Cascade and Effective Syntheses of Functionalized Tellurophenes

Article abstract of DOI:10.1021/acs.orglett.8b00279

A one-pot and transition-metal-catalyst-free synthetic strategy to construct functionalized tellurophenes has been developed. Substituted 1,1-dibromo-1-en-3-ynes are smoothly converted to tellurophenes with telluride salts in high yield via a series of cascade reactions through reductive debromination, hydrotelluration, nucleophilic cyclization, and aromatization. Close inspection of the results clearly showed that the reactivity of in situ prepared telluride salts are significantly influenced by the polarity of the solvent system and the electronic nature of the substituent on the enyne substrate. This method reports the first direct synthesis of 3-aryltellurophenes in high yields at room temperature. This novel reaction strategy is also found to be a promising synthetic method for multisubstituted tellurophenes and selenophenes.

Full text of DOI:10.1021/acs.orglett.8b00279

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cascade and Effective Syntheses of Functionalized Tellurophenes
Vamsi Krishna Karapala,† Hong-Pin Shih,† and Chien-Chung Han*,†
†Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan, Republic of China
S
* Supporting Information
ABSTRACT: A one-pot and transition-metal-catalyst-free synthetic
strategy to construct functionalized tellurophenes has been developed.
Substituted 1,1-dibromo-1-en-3-ynes are smoothly converted to tellur-
ophenes with telluride salts in high yield via a series of cascade reactions
through reductive debromination, hydrotelluration, nucleophilic cycliza-
tion, and aromatization. Close inspection of the results clearly showed
that the reactivity of in situ prepared telluride salts are significantly influenced by the polarity of the solvent system and the
electronic nature of the substituent on the enyne substrate. This method reports the first direct synthesis of 3-aryltellurophenes in
high yields at room temperature. This novel reaction strategy is also found to be a promising synthetic method for
multisubstituted tellurophenes and selenophenes.
unctionalized chalcogenophenes are an important class of
yn-2-ol intermediate by a ring-closing reaction with NaHTe/
Fcompounds due to their wide range of applications in the Na2Te that can place the desired alkyl group directly in the 3-
position with low overall yields (<35%).7b However, these
methods require the use of an expensive Weinreb amide (i.e., 2-
fields of biochemistry, electrochemistry, and materials sci-
ence.1,2 Swapping of lighter chalcogenophenes with heavier
chloro-N-methoxy-N-methylacetamide) as the starting material
(Scheme S1c, SI).7 Thus, there is still a lack of rapid and facile
synthetic methods for the highly demanding 3-alkyl- and 3-aryl-
substituted tellurophenes. Herein, a novel method is presented
for the synthesis of 3-functionalized and 2,4-difunctionalized
tellurophenes from easily made substituted 1,1-dibromo-1-en-3-
ynes and telluride salts (Na2Te/NaHTe) under mild conditions
with high yields.
