3176
J. Org. Chem. 1997, 62, 3176-3182
Formation of Bromocarbenium Bromide Ion Pairs in the Electrophilic Bromination of Highly Reactive Olefins in Chlorinated Aprotic Solvents† Giuseppe Bellucci, Cinzia Chiappe,* and Giacomo Lo Moro Dipartimento di Chimica Bioorganica, via Bonanno 33, 56126 Pisa, Italy Received November 1, 1996X
The kinetics and the products of bromination of several substituted stilbenes with tetrabutylammonium tribromide (TBAT) have been investigated in aprotic solvents at different temperatures. Stilbenes bearing electron-withdrawing or moderately electron-donating substituents gave stereospecifically the anti addition products. The reactions followed a second-order rate law, and an inverse kinetic isotope effect (KIE), kH/kD ) 0.85(0.05), was found for the bromination of cis-stilbene. The reactions of cis- and trans-4,4-dimethoxystilbenes yielded mixtures of meso and d,l dibromides both in chloroform and 1,2-dichloroethane. The rate constants (kBr3-) measured for the latter olefins deviated considerably from the Hammett correlations, and added bromide had a significant effect on the rates. The reactions of these activated stilbenes with molecular Br2, carried out at low Br2 concentration, followed a mixed second/third-order rate law. The kinetic and product distribution data for the reaction, with TBAT, of stilbenes bearing electron-withdrawing or moderately electrondonating substituents are interpreted on the basis of the known mechanism involving a productand rate-determining nucleophilic attack by bromide on the olefin-Br2 π-complex. The data related to the bromination of the more activated methoxystilbenes are rationalized considering that, for these olefins, even in aprotic solvents, the ionization of the initially formed 1:1 π-complex to a bromocarbenium bromide ion pair can compete both with the formation of a bromonium-tribromide ion pair and with the nucleophilic attack by Br-. For this second-order process (first order in Br2), the kinetic constants and the activation parameters have been measured in chloroform and 1,2dichloroethane and the activation parameters have been compared with those related to the thirdorder Br2 addition and to the reaction with TBAT. Introduction Sixty years after the postulation of bromonium ions as the intermediate of the electrophilic bromination of olefins by Roberts and Kimball,1 important features concerning the formation of transient species are still under discussion and the object of extensive investigations.2 Not only has the involvement of olefin-Br2 π-complexes prior to the rate-determining step leading to the ionic intermediate been established3 but also the nature of the interaction has recently been revealed.4 Furthermore, evidence for the reversibilty of the ionization step has been presented, and the factors affecting this process have been extensively discussed.5 The effect †Dedicated to the memory of Professor Giuseppe Bellucci (d. March 3, 1996). X Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Roberts, I.; Kimball, G. E. J. Am. Chem. Soc. 1937, 59, 947. (2) (a) Schmid, G. H. The Chemistry of Double-Bonded Functional groups; Patai, S., Ed.; Wiley: New York, 1989; Suppl. A, Vol. 2, part 1, p 699. (b) Ruasse, M. F. Adv. Phys. Org. Chem. 1993, 28, 207. (3) Bellucci, G.; Bianchini, R.; Ambrosetti, R. J. Am. Chem. Soc. 1985, 107, 2464. (4) Bellucci, G.; Bianchini, R.; Chiappe, C.; Lenoir, D.; Herges, R. J. Am. Chem. Soc. 1995, 117, 12001. (5) (a) Brown, R. S.; Gedye, R.; Slebocka-Tilk, H.; Buschek, J. M.; Kopecky, K. R. J. Am. Chem. Soc. 1984, 106, 4515. (b) Bellucci, G.; Chiappe, C.; Marioni, F. J. Am. Chem. Soc. 1987, 109, 515. (c) Bellucci, G.; Bianchini, R.; Chiappe, C.; Marioni, F.; Spagna, R. J. Am. Chem. Soc. 1988, 110, 546. (d) Bellucci, G.; Chiappe, C.; Marioni, F.; Marchetti, F. J. Phys. Org. Chem. 1991, 4, 387. (e) Bellucci, G.; Bianchini, R.; Chiappe, C.; Brown, R. S.; Slebocka-Tilk, H. J. Am. Chem. Soc. 1991, 113, 8012. (f) Bellucci, G.; Bianchini, R.; Chiappe, C.; Ambrosetti, R.; Catalano, D.; Bennet, A. J.; Slebocka-Tilk, H.; Aartsand, G. H. M.; Brown, R. S. J. Org. Chem. 1993, 58, 340. (g) Zheng, C. Y.; Slebocka-Tilk, H.; Nagorski, R. W.; Alvarado, L.; Brown, R. S. J. Org. Chem. 1993, 58, 2122. (h) Bellucci, G.; Bianchini, R.; Chiappe, C.; Gadgil, V. R.; Marchand, A. P. J. Org. Chem. 1993, 58, 3575. (i) Bellucci, G.; Bianchini, R.; Chiappe, C.; Lenoir, D.; Attar, A. J. Am. Chem. Soc. 1995, 117, 6243.
