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Electrophilic Mercuration and Thallation of Benzene and Substituted Benzenes in Trifluoroacetic Acid Solution* (Electrophilic Substitution/Selectivity) GEORGE A

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Electrophilic Mercuration and Thallation of Benzene and Substituted Benzenes in Trifluoroacetic Acid Solution* (Electrophilic Substitution/Selectivity) GEORGE A Proc. Natl. Acad. Sci. USA Vol. 74, No. 10, pp. 4121-4125, October 1977 Chemistry Electrophilic mercuration and thallation of benzene and substituted benzenes in trifluoroacetic acid solution* (electrophilic substitution/selectivity) GEORGE A. OLAH, IWAO HASHIMOTOt, AND HENRY C. LINtt Institute of Hydrocarbon Chemistry, Department of Chemistry, University of Southern California, Los Angeles, California 90007; and the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44101 Contributed by George A. Olah, July 18, 1977 ABSTRACT The mercuration and thallation of benzene and iments at 250 and quenched the mixtures after 5 min, whereas substituted benzenes was studied with mercuric and thallic Brown and Nelson carried out the reaction for 6.5 hr. In sub- trifluoroacetate, respectively, in trifluoroacetic acid. With the in view of the importance of both the mechanistic shortest reaction time (1 sec) at 00, the relative rate of mercu- sequent work, ration of toluene compared to that of benzene was 17.5, with and practical implications of the orientation-rate correlation, the isomer distribution in toluene of: ortho, 17.4%; meta, 5.9%; Brown and McGary (7), carried out a more detailed study of and para, 76.7%. The isomer distribution in toluene varied with the mercuration reaction. They concluded: "A redetermination the reaction time, significantly more at 25° than at 00. The of the isomer distributions and relative rates indicates excellent competitive thallation of benzene and toluene with thallic tri- agreement with the linear relationship of orientation and rel- fluoroacetate in trifluoroacetic acid at 150 showed the relative ative however, that the isomer distri- rate, toluene/benzene, to be 33, with the isomer distribution in rate." They recognized, toluene of: ortho, 9.5%; meta, 5.5%; and para, 85.0%. With butions change with time but still claimed that the corrected increasingly higher reaction temperatures in both mercuration meta isomer distribution in the mercuration of toluene is 9.5 and thallation reactions of aromatics, isomerization (both in- + 0.5% and remarked, "there remains a relatively minor dis- tramolecular and intermolecular) within the relevant ortho- and agreement with the values reported by Klapproth and West- para-metallated intermediate ions and/or of the isomers be- heimer (6% meta) for which we are unable to account." comes more important. Competitive rates and isomer distri- In we have that, regardless of butions of mercuration and thallation of benzene and substi- preceding work (8) suggested tuted benzenes were also determined. The predominant para the substrate selectivity in electrophilic aromatic substitutions substitution in both mercuration and thallation of methylben- (which generally can be most conveniently expressed by kT/kB zenes reflects, besides some steric factors, the strong stabilizing rate ratios), the regioselectivity (positional) of the reactions effect of para methyl groups on the arenium ion intermediates. remains high, with usually predominant ortho-para substitu- Under predominantly kinetically controlled conditions, no tion, the amount of meta isomer being 2-5%. anomalous increase in the amount of meta substitution was We considered it possible that the reported relatively high observed. meta isomer ratio observed in preceding studies of mercuration The mercuration of aromatic hydrocarbons with mercuric of toluene (as well as related alkylbenzenes) was still the result trifluoroacetate in trifluoroacetic acid at 250 has been studied, of ongoing faster isomerization and/or disproportionation of predominantly by Brown and coworkers (1-3). The rate ratio the ortho and para isomers, compared to the more stable meta of toluene to benzene (kT/kB) was found to be 9.89 and the isomer (7) or related rearrangements in the a-complex inter- isomer distribution in toluene was: ortho, 12.2%; meta, 8.6%; mediates of the reactions. These would fully explain the dis- and para, 79.2%. From these results, as well as from their pre- agreements with Klapproth and Westheimer's work. Conse- ceding studies in acetic acid solution with perchloric acid cat- quently, it appeared desirable to reinvestigate in detail the alyst giving kT/kB = 3.6-7, and 12-16% meta isomer, it was mercuration of benzene and substituted benzenes under pre- concluded that the relatively high meta isomer ratio observed dominantly kinetically controlled conditions, minimizing the was due to the high electrophilic reactivity of the mercurating possibility of isomerization and disproportionation affecting agent causing the observed low selectivity substitutions. These the results. results were correlated by the so-called Brown selectivity re- The thallation of benzene and