J. Am. Chem. SOC. 1992,114,1113-1179 per host are indicated on a difference electron density map. Position parameters have been assigned for only 30 of the solvent atoms. The hemicarcerand is in the wrapped conformation. The distance betwecn the planes of the four oxygen atoms of each cavitand component is about 2.4 A. The angle between normals to these two least-squares planes is 0.6O. The four oxygen atoms are h0.w A from the leastsquares plane through these four atoms. The distance between the oxygen atoms within a xylyl bridge ranges from 2.76 to 2.95 A. The approximatesquares formed by the four oxygen atoms of each cavitand
1113
are twisted with respect to each other by about 21° (defined by &square midpoint-square midpoint*). supplemenCuy M.terinl Available: Experimental details of the Crystal StrUctUredetermination, including tables of atom positions and their thermal parameters and bond lengths and angles (13 pages). Ordering information is given on any current masthead
page.
Electrophilic Addition of Br2 to Olefins in the Presence of Nucleophilic Trapping Anions. Implications for the Lifetimes of Bromonium Ion Intermediates Produced from Electrophilic Bromination of Olefins in Methanol R. W. Nagorski and R. S. Brown* Contribution from the Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2. Received March 20, I992
AbsbPct: The product ratios for Br, or NBS additions to cyclopentene,cyclohexene,tetramethylethylene, and styrene in MeOH
containing varying concentrations of added N3- or Br- have been determined with an aim of determining the lifetimes of the bromonium ion intermediates. On the basis of the ratio of trans bromo azide to methoxy bromide products, the partitioning for the four olefins are 5.9,4.9,9.3, and 2.1 M-I, respectively. That the far better nucleophile rate constant ratios (kN,/kCHjOH) (N3-) does not lead to a marked increase in product formation relative to solvent suggests that both species capture a highly reactive intermediate in a non-activation-limited process. Assuming that the N3- reacts with the intermediate with a diffusion-limited rate constant of 1Olo M-l s-I, the respective lifetimes of the ions produced from bromination of the four olefins s, respectively. On the basis of existing comparisons, these values are 5.9 X 1O-Io, 5.0 X 1O-Io,9.3 X 1O-Io, and 2.7 X indicate the following: the cyclic olefins produce ions that live about 100 times longer than a secondary carbocation; tetramethylethylene gives a bromonium ion that lives 10 times longer than a tertiary cation; and styrene gives an ion (bromonium or @-bromocation) that is -40-fold longer lived than the 1-phenylethylcation. In the case of Br, or NBS addition to cyclohexene in the presence of varying [Br-1, the ratio of the trans dibromide to methoxy bromide product tends to zero as [Br-] 0. This indicates that the trans dibromide cannot be formed by ion pair collapse. The solvolysis of trans-2-bromocyclohexyl trifluoromethanesulfonate in MeOH containing N3-or Br- produces significantly less azide or bromide capture product than does electrophilic addition of Br, or NBS to cyclohexene under the same conditions, suggesting that the ions produced in the two cases are not identical.
-
Introduction The electrophilic bromination of olefins is a well-studied reaction' that has recently attracted renewed interest. The generally accepted mechanism is given in eq 1. In protic solvents at low [Br2], the reaction is first order in [Br,] and proceeds via a 1:l charge-transfer complex (CTC). At higher [Br,] or in nonprotic
Br2
It
2:1 CTC
solvents, terms second order in [Br,] are seen and interpreted as a Br,-assisted ionization of the 1:l CTC (perhaps via a 2:l CTC) to produce a bromonium ion tribromide.2 The more recent work (1) (a) Schmid, G. H.; Garrat, D. G. The Chemistry of Double-Bonded Functional Groups; Patai, S., Ed.; Wiley: New York, 1977; Suppl. A, Part 2, p 725. (b) De la Mare, P. B. D.; Bolton, R. Electrophilic Additions to UmaturatedSystems, 2nd cd.;Elsevier: New York, 1982; pp 136-197. (c) V'yunov, K.A.; Guniak, M. I. Russ. Chem. Reu. (Engl. Transl.) 1981, 50, 151-163. (d) Ruasse, M. F. Acc. Chem. Res. 1990, 23, 87.
