Halogenations of Anthracenes and Dibenz[a,c]anthracene with N-Bromosuccinimide and N-Chlorosuccinimide¹

Abstract

Halogenation of dibenz[a,c]anthracene (1) by NBS in CCl(4) affords the products of 9- and 10-monobromination in the ratio of 9:1. The reaction is accelerated by iodine, and HBr effects rearrangement of 9-bromo product to the sterically less crowded 10-bromo isomer. The mechanism is proposed to involve reversible addition of Br(2), followed by elimination of HBr. Reaction of NCS with 1 in CCl(4) requires addition of HCl and affords exclusively 9-chlorination. The different reactivities of NBS and NCS are ascribed to the relative amounts of free halogen produced (due to differences in N-X bond strengths involving Br and Cl), and the different sizes of the halogens. Under similar conditions, NCS chlorinates 9-bromoanthracene (2a) to afford 9,10-dichloroanthracene and 9-bromo-10-chloroanthracene in the ratio of 65:35. This reaction ostensibly occurs by addition of Cl(2) to 2a, followed by preferential loss of HBr rather than HCl. 9-Methylanthracene (3) affords exclusively 9-(bromomethyl)anthracene with NBS in the absence of iodine, but mainly (67%) 9-bromo-10-methylanthracene in the presence of iodine. Chlorination of 3 with NCS in the presence of HCl also affords mostly (65%) nuclear halogenation. Nuclear bromination of anthracene, 9-methylanthracene, and dibenz[a, c]anthracene by NBS in the absence of added HBr is accelerated by iodine. This effect is probably due to an increase in the amount of bromine produced from NBS in the presence of iodine.

Addition–Elimination versus Direct Substitution Mechanisms for Arene Chlorination

“…Our previous theoretical studies on the bromination of arenes10 revealed that the lowest‐energy halogen addition routes are more favourable than direct substitution pathways. As noted above, numerous experimental studies report the bromination and chlorination addition products of aromatic hydrocarbons 21,22. Further transformations involving low‐energy barriers and hydrogen halide elimination can lead to substitution products.…”

Arenium ions are not obligatory intermediates in electrophilic aromatic substitution

“…This addition-elimination (AE) mechanism (the historical antecedent to Fig. 1) (17)(18)(19)(20)(21)(22)(23)(24)(25)(26) predicts the same positional orientation as the usually assumed direct substitution ("S E Ar") alternative. Instead of this classic S E Ar mechanism (Fig.…”

“…Thus, de la Mare (21,25,38,39) demonstrated the formation of halogen adduct intermediates. Polybenzenoid hydrocarbons (PBHs) react with halogens to give isolable addition products, which then give substitution products easily by hydrogen halide elimination (23). Our computational investigations of arene bromination with molecular bromine (12) and sulfonation with SO 3 (13) provided clear evidence that the mechanisms of the inherent substitution reactions (i.e., uncatalyzed, gas phase, or weakly solvated) are concerted and do not involve the conventional σ-complex (or any other) intermediates.…”

(Anthracen-9-yl)methyl sulfides as a mechanistic probe for chemical as well as photochemical electron-transfer reactions

“…Various fractions were collected and analyzed using spectroscopic measurements. The products formed were identified as 9-anthraldehyde (3), 63 9,10-anthraquinone (6), 64 9-methylanthracene (12), 65 1,2-bis(9-anthracenyl)ethane (13), [66][67][68][69] lepidopterene (14), 66,[70][71][72][73][74] biplanene (15), [75][76][77] anthrone (16) 78 along with products arising through the S-alkyl residues present in parent anthracenemethyl sulfides. Common products formed in the photoirradiation of (anthracen-9-yl)methyl sulfides 1a-f are shown in Chart 2 and details of all the products isolated are presented in Scheme 3.…”