Preparation, 13C NMR/DFT/IGLO study of benzylic mono- and dications, and attempted preparation of a trication

Article abstract of DOI:10.1021/ja971287b

Substituted benzylic mono- and dications were prepared and investigated by ¹H and ¹³C NMR spectroscopy and DFT/IGLO calculations. Combined experimental and theoretical study suggest that the structure 1a is the major resonance contributor to the 2,4,6-trimethylbenzyl cation 1. Similar results were also found for the 2,4,6-dimethyl-4-tert-butylbenzyl 2 and 2,3,4,5,6- pentamethylbenzyl cation 3. It was found that the structure 4a is the predominant resonance contributor to the overall structure of 2,6- dimethylmesityldiyl dication 4 wherein the dienyl and allylic cation units are insulated from each other. Similar studies indicate structure 5a as the predominant canonical structure for 5-methoxy-2,6-dimethyl-m-xylyldiyl dication 5 wherein the dienyl and oxoallylic cation units are insulated from each other. Attempts to generate the 2,3,5,6-tetramethyl-1,4- dimethylbenzenediyl dication 8 was, however, not successful as were the generation of the 2,4,6-trimethylmesityltriyl trication 10 by ionization of 2,4,6-bis(chloromethyl)-mesitylene. The resulting ion was characterized as a chloromethyl substituted dication 9.

Full text of DOI:10.1021/ja971287b

J. Am. Chem. Soc. 1997, 119, 12923-12928
12923
1
3
Preparation, C NMR/DFT/IGLO Study of Benzylic Mono-
and Dications, and Attempted Preparation of a Trication1
George A. Olah,* Tatyana Shamma, Arwed Burrichter, Golam Rasul, and
G. K. Surya Prakash*
Contribution from the Donald P. and Katherine B. Loker Hydrocarbon Research Institute and
Department of Chemistry, UniVersity of Southern California, UniVersity Park,
Los Angeles, California 90089-1661
X
ReceiVed April 23, 1997. ReVised Manuscript ReceiVed October 21, 1997
Abstract: Substituted benzylic mono- and dications were prepared and investigated by H and 13C NMR spectroscopy
and DFT/IGLO calculations. Combined experimental and theoretical study suggest that the structure 1a is the major
resonance contributor to the 2,4,6-trimethylbenzyl cation 1. Similar results were also found for the 2,4,6-dimethyl-
1
4
-tert-butylbenzyl 2 and 2,3,4,5,6-pentamethylbenzyl cation 3. It was found that the structure 4a is the predominant
resonance contributor to the overall structure of 2,6-dimethylmesityldiyl dication 4 wherein the dienyl and allylic
cation units are insulated from each other. Similar studies indicate structure 5a as the predominant canonical structure
for 5-methoxy-2,6-dimethyl-m-xylyldiyl dication 5 wherein the dienyl and oxoallylic cation units are insulated from
each other. Attempts to generate the 2,3,5,6-tetramethyl-1,4-dimethylbenzenediyl dication 8 was, however, not
successful as were the generation of the 2,4,6-trimethylmesityltriyl trication 10 by ionization of 2,4,6-bis(chloromethyl)-
mesitylene. The resulting ion was characterized as a chloromethyl substituted dication 9.
Introduction
We now wish to report the preparation, 13C NMR spectro-
scopic characterization, and DFT/IGLO study of several benzylic
monocations and dications. These include the preparation and
study of dienylic-allylic 2,6-dimethylmesityldiyl dication and
dienylic-oxoallylic 3-methoxy-2,6-dimethyl-m-xylyldiyl dication
and comparison of their structure with that of the bisallyl
benzene dication. The study also includes comparison of
calculated data with experimentally observed results. No such
comparisons have so far been reported for benzylic cations. We
also report attempted preparation of the 2,4,6-trimethylmesi-
tyltriyl trication.
