Iron-Promoted Tandem Cyclization of 1,3-Diynyl Chalcogen Derivatives with Diorganyl Dichalcogenides for the Synthesis of Benzo[b]furan-Fused Selenophenes

Article abstract of DOI:10.1002/adsc.201600759

A double intramolecular 5-endo-dig cyclization of butyl[2-(phenylbuta-1,3-diynyl)phenyl]chalcogens has been employed in a selective preparation of benzo[b]chalcogenopheno[2,3-d]furans. Several reaction parameters were studied to determine the best reaction conditions and we observed that the reaction of butyl[2-(phenylbuta-1,3-diynyl)phenyl]chalcogens (0.25 mmol) with iron(III) chloride hexahydrate (2.0 equiv.) and diorganyl diselenides (1.75 equiv.) at reflux of dichloromethane was the most appropriate to give the products in 35–89% yields. These standard reaction conditions were compatible with many functional groups in the substrates, such as methyl, chlorine, fluorine, methoxy and heteroaryl. This protocol was also efficient for diorganyl diselenides but it was ineffective with diorganyl disulfides and ditellurides. In a competition among selenium, sulfur and oxygen nucleophiles we observed that the nucleophilicity and steric effects of the competing functional groups were determinant factors for the selectivity of the cyclization. The benzo[b]chalcogenopheno[2,3-d]furans had absorptions in the UV region (300–350 nm range) with molar absorptivity coefficient values ascribed to spin and symmetry allowed π–π* electronic transitions. An emission located in the purple region (380–440 nm range), with a Stokes shift of between 65–100 nm, is probably associated to the charge transfer character of the excited state. (Figure presented.).

Full text of DOI:10.1002/adsc.201600759

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DOI: 10.1002/adsc.201600759
Very Important Publication
Iron-Promoted Tandem Cyclization of 1,3-Diynyl Chalcogen
Derivatives with Diorganyl Dichalcogenides for the Synthesis of
Benzo[b]furan-Fused Selenophenes
Josꢀ S. S. Neto,a Bernardo A. Iglesias,b Davi F. Back,b and Gilson Zenia,*
a
Laboratꢁrio de Sꢂntese, Reatividade, Avaliażo Farmacolꢁgica e Toxicolꢁgica de OrganocalcogÞnios CCNE, UFSM, Santa
Maria, Rio Grande do Sul, Brazil 97105-900
Fax : (+55)-55-3220-8978; Phone: (+55)-55-3220-9611; e-mail: gzeni@ufsm.br
Laboratꢁrio de Materiais Inorgꢃnicos, Departamento de Quꢂmica, UFSM, Santa Maria, Rio Grande do Sul, Brazil
b
97105-900
Received: July 14, 2016; Revised: August 31, 2016; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600759.
Abstract: A double intramolecular 5-endo-dig cycli-
ACHTUNGTRENoNGNU rganyl disulfides and ditellurides. In a competition
zation of butyl[2-(phenylbuta-1,3-diynyl)phenyl]- among selenium, sulfur and oxygen nucleophiles we
chalcogens has been employed in a selective prepa- observed that the nucleophilicity and steric effects of
ration of benzo[b]chalcogenopheno[2,3-d]furans. the competing functional groups were determinant
Several reaction parameters were studied to deter- factors for the selectivity of the cyclization. The ben-
mine the best reaction conditions and we observed zo[b]chalcogenopheno[2,3-d]furans had absorptions
that the reaction of butyl[2-(phenylbuta-1,3-diynyl)- in the UV region (300–350 nm range) with molar ab-
phenyl]chalcogens (0.25 mmol) with iron(III) chlo- sorptivity coefficient values ascribed to spin and
ride hexahydrate (2.0 equiv.) and diorganyl disele- symmetry allowed p–p* electronic transitions. An
nides (1.75 equiv.) at reflux of dichloromethane was emission located in the purple region (380–440 nm
the most appropriate to give the products in 35–89% range), with a Stokes shift of between 65–100 nm, is
yields. These standard reaction conditions were com- probably associated to the charge transfer character
patible with many functional groups in the sub- of the excited state.
strates, such as methyl, chlorine, fluorine, methoxy
and heteroaryl. This protocol was also efficient for Keywords: cyclization; furans; heterocycles; iron
diorganyl diselenides but it was ineffective with di- salts; selenophenes; thiophenes
Introduction
inal and pharmaceutical fields. A number of seleno-
phene derivatives are known to display a wide range
Selenium plays important roles in many physiological of biological activities, such as antidepressant,[5] anti-
and pathological processes and is an essential nutrient oxidant,[6] anticonvulsant,[7] antibacterial,[8] antitumor-
for the human body.[1] Organoselenium compounds al,[9] antinociceptive[10] and antiapoptotic.[11] A few of
have been of great interest to both synthetic and me- these compounds exhibit an interesting profile of
dicinal chemists due to their exceptional biological anti-inflammatory activity with a significant analgesic
properties defined by the chemical and physical char- effect.[10] Besides biological activity, compounds con-
acteristics of the selenium atom.[2] Besides the phar- taining the selenophene nucleus have found applica-
macological properties, the presence of a selenium tions, for example, in the preparation of materials
atom in a potentially bioactive molecule can dramati- that show potentially useful optical properties,[12] as
cally increase the native biological activity of the sub- well as semiconductors,[13] solar cells,[14] film transis-
strate.[3] This is mainly attributed to the fact that the tors[15] and energy storage applications.[16] Generally,
selenium atom can serve as hydrogen-bond acceptor methods for preparing these compounds rely on the
or electron donor altering the chemical characteristics addition of either electrophilic or nucleophilic seleni-
of enzyme active sites.[4] Among organoselenium com- um reagents to active acyclic substrates to promote
pounds, selenophenes are the focus of research be- the intramolecular cyclization[17] or to apply suitable
cause some of them have found applications in medic- cyclization methodology to appropriate, previously
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prepared organoselenium substrates.[18] Although sev-
eral procedures have been developed for the synthesis
of selenophenes, no examples involving the prepara-
tion of the corresponding fused-selenophene hetero-
cycle derivatives, in sequential cyclization/chalcogena-
tion reactions, have been reported in the literature.
Herein, we wish to extend the concept of tandem re-
actions and report the synthesis of benzo[b]seleno-
pheno[2,3-d]heterocycles 3 by using an iron-promoted
tandem cyclization, starting from 1,3-diynes 1 and di-
ACHTUNGTRENNUNGorganyl diselenides 2 (Scheme 1). The main feature of
Scheme 1. General scheme.
our methodology is the use of diorganyl diselenide re-
agents in the cyclization reaction because of their
dual role by acting as cyclizing agent and to incorpo-
rate both portions of diorganyl diselenides (2 RSe) in
Scheme 2. Reaction conditions: i) n-BuSeLi (1.0 equiv.),
THF, 608C,12 h; or EtSH (1.0 equiv.), KOH (1.2 equiv.), (n-
Bu)4NBr (20 mol%), H2O, 82 8C, 12 h, 74–95% yields; ii)
the final product, thus providing a more useful and CBr4 (1.5 equiv.), PPh3 (2.0 equiv.), CH2Cl2, 08C, 2 h, 88–
91% yields; iii) n-BuLi (2.0 equiv.), I2 (1.0 equiv.), THF, 08C,
4 h, 60–70% yields; iv) PdCl2(PPh3)2 (2 mol%), CuI
(1 mol%), Et3N, r.t., 10 h, 68–82% yields; v) hexane, KOH
(3.0 equiv.), reflux, 12 h, 70–85% yields; vi) n-BuLi
(1.0 equiv.), I2 (1.0 equiv.), THF, 08C, 2 h, 74–98% yields;
vii) CuI (20 mol%), pyrrolidine, 08C, 4 h, 95% yield.
