Two routes of tantalum-catalyzed alkene carbomagnesiation with ethyl Grignard reagents

Article abstract of DOI:10.1016/j.jorganchem.2012.05.023

Carbomagnesiation of 1-alkenes with EtMgX (X = Cl, Et) has been implemented in the presence of TaCl₅ as a catalyst to give a mixture of 2-(R-substituted)- and 3-(R-substituted)-n-butylmagnesiums (5:4 ratio) ina total yield of 98%. The probable

Full text of DOI:10.1016/j.jorganchem.2012.05.023

Journal of Organometallic Chemistry 715 (2012) 5e8
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Communication
Two routes of tantalum-catalyzed alkene carbomagnesiation with ethyl
Grignard reagents
*
Rifkat M. Sultanov , Usein M. Dzhemilev, Elena V. Samoilova, Ruslan R. Ismagilov, Leonard M. Khalilov,
Natal’ya R. Popod’ko
Institute of Petrochemistry and Catalysis, Russian Academy of Sciences, 141 Prospekt Oktyabrya, Ufa 450075, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbomagnesiation of 1-alkenes with EtMgX (X ¼ Cl, Et) has been implemented in the presence of TaCl5
as a catalyst to give a mixture of 2-(R-substituted)- and 3-(R-substituted)-n-butylmagnesiums (5:4 ratio)
ina total yield of 98%. The probable mechanism of their formation through tantalacyclopentanes as key
intermediates is discussed.
Received 22 December 2011
Received in revised form
24 April 2012
Accepted 21 May 2012
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Organomagnesium compounds
Tantalum complexes
Catalysis
1-Alkenes
Carbomagnesiation
1. Introduction
2. Results and discussions
As we have earlier shown, the Zr-catalyzed regioselective car-
bomagnesiation of monosubstituted unactivated alkenes with
EtMgX (X ¼ Et, Cl, Br) is a convenient and efficient route to different
organomagnesiums (OMCs) (Scheme 1) [1].
Later, this reaction became a research subject in a few
research groups of chemists, who have developed its powerful
synthetic potential [2] and have proposed its reaction mechanism
(Scheme 2) [3].
Our experiments showed that tantalum pentachloride TaCl5 has
high catalytic activity in the reaction of 1-alkenes with ethyl-
magnesiums. The reaction products, in this case, surprisingly differ
from those produced in this reaction involving Cp2ZrCl2.
Thus, the model reaction between 1-octene and EtMgCl (1:2
ratio) in the presence of the TaCl5 catalyst (octene:[Ta] ¼ 100:5) in
THF (20 ꢀC, 3 h) affords a mixture of two OMCs 1b and 2b in a total
yield of 90% towards initial 1-octene (Scheme 3).
Zirconium cyclopentadienyl derivatives were found to be the
most active and effective catalysts for this reaction.
The only product 3-methylnonane 3b [1a] has been obtained
after hydrolysis of 1b and 2b with dilute (8%) hydrochloric acid,
while deuterolysis of the reaction mixture led to two mono-
deuterated hydrocarbons 4b and 5b (5:4 ratio). The yield and the
molar ratio of the resultant regioisomers were found by calculating
their g.l.c. peak area ratios.
Niobium and tantalum complexes are known to be active
catalysts for alkene dimerization and codimerization [4], metath-
esis [5], ethylene polymerization [6], etc. However, there are no
published data on the catalytic behavior of transition metal (V, Nb,
Ta) salts and complexes in the reactions with unsaturated hydro-
carbons involving organic derivatives of nontransition metals
(eg. Mg, Al, Zn).
The 13C NMR spectrum of the predominant regioisomer 4b
exhibits the signal at d
(C10) 18.85 ppm (see Section Experimental)
with characteristic spinespin coupling constant (SSCC)
In this paper, we report the first use of TaCl5 as a catalyst for
carbomagnesiation of 1-alkenes with ethyl Grignard reagents.
1JCeD ¼ 19.0 Hz and a diagnostic signal at
d
(C1) 11.34 Hz. The
deuterium isotope effect on the 1H chemical shift of the C10-methyl
group causes its shielding as compared to that of compound 5b.
This shielding effect results in the chemical shift of 0.31 ppm. The
13C NMR spectrum of regioisomer 5b also contains the character-
* Corresponding author. Tel./fax: þ7 347 2842750.
E-mail address: Sultanov55@mail.ru (R.M. Sultanov).
istic triplet signal at
d
(C1) 11.04 ppm (1JCeD ¼ 19.0 Hz).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.05.023