The study of long-lived aliphatic and aromatic carbocations
and carbodications is of considerable interest.2 A number of
ring-substituted primary benzylic monocations (aryl carbenium
ions) were observed as long-lived ions by Olah et al. in 1966
and were characterized by H NMR spectroscopy. No long-
lived phenyl-1,3-dimethyldiyl dication wherein the carbocation
centers are both primary, however, has been reported. Previous
studies on carbodications have shown that in the absence of
aryl stabilization of the carbocation centers, long-lived dipositive
ions can be generated only if the carbocationic centers are
separated by at least two carbon atoms and the carbenium
centers are tertiary. We reported successful characterization
of 2,10-para-[3 .5 ]octahedranedimethyl dication a novel bis-
cyclopropylmethylium dication. Recently we also reported in
a preliminary communication the preparation and NMR/DFT/
IGLO study of the 2,6-dimethylmesityldiyl dication. The
dication can also be considered a substituted benzene dication,
a unique dienylic and allylic dication system.
-4
5
1
Results and Discussion
6
2
6
The benzylic mono and dications were prepared in superacid
7
solutions at -78 °C by the ionization of their respective halides
1
1
under previously established conditions and studied by H and
1
3
C NMR spectroscopy. The structures of the benzyl mono-
8
cation and dications were fully optimized at the DFT B3LYP/
9
3
-21G level using the GAUSSIAN-94 package of programs.
1
3
10
C NMR chemical shifts were calculated by IGLO methods
X
Abstract published in AdVance ACS Abstracts, December 15, 1997.
using B3LYP/3-21G geometries (i.e., at the IGLO/DZ//B3LYP/
-21G level). Benzylic dications were further optimized at the
(1) Stable Carbocations Part 303. For Part 302, see: Olah, G. A.;
3
Shamma, T.; Burrichter, A.; Rasul, G.; Prakash, G. K. S. J. Am. Chem Soc.
997, 119, 3407.
2) For major reviews, see: (a) Olah, G. A.; Pittman, C. U., Jr.; Symons,
1
3
higher B3LYP/6-31G* level, and the C NMR chemical shifts
were calculated at the IGLO/DZ using B3LYP/6-31G* geom-
etries (i.e., IGLO/DZ//B3LYP/6-31G* level). L o¨ wdin bond
1
(
M. C. R. Carbonium Ions; Olah, G. A., Schleyer, P. v. R., Eds.;
Interscience: New York, NY, 1968; Vol. 1. (b) Olah, G. A. Angew. Chem.,
Int. Ed. Engl. 1973, 12, 173. (c) Olah, G. A. Acc. Chem. Res. 1976, 9, 410.
1
1
12 a
orders and natural bond orbital (NBO)
charges were
(
d) Olah, G. A. Top. Curr. Chem. 1979, 80, 21.
calculated at the B3LYP/6-31G*//B3LYP/6-31G* level.
(3) Olah, G. A. Chem. Scr. 1981, 18, 97.
Benzylic Monocations 1-3. Benzylic monocations 1-3
were obtained by dissolving the corresponding benzyl chlorides
(4) Prakash, G. K. S.; Rawdah, T.; Olah, G. A. Angew. Chem., Int. Ed.
Engl. 1983, 22, 390.
5) (a) Cupas, C. A.; Comisarow, M. B.; Olah, G. A. J. Am. Chem Soc.
966,. 88, 361. (b) Bollinger, J. M.; Comisarow, M. B.; Cupas, C. A.; Olah,
(
1
(8) Ziegler, T. Chem. ReV. 1991, 91, 651.
G. A. J. Am. Chem. Soc. 1967, 89, 5687. (c) Olah, G. A.; Porter, R. D.;
Jeuell, C. L.; White, A. M. J. Am. Chem Soc. 1972, 94, 2044.
(9) Gaussian 94 (Revision A.1); Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T. A.; Peterson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-
Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski,
J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart,
J. J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian, Inc.:
Pittsburgh, PA, 1995.
(6) (a) Olah, G. A.; Grant, J. L.; Spear, R. J.; Bollinger, J. M.; Serianz,
A.; Sipos, G. J. Am. Chem Soc. 1976, 98, 250. (b) Olah, G. A.; Cupas, C.