valuable method in terms of atom economy. In the or-
ganoselenium chemistry there is growing interest in
the formation of new C(sp2)–chalcogen bonds, such as
chalcogen-containing heterocycles, especially due to
their biological properties[19] as well as their versatility
as intermediates in organic synthesis.[20] For instance,
organoselenium groups contain a very interesting re-
active site because the carbon–chalcogen bond can be
readily replaced by carbon–hydrogen, carbon–halogen reaction of 2-halobenzaldehydes with alkyl thiols in
and carbon–carbon bonds.[21] Another innovative fea- the presence of base. The Wittig reaction led to the
ture of this tandem reaction is the use of iron reagents dibromoalkenes that gave alkynyl iodides 4 after reac-
as an alternative to other transition metals due to tion with n-butyllithium followed by the addition of
their low price and toxicity, ecological advantages and iodine.[24] The alkynyl iodides 5 were prepared by re-
excellent tolerance towards various functional acting 2-iodoanisole with terminal alkynes via the So-
groups.[22]
nogashira reaction.[25] The retro-Favorskii[26] reactions
followed by reaction with n-butyllithium and iodine
afforded the alkynyl iodides 5. The Cadiot–Chodkie-
wicz coupling of terminal alkynes with alkynyl iodides
4 or 5 gave 1,3-diynes 1.[23]
Results and Discussion
The starting material, 1,3-diynes 1, were prepared by
Cadiot–Chodkiewicz coupling involving terminal al-
As shown in Scheme 1, the two halves of the di-
ACHTUNGTRENNUNG
kynes reacting with alkynyl iodides
4
or
5
(Scheme 2).[23] The alkynyl iodides 4 were synthesized phenes 3. This implies that some promoter must be
in three steps from commercially available 2-haloben- used to reduce the diorganyl diselenides (RSeSeR) to
zaldehydes. Thus, with the aim to introduce the orga- selenides (RSe). Therefore, the same parameters,
noselanyl group we prepared the lithium alkylseleno- such as type of promoter (Lewis or Brønsted acids),
late by reacting elemental selenium (1 equiv.) with al- amount of dibutyl diselenide, reaction temperature
kyllithium reagents (1 equiv.), in THF at À788C for and solvent were expected to affect the cleavage of
30 min. The nucleophilic aromatic substitution reac- the selenium–selenium bond. Another observation is
tions of 2-halobenzaldehydes with lithium alkylsele- that the accomplishment of this reaction probably is
nolate afforded the 2-(n-butylselanyl)benzaldehydes. dependent on the nature of the group directly linked
The introduction of the thiolate group was carried by to the selenium atom, and it should occur only with
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diselenides having at least one Se–C(sp3) bond. For
these reasons we began our screening of the reaction
condition parameters by the addition of dibutyl dise-
Table 1. Effect of different reaction parameters on the prep-
aration of selenonophene 3a[a]
lenide (2.0 equiv.) to
a
solution of promoter
(2.0 equiv.) in CH2Cl2 (4 mL) at room temperature in
an open tube. After 15 min, 1,3-diyne 1a (0.25 mmol)
was added and the reaction mixture was refluxed for
12 h. We found that the use of Lewis acids (CuI,
PdCl2, ZnCl2, BF3·OEt2) and Brønsted acids (p-tolue-
nesulfonic acid and H3PO4)[27] did not promote the
cyclization of 1,3-diyne 1a (Table 1, entries 1–6). We
believe that the selenium–selenium bond breakage
(202 KJmolÀ1) promoted by the addition of ferric
chloride forms a reactive intermediate, which allows
the carbon-carbon triple bond activation, promoting
the cyclization. Probably, the use of other transition
metals was inefficient to promote the cyclization be-
cause the intermediate would have higher thermody-
namic stability and low activation energy, which ren-
ders it ineffective to promote the activation of the
carbon-carbon triple bond. Usually iron salts react
smoothly with diorganyl diselenides promoting the
Se–Se bond heterogeneous cleavage.[28] Thus, the
effect of different iron catalysts on the cyclization of
1,3-diyne 1a was also examined. The reaction with
iron(II) chloride tetrahydrate permitted the cycliza-
tion reaction to proceed, although a very poor yield
of selenonophene 3a was obtained (Table 1, entry 7).
With this result in hand, we carried out some experi-
ments using different iron sources to determine if the
activity of the iron salt could be improved by chang-
ing the ligand partner type on the iron complex or by
using dry iron salts. It was found that no reaction oc-
curred when FeSO4·7H2O, Fe2O3 and Fe(0) were used
as iron source, even under reflux conditions, and in
these experiments the starting 1,3-diyne 1a was recov-
ered quantitatively (Table 1, entries 8–10). Carrying
out the reaction using FeCl3 and FeCl3·6H2O led to
the formation of the desired fused selenonophene 3a
in 78% and 89% yields, respectively (Table 1, en-
tries 11 and 12). Although a good result was observed
by using anhydrous iron(III) chloride (Table 1,
entry 11), owing to its limitation with respect to eco-
nomic aspects, more tolerance to air and moisture,
iron(III) chloride hexahydrate (Table 1, entry 12) was
adopted for further studies. Further reactions varying
the equivalents of FeCl3·6H2O produced mixed yields
revealing that the use of FeCl3·6H2O (2 equiv.) is nec-
essary to give the cyclized product in acceptable
yields (Table 1, entries 13 and 14). Moreover, the re-
action behavior under an inert atmosphere had the
same activity as that under aerobic conditions, conse-
quently, we chose to continue this study carrying out
En- Promoter
(n-BuSe)2 Solvent
(equiv.)
Yield of
try
(equiv.)
3a [%][b]
1
CuI (2.0)
2.0
2.0
2.0
2.0
2.0
2.0
2.0
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
dioxane
N.R
N.R
N.R
N.R
N.R
N.R
<7
N.R.
N.R.
N.R.
78
2
PdCl2 (2.0)
3
4
5
6
ZnCl2 (2.0)
BF3OEt2 (2.0)
PTSA (2.0)
H3PO4 (2.0)
FeCl2·4H2O (2.0)
7
8
FeSO4·7H2O (2.0) 2.0
9
Fe2O3 (2.0)
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.5
1.75
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.75
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Fe8 (2.0)
FeCl3 (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (3.0)
FeCl3·6H2O (1.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3·6H2O (2.0)
FeCl3.6H2O (2.0)
89
85
25
87[c]
53]d]
80
89 (83)[e]
20
84
CH3NO2 44
CH3CN
DMF
DMSO
EtOH
hexane
CH2Cl2
48[f]
<5[f]
0[f]
16[g]
34[h]
89 (83)[e,i]
[a]
The reaction was performed by addition of dibutyl dise-
lenide to a solution of promoter in solvent (4 mL), under
an air atmosphere, at room temperature. After 15 min at
this temperature, 1,3-diyne 1a (0.25 mmol) was added.
The resulting mixture was refluxed for 12 h.
