Synthesis of zinc dialkyls from zinc and alkyl bromides

Article abstract of DOI:10.1023/A:1013703707139

Zinc dialkyls with linear radicals were prepared from zinc and alkyl bromides in the presence of stimulating systems based on a transition metal derivative and an organometallic compound capable of reducing the transition metal derivative under the reaction conditions.

Full text of DOI:10.1023/A:1013703707139

Russian Journal of Applied Chemistry, Vol. 74, No. 8, 2001, pp. 1410 1412. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 8, 2001,
pp. 1371 1373.
Original Russian Text Copyright
2001 by Eremeev, Danov, Sakhipov, Skudin.
BRIEF
COMMUNICATIONS
Synthesis of Zinc Dialkyls from Zinc and Alkyl Bromides
I. V. Eremeev, S. M. Danov, V. R. Sakhipov, and A. G. Skudin
Dzerzhinsk Branch, Nizhni Novgorod State Technical University, Dzerzhinsk, Nizhni Novgorod oblast, Russia
Received March 22, 2001
Abstract Zinc dialkyls with linear radicals were prepared from zinc and alkyl bromides in the presence
of stimulating systems based on a transition metal derivative and an organometallic compound capable of
reducing the transition metal derivative under the reaction conditions.
Zinc alkyl compounds are widely used in labora-
tory studies and industry as starting compounds in
inorganic, organic, and organometallic synthesis and
as catalysts for polymerization of unsaturated com-
pounds [1 7]. However, one of the major application
fields of alkyl derivatives of nontransition metals and
zinc is preparation of materials for electronics [7 22].
preparing dipropyl- and dibutylzinc resulted in low
yields of zinc dialkyls. Therefore, we carried out a
study aimed at extending the synthetic potential of the
procedure for preparing zinc dialkyls.
It is known [2, 3] that organic halides can react
with OMCs. This is a complex process, and its mech-
anism is not still fully understood; the reactions are
often accompanied by exchange, hydride reduction,
and dehydrohalogenation of the organic halide. As a
result, a complex mixture of products is usually
formed. The reaction catalysts are commonly transi-
tion metals or their derivatives. Hence, it could be
expected that the most probable reasons for low yields
of dipropyl- and dibutylzinc in the preliminary exper-
iments are side reactions of the initial alkyl bromides
with the forming mono- and diorgano derivatives of
zinc, yielding a mixture of hydrocarbons and zinc
bromide. The other possible side reactions [reduction
of copper(I) iodide with organozinc compounds or
their thermal decomposition] can hardly be responsi-
ble for the poor yields of zinc dialkyls. Indeed, even
complete reduction of the catalytic amounts of cop-
per(I) iodide as a component of the stimulating system
can decrease the yield of zinc dialkyl only to approx-
imately 93%. The insignificant effect of thermal de-
composition of zinc dialkyls on their synthesis fol-
lows, e.g., from the approximately equal yields of
A procedure for preparing diethylzinc in 92% yield,
involving reaction of zinc powder with ethyl bromide,
was proposed in [23]:
Zn + AlkBr
AlkZnBr,
(1)
(2)
2AlkZnBr Alk2Zn + ZnBr2.
Stage (1) is performed in the liquid phase under at-
mospheric pressure without solvent at 38 45 C in
the presence of catalytic amounts of stimulating sys-
tems whose necessary components are a transition
metal derivative and an organometallic compound
(OMC). The latter should be capable of reducing the
transition metal derivative under the reaction condi-
tions. Disproportionation of alkylzinc bromide (2)
with simultaneous distillation of dialkylzinc in a vac-
uum is performed in the same vessel at elevated tem-
perature under isothermal conditions, or with a more
sophisticated temperature schedule. This procedure
eliminates the labor-consuming procedure for prepar- diethyl-, dipropyl-, and dibutylzinc (78 86%) in
ing a zinc copper couple, the use of relatively expen-
sive ethyl iodide, and the uncontrollable induction
period in the first stage. Among a broad range of
tested stimulating systems based on a transition metal
derivative and an OMC (some of them are listed in
patent [23]), the system copper(I) iodide ethylzinc
bromide showed high performance in synthesis of di-
ethylzinc. However, attempts to use this system for
their synthesis by Noller’s procedure from a zinc
copper couple and equimolar mixtures of the corre-
sponding alkyl iodides and alkyl bromides under con-
ditions similar to those of our experiments [1]. If this
is the case, then, if unchanged alkyl bromide or a
solid residue containing a transition metal and its de-
rivative is removed from the reaction mixture before
thermal disproportionation of alkylzinc bromide, the
1070-4272/01/7408-1410 $25.00 2001 MAIK Nauka/Interperiodica