tellurophenes for conjugated polymers has become popular in
recent years, attracted by the advantages gained on red-shifted
optical absorption and enhanced polarizability.3 It is also
evident, from the emerging reports, tellurophenes are
captivating elevated recognition, over the widely accepted
thiophene counterparts, as potential new and better optoelec-
tronic materials due to their exceptional LUMO energy level
stabilizations, lower optical band gaps, strong π−π interactions,
and high charge carrier mobility.4 Although functionalized
tellurophenes are highly desired by the scientific community,
there remains a scarcity for a direct and customized method to
prepare polysubstituted tellurophenes. Most commonly used
synthetic methods for substituted tellurophenes are metal-
catalyzed cross-coupling reactions with 2- and/or 5-function-
alized tellurophenes, cyclization of substituted 1,3-diynes with
telluride salts, and cyclization reaction of (Z)-telluroenynes
with diorganoyl dichalcogenides.5 However, these methods are
only useful for making 2- and/or 5-substituted tellurophenes,
but not for 3-substituted tellurophenes, which are greatly
desirable for making highly conductive, conjugated poly(3-
substituted tellurophene)s. It is conceivable that 3-alkyl- or 3-
aryl-substituted tellurophenes can be prepared from 3-bromo-
or 3-iodo-tellurophenes via metal-catalyzed cross-coupling
reactions. However, the preparation of 3-bromotellurophene
requires a complicated and multistep procedure via a series of
novel intermediates such as 2,3,4,5-tetrakis(pinacolboronate)-
zirconacyclopentadiene, 2,3,4,5-tetrakis(pinacolboronate)-
tellurophene, and then 3-(pinacolboronate)tellurophene
(Scheme S1a, Supporting Information (SI)).6 Likewise, 3-
iodo-tellurophene has to be prepared from 1,3-butadiyne and
1,2-dibutylditellane via butyltelluroenyne (Scheme S1b, SI).5f
Alternatively, Catel et al. and Seferos et al. have demonstrated
the synthesis of 3-alkyltellurophenes via 2-alkyl-1-chlorobut-3-
We have conceived that versatile 3-alkyl and 3-aryl
tellurophenes can be prepared easily from the functionalizable
enynes 1, via cyclization with telluride salts. The enyne
precursor, e.g., (3-(dibromomethylene)oct-1-yn-1-yl)trimethyl-
silane 1i, can be synthesized by the modified literature
methods,8 and the telluride salts (Na2Te/NaHTe) can be
prepared from the reduction of Te metal with sodium
borohydride in an alcoholic solvent, such as ethanol.7 Our
initial attempts to react the precursor 1i with the telluride salts
in ethanol at 75 °C, however, failed to yield 3-n-pentyl-
tellurophene, but only resulted in the desilylated and
monodebrominated product 3i′ (entries 1 and 2, Table 1).
As desilylations can happen easily in alcoholic solvents under
basic conditions, it is obvious that deprotection of the TMS
group might be the first step in the reaction of 1i with Na2Te/
NaHTe in ethanol. Furthermore, monodebromination of
geminal dibromo-cyclopropanes and 1,1-dibromoalkenes can
occur smoothly with telluride salts (Na2Te/NaHTe) in ethanol
even at 50 °C.9,10 It is expected that monodebromination might
be the next step in the reaction of 1i with telluride salts at 75
°C. Therefore, desilylated and monodebrominated product 3i′
would be the key intermediate in the reaction of 1i and telluride
Received: January 26, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00279
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Organic Letters
Letter
strong base (KOH) are needed to trigger the reactions between
telluride salts and 3i′ (for both cyclization and dimerization
reactions).
Table 1. Optimization Studies for 3-Substituted
Tellurophenes
a
To further understand the results and reaction behaviors, a
preliminary mechanistic rationalization is depicted in Scheme 1.
As discussed above, the first stage reactions for the formation of
the compound 3 (via desilylation and reductive debromination)
can be easily accomplished at 75 °C in ethanol with the
telluride salts alone. While, the second stage reactions for the
formation of the cyclized product 2 (via hydrotelluration,
cyclization, and aromatization) and the dimerized byproduct 4
(via hydrotelluration and dimerization) have to rely on the
H2O cosolvent plus KOH to turn on the reactions. Since Te
and H have the same electronegativity of 2.1 and H2O has a
similar pKa (15.7) as ethanol (pKa = 16.0), we have
hypothesized that the main benefit provided by the H2O
cosolvent might be due to its much higher dielectric constant (ε
= 79 vs 25 of ethanol), which helps to polarize the Te−H
covalent bond in the telluride salt (NaTeH) or in the organic
tellurol (e.g., intermediate 3-1, Scheme 1), which in turn leads
to the increased negative charge (thus with enhanced
nucleophilicity) on the telluride center and the increased
positive charge (thus with increased acidity) on the tellurol
proton, similar to the renowned polarization effect of an
approaching nucleophile has on a C−I bond (cf. both C and I
have the same electronegativity of 2.5). The increased
nucleophilicity of the telluride salt (NaTeH) could trigger the
hydrotelluration with the alkyne group of 3, to afford the
organotellurol (intermediate 3-1, Scheme 1), while the
increased acidity of the tellurol proton should allow the
KOH base to deprotonate intermediate 3-1 more effectively to
form the organotelluride anion 3-2, which in turn triggers the
nucleophilic cyclization (forming 3-3, Scheme 1) or the
dimerization (forming 3-5, Scheme 1). As the amount of
H2O cosolvent increased to 30 vol %, the resultant solution
became cloudy caused by the aggregation of water-insoluble
enyne 3. The enriched local concentration of enyne 3 would
naturally lead to a higher amount of dimerized byproduct 4
over the cyclized tellurophene 2.
