Rate study of haloadamantane reduction by samarium diiodide

Article abstract of DOI:10.1002/jccs.200200138

Rate constants directly measured by GC/MS-analyzed method for reduction of haloadamantanes by SmI₂ in presence of HMPA and H₂O were obtained. HMPA exhibits stronger catalytic effect than H₂O does. The result of faster reaction rate of 1-bromoadamantane than that of 2-bromoadamantane can be used to confirm the formation of alkyl radical as the rate limiting step of this reduction.

Full text of DOI:10.1002/jccs.200200138

Journal of the Chinese Chemical Society, 2002, 49, 969-973
969
Rate Study of Haloadamantane Reduction by Samarium Diiodide
Tzuen-Yeuan Lin* (
), Ming-Ren Fuh (
) and Yu-Yu Chen (
)
Department of Chemistry, Soochow University, Taipei, Taiwan, R.O.C.
Rate constants directly measured by GC/MS-analyzed method for reduction of haloadamantanes by SmI
in presence of HMPA and H O were obtained. HMPA exhibits strong e r catalytic effect than H O does. The re-
2
2
2
sult of faster reaction rate of 1-bromoadamantane than that of 2-bromoadamantane can be used to confirm the
formation of alkyl radical as the rate limiting step of this reduction.
INTRODUCTION
A series of rate examinations on reduction of organic
sions showed a coefficient of variation (C.V.) rangi n g from
1
.0% to 2.4% and 1.1% to 3.4%, respectively.
In addition, we also examined the recovery of samp l e
compounds including ketones, sulfoxides, alkyl halides,
epoxides and esters by samarium diiodide has been accom-
preparation procedure. Known amounts (2.0, 1.0 and 0.3
mM) of 1-bromoadamantane and 1-iodoadamantane were
added in a tetrahyrofuran solution and then extracted by the
procedure described in the experimental section. Good re-
coveries rangi n g from 91.7% to 98.5% were determined.
For the reaction of haloadamantanes reduced by samar-
ium diiodide, pseudo-first-order rate constants were obtained
1
-5
plished previously. GC analysis was used as the reliable
method to achieve the kinetic measurement in the presence of
2
oxygen and moisture-sensitive SmI . For having a better
analysis of reaction products, we furthermore use GC-Mass
to monitor such reactions, and the structures of the products
are confirmed. Various haloadamantanes with strains were
chosen as the target compounds. Besides measuring the rates
of reduction for the strained alkyl halides, the reaction mech-
anism is clarified according to the structural effect on reduc-
tion rates.
by plotting log(C
t
-C ) versus time for more than two half-
lives. Here C and C represent the concentration of haloa d a -
t
mantanes at time t and infinity, respectively (Fig. 1). The
pseudo-first-order rate constants were also tested by using
different initial concentrations of the reactant (Table 1). Both
RESULTS AND DISCUSSION
A GC-MS method was developed to determine the con-
centration of haloadamantanes and measure the reduction
rates. In order to minimize the errors of sample preparation
and GC-MS analysis, decane was used as an internal stan-
dard. The retention times of haloadamantane and decane
were approximately 12.8 and 5.0 min., respectively. The lin-
earity of GC-MS assay was established by the peak area ratio
2
of haloadamantane/decane. Good linearity (r = 0.99) from
0
.3 to 2.0 mM was measured. The detection limit of this
newly developed method was about 0.5 M based upon three
times signal-to-noise ratio. The analytical precision of this
method was evaluated by replicated analysis of haloada-
mantane standard solutions. A total of three series of samples
were analyzed over a one-week period and each sample was
determined in triplicate. The intra-day and inter-day preci-
Fig. 1. Pseudo first-order rate plot of SmI
2
reduction of
-
2
1-iodoadamantane; [SmI ] = 7.2
2
10 M,
o
O] = 4.4 10 M, [1-iodoadamantane] =
-
1
[H
.0 10 M, T = 25 C.
2
-
3
2
*
Corresponding author. Tel: +886-2-2881-9471 ext. 6810; fax: +886-2-2881-1053