6
R.M. Sultanov et al. / Journal of Organometallic Chemistry 715 (2012) 5e8
R
+
+
EtMgX
[Ta]
Cp2ZrCl2
+
EtMgX
20 oC
3 h
R
R
MgX
ether (THF), 20 25 oC
MgX
X = Et, Cl, Br
Scheme 1.
R
R
MgX
In an analogous fashion, the deuterium isotope effect in
compound 5b causes the shielding of the C1-methyl group. The
shielding effect, in this case, resulted in the chemical shift of
0.30 ppm.
1a, b
2a, b
H3O+
D3O+
O2
We should also note the presence of b-isotope effects in both
compounds 5b (Ddb
C2 ¼ 0.07 ppm) and 4b (Ddb
-isotop
-isotop
C3 ¼ 0.06 ppm).
Oxidation of the reaction mixture with dry oxygen affords
regioisomeric alcohols 6b and 7b, in which the hydroxyl groups are
attached to different carbon atoms (see Section Experimental).
Thus, in the 13C NMR spectrum of regioisomer 7b, the carbinol
signal appeared at higher field (
corresponding signal in the spectrum of regioisomer 6b (
65.18 ppm). Moreover, one can observe the significant difference
between positions of the low field methyl carbon resonance (
C10
19.65 ppm) and the shielded methyl carbon signal (
C1 11.08 ppm)
R
OH
OH
R
R
D
3a, b
4a, b
6a, b
7a, b
+
+
D
d
C1 61.07 ppm) as compared to the
C10
n
R = Bu (a)
d
n
Hex (b)
5a, b
d
X = Cl, Et
R
R
d
in the spectra of compounds 7b and 6b respectively. The 5:4 ratio of
regioisomers 6b and 7b has been calculated through 1H NMR
experiments, from integration of unambiguously different proton
Scheme 3.
signals belonging to the hydroxymethylene groups (
C1H2 3.64 ppm for compounds 6b and 7b resp.).
For a study of tantalum-catalyzed alkene carbomagnesiation in
d
C1H2 3.51 and
OMC:olefin ratios (2:1) are required to achieve the same results
with EtMgCl (Table 1, entries 2 and 5).
d
It should be noted that we could not obtain the desired result
while using the diethyl ether as a solvent. The above reaction can
proceed satisfactorily only in the presence of tetrahydrofuran
(Et2O:THF ¼ 1:1), but a total yield of 1b and 2b, in this case, was
much smaller than in pure THF (Table 1, entry 8).
Carbomagnesiation of other 1-alkenes, e.g. 1-hexene, in the
presence of TaCl5 occurs in a similar manner. The yields of the
target products and their molar ratios remain the same.
more detail, we compared the reactivity of EtMgCl and Et2Mg in the
reaction with 1-octene (Table 1).
As seen in Table 1, carbomagnesiation of 1-octene with Et2Mg
(1:1.1 M ratio) in THF for 3 h afforded organomagnesiums 1b and 2b
in almost quantitative yield (Table 1, entry 4), whereas the higher
Further, we intend to study this reaction involving various
tantalum complexes, other unsaturated hydrocarbons and alkyl-
magnesiums as well. The stoichiometric variants of the reaction
discovered will be also explored. Special efforts will be focused on
developing selective synthesis of the unusual products 2a,b.
The detailed mechanism of the interaction between 1-alkene
and EtMgCl (or Et2Mg) requires additional research. Nevertheless,
our experimental findings as well as the literature data [4,7] allow
us to suggest possible ways for the simultaneous formation of the
OMCs 1 and 2 (Scheme 4).
Cp2ZrCl2
2Et2Mg
2EtMgCl
H
Cp2Zr
Et
C2H6
R
Cp2Zr
We suppose that carbomagnesiation of 1-alkene with an excess
of EtMgCl (or Et2Mg) in the presence of TaCl5 proceeds along two
parallel reaction pathways. The first step of both routes involves the
formation of the labile tantalacyclopentane intermediate 8. The
subsequent stepwise transmetallation of 8 through simultaneous
formation of two bimetallic complexes 9 (route 1) and 10 (route 2)
finally affords 2-(R-substituted)-n-butylmagnesium (product 1)
and 3-(R-substituted)-n-butylmagnesium (product 2) compounds
R
MgEt
H'
R
Zr
Cp2
respectively as a result of b-hydride transfer.
MgEt
R
3. Conclusions
Et2Mg
Thus, we represent a new approach to the synthesis of mono-
organomagnesiums from 1-alkenes mediated by ethyl Grignard
reagents and TaCl5 as a catalyst. In contrast to the known Zr-
catalyzed carbomagnesiation, the new catalytic method makes it
Cp2Zr
H'
Scheme 2.

