Synthesis of protected benzenepolyselenols

Article abstract of DOI:10.1021/jo301447h

Previously unknown benzenepolyselenols have been synthesized and isolated in their acetyl-protected form. The two molecules 1,3,5-tris(acetylseleno) benzene and 1,2,4,5-tetrakis(acetylseleno)benzene were synthesized by the reductive dealkylation in Na/NH<

Full text of DOI:10.1021/jo301447h

Note
pubs.acs.org/joc
Synthesis of Protected Benzenepolyselenols
Dayna L. Turner† and Thomas P. Vaid*
Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States
S
* Supporting Information
ABSTRACT: Previously unknown benzenepolyselenols have been
synthesized and isolated in their acetyl-protected form. The two molecules
1,3,5-tris(acetylseleno)benzene and 1,2,4,5-tetrakis(acetylseleno)benzene
were synthesized by the reductive dealkylation in Na/NH3 of 1,3,5-
tris(tert-butylseleno)benzene and 1,2,4,5-tetrakis(tert-butylseleno)benzene,
respectively. Hexakis(tert-butylseleno)benzene was also synthesized and
structurally characterized by single-crystal X-ray diffraction, but it was not
possible to isolate hexakis(acetylseleno)benzene. The synthetic method-
ology is likely to be useful in the synthesis of other areneselenols.
enzenepolythiols have been fairly widely studied as
by ortho-lithiation of benzeneselenol followed by reaction with
Belectrically active components of materials and molecules. 1 equiv of elemental selenium,34 by the reaction of 1,2-
dilithiobenzene with 2 equiv of selenium,30 or by reduction of a
For example, 1,4-benzenedithiolate is the prototypical moiety
1,2-phenylenediselenide polymer.32,33 It might be possible to
for studies of single-molecule conductivity, usually between two
gold electrodes.1−3 Polynuclear metal complexes have been
iterate the ortho-lithiation route to produce 1,2,3-benzene-
synthesized from 1,4-benzenedithiol,4 1,3,5-benzenetrithiol,4
triselenol, 1,2,3,4-benzenetetraselenol, etc., but such a route
would be inefficient and has been little explored.44 The other
reported benzenediselenol, 1,4-benzenediselenol, was synthe-
sized by lithium−halogen exchange between 1,4-dibromoben-
zene and tert-butyllithium followed by treatment with elemental
selenium.28,29 However, that method is not viable for installing
three or more selenol groups.45
1,2,4,5-benzenetetrathiol,5−11 and benzenehexathiol.12−15
Metal−organic polymers or networks, some of them semi-
conducting, have been synthesized with 1,4-benzendithio-
late,16−18 1,2,4,5-benzenetetrathiolate,17,19 and benzenehexa-
thiolate17 as the organic component. While both experimen-
tal20,21 and theoretical22−25 studies indicate that replacing the
sulfur linking atoms with selenium would lead to higher single-
molecule conductivity (although another theoretical study
contradicts that finding26), and replacing sulfur by selenium
in a hybrid organic−inorganic semiconductor is predicted to
decrease the electronic band gap,27 the chemistry of
benzenepolyselenols is much less developed than that of
benzenepolythiols. Para-substituted 1,4-benzenediselenol has
been synthesized in both its unprotected form28 and as the
protected 1,4-di(acetylseleno)benzene.29 Ortho-substituted 1,2-
benzendiselenol (or 1,2-benzenediselenolate) has been synthe-
sized by a variety of methods.30−34 However, no benzenese-
lenols with three or more selenol groups (protected or
unprotected) have been reported.
The most common route to arenepolythiols, the reductive
cleavage of arenepoly(alkylthioethers) by sodium in liquid
NH3, was established in 1959.35 The starting arenepoly-
(alkylthioethers)36 are readily accessible through the nucleo-
philic aromatic substitution of arenepolyhalides with sodium
alkylthiolates in solvents such as hexamethylphosphoramide
(HMPA),37−40 1,3-dimethyl-2-imidazolidinone (DMI),41 or
dimethylformamide (DMF).42,43 Scheme 1 shows this set of
reactions for the benzenepolythiols.
