Ruthenium complex catalyzed oxidative conversion of aliphatic amines to carboxylic acids using bromamine-T: Kinetic and mechanistic study

Article abstract of DOI:10.1016/j.molcata.2009.05.025

Ruthenium(III) complex catalyzed oxidation of aliphatic amines with bromamine-T under alkaline condition proceeds efficiently to afford carboxylic acids in high conversion. Hexa-coordinated ruthenium(III) complex of the type [RuCl₂(PPh₃

Full text of DOI:10.1016/j.molcata.2009.05.025

Journal of Molecular Catalysis A: Chemical 311 (2009) 23–28
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ruthenium complex catalyzed oxidative conversion of aliphatic amines to
carboxylic acids using bromamine-T: Kinetic and mechanistic study
C.H. Vinod Kumara, K.N. Shivanandab, Rajenahally V Jagadeeshc,∗, C. Naga Rajua,∗
a Department of Chemistry, Sri Venkateswara University, SV University Campus, Tirupati 517502, India
b Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Technion City, Haifa -32000, Israel
c Division of Organic Chemistry, SSH, VIT University, Vellore-632014, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Ruthenium(III) complex catalyzed oxidation of aliphatic amines with bromamine-T under alkaline con-
dition proceeds efficiently to afford carboxylic acids in high conversion. Hexa-coordinated ruthenium(III)
complex of the type [RuCl2(PPh3)(L)] (L, tridentate ligand derived by the condensation of o-phenylene
diamine with salicylaldehyde) has been synthesized and it was used as a catalyst for the oxidative conver-
sion of amines to carboxylic acids. The detailed mechanistic and kinetic investigations have been made
for the oxidation reactions. Under similar experimental conditions all the amines proceed with a common
oxidation mechanism and follows an identical kinetics with first-order dependence each on [Oxidant]o
and [Amine]o, and fractional order with respect to [Catalyst] and [OH−]. To understand the detailed
kinetics and mechanism of the reactions, the reactions have been subjected to changes in (i) dielectric
permittivity, (ii) primary salt effect, (iii) halide ions and (v) temperature. The reactions were carried out at
different temperature and the activation parameters have been calculated. From enthalpy–entropy rela-
tionships and Exner correlations, the isokinetic temperature (ˇ) of 382 K, calculated is much higher than
the experimental temperature (313 K), indicating that, the enthalpy factor controls the rate. The observed
results have been explained by a plausible mechanism and the related rate law has been deduced. The
present method developed for the oxidation of amines to carboxylic acids by bromamine-T offers sev-
eral advantages including high conversion, short reaction times, and stable, cost effective and relatively
non-toxic reagents which make the reaction process simple and smooth.
Received 18 March 2009
Accepted 31 May 2009
Available online 10 June 2009
Keywords:
Ru(III) catalysis
Amine oxidation
Alkaline medium
Bromamine-T
© 2009 Published by Elsevier B.V.
1. Introduction
haloamine such as bromamine-T is particularly attractive from an
economic and environmental point of view. The development of
The oxidation of amines plays a central role in organic chem-
istry both at the laboratory and industry level. Several methods
[1–6] are available to effect the oxidation, and continuous attention
is drawn to newer and more selective methods of amine oxida-
tion. Different methods were designed to give different oxidation
products in the amine oxidation [1–6]. Oxidation of amines is of
importance, as it adds to the body of knowledge of redox chem-
istry. The oxidation products depend on the type of oxidant, on
the reaction medium and on the nature of the alkyl groups pre-
sented [6]. The amines selected for the oxidation-kinetic studies are
ethylenediamine (EDA), diethylenetriamine (DETA), triethylenete-
tramine (TETA), tetraethylenepentamine (TEPA), bishexamethylen-
etriamine (BHMT), 2-methylpentamethyldiamine (2-MPDA) and
2,2,4-trimethyldiamaine (2,2,4-TMHDA). Catalytic oxidation with
new processes for the selective oxidations with environmentally
friendly oxidants has potential practical applications in organic
synthesis. In this regard, we used sodium N-bromo-p-toluene
sulfonamide or bromamine-T (p-CH3C6H4SO2NBrNa·3H2O; abbre-
viated as BAT), a member of organic haloamine as an oxidant in
NaOH medium for the selective oxidative conversion of aliphatic
amines to carboxylic acids using hexa-coordinated ruthenium com-
plex as catalyst with a view to explore the detailed mechanistic and
kinetic aspects of the reactions.
