Synthesis, charge distribution, and dimerization behavior of lithium alkynylselenolates

Article abstract of DOI:10.1002/hc.20088

The reactivity of alkynyl lithium compounds toward elemental gray selenium yields alkynylselenolates, which show significant interaction with the lithium counter ions, depending on solvent polarity and the electronic nature of the adjacent organic substit

Full text of DOI:10.1002/hc.20088

Heteroatom Chemistry
Volume 16, Number 2, 2005
Synthesis, Charge Distribution,
and Dimerization Behavior
of Lithium Alkynylselenolates
Rudolf Pietschnig,1 Klaus Merz,2 and Sven Scha¨fer2
1Institut fu¨ r Chemie, Karl-Franzens-Universita¨t Graz, Schubertstraße 1, A-8010 Graz, Austria
2Fakulta¨t fu¨ r Chemie, Ruhr-Universita¨t Bochum, Universita¨tsstraße 150, D-44780 Bochum,
Germany
Received 2 August 2004; revised 29 November 2004
rise to either organic chalcogenides (selenanes) or
ABSTRACT: The reactivity of alkynyl lithium com-
to chalcogenoketenes (Scheme 1). In the case of
pounds toward elemental gray selenium yields alkynyl-
alkynylthiolates, this reactivity pattern has been
selenolates, which show significant interaction with
successfully employed to generate thioketenes
the lithium counter ions, depending on solvent polarity
[1–5]. In contrast to the well-known ketenes and
and the electronic nature of the adjacent organic sub-
thioketenes, their heavier congeners, i.e. seleno and
stituent. Likewise the dimerization behavior is differ-
telluroketenes remain unknown up to now. Recent
ent for alkyl- and aryl-substituted alkynylselenolates.
theoretical investigations suggest that in terms of re-
From the latter, unsymmetrical 1,3-selenol is formed,
activity thioketene is more similar to selenoketene
the molecular structure of which was confirmed by
than to ketene [4]. Nevertheless, experimental data
ꢀ
C
X-ray structure analysis. 2005 Wiley Periodicals, Inc.
to corroborate this hypothesis are not available for
Heteroatom Chem 16:169–174, 2005; Published online
selenoketenes. Therefore, we were interested to in-
in Wiley InterScience (www.interscience.wiley.com). DOI
vestigate the reactivity of selenolates, due to their im-
10.1002/hc.20088
portance as potential precursors for the generation
of selenoketenes.
INTRODUCTION
RESULTS AND DISCUSSION
Alkynylchalcogenolates are known to show typi-
To investigate the ambident reactivity of alkynylse-
cal ambident nucleophilic behavior. For the heav-
lenolates, we prepared some hitherto unknown
ier chalcogens of such compounds, the ꢀ-carbon
alkynylselenolates following an adapted approach
atom represents the hard and the chalcogens atom
of the general procedure by Brandsmaa [6,7]. The
the soft site of the unsaturated unit [1]. Therefore,
instability of ethynylselenolate earlier described
addition of positively charged electrophiles gives
by the same authors prompted us to consider
substituents with different electronic properties.
Therefore, we focused in our investigation on
a comparison of phenyl- and pentyl-substituted
Correspondence to: Rudolf Pietschnig; e-mail: rudolf.pietschnig
@uni-graz.at.
alkynylchalcogenolates. 1-Heptyne and phenylacety-
lene were deprotonated with n-BuLi in THF solution
at low temperature (−78◦C) to the corresponding
alkynyllithium derivatives, which subsequently were
Contract grant sponsor: Fonds der Chemischen Industrie
(Liebig Fellowship).
Contract grant sponsor: Ruhr-Universita¨t Bochum (Anschub-
programm fu¨r den wissenschaftlichen Nachwuchs).
c
ꢀ
2005 Wiley Periodicals, Inc.
169