There is one published investigation in which a route
analogous to the one in Scheme 1 was applied to the synthesis
of benzeneselenols.46 The alkyl selenide nucleophile was
LiSeCH3, and that led to a side reaction in which the
methylselenide nucleophile adds to the methyl carbon of an
already-formed aryl-SeCH3 group, as shown in Scheme 2. For
LiSeCH3 ,this dealkylation is significantly faster than the second
nucleophilic aromatic substitution,46 while in a similar reaction
of NaSCH3 with 1,4-dichlorobenzene the two reactions occur
at a comparable rates.38 Once a dealkylation has occurred to
form an arylselenide anion, the arene is deactivated and further
nucleophilic aromatic substitution does not occur. We have
found that the bulkier alkyl group of LiSetBu prevents the
dealkylation reaction and allows the synthesis of benzenepoly-
(tert-butyl selenoethers), which can be subsequently deal-
kylated in Na0/NH3 to form (protected) benzenepolyselenols.
It was most convenient and effective to synthesize, isolate,
and store a form of LiSetBu for use in subsequent reactions.
Treatment of elemental selenium with 1 equiv of tBuLi in THF
at −78 °C yields47 LiSetBu(THF)x, which is highly soluble in
THF and too soluble in hexane for efficient recrystallization.
We reasoned that 1,4-dioxane would form a less soluble,
polymeric complex with LiSetBu. Evaporation of the volatiles
The known benzenediselenols, 1,2-benzendiselenol and 1,4-
benzenediselenol, were not synthesized by a route analogous to
that in Scheme 1. Instead, 1,2-benzendiselenol (or 1,2-
benzenediselenolate as an intermediate) has been synthesized
Received: August 14, 2012
Published: September 24, 2012
© 2012 American Chemical Society
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The Journal of Organic Chemistry
Note
Scheme 1. Synthesis of Benzene Polythiols
Scheme 2. Reaction of LiSeCH3 with 1,4-Dichlorobenzene
Table 1. Halobenzene Reactants, Reaction Conditions, and Yields
from a solution of LiSetBu(THF)x followed by addition of
dioxane yields a homogeneous solution. Removal of excess
dioxane and addition hexanes yields a white suspension of
LiSetBu(dioxane), which was isolated by filtration and stored in
a nitrogen-filled glovebox.
by Na0/NH3 and then treated with acetic anhydride to give
1,3,5-benzenetri(selenoacetate) and 1,2,4,5-benzenetetra-
(selenoacetate), respectively, in good yield. Acetyl is a very
convenient protecting group for areneselenols: it imparts air-
stability on the areneselenol group, while it is easily hydrolyzed
in the presence of an alkylamine (or stronger) base.
Reactions of polyhalobenzenes with the appropriate
stoichiometry of LiSetBu(dioxane) were conducted in DMI
or DMF at either 22 or 80 °C; reaction conditions and yields
are summarized in Table 1. The identity of the halogen in the
polyhalobenzenes was important. The synthesis of 1,3,5-
benzenetri(tert-butyl selenide) from 1,3,5-trichlorobenzene in
DMI gave an isolated yield of only 17%; the use of 1,3,5-
trifluorobenzene improved the yield to 41%. The synthesis of
1,2,4,5-benzenetetra(tert-butylselenide) was possible only from
1,2,4,5-tetrabromobenzene: reactions using 1,2,4,5-tetrachlor-
obenzene or 1,2,4,5-tetrafluorobenzene in either DMF or DMI
did not yield isolable product. Overall, yields of the
benzenepoly(tert-butylselenoethers) were low to moderate.
The products were isolated in pure form by simply filtering
the DMI or DMF reaction mixture, from which the product
precipitated over the course of the reaction. Analysis of the
DMI/methanol or DMF/methanol filtrate showed the presence
of a small amount of additional product in each case, but it was
not practical to isolate and purify this additional product.