The chemistry of N-metallo-N-arylhalosulfonamides, generally
known as organic haloamines, is of interest due to their diverse
behaviour. Their versatile nature is due to their ability to act
as sources of halonium cations, hypohalite species and nitrogen
anions. The monohaloamines are two-electron change oxidants
while dihaloamines undergo four-electron changes. As a result,
these compounds react with a range of functional groups and affect
a variety of molecular changes. The prominent member of this
group is chloramine-T (CAT), which contains chlorine in +1 oxi-
dation state and is a by-product in the manufacture of saccharin.
∗
Corresponding authors.
E-mail addresses: rvjdeesh@yahoo.com (R.V. Jagadeesh), chvinod20@yahoo.com
(C.N. Raju).
1381-1169/$ – see front matter © 2009 Published by Elsevier B.V.
doi:10.1016/j.molcata.2009.05.025

24
C.H.V. Kumar et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 23–28
Bromamine-T (p-MeC6H4SO2NBrNa·3H2O or TsNBrNa or BAT), the
bromine analogue of CAT, is gaining importance as a mild oxidant
and is found to be a better oxidizing agent than the chloro com-
pound. It can be easily prepared from chloramine-T. Although the
mechanistic aspects of many of the haloamine reactions have been
well documented [7–12], similar studies on bromine analogues are
sparse. In view of these facts, there is a considerable scope for the
study with BAT to get a better insight of the speciation of BAT reac-
tion models and to understand its redox chemistry in solutions.
Ruthenium has various valences (0–8 valences) and is not so
expensive transition metal; therefore, various useful catalytic reac-
tions for organic synthesis have been explored [13]. It is noteworthy
that the medical property of ruthenium is now being recognized,
and ruthenium anticancer agents have recently entered the clinic,
showing promising activity on resistant tumors. There has been
considerable interest in ruthenium complexes in recent years
because of their redox stability [14]. Metal complexes of ruthenium
containing nitrogen and oxygen donor ligands are found to be effec-
tive catalysts for oxidation, reduction, hydrolysis, and other organic
transformations [15–16]. In the present work, we have chosen a
ruthenium complex with group of three tridentate O, N, N donor
ligands. The ligand is abbreviated in general as L. The ligand has
three potential donor sites, viz., a phenolate oxygen, an azomethine
nitrogen and an amine nitrogen.
tion mixture was evaporated to a small volume and left to cool. The
resulting Schiff’s base ligand precipitated on cooling and then was
it filtered off, washed with ethanol and recrystallized from ethanol.
The purity of the Schiff’s base ligand was monitored on TLC using
eluants 1:1 ethyl acetate and petroleum ether and separated by
column chromatography (yield = 90%). The ligand was character-
ized by FT-IR spectral analysis. IR spectra of Schiff’s base showed
a strong band around 1580–1600 cm−1 for the free azomethine
(
CH N ) group. In the complex, this band is shifted to the region
1522–1543 cm−1, indicating the coordination of the Schiff’s base
through the azomethine nitrogen [18]:
(1)
The starting complex [RuCl3(PPh3)3] was prepared according
to the reported literature procedures [19]. The ruthenium com-
plex was prepared by the reaction between [RuCl3(PPh3)3] and
Schiff’s base. The mixture (1:1 ratio) of [RuCl3(PPh3)3] (0.1 mmol)
and Schiff’s base ligand (0.1 mmol) in 20 mL benzene was heated
under reflux for 5 h. After completion of the reaction, the reaction
mixture was concentrated and cooled to room temperature fol-
lowed by the addition of ether. The solid complex was removed by
filtration, washed with petroleum ether, recrystallized from CH2Cl2
and dried in vacuum. The yield of the complex is found to be around
70%:
The applications in the oxidative conversion of amines to car-
boxylic acids and the usefulness of ruthenium catalysis in organic
reactions instigate us to carryout the title reaction to develop a
protocol for the oxidation of amines with an interest of studying
the oxidation mechanism with kinetic interpretations. The main
(2)
objectives of the present investigation are to: (i) develop an appro-
priate experimental condition for the oxidation process, (ii) study
the catalytic activity of catalyst, (iii) elucidate plausible mecha-
nism, (iv) deduce appropriate rate law and (v) ascertain the various
reactive species.
