Structure/Reactivity Relationships in the Benzo[c]phenanthrene Skeleton: Stable Ion and Electrophilic Substitution (Nitration, Bromination) Study of Substituted Analogs; Novel Carbocations and Substituted Derivatives

Abstract

A series of novel carbocations were generated by low temperature protonation of substituted benzo[c]phenanthrenes B[c]Phs and their charge delocalization pathways were elucidated by NMR based on the magnitude of Δδ¹³C values. It has been shown that the protonation regioselectivity is strongly controlled by methoxy and hydroxyl substituents, whose directive effects override methyl substitution effects. Regiocontrol by –OMe and –OH substituents, and its stronger influence relative to methyl groups, was also observed in the nitration and bromination reactions. Charge distribution modes in the regioisomeric protonated carbocations via parent B[c]Ph as well as in the benzylic carbocation formed via fjord-region epoxide ring opening were deduced by GIAO-DFT, and from the NPA-derived changes in charges over CHs. These patterns were compared with those derived from NMR experiments in the substituted derivatives. NMR-based charge delocalization mapping provided insight into structure/activity relationships in the methylated and fluorinated B[c]Phs. Regioselectivities observed in the nitration and bromination reactions in representative cases are the same as those via protonations. Among a group of novel nitro and bromo derivatives synthesized in this study are examples where nitro group is introduced into the fjord-region, for which X-ray structure could be obtained in one case.

Introduction

Parent benzo[c]phenanthrene (B[c]Ph) 1 is the smallest polycyclic aromatic hydrocarbon (PAH) with a fjord region (Fig. 1). As the first member of [n]-helicenes family,¹ 1 has a twisted framework with an angle of 27° between A/D rings from the X-ray structure.²

Figure 1.

Substituted benzo[c]phenanthrenes studied.

B[c]Ph is produced in the environment by incomplete combustion, and has been identified in food, cigarette smoke, and wastewater.^3a Parent 1 is weakly carcinogenic in animal tests, in which B[c]Ph-5,6-dihydrodiol is produced as the major oxidized product and B[c]Ph-3,4-dihydrodiol formation is minor.^3a Interestingly, the opposite regioselectivity is exhibited in human metabolism, whereby B[c]Ph-3,4-dihydrodiol (proximate carcinogen) is the main product (B[c]Ph-3,4-dihydrodiol is the minor pathway), which is further oxidized to B[c]Ph-3,4-diol-1,2-epoxides (ultimate carcinogens), with the (−)-anti-diol epoxide being more potent.^3a Dipple and associates^3b reported that B[c]Ph-3,4-diol-1,2-epoxides are exceptionally good DNA alkylating agents, with each configurational isomer exhibiting marked preference for covalent binding to DNA over hydrolysis, and each reacting extensively with deoxyadenosine. Metabolic studies by Baird and coworkers,^3c and by Eisenbrand and associates^3a demonstrated that B[c]Ph-3,4-diol-1,2-epoxides are formed from B[c]Ph in human mammary carcinoma cell line (MCF-7), and via oxidative metabolism in human liver and lung. The results underscored that B[c]Ph is potentially more toxic in humans than in rodents.

Despite its remarkably high tumorigenicity, the solvolytic reactivity (both in acid catalyzed and neutral hydrolysis) of the anti-diol epoxide of B[c]Ph was reported to be relatively low,^3d and it is plausible that the observed high levels of DNA alkylation could stem from longer life-time of the metabolites.

Given the significance of fjord-region epoxide of B[c]Ph in human metabolism, studies focusing directly on the benzylic carbocation (12 → 12⁺) (Fig. 2) are quite relevant.

Figure 2.

Benzylic carbocation formation in metabolic activation of B[c]Ph.