170 Pietschnig, Merz, and Scha¨fer
TABLE 1 Heteronuclear NMR Data for 2a in Solvents of Dif-
ferent Polarity [8]
C6D6
Et2O
THF
1.63
7.52
−114.6
−2.1
Dipole moment
Dielectric constant
δ(77Se)
0
2.28
1.15
4.34
−15.1
−111.6
SCHEME 1 Reactivity pattern of alkynylchalcogenolates
(E = chalcogens, M = alkali metal, R/Rꢁ = organic
substituent).
δ(7Li)
−0.8
−1.7
ence is found for the 77Se shifts of the aryl-substituted
alkynylselenolates 2b,c that appear to resonate gen-
erally at lower field than in the alkyl derivative. The
13C resonances of the alkynyl carbon atoms do not
differ by more than 10 ppm in all cases, and also the
reacted without further purification with elemen-
tal gray selenium affording the respective alkynylse-
lenolates 2a,b in good yield (Scheme 2).
Besides 1H and 13C NMR, the resulting products
7
have also been characterized by Li and 77Se NMR
2
13C-77Se-coupling constants (1 J and J) are not sig-
spectra. These heteronuclear NMR measurements
show that the chemical shift values differ substan-
tially with the polarity of the solvent and the sub-
stituent attached to the alkynyl group. For instance
with R = pentyl, the 77Se and 7Li nuclei become more
shielded going from lower to higher solvent polarity
(Table 1).
nificantly affected by substituents. On the first sight,
from these spectroscopic data, no marked differ-
ence for the electron distribution for aryl- and alkyl-
substituted alkynylselenolates would be deduced.
In contrast to this conclusion, we found sub-
stantial differences in the reactivity for alkyl- and
aryl-substituted systems of this type. On protona-
tion of the selenolates 2a,b one would expect the
formation of the corresponding selenols. However,
these have not been isolated as the final products of
this reaction. As we found out, the nature of the re-
sulting product depends strongly on the nature of
the substituent R in these selenolates. For instance,
protonation of the pentyl-substituted alkynylseleno-
late 2a results in the formation of the corresponding
dialkynylselenide 3 (Scheme 4).
In contrast to this result, the protonation of the
phenyl-substituted alkynylselenolate 2b also leads to
a dimeric product, however with a different struc-
ture. In the latter case, an unsymmetrically substi-
tuted 1,3-diselenole (4) is formed as the main prod-
uct (Scheme 5).
To explain the formation of these products an ini-
tial protonation of the alkynylselenolate seems likely.
For the resulting product, two tautomeric forms have
to be considered: the alkynylselenol and the corre-
sponding selenoketene. Since a direct nucleophilic
displacement of an HSe− group by a selenol or a
selenolate appears highly unlikely at an sp-carbon
center, the formation of 3 is more likely to involve a
These findings may be caused by the influence
of the solvent polarity on the separation of the ions
in the lithium 1-heptynylselenolate. The observation
that an increase in the solvent polarity leads to a
7
shielding of the 77Se and the Li nuclei is in line
with the assumption of a contact ion pair at lower
solvent polarity, which becomes more separated the
better the coordination properties of the solvent are
(Scheme 3). This should increase the negative charge
on the unsaturated CCSe unit and consequently on
the selenium atom. Likewise the coordination num-
ber for the solvated lithium cation should increase,
accompanied by a reduced positive charge on this
metal ion.
Besides the role of the solvent in the separation
of the ion pair of alkynylselenolates 2a,b, we also
wanted to study the influence of the adjacent organic
substituent on the negatively charged CCSe unit.
In addition to the already mentioned pentyl- and
phenyl-substituted alkynylselenolates 2a,b, we also
prepared the bulky 2,4,6-triisopropyl (=Tip) substi-
tuted congener 2c [9]. All shifts have been measured
in THF solution to maximize the negative charge on
7
the anion. The Li, 77Se, and 13C NMR data for the
relevant CC-SeLi unit of alkynylselenolates 2a–c are
summarized in Table 2. The most pronounced differ-
SCHEME 2 Synthesis of alkynylchalcogenolates 2a,b.
SCHEME 3 Equilibria of lithium alkynylchalcogenolates.