The complex was characterized spectroscopically as follows.
Strong band was observed in the region 1270–1300 cm−1 for
free Schiff’s base has been assigned to phenolic C O stretching.
After the complexation, this band was shifted to higher frequency
region 1290–1330 cm−1, indicates that the other coordination is
through the phenolic oxygen atom [20]. The characteristic absorp-
tion band for primary amine N H stretching is observed in the
region 3450–3500 cm−1. In the spectra of the new ruthenium(III)
complex, the absorptions due to the NH2 group has been observed
at a lower region 3400–3470 cm−1, indicating that the other coor-
dination site is amine nitrogen atom to the metal ion [21]. The other
characteristic bands due to triphenylphosphine were also present
in the expected region. Elemental analysis of the complex was also
made and the percentage of carbon, hydrogen and nitrogen were
found to be 58.25, 4.62 and 4.46 respectively.
2. Experimental
2.1. Preparation of oxidant
Bromamine-T was prepared [17] using chloramine-T. To a solu-
tion of CAT (20 g in 400 mL of water), about 4 mL of liquid bromine
was added drop wise with constant stirring at room temperature to
yield dibromamine-T (DBT). The solid DBT was filtered under suc-
tion, washed thoroughly with ice cold water until all the absorbed
bromine was removed and then vacuum-dried for 24 h. About 20 g
of DBT thus obtained was dissolved in 30 mL of 4 mol dm−3 NaOH
with constant stirring at room temperature and the resultant aque-
ous solution was cooled in ice. Pale yellow crystals of BAT formed
was filtered under suction, washed quickly with the minimum
amount of ice cold water and dried over P2O5. Aqueous solution
of the oxidant was standardized by iodometric procedure and pre-
served in brown bottles to prevent photochemical deterioration.
The electronic spectrum of ruthenium(III) complex showed
charge transfer bands [22]. Due to the relatively high oxidizing
properties of ruthenium(III), the charge transfer bands of type
ꢀ
L
y → t2g are prominent in the lower energy region. The electronic
spectrum of the complex is showed in the 360 nm region.
The EPR spectrum (Fig. 1) in solid state at X-band frequen-
cies of ruthenium(III) complex was recorded at room temperature.
The complex showed a single isotropic line with g value in the
2.34–2.44 range (Fig. 1). This type of isotropic lines is observed due
to intramolecular spin exchange.
2.2. Preparation of catalyst
Based on the above spectroscopic data the complex is hexa-
coordinated, with the donor atoms distributed about the metal
center in an octahedral fashion as shown in Fig. 2.
Ethylenediamine, diethylenetriamine and triethylenetetramine
(SD Fine Chem. Ltd. Ind.), tetraethylenepentamine, bishex-
The Schiff’s base ligand was prepared by the reaction between
salicylaldehyde and o-phenylene diamine. Ethanolic solutions
of salicylaldehyde (0.01 mol) in 20 mL and o-phenylenediamine
(0.01 mol) in 20 mL were mixed and refluxed for about 4 h. The reac-

C.H.V. Kumar et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 23–28
25
ThereactionwascarriedoutinstopperedPyrexboilingtubeswhose
outer surfaces were coated with black to eliminate photochemical
effects. For each run, requisite amounts of solutions of substrate,
NaOH, Ru(III) complex and water (to keep the total volume con-
stant for all runs) were taken in the tube and thermostated at 313 K
until thermal equilibrium was attained. A measured amount of BAT
solution, which was also thermostated at the same temperature,
was rapidly added with stirring to the mixture in the tube. The
course of the reaction was monitored by the iodometric determi-
nation of unreacted BAT in 5 mL of aliquots of the reaction mixture
withdrawn at different intervals of time. The course of the reaction
was studied for at least two half-lives. The pseudo first-order rate
constants (kꢀ) calculated from the linear plots of log [BAT] vs. time
were reproducible within 5%. Regression analysis of experimen-
tal data to obtain regression coefficient, r, was performed using a
fx-100W scientific calculator.