Yields were determined by gas chromatography using do-
decane as internal standard.
Reaction was carried under argon atmosphere.
Reaction was carried out at room temperature.
Isolated yields in parentheses.
Reaction was carried out at 808C.
Reaction was carried out at 788C.
Reaction was carried out at 688C.
The progress of the reaction was monitored by TLC to
determine the necessary reaction time for the complete
consumption of the starting material. We observed that
4 h was the required time for the reactions to be com-
pleted.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
the reaction in an open tube (Table 1, entry 15). Next, butyl diselenide affords four portions of butyl sele-
lowering the reaction temperature to 258C had no nide, we reasoned that one equiv. could be used as
positive effect, giving the product only in 53% yield a nucleophile in an SN2 reaction to remove the alkyl
(Table 1, entry 16). Recognizing that two equiv. of di- group directly bonded to the heteroatom and to be in-
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corporated in the final product. The second equiv.
acts as a nucleophile in an SN2 reaction to remove the
n-butyl group directly bonded to the selenium atom
and as a promoter of the second cyclization step. To
confirm this assumption we carried out the reaction
using 1.75, 1.5 and 1.0 equiv. of dibutyl diselenide
(Table 1, entries 17–19). The fact that the yields of se-
lenonophene 3a decreased with 1.5 and 1.0 equiv. in-
dicates that the reaction requires at least 1.75 equiv.
of dibutyl diselenide. That two portions of diorganyl
diselenide are incorporated in the final product is an
important aspect to note with diorganyl diselenides,
and is useful and valuable in terms of atom economy.
Furthermore, an examination of the efficiency of
FeCl3·6H2O with different solvents showed that diox-
ane, CH3NO2 and CH3CN delivered moderate cycliza-
tion conversion, while dichloromethane, with respect
to the lower reaction time and higher reaction yields,
gave the best result in the formation of selenophene
Scheme 3.
3a (Table 1, entries 20–27). From these studies, we nucleophilic substitution is hampered, consequently
concluded that the reaction of 1,3-diyne 1a the selenophene 3 is not formed (Scheme 3). In the
(0.25 mmol) in dichloromethane (4 mL) with diorgan- second step we evaluated the 1,3-diynes having sub-
yl diselenides (1.75 equiv.) and FeCl3·6H2O (2 equiv.), stituents electronically divergent at the alkyne
under reflux for 4 h are the optimized conditions to (Table 2, entries 5–11). Thus, the reactions carried out
obtain the selenophene 3a in higher yield (Table 1, with an electron-rich aryl-substituted substrate pro-
entry 27).
duced good isolated yield of the selenophene 3e
(Table 2, entry 5). The cyclization of 1,3-diynes with
electron-deficient aryl groups having o- and p-Cl and
p-Br proceeded smoothly to give the selenophenes
3f–h in 65–73% yields (Table 2, entries 6–8). Addi-
Scope of the Reaction
To examine the applicability of the present protocol, tionally, the use of a naphthyl group decreased the
various functionalized 1,3-diynes 1 were subjected to isolated yields of the product 3i, which may be ex-
the cyclization with diorganyl dichalcogenides con- plained by steric hindrance (Table 2, entry 9). A het-
taining a wide array of functional groups (Table 2). eroaromatic thienyl group bonded to the alkyne also
Because the accomplishment of this reaction probably gave an excellent partner for this cyclization giving
is dependent on the nature of the group directly the product 3j in 74% yield (Table 2, entry 10). The
linked to the selenium atom, the first step in the gen- presence of a deactivated carbon-carbon triple bond
eralization of the cyclization involved the study of di- in the structure of 1,3-diyne 1h affected the reactivity
organyl dichalcogenides containing different groups resulting in a severe decline in the yield of seleno-
bonded at the selenium atom. The yields were good phene 3k (Table 2, entry11). In this case, the starting
for all alkyl groups, however, there was a significant material 1h was recovered and no product of the first
difference in the reactivity between butyl and the cyclization step was observed. Probably, the absence
other groups (Table 2, entries 1–4). In contrast, the of p bonds next to the alkyne decreased the reactivity
cyclization with diphenyl diselenide, which has for the electrophilic attack at the carbon-carbon triple
a phenyl group bonded at the selenium atom, gave bond. In the third step of the substrate evaluation, we
the benzofuran 7, the product of the first cyclization examined the reactivity of 1,3-diynes 1 with substitu-
step (Scheme 3). The formation of benzofuran 7 can ents on the aromatic ring of the anisole. The experi-
be accounted for the cooperative action between di- mental results showed that the electronic effect of
phenyl diselenide and iron salt, whereas one half of para-substituents is an important factor in the out-
the diselenide is incorporated in the structure of the come of the cyclization. Although both electron-do-
benzo[b]furan and the other one acts as nucleophile nating and electron-withdrawing groups afforded the
to remove the methyl group from intermediate 6. The desired products in good yields, substrates with an
key stage of the selenophene 3 formation involves electron-donating group required longer reaction
a nucleophilic substitution on the group bonded at times (Table 2, entries 12 and 13). In the next series
the selenium atom in the intermediate 8 of the second of experiments, the reactivity of diorganyl disulfides
cyclization step. When this group has a carbon with and ditellurides with 1,3-diynes 1, under the opti-
sp2 hybridization, as is the case of phenyl group, the mized iron protocol conditions, was tested. The results
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Table 2. Synthesis of benzo[b]selenopheno[2,3-d]heterocycles 3.[a]
[a]
The reaction was performed by the addition of diphenyl diselenide (1.75 equiv.) to a solution of iron salt (2.0 equiv.) in di-
chloromethane (3.0 mL), at room temperature, under an air atmosphere. After 15 min at this temperature, 1,3-diyne
1 (0.25 mmol) was added. The resulting mixture was stirred for the time indicated at 408C.
Yields of purified products.
The product was not formed and the starting material was recovered.
An inseparable mixture of the starting material, first cyclization product and second cyclization product was obtained.
[b]
[c]
[d]
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showed that no product was obtained in the reaction
of 1,3-diynes 1a and any of the disulfides and ditellur-
ides tested (Table 2, entries 14–17). It is currently not
clear why these diorganoyl dichalcogenides are un-
reactive under this protocol. However, we believe
that in the case of diorganyl disulfides the strength of
the sulfur–sulfur bond prevents its cleavage by the
iron salt. In case of dibutyl ditelluride, we suppose
that its coordination with iron salt forms a polymeric
iron tellurolate complex, which is very instable de-
composing via telluroxide elimination by oxidation.[29]
The nucleophilic effect of a heteroatom other than
oxygen was further studied. This has been accom-
plished by the introduction of sulfur, selenium and ni-
trogen atoms at the 2-position of the aryl group
bonded to the alkyne. On the one hand, the cycliza-
tion reaction of sulfur and selenodiynes 1k–1p with
dibutyl diselenide furnished moderate to good yields
of the corresponding selenophenes 3n–3u.
On the other hand, the cyclization of 1,3-diynes 1q
and 1r bearing a nitrogen as nucleophile gave a mix-
ture of side-products (Table 2, entries 18–27). We ex-
pected that, due to the higher nucleophilicity of sele-
nium compared to oxygen, sulfur and nitrogen be-
cause of its lower electronegativity and higher polariz-
ability, the selenodiynes 1n–1p should be more reac-
tive.[30] Because there is no relationship between
reaction time and yields, it is quite difficult to ration-
alize the reactivity of these susbtrates to any factor
that affects the cyclization.
