Quinolinium Bichromate Oxidation of Diols: A Kinetic Study

Article abstract of DOI:10.1021/jo961079m

Quinolinium dichromate (QDC) in water oxidizes vicinal and nonvicinal diols to the corresponding α-hydroxy carbonyl compound. The rate is proportional to the concentrations of diol, oxidant and hydrogen ions, and the reactions proceed almost twice as fast in D₂O than in H₂O. These data can be correlated with a mechanistic pathway involving the intermediate formation of a chromate ester, which undergoes decomposition to give the product.

Full text of DOI:10.1021/jo961079m

J . Org. Chem. 1996, 61, 8875-8877
8875
Qu in olin iu m Dich r om a te Oxid a tion of Diols: A Kin etic Stu d y
Bernadette Kuotsu, Enboklin Tiewsoh, Abhijit Debroy, and Mahendra K. Mahanti*
Department of Chemistry, North-Eastern Hill University, Shillong 793003 India
Received J une 7, 1996X
Quinolinium dichromate (QDC) in water oxidizes vicinal and nonvicinal diols to the corresponding
R-hydroxy carbonyl compound. The rate is proportional to the concentrations of diol, oxidant and
2 2
hydrogen ions, and the reactions proceed almost twice as fast in D O than in H O. These data can
be correlated with a mechanistic pathway involving the intermediate formation of a chromate ester,
which undergoes decomposition to give the product.
In tr od u ction
of the two mechanisms, eq 1 or 2 above, would be
prevalent in the oxidation reactions of diols by QDC.
We report here the kinetic aspects of the selective
conversion of trans-1,2-cyclohexanediol and 1,5-pen-
tanediol to the corresponding R-hydroxy carbonyl com-
pound, using quinolinium dichromate (QDC) in acid
medium.
The unique feature of chromic acid is that it is able to
effect both normal oxidation and cleavage, in so far as
its reactions with diols. For example, Waters had
reported 1-2% cleavage in the case of ethylene glycol,
1
and 20-30% cleavage of 2,3-butanediol, while Chatterjee
and Mukherjee had shown that pinacol was readily
cleaved to give acetone.2 Two mechanistic pathways were
thus available:
Exp er im en ta l Section
(
i) A normal oxidation to yield an R-hydroxy carbonyl
(
a ) Ma ter ia ls. The diols (trans-1,2-cyclohexanediol and 1,5-
compound such as
pentanediol) were obtained from the Aldrich Co. and were used
after recrystallization and distillation, respectively. Quino-
linium dichromate (QDC) was prepared by the reported
method,9 and its purity was checked by estimating Cr(VI)
iodometrically. Sulfuric acid (E. Merck) was used as such after
a check of its physical constants. Acetic acid (E. Merck) was
distilled, and the fraction distilling at 116 °C was used.
-
+
HOCH CH OH + HCrO + H h
2
2
4
HOCH CH OCrO H + H O
2
2
3
2
HOCH CH OCrO H f HOCH CHO + Cr(IV) (1)
2
2
3
2
(
b) Kin etic Mea su r em en ts. The reactions were performed
(
ii) Cleavage of the carbon-carbon bond,
under pseudo-first-order conditions, maintaining a large excess
of the diols with respect to QDC. The reactions were carried
out at constant temperature ((0.1 K) and were followed by
HO
CH2
CH2
O
CrO3H
2CH2 O + Cr(IV) (2)
monitoring the UV-vis absorption band at 440 nm, spectro-
photometrically, as described in earlier papers.1
0-11
All the
Rocek and Westheimer had used energy considerations
to ascertain the pathway of oxidation and had established
that the oxidation to a hydroxy aldehyde (or ketone) was
reactions were performed under nitrogen. The rate constants
were evaluated from the linear (r > 0.995) plots of log [QDC]
against time. The values reported were the mean of two or
more runs and were reproducible to within (3%. The solvent
was water or a water-acetic acid mixture. The reaction
mixture remained homogeneous in the solvent systems used.
3
energetically favored by 15 kcal/mol or more. The
obvious conclusion was that when a mechanistic pathway
was available, the oxidation of a primary or secondary
alcohol group to a carbonyl would be preferred to cleavage
of the diol.
The stereoselective synthesis of enantiomerically pure
R-hydroxy carbonyl compounds has become significant,4
especially since these compounds have been established
as important synthons in the asymmetric synthesis of
natural products and fine chemicals. It has been shown
that the osmium-catalyzed asymmetric dihydroxylation
of enol ethers or silyl enol ethers had yielded R-hydroxy
ketones in high enantiomeric excess.
(
c) P r od u ct An a lysis. Using the same experimental
conditions as were employed for the kinetic determinations,
the reaction mixture was kept overnight for completion of the
reaction.