R.M. Sultanov et al. / Journal of Organometallic Chemistry 715 (2012) 5e8
7
Table 1
The effect of the OMC structure, solvent nature, and molar ratio of the starting reagents on the yield and composition of the carbomagnesiation products of 1-octene in the
presence of TaCl5.
Nos.
OMC
OMC/1-octene molar ratio
1-Octene/TaCl5 molar ratio
Solvent
Total yield of 1b þ 2b, %
1b/2b ratio
1
2
3
4
5
6
7
8
EtMgCl
EtMgCl
Et2Mg
Et2Mg
EtMgCl
EtMgCl
Et2Mg
EtMgCl
1:1
2:1
1:1
1.1:1.0
2:1
2:1
100:5
100:5
100:5
100:5
100:10
100:5
100:5
100:5
THF
THF
THF
THF
51
90
94
98
98
e
5:4
5:4
5:4
5:4
5:4
e
THF
Diethyl ether
Diethyl ether
Diethyl ether/THF (1: 1)
1:1
2:1
e
e
39
5:4
Reaction conditions: concentration of the OMC 0.6 mmol/mL, 20 ꢀC, 3 h.
possible to obtain unusual 3-(R-substituted)-n-butylmagnesium
compound, and its synthetic potential, undoubtedly, requires
further study.
the compounds were recorded using
spectrometer.
a
Bruker VERTEX 70v
4.1. Synthesis of 1-deutero-2-ethyloctane, 4b, 1-deutero-2-
methylnonane, 5b
4. Experimental
4.1.1. General procedure
All reactions were carried out under dry argon atmosphere.
Alkenes having a purity of no less than 99.8% were distilled over Al
in a stream of argon prior to use. Etherial solvents (Et2O, THF)
employed were pre-dried over KOH, refluxed over sodium-wire for
2 h and Etherial solvents distilled from LiAlH4 in a stream of argon.
EtMgCl was synthesized as described in Ref. [8]. Solutions of
diethylmagnesium were prepared by dissolving solid Et2Mg of high
purity, obtained as described in Ref. [9], in diethyl ether or tetra-
hydrofuran. Commercially available TaCl5 (purity 99.8%) was
purchased from Aldrich.
The hydrolysis and deuterolysis products were analyzed with
a Chrom-5 gas chromatograph in a stream of helium, column
1200 ꢁ 3 mm or 3000 ꢁ 3 mm, 5% SE-30 on Chromaton N-AW-
DMCS (0.16e0.20 mm), oven temperature 50e300 ꢀC. The 1H and
13C NMR spectra were recorded in CDCl3 on a Bruker-Avance-400
spectrometer [400.13 MHz (1H) and 100.62 MHz (13C)]. A Fin-
nigan Model 4021 gas chromatography-mass spectrometer system
(electron ionization 70 eV, an ion source temperature 280 ꢀC)
provided analysis of final products. Elemental analysis of samples
was performed on Karlo Erba model 1106 instrument. IR spectra of
A glass reactor, under a dry argon atmosphere at 0 ꢀC, was
charged under stirring with TaCl5 (2.5 mmol, w0.9 g), 1-octene
(50 mmol, 5.6 g), and ethylmagnesium chloride (100 mmol, 0.6 M
solution in THF). The temperature was raised to 20 ꢀC and the
mixture was stirred for additional 3 h. Then, the reaction mixture
was quenched by a 8% DCl in D2O to identify OMCs obtained. The
organic layer was separated. The aqueous layer was extracted with
diethyl ether (3 ꢁ 50 mL). The combined organics were washed
with NaHCO3 (until neutral) and dried over MgSO4. The mixture of
products 4b and 5b was separated by fractional distillation.
Compound 3b (3-methylnonane), additionally, was identified by