The two selenoethers 1,3,5-benzenetri(tert-butyl selenide)
and 1,2,4,5-benzenetetra(tert-butyl selenide) were dealkylated
All attempts to synthesize benzenehexa(selenoacetate) from
benzenehexa(tert-butylselenide) led to an insoluble orange
solid. Treatment of the putative Na6C6Se6 intermediate with
CH3I led to a mixture of products. Attempted crystallization of
Na6C6Se6 from water (the structure of the analogous
Na6C6S6·16H2O is known48) yielded single crystals of
Na2Se·9H2O. It is possible that the steric congestion of six
selenolate anions on Na6C6Se6 leads to its instability, so we
have investigated the single-crystal X-ray structure of
benzenehexa(tert-butyl selenide).
A sample of C6(SetBu)6 was dissolved in hot heptane to form
a red solution, which was slowly cooled to 22 °C. Data
collection from a resulting red crystal yielded the structure
shown in Figure 1. The bond distances for Se−C(arene) range
from 1.93(1) to 1.95(1) Å and for Se−C(tBu) range from
1.98(1) to 2.01(1) Å. The C−Se−C bond angles range from
100.2(4) to 101.9(4)°. Steric interactions force the tert-butyl
groups to alternating sides of the C6 plane, as seen in Figure 1.
In addition, the selenium atoms themselves deviate, on average,
0.18 Å from the mean plane of the C6 core, all in the same
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The Journal of Organic Chemistry
Note
1,3,5-Tris(tert-butylseleno)benzene. DMI (2 mL) was added
via syringe to LiSetBu(dioxane) (500 mg, 2.16 mmol) to form an
orange solution. Next 1,3,5-trifluorobenzene (80 mg, 0.61 mmol) was
added, and the solution was heated to 80 °C for 15 h. The solution
was cooled to 22 °C, and a white precipitate formed. Methanol (2 mL)
was added, the suspension was cooled to −12 °C and stirred for 15
min, and the product was isolated by filtration and washed with 1 mL
1
of cold methanol. Yield: 120 mg, 41%. Mp: 138−140 °C. H NMR
(CDCl3): δ 7.97 (s, 3H), 1.42 (s, 9H). 13C NMR (CDCl3): δ 147.4,
128.8, 44.1, 32.4. UV−vis in acetonitrile, 5.25 × 10−6 M: λmax = 214
nm, ε = 1.7 × 105 L·mol−1·cm−1, λmax = 291 nm, ε = 8.5 × 103
L·mol−1·cm−1. IR (Nujol cm−1): 1536 (m), 1146 (s), 1020 (w), 882
(m), 801 (w), 742 (w), 691 (m), 666 (w). Anal. Calcd for C18H30Se3:
C, 44.73; H, 6.26; N, 0.00. Found: C, 44.76; H, 6.30; N, 0.23.
1,3,5-Tris(acetylseleno)benzene. Dry liquid ammonia was
added to sodium (0.324 g, 14.1 mmol) and 1,3,5-tris(tert-butylseleno)-
benzene (0.680 g, 1.41 mmol). The liquid ammonia was allowed to
boil off through a bubbler over about 1 h. Degassed methanol (5 mL)
was added via syringe, and the solution was stirred for 20 min. The
methanol was then removed under vacuum and THF added to the
orange residue. The suspension was stirred for 10 min, and then the
THF was removed under vacuum. Fresh THF was added the residue,
and then an excess of degassed acetic anhydride (2 mL) was added to
the solution via syringe. After the solution was stirred for 1 h, the
volatiles were removed under vacuum. Water (25 mL) was added to
the residue, and the product was extracted with dichloromethane (3 ×
25 mL). The combined organic layers were washed with water, dried
over MgSO4, and filtered. The solvent was removed under vacuum
leaving a yellow oil. Yield of 1,3,5-tris(acetylseleno)benzene: 0.456 g,
Figure 1. Single-crystal X-ray structure of C6(SetBu)6. Key: carbon,
gray; selenium, red; hydrogen, not shown.
direction as their respective tert-butyl groups. The average Se−
C−C−Se dihedral angle is 10°. It is most likely that those
deviations from planarity are due to Se−Se interactions rather
than interactions of selenium atoms with neighboring tert-butyl
groups, as the compound C6(SePh)6 (Ph = phenyl) exhibits a
similar alternating displacement of the selenium atoms from the
C6 plane.49 Interestingly, in contrast, in hexaiodobenzene the
iodine atoms deviate from the C6 mean plane by only 0.046 Å
on average.50 In any case, when the selenoethers of C6(SetBu)6
are reduced to selenides in C6Se66− the mutual repulsions of the
anionic selenides may destabilize it to the point that it
decomposes.
