First examples of fluorinated and chlorinated polycyclic aromatic hydrocarbon (PAH) dications from benzo[a]pyrene, pyrene, and their alkyl- substituted derivatives

Article abstract of DOI:10.1021/jo980839l

Two-electron oxidations (SbF₅/SO₂ClF) of 6-fluoro-benzo[a]pyrene, 3, 6-chloro-benzo[a]pyrene 4, and their corresponding 7-methyl derivatives 5 and 6 as well as 1-fluoropyrene, 7, 1-chloropyrene, 8, and 1-fluoro-2,7-di-tert- butylpyrene, 9, allowed the charge delocalization mode, substituent effects, one-bond and long-range C/F coupling constants, and the tropicity of the resulting persistent PAH dications to be examined by NMR for the first time. For comparison, detailed NMR studies of the dications of 7- methylbenzo[a]pyrene, 2, and the parent benzo[a]pyrene, 1, are also reported. Total deshieldings ΣΔδ ¹³C in the resulting dications are ca. 195-211 ppm/e for the benzo[a]pyrenium and ca. 200-209 ppm/e for the pyrenium dications, respectively. Their AM1-calculated ΔΔH(f)(o) values are between 421 and 427 kcal/mol and 444 and 449 kcal/mol, respectively, well within the previously predicted dication formation limit of 463 ± 4 kcal/mol.

Full text of DOI:10.1021/jo980839l

J . Org. Chem. 1998, 63, 8217-8223
8217
F ir st Exa m p les of F lu or in a ted a n d Ch lor in a ted P olycyclic
Ar om a tic Hyd r oca r bon (P AH) Dica tion s fr om Ben zo[a ]p yr en e,
P yr en e, a n d Th eir Alk yl-Su bstitu ted Der iva tives
Kenneth K. Laali,*,† Mutsuo Tanaka,† and Poul Erik Hansen*,‡
Department of Chemistry, Kent State University, Kent, OH 44242, and Department of Life Sciences and
Chemistry, Roskilde University, DK-4000, Roskilde, Denmark
Received May 4, 1998
Two-electron oxidations (SbF5/SO2ClF) of 6-fluoro-benzo[a]pyrene, 3, 6-chloro-benzo[a]pyrene 4, and
their corresponding 7-methyl derivatives 5 and 6 as well as 1-fluoropyrene, 7, 1-chloropyrene, 8,
and 1-fluoro-2,7-di-tert-butylpyrene, 9, allowed the charge delocalization mode, substituent effects,
one-bond and long-range C/F coupling constants, and the tropicity of the resulting persistent PAH
dications to be examined by NMR for the first time. For comparison, detailed NMR studies of the
dications of 7-methylbenzo[a]pyrene, 2, and the parent benzo[a]pyrene, 1, are also reported. Total
deshieldings Σ∆δ 13C in the resulting dications are ca. 195-211 ppm/e for the benzo[a]pyrenium
and ca. 200-209 ppm/e for the pyrenium dications, respectively. Their AM1-calculated ∆∆Hf° values
are between 421 and 427 kcal/mol and 444 and 449 kcal/mol, respectively, well within the previously
predicted dication formation limit of 463 ( 4 kcal/mol.
In tr od u ction
Because of the well-established relationship between
13C NMR chemical shift and charge in structurally
analogous ions, it became the primary tool for probing
dications.1,2,24 However, in many instances, only partial
assignments were made. Moreover, except for anthra-
cenium dications, whose alkyl (aryl) and chloro deriva-
tives were reported,14 and some polyalkylated naphtha-
lenes16 and biphenylenes,22,11 the majority of the reported
examples are those of the parent hydrocarbons. There-
fore, substituent effects on charge delocalization and on
tropicity in dications remained to be explored. Proton
NMR data, which may allow an evaluation of ring current
effects, were considerably less emphasized for dications.
Dications are theoretically interesting for comparison
with solution NMR, since counterion effects are expected
to be greatly reduced in SbF5/SO2ClF medium as com-
pared to metal/THF for dianions and polyanions that are
contact ion pairs.4-6,11
Numerous examples of π-conjugated dianions of alter-
nant and nonalternant polycyclic, heterocyclic, and bridged
annulenes are known. Contributions by several groups,
notably those of Rabinovitz,1-7 Mu¨llen,8-10 and Olah,11
laid a strong foundation for understanding their NMR
features in relation to topology, charge delocalization,
tropicity, and ion pairing.
