Complexes PhMgBr·nMgBr2·mNEt3 in the reaction with acetylene

Article abstract of DOI:10.1016/S0022-328X(99)00136-9

Kinetics of the reaction of acetylene with two phenylmagnesium bromide reagents of different magnesium bromide content (Br/Mg 1.08 and 1.27) in diethyl ether in the presence of various additions of triethylamine (TEA) were investigated. The reagent of average stoichiometry PhMgBr·2TEA reacts about 20 000 times faster than in the absence of the amine. Elevated content of magnesium bromide in the reagent enhances the accelerating effect of the amine. The complicated relationships obtained enabled us to draw conclusions on the structure and reactivity of the title complexes. At moderate magnesium bromide content species 2PhMgBr·MgBr₂ dominate and preferentially complex with TEA giving solvates more reactive than PhMgBr·TEA, whose formation takes place at a higher amine content.

Full text of DOI:10.1016/S0022-328X(99)00136-9

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 584 (1999) 185–189
Complexes PhMgBr·nMgBr ·mNEt in the reaction with acetylene
2
3
Vello P a¨ llin, Ants Tuulmets *
Institute of Organic Chemistry, Uni6ersity of Tartu, Tartu, Estonia
Received 3 February 1999
Abstract
Kinetics of the reaction of acetylene with two phenylmagnesium bromide reagents of different magnesium bromide content
(
Br/Mg 1.08 and 1.27) in diethyl ether in the presence of various additions of triethylamine (TEA) were investigated. The reagent
of average stoichiometry PhMgBr·2TEA reacts about 20 000 times faster than in the absence of the amine. Elevated content of
magnesium bromide in the reagent enhances the accelerating effect of the amine. The complicated relationships obtained enabled
us to draw conclusions on the structure and reactivity of the title complexes. At moderate magnesium bromide content species
2
PhMgBr·MgBr2 dominate and preferentially complex with TEA giving solvates more reactive than PhMgBr·TEA, whose
formation takes place at a higher amine content. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Grignard reagent; Acetylene; Kinetics; Solvation effects
1
. Introduction
Besides other findings a conspicuous effect of magne-
sium bromide additions was intriguing. Similarly to the
one reported by Wotiz et al. [5] for the reaction of
ethylmagnesiumbromide with 1-hexyne, additions of
magnesium bromide to the Grignard reagent enhanced
considerably the catalytic effect of triethylamine in the
reaction with acetylene [8]. Such a result was bewilder-
ing on the background of a well-known suppressing
effect of magnesium halides on the reaction rates of
Grignard reagents with 1-hexyne [9], acetylene [8], ke-
tones [10–13], esters [12], nitriles [14] and phosphonates
Acetylenic Grignard compounds or the correspond-
ing organoalkali metal derivatives are important inter-
mediates in many syntheses of acetylenic compounds.
Although deprotonation of terminal acetylenes by
organolithium compounds in organic solvents or by
alkali metal amides have been most frequently used, in
certain cases [1,2] converting of acetylenes into Grig-
nard reagents has advantages over these methods.
Reactions of acetylenic compounds with Grignard
reagents have been investigated sporadically over many
years [3]. The reaction of acetylenes is an extremely
slow process, however, it was found that addition of
tertiary amines accelerates remarkably the reaction of
alkylmagnesium halides with 1-hexyne [4,5]. Formation
of bis-bromomagnesiumacetylene in conventional
ethers also proceeds sluggishly [1].
[15]. Namely, it was found that in the presence of a
small amount of TEA (about 0.2 equivalents per Ph–
Mg) the reaction rate in diethyl ether increased almost
linearly and by more than 10 times when passing from
pure diphenylmagnesium (no catalytic effect) to the
complex of stoichiometry 2PhMgBr·MgBr . Although
2
complexation and solvation effects were evident, a de-
tailed analysis of the phenomenon failed in that work.
In connection with our continuous interest in solva-
tion effects in the Grignard chemistry, we have resusci-
tated our earlier investigation of the reaction of
acetylene with phenylmagnesium bromide. In this work
the rate constants of the reaction were determined for a
wide range of triethylamine additions in the presence of
both small and considerable amounts of magnesium
bromide. The unexpectedly complicated relationships
The kinetics of the reaction of acetylene with phenyl-
magnesium bromide were previously investigated in this
laboratory [6–8]. A catalytic effect of organic bases was
established, whereas secondary and aliphatic tertiary
amines were found to accelerate considerably the for-
mation of bis-bromomagnesiumacetylene. Triethy-
lamine (TEA) appeared to be the most effective catalyst
among the investigated donors [6].
*
Corresponding author. Fax: +372-7-375-264.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00136-9

