Mechanism of catalytic cycloboration of Α-olefins with boron trichloride: the synthesis of hardly obtainable boriranes and the mechanistic DFT study of transmetalation of titanacyclopropane intermediates

Article abstract of DOI:10.1134/S0023158417050226

A theoretically justified mechanism of transmetalation of 2-alkyl substituted titanacyclopropanes with BCl₃ is proposed based on the DFT calculations of the thermodynamic and activation parameters of possible reaction pathways. Based on the data obtained, phenyl- and alkyldichloroboranes were proposed to be used as transmetalating agents along with BCl₃ in the catalytic cycloboration in the presence of Cp₂TiCl₂ and Mg metal. It was shown that the barely accessible 1-phenyl-2-hexylborirane can be synthesized using PhBCl₂.

Full text of DOI:10.1134/S0023158417050226

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 5, pp. 549–555. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © T.V. Tyumkina, L.O. Khafizova, S.M. Idrisova, L.I. Khusainova, L.M. Khalilov, U.M. Dzhemilev, 2017, published in Kinetika i Kataliz, 2017, Vol. 58,
No. 5, pp. 560–566.
Mechanism of Catalytic Cycloboration of α-Olefins with Boron
Trichloride: the Synthesis of Hardly Obtainable Boriranes
and the Mechanistic DFT Study of Transmetalation
of Titanacyclopropane Intermediates
T. V. Tyumkina*, L. O. Khafizova, S. M. Idrisova, L. I. Khusainova,
L. M. Khalilov, and U. M. Dzhemilev
Institute of Petrochemistry and Catalysis, Russian Academy of Sciences, Ufa, 450075 Russia
*
e-mail: ttvnmr@gmail.com
Received October 25, 2016
Abstract—A theoretically justified mechanism of transmetalation of 2-alkyl substituted titanacyclopro-
panes with BCl is proposed based on the DFT calculations of the thermodynamic and activation
3
parameters of possible reaction pathways. Based on the data obtained, phenyl- and alkyldichloroboranes
were proposed to be used as transmetalating agents along with BCl in the catalytic cycloboration in the
3
presence of Cp TiCl and Mg metal. It was shown that the barely accessible 1-phenyl-2-hexylborirane
2
2
can be synthesized using PhBCl2.
Keywords: α-olefin cycloboration, transmetalation, 1-phenyl-2-hexylborirane, reaction mechanism, titana-
cyclopropanes, metal complex catalysis, Cp TiCl , DFT study
2
2
DOI: 10.1134/S0023158417050226
Three-membered boron-containing carbocycles, resulting in the formation of 1-fluoro- and 1-chlorob-
boriranes and borirenes containing conjugated double oriranes, respectively [12, 13]. Similar reactions have
bonds in their structures, find application as donor- been described earlier for the synthesis of aluminacyclo-
acceptor materials in nonlinear optics (organic light- propanes [14] and aluminacyclopropenes [15]. It is obvi-
emitting diodes, field-effect transistors, and photovol- ous that, at the first step under the reaction conditions in
taic converters) [1, 2]. In the literature, there are only the presence of Mg metal (as the halogen ion acceptor)
examples of the noncatalytic methods for the synthesis in THF, the coordinatively unsaturated titanocene Cp
2
-
of boriran(en)es, for example, by the isomerization of
unstaturated compounds on exposure to heat or UV
light [3–11]. Prior to our studies, no catalytic methods
for the synthesis of boriran(en)es were known.
2+
Ti forms, which coordinates the arylolefin or acetylene
molecule to form titanacyclopropan(en)e intermediates.
This assumption is based on the published data for the
synthesis of a crystalline ethylene–bis-(pentamethylcy-
Recently, we have performed for the first time the clopentadiene)titane adduct by the reduction of the cor-
Сp TiCl -catalyzed cycloboration of α-olefins using responding dichloride (η-Cp*) TiCl with sodium
2
2
2
2
BF ·THF (THF is tetrahydrofuran) and BCl ·SMe , amalgam in an ethylene atmosphere [16] (Scheme 1):
3
3
2
CH2=CH2
Na
NaCl
(η-Cp*)2
Ti
(
η-Cp*) TiCl2
(η-Cp*) Ti:
(η-Cp*) Ti
2
2
2
−
80%
Scheme 1.
