Mechanism of co-dimerization of vinylboronates with terminal alkynes catalyzed by ruthenium-hydride complex

Article abstract of DOI:10.1016/j.molcata.2014.09.040

The co-dimerization reaction of vinylboronates with terminal alkynes, which was discovered in our group, is an efficient method for the synthesis of boryl- and borylsilylsubstituted buta-1,3-dienes. This study is a continuation of our previous and present results concerning the mechanism of this new catalytic reaction in boron chemistry. DFT calculations, as well as kinetic measurements carried out at several different temperatures in the range of 70-100 °C, provided rational grounds for the mechanism proposed previously on basis of stoichiometric reactions monitored by ¹H NMR. The activation energy and the rate determining step were also estimated.

Full text of DOI:10.1016/j.molcata.2014.09.040

Journal of Molecular Catalysis A: Chemical 396 (2015) 239–244
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Mechanism of co-dimerization of vinylboronates with terminal
alkynes catalyzed by ruthenium-hydride complex
Jadwiga Pyziaka, Je˛drzej Walkowiakb, Marcin Hoffmanna, Bogdan Marcinieca,b,∗
a Faculty of Chemistry, Adam Mickiewicz University in Poznan´, Umultowska 89b, 61-614 Poznan´, Poland
b Centre for Advanced Technologies, Adam Mickiewicz University in Poznan´, Umultowska 89c, 61-614 Poznan´, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The co-dimerization reaction of vinylboronates with terminal alkynes, which was discovered in our group,
is an efficient method for the synthesis of boryl- and borylsilylsubstituted buta-1,3-dienes. This study
is a continuation of our previous and present results concerning the mechanism of this new catalytic
reaction in boron chemistry. DFT calculations, as well as kinetic measurements carried out at several
different temperatures in the range of 70–100 ◦C, provided rational grounds for the mechanism proposed
previously on basis of stoichiometric reactions monitored by 1H NMR. The activation energy and the rate
determining step were also estimated.
Received 3 April 2014
Received in revised form
26 September 2014
Accepted 30 September 2014
Available online 23 October 2014
Keywords:
© 2014 Elsevier B.V. All rights reserved.
Silylalkynes
Co-dimerization
Vinylboronates
DFT calculations
Kinetics
1. Introduction
of the reaction (see Fig. 1). The ruthenium hydride complexes
[Ru(CO)ClH(PCy3)2] or [Ru(CO)ClH(PPh3)3] are especially useful.
Synthesis of unsaturated compounds functionalized with metal-
loids from groups 13 and 14 of the periodic table is a very important
branch of organometallic chemistry because of the potential appli-
cations of these compounds to organic syntheses and catalysis
processes. The possibility for easy exchange of metalloids from
these compounds and the introduction of various functional groups
are very attractive for chemists. Hence, olefins and alkynes substi-
tuted with silyl, boryl, germyl or stannyl moieties have found broad
application in the formation of specialty and fine chemicals [1–7].
We specialize in syntheses of olefins and alkynes with silyl,
germyl and boryl functions on the basis of trans-metallation reac-
tions discovered previously [8,9]. These catalytic transformations
are based on the activation of the C E bond in the vinylmetalloid
compound and the Csp2 H bond in an olefin or the Csp H bond in an
alkyne. The reactions are catalyzed by transition metal (TM) com-
plexes that contain or generate in situ the TM H or TM E bonds
and occur with the simultaneous evolution of ethylene in the course
In trans-metallation of olefins, we are able to obtain selectively
products with trans, cis configuration or a geminal product with
both functional groups attached to the same atom of a vinyl group,
depending on which bond in the olefin is activated. This mode of
reactivity is general for olefins when vinylsilanes, vinylgermanes
[10,11] or vinylboranes [12,13] are used, while with alkynes the
reaction occurs only with vinylmetalloids from the 14th group of
the periodic table [14].
The mechanism of silylative and borylative coupling of vinylsi-
lanes and vinylboronates with olefins was proposed on the basis
of stoichiometric reactions as well as DFT calculations [15,16]. We
have previously reported that the rate-determining step in the cat-
alytic cycle of silylative coupling is an insertion of an alkene into
the Ru Si bond coupled with a silyl moiety migration from Ru to C
atoms. The trans-borylation reaction of olefins with vinylboronates
was analyzed in detail with the use of DFT calculation by Lin and
Marder [16]. Computational results support the mechanism which
was proposed on the basis of a stoichiometric reaction monitored
by 1H NMR as well as experiments with deuterated styrene, which
permitted a distinction between the non-metalcarbene mechanism
of trans-borylation and metathesis using a GC–MS analysis of prod-
ucts enriched with deuterium [17].
∗
Corresponding author at: Adam Mickiewicz University, Centre for Advanced
Technologies, Umultowska 89c, 61-614 Poznan´ , Poland. Tel.: + 48 61 829 19 88;
fax: +48 61 829 19 87.
The reaction of vinylboronates with terminal alkynes took,
unexpectedly, a different course, giving borylsubstituted buta-1,3-
dienes as products instead of alkynylboranes [18].
E-mail addresses: Bogdan.Marciniec@amu.edu.pl, marcinb@amu.edu.pl
(B. Marciniec).
http://dx.doi.org/10.1016/j.molcata.2014.09.040
1381-1169/© 2014 Elsevier B.V. All rights reserved.

