13C NMR/DFT/GIAO studies of phenylene bis(1,3-dioxolanium) dications and 2,4,6-triphenylene tris(1,3-dioxolanium) trication

Article abstract of DOI:10.1021/jo020753z

The o-, m-, and p-phenylene bis(1,3-dioxolanium) dications (4-6) and 2,4,6-triphenylene tris(1,3-dioxolanium) trication (7) have been prepared by the ionization of the corresponding 2-methoxyethyl benzoates in FSO₃H or CF₃SO₃H at 40 and 60 °C, respectively. The charge delocalization in these carbocations was probed by ¹³C NMR chemical shifts and substantiated by GIAO/DFT calculations. Relatively less charge is delocalized into the aromatic ring of the carbotrication 7. The rotational barrier around the C⁺-Ar bond for carbodications 4 and 5 was also estimated to be 8-10 kcal/mol.

Full text of DOI:10.1021/jo020753z

13C NMR/DF T/GIAO Stu d ies of P h en ylen e Bis(1,3-d ioxola n iu m )
Dica tion s a n d 2,4,6-Tr ip h en ylen e Tr is(1,3-d ioxola n iu m ) Tr ica tion
V. Prakash Reddy,*,† Golam Rasul,‡ G. K. Surya Prakash,*,‡ and George A. Olah*,‡
Department of Chemistry, University of MissourisRolla, 142 Schrenk Hall,
Rolla, Missouri 65409-0010, and Loker Hydrocarbon Research Institute and Department of Chemistry,
University of Southern California, University Park, Los Angeles, California 90089-1661
olah@usc.edu
Received December 20, 2002
The o-, m-, and p-phenylene bis(1,3-dioxolanium) dications (4-6) and 2,4,6-triphenylene tris(1,3-
dioxolanium) trication (7) have been prepared by the ionization of the corresponding 2-methoxyethyl
benzoates in FSO3H or CF3SO3H at 40 and 60 °C, respectively. The charge delocalization in these
carbocations was probed by 13C NMR chemical shifts and substantiated by GIAO/DFT calculations.
Relatively less charge is delocalized into the aromatic ring of the carbotrication 7. The rotational
barrier around the C+-Ar bond for carbodications 4 and 5 was also estimated to be 8-10 kcal/mol.
In tr od u ction
type polymers, respectively. Tomalia and Hart reported
1
the synthesis and H NMR-characterization of the diox-
The dioxolanium carbocations are among the most
stable among the heteroatom-stabilized carbocations.
These carbocations along with their sulfur analogues
have been extensively investigated for synthetic applica-
tions as well as for mechanistic studies. They can be
readily prepared by hydride abstraction of the corre-
sponding 1,3-dioxolanes using trityl tetrafluoroborates or
dehydrative cyclization of 2-hydroxyethyl esters or by
Meerwein’s method using equimolar amounts of 2-bromo-
ethyl esters and silver tetrafluoroborate.1-7 The dioxola-
nium cations were also used as initiators for the polym-
erization of 1,3,5-trioxanes,8 cyclic siloxanes,9 tetrahydro-
furan,10 and cationic graft copolymerization of tetrahy-
drofuran to polystyrene.11
olanium carbodications and trications.12 The structures
1
of these carbocations were, however, inconclusive as H
NMR chemical shifts could not reliably distinguish mono-
vs dications. Our interests in the cationic polymerization
using carbopolycations prompted us to develop conve-
nient synthetic methods for the synthesis of these car-
bodications and trications. Here, we report the 13C NMR
characterization of the selected dioxolanium carbo-
dications and a carbotrication. We have also calculated
the structures and 13C NMR chemical shifts of these
carbocations using the GIAO (gauge-including atomic
orbital)/DFT (density functional theory) method for com-
parison.
Resu lts a n d Discu ssion
The dioxolanium carbocations can be readily obtained
by hydride exchange of the corresponding dioxolanes
using trityl cation salts. However, this method could not
be used for these carbodications, as the initially formed
carbomonocation can initiate the polymerization of the
unreacted dioxolane ring. Synthetic methods using silver
tetrafluoroborate are also inconvenient for the large-scale
preparations. We have found that the o-, m-, and p-
phenylene bis(1,3-dioxolanium) dications (4-6) and 2,4,6-
triphenylene tris(1,3-dioxolanium) trication (7) can be
Analogous carbodications and trications would be
ideally suited for the preparation of diblock- and star-
(6) Childs, R. F.; Frampton, C. S.; Kang, G. J .; Wark, T. A. J . Am.
