5680 J . Org. Chem., Vol. 64, No. 15, 1999
Notes
The bromonium ion path of Scheme 3 is also consistent
with the second-order kinetics observed for the debro-
mination of 4 with 1. The concentration of bromonium
ion from a first-order process in vic-dibromide 4 would
change proportionally with the concentration of 4. Thus,
the second-order kinetics would still be first order in both
telluride 1 and vic-dibromide 4.
Su m m a r y
In summary, the bromination of olefins and subsequent
debrominations of the resulting vic-dihalides offer a
means for protection of the carbon-carbon double bond
in synthesis. With strongly nucleophilc debrominating
agents such as iodide and and, perhaps, aryltelluride
anions, debrominations of vic-dibromides are initiated by
nucleophilic attack at one bromine. With less nucleophilic
debrominating agents such as dihexyltelluride (1), the
propensity of the vic-dibromide to form bromonium ion
intermediates determines relative reactivity. The diver-
gence in mechanistic paths should permit the selective
protection/deprotection of olefins with different substitu-
tion patterns.
F igu r e 2. Arrhenius activation parameters from a plot of ln
k vs 1000/T, where k is the second-order rate constant at
temperature T. The slope is equal to -Ea/R, and the intercept
is equal to ln A. Values are compiled in Table 3. Error limits
for the second-order rate constants k are contained in Table 3
(all values are e0.07 on the natural log scale of the figure).
Filled circles: debromination of 1,2-dibromodecane (3) with
dihexyltelluride (1). Open circles: debromination of 3 with Bu4-
NI. Filled triangles: debromination of 2,3-dibromo-2-methyl-
pentane (4) with 1. Open triangles: debromination of 4 with
Bu4NI.
Exp er im en ta l Section
Gen er a l Meth od s. Telluride 1 and vic-dibromides 3-6 were
prepared according to ref 4a. Nuclear magnetic resonance (NMR)
spectra were recorded on a Varian 400 MHz instrument with
residual solvent signal as internal standard (δ 7.26 for proton).
Tetrabutylammonium iodide and CDCl3 were used as received
from Aldrich Chemical Co.
Sch em e 3
Gen er a l P r oced u r e for Kin etics Exp er im en ts. A. Rea c-
tion Or d er Exp er im en ts. Stock solutions containing 0.40 M
1,2-dibromodecane (3) and either 0.40 M telluride 1 or 0.40 M
Bu4NI were prepared in CDCl3 as were stock solutions contain-
ing 0.40 M 2,3-dibromo-2-methylpentane (4) and either 0.40 M
telluride 1 or 0.40 M Bu4NI in CDCl3. The stock solutions and
2-fold (0.2 M in each reagent) and 4-fold (0.1 M in each re-
agent) dilutions were sealed in NMR tubes and placed in a
constant-temperature bath at 338 K. The solutions were sampled
periodically by 1H NMR spectroscopy and data was collected
over the initial 10% of reaction for 1,2-dibromodecane (3) and
over the initial 20% of reaction for 2,3-dibromo-2-methylpentane
(4) using residual CHCl3 as an internal standard. For these
systems and the other systems described below, the olefinic
protons, the protons on carbon-bearing bromine in the vic-
dibromides, the R-methylene protons of both telluride 1 and Te-
(IV) dibromide 2, and the R-methylene protons of Bu4NI were
well separated at 400 MHz. A plot of [A]t/[A]0, where [A] is the
concentration of substrate, as a function of time gave the initial
slopes, which were multiplied by [B]0, where [B]0 is the initial
concentration of debrominating agent, to give the reaction ve-
locities compiled in Table 1. Values are the average of duplicate
runs.
B. F or P seu d o-F ir st-Or d er Rea ction s. Stock solutions of
telluride 1 and Bu4NI were prepared at 0.2, 0.4, 0.6, and 0.8 M.
Ten-milliliter aliquots of the stock solution were transferred to
a septum-sealed vessel containing 0.25 mmol of the vic-dibro-
mide substrate (0.025 M in vic-dibromide). The solution was
outgassed under a stream of argon bubbles. The reaction vessel
was placed in a constant-temperature bath. At appropriate time
intervals over 1-2 half-lives, 0.8-mL aliquots were withdrawn
from the reaction mixture and the samples were analyzed by
1H NMR spectroscopy. Duplicate runs were repeated at each
concentration of debrominating agent and temperature. The
calculated pseudo-first-order rate constants for debromination
were plotted as a function of telluride or iodide concentration
with the slope of the resulting line giving the second-order rate
constants compiled in Tables 2 and 3. Values of k and 2σ are
based on all runs as calculated by the program Sigma Plot
(Version 4.16, J andel Scientific).
process (∆Sq ) -42.4 cal mol-1 K-1) with low barriers
for both ∆Hq and Ea (12.7 and 13.4 kcal mol-1, respec-
tively). These data suggest a highly ordered transition
state in which partial bond breaking has occurred prior
to approach of the debrominating agent. Earlier studies
of vic-dihalides derived from steroids argue that halo-
nium ion intermediates are readily formed under mild
conditions.7 Either nucleophilic attack of 1 on a dibromide
with partial C-Br cleavage and partial bromonium ion
character (as in 9) or nucleophilic attack of 1 on a
bromonium ion intermediate (as in 10 in Scheme 3) offers
a scenario for a large negative value of ∆Sq and small
values of ∆Hq and Ea. In both scenarios, significant bond
breaking has occurred prior to nucleophilic attack of
debrominating agent, which would contribute to the
small values of ∆Hq and Ea observed. The added ordering
in transition state 9 relative to 7 is apparent with respect
to rotational freedom. Transition state 10 imposes the
order derived from partial bonding of the bromonium ion
as opposed to a transition state from a less ordered
carbocation as in 11.
(7) (a) Grob, C. A.; Winstein, S. Helv. Chim. Acta 1952, 35, 782. (b)
Kwart, H.; Weisfield, L. B. J . Am. Chem. Soc. 1956, 78, 635. (c) Barton,
D. H. R.; King, J . F. J . Chem. Soc. 1958, 4398. (d) Alt, G. H.; Barton,
D. H. R. J . Chem. Soc. 1954, 4284. (e) Barton, D. H. R.; Rosenfelder,
W. J . J . Chem. Soc. 1951, 1048.
Butcher, Timothy S.