In comparison, the field of stable dication chemistry
of alternant and nonalternant PAHs and bridged annu-
lenes is relatively less extensive. Following initial fea-
1
sibility studies using early, low-field H NMR by Brou-
wer,12 many of the subsequent investigations were
reported by Olah and associates.13-17 More recent ex-
amples were provided by Michl,18 Laali,19-22 Mu¨llen,8,9
and their associates; the topic has been reviewed.23
† Kent State University.
‡ Roskilde University.
Benzo[a]pyrene (BaP) (1), 7-MeBaP (2), and 6-FBaP
(3) are carcinogenic, with 2 and 3 being less active, but
6-ClBaP (4) (and 6-BrBaP) are very weakly active.25-28
Because of the significance of the radical cation (RC)
pathway in metabolic activation, the reactions of chemi-
cally generated RCs derived from the 6-halo and 6-methyl
derivatives of 1 have been studied with model nucleo-
(1) Review: Rabinovitz, M.; Cohen, Y. Tetrahedron 1988, 44, 6957.
(2) Review: Rabinovitz, M. Top. Curr. Chem. 1988, 146, 99.
(3) Rabinovitz, M.; Ayalon, A. Pure Appl. Chem. 1993, 65, 111.
(4) Cohen, Y.; Klein, J .; Rabinovitz, M. J . Am. Chem. Soc. 1988, 110,
4634.
(5) Zelikovitch, L.; Hoffman, R. E.; Ayalon, A.; Rabinovitz, M. J .
Chem. Soc., Perkin Trans. 2 1992, 1785.
(6) Frim, R.; Goldblum, A.; Rabinovitz, M. J . Chem. Soc., Perkin
Trans. 2 1992, 267.
(7) Ioffe, A.; Ayalon, A.; Rabinovitz, M. J . Chem. Soc., Perkin Trans.
2 1994, 1115.
(8) Mu¨llen, K.; Meul, T.; Schade, P.; Schmickler, H.; Vogel, E. J .
Am. Chem. Soc. 1987, 109, 4992.
(9) Review: Mullen, K. Chem. Rev. 1984, 84, 603.
(10) Mu¨llen, K. Helv. Chim. Acta. 1978, 61, 2307.
(11) Bausch, J . W.; Gregory, P. S.; Olah, G. A.; Prakash, G. K. S.;
Schleyer, P. v. R.; Segal, G. A. J . Am. Chem. Soc. 1989, 111, 3633.
(12) Brouwer, D. M.; Van Doorn, J . A. Recl. Trav. Chim. Pays-Bas
1972, 91, 1110.
(13) Forsyth, D. A.; Olah, G. A. J . Am. Chem. Soc. 1976, 98, 4036.
(14) Olah, G. A.; Singh, B. P. J . Org. Chem. 1983, 48, 4830.
(15) Review: Prakash, G. K. S.; Rawdah, T. N.; Olah, G. A. Angew.
Chem., Int. Ed. Engl. 1983, 22, 390.
(19) Laali, K. K.; Tanaka, M.; Fetzer, J . C. J . Chem. Soc., Perkin
Trans. 2 1997, 1315.
(20) Laali, K. K.; Hansen, P. E.; Gelerinter, E.; Houser, J . J . J . Org.
Chem. 1993, 58, 4088.
(21) Laali, K. K.; Houser, J . J .; Zander, M. J . Chem. Soc., Perkin
Trans. 2, 1996, 1265.
(22) Laali, K. J . Chem. Res. Synop, 1988, 378.
(23) Laali, K. K. Chem. Rev. 1996, 96, 1873.
(24) Eliasson, B.; J ohnels, D.; Sethson, I.; Edlund, U. J . Chem. Soc.,
Perkin Trans. 2 1990, 897.
(25) Cavalieri, E. L.; Rogan, E. G.; Roth, R. W.; Saugier, R. K.;
Hakam, A. Chem.-Biol. Interact. 1983, 47, 87.
(26) Kinoshita, T.; Konieczny, M.; Santella, R.; J effrey, A. M. Cancer.
Res. 1982, 42, 4032.
(27) Cavalieri, E. L.; Rogan, E. G.; Cremonesi, P.; Devanesan, P. B.
Biochem. Pharmacol. 1988, 37, 2173.