The next step assumes the titanacyclopropane
intermediate to undergo transmetalation in the alumi- transmetalation is realized not only in acyclic but also
num atom using aluminum alkyl chlorides. in cyclic systems. For example, the catalytic cycloalu-
There is a sufficient amount of data indicating that
549

5
50
TYUMKINA et al.
mination of α-olefins using AlEt [17] was found to
Methods of Analysis and Physicochemical Studies
3
form 1-ethyl-3-substituted alumolanes [18] due to the
transmetalation of the corresponding zirconacyclo-
pentane intermediates [19]. In addition, the alumi-
num atom in the five-membered metallacarbocycles
can be replaced with the boron or phosphorus atom on
treatment with boron halides or dichlorosphosphines
to yield 1-halo-3-substituted borolanes [20, 21] and
1
13
11
1
D ( H, C, and B) and 2D homo- (COSY) and
heteronuclear (HSQC and HMBC) NMR spectra
were recorded in CDCl on a Bruker Avance 400 spec-
trometer (Bruker, Germany) with working frequencies
of 400.13 ( H), 100.62 ( C), and 128.33 MHz ( B).
The internal standard was Me Si for H and
C NMR spectra and BF ⋅ Et O for B NMR spec-
tra. Boriranes were oxidized by hydrogen peroxide in
an alkaline medium according to the procedures
described in [12, 13, 20, 21, 31]. The spectral data
and physical characteristics of compounds X–XII
have been reported in [32–34].
3
1
13
11
1
4
13
11
1,3-disubstituted phospholanes [22], respectively. The
3
2
first one-pot synthesis of macroaluminahet-
ero(N,S)cycles by the example of 4-aryl-2,6,8,11-tet-
raethyl-1,7-dithia-4-aza-2,6,8,11-tetraaluminacycloun-
decanes from 3-aryl-1,5,3-dithiazepanes involving
metalla(Ti,Zr)cene catalysts [23] is also based on the
transmetalation of titanaceneheterocycles with the
formed EtAlCl , where the above-mentioned titana-
2
Quantum Chemical Calculations
ceneheterocycles were generated in situ by the well-
known inclusion reaction of titanacene Cp Ti in the
Quantum chemical calculations were performed
using the Priroda 6.0 program [35]. Optimization of
the geometric parameters, solution of the vibrational
problem of searching for transition states, scan over
the intrinsic reaction coordinate, and calculation of
the thermodynamic corrections to the total energy
2
C–S bond [24].
At the time of our studies, there were no published
data on the transmetalation mechanism for three-
membered rings, including the catalytic one-step for-
mation of boriranes in the presence of Cp TiCl . A were performed at the DFT level by the PBE method
2
2
considerable amount of the published data concerns [36] (3ζ basis set [37]). The method has been success-
the theoretical and experimental substantiation of the fully applied for calculating the energy parameters
involving Ti-containing systems [38]. The thermody-
namic parameters and the free activation energies
were determined at 298.15 K. The quantum chemical
data were visualized by the Chemcraft program [39].
transmetalation mechanism being a key step in cross-
coupling reactions (Stille [25], Suzuki [26], and Neg-
ishi cross couplings [27]) which find wide application
in synthetic practice [28–30]. The ligand exchange
reactions in acyclic organometallic compounds pro-
ceed via the formation of a bimetallic associate with-
out the preliminary dissociation of one of the ligands.
Synthesis of 2-hexyl-1-phenylborirane IX
Et O (10 mL), Mg powder (2 mg-at.), 1-octene
2
The aim of this work is to study by DFT the trans-
metalation mechanism of titanacyclopropane inter-
mediates using boron chloride, which is a key step in
the catalytic cycloboration of α-olefines. The mecha-
nism of substituting boron atoms by the titanium
atoms in three-membered cyclic systems proposed
based on the theoretical data allows us to develop a
strategy for the synthesis of hardly obtainable boriranes
involving boron chlorides.
