Kinetics and Mechanism of the Oxidation of Diols by Pyridinium Bromochromate

Article abstract of DOI:10.1002/(SICI)1097-4601(1998)30:4<285::AID-KIN7>3.0.CO;2-O

The kinetics of oxidation of four vicinal diols, four nonvicinal diols, and one of their monoethers by pyridinium bromochromate (PBC) have been studied in dimethyl sulfoxide.The main product of oxidation is the corresponding hydroxyaldehyde.The reaction is first-order with respect to each the diol and PBC.The reaction is acid-catalyzed and the acid dependence has the form: k_obs = a + b+>.The oxidation of <1,1,2,2-2H4>ethanediol exhibited a primary kinetic isotope effect (k_H/k_D = 6.70 at 298 K).The reaction has been studied in 19 organic solvents including dimethyl sulfoxide and the solvent effect has been analyzed using multiparametric equations.The temperature dependence of the kinetic isotope effect indicates the presence of a symmetrical transition state in the rate-determining step.A suitable mechanism has been proposed.

Full text of DOI:10.1002/(SICI)1097-4601(1998)30:4<285::AID-KIN7>3.0.CO;2-O

Kinetics and Mechanism of
the Oxidation of Diols by
Pyridinium Bromochromate
P. SURYA CHANDRA RAO, DEEPA SURI, SEEMA KOTHARI, KALYAN K. BANERJI
Department of Chemistry, J.N.V. University, Jodhpur 342 005, India
Received 8 January 1997; accepted 5 July 1997
ABSTRACT: The kinetics of oxidation of four vicinal diols, four nonvicinal diols, and one of
their monoethers by pyridinium bromochromate (PBC) have been studied in dimethyl sulf-
oxide. The main product of oxidation is the corresponding hydroxyaldehyde. The reaction is
first-order with respect to each the diol and PBC. The reaction is acid-catalyzed and the acid
ϩ
2
dependence has the form: kobs ϭ a ϩ b[H ]. The oxidation of [1,1,2,2- H ]ethanediol exhibited
4
a primary kinetic isotope effect (k /k ϭ 6.70 at 298 K). The reaction has been studied in 19
H
D
organic solvents including dimethyl sulfoxide and the solvent effect has been analyzed using
multiparametric equations. The temperature dependence of the kinetic isotope effect indicates
the presence of a symmetrical transition state in the rate-determining step. A suitable mech-
anism has been proposed. ᭧ 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 285–290, 1998.
INTRODUCTION
EXPERIMENTAL
Material
Pyridinium bromochromate (PBC) has been used as a
mild and selective oxidizing reagent in synthetic or-
ganic chemistry [1]. It is structurally similar to well-
known Corey’s reagent-pyridinium chlorochromate
The diols and the monoether (BDH or Fluka) were
distilled under reduced pressure before use. PBC was
prepared by the reported method [1]. [1,1,2,2-
(PCC) and pyridinium fluorochromate (PFC). There
2
H ]ethanediol (DED) was prepared by reducing die-
4
are only few reports on the mechanistic aspects of ox-
idation reactions of PBC [2–5]. There seems to be no
report on the kinetics of oxidation of diols by PBC. It
has been observed that the kinetics of oxidation of
diols by PFC [6] and PCC [7] are different. In this
article we report the kinetics of oxidation of some vic-
inal diols, nonvicinal diols, and one of their monoeth-
ers by PBC in dimethyl sulfoxide (DMSO). The mech-
anistic aspects are discussed.
thyl oxalate with lithium aluminium deuteride [8]. Its
isotopic purity, determined by its H NMR spectrum,
was 88 Ϯ 4%. Solvents were purified by the usual
methods [9]. Toluene-p-sulfonic acid (TsOH) was
used as a source of hydrogen ions.
1
Product Analysis
Product analysis was carried out under kinetic condi-
tions. In a typical experiment, the diol (0.1 mol) and
PBC (0.01 mol) were taken in DMSO (100 ml) and
the mixture was allowed to stand in the dark for ca.
10 h to ensure completion of the reaction. Most of the
solvent was removed under reduced pressure and res-
idue treated overnight with an excess (250 ml) of a
saturated solution of 2,4-dinitrophenylhydrazine in
Correspondence to: K. K. Banerji
Contract grant Sponsor: University Grants Commission (India)
Contract grant Sponsor: Council of Scientific and Industrial Re-
search (India)
᭧
1998 John Wiley & Sons, Inc.
CCC 0538-8066/98/040285-06

2
86
RAO ET AL.
