Radical catalyzed debromination of bromo-alkanes by formate in aqueous solutions via a hydrogen atom transfer mechanism

Article abstract of DOI:10.1016/j.tetlet.2003.11.091

CO₂^·- radicals catalyze the dehalogenation of bromo-alkanes by formate via a hydrogen atom transfer mechanism.

Full text of DOI:10.1016/j.tetlet.2003.11.091

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 989–992
Radical catalyzed debromination of bromo-alkanes by formate
in aqueous solutions via a hydrogen atom transfer mechanism
Elisabetha Shandalov,a Israel Zilbermann,a,b Eric Maimon,a,b Yeoshua Nahmani,b
Haim Cohen,a,b Eilon Adarc and Dan Meyersteina,d,
*
aDepartment of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
bNuclear Research Centre Negev, Beer-Sheva 9001, Israel
cDepartment of Environmental Hydrology and Microbiology, Institute of Water Sciences and Technologies,
Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
dDepartment of Biological Chemistry, The College of Judea and Samaria, Ariel 44837, Israel
Received 15 September 2003; revised 10 November 2003; accepted 21 November 2003
Åꢀ
Abstract—CO2 radicals catalyze the dehalogenation of bromo-alkanes by formate via a hydrogen atom transfer mechanism.
ꢀ 2003 Elsevier Ltd. All rights reserved.
1. Introduction
Surprisingly enough blank experiments pointed out
that the metal complexes partially inhibit the debro-
mination reactions, which occur via a chain process. The
mechanism of the latter process is the subject of this
study.
Organohalides are known as very important and
unyielding environmental pollutants. The main sources
of these pollutants are industrial installations. Industrial
wastes containing these pollutants released from the
installations into water courses can have long-term toxic
effects. Thus, dehalogenation of organohalides is of
increasing interest. Recently, a number of methods for
decontamination of zones polluted by organohalides
have been proposed. These include photolysis,1 ionizing
radiation,2 chemical dehalogenation,3 ultrasonic irradi-
ation,4 pyrolysis,5 electrochemical6 and biological7
methods.
–
2. Formation of CO2 radicals
Å
When ionizing radiation is absorbed by a dilute aqueous
solution the following initial products are formed:8
e
ꢀ;c
H2O ! ÅOH ð2:65Þ; H ð0:60Þ; eaꢀq ð2:65Þ;
Å
It seemed of interest to study the effect of the redox
potential of several Mnþ1=nL couples on the rate of the
reaction
H2 ð0:45Þ; H2O2 ð0:75Þ; H3Oþ; OHꢀ
ð3Þ
MnL þ RBr ! Mnþ1L þ RÅ þ Brꢀ
It was decided to use the pulse-radiolysis technique for
this purpose preparing the MnL complexes in situ via
ð1Þ
The G values are given in parentheses (G values are
defined as the number of molecules of each product per
100 eV of radiation absorbed by the solution).
CO2 Åꢀ þ Mnþ1L ! MnL þ CO2
ð2Þ
The hydrated electrons react with N2O, via
eꢀaq þ N2O ! N2 þ Å OH þ OHꢀ
k ¼ 8:7 ꢁ 109 Mꢀ1 sꢀ1 9
ð4Þ
Keywords: Dehalogenation; Bromo-alkanes; Catalysis.
* Corresponding author. Tel.: +972-86472737; fax: +972-86472943;
e-mail: sukhov@bgumail.bgu.ac.il
In the presence of formate (HCOOꢀ), both HÅ and ÅOH,
react via
0040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.11.091