-
has centered on structural and NMR characterization of a stable bromonium ion,3determining the circumstances where reversible formation of the ions occur^^^^ and determining the importance of nucleophilic solvent assistance in br~mination.~' In some cases, bromonium ions can be studied by NMR under stable ion conditi~ns.~ However, because of the general instability of three-membered bromonium ions toward nucleophilic attack, very little information exists about the characteristics of these ions in solution. One of these is the lifetime, about which virtually no data exist other than the fact that the ion must live sufficiently long to allow a translocation of Br- from where it is formed after the ionization of the CTC to where it attacks the carbon to form (2) Brown R. S.;Slebocka-Tilk,H.; Bennet, A. J.; Bellucci, G.;Bianchini, R.; Ambrosetti, R. J. Am. Chem. Soc. 1990,112,6310 and reference therein. (3) (a) Slebwka-Tilk,H.; Ball, R. G.; Brown, R. S.J . Am. Chem. Soc. 1985,107,4505. (b) Bennet, A. J.; Brown, R. S.;McClung, R. E. D.; Aarts, G. M.; Klobukowski, M.; Santarsiero, B. D.; Bellucci, G.; Bianchini, R. J. Am. Chem. SOC.1991, 113, 8532. (4) (a) Brown, R. S.;Gedye, R.; Slebocka-Tilk, H.; Buschek, J.; Koptcky, 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.; Bianchini, R.; Chiappe, C.; Marioni, F.; Ambrosetti, R.; Brown, R. S.;Slebocka-Tilk, H. J. Am. Chem. Soc. 1989, 1 1 1 , 2640. (e) Ruasse, M.-F.; Motallebi, S.;Galland, B.; Lomas, J. S.J . Org. Chem. 1990, 55, 2298. (0 Ruasse, M.-F.; Motallebi, S.;Galland, B. J. Am. Chem. Soc. 1991, 113, 3440. (5) Olah, G. A. Halonium Ions; Wiley Interscience: New York, 1975 and references therein.
0002-1863/92/15 14-1113%03.00/0 0 1992 American Chemical Society
Nagorski and Brown
1114 J. Am. Chem. SOC., Vol. 114, No. 20, 1992
a trans dibromide product. In the case of the bromination of cis-stilbene in HOAc or ClCH2CH2Cl, the intermediate ions (bromonium or 8-bromocarbocation) must last long enough to allow rotation about the central C-C bond and reejection of Br+ to form isomerized starting materiaL6 We seek in this report to determine whether information about the lifetime of bromonium in solution can be probed utilizing the azide "clock" methodology developed by Jencks and Co-workers.'** There, partitioning of a cationic intermediate between solvent capture and capture by an added nucleophile (Ny) that reacts with R+ at the diffusion limit is determined from quantitative assessment of the product ratios as a function of [N,-]. We report herein that it can be applied to the bromonium ion problem by consideration of the competing product-forming steps given in eq 2.
I IkB, cBr-1
B
1
2
In what follows, we report such studies for the bromonium ions cyclohexene, cyclopentene, and tetramethylethylene, as well as that of the ion (bromonium or 8-bromocarbocation) of styrene in MeOH. In addition, we provide evidence that in MeOH the collapse of intimate or solvent-separatedbromonium-Br- ion pairs to form dibromide does not represent a significant route to products. Finally, we consider the products formed from the solvolysis of the trans-Zbromotriflate of cyclohexane in MeOH containing Br- and N c .
