Kinetic role of tert-amines in the osmium tetroxide catalyzed trimethylamine N-oxide dihydroxylation of cyclohexene

Article abstract of DOI:10.1002/(SICI)1097-4601(1997)29:5<359::AID-KIN6>3.0.CO;2-T

Effect of some tert-amines on the catalytic osmium tetroxide dihydroxylation of cyclohexene in aqueous tert-butyl alcohol has been investigated. All amines have been found to retard the catalysis greatly and beyond a definite concentration of amine, the rate reaches a minimal and remains constant. The oxidation of cyclohexene is inhibited by pyridine, 2,2′-bipyridyl, and DABCO with an inverse first-order dependence whereas inhibition by triphenylamine, N,N-diethylaniline, picoline, pyrazine, hexamethylenetetraamine, and TMEDA shows an inverse partial order dependence. The involvement of dioxomonoglycolatoosmium(VI) esters and their monoamine adducts in the rate determining oxidation step was established by the linear plots of 1/Δk₂ vs. 1/[L] where Δk₂ is the decrease in the second-order rate constant in the presence of [L] concentration of tert-amine. The ligand-accelerated or ligand-decelerated catalysis of tert-amines in the catalytic osmium tetraoxide dihydroxylation of alkenes may vary depending on the secondary oxidant, on the alkene, and on the structure and concentration of the tert-amine.

Full text of DOI:10.1002/(SICI)1097-4601(1997)29:5<359::AID-KIN6>3.0.CO;2-T

Kinetic Role of Tert-Amines
in the Osmium Tetroxide
Catalyzed Trimethylamine
N-Oxide Dihydroxylation of
Cyclohexene
ENDER ERDIK, DIDEM KÂHYA
Ankara University, Science Faculty, Be s¸ evler, Ankara 06100, Turkey
Received 6 May 1996; accepted 9 August 1996
ABSTRACT: Effect of some tert-amines on the catalytic osmium tetroxide dihydroxylation of
cyclohexene in aqueous tert-butyl alcohol has been investigated. All amines have been found
to retard the catalysis greatly and beyond a definite concentration of amine, the rate reaches
a minimal and remains constant. The oxidation of cyclohexene is inhibited by pyridine, 2,2Ј-
bipyridyl, and DABCO with an inverse first-order dependence whereas inhibition by triphenyl-
amine, N,N-diethylaniline, picoline, pyrazine, hexamethylenetetraamine, and TMEDA shows
an inverse partial order dependence. The involvement of dioxomonoglycolatoosmium(VI) es-
ters and their monoamine adducts in the rate determining oxidation step was established by
the linear plots of 1/⌬k vs. 1/[L] where ⌬k is the decrease in the second-order rate constant
2
2
in the presence of [L] concentration of tert-amine. The ligand-accelerated or ligand-deceler-
ated catalysis of tert-amines in the catalytic osmium tetraoxide dihydroxylation of alkenes
may vary depending on the secondary oxidant, on the alkene, and on the structure and con-
centration of the tert-amine. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 359–366, 1997.
INTRODUCTION
There have been several kinetic and mechanistic
studies of plain dihydroxylation [1–8] and Sharpless’
asymmetric dihydroxylation of alkenes [9–13] by
The kinetic behavior with respect to tert-amines as
ligand catalysts in the osmium tetraoxide catalyzed
dihydroxylation of cyclohexene with trimethylamine
N-oxide in aqueous tert-butyl alcohol was reexam-
ined to obtain further insight into the ligand decelera-
tion mechanism of tert-amines.
stoichiometric OsO oxidation [1–5,10,12,13] or by
4
catalytic OsO oxidation using a secondary oxidant
4
[6,7,9,10]. Certain tert-amines [2,4,5,8] and chiral al-
kaloids [9,10,12–14] are reported as ligand acceler-
ated catalysts [15] in the stoichiometric and catalytic
OsO4 dihydroxylation of alkenes. Stoichiometric
OsO4 dihydroxylation [2–5,9,10,12–14] and cat-
Correspondence to: E. Erdik
Contract grant Sponsor: Scientific and Technical Research
Council of Turkey
alytic OsO dihydroxylation using perchlorate [6] and
4
N-methylmorpholine-N-oxide [10] as oxidant were
©
1997 John Wiley & Sons, Inc.
CCC 0538-8066/97/050359-08
found typically first-order in alkene and first-order

3
60
ERDIK AND KÂHYA
in OsO . In the stoichiometric OsO plain dihy-
EXPERIMENTAL
Chemicals
4
4
droxylation catalyzed by tert-amines, rate laws with
additional terms which are first-order [2,4,5] and sec-
ond-order in the tert-amine [4,5], were reported.
