Substituent Effects in Benz[a]anthracene Carbocations:  A Stable Ion, Electrophilic Substitution (Nitration, Bromination), and DFT Study

Abstract

A series of novel carbocations were generated from isomeric monoalkylated and dialkylated benz[a]anthracenes (BAs) by low-temperature protonation in FSO(3)H/SO(2)ClF. With the monoalkyl derivatives (5-methyl, 6-methyl, 7-methyl, and 7-ethyl) as well as the D-ring methylated analogues (9-methyl, 10-methyl, and 11-methyl), the C-7 or the C-12 protonated carbocations were observed (as the sole or major carbocation) in all cases. Protonation of the 12-methyl derivative (9) gave the C-7 protonated carbocation (9H+) as the kinetic species and the ipso-protonated carbocation (9aH+) as the thermodynamic cation. With the 12-ethyl derivative (10), relief of steric strain in the bay-region greatly favors ipso-protonation (10aH+). With 3,9-dimethyl (14), C-7 protonation (14H+) is strongly favored (with <10% protonation at C-12), and with 1,12-dimethyl (15) the sole species observed is the C-7 protonated carbocation (15H+). For 7-methyl-12-ethyl, 7-ethyl-12-methyl, and 7,12-diethyl derivatives (16, 17, and 18), two ipso-protonated carbocations were initially formed (C-7/C-12), rearranging in time to give the C-12 protonated carbocations exclusively (16aH+, 17aH+, and 18aH+). Protonation outcomes are compared with the computed relative energies by DFT. Charge delocalization paths in the resulting carbocations were deduced based on the magnitude of Deltadelta13C values. For the thermodynamically more stable C-12 protonated carbocations, the charge delocalization path is analogous to those derived based on computed NPA charges for the benzylic carbocations formed by 1,2-epoxide (bay-region) and 5,6-epoxide (K-region) ring opening. Nitration (and bromination) of the 4-methyl, 7-methyl, 7-ethyl, 3,9-dimethyl, and 1,12-dimethyl derivatives resulted in isolation and characterization of several novel derivatives. Excellent agreement is found between low-temperature protonation selectivities and the regioselectivities observed in model substitution reactions.

Carbocations from dibenz[a,j]anthracene and dibenz[a,h]anthracene, their methylated derivatives, and oxidized metabolites: A stable ion and DFT study

“…A major difference between the overall charge delocalization modes in the DBA carbocations, as revealed from this work, and the previously studied alkyl-BA carbocations 9,10 is that in the BA carbocations a more distinct anthracenium ion character exists, whereas with DBA carbocations the delocalization path is less extended and less homogeneous. This could stem from increased twisting of the framework in the DBA carbocations.…”

Revisiting Polyarenes and Related Molecules: An Update of Synthetic Approaches and Structure‐Activity‐Mechanistic Correlation for Carcinogenesis

“…Compound 194 protonation at C‐7 giving 194 H + was favored due to stabilization of the carbocation which was similar to compound 195 , giving 195H + with a protonation at C‐7 . For compounds 196 , 197 , and 198 , two ipso ‐protonated carbocations at C‐7 and C‐12 were produced, but ultimately gave C‐12 protonated carbocations as the dominant species giving 196 aH + , 197 aH + , and 198 aH + , which was indicative of allowing more localization of charge delocalization within the C‐ring.…”

“…Laali et al . were further able to take his studies with benz[ a ]anthracenes (BA) and its methyl and ethyl substituted derivatives by generating a series of carbocations from isomeric monoalkylated and dialkylated benz[ a ]anthracenes via protonation in FSO 3 H/SO 2 CIF and analyzing thermodynamic data using density functional theory (DFT) and stable ion NMR.…”

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]. …”