Analysis of substituent effects on C-13 NMR parameters of substituted arylacetylene derivatives. Linear free energy relationships and PM3 semiempirical calculations

Article abstract of DOI:10.1016/S0040-4020(00)00920-0

The substituent-induced chemical shifts (SCS) of C(α,β) on the ¹³C NMR spectra of arylalkynes (i.e. containing H, CH₃, Cl and Br) were studied. The correlation between SCS and Hammett constants shows that the tendency of the effect by the substituents on the phenyl ring is Br (ρ=8.15)>Cl (ρ=7.27)>CH₃ (ρ=6.79)>H (ρ=5.78). This order can be rationalized as due to the ability of the group on the alkyne to stabilize the partial positive charge on C(β) resulting from polarization with π electron transfer from C(β) to the phenyl ring. The SCS values are also well correlated with the electron densities obtained from PM3 calculations. The solvent effect on the ¹³C chemical shifts of phenylalkynyl bromide demonstrates a strong dependence on the relative permittivity as well as the shielding character of the solvents. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00920-0

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 9619±9623
Analysis of Substituent Effects on C-13 NMR Parameters of
Substituted Arylacetylene Derivatives. Linear Free Energy
Relationships and PM3 Semiempirical Calculations
b
a
Shaw-Tao Lin,a, Chuan-Chen Lee and David W. Liang
*
aDepartment of Applied Chemistry, Providence University, Sha-Lu, Taichung Hsien 433, Taiwan
bDepartment of Chemical Engineering, Hsiuping Institute of Technology, Taichung 404, Taiwan
Received 23 June 2000; revised 18 September 2000; accepted 28 September 2000
AbstractÐThe substituent-induced chemical shifts (SCS) of C(a,b) on the 13C NMR spectra of arylalkynes (i.e. containing H, CH3, Cl and
Br) were studied. The correlation between SCS and Hammett constants shows that the tendency of the effect by the substituents on the phenyl
ring is Br (rˆ8.15).Cl (rˆ7.27).CH3 (rˆ6.79).H (rˆ5.78). This order can be rationalized as due to the ability of the group on the
alkyne to stabilize the partial positive charge on C(b) resulting from polarization with p electron transfer from C(b) to the phenyl ring. The
SCS values are also well correlated with the electron densities obtained from PM3 calculations. The solvent effect on the 13C chemical shifts
of phenylalkynyl bromide demonstrates a strong dependence on the relative permittivity as well as the shielding character of the solvents.
q 2000 Elsevier Science Ltd. All rights reserved.
Introduction
in 1973,5a there is, however, still no such study for aryl-
acetylene derivatives. In this work, we determined the 13C
NMR spectra of arylacetylene halides (Br (1) and Cl (2))
and compiled the spectra of arylacetylenes (3)8 and
propynylarenes (4)9 for an analysis of the substituent effects.
Nine solvents were used to study their effect on 4-methyl-
phenylacetylene bromide (1C). Furthermore, semiempirical
calculations were successfully applied for simulating the
thermodynamic data and electron densities of atoms.10 In
this work, the correlation between the electronic parameters
thus calculated from an extensive semiempirical PM3 calcu-
lation of substituted arylacetylene derivatives with 13C
NMR chemical shifts were also studied.
Carbon-13 NMR spectroscopy is an especially valuable tool
for studying the electronic properties of aromatic systems.1
Hammett-type correlation with the rate constant is also able
to reveal the nature of the reaction center.2 In spite of some
theoretical limitations, the use of linear free energy relation-
ships (LFER) to correlate NMR data with Hammett
constants is a useful technique to determine the transmission
of inductive and resonance electronic effects through
benzene rings, and some reviews are available.3,4 Good
linear correlations have been found between differences in
chemical shifts with s constants or with dual substituent
parameters (Dd vs rIsI1rRsR) for atoms directly linked
to benzene ring. There are a number of reports in which the
13C NMR spectra for various para-substituted styrene
derivatives present as the positive r values.5 On the other
hand, the correlation for b carbons of the substituted ethyl-
benzene appear as the negative r values (reverse effect).6 In
our previous work, we applied this concept to obtain a corre-
lation between substituent induced chemical shifts (SCS)
and Hammett constants in a series of halocyclopropyl-
arenes,7 which behave as ethylbenzene derivatives leading
to a reverse effect.
Results and Discussion
Preparation of arylacetylene halides
Arylacetylene halides have been prepared by (1) halogena-
tion of metal acetylides or surrogates,11 (2) oxidative halo-
genation of terminal alkynes,12 and (3) dehydrohalogenation
of 1,1-dihaloole®ns by using strong base, such as amide,
butoxide, DBU etc.13 In this work, arylacetylene halides
can be simply prepared from dehydro-halogenation of b,
b-dihalostyrenes using KOH in the presence of benzyl-
triethylammonium chloride (Eq. (1)). Dehydrobromination
and dehydrochlorination from the corresponding styrenes
occur readily to form arylacetylene halides with nearly
quantitative yields in CH2Cl2 solution within 2 h. This
reaction fails for the preparation of arylacetylene ¯uorides
Although the relationship between SCS and Hammett
constants has been utilized to study the possible electronic
properties of various aromatic systems since the ®rst report
* Corresponding author. Tel.: 1886-4-632-9901; fax: 1886-4-632-2293;
e-mail: sdlin@pu.edu.tw
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00920-0