(
i) Oxid a tion of 1,2-Cycloh exa n ed iol. The reaction
product was extracted with ether, and the solvent ether was
removed. The product was recrystallized from ethanol (mp
113 °C, yield ∼90%). IR spectrum of this product showed a
5
,6
-
1
sharp band at 1687 cm , suggesting the presence of a
hydrogen bonded carbonyl group. The absorption band at 3050
7
8
-
1
cm was due to the presence of the OH group. The lowering
in the wave-number of the O-H absorption band was probably
due to the presence of hydrogen bonding between the hydroxyl
and the carbonyl groups, thus
While supplementing the earlier work in this area, we
have attempted to use kinetic data to help decide which
X
Abstract published in Advance ACS Abstracts, November 1, 1996.
O
H
O
(
1) Slack, R.; Waters, W. A. J . Chem. Soc. 1949, 594.
(2) Chatterjee, A. K.; Mukherjee, S. K. Z. Phys. Chem. 1958, 208,
2
81; 1959, 210, 166.
(
(
(
3) Rocek, J .; Westheimer, F. H. J . Am. Chem. Soc. 1962, 84, 2241.
4) Davis, F. A.; Chen, B.-C. Chem. Rev. 1992, 92, 919.
5) Hannessian, S. Total Synthesis of Natural Products: The Chiron
The product was thus confirmed as 2-hydroxycyclohexanone.
The phenylhydrazone derivative of the product was prepared,
recrystallized from ethanol, and dried (mp 121 °C).
Approach; Pergamon Press: New York, 1983; Chp. 2.
6) Samuels, W. D.; Nelson, D. A.; Hallen, R. T. Tetrahedron Lett.
986, 27, 3091.
7) Hashiyama, T.; Morikawa, K.; Sharpless, K. B. J . Org. Chem.
992, 57, 5067.
8) Xu, D.; Crispino, G. A.; Sharpless, K. B. J . Am. Chem. Soc. 1992,
14, 7570.
(
1
1
1
(
(9) Balasubramanian, K.; Prathiba, V. Ind. J . Chem. 1986, 25B, 326.
(10) Dey, D.; Mahanti, M. K. J . Org. Chem. 1990, 55, 5848.
(11) Nongkynrih, I.; Mahanti, M. K. J . Org. Chem. 1993, 58, 4925.
(
S0022-3263(96)01079-1 CCC: $12.00 © 1996 American Chemical Society

8
876 J . Org. Chem., Vol. 61, No. 25, 1996
Kuotsu et al.
Ta ble 1. Dep en d en ce of Ra te Con sta n ts on Oxid a n t
Con cen tr a tion ([Diol] ) 0.01 M; [H2SO4] ) 1.0 M; T ) 323
K)
4
1
1
0 [QDC] (M)
1.0
5.0
7.5
10.0
20.0
4
-1
0 k1 (s ) for
1
1
,2-cyclohexanediol
,5-pentanediol
7.75
3.96
7.52
3.79
7.36
3.86
7.67
3.93
7.62
3.76
Ta ble 2. Dep en d en ce of Ra te Con sta n ts on Su bstr a te
Con cen tr a tion ([QDC] ) 0.001 M; [H2SO4] ) 1.0 M; T )
3
23 K)
2
1
1
0 [diol] (M)
1.0
2.5
5.0
7.5 10.0
20.0
4
-1
0 k1 (s ) for:
a
1
1
,2-cyclohexanediol 7.67 19.2
,5-pentanediol
38.5 58.0 77.1 155
b
3.93
9.81 19.7 29.7 39.6 79.0
a
02k2 (M-1 s-1) ) 7.68 (av). b 10 k2 (M s-1) ) 3.94 (av). k2
2
-1
1
)
k1/[diol].
(
ii) Oxid a tion of 1,5-P en ta n ed iol. The reaction product
was extracted with ether, and the solvent ether was removed.
The product (5-hydroxypentanal) was treated with 2,4-dini-
trophenylhydrazine in 2 M HCl and left overnight to obtain
the corresponding 2,4-dinitrophenylhydrazone (DNP) deriva-
tive, which was then filtered and recrystallized from ethanol
(mp 80 °C, yield ∼90%).
Resu lts
The oxidation of the diols by QDC resulted in the
formation of the corresponding R-hydroxy carbonyl com-
pound. Under the present experimental conditions, there
was no further oxidation of the product.
Stoich iom etr y. The stoichiometric experiments were
performed under nitrogen at 323 K, under the conditions
F igu r e 1. Plot of k
cyclohexanediol (A) and 1,5-pentanediol (B).