A.; Friday, K. J.; Bollinger, J. M.; Woolfe, M. L. J. Am. Chem Soc. 1967,
8
9, 156. (c) Olah, G. A.; Reddy, V. P.; Lee, G.; Casanova, J.; Prakash, G.
K. S. J. Org. Chem. 1993, 58, 1639. (d) Olah, G. A.; Hartz, N.; Rasul, G.;
Prakash, G. K. S.; Burkhart, M.; Lammertsma, K. J. Am. Chem Soc. 1994,
1
16, 3187.
7) Olah, G. A; Buchhilz, B.; Prakash, G. K. S.; Rasul, G.; Sosnowski,
(
(10) (a) Schindler, M. J. Am. Chem. Soc. 1987, 109, 1020. (b) Kutzelnigg,
W.; Fleischer, U.; Schindler, M. NMR Basic Princ. Prog. 1991, 23, 165.
(11) L o¨ wdin, P. O. Phys. ReV. 1955, 97, 1474.
J. J.; Murray Jr. R. K.; Kusenetsov, M. A.; Liang, S.; de Meijere, A. Angew.
Chem., Int. Ed. Engl. 1996, 35, 13.
S0002-7863(97)01287-0 CCC: $14.00 © 1997 American Chemical Society

12924 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Olah et al.
Scheme 1
Dibenzyldiyl Dications 4 and 5. 2,6-Dimethylmesityldiyl
dication 4 and 2,6-dimethyl-5-methoxy-m-xylyldiyl dication 5
were prepared by the ionization of their precursors 2,6-bis-
(
chloromethyl)mesitylene and 2,4-bis(chloromethyl)-3,5-di-
methylanisole, respectively, in 5-fold excess of SbF5 in SO2-
ClF at -78 °C (Scheme 2).
Scheme 2
in excess of SbF5-SO2ClF solutions at -78 °C (Scheme 1).
All ions were stable at -78 °C, and the 13C NMR spectra of
the solutions of these ions were well resolved. Previously the
1
4
cations 1-3 were characterized by only H NMR spectroscopy.
Scheme 3
13
Presently obtained C NMR data of the cations are given in
Table 1.
The 75 MHz 13C NMR spectrum of the 2,4,6-trimethylbenzyl
cation 1 exhibited seven peaks. The peak at δ C 169.5 (triplet
in the proton coupled 13C NMR spectrum) is due to the
positively charged benzylic carbon C1. The most deshielded
peak in the spectrum at δ 13C 188.5 is assigned to the C5 carbon,
which is 19.0 ppm more deshielded than that of benzylic carbon
C1. This suggests that the resonance structure 1a makes an
important contribution to the overall stabilization of the ion 1.
13
The 75 MHz 13C NMR spectrum of the ion 4 shows seven
1
3
well-resolved peaks at δ C 218.5 (s), 198.2 (t, JC,H ) 170.3
Hz) 195.4 (s), 143.9 (s), 140.0 (d, JC,H ) 177.3 Hz), 25.6 (q,
1
JC,H ) 131.9 Hz), 23.9(q, JC,H ) 132.3 Hz). The 300-MHz H
1
NMR showed absorptions at δ H 8.77 (br, 2H), 8.51 (br, 2H),
7
.10 (singlet, 1H), 2.62 (singlet, 3H), and 2.10 (singlet, 6H).
Based on the observed NMR data the dienylic allylic dication
structure 4a appears to be the predominant resonance contributor
to the overall structure 4. The ion is remarkably stable even at
This is also in agreement with the calculated structure of 1
-
10 °C.
(
Figure 1). The C1-C2 (1.361 Å) and C3-C4 (1.375 Å) bond
distances are close to that of a double bond, but the C2-C7
1.471 Å) and C4-C5 (1.416 Å) bond distances are significantly
1
13
The H (in brackets) and C NMR chemical shift assignments
are also shown on the structure 4a in Scheme 3. The unusual
stability of the dication can be attributed to the two isolated,
highly stabilized dienylic and allylic cationic fragments in the
structure 4a.