substituted benzenes with lationship (for a review, see ref. 4). thallium trifluoroacetate in trifluoroacetic acid, the mechanism It should be pointed out that, in their original paper on of which was considered to be similar to that of related mer- mercuration of toluene and benzene, Brown and Nelson, (5) curations (9-11), was also studied under similar conditions. extending the claimed quantitative selectivity relationship MATERIALS AND METHODS governing isomer distributions in aromatic substitution, com- mented on the preceding work of Klapproth and Westheimer Materials. Benzene, toluene, and alkylbenzenes were of (6) who found that, in glacial acetic acid with perchloric acid highest available purity. Trifluoroacetic acid (Cationics, Inc.) as catalyst, toluene was mercurated by mercuric acetate at 250 was obtained in sufficient purity to be used without further to give 6% of the meta isomer. Brown and Nelson claimed to purification (boiling point 730). Mercuric and thallium triflu- have observed 12% meta isomer under similar conditions and oroacetate were commercial materials (Aldrich Chemical Co., stated "we are, however, unable to account for the differences Inc.). between the values of the meta isomer." In the former case (6), a precise radiochemical technique was used in establishing Abbreviation: kT/kB, ratio of reaction rates for toluene and ben- orientation, whereas infrared analysis was utilized in the latter. zene. * Paper no. 40 in the series "Aromatic Substitution." Paper no. 39 was Comparison of the experimental sections of Brown and Nelson's Olah, G. A., Pelizza, F. Kobayashi, S. & Olah, J. A. (1976) J. Am. and Klapproth and Westheimer's papers, however, indicates Chem. Soc. 98,296. that the claim of reinvestigation "under similar conditions" is t Visiting Scientist from the Wakayama Technical College, Japan. incorrect. Klapproth and Westheimer carried out their exper- * Senior Research Associate (1974-1975). 4121 Downloaded by guest on October 2, 2021 4122 Chemistry: Olah et al. Proc. Natl. Acad. Sci. USA 74 (1977) r - - -- 1 Table 1. Gas/liquid chromatographic parameters Conditionst of retention Bromo compound Column* Column Time, min E ~G Bromobenzene D I 5.9 Quench o-Bromotoluene D I 10.0 m-Bromotoluene D I 11.0 p -Bromotoluene D I 12.3 FIG. 1. Flow-quenching apparatus for the mercuration of o-Bromoethylbenzene D I 10.3 polymethylbenzenes. Two 50-ml syringes (A and B) are driven by a m-Bromoethylbenzene D I 12.0 syringe pump (C; Sage Instruments, model 351). The separate solu- p-Bromoethylbenzene D I 14.2 tions came together via two flexible Teflon tubes (D and E, inside o-Bromoisovronvlbenzene D I 11.7 diameter 1.4 mm; length, 60 cm), mix at F, react during passage G, and D I 13.8 are quenched at the end of G. p-Bromoisopropylbenzene D I 18.3 o-Bromo-tert- butylbenzene D I 16.2 Competitive Mercuration of Benzene and Substituted m-Bromo-tert- butylbenzene D I 19.3 Benzenes. Mercuric trifluoroacetate (25 ml of 0.2 M solution p-Bromo-tert-butylbenzene D I 22.3 in trifluoroacetic acid) was added rapidly to 25 ml of 1 M so- 4-Bromo-o-xylene C I 2.9 lutions of the aromatics in the same solvent with vigorous stir- 4-Bromo-m-xylene C I 2.7 ring, both precooled and kept at 00. After specific time inter- 2-Bromo-p-xylene C I 3.0 vals, the reaction mixture was quenched with aqueous sodium Bromoesitylene C III 3.2 bromide, maintaining a bromide ion to mercuric ion ratio of 4-Bromo-1,2,3-trimethylben- 3:1. The precipitated arylmercuric bromides were filtered and zene C I 5.1 thoroughly dried under reduced pressure. Then, the mixture 5-Bromo-1,2,4-trimethyl- of arylmercuric bromides was slowly treated with bromine in benzene C I 4.8 chloroform at 00 until a permanent red color developed. Stir- 5-Bromo-1,2,3,4-tetramethyl- ring was continued for 5 hr at 0° and then for an additional hour benzene C II 4.5 at room temperature. Conversion of arylmercuric bromides to 4-Bromo-1,2,3,5-tetramethyl- the corresponding bromoarenes under these conditions was benzene C II 4.5 found to be quantitative for all practical purposes (the slower Bromodurene C III 6.6 conversion of the parent phenylmercuric bromide without Bromopentamethylbenzene C III 17.8 to is not o-Bromofluorobenzene B IV 12.2 raising the temperature ambient always quantitative). m-Bromofluorobenzene B IV 9.8 The reaction mixture was washed with sodium bisulfite solution p-Bromofluorobenzene B IV 10.5 and water and then dried over sodium sulfate. Bromoarenes o-Bromochlorobenzene B V 15.1 were subsequently analyzed by gas/liquid chromatography. m-Bromochlorobenzene B V 13.0 Competitive Mercuration of Polymethylbenzenes. The p-Bromochlorobenzene B V 13.2 rates of mercuration of polymethylbenzenes are so fast that it o-Dibromobenzene B V 25.8 is difficult to carry out the reactions by the usual methods. m-Dibromobenzene B V 21.9 Therefore, a modified flow-quench apparatus (12) was used p-Dibromobenzene B V 22.2 to allow short reaction times (of the order of 1 sec) (Fig. 1). o-Bromoanisole A VI 10.9 Quenching was with aqueous sodium bromide surrounded by m-Bromoanisole A VI 9.6 ice.
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