b
yield (%)
Na2Te/NaHTe
(equiv)
H2O
entry
base (equiv)
(ratio)
2i′
3i′
4i′
−
−
−
26
27
18
32
19
1
2
3
4
5
6
1.2
2.5
2.5
2.5
3.0
3.0
3.0
3.0
−
−
−
−
−
−
−
−
81
84
85
27
−
−
−
−
KOH (2.0)
KOH (2.0) H2O (10%) 32
KOH (2.0) H2O (10%) 63
KOH (2.0) H2O (20%) 76
KOH (2.0) H2O (30%) 62
c
7
8
NaOH
(2.0)
H2O (20%) 69
d
9
3.0
3.0
KOH (2.0) H2O (20%)
− H2O (20%)
−
−
77
82
−
−
10
a
Reaction conditions: 1i (1.0 equiv), Na2Te/NaTeH, base, and H2O
(ratios are as mentioned in the table), 75 °C, 3 h, N2. Isolated yield.
b
c
d
Reaction mixture was opaque. At room temperature.
salts in ethanol at 75 °C (3, Scheme 1). To further promote the
subsequent hydrotelluration and cyclization of the intermediate
Scheme 1. Mechanistic Rationalization
Although substantial studies are needed to confirm and
finalize the actual reaction mechanism, the current mechanistic
deduction did provide us with many useful guidelines for
understanding and even improving the synthetic strategy. For
example, based on the preliminary mechanistic scheme, it is
conceivable that the efficiency of the cyclization path can be
further improved, if the stability of the initially formed allylic
anion (intermediate 3-3, Scheme 1) can be further enhanced,
e.g., by introducing an additional electron resonance effect, via
replacing the alkyl group of 1i with an aryl or a heteroaryl
group. Indeed, when the n-pentyl group of 1i was replaced with
a phenyl group (as in 1a), the desired cyclized product 2a′ was
obtained exclusively in 96% yield, consistent with our
hypothesis. Furthermore, we found that this synthetic method
is also extendable to other aryl and heteroaryl substituted
enynes, providing a simple and efficient way for making 3-alkyl
and 3-aryltellurophenes (Scheme 2, 2a′−j′). With the
aforementioned results in hand, it is easy to envisage that the
reactivity of telluride salts with compound 1 could be greatly
influenced by the polarity of the solvent and the reaction
temperature. To further understand and expand the scope of
the reaction, we chose DMF as the reaction solvent, which can
better dissolve enynes compared H2O and has a higher
dielectric constant (ε = 37) and a higher boiling point than
3i′, a stronger base like KOH was added in the reaction; and to
our disappointment, it somewhat failed to promote any
observable reaction between telluride salts and intermediate
3i′ (entry 3, Table 1). Interestingly, when we added a small
amount of water (10%) as a cosolvent in the reaction medium
along with KOH, the reaction between telluride salt(s) and
intermediate 3i′ can be effectively triggered on to furnish the
desirable cyclized compound, 3-n-pentyltellurophene 2i′, in
32% yield together with a competing dimeric byproduct 4i′
(26%) (entry 4, Table 1). By increasing the telluride reagent
usage from 2.5 to 3 equiv, the reaction of 3i′ can be driven to
completion and could afford 63% of 3-n-pentyltellurophene 2i′
(entry 5, Table 1). The use of H2O cosolvent in 20 vol % gave
the best yield of 2i′ (76%; entry 6, Table 1). A Further increase
in H2O cosolvent up to 30 vol % caused the reaction solution
to be opaque and somehow led to an increase in dimeric
product 4i′ (32%) by sacrificing the desired cyclized product
2i′ (62%; entry 7, Table 1). Apparently, the cyclized product