9
70 J. Chin. Chem. Soc., Vol. 49, No. 6, 2002
Lin et al.
a
Table 1 . Dependence of Pseudo-first-order Rate constants on Initial Concentration of
Haloadamantanes
a
-1
-3
b
-1
-3
[
1-Bromoadamantane] , M
kps , sec
[1-Iodoadamantane] , M
kps , sec
0
0
-
-
-
3
3
3
-3
4
2
1
a
.0 10
.0 10
.0 10
1.2 10
1.3 10
1.3 10
4.0 10
2.0 10
3.3 10
3.2 10
3.3 10
-3
-3
-3
-3
-3
-3
1.0 10
-2
-2
[
[
SmI ] = 6.7 10 M; [HMPA] = 9.2 10 M; T = 25 C.
2
SmI ] = 6.2 10 M; [H O] = 8.9 10 M; T = 25 C.
b
-2
-2
2
2
kps represents pseudo-first-order rate constant.
7
cases of bromoadamantane and iodoadamantane reductions
exhibit the same kps with different initial concentrations of
haloadamantane, i.e., these reductions fit perfectly pseudo-
first-order in these substrates. Here catalysts such as water or
hexamethylphosphoramide were necessary for effecting the
reduction. In ord e r to minimize the effect of Sm metal pow-
der on reduction rate, samarium diiodide solution was centri-
fuged prior to reaction. The product was traced and character-
ized as adamantane with an authentic sample by GC-MS
chromatography.
Curran’s work which presented evidence that water serves
not only as a proton source, but can also accelerate certain
classes of samarium reductions. We, however, are able to ob-
tain absolute rates instead of relative rates which were mea-
sured from reactant to product ratios. Fig. 3 shows the cata -
lytic effect of HMPA on reduction of 1-bromoadamantane by
SmI
2
. The induction period of this figure can be explained by
and HMPA, i.e., a small
. The following rel a -
the slow reaction between SmI
2
amount of HMPA is consumed by SmI
tionship can be derived as
2
Both water and hexamethylphosphoramide (HMPA)
catalyze this reduction. Yet as expected, HMPA is more eff i -
cient than wat e r. However, in the presence of HMPA, the
pseudo-first-order linear plot can be obtained only for fast re-
actions (within 10 minutes) while the curvature appeared for
k
2
ps = k [HMPA]
for the linear part of this graph.
Since
slow reactions. This can be accounted for by the slow rea c -
6
tion between SmI
2
and HMPA. Und e r our reaction condition,
Rate = kps[1-bromoadamantane]
therefore,
we solved this problem simp l y by taki n g the initial rates for
slow reactions since at the beginning of the reaction, the lin-
earity for the first-order relationship still can be obtained.
Fig. 2 shows the catalytic effect of H
iodoadamantane by SmI . Good linearity was achieved to
give a relationship as follows:
2
O on reduction of 1-
2
k
1 2
ps = k [H O]
Since
Rate = kps[1-iodoadamantane]
therefore,
Rate = k
The slope of the straight line obtained in Fig. 2 can be used to
1
2
[H O][1-iodoadamantane]
Fig. 2. Dependence of water concentration on pseudo
-
2
-
3
-1 -1
2
first-order rate constant [SmI ] = 7.2 10 M,
calculate k
1 2
as 6.3 10 sec M that is dependent on [SmI ]
-
3
[
0
1-iodoadamantane] = 2.0 10 M, T = 25 C.
and temperature. This result is consistent with Hasegawa and