Table I Analysis of Products in the Oxidation of Diols and 3-Methoxybutan-1-ol by PBC
m.p. of
DNP,
ЊC
Yielda
%
Diol
Product
Ethanediol
HOCH CHO
MeCH(OH)CHO
MeCOCH(OH)Me
153
89
91
85
92
92
90
88
82
88
2
Propane-1,2-diol
Butane-2,3-diol
Butane-1,2-diol
Propane-1,3-diol
Butane-1,3-diol
Butane-1,4-diol
Pentane-1,5-diol
94b
114
132c
133
91
117
79
MeCH CH(OH)CHO
2
HOCH CH CHO
2
2
MeCH(OH)CH CHO
2
HOCH CH CH CHO
2
2
2
HOCH (CH ) CHO
2
2 3
3
-Methoxybutan-1-ol
MeCH(OMe)CH CHO
104
2
a
The yield is of DNP derivative after recrystallization.
Instead of DNP, phenylhydrazone derivative was prepared.
Instead of DNP, 4-nitrophenylhydrazone derivative was prepared.
b
c
Ϫ3
2
mol dm HCl. The precipitated 2,4-dinitropheny-
ing the relation: k ϭ k /[redutant]. All experiments,
2 obs
lhydrazone(DNP) was filtered off, dried, recrystallized
from ethanol, and weighed. The DNP derivatives were
found to be homogeneous by TLC. The identity of the
products were established by comparing the m.p. of
the DNP derivatives with the literature values [10]. In
the oxidation of ethanediol and butane-1,3-diol, the
identity of the products were confirmed by determin-
ing the m.m.p. with authentic samples of DNP of hy-
droxyethanal and 3-hydroxybutanal, respectively. The
melting points were obtained in open capillaries and
are uncorrected. The results are summarized in Ta-
ble I.
other than those for studying the effect of hydrogen
ions, were carried out in the absence of TsOH.
RESULTS
The rate laws and other experimental data were ob-
tained for all the diols investigated. As the results were
similar, only representative data are given here. There
The oxidation state of chromium in completely re-
duced reaction mixtures, determined by an iodometric
method, was 3.95 Ϯ 0.09. The overall reaction may,
therefore, be written as the following equation:
Table II Rate Constants for the Oxidation of
Ethanediol by PBC at 298 K in DMSO
03 [PBC]
4
1
[diol]
[TsOH]
10 k
obs
Ϫ3
Ϫ3
Ϫ3
Ϫ1
mol dm
mol dm
mol dm
s
HOCH 9CH OH ϩ O CrBrOpyH ۙۙƒ
2
2
2
HOCH 9CHO ϩ H O ϩ OCrBrOpyH (1)
1.0
0.10
0.20
0.30
0.60
1.00
1.50
2.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.2
0.3
0.5
0.8
1.0
4.00
7.95
12.7
2
2
1
1
1
1
1
1
1
2
4
.0
.0
.0
.0
.0
.0
.0
.0
.0
Kinetic Measurements
24.7
40.4
63.1
76.9
40.6
41.0
40.2
40.6
41.4
46.7
53.9
62.5
71.4
95.0
The reactions were followed under pseudo-first-order
conditions by keeping a large excess (ϫ15 or greater)
of the diol over PBC. The temperature was kept con-
stant to ϫ0.1 K by means of a refrigerated bath cir-
culator (Haake, GD-8). The solvent was DMSO, un-
less specified otherwise. The reactions were followed
by monitoring the decrease in the concentration of
PBC spectrophotometrically (Spectrochem model MK
II) at 358 nm for upto 80% of the reaction. The
a
8
.0
10.0
1.0
1.0
pseudo-first-order rate constants, k , were evaluated
1.0
obs
1
1
1
.0
.0
.0
from the linear (r ϭ 0.9960–0.9990) plots of log
PBC] against time. Duplicate kinetic runs showed that
the rate constants were reproducible to within Ϯ3%.
[
110
a
Contained 0.005 mol dm 3 acrylonitrile.