(
longer than a double bond. Similarly the 2,6-dimethyl-4-tert-
butylbenzyl cation 2 was also prepared from its precursor 2,6-
dimethyl-4-tert-butylbenzyl chloride. The resonance due to
benzylic carbon C1 of 2 at δ 13C 170.1 is only 0.6 ppm more
The structure 4a resembles the bisallylic benzene dication 6.
The benzene dication 6 is experimentally still unknown but di-
and polycyclic benzoanalogs were obtained by two-electron
1
3
deshielded than that of 1. However, the C5 peak of 2 at δ
98.4 is 9.9 ppm more deshielded than that of 1 due to tert-
butyl substitution. The calculated structure of 2 is given in
C
1
13
oxidation of the corresponding arenes by SbF5. MINDO/3
14
calculations by Dewar and co-workers showed that the benzene
1
3
13
Figure 1. δ C of C1 and C5 in the C NMR spectrum of
,3,4,5,6-pentamethylbenzyl cation 3 is similar to those of 1.
Calculated structure of 3 is also very similar to the calculated
dication 6 favors a C2h chair conformation as the most stable
2
form, with essentially isolated allyl cation units. Subsequently
15
Schleyer et al. showed that 6 is subject to Jahn-Teller distortion
13
structure of 1. IGLO calculated C NMR chemical shifts of
cations 1-3 correlate well with the experimentally obtained data
forming a double-allyl dication.
(
Table 1). The calculated 13C NMR chemical shifts of car-
bocationic centers (CH2 carbon) are 10.6-12.5 ppm more
deshielded than the experimentally observed results. The
agreement between experimental and calculated values may be
improved by using correlated level calculations such as GIAO-
1
2b
MP2 method.
However, GIAO-MP2 calculations using
ACES II program1 are presently limited to only small size
2c
molecules.
(13) Olah, G. A.; Forsyth, D.; J. Am. Chem Soc. 1976, 98, 4086. Olah,
(
12) (a) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys.
G. A.; Sing, B. P.; J. Org. Chem. 1983, 48, 4830.
(14) Dewar, M. J. S.; Holloway, M. K. J. Am. Chem. Soc. 1984, 106,
6619.
1
8
985, 83, 735.; Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988,
8, 899. (b) Gauss, J. J. Chem. Phys. Lett. 1992, 191, 614. Gauss, J. J.
Chem. Phys. 1993, 99, 3629. (c).ACES II, an ab initio program system by
Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.; Bartlett, R. J.;
Quantum Theory Project, University of Florida, 1991 and 1992.
(15) (a) Schleyer, P. v. R.; Lammertsma, K. J. Am. Chem. Soc. 1983,
105, 1049. (b) Schleyer, P. v. R.; Lammertsma, K.; Schwarz, H. Angew.
Chem., Int. Ed. Engl. 1989, 28, 1321.

Substituted Benzylic Mono- and Dications
J. Am. Chem. Soc., Vol. 119, No. 52, 1997 12925
Table 1. Calculated and Experimental 13C NMR Data for 1-6a
benzyl
cation
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
C12
C13
1
182.0
169.5)
180.7
170.1)
179.5
167.0)
227.5
218.5)
205.0
187.0)
180.3
135.8
(142.9)
135.7
(143.1)
138.5
(144.5)
136.0
(143.9)
133.4
176.4
(167.0)
173.4
(167.4)
173.2
(161.6)
203.9
(195.4)
193.4
130.0
(133.8)
129.0
(130.6)
132.8
(140.5)
131.8
(140.0)
115.5
201.0
(188.5)
214.5
(198.4)
200.6
(186.7)
203.9
(195.4)
198.7
130.0
(133.8)
127.3
(130.6)
132.8
(140.5)
136.0
(143.9)
131.0
176.4
(167.1)
178.1
(167.4)
173.2
(161.6)
21.3
(23.9)
227.1
(219.1)
20.4
(19.0)
20.8
(19.5)
18.6
20.4
(19.0)
27.4
(26.6)
15.6
(14.7)
24.4
(25.6)
20.6
(23.6)
26.8
(25.6)
27.6
(27.0)
23.4
(22.3)
24.4
(25.6)
213.4
(192.8)
(
(
(
(
(
2
3
27.6
(27.0)
15.6
27.6
(27.0)
18.6
20.2
(19.5)
(16.8)
211.9
(198.2)
23.2
(14.7)
211.9
(198.2)
66.4
(16.8)
4
ab
ab
5
(142.1)
224.9
(187.0)
224.9
(119.0)
180.3
(184.2)
224.9
(138.3)
224.9
(29.0)
(65.4)
6b
a
At IGLO/DZ//3-21G level. b At IGLO/DZ/B3LYP/6-31G* level. The values in parentheses correspond to experimental values.