Focusing on structure/activity relationships, the 3-, 4-, 5-, and 6-methylated derivatives of B[c]Ph, were found to be tumorigenic in mouse skin, but the 1-Me and 2-Me derivatives were less active than the parent B[c]Ph.⁴ Fluorine substitution on the benzo-ring enhanced tumorigenicity relative to parent 1, except for the 2-fluoro-derivative which was less active.^5a Fluorine substitution at C-6 increased tumorigenic activity by over 4 fold.^5b

Nitration, acetylation and bromination of parent 1 was studied by Newman and Kosak in the 1940’s, showing preferential substitution at C-5.⁶

Methyl substitution into the highly congested fjord-region increases skeletal distortion from 27° for the A/D ring angle in B[c]Ph to 37° for that in 1,4-dimethyl-B[c]Ph (compound 10 in Fig. 1).² Increased non-planarity in 10 lowers its metabolic activation, relative to parent B[c]Ph, to form DNA alkylating species with MCF-7 cells.² More recently, the 1,4-difluoro-B[c]Ph 9 and its dihydrodiol have been synthesized.⁷ The A/D angle in 9 is about 33°, but the dihydrodiol is less twisted (A/D angle 22°). There are some notable features in the dihydrodiol and diol epoxide derivatives of B[c]Ph 1, 9, and 10. Whereas in each case the dihydrodiols exhibit a quasi-diequatorial arrangement of the hydroxyl groups, conformational preference in the diol epoxides of 1 and 9 are different. For 1, both syn- and anti-diol epoxides exhibit a predominantly quasi-diequatorial arrangement of the hydroxyls, but for 9, the OH groups are quasi-diaxial in the syn-diol epoxide and quasi-diequatorial in the anti-diol epoxide.^5a,7

As part of a broader stable ion and electrophilic substitution study,⁸ we showed that nitration and benzoylation of parent 1 gave the corresponding 5-substituted derivatives, a methoxy- or a hydroxyl- substituent at C-3 directed the electrophilic attack to C-4 (in stable ion protonation and/or protic nitration).

In an effort to gain more insight into structure/reactivity relationships in the B[c]Ph skeleton, the present study focuses on stable ion and electrophilic chemistry of a relatively large group of substituted derivatives, including those with substituents in the fjord region (see Fig. 1).

Results and Discussion

NMR of Neutral PAHs

Complete NMR assignments were made with the aid of 2D-NMR and NOE for the B[c]Ph derivatives listed in Fig. 1 (except for compound 5; see later) and the data are gathered in Figure 3. The following general features are noteworthy. The fjord-region protons (H-1 and H-12) are highly deshielded (appear around 9 ppm). The presence of a substituent at position 1 shields the H-12 by about 1 ppm. In the 1,4-difluoro-substituted derivative (9) the fjord-region fluorine is deshielded by 23 ppm relative to the other fluorine.

Figure 3.

Specific ¹H (italic) and ¹³C NMR assignments for the B[c]Ph derivatives.

Generation and Direct NMR Study of the Carbocations (Schemes 1–3, Figures 4–5)

Scheme 1.

Protonation of 2–4 in FSO₃H/SO₂ClF.

Scheme 3.

Protonation of 11.

Figure 4.

¹H (italic) and ¹³C NMR data for the carbocations.

Figure 5.

Charge delocalization modes derived from experimental Δδ¹³C values (carbocation minus neutral) (threshold 8 ppm).

Low temperature protonation of the 6-Me derivative 2 (Scheme 1) with FSO₃H in SO₂ClF at −65 °C led to the formation of 2H⁺ (protonation at C-5) as the only detectable carbocation. The most deshielded carbon resonances in 2H⁺ are due to C-6 (δ 203.8) and C-12b (δ 160.3) (Fig. 4), with extensive charge delocalization into rings B and C (Fig. 5).

Introduction of a second methyl substituent at C-7 (compound 3) did not change the protonation regioselectivity, generating carbocation 3H⁺ (attack at C-5), leading to loss of symmetry (Scheme 1). Despite the presence of two methyl groups, the positive charge remains strongly localized on C-6 (δ 197.0). It is interesting to note that the fjord-region protons in both 2 and 3 (Fig. 3) are more deshielded than those in the corresponding carbocations 2H⁺ and 3H⁺ (Fig. 4), reflecting conformational change as a way to lower steric strain.