1 2 4
vs concentration of H SO for 1,2-
Ta ble 3. Solven t Isotop e Effect ([Diol] ) 0.01 M; [QDC] )
0
.001 M; [H2SO4] ) 1.0 M; T ) 323 K)
of [QDC]
disappearance of Cr(VI) was followed until the absor-
bance values became constant. The [QDC] was esti-
0
0
> [diol] , at varying acid concentrations. The
4
10 k1 (s-1)
4
10 k1 (s-1)
compound
(H2O)
(D2O)
kD O/kH2
O
∞
2
mated. Stoichiometric ratios, ∆[QDC]/∆[diol], in the
range 0.65-0.69 were observed, which conformed to the
overall equations:
1,2-cyclohexanediol
1,5-pentanediol
7.67
3.93
15.5
7.81
2.02
1.99
Ta ble 4. Dep en d en ce of Ra te Con sta n ts on Solven t
Com p osition ([Diol] ) 0.01 M; [QDC] ) 0.001 M; [H2SO4 )
VI
3
C H O
+ 2Cr f 3 C H O +
1.0 M; T ) 323 K)
6
12
2
6
10
2
1
1
,2-cyclohexanediol
H2O:AcOH (%, v/v)
100:0
95:5
90:10 85:15 80:20
III
+
dielectric constant (D)
69.94 66.76 63.59 60.41 57.23
2
Cr + 6H (3)
4
-1
1
0 k1 (s ) for:
,2-cyclohexanediol
1,5-pentanediol
1
7.67
3.93
7.93
4.06
8.25
4.22
8.80
4.50
9.46
4.84
VI
III
+
3
C H O
+ 2Cr f 3C H O + 2Cr + 6H
5 10 2
5
12
2
,5-pentanediol
(4)
Ta ble 5. Effect of Tem p er a tu r e a n d Activa tion
P a r a m eter s ([Diol] ) 0.01 M; [QDC] ) 0.001 M; [H2SO4] )
.0 M)
1
Ra te La w . Under pseudo-first-order conditions, the
individual kinetic runs were first order with respect to
QDC. Further, the pseudo-first-order rate constants (k
did not vary with the initial concentration of QDC (Table
). The order with respect to the concentrations of diols
1,2-cyclohexanediol
1,5-pentanediol
1
)
T ( 0.1 K
313
318
104k1 (s-1)
3.81
5.75
7.67
11.4
15.5
1.94
2.88
3.93
5.79
7.91
1
3
3
3
23
28
33
was unity (Table 2). The reaction was catalyzed by acid,
and the acid-catalyzed reaction showed a first-order
dependence on acidity (Figure 1). These data suggest the
following rate law:
∆
∆
∆
H (kJ mol-1)
q
57
-135
101
60
-129
102
S (J K mol-1)
q
-1
q
-1
G (kJ mol
)
+
-d[QDC]/dt ) k[diol][QDC][H ]
(5)
Error limits: ∆H (2 kJ mol ; ∆S (3 J K mol-1
q
-1
q
-1
;
q
-1
∆
G (2 kJ mol
Solven t Isotop e Effect. All the oxidation reactions
have been carried out in aqueous medium. The effect of
proportions of water and acetic acid. A decrease in the
dielectric constant of the medium increased the rate of
the reaction (Table 4).
a change in the solvent from H
2 2
O to D O was studied in
order to ascertain the extent of the solvent isotope effect.
The rates of oxidation of the diols were increased in D
medium by a factor of 2.0 (Table 3).
2
O
Effect of Tem p er a tu r e. The oxidation of diols was
studied at different temperatures, and the activation
parameters were evaluated (Table 5).
Effect of Solven t Com p osition . The oxidation of
diols by QDC was studied in solutions containing varying

Quinolinium Dichromate Oxidation of Diols
J . Org. Chem., Vol. 61, No. 25, 1996 8877
Discu ssion
Sch em e 1
The activation parameters obtained were in agreement
1
,3
with literature values. The values for the free energies
of activation were nearly constant, indicating that the
same mechanism operated for the oxidation of these diols.
The linear increase in the rate of oxidation with acidity
suggested the involvement of a protonated Cr(VI) species
in the rate-determining step. Earlier reports have
There was no kinetic evidence for the formation of a
cyclic intermediate in any appreciable concentration
between the diols and QDC. The rate of oxidation, for
example, was first order with respect to each reagent,
for all the diols studied. The only evidence for the
formation of a cyclic intermediate would depend upon the
relation between the reaction rate and the configuration
of the diol. An attempt was made to decipher a general
correlation between the rate of oxidation by QDC and
the proximity of the hydroxyl groups. Inspection of the
kinetic data revealed that trans-1,2-cyclohexanediol re-
acted faster than 1,5-pentanediol by a factor of 1.95. The
geometry of trans-1,2-cyclohexanediol was such that the
two hydroxyl groups were in equatorial positions. The
chromate substituent group was quite strained. There
would be a rate-enhancing relief of steric strain in the
transition state, and hence the decomposition of the
chromate ester would be facilitated. The factor of 1.95
would thus represent the difference in the ease of ester
decomposition. In 1,5-pentanediol, the hydroxyl groups
were quite far apart. Hence, there would not be any
possibility of a cyclic mechanism operating in the oxida-
tion reactions of the diols under investigation.