Scheme 4.
In addition to the above results, unsymmetrical 1,3-
diynes 1s–v bearing two potential nucleophiles have a good 74% yield [Scheme 4, Eq. (3)]. Although the
been used as starting material. Due to the differences nucleophilicity of the selenium atom is higher than
in reactivity of nucleophiles, the cyclization reaction that of the sulfur atom, the steric hindrance of the
using these 1,3-diynes could lead to competitive cycli- butyl group was determinant to this selectivity. The
zation. The ratio of products depends on several fac- reaction of benzofuran derivative 9, which is the inter-
tors, including the nucleophilic character, the steric ef- mediate of the first cyclization step of symmetrical
fects and polarization of the alkyne triple bond.[30] We 1,3-diyne 1v, was also investigated. This reaction
performed a first competitive cyclization reaction be- could give the cyclized product 10, instead seleno-
tween nucleophilic oxygen at the ortho position of phene 3y was isolated as the sole product [Scheme 4,
aryl group and nucleophilic sulfur directly bonded to Eq. (4)]. This result showed that this cyclization was
the ortho position of opposite aryl group. In this case, also governed by the nucleophilic effect of the seleni-
running the reaction of 1,3-diyne 1s under standard um atom. In all experiments described in Scheme 4,
conditions the selenophene 3v was obtained as the the presence of a unique isomer was confirmed by
unique product [Scheme 4, Eq. (1)]. This result is in 1H NMR spectral data and mass spectroscopic analy-
agreement with the higher nucleophilicity of sulfur sis from crude reaction mixture to avoid errors in the
compared to that of oxygen. For the cyclization of detection isomer.
1,3-diyne 1t, which has competition between selenium
and oxygen nucleophiles, the selenophene 3w was ob-
tained exclusively in 66% yield [Scheme 4, Eq. (2)]. Mechanism Discussion
This selectivity is also governed by the difference of
the nucleophilicity between selenium and oxygen Having examined the reactivity of 1,3-diynes 1 in the
atoms. The cyclization of 1,3-diyne 1u involves the preparation of selenophenes 3, we then investigated
competition of selenium versus sulfur nucleophiles the detailed aspects that could help to explain the
and taking into account the nucleophilicty, it should mechanism of this process. First, we examined wheth-
give the product of selenium nucleophilic attack, how- er the combination of iron salt and diorganyl disele-
ever, the unexpected product 3x was obtained in nide could lead to an in situ formation of RSeCl, an
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electrophilic selenium species, which could be the cy- of 1,3-diyne 1a with diphenyl diselenide was conduct-
ACHTUNGTRENNUNGclization promoter. Towards this end, we carried out ed in the presence of dry HCl gas but no trace of sele-
the reaction of 1,3-diyne 1a with BuSeCl[31] in the ab- nophene 3a was detected in this experiment
sence of iron salt and dibutyl diselenide. These condi- [Scheme 4, Eq. (3)]. As previously reported, in the re-
tions led to the formation of the product 3a in 42% actions promoted by transition metals, occasionally
yield [Scheme 5, Eq. (1)]. In addition, when the 1,3- trace metal impurities are the active catalytic species.
diyne 1a was added to a solution of FeCl3 and BuSeCl When iron salts are used they may be contaminated
in CH2Cl2 the product 2a was obtained in 63% yield by a trace amount of Cu2O and this could be the cata-
[Scheme 5, Eq. (2)]. These last two experiments indi- lyst.[32] However, the selenophene 3a was not obtained
cate that the BuSeCl species alone is not sufficient to in the reaction of 1,3-diyne 1a with diphenyl disele-
promote the cyclization and that a cooperative action nide in the presence of a catalytic or equimolar
between diphenyl diselenide and the iron salt is essen- amount of Cu2O [Scheme 5, Eq. (4)]. From these re-
tial to provide a good yield of the cyclized product. A sults, we eliminate the possible effect of Cu2O as cata-
second possibility investigated was to determine lyst in the formation of selenophene 3a. When radical
whether the HCl liberated from the iron salt could inhibitors, such as hydroquinone and TEMPO were
act as a catalyst. To answer this question the reaction added to the reaction of 1,3-diyne 1a with dibutyl di-
AHCTUNGTRENsGNUN elenide and FeCl3·6H2O under the standard condi-
tions, the desired product 3a was obtained in 79 and
66% yields, respectively. These results indicate that
this cyclization does not follow a radical mechanism
in which diorganyl diselenide is the radical source
[Scheme 5, Eqs. (5) and (6)].
Based on these experimental results and the knowl-
edge that iron salts react with diorganyl diselenides
promoting heterogeneous cleavage of the Se–Se bond
to give an organoselenyl cation and an organoselenyl
anion,[28,29] we propose that this cyclization most likely
proceeds via the pathway outlined in Scheme 6. The
iron(III) coordination with one selenium atom from
dibutyl diselenide activates the other towards nucleo-
philic attack by the alkyne giving the seleniranium
ion I. Nucleophilic anti-attack of the selenium atom
on the activated seleniranium ion I leads to the cy-
clized species II and selenolate anion. The removal of
the alkyl group bonded to the selenium atom via SN2
displacement by the selenolate anion present in the
reaction mixture affords the benzoselenophene III. A
second cyclization process occurs with benzoseleno-
phene III to give the final product 3.
Characterization of Benzo[b]chalcogenopheno [2,3-
d]furan Derivatives
The structures of all products 3 were unambiguously
elucidated by their NMR data and confirmed by
single crystal X-ray diffraction. The molecular struc-
ture of 3f is represented in Figure 1. The bond distan-
ces C-7–Se-1 [1.856(3) ꢅ], Se-1–C-10 [1.916(3) ꢅ] are
equal to the characteristic values for a C–Se bond in
a ring(1.861–1.887 ꢅ)[33] while the distances O–C-
1 [1.389(4) ꢅ] and O–C-8 [1.367(4) ꢅ] are in agree-
ment with that reported in the literature (1.376 ꢅ).[34]
The angles involving the bonds C-7–Se-1–C-10 and C-
1–O–C-8 are 86.96(14)8and 104.59(25)8, respectively.
The bond distances and angles of the molecule 3f
are virtually the same as those described for 3h
Scheme 5. Control experiments for the mechanism study.
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Scheme 6. Proposed mechanism.