Chem. Soc. 1994, 116, 8499-8505.
† University of MissourisRolla.
‡ University of Southern California.
(7) Allen, A. D.; Kitamura, T.; McClelland, R. A.; Stang, P. J .;
Tidwell, T. T. J . Am. Chem. Soc. 1990, 112, 8873-8878.
(8) J edlinski, Z.; Gibas, M. Macromolecules 1980, 13, 1700-1703.
(9) Chojnowski, J .; Penczek, S.; Mazurek, M.; Sciborek, M.; Wilczek,
L. Pol. 1977, 85, 600; Chem. Abstr. 1977, 90, 122270.
(10) Matyjaszewski, K.; Slomkowski, S.; Penczek, S. J . Polym. Sci.,
Polym. Chem. Ed. 1979, 17, 2413-2422.
(1) Matskovskaya, E. S.; Mezheritskaya, L. V.; Dorofeenko, G. N.
Zh. Org. Khim. 1978, 14, 1119-1120.
(2) Dorofeenko, G. N.; Mezheritskaya, L. V.; Matskovskaya, E. S.
Zh. Org. Khim. 1975, 11, 2424-2427.
(3) Mezheritskaya, L. V.; Matskovskaya, E. S.; Dorofeenko, G. N.
Zh. Org. Khim. 1980, 16, 183-188.
(4) Akhmatdinov, R. T.; Chalova, O. B.; Kantor, E. A.; Rakhman-
kulov, D. L. Zh. Org. Khim. 1980, 16, 962-964.
(5) Tomalia, D. A.; Hart, H. Tetrahedron Lett. 1966, 3389-3394.
(11) Okada, M.; Sumitomo, H.; Kakezawa, T. Makromol. Chem.
1972, 162, 285-289.
(12) Hart, H.; Tomalia, D. A. Tetrahedron Lett. 1967, 1347-1351.
10.1021/jo020753z CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/09/2003
J . Org. Chem. 2003, 68, 3507-3510
3507

Reddy et al.
SCHEME 1. P r ep a r a tion of P h en ylen e Bis(1,3-d ioxola n iu m ) Dica tion s a n d Tr is(1,3,5-d ioxola n iu m Tr ica tin
1
conveniently prepared by the ionization of the corre-
sponding 2-methoxyethyl benzoates (e.g., 9).
We could not obtain satisfactory H NMR spectra of
the cations because of enormous proton signal of the acid
media. Furthermore, in neat acid solutions the peaks
were relatively broad.
We have calculated the structures and 13C NMR
chemical shifts of the ions by the GIAO/DFT method and
compared the results with the experimental data. The
structures were fully optimized at the B3LYP/6-31G*
level using the Gaussian-98 package of programs. The
bond lengths of the minimum energy structures are given
in Table 2. The 13C NMR chemical shift of the ions were
calculated by the GIAO/DFT method using B3LYP/6-
31G* geometries.
The experimental 13C NMR chemical shifts for these
carbocations can be readily assigned by comparison with
those calculated using GIAO/DFT method (Table 1). It
is interesting to note that the o-, m-, and p-phenylene
dications have very similar 13C chemical shifts for the
carbocationic centers (δ13C 182.7, 182.3, and 182.2,
respectively). The equivalence of the NMR chemical shifts
is also in accordance with those calculated by the GIAO/
DFT method (δ13C 188.5, 187.4, and 188.3, respectively,
for 4, 5, and 6). The corresponding δ13C for the car-
bomonocation, 2-phenyl-1,3-dioxolan-2-ylium cation (182.9
ppm) is also strikingly similar, suggesting the extensive
charge delocalization into the neighboring oxygens for the
mono-, di-, and tricarbocations.