Experimental Section (a) General. Routine NMR and IR spectra were obtained with Bruker WP-80 NMR and Nicolet FTIR machines; 200-MHz NMR spectra were obtained with a Bruker WP-200 machine. HPLC analysis of the product mixtures was performed with a Hewlett-Packard 1081A HPLC utilizing a Waters RCM 8 X 10 radial compression module containing a Waters IGcm C-18 p Bondapak 8 mm X 100 mm column and Waters Differential Refractometer R401 detector. The eluent was 70:30 methanol/H20 for all systems except tetramethylethylene where the eluent was 55:45 methanol/H20. GLPC analysis of the product mixtures from solvolysis experiments was done with a Hewlett-Packard in. 10%DEGS on 80-100 5830A gas chromatograph using a 6 ft X mesh Chromosorb W column. Temperature 1 = 100 OC for 3 min, programmed at AT = 9 "C/min to 180 OC; He flow = 38 cc/min. (b) Materials. trans-l,2-Dibromocyclohexane,9trans-l,2-dibromocyclopentane,9 2,3-dibromo-2,3-dimethylb~tane,'~ and 1,2-dibromo-lphenylethane" were prepared and purified as described. truns-lBromo-2-methoxycyclohexaneL2and trans-1-bromo-2-methoxycyclopentane" were prepared by a method outlined by Erickson and Kimi4 ~~~~
~
(6) (a) Bellucci, G.; Bianchini, R.; Chiappe, C.; Brown, R. S.; SlebockaTilk, H. J. Am. Chem. SOC.1991, 113, 8012. (b) Buckles, R. E.; Bader, J. M.;Thurmaier, R. J. J. Org. Chem. 1%2,27,4523. (c) Yates, K.; McDonald, R. S. J. Org. Chem. 1973, 38, 2465. (7) (a) Richard, J. P.; Rothenberg, M. E.; Jencks, W. P. J . Am. Chem. Soc. 1984, 106, 1361. (b) Richard, J. P.; Jencks, W. P. J . Am. Chem. SOC. 1984,106, 1373. (c) Richard, J. P.; Jencks, W. P. J . Am. Chem. Soc. 1984, 106, 1383. (d) Richard, J. P.; Jencks, W. P. J . Am. Chem. Soc. 1984, 106, 1396. (e) Young, P. R.; Jencks, W. P. J. Am. Chem. Soc. 1977, 99,8238. (f) Amyes, T. L.;Jencks, W. P. J . Am. Chem. Soc. 1989, 1 1 1 , 7888. (g) Jencks, W. P. Acc. Chem. Res. 1980, 13, 161. (8) (a) Richard, J. P.; Amyes, T. L.; Bei, L.;Stubblefield, V. J . Am. Chem. Soc. 1990, 112, 9513. (b) Amyes, T. L.; Richard, J. P. J . Am. Chem. SOC. 1990, I 12, 9507. (9) Weinstock, J.; Lewis, S. N.; Bordwell, F. G. J . Am. Chem. SOC.1956, 78, 6072. (10) Dubois, J. E.; Chretien, J. R. J . Am. Chem. SOC.1978, 100, 3506. (11) Rolston, J.; Yates, K. J . Am. Chem. Soc. 1%9,91, 1469, 1477, 1483. (12) Winstein. S.; Henderson, R. B. J . Am. Chem. SOC.1943, 65, 2196. (13) Kitching, W.; Olszowy, H. A.; Harvey, K. J . Org. Chem. 1982, 47, 1893. (14) Erickson, K. L.; Kim, J. L. J . Org. Chem. 1971, 36, 295.
and purified by vacuum distillation. 2-Bromo-3-methoxy-2,3-dimethylbutaneLswas prepared analogo~sly'~ and was purified by spinning band distillation using a water aspirator (bp 49 OC, -20 Torr; lit.I5 bp 52 OC, 17 Torr). 2-Bromo-1-methoxy-1-phenylethanei6 was preparedL4and purified by distillation (bp 50 OC, 0.05 Torr; liti6 bp 52 OC, 0.05 Torr). All bromo azides were prepared by a general procedure described by Van Ende and Krief." The crude materials were purified by column chromatography using silica gel (70-230 mesh) and an eluent of petroleum ether. The last traces of hydrocarbon solvent were removed by freeze-thaw cycles under 0.5 Torr vacuum. So prepared were trans-1azid0-2-bromocyclohexane,'~trans-1-azido-2-bromocyclopentane[IH NMR (CDCI,, 200 MHz) 6 4.11 (m, 2 H), 2.40-1.60 (m, 6 H); FTIR (neat) 2104 cm-l; exact mass calcd for CSHBBrN3188.9901, found 188.99001, and 2-azido-3-bromo-2,3-dimethylbutane19[IH NMR (CDCI,, 200 MHz) 6 1.79 (s, 6 H), 1.49 (s, 6 H); exact mass calcd for C6HI2Br(M+ - N3) 165.0102, 163.0122, found 165.0102, 163.0107. Anal. Calcd for C6HI2BrN3:C, 34.97; H, 5.87; N, 20.39; Br, 38.77. Found: C, 34.95; H, 5.94; N, 20.31; Br, 38.701. 