Compound 4 with a methoxy group in the fjord-region, and a methyl group at C-6, provided the opportunity to explore relative effectiveness of these groups in directing electrophilic attack. Protonation of 4 with FSO₃H/SO₂ClF at −65 °C (Scheme 1) yielded a 1:1 mixture of the two regioisomers 4aH⁺ and 4bH⁺ (protonation ortho/para to methoxy). Warming the mixture to −20 °C during 15–20 minutes, resulted in complete isomerization of 4aH⁺ to 4bH⁺, thus establishing 4aH⁺ as the kinetic and 4bH⁺ as the thermodynamic carbocations. This isomerization is irreversible, thus allowing complete NMR characterization of 4bH⁺ alone to be made at −50 °C. Specific proton NMR assignments for each regioisomer were made via NOE enhancements.

The methylene protons are diastereotopic in both 4aH⁺ and 4bH⁺ giving rise to doublets with 27–30 Hz coupling, corresponding to a gem-relationship. Whereas in 4bH⁺, the ¹H NMR chemical shift values for the two geminal protons are quite close (Δppm = 0.08), this difference is larger in the case of 4aH⁺ (Δppm = 0.30). The methoxy group in 4aH⁺ is in a sterically congested environment due to both the fjord-region and the ortho methylene group, and is forced to adopt an out-of-plane conformation for the O-CH₃ bond. Consequently, the two protons of the methylene group in 4aH⁺ appear in different environments, with a larger difference in chemical shifts. Positive charge in both 4aH⁺ and 4bH⁺ is delocalized within a naphthalenium moiety plus one conjugated carbon (Fig. 5).

The outcome of low temperature protonation of compound 5 (this compound was available in very small quantity) was similar to 4, leading to a mixture of two regioisomers 5aH⁺ and 5bH⁺ (Scheme 2). However in this case, the proton resonances were broad and this prevented specific assignments. As with 4, isomerization of 5aH⁺ into 5bH⁺ was observed upon raising the temperature to −30 °C for 20 minutes. Only partial NMR assignments for 5bH⁺ could be achieved at −40 °C (Fig. 4). Skeletally intact 5 was recovered upon quenching and the substrate was used for bromination (see later).

Scheme 2.

Protonation of compounds 5–7.

Low temperature reaction of compound 6 with FSO₃H/SO₂ClF at −65 °C (Scheme 2) resulted in quantitative formation of carbocation 6H⁺ (protonation at C-4), demonstrating that an OH group in the A-ring is much more effective in directing electrophilic attack compared to a methyl group in the K-region. Positive charge in 6H⁺ is heavily localized in the A-ring plus a conjugated carbon in the B-ring (Fig. 5). The fjord-region protons in 6H⁺ have very different chemical shifts (with H-1 at 9.74 ppm and H-12 at 8.33 ppm).

A similar directive effect is exhibited in the low temperature protonation of 7 (Scheme 2), which led to quantitative formation of 7H⁺ (protonation at C-4). The NMR characteristics and charge delocalization modes in 6H⁺ and 7H⁺ are very similar. Thus methyl substitution in the K-region has no control over the protonation outcome, when an OH group is present in the A-ring.

Low temperature protonation of the 3,4-dimethoxy-derivative 8 under the same set of conditions resulted in a complex mixture that could not be specifically analyzed. Similarly, the low temperature reaction of the 1,4-difluoro-derivative 9 gave a complex spectrum with indication for gradual decomposition. Nevertheless, skeletally intact 9 could be recovered in relatively good yield upon quenching of the superacid solution.

Initial NMR spectra recorded immediately after reacting the 1,4-DMB[c]Ph 10 with FSO₃H/SO₂ClF at −65 °C showed evidence for C-protonation due to the presence of a methylene group appearing as a pair of doublets with geminal coupling. However, minutes later broad signals appeared and the spectral resolution greatly diminished. These features suggested competing oxidation with the formation of radical cation, rather than side reactions, since the starting material 10 was recovered intact upon quenching (together with a small amount of side products).

