Stereochemical study of the sterically crowded phenylselanylalkenes by means of 77Se-1H spin-spin coupling constants

Article abstract of DOI:10.1002/mrc.2784

Stereochemical study of five sterically crowded phenylselanylalkenes obtained via the hydroselenation of either terminal or internal alkynes with benzeneselenol catalyzed by the nanosized Ni complexes has been carried out based on the experimental HMBC me

Full text of DOI:10.1002/mrc.2784

Research Article
Received: 23 March 2011
Accepted: 30 May 2011
Published online in Wiley Online Library: 5 August 2011
(wileyonlinelibrary.com) DOI 10.1002/mrc.2784
Stereochemical study of the sterically crowded
phenylselanylalkenes by means of 77Se–1H
spin–spin coupling constants
Yury Yu. Rusakov,a Leonid B. Krivdin,a∗ Nikolay V. Orlovb
and Valentine P. Ananikovb
Stereochemical study of five sterically crowded phenylselanylalkenes obtained via the hydroselenation of either terminal or
internal alkynes with benzeneselenol catalyzed by the nanosized Ni complexes has been carried out based on the experimental
HMBC measurements and theoretical second order palarization propagator approach (SOPPA) calculations of their 77Se–1H
spin–spin coupling constants across double bond in combination with the energy-based theoretical conformational analysis
performed at the MP2/6-311G∗∗ level. It has been found that studied phenylselanylalkenes adopt mainly skewed s-cis
c
conformation with the noticeable out-of-plane deviations of the phenylselanyl and phenyl groups. Copyright ꢀ 2011 John
Wiley & Sons, Ltd.
Keywords: NMR;77Se–1Hspin–spincouplingconstants;SOPPA;HMBC;stereochemicalanalysis;stericallycrowdedphenylselanylalkenes
Introduction
plings measured in the 77Se–1H HMBC spectra of 1–5, as
exemplified for compound 3 in Fig. 1. It is noteworthy that tran-
soidal couplings across double bond (ca. 7–8 Hz) are somewhat
larger than cisoidal ones (ca. 4–5 Hz) in this series, being in line
with our previous findings in divinyl selenide[4] and selenium-
containing heterocycles.[7] The only exception is compound 4
with cisoidal coupling 3J(Se,HB) = 10 Hz, at least twice as large as
in the rest of compounds under study, being even larger than the
transoidal couplings in this series. On the other hand, the absolute
value of the difference between 3J(Se,HA) and 3J(Se,HB) is pretty
moderate (only ca. 3–4 Hz), and these two unexpected facts are
thesubjectofamoredetaileddiscussiongivenbelowbasedonthe
theoretical calculation of the model dihedral angle dependences
of 3J(Se,HA) and 3J(Se,HB) shown in Fig. 2.
All calculations of J(Se,H) were performed at the Second
order polarization propagator approach, SOPPA,[8] taking into
account all four non-relativistic coupling contributions to the total
coupling,namely,Fermicontact,JFC,spin-dipolar,JSD,diamagnetic
spin–orbital JDSO, and paramagnetic spin–orbital, JPSO, terms.
For selenium, we used Dunning’s correlation-consistent triple
zeta basis set, cc-pVTZ,[9] with decontracted s-functions and
augmented with two tight s-functions (ξ1 = 63865962.54 and
ξ2 = 426498512.2)referredtoascc-pVTZ-su2,asreportedrecently
bysomeofus.[5b] Sauer’scontractedaug-cc-pVTZ-Jbasisset[10] was
employed for the hydrogens involved in the spin–spin coupling
Functionalized selanylalkenes, providing a wide spectra of their
application in material science, organic synthesis and being the
potential precursors of the selenium-based biologically active
molecules,[1,2] have now become accessible through the recently
developed regio- and stereoselective addition of selenoles to
alkynes catalyzed by the nanosized Pd and Ni particles stabilized
withorganicsulfurorseleniumligands.[3] Apartfromtheirpractical
application, polyfunctionalized selanylalkenes demonstrate an in-
teresting conformational behavior due to the marked steric effects
in the sterically crowded olefinic moiety which is the main subject
of the present communication studied using 77Se–1H spin–spin
coupling constants across double bond. Considered herewith
are five most typical representatives of this series, compounds
1–5 (Scheme 1), obtained via heterogeneous Ni-based catalytic
regioselective hydroselenation of terminal alkynes (to give
compounds 1–3) and stereoselective hydroselenation of internal
alkynes (to give compounds 4 and 5), as described elsewhere[3b]
.
