Electrospray mass spectra were recorded on a Bruker
Esquire-LC instrument with MS/MS capability. MeOH or
MeOH–MeCN was used as solvent to which 0.01% NH4NO3
was added to protonate the PAH.
for technical assistance with NMR and ES-MS instruments.
Support of our work on mechanistic carcinogenesis by the NCI
of NIH (R15 CA 78235–01A1) is gratefully acknowledged.
SEG acknowledges the Brazilian foundations FAPESP, CAPES
and CNPq for financial support and LCCA-USP for generous
allocation of computational resources.
AM1 calculations
These were carried out using standard methods implemented in
the Hyperchem package version 5.11 (Hypercube Inc, 1999) or
Insight II Release 97.0 (MSI, 1999). Frequency calculations
confirmed that in all cases the carbocations were bona fide
minima (no imaginary frequency) (see Supplementary inform-
ation). DFT calculations were performed using Gaussian-98
software.26 Optimized geometries were obtained at the B3LYP/
6-31G(d) (Cartesian coordinates are provided as Supplemen-
tary information). NMR shifts were computed according to the
GIAO method.27 NICS calculations were carried out by GIAO/
HF/3–21G at the ring centroid (NICS(0)) at 1.0 Å above this
point.21
References
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2 R. G. Harvey, Polycyclic Aromatic Hydrocarbons, Wiley-VCH, New
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3 (a) B. P. Cho and R. G. Harvey, Tetrahedron Lett., 1987,
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4 W. H. Rastetter, R. B. Nachbar, Jr., S. Russo-Rodriguez, R. V.
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9 M. J. S. Dewar and R. D. Dennington II, J. Am. Chem. Soc., 1989,
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10 For the most recent publications see: (a) K. K. Laali, T. Okazaki,
S. Kumar and S. E. Galembeck, J. Org. Chem., 2001, 66, 780;
(b) K. K. Laali, T. Okazaki and M. M. Coombs, J. Org. Chem., 2000,
65, 7399; (c) K. K. Laali, T. Okazaki and P. E. Hansen, J. Org.
Chem., 2000, 65, 3816; (d ) K. K. Laali, S. Hollenstein, S. E.
Galembeck and M. M. Coombs, J. Chem. Soc., Perkin Trans. 2,
2000, 211.
11 A. J. Jones, T. D. Alger, D. M. Grant and W. M. Litchman, J. Am.
Chem. Soc., 1970, 92, 2386.
12 L. Ernst, Org. Magn. Reson., 1976, 8, 161.
13 B. P. Cho and R. G. Harvey, J. Org. Chem., 1987, 52, 5679.
14 D. J. Sardella and E. Boger, Magn. Reson. Chem., 1989, 27,
13.
15 J. J. Gonzales, N. Garcia, B. Gomez-Lor and A. M. Echavarren,
J. Org. Chem., 1997, 62, 1286.
General procedure for stable ion generation
SO2ClF (ca. 0.4 mL) was distilled into a 5 mm NMR tube
containing the substrate (5–10 mg) cooled to dry ice–acetone
temperature. To the resulting suspension cold FSO3H (2–3
drops) was added with efficient mixing until homogeneous
(vortex). Subsequently, 2 drops of CD2Cl2 were added on top of
the cold solution and the mixture was thoroughly mixed.
Quenching experiments
The superacid solution was carefully poured into ice–
bicarbonate and the mixture was extracted with CH2Cl2. The
organic extract was washed (10% NaCl) and dried (MgSO4).
The solvent was removed under reduced pressure and the resi-
due was analyzed by NMR. In the case of 1Hϩ, apart from
intact 1, ca. 10% of 11 was formed (NMR in Fig. 3; ES-MS: see
below). In the case of 6, dimer 11 was present in ca. 33% yield.
Separation of 11 from 6 by silica chromatography proved
unsuccessful; the dimer was analyzed by NMR (Fig. 3a) and
ES-MS (see below) directly in the mixture.
16 B. P. Cho, M. Kim and R. G. Harvey, J. Org. Chem., 1993, 58,
5788.
17 I. B. Berlman, H. O. Wirth and O. J. Steingraber, J. Am. Chem. Soc.,
1968, 90, 566.
