Kinetic study on the reaction of tributylphosphine with methylviologen. Reactivity of the phosphine radical cation intermediate towards nucleophiles

Article abstract of DOI:10.1039/a807722c

Tributylphosphine, Buⁿ3P (BP), was reacted with 1.1′-dimethyl-4,4′-bipyridinium (methylviologen; MV²⁺) in the presence of an alcohol or thiol (RXH; X = O, S) in acetonitrile under an argon atmosphere at 50 °C, which resulted in the gradual formation of the one-electron reduced form of the MV²⁺, MV⁺. Meanwhile, BP was oxidized to tributylphosphine oxide (BP-O). The increase in the amount of MV⁺, which was followed spectrophotometrically with BP and RXH being in large excess, did not obey first-order kinetics. The observation, along with the results from product analysis, shows that single-electron transfer (SET) takes place from BP to MV²⁺ to generate tributylphosphine radical cation BP^·+, as well as MV⁺, and the resulting BP^·+ undergoes ionic reaction with RXH and back electron transfer from MV⁺ in comparable efficiency. A regression analysis of the kinetic data gave the relative value of the second-order rate constant, k_Nu^rel, for the ionic reaction of BP^·+ with RXH. Comparison of the k_Nu^rel values thus obtained for reactions with various RXH's shows that the reaction of BP^·+ with nucleophile RXH is governed by both steric and electronic factors of RXH. The activation energy E_a of the reaction was found to be significantly large, which is in contrast to previous observations that ionic reactions of carbon radical cations with nucleophiles usually have very small values of E_a.

Full text of DOI:10.1039/a807722c

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Kinetic study on the reaction of tributylphosphine with
methylviologen. Reactivity of the phosphine radical cation
intermediate towards nucleophiles
a
b
a
b
Shinro Yasui,* Kosei Shioji, † Munekazu Tsujimoto and Atsuyoshi Ohno
a
Tezukayama College, Gakuen-Minami, Nara 631-8585, Japan
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
b
Received (in Cambridge) 5th October 1998, Accepted 2nd February 1999
n
2؉
Tributylphosphine, Bu P (BP), was reacted with 1,1Ј-dimethyl-4,4Ј-bipyridinium (methylviologen; MV ) in the
3
presence of an alcohol or thiol (RXH; X = O, S) in acetonitrile under an argon atmosphere at 50 ЊC, which resulted
2
؉
؉
in the gradual formation of the one-electron reduced form of the MV , MV . Meanwhile, BP was oxidized to
؉
tributylphosphine oxide (BP-O). The increase in the amount of MV , which was followed spectrophotometrically
with BP and RXH being in large excess, did not obey first-order kinetics. The observation, along with the results
2
؉
from product analysis, shows that single-electron transfer (SET) takes place from BP to MV to generate tributyl-
ϩ
᝽
؉
ϩ
phosphine radical cation BP , as well as MV , and the resulting BP undergoes ionic reaction with RXH and back
᝽
؉
electron transfer from MV in comparable efficiency. A regression analysis of the kinetic data gave the relative value
rel
ϩ
rel
of the second-order rate constant, kNu , for the ionic reaction of BP with RXH. Comparison of the k
thus obtained for reactions with various RXH’s shows that the reaction of BP with nucleophile RXH is governed
by both steric and electronic factors of RXH. The activation energy E of the reaction was found to be significantly
values
᝽
Nu
ϩ
᝽
a
large, which is in contrast to previous observations that ionic reactions of carbon radical cations with nucleophiles
usually have very small values of Ea.
1
–4
Anodic oxidation of trivalent phosphorus compounds
irradiation of these compounds with γ-rays
or
generates the
corresponding trivalent phosphorus radical cations. Such radi-
trivalent phosphorus compounds, it is essential to understand
the reactivity of the putative intermediate, trivalent phosphorus
5–7
ϩ
radical cation (Z P ). In addition, such radical cations have
3
᝽
8,9
cal cations are generated also as intermediates of thermal and
potential uses in organic syntheses; for example, the reaction of
such radicals can be used in the synthesis of a cyclic adenosine
10–12
photochemical
reactions of precursor trivalent phosphorus
compounds with electron-deficient compounds. Meanwhile, it
has been accepted that trivalent phosphorus compounds (Z P)
act as nucleophiles when reacted with electrophiles; that is, the
proposed mechanism includes single-step nucleophilic attack
14
monophosphate (AMP) analogue. Elucidation of the reactiv-
ity of trivalent phosphorus radical cations is of importance for
this reason, as well.
3
While trivalent phosphorus radical cations have in principle
both cationic and radical characteristics, many reports have
shown that these radicals readily undergo ionic reaction with
by the phosphorus atom in Z P on the electrophilic center of
electrophile E (mechanism A in Scheme 1). However, that tri-
3
ϩ
9,11,12,15
valent phosphorus radical cations are produced readily under
certain circumstances suggests that this “nucleophilic” mechan-
ism should be reconsidered; the reaction could take placϩe
various kinds of nucleophiles.
