General Base Catalysis by Organic Bases of the Reaction of Diphenylphosphinic Hydrazide with Phenyl Isocyanate in Benzene

Article abstract of DOI:10.1023/A:1024727618620

The kinetics of the reaction of diphenylphosphinic hydrazide with phenyl isocyanate in the presence of organic bases in benzene at 25°C were studied. The catalytic activity of the bases correlates with spectroscopic and thermodynamic Taft (pK_HB), Koppel-Palm (B), and Gutmann (DN) parameters. A common general base mechanism of the catalysis of semicarbazide formation by bases belonging to different classes of organic compounds is offered and discussed.

Full text of DOI:10.1023/A:1024727618620

Russian Journal of General Chemistry, Vol. 73, No. 2, 2003, pp. 192 196. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 2, 2003,
pp. 211 215.
Original Russian Text Copyright
2003 by N. Yanchuk, Ivanets, S. Yanchuk.
General Base Catalysis by Organic Bases of the Reaction
of Diphenylphosphinic Hydrazide with Phenyl Isocyanate
in Benzene
N. I. Yanchuk, L. N. Ivanets, and S. N. Yanchuk
Gnatyuk Ternopol State Pedagogical University, Ternopol, Ukraine
Received January 30, 2001
Abstract The kinetics of the reaction of diphenylphosphinic hydrazide with phenyl isocyanate in the
presence of organic bases in benzene at 25 C were studied. The catalytic activity of the bases correlates with
spectroscopic and thermodynamic Taft (pKHB), Koppel Palm (B), and Gutmann (DN) parameters. A common
general base mechanism of the catalysis of semicarbazide formation by bases belonging to different classes of
organic compounds is offered and discussed.
Organic catalysts exhibit a high catalytic activity
in nucleophilic additions of hydrazine derivatives
[1, 2]. At the same time, the catalytic activity of such
compounds in reactions of phosphorus-containing
hydrazides with isocyanates have scarcely been
studied. The problem of searching for effective cata-
lysts for such practically important processes is actual.
In the purposeful search for highly active organic
catalysts for systems in which starting compounds
enhance their reactivity via donor acceptor interaction
with catalysts of special interest are organic bases
containing different basic centers and belonging to
different classes of compounds. It is important to find
out the possibility of realization of the above me-
chanism and to correlate the catalytic properties of
bases with the nature and structure of electrophilic
reagents.
(C6H5)2(O)PNHNHCONHC6H5.
XXI
Control experiments showed that the catalytic re-
action proceeds quantitatively and is not complicated
by side processes. The substituted semicarbazide
formed by the reaction has no effect on the catalytic
process. Small additives of organic bases to the reac-
tion mixture accelerate the reaction significantly, i.e.
act as catalysts.
The rate of the reaction in benzene in the presence
of organic catalysts is described by Eq. (1).
dx/dt = k0(a
x)(b
x) + kb(a
x)(b
x)m. (1)
Here k0 and kb are the rate constants of the non-
1
1
catalytic (l mol 1 s ) and catalytic (l2 mol 2 s ) reac-
tions, respectively; a, b, and m are the initial con-
centrations of hydrazide, phenyl isocyanate, and ca-
talyst (M); x is the yield of the reaction product (M);
and t is time (s). From Eq. (1) it follows that the
overall apparent rate constant is described by Eq. (2).
To this end, in the present work we studied the
catalytic activity of benzenes and nitrogen- and
oxygen-containing compounds I XVIII that belong
to different classes and capable of specific interactions
with reagents in the reaction of diphenylphosphinic
hydrazide (XIX) with phenyl isocyanate (XX) in
benzene at 25 C. We correlated the catalytic activity
with the class of organic bases, their structure, the
nature and structure of electrophilic reagents, and also
different basicity parameters of catalysts.
k = k0 + kbm.
(2)
The apparent rate constant k (l mol 1 s ) was
found by a second-order reaction equation, since in
was shown that the reciprocal current reagent con-
centrations linearly vary with time at varied catalyst
concentrations and the rate constants are invariable in
the course of the process. The catalytic rate constants
kb listed in Table 1 were found by Eq. (2) {k0 0.449
1
It was established that semicarbazide XXI forma-
tion in the presence of the catalysts under investiga-
tion proceeds irreversibly according to the following
scheme.
1
(C6H5)2(O)PNHNH2 + O=C=N C6H5
0.012 l mol 1 s ) [3]}.