Nitration of 11
The methoxy derivative (10 mg) was reacted with NO2BF4
(2 equiv.) in CHCl3 at rt. After 5 minutes’ stirring, the reaction
was quenched (ice–bicarbonate). The organic phase was
separated, the solvent was removed and the residue was
chromatographed on silica (hexane–CH2Cl2 1 : 1). NMR analy-
sis indicated the formation of the 9-nitro isomer (∼90%) 11
together with other minor isomers (or by-products) which
could not be further separated [NMR data (Fig. 3a); ES-MS
(see below)].
18 (a) K. K. Laali and P. E. Hansen, Res. Chem. Intermed., 1996,
22, 737; (b) S. Hollenstein and K. K. Laali, J. Chem. Soc., Perkin
Trans. 2, 1999, 895.
19 See for example: P. v. R. Schleyer, C. Maerker, P. Buzek and
S. Sieber, in Stable Carbocation Chemistry, ed. G. K. S. Prakash
and P. v. R. Schleyer, Wiley, New York, 1997; H.-U. Siehl, M. Fuss
and J. Gauss, J. Am. Chem. Soc., 1995, 117, 5983; G. A. Olah,
A. Burrichter, T. Mathew, Y. D. Vankar, G. Rasul and G. K. S.
Prakash, Angew. Chem., Int. Ed. Engl., 1997, 36, 1875.
20 K. K. Laali, S. Hollenstein, S. E. Galembeck and M. M. Coombs,
J. Chem. Soc., Perkin Trans. 2, 2000, 211; K. K. Laali, T. Okazaki,
S. Kumar and S. E. Galembeck, J. Org. Chem., 2001, 66, 780.
21 (a) P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao and
N. J. R. v. E. Hommes, J. Am. Chem. Soc., 1996, 118, 6317; (b) J. M.
Schulman, R. L. Disch, H. Jiao and P. v. R. Schleyer, J. Phys.
Chem. A, 1998, 102, 8051.
22 K. K. Laali and S. Hollenstein, J. Chem. Soc., Perkin Trans. 2, 1998,
897; K. K. Laali, S. Hollenstein and P. E. Hansen, J. Chem. Soc.,
Perkin Trans. 2, 1997, 2207.
23 K. K. Laali and P. E. Hansen, J. Chem. Soc., Perkin Trans. 2,
1994, 2249; K. K. Laali and P. E. Hansen, J. Org. Chem., 1993, 58,
4096.
Summary of ES-MS data
Compound 1: m/z 203 (M ϩ H)ϩ; m/z 220 (M ϩ NH4)ϩ. Com-
pound 5: m/z 253 (M ϩ H)ϩ; m/z 270 (M ϩ NH4)ϩ. Compound
6: m/z 283 (M ϩ H)ϩ [MS/MS: m/z 263 (methyl loss)]; m/z 300
(M ϩ NH4)ϩ; m/z 391 (M ϩ Ag)ϩ [MS/MS: Agϩ]. Compound 7:
m/z 271 (M ϩ H)ϩ; m/z 288 (M ϩ NH4)ϩ. Compound 8: m/z
298 (M ϩ H)ϩ [MS/MS: m/z 252 (loss of nitro)]; m/z 315 (M ϩ
NH4)ϩ. Compound 9: m/z 277 (M ϩ H)ϩ; m/z 294 (M ϩ NH4)ϩ.
Compound 10: m/z 282 (M ϩ 2H)ϩ2; m/z 580 (M ϩ NH4)ϩ.
Compound 11: m/z 563 (M ϩ H)ϩ; m/z 580 (M ϩ NH4)ϩ. Com-
pound 12: m/z 328 (M ϩ H)ϩ [MS/MS: m/z 311 and m/z 282];
m/z 345 (M ϩ NH4)ϩ.
24 K. K. Laali, T. Okazaki, S. E. Galembeck and J. S. Siegel, J. Org.
Chem., 2002, 66, 8701.
25 V. P. Reddy, D. R. Bellow and G. K. S. Prakash, J. Fluorine Chem.,
1992, 56, 195.
26 Gaussian 98, Revision A.9, M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery, Jr., R. E. Stratmann, J. C. Burant,
S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C.
Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi,
B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
Acknowledgements
We thank Dr S. Hollenstein for preliminary protonation
experiments using 300 MHz NMR and Dr M. Gangoda
628
J. Chem. Soc., Perkin Trans. 2, 2002, 621–629