Figure 1. Thermal ellipsoid model of the asymmetric unit in
the crystal structure of 3f molecule with the atomic number-
ing Scheme used. All atoms are drawn with displacement el-
lipsoids at the 50% probability level.[35]
Figure 2. The structural representation of the dianionic
moiety of 3h with 50% thermal ellipsoids using ORTEP.[35]
cial even for the organization of macromolecules,
ligand, showing that there were no significant changes such as proteins[39] in a crystalline solid state. Accord-
due to the replacement of halogen in the para posi- ing to quantum mechanical calculations, the energy
tion on the aromatic ring. The bond distances C-7–Se- associated with this interaction is approximately 1.5–
1, Se-1–C-10 and C-1–O, C-8–O of 3h are 1.856(5) ꢅ, 2.5 kcalmolÀ1,[40] however, this evaluation out of a the-
1.904(5) ꢅ and 1.391(6) ꢅ, 1.378(6) ꢅ, respectively. oretical context becomes compromised because the
The angles involving the bonds C-7–Se-1–C-10 and C- nature of these connections is not clear. The C–H···p
1–O–C-8 are 87.12(20)8 and 104.44(36)8, respectively interactions cannot be treated as conventional hydro-
(Figure 2).
gen bonds because the directionality of C–H···p is
Although there are no significant changes in the weak[41] and there are dispersive and electrostatic
angles and distances between the chalcogenide and forces that may be involved in this type of interac-
the two central rings, there is a change of approxi- tions (Table S1, Supporting Information).
mately 12% in the dihedral angles for molecules 3f
(958, 3.310) and 3h (1248, 3.731) (Figure 3). Figure 4
shows the interactions (secondary) between the mole- General Absorption and Emission Properties of
cules of the free ligand 3f (also with the molecule 3h) Benzo[b]chalcogenopheno[2,3-d]furans 3a–3u
by the crystallographic ac plane. There is a predomi-
nance of non-classic hydrogen interactions (C– The comparative absorption spectra of the benzo[b]-
H···p)[36] between C-5–H-5···C4# with distances of chalcogenopheno[2,3-d]furans, using acetonitrile as
2.886(5) ꢅ showing a supramolecular structure in solvent, are shown in Figure 5. The optical properties
a zig-zag pattern.
are listed in Table 3 and additional data (absorption/
Some processes that involve the directions of crys- emission spectra) are presented in Figures S3–S23 in
talline packings[37] as well as, molecular recognition[38] the Supporting Information. The benzo[b]selenophe-
have been attributed to weak interactions of the type no[2,3-d]heterocycle derivatives show absorption
C–H···p. These interactions, although weak, are cru- maxima located around the 300–330 nm range, in the
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Figure 5. Comparative electronic UV-vis absorption spectra
of benzo[b]chalcogenopheno[2,3-d]furan containing differ-
ent substituent groups in acetonitrile solutions.
Table 3. Optical properties of benzo[b]chalcogenopheno[2,3-
d]furans 3a–3u.
Com- UV-vis l [nm]
Emission Ffl fl Stokes
E0–0
pound (MÀ1 cmÀ1)[a]
[nm][b]
[%][c] shifts
[nm][d]
[eV][e]
3a
3b
3c
3d
3e
3f
3g
3h
3i
316 (8940)
319 (9239)
320 (10135)
315 (7151)
321 (9824)
325 (10915)
309 (9626)
325 (7191)
398
400
416
386
408
401
394
398
0.011 82
0.010 81
0.008 96
0.002 71
0.020 87
0.017 76
0.002 85
0.004 73
0.021 72
3.397
3.378
3.387
3.542
3.324
3.306
3.378
3.342
3.297
Figure 3. Differentiation of dihedral planes between the
planes of the molecules 3f (top) and 3h (bottom).
296 (7985); 336 408
(10037)
3j
295 (8670); 334 403
(10868)
0.004 69
3.325
3k
3l
321 (10857)
327 (5707)
319 (11482)
315 (8946)
314 (8885)
320 (6740)
324 (10915)
316 (7754)
317 (9199)
314 (8007)
331 (7191)
396
398
400
397
386
403
404
396
392
394
418
n.d.* 75
0.002 71
0.005 81
0.002 82
0.002 72
0.017 83
0.016 80
0.003 80
0.003 75
0.004 80
n.d.* 87
3.323
3.342
3.406
3.463
3.532
3.387
3.271
3.454
3.492
3.351
3.271
3m
3n
3o
3p
3q
3r
3s
3t
3u
Figure 4. Crystal structures of the molecules 3f and 3h with
evidence for intermolecular interactions C–H···p along the
crystallographic plane ac illustrated by dashed lines (red
line).Symmetry operation: (#) 0.5 x, y, 1.5-z, (#2) 0.5-x, y,
1.5-z.
[a]
[b]
Measured in acetonitrile ([ ]=1.0ꢆ10À4 M).
Measured in deaerated acetonitrile solution ([ ]=2.70ꢆ
10À7 M) at 298 K (lexcitation =320 nm).
9,10-Diphenylanthracene (DPA) in CHCl3 as standard
(Ffl =0.65).
[c]
[d]
[e]
Stokes shift equation: Dl’=lemissonÀlabsorption (Dl’=1/labs
À1/lem ꢆ107 cmÀ1).
UV region. The values for the molar absorptivity co-
efficient for these compounds indicated spin and sym-
metry allowed electronic transitions which could be
E
0–0 =1240/l (in eV); n.d.* not determined (values below
2%).
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related to p–p* transitions (Figure 5). We did not ob- maxima wavelength of lexc =320 nm. The benzo[b]-
serve significant changes in the absorption maxima lo- chalcogenopheno[2,3-d]furan compounds presented
cation associated with different substituent groups in emission in the purple region (380–440 nm). As al-
the benzo[b]chalcogenopheno[2,3-d]furans. The same ready observed in the ground state, they presented
behavior was observed when the oxygen atom was comparable locations to the emission maxima, which
changed to sulfur or selenium (Figure 6). In this way, indicates that different organic moieties in the ben-
zo[b]selenopheno[2,3-d]heterocycles did not play
a fundamental role in the excited state of these com-
pounds. When the nature of the chalcogen in the ex-
cited state was compared, the same behavior was ob-
served in the emission spectra (Figure 7). These re-
sults can probably also be attributed to the nature of
the p–p* electronic transition, indicating that these
compounds allow better electronic delocalization in
the excited state.[42] The Stokes shifts observed in the
excited state indicate that benzo[b]chalcogenophe-
no[2,3-d]furans present some charge separation,
which can be assigned as an intramolecular charge
transfer character in the excited state (Table 3). More-
over, they showed lower fluorescence quantum yields
when compared to the DPA (9,10-diphenylanthra-
cene) standard or other chalcogenyl compounds
(Table 3). This suggests that the presence of the sulfur
or selenium atom seems to be a non-efficient non-ra-
diactive deactivation channel in these structure types
(heavy atom effects).
Figure 6. Comparative electronic UV-vis absorption spectra
of benzo[b]chalcogenopheno[2,3-d]furans 3a, 3n and 3r in
acetonitrile solutions.
Electrochemical Properties of Benzo[b]chalcogeno-
changes of the substituent in the benzo[b]chalcogeno- pheno[2,3-d]furans 3a–3u
pheno[2,3-d]furans did not affect the ground state of
these compounds. The fluorescence emission spectra In order to better evaluate the redox properties of
of some benzo[b]chalcogenopheno[2,3-d]furans are benzo[b]chalcogenopheno[2,3-d]furans. cyclic voltam-
demonstrated in Figure 7. The emission curves were metry experiments were performed (Table 4). In gen-
obtained by exciting the compounds at the absorption eral, the cyclic voltammograms of selenopheno-deriv-
atives displayed one reversible oxidation peak (Epa)
between +0.80 V and +1.00 V vs. Fc/Fc+ redox
couple (Figures S24–S44, Supporting Information). In
aerobic conditions and dry acetonitrile solution, the
anodic oxidation of these compounds can be associat-
ed with an irreversible process that involves the for-
mation of an intermediate radical cation species rap-
idly followed by a reaction with molecular oxygen
giving a sulfoxide or selenoxide species (S=O/Se=
O).[42] At the cathodic region, all derivatives exhibit
one irreversible reduction wave in the reduction
range (Epc), at À1.30 V to À1.70 V (Table 4). The
cathodic peak observed in these compounds can be
assigned to the radical species of benzo[b]chalcogeno-
pheno[2,3-d]furans. Figure 8 illustrates the redox po-
tential
data for benzo[b]chalcogenopheno[2,3-
d]furans containing different groups in their struc-
tures. Analyzing the donor/acceptor groups in these
derivatives (Figure 8), it is possible to suggest that
they did not influence the redox potential of the stud-
ied benzo[b]chalcogenopheno[2,3-d]furan compounds.