SYNTHESIS OF ZINC DIALKYLS FROM ZINC AND ALKYL BROMIDES
1411
selectivity of zinc dialkyl synthesis must increase con-
siderably.
Synthesis of zinc dialkyls from zinc and alkyl bromides
in the presence of the copper(I) iodide ethylzinc bromide
stimulating system
Our experiments showed that the selectivity of syn-
thesis of alkylzinc bromides at T 50 C is very high:
The reaction mixtures contained only unchanged ini-
tial compounds (zinc and alkyl bromide) and alkylzinc
bromide. The release of gaseous products was moni-
tored with a gas meter connected to the reactor, and
was insignificant. However, the conversion of alkyl
halides in 4 8 h was as low as 50 80%. Subsequent
thermal disproportionation of alkylzinc bromides with
simultaneous vacuum distillation of zinc dialkyls from
such reaction mixtures resulted in low yields of zinc
dialkyls. The results of two of such experiments
(nos. 1 and 3) are listed in the table. Removal of
unchanged alkyl bromides from the reaction mixtures
by vacuum distillation at temperatures below 50 C
prior to thermal decomposition noticeably increased
the yield of zinc dialkyls. Also, we found that, in the
absence of other components of the reaction mixtures,
the alkyl bromides in hand do not react with the cor-
responding organozinc compounds under the experi-
mental conditions. Therefore, as expected, when the
solid residue was removed from the reaction mixture
prior to disproportionation, the yield of zinc dialkyls
also increased (see table, run nos. 2 and 4). Since
5 15% of the formed organozinc compounds always
remains in the reactor after the final stage of synthesis,
our results show that, in the absence of alkyl bromides
or substances catalyzing reaction of organic halides
with OMC, disproportionation of alkylzinc bromides
into zinc dialkyls is highly selective.
Yield, %
Run
no.
*
Alk2Zn
,
h
AlkZnBr
by analysis
Alk2Zn
1
2
3
4
Pr2Zn
Pr2Zn
Bu2Zn
Bu2Zn
7
75
81
52
78
30
7
4
8
68**
0
66**
*
( ) Time of AlkZnBr synthesis.
**
The solid residue was removed from the reaction mixture
prior to disproportionation of alkylzinc bromide.
was separated from unchanged zinc, and excess ethyl
bromide was distilled off in a vacuum. The remaining
crystalline ethylzinc bromide was dissolved in an
appropriate alkylbromide for subsequent use as a com-
ponent of the stimulating system.
Synthesis of zinc dialkyls. A mixture of 8.76 g
(0.046 mol) of copper(I) iodide and 50 g (0.765 mol)
of zinc powder was vacuum-treated for 10 min, dry
oxygen-free argon was passed, and alkyl bromide and
a solution of ethylzinc bromide in the corresponding
alkyl bromide were added. The total amount of alkyl
bromide and the amount of ethylzinc bromide in the
mixture were 0.66 and 0.04 0.05 mol, respectively.
In the process, the reactor was protected from atmos-
pheric moisture. Then the system was connected to a
gas meter. The first stage was performed with stirring
at 50 C. Then the stirring was stopped, the mixture
was cooled to room temperature, and thermal dis-
proportionation of alkylzinc bromide formed in the
first stage was performed, with simultaneous vacuum
distillation of zinc dialkyl. In certain experiments
unchanged alkyl bromide was distilled off in a vacu-
um prior to disproportionation, or unchanged zinc
was separated by decanting and washed with several
portions of appropriate alkyl bromide. The liquid
phase was separated by decanting after each washing.
After combining all the liquid phases from washings
and the liquid fraction of the reaction mixture, alkyl
bromide was distilled off in a vacuum, and alkylzinc
bromide was subjected to thermal disproportionation.
The constants of the resulting dipropylzinc and di-
butylzinc agreed with published data [1]. The yield of
zinc dialkyls was determined from the amount of the
isolated reaction products. The content of alkyl bro-
mide in the reaction mixture was determined by GLC
Thus, our study supports the above assumption
concerning the reasons for the low yield of zinc dial-
kyls in their synthesis from zinc and alkyl halides in
the presence of stimulating systems based on a tran-
sition metal derivative and an OMC. It should be
noted that, to prepare high-purity zinc dialkyls, it is
appropriate to use alkylzinc bromide formed in the
first stage or the corresponding zinc dialkyl as the
organometallic component of the stimulating system.
EXPERIMENTAL
In our experiments, we used PTs-10 zinc powder
[TU (Technical Specifications) 48-4015-1/0 81] and
pure grade copper(I) iodide (TU 6-09-02-437 87).
Pure grade alkyl bromides were dried over calcium
chloride and distilled before use. Ethylzinc bromide
was prepared from zinc powder and ethyl bromide in
the presence of the copper(I) iodide ethylzinc bromide
stimulating system, as described in [23]. The result-
ing solution of ethylzinc bromide in ethyl bromide
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 8 2001

1412
EREMEEV et al.
on a Tsvet-110 chromatograph after hydrolysis of
alkylzinc bromide and extraction of alkyl bromide
with toluene [24]. The yield of alkylzinc bromide in
the first stage of the synthesis was evaluated from the
content of zinc ions (EDTA titration [25]) after hy-
drolysis of alkylzinc bromide in a solid particle-free
sample of the reaction mixture and dissolution of
the resulting zinc hydroxide precipitate in aqueous
HCl.
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(1) Stimulating systems based on a transition
metal derivative and an organometallic compound
allow synthesis of zinc dialkyls with linear radicals in
high yields from zinc and alkyl halides.
(2) In thermal disproportionation of alkylzinc
halides, the reaction mixture should not contain sig-
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