1214
C. K. Lee, J. S. Yu and H.-J. Lee
Vol. 39
Table 8
2.80, and para 4.79 ppm in chloroform-d. In this case the
effect on the para-C is about half of that on the ortho-C. It
is also quite unusual that the ipso-Cs show upfield shift in
both solvents. In case of 2-thienoyl ester 2 the effects of
the ester group on the chemical shift of the carbons are
most significant: ipso 5.91, ortho 8.32, meta 1.70, and
Chemical Shift Values of 0.1 M Solutions of Benzene, Thiophene,
and Furan in Dimethyl Sulfoxide-d and in Chloroform-d
6
-H
-H D(a -b)
-C
-C
D(a -b)
Benzene
[a]
[b]
7.37
7.36
diff. 0.01
7.57
7.35
diff. 0.22
7.67
7.45
diff. 0.22
7.37
0
0
0
0.42
0.22
0.20
1.19
1.06
0.13
128.80
128.33
0.47
126.20
125.11
1.09
143.37
142.52
0.85
128.80
128.33
0.47
127.56
126.86
0.70
110.17 33.20
109.44
0.73
0
0
0
-1.36
-1.75
0.39
para 9.75 in dimethyl sulfoxide-d ; and ipso 7.38, ortho
7.36
0.01
7.15
7.13
0.02
6.48
6.39
0.09
6
8.03, meta 1.22, and para 8.65 ppm in chloroform-d. Here,
the effect of the ester group on the ortho- and the para-Cs
are relatively similar in magnitude, and the effect on the
ipso-C is significantly deshielding.
Thiophene [a]
[b]
Furan
[a]
[b]
The small downfield shift in the benzoate carbon signals
can be readily explained by the relatively insignificant contri-
bution of the resonance structures VIII and IX. Such struc-
tures should lose the requirement of 6p electrons for
aromaticity. Among the resonance structures of the benzoates
VII is the most significant contributor. In this structure the
positive end of the dipole passes through the para-C and,
consequently, thepara-C should appear farthest downfield.
On the other hand, the contribution of X-XII are
relatively important in the 2-thienoates 2 and the
2-furoates 3. In case of 3 further conjugation like XIII (X
= O) is possible, which should diminish the positive
character of the para-C so that the magnitude of the
downfield shift of the para-C is about half of that of the
ortho-C. But the size of the sulfur atom should disfavor
similar conjugation with 2. Therefore, the magnitude of
the shift of the ortho- and the para-C's are about the same.
33.08
0.12
[a] Dimethyl sulfoxide-d ; [b] Chloroform-d.
6
The averaged chemical shift values of ortho and para-
H's of 2 are very close, d 8.04 and 8.11, respectively, in
dimethyl sulfoxide-d as shown in Table 7. But the values
are not only far apart but the order is reversed in chloro-
form-d, showing ortho-H at d 7.98 and para-H at d 7.69.
The assignment was made based on the coupling
6
constants. They are J = 3.57 Hz and J
average. These values are consistent with the values in
= 4.94 Hz in
3,4
4,5
literature [12].
The a -H of furan is shifted downfield by 0.10 ppm from
that of thiophene in both solvents, but the b-H of furan is
shifted upfield by 0.67 ppm from that of thiophene in
dimethyl sulfoxide-d and 0.74 ppm in chloroform-d
6
(Table 8). Therefore, the close, if not the same, chemical
shift values (d 8.11-8.12 in dimethyl sulfoxide-d and
6
7.68-7.69 in chloroform-d) for para-H of 2 and 3 (Table 7)
may be strong evidence that the polar electronic effect of
the ester group should be very similar. In other words, both
2 and 3 prefer similar rotameric conformation, such as IV.
13
The C signals of 2 and 3 are also influenced to a
greater extent (1.06-2.64 ppm toward downfield) by
changing the solvent from chloroform-d to dimethyl
sulfoxide-d than those of 1 (0.01-0.20 ppm upfield except
6
ortho-C which shows 0.14 ppm downfield). The difference
in the averaged chemical shift of ortho- and para-Hs of 2
The contrasting observations of the substituent effect on
the chemical shift (which is quite significant on the para
position in 1 and the ortho position in 3) may be explained
by two different mechanisms of transmission of the
substituent effect. The aryloxy group may induce local
polarization in benzoyl group like XIV.
is merely 0.07 ppm in dimethyl sulfoxide-d , ortho-H
6
appearing upfield. In chloroform-d, however, the
difference is relatively large (1.13 ppm) and, in contrast
with the ortho-H appears downfield.
The ester group causes a downfield shift of ipso-, ortho-
, meta- and para-Cs of the benzoyl ring in 1 by 0.21, 1.52,
0.33, and 4.83 ppm, respectively, in dimethyl sulfoxide-d
6
and 0.81, 1.85, 0.27, and 5.43 ppm, respectively, in chloro-
form-d. The magnitude of the effect on the para-C is about
three times of that on the ortho-C in both solvents. On the
other hand, similar effects of 2-ester group on furanyl ring
are: ipso -0.03, ortho 10.88, meta 3.16 and para 5.91 ppm
in dimethyl sulfoxide-d and ipso -1.82, ortho 10.33, meta
6
Lee, Chang Kiu