XIX
XX
Small bases additives significantly accelerate the
1070-3632/03/7302-0192$25.00 2002 MAIK Nauka/Interperiodica

GENERAL BASE CATALYSIS BY ORGANIC BASES OF THE REACTION
193
Table 1. Effect of organic bases on the rate of the reaction of diphehylphosphinic acid hydrazide with phenyl isocyanate
in benzene at 25 C
Comp.
no.
1
pKBH [4, 5] pKHB [4] DN [6, 7] B [7, 8] kb, l2 mol 2 s
kb/k0, M
+
Catalyst
I
II
III
IV
V
Toluene
Mesitylene
Nitromethane
Nitrobenzene
Anisole
58
77
65
0.498 0.063
0.511 0.067
0.539 0.061
0.544 0.056
1.58 0.35
2.51 0.25
2.64 0.34
4.08 0.31
4.72 0.24
6.60 0.19
12.5 1.5
6.71 0.42
7.80 0.28
23.0 2.3
31.7 2.4
47.9 3.2
1.11
1.14
1.20
1.21
3.52
5.59
5.87
9.08
10.5
14.7
27.8
14.9
17.4
51.2
70.6
106
2.7
4.4
12.14
6.82
10.70
0.73
0.02
0.80
0.90
1.00
1.09
1.01
0.73
1.02
1.26
2.06
2.38
2.47
2.53
1.88
67
155
155
160
170
181
280
237
285
287
291
343
335
362
472
VI
Benzonitrile
Acetonitrile
11.9
14.1
16.5
17.1
19.2
14.6
VII
VIII
IX
X
XI
XII
XIII
XIV
XV
XVI
Methyl acetate
Ethyl acetate
Diethyl ether
Dioxane
Dibutyl ether
Tetrahydrofuran
N,N-Dimethylformamide
N,N-Dimethylacetmide
N,N-Diethylacetamide
5.2
3.96
2.42
3.22
5.40
2.08
1.50
0.36
20.0
26.6
27.8
32.2
29.8
33.1
XVII Dimethyl sulfoxide
XVIII Pyridine
1.04
5.25
40.9 1.6
66.2 4.1
91.0
147
reaction, and the reaction rate significantly increases
with increasing catalyst concentration. The apparent
rate constants k linearly vary with catalyst concentra-
tion (up to the concentration 0.05 M).
lowest. The catalytic activity of bases belonging to
different classes of organic compounds increases in
the order aromatic hydrocarbons < nitro compounds
< nitriles < ethers carboxylic esters < carboxamides
< sulfoxides < tertiary amines. This trend is evidently
explained by the electron-donor properties and ability
of these compounds to hydrogen-bond formation,
which, too, enhance in the above order.
From Table 1 it follows that organic bases I XVIII
are effective catalysts for the process of formation of
phosphorus-containing semicarbasides. At the same
time, noteworthy are great differences in the activity
of bases belonging to differerent classes of com-
pounds. The catalytic activity measured by the kb/k0
ratio span the range 1.11 147. Hence, the catalytic
activity of the catalysts studied varies 130 times. The
Note that the character of the structural effect of
electrophilic reagents on the catalytic activity of
organic bases in this reaction is similar [9]. This fact
shows that hydrogen-bond formation on catalysis
most active catalyst is pyridine (XVIII) whose nucleo- plays a decisive role in the reactions with all the
philic solvation ability is the highest. It is known that
nitrogen- and oxygen-containing bases act as strong
hydrogen acceptors in hydrogen-bond formation in
donor acceptor complexes. The formation of fairly
strong hydrogen bonds of N H O(N) type increases
the electron density on the nitrogen atom of the amino
group of the hydrazide, thus enhancing the nucleo-
philicity of the latter and ensuring the highest reaction
rates in the presence of these bases. It is known that
the aromatic ring can exhibit nucleophilic properties
and forms weak hydrogen bonds with proton donors.
But, as the specific solvation with aromatic hydro-
carbons is not so effective as with nitrogen- and
oxygen-containing compounds, the reaction rates in
the presence of toluene (I) and mesitylene (II) are the
substrates. In the series of the electrophilic reagents
we studied previously and in the present work, the
sensitivity to the catalytic effect of organic bases in-
creases in the order phenyl isocyanate < picryl chlo-
ride < phenyl isothiocyanate < mesityl isothiocyanate,
even though the rate of a noncatalytic reaction of di-
phenylphosphinic hydrazide with the above-mentioned
substrates varies in the opposite order. The fact that
the catalytic activity of organic bases increases in
going from phenyl isocyanate to mesityl isothio-
cyanate agrees with the commonly accepted postulate
that catalysts act more effectively in slower reactions.