So, in the evaluation of
the reactivity of trivalent phosphorus radical cations, investig-
ation of the ionic reaction of these radical cations with nucleo-
philes is a first step. However, such investigations have been
qualitative so far, and in particular, no kinetic study has been
reported. Measurement of the amount of such a short-lived
radical generated during the reaction may be the main difficulty.
through initial single-electron transfer (SET) from Z P to E
3
followed by coupling of the resulting radical species (mechan-
ism B in Scheme 1). In fact, an SET process has been proposed
1
,1Ј-Dimethyl-4,4Ј-bipyridinium dication (methylviologen;
A. Direct Nucleophilic Attack on an Electrophile
2؉ 16–22
MV ) is a good one-electron acceptor.
MV is gradually reduced to its one-electron reduced form,
MV , when treated with tributylphosphine (BP) in the presence
of an alcohol or thiol. Product analysis as well as other
evidence showed that the accumulation of MV results from
the initial SET from BP to MV followed by an ionic reaction
of the resulting radical cation BP with the alcohol or thiol.
In this reaction system, quantitative evaluation of the reactivity
We found that
2
؉
؉
+
Z3P:
+
E+
Z3P-E
23
؉
B. Via Initial SET
E+
2؉
ϩ
SET
+
᝽
Z3P•+
E•
Z3P
+
+
Z3P-E
of trivalent phosphorus radical cations toward nucleophiles is
Scheme 1 Possible mechanisms for nucleophilic attack by a trivalent
ϩ
possible without measurement of the amount of BP . Thus,
ϩ
᝽
phosphorus compound Z P on an electrophile E .
3
؉
the increase in the amount of MV can be monitored spectro-
photometrically, and analysis of the kinetic data based on the
proposed reaction mechanism gives the relative value of the
in a Mitsunobu reaction on the basis of results of EPR, giving
another mechanism for this conventional reaction. To inspect
the possibility of mechanism B in “nucleophilic” reactions of
13
rel
ϩ
second-order rate constant, kNu
, for the reactions of BP᝽
with nucleophiles.
In this article, we describe the mechanism of the reaction
2
؉
of BP with MV in the presence of an alcohol or thiol. We
†
Present address: Department of Chemistry, Faculty of Science,
Fukuoka University, Jonan-ku, Fukuoka 814-0180, Japan.
hereafter present the relative second-order rate constants for
J. Chem. Soc., Perkin Trans. 2, 1999, 855–862
855

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at 50 °C
n
Bun3P=X
Bu 3P
+
2 Me
N
+
+ N Me
+
2 RXH
RXH
+
2 Me N •
+ N Me
+
RXR
(1)
in MeCN–Ar
BP
BP-X
(X = O or S)
MV2+
MV+
H2O
MeOH
EtOH
n
Bu OH
i
PrOH
t
Bu OH
HO(CH2)3OH
HO(CH2)4OH
n
Bu SH
t
Bu SH
؉
reduction to MV without further reduction to its two-electron
0
reduced form, MV . The reaction mixture in acetonitrile-d was
3
1
31
analyzed with H and P NMR spectroscopy, which showed
the formation of tributylphosphine oxide (BP-O) together with
ether ROR. The formation of BP-O was also monitored by
GC-MS. The yield of MV and the conversions of BP and
ROH were determined by spectrophotometric and H NMR
؉
1
spectroscopic analyses after an appropriate reaction time. Table
1
lists the results along with the initial concentrations of the
starting materials. Clearly, each mole of BP reduces 2 equiv-
2
؉
؉
alents of MV to MV with consumption of 2 equivalents of
ROH [eqn. (1)].
2
؉
When BP was reacted with MV in the presence of a large
excess of thiol (RSH) instead of alcohol under otherwise iden-
2
؉
؉
tical conditions, MV was gradually reduced to MV . Results
of GC-MS showed that tributylphosphine sulfide (BP-S) was
formed together with a small amount of BP-O. The oxygen in
26
BP-O resulted from water contaminating the solvent. Thiol
2
؉
؉
itself did not reduce MV to MV .
Fig. 1 Changes in the UV–visible spectrum during the reaction of BP
2
with MV in the presence of MeOH in acetonitrile at 50 ЊC under an
Ϫ1
Ϫ3
Reaction mechanism
argon atmosphereϪ4in the dark. [BP] = 1.50 × 10
mol dm ,
0
2
؉
Ϫ3
Ϫ3
[
MV ]0 = 1.00 × 10 mol dm , and [MeOH] = 12.5 mol dm . Spec-
0
tra recorded at 15, 30, 60, 120, 180, 240, 300, 360, 420, and 480 minutes
A key observation in the present reaction is one-electron reduc-
2؉
؉
tion of MV to MV , which certainly results from the transfer
are shown.
2
؉
of an electron from BP to MV . This process is composed of
27
2؉
three successive steps; formation of an encounter complex
2؉
Table 1 Reaction of tributylphosphine BP with methylviologen MV
a
in the presence of EtOH
between MV and BP, single-electron transfer (SET) within
the complex, and dissociation of the pair of the resulting radi-
cal species [eqn. (2)]. Results of cyclic voltammetry showed that
b
c
Yield (%)
Conversion (%)
؉
d
d
d
t/min
MV
[eq.]
BP [eq.]
EtOH [eq.]