Ϫ
The second-order rate constant, k , was determined us-
2

OXIDATION OF DIOLS
287
Table III Rate Constants and Activation Parameters of the Oxidation of Diols and 3-Methoxybutan-1-ol by PBC in
DMSO
3
Ϫ1
3
Ϫ1
1
0 k /mol dm s
⌬ H*
⌬ S*
⌬ G*
2
Ϫ1
Ϫ1
Ϫ1
kJ mol 1
Ϫ
Diol
298 K
308 K
318 K
328 K
kJ mol
J mol
K
Ethane-1,2-diol
Propane-1,2-diol
Butane-2,3-diol
Butane-1,2-diol
Propane-1,3-diol
Butane-1,3-diol
Butane-1,4-diol
Pentane-1,5-diol
4.04
17.0
24.5
71.9
8.58
10.5
13.0
14.1
24.0
0.603
6.70
8.50
17.0
61.5
86.1
215
31.4
43.0
49.2
64.0
86.6
2.70
6.30
32.9
114
160
54.2 Ϯ 0.2
49.0 Ϯ 0.3
48.2 Ϯ 0.5
43.8 Ϯ 1.3
50.5 Ϯ 0.9
53.5 Ϯ 0.3
51.1 Ϯ 0.5
57.3 Ϯ 0.1
49.2 Ϯ 0.6
59.1 Ϯ 1.1
Ϫ109 Ϯ 1
Ϫ115 Ϯ 1
Ϫ115 Ϯ 2
Ϫ121 Ϯ 4
Ϫ116 Ϯ 3
Ϫ104 Ϯ 1
Ϫ110 Ϯ 2
Ϫ 89 Ϯ 1
Ϫ112 Ϯ 2
Ϫ109 Ϯ 3
86.7 Ϯ 0.1
83.1 Ϯ 0.3
82.2 Ϯ 0.4
79.6 Ϯ 1.0
84.9 Ϯ 0.7
84.3 Ϯ 0.2
83.8 Ϯ 0.4
83.6 Ϯ 0.1
82.3 Ϯ 0.4
91.5 Ϯ 0.8
32.8
46.5
127
16.2
22.1
25.6
30.4
45.6
404
60.7
83.8
94.5
128
162
5.90
5.58
3
-Methoxybutan-1-ol
a
DED
1.28
6.64
k /k
H
D
[
a
diol] Ϫ 0.1–1.0 mol dmϪ3; [PBC] Ϫ 0.001 mol dmϪ3.
2
[
1,1,2,2- H ]Ethane-1,2-diol.
4
is no noticeable oxidation of pinacol by PBC under
our reaction conditions.
Effect of Temperature
The rate constants of the oxidation of diols were ob-
tained at different temperatures and the activation pa-
rameters were evaluated (Table III).
The reactions were found to be first-order with re-
spect to PBC. Individual kinetic runs were strictly
first-order in PBC. Further, the first-order rate coeffi-
cients did not vary with the initial concentrations of
PBC. The order with respect to diol was also found to
be one. The rate increased with an increase in the con-
centration of hydrogen ions. The hydrogen-ion depen-
Kinetic Isotope Effect
To ascertain the importance of the cleavage of the ␣-
C9H bond in the rate-determining step, the oxidation
of DED was studied. The results recorded in Table III
showed that the reaction exhibited a substantial kinetic
isotope effect (k /k ϭ 6.70 at 298 K).
ϩ
dence had the form kobs ϭ a ϩ b [H ]. The results are
recorded in Table II.
H
D
Induced Polymerization of Acrylonitrile
Solvent Effect
The oxidation of diols by PBC, in an atmosphere of
nitrogen, failed to induce the polymerization of acry-
lonitrile. Further, the addition of acrylonitrile had no
effect on the rate (Table II).