Interestingly, introduction of a methoxy group into the other outer ring of 10 produced a different protonation outcome. Thus the low temperature reaction of 11 with FSO₃H/SO₂ClF at −65 °C (Scheme 3) gave a 4:1 mixture of 11aH⁺ and 11bH⁺ as distinct conformers, with protonation directed ortho to methoxy (at C-9). Based on NOE studies, the major conformer 11aH⁺ corresponds to a less hindered conformation with OMe pointing towards H-11, whereas the minor conformer 11bH⁺ corresponds to the conformation with OMe facing CH₂. On raising temperature to −30 °C, these merged into a single averaged species 11H⁺. Charge delocalization mode in 11H⁺, as well as in 11aH⁺ and 11bH⁺, is analogous to those of 6H⁺ and 7H⁺. The protonation outcome underscores the directive effect of the methoxy group in 11.

Computational Study

As reported previously, attempts to generate a stable carbocation from parent 1 were unsuccessful. But protic nitration (HNO₃/HOAc) and benzoylation (PhCOCl/AlCl₃) as prototypic electrophilic substitution reactions resulted in C-5 substituted derivatives.⁸ According to DFT calculations by B3LYP/6-31G(d),⁹ the arenium ion of protonation of B[c]Ph at C-5 is more stable than those derived from protonation at C-1, C-4, C-6, C-2, and C-3 by 2.7, 2.7, 3.4, 3.5, and 3.7 kcal/mol respectively (Table S1 in supporting information.)

The charge delocalization modes in benzo[c]phenanthrenium cations 1aH⁺ (C-2 protonated), 1bH⁺ (C-4 protonated) and 1cH⁺ (C-5 protonated), which are not accessible via experimental stable ion studies, were derived based on the magnitude of Δδ¹³C values, computed by GIAO-DFT,⁹ for comparison with the NMR-derived values summarized in Fig. 5. In addition, charge delocalization mode for the benzylic carbocations formed via fjord-region epoxide ring opening (13a⁺) was also calculated for comparison (see Fig. 6).

Figure 6.

GIAO-NMR chemical shifts for model benzo[c]phenanthrenium ions 1aH⁺, 1bH⁺, 1cH⁺ and 13a⁺, and Δδ¹³C values (in parentheses) relative to computed GIAO data for parent B[c]Ph or 2-hydroxy-B[c]Ph (dark circles signify the sites with notable Δδ¹³C values)

To examine the charge pattern by another approach, the NPA-derived changes in charges over CHs were computed by DFT for 12a⁺/12b⁺ and 13a⁺/13b⁺ (see Fig. 7).

Figure 7.

NPA-derived changes in charges over CHs relative to neutral precursors (dark circles are roughly proportional to the magnitude of changes; threshold = 0.06).

NPA-derived changes in charges in benzylic carbocations formed by epoxide ring opening indicate no significant positive charge retention at the benzylic carbocation center and charge accumulation at C-3, C-4a, C-6, C-10, and C-12b, with C-6 exhibiting the largest positive localization. Introduction of carbocation stabilizing substituents (methyl and fluorine) at C-3/C-6/C-10 will stabilize the carbocation, and this concurs with the observed increased bioactivity in the 6-Me, 3-Me, and the 6-F derivatives, but does not provide a basis for understanding the increased activity in the 4-Me and 5-Me derivatives. To test the effect of fluorine/methyl substitution, the NPA-derived change of charges in the fluorinated carbocations 14⁺, 15⁺ and 16⁺ (Fig. 8) and the carbocations 17⁺, 18⁺ and 19⁺ (Fig. 9) were computed. These substitutions did not cause notable changes in the charge delocalization mode (positive charge is still primarily delocalized into C-3, C-4a, C-6, C-10, and C-12b). In 19⁺, in addition to the indicated ring positions, the methyl group also becomes notably positive.

Figure 8.

NPA-derived changes in charges over CHs relative to neutral precursors (dark circles are roughly proportional to the magnitude of changes; threshold = 0.06).

Figure 9.

NPA-derived changes in charges over CHs relative to neutral precursors (dark circles are roughly proportional to the magnitude of changes; threshold = 0.06).

Comparing the GIAO-derived Δδ¹³C patterns (Fig. 6) with experimental Δδ¹³C values (Fig. 5) and the NPA-derived charge maps (Fig. 7), it can be seen that: 1aH⁺ and 13⁺ are analogous to 4aH⁺, 1bH⁺ is similar to 4bH⁺, 6H⁺, 7H⁺, and 11H⁺, and 1cH⁺ is similar to 2H⁺ and 3H⁺. Therefore, the arenium ions can be viewed as rational models for studying substituent effects and charge delocalization modes via the epoxides.