SET
BP + MV2+
BP ••• MV2+
BP•+••• MV+
BP•+ + MV+
(2)
1
3
5
5
4.5
5.5
[2.0]
[2.0]
2.2 [1]
2.8 [1]
4.2
5.6
[1.9]
[2.0]
2
؉
peak potentials E for BP and MV are 1.45 and Ϫ0.43 V,
a
2؉
Ϫ5
Ϫ3
Ϫ5
p
[
BP] = [MV ]0 = 1.00 × 10 mol dm , [EtOH] = 2.00 × 10 mol
0 0
Ϫ3
respectively, vs. Ag/AgCl in acetonitrile. This large difference in
(1.88 V) predicts that the SET is highly endothermic.
Such a highly endothermic SET could take place when the
dm . In acetonitrile at 50 ЊC under an argon atmosphere in the dark.
b
c
E
p
Determined on a spectrophotometer. Determined on an NMR
d
spectrometer. Numbers in square brackets denote equivalence of the
yield or the conversion against the conversion of BP.
process is associated with an exothermic chemical reaction of
the resulting radical species. For example, an endothermic SET
from a one-electron donor to an alkyl halide is accomplished by
ϩ
rel
the reactions of BP with nucleophiles, kNu , to discuss the
᝽
virtue of energy-gaining cleavage of the carbon–halogen bond
ϩ
reactivity of BP᝽ towards nucleophiles.
28–31
in the resulting alkyl halide radical anion.
shown, by presenting several examples,
Schuster has
32
33–36
that SET pro-
Results and discussion
Reaction of BP with MV in the presence of nucleophile
cesses, which are energetically unfavorable and would otherwise
be unlikely to occur, in fact take place easily when the resulting
radical species undergo follow-up chemical reactions rapidly.
2؉
2
؉
Tributylphosphine (BP) was reacted with MV (tetrafluoro-
borate salt) in acetonitrile at 50 ЊC under an argon atmosphere
in the dark in the presence of a large excess of alcohol (ROH).
Spectrophotometric measurements showed gradual increases in
the absorption at 398 and 605 nm, wavelengths characteristic of
As for reactions with trivalent phosphorus compounds Z P, we
3
have found that a highly endothermic SET from Z P to an
3
iron() complex takes place in association with a rapid reaction
of the resulting trivalent phosphorus radical cations with alco-
3
7
؉
hol. We therefore conclude that the accumulation of MV
observed in the present reaction results from a follow-up reac-
2
؉
؉ 16–18
the one-electron reduced form of MV , MV .
The form-
؉
ϩ
ation of MV was found to be nearly theoretical based on its
tion of BP
.
᝽
16,24
extinction coefficient at 605 nm.
Fig. 1 shows changes in the
The present reaction gives BP-O as a final product from BP
ϩ
UV–visible spectrum during the reaction in the presence of
in the presence of ROH, which shows the fate of BP᝽ ; a P–O
25
2؉
ϩ
MeOH. The changes show that MV undergoes one-electron
bond is formed through nucleophilic attack by ROH on BP᝽ ,
8
56 J. Chem. Soc., Perkin Trans. 2, 1999, 855–862

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ؒ
9,11,12,15
yielding phosphoranyl radical BP -OR [eqn. (3)].
In
analogy with alcohol, thiol RSH undergoes an ionic reaction
kNu
•
BP•
+
Bu 3P-XR
n
+
RXH
(3)
–
H+
X = O, S
BP•-XR
ϩ
ؒ
with BP giving the corresponding phosphoranyl radical BP -
᝽
SR. More evidence that nucleophilic attack by RXH (X = O, S)
on BP is needed if MV is to accumulate was provided by the
reaction with mesityldiphenylphosphine (MesPh P) used in
place of BP. The concentration of MV did not increase in the
reaction of MesPh P with MV in the presence of MeOH؉,
despite the prediction that MesPh P would give more MV
than BP did because the oxidation peak potential E (1.14 V
ϩ
؉
᝽
2
؉
2؉
2
2
ox
p
4
vs. Ag/AgCl) is lower than that of BP. Radical cation
ϩ
MesPh P is generated, but, as suggested by examination of
2
᝽
Corey–Pauling–Koltun (CPK) molecular models, its cationic
center is protected sterically by the mesityl ligand from
nucleophilic attack by MeOH.
Phosphoranyl radicals are known to preferentially cleave the
O–C bond to give alkyl radicals when the radical is stable
؉
Fig. 2 Logarithmic plot of increase in the absorption of MV at 605
nm against time. ᭿, ᭡, ᭹, and ᭺ denote the points for the reactions
38,39
i
t
enough.
However, we saw no evidence for the products
with MeOH, EtOH, Pr OH, and Bu OH, respectively.
derived from the tert-butyl radical in the reaction in the pres-
t
2؉
4
3
ence of Bu OH. In the present system, therefore, MV is likely
tions. When the reaction was carried out with dibutyl sulfide
ؒ
n
n
to very rapidly oxidize the phosphoranyl radical BP -XR
(
1
Bu S) instead of BP and in the presence of MeOH ([Bu S] =
2
2
0
؉
Ϫ1
Ϫ3
2؉
Ϫ4
Ϫ3
(
X = O, S) to the phosphonium ion BP -XR [eqn. (4)]; the half-
.50 × 10 mol dm , [MV ] = 2.00 × 10 mol dm , and
0
Ϫ3 ؉
[
MeOH] = 12.0 mol dm ), no MV was detected. Further-
0
2؉
+
more, kinetic parameters of the reaction of BP and MV in the
•
MV2+
n
MV+
BP -XR
+
Bu 3P-XR
+
(4)
n
Ϫ3
presence of MeOH were unchanged when Bu S (2.00 × 10
mol dm ) was added to the reaction mixture.