Based on our previous results on the marked stereospecificity
of geminal and vicinal J(Se,H) couplings in respect with the
mutual orientation of the selenium lone pair and the correspond-
ing coupling pathway in divinyl selenide,[4] selenophenes,[5]
[6]
selenosugars
and selenium-containing heterocycles,[7] in this
communication we employ vicinal selenium–proton couplings
across double bond measured experimentally and calculated
theoretically for the stereochemical study of 1–5 dealing with
the internal rotation around the Se–C bonds. These results are
supplemented with the energy-based theoretical conformational
analysis performed at the MP2/6-311G∗∗ level.
∗
Correspondence to: Leonid B. Krivdin, A. E. Favorsky Irkutsk Institute of
Chemistry, Siberian Branch of the Russian Academy of Sciences, Favorsky
St. 1, 664033 Irkutsk, Russia. E-mail: krivdin office@irioch.irk.ru
a
A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian
Academy of Sciences, Favorsky St. 1, 664033 Irkutsk, Russia
Results and Discussion
Collected in Table 1 are the experimental values of transoidal,
3J(Se,HA), and cisoidal, 3J(Se,HB), vicinal selenium–proton cou-
b
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
Leninsky Prospect 47, 119991 Moscow, Russia
c
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Stereochemical study of the sterically crowded phenylselanylalkenes
Figure 1. The olefinic region of the 77Se–1H HMBC spectrum of 2-methyl-3-(phenylselanyl)but-3-en-2-yl acetate (3) in CDCl3 (1H, 400.13 MHz;
77Se, 76.34 MHz).
the dihedral angle itself, the former falling into the range of ca.
4–5 Hz for ϕ = 0–60◦ to ca. 15–20 Hz for ϕ = 120–180◦ with the
transoidal couplingbeinglargerinthewholerangeofϕ = 0–180◦.
Now it becomes clear why experimental cisoidal 3J(Se,HB) and
especially transoidal 3J(Se,HA) are so small and why experimental
difference of those couplings does not exceed 3–4 Hz in most
cases.
Se
Se
Se
OMe
O
OH
O
3
1
2
Theanswerliesintheveryfactthatthepreferableconformations
in this series are skewed s-cis (which corresponds to the range
of ϕ = 0–60◦). Indeed, in this range of ϕ the value of
3J(Se,HA) is markedly decreased due to the diminished selenium
lone pair effect in the cisoidal conformation resulting in the
Se
Se
OH
3
3
HO
unusually small difference of J(Se,HA) and J(Se,HB) mentioned
above.
To support this finding based on the experimental and
calculated 77Se–1H couplings, we have carried out the energy-
based theoretical conformational analysis of 1–5 performed at
the MP2/6-311G∗∗ level. For each of the five compounds under
scrutiny, two true-minimum conformers, skewed s-cis (A) and
skewed s-trans (B) were localized, as schematically depicted in
Scheme 2. All localized true-minimum conformers of 1–5 are
showninFigs. 3and4whiletheresultsoftheperformedtheoretical
conformational analysis adopting Boltzmann distribution of
conformations are collected in Table 3.
4
5
Scheme 1. Numeration of phenylselanylalkenes.
with selenium while standard Dunning’s cc-pVDZ basis set[9] was
used for all uncoupled atoms.
Fermi contact term was found to be dominant in all cases
with the overall contribution of the non-contact terms being
almost next to negligible in most cases. Out of the three non-
contact terms, JSD, JDSO and JPSO, only paramagnetic spin–orbital
contribution, JPSO, is of any practical significance, and this is
exemplified by the data presented in Table 2 for a model
selanylalkene.