(16) Lammertsma, K.; Olah, G. A.; Berke, C. M.; Streitwieser, A.,
J r. J . Am. Chem. Soc. 1979, 101, 6685.
(17) Olah, G. A.; Liang, G. J . Am. Chem. Soc. 1977, 99, 6045.
(18) Wallraff, G. M.; Vogel, E.; Michl, J . J . Org. Chem. 1988, 53,
5807.
(28) Cremonesi, P.; Cavalieri, E. L.; Rogan, E. G. J . Org. Chem.
1989, 54, 3561.
10.1021/jo980839l CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/16/1998

8218 J . Org. Chem., Vol. 63, No. 23, 1998
Laali et al.
Ta ble 1. AM1-∆∆Hf° a n d Tota l Desh ield in g Σ∆δ13C
(p p m /e) for th e Dica tion s
dication
color
blue
∆∆Hf°
Σ∆δ13C (ppm/e)
12+
22+
32+
42+
52+
62+
72+
82+
92+
412.4
426.7
427.2
427.6
422.2
422.2
448.7
448.0
443.9
210.7
210.1
195.1
207.4
203.0
199.8
202.9
209.4
nda
blue
purple
green
blue
green
green
black
red-black
a
nd, not determined.
sample temperature. With halopyrenes, dication forma-
tion was slower and required warming to -30 °C;
increasing the oxidizing power of the medium by addition
of SbCl5 to bring about these oxidations at lower tem-
peratures was ineffective. Dication generation from 8
and 9 was quite sluggish, requiring prolonged reaction
times (several days) for completion. Attempted genera-
tion of persistent dications from halopyrenes 10-12 was
unsuccessful (black solids were formed).
In agreement with these observations, the AM1-
predicted ∆∆Hf° values (Table 1) for 8 and 9 are ca. 20
kcal/mol higher than those of benzo[a]pyrenium dica-
tions. The ∆∆Hf° for 10-12 are still higher (ca 451-
458 kcal/mol range), close to the predicted set limit (463
( 4 kcal/mol) for dication formation.30a
Monitoring the process of two-electron oxidation with
1H NMR in the case of 11 showed broad aromatic
resonances, which did not sharpen, even after 3 weeks.
Solutions of 7 and 11 exhibited unresolved ESR signals,
even after 2 months of storage at dry-ice/acetone tem-
perature (7: g ) 2.002, ∆Hpp ) 7.4 G at -57 °C. 11: g
) 2.001; ∆Hpp ) 13.6 G at -60 °C.), consistent with the
concomitant presence of their radical cations (RC). At-
tempts to remove the RC by the introduction of oxygen
or air into the sample were unsuccessful. A broad and
shielded 19F resonance was observed in the dication
spectra for all of the fluorinated PAHs.
F igu r e 1. Substrates.
philes(i.e., OAC-).27-29 With 3 and 2, there is a strong
tendency for nucleophilic attack at C-6, whereas with 4
(and with 6-BrBaP), attack at C-1/C-3 also occurred
(positions of second highest charge localization in the
radical cation). With 6-methyl-BaP, benzylic cations
were formed and nucleophilic attack occurred on the side
chain. Ipso attack leading to defluorination was not
observed under electrophilic substitution conditions (with
TFAD and PyHBr3).28
1
For 7, the initial H NMR spectrum (recorded after 1
In our previous studies, we reported on dications from
h) indicated the presence of two very similar species in
a ca. 1:1 ratio; their chemical shifts can tentatively be
assigned as follows: H-1, 6.2; H-3, 9.0; H-4, 7.8; H-5, 7.8;
H-6, 9.3; H-7, 7.3; H-8, 8.9; H-9, 8.4; H-10, 8.2 ppm.
There is a clear four-bond H/F coupling in both species.
In relation to a previous study by Cornelisse et al.30b on
the alkylation of pyrene using AlCl3 in CS2 solvent, where
the formation of the phenalenium cation/allyl anion-
AlCl3 complex was suggested to explain the regioselec-
tivity of alkylation, the intial spectrum via 7/SbF5/SO2ClF
may stem from two regioisomeric SbF5 complexes (syn
and anti to F); these disappear with time and are
replaced by resonances for the dication.
Quenching of the dication solutions in all cases re-
turned the skeletally intact PAH together with black
unidentified oily byproducts, which were produced more
extensively for the halobenz[a]pyrenes than for the
pyrenes, especially in the case of 4.