Hence, the trend in variation of the catalytic activity
of bases with the nature and structure of electrophilic
reagents provides evidence for hydrazide catalyst
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194
YANCHUK et al.
complex formation and thus for a general base
mechanism of the catalysis.
parameter of Lewis basicity. It characterizes the
overall strength of interaction of an electron donor
with an electron acceptor. The catalytic activity of
bases considerably depends on their nature, and, as
follows from Table 1, it increases with increasing
donor number. We found that the dependence of the
logarithms of catalytic rate constants on the donor
numbers of catalysts is quantitatively described by the
linear equation (4) (data for dioxane were not used).
To find out the mechanism of the catalytic action
of various organic bases, it is expedient to quanti-
tatively correlate the activity of catalysts with para-
meters characterizing their properties. With this
purpose we correlated the logarithms of catalytic rate
constants kb with the basicity of catalysts (pKBH+) in
water (Table 1). However, no satisfactory correlation
was obtained. This implies lack of ionic dissociation
with complete proton transfer from hydrazide to
catalyst. It was recently showed [5] that the strength
of organic bases in protoinert media is best evaluated
on the Taft pKBH scale which reflects the ability of
the nucleophilic center of a catalyst to hydrogen-bond
formation [4, 10 12]. The corresponding data analysis
showed that the catalytic rate constants vary in pa-
rallel with the ability of bases to hydrogen-bond
formation. The effect of the catalysts on the reaction
rate was quantitatively described in terms of pKHB
values by means of Eq. (3) (data in dioxane XI were
not used, because they much deteriorated the correla-
tion coefficient).
log kb = ( 0.514 0.031) + (0.0702 0.0014)DN;
n 13, r 0.997, s 0.049.
(4)
It is known that the nucleophilicity of compounds
can be quantitatively characterized in terms of a
Koppel Palm general basicity (B) [8, 18]. The de-
pendence of the logarithms of catalytic rate constants
on B is described by Eq. (5).
log kb = ( 0.563 0.105) + (0.00576 0.00042)B;
n 18, r 0.960, s 0.206.
(5)
Considering dependences (3) (5) it is easy to pro-
pose that both the spectoscopic Taft (pKBH) and
Koppel Palm (B) basicity parameters and the thermo-
dynamic basicity scale of Gutmann (DN) reflect the
same nucleophilic effects of organic bases in the reac-
tion of hydrazine derivatives with isocyanates and,
therefore, are interchangeable. This is also confirmed
by the conclusion of Makitra et al. [19] about equi-
valence of B and DN values and their possible inter-
changeability in treatment kinetic data.
log kb = ( 0.0985 0.1631) + (0.728 0.105)pKHB
;
n 14, r 0.900, s 0.289.
(3)
The resulting data suggest that catalysts of different
nature act by a common mechanism. Most probably,
this is a general base catalysis mechanism that in-
volves formation of a hydrogen-bonded complex like
XXII. Really, we can hardly develop an alternative
mechanism for such weak bases as aromatic hydro-
carbons, nitro compounds, nitriles, ethers, and car-
boxylates. Their catalytic activity can be explained in
no other way than by hydrogen-bond formation,
which is characteristic of general base catalysis [5].
Similarly, the catalytic activity of carboxamides and
sulfoxides is in the first turn determined by their
ability to hydrogen-bond formation [5]. It was pre-
viously shown [13] that the high catalytic activity of
tertiary amines in thiosemicarbazide formation pri-
marily contributed by the electron-donor properties of
the catalysts and their ability to hydrogen-bond forma-
tion. The latter fact better agrees with base rather than
nucleophilic catalysis. Correlation (3) that includes
both strong and weak bases, too, provides evidence
for the proposed mechanism. Note also that iso-
cyanates exhibit weak coordination properties and do
not form complexes even with very strong electron
donors [14 16].
The aforesaid gives us grounds to suggest that the
catalysis mechanism is preassociative (within the
frames of the general base mechanism) and common
for all the bases, even though the latter belong to dif-
ferent classes of organic compounds. On the first
stage [scheme (6)] hydrazide XIX associates with base
B (I XVIII) to form complex XXII via hydrogen
bonding. The association is a fast and an equilibrium
process.