Probing Substituent Effects Through Electrophilic Substitution Reactions

Nitration and bromination were used as prototypic reactions to examine substituent effects in the B[c]Ph derivatives, with the aim to compare the resulting regioselectivities with the protonation outcomes. The results of these reactions are summarized in Schemes 4 and 5.

Scheme 4.

Nitration/bromination reactions of compounds 2, 4, and 5.

Scheme 5.

Nitration/bromination reactions of compounds 8, 9 and 11.

Two sets of conditions were typically employed in protic nitration, namely HNO₃ (50% aqueous)/AcOH at room temperature (method A), and HNO₃ alone (20% aqueous) at low temperature (method B).¹⁰ The milder conditions were used in the nitration of the methoxy-derivatives. For bromination reactions, a modified procedure reported by Mitchell et al was used (method C).¹¹ Limited availability of the PAHs was the determining factor in the number of derivatives that could be synthesized.

Nitration of 2 using method A yielded 2NO₂ (substitution at C-5) in 55% isolated yield. The starting material was totally consumed, and some side products were also present (probably corresponding to various isomeric dinitrated compounds). Mild bromination of 2 gave the 5-bromo derivative 2Br in 61% yield. It follows that the regioselectivity in nitration and bromination of 2 is the same (substitution at C-5) as that observed in the stable ion study.

Bromination of 4 (OMe group in the fjord-region) yielded 4Br in 88% yield by substitution at C-4 (para to methoxy), whereas, as described earlier, protonation of 4 yielded both 4aH⁺ (ortho) and 4bH⁺ (para) carbocations. Absence of a C-2 brominated product reflects the higher energy of its arenium ion due to steric crowding.

Bromination of 5 (OH group in the fjord-region) yielded the monobromo-derivative 5aBr (substitution at C-4; para to OH) in 48% isolated yield, and an equimolar mixture of 5bBr (substitution at C-2; ortho to OH) and the 2,4-dibromo-derivative 5cBr. The latter two compounds (combined yield 43%) could not be separated by preparative TLC, but their constitution was confirmed by electrospray mass spectrometry. Formation of 5cBr was likely due to the use of a small excess of NBS (1.3 eq.).

By employing method A in the nitration of the dimethoxy-derivative 8, the dinitro- and trinitro-substituted derivatives 8bNO₂ and 8cNO₂ were obtained (in 25% and 27% yields respectively), together with other unidentified products (likely polynitrated compounds that could not be isolated). But mild nitration of 8 (using method B) gave the novel mono-nitrated product 8aNO₂ by nitration in the fjord-region in 94% isolated yield. Re-crystallization of 8aNO₂ produced yellow crystals suitable for X-ray analysis (Fig. 10). It is noteworthy that the presence of nitro group in the fjord-region increases the helicity of the system relative to parent B[c]Ph so that the torsion angle between the rings A and D increases from 26.7°² to 29.3°The average dihedral angle formed between the nitro group and the aromatic plane of the A-ring is 28.3°. Relative to the A-ring, the nitro group is severely buttressed with an out-of-plane angle of 16.2°.

Figure 10.

Thermal ellipsoid plot of 8aNO₂ (ellipsoids are drawn at the 30% level).

Attempted selective protic nitration of 1,4-DFB[c]Ph 9 was unsuccessful under a variety of conditions. Nitration of 9 with stoichiometric amount of the nitronium tetrafluoroborate (NO₂BF₄) in CH₃CN at room temperature overnight led to a mixture of two mononitro-isomers (total 24%), with 36% of 9 remaining unreacted (corresponding to 64% of the total conversion). Formation of the isomeric mononitro-derivatives was corroborated by ES-MS and by ¹⁹F NMR (see experimental). Regioselectivity in nitration could not be established via NOE due to close proximity of the aromatic ¹H NMR signals. Attempted bromination of 9 with NBS/CH₃CN gave no conversion even at reflux in CH₃CN!