NMR Assign m en ts. For the precursors, the 13C NMR
spectra of the proton-carrying carbons were assigned with
HETCOR experiments and those of quaternary carbons
a
series of alkyl(cycloalkyl)pyrenes20 and on alkyl-
perylenes.19 In relation to our continuing interest in
arenium ions and dications derived from different classes
of PAHs and the significance of electrophiles derived from
carcinogenic PAHs in adduct formation with DNA bases
that could lead to cell damage, we report here the first
examples of benzo[a]pyrenium dications, 2-6, and pyre-
nium dications, 7-9. Detailed assignments for parent 1
are also included for comparison.
On the basis of 13C, 19F, and H NMR data, the charge
1
delocalization mode, total deshieldings, one-bond and
long-range C-F coupling constants, and tropicity of the
systems have been evaluated. The AM1-calculated ∆∆Hf°
values and carbon charges were also compared.
Resu lts a n d Discu ssion
Dica tion Gen er a tion , ∆∆Hf° a n d P r esen ce of
Ra d ica l Ca tion . Using SbF5/SO2ClF, two-electron oxi-
dation of 1-6 (Figure 1) took place rapidly at dry-ice/
acetone temperature (NMR) without the need to raise the
(30) (a) Mills, N. S. J . Org. Chem. 1992, 57, 1899. (b) Rodenburg,
L.; deBlock, R.; Lugtenburg, J .; Cornelisse, J . Recl. Trav. Chim. Pays-
Bas 1988, 107, 529.
(29) Cremonesi, P.; Stack, D. E.; Rogan, E. G.; Cavalieri, E. L. J .
Org. Chem. 1994, 59, 7683.

Polycyclic Aromatic Hydrocarbon Dications
J . Org. Chem., Vol. 63, No. 23, 1998 8219
were assigned by analyzing the C-F containing com-
and three-bond C/F couplings have previously been
observed in fluorobenzenium,34 fluoronaphthalenium cat-
ions,34 and in fluoroallyl cations.35,36 The absence of C/F
couplings at the C-7 for 5 and 52+ is unexpected, as
3J (C,F) values greater than 3 Hz are seen at the C-8 of
1-fluoro-8-methylnaphthalene.37
pounds and the others by analogy. The 13C NMR data
1
for 7 and the H and 13C data for parent 1 had already
been assigned;31a,b 3 was assigned by analogy. For the
remaining compounds, the assignments were based on
C/F coupling constants and comparisons of the chemical
shifts with unsubstituted compounds.32 For 5, the as-
signments are tentative due to low solubility and the
resulting low S/N ratio in the 13C spectra.
For 5 and 9, relatively large fluorine couplings are
observed at Me and tBu(Me) both in the carbon and in
the proton spectra. Similar couplings have been observed
in 1-fluoro-8-methylnaphthalene37 and 8-fluoro-N-meth-
ylquinoline.38
For the dications, the proton-carrying resonances were
assigned by HETCOR experiments. For the quaternary
carbons of 72+, 92+, 32+, and 52+, the assigments were
based on C/F couplings and by analogy with the remain-
ing dications.
The 1H NMR assignments for the dications and the
precursors were based on the chemical shifts, H/H and
H/F coupling constant, multiplicities, and H/H COSY and
C/H HETCOR correlations.
For the methylated benzo[a]pyrene dications 2, 5, and
6, NOED spectra assisted with the assignments of ortho
and peri ring protons. Likewise, NOED spectra assisted
in the assignment of H-3 for both the dication and the
precursor.
2J (C,F ). The two-bond couplings in noncharged mol-
20,39
ecules are thought to roughly depend on bond order
or bond length. This is confirmed in the present study.
For the charged species, much less information is
2
available, but J (C,F) is seen to decrease as compared to
the uncharged compound, especially in cases where
3J (C,F) increases in the charged species (see below).