2
+
Ϫ3
BP -XR
In conclusion, the present reaction proceeds according to the
mechanism represented by eqns. (2)–(6), which is summarized
in Scheme 2. This mechanism represents the stoichiometry
observed (see Table 1).
wave potential E1/2 of a related compound, tetraphenylphos-
ϩ
phoranyl radical, is Ϫ1.73 V vs. Ag/Ag (=Ϫ1.51 V vs. Ag/
4
0
ؒ
AgCl). In addition, the oxidation of BP -XR is so rapid that a
possible α-scission of a P–C bond in this phosphoranyl radical
to liberate n-butyl radical would be negligible, if present at all.
BP +
MV2+
ϩ
An alkyl cation equivalent is eliminated from BP -XR by
attack by alcohol ROH or thiol RSH. In reactions with ROH,
k21 k12
41
ROH, which is classified as a hard base, attacks the phos-
؉
phorus atom, a harder electrophilic center in BP -OR, to form
k23
k32
k34
k43
42
BP ••• MV2+
BP•+ ••• MV+
BP•+ MV+
+
a phosphorane intermediate, which collapses to yield BP-O as
9d
well as ether ROR [eqn. (5)]. In reactions with RSH, on the
kNu + RXH / – H+
+
+
n
n
+
BP -OR
ROH
Bu 3P(OR)2
Bu 3P=O
ROR
(5)
H+
•
–
BP-XR
BP-O
MV2+
4
1
other hand, RSH is a soft base and attacks in S 2 fashion the
N
k50
carbon atom in the alkylthio ligand, a softer electrophilic center
in BP -SR, to give BP-S as well as sulfide RSR in a single step
eqn. (6)]. It should be emphasized that the decomposition
MV+
؉
+ RXH / – H+
[
+
BP-X
BP-XR
–
RXR
+
•
•
Scheme 2 The mechanism of the reaction of tributylphosphine (BP)
n
n
Bu 3P
S
R
+
RSH
Bu 3P=S
+
RSR
(6)
2؉
with methylviologen (MV ) in the presence of alcohol or thiol (RXH).
– H+
+
BP -SR
BP-S
Kinetics
occurs in any case after a rapid and practically irreversible
2
؉
reaction, reaction (4). As a result, the rate of increase in the
The reaction of BP with MV was carried out in the presence
of a nucleophile RXH under pseudo-first-order conditions,
with the concentrations of BP and RXH being 750 and 7500
؉
concentration of MV , monitored for calculation of rate
؉
constants, is completely independent of the fate of BP -XR.
ϩ
2؉
The phosphine BP acting as the nucleophile to trap BP is
times higher, respectively, than the concentration of MV . The
increase in the absorption at 605 nm from MV was monitored
᝽
1,2a,5,6
؉
in principle possible.
However, reaction in acetonitrile
without added nucleophile resulted in very sluggish formation
spectrophotometrically. Fig. 2 shows the plots of Ϫlog{(A Ϫ
∞
؉
of MV (less than 4% after 3.5 h, resulting probably from the
A )/A } against time for the selected reactions, where A is the
t
∞
∞
ϩ
reaction of BP with a small amount of water in the solvent).
final absorption at 605 nm (= 2.78 under our experimental con-
᝽
So, this pathway clearly does not contribute significantly to the
reaction observed.
ditions) and A is the absorption at a given time t. If the reaction
t
obeys first-order kinetics, this type of a logarithmic plot should
give a linear line for each reaction. However, each plot shows
downward deviation.
Control experiments with added sulfide eliminated the possi-
2؉
bility that this reaction product reduces MV under our condi-
J. Chem. Soc., Perkin Trans. 2, 1999, 855–862
857

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Table 2 Kinetic parameters in the reaction of tributylphosphine BP
Table 3 Activation energy of the reaction of tributylphosphine radical
cation BP᝽
2؉ a
ϩ
a
with methylviologen MV
with EtOH
ϩ
Ϫ4
Ϫ3
Ϫ
rel b
(
k /a)/10
10 (k /a)/
T/ЊC
kNu
3
Ϫ1 Ϫ1 b
b
rel c
entry
RXH
dm mol
s
kNu
kNu
3
40
50
0
0.074
0.23
1
1
2
3
H O
8.9
3.3
3.2
7.0
7.4
7.1
7.2
6.8
7.5
85
3.3
4.0
3.8
2.0
2.1
2.1
1.2
3.7
26
0.61
0.50
0.53
1
0.95
0.95
1.7
0.54
0.077
0.56
0.57
2
MeOH
MeOH
EtOH
EtOH
d
Ϫ1
4
5
E /kJ mol
Ϫ1
107 ± 15
a
e
a
Ϫ3
2؉
Ϫ4
Ϫ3
f
[BP]
0
= 1.50 × 10
mol dm
,
[MV
]
0
= 2.00 × 10
mol dm ,
6
7
8
9
0
1
EtOD
Ϫ3
n
[EtOH] = 12.3 mol dm . In acetonitrile under an argon atmosphere in
0
b EtOH
Bu OH
i
the dark. Relative rate constant with kNu
(50 ЊC) = 1. Errors are
Pr OH
t
within 3%.