Sharpless’ group reported that the rate determining
step is the same in stoichiometric [9,10,12,13] and
catalytic [9,10] dihydroxylation of alkenes using N-
methylmorpholine-N-oxide in tert-butyl alcohol at
tert-Butyl alcohol was fractionated. Osmium tetrox-
ide and trimethylamine N-oxide were obtained from
the Aldrich Chemical Co. Cyclohexene and tert-
amines were obtained from various sources and puri-
fied according to the published procedures [16].
2
5°C in the presence of chiral alkaloids [9,10,12,13]
or some tert-amines [9,10,12] and consists of the ini-
tial addition reaction forming dioxomonoglycolato
osmium(VI) esters, 1 and their monoamine adducts,
Kinetic Measurements
2
(Scheme I, step a and step b). They also found that
the rate reaches a maximal and constant value beyond
a definite tert-amine concentration and the onset of
this rate saturation corresponds to a preequilibrium
The kinetics of catalytic OsO dihydroxylation of cy-
4
clohexene using trimethylamine N-oxide as sec-
ondary oxidant were followed by measuring the con-
centration of the remaining alkene by GC with a
silicone fluid column on a Perkin Elmer F-11 gas
chromatograph equipped with an electronic integrator
[7]. In a jacketed reaction vessel of approximately
50 mL capacity capped with two rubber septa,
equipped with a magnetic stirrer, and flushed with
nitrogen, a solution of trimethylamine N-oxide and
cyclohexene in 6 mL of 90% aqueous tert-butyl
alcohol solution was thermostated at 50°C. A solu-
between OsO and ligand tert-amine with a binding
4
constant, K [9,10,12,13]. In the catalytic OsO oxi-
o
4
dation of alkenes with ferricyanide in aqueous tert-
butyl alcohol at 25°C, amines were reported to accel-
erate the reaction by promoting hydrolysis rater than
osmylation step [8]. OsO oxidation of alkenes with
4
ferricyanide in aqueous tert-butyl alcohol at 25°C,
amines were reported to accelerate the reaction by
promoting hydrolysis rather than osmylation step [8].
O
O
O
O
O
O
O
HO
[
O]
c
H O
e
2
OsO4
Os
O
O
Os
ϩ OsO4
a
HO
1
2
3
K0
L
K1
L
O
O
O
O
O
L
O
HO
HO
[
O]
d
H O
2
OsO L
L
Os
Os
O
ϩ OsO ϩ L
4
4
и
b
f
O
O
K2
L
O
L
O
O
[O]
Os
O
L
Scheme I Kinetic scheme for the catalytic OsO oxidation of cyclohexene in the presence of a tert-amine, L
4

KINETIC ROLE OF TERT-AMINES
361
tion of tert-amine in tert-butyl alcohol was added
followed by internal standard and 4% aqueous solu-
tion of OsO . Beginning two minutes after injection
4
N
N
N
N
of the OsO solution, aliquots were withdrawn at ap-
4
propriate times by removing 0.2–0.3 mL of the reac-
tion mixture by syringe and adding it slowly to a vial
containing a quenching solution of 10% aqueous
sodium bisulfite and ether. By capping and shaking
the vial, cyclohexene and the internal standard was
quantitatively extracted to the ethereal phase. The re-
action was generally followed for four half-lives. The
data were taken with almost equal concentrations
of cyclohexene and trimethylamine N-oxide under
4
5
6
N
CH2 N(CH3)2
CH2
N(CH3)2
N
CH3
N
7
8
9
N(C H )
2
5 2
N
limiting OsO concentrations, i.e., [cyclohexene] ϭ
N
4
N
N
N
0
.436 M, [trimethylamine N-oxide] ϭ 0.451 M,
Ϫ3
Ϫ3
[OsO ] ϭ 0.7 ϫ 10 M Ϫ 1.2 ϫ 10 M. The ob-
4
N
served pseudo-first-order rate constants, k were cal-
1
1
0
11
12
culated by the method of least squares from the
slopes of the linear plots of log c vs. time where c is
t
t
the concentration of cyclohexene at time t. Rate con-
stants, k were calculated as k /[OsO ]. With all ex-
amined tert-amines first-order plots proved linear
2
1
4
N
over three half-lives.