The skewed s-cis conformer dominates for compounds 1–3 and
5 (92–98%) while selanylalkene 4 exists mainly in the form of the
skewed s-trans conformation B (73%). This is the answer on why
3J(Se,HB) = 10 Hz in 4 is so large as compared to that in the rest
of compounds in this series (4–5 Hz), see Table 1. Indeed, due to
3
3
Coming back to the dihedral angle dependences of J(Se,HA)
the dihedral angle dependence of J(Se,HB) shown in Fig. 2, the
and 3J(Se,HB) calculated for the same model selanylalkene and
shown in Fig. 2, it is evident that the difference between cisoidal
and transoidal couplings is strongly affected by the value of
value of this coupling should actually be around 10–12 Hz for the
s-trans conformer with the dihedral angle ϕ falling into range of
120–150◦.
c
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Y. Y. Rusakov et al.
Table 1. Experimental values of 3J(Se,HA) and 3J(Se,HB) in the series
of 1–5
Compound
Notation of atoms
3J(Se,HA) 3J(Se,HB)
1
7.4
4.7
HB
HA
Se
OH
2
3
7.2
4.0
HB
HA
Se
OMe
Figure 2. Dihedralangledependencesof3J(Se,HA)and3J(Se,HB)calculated
in the model phenylselanylalkene at the SOPPA level. The value of ϕ = 0◦
is assigned to the ideal s-cis conformation, as shown.
8.4
4.3
HB
HA
Se
O
O
4
5
10.0
HB
Se
HO
5.2
HB
Se
OH
Figure 3. Equilibriumstructuresofthelocalizedtrue-minimumconformers
of 1–3 optimized at the MP2/6-311G∗∗ level. Element colors: sele-
nium – brown, oxygen – red, carbon – gray; hydrogen – light gray. Shown
on structures are the out-of-plane deviations of the phenylselanyl and
phenyl groups in respect with the rotation around the Se–C bonds.
CDCl3, 25 ◦C; All couplings are in Hertz.
As follows from the data presented in Figs. 3 and 4, both true-
minimum conformers, A and B, in the whole series of 1–5 show
markedout-of-planedeviationsinrespectwiththerotationaround
both Se–C bonds which is due to the strong steric effects involving
thephenylring of thephenylselanylgroupandsubstitutedolefinic
moiety in both conformations of 1–5.
HB
HA
Se
R
HB
HA
Se
R
(b)
(a)
Scheme 2. True-minimum conformers, skewed s-cis (a) and skewed s-trans
(b), of phenylselanylalkenes 1–5.
Concluding Remarks
The main conclusion drawn from the present results deals
with the practical utilization of vicinal 77Se–1H couplings across
double bond for a configurational assignment of selanylalkenes.
It follows that experimental measurements of 3J(Se,H) should be
supplementedwithadetailedtheoreticalstudytoavoiderroneous
configurational assignment at the double bond of unsaturated
organoselenium compounds (as can be mistakenly done, e.g., for
Another interesting question is why the preferable confor-
mation of selanylalkene 4 is s-trans in contrast to the rest of
selanylalkenes 1–3 and 5 existing in the form of the predominant
s-cis conformer, see Table 3. The most reasonable explanation of
this fact is the favorable π-stacking of two phenyl rings oriented
in a ‘stack of coins’ fashion in the s-trans conformation of 4, as
can be seen in Fig. 4. The stabilizing effect of aromatic π-stacking
is well known[11] and has received a good deal of theoretical
3
compound 4, provided only experimental value of J(Se,H) was
used for this purpose). Traditional Karplus-type dihedral angle
dependence of 3J(Se,H) may be heavily affected by the orientation
of the terminal substituent bearing selenium atom, as was found
interest[12]
.
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Stereochemical study of the sterically crowded phenylselanylalkenes
Table 2. Individual coupling contributions to spin–spin coupling constants 3J(Se,H) across double bond in a model selenoalkene calculated at the
SOPPA level
HB
HA
Se
CH3
a
Spin–spin coupling constant
JDSO
JPSO
JSD
JFC
Jcalc
Jexp
3J(Se,HA)
3J(Se,HB)
−0.26
−0.81
−0.66
0.07
9.27
5.44
8.27
4.57
7.4
4.7
0.04
−0.25
Preferable skewed s-cis conformation is adopted as shown schematically in the table. All couplings and coupling contributions are in Hertz.
a Measured in compound 1, this work.