established the involvement of such Cr(VI) species in
chromic acid oxidations.12 Since the concentrations of
acid used were in the range of 0.5 M to 2.0 M, the
dichromate ion (and its protonated form) would be the
predominant species. Moreover, the protonated Cr(VI)
species would be a more reactive electrophile capable of
increasing its rate of coordination to the diol.
The dielectric constants for water-acetic acid mixtures
have been estimated approximately from the dielectric
constants of the pure solvents. Plots of log k
the inverse of the dielectric constant were linear (r )
.993) with positive slopes, suggesting an interaction
1
against
0
1
3
between a positive ion and a dipole. The data in Table
indicated that the dielectric constants for water-acetic
4
acid mixtures were a linear function of the solvent
composition used in this investigation. This relationship
between log k and 1/D was thus obeyed in the range of
1
dielectric constants used.
The rate of the acid-catalyzed reaction was greater in
The formation of R-hydroxy carbonyl compounds would
suggest that the oxidation of diols by QDC simulated that
D O than in water (kD O/kH O ) 2.0). The rate of an acid-
2
2
2
2
catalyzed reaction would be expected to be faster in D O
1
2
of the oxidation of monohydric alcohols by chromic acid.
2
than in H O, when a preequilibrium protonation was
involved.1
4,15
Cyclic ester formation involving both the hydroxyl groups
was unlikely, and the mechanistic pathway would be an
acyclic process through a chromate ester, which would
then undergo decomposition (Scheme 1). This mecha-
nistic sequence drew ample support from two excellent
correlations:
The value of the solvent isotope effect
suggested that the hydroxyl group was not involved
either in the preequilibria or in the rate-determining step.
Since the effect of changing the solvent was large, it
effectively precluded the possibility of breaking an O-H
bond in the rate-determining step. Hence, an ester
mechanism became very probable.
(a) Periodic acid, lead tetraacetate and phenyliodo-
soacetate (used primarily to cleave 1,2-diols) do not
There was no evidence for carbon-carbon bond fission
upon the QDC oxidation of diols, in aqueous medium or
aqueous acetic acid solutions. In the absence of products
which would have resulted from the carbon-carbon bond
fission, it would be reasonable to suggest that an acyclic
mechanism must perforce operate in these oxidation
reactions. The major portion of the diols was oxidized
in the normal manner, CH(OH) f CdO. The fission of
diols could not be effected in the presence of added free
acid. Thus, the diols reacted with QDC to produce the
corresponding R-hydroxy carbonyl compound, a pathway
readily oxidize simple alcohols or oxidize diols to R-hy-
3
droxy carbonyl compounds (as do chromic acid or per-
manganate).
(b) The oxidative cleavage of 1,2-diols by periodate (the
Malaprade reaction) is an allowed electrocyclic process,
whereas the corresponding reaction of 1,2-diols by Cr-
(
VI) is a forbidden process.16 This excludes a cyclic
mechanism and supports the view that the oxidation of
diols by QDC would involve the conversion CH(OH) f
CdO.
The data collected demonstrated that application of
QDC to the oxidation of diols (vicinal and nonvicinal) led
to the formation of R-hydroxy carbonyl compounds in
good yields. This efficient reaction could thus prove to
be a useful and general route in the synthesis of R-hy-
droxy carbonyl compounds.
which has already been established to be energetically
more favorable than that yielding the fission product.3
The rates and the enthalpies would favor the formation
of the hydroxy carbonyl product, similar to earlier
investigations,1,3 suggesting that the structure of the
transition state was quite near to that of the products.
Ack n ow led gm en t. Financial assistance from the
University Grants Commission, New Delhi, under the
Special Assistance Program, is gratefully acknowledged.
(
12) Wiberg, K. B. Oxidation in Organic Chemistry; Academic
Press: New York, 1965, Part A, p 69.
13) Amis, E. S. Solvent Effects on Reaction Rates and Mechanisms;
Academic Press: New York, 1967; p 42.
(
J O961079M
(14) Wiberg, K. B. Chem. Rev. 1955, 55, 713.
(
15) Collins, C. J .; Bowman, N. S. Isotope Effects in Chemical
Reactions; van Nostrand-Reinhold: New York, 1970; p 267.
(16) Littler, J . S. Tetrahedron 1971, 27, 81.