Bu OH
n
1
1
Bu SH
3.6
3.5
t
Bu SH
68
a
Ϫ1
Ϫ3
2؉
Ϫ4
Ϫ3
[
BP] = 1.50 × 10
mol dm
,
[MV ]0 = 2.00 × 10
mol dm ,
0
Ϫ3
[
RXH] = 1.50 mol dm . In acetonitrile at 50 ЊC under an argon
0
b c
atmosphere in the dark. Errors are within 3%. Relative value of
d
Ϫ3
rate constant kNu. In the presence of dibutyl sulfide (2.00 × 10 mol
dm ). [EtOH] = 1.18 mol dm . Ethanol-O-d was used.
Ϫ3
e
Ϫ3 f
0
1
Such kinetic behavior is interpreted in terms of the mechan-
ism shown in Scheme 2. Taking the steady-state approximation
2
؉
ϩ
؉
with respect to concentrations of BP ؒ ؒ ؒ MV , BP ؒ ؒ ؒ MV ,
᝽
ϩ
ؒ
؉
BP , and BP -OR, the increase in the concentration of MV is
᝽
ϩ
Ϫ
expressed by eqn. (7), where k = k k k , k = k k k ,
12
23 34
21 32 43
ϩ
2؉
2
(k /a)k [BP][MV ][ROH]
Nu
؉
d[MV ]/dt =
(7)
Ϫ
؉
(
k /a)[MV ] ϩ kNu[ROH]
and a = (k21 ϩ k )(k ϩ k ) Ϫ k k . This equation predicts
23
32
34
23 32
downward deviation in the logarithmic plot when two terms in
Ϫ
؉
the denominator, (k /a)[MV ] and kNu[ROH], are of similar
44
importance. That is, the observed “bent” kinetics show that
ϩ
BP undergoes the forward reaction leading to the products
᝽
and the backward reaction giving back the starting materials
with comparable efficiency. Similar “bent” kinetics have been
observed for the oxidation of toluene derivatives with metal
45–47
complexes.
The kinetics have been analyzed quantitatively
by multiple nonlinear regression; the reaction scheme in that
analysis, as well as the rate expression derived, is essentially the
same as ours. Then, our kinetic data were analyzed by this
ϩ
Ϫ
method, affording the parameters k /a and (k /a)/k as sum-
Nu
marized in Table 2. Curve-fitting of the regression was satis-
factory for each reaction (correlation coefficient, r > 0.9990).
Examples are shown in Fig. 3 for the reactions with EtOH and
i
Pr OH. These kinetic parameters were cross-checked by
experiments with different initial concentrations of BP [entries
4
and 5 in Table 2]. Table 2 lists also the relative values of kNu,
؉
rel RXH EtOH
Fig. 3 Time-course of increase in the absorption of MV at 605
kNu (= kNu
/kNu
), which were estimated taking into
i
Ϫ
nm for the reactions with EtOH (top) and Pr OH (bottom). The crosses
ϩ) represent the experimental points. The lines represent the best fit
account that k /a is the same for all of the reactions. Appar-
ently, kNu represents relative reactivity of BP towards a
nucleophile RXH. For the reaction with EtOH, activation
(
rel
ϩ
᝽
curve based on eqn. (7).
parameters for the kNu step were determined as shown in Table
2؉
nucleophile. The peak potentials of BP and MV changed
little if at all when ethanol was added to the acetonitrile solu-
tion at the same concentration as that for kinetics, but the add-
ition of the same concentration of butane-1-thiol lowered the
3
, from the Arrhenius plot shown in Fig. 4.
As expected from the premise that the process from
2
؉
ϩ
᝽
؉
BP ϩ MV to BP ϩ MV is independent of any nucleophile
ϩ
being added, there is little variation in k /a depending on the
2؉
peak potential of BP and raised that of MV to a significant
extent. This trend in the changes of the peak potentials is in the
expected direction.
nucleophile within a series of the reactions with alcohols, which
verifies our kinetic treatment. An exception is seen for the reac-
ϩ
tion with MeOH, in which the k /a value is slightly smaller than
expected. A possible explanation is that the complexation
ϩ
Reactivity of BP᝽
towards nucleophiles
2؉
between MV
and MeOH, which has been proposed
makes k12 smaller, resulting in the smaller value
48–50
ϩ
previously,
Table 2 shows that the reaction of BP with alcohol becomes
slower as the alcohol goes from the primary to the secondary
and to the tertiary [entries 7, 8, and 9]. Meanwhile, as seen in a
series of reactions with primary alcohols, kNu increases with
increasing electron-releasing ability of the alcohol [entries 2, 4,
and 7]. These observations simply provide a conclusion that the
᝽
ϩ
ϩ
of k /a. The reactions with thiols gave larger k /a values than
the reactions with alcohols, which results at least partly from
participation of the nucleophile as a solvent in k23 step (the SET
rel
2
؉
step). Table 4 lists the peak potentials of BP and MV meas-
ured by cyclic voltammetry in the presence or absence of a
8
58
J. Chem. Soc., Perkin Trans. 2, 1999, 855–862

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Table 4 Peak potentials of tributylphosphine BP and methylviologen
2
؉ a
MV
c
Ep/V
b
e
2؉
d
RXH
None
BP
MV
∆Ep
1.45
1.45
1.34 ± 0.04
Ϫ0.43
Ϫ0.43
Ϫ0.39
1.88
1.88
EtOH
Bu SH
n
1.73 ± 0.04
a
Measured by cyclic voltammetry in acetonitrile using tetraethylam-
b
monium tetrafluoroborate as a supporting electrolyte. [RXH] = 1.50
mol dm
0
Ϫ3
c
d
2؉
e
. vs. Ag/AgCl. E (BP) Ϫ E (MV ). No RXH added.