1
3
RESULTS AND DISCUSSION
Formulae for compounds 4–13.
Our previous kinetic studies on the catalytic OsO ox-
4
Reaction Order in Osmium, Alkene, and
Secondary Oxidant
idation of cyclohexene and ␣-pinene with trimethyl-
amine N-oxide as oxidant in aqueous tert-butyl alco-
hol at 50°C and 70°C, respectively, led to a rate law
The data showed that the reaction is first-order in to-
which is first-order in OsO , first-order in trimethyl-
tal OsO , first-order in trimethylamine N-oxide, and
4
4
amine N-oxide, and zeroth-order in alkene, and the
rate determining step is the oxidation of 1 and 2 by
trimethylamine N-oxide (Scheme I, step c and step d)
zeroth-order in alkene. The second-order rate con-
stants, k2 were found from pseudo-first-order rate
constants, k of the reactions carried out at almost
1
[7]. The reaction of cyclohexene is inhibited by pyri-
equal concentrations of cyclohexene and trimethyl-
dine, 4 and 2,2Ј-bipyridyl, 5 with an inverse first-
order dependence. ␣-Pinene behaves differently.
Pyridine increases the rate of ␣-pinene dihydrox-
ylation whereas 2,2Ј-bipyridyl has a slight retarding
effect.
amine N-oxide in the presence of catalytic concentra-
tions of OsO and a tert-amine ligand:
4
d[Os(VI) esters]
d[cyclohexene]
Ϫ
ϭ Ϫ
dt
dt
o
We report here the ligand decelerated catalysis of
ϭ k [OsO ] [Me NO] [cyclohexene] (1)
2 4 3
various tert-amines in the catalytic OsO dihydroxy-
4
o
rate ϭ k [Me NO] [cyclohexene]
(2)
(3)
1
3
lation of cyclohexene with trimethylamine N-oxide
in aqueous tert-butyl alcohol at 50°C. We tested
quinoline, 6, 2-picoline, 7, pyrazine, 8, TMEDA
k1
k2 ϭ
[
OsO4]
(
N,N,N,NЈ-tetramethyl
DABCO (1,4-diazabicyclo [2,2,2] octane), 10, hexa-
methylenetetraamine (1,3,5,7-tetraazaadamantane,
ethylene
diamine),
9,
Typical first-order plots for triphenylamine, picoline,
and urotropin decelerated catalytic OsO dihydroxy-
4
urotropin), 11, N,N-diethylaniline, 12, and triphenyl-
amine, 13 as ligand catalysts.
lation of cyclohexene obtained over three half-lives
are given in Figure 1.

3
62
ERDIK AND KÂHYA
[10–13]. All tert-amine ligands are known to acceler-
ate the osmylation step [2,4,5,9,10,12,13]. However,
if they bind too strongly to the resulting osmium(VI)
esters, 1 to form 2, they cause resistance to oxidation.
In our catalytic OsO oxidation conditions, both os-
4
mium(VI) esters, 1 and their monoamine adducts, 2
are expected to react with trimethylamine N-oxide
until ligand saturation concentration of amines, [L]
(sat); however at concentration of amines higher than
[
L] (sat), mostly osmium(VI) esters, 2 will react.
Then saturation rate constant, k (sat) seems to be the
2
rate constant of the catalytic OsO oxidation of only
4
2
, i.e., k (sat) ϭ k . However, to explain the rate re-
2 d
tardation, now we can not exclude the formation of
coordinatively saturated bisamine adducts of diox-
omonoglycolatoosmium(VI) esters, 3, which are
completely unreactive toward OsO4 oxidation.
Chelating diamines and monoamines with substan-
Figure 1 Typical first-order plots of the catalytic OsO4
oxidation of cyclohexene with trimethylamine N-oxide in
9
0% aqueous tert-butyl alcohol at 50°C in the presence of a
tially higher binding constants, K are expected to in-
tert-amine; [cyclohexene] ϭ 0.436 M, [Me NO] ϭ 0.451
2
3
Ϫ3
hibit the oxidation at a lower ligand concentration by
forming stable 3 easily.