The oxidation of ethanediol was studied in 19 different
solvents. The choice of solvent was limited due to the
solubility of PBC and its reaction with primary and
Table IV Rate Constants for the Oxidation of Ethanediol by PBC in Different Solvents at 298 K
03 k2
3
1
10 k2
Ϫ
1
3
Ϫ1
Ϫ
1
3
Ϫ1
Solvent
mol dm s
Solvent
mol dm s
Chloroform
Carbon disulfide
1.06
0.205
1.35
1.32
4.04
1.26
2.25
0.972
1.61
0.498
Cyclohexane
Toluene
Acetophenone
Tetrahydrofuran
t-Butylalcohol
1,4-Dioxane
1,2-Dimethoxyethane
Acetic acid
Ethyl acetate
0.054
0.395
1.72
0.691
0.424
0.686
0.354
0.130
0.514
1
,2-dichloromethane
Dichloromethane
DMSO
Acetone
N,N-Dimethylformamide
Butanone
Nitrobenzene
Benzene
Diol] Ϫ 1.0 mol dm 3; [PBC] Ϫ 0.001 mol dm 3
Ϫ
Ϫ
.
[

2
88
RAO ET AL.
Table V Reaction Constants of the Oxidation of
Vicinal Diols by PBC in DMSO
The rate constant of the oxidation of ethanediol, k2,
in the nineteen solvents were analyzed in terms of
Swain’s equation [14] of cation- and anion-solvating
concept of the solvents also.
r2
T/K
⌹*
sd
⌿
2
3
3
3
98
08
18
28
Ϫ1.28 Ϯ 0.03
Ϫ1.20 Ϯ 0.03
Ϫ1.13 Ϯ 0.03
Ϫ1.12 Ϯ 0.03
0.9991
0.9989
0.9982
0.9987
0.02
0.02
0.03
0.02
0.02
0.03
0.03
0.03
log k ϭ aA ϩ bB ϩ C
(3)
2
A represents the anion-solvating power of the solvent
and B the cation-solvating power. C is the intercept
term. (A ϩ B) is postulated to represent the solvent
polarity. The rates in different solvents were analyzed
in terms of eq. (3), separately with A and B and with
(A ϩ B).
secondary alcohols. There was no reaction with the
solvents chosen. The kinetics were similar in all the
solvents. The values of k are recorded in Table IV.
2
log k ϭ 0.29(Ϯ0.01)A
2
DISCUSSION
ϩ 1.75(Ϯ0.01)B Ϫ 4.38 (4)
2
The values of log k at 298 K for the nine compounds
R ϭ 0.9996; sd ϭ 0.01; n ϭ 19; ␺ ϭ 0.01
2
are linearly related (r ϭ 0.9911) to the corresponding
values at 328 K (Table III). The isokinetic temperature
evaluated from this plot is 788 Ϯ 45 K [11]. The lin-
ear isokinetic correlation implies that all the com-
pounds are oxidized by the same mechanism and the
changes in the rate are governed by the changes in both
the enthalpy and entropy of the activation [11]. The
linear correlation here involves a typical monohydric
alcohol, 3-methoxybutan-1-ol. Thus, it seems likely
that the diols also behave like monohydric alcohols
towards PBC. This is further supported by the isolation
of the hydroxyaldehydes as the products (Table I) and
the resistance of pinacol towards oxidation by PBC.
In correlation analyses, we have used coefficient of
log k ϭ 0.05(Ϯ0.58)A Ϫ 4.82
(5)
(6)
(7)
2
r2 ϭ 0.0004; sd ϭ 0.47; n ϭ 19; ␺ ϭ 1.02
log k ϭ 1.73(Ϯ0.05)B Ϫ 4.29
2
r2 ϭ 0.9844; sd ϭ 0.06; n ϭ 19; ␺ ϭ 0.09
log k ϭ 1.27 Ϯ 0.19(A ϩ B) Ϫ 4.34
2
r2 ϭ 0.7318; sd ϭ 0.24; n ϭ 19; ␺ ϭ 0.39
Here n is the number of data points, i.e., the number
of solvents studied.
2
2
determination (R or r ), standard deviation (sd), and
Exner’s parameter [12]. ␺, as the measures of good-
ness of fit.
The rate constants of oxidation of ethanediol in dif-
ferent solvents show an excellent correlation in
Swain’s equation with the cation-solvating power
playing the major role. In fact, the cation-solvation
alone accounts for ca. 98% of the data. The solvent
polarity, represented by (A ϩ B), accounted for ca.
