Structure/Reactivity Relationships in the Benzo[c]phenanthrene Skeleton:  Stable Ion and Electrophilic Substitution (Nitration, Bromination) Study of Substituted Analogues, 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 on the basis of the magnitude of Deltadelta13C 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 formed via parent B[c]Ph as well as in the benzylic carbocation formed via fjord-region epoxide ring opening were deduced by gauge-invariant atomic orbital density functional theory (GIAO-DFT) and from the natural population analysis (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 the nitro group is introduced into the fjord region, for which the X-ray structure could be obtained in one case.

Synthesis, Structural, and Photophysical Properties of the First Member of the Class of Pyrene‐Based [4]Helicenes

“…The 1 H NMR spectrum (300 MHz, CDCl 3 ) of 8 shows four distinct singlets at δ = 1.82 (14‐Me), 2.06 (12‐Me), 2.59 (1‐Me), and 2.96 (3‐Me) ppm for the protons of four methyl groups, two sets of doublets ( J = 8.7 Hz) at δ = 7.98 and 8.74 ppm for 10‐H and 9‐H, and two sets of doublets ( J = 9.1 Hz) at δ = 8.04 and 8.21 ppm for 5‐H and 4‐H, respectively (Figure 2). As expected, the protons of the two methyl groups in the fjord region of the [4]helicene structure are significantly shielded at δ = 1.82 (14‐Me) and 2.59 (1‐Me) ppm by the adjacent rings, and there is an absence of meta coupling in the aromatic protons, a common consequence of the distortion from planarity of peripheral rings 20…”

“…To investigate this helicity in more detail, two methyl groups were introduced into the fjord regions of the synthesized [4]helicene. An additional methyl group at the 14‐position could lead to increased skeletal distortion, even more than that reported in benzo[ c ]phenanthrene 20. Thus, the reaction of 2 and (3,5‐dimethylbenzyl)triphenylphosphonium bromide ( 6 ) with n ‐butyllithium in THF gave the desired 7‐ tert ‐butyl‐1,3‐dimethyl‐5‐(3,5‐dimethylphenylethynyl)pyrene [( E )‐ 7 ] in 75 % yield (Scheme ).…”

Application of Electron Delocalization Indicators in the Study of Electrophilic Aromatic Substitution Reactions

“…Regarding larger arenes, Laali and coworkers [52][53][54][55][56] have considered the relation between multinuclear NMR properties and their GIAO-derived 9 charge delocalization modes. For example, changes in charges,…”

“…13 C NMR-derived charge delocalization modes in the carbocation resulting from the protonation of 3-isopropenylperylene [59]. The circles are related to the magnitude of 13 C. 9 The Gauge invariant atomic orbital (GIAO) method [57; 58] allows the obtention of gauge invariant wavefunctions from which NMR chemical shifts can be calculated and interpreted in terms of charge delocalization modes as in references [52][53][54][55][56]. …”

Superacid‐Catalyzed Dimerization/Cyclization of Isopropenyl‐PAHs – Novel Pathways to PAH Dimers, Phenalenes and Their Stable Carbocations

“…Facile conversion of 12b to 12c under equilibrium protonation conditions may be traced to the highly favorable energetics of C-5 protonation in the BcPh moiety. [10,11] The DFT-optimized structure of 12b is shown in Figure S5. In this case, both BcPh units are severely distorted, the largest angles are found between A/D, A/C and B/D rings (see Table S1).…”