M, [OsO ] ϭ 1.2 ϫ 10
M, [triphenylamine] ϭ 4.6 ϫ
M, [picoline] ϭ 0.1 M, and [urotropin] ϭ 5 ϫ 10
4
Ϫ3
Ϫ2
1
0
M. (A /A is the ratio of the peak area of cyclohexene to
As Sharpless and co-workers have already re-
ported that saturation rate constants of the ligand ac-
m
s
that of internal standard and is proportional to c .).
t
celerated stoichiometric OsO dihydroxylation are in-
4
fluenced principally by the nature of the alkaloid
ligands and they are not correlated with K constants
o
Reaction Order in Tert-Amine
[13], we have not tried a systematic study between k2
k values of catalytic OsO dihydroxylation of cyclo-
(sat) values and K and K constants and we did not
2
4
o
2
hexene with trimethylamine N-oxide in the presence
of varying concentrations of triphenylamine, 13, N,N-
diethylaniline, 12, 2-picoline, 7, pyrazine, 8,
urotropin, 11, TMEDA, 9, DABCO, 10, pyridine, 4,
and 2,2Ј-bipyridyl, 5 are given in Table I. All amines,
except quinoline, 6 greatly retard the catalysis and
beyond a definite concentration of amine, the rate
reaches a minimal and remains constant. Saturation
behaviors of ligands pyrazine and TMEDA are shown
in Figure 2. As expected, quinoline did not change
measure these binding constants. We compared k2
(sat) values and [L] (sat) values and showed that k2
(sat) values may be considered as the constants for
the catalytic OsO oxidation of osmium(VI) esters, 2.
4
DISCUSSION
According to our mechanism given in the scheme, the
overall rate of catalytic OsO dihydroxylation of cy-
4
the rate constant since chelation of OsO by the
4
clohexene is the rate of oxidation of osmium(VI) es-
quinoline N is geometrically impossible [10]. Range
of amine concentrations, [L], saturation rate con-
ters, C H O OsO , 1 and C H O OsO L, 2:
6
10
2
2
6
10
2
2
и
stants, k (sat), and the concentration of amines, [L]
2
(
sat), beyond with k (sat) values remain constant, are
d[Os(VI) esters]
2
rate ϭ Ϫ
ϭ k [Os(VI) esters]
2
given in Table II to compare the ligand-decelerated
dt
effect of amines. In the catalytic OsO dihydroxyla-
4
[Me NO]
3
tion of cyclohexene using trimethylamine N-oxide as
secondary oxidant, we may attribute the rate retarda-
tion by tert-amines to the formation of monoamine
adducts of dioxomonoglycolatoosmium(VI) esters, 2
since 2 is not as easily oxidized as dioxomonoglyco-
latoosmium (VI) esters, 1 in the rate determining
step. Sharpless’ group have shown that 2 is formed
ϭ {k [C H O OsO ]
c
6
10
2
2
ϩ k ([C H O OsO .L]
d
6
10
2
2
Ϫ K [C H O OsO .L][L])}[Me NO]
(4)
2
6
10
2
2
3
and this assumption requires that essentially all of the
osmium be present as osmium(VI) esters, 1, 2, or 3,
so that [osmium(VI) esters] ϭ [OsO4 ]total . As OsO4
and OsO L yield 1 and 2, respectively, the rate of
from OsO .L and alkene in the osmylation step