This fact may be attributed to the easy oxidation of
Figure 7. Comparative fluorescence emission spectra of
some benzo[b]chalcogenopheno[2,3-d]furans in deareated
acetonitrile solutions.
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Table 4. Electrochemical values of benzo[b]chalcogenophe-
no[2,3-d]furans 3a–3u.
Conclusions
In summary, benzo[b]chalcogenopheno[2,3-d]furans
have been synthesized by reaction of butyl[2-(phenyl-
buta-1,3-diynyl)phenyl]chalcogens with diorganyl dis-
elenides and FeCl3·6H2O. It is shown that the opti-
mized reaction conditions tolerated substrates with
a variety of functional groups, such as methyl, chlor-
ine, fluorine, methoxy and heteroaryl. The same reac-
tion conditions were also compatible with dialkyl di-
Com- Eox
Ered
HOMO
[eV][c]
LUMO
[eV][c]
DE
[eV]
pound [V][a]
[V][b]
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
+0.866 À1.398 À5.716
+0.885 À1.430 À5.735
+0.885 À1.426 À5.735
+0.941 À1.474 À5.791
+0.861 À1.398 À5.711
+0.905 À1.457 À5.755
+1.000 À1.450 À5.850
+0.897 À1.448 À5.747
+0.849 À1.450 À5.699
+0.833 À1.502 À5.683
+0.894 À1.450 À5.744
+0.822 À1.410 À5.672
+0.929 À1.490 À5.779
+0.854 À1.631 À5.704
+0.899 À1.586 À5.750
+0.877 À1.386 À5.727
+0.937 À1.482 À5.788
+0.838 À1.644 À5.688
+0.834 À1.645 À5.684
+0.837 À1.402 À5.687
+0.849 À1.503 À5.699
À3.452
À3.420
À3.424
À3.376
À3.452
À3.393
À3.400
À3.402
À3.400
À3.348
À3.400
À3.440
À3.360
À3.219
À3.262
À3.464
À3.368
À3.206
À3.207
À3.448
À3.347
2.264
2.315
2.311
2.415
2.259
2.362
2.450
2.345
2.299
2.335
2.344
2.232
2.419
2.485
2.482
2.263
2.420
2.482
2.481
2.239
2.352
AHCTUNGTREGsNNNU elenides but incompatible with dialkyl disulfides and
ditellurides. Based on the data obtained with control
experiments, we found that in this double intramolec-
ular 5-endo-dig cyclization an initial formation of a se-
leniranium ion and subsequent removal of the alkyl
group bonded to the chalcogen atom via SN2 displace-
ment were the key steps to the product formation. In
a competitive study among selenium, sulfur and
oxygen nucleophiles, the effect of the nucleophilicity
and steric hindrance determined the rate of the prod-
ucts obtained. The obtained compounds absorb in the
ultraviolet region with molar absorptivity coefficient
values allowed in the selection rules. The emission lo-
cated in the purple region with the Stokes shifts (65–
100 nm range), observed in the excited state, indicates
that benzo[b]chalcogenopheno[2,3-d]furans present
some charge separation, probably assigned as an in-
tramolecular charge transfer character in the excited
state.
3s
3t
3u
[a]
[b]
[c]
Epa =anodic peak.
Epc =cathodic peak.
E
HOMOÀLUMO =À[EoxÀred(vs. Fc/Fc+)+4.85 eV.
Experimental Section
General Procedure for Compounds 3a–n
In a Schlenk tube, under an argon atmosphere, containing
dichloromethane (3 mL) was added FeCl3·6H2O (0.135 g,
0.5 mmol) and diorganyl dichalcogenide (1.75 equiv.). The
resulting solution was stirred at room temperature for
15 min. After this time, the appropriate diyne 1 (0.25 mmol)
was added and the reaction mixture was stirred under reflux
for the times indicated in Table 2. The mixture was dissolved
in ethyl acetate, washed with a saturated solution of NH4Cl,
dried with MgSO4, and concentrated under vacuum. The
residue was purified by column chromatography over silica
gel to provide the products 3 on eluting whith hexane
3-(Butylselanyl)-2-phenylbenzo[b]selenopheno[2,3-
d]furan (3a): Isolated by column chromatography as
a yellow oil; yield: 0.097 g (89%). 1H NMR (400 MHz,
CDCl3): d =7.64–7.58 (m, 4H), 7.44–7.24 (m, 5H), 2.95 (t,
J=7.3 Hz, 2H), 1.53 (quint, J=7.3 Hz, 2H), 1.28 (sex, J=
7.3 Hz, 2H), 0.78 (t, J=7.3 Hz, 3H); 13C{1H} NMR
(100 MHz, CDCl3): d=161.0, 157.1, 149.4, 136.7, 129.5,
128.3, 128.2, 126.4, 124.4, 123.1, 119.6, 116.5, 112.32, 107.0,
32.2, 27.7, 22.5, 13.4; MS (EI, 70 eV): m/z (relative intensi-
ty)=434 ([M+1], 14), 433 (70), 427 (13), 377 (58), 375 (59),
346 (100), 268 (36), 189 (60), 163 (12), 63 (5), 57 (3); HR-
MS (ESI-TOF): m/z=434.9771, calcd. for C20H19OSe2 [M+
H]+: 434.9766.
Figure 8. Comparative redox potentials of some benzo[b]-
chalcogenopheno[2,3-d]furans containing different groups in
their structures in dry CH3CN solutions, using TBAPF6 as
supporting electrolyte.
selenium atoms present in all molecules and the
almost same basic conjugated heterocyclic structures.
All the cyclic voltammograms of benzo[b]chalcogeno-
pheno[2,3-d]furan derivatives are presented in the
Supporting Information.
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3-(Ethylselanyl)-2-phenylbenzo[b]selenopheno[2,3-
130.7, 128.7, 126.29, 124.7, 123.3, 119.7, 116.8, 112.5, 107.5,
32.3, 27.9, 22.6, 13.4; MS (EI, 70 eV): m/z (relative intensi-
ty)=468 ([M+1], 10), 467 (50), 463 (23), 430 (13), 378 (30),
376 (100), 373 (81), 348 (11), 333 (17), 302 (15), 267 (12),
252 (21), 222 (22), 186 (12), 161 (16), 73 (10), 55 (7); HR-
MS (ESI-TOF): m/z=468.9381, calcd. for C20H18ClOSe2
[M+H]+: 468.9377.
