Metal-Free Acetylene Coupling by the (C6F5)2B?X 1,2-Halogenoboration Reaction

Article abstract of DOI:10.1002/chem.201802084

(C₆F₅)₂B-halides were conveniently prepared by treatment of (C₆F₅)₂BH with tritylchloride or -bromide, respectively. With cyclopropylacetylene, (C₆F₅)₂BBr underwent sequential cis-1,2-halogenoboration followed by 1,2-carboboration to give the 4-bromo-2,4-dicyclopropylbutadienyl-B(C₆F₅)₂ product. It reacted further with additional cyclopropylacetylene to give the linear triene and tetraene products in a metal-free alkyne oligomerization reaction. The pyridine adduct of the initial diene product was characterized by X-ray diffraction. (C₆F₅)₂BCl reacted analogously. Similar (C₆F₅)₂BX induced oligomerization reactions were carried out with two conjugated enynes.

Full text of DOI:10.1002/chem.201802084

A Journal of
Accepted Article
Title: Metal-free Acetylene Coupling by Means of the (C6F5)2B-X 1,2-
Halogenoboration Reaction
Authors: Gerhard Erker, Atsushi Ueno, Jun Li, Constantin G Daniliuc,
and Kehr Gerald
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To be cited as: Chem. Eur. J. 10.1002/chem.201802084
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COMMUNICATION
Metal-free Acetylene Coupling by Means of the (C
Halogenoboration Reaction
6
F
5
2
) B-X 1,2-
Atsushi Ueno, Jun Li, Constantin G. Daniliuc, Gerald Kehr, and Gerhard Erker*
Dedicated to the memory of Professor Takao Ikariya, Tokyo Institute of Technology, deceased April 21, 2017.
Abstract: (C
of (C BH with tritylchloride or -bromide, respectively. With
cyclopropylacetylene (C BBr underwent sequential cis-1,2-
halogenoboration followed by 1,2-carboboration to give the 4-bromo-
,4-dicyclopropylbutadienyl-B(C product. It reacted further with
6
F
5
)
2
B-halides were conveniently prepared by treatment
6 5 2
This gave the (C F ) BX products, which we isolated in 79% (X:
Cl) and 77% (X: Br) yield, respectively. Typically, one molar
equivalent of triphenylmethyl bromide was added to a suspension
6
F
)
5 2
6 5 2
F )
6 5 2
of (C F ) BH in toluene. The mixture was stirred for 18 hours at
2
F )
6 5 2
r.t. Workup including vacuum distillation at 100 °C gave
compound 4b as a pale orange solid (1.9 g prepared in a typical
procedure). We assume that the reaction is initiated by halide
addition to the boron Lewis acid from the dissociative trityl halide
equilibration, followed by hydride abstraction by the remaining
additional cyclopropylacetylene to give the linear triene and tetraene
products in a metal free alkyne oligomerization reaction. The pyridine
adduct of the initial diene product was characterized by X-ray
6 5 2 6 5 2
diffraction. (C F ) BCl reacted analogously. Similar (C F ) BX induced
oligomerization reactions were carried out with two conjugated
enynes.
trityl cation to give the respective (C
(see Scheme 1), although this mechanistic description is not
strictly proven. The (C BH starting material for this synthesis
was conveniently prepared by treatment of commercially
available (C B with triethylsilane by means of a variant of the
procedure published by Piers et al.
6 5 2 3
F ) BX products and Ph CH
6 5 2
F )
Boranes undergo two major types of reactions with acetylenes,
both giving rise to the formation of alkenylborane derivatives.
There are the variants of the 1,1-carboboration reaction.[1,2] These
are characterized by attachment of a substituent from the borane
and the remaining boryl group at the same acetylene carbon atom.
Consequently, an acetylene substituent will undergo a 1,2-
migration along the alkyne framework. More often encountered
are the genuine 1,2-additions, most commonly the hydroboration
reaction.[3] This is usually a cis-1,2-H[B] addition, but there are
6 5 3
F )
[
9e]
trans-1,2-H/BR
2
addition variants, as well.[ 1,2-Diboration , 1,2-
4]
[5]
halogeno-[6,7] and 1,2-cyanoboration[8] reactions of alkynes are
becoming increasingly important. These usually involve only
Scheme 1. Synthesis of the bis(pentafluorophenyl)boron halides 4.
XBR
have now found a novel very simple way of preparing the
BX reagents (X = Cl, Br)[9c,d,g] and found that they undergo
2
addition reactions to acetylenes in a 1:1 stoichiometry. We
6 5 2
(C F )
cis-1,2-halogenoboration reactions that do not stop after the
reaction with one alkyne equivalent but continue and add
additional alkyne equivalents to give the respective linear
oligomers. First examples of such sequences of halogenoboration
reactions combined with subsequent alkyne/alkyne coupling will
be presented in this account.
6 5 2
Chloro-bis(pentafluorophenyl)borane [(C F ) BCl, 4a] is a useful
6 5 2
Figure 1. A view of the molecular structure of (C F ) BBr (4b) (thermal ellipsoids
are shown at the 30% probability level).
boron reagent. It had been synthesized in rather cumbersome
ways using problematic organotin or organomercury reagents.[9]
We have now developed a more convenient synthesis of 4a and
the related (C
6 5 2
F ) BBr reagent 4b by reacting Piers’ borane
In CD
2
Cl
2
solution, compound 4b shows a 11B NMR signal at
[
(C
6 5 2
F )
BH][9e] with trityl chloride or trityl bromide, respectively.
60.5 (cf. 4a: 58.1) and a set of three 9F NMR resonances with
1