2-Azido-1-bromo-1-phenylethane and 1-azido-2-bromo-1-phenylethane were preparedL7and purified by column chromatography as an inseparable 8:s mixture (by 'H NMR) of isomers: exact mass calcd for C8HliBrN, 226.9881, found 226.9882. Anal. Calcd for C8H8BrN,: C, 42.50; H, 3.57; N , 18.59. Found: C, 42.64; H, 3.45; N, 18.68. The identities of the two regioisomers were ascertained by comparison with 'H NMR data given by Boerwinkle and Hassner." 2-Azido-lbromo-I-phenylethane: 'H NMR (CDCI,, 200 MHz) 6 7.40 (m, 5 H), 5.00 (t, 1 H, J = 7.6 Hz), 3.88 (d, 2 H, J = 7.6 Hz). l-Azido-2bromo-1-phenylethane: IH NMR (CDCI,, 200 MHz) 6 7.40 (m, 5 H), 4.76 (t, 1 H, J = 7.6 Hz), 3.55 (d, 2 H, J = 7.6 Hz). trans-2-Bromocyclohexyltrifluoromethanesulfonate (I) was prepared several times as needed from the corresponding trans-2-bromocyclohexano12' (1-2 g) in a solution of dry CH2C12 (10 mL) containing equimolar pyridine cooled to 0 "C in an ice bath.22 To this was added dropwise 10-2076 excess of neat trifluoromethanesulfonic anhydride at such a rate that the temperature did not exceed 5 OC. Immediately after the addition, the reaction mixture was washed with two 2-mL aliquots of ice cold 0.01 N HCI solution, and then the CH2CI2layer was dried over MgS04 in the cold, filtered through glass wool, and immediately placed in a stoppered flask in a dry ice chest. The material is stable at -78 OC for several days, but becomes discolored when kept above 0 "C for prolonged periods: 'H NMR (CD,CI,) 6 5.0-4.8 (m, 1 H), 4.2-4.0 (m, 1 H), 2.5-2.3 (m, 2 H), 2.0-1.6 (m, 4 H), 1.5-1.2 (m, 2 H).3b The material so-produced is -90% pure by 'H NMR, the remaining 10% being the 2-bromocyclohexanolthat is probably formed by hydrolysis of I during the aqueous workup. (c) Bromination. Reagent grade methanol (Mallinckrodt) was purified by distillation from CaH, and used within 1 day. Bromine (Fluka), cyclohexene (BDH), 2,3-dimethyl-2-butene (Aldrich), and styrene (Fischer) were used as supplied. Cyclopentene was prepared by dehydration2, of cyclopentanol and purified by distillation (bp 44 OC, ambient pressure). NBS (MCB) was recrystallized from water. A typical bromination was conducted under conditions where the olefin and brominating agent'were maintained at as low a concentration as possible. A solution of NaN, (0.03-0.4 M) in MeOH (minimum volume of 70 mL) was placed in a three-necked flask equipped with a magnetic stir bar and two addition funnels. The olefin ((3.5 X 10-4)-(9.7 X lo-,) mol) in 1.5-3.0 mL of MeOH was placed in one addition funnel, and an equimolar amount of Br, or NBS in a similar amount of MeOH was placed in the other. (The relative concentrations of olefin (01) and azide were chosen such that [NJ 1 lO[ol], and for purposes of calculation the [N,-] was determined at the final volume which includes all added MeOH.) The mixture was vigorously stirred, and both the olefin and Br, solutions were added dropwise over a period of several minutes at such a rate that the Br, color was discharged before the next drops were added. All reactions were conducted at ambient temperature (22 2 " C ) . Control experiments at temperatures of 15.5 and 27.5 OC
*
(15) Kuivila, H. G.;Choi, Y . M. J . Org. Chem. 1979, 44, 4774. (16) Janda, M.; Srogl, J.; Holy, P.; Nemec, M. Z . Chem. 1977, 17, 226. (17) Van Ende, D.; Krief, A. Angew. Chem., Int. Ed. Engl. 1974,13,279. (18) Maxa, E.; Zbiral, E.; Schulz, G.; Haslinger, E. Justus Liebigs Ann. Chem. 1975, 9, 1705. (19) Lion, C.; Boukou-Poba, J.-P.; Saumtally, 1. Bull. SOC.Chim. Belg. 1987, 96, 711. (20) Boerwinkle, F.; Hassner, A. J . Am. Chem. SOC.1968, 90, 216. (21) (a) Guss, C. 0.;Rosenthal, R. J . Am. Chem. SOC.1955, 77, 2549. (b) Winstein, S.; Buckles, R. E. J . Am. Chem. SOC.1942, 64, 2780. (22) An adaptation of the procedure of Stang, P. J.; Hancock, M.; Subramanian, L. R. Synthesis 1982, 2, 85. (23) Vogel, A. 1. A Textbook of Practical Organic Chemistry, 3rd ed.; Lowe and Brydone Ltd.: London, 1956.