Nitration of 1,4-DMB[c]Ph 10 was attempted under various conditions. However in all cases the starting material underwent degradation. Therefore, no nitro-derivative of 10 could be synthesized via conventional means. Attempted bromination of 10 (NBS/CH₃CN) led to low conversion (even after 2 days reflux). Among the formed products, a mono-brominated compound was detected by TLC and by ES-MS.

Contrary to the unexpected reactivity pattern in 10, the methoxy-substituted analog 11, was successfully nitrated by using 30% HNO₃ at −10 °C, yielding a 3:2 mixture of regioisomeric mononitrated derivatives 11aNO₂ and 11bNO₂ respectively (in 53% yield). Since the isomeric mixture could not be separated by preparative TLC, they were analyzed as a mixture by ¹H NMR and ES-MS. The position of nitro group could only be determined unambiguously in the case of 11aNO₂ via NOE experiments. Bromination of 11, resulted in the isolation of 11Br in quantitative yield (bromine at C-9). Overall, electrophilic reactivity studies on 11 establish a common pattern in protonation, nitration and bromination, showing that the regioselectivity is controlled by the methoxy group, leading in all cases to substitution at C-9.

Comparative Discussion and Summary

Protonation, nitration, and bromination data for the K-region monomethylated compound 2 and protonation data for the dimethyl-derivative 3, indicate that in all cases electrophilic addition is directed to C-5. However, the K-region methyl groups are no longer in control of regioselectivity, once methoxy or hydroxyl groups are introduced into the outer ring. This is seen in protonation and bromination of 4, in bromination and protonation of 5, in protonation of 6 and 7, in nitration of 8, and in protonation, nitration and bromination of 11. In such cases, initial electrophilic attack is directed to the outer ring, ortho/para to OH or OMe substituents.

The charge delocalization pattern established in the resulting carbocation intermediates based on Δδ¹³C values (Fig. 5) is analogous to that derived from GIAO-DFT for the benzylic carbocation formed via fjord-region epoxide ring opening.⁸ Positive charges in the substituted 5-benzo[c]phenanthrenium ions are delocalized extensively into the B and C rings, and charges in the 2- or 4-benzo[c]phenanthrenium ion derivatives are delocalized into the A rings and conjugated carbons in the B rings. All carbocations have significant charge localization at C-3 and C-6 positions. It is, therefore, reasonable to consider these model carbocations and their charge delocalization maps as a basis for predicting substituent effects, for comparison with the available biological data. This approach rationalizes the increased activity of the 3-Me-, 6-Me- and the 6-F-derivatives, as these strategic substitutions stabilize the benzylic carbocation intermediate. Methyl substitution at C-2 creates steric hindrance to bay-region epoxidation and inhibits tumorigenic activity, despite the fact that epoxidation could still occur in the other benzo-ring. Similarly, fluorine-substitution at C-2 has been shown to decrease tumorigenic activity, even though in principle epoxidation could still occur in the other benzo-ring. The DFT study infers that if this could occur, charge delocalization in the resulting carbocation is not influenced by the presence of fluorine at C-11. Further studies (synthesis of DNA adducts and computational modeling) are warranted to help rationalize these points.

The greatly increased tumor-initiating activity of the 6-Me and the 6-F derivatives are clear indications for the importance of fjord-region diol-epoxides, since these substituents can inhibit metabolism at the K-region. The present study underscores the higher stability of the benzylic carbocation, when carbocation stabilizing groups are present at C-6 and C-3. Obviously, the bigger picture needs to take into account steric and planarity factors, in particular in the case of 9, 10 and 11, since it was shown previously that 10 produces only low levels of DNA adducts in MCF-7 cells.^2a

Finally, the present study has resulted in the synthesis, isolation and characterization of a host of nitro- and bromo-substituted derivatives of B[c]Ph, including 8aNO₂, 8bNO₂ and 8cNO₂ with a nitro group in the fjord-region. The X-ray structure of 8aNO₂ is consistent with severe nitro-buttressing.