Table 3. MP2/6-311G∗∗ theoretical energy-based conformational analysis of 1–5
Compound
Structure
Conformer
Relative energy (kJ/mol)
Population (%)a
1
A
B
0.0
9.5
98
2
Se
OH
2
3
A
B
0.0
99
1
Se
15.8
OMe
A
B
0.0
9.0
97
3
Se
O
O
4
5
A
B
2.5
0.0
27
73
Se
Se
HO
A
B
0.0
6.0
92
8
OH
a Derived from the relative energies of the individual conformers adopting Boltzmann distribution of conformations.
here in the series of sterically crowded phenylselanylalkenes.
Based on the combined theoretical and experimental study of
the vicinal selenium–proton coupling constants across double
bond, the compounds under scrutiny were shown to adopt
mainly skewed s-cis conformation (except the one with a strong
π-stacking stabilization of s-trans conformer) with the noticeable
out-of-plane deviations of the phenylselanyl and phenyl groups
due to the marked steric effects.
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Magn. Reson. Chem. 2011, 49, 570–574
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Y. Y. Rusakov et al.
was formed (ca. 2–5 min). PhSeH (1.2 × 10−3 mol) was added to
the stirred mixture at ca. 5 ◦C (water/ice bath). The stirring was
continued for additional 10 min and the color of the suspension
changed from green to dark. The reaction was stirred 1.5 h at 20 ◦C
for 1 and 2–4 h at 60 ◦C for 2–5 (complete conversion of the
alkyne was checked by NMR monitoring).
After completion of the reaction the products were purified by
flash chromatography on silica with hexane/ethylacetate gradient
elution. After drying in vacuum the pure products were obtained
with the yields: 1 – 74%, 2 – 88%, 3 – 85%, 4 – 29%, 5 – 62%. All
products were identified according to the published NMR data.[3b]
Acknowledgement
Financial support from the Russian Foundation for Basic Research
(Grant No. 11-03-00022) is acknowledged.
Figure 4. Equilibriumstructuresofthelocalizedtrue-minimumconformers
of 4 and 5 optimized at the MP2/6-311G∗∗ level. Element colors:
selenium – brown, oxygen – red, carbon – gray; hydrogen – light gray.
Shown on structures are the out-of-plane deviations of the phenylselanyl
and phenyl groups in respect with the rotation around the Se–C bonds.
References
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NMR Measurements
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77Se, 76.34 MHz) in a 5 mm broadband probe at 25 ◦C in CDCl3
with hexamethyldisiloxane (1H,13C) and dimethylselenide (77Se)
as internal standards. Experimental measurements of 77Se–1H
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and 8 scans for each increment, resulting in the total experiment
time of 60 min for each sample.
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Computational Details
Localization of stationary points and geometry optimizations of
1–5 were performed with the GAMESS code[13] at the MP2/6-
311G∗∗ level without symmetry constraints. All calculations of
spin-spin coupling constants have been carried out taking into
account all four non-relativistic coupling contributions with the
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General Synthetic Part
Ni(acac)2 was dried in vacuum (0.01–0.02 Torr, 60 ◦C, 30 min) be-
fore use. Solvents were purified according to published methods.
Products were purified by dry column flash chromatography.[15]
The isolated yields given below were calculated based on ini-
tial amount of the alkyne. Nanosized Ni catalyst was prepared
in situ from Ni(acac)2 during the synthetic procedure as described
earlier.[3b]
[14] DALTON, A Molecular Electronic Structure Program, Release 2.0 2005,
see http://www.kjemi.uio.no/software/dalton/dalton.html.
[15] D. S. Pedersen, C. Rosenbohm, Synthesis 2001, 2431.
Synthetic Procedure (1–5)
Alkyne (1.0 × 10−3 mol) was added to Ni(acac)2 (2.0 × 10−5 mol)
and stirred at room temperature until uniform green suspension
c
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Magn. Reson. Chem. 2011, 49, 570–574