In conclusion, the synthesis of three benzene(selenoethers)
and two acetyl-protected benzene(polyselenols) has been
established. The synthetic methodology is likely to be useful
in the synthesis of other areneselenols. Even when the halogen
atoms of the haloarenes are not strongly mutually activating, as
in 1,3,5-trifluorobenzene, the nucleophilic aromatic substitution
to form the selenoethers is successful, which indicates that it is
likely that this method can be extended to larger fluoroarenes in
which the fluorine atoms are electronically isolated from each
other.
1
85%. H NMR (CDCl3): δ 7.70 (s, 3H); 2.50 (s, 9H). 13C NMR
(CDCl3): δ 195.6, 142.6, 128.6, 34.3. HRMS (EI, magnetic sector
mass analyzer): m/z calcd for C12H12O3Se3 441.8290, found 441.8288.
1,2,4,5-Tetrakis(tert-butylseleno)benzene. DMF (5 mL) was
added to 1,2,4,5-tetrabromobenzene (0.406 g, 1.03 mmol) and
LiSetBu(dioxane) (1.000 g, 4.33 mmol). The resulting orange solution
was heated to 80 °C for 16 h and then cooled to room temperature.
The white precipitate was isolated by filtration and washed sparingly
with methanol. Yield of white 1,2,4,5-tetrakis(tert-butylseleno)-
benzene: 0.188 g, 29%. Product can be recrystallized from hot 1:1
1
ethanol−methanol. Mp: 147−149 °C. H NMR (CDCl3): δ 1.50 (s,
36H), 8.12 (s, 2H). 13C NMR (CDCl3): δ 145.5, 137.6, 45.9, 32.6.
UV−vis in acetonitrile, 3.11 × 10−5 M: λmax = 229 nm, ε = 2.5 × 104
L·mol−1·cm−1, λmax = 286 nm, ε = 1.1 × 104 L·mol−1·cm−1. IR (Nujol,
cm−1): 1407 (m), 1275 (w), 1231 (w), 1151 (s), 1099 (w), 1027 (s),
882 (m), 801 (w), 666 (w). Anal. Calcd for C22H38Se4: C, 42.73; H,
6.19; N, 0.00. Found: C, 42.61; H, 6.33; N, 0.11.
EXPERIMENTAL SECTION
■
General Procedures and Materials. All reactions were carried
out under N2 in dry, oxygen-free solvents. Reagents and solvents were
purchased from commercial suppliers and used as received, unless
noted as follows. THF and dioxane were distilled from a purple
sodium benzophenone solution, while hexane was distilled from
sodium benzophenone tetraglyme. Dimethylformamide (DMF) and
1,3-dimethyl-2-imidazolidinone (DMI) were both distilled from P2O5
and stored over 3 Å molecular sieves. The benzenepoly(tert-butyl
selenoethers) are air-stable, and the workup and subsequent handling
were done without protection from air. The acylated products were
worked up without protection from air but were stored under nitrogen
after purification.
1,2,4,5-Tetrakis(acetylseleno)benzene. Dry liquid ammonia
was added to sodium (0.112 g, 4.85 mmol) and 1,2,4,5-tetrakis(tert-
butylseleno)benzene (0.300 g, 0.485 mmol). The liquid ammonia was
allowed to boil off through a bubbler over about 1 h. Degassed
methanol (5 mL) was added via syringe to quench the excess sodium.
The methanol was then removed under vacuum, and THF was added
to the light pink residue. The suspension was stirred for 10 min, and
then the THF was removed under vacuum and fresh THF was added.