(
1 mmol), a Cp TiCl catalyst (0.2 mmol), and PhB-
2 2
Cl (1.2 mmol) were loaded successively in a 50 mL
2
glass vessel in a dry argon atmosphere while being
stirred on a magnetic stirrer at 0°C. The mixture was
stirred for 14 h at 20–22°C. The excess magnesium
was filtered off, diethyl ether was evaporated, and pyr-
idine (1.2 mmol) was added. The mixture was stirred
for 30 min and placed in a sublimation chamber. Pyr-
idine was evaporated and the residue was subjected to
vacuum sublimation in an argon atmosphere.
EXPERIMENTAL
2
-Hexyl-1-phenylborirane (IX) was obtained as a
1
white solid with a yield of 75%. H NMR, CDCl , δ:
.83–0.98 m (3Н, C Н ), 1.20–1.53 m (8Н, С Н ),
3
Initial Reagents and Materials
9
5–8
0
3
A
2
B
4
4
All the reactions were performed in an argon atmo- 1.53–1.58 m (1Н, C Н ), 1.75–1.78 m (1Н, C Н ),
sphere. The commercially available PhBCl , Cp Ti- 7.21–7.32 m (2Н, Ar), 7.52–7.63 m (1H, Ar), 8.24 m
2
2
13
Cl , Mg, and 1-octene were used. Diethyl ether was (2H, Ar). C NMR, CDCl
, δ: 14.34, 22.90, 29.53,
2
3
4
11
dried by refluxing over sodium metal and freshly dis- 29.60, 32.09, 38.41 (C ), 127.30, 131.95, 135.24. B
tilled prior to use.
NMR, CDCl , and δ: 31.10.
3
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MECHANISM OF CATALYTIC CYCLOBORATION
551
RESULTS AND DISCUSSION
tion to boron-containing products; and (3) recyclization
leading to the target boriranes (Scheme 2). The insertion
of the boron halide molecule in the titanium–carbon
The mechanistic study of the transmetalation of the
substituted titanacyclopropanes assumes consideration bond can occur along paths a and b due to the fact that
of the following steps: (1) ring opening; (2) transmetala- the transmetalating agent can come from two sides:
R1
R2
R1
R1
R3
R2
+BHal3
+BHal3
−Cp2TiHal2
Hal B
BHal2
Ti
Hal B
TiCp2
2
2
а
b
Hal
IIа, b
Cp
Cp
I
III
R = H, R = Et (а)
−Cp2TiHal2
−BHal3
1
2
R = Et, R = H (b)
1
2
R1
R2
B
Hal IV
Scheme 2.
It is obvious that the reaction of reagents I + BHal3 mations are shown in Scheme 3 and the energy dia-
must result in the formation of open- or closed-struc- gram is shown in Fig. 1. The optimized structures of
ture intermediates IIa, b. Afterwards, one can propose the transition states and intermediates are summarized
two alternative transformation pathways: the path of in Fig. 2.
direct one-step transformation IIa, b → IV and IIa, b →
The analysis of the resulting calculation data
III → IV. According to the latter, the reaction of II showed the insertion of the BCl molecule in titanacy-
3
with the second boron halide molecule must lead to
the formation of the target borirane IV through the
clopropane I to occur predominantly in the Ti–CH2
bond due to the relatively lower energy barrier. The
comparative analysis of the activation parameters sug-
gests the energy barrier of this step along path a to be
1
,2-diboron derivative III. This path contemplates an
additional step of cyclization. Scheme 2 was taken as a
basis upon the quantum chemical simulation of the
replacement mechanism of the transition metal atom
in the titanacyclopropane intermediate with the boron
~
20 kcal/mol (the process can even occur at room
temperature), whereas path b (approach made from
the substituted side) is 4 kcal/mol higher in energy.
atom with the participation of BCl .