S0022-3263(96)02052-X CCC: $14.00
of the addition of bromide ion in protic and aprotic solvents has also been investigated.2,6 At least three alternative nonradical mechanistic pathways, all of which involve olefin-Br2 π-complexes, were identified for the bromination of olefins (Scheme 1). In particular the first pathway (path a) has been proposed for the reaction with molecular bromine in aprotic solvents of modest polarity and consists in the ionization of a 1:2 π-complex to give a bromonium-bromocarbenium tribromide ion pair intermediate, which then collapses to dibromide and molecular Br2.3,5b,6,7 This pathway, involving a second-order dependence of the rate on Br2, has been clearly demonstrated for brominations in several chlorinated hydrocarbon solvents. A similar ionic mechanism, where a solvent-assisted bromine-bromine bond breaking occurs in a 1:1 π-complex and the reaction is first-order in Br2 (path b), has been established for the bromination at low Br2 concentrations in hydroxylic solvents8,9 which provide specific electrophilic solvation by hydrogen bonding to the leaving bromide ion.10 At higher Br2 concentration, a third-order process of type a, has been reported also in acetic acid.9b In the presence of added bromide salts, which in low polarity nonprotic solvents bind Br2 as the highly stable (6) Bellucci, G.; Bianchini, R.; Ambrosetti, R.; Ingrosso, G. J. Org. Chem. 1985, 50, 3313. (7) Modro, A.; Schmid, G. H.; Yates, K. J. Org. Chem. 1977, 42, 3673. (8) Dubois, J.-E.; Mouvier, G. Tetrahedron Lett. 1963, 1325. Dubois, J.-E.; Mouvier, G. Bull. Soc. Chim. 1968, 1426. (9) Roston, J. H.; Yates, K. J. J. Am. Chem. Soc. 1969, 91, 1469. Yates, K.; McDonald, R. S.; Shapiro, S. A. J. Org. Chem. 1973, 38, 2460. (10) Garnier, F.; Donnay, R. H.; Dubois, J.-E. Chem. Commun. 1971, 829. Modro, A.; Schmid, G. H.; Yates, K. J. Org. Chem. 1979, 44, 4221.
© 1997 American Chemical Society
Formation of Bromocarbenium Bromide Ion Pairs Scheme 1
Table 2. Product Distribution for the Bromination of 1a and 2a with Br2 and TBAT in DCE and Chloroform at 25 °C
Table 1. Rate Constants for the Bromination of Stilbenes 1b-g and 2b-g in DCE with TBAT at 25 °C olefin
kBr3- (M-1 sec-1)
3:4
1a 1b 1c 1d 1e 1f 1g 2a 2b 2c 2d 2f 2g
6.00(0.05) × 10-2 3.30(0.05) × 10-3 7.90(0.1) × 10-4 3.64(0.05) × 10-4 3.20(0.05) × 10-4 1.35(0.05) × 10-4 6.25(0.1) × 10-5 3.30(0.05) × 10-2 1.05(0.05) × 10-3 3.20(0.05) × 10-4 2.30(0.07) × 10-4 9.95(0.15) × 10-5 6.20(0.1) × 10-5
a >99:1 >99:1 >99:1 >99:1 >99:1 >99:1 a 99.5%), and chloroform (Fluka > 99.5%) were used as supplied. Olefins. cis- and trans-stilbenes 1a-g and 2a-g were synthesized by the Wittig reaction as previously reported.17 4,3′-Bis(trifluoromethyl)-1,1-diphenylethylene(L ) H) was prepared by reacting methylmagnesium bromide with 4,3′-bis(17) Bellucci, G.; Chiappe, C.; Lo Moro G. Tetrahedron Lett. 1996, 37, 4225.
3182 J. Org. Chem., Vol. 62, No. 10, 1997 (trifluoromethyl)benzophenone, followed by dehydration with p-toluenesulfonic acid in refluxing benzene. 1,1-Diphenylethylene (L ) D) and 4,3′-bis(trifluoromethyl)-1,1-diphenylethylene (L ) D) were obtained with the same procedure using (methyl-d3)magnesium iodide. 1H NMR analysis indicated the presence of ca. 2% of H at the vinylic position. Olefin 1a (L ) D) was prepared by reduction of the 1,2-diphenylacetylene with LiAlD4.18 1H NMR analysis indicated the presence of ca. 10% of H at the vinylic positions. Commercial 1,1-diphenylethylene (Aldrich, 97%) and cis-stilbene (Aldrich >97%) were distilled before use. All olefins were finally checked by HPLC and NMR and were found to be >99% pure. Bromination Procedure: Product Distribution. 1,2Dichloroethane or chloroform solutions of Br2 (1 mL, 10-2 M) or TBAT (1 mL, 10-2 M) were rapidly mixed with 1 mL of 10-2 M solutions of olefins 1a or 2a in the same solvent, and the reaction mixtures were stored in the dark at 25 °C. At preset times samples were withdrawn and stopped by washing with saturated aqueous NaHSO3, and the mixtures were analyzed by HPLC and NMR. The 3:4 ratios were determined using appropriate calibration curves. All the reactions were carried out in triplicate. The ratios reported in Tables 1 and 2 were reproducible within (2%. The stability of dibromides 3a and 4a in the presence of the halogen and TBAT was checked by exposing all dibromides to Br2 and TBAT under conditions identical with those employed in the bromination reactions, following by HPLC analysis. 3d: mp 119-121 °C; 1H NMR δ 5.35 (s, 2H, CH2Br), 7.307.45 (AA'BB' system, 10H, aromatic protons). Anal. Calcd for C14H10Cl2Br2: C, 41.12; H, 2.46. Found: C, 41.03; H, 2.45. 3e: mp 161-163 °C; 1H NMR δ 5.25 (s, 2H, CH2Br), 7.007.30 (m, 10H, aromatic protons). Anal. Calcd for C14H10Br4: C, 33.78; H, 2.02. Found: C, 33.80; H, 2.05. 3f: mp 100-101 °C; 1H NMR δ 5.33 (s, 2H, CH2Br), 7.307.50 (m, 10H, aromatic protons). Anal. Calcd for C14H10Cl2Br2: C, 41.12; H, 2.46. Found: C, 41.15; H, 2.50. 3g: mp 210-211 °C (lit.5e mp 210-211 °C). 4d: mp 236-238 °C; 1H NMR δ 5.41 (s, 2H, CH2Br), 7.107.20 (AA′BB′ system, 10H, aromatic protons). Anal. Calcd for C14H10Cl2Br2: C, 41.12; H, 2.46. Found: C, 41.20; H, 2.37. 4e: mp 256-258 °C; 1H NMR δ 5.32 (s, 2H, CH2Br), 7.007.30 (m, 10H, aromatic protons). Anal. Calcd for C14H10Br4: C, 33.78; H, 2.02. Found: C, 33.67; H, 2.05. (18) Ashby, E. C.; Lin, J. J. J. Org. Chem. 1978, 43, 2567.
Bellucci et al. 4f: mp 208-210 °C; 1H NMR δ 5.37 (s, 2H, CH2Br), 7.307.45 (m, 10H, aromatic protons). Anal. Calcd for C14H10Cl2Br2: C, 41.12; H, 2.46. Found: C, 41.18; H, 2.35. 4g: mp 237-238 °C (lit.5e mp 237-238 °C). Kinetic Measurements and Product Analysis. 1,2Dichloroethane or chloroform Br2 solutions, prepared shortly before use, were protected from daylight and adjusted to twice the desired initial concentrations in the kinetic runs. Aliquotes of these prethermostated solutions were mixed with equal volumes of prethermostated solutions of olefins 1a or 2a of suitable concentrations. The following olefin and Br2 concentrations (molar), pathlength (centimeters), and monitored wavelengths (nanometers) were used. 1a: 5 × 10-3-5 × 10-4 and 5 × 10-3-5 × 10-4, 2, 480, and 410. 2a: (5-2.5) × 10-1 and 2.5 × 10-2, 0.1, 410. The brominations with TBAT of 1bg, 2b-g, and 3a-c were carried in the conventional spectrophotometer using the following olefin and TBAT concentrations (molar): olefins 10-2-10-1 M, TBAT 10-2-10-1 M. The absorbance/time data were fitted to the appropriate secondorder or pseudo-first-order rate law. The rate constants for the bromination of 1a and 2a with Br2 in the presence of TBAB were measured both in DCE and chloroform at three different temperatures, 10 ( 0.05, 25 ( 0.05, and 40 ( 0.05 °C, using the following reagent concentrations: olefin 10-2 M, Br2 10-3 M, and TBAB between 1 × 10-2 and 10-1. The temperature inside the reaction mixtures was periodically measured as precise as possible by inserting a termocouple into the cell. The kinetic constants k2 and kBr3were obtained by plots of kobsd(1 + K[Br-]) against the added [Br-]. All reactions were carried our at least in triplicate. The kinetic constants and the activation parameters are reported in Tables 1 and 3.
Acknowledgment. This work was supported in part by grants from Consiglio Nazionale delle Ricerche (CNR, Roma) and Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (MURST, Roma). We thank Prof. M.-F. Ruasse for the helpful discussion. Supporting Information Available: Tables of kinetic constants measured in each experiment and some kinetic and Arrhenius plots (11 pages). This material is contained in libraries on microfiche, immediately follows this article in the microfilm version of the journal, and can be ordered from the ACS; see any current masthead page for ordering information. JO9620526