p p
Fig. 4 The Arrhenius plot for the ionic reaction of radical cation
ϩ
BP᝽ with EtOH.
reaction is governed by both steric and electronic factors of
rel
ROH. In fact, attempts to correlate kNu with a single steric or
electronic parameter of R in ROH failed. Fig. 5a and b, for
rel
example, show scattered plots of log kNu against the electronic
and steric substituent constants σ* and E , respectively. The
s
rel
data set of kNu fits Taft’s equation [eqn. (8)] better; in this
equation, the constants σ* and E are used simultaneously.
51
s
rel
Fig. 5 Dependence of log kNu on the Taft’s electronic parameter
σ* (a) and on the steric parameter E (b) of R in ROH.
ROH
MeOH
s
log(kNu /kNu
) = ρ*σ* ϩ δEs
(8)
Thus, the correlation improves as shown in Fig. 6, when
52
ρ* = Ϫ3.08 and δ = 1.03. Yet, the correlation is not significant
correlation coefficient, r > 0.87 with all points; r > 0.97 when
(
n
the point for Bu OH is ignored). This lack of significance is
partly because the parameters used for this analysis have been
defined in carbon-centered reactions. In addition, only a few
datum points are available in this study. So, our discussion must
be qualified, but the regression coefficients obtained here allow
us, at least in a qualitative way, to depict the structure of the
ϩ
transition state of the reaction of BP with ROH. The large
᝽
negative value of ρ* suggests that a positive charge develops
largely on the oxygen at the transition state. Meanwhile, the
value of δ is close to unity, which indicates proximity of both
reactants at the transition state. The value δ = 1.482 has been
obtained for the Menschutkin reaction between dimethyl(ortho-
53
substituted phenyl)amine and methyl iodide, the transition
state of which must be crowded.
Correlation analysis therefore predicts the structure of the
ϩ
transition state for the reaction of BP with ROH; at the tran-
᝽
sition state, formation of the P–C bond is almost completed
without a great extent of O–H bond cleavage, and as a result, the
positive charge resides mostly on the oxygen atom (Scheme 3).
ROH
MeOH
Fig. 6 Correlation of log (kNu
dual-parameter equation.
/kNu
) with ρ*σ* ϩ δE
in Taft’s
s
Compatible with this structure of the transition state is the
ϩ
2؉
observation that the reaction of BP with ethanol exhibits no
general acid-catalysis. When the reaction of BP with MV was
carried out in the presence of propane-1,3-diol, the rate of the
kNu step was higher than predicted from electronic and steric
contributions of the propanediol (Table 5). This increase in the
᝽
H
D
appreciable kinetic isotope effect k /kNu (entries 4 and 6 in
Nu
Table 2).
ϩ
The nucleophilic attack by alcohol on BP᝽ is governed by
J. Chem. Soc., Perkin Trans. 2, 1999, 855–862
859

View Article Online
2؉
Table 5 Reaction of tributylphosphine BP with methylviologen MV
taken as evidence against the configuration mixing (CM) model
developed by Pross; according to this model, nucleophilic
a
in the presence of diols
60
attack on a certain radical cation is a high-energy pathway and
hence a forbidden process, whereas the corresponding attack on
ϩ
Ϫ4
Ϫ3
Ϫ
(
k /a)/10
10 (k /a)/
3
Ϫ1 Ϫ1 b
b
rel c
ROH
dm mol
s
kNu
kNu
cation species is allowed. The present study has revealed that
ϩ
BP undergoes nucleophilic attack by an alcohol with a quite
᝽
EtOH
HO–(CH ) –OH
HO–(CH ) –OH
7.0
10
6.5
2.0
0.32
2.0
1
6.3
1.0
large activation energy. This fact thus suggests that the reaction
of phosphine radical cations with nucleophiles is forbidden as
predicted by the CM model.
2
3
2
4
a
Ϫ1
Ϫ3
2؉
Ϫ4
Ϫ3
[
BP] = 1.50 × 10
mol dm
,
[MV ]0 = 2.00 × 10
mol dm ,
0
Ϫ3
[
ROH] = 12.3 mol dm . In acetonitrile at 50 ЊC under an argon
0
b c
atmosphere in the dark. Errors are within 3%. Relative rate constant,
ROH
EtOH
Experimental
kNu /kNu
.
Instruments
Bu
Bu
Bu
R
H
UV–visible spectra were recorded on a Shimadzu UV-2200A
UV–vis recording spectrophotometer. GC analysis was done
with a Shimadzu GC-14A gas chromatograph. Mass spectra
were obtained on a Shimadzu GCMS-QP2000A gas chromato-
graph-mass spectrometer equipped with a Shimadzu GC-
+
•
P
O
ϩ
Scheme 3
ROH.