1
9
a 
Fm,p = 16.3 ppm chemical shift difference. Both these NMR
features are typical for a strongly Lewis acidic planar-tricoordinate
boron compound. Single crystals suited for the characterization of
[*]
Dr. A. Ueno, Dr. J. Li, Dr. C. G. Daniliuc, Dr. G. Kehr, Prof. G. Erker
Organisch-Chemisches Institut der Universität Münster
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster (Germany)
E-mail: erker@uni-muenster.de
(C
6
F
5
)
2
BBr (4b) by X-ray diffraction were obtained from slow
Cl solution of 4b. The X-
diffusion of pentane into a saturated CH
2
2
ray crystal structure analysis shows the presence of a planar-
tricoordinate geometry around boron (see Figure 1). The structure
Supporting information for this article is given via a link at the end of
the document. CCDC 1821915 (8a), 1821916 (4b), 1821917 (8b),
is close to C
B1Br1 bond length amounts to 1.908(2) Å. The pair of
B1C11/C21 bonds to the C rings are equal in length (both
1.566(3) Å). The sum of bond angles at the boron atom B1 is
2
-symmetric (but not crystallographically exact). The
1834994 (8f),1834995 (8c), and 1834996 (8e) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
6 5
F
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Chemistry - A European Journal
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COMMUNICATION
3
60.0° with the individual bond angles found within a small range
C3=C4 are markedly rotated from conjugation (see Table 2).[10]
The cyclopropyl group at C4 is found in a bisected orientation to
allow -conjugation of the cyclopropyl Walsh orbitals with the
adjacent olefinic -bond. The other cyclopropyl group was found
disordered. The boron atom attached at carbon atom C1 is
tetracoordinated. The stereochemistry at the olefinic double
bonds indicates a pathway of formation of compound 7b that
involves an initial cis-1,2-bromoboration to give the intermediate
6b, followed by a rare example of a cis-1,2-carboboration
reaction[11] to give the observed product 7b (see Scheme 2 and
Figure 2).
[
C11-B1-Br1: 119.0(2)°, C21-B1-Br1: 118.7(2)°, C11-B1-C21:
6 5
122.3(2)°]. The angle between the two C F planes amounts to
60.3° in compound 4b.
We chose cyclopropylacetylene (5a) to investigate the
halogenoboration reactions of the strongly electrophilic
halogenoboranes 4a and 4b. It turned out that in both cases the
reaction took an uncommon course. We first treated the
(
C
6
F
5
)
2
BBr borane 4b with one molar equiv. of cyclopropyl-
acetylene (5a, d -benzene, r.t., 1h) and observed the clean
formation of the cis-1,2-bromoboration product 6b, which was
characterized by NMR spectroscopy [ H:  6.85 (s, =CH),  1.46,
6
1
1
3
0
.80, 0.44 (cyclopropyl-H); C:  163.5 (=CBr),  130.6 (=CH),
1
1
B:  57.4]. The reaction of compound 4b with two molar equiv.
of 5a (or alternatively the addition of one equiv. of 5a to the in situ
formed halogenoboration product 6b) under analogous conditions
2
resulted in “alkyne-insertion” into the C(sp )B bond of 6b to give
the carbon-carbon coupling product 7b. The reaction was first
carried out in C
spectroscopy without workup. The 2:1 mixture of 5a and 4b in
was stirred for 2 h at room temperature. The NMR spectra
6 6
D and the outcome investigated directly by NMR
6 6
C D
showed that two acetylene equivalents had become coupled to
give the 2,4-bis-cyclopropyl substituted butadiene, bearing a
bromide and a -B(C
b apparently features an extended conformation that prohibits
internal -Br/-B(C interaction. The NMR features of the -
B(C group in 7b point to a Lewis acidic planar-tricoordinate
geometry at boron (see Table 1).
6 5 2
F ) functional group at its ends. Compound
7
6 5 2
F )
6 5 2
F )
Figure 2. A view of the molecular structure of compound 8b (thermal ellipsoids
are shown at the 30% probability level).
Table 1. Selected NMR dataa of the acetylene coupling products 7a,b and
their pyridine adducts 8a,b
7
a (X: Cl)b
8a (Pyr)c
-3.2
7b (X: Br)b
55.3
8b (Pyr)c
-3.4
11B
56.3
1
-H
-H
6.58
6.09
6.57
6.08
3
5.53
5.56
5.73
5.84
13C1
13C2
13C3
13C4
130.6
175.3
121.2
141.3
11.9
132.3
146.9
124.3
136.5
5.5
130.7
176.8
124.3
135.8
12.0
131.8
147.6
127.6
130.9
5.5