9620
S.-T. Lin et al. / Tetrahedron 56 (2000) 9619±9623
Table 1. The 13C chemical shifts for arylacetylene bromides and arylace-
tylene chlorides in deuteriochloroform (ppm form TMS as an internal stan-
dard)
C1
C2,6
C3,5
C4
Ca
Cb
Others
1a
122.62
131.91
133.31
131.81
133.11
132.81
131.95
133.19
131.82
133.15
132.77
128.25
113.80
129.04
128.60
123.60
128.56
113.77
129.10
128.69
123.59
128.59 80.05
159.69 79.85
138.83 80.08
134.68 78.92
147.25 78.37
128.44 69.39
159.64 69.27
138.75 69.64
134.68 68.34
147.27 67.74
49.78
1b 114.55
1c 119.54
1d 121.02
1e
2a
2b 113.62
2c 119.04
2d 120.59
2e 128.97
47.86 55.10
48.75 21.48
51.06
56.40
67.99
66.16 54.90
67.16 21.50
69.12
129.45
122.15
73.88
from di¯uorostyrenes. All pure compounds for NMR
studies were obtained from column chromatographic
separation.
Figure 1. Carbon-13 SCS(a,b) values in ppm for arylacetylene bromide
(X, dotted line) and arylacetylene chloride (O, solid line) vs Hammett
substituent constants (s).
(r2ˆ0.984), 6.79 (r2ˆ0.925), 4.47 (r2ˆ0.907), and 21.73
(r2ˆ0.845), 21.73 (r2ˆ0.886), 21.91 (r2ˆ0.894) for C(b)
of compounds 1, 2, 3, and 4, and C(a) of compounds 1, 2,
and 3, respectively. Very poor correlation was obtained for
C(a) of compounds 4.
ꢀ1†
These correlations resemble those obtained from styrene
derivatives and contrast with those of arylcyclopropanes
and ethylbenzene derivatives.6,7 For the acetylene halides,
the slopes of the Hammett-plot are larger than that for
styrenes (Table 3). 13C NMR provides information which
shows that the interaction of the substituent on C(b) through
phenyl rings should be accentuated by the ®eld-induced
polarization of the arylacetylene p electron system.
Carbon-13 NMR spectra of arylacetylene halides
The 13C chemical shifts of arylacetylene bromides and
arylacetylene chlorides are summarized in Table 1. The
C(b) chemical shifts of compounds 1 appear much further
up®eld than those of compounds 2,3, and 4 due to the heavy
atom factor.14 In general, SCS(b) values are shifted down-
®eld when the substituents on the aromatic rings are elec-
tron-donating groups, and the SCS(a) work in the other way
(Table 2 and Fig. 1). The linear regressions of compounds 3,
4 are also compiled from the literature for comparison.8,9
There appears to be an in¯ection in the correlation line
which corresponds to the slope of 8.15 (r2ˆ0.9847), 7.27
The soft bromine atom is able to stabilize the partial charge
on C(b) leading to a greater shift difference for C(a) and
C(b) and higher slope. The incremental shifts of the
aromatic carbon atoms of compounds 1, 2, and 3 along
with phenylacetylene appear in Table 4. It shows that the
incremental shifts for all the carbon atoms are similar to the
corresponding phenylacetylene.15 The incremental shifts for
C-1 are 25.7, 26.3, and 25.8 ppm for compounds 1, 2, and
3, respectively, due to the shielding character of the triple
bond. On the other hand, these values for C-2 are 13.5,
13.5, and 13.8 ppm in the same series of compounds.
This implies that the higher electronegativity of the sp-
hybrid carbon reduces the electron density on the C-2
atom. The presence of halogen on the acetylene group
Table 2. SCS (in ppm) of arylacetylene bromides and arylacetylene chlor-
ides in deuteriochloroform solution (positive values represent down®eld
shifts)
Compounds
s
Ca
Cb
1b
1c
1a
1d
1e
2b
2c
2a
2d
2e
20.27
20.17
0
0.23
0.76
20.27
20.17
0
20.20
0.03
21.92
21.03
49.78(0)
1.28
Table 3. Slopes for Cb of styrenes and arylacetylenes obtained from the
plots of SCS vs Hammett constants
80.05(0)
21.13
21.68
20.12
0.07
69.39(0)
21
21.64
X
R±C6H4±CuCX
R±C6H4±CHvCX2
6.62
21.83
20.83
67.99(0)
1.13
Br
Cl
F
Me
H
8.15
7.27
,
5.94
5.76
1.2019
6.265g
6.5020
0.23
0.76
6.799
4.478
5.88