3. Results
3.1. Oxidation of amines
Selective oxidation of amines to carboxylic acids was made
with bromamine-T oxidant in alkaline medium in presence of cat-
alytic amount of ruthenium(III) complex. Stoichiometry of each
amine oxidation was determined by equilibrating each amine
with excess of oxidant in presence of 0.01 mol dm−3 NaOH and
1.0 × 10−4 mol dm−3 catalyst at 313 K for 48 h. Determination of the
unreacted BAT reveals the stoichiometry (amine:oxidant) of each
amine oxidation reaction and the results were presented in Table 1.
Oxidation of amines was achieved by stirring 1 mmol of amine
with stoichiometric amount of oxidant in presence of 0.01 mmol
of ruthenium catalyst and 0.1 mmol NaOH medium at 313 K. After
completion of the reaction (monitored by TLC), the reaction prod-
ucts were neutralized by acid and extracted with ether. The organic
products were subjected to GC-Mass analysis and products were
characterized by mass spectral analysis. The oxidation products
obtained and stoichiometry of the reactions were presented in
Table 1.
Fig. 1. EPR spectrum of complex [RuCl2(PPh3)(L)].
Fig. 2. Structure of complex RuCl2(PPh3)(L).
amethylenetriamine, 2-methylpentamethyldiamine and 2,2,4-
trimethyldiamaine (Aldrich) were of acceptable grades of purity
and were used as received. Aqueous solutions of these compounds
in 0.001 mol dm−3 HCl are employed. A solution of ruthenium(III)
complex in 0.02 mol dm−3 HCl was used as the catalyst. Allowance
was made for the amount of HCl present in catalyst solution, while
preparing solution for kinetic runs. Solvent isotope studies were
made in D2O (99.4% purity) supplied by Bhabha Atomic Research
Center, Mumbai, India. Reagent grade chemicals and doubly dis-
tilled water were used throughout the experiments.
3.2. Kinetics of oxidation of amines
The kinetics of oxidation of amines by BAT were investigated at
several initial concentrations of the reactants in alkaline medium.
With the substrate in excess, at constant [NaOH], [Amine]o and
temperature, plots of log [BAT] vs. time were linear (r > 0.9976) indi-
cating a first-order dependence of rate on [BAT]o. The values of
pseudo first-order rate constants (kꢀ) are given in Table 2. Further,
2.3. Kinetic measurements
The reactions were carried out under pseudo first-order condi-
tions by taking a known excess of [Amine]o over [oxidant]o at 313 K.
Table 1
Ruthenium complex catalyzed oxidative conversion of amines to carboxylic acids usingbromamine-T in NaOH medium.
Entry
A
Amine
Product(s)
Stoichiometry (Amine:BAT)
1:2
Conversion (%)
88
H2N-(CH2)2-NH2
H2N-CH2-COOH
B
1:4
89
C
1:4
88
D
E
F
H2N-(CH2)6-NH-(CH2)6-NH2
H2N-(CH2)2-NH-(CH2)2-NH2
H2N-(CH2)2-NH-(CH2)2-NH-(CH2)2-NH2
H2N-(CH2)2-NH-(CH2)2-NH-(CH2)2-NH-(CH2)2-NH2
2HOOC-(CH2)4-COOH
2H2N-CH2-COOH
2H2N-CH2-COOH + HOOC-COOH
3H2N-CH2-COOH + HOOC-COOH
1:7
1:7
1:8
1:10
90
92
90
91
G

26
C.H.V. Kumar et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 23–28
Table 2
Effect of varying reactant concentrations on the reaction rate.