990
E. Shandalov et al. / Tetrahedron Letters 45 (2004) 989–992
Table 1. Yield of bromide
Compound
[RBr], mol dmꢀ3
[NaOOCH],
mol dmꢀ3
Dose, Gy
[Brꢀ], mol dmꢀ3
G-value,
molecules/100 eV
Br3CCOOH
7 · 10ꢀ3
7 · 10ꢀ3
0.1
0.1
0.1
0.1
0
594
594
594
594
594
396
264
66
0.00082
0.0014
13.8
23.3
13.5
136
9.7
CH2BrCH2SOONa
CH2BrCH2COOH
(CH2Br)2C(CH2OH)2
(CH2Br)2C(CH2OH)2
BrCH2COOH
7 · 10ꢀ3
0.00079
0.0081
7 · 10ꢀ3
7 · 10ꢀ3
0.00058
0.0052
1.4 · 10ꢀ2
1.4 · 10ꢀ2
1.4 · 10ꢀ2
1.4 · 10ꢀ2
1.4 · 10ꢀ2
1.4 · 10ꢀ2
1.6 · 10ꢀ3
4.2 · 10ꢀ2
7 · 10ꢀ3
0.1
0.01
0.5
0.1
0.1
0.1
0.1
0.1
0.1
0.1
131
14.2
589
125
<10.8
70
BrCH2COOH
BrCH2COOH
0.00037
0.0039
0.0033
BrCH2COOH
264
260
300
300
396
594
396
BrCH2COOHa
BrCH2COOHb
BrCH2COOH
0.00028
0.0021
0.00077
0.0032
25.5
81
BrCH2COOH
3-BrC6H4COOHc
ClCH2COOH
0.00010
0.000038
1.8
0.96d
1.4 · 10ꢀ2
N2O saturated solutions of RBr+HCOONa, pH 6.0 (at a dose rate of 3.3 Gy minꢀ1).
a Dose rate of 20 Gy lsꢀ1 (5 MeV electron beam).
b Dose rate of 30 Gy minꢀ1
c pH 10.0.
d Yield of chloride.
.
2RÅ ! R2
ð8Þ
ð9Þ
ÅH=ÅOH þ HCOOꢀ ! CO2 Åꢀ þ H2=H2O
ꢀ1
k OH ¼ 3:2 ꢁ 109 M sꢀ1 10
Å
Åꢀ
2ꢀ
2CO2 ! C2O4
ꢀ1
k H ¼ 2:1 ꢁ 108 M sꢀ1 10
ð5Þ
Å
Thus all the primary radicals are transformed into
Åꢀ
Å
ꢀ
CO2 Åꢀ þ R ! RCO2
ð10Þ
CO2 radicals, with a yield of
The length of the chain depends on the relative rates
of the propagation reactions 6 and 7, versus the rates of
the termination reactions (8)–(10), which all have rate
GðCO2 ÅꢀÞ ꢂ 6:0:8
constants of ca. 5 · 108 molꢀ1 dm3 sꢀ1 11
.
When N2O-saturated aqueous solutions containing
0.1 mol dmꢀ3
HCOONa
and
7 · 10ꢀ3–1.4 · 10ꢀ2
mol dmꢀ3 of several bromo-organic compounds at
pH 6.0 (for bromobenzoic acid pH 10) are irradiated in
The results for bromobenzoic acid suggest that reac-
tions 6 and 7 for this system are too slow to compete
with the termination reactions. This result is not sur-
a
60Co c source with a dose rate 3.3 Gy minꢀ1 to an
Å
overall dose of 66–594 Gy, the formation of bromide is
observed. The yields of bromide determined by ion
chromatography, are summarized in Table 1. [The
choice of 2,2-dibromomethyl-1,3-propanediol (C(CH2-
Br)2(CH2OH)2) as one of the bromo-alkanes stems from
the fact that this compound is a major pollutant in the
Beer–Sheva region.]
prising as the C6H4CO2H radicals are resonance stabi-
lized.
The result for chloroacetic acid indicates that the
dechlorination reaction, the reaction analogous to
reaction 6, is too slow to compete with the termination
reaction.
The results for the bromo-alkanes clearly demonstrate
that the bromide yield is considerably larger than the
primary yield of radicals, G ¼ 6:0. Thus it has to be
It is reasonable to assume that k6 > k7 for the com-
pounds studied, that is, the Ôchain lengthÕ depends
mainly on the competition between reaction 7 and the
termination steps. This assumption is based on the
observation that the yield of Brꢀ is nearly proportional
to [HCO2ꢀ], that is, reaction 7 is the rate determining
step of the process. This assumption is also in accord
with the observation that the chain length for
BrCH2CO2H is considerably longer than for Br3CCO2H
as one expects k6 to be higher for Br3CCO2H. The
slower k7 for Br3CCO2H is probably due to stabilization
Åꢀ
concluded that the CO2 radicals initiate a catalyzed
chain debromination process. The results are in accord
with the following mechanism:
.
.
−
k6
k7
−
(6)
(7)
RBr + CO2
R + CO2 + Br
.
.
−
−
Å
R + HCO2
RH + CO2
of the radical CBr2CO2H by resonance and/or to steric
hindrance. However, a comparison of the results for
(CH2Br)2C(CH2OH)2 and BrCH2CH2CO2H suggests
and the termination steps involved:

E. Shandalov et al. / Tetrahedron Letters 45 (2004) 989–992
991
H2CBr
BrH2C
BrH2C
- H2O
.CHOH
+
. OH
BrH2C
C
CH2OH
C
CH2OH
CH2OH
H2C.
C
H3C
C
H2CBr
BrH2C
BrH2C
Br-
BrH2C
CH OH BrH C
+
H
+
.CHOH
CHO
+
CH2OH
+
BrH2C
2
2
C
+
C
CH2OH
CH2OH
CH2OH
CH2OH
H2C.
H2CBr
C
BrH2C
BrH C
H3C
C
.CHOH
CH2OH
BrH2C
+ BrH2C
CH2OH
C
+
CH2OH
BrH2C
2
C
CH2OH
CH2OH
CH2OH
CH2OH
Scheme 1.
that k6 might contribute to the rate determining step of
the chain reaction.
Thus the steady-state radical concentration, where
[Radical] ¼ [RÅ]+[CO2Åꢀ], is
The decrease in the Brꢀ yield when the concentration of
BrCH2CO2ꢀ is increased is attributed to the competition
of the side reaction 11
2
3:3 ꢁ 10ꢀ8 ¼ 5 ꢁ 108½Radicalꢃ
½Radicalꢃ ꢄ 8 ꢁ 10ꢀ9 mol dmꢀ3
For the bromoacetic system ([BrCH2CO2ꢀ] ¼
1.4 · 10ꢀ2 mol dmꢀ3 and [HCO2ꢀ] ¼ 0.1 mol dmꢀ3) the
Ôchain lengthÕ or the turn-over number equals 131/
6 ¼ 21.8. Therefore, assuming that reaction 7 is the rate
determining step:
RÅ þ BrCH2CO2 ! RH þ Å CHBrCO2
ð11Þ
ꢀ
ꢀ
ꢀ
with reaction 7. The lower Brꢀ yield at low BrCH2CO2
concentrations proves that under these conditions the
rate of reaction 6 also limits the chain length.
2
k7½RÅꢃ½HCO2 ꢀꢃ ¼ 21:8 ꢁ 5 ꢁ 108 ꢁ ½RÅꢃ
When solutions of (CH2Br)2C(CH2OH)2 are irradiated
in the absence of NaOOCH, GðBrꢀÞ ¼ 9.7 was obtained.
However, GðBrꢀÞ ¼ 0 for solutions of BrCH2CO2H in
the absence of formate. This result suggests that a short
catalytic process occurs in the former system even in the
absence of formate. A plausible mechanism for this
process is depicted in Scheme 1.
and as ½RÅꢃ ꢄ 8 ꢁ 10ꢀ9 mol dmꢀ3
For [HCOONa] ¼ 0.1 mol dmꢀ3
:
21:8 ꢁ 5 ꢁ 108 ꢁ 8 ꢁ 10ꢀ9
k7 ¼
¼ 872molꢀ1 dm3 sꢀ1
0:1
For [HCO2Na] ¼ 0.01 and 0.5 mol dmꢀ3 one thus
obtains k7 ¼ 947 and 785 molꢀ1 dm3 sꢀ1, respectively. It
can be concluded that k7 ¼ 900 ꢅ 200 molꢀ1 dm3 sꢀ1
,
which is somewhat smaller than rate constants of similar
reactions.11
3. Rate constant estimation
From the yield ofꢀbromide, that is, the chain length,
which equals GðBr Þ/6.0, the dose rate, [HCOꢀ2 ] and 2k
termination ꢂ5 · 108 molꢀ1 dm3 sꢀ1, one can estimate the
rate constant of reaction 7 assuming that this is the rate
determining step of the catalytic process as follows:
The effect of the dose rate observed on the bromide yield
is in accord with expectations.
In conclusion the results point out that:
1. Formate can be used as a reductant of bromo-alkanes
in a radical catalyzed process.
2. Formate can be used as a reducing agent via a mech-
anism involving hydrogen atom transfer and not only
via a hydride transfer mechanism as is commonly
accepted.12
d½CO2 Åꢀꢃ 6 ꢁ 3:3 ꢁ 10ꢀ7
ꢄ
¼ 3:3 ꢁ 10ꢀ8 mol dm3 sꢀ1
dt
60
[as for each Gy absorbed by the solution the yield of
radicals is 1 · 10ꢀ7 mol dmꢀ3 if G(radical) ¼ 1.0].

992
E. Shandalov et al. / Tetrahedron Letters 45 (2004) 989–992
4. Gaddam, K.; Cheung, H. M. Ultrason. Sonochem. 2001, 8,
103–109.
Acknowledgements
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This study was supported in part by a grant from the
Budgeting and Planning Committee of The Council of
Higher Education and the Israel Atomic Energy Com-
mission. D.M. wishes to express his thanks to Mrs. Irene
Evens for her ongoing interest and support.
8. Matheson, M. S.; Dorfman, L. M. Pulse Radiolysis; MIT:
Cambridge, MA, 1969.
9. Anbar, M.; Bambeneck, M.; Ross, A. B. Natl. Bur. Stand.
Ref. Data Ser. 1973, NSRDS-NBS 43.
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