73% of the data. In view of the fact that solvent po-
larity is able to account for ca. 73% of the data, an
attempt was made to correlate the rate with the relative
permittivity of the solvent. It was observed that a plot
of log (rate) against the inverse of the relative permit-
The rate constants of oxidation of ethanediol, k2
(
Table IV), in eighteen solvents (CS was not consid-
2
ered as the complete range of solvent parameters was
not available) were correlated in terms of linear sol-
vation energy relationship of Kamlet et al. [13].
log k ϭ A ϩ p␲* ϩ b␤ ϩ a␣
(2)
2
0
In this equation, ␲* represents the solvent polarity, ␤
the hydrogen bond acceptor basicities, and ␣ is the
2
tivity is not linear (r ϭ 0.4560; sd ϭ 0.34; ␺ ϭ
hydrogen bond donor acidity. A is the intercept term.
0.59). In earlier studies with PBC also [2–5], it was
observed that the effect of the 19 solvents on the re-
action rate did not exhibited a significant correlation
in terms of Kamlet’s equation whereas the correlations
with Swain’s equation were excellent.
0
The results of correlation analyses in terms of eq. (2),
a biparametric equation involving ␲* and ␤, and sep-
arately with ␲* and ␤ showed that Kamlet’s tripara-
metric equation explain ca. 85% of the effect of sol-
vents on the oxidation. However, by Exner’s criterion
the correlation is not satisfactory [12]. The major con-
tribution is of solvent polarity. It alone accounted for
ca. 80% of the data. Both ␤ and ␣ play relatively minor
role.
Correlation Analysis of Reactivity
The rate constants of oxidation, k , of the four vicinal
2
diols in DMSO (from Table III) showed an excellent

OXIDATION OF DIOLS
289
correlation with Taft’s ͚␴* values [15] with negative
reaction constants, ␳* (Table V). Here ͚␴* represents
the sum of the substituent constant for the substituents
present on the two alcoholic carbons of the vicinal
diols.
Mechanism
The absence of any effect of a radical scavenger, ac-
rylonitrile (Table II), indicated that a hydrogen ab-
straction mechanism, giving rise to free radicals, is
unlikely. The presence of a substantial kinetic isotope
effect (Table III) confirms the cleavage of an
␣-C9H bond in the rate-determining step. The neg-
ative value of the polar reaction constant (Table V)
indicated that the transition state approaches a carbo-
cation in character. Therefore, transfer of a hydride-
ion from the diol to the oxidant is suggested. The hy-
dride-ion transfer mechanism is also supported by the
greater role of cation-solvating power of the solvents
in the reactions.
The hydride-ion transfer may take place either by
a cyclic process via an ester intermediate or by an acy-
clic one-step bimolecular process. Kwart and Nickle
[
16] have shown that a dependence of k /k on tem-
H D
perature can be successfully employed to determine
whether the loss of hydrogen proceeds through a con-
certed cyclic process or by an acyclic one. The data
for protio and deuterioethanediols (Table III),
fitted to the familiar expression k /k ϭ A /A
Scheme I
posed to involve the formation of a chromate ester in
a fast preequilibrium and then a decomposition of the
ester in a subsequent slow step via a cyclic concerted
symmetrical transition state leading to the product.
The observed hydrogen-ion dependence (Table II)
suggests that the reaction follows two mechanistic
pathways, one acid-independent and another acid-de-
pendent. The acid-catalysis may well be attributed to
a protonation of either PBC to yield a protonated
Cr(VI) species which is a stronger oxidant and elec-
trophile or the chromate ester, with both the protonated
and unprotonated forms being reactive. Similar obser-
vations were recorded earlier in the reactions of struc-
turally similar PFC [6].
H
D
H
D
exp(Ϫ⌬E /RT) show a direct correspondence with the
a
properties of a symmetrical transition state in which
the activation energy difference (⌬E ) for k /k is
a
H
D
equal to the zero-point energy difference for the re-
spective C9H and C9D bonds (Ϸ4.5 kJ/mol) and
the frequency factors and entropies of activation of the
respective reactions are nearly equal [17,18]. Similar
phenomena were observed earlier in the oxidation of
benzhydrol by PBC [1] and that of diols by PFC [6].