Synthesis, Charge Distribution, and Dimerization Behavior of Lithium Alkynylselenolates 171
TABLE 2 Survey of NMR Data for the CCSe Unit of Lithium
Alkynylchalcogenolates 2a–c in the THF solution
R = Pentyl
R = Phenyl
R = Tip
δ(77Se)
−114.6
−2.1
70.4
78.5
219
−59.3
−2.2
91.1
81.6
231
−64.6
−0.8
83.0
84.6
–
δ(7Li)
δ(13C), Se C C
δ(13C), Se C C
1J CSe (Hz)
2J CSe (Hz)
SCHEME 5 Synthesis of 4.
34
32
–
ing a Becke–Lee–Yang–Parr hybrid functional and a
6-31G(d) basis set [12–14]. For the sake of simplic-
ity, we replaced the pentyl group in 2a with a methyl
group (≡2d).
selenoketene. Addition of either a selenol or a seleno-
late to the selenocarbonyl group of the selenoketene
appears plausible. The resulting intermediate may
then lose HSe− or H2Se giving the finally observed
product 3 (Scheme 6).
With a phenyl-substituted alkynylselenolate in
principle the same initial intermediates should be
likely. In the second step, however the 1,3-dipolar cy-
cloaddition of two selenoketene units would directly
furnish the final product, which appears more likely
than a stepwise addition mechanism.
We were able to obtain suitable crystals for an
X-ray diffraction analysis of compound 4. From this
structure the constitution of the unsymmetrically
substituted 1,3-diselenole is evident (Fig. 1). Actually,
this is the first experimental proof for an unsymmet-
rical substituted 1,3-diselenole that had been postu-
lated without any evidence in older work [10]. All
rings as well as the exocyclic double bond of the
molecule lie in one plane. Due to librational effects,
the C C bonds in and adjacent to the central ring
appear shortened as was discussed by Dunitz earlier
[11]. Otherwise, the relevant bond parameters and
angles show no peculiarities and are summarized in
Table 3. Details of the data collection and structure
refinement are summarized in Table 4 and in the ex-
perimental section.
In the case of the methyl-substituted alkynylse-
lenolate, the charge distribution in terms of an NBO
analysis shows that the negative charge is distributed
over the whole molecule but still predominantly lo-
cated on the selenium atom. Replacing methyl by
phenyl results in a lowered charge density on the se-
lenium atom as well as the alkynyl unit that in both
cases seems to be rather unpolar (Fig. 2).
A more significant difference between these two
substitution patterns can be derived from the MO
coefficients of the HOMO. While the HOMO in the
methyl-substituted case has the largest coefficient
on the Cβ atom. The largest HOMO coefficient of
the phenyl-substituted system is found on the se-
lenium atom. Since in orbital-controlled reactions
an external electrophiles is likely to interact with
the HOMO, these findings are is especially interest-
ing with respect to the generation of a selenoketene
from an alkynylselenolate. Moreover, in the case of
the phenyl-substituted alkynylselenolate, the coef-
ficients of the HOMO in the 1,3 positions of the
CCSe unit are substantially larger than in the methyl
derivative. This might be a one of the reasons for the
increased reactivity of 2b toward dimerization via
1,3-dipolar cycloaddition.
In order to explain the difference in reactiv-
ity of alkyl- and aryl-substituted alkynylselenolates
and to get an insight into the electron distribution
within the CCSe unit, we performed quantum chem-
ical calculations on DFT level. The results were ob-
tained with the program package Gaussian 98 us-
CONCLUSION
In summary, we investigated synthesis, characteri-
zation, and reactivity of alkyl versus aryl-substituted
alkynylselenolates. We explored their dissociation
SCHEME 4 Formation of 3.
FIGURE 1 Molecular structure of 4 in the solid state.

172 Pietschnig, Merz, and Scha¨fer
SCHEME 6 Tentative mechanism for the formation of 3.
behavior in solvents of different polarity, their spec-
troscopic properties, their different dimerization be-
havior, as well as their different electronic situation
on the basis of ab initio calculations. As a result, we
found a substantial difference caused by the different
electronic interaction of the CCSe unit with adjacent
substituents. This leads to a marked contrast in the
dimerization behavior for 2a,b, which in both cases
might involve selenoketene intermediates. Moreover,
we report the first experimental evidence for an un-
symmetrically substituted 1,3-diselenole, for which
we present a crystal structure.
Synthesis of Lithium 1-Heptynylselenolate 2a
To a solution of 0.96 g (10 mmol) of 1-heptyne in
40 mL of the THF cooled to −78◦C, a solution of
n-BuLi (11 mmol) in hexane is slowly added while
stirring. Stirring is continued for 90 min at this tem-
perature, after which the mixture is warmed to room
temperature resulting in a yellow solution. This so-
lution is again cooled to −78◦C and gray selenium
powder (0.79 g, 10 mmol) is added in small por-
tions. Stirring is continued and the reaction mix-
ture is warmed to room temperature over night upon
TABLE 4 Crystal Data and Structure Refinement for 4
EXPERIMENTAL
Formula
Formula weight
Temp. (K)
C16H12Se2
362.18
203(2)
0.71073
Orthorhombic
Pca2(1)
The chemical experiments were performed in an
argon atmosphere and solvents were freshly distilled
prior to use. NMR spectra were recorded on a Bruker
DPX 250 and referenced using tetramethylsilane
(1H/13C), LiCl/H2O (7Li), and H2SeO3/H2O (77Se) as
external standards. Mass spectra were recorded on a
VG Instruments Autospec/EBEE-geometry.
Wavelength
Crystal system
Space group
Unit cell dimensions
˚
a (A)
8.2207(16)
5.7511(12)
27.936(6)
90
90
90
1320.8(5)
4
1.821
5.576
2.92–25.18
1.122
˚
b (A)
˚
c (A)
α (◦)
β (◦)
γ (◦)
TABLE 3 Selected Bond Lengths and Angles of 4 in degrees
˚
and A
3
˚
Volume (A )
Se(1) C(16) 1.872(14) C(16) Se(1) C(7)
90.4(8)
94.0(7)
Z
Se(1) C(7)
2.03(3)
C(16) Se(2) C(8)
Density (calcd.) (mg/m3)
Se(2) C(16) 1.821(17) C(7) C(8) Se(2)
117.7(14)
129(2)
µ (mm−1
)
Se(2) C(8)
C(1) C(7)
C(7) C(8)
C(9) C(15)
1.96(2)
1.56(2)
1.28(3)
1.56(2)
C(16) C(15) C(9)
θ range for data collection (deg)
goodness-of-fit on F
R1 (observed data)
wR 2 (all data)
C(15) C(16) Se(2) 122.4(15)
C(15) C(16) Se(1) 118.7(15)
Se(2) C(16) Se(1) 118.8(9)
2
0.0826
0.2113
C(15) C(16) 1.24(3)