103 [BAT]o (mol dm−3
)
102 [Amine]o (mol dm−3
)
104 kꢀ (s−1
)
A
B
C
D
E
F
G
0.25
0.50
1.00
2.00
4.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.25
0.50
1.00
2.00
4.00
3.20
3.15
3.10
3.16
2.96
0.75
1.50
3.10
6.24
12.6
4.10
4.00
4.20
4.30
4.15
1.02
2.11
4.20
8.42
5.20
5.15
5.10
5.21
4.95
1.30
2.60
5.10
7.0
6.8
6.9
6.85
7.01
1.70
3.50
6.9
7.90
7.75
7.85
7.90
7.95
2.00
4.00
7.85
10.3
10.2
10.4
10.1
10.5
2.7
12.1
12.2
12.0
11.9
12.1
3.10
6.05
12.0
5.20
10.4
20.9
42
10.2
20.6
14.0
28.0
15.9
31.2
24.1
48.4
17.0
[NaOH] = 1.0 × 10−2 mol dm−3; Ru(III) = 1.0 × 10−5 mol dm−3; T = 313 K
Table 3
Effect of varying NaOH and Ru(III) concentrations on the reaction rate.
102 [NaOH] (mol dm−3
)
105 [Ru(III)] (mol dm−3
)
104 kꢀ (s−1
)
A
B
C
D
E
F
G
0.25
0.50
1.00
2.00
4.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.25
0.50
1.00
2.00
4.00
1.41
2.10
3.10
4.51
6.85
1.00
1.81
3.10
5.31
8.90
1.90
2.85
4.20
6.31
9.21
1.31
2.40
4.20
7.10
2.35
3.41
5.10
7.62
11.3
1.70
3.02
5.10
8.91
2.99
4.51
6.9
10.0
14.4
2.39
4.01
6.9
11.5
20.0
3.61
5.41
7.85
11.9
17.8
2.91
4.79
7.85
13.8
23.0
4.51
7.00
10.4
15.2
22.4
3.59
6.10
10.4
17.8
29.0
5.55
8.4
12.0
18.6
28.1
4.31
7.51
12.0
21.8
37.0
12.0
14.8
[BAT]o = 1.00 × 10−3; [Amine]o = 1.00 × 10−2; T = 313 K.
the values of kꢀ are unaffected with variation of [BAT]o, confirming
first-order dependence on [BAT]o. The rate increases with increase
in [Amine]o (Table 2) and plot of log kꢀ vs. log [Amine] were lin-
ear (r > 0.9909) with unit slope indicating first-order dependence
of rate on [Amine]o. The rate of reaction increases with increase
in [NaOH] (Table 3) and plots of log kꢀ vs. log [NaOH] were lin-
ear (r > 0.9909) with slopes of 0.5–0.55 indicating fractional-order
dependence of rate on [NaOH]. Further, the rate of the reaction
increases with increasing [Ru(III)] (Table 3) and a plots of log kꢀ
vs. log [Ru(III)] were linear (r > 0.9957) with slopes less than unity
(0.7–0.75) indicating a fractional-order dependence of rate on
[Ru(III)].
Addition of p-toluenesulfonamide (PTS or TsNH2) and
halide ions, Cl− or Br− ions, in the form of NaCl or NaBr
(1–10 × 10−3 mol dm−3) or variation of the ionic strength (I)
of medium using NaClO4 solution (0.1–0.5 mol dm−3) showed
negligible effect on the rate. The dielectric constant of the solvent
medium was varied by adding methanol (0–30%, v/v), which had
negative influence on the rate (Table 4). Solvent isotope studies
in D2O medium (0–100%) with ethylenediamine as a probe gave
Table 4
Effect of varying dielectric constant (D) of the medium on the rate of reaction.
%MeOH (v/v)
D
104 kꢀ (s−1
)
A
B
C
D
E
F
G
0.0
10.0
20.0
30.0
76.73
72.37
67.48
62.71
3.10
2.81
2.41
2.00
4.20
3.61
3.20
2.70
5.10
4.41
3.80
3.11
6.90
5.71
5.00
4.10
7.85
6.71
5.80
4.61
10.4
8.70
7.41
5.91
12.0
10.9
8.91
7.11
[BAT]o = 1.0 × 10−3
;
[Amine]o = 1.0 × 10−2
;
[NaOH] = 1.0 × 10−2 mol dm−3
;
Ru(III) = 1.0 × 10−5 mol dm−3; T = 313 K.
the values: kꢀ(H2O) = 3.10 × 10−4 s−1 and kꢀ(D2O) = 3.88 × 10−4 s−1
.
The solvent isotope effect kꢀ(H2O)/kꢀ(D2O) = 0.81 indicates a slight
increase of rate in D2O medium.