Figure 1. B3LYP/3-21G calculated structures of 1-3.
The major resonance contribution of 4a was also indicated
by density functional theory (DFT) calculations. The B3LYP/
-31G* calculated (Figure 2) C6-C11 bond distance is 1.377
Å, which is slightly longer than that of a double bond (1.34 Å).
On the other hand, C1-C2 bond distance is 1.451 Å, which is
between those of a single (1.54 Å) and a double bond. Thus,
6

12926 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Olah et al.
Figure 2. B3LYP/6-31G* calculated (a) bond distance and IGLO 13C NMR chemical shifts (in parentheses). (b) L o¨ wdin bond order and NBO
charges (in parentheses) of 4a.
Figure 3. B3LYP/6-31G* calculated (a) bond distance and IGLO 13C NMR chemical shifts (in parentheses). (b) L o¨ wdin bond order and NBO
charges (in parentheses) of 6.
one of the positive charges of the dication is asymmetrically
delocalized over the C8-C2-C1-C6-C11 atoms (i.e., dienyl
cation). The other positive charge is delocalized among C5-
C4-C3 atoms (i.e., allyl cation) as the bond distance of C3-C4
(singlet, 3H). The ion was found to be indefinitly stable at -78
°C. The assignment of the chemical shifts of the carbon atoms
was based on their position, integrated areas, the 13C NMR
spectral multiplicity patterns, and the C-H coupling constants
of the dication.
(1.393 Å) is between those of single and double bond. So, the
dication can be described as dienyl-allyl dication. These results
Dienylic oxoallylic dication structure 5a seems to be the
predominant contributor of the 2,6-dimethyl-5-methoxy-m-
11
are also in agreement with the calculated L o¨ wdin bond orders
12
13
and natural bond orbital (NBO) charges (Figure 2). IGLO
calculated 13C NMR chemical shifts of 4a (Figure 2) correlate
well with the experimentally obtained data as shown in Scheme
xylyldiyl dication 5 based on observed NMR data. The
C
1
and H (in brackets) NMR chemical shift assignments are shown
on the structure 5a in Scheme 4. The stability of the dication
can also be due to the two isolated, highly stabilized dienylic
and oxoallylic cationic segments in the structure 5a.
3
.
To support this argument, we also calculated the benzene
dication at the B3LYP/6-31G* level. Comparison of the
geometries calculated for 4a (Figure 2) and those of calculated
for 6 (Figure 3) shows considerable similarities between the
two structures, although, former cation involves with a dienyl-
allyl and later involves with an allyl-allyl interactions.