3J (C,F ). Three different types of three-bond carbon-
fluorine couplings can be observed as exemplified in the
following cases: for 7, 3J (C-3,F) and 3J (C-10b,F) both
with bonds in an anti relationship but with widely
3
different bond orders and J (C-10,F) with a syn relation-
Ch a r ge Deloca liza tion Mod e. Detailed 13C chemical
shift assignments, the ∆δ values, and the one-bond and
long-range C/F couplings (for fluorinated dications) are
gathered in Figure 2 and Figure 3 for the benzo[a]-
pyrenium and pyrenium dications, respectively. In the
benzo[a]pyrenium dications, the charge is extensively
delocalized throughout the periphery including the an-
nelated ring, with C-6, C-1/C-3, and C-9 carrying the
highest positive charge density, whereas C-12b and C-12c
remain relatively unchanged. There is a pronounced
ship and a short distance between C-10 and the fluorine
3
and for 9, the presence of J ((tBu/C)-F).
3
The marked increase in J (C,F)anti in the dications can
be related to the positive charge on fluorine as discussed
in fluoroallyllic ions.35,36 An increase in 3J (C-3,F) for
protonated 1-fluoronaphthalene and fluorobenzenes was
found by Olah et al.34 An increase in 3J (C-3a,F) of the
2-fluoropyrenium cation has also been reported.40 Olah
3
et al.34 correlated J (C,F) of para substituted fluoroben-
1
zenes with δF, but as J (C,F) also correlates with δF, a
correlation between 1J (C,F) and 3J (C,F) can also been
expected. In Figure 4, we have plotted the observed
three-bond C/F (anti) couplings versus 1J (C,F) for a series
of fluoroarenium cations and oxidation dications. The
graph defines the maximal values. 3J (C,F)anti depends
methyl substituent effect in comparing 22+ with 12+
.
The fluoronium ion character in the fluorinated benzo-
[a]pyrene dications is evident from significant ∆δ19F
values (given in Figure 3; for additional discussion see
later sections) and enhanced one-bond C/F couplings.
Back-bonding can also be inferred by a much larger ∆δ13C
for 32+ compared to 42+ (39.3 vs 68.9 ppm) and 52+
compared to 62+ (35.6 vs 57.0 ppm). The fluoronium ion
character is also reflected in the decreased total carbon
1
on J (C,F) but possibly also on the bond order as seen by
comparing different coupling paths within the same
molecule. For 72+, three-bond couplings of 32.7 and 15.1
Hz are observed. The larger coupling clearly relates to
the higher bond order of the C-3-F bond.
deshielding for 32+ and 52+ as compared to that for 12+
2
,
Semiempirical calculations41 on 1J (C,F) showed that
this type of coupling is largely dominated by the Fermi
contact term for neutral PAHs and the variations are
relatively small, whereas for the charged species with
large one-bond couplings, these in addition have a large
orbital term. A similar trend is seen for 3J (C,F). The
3J (C,F)syn couplings have been observed in a series of
2+, and 42+ (see Table 1).
For the pyrenium dications, in line with previous
examples,20 the charge is extensively delocalized through-
out the entire periphery, with those of C-1/C-3 and C-6/
C-8 being most positive. Fluoronium ion character is
similarly reflected in ∆δ19F and a lower Σ∆δ13C for 72+
versus 82+
.
3
Using the data for 3, 4, and 6, a plot of ∆δ13C values
versus the AM1-calculated Mulliken carbon charges ∆qc
[qc(dication)-qc (PAH)] gave a good correlation for car-
bons with ∆qc > 0.1.33
fluorinated polycylic aromatics with the result that J (C-
4,F) of 1-fluoronaphthalene and couplings in pyrenes,
phenanthrenes,31a and benz[a]pyrenes39 were of the order
(34) Spear, R. J .; Forsyth, D. A.; Olah, G. A. J . Am. Chem. Soc. 1976,
98, 2493.
(35) Galakhov, M. V.; Petrov, V. A.; Bakhmutov, V. I.; Belenkii, G.
G.; Kiasov, B. A.; German, L. S.; Fedin, E. I. Izv. Akad. Nauk, Ser.
Khim. 1985, 306.
(36) Galakhov, M. V.; Petrov, V. A.; Belenkii, G. G.; German, L. S.;
Fedin, E. I.; Snegirev, V. F.; Bakhmutov, V. I. Izv. Akad. Nauk., Ser.
Khim. 1986, 1063.
(37) Gribble, G. W.; Keany, D. J .; Olson, E. R.; Rae, I. D.; Staffa, A.;
Herr, T. E.; Ferraro, M. B.; Contreras, R. H. Magn. Reson. Chem. 1991,
29, 422.