Experimental Section

General

NMR spectra were recorded at 500 MHz for low temperature stable ion studies and at 500 MHz or at 400 MHz in room temperature studies. Electrospray-MS (ES-MS) spectra were obtained by infusion mode by mixing a dilute MeOH solution of the product (10 μM) with AgOTs (30 μM) in MeOH to form PAH/Ag⁺ adducts.¹² Because of the existence of two stable isotopes for both silver and bromine atoms, only the averaged m/z values are reported to simplify the description of mass spectra. IR spectra were recorded on a FT-IR instrument.

FSO₃H was distillated twice under argon in an all-glass distillation apparatus at atmospheric pressure and stored under argon at −20 °C in Teflon bottles with Teflon seals. SO₂ClF was prepared according to a modified procedure of Prakash et al.¹³

Typical procedure for stable carbocation generation

The substrate (10–15 mg) was charged into a 5mm NMR tube, flushed with argon, and cooled to dry ice-acetone temperature. SO₂ClF (~0.3 mL) was condensed directly into the tube. Then 4–5 drops of FSO₃H were slowly added under argon to prevent local overheating, whereupon immediate color change took place (variable from compound to compound). After vigorous (vortex) stirring at −78 °C, 3–4 drops of CD₂Cl₂ were slowly introduced into the NMR sample with further vigorous stirring to give a homogeneous solution.

Quenching procedure

The superacidic solution was carefully poured into a cold aqueous solution of sodium bicarbonate, and extracted three times with dichloromethane. The organic extract was dried over magnesium sulfate, filtered, concentrated under reduced pressure and the resulting solid residue was assayed by ¹H NMR.

X-ray crystallographic data

X-ray crystallography was performed by mounting each crystal onto a thin glass fiber from a pool of Fluorolube™ and immediately placing it under a liquid N₂ stream, on an X-ray diffractometer. The radiation used was graphite monochromatized Mo K_α radiation (λ = 0.7107 Å). The lattice parameters were optimized from a least-squares calculation on carefully centered reflections. Lattice determination, data collection, structure refinement, scaling, and data reduction were carried out using APEX2 version 1.0–27 software package. Each structure was solved using direct methods. This procedure yielded a number of the C, N, and O atoms. Subsequent Fourier synthesis yielded the remaining atom positions. The hydrogen atoms were fixed in positions of ideal geometry and refined within the XSHELL software. These idealized hydrogen atoms had their isotropic temperature factors fixed at 1.2 or 1.5 times the equivalent isotropic U of the C atoms to which they were bonded. The final refinement of each compound included anisotropic thermal parameters on all non-hydrogen atoms.

Computational protocols

Structures were optimized using a C₁ molecular point group, except for B[c]Ph (C₂), by the density function theory (DFT) method at B3LYP/6-31G(d) level using the Gaussian 03 package.¹⁴ All computed geometries were verified by frequency calculations to have no imaginary frequencies. Energies of the optimized structures for benzo[c]phenanthrene (1), 2-hydroxybenzo[c]phenanthrene, and their protonation cations are summarized in Table S1 in supporting information. NMR chemical shifts were calculated by the GIAO¹⁵ method at the B3LYP/6-31G(d) level. NMR chemical shifts were referenced to TMS (GIAO magnetic shielding tensor = 189.8 ppm in TMS; these values are related to the GIAO isotropic magnetic susceptibility for ¹³C), calculated with molecular symmetry of T_d at the same level of theory (Fig 6). Natural population analysis (NPA)-derived charges were computed at the same level (Figs 7 and 8).

Global minima for 2-hydroxy-B[c]Ph and 2-hydroxy-2-benzo[c]phenanthrenium ion were located by changing the direction of the O-H groups and H-C-O geometry and by comparing the resulting optimized structures and their energies. Changes in the NPA-derived charges relative to the corresponding neutrals were found to be very close (almost the same) among the isomers. Therefore, only representative conformational isomers for the diolepoxides and their corresponding ring-opened carbocations were computed in order to derive charge delocalization modes via changes in the NPA-derived charges.

Supplementary Material

si20070228_015. Supporting information available.

Experimental method, compound characterization, and energies and Cartesian coordinates for the optimized structures (for selected carbocations) by B3LYP/6-31G(d) (Tables S1-S6). Crystallographic Information File (CIF) for compound 8aNO₂. These material are available free of charge via the Internet at http://pubs.acs.org.