3
As seen in Fig. 1, the product of the first elemen-
tary step is intermediate II whose structure contains
chlorotitanacene and dichloboron fragments in posi-
The data from the study of the potential energy sur-
face of titanocyclopropane transmetalation using BCl
3
according to the developed algorithm of the transfor- tions 1 and 2. It is obvious that the structure of inter-
mediate formed is acyclic, since the covalent B–Cl
bond (d = 1.78 Å) is considerably shorter than the
calc
ΔG298, kcal/mol
distance between the boron and chlorine atoms shown
TS(I–Vb)
2
5
0
in Fig. 2 for intermediate II. However, the closeness in
space of the reactive fragments creates favorable con-
ditions for the intramolecular transformation resulting
from the ligand exchange between the central atoms
of the catalyst and the boron atom. Indeed, the cal-
culation of thermodynamic and activation parame-
TS(I–Va)
2
TS(Va–VIa)
TS(Vb–VIb)
15
10
5
0
5
ters showed such a possibility at room temperature,
I + BCl
3
#
98
Va
Vb
since the energy barrier is ΔG2 = 19.4kcal/mol. Then,
VIa
VIb
–
the Cp TiCl catalyst is reactivated due to the dissociation
2
2
of the B–Cl bond in the intermediate complex VI and the
target 1-chloro-2-ethylborirane VII forms (Scheme 3). In
Scheme 3, the values of Gibbs free energy and the activa-
tion barriers expressed in kcal/mol are given for each step
of pathway a:
–
10
2 2
VII + Cp TlCl
Reaction coordinate
Fig. 1. Energy diagram for transmetalation of 2-ethyltita-
nacyclopropane using BCl .
3
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

5
52
TYUMKINA et al.
R = H, R = Et (а)
1
2
+
R1
Cl2B
R2
R = Et, R = H (b)
1
2
TiCp2
R1
R2
Cl
R1
R2
TS( I−V)
+
BCl3
Cl B
2
TiCp2
Ti
Cl
Cp
Cp
I
Vа, b
+BCl3
−Cp2TiCl2
+
Cl2
+
R1
Cl
R2
R1 R2
Cl2B
B
Cl
B
ΔG298 =−39.2
ΔG298 = −2.4
TiCp2
Ti
Cp2
#
#
Cl
Cl
ΔG298 = 48.0
ΔG298 = 19.4
Cl
TS( II−VIII)
TS( V−VI)
R1
R2
R1
Cl B
R2
B
BCl2
2
Cl
Cl
VIII
TiCp2
+
VI Cl
+
R1
R2
Cl
B
BCl2
Cl
ΔG298 = 29.1
R1
R2
#
ΔG298 = 47.8
ΔG298 =−7.6
−Cp2TiCl2
TS( VIII−VII)
−BCl3
B
Cl
VII
Scheme 3.
1
13
11
To estimate the thermodynamic and activation multinuclear H, C, and B NMR spectroscopy is
parameters of the elementary steps upon realization of difficult due to the absence of signals for the endocy-
an alternative pathway for the formation of 1-chloro- clic carbon and hydrogen atoms, and the chemical
2
-ethylborirane involving two BCl molecules via the
3
shifts of the boron atom for the described boriranes
diboron-containing intermediates IIa, b → III → IV, vary in a wide range from δ –1 to +38 [3, 4, 12, 13].
we studied the reaction path а. The barriers on this Most of the known boriranes are not suited for analysis
pathway were found to be high. For example, only by mass spectrometry.
cyclization requires 48 kcal/mol and is energetically
The theoretical data given in this work are unprec-
edented for the transmetalation reactions in cyclic sys-
tems. The mechanism of substituting the boron atom
by the transition metal atom in three-membered rings,
unfavorable (ΔG2 = 29.1 kcal/mol). In addition, the
98
same amount of energy is required to replace titanium
with the second boron atom (Scheme 3).