3-(Butylselanyl)-2-(2-chlorophenyl)benzo[b]selenophe-
no[2,3-d]furan (3g): Isolated by column chromatography as
an orange oil; yield: 0.079 g (68%). 1H NMR (400 MHz,
CDCl3): d=7.67–7.61 (m, 2H), 7.52–7.47 (m, 1H), 7.45–7.41
(m, 1H), 7.38–7.27 (m, 4H), 2.93 (t, J=7.3 Hz, 2H), 1.57
(quint, J=7.3 Hz, 2H), 1.29 (sex, J=7.3 Hz, 2H), 0.81 (t,
J=7.3 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): d=160.1,
157.2, 145.6, 135.4, 134.4, 132.7, 130.0, 129.7, 126.4, 126.3,
124.7, 123.2, 119.6, 118.1, 112.5, 110.4, 32.4, 27.4, 22.6, 13.4;
MS (EI, 70 eV): m/z (relative intensity)=468 ([M+1], 11),
467 (51), 465 942), 411 (10), 377 (33), 376 (100), 371 (42),
348 (15), 346 (21), 302 (15), 295 (24) 267 (12), 224 (22), 206
(29),187 (84), 186 (12), 161 (16), 73 (10); HR-MS (ESI-
TOF): m/z=468.9383, calcd. for C20H18ClOSe2 [M+H]+:
468.9377.
d]furan (3b): Isolated by column chromatography as
1
a yellow solid; yield: 0.061 g (60%); mp 87–898C. H NMR
(400 MHz, CDCl3): d=7.68–7.56 (m, 4H), 7.45–7.28 (m,
5H), 2.98 (quart, J=7.5 Hz, 2H), 1.30 (t, J=7.5 Hz, 3H);
13C{1H} NMR (100 MHz, CDCl3): d=160.9, 157.1, 149.6,
136.6, 129.5, 128.4, 128.3, 126.4, 124.5, 123.2, 119.6, 116.5,
112.4, 106.6, 21.6, 15.6; MS (EI, 70 eV): m/z (relative inten-
sity)=406 ([M+1], 45), 403 (42), 377 (16), 374 (23), 346
(21), 298 (28), 269 (59), 267 (33), 189 (100), 163 (27), 149
(10), 89 (12), 74 (11); HR-MS (ESI-TOF): m/z=406.9460,
calcd. for C18H15OSe2 [M+H]+: 406.9453.
2-Phenyl-3-(propylselanyl)benzo[b]selenopheno[2,3-
d]furan (3c): Isolated by column chromatography as a gray
solid; yield: 0.080 g (76%); mp 83–858C. 1H NMR
(400 MHz, CDCl3): d=7.68–7.57 (m, 4H), 7.47–7.24 (m,
5H), 2.94 (t, J=7.3 Hz, 2H), 1.59 (quart, J=7.3 Hz, 2H),
0.86 (t, J=7.3 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3):
d=161.0, 157.1, 149.5, 136.7, 129.6, 128.4, 128.3, 127.7, 126.4,
124.5, 123.2, 119.6, 116.5, 114.2, 112.4, 106.9, 30.2, 23.6, 14.1;
MS (EI, 70 eV): m/z (relative intensity)=420 ([M+1], 6),
419 (39), 346 (16), 297 (78), 266 (29), 218 (23), 189 (100),
163 (32), 150 (9), 87 (6); HR-MS (ESI-TOF): m/z=
420.9615, calcd. for C19H17OSe2 [M+H]+: 420.9610.
2-(4-Bromophenyl)-3-(butylselanyl)benzo[b]selenophe-
no[2,3-d]furan (3h): Isolated by column chromatography as
3-(Octylselanyl)-2-phenylbenzo[b]selenopheno[2,3-
a yellow solid; yield: 0.083 g (65%); mp 105–1078C.
d]furan (3d): Isolated by column chromatography as
a yellow oil; yield: 0.080 g (65%). 1H NMR (400 MHz,
CDCl3): d=7.67–7.58 (m, 4H), 7.47–7.28 (m, 5H), 2.96 (t,
J=7.3 Hz, 2H), 1.61–1.50 (m, 2H), 1.30–1.13 (m, 10H), 0.83
(t, J=7.3 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): d=
161.0, 157.1, 149.5, 136.7, 129.6, 129.1, 128.4, 128.30, 126.5,
124.5, 123.2, 119.6, 116.5, 112.4, 107.0, 31.7, 30.2, 29.4, 29.1,
28.92, 28.2, 22.6, 14.0; MS (EI, 70 eV): m/z (relative intensi-
ty)=491 ([M+1], 17), 490 (13), 489 951), 487 (46), 485 (28),
483 (11), 379 (19), 377 (60), 373 (39), 371 (16), 346 (11), 299
(19), 297 (100), 295 (54), 268 (30), 218 (25), 187 (18), 163
(12), 76 (6), 57 (17); HR-MS (ESI-TOF): m/z=491.0398,
calcd. for C24H27OSe2 [M+H]+: 491.0392.
1H NMR (400 MHz, CDCl3): d=7.65–7.59 (m, 2H), 7.54 (d,
J=8.6 Hz, 2H), 7.48 (d, J=8.6 Hz, 2H), 7.36–7.26 (m, 2H),
2.97 (t, J=7.3 Hz, 2H), 1.55 (quint, J=7.3 Hz, 2H), 1.31
(sex, J=7.3 Hz, 2H), 0.80 (t, J=7.3 Hz, 3H); 13C{1H} NMR
(100 MHz, CDCl3): d=161.0, 157.2, 147.6, 135.7, 131.6,
131.0, 126.3, 124.7, 123.3, 122.6, 119.7, 116.9, 112.42, 107.7,
32.4, 27.93, 22.6, 13.3; MS (EI, 70 eV): m/z=513 ([M+1],
32), 512 (10), 511 (48), 509 (33), 507 (17), 457 (27), 455 (41),
453 (30), 377 (57), 375 (100), 373 (70), 371 (32), 348 (12),
346 (21), 297 (15), 268 (15), 216 (51), 207 (67), 188 (85), 161
(11), 73 (23), 57 (13); HR-MS (ESI-TOF): m/z=512.8882,
calcd. for C20H18BrOSe2 [M+H]+: 512.8871.
3-(Butylselanyl)-2-(naphthalen-2-yl)benzo[b]selenophe-
no[2,3-d]furan (3i): Isolated by column chromatography as
3-(Butylselanyl)-2-p-tolylbenzo[b]selenopheno[2,3-
1
d]furan (3e): Isolated by column chromatography as
a yellow solid; yield: 0.069 g (57%); mp 69–718C. H NMR
1
a yellow solid; yield: 0.075 g (67%); mp 80–828C. H NMR
(400 MHz, CDCl3): d=8.12 (s, 1H), 7.99–7.87 (m, 3H),
7.86–7.80 (m, 1H), 7.74–7.66 (m, 2H), 7.59–7.53 (m, 2H),
7.44–7.30 (m, 2H), 3.04 (t, J=7.3 Hz, 2H), 1.61 (quint, J=
7.3 Hz, 2H), 1.34 (sex, J=7.3 Hz, 2H), 0.82 (t, J=7.3 Hz,
3H); 13C{1H} NMR (100 MHz, CDCl3): d=161.1, 157.2,
149.4, 134.2, 133.2, 133.0, 128.7, 128.3, 128.0, 127.8, 127.3,
126.6, 126.5, 124.6, 123.2, 119.7, 117.9, 116.8, 112.4, 107.3,
32.3, 27.9, 22.6, 13.4; MS (EI, 70 eV): m/z (relative intensi-
ty)=484 ([M+1], 16), 483 (68), 481 (63), 479 (38), 428 (31),
426 (91), 424 (85), 420 (20), 398 (31), 349 (17), 318 (58), 268
(31), 239 (100), 237 (67), 212 (12), 206 (37), 163 (12), 150
(10), 119 (12), 73 (11), 57 (96); HR-MS (ESI-TOF): m/z=
484.9930, calcd. for C24H21OSe2 [M+H]+: 484.9923.