An excess of acetic anhydride (1 mL) was added to the solution via
syringe, and after the suspension was stirred for 1.5 h, the solvents
were removed under vacuum. Water (25 mL) was added, and the
product was extracted with dichloromethane (4 × 25 mL). The
combined organic layers were washed with water, dried over Na2SO4,
and filtered. The solvent was removed under vacuum leaving a yellow
solid. Yield of 1,2,4,5-tetrakis(acetylseleno)benzene: 0.189 g, 69%.
Product can be recrystallized by partial evaporation of a solution in 1:1
LiSetBu(dioxane). To a suspension of selenium powder (3.00 g,
38.0 mmol) in 25 mL of THF at −78 °C was added BuLi (1.7 M in
t
pentane, 24 mL, 41 mmol) via syringe. An orange suspension began to
form immediately. After 30 min, the cold bath was removed, and
suspension was allowed to warm to 22 °C and stirred for 30 min. The
volatiles were removed under reduced pressure, and the solid residue
was dissolved in 25 mL of 1,4-dioxane to form a pale orange solution.
The dioxane was removed under reduced pressure. Hexanes (∼50
mL) was added, and stirring yielded a white suspension. The white
solid was isolated by filtration and washed with hexanes. Yield of
1
CH2Cl2−heptane. Mp: 164−166 °C. H NMR (CDCl3): δ 2.49 (s,
12H), 8.09 (s, 2H). 13C NMR (CDCl3): δ 194.9, 144.8, 135.6, 34.1.
UV−vis in acetonitrile, 1.96 × 10−5 M: λmax = 213 nm, ε = 2.1 × 104
L·mol−1·cm−1, λmax = 235 nm, ε = 2.9 × 104 L·mol−1·cm−1. IR (Nujol
cm−1): 3414 (w), 1280 (m), 1096 (s), 1034 (m), 999 (w), 940 (m),
877 (m), 818 (w), 666 (w). Anal. Calcd for C14H14O4Se4: C, 29.92; H,
2.51; N, 0.00. Found: C, 29.53; H, 2.22; N, 0.12.
1
LiSetBu(dioxane): 5.72 g, 65%. H NMR (CD3CN): δ 3.60 (s, 8H),
1.47 (s, 9H).
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The Journal of Organic Chemistry
Note
Hexakis(tert-butylseleno)benzene. DMI (8 mL) was added via
syringe to hexachlorobenzene (0.317 g, 1.11 mmol) and LiSetBu-
(dioxane) (1.800 g, 7.78 mmol). After being stirred for several minutes
at room temperature, the resulting brown solution became red-orange
and a pink precipitate formed. After the solution was stirred for 16 h at
room temperature, 100 mL of methanol was added and the solution
was stirred at 0 °C for 30 min. The pink precipitate was isolated by
filtration and washed with methanol. Yield of pink hexakis(tert-
butylseleno)benzene: 0.590 g, 60%. The product is somewhat light-
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1
sensitive. Mp (under N2): ∼212−215 °C dec. H NMR (C6D6): δ
1.56. 13C NMR (C6D6): δ 33.06, 48.79, 128.31. UV−vis in hexanes:
λmax = 248 nm, ε = 3.8 × 104 L·mol−1.cm−1; 276 nm (shoulder), ε ∼
1.0 × 104 L·mol−1·cm−1; λmax = 392 nm, ε = 1.0 × 103 L·mol−1·cm−1;
λmax = 542 nm, ε = 1.8 × 102 L·mol−1·cm−1. IR (Nujol cm−1): 1295
(w), 1218 (w), 1150 (s), 1036 (w), 1010 (w), 666 (w). Anal. Calcd for
C30H54Se6: C, 40.55; H, 6.13; N, 0.00. Found: C, 40.10; H, 5.88; N:
0.13.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data in CIF and tabular formats; H and 13C
1
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tpvaid@ua.edu.
■
Present Address
†Novus International, 20 Research Park Drive, St. Charles, MO
63304.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Steven Kelley for assistance in single-crystal X-ray
data collection and reduction. This work was supported by the
University of Alabama.
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