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J. Pyziak et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 239–244
functional theory (DFT) calculations using several popular func-
tionals: BLYP [22], B3LYP [23–25], M06 [26]. Basis sets of triple
valence and double valence quantity SDD [27,28] and LANL2DZ
were checked to see if they provided similar results. Frequency
calculations at the same level of theory were also performed to
confirm the characteristics of all of the optimized structures as
minima or transition states. Thermal corrections to Gibbs free
energies calculated at 298 K were included. All computations were
carried out with Gaussian09 [29].
Fig. 1. General scheme of trans-metallation of olefins and alkynes with vinylmet-
alloids.
2.3. Kinetic examinations
Co-dimerization of vinylboronates with terminal alkynes
(organic and silylacetylenes) occurs in presence of complexes with
a Ru H bond (Fig. 2.). The best results were obtained for the
most active complex [Ru(CO)ClH(PCy3)2] [18]. Lower reactivity of
vinylboronates in reactions with terminal alkynes in comparison
to metalative coupling of alkynes with vinylsilanes and vinylger-
manes is responsible for different pathway. The aim of this paper
is to present the mechanism of this new catalytic transformation
in boron chemistry on the basis of quantum chemical calculations
using density functional theory (DFT) as well as kinetic measure-
ments.
Kinetic measurements were carried out in a reactor (4 mL)
equipped with a reflux condenser, an inlet for inert gas and
a magnetic stirrer. An oil bath was employed as a heating
medium. Appropriate amounts of the catalyst and reagents were
placed into the reactor. GC analyses were carried out during
the course of the reactions. Conversion of substrates and the
yield of products were calculated using the internal standard
method.
2.3.1. Kinetic reaction of 2-vinyl-1,3,2-dioxaborinane with
triethylethynylsilane:
2. Experimental
[ethynyltrimethylsilane]:[vinylboronate] = 1:2; (70–100 ◦C)
A
kinetic reactor (Schlenk tube) was loaded with 16 mg
2.1. General methods
(0.022 mmol) of [Ru (CO)ClH(PCy3)2] complex, 125 mg of
ethynyltrimethylsilane (1.13 mmol), 0.38 mL (2.26 mmol) of
2-vinyl-1,3,2-dioxaborinane, 2 mL of toluene and 0.32 mL decane
under argon atmosphere. The reactor was placed in an oil bath
and heated to the desired temperature. Changes in the reagent
and product concentrations were monitored by GC analysis using
the internal standard method. Reactions were conducted at five
different temperatures: 70, 75, 80, 90 and 100 ◦C. The samples for
GC analysis were frozen in dry ice. Each reaction was repeated
four times. The average conversion and reaction kinetics were
calculated with four repetitions.
All syntheses and manipulations were carried out under argon
atmosphere using standard vacuum line techniques. The 1H NMR
and 31P NMR spectra were recorded using a Bruker Ultra Shield
NMR (600 MHz) in toluene-d8. Volatile compounds were deter-
mined by a GC–MS (Varian Saturn 2100 T equipped with a capillary
column, Varian VF-1 Factor Four, 0.26 mm, 30 m). GC analyses were
carried out on a Varian 3400 CX series gas chromatograph equipped
with a capillary column, Varian VF-5 Factor Four, 30 m and TCD.
The chemicals were obtained from the following sources: toluene,
triethylethynylsilane from Sigma-Aldrich, trimethylethynylsilane
from ABCR, decane, pentane, hexane from POCH Gliwice (Poland).
All solvents and liquid reagents used for catalytic as well as stoi-
chiometric experiments were dried with standard procedures and
distilled under argon atmosphere prior to use.
2.4. Stoichiometric reaction of the Ru H complex with
triethylethynylsilane
2-Vinyl-1,3,2-dioxaborinane was synthesized according to liter-
ature procedures with some modifications [19,20]. The ruthenium
complex [Ru(CO)ClH(PCy3)2] (I) was synthesized on the basis of a
literature procedure [21].
The complex [Ru(CO)ClH(PCy3)2] (66 mg, 0.138 mmol), tri-
ethylsilylacetylene (13 mg, 0.138 mmol) and toluene-d8 (0.6 mL)
were placed in an NMR tube under an argon atmosphere. The
reaction mixture was heated from room temperature to 80 ◦C and
monitored by 1H NMR and 31P NMR spectroscopy. Free tricyclo-
hexylphoshpine was observed at 10.6 ppm in 31P NMR even at room
temperature, therefore it can be confirmed that the dissociation of
phosphine and the formation of the 14-electron proper ruthenium
catalyst began this catalytic cycle.
2.2. DFT calculations
For all of the studied molecules, geometries of the potential
energy minima and saddle points were optimized by density
Fig. 2. Co-dimerization of vinylboronates with terminal alkyne.