Transition state of the reaction of radical cation BP᝽
with
1
31
MSPAC 200S data processor. H and P NMR spectra were
obtained on a Varian XL 200 NMR spectrometer operating at
rate results from intramolecular general acid-catalysis; the tran-
sition state of the reaction of BP with the propanediol is
stabilized by the formation of a six-membered ring as shown in
Scheme 4. On the other hand, the terminal hydroxy hydrogen in
2
00 and 81 MHz, respectively. Cyclic voltammetry was done on
ϩ
᝽
a Cypress Systems OMNI 90 potentiostat in acetonitrile with
platinum and Ag/AgCl electrodes as the working and the
reference electrodes, respectively, and with tetraethylam-
Ϫ3
monium tetrafluoroborate (0.10 mol dm ) as the supporting
Bu
electrolyte.
δ+
O
•
P
Bu
Bu
Materials
H
O δ+
Tributylphosphine (BP) was commercially available (Tokyo
H
Chemical Industry) and distilled before use. Mesityldiphenyl-
ϩ
4
Scheme 4 Transition state of the reaction of radical cation BP᝽
propane-1,3-diol.
with
phosphine was prepared according to the literature procedure.
2؉
1
,1Ј-Dimethyl-4,4Ј-bipyridinium
(methylviologen;
MV )
tetrafluoroborate was obtained from the corresponding chlor-
ide salt (Tokyo Chemical Industry) as follows. The chloride salt
was treated with silver tetrafluoroborate in methanol, and after
filtration of the resulting precipitate, the filtrate was concen-
trated under reduced pressure. Extraction of the residue with
acetonitrile and evaporation of the solvent gave the tetrafluoro-
butane-1,4-diol has no effect on the rate. Certainly, there is
an entropic or configurational disadvantage (or both) in the
formation of a ring structure at the transition state for the
ϩ
54
reaction of BP with the butanediol. The nucleophilicity
᝽
ϩ
of water toward BP
is smaller than expected from its
᝽
basicity (entry 1 in Table 2). The magnitude is almost the
same as that of propan-2-ol, as has been observed before in
reactions of carbocations with nucleophiles.
2؉
borate salt of MV as a crude solid, which was purified by
recrystallization from acetonitrile–diethyl ether. Alcohols,
thiols, and acetonitrile were purchased (Tokyo Chemical
Industry) and distilled before use.
55–57
Hydrogen
bonding among water molecules is so strong that water cannot
act as a general acid-catalyst, and instead the hydrogen bond-
ing contributes to cluster formation lowering the nucleo-
philicity.
General procedures
The steric bulk of the thiol has little effect on the rate con-
To a UV-cell equipped with a septum and filled with argon
ϩ
2؉
stants kNu in reactions with BP unlike with alcohol (entries 10
gas were added successively solutions of MV and of RXH
᝽
and 11). It would appear that the results are accounted for by
(X = O, S) in acetonitrile, and then tributylphosphine (BP) (as a
2
؉
a sulfur atom being larger than an oxygen atom; the distance
neat material). The initial concentrations of BP, MV , and
Ϫ1 Ϫ4 Ϫ3
ϩ
between the phosphorus atom in BP and the R group in
RXH were 1.50 × 10 , 2.00 × 10 , and 1.50 mol dm ,
respectively. The UV-cell was kept in a cell holder of a spectro-
photometer maintained at 50 ЊC, and the UV–visible spectrum
᝽
a nucleophile at the transition state may be longer in the reac-
tion with thiol RSH than in the reaction with alcohol ROH.
Alternatively, the lack of a steric effect in the k step with
was taken every 18 seconds. The reaction was carried out also
Nu
ϩ
2؉
RSH may mean that the reaction between BP and RSH
in an NMR tube with acetonitrile-d at [BP] = [MV ] =
3 0 0
Ϫ5 Ϫ3 Ϫ5 Ϫ3
᝽
proceeds through single-electron transfer (SET) from RSH to
1.00 × 10 mol dm and [RXH] = 2.00 × 10 mol dm , and
0
1
ϩ
BP , giving neutral phosphine BP and radical cation
the H NMR spectrum was recorded after an appropriate
reaction period.
᝽
ϩ 43
RSH
. Nevertheless, that phosphine sulfide BP-S is the final
᝽
product of this reaction shows that, if the SET occurs, radical
attack by RSH on BP takes place rapidly, eventually giving
the phosphoranyl radical intermediate BP -SR.
ϩ
Kinetics
᝽
ؒ
The reaction was carried out in a UV-cell, and the growth of
the absorbance at 605 nm was monitored on a spectro-
photometer every 18 seconds. Kinetic data obtained were
The value of activation energy E for the nucleophilic attack
a
ϩ
on phosphine radical cation BP is worth comparing with
᝽
values reported for reactions of carbon radical cations (see
Table 3). The reactions of anthracene radical cations with
amine nucleophiles take place rapidly with extremely small
45–47
analyzed as described in the literature.