and the dimerized product went through a common
intermediate. Also a change in the usage of base did not
show any considerable effect in the conversion (entry 8, Table
1). In the presence of 20 vol % of H2O cosolvent, control
experiments were performed (entries 9−10, Table 1), and it is
confirmed that use of both elevated temperature (75 °C) and
B
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Organic Letters
Letter
a
As anticipated, compound 1a with an electron-resonance
phenyl group at the 2-position could stabilize the intermediate
3-3 (Scheme 1) and, thus, facilitate the formation of the desired
product 2a′ in 97% yield under the same reaction conditions in
a shorter time (0.5 h; entry 4, Table 2). As deprotection of the
acetylenic TMS group always occurred in the initial stage under
the above reaction conditions, it is understood that terminal
dibromo-enyne can undoubtedly undergo reactions with
telluride salts under the appropriate conditions to afford 3-
substituted tellurophenes. However, desilylation is not the
requisite step for the cyclization with telluride salts to occur,
because a structurally similar dibromo-enyne of 1s, having a 2-
phenyl substituent and a 4-tert-butyl group, could readily react
with telluride salts under the same reaction conditions to give
cyclized 2-tert-butyl-4-phenyltellurophene 2s in 91% yield
(entry 5, Table 2). The studies further indicated that the
cyclization of 1s with telluride salts was achievable even at room
temperature in the presence of KOH to afford 2s in 52%−87%
yields (0.5−20 h) (entries 6 and 7, Table 2). Apparently, the
electron resonance effect of the 2-phenyl group had greatly
facilitated the cyclization, and the steric effect of the 4-tert-butyl
group could at most only slow down the cyclization rate but
not prevent it from happening. Most interestingly, we found
that the cyclization of 1s with the telluride salts could also
proceed at 25 °C even before the addition of KOH, though at a
slower pace. Further detailed studies indicated that telluride
salts can cyclize 1s at room temperature in the absence of KOH
with prolonged time (2−48 h) to form 2s in 31%−81% yields
(entries 8 and 9, Table 2). The same favorable reaction
behaviors however failed to occur in the case of 1i. Even with a
prolonged reaction time (20 h) at rt in the absence of KOH, it
totally failed to undergo cyclization, but only yielded the
monodebrominated and silylated compound in 71% yield
(entry 10, Table 2). Although the cyclization reaction of 1i in
the absence of KOH could be promoted at the elevated
temperature of 140 °C, the yield was still rather poor (41%;
entry 11, Table 2). These results reconfirmed that the
electronic nature of the substituent group at the 2-position of
the dibromo-enynes also plays a very important role. With
these findings, we have further conceived that syntheses of 2-
silyl-4-aryltellophenes might be feasible in the DMF medium at
low temperatures in the absence of a strong desilylating reagent
such as KOH. Thus, the cyclization of the 2-aryl-substituted
and 4-silyl-substituted enynes of 1a (R1 = phenyl), 1e (R1 = 4-
methoxyphenyl), and 1f (R1 = 2-naphthyl) were conducted,
which all gave fair yields (40−50%) of 4-aryl-2-silyltellur-
ophenes (2a, 2e, and 2f; entries 12−14, Table 2) within 2 h at
rt.