The reactions were carried out in a range of temperature
303–323 K, keeping other experimental conditions constant. From
the linear Arrhenius plot of log k vs. log(1/T) (r = 0.9901), values of
composite activation parameters Ea, ꢀH#, ꢀG# and ꢀS# were eval-
uated. These results are summarized in Table 5. Addition of reaction
Table 5
Temperature dependence on the reaction rate and values of composite activation parameters for the oxidation of amines by BAT.
Temperature (K)
104 kꢀ (s−1
)
A
B
C
D
E
F
G
303
308
313
318
323
0.90
1.80
3.10
6.20
12.4
1.20
2.20
4.20
8.00
1.80
3.00
5.10
9.50
2.50
4.00
6.90
11.0
20.0
3.20
5.00
7.85
13.0
22.0
4.50
6.40
10.4
15.0
27.1
5.50
9.00
12.0
17.1
29.0
14.0
18.1
Ea (kJ mol−1
)
100
93.7
90.8
97.5
−18.8
83.7
80.7
96.8
−49.0
77.4
74.5
95.5
−62.7
71.1
68.2
95.0
−87.0
62.7
59.8
94.5
−110
56.5
53.3
94.0
−129
ꢀH# (kJ mol−1
ꢀG# (kJ mol−1
)
)
97.5
98.2
−5.00
ꢀS# (J K−1 mol−1
)
[BAT]o = 1.0 × 10−3; [Amine]o = 1.0 × 10−2; [NaOH] = 1.0 × 10−2 mol dm−3; Ru(III) = 1.0 × 10−5 mol dm−3
.

C.H.V. Kumar et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 23–28
27
mixture to acrylamide in an inert atmosphere did not initiate poly-
merization of the latter showing the absence of any free radical in
the reacting system.
4. Discussion
4.1. Reactive species of oxidant
Pryde and Soper [23], Morris et al. [24] and Bishop and Jennings
[25] have shown the existence of similar equilibria of N-metallo-
N-arylhalosulfonamides in aqueous media. Bromamine-T, like CAT
acts as an oxidizing agent in both acidic and alkaline solutions. The
oxidation potential of BAT/TsNH2 is pH dependent and decreases
with increase in the pH of the solution with 1.14 V at pH 0.65 and
0.50 V at pH 12. Depending on the pH, BAT exhibits the following
equilibria in aqueous solutions [23–26].
Scheme 1.
[31–33] the positive influence of OH− ion on the rate of haloamine
reactions with a number of substrates have been observed and sug-
gested RNX− (R = Ts or PhSO2; X = Cl or Br) is the reactive oxidizing
species. In the present investigations, rate of the reaction is accel-
erated by OH− ions clearly indicates that the anion TsNBr− is the
significant oxidizing species involved in the oxidation of FA by BAT
in all the four catalyzed reactions.
TsNBrNa ꢀ TsNBr− + Na+
TsNBr− + H+ ꢀ TsNHBr
(3)
(4)
(5)
(6)
(7)
(8)
(9)
2TsNHBr ꢀ TsNH2 + TsNBr2
TsNHBr + H2O ꢀ TsNH2 + HOBr
TsNBr2 + H2O ꢀ TsNHBr + HOBr
HOBr ꢀ H+ + OBr−
4.2. Mechanism and rate law for the oxidation of amines
Under the experimental conditions, [OH−] ꢁ [Ru(III)] and the
fact that [OH−] increases the rate, ruthenium(III) is mostly present
as the hydroxylated species [Ru(OH)2(PPh3)(L)] and its formation
is given in the following equilibrium:
HOBr + H+ ꢀ H2OBr+
[RuCl2(PPh3)(L)] + 2OH− ꢀ [Ru(OH)2(PPh3)(L)] + 2Cl−
(10)
where Ts = p-CH3C6H4SO2.
The possible oxidizing species in acidified BAT solutions are
TsNHBr, TsNBr2 HOBr and possibly H2OBr+ and possible oxidiz-
ing species in alkaline medium would be TsNBr− and OBr− anions.