The 13C NMR chemical shifts for the cationic centers
often reflect their stability for closely related carboca-
tions. In other words, the more stable carbocations show
relatively shielded chemical shifts. Interestingly, for
these carbodications, the calculated (Table 2) relative
energies of the meta and para isomers (5 and 6), as
obtained from the DFT (at B3LYP/6-31G*//B3LYP/6-
31G* + ZPE level), are very close to each other; the meta
isomer (5) is only 0.3 kcal/mol more stable than that of
the para isomer (6). The energy difference for the
carbodications 4 (ortho isomer) and 5 (meta isomer) at
The reaction of the 1,2-, 1,3-, and 1,4-benzenedicarbo-
nyl chloride or the 1,3,5-benzenetricarbonyl chloride with
2-methoxyethanol in the presence of triethylamine gave
the corresponding esters, which could be directly ionized
using fluorosulfonic acid or triflic acid to give the car-
bodications 4-6 and the carbotrication 7 at moderate
temperatures as illustrated in Scheme 1 for the prepara-
tion of carbodication 4. The ionization in FSO3H takes
place under relatively milder conditions than that for the
triflic acid. Typically, the ionizations were carried out at
40 °C in neat FSO3H and 60 °C in neat triflic acid. The
formation of these carbocations is complete in about 30
min under these conditions. The progress of these reac-
tions can be readily followed by 13C NMR spectroscopy
using a variable-temperature probe. These carbodications
and the carbotrication were stable in superacidic media
for extended times, even up to 100 °C.
3508 J . Org. Chem., Vol. 68, No. 9, 2003

13C NMR/ DFT/ IGLO Studies of Phenylene Bis(1,3-dioxolanium) Dications
TABLE 1. 13C NMR Da ta (Mu ltip licity a n d J , Hz, in P a r en th eses) a n d GIAO/DF T δ13C Va lu es in Br a ck ets) for
Ca r bom on oca tion 2′, Ca r bod ica tion s 4, 5, a n d 6, a n d Ca r botr ica tion 7
ions
C+
C1
116.7
[117.2]
118.2 (s)
[121.6]
C2
C3
C4
C5
C6
CH2
75.6
[76.4]b
2′a 182.9
134.3
[142.0]
131.0
[137.6]
142.2
[154.7]
131.0
[137.6]
134.3
[142.0]
[188.0]
182.2
[188.5]
4
5
6
7
118.2 (s)
[121.6]
141.3 (dd, 173.7) 139.7 (dd, 178.6) 139.7 (dd, 178.6) 141.3 (dd, 173.7) 77.9 (t, 169)
[150.6] [152.4] [152.4] [150.6]
[80.8]b
182.3 (s) 119.8 (d, 8) 133.6 (d, 174.9) 119.8 (d, 8)
144.3 (dt, 172.7) 138.1 (dt, 174.7) 144.3 (dt, 172.7) 77.5 (t, 165)
[187.4]
182.2 (s) 125.7 (t, 10) 133.9 (d, 175)
[188.3] [131.3] [142.6]
188.6 (s) 130.4 (s)
[123.2]
[145.7]
[123.2]
[154.6]
[141.7]
[154.6]
[81.1]b
133.9 (d, 175)
[142.6]
125.7 (t, 10)
[131.3]
153.8 (dt, 178.6) 130.4 (s)
[154.4] [128.3]
133.9 (d, 175)
[142.6]
133.9 (d, 175)
[142.6]
153.8 (dt, 178.6) 86.9 (t, 167)
77.5 (t, 165)
[81.5]b
153.8 (dt, 178.6) 130.4 (s)
[154.4] [128.3]
[185.6]
[128.3]
[154.4]
[85.5]b
a
b
From ref 6. Average.
TABLE 2. Tota l En er gies (-a u ), ZP Ea (k ca l/m ol),
Rela tive En er giesb (k ca l/m ol), a n d Rota tion a l Ba r r ier b
(k ca l/m ol)
barriers around the carbocationic center and the aromatic
ring (C+-Ar bond). At the B3LYP/6-31G*//B3LYP/6-31G*
+ ZPE level these barriers are estimated as 4.7 and 9.8
kcal/mol for the carbodcations 4 and 5, respectively.