Scheme 4
1
3
The C NMR spectrum of the dication 2,6-dimethyl-5-
methoxy-m-xylyldiyl dication 5 consists of 11 well-resolved
1
3
peaks at δ C 219.1 (s), 192.4 (t, JC,H ) 170.1 Hz), 186.7 (t,
JC,H ) 169.4 Hz), 184.0 (s), 181.9 (s), 142.0 (s), 138.2 (s), 118.7
(
)
d, JC,H ) 176.2 Hz), 65.3 (q, JC,H ) 155.4 Hz), 23.4 (q, JC,H
1
133.8 Hz), 23.5 (q, JC,H ) 133.8 Hz). The 300 MHz H
1
NMR showed absorptions at δ H 8.45-8.11 (br, 4H), 6.42
singlet, 1H), 3.61 (singlet, 3H), 2.49 (singlet, 3H), and 2.04
(
DFT/IGLO calculations also indicated that the resonance

Substituted Benzylic Mono- and Dications
J. Am. Chem. Soc., Vol. 119, No. 52, 1997 12927
Figure 4. B3LYP/6-31G* calculated (a) bond distance and IGLO 13C NMR chemical shifts (in parentheses) (b) L o¨ wdin bond order and NBO
charges (in parentheses) of 5a.
Scheme 5
structure 5a is the predominant contributor of the dication 5.
DFT B3LYP/6-31G* calculated Cs symmetrical structure is
shown in Figure 4. Both C6-C11 and C2-C8 bond distances
are 1.374 Å, which are slightly longer than that of a double
bond (1.34 Å). The C1-C2 and C1-C6 bond distances are 1.443
and 1.452 Å, respectively, which are between those of a single
C11 atoms. The C2-C3 and C6-C5 bond orders of 1.07 and
1.10, respectively, indicate little interaction between the dienyl
and the oxoallyl parts of the dication. IGLO calculated 13
C
NMR chemical shifts of 5a are also correlate very well with
the experimentally obtained data (Figure 4).
Comparison of the geometries calculated for benzene dication
(
1.54 Å) and a double bond. Thus, one of the positive charge
of the dication is delocalized over C8-C2-C1-C6-C11 atoms
i.e., dienyl cation). The other positive charge is delocalized
(chair conformation) 6 (Figure 3) and for 5a (Figure 4) also
shows considerable similarities between the two structures,
although the former cation involves a dienyl-allyl system
whereas the latter involves an allyl-oxoallyl isystem.
Attempted Preparation of 2,3,5,6-Tetramethylbenzene-1,4-
dimethyldiyl Dication 8. In an attempt to prepare dication 8,
(
among C5-C4-C3-O atoms (i.e., oxoallyl cation) as the bond
distances of C3-C4 and C4-C5 (1.409 Å and 1.376 Å) are
between those of a single and a double bond and the C3-O
bond is close to that of the CdO bond of carbonyl compounds
2
,3,5,6-tetramethyl-1,4-bis(chloromethyl) benzene was ionized
(
i.e., a carboxonium ion). So, the dication 5a can be described
as dienyl-oxoallyl dication. Unlike dication 4a, the structure
a is perfectly planar. There is very little interaction between
1
3
in SbF5/SO2ClF solution at -78 °C. C NMR spectrum of
the solution indicated, however, the formation of only mono-
cation 7 (Scheme 5). No evidence was obtained for the
formation of dication 8. The un-ionized chloromethyl group
of the monocation 7 forms a donor-acceptor complex with
5
the dienyl and the oxoallyl parts of the dication as the C2-C3
and C6-C5 bond distances (1.487 and 1.482 Å) are close to
that of a single bond. These results are also consistent with
1
3
13
SbF5. Thus, the observed C NMR chemical shifts of δ
C
11
calculated L o¨ wdin bond orders and natural bond orbital (NBO)
charges12 of 5a (Figure 4). Thus, the bond orders of C8-C2,
of positively charged benzylic carbon C1 and the CH2Cl carbon
13
are expectedly deshielded at δ C 170.2 and 82.9, respectively.
C2-C1, C1-C6, and C6-C11 are 1.65, 1.26, 1.23, and 1.66,
respectively, and the atomic charges of C8, C2, C1, C6, and
C11 are -0.02, -0.21, +0.31, -0.19, and -0.05 au, respec-
tively, indicating charge delocalization over C8-C2-C1-C6-
Attempted Preparation of 2,4,6-Trimethylmesityltriyl Tri-
cation 10. It was also attempted to prepare benzyl trication 10
by the ionization of 2,4,6-tris(chloromethyl)mesitylene in excess
1
3
of SbF5 in SO2ClF at -78 °C.