(C6H5)2(O)PNHNH2 + B
XIX
I XVIII
(C6H5)2(O)PNHHN H B.
(6)
XXII
The second stage [scheme (7)] is the rate-limiting
reaction of associate XXII with phenyl isocyanate XX.
Increased nucleophilicity of hydrazide facilitates
formation of a new bond with the isocyanate carbon
atom to form a cyclic transition state XXIII which
Donor number (DN) is an important characteristic
of electron-donor compounds. This quantitative cha-
racteristic has been offered by Gutmann [6, 17] as a
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GENERAL BASE CATALYSIS BY ORGANIC BASES OF THE REACTION
195
Table 2. Noncatalytic k0 and catalytic kb rate constants of the reactions of diphenylphosphinic hydrazide and p-toluidine
with phenyl isocyanate and picryl chloride in benzene in the presence of catalysts at 25 C
Phenyl isocyanate
Picryl chloride
Nucleophile
Catalyst
k0,
kb,
kb/k0,
l mol
k0,
kb,
kb/k0,
l mol
1
1
1
1
1
1
l mol 1 s
l2 mol 2 s
l mol 1 s
l2 mol 2 s
(C6H5)2(O)PNHNH2 Pyridine
0.449 [3]
66.2 4.1
23.0 2.3
147
0.0418 [20]
7.12 0.28 170
2.77 0.14 66.2
N,N-Dimethylform- 0.449 [3]
51.2 0.0418 [20]
amide
Dioxane
0.449 [3]
12.5 1.5
27.8 0.0418 [20] 0.880 0.031
21.0
0
0
p-CH3C6H4NH2
Dimethyl sulfoxide 0.00386 [21] 0.196 [22]
N,N-Dimethylform- 0.00386 [21]
49.4 0.745 [23]
27.2 0.745 [23]
0
0
0.108 [22]
amide
Dioxane
0.00386 [21] 0.00800 0.00037
2.07 0.745 [23]
0
0
then converts to the reaction product, semicarbazide
XXI, with regeneration of catalyst I XVIII.
suggests that the catalyst interacts with the hydrogen
atom of the starting hydrazide.
Hence, the accelerating effect of organic bases on
the rate of the reaction of diphenylphosphinic hydra-
zide with phenyl isocyanate is evidently explained by
formation of a hydrogen bond involving the amino
hydrogen atom of the starting hydrazide.
EXPERIMENTAL
Benzene for kinetic studies was prepared according
to [25]. Diphenylphosphinic hydrazide was synthesi-
zed and purified according to the procedures described
in [26, 27]. Phenyl isocyanate was distilled in a
vacuum directly before use. Organic catalysts were
purified by known procedures. The purity of the
compounds used was determined by the coincidence
of their physicochemical characteristics with pub-
lished data and by the constancy of the kinetic para-
meters of a reaction involving a particular substance,
measured after several consequtive cycles of purifica-
tion.
(6)
The close catalytic activities of organic bases in the
reactions of diphenylphosphinic hydrazide with
phenyl isocyanate and picryl chloride (Table 2)
suggest that activation of substrate molecules does not
play a significant role, and the different effects of
these bases are evidently explained by interaction with
the starting hydrazide or the transition state. From this
point of view it is interesting to compare the effect of
these catalysts in the reactions of diphenylphosphinic
hydrazide and p-toluidine with phenyl isocyanate and
picryl chloride. The absence of accelerating effect of
DMSO, DMF, and dioxane in the reaction of p-tolui-
dine with picryl chloride can be explained, by analogy
with the reactions of aromatic amines with p-nitro-
benzenesulfonyl bromide and picryl chloride [24], by
activation of the amino group in the transition state,
since the contributions of activation of the starting
amine into the rates of the reactions with phenyl iso-
cyanate and picryl chloride should be the same. The
fact that organic bases exhibit almost equal catalytic
activities in the reactions of diphenylphosphinic hydra-
zide with phenyl isocyanate and picryl chloride
The reaction progress was followed as described in
[28]. The kinetic measurements were performed in
benzene at 25 0.05 C. The initial concentrations of
diphenylphosphinic hydrazide and phenyl isocyanate
3
were 0.625 10 M each, and the concentrations of
catalysts were 0.0001 0.05 M. The accuracy of result-
ing data was evaluated by mathematical statistics
methods (confidence level 0.95) [29].
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