Attempts of measuring these barriers experimentally
were not successful as, even at -70 °C, the methylene
carbons show only broadened signals, but do not merge
with the baseline. An upper limit of 8-10 kcal/mol can
be estimated for these rotational barriers, based on the
significant line broadening at -70 °C.
The carbotrication 7, obtained from the ionization of
the tris(2-methoxyethyl)-1,3,5-benzene tricarboxylate in
FSO3H at 40 °C, showed much higher deshielded δ13C
for the methylene carbons (δ13C 86.7), as compared to the
carbodications 4-6. This points to the enhanced reso-
nance stabilization of the cationic centers by the adjacent
oxygen atoms, to minimize the electrostatic repulsion
between the carbocationic centers. The relatively less
delocalization of the positive charge into the aromatic
ring is also evidenced by the fact that the ipso-carbons
appear at δ13C 130.4, which is 10 ppm more deshielded
from those of the related carbodications. The enhanced
deshielding of the ispso-carbons can be explained by the
more efficient electron-withdrawing inductive effect of the
dioxolanium rings. The minimum energy structure for
the trication was found to be of D3h symmetry (Table 3)
with the C+-C1 bond distance of 1.471 Å.
rel
rotational
ion B3LYP/6-31G*//B3LYP/6-31G* ZPE energy
barrier
2
4
5
6
7
498.57947
764.77582
764.79989
764.79959
1030.92948
101.1
139.9
140.0
139.9
178.3
14.4
4.7
9.8
8.4
8.3
15.0
0.0
0.3
a
Zero-point vibrational energy (ZPE) at B3LYP/6-31G*//B3LYP/
b
6-31G* scaled by a factor of 0.98; Relative energy and rotational
barrier at B3LYP/6-31G*//B3LYP/6-31G* + ZPE level.
this level of calculation is, on the other hand, much
higher; i.e., cabodication 4 is 15.0 kcal/mol higher in
energy as compared to the carbodication 5 (Table 2). This
can be explained in terms of higher steric constraints in
the carbodication 4, as well as charge-charge repulsion
due to conjugation.
It is, however, difficult to rationalize the extent of
charge-charge delocalization into the aromatic ring in
dications 4-6 based on 13C NMR chemical shifts of the
ring carbons alone. The meta-substituted carbocations 5
should be able to delocalize more positive charge into the
aromatic ring compared to ortho- and para-substituted
analogues 4 and 6 (vide supra). The minimum energy
structures of the dications 4, 5 and 6 were found to be of
C2, C2v, and D2h symmetry (Table 3) with the C+-C1 bond
distances of 1.456, 1.453, and 1.460 Å, respectively. For
comparison, the structure of the monocation 2 was also
computed and listed in Table 3. Optimized structure
indicates that the oxolanium ring of dication 4 is rotated
out of the benzene ring plane around the C+-C1 bond
by about 20°.
As can be seen from Table 1, the GIAO/DFT-derived
δ13C at the B3LYP/6-311G*///B3LYP/6-31G* level of
theory match closely with the experimental values. The
δ13C values for the carbocationic center are deshielded
from those of the experimental values by 6.3, 5.1, and
6.1 ppm for the ortho-, meta-, and para-substituted
carbodications (4, 5, and 6), respectively. At higher levels
(e.g., ab initio correlated GIAO-MP2 method), the dis-
crepancies may be eliminated. Due to the relatively large
number of basis sets involved for these molecules, such
calculations could not be carried out. However, the
relative trends in these chemical shifts are consistent
with the experimental values. Similar deviations are also
obtained at this level of calculations for other carboca-
tions.13,14 In ions 4 and 5, there are significant rotational
We have also computed 17O NMR chemical shifts at
the GIAO/DFT B3LYP/6-311G*//B3LYP/6-31G* level.
The monocation 2 shows the most shielded shift at δ17O
223.1 (with respect to δ17O H2O). On the other hand, the
ortho- and para-substituted dications 4 and 6 indicate
comparable deshielded shifts at δ17O 254.7 and 255.8,
respectively. This indicates that there is more positive
charge localization in these systems (charge delocaliza-
tion into the aromatic ring results in vicinal dicationic
resonance structure). Interestingly, in the meta isomer,
5, wherein delocalization can only lead to 1,3-dicationic
resonance structure, the charge localization on oxygen
is relatively less as reflected by its 17O NMR chemical
shift (δ17O 248.4). Finally, the trication 7 shows the most
localization of the positive charge at δ17O 265.6.
