5978 J . Org. Chem., Vol. 63, No. 17, 1998
Neverov and Brown
Sch em e 1
of a program exploring the possibility for asymmetric
transfer of halogen from derivatives of 2 where the 2-
and 6-positions are substituted with chiral groups, we
required a firm understanding of the various steps
involved in the transfer mechanism. The following
describes our findings from a study of the reaction of bis-
(sym-collidine)bromonium triflate (2-Br+/OTf-) with vari-
ous olefins including adamantylideneadamantane (Add
Ad, 3), cyclohexene, and 4-penten-1-ol (4).
Exp er im en ta l Section
Kin etics. The kinetics of the reactions of bromonium bis-
(sym-collidine)bromonium triflate with AddAd, cyclohexene,
and 4-penten-1-ol were monitored by observing the rate
disappearance of the bromonium ion at 240 nm (the wave-
length of maximum change) in 1,2-dichloroethane (DCE) at
25 °C under conditions of excess olefin, both in the presence
and absence of added collidine. The concentration of 2-Br+/
OTf- was 5 × 10-5M. The pseudo-first- and pseudo-second-
order rate constants were evaluated by nonlinear least-squares
fitting of the absorbance vs time traces for disappearance of
2-Br+/OTf- to the appropriate kinetic equation. The kobs
values reported in the tables are the averages of four or five
runs.
Ma ter ia ls a n d Meth od s. Dichloromethane and hexane
were purified as described.6 1,2-Dichloroethane (Aldrich,
HPLC grade) was used as supplied. Chemicals, including
tetrabutylammonium bromide and tetrabutylammonium ac-
etate, were purchased from Aldrich and used without further
purification. Adamantylideneadamantane (AddAd, 3) was
prepared as previously described.7
1H NMR and 13C NMR spectra were obtained using Bruker
WH-200 and Bruker AM-400 instruments. Spectrophotomet-
ric kinetic measurements were obtained using an OLIS
modified Cary 17 UV-vis spectrophotometer or an Applied
Photophysics SX-17MV stopped-flow reaction analyzer.
Bis(sym -collid in e)br om on iu m Tr ifla te (2-Br+/OTf-).
In 20 mL of dry dichloromethane was suspended 1.41 g of
silver triflate (5.48 mmol) along with 1.43 mL of 2,4,6-collidine
(1.33 g, 10.97 mmol). Almost immediately all solids disap-
peared. A solution of 0.875 g of bromine (5.47 mmol) in 5 mL
of dry dichloromethane was added to the mixture with stirring.
After 10 min the solution was filtered through a Celite pad
and most of the solvent removed with an Ar stream. Addition
of two volumes of dry hexane under stirring resulted in
formation of a white crystalline material which was filtered,
washed with dry hexane, and later recrystallized from a
dichloromethane/hexane system. This procedure yielded 2.32
g of a white crystalline product (90%), mp 105.5-106.5 °C; 1H
NMR (200 MHz, CD2Cl2) δ 7.20 (s, 2H), 2.81 (s, 6H), 2.42 (s,
3H); 13C NMR (400 MHz, CD2Cl2) δ 155.80, 154.41, 126.13,
25.37, 21.15
Resu lts a n d Discu ssion
Adamantylideneadamantane is an ideal olefin for
simplifying this investigation because the transfer of Br+
to it only progresses as far as the AddAd-bromonium
ion.4a The H and 13C NMR spectra of 2-Br+/OTf- show
1
a symmetrical system on the NMR time scale. When a
NMR solution of 2-Br+/OTf- is treated with equimolar
AddAd and the 13C NMR spectrum rerecorded, the Add
Ad olefinic carbons at 133.61 ppm are replaced by a
slightly broadened singlet at 156.38 ppm. The latter
value is very close to that of the central carbons in
authentic AddAd-bromonium ion triflate, (156.24 ppm
in dichloromethane4a). This observation, coupled with
the UV/vis results, indicates that the product of the
reaction between equimolar 2-Br+/OTf- and AddAd is
a an AddAd-bromonium ion complex with one associ-
ated collidine.
Rea ction of 2-Br +/OTf- w ith Olefin s. The reactions of
2-Br+/OTf- with various olefins (AddAd, cyclohexene, 4-penten-
1-ol, and 1-octene) have been followed using NMR and UV/vis
spectrophotometry. The UV/visible spectrum of a 1 × 10-4
M
solution of 2-Br+/OTf- in 1,2-dichloroethane has a broad
The reaction between 5 × 10-5 M 2-Br+/OTf- and
varying, but excess, amounts of AddAd (2.5-12.2 × 10-4
M) leads to a decrease in optical absorbance of the
solution, the final absorbance of which is independent of
the [AddAd]. This result also indicates that at these
concentrations the reaction proceeds completely to the
side of the AddAd-bromonium ion which is indicated
to be complexed with collidine in Scheme 1.
The reaction displays excellent pseudo-first-order ki-
netics under conditions of excess [AddAd]. In the pres-
ence of added collidine, (0-1) × 10-3 M, the reaction
exhibits a common species rate depression. Shown in
Figure 1 is the dependence of the observed pseudo-first-
order rate constants (kobs) vs [collidine] at different [Add
Ad], the kobs values being given in Table 1. Given in eq
2 is the kinetic expression corresponding to the process
outlined in Scheme 1 assuming a steady state in [coll-
Br+]. Notably, nonlinear least squares (NLLSQ) fits of
the data to eq 2
absorbance with peak at 267 nm (ꢀ267 ) 11 000 M-1 cm-1), on
the side of a prominent band (ꢀ240 ) 13 500 M-1 cm-1
)
approaching the solvent cutoff. In a set of experiments, the
UV/vis spectrum of a 1 × 10-4 M sample solution of authentic
AddAd bromonium triflate3a was monitored and shows a broad
featureless band from 300 to 230 nm (ꢀ260 ) 3200 M-1 cm-1);
the sample and reference solutions were treated with equimo-
lar collidine and the UV/vis spectrum monitored again, this
time showing a broad band with λmax ) 267.6 and 274 nm,
ꢀ267.6 ) 5800 M-1 cm-1, ꢀ274 ) 5330 M-1 cm-1. Addition of a
second aliquot of 1 × 10-4 M collidine to the sample and
reference solutions gave no change in the appearance of the
spectrum. This indicates that, at a concentration of 10-4 M,
in the presence of collidine, the AddAd bromonium ion is
completely associated with a single collidine molecule.
In a standard NMR experiment, 0.04-0.06 mmol of 2-Br+/
OTf- was dissolved in 0.5 mL of CD2Cl2 and placed in an NMR
tube along with an equimolar amount of the olefin. The 1H
NMR spectrum was recorded within 5 min. In the cases of
cyclohexene and 1-octene, subsequent experiments were per-
formed by adding a slight excess of tetrabutylammonium
bromide or acetate to the above mixture and then recording
the 1H NMR spectra within 5 min.
kobs ) kdk2[AddAd][coll2-Br+]/(k-d[collidine] +
k2[AddAd]) (2)
(6) Perrin, D. D.; Armagero, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, 1980; p 204.
(7) Flemming, M. P.; McMurry, J . E. Org. Synth. 1981, 60, 113.
require only an inverse first-order common species rate
depression with [collidine] and not an inverse second-