Rate Study of Haloadamantane Reduction by Samarium Diiodide
J. Chin. Chem. Soc., Vol. 49, No. 6, 2002 971
a
a
Table 2 . SmI Reduction of 1-Haloadamantane
Table 3 . Structural Effect of Bromoadamantane on Reduction
Rates
2
b
c
1
-Haloadamantane kps (H O catalyzed) k (HMPA catalyzed)
2 ps
-
1
d
-6
-3
-2
Bromoadmantane
kps, sec
RCl
RBr
RI
--
5.1 10
1.3 10
8.0 10
-
-
5
3
-4
-4
7.5 10
5.3 10
1-bromoadamantane(tert-RBr)
2-bromoadamantane(sec-RBr)
5.4 10
3.1 10
a
-2
-3
a
-2
-3
[
SmI ] = 7.3 10 M; [RX] = 2.0 10 M; T = 25 C; k
[SmI ] = 6.2 10 M; [RX] = 2.0 10 M; [HMPA] = 6.9
2 0
2
0
ps
-
2
represents pseudo-first-order rate constant.
10 M; T = 25 C; k represents pseudo-first-order rate
ps
b
c
d
[
[
H O] = 1.1 M.
constant.
2
-2
HMPA] = 6.9 10 M.
too slow to measure.
confirms that the rate determining step involves the form a -
tion of adamantane radical instead of an adamantane anion.
The mechanism thus can be proposed as follows:
2
Rate = k [HMPA][1-bromoadamantane]
for the linear part of this graph, and the slope obtained can be
used to calculate k which is dependent on [SmI ] and temper-
ature. Under our reaction condition, k is approximated to be
2
2
2
-
3
-1 -1
5
.3 10 sec M .
Either H O or HMPA catalyzed reduction exhibits the
reaction rates in the increasing ord e r of 1-chloroadaamantane
-bromoadamantane 1-iodoadamantane (Table 2). This
2
1
reasonable result is fully consistent with that of unstrained
3
alkyl halides.
The rates of 1-bromoadamantane and 2-bromoada-
mantane were compared carefully in order to examine the
mechanism. Table 3 indicates 1-bromoadamantane is re-
duced faster than 2-bromoadamantane, i.e., tertiary bromo-
adamantane reacts with samarium diiodide faster than sec-
ondary bromoadamantane does. This piece of information
Finally, temperature effect was examined (Table 4)
4
,5
which shows a similar result with that of our previous work,
i.e., this reduction is favored at higher temperatures.
CONCLUSION
In summary, by GC/MS analyzing method, we have ob-
tained the pseudo-first-order rate constants of the reduction
of strained molecules of haloadamantanes that are less rea c -
3
tive toward SmI
2
than unstrained alkyl halides. Radical for-
mation is confirmed as the rate determining step.
a
Table 4 . Temperature Effect on Reaction Rate
b
-1
T, C
kps , sec
-
-
-
-
3
4
4
4
4
3
2
1
a
0
4
4
6
1.3 10
9.3 10
5.0 10
2.7 10
Fig. 3. Dependence of HMPA concentration on pseudo
-3
-
2
[SmI ] = 0.11 M; [1-bromoadamantane] = 2.0 10 M;
2 0
2
first-order rate constant [SmI ] = 7.0 10 M,
-
2
[
HMPA] = 6.9 10 M.
-
3
[
1-bromoadamantane]
C.
0
= 2.0 10 M, T = 25
b
kps represents pseudo-first-order rate constant.

9
72 J. Chin. Chem. Soc., Vol. 49, No. 6, 2002
Lin et al.
EXPERIMENTAL
Chemicals
substrate to the peak area of deca n e vs. the concentration of
substrates. Extraction recoveries were also done. Pseudo-
first-order rate constants were obtained by plotting log(C
t
-
Samarium iodide, decane, 1-chloroadamantane, 1-bro-
C ) versus time for more than two half-lives. Here C and C
t
moadamantane, 1-iodoadamantane, 2-bromoadamantane and
hexamethyl phosphoramide (HMPA) were purchased from
Aldrich Chemical Company, Inc. (Milwaukee, WI, USA).
Hydrochloric acid and iodine were purchased from Nacalai
Tesque, Inc. (Kyoto, Japan) and Wako Pure Chemical Indus-
tries, Ltd. (Osaka, Japan). Hexa n e was purchased from Tedia
Company (Fairfield, OH, USA). Tetrahydrofuran (THF) was
purchased from Labscan Ltd. (Dublin, Ireland). The chemi -
cals were used as received without further purification. THF
was freshly distilled and immediately transferred to a glove
box before use.
represent the concentration of haloadamantanes at time t and
infinity, respectively.
ACKNOWLEDGMENT
This work was financially supported by the National
Science Council of the Republic of China (NSC 89-2113-
M-031-007).
Received June 4, 2002.
General Method
Both external and internal standards were necessary for
quantitative measurements of haloadamantanes. A HP 6890
series GC system with a flame ionization detector (FID), a
HP autosampler (Hewlett Packard Company, CA, USA),
HP-5890 seriesII GC system (Hewlett Packard Company,
CA, USA), HP 5971A MS (Hewlett Packard Company, CA,
USA), and a split/splitless injector were used for analysis.
The SISC chromatography data system (Scientific Informa-
tion Service Corp., Taipei, Taiwan) and a personal computer
were used for data acquisition and processing. A HP-5 col-
umn (30 m., 0.53 mm i.d., 1.5 m film thickness, Hewlett
Packard Company, CA, USA) was used for separation. Most
operations were carried out in the glove box. All the reactions
proceeded in a temperature controlled compartment in the
glove box. In ord e r to minimize the effect of free Sm metal in
Key Words
Samarium diiodide; Haloadamantane.
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1