Thus, the experimental data for the production of where each step can define the reaction pathway, is
hardly accessible 1-chloroboriranes agree closely with proposed for the first time. The complexity and multi-
the data from the theoretical mechanistic simulation step pattern of transmetalation in rings distinguish it
of this reaction involving BCl . Without the theoreti- from the analogous process in acyclic systems for
3
cal studies, the conclusion that an exclusively cyclic which the difference between the electronegativities of
structure rather than a diboron derivative is formed is the atoms being exchanged with ligands is considered
not quite obvious, since the oxidation products (for as the driving force for the reaction. In this investiga-
example, in a H O /NaOH system) will be identical in tion phase, additional experiments are needed to
2
2
both cases. In addition, interpreting the boriranes by obtain far-reaching conclusions on the feasibility of
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MECHANISM OF CATALYTIC CYCLOBORATION
553
Cl
3.06
Ti Cp2
.30
2
.24
Cl
2
Cp2 Ti
B
2.76
1.45
2.20
Cl
Et
I
TS(I–Va)
Cl
Cl
Cp2 Ti
Cl
3.17
2.99
Et
B
Cl
Cl
2.01
B
Et
Cp2 Ti
Cl
Va
TS(Va–VI)
Cl
Cl
B
Et
B
Et
1.95
1.52
1.52
Cl
Cl
1.58
Cp2 Ti
VI
VII
Cp2
2
.67
Cl
Ti
.61
Cl
Cl
2
Et
Cl
2.04
Cl
2
.02
B
Cl
B
B
Cl
B
Cl
Cl
Cl
Et
TS(Va–VIII)
VIII
Cl
Cl
B
2.91
Cl
.33
Cl
2
TS(VIII–VII)
Fig. 2. Optimized structures of transition states and intermediates for transmetalation of 2-ethyltitanacyclopropane using BCl .
3
Bond lengths are expressed in Å.
the transmetalation of titanacyclopropanes involving of the metallacycle results in the ring opening followed
other boron halides. In this work, we found unambig- by the intramolecular transformation of the interme-
uously that the considered transmetalation can be diate that was formed with the simultaneous formation
realized by a two-step mechanism. The initial attack of of the target 1-chloro-substituted borirane. The possi-
the boron halide molecule from the unsubstituted side ble migration of the chlorine atom from boron to a
KINETICS AND CATALYSIS Vol. 58 No. 5 2017

5
54
TYUMKINA et al.
transition metal (titanium) in intermediate II is a key
To confirm this assumption, we performed a model
step in terms of this reaction pathway. The calculated experiment on the reaction between 1-octene and
energy parameters of the intramolecular exchange PhBCl in the presence of magnesium metal (a chlo-
2
between the spatially close titanium- and boron-con- rine ion acceptor) and catalytic amounts of Cp TiCl .
2
2
taining fragments as a part of intermediate II suggest The reaction between 1-octene and PhBCl was found
2
that boron dihalides should also transmetalate titanacy-
to lead to the formation of 2-hexyl-1-phenylborirane
IX under the following conditions: 1-octene :
PhBCl : Mg : [Ti] = 1 : 1.2 : 2 : 0.2, Et O, ~20°C,
clopropane I. Then, 1-phenyl- or 1-alkylboriranes can
be synthesized in one preparative step using the PhBCl
2
2
2
or AlkBCl reagents in the presence of Cp TiCl as a 14 h (Scheme 4). The reaction proceeds to full conver-
2
2
2
catalyst.
sion of the original 1-octene:
Hex
OH
OH
B
H2O2, OH−
°C
Hex
OH
Mg, Cp2TiCl2
OH
Hex
+
+
+
Hex
+
0
Hex
Et2O
HO
PhBCl2
20−22°C
IX
X
XI
XII
Scheme 4.
13
In the C NMR spectrum the signals of carbon synthesized by the Cp TiCl -catalyzed cycloboration
2 2
atoms of borirane ring do not occur, apparently, due to of 1-octene involving PhBCl2.
the quadrupole broadening effects of the boron nuclei
4
[
36, 40]. The signal for the methylene C atom of the
ACKNOWLEDGMENTS
substituent is also broadened and assigned according
to the HSQC spectral data to the signal at δ 38.41
This work was financially supported by the grant
which corresponds to the cross peaks of the diastereo- from the Russian Science Foundation (project no. 17-
4
A
4
B
topic protons at δ(C H ) = 1.53 and δ(C H ) = 1.75. 73-10124). The structural studies of compounds IX–
11
XII were performed at the Core Facility Agidel at the
Ufa Scientific Center of the Russian Academy of Sci-
ences.
The B NMR spectrum displays a broadened signal at
δ 31.1. The organoboron compound was additionally
identified by the oxidation products. For example, the
treatment of the reaction mass with H O in the alka-
2
2
line medium leads to the formation of diol X and
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Translated by K. Utegenov
KINETICS AND CATALYSIS Vol. 58 No. 5 2017