d

 Fm,p
a 1
H/13C NMR chemical shifts rel. TMS, -scale, 11B rel. BF .
3 2 6 6
Et D (299
O; b in C
Scheme 2. (C
acetylenes 5a-c.
6
F
5 2
) Bhalide induced linear oligomerization reaction of terminal
K); c in CD (299 K); d in ppm.
2
Cl
2
In solution the pyridine adduct 8b shows the typical 1H NMR
signals of a pair of inequivalent cyclopropyl substituents. They
Treatment of 7b with pyridine resulted in the formation of the
pyridine adduct 8b. This was synthesized on a preparative scale
and isolated as a white solid in 82%. The X-ray crystal structure
analysis showed the core structure that was formed by head to
tail coupling of two cyclopropylacetylene units. Carbon atoms C2
and C4 have the pair of cyclopropyl substituents attached.
Noteworthy is that the individual olefinic -systems C1=C2 and
1
3
show C NMR resonances at  20.7/6.8 and  19.2/7.3,
respectively. The 11B NMR resonance is in the typical range of a
tetracoordinate boron center. We note that pyridine coordination
13
to boron has resulted in a marked shift of the - C NMR feature
of the directly attached alkenyl moiety to smaller -values relative
to its active alkenylborane Lewis acid precursor 7b [(C2) = 29.2
ppm, see Table 1].
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COMMUNICATION
The cyclopropylacetylene coupling product 7b reacted further
with additional monomer 5a. When one more equiv. of the alkyne
was added to a solution of 7b (r.t., 1h) we observed the formation
of a mixture of the respective linear threefold (9b) and tetrafold
acetylene coupling products (10b) in addition to some remaining
The reaction of (C
ratio gave the diene 7a, which was apparently formed by
sequential cis-1,2-chloroboration followed by cis-1,2-
carboboration. We characterized the cis-1,2-chloroboration/
alkyne coupling product 7a spectroscopically from an in situ
experiment (see Table 1) and isolated the corresponding pyridine
adduct 8a in 81% yield from an experiment on a preparative scale.
Compound 8a was characterized by X-ray diffraction (see Table
2). The structure is depicted in the Supporting Information.