4
4
и

KINETIC ROLE OF TERT-AMINES
363
Table I Effect of Tert-amines, L on the Rate of the Catalytic OsO Oxidation of Cyclohexene with Trimethylamine
4
N-Oxide in 90% Aqueous Tert-Butyl Alcohol at 50°C (Data for the Reaction in the Presence of Pyridine, 2,2Ј-Bipyridyl,
and DABCO (Unpublished Material) are Taken from Ref. [7])
L:Triphenylamine
03 [L] (M) k2(MϪ1sϪ1
L:N,N-Diethylaniline
L:Picoline
k2(MϪ1sϪ1
)
3
k2(MϪ1sϪ1
1
)
10 [L] (M)
)
[L] (M)
0.5
1.6
2.2
3.0
4.6
1.96
1.78
1.67
1.34
1.24
3.9 ϫ 10Ϫ6
1.3 ϫ 10Ϫ4
5.4 ϫ 10
1.0 ϫ 10Ϫ2
1.43
1.35
1.39
0.90
0.83
0.77
0.86
0.02
0.04
0.1
0.2
0.5
1.41
1.24
1.42
0.91
0.90
0.63
0.64
Ϫ4
0.1
0
1
.5
.0
0.75
1.0
L:Pyrazine
L:Urotropin
L:TMEDA
03 [L] (M)
k2(MϪ1sϪ1
)
10 [L] (M)
2
k2(MϪ1sϪ1
)
10 [L] (M)
6
k2(MϪ1sϪ1
)
1
4
9
9
1
90
00
50
.8
.1
1.58
1.22
0.49
0.63
0.26
0.19
0.26
2.0
3.5
5.0
1.84
1.63
1.32
0.97
0.73
0.39
0.13
0.15
4
5
1.13
1.02
0.81
0.87
0.60
0.75
0.52
0.54
0.50
0.29
0.34
4
9
10
12
15
20
30
60
00
25
00
9.0
1
3
4
15.0
20.0
35.0
5
0.0
1
1
3
L:DABCO
L:Pyridine
L:2,2Ј-Bipyridyl
03 [L] (M)
k2(MϪ1sϪ1
)
10 [L] (M)
2
k2(MϪ1sϪ1
)
10 [L] (M)
3
k2(MϪ1sϪ1
)
1
2
4
9
.3
.6
.2
0.27
0.15
0.08
0.04
1.8
3.6
5.4
7.2
0.29
0.12
0.11
0.07
7.6
11.7
15.2
22.8
0.040
0.025
0.0185
0.014
1
8.4
8
9
8.4
.6
.2
0.055
0.032
0.030
1
osmylation may be written as:
[OsO4]total
[
OsO ] ϭ
and
4
1
ϩ K [L]
o
rate ϭ {k [OsO ]
c
4
K [L][OsO ]
o
4
total
ϩ k [OsO L](1 Ϫ K [L])}[Me NO] (5)
d
4
и
2
3
[OsO L] ϭ
(8)
4
и
1
ϩ K [L]
o
[
OsO4 ]total can be defined as
Substituting eq. (8) into eq. (5)
OsO4]total
[OsO4 ]total ϭ [OsO ] ϩ [OsO L]
(6)
4
4
и
[
rate ϭ {kc
1
ϩ K [L]
o
and in the presence of tert-amine ligand, L, OsO and
4
OsO L are in rapid equilibrium with one another:
K [L][OsO ]
4
и
o
4
total
ϩ kd
1
ϩ K [L]
o
[
OsO4 L]
и
K ϭ
o
(7)
(1 Ϫ K [L])}[Me NO] (9)
2 3
[
OsO ][L]
4
is obtained. The second-order rate constant, k is
2
Combining eqs. (6) and (7), we may define [OsO4 ]
and [OsO L] as:
rate ϭ k [OsO ] [Me NO]
(10)
4
и
2
4
total
3

3
64
ERDIK AND KÂHYA
rate ϭ (k [C H O OsO ]
c
6
10
2
2
ϩ k [C H O OsO L])[Me NO] (12)
d
6
10
2
2
и
3
and k may be given as
2
k ϩ k K [L]
c
d
o
k2 ϭ
(13)
1
ϩ K [L]
o
and this equation is the same as given by Sharpless
for the ligand accelerated OsO dihydroxylation of
4
alkenes [10]. The difference between k and k can be
c
2
written as
Figure 2 Plot of second-order rate constants, k , vs.
2
amine concentration, [L] for the catalytic OsO oxidation
of cyclohexene with trimethylamine N-oxide in 90% aque-
ous tert-butyl alcohol at 50°C in the presence of a tert-
K [L](k Ϫ k )
4
o
c
d
k Ϫ k ϭ ⌬k ϭ
(14)
(15)
c
2
2
1
ϩ K [L]
o
amine; [cyclohexene] ϭ 0.436 M, [Me NO] ϭ 0.451 M,
3
or
Ϫ4
and [OsO ] ϭ 7.3 ϫ 10 M.
4
1
1
1
1
ϭ
ϩ
⌬
k2
K (k Ϫ k ) [L]
(k Ϫ k )
c d
o
c
d
where
and plots of 1/⌬k vs. 1/[L] should be linear.