(38) Barfield, M.; Walter, S. R.; Clark, K. A.; Gribble, G. W.; Haden,
K. W.; Kelly, W. J .; Le Houlier, C. S. Org. Magn. Res. 1982, 20, 92.
(39) Sardella, D. J .; Boger, E. Magn. Reson. Chem. 1986, 24, 422.
(40) Laali, K. K.; Hansen, P. E. J . Org. Chem. 1993, 58, 4096.
(41) Duangthai, S.; Webb, G. A. Org. Magn. Res. 1979, 12, 98.
An alysis of th e C/F Cou plin gs. Whereas the carbon-
fluorine couplings for the PAHs 3, 5, and 9 are similar
3
to those of other F-PAHs, very large 1J (C,F) and J (C,F)
values can be seen in their dications. Large one-bond
(31) (a) Hansen, P. E.; J akobsen, H. J .; Manzara, A. P.; Michl, J .
Org. Magn. Reson. 1977, 10, 179. (b) Unkefer, C. J .; London, R. E.;
Whaley, T. W.; Daub, G. H. J . Am. Chem. Soc. 1983, 105, 733.
(32) Buchanan, G. W.; Ozubko, R. S. Can. J . Chem. 1975, 53, 1829.
(33) Caution in using carbon charges from semiempirical calcula-
tions and the approximate/qualitative nature of such comparisons have
been discussed on several previous occasions. See: Laali, K. K.;
Hollenstein, S. J . Chem. Soc., Perkin Trans. 2 1998, 897 and related
references therein.

8220 J . Org. Chem., Vol. 63, No. 23, 1998
Laali et al.
F igu r e 2. 13C NMR chemical shifts data (∆δ13C) {J C-F} for the dications.

Polycyclic Aromatic Hydrocarbon Dications
J . Org. Chem., Vol. 63, No. 23, 1998 8221
F igu r e 4. Plot of 3J (C,F) anti versus 1J (C,F) (coupling
constants are from the present study and from refs 31a,
38-40, and 52).
F igu r e 5. Plot of 3J (C,F) syn versus 1J (C,F) (coupling
constants are from the present study and from refs 31a, 34,
39, 40, and 52).
The 3J (C,F)syn couplings are unusually large for
3J (C-5,F) for the dications 32+ and 52+ and for 1-fluo-
ronaphthalenium ion.34 For a number of neutral fluoro-
PAHs, the 1J (C,F) value is close to 251 Hz. The variation
in their 3J (C,F)syn (Figure 5) can, as mentioned above,
be in part due to a TS contribution. The very large values
obtained for the dications cannot be ascribed to an
increase in the TS contribution, as the F‚‚‚H distance is
at best the same as that in the neutral compounds.
Furthermore, the positive charge at fluorine should lead
to reduced C/F coupling as shown by Hsee and Sardella42
who found that presence of an electron-withdrawing
substituent para to fluorine (more positive charge at
fluorine) had the effect of reducing the TS contribution.
For the dications, the likely cause of the increase can best
be ascribed to a larger bond order as also suggested for
neutral molecules.39 For charged species, a reasonable
F igu r e 3. 1H NMR chemical shifts data (∆δ1H) {∆δ19F}for
the dications.
1
3
correlation between J (C,F) and J (C,F)cis is seen (Figure
4).
of 4.6-6.3 Hz, with one exception, that of 5 in this study,
showing a coupling of 7.7 Hz. In contrast to these
couplings, very small values were seen for 3J (C-5,F) in
The J (CH3,F) coupling in 9 is formally a coupling over
four bonds, but this coupling most likely has a large TS
contribution. In a similar manner, a coupling is observed
in 5 and 52+ whose magnitude is actually similar in the
dication and the neutral PAH.
3
4-fluoroacenaphthenes and acenaphthylenes and a J (C-
4,F) in 3-fluorofluoranthene. The small couplings in the
latter three compounds were ascribed to a small through
space (TS) contribution due to a large distance (large
1
Tr op icity in th e Dica tion s. The H NMR chemical
shifts and ∆δ1H values for the benzo[a]pyrenium and
pyrenium dications are gathered in Figure 3, which also
contains the 19F NMR data for the fluorinated dications
3, 5, 7, 9. The concomitant presence of the RC did not
3
C-C-C angle).31a Larger J (C,F) values in 5 can possibly
be related to steric strain imposed by the peri methyl
group. A similar situation is seen in 9, where the
buttressing effect of ortho tBu group forces the C-F bond
toward the C-10-H bond, leading to better orbital
overlap.
