to give the thiavinyl
carbene complex in low yields and regioselectivity.13 It has been
reported13c that one of these symmetric carbene complexes
could undergo cycloaddition with diphenylacetylene toward
tetraphenylthiophene (eq 5). The formation of thiirene has also
been postulated upon photolytic decomposition of 1, which in
the presence of hexafluoro-2-butyne led to formation of notable
amounts of the corresponding thiophene (eq 6).14
Transannulation of 1,2,3-Thiadiazoles with Alkynes
We hypothesized that if the generation of a putative Rh
thiavinyl carbene15 C from 1,2,3-thiadiazole 1 is possible, it could
potentially become a useful intermediate for a number of
transformations, including the annulation reaction with alkynes
toward thiophenes 3.16 To this end, we examined a possible
B
DOI: 10.1021/acs.orglett.6b00541
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Organic Letters
Letter
transannulation reaction of 1,2,3-thiadiazole-4-carboxylate 1a
with phenylacetylene (Table 1). Initially, Rh(II) complex,
efficiently employed in the formation of Rh oxavinyl carbene
A4 and Rh azavinyl carbene B,6 has been tested in this
transformation. However, the starting thiadiazole decomposed
under these conditions (entry 1). Employment of Rh(I)
complex,17 however, produced a 5% of thiophene 3a (entry 2).
Gratifyingly, addition of Josiphos ligand led to a dramatic
improvement of the reaction outcome producing thiophene 3a in
isolated 70% yield in 10:1 ratio with its minor regioisomer 3a′
(entry 3). Other bulky bidentate ligands tested were efficient but
less selective.18 DPPF appeared to be a superior ligand for this
reaction both in terms of reactivity and selectivity, quickly
producing 3a in a nearly quantitative yield and excellent
regioselectivity (entry 4). Decreasing the reaction temperature
to 60 °C allowed for further improvement of the selectivity,
though the reaction required 7 h for completion (entry 5).
Wilkinson’s catalyst was less efficient in this reaction (entry 6).
Interestingly, employment of Rh(III)19 catalyst in a combination
with DPPF was also efficient, though less selective (entry 7).
Notably, Rh(II) complex while combined with DPPF ligand still
did not show substantial improvement (entry 8).
With the best optimized conditions in hand, the scope of this
novel transannulation reaction has been examined (Table 2).
First, the reaction of ethyl-1,2,3-thiadiazole-4-carboxylate (1a)
with different alkynes has been tested. It was found that
arylalkynes possessing p-MeO (2b), o-MeO (2c), p-F (2d), and
p-CF3 (2e) groups underwent a smooth transannulation reaction
with 1a to produce thiophenes 3 in a good yield with complete
regiocontrol. Notably, electron-defficient aryl alkynes 2d,e were
less reactive compared to their more electron-rich analogs 2a−c
(entries 1−5). Heterylalkyne 2f and enyne 2g reacted
uneventfully (entries 6−7). It deserves mention that some of
the produced carbethoxy thiophenes were not easily separable
from the byproducts of alkyne trimerization; therefore, these
thiophenes were isolated as the corresponding acids 4 after
hydrolysis. Importantly, primary (2h), secondary (2i), and
tertiary (2j) alkylacetylenes (entries 8−10) underwent trans-
annulation with 1a at 80 °C with high yields and
regioselectivities. Alkylacetylenes possessing functional groups
at the side chain, including cyano (2k) and chloro (2l)
substituents, as well as (triethylsilyl)acetylene (2m), successfully
reacted with 1a to produce the corresponding 2,4-disubstituted
thiophenes in good yields (entries 11−13).
complementary method toward thiophenes, and it also features
several advantages over the existing methods, such as employ-
ment of easily available and stable thiadiazoles and a wider scope
of alkynes. For instance, both terminal and internal alkynes
possessing important functional groups could be efficiently
employed in this transannulation reaction toward densely
substituted and functionalizable thiophenes.
We propose the following plausible pathways for this new
transannulation reaction (Scheme 3). First, the rhodium carbene
species C is generated upon the denitrogenative reaction of Rh
with the open form of 1,2,3-thiadiazole 1′. A [2 + 1]
cycloaddition of C with alkyne (path a) would lead to
cyclopropene intermediate D, which then cycloisomerizes21
into thiophene 3. On the other hand, C may undergo a
nucleophilic attack by alkyne (path b) to produce an ylide F,
which then cyclizes into zwitterion E.22 Alternatively, the latter
can form via a direct [3 + 2] cycloaddition of C with 2 (path c).23
A release of Rh catalyst from E produces reaction product,
thiophene 3. Apparently, the observed higher reactivity of
electron-rich alkynes fits the ylide mechanism well (path b). On
the other hand, based on the observed high regioselectivity of the
process, we tentatively consider path a as less likely.
Unmistakably, more detailed studies are required to establish
the precise mechanism for this transformation.
In summary, we have demonstrated for the first time that
reactive Rh thiavinyl carbene species can be effectively generated
from 1,2,3-thiadiazoles. Useful analogues of the widely known Rh
oxa- and -azavinyl carbenes, Rh thiavinyl carbenes enabled a new
modular approach for the construction of a multisubstituted
thiophene ring by transannulation of 1,2,3-thiadiazoles with
alkynes. The developed method features a wide scope and high
efficiency and selectivity.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00541.
Experimental details, characterization data for the
products, and copies of NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: vlad@uic.edu.
Next, the scope of 1,2,3-thiadiazoles (Table 2, 1b−e) was
examined. All of these compounds appeared to be competent
substrates in the transannulation reaction, revealing the following
reactivity trend toward 2a: 1c > 1b > 1d > 1e ≥ 1a. Hence,
reactions of 1b and 1c were completed in 30 min only, producing
the corresponding thiophenes in high yields (entries 14 and 19).
Likewise, the reactions of 1b and 1c with alkylacetylenes (2n-
p,h) were completed in just 1 h at 80 °C (entries 15, 16) or in 7 h
at 60 °C (entries 17, 20). Moreover, 1,2,3-thiadiazole 1b was able
to react with internal alkyne 2q to produce the fully substituted
thiophene 2bq in a reasonable yield (entry 18). 1,2,3-
Thiadiazoles 1d and 1e possessing a phenyl group at the C-4
position also underwent transannulation with aryl- (2a) and
alkylacetylenes (2j,r) to give the corresponding thiophenes with
good yields (entries 21−24).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank National Science Foundation (CHE-1362541) for
financial support of this work. We also thank Dr. Olga
Zatolochnaya for initial experiments and fruitful discussions.
DEDICATION
■
This work is dedicated to the memory of Prof. Alexander Butin.
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C
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Letter
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D
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