2
k ϩ k K [L] (1 Ϫ K [L])
For the catalytic OsO dihydroxylation of cyclo-
hexene in the presence of triphenylamine, 2-picoline,
pyrazine, urotropin, TMEDA, DABCO, pyridine and
c
d
o
2
4
k2 ϭ
(11)
1
ϩ K [L]
o
If we assume 1 ϾϾ K [L] at our experimental condi-
2,2Ј-bipyridyl, 1/⌬k vs. 1/[L] plots were found to be
2
2
tions (for a chelating amine, TMEDA, [L] is varied
between 4 ϫ 10 M Ϫ 300 ϫ 10 M), the rate
linear establishing the involvement of 1 and 2 in the
rate determining oxidation step. We have not ob-
Ϫ6
Ϫ3
may be given as
served a regular decrease in k constants with increas-
2
Table II Range of Amine Concentrations, [L], Saturation Rate Constants, k (Sat), Concentration of Amines, [L](Sat),
2
Beyond Which k (Sat) Values Remain Constant, Reaction Order in Amines, and Calculated k Constants in the
2
d
Catalytic OsO Dihydroxylation of Cyclohexene with Trimethylamine N-Oxide in the Presence of Tert-Amines in
4
a
Aqueous 90% Tert-Butyl Alcohol at 50°C
1
03[L](Sat)
(M)
k (Sat)
Reaction
Order
Calculated kd
(M s )
2
3
Ϫ1 Ϫ1 b
Ϫ1 Ϫ1 c
Tert-Amine, L
10 [L] (M)
(M
s
)
Triphenylamine
N,N-Diethylaniline
0.5–4.6
4.6
0.1
750
300
350
125 ϫ 10Ϫ3
1.24
0.90
0.63
0.19
0.13
0.29
0.04
0.03
0.01
Ϫ0.35
1.14
–
0.71
3.9 ϫ 10Ϫ6 –1
d
2
-Picoline
20–1000
4.8–450
20–500
Ϫ0.20
Ϫ0.55
Ϫ0.80
Ϫ0.35
Ϫ0.90
Ϫ1.04
Ϫ1.00
Pyrazine
Urotropin
TMEDA
0.24
e
4 ϫ 10Ϫ3 –300 ϫ 10Ϫ3
2.3–18.4
0.43
0.00
0.00
0.00
f,g
DABCO
Pyridine
2
18
92
23
g
18–184
7.6–23
,2Ј-Bipyridylg
a
Cyclohexene] ϭ 0.436 M, [trimethylamine N-oxide] ϭ 0.451 M, and [OsO ] ϭ 0.7 ϫ 10 M Ϫ 1.2 ϫ 10Ϫ3 M.
Ϫ3
[
4
b
c
d
e
f
Ϫ1 Ϫ1
Ϫ1 Ϫ1
kc ϭ 2.24 M
s
(i.e., k at [L] ϭ 0). (k ϭ 1.31 M
s
for pyridine, 2,2Ј-bipyridyl, and DABCO.)
2
c
Calculated from the intercept of the linear 1/(k –k ) vs. 1/[L] plots according to eq. (15).
log k vs. log [L] was not found to be linear.
kd was found as a negative value.
From ref. [7] (unpublished material).
From ref. [7] for comparison.
c
2
2
g

KINETIC ROLE OF TERT-AMINES
365
kc
k2 ϭ
(16)
K [L]
o
and linear plots of k vs. 1/[L] should be obtained for
2
the tert-amine decelerated catalytic OsO oxidation of
4
cyclohexene yielding almost zero k (sat) values. As
2
seen in the Table II, DABCO, pyridine, and 2,2Ј-
bipyridyl are the tert-amines which lead to very low
k (sat) values (calculated k values are zero) among
2
d
nine tested amines as ligand-decelerated catalysts and
slopes of log k vs. log [L] plots for these three
2
amines were found to be almost Ϫ1 supporting the
correctness of the above assumptions.
The effectiveness of amines as ligand decelerated
catalysts in catalytic OsO dihydroxylation of cyclo-
4
hexene may be considered dependent upon the fol-
lowing factors: (i) The coordination ability of the
amine to OsO to form OsO L, i.e., the K constant;
Figure 3 Plot of 1/(k Ϫ k ) vs. 1/[L] for the catalytic
OsO4 oxidation of cyclohexene with trimethylamine N-
oxide in 90% aqueous tert-butyl alcohol at 50°C in the
c
2
4
4
и
o
(
ii) The ability of the amine to coordinate to 2 to
form 3, i.e., the K constant; and (iii) The oxidation
presence of
a
tert-amine; [cyclohexene] ϭ 0.436 M,
2
Ϫ4
[
Me NO] ϭ 0.451 M, and [OsO ] ϭ 7.3 ϫ 10 M.
ability of 2. k (sat) constants seem to be controlled
3
4
2
mainly by (i) and (ii). The higher K and K con-
o
2
stants will lead to the formation of more osmium(VI)
esters 2 and 3 and consequently lower k (sat) values
2
will be obtained. Chelating amines, TMEDA and
2,2Ј-bipyridyl as well as pyridine which is known to
give a bis amine adduct [17] lead to the lowest k2
(sat) values as expected. The presence of DABCO re-
sults in a k2 (sat) constant as low as pyridine.