186
V. P a¨ llin, A. Tuulmets / Journal of Organometallic Chemistry 584 (1999) 185–189
60
obtained enabled us to draw significant conclusions on
the structure and reactivity of the system Ph-
kI=
[
C H ]
2 2 0
MgBr·nMgBr ·mTEA in diethyl ether.
2
where 6 is the slope of the tangent to the kinetic curve
0
−
1
in its initial point in mm s
and the initial concentra-
tion of acetylene, [C H ] , is expressed as the total heat
2
2 0
2. Experimental
of the reaction measured in mm from the thermogram
corrected for the heat exchange.
2.1. Materials
All the operations with the reagents and solutions
3. Results and discussion
were carried out under dry argon. The Grignard
reagents were prepared in the conventional manner.
Elevated magnesium bromide content in the reagent
was attained by partial replacing of bromobenzene with
The two-step mechanism of the reaction of acetylene
with a Grignard reagent
HCꢀCH+RMgX“HCꢀCMgX+RH
HCꢀCMgX“XMgCꢀCMgX+HCꢀCH
(1)
(2)
1,2-dibromoethane in the synthesis. The reagents were
analysed for basic magnesium concentration acidimetri-
cally and for bromide ion concentration by the Mohr
method in the same solution after the acidimetric analy-
sis. The total magnesium content was determined by
complexation titration. The material balance was al-
ways good to excellent.
2
was suggested by Kleinfeller and Lohmann [17] and
confirmed inter alia by our investigations [7]. The first
step of the reaction is rate limiting. Although amines
catalyse the second step more intensively [7], under a
1
00-fold excess of the Grignard reagent and at low
2
.2. Kinetic measurements
acetylene concentration applied in our experiment only
reaction (1) could be observed at the beginning of the
process. However, the total thermal effect of the pro-
cess recorded at the end of the reaction consists of heats
DH and DH of two consecutive reactions (1) and (2),
The reaction was carried out in a cylindrical glass
3
vessel (34 cm ) mantled with foam plastic and placed in
a thermostatted housing. The equipment was sealed
with a thermostatted lid. The reaction cell was provided
with three tubes mounting through the lid. Two inlets
were employed for a mechanical stirrer and for a ther-
mistor, respectively, the third inlet serving for the injec-
tion of the reagents. The thermistor was connected
through a bridge circuit to a recording potentiometer
and the device was calibrated for the temperature range
from 19.8 to 20.3°C.
All parts of the equipment as well as the reagents
were thermostatted at 19.8°C. The reaction vessel was
purged thoroughly with pure argon, about 30 ml of the
Grignard reagent was cannulated into the cell and the
stirring was started. After the thermal equilibrium was
set, the temperature of the solution was adjusted to
1
2
respectively. Therefore, the pseudo-first-order rate con-
stants calculated as described in the experimental sec-
tion should be corrected by a factor (DH +DH )/DH ,
1
2
1
but the reaction heats are not known. We could only
establish that the total heat of the process was propor-
tional to the amount of acetylene regardless of the
changes in Grignard reagents or in the amine concen-
tration. Hence, two corrections were made. The ob-
served pseudo-first-order rate constants divided by the
Grignard reagent concentration gave the second-order
rate constants kII and the relative rate constants krel
were calculated by taking the system Ph-
MgBr·0.087MgBr ·0.01TEA for the reference point
2
(k =1). The obtained reactivity values for the Grig-
rel
1
9.8°C by introducing a thin jet of dry argon. Then
nard–TEA systems are presented in Tables 1 and 2 and
in Fig. 1.
3
about 0.6 cm of dry ether saturated with acetylene was
introduced, and the temperature change of the reaction
solution (usually 0.4°C by the end of the reaction) was
recorded as a plot of temperature (in mm) versus time.
Thus the mean reaction temperature was about 20°C.
Because of short reaction times ranging from tens to a
few hundreds of seconds, the system was nearly adia-
batic and the heat exchange with the internal part of
the calorimeter caused a heat loss to an extent of a few
per cent. The temperature of the reaction mixture was
corrected as described earlier [16].
True rate constants can be estimated from the follow-
ing experimental data. The second-order rate constant
for 1 molar per cent amine content in Table 1 exceeds
by about 50 times the one obtained by the same method
for the same reagent without amine addition [7]. Ki-
netic measurements both by acetylene absorption and
benzene production gave coinciding results and indi-
cated about a 100-fold rise of the rate by adding the
same relative amount (1 molar per cent) of TEA [18].
Thus, the correction factor for thermographic data is
about 2.0, indicating at the same time close values for
DH and DH .
The pseudo-first-order rate constants were calculated
from the initial rate of the reaction as
1
2