These anionic species were transformed into more reactive species,
TsNHBr and HOBr during the course of the reaction in alkaline and
acidic media. Several workers have observed the retarding effect of
OH− ions on the rate of haloamines with a number of substrates
[27–30] and have suggested that the reactivity of weakly alkaline
solutionsofhaloaminesisduetotheformationoftheconjugateacid
TsNHBr from TsNBr− in OH− retarding step. But in the present inves-
tigations, the OH− ions increases the rate of the reaction clearly
indicates TsNBr− is the reactive oxidizing species. In earlier work
The existence of intermediate between Ru(III) and oxidant was
evidenced from the UV–vis spectra of both Ru(III) and Ru(III)-BAT
mixture, in which a shift of Ru(III) from 360 to 350 nm was observed,
indicating the intermediate species between catalyst and oxidant.
Based on the above discussion and the observed kinetic results,
mechanism as shown in Scheme 1 is proposed for the ruthenium
catalyzed oxidation of amines by BAT in alkaline medium.
In Scheme 1, X and Xꢀ present the intermediate species whose
structures are shown in Scheme 2. The detailed mechanism of
ruthenium complex catalyzed oxidative conversion of amines is
shown in Scheme 2.
Scheme 2.

28
C.H.V. Kumar et al. / Journal of Molecular Catalysis A: Chemical 311 (2009) 23–28
ative values of ꢀS# indicate a more ordered, rigid transition state
The total effective concentration of BAT is:
for each substrate. Consistent values of ꢀG# is in favor of common
[BAT]t = [TsNHBr] + [TsNBr−] + [X]
By substituting for [TsNHBr] and [TsNBr−] from steps of (i) and (ii)
of Scheme 1 in Eq. (11) and solving for [X], we get:
(11)
mechanism operates for all the amine oxidation reactions.
5. Conclusions
K1K2[BAT]t[Ru(III)][OH−]
[X] =
(12)
Oxidative conversion of aliphatic amines to carboxylic acids was
achieved efficiently using bromamine-T oxidant with Ru(III) com-
plex catalyst. Hexa-coordinated ruthenium complex was prepared
and used and its catalytic efficiency was observed in the amine
oxidation. All the seven reactions followed identical kinetics with
similar mechanism operation Activation parameters and isokinetic
temperature were deduced. The reactions were carried out at differ-
ent temperatures and the activation parameters were evaluated for
each reaction. The present method developed for the oxidation of
amines to carboxylic acids by BAT offers several advantages includ-
ing good conversion, short reaction times, and stable, cost effective
and relatively non-toxic reagents which make the reaction process
simple and smooth. The observed results have been explained by a
plausible mechanism and the related rate law has been deduced. It
can be concluded that Ru(III) complex/bromamine-T system effec-
tively performs the oxidative conversion of amines in alkaline
medium.
[H2O] + K1[OH−] + K1K2[Ru(III)][OH−]
From the slow step of Scheme 1:
rate = −d[BAT]t/dt = k3[X][Amine]
(13)
By substituting for [X] from Eq. (12) into Eq. (13), the following rate
law was obtained:
K6K7k8[BAT]t[Amine][Ru(III)][OH−]
[H2O] + K1[OH−] + K1K2[Ru(III)][OH−]
rate =
(14)
Scheme 1 and the rate law (14) are consistent with the observed
experimental results and supported by the following facts.
For reaction involving fast pre-equilibrium H+ or OH− ion trans-
fer, the rate increases in D2O since D3O+ and OD− are 2–3 times
stronger acids and stronger bases [34,35], respectively, than H3O+
and OH− ions. In the present studies the observed solvent isotope
effect of kꢀ(H2O)/kꢀ(D2O) < 1 is due to the greater basicity of OD−
compared to OH−. The magnitude of increase of rate in D2O is
small (kꢀ(H2O)/kꢀ(D2O) = 0.81) compared to the expected value of
2–3 times greater, which can be attributed to the fractional-order
dependenceofrateon[OH−]. Thenegligibleinfluenceofvariationof
the ionic strength and addition of PTS and halide ions on the rate of
the reaction and also activation parameters are in good agreement
with the mechanism proposed and the rate law derived.
A decrease in the rate with a decrease in D of the medium sup-
ports the proposed mechanism. Amis and Jaffe [36] have shown
that:
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