In summary, we have prepared the dioxolanium car-
bodications (4-6) and dioxolanium trication (7) by a novel
and convenient method and characterized their struc-
(13) Olah, G. A.; Reddy, V. P.; Rasul, G.; Prakash, G. K. S. J . Am.
Chem. Soc. 1999, 121, 9994-9998.
(14) Prakash, G. K. S.; Reddy, V. P.; Rasul, G.; Casanova, J .; Olah,
G. A. J . Am. Chem. Soc. 1998, 120, 13362-13365.
J . Org. Chem, Vol. 68, No. 9, 2003 3509

Reddy et al.
TABLE 3. Selected Op tim ized Bon d Len gth s (Å)
ion
C+-C1
C+-O
H2C-O
H2C-CH2
C1-C2
C2-C3
C3-C4
C4-C5
2 (C2v
4 (C2)
5 (C2v
6 (D2h
7 (D3h
)
)
)
)
1.429
1.456
1.453
1.460
1.471
1.302
1.292a
1.285a
1.288
1.283
1.469
1.485a
1.486a
1.486
1.499
1.540
1.538
1.541
1.541
1.543
1.414
1.435
1.402
1.408
1.404
1.386
1.402
1.402
1.389
1.404
1.401
1.402
1.413
1.408
1.404
1.390
1.393
1.389
1.404
a
Average.
tures by comparing the GIAO/DFT-derived δ13C with the
experimental values. The meta- and para-substituted
carbodications (5 and 6) are similar in energy, while the
ortho-substituted carbodication (4) is much higher in
energy, reflecting increased steric hindrance in the latter
cation, which precludes significant resonance stabiliza-
tion from the aromatic ring. The carbotrication 7, on the
other hand, derives much less stabilization through
delocalization into the aromatic ring.
CH2), 64.31 (t, J ) 148.1 Hz, CH2O), 58.9 (q, J ) 141 Hz,
OCH3), 165.65 (s, >CdO); δ1H 7.22 (s, 4 H, aromatic H), 3.6
(t, 4 H, J ) 4.7 Hz, CO2CH2), 2.84 (t, 4 H, J ) 5.4 Hz, CH2-
OMe), 2.53 (s, 6 H, OCH3).
Compound 15:19 δ13C 131.03 (s, Ar-C1, C3, C5), 134.85 (d, J
) 168.2 Hz, Ar-C2, C4, C6), 70.27 (t, J ) 144.5 Hz, CO2CH2),
64.66 (t, J ) 148 Hz, CH2O), 59.02 (q, J ) 141.52 Hz, OCH3),
164.94 (s, >CdO); δ1H 8.90 (s, 3 H, aromatic H), 4.53 (t, 6 H,
J ) 4.6 Hz, CH2OMe), 3.76 (6 H, t, J ) 4.6 Hz, CH2OMe),
3.44 (s, 9 H, OCH3).
P r ep a r a tion of th e Ca r boca tion s. Approximately 10 mg
of the sample was dissolved in triflic acid or FSO3H in a 5
mm NMR tube at -78 °C (dry ice/acetone bath) and was
followed by warming in the NMR probe to desired tempera-
tures. The formation of the carbodications and the trication is
quantitative as shown by the complete disappearance of the
starting materials.
Exp er im en ta l Section
Triflic acid and FSO3H were freshly distilled prior to use.
Diethyl ether, triethylamine, 2-methoxyethanol, and the aro-
matic acid chlorides were used as received. H and 13C NMR
1
were recorded on a 300 MHz superconducting NMR spectrom-
eter, equipped with a variable-temperature probe, and were
referenced with respect to the residual solvent absorptions
(δ1H CHCl3 ) 7.27, and δ13C CHCl3 ) 77.0). The spectra were
referenced with respect to the external capillary tetrameth-
ylsilane for carbocations. Interpretation of the 13C NMR data
was facilitated by the acquisition of both proton-decoupled and
the proton-coupled 13C NMR spectra, as well as the APT
(attached proton test) experiments. Where applicable, the
smaller J values, indicated next to the directly coupled J
values, in the 13C NMR correspond to the long-range J values.