C NMR spectrum of the
(
16) Fuson, R. C.; Denton, J. J.; Kneisley, J. W. J. Am. Chem. Soc. 1941,
resulting deep orange-red colored solution indicated, however,
only the formation of only dication 9 (Scheme 6). There is no
indication for the formation of trication 10. The un-ionized
6
3, 2652.
(
31.
17) Aitken, R. R.; Badger, G. M.; Cook, J. W. J. Chem. Soc. 1950,
3

12928 J. Am. Chem. Soc., Vol. 119, No. 52, 1997
Olah et al.
Scheme 6
probe at 300 and 75.4 MHz, respectively. 1H and 13C NMR spectra
were obtained with respect to TMS by using an acetone-d capillary as
6
external standard.
chloromethyl group of dication 9 seems to form a very weak
donor-acceptor complex with SbF5. Similar results were
obtained when the ionization was carried out in more acidic
HF:SbF5/SO2ClF medium.
2
,6-Dimethyl-4-tert-butylbenzyl Chloride.16 To 7.3 g (45 mmol)
13
of 5-tert-butyl-m-xylene in 38 mL of concentrated hydrochloric acid
was added 15 g of formalin (37% formaldehyde in water). This mixture
was heated at 80°C for 48 h with vigorous stirring. The solution was
allowed to cool, and the solid material was filtered and washed with
water. The solid material was then taken up in dichloromethane,
washed with sodium carbonate solution, and dried over anhydrous
potassium carbonate. Removal of dichloromethane left 6.97 g of a
white solid. Three recrystallizations from hexanes gave 4.39 g of pure
The C NMR spectrum of the dication 9 consists of eight
peaks at δ 13C 217.8 (s), 197.7 (t), 194.8 (s), 145.9 (s), 144.8
1
(
s), 35.3 (t), 24.3 (q), 22.9 (q). The H NMR showed
1
absorptions at δ H 8.63-8.49 (br, 4H), 3.54 (singlet, 2H), 2.56
singlet, 3H), and 2.15 (singlet, 3H). Based on the observed
(
NMR data the dienylic allylic dication structure 9a seems to
be the major contributor of the dication 9 (Scheme 7). 9a is
very similar to structure 4a.
+
compound, 46%. MS (m/z, 70 eV, EI): 210 (M , 15.0), 195 (66.2),
1
1
75 (49.3), 119 (63.1), 77 (65.2), 51 (100); H-NMR (CDCl
3
) δ )
), 7.06 (s, 2H,
CH); 13C NMR 151.46, 136.99, 131.01, 125.42, 41.13,.34.32, 31.19,
9.50.
,6-Bischloromethyldurene.17 A mixture of 2,3,5,6-tetramethyl-
Scheme 7
3 3 2
1.28 (s, 9H, CH ), 2.42 (s, 6H, CH ), 4.64 (s, 2H, CH
1
3
benzyl chloride (4.11 g, 23 mmol), concentrated hydrochloric acid (18.8
mL), and formalin (7.5 g) was heated at 90°C for 12 h with vigorous
stirring. The solution was then filtered and washed with water. The
solid material was taken up in dichloromethane, washed with sodium
carbonate solution followed with water, and dried over anhydrous
potassium carbonate. Removal of dichloromethane left 3.59 g of a
white solid. Three recrystallizations from hexanes gave 1.36 g of pure
+
compound, 26%, mp 194-195°C. MS (m/z, 70 eV, EI): 230 (M ,
2
.9), 195 (75.5), 129 (100), 115 (89.8), 91 (89.8), 77 (80), 51 (78.9);
Conclusions
1
13
H-NMR (CDCl
134.54, 134.02, 42.21, 15.89.