S.-T. Lin et al. / Tetrahedron 56 (2000) 9619±9623
Table 4. The incremental shifts of the aromatic carbon atoms of monosub-
stituted benzene (ppm from benzene at 128.5 ppm, 1 to the left, 2 to the
9621
right)
R±C6H4±CuCX
C1
C2
C3
C4
1 (Br)
2 (Cl)
4 (H) 15
25.7
26.3
26.1
3.5
3.5
3.8
20.3
0
0.4
0.5
0.1
20.2
does not alter those incremental shifts from the unsubsti-
tuted compound.
Correlation between SCS and charge densities of carbon
atoms
Figure 2. Correlation of calculated net atomic charge densities of C(b) and
their SCS(b).
An optimization of the electronic structure and geometry of
arylacetylene derivatives by PM3 semiempirical molecular
orbital calculations has been carried out. Molecular charac-
teristics of the arylacetylenes described below correspond-
ing to the energy minima obtained by a geometry
optimization and a full all degrees of freedom analysis
were investigated using the gradient optimization routine
in the programs. The correlation between the net atomic
densities and the 13C NMR chemical shifts is well estab-
lished in the literature.3,4 The most signi®cant molecular
parameters to be related with NMR chemical shifts are the
net atomic densities on C(a) and C(b). The correlation
between the net atomic densities and the 13C NMR chemical
shifts are summarized in Tables 5 and 6 and shown in Figs. 2
and 3. Fig. 2 shows good regression lines both for
compounds 1 (rˆ179.22, R2ˆ0.9931) and compounds 2
(rˆ174.47, R2ˆ0.9921). The correlations of dC(b) vs. qC(b)
are more sensitive than that of C(a), is agreement with the
experimental resultants.
tionship with DHr. This indicates that the van der Waals
interaction between analyte and solvent plays an important
role.14,17 The variation of the chemical shifts resulting from
different solvents can reveal structural information in the
solution phase.
The chemical shifts of 4-methylphenylacetylene bromide in
nine solvents are listed in Table 7 and shown in Fig. 4. From
Fig. 4 we found that the dish-shaped molecules16 (C6D6,
[2H5]pyridine, and [2H3]nitromethane) and polyhalo-
methanes (i.e. CDCl3) form a group, while the oxygen-
containing solvents ([2H8]dioxane, CD3OD, [CD3]2SO, and
[2H6]acetone) and rod-like molecule16 ([2H]acetonitrile) are
belonging in another group. In general, the carbon-13
chemical shifts of the whole molecule move up®eld in
polar solvents. We conclude from Fig. 4 that the chemical
shifts are mainly affected by two factors, i.e. the shielding
ability and relative permittivity of solvents. The dish-shaped
molecules and polyhalomethanes are possess as a deshield-
ing nature. It is surprising to observe the reverse in¯uence
for the arylalkyne series in this work. The difference in
solvent-induced shift for C(b) is 4.6 ppm for nitromethane
(deshielding solvent) and dimethylsulfoxide (more polar).
This suggests that the polar solvent will stabilize the
polarization of this alkyne and result in more positive
character of C(b). This value is comparable to that of
ethyl iodide, for which the solvent-induced shift is
Chemical shifts in various solvents
For the interaction ®eld of a polar molecule in a medium of
relative permittivity, er, the 13C screening constants in
substituted methanes should depend linearly on the func-
tion, (er21)/(212n), where n is the refractive index of
the solvent molecule.16 Also, the difference between the
methane shift in a given solvent is found in the linear rela-
Table 5. Calculated net atomic charge densities of C(b) and SCS(b) for
R±C6H4±CuC±X
R
1-Cb (SCS)
1-Cb charge
2-Cb (SCS)
2-Cb charge
4-OMe
4-Me
H
4-Cl
4-NO2
21.92
21.03
0
1.28
6.62
20.2843
20.2798
20.2767
20.2698
20.2375
21.83
20.83
0
1.13
5.88
20.3073
20.3031
20.3004
20.2938
20.2644
Table 6. Calculated net atomic charge densities of C(a) and SCS(a) for
R±C6H4±CuC±X
R
1-Ca (SCS)
1-Ca charge
2-Ca (SCS)
2-Ca charge
4-OMe
4-Me
H
4-Cl
4-NO2
20.204
0.034
0
21.134
21.681
20.0581
20.063
20.0661
20.0724
20.1017
20.12
0.07
0
21
21.64
20.0918
20.0967
20.0998
20.1062
20.1368
Figure 3. Correlation of calculated net atomic charge densities of C(a) and
their SCS(a).