Gen er a l P r oced u r e for th e P r ep a r a tion of 2-Meth oxy-
eth yl Ben zoa tes. The acid chloride was dissolved in diethyl
ether, 2 molar equiv of triethylamine was added to the
contents, and the mixture was cooled to 0 °C. The 2-methoxy-
ethanol was then added dropwise to the contents using a
hypodermic syringe. A white precipitate of triethylamine
hydrochloride separated during the addition. Workup involving
washing with 5% HCl and saturated sodium bicarbonate
solutions provided the esters in nearly quantitative yields,
essentially pure by NMR analysis. Further purification was
achieved by distillation at reduced pressures. Compound 9:15
δ13C 131.93 (s, Ar-C1, C2), 131.12 (d, J ) 158.6 Hz, Ar-C3,
C6), 129.02 (d, J ) 160.4 Hz, Ar-C4, C5), 70.23 (t, CO2CH2),
64.67 (t, CH2O), 58.97 (q, OCH3), 167.55 (s, >CdO); δ1H 7.76
(dd, 2 H, J ) 5.8, 3.0 Hz, aromatic C3-H, C6-H), 7.53 (dd, 2 H,
J ) 5.8 Hz, 3.0 Hz, aromatic C4-H, C5-H), 4.46 (t, 4 H, J ) 4.6
Hz, CO2CH2), 3.69 (t, 4 H, J ) 4.6 Hz, CH2OMe), 3.40 (s, 6 H,
OCH3).
Ca lcu la tion a l Meth od s. The structures, energies and 13C
NMR chemical shifts of the carbocations were calculated using
Gaussian-9820 series of programs. The structures of the
carbocations were fully optimized at the B3LYP/6-31G* level.
Vibrational frequencies were calculated at this level to char-
acterize the stationary points as minima (NIMIG ) 0), and to
obtain the zero point energies, which were scaled by 0.98.21
The calculations of 13C NMR chemical shifts were performed
using GIAO/DFT method22 at the B3LYP/6-311G* level using
B3LYP/6-31G* geometries. 13C NMR chemical shifts were
referenced to tetramethylsilane (TMS) (calculated absolute
shift, i.e., σ(C) ) 183.8). 17O NMR chemical shifts were
referenced to H2O (calculated absolute shift, i.e., σ(O) ) 332.4).
Ack n ow led gm en t. Support of our work by the
Loker Hydrocarbon Research Institute and the National
Science Foundation is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Cartesian coordi-
nates and energies of the optimized structures 2 and 4-7. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O020753Z
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(18) Wolf, K. H.; Herlinger, H. Angew. Makromol. Chem. 1977, 65,
133-145.
Compound 11:16 δ13C 130.38 (d, J ) 7.8 Hz, Ar-C1, C3),
128.45 (d, J ) 163.5 Hz, Ar-C2), 133.89 (dt, J ) 157.9, 7.3 Hz,
Ar-C4, C6), 130.82 (dt, J ) 167.1, 5.9 Hz, Ar-C5), 70.31 (t, J
) 141 Hz, CO2CH2), 64.26 (t, J ) 147.4 Hz, CH2O), 58.94 (q,
J ) 141 Hz, OCH3), 165.62 (s, >CdO); δ1H 8.74 (d, 1 H, J )
1.22 Hz, aromatic C2-H), 8.26 (dd, 2 H, J ) 7.8, 1.83 Hz,
aromatic C4-C6-H), 7.54 (t, 1 H, J ) 7.6 Hz, aromatic C5-H),
4.51 (t, 4 H, J ) 4.7 Hz, CO2CH2, 3.75 (t, 4 H, J ) 4.7 Hz,
CH2OMe), 3.44 (s, 6 H, OCH3), 3.44 (s, 6 H, OCH3).
(19) Korshak, V. V.; Pavlova, S. A.; Chernomordik, Y. A. Vysokomol.
Soedin., Ser. A 1967, 9, 1033-1038; Chem Abstr. 1967, 68, 87636).
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