V. P a¨ llin, A. Tuulmets / Journal of Organometallic Chemistry 584 (1999) 185–189
187
Table 1
Kinetic data for the reagent PhMgBr·0.087MgBr2 in the presence of
different additions of triethylamine
PhMgBr
(
Amine a
(mol%)
kI (102 s−1
)
kII (102) b
krel
mol 1 1)
−
0.707
0.690
0.662
0.702
0.707
0.678
0.668
0.627
0.515
0.540
0.660
0.515
0.606
0.636
1.00
1.74
3.68
6.87
18.1
33.0
51.0
59.5
75.0
81.5
94.3
103.0
121.0
132.0
0.176
0.253
0.550
1.24
3.30
4.70
6.00
6.67
6.12
7.00
9.20
7.60
11.4
16.0
0.250
0.366
0.831
1.77
4.67
6.91
8.99
10.6
11.8
12.9
13.9
14.7
18.8
25.1
1.00
1.46
3.32
7.06
18.7
27.6
36.0
42.5
47.5
51.8
55.7
59.0
75.2
100.6
a
(
[TEA]/[PhMgBr])×100.
Fig. 1. Dependence of the reaction rate on the relative content of
b
−1 −1
l mol
s
.
triethylamine. (A) The reagent PhMgBr·0.087MgBr ; (B) the reagent
2
PhMgBr·0.37MgBr2.
Two Grignard reagents with different contents of
observed also for the ethylmagnesium bromide–magne-
sium bromide system [10]. Thus, at lower values of the
ratio Br/Mg, complexes of lower magnesium bromide
magnesium bromide were used. Conventional Grignard
reagents usually contain a slight excess of magnesium
halide because of some unavoidable Wurtz coupling. In
this experiment the ratio Br/Mg was equal to 1.08 and
for the reagent with enhanced magnesium bromide
content it equalled 1.27 Thus, the stoichiometric com-
content, e.g. 2PhMgBr·MgBr , should be preponderant.
2
Quite rational structures I or II, where E stands for
diethyl ether and X for the bromine atom, could be
attributed to the complex.
position of the reagents was PhMgBr·0.087MgBr (A)
2
and PhMgBr·0.37MgBr (B), respectively.
2
A Grignard reagent in diethyl ether is an extremely
complex system [3]. Because of low solubility the mag-
nesium bromide present must be complexed with the
organomagnesium compound. At the ratio Br/Mg=1.5
the stoichiometric complex PhMgBr·MgBr forms and
2
layering of the solution occurs. The same has been
If this is true, in the Grignard reagent of the ratio
Br/Mg=1.27 (B) about 74% of PhMgBr pertains to the
complex with magnesium bromide and 26% of it is
unbound. For the conventional Grignard reagent (A)
the figures are 17 and 83, respectively.
Table 2
Kinetic data for the reagent PhMgBr·0.37MgBr2 in the presence of
different additions of triethylamine
PhMgBr
(
Amine a
(mol%)
kI (102 s−1
)
kII (102) b
krel c
Let us consider now the influence of triethylamine
additions on the reactivity of Grignard reagents A and
B towards acetylene. It is seen from Fig. 1 that small
amine additions enhance reactivity proportionally and
the plots for both reagents are parallel. However, be-
ginning from certain points on the graph the growth of
reactivity decelerates. It is remarkable that the range of
the bend is close to 17 mol% of amine for reagent A
and to 74 mol% for reagent B, which is just in corre-
spondence with the relative content of complexes I or II
in the reagents. Further increase in reactivity is moder-
ate and lasts for a short interval in the amine content
scale for reagent B but does for quite a long one for
reagent A. Then, from 120 to 130 mol% of amine the
reactivity graphs of both reagents are steep again. An
mol 1 1)
−
0.380
0.380
0.372
0.370
0.366
0.363
0.360
0.356
0.352
0.346
0.345
1.00
5.00
9.40
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
0.262
0.690
1.120
2.26
4.36
6.70
9.10
9.95
10.9
13.9
0.689
1.82
3.22
6.11
11.9
18.5
25.3
27.9
31.0
39.8
50.7
2.75
7.26
12.9
24.4
47.6
73.8
101.1
111.8
123.9
159.3
202.9
17.5
a
(
[TEA]/[PhMgBr])×100.
b
c
−1 −1
l mol
s
.
For krel=1 see first entry in Table 1.