(42) Hsee, L. C.; Sardella, D. J . Magn. Reson. Chem. 1990, 28, 688.

8222 J . Org. Chem., Vol. 63, No. 23, 1998
Laali et al.
1
interfere with the assignments of the H NMR spectra
can bind to DNA. Radical cations of isomeric monom-
ethyl-BaP have been generated electrochemically, and
their stabilities (estimated from rate constanst for reac-
tion with water) showed some parallel with animal
carcinogenicity data.44 Although chemically generated
RCs of several halo-PAHs have been studied as models
for metabolic activation,28 their radical cations have not
been directly examined. The present study provides the
first insight into their doubly charged analogues.
of the dications, which usually contained additional broad
features. Whereas the H spectra of the benzo[a]pyrenes
1
are rather similar, those of the dications vary consider-
ably. A common feature in the benzo[a]pyrenium dica-
tion is a significant shielding of the bay-region protons
(H-10/H-11) by about 1 ppm. With the pyrenium dica-
tions 7-9, a more or less general proton shielding trend
is apparant.
The parent benzo[a]pyrenium dication is an 18π sys-
tem with a 16π periphery and would sustain a paramag-
netic ring current, which oppossess the deshielding effect
due to charge; it is hence paratropic. Among the studied
benzo[a]pyrenium dications, several hydrogens (H-2, H-7,
H-8, H-10, and H-11) are consistently shifted upfield;
others are mainly deshielded. The latter are mostly
attached to carbons with large ∆δ13C values.
Exp er im en ta l Section
P AHs. High purity samples of parent 1 and its
7-methyl derivative 2 were purchased from Aldrich and
used without further purification. The isomeric
fluoro-45 and chloropyrenes46 were synthesized according
to the literature. Synthesis of 3 by XeF2 fluorination of
1 has been reported.47 In the present study, compounds
3, 5, and 9 were prepared either by XeF2 fluorination,
using the literature procedure, or by fluorination with
N-fluoro-2,4-dinitroimidazole in refluxing dichloroethane,
for which the procedures are been reported elsewhere.48
Synthesis of 4 by chlorination of 1 with sulfuryl chloride
has been reported by Cho and Harvey.49 In the present
study, 4 and 6 were prepared accordingly.
Typically, sulfuryl chloride (0.11 mmol) was added to
a solution of 1 or 3 (0.1 mmol) with 20 mL of dry
dichloroethane in a 50 mL three-necked flask. The
reaction mixture was stirred for 1 h at room temperature,
after which it was poured into water and extracted with
chloroform. After removal of solvent under pressure, the
residue was separated by column chromatography using
hexane as an eluent. The yields were 88% and 83%
respectively.
It has been shown that the HOMO-LUMO gap and
tropicity correlate.1-3 In dianions, smaller paratropic
shifts are associated with larger HOMO-LUMO gaps.
In the present study, the total ∆δ1H values (difference
in the center of gravities between the dication and the
neutral PAH) in the parent 12+ amounts to -4.73 ppm.
The substituted dications exhibit a pronounced substitu-
ent dependency, with paratropicity decreasing in the
order 12+ (-4.73 ppm) > 42+ (-2.79 ppm) > 22+ (-1.64
ppm) > 32+ (0.85 ppm) > 62+ (2.31 ppm) > 52+ (2.65 ppm),
while the AM1-predicted HOMO-LUMO gaps remain
very close, for example, 12+ (5.77 eV), 22+ (5.65 eV), and
52+ (5.67 eV). Whereas a paratropic f diatropic switch
for 62+ and 32+ may well have a steric origin, the
difference between 12+ and 32+ is not clear-cut. It is
unlikely that variations in the total ∆δ1Hs are due to the
presence of the RC, as the resonances for the dications
remain sharp. Moreover, since chemical shift changes
due to radicals result form contact, charge repulsion
between the dication and the RC should minimize such
interactions.
The PAHs were subjected to a complete NMR analysis
(13C, 1H, 19F, H/H COSY, C/H HETCOR, and NOED
experiments). The data are incorporated into the data
for the dications gathered in Figures 2 and 3.