Scheme 2. Preparation of 3-Substituted Tellurophenes
a
Reaction condition: 1 (1.0 equiv), Na2Te/NaHTe (3.0 equiv),
EtOH/H2O (8:2), KOH (2.0 equiv), 75 °C, 3 h. For 8 h. See SI for
b
detailed procedures. Isolated yields of products are given.
ethanol, and using t-BuOH (in 1 vol %) as the acidic proton
source for making the telluride salts. Our initial trials with
compound 1i in the presence of KOH were indeed free of the
dimeric byproduct 4i′ (due to its better dissolution power than
H2O), but a higher reaction temperature is required (probably
due to its lower dielectric constant than that of H2O) for
obtaining a moderate yield of tellurophene 2i′ (0% at rt, 32% at
75 °C, 78% at 140 °C; entries 1−3, Table 2).
Table 2. Optimization Studies for 2,4-Substituted
a
Tellurophenes
bc
,
yield (%)
temp/time/KOH
(°C/h/equiv)
entry
1 (R1/R2)
2
3
1
2
3
4
5
1i (n-pentyl/TMS)
1i (n-pentyl/TMS)
1i (n-pentyl/TMS)
1a (Phenyl/TMS)
25/2/2
−
3i′ (82)
2i′ (32) 3i′ (44)
75/2/2
140/2/2
140/0.5/2
140/0.5/2
2i′ (78)
2a′ (97)
2s (91)
−
−
−
1s (Phenyl/
tert-butyl)
6
7
8
9
1s (Phenyl/
tert-butyl)
25/0.5/2
25/20/2
25/2/−
2s (52) 3s (36)
2s (87)
2s (31) 3s (54)
1s (Phenyl/
tert-butyl)
−
1s (Phenyl/
tert-butyl)
Interestingly, when both substituents are phenyl groups, as in
1k, telluride salts can smoothly cyclize 1k at rt without KOH in
1 h to yield 2k in 98% yield (entry 15, Table 2). The scope of
dibromo-enynes was further examined (Scheme 3), revealing
the cyclization of diaryl-substituted dibromo-enynes with
telluride salts occurred very smoothly at rt without KOH,
regardless of the electronic nature of the substituents on the
aryl groups; the corresponding products were obtained in good
to excellent yields in 1 h (89−98%, 2k−r, Scheme 3). In the
case of the 2-aryl and 4-alkyl disubstituted enynes, the
cyclization could still proceed in DMF at rt without KOH,
but at a slower pace to give moderate yields (39%−81%, 2a, 2e,
2f, and 2s; 2−48 h, Scheme 3). Likewise, the monosubstituted
3-aryltellurophenes can also be prepared in moderate yields
(82−84%, 2t−u, Scheme 3) from the corresponding 2-aryl
1s (Phenyl/
tert-butyl)
25/48/−
2s (81)
−
10
11
12
13
1i (n-pentyl/TMS)
1i (n-pentyl/TMS)
1a (Phenyl/TMS)
25/20/−
140/6/−
25/1/−
25/2/−
−
3i′ (71)
2i′ (41) 3i′ (32)
2a (47)
−
−
1e (4-MeOPh/
TMS)
2e (39)
14
15
1f (2-naphth/
25/2/−
25/1/−
2f (42)
2k (98)
−
−
TMS)
1k (Phenyl/
Phenyl)
a
Reaction conditions: 1 (1.0 equiv), Na2Te/NaTeH (2.1 equiv),
KOH, time and temp (values are as mentioned in the table), N2.
Isolated yield. Products with prime symbol (′) are TMS deprotected
b
c
(i.e., with R2 = H).