188
V. P a¨ llin, A. Tuulmets / Journal of Organometallic Chemistry 584 (1999) 185–189
increase in reactivity was evident up to at least 300
mol% of amine, but our experimental facilities did not
allow the performance of reliable kinetic measurements
in this region.
The observed experimental facts impose some conclu-
sions on the structure and the reactivity of species
comprising the systems under investigation. Obviously,
the first portions of amine become bound to PhMgBr–
considerably exceeds that of alkyl halides. When the
magnesium centre carries a ligand like TEA, basicity of
the bromine atom may be comparable to or higher than
that of alkyl ethers. Owing to low steric requirements
the bromine atom in complex III can gain an unexpect-
edly great donating power. Similar circumstances seem
to be liable for the association of Grignard reagents in
the media of moderate solvating ability. We intend to
publish further details on this issue elsewhere.
At an amine content of 100 mol% each PhMgBr
entity is solvated by one molecule of TEA on the
average. In this region complexes like V and VI should
be formed.
MgBr complexes with the formation of species more
2
reactive than PhMgBr·TEA complexes appearing at
further amine additions. A reverse order of the forma-
tion of the complexes or their opposite reactivity would
not bring about the shapes of the curves shown in Fig.
1.
The conclusions drawn above can be rationalized on
the basis of effective (Lewis) acidity of solvation centres
and effective basicity of donors (see, e.g. discussion in
Ref. [19]). It is appropriate to note here that a strong
Brønstedt base TEA appeared to be a much better
donor than diethyl ether and slightly better than THF
for phenylmagnesium bromide but not for diphenyl-
magnesium. In the case of sterically more hindered
diethylmagnesium TEA is even a weaker donor than
diethyl ether [6,19].
In complex II the central magnesium atom is the
most favourable centre to be solvated by TEA. As
regards free phenylmagnesium bromide, simultaneous
solvation by two donors with great steric demands,
TEA and diethyl ether, is energetically less advanta-
geous and therefore this complex forms at higher amine
concentrations. Replacement of diethyl ether by TEA
in complex II can lead to complexes III or IV, where A
stands for the amine molecule.
Further increase in the amine content involves the
formation of complexes including more amine
molecules and thus brings about higher reactivity of the
system. Finally, at high amine contents only disolvated
species PhMgBr·2TEA and MgBr ·2TEA should occur.
2
However, the equilibrium
PhMgBr·Et O·TEA+TEA X PhMgBr·2 TEA+Et O
2
2
is not necessarily shifted to the far right. It is known
that the second equilibrium constant of recoordination
between the complex of ethylmagnesium bromide with
1
-ethoxy-2-methylbutane and TEA is about 25 times
less than the first equilibrium constant [24]. This is an
obvious reason for the continuing increase of krel in the
region of amine additions exceeding two equivalents
per phenylmagnesium bromide. The powerful effect of
amine is manifested by the fact that the reagent of
average stoichiometry PhMgBr·2TEA reacts with
acetylene about 20 000 times faster than in the absence
of the amine.
Although complex III seems to be energetically more
favourable, complex IV should be more reactive be-
cause of a proximate effect of the donor on the nucle-
ophilicity of the organic group. However, a possible
long-range effect of the donor in complex III should
not be overlooked. According to Charton parameters
Acknowledgements
This work was supported by the Estonian Science
Foundation (ESF Grant no. 3058).
[
20] steric demands of the ligands line up in a sequence
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