J. Pyziak et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 239–244
241
Fig. 3. Catalytic cycle of the co-dimerization of terminal ethynylsilanes with vinylboronates catalyzed by [Ru] H complexes.
3. Results and discussion
two-step insertion into the [Ru]
new [Ru] C bond.
C C bond with formation of a
C
The mechanism proposed in our previous work was based on
catalytic and stoichiometric studies (Fig. 3). We were able to verify
the presence of A2, A3 and A4 intermediate steps, but the transition
states were to be determined. We could not earlier identify rate
determining step of the mechanism of the catalytic cycle.
According to the proposed mechanism [18], the catalytic cycle
reported by Lin and Marder [16] starts with the dissociation of
tricyclohexylphosphine and the formation of a four-coordinative
catalyst as suggested by calculations of the trans-borylation of an
olefin with boronates. Such 14-electron complex can easily coor-
dinate a terminal alkyne (A1). In our current studies we focused
on the detailed mechanism. The freed PCy3 was monitored by its
31P NMR spectrum (ı = 10.6 ppm) in the first stage of the stoichio-
metric reaction of the [Ru] H complex with triethylethynylsilane.
Insertion of terminal silylacetylene into the [Ru] H bond with the
formation of the vinylene complex (A2) is the next step of the cat-
alytic process. 31P as well as 1H NMR spectra showed the formation
of a new Ru–vinylene complex.
␤-H elimination of borylsilylsubstituted buta-1,3-diene gives
product A5 with the regeneration of [Ru(CO)ClH(PCy3)] and closes
the catalytic cycle.
In our DFT calculations we used the ruthenium catalyst
[Ru(CO)ClH(PCy3)] (Cy, cyclohexyl), in which the strong ␴-donating
PCy3 ligand was replaced with a smaller PH3 group serving as a
model catalyst, which is a standard approximation (Fig. 4.) [15,16].
The ꢀ2-ethynylsilane complex [(PH3)RuCl(CO)(ꢀ2-CH2 = CHSi
(C2H5)3] A1, was formed by binding ethynylsilane to ruthenium.
Fig. 5 shows the energy profile for the A1 A2 A3 A4 A5 process. The
relative free energies are given in [kcal/mol]. Structural details of
selected intermediates and transition states are shown in Fig. 3.
As shown in Fig. 6, the carbon–carbon triple bond coordinates
to the Ru central atom prior to the terminal silylacetylene insertion
into the [Ru] H single bond in A1. In the transition state TSA1–A2
,
the hydrogen atom migrates from the ruthenium centre to the
␣-carbon atom of the terminal silylacetylene. The product of this
hydrogen atom migration is the formally 16-electron Ru–vinylene
The generation of a vinylene ruthenium complex by the inser-
tion of an alkyne into the Ru H bond is known in the catalytic
dimerization of acetylene [30] and proceeds very rapidly even at
room temperature [31–35]. We have shown that this type of com-
plex is formed as an intermediate in the codimerization reaction of
vinylboronates with terminal organic alkynes and ethynylsilanes
[18].
A3 complex is formed by the coordination of vinylboroate to
metal centre, which then is transformed in complex (A4) by its
Fig. 4. Ruthenium catalyst [Ru] H, (a) ethynylsilane (b) and vinylboronate (c) used
in DFT calculations.