C
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Organic Letters
Letter
a
species at C2 (intermediate 3-3, Scheme 4), which would be
destabilized by an electron-donating 2-alkyl group, but
stabilized by an electron-resonance 2-aryl group. Thus, the
cyclization of a 2-alkylated-enyne required the use of a much
stronger nucleophile, i.e., the organotelluride 3-2 (Scheme 4),
while the same cyclization of a 2-arylated-enyne can be done
with a much weaker nucleophile, such as organotellurol 3-1
(Scheme 4b and 4c).
Scheme 3. Preparation of Substituted Tellurophenes at rt
The DFT calculations also clearly indicated that the
replacement of the 2-alkyl group (as in AI; Figure S1a, SI)
with a 2-phenyl group (as in BI; Figure S1b, SI) would help to
lower the ΔG of the cyclization reaction by 1.8 kcal/mol. The
DFT calculations for the hypothetical transition states TS1 and
TS2 (Figure S2, SI; starting from the corresponding organo-
tellurols to the respective tellurophenes) indeed confirming
that the 2-alkyl-substituted TS1 is far more unstable than the 2-
phenyl-substituted TS2 by 5.5 kcal/mol in gas phase.
Alternatively, the cyclization reaction can also be well explained
by considering SNVσ and SNVπ mechanism modes (see SI for
detailed explanation and references).
In conclusion, we have successfully developed a new and
effective method for the syntheses of a wide variety of 3-
substituted and 2,4-disubstituted tellurophenes. Our prelimi-
nary studies indicated that this methodology can also be
extended to prepare selenophene analogs (see Scheme S2, SI).
Furthermore, research in synthesizing other chalcogeno-hetero-
cycles by using this novel methodology is underway in our
laboratory.
a
Reaction conditions: 1 (1.0 equiv), Na2Te/NaHTe (2.1 equiv),
b
c
d
e
DMF/t-BuOH (99:1), rt, 1 h. For 2 h. For 48 h. For 24 h. KOH
(2.0 equiv), 140 °C for 3 h. See SI for detailed procedures. Isolated
yields of products are given.
monosubstituted enyne in DMF without KOH at rt in 24 h. In
contrast, the cyclization of 2-alkyl and 4-aryl disubstituted
enyne (1v) to form 2v (Scheme 3), however, requires the use
of both KOH and elevated temperature (e.g., 140 °C) in DMF.
These results suggest that the electron resonance stabilization
effect for the initially formed anion at C2 is the key control
factor and the additional electron resonance stabilization effect
at C4 can also further facilitate the cyclization.
In summary, the cyclization of 2-alkyl-substituted enynes
with NaTeH can be best performed by the use of a polar
solvent (i.e., DMF or the cosolvent of H2O/EtOH) at high
temperature (140 or 75 °C respectively) with strong base KOH
(Scheme 4; conditions A−D), while the same cyclization of 2-
ASSOCIATED CONTENT
■
S
* Supporting Information
Scheme 4. Reaction Conditions and Mechanistic
Implications
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00279.
Experimental procedures and characterization of prod-
ucts (PDF)
Accession Codes
CCDC 1581863−1581864 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: cchan@mx.nthu.edu.tw.
ORCID
Vamsi Krishna Karapala: 0000-0003-4327-6704
Chien-Chung Han: 0000-0002-1045-0270
Notes
aryl-substituted enynes can be readily performed in DMF at rt
without KOH (Scheme 4; conditions E and F). The vastly
different reaction behaviors may be accountable by our
preliminarily proposed mechanisms (Schemes 1 and 4). The
cyclization between enynes and NaTeH may be preceded first
by a hydrotelluration to form the organotellurol (intermediate
3-1, Scheme 4), followed by the nucleophilic addition of the
tellurol to the C1 of the vinyl bromide moiety to form an anion
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the financial support from MOST, ROC,
Taiwan. We thank Ker-Yin Liu (National Tsing Hua
University) for assisting H.-P.S. in performing Gaussian DFT
calculations.
D
DOI: 10.1021/acs.orglett.8b00279
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Letter
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DOI: 10.1021/acs.orglett.8b00279
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