Eqn. (7) is derived as follows, from eqns. (9)–(13), when
58
or sometimes negative activation energy. It has also been
reported that styrene radical cations react with nucleophiles
2
؉
d[BP ؒ ؒ ؒ MV ]/dt =
k [BP][MV ] ϩ k [BP ؒ ؒ ؒ MV ] Ϫ
2
؉
ϩ
؉
59a,b
59b
59b
59c
such as azide,
halides,
alcohols,
and alkenes
as
12
32
᝽
2
؉
rapidly as the related carbocations do. These findings have been
(k21 ϩ k )[BP ؒ ؒ ؒ MV ] = 0 (9)
23
8
60
J. Chem. Soc., Perkin Trans. 2, 1999, 855–862

View Article Online
ϩ
ϩ
؉
d[BP᝽
ؒ ؒ ؒ MV ]/dt =
20 T. Endo, Y. Saotome and M. Okawara, J. Am. Chem. Soc., 1984,
2
؉
ϩ
؉
106, 1124.
k [BP ؒ ؒ ؒ MV ] ϩ k [BP ][MV ] Ϫ
23
43
᝽
᝽
2
1 E. H. Yonemoto, G. B. Saupe, R. H. Schmehl, S. M. Hubig,
R. L. Riley, B. L. Iverson and T. E. Mallouk, J. Am. Chem. Soc.,
ϩ
؉
(
k32 ϩ k )[BP ؒ ؒ ؒ MV ] = 0 (10)
34
1
994, 116, 4786.
22 X. Xu, K. Shreder, B. L. Iverson and A. J. Bard, J. Am. Chem. Soc.,
996, 118, 3656.
23 S. Yasui, K. Shioji, M. Tsujimoto and A. Ohno, Chem. Lett., 1995,
83.
ϩ
؉
؉
d[BP᝽
]/dt = k [BP
ؒ ؒ ؒ MV ] Ϫ (k [MV ] ϩ
34
᝽
43
1
ϩ
] = 0
kNu[RXH])[BP᝽
(11)
7
ؒ
24 The absorption maximum of the longer wavelength observed in our
study was slightly blue-shifted compared with the maximum (608
d[BP -XR]/dt =
ϩ
ؒ
2؉
kNu[RXH][BP᝽
] Ϫ k [BP -XR][MV ] = 0 (12)
Ϫ
50
nm) observed for the PF6 salt in an aprotic solvent such as
acetonitrile, dichloromethane, or THF (ref. 16). This shift could
result either from the large amount of alcohol present in our
؉
ϩ
؉
ؒ
2؉
d[MV ]/dt = k [BP ؒ ؒ ؒ MV ] ϩ k [BP -XR][MV ] Ϫ
3
4
᝽
50
؉
ϩ
؉
reaction system or from the difference in the counter anion of MV .
5 The UV–visible spectrum showed isosbestic points at 260 and 294
nm.
k [BP ][MV ] (13)
4
3
᝽
2
2؉
ϩ
᝽
؉
ϩ
ؒ
[
BP ؒ ؒ ؒ MV ], [BP ؒ ؒ ؒ MV ], [BP ], and [BP -XR] in
Scheme 2, respectively, are in the steady-state concentrations.
The rate expression is given in eqn. (13). Eliminating
BP ؒ ؒ ؒ MV ], [BP -XR], and [BP ] in eqn. (13) through
eqns. (9)–(12) and taking k = k k k , k = k k k , and
26 In spite of repeated distillation on calcium hydride, the acetonitrile
᝽
Ϫ2 Ϫ3
used contained about 5 × 10 mol dm water (ref. 11c).
7 C. P. Andrieux, C. Blocman, J.-M. Dumas-Bouchiat and J.-M.
Savéant, J. Am. Chem. Soc., 1979, 101, 3431.
8 For a comprehensive work on dissociative SET, see J.-M. Savéant,
Acc. Chem. Res., 1993, 26, 455.
2
2
ϩ
؉
ؒ
ϩ
[
᝽
᝽
ϩ Ϫ
1
2
23 34
21 32 43
a = (k21 ϩ k )(k ϩ k ) Ϫ k k , we obtain eqn. (7).
23
32
34
23 32
29 J. Grimshaw, J. R. Langan and G. A. Salmon, J. Chem.Soc., Faraday
Trans., 1994, 90, 75.
3
0 M. S. Workentin and R. L. Donkers, J. Am. Chem. Soc., 1998, 120,
2664.
Acknowledgements
3
3
3
1 S. Antonello and F. Maran, J. Am. Chem. Soc., 1998, 120, 5713.
2 G. B. Schuster, J. Am. Chem. Soc., 1979, 101, 5851.
3 S. Bank and D. A. Juckett, J. Am. Chem. Soc., 1975, 97, 567.
This work was financially supported in part by a Grant-in-Aid
for Scientific Research (C) (No. 09640654) from the Ministry of
Education, Science, Sports, and Culture, Japan. One of
the authors (S. Y.) acknowledges the financial support of a
Tezukayama Research Grant in 1997.
34 H. C. Gardner and J. K. Kochi, J. Am. Chem. Soc., 1975, 97, 1855.
35 T. W. Chan and T. C. Bruice, J. Am. Chem. Soc., 1977, 99, 7287.
3
3
6 M. J. Thomas and C. S. Foote, Photochem. Photobiol., 1978, 27, 683.
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1
019.
3
8 A. G. Davies, D. Griller and B. P. Roberts, J. Chem. Soc., Perkin
Trans. 2, 1972, 2224.
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32
34
؉
1
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43 43
؉
[MV ] is much smaller than [ROH] under the experimental
؉
2
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3
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(not k
) as the
Nu
rel
standard in calculating the relative value of kNu, kNu
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.
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/kNu
was
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rel
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EtOH
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.
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1
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Paper 8/07722C
8
62
J. Chem. Soc., Perkin Trans. 2, 1999, 855–862