Oth er Ma ter ia ls. SbF5 (Aldrich or Fluorochem) was
distilled twice under a dry argon atmosphere in an all-
glass distillation unit. SO2ClF was prepared by fluorina-
tion of SO2Cl2 with NH4F and CF3COOH according to a
modified procedure of Prakash et al.50
AM1 Ca lcu la tion s. Energy minimizations and AM1
calculations were performed with the Hyperchem Pack-
age (Hypercube, Inc, 1994).
NMR spectra were recorded on a wide-bore GE-GN300
instrument using a 5 mm C/H switchable probe using
very short (ca. 3 cm) external sealed capillary tubes
(made from melting point tubes) added to 5 mm NMR
tubes containing the dication solution. The reported1H
As shown previously,10,20 a pyrenium dication having
a 12π perimeter sustains a paramagnetic ring current,
which compensates the deshielding due to charge to the
extent that the center of gravity remains more or less
unchanged, with only H-â shifted upfield. For the
substituted derivatives, considerable variation in the
∆δ1H values was detected within the series pyrene,
4-isopropylpyrene, 2,7-di-tert-butylpyrene, and 1,3,6,8-
tetrasubstituted- and 1,3,5,7,9-penta-isopropylpyrene.20
The present 1H NMR data on dications 7-9 provide
additional substituent effect information in 1-substituted
and 1,2,7-trisubstituted dications. The 1-substituted
derivatives exhibit small and similar effects at the H-R
protons, very large and negative effects at H-â and
medium negative effects for H-Râ. The pattern is similar
to that seen in the 4-isopropylpyrenium dication.20 The
pattern for 92+ is analogous to that of 4-isopropylpyrene
and quite different from that for the 2,7-di-tert-butylpyre-
nium dication.
In summay, we have provided a detailed comparative
NMR analysis for the first series of persistent fluorinated
and chlorinated benzo[a]pyrene dications and pyrenium
dications. For parent 1, the similarity in the positions
of highest spin density localization in the radical
cation 43,28 and highest charge localization in the dication
(C-6, C-1, C-3; followed by C-9, C-7, C-12, C-4, C-5)
suggests that dications may be suitable models for NMR
studies of large carcinogenic PAHs whose radical cations
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In Free Radicals in Biology; Pryer, W. A., Ed.; Academic Press: New
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Radicals in Biology; Pryer, W. A., Ed.; Academic Press: Orlando, 1984;
Vol. 6, Chapter 10.
(44) Tryk, D. A.; Park, S.-U.; Daub, G. H. J . Electrochem. Soc. 1983,
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(45) Lund, H.; Berg, A. K. Dan. Vidensk. Selsk. Mater. Fys. Medd.
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(46) Vollmann, H.; Becker, H.; Corell, M.; Streek, H. Leibigs. Ann.
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Polycyclic Aromatic Hydrocarbon Dications
J . Org. Chem., Vol. 63, No. 23, 1998 8223
NMR shifts are corrected for the use of an external
capillary as previously described.51
with vigorous mixing (the resulting colors are included
in Table 1).
Gen er a l P r oced u r e for Sta ble Dica tion Gen er a -
tion . SbF5 (1 mL) was charged into a 10 mm NMR tube
under argon. SO2ClF (2 mL) was condensed into a 5 mm
NMR tube using a high vaccum line and was subse-
quently poured directly into SbF5 in the 10 mm tube at
dry-ice/acetone temperature. Upon vigorous mixing (vor-
tex) and slight periodic warming, a clear homogeneous
solution resulted. A slurry of the PAH substrate (30 mg)
in SO2ClF (0.3 mL) was prepared in another 5 mm NMR
tube at dry-ice/acetone temperature to which the solution
of SbF5/SO2ClF was subsequently added under argon
Ack n ow led gm en t. Support of our work in the area
of reactive intermediates of carcinogenesis of PAHs by
the NCI of NIH (R15 CA63595-01A1) is gratefully
acknowledged. We thank the Danish National Research
Council for summer research support.
Su p p or tin g In for m a tion Ava ila ble: Selected 1D and 2D
NMR spectra for the dications (18 pages). This material is
contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
(51) Laali, K. K.; Hollenstein, S.; Hansen, P. E. J . Chem. Soc., Perkin
Trans. 2 1997, 2207.
(52) Laali, K. K.; Hollenstein, S.; Harvey, R. G.; Hansen, P. E. J .
Org. Chem. 1997, 62, 4023.
J O980839L