(400 MHz, CDCl3): d=7.65–7.59 (m, 2H), 7.51 (d, J=
8.1 Hz, 2H), 7.35–7.26 (m, 2H), 7.23 (d, J=8.1 Hz, 2H),
2.96 (t, J=7.3 Hz, 2H), 2.39 (s, 3H), 1.54 (quint, J=7.3 Hz,
2H), 1.30 (sex, J=7.3 Hz, 2H), 0.79 (t, J=7.3 Hz, 3H);
13C{1H} NMR (100 MHz, CDCl3): d=161.0, 157.1, 149.8,
138.3, 133.9, 129.38, 129.1, 126.5, 124.4, 123.1, 119.5, 118.2,
116.1, 113.5, 112.3, 109.8, 106.6, 32.3, 27.8, 22.6, 21.3, 13.4;
MS (EI, 70 eV): m/z (relative intensity)=448 ([M+1], 12),
447 (57), 443 (31), 393 (12), 389 (43), 375 (15), 360 (11), 313
(18), 311 (100), 309 (56), 360 (11), 313 (15), 311 (100), 282
(25), 232 (23), 206 (30), 202 (69), 200 (17), 189 (10), 163
(11), 115 (8), 73 (10), 55(4): HR-MS (ESI-TOF): m/z=
448.9927, calcd. for C21H21OSe2 [M+H]+: 448.9923.
3-(Butylselanyl)-2-(thiophen-3-yl)benzo[b]selenophe-
3-(Butylselanyl)-2-(4-chlorophenyl)benzo[b]selenophe-
no[2,3-d]furan (3f): Isolated by column chromatography as
a white solid; yield: 0.085 g (73%); mp 103–1058C. H NMR
(400 MHz, CDCl3): d=7.68–7.60 (m, 2H), 7.56 (d, J=
8.6 Hz, 2H), 7.41 (d, J=8.6 Hz, 2H), 7.37–7.28 (m, 2H),
2.97 (t, J=7.3 Hz, 2H), 1.54 (quint, J=7.3 Hz, 2H), 1.30
(sex, J=7.3 Hz, 2H), 0.80 (t, J=7.3 Hz, 3H); 13C{1H} NMR
(100 MHz, CDCl3): d=160.9, 157.1, 147.8, 135.1, 134.4,
no[2,3-d]furan (3j): Isolated by column chromatography as
yellow oil; yield: 0.081 g (74%). 1H NMR (400 MHz,
CDCl3): d=7.69 (dd, J=3.0 Hz, J=1.3 Hz, 1H), 7.62–7.58
(m, 2H), 7.47 (dd, J=5.0 Hz, J=1.3 Hz, 1H), 7.39–7.25 (m,
3H), 3.00 (t, J=7.3 Hz, 2H), 1.58 (quint, J=7.3 Hz, 2H),
1.33 (sex, J=7.3 Hz, 2H), 0.81 (t, J=7.3 Hz, 3H); 13C {1H}
NMR (100 MHz, CDCl3): d=161.1, 157.3, 143.7, 137.3,
128.3, 126.5, 125.6, 124.5, 123.7, 123.2, 119.6, 115.6, 112.4,
1
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106.6, 32.5, 28.0, 22.7, 13.4; MS (EI, 70 eV): m/z (relative in-
tensity)=440 ([M+1], 20), 439 (100), 437 (89), 380 (60), 354
(20), 305 (22), 303 (94), 275 (42), 224 (69), 195 (80), 163
(10), 151 (48), 137 (9), 87 (6), 57 (6); HR-MS (ESI-TOF):
m/z=440.9339, calcd. for C18H17OSSe2 [M+H]+: 440.9331.
2-Butyl-3-(butylselanyl)benzo[b]selenopheno[2,3-d]furan
(3k): Isolated by column chromatography as a yellow oil;
187 (20), 132 (12), 73 (21), 57 (5); HR-MS (ESI-TOF):
m/z=450.9541, calcd. for C20H19SSe2 [M+H]+: 450.9538.
Acknowledgements
1
yield: 0.040 g (39%). H NMR (400 MHz, CDCl3): d=7.62–
We are grateful to FAPERGs, CAPES and CNPq for the fi-
nancial support. CNPq is also acknowledged for the fellow-
ships.
7.55 (m, 2H), 7.31–7.23 (m, 2H), 3.13 (t, J=7.3 Hz, 2H),
2.96 (t, J=7.3 Hz, 2H), 1.75–1.58 (m, 4H), 1.51–1.36 (m,
4H), 0.97 (t, J=7.3 Hz, 3H), 0.87 (t, J=7.3 Hz, 3H);
13C{1H} NMR (100 MHz, CDCl3): d=160.5, 157.1, 155.6,
126.7, 123.9, 123.0, 119.1, 114.1, 112.3, 107.1, 34.7, 33.15,
32.6, 27.9, 22.7, 22.2, 13.8, 13.5; MS (EI, 70 eV): m/z (rela-
tive intensity)=414 ([M+1], 9), 413 (47), 411 (42), 409 (25),
407 (11), 372 (10), 370 (36), 368 (33), 356 (19), 354 (20), 316
(18), 314 (58), 310 (34), 277 (48), 275 (25), 236 (12), 234
(100), 231 (20), 206 (27), 190 (13), 168 (27), 155 (37), 139
927), 126 (37), 115 (17), 77 (7), 57 (5); HRMS (ESI-TOF):
m/z=415.0085, calcd. for C18H23OSe2 [M+H]+: 415.0079.
3-(Butylselanyl)-7-methyl-2-phenylbenzo[b]selenophe-
no[2,3-d]furan (3l): Isolated by column chromatography as
an orange oil; yield: 0.086 g (77%). 1H NMR (400 MHz,
CDCl3): d=7.65–7.57 (m, 2H), 7.48 (d, J=8.4 Hz, 1H),
7.44–7.32 (m, 4H), 7.12 m, 1H), 2.95 (t, J=7.3 Hz, 2H), 2.46
(s, 3H), 1.55 (quint, J=7.3 Hz, 2H), 1.30 (sex, J=7.3 Hz,
2H), 0.79 (t, J=7.3 Hz, 3H); 13C{1H} NMR (100 MHz,
CDCl3): d=161.3, 155.6, 149.0, 136.9, 132.8, 129.5, 128.3,
128.3, 126.5, 125.6, 119.6, 116.3, 111.8, 107.2, 32.3, 27.7, 22.6,
21.3, 13.3; MS (EI, 70 eV): m/z (relative intensity)=448
([M+1], 13), 447 (59), 393 (16), 391 (46), 360 (18), 311
(100), 281 (21), 232 (19), 202 (89), 176 (8), 150 (10), 57 (5);
HR-MS (ESI-TOF): m/z=448.9927, calcd. for C21H21OSe2
[M+H]+: 448.9923.
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Iron-Promoted Tandem Cyclization of 1,3-Diynyl Chalcogen
Derivatives with Diorganyl Dichalcogenides for the
Synthesis of Benzo[b]furan-Fused Selenophenes
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Josꢀ S. S. Neto, Bernardo A. Iglesias, Davi F. Back,
Gilson Zeni*
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