comparison with the known sample [1a]. Spectral data for 4b: Bp
75 ꢀC (30 mm Hg). IR nCO (cmꢂ1 in thin film): 2175 (CeD). 1H NMR
(CDCl3, in ppm, 400.13 MHz):
d
¼ 1.12e1.45 (m, 13H, CH2, CH),
0.86e0.97 (m, 8H, CH3, CH2D). 13C NMR (CDCl3, in ppm,
100.62 MHz):
d
¼ 36.78 (C4), 34.51 (C3), 32.09 (C7), 29.83 (C6), 29.57
(C2), 27.20 (C5), 22.79 (C8), 18.85 (C10, t, 1JCeD ¼ 19 Hz), 14.05 (C9),
11.34 (C1). MS, m/z: 143 (Mþ). Elemental Anal. Calc. for: C, 83.92; H,
14.68; D, 1.40. Found: C, 84.20; H þ D, 15.49%. Spectral data for 5b:
Bp 75 ꢀC (30 mm Hg). IR nCO (cmꢂ1 in thin film): 2175 (CeD). 1H
NMR (CDCl3, in ppm, 400.13 MHz):
d
¼ 1.12e1.45 (m, 13H, CH2, CH),
0.86e0.97 (m, 8H, CH3, CH2D). 13C NMR (CDCl3, in ppm,
R
TaCl5 + EtMgX +
R
100.62 MHz):
d
¼ 36.78 (C4), 34.45 (C3), 32.09 (C7), 29.83 (C6), 29.50
(C2), 27.20 (C5), 22.79 (C8), 19.16 (C10), 14.05 (C9), 11.04 (C1, t,
1JCeD ¼ 19 Hz). MS, m/z: 143 (Mþ). Elemental Anal. Calc. for: C,
83.92; H, 14.68; D, 1.40. Found: C, 84.20; H þ D, 15.49%.
[Ta] = TaLn
[Ta]
8
4.2. Synthesis of 2-ethyloctane-1-ol, 6b, 3-methylnonane-1-ol, 7b
route 1
route 2
EtMgX
excess
4.2.1. General procedure
MgX
R
Oxidation of the OMCs 1b and 2b, obtained by the reaction of 1-
octene with Et2Mg or EtMgCl in THF in the presence TaCl5, was
carried out in a thermostated glass reactor (100 mL) by passing
bubbles of pure dry oxygen through the solution. The temperature of
solution was maintained at 5e8 ꢀC. After 60 min, the reaction
mixture was poured into a 5% HCl solution. The resultant alcohols
were extracted with diethyl ether (3 ꢁ 20 mL), dried over MgSO4,
and, after removal of ether, were analyzed by GLC. The alcohol
mixture (6b and 7b) was separated by vacuum distillation. Spectral
data for 6b: Bp 76e77 ꢀC (3 mm Hg). IR nCO (cmꢂ1 in thin film): 3420
(OeH),1045 (CeO),1465,1375.1H NMR (CDCl3, inppm, 400.13 MHz):
[Ta]
R
10
9
H
H
[Ta]
R
MgX
R
R
R
XMg
MgX
1
2
d
¼ 3.51 (d, JHeH ¼ 5 Hz, 2H, CH2O), 1.12e1.41 (m, 13H, CH2, CH),
X = Cl, Et
0.87e0.89 (m, 6H, CH3). 13C NMR (CDCl3, in ppm, 100.62 MHz):
¼ 65.18 (C1), 31.93 (C6), 30.49 (C2), 29.79 (C3), 29.66 (C5), 26.96 (C4),
Scheme 4.
d

8
R.M. Sultanov et al. / Journal of Organometallic Chemistry 715 (2012) 5e8
23.37 (C9), 22.71 (C7), 14.10 (C8), 11.08 (C10). MS, m/z: 158 (Mþ).
Elemental Anal. Calc. for: C, 75.95; H,13.92; O,10.13. Found: C, 75.91;
H,13.69%. Spectral data for 7b: Bp 76e77 ꢀC (3 mm Hg). IR nCO (cmꢂ1
in thin film): 3420 (OeH),1045 (CeO),1465,1375. 1H NMR (CDCl3, in
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ppm, 400.13 MHz):
d
¼ 3.64 (m, 2H, CH2O), 1.59 and 1.35 (m, 2H,
H2C2), 1.57 (m, 1H, HC3), 1.18e1.31 (m, 10H, CH2), 0.87e0.89 (m, 6H,
CH3).13C NMR (CDCl3, inppm,100.62 MHz):
d
¼ 61.07 (C1), 39.98 (C2),
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Acknowledgement
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This work was financially supported by a Grant of the RF President
(Sci.Sh.-2349.2008.3).
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