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J. Pyziak et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 239–244
Fig. 5. Energy profile calculated for the proposed pathway of co-dimerization of terminal silylacetylene with vinylboronate. The relative free energies are given in [kcal/mol].
Fig. 6. The first step of co-dimerisation of terminal silylacetylene with vinylboronates: coordinated terminal triethylethynylsilane inserts into [Ru] H bond. All distances
are in Ångstroms.
Fig. 7. The coordination of vinylboronate and its insertion into the [Ru]
C C bond. All distances are in Ångstroms.
complex A2 (see Fig. 6). In the TSA1–A2 transition state Ru. . .H and
Initial coordination of a carbon–carbon double bond from vinyl-
boronate into the ruthenium atom of complex A3 leads to formation
of the TSA3–A4 transition state. The new carbon–carbon bond
between the ␤-carbon bound to the boron atom in the vinyl-
boronate and the ␤-C atom bound to the silicon atom in the A3
˚
˚
H. . .C distances are 1.674 A and 1.514 A, respectively. The energy
barrier for ethynylsilane insertion is low 17.6 [kcal/mol]. That is in
agreement with experimental results on both the co-dimerization
reaction as well as the Ru-catalyzed dimerization of acetylenes.

J. Pyziak et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 239–244
243
Fig. 8. Structures of molecules involved in the last step of the examined reaction. All distances are in Ångstroms.
Fig. 9. Arrhenius plot of ln kobs on the 1/T.
Table 1
complex is formed in this transition state At the same time, the ␣-C
atom of the vinylboronate is getting closer to the central ruthe-
nium atom. The shortening of the distance between the ␣-carbon
atom of the vinylboronate and ruthenium, as well as the length-
ening of the bond between Ru and the C atom directly attached
to the metal centre in the vinylene complex were observed. As a
result of a rearrangement involving the vinylboronate insertions
Co-dimerization of 2-vinyl-1,3,2-dioxaborinane with ethynyltrimethylsilane at
given temperatures.
T (◦C)
Time (h)
Conversion of ethynylsilane (%)
kobs
70a
75
80
90
100
11.0
11.0
11.0
11.5
11.0
81
84
87
96
98
0.010 0.001
0.015 0.002
0.031 0.002
0.117 0.014
0.198 0.042
into the [Ru]
C C bond, the TSA3–A4 transition state is converted
into complex A4. As shown in Fig. 7, the distances between the
Ru. . .C ␣-carbon of vinylboronate and the Ru. . .C for ethynylsilane
ꢁEA = 26.9 1.6 [kcal/mol].
a
Reaction conditions unless otherwise stated: [Ru]:[Si]:[B] = 2 × 10−2:1:2, 24 h,
˚
˚
are 2.130 A and 2.110 A, respectively, in the TSA3–A4 transition state.
The energy barrier for this step is very small 6.7 [kcal/mol].
In last step of examined reaction, complex A4 undergoes ␤-
hydride elimination, which occurs through the TSA4–A5 transition
state. Finally, the borylsilyl-substituted buta-1,3-diene A5 and the
regenerated [Ru(CO)ClH(PCy3)] are obtained. The relative energies
of these species are presented in Fig. 8. This is the rate-determining
step in the catalytic cycle of terminal silylacetylene with vinyl-
boronates co-dimerization. The energy barrier for this step is
28.4 [kcal/mol]. This value is so high probably because of the
appearance of an additional positive charge on the boron atom,
which is already an electron deficient element, and fully supports
earlier findings of Z. Lin et al. that the “empty” p orbital of B does
not participate in the reaction studied.
toluene, argon.
These kinetic data were obtained by measurements of the
consumption of the substrate (ethynyltrimethylsilane) by gas
chromatography using decane as the internal standard. The effect
of temperature on the respective kobs gave an activation energy of
Ea = 26.9 1.6 [kcal/mol]. This value is in very good agreement with
the theoretical calculations.
4. Conclusions
We have studied in silico the mechanism of the co-dimerization
reaction of 2-vinyl-1,3,2-dioxaborolane and triethylethynylsilane
employing three density functional theories (DFT): BLYP, B3LYP
and M06 functionals. All of the methods gave comparable results
indicating that the ␤-hydride elimination of borylsilylbuta-1,3-
diene with the regeneration of the Ru H complex is the rate
determining step. The energy barrier calculated by the DFT
method is in agreement with the kinetic measurements from
In order to confirm the results of the DFT studies, a sep-
arate series of kinetic measurements were performed using
2-vinyl-1,2,3-dioxaborinane and ethynyltrimethylsilane for co-
dimerization in the temperature range of 70–100 ◦C. The
pseudo-first-order rate constants were calculated for the reaction
(see Table 1, Fig. 9).

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J. Pyziak et al. / Journal of Molecular Catalysis A: Chemical 396 (2015) 239–244
in vitro experiments, which confirms our DFT calculations as well
as the stoichiometry of the reactions.
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This work was supported by the National Science Centre
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