3 3 2
) δ ) 2.34 (s, 12H, CH ), 4.69 (s, 4H, CH ); C NMR
Several substituted benzylic mono- and dications were
prepared and investigated by H and C NMR spectroscopy as
1
13
2,4-Bis(chloromethyl)-3,5-dimethylanisole. A mixture of 3,5-
dimethylanisole (6.13 g, 45 mmol), concentrated hydrochloric acid (38
mL), and formalin (15 g) was heated at 30 °C for 12 h with vigorous
stirring. The solution was then filtered and washed with water. The
visible solid material was taken up in dichloromethane, washed with
sodium carbonate solution followed with water, and dried over
anhydrous potassium carbonate. Removal of dichloromethane left 4.68
g of a white solid. Two recrystallizations from hexanes gave 3.57 g
13
well as by DFT/IGLO calculations. IGLO calculated C NMR
chemical shifts agree well with the experimental data. Com-
bined experimental and theoretical study suggest that 1a is the
major resonance contributor to the overall structure of the 2,4,6-
trimethylbenzyl monocation 1. Similar results were also found
for the 2,4,6-dimethyl-4-tert-butylbenzyl 2 and 2,3,4,5,6-pen-
tamethylbenzyl cation 3. The studies also suggest that 4a is
the predominant canonical contributor to the overall structure
of the 2,6-dimethylmesityldiyl dication 4 wherein the dienyl
and allylic cation units are insulated from each other. Similar
studies indicated that 5a is the predominant canonical contributor
to the overall structure of 2,6-dimethyl-5-methoxy-m-xylyldiyl
dication 5 wherein the dienyl and oxoallylic cation units are
again insulated from each other. Attempts to generate 2,3,5,6-
tetramethylbenzene-1,4-dimethyldiyl dication 8 was, however,
not successful as was that to generate the 2,4,6-trimethylmesi-
tyltriyl trication 10 by ionization of 2,4,6-bis(chloromethyl)-
mesitylene. The resulting ion was characterized as chloromethyl
substituted dication 9.
of pure compound, 43%, mp 129-130 °C. MS (m/z, 70 eV, EI): 232
+
(
M , 6.2), 197 (72.4), 117 (48.9), 91 (100), 77 (73), 65 (46.3), 51 (87.1);
1
H-NMR (CDCl
3
) δ ) 2.44 (s, 3H, CH
3 3
), 2.47 (s, 3H, CH ), 3.85 (s,
1
3
3H, CH ), 4.67 (s, 2H, CH ), 4.75 (s, 2H, CH ), 6.62 (s, 1H, CH);
NMR 157.43, 139.73, 138.91, 126.90, 122.66, 110.70, 55.68, 41.60,
38.26, 20.11, 14.63.
3
2
2
C
Preparation of Ions. Approximately 1.5 mL of 50% v/v solution
in SO ClF was placed into a 5 mm NMR tube and cooled to
5 2
78 °C in a dry ice/acetone bath. The appropriate benzyl chloride
of SbF
-
(
∼30 mg) was carefully added to the solution at -78 °C. The ensuing
mixture was vigorously stirred (Vortex stirrer) under periodic cooling,
and samples were transferred to NMR studies.
NMR Spectroscopy. 1H and 13C NMR spectra were recorded on a
Varian Associates Model VXR-300 spectrometer, equipped with a 5
mm variable temperature broad band probe. All spectra were obtained,
Experimental Section
operating with an internal lock provided by an acetone-d
6
capillary.
H, C resonances were referenced to external (capillary) tetrameth-
ylsilane.
1
13
5 2
SbF and SO ClF were doubly distilled prior to use in the preparation
of the ions. 2,4,6-Trimethylbenzyl chloride, 2,3,4,5,6-pentamethyl-
benzyl chloride, 2,6-bis(chloromethyl)mesitylene, 5-tert-butyl-m-xylene,
2
,3,5,6-tetramethylbenzyl chloride, 3,5-dimethylanisole, and 2,4,6-bis-
Acknowledgment. Support of our work by the Loker
Hydrocarbon Research Institute is gratefully acknowledged.
(chloromethyl)mesitylene are commercial samples of the highest purity
1
13
and were used without further purification. H and C NMR spectra
were obtained on a spectrometer equipped with a variable-temperature
JA971287B