Bordwell [19] has given very cogent evidence
against the occurrence of concerted one-step bimolec-
ular processes by hydrogen transfer and it is evident
that in the present reaction also the hydrogen transfer
does not occur by an acyclic bimolecular process. It is
well established that intrinsically concerted sigma-
tropic reactions, characterized by transfer of a hydro-
gen in a cyclic transition state, are the only truly sym-
metrical processes involving linear transfer of
hydrogen [20]. Littler [21] has also shown that a cyclic
hydride-ion transfer, in the oxidation of alcohols by
Cr(VI), involves six electrons and, being a Huckel-
type system, is an allowed process. Thus, a transition
state having a planar, cyclic, and symmetrical structure
can be envisaged for the decomposition of the ester
intermediate. Hence, the overall mechanism is pro-
+
ϩ
ۙۙƒ
O CrBrOPyH ϩ H ۙ´ ۙ OBrCr(OH)OPyH (12)
2
The observed negative values of entropy of activation
(Table III) also supports a polar transition state. As the
charge separation takes place in the transition state,
the charges become highly solvated. This results in an
immobilization of a large number of solvent mole-
cules, reflected in the loss of entropy. The mechanism
depicted in Scheme I accounts for all the observed
data.

2
90
RAO ET AL.
It is of interest to compare the mode of oxidation
BIBLIOGRAPHY
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while the oxidations by PCC and PBC exhibited first-
order kinetics with respect to diols. This may be due
to the higher electronegativity of fluorine atom than
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paths, one acid-dependent and another acid-indepen-
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Swain’s equation with the cation-solvating power of
the solvents playing the major role. Solvent effect was
not studied in the oxidation by PCC. The oxidations
by all the three oxidants showed an excellent corre-
lation by Taft’s ⌺␴* values with negative polar re-
action constant. The correlation analysis, in the case
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0. S. Coffey (ed.), Rodd’s Chemistry of Carbon Com-
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2
of PCC, was performed by us for this study (r 0.9996;
␳
* Ϫ1.18 Ϯ 0.02). Though the data are not conclu-
sive, the difference in the observed kinetics suggest
that the mode of oxidation depends on the nature of
the halogen atom present in the Cr(VI) species.
1
1. O. Exner. Collect. Chem. Czech. Commun., 29, 1094
(1964).
12. O. Exner. Collect. Chem. Czech. Commun., 31, 3222
It may be mentioned here that pinacol is oxidized
by chromic acid but not by PBC. Chatterjee and Mu-
kherji [22] reported an abrupt change from butane-2,3-
diol to pinacol, the latter reacting very fast. As pointed
out by Littler [21] a cyclic ester mechanism is forbid-
den in the diol-Cr(VI) reaction. Chromic acid oxida-
tion of pinacol may therefore involve two one-electron
steps. Chromic acid oxidations are known to induce
polymerization of acrylamide under certain conditions
(1966).
1
1
1
1
1
1
3. M. J. Kamlet, J. L. M. Abboud, M. H. Abraham, and R.
W. Taft, J. Org. Chem., 48, 2877 (1983).
4. C. G. Swain, M. S. Swain, A. L. Powell, and S. Alumni.
J. Am. Chem. Soc., 105, 502 (1983).
5. K. B. Wiberg, Physical Organic Chemistry, Wiley, New
York, 1963, p. 416.
6. H. Kwart and J. H. Nickel, J. Am. Chem. Soc., 95, 3394
(
1973).
7. H. Kwart and M. C. Latimer, J. Am. Chem. Soc., 93,
971 (3770).
8. H. Kwart and J. Slutsky, J. Chem. Commun., 1182
1972).
[23]. No such observation has yet been recorded with
1
PBC. Thus, the capability of chromic acid and the in-
ability of PBC to act as a one-electron oxidant may
explain the different behavior of pinacol towards these
two oxidants.
(
19. F. G. Bordwell, Acc. Chem. Res., 5, 374 (1974).
20. R. W. Woodward and R. Hoffmann, Angew. Chem. Int.
Ed. Eng., 8, 781 (1969).
2
2
1. J. S. Littler, Tetrahedron, 27, 81 (1971).
2. A. C. Chatterjee and S. K. Mukherjee, Z. Phys. Chem.,
2
07, 372 (1957); 208, 281 (1958); 209, 166 (1959).
Thanks are due to the University Grants Commission (India)
and the Council of Scientific and Industrial Research (India)
for financial support.
23. M. Rahman and J. Rocek, J. Am. Chem. Soc., 93, 5462
(1971); F. Hasan and J. Rocek, J. Am. Chem. Soc., 94,
3181 (1972).