9622
S.-T. Lin et al. / Tetrahedron 56 (2000) 9619±9623
Table 7. Solvent dependence of the 13C chemical shifts (in ppm) of 4-methylphenylacetylene bromide
Solvent
m(D)
C1
C2,6
C3,5
C4
Ca
Cb
Me
CDCl3
1.01
2.88
1.7
0
2.2
3.96
3.46
0.45
3.92
119.539
120.241
120.899
120.083
119.389
119.591
118.320
121.111
120.123
131.812
132.519
132.791
132.172
131.585
132.517
130.829
133.306
132.655
129.044
130.033
130.154
129.315
128.996
130.283
128.32
138.826
139.928
140.204
138.856
138.658
140.02
138.801
140.399
140.387
80.084
80.720
80.890
80.836
80.068
80.853
79.088
81.574
80.759
48.751
50.062
49.831
49.532
51.069
52.094
47.513
50.933
50.086
21.477
21.370
21.486
21.227
20.631
21.881
19.351
22.136
21.469
CD3COCD3
CD3OD
C6D6
C5D5N
CD3SOCD3
CD3NO2
C4D8O2
CD3CN
130.595
130.189
Figure 4. Correlation between the 4-methylphenylacetylene bromide with dipole moment of solvents.
5.2 ppm for cyclohexane and nitromethane.18 The lack of a
correlation between the difference in solvent-induced shifts
for C(b) and the nature of the solvent might be due to the
combination of the nature of solvent and the polarizability
of C(b)-bromine bond.
and then dried over MgSO4. After ®ltrating and removing
solvent, the residue was puri®ed by column chromatography
(silica gel) with n-hexane as an eluent to give 0.076 g of 2a
(yield 96%).
All of the arylacetylene bromides (and chlorides) used in
this work are known compounds and were prepared using
this method to give nearly quantitative yields. All
compounds for NMR studies were puri®ed by column
chromatography and purity was further veri®ed by GC-
MS analysis.21
Experimental
13C (or C-13) measurements
The NMR data of 15 (w/v)% of analyte in CDCl3 (or other
speci®ed solvent) were recorded on a Bruker AC-250 spec-
trometer at 62.9 MHz; 64 K data points were collected
within a range of 15 kHz. All chemical shifts were measured
relative to TMS in proton noise decoupled spectra. The
digital resolution was generally 0.02 ppm. In general, 500
scans were accumulated for each compound in this study.
The calculations were carried out with the spin-unrestricted
version of the PM3 method available in the MOPAC
program system implanted in Alchemy 2000 software
(Windows version). Geometry optimization of compounds
was performed through the eigenvector following (EF)
routine incorporated into the PM3 program. To obtain the
minimization, the gradient convergence was set to 0.01,
gradient_min_Type to NLLSQ, and minimize_type to
BFGS.
Preparation of arylacetylene halidesÐtypical procedure
A mixture of dichlorostyrene (0.1 g, 5.78£1024 mol) and
benzyltriethylammonium chloride (0.1 g) in 2.0 mL
CH2Cl2 and aqueous KOH (60%, 1.0 mL) in a round-
bottomed ¯ask was stirred on an ice-bath for 2 h. The
resultant mixture was extracted with CH2Cl2 (3£10 mL)
Acknowledgements
Financial support by the National Science Council of the

S.-T. Lin et al. / Tetrahedron 56 (2000) 9619±9623
9623
Â
153. (b) Alvarez-Ibarra, C.; Quiroga-Feijoo, M. L.; Toledano, E.
Republic of China (NSC89-2113-M-126-006) is gratefully
acknowledged.
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(c) Wagner, A.; Heitz, A.; Mioskowski, C. Tetrahedron Lett.
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