Synthesis, Charge Distribution, and Dimerization Behavior of Lithium Alkynylselenolates 173
Synthesis of
2-Benzylidene-4-phenyl-1,3-diselenole (4)
In a schlenk flask, 2a (0.5 g, 2.7 mmol) is dissolved in
50 mL of ether and subsequently treated with 10 mL
of half-concentrated aqueous hydrochloric acid. The
mixture is stirred for 30 min and then the layers are
separated, and the aqueous layer is extracted twice
with 20 mL of pentane. The combined organic layers
are dried over magnesium sulfate, filtered, and the
solvents are removed in vacuum. 4 is obtained as
orange pasty residue that can be recrystallized from
FIGURE 2 Natural charges derived from an NBO analysis
MTBE/toluene (0.32 g, 64%).
(upper section) and illustration of the MO coefficients of the
1H: 7.5–7.2 (m). 13C: 138.6, 137.5, 136.0, 129.3
(CH), 129.0 (CH), 128.2, 127.2 (CH), 126.9 (CH),
126.2, 125.9, 121.8, 126.3. 77Se: 684 (s), 544 (s). MS
(FAB): 363.9, M+, 73%; 182, M+/2, 20%. C16H12Se2:
calcd.:C: 53.06, H: 3.34; found: C: 52.67, H: 3.08.
π-system (lower section) of 2b and 2d.
which the selenium is dissolved completely. Evapo-
ration of the solvent in vacuum furnishes a orange-
brown viscous oil, which is washed twice with 10 mL
1
of pentane. Yield: 1.71 g (94%). H: 0.8 (3H), 1.1–
Crystallographic Details for 4
1.5 (6H), 2.0 (2H). 13C (C6D6): 14.15 (CH3), 21.86
(CH2), 22.77 (CH2), 30.72 (CH2), 31.54 (CH2), 70.35
(1 JCSe = 219 Hz, Se-CC), 78.45(2 JCSe = 34 Hz, Se-
CC). 77Se (C6D6): −15.1. 7Li (C6D6): −0.8. MS (FAB):
176, M+, 20%; 133, M+-propyl, 30%; 95, propyl+,
55%.
A colorless crystal of 5 with dimensions 0.1 × 0.2 ×
0.2 mm was coated in paraffin oil, mounted on a glass
fiber, and placed under a cold stream of nitrogen. All
the measurements were performed using graphite-
monochromatized Mo Kꢁ radiation at 203 K. A total
of 4983 reflections were collected (ꢀmax = 25◦), from
which 2220 were unique (R = 0.0740). Additional
int
Synthesis of Lithium Phenylalkynylselenolate 2b
experimental details are given in Table 4. The struc-
ture was solved by direct methods (SHELXS-97)
and refined by full-matrix least-squares techniques
against F2 (SHELXL-97) [15,16]. The non-hydrogen
atoms were refined with anisotropic displacement
parameters without any constraints. For 140 param-
eters, final Rindices of R = 0.0826 and wR2 = 0.2113
(GOF = 1.122) were obtained.
2b was prepared similar to 2a. Starting from of 1.02 g
(10 mmol) of phenylacetylene 1.65 g (88%) of lithium
1
phenylalkynylselenolate 2b was obtained. H (THF-
d8): 6.9–7.5 (m). 13C (THF-d8): 81.6 (1 JCSe = 31.5 Hz,
Se-CC), 91.1 (1 JCSe = 231 Hz, Se-CC), 122.9 (p-CH),
127.9 (CH), 130.6 (Cq), 131.0 (CH). 77Se (THF-d8):
7
−59.2. Li (THF-d8): −2.18. MS (EI): 187, M+, 2%;
98, M+-H2Se, 17%, 60, 100%.
ACKNOWLEDGMENT
Synthesis of Di(1-heptynyl)selenide 3
The authors would like to thank Prof. Matthias
Driess for helpful discussions.
In a schlenk flask, 2a (0.5 g, 2.8 mmol) is dissolved
in 50 mL of ether and subsequently treated with
10 mL of half-concentrated aqueous hydrochloric
acid. The mixture is stirred for 30 min and then
the layers are separated, and the aqueous layer is
extracted twice with 20 mL of pentane. The com-
bined organic layers are dried over magnesium sul-
fate, filtered, and the solvents are removed in vac-
uum. 3 is obtained as an orange pasty residue (0.27g,
72%).
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