Pyrazine and urotropin are the other two ligands that
ing amine concentration in the N,N-diethylaniline de-
celerated catalytic OsO oxidation of cyclohexene.
4
Linear plots of 1/⌬k2 vs. 1/[L] for pyrazine and
TMEDA decelerated reactions are given in Figure 3.
Additionally, we calculated k values from the inter-
d
cept of these plots and compared with k (sat) values
lead to substantially lower k (sat) constants. Pyrazine
2
2
to substantiate the assumption of k (sat) ϭ k . As
and cage ligands DABCO and urotropin are known to
2
d
seen from Table II, calculated k values are almost
give also L 2 OsO type adducts and DABCO may
d
и
4
equal to k (sat) values for catalytic OsO dihydroxy-
retain its bridging role between two osmium(VI) es-
ters, 1 while forming monoamine adducts, 2 [18].
Possibly due to this capability, DABCO decreases the
rate by forming two times excess 2 compared to a
mono amine and reduces the catalyst turnover easily
2
4
lation of cyclohexene performed in the presence of
triphenylamine, 2-picoline, pyrazine, DABCO, pyri-
dine, and 2,2Ј-bipyridyl supporting the above as-
sumption.
We also tried to find the reaction order in tert-
resulting in one of the lowest k (sat) values as a
2
amines from the slopes of log k vs. log [L] plots.
Since we already found almost inverse first-order de-
chelating amine. The lowest [L] (sat) value was ob-
tained in the ligand decelerated catalysis of N,NЈ-di-
ethylaniline, but not that of chelating amines, pyri-
dine and 2,2Ј-bipyridyl, as expected. This can be
explained by the availability of lone pair on the N
atom for strong N!Os coordination in N,N-diethyl-
aniline since the N atom is included in the conjugated
system of pyridine and 2,2Ј-bipyridyl. So monoamine
adducts of pyridine and 2,2-bipyridyl can be more
easily oxidized than that of N,N-diethylaniline. By
the same reason, TMEDA leads to rate saturation at a
much lower [L] (sat) value than that obtained in the
presence of other chelating amines, pyridine and 2,2Ј-
bipyridyl. DABCO, due to its above discussed capa-
2
pendence of k on pyridine and 2,2Ј-bipyridyl con-
2
centration [7]. However, only in the case of DABCO
decelerated OsO dihydroxylation, we observed a re-
4
action order of Ϫ 0.9 in the amine. Partial reaction or-
ders were obtained for other amines (Table II). This
result further substantiates eq. (13) as the rate law for
the ligand decelerated catalytic OsO oxidation of cy-
4
clohexene. Since binding of tert-amine ligand to the
osmium(VI) ester decelerates highly or completely
inhibits its oxidation, i.e., k ϾϾ k K [L] (or simply
c
d
o
k ϭ 0) and K [L] ϾϾ 1 is assumed, eq. (13) is sim-
d
o
plified to give

3
66
ERDIK AND KÂHYA
bility, shuts down the oxidation at a [L] (sat) value as
low as that obtained in the presence of 2,2Ј-bipyridyl.
Triphenylamine gives a [L] (sat) value, which is
much higher than that found with N,N-diethylaniline,
but much lower than that found with pyridine as ex-
pected from the strength of N!Os coordination. It
seems that the controlling factor for reaching [L] (sat)
value is (iii) rather than (i) or (ii).
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(
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4
Ϫ9
Ϫ3
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1
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4
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4
droxylation of alkenes may vary depending on the
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1
6. D. D. Perrin and W. L. F. Armarego, Purification of
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1
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We thank the Scientific and Technical Research Council of
Turkey for Grant No. TBAY-80. We also thank Prof. D. S.
Matteson for his continuing interest in this work.
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