Electrophilic Addition of Br2 to Olefins
J . Am. Chem. SOC.,Vol. 114, No. 20, 1992 1115
scheme I'
10.0 2
6.0
1
I
* * i f *Brz Doto Oxco NBS Doto
2.0
a
kHos incorporates the solvent concentration.
established that the dibromide/methoxy bromide/bromo azide product ratios for cyclohexene and styrene were invariant to these temperature changes ([NaBr] * [NaN,] = 0.1 M). At the completion of the addition, the reaction mixture was poured into -800 mL of water and then extracted with 4 X 75 mL of CH2CI2. The CH,C12 solution was dried (MgSO,), and the solvent was removed by rotary evaporation. The residue was diluted with a small amount of the HPLC solvent (70:30, mcthanol/H,O or CH2C12)to give a final concentration of 0.01-0.7 M in total products. This sample mixture was analyzed in 5-7 separate runs by HPLC, and the values of the integrated areas were averaged for the computations. In the case of 2,3-dimethyl-2-butene,the above extraction procedure proved to give nonreproducible results, so that an alternative workup procedure was employed. Following the bromination (as above), the reaction mixture was poured into saturated NaCI, (300-500 mL) and additional solid NaCl was added to saturate the MeOH. This mixture was extracted with hexane (5 X 50 mL), and the combined hexane extracts were dried over CaCO,. The hexane was removed by distillation through a 20-cm Vigreux column, and the last traces were "chased" by codistillation with added CH2CI2. HPLC and GLPC analyses were calibrated using known amounts of the authentic materials to determine the response factors. In the case of the bromo azidcs of styrene, only 1-azido-2-bromo-1-phenylethanewas detected during the bromination in MeOH. For each series of reactions, the extraction procedures were checked using known amounts of the authentic expected products in MeOH and subjecting this to the workup procedure. In all cases, the products were recovered in amounts that agreed with the initial ratios.
(a) Solvolysis of trnm-2-BromocyclohexylTrifluoromethanesdfonate (I). The solvolysis was conducted by adding a small aliquot of the tritlate in CH2C12(1.5-2.5 mL containing (3.5 X 104)-(9.5 X lo-,) mol) dropwisc to 100-mL MeOH solutions containing04.3 M NaN, or NaBr
at ambient temperatures. The vigorously stirred solution was allowed to react for 10 min, after which time the products were isolated in the same fashion as in the bromination experiments. Analysis of the products for the solvolysis in the presence of N< was conducted using the same HPLC protocol as in the brominations. For the solvolysis in the presence of Br-, the analyses were conducted by GLPC. (e) W Studies. To ascertain the presence of new species formed from N,- and Br2or NBS, qualitative UV experiments in MeOH (purified by distillation from Br2 as described by Dubois et aL2') were undertaken. M) a very rapid forWith Br, (1.2 X 10-3 M) and NaN, (5.3 X mation of a new species having a A, at 312 nm was observed with s-I) to generate a faint rose subsequent decomposition (kobd = 1.5 X colored solution that was unreactive toward the addition of subsequently added cyclohexene. With added NBS ((3-6) X lo-' M) and NaN, M) a slower buildup of the 312-nm transient was ((3.5-5.3) X observed (approximate k2 = 0.1 M-I s-)), followed by its decomposition (kOu N 1.5 X lo-, s-l). The nature of the 312-nm species is unknown, but can be surmised to be BrN3.25 When a MeOH solution of 3.3 X lo-) M NBS and 0.10 M NaBr was monitored by UV-visible spectroscopy, the slow buildup (- 10 min) of an absorbance at 280 nm was observed (k, = 0.064 M-I s-I). The resultant solution was stable for several hours. Addition of cyclohexene (24) Dubois, J. E.; Hegarty, A. F.; Bergmann, E. D. J . Org. Chem. 1972, 37, 2218. (25) (a) Dehnicke, K.; Ruschke, P. Z . Nuturforsch. 1978,336,750. (b) Cambie, R. C.; Robertson, J. D.; Rutledge, P. S . ; Woodgate, P. D. Aust. J. Chem. 1982, 35, 863. ( c ) Hassner, A. Acc. Chem. Res. 1971, 4, 9 and references therein. (d) Hassner, A.; Boenuinkle, F. P.; Levy, A. B. J . Am. Chem. Soc. 1970, 92,4819.
0.0
I I I I I I I I ~ I , I I " " " ' / ~ " ' ~ ' ' ~ ~ 1 ~ ~ ' " " " I
0.00
10.00
30.00
40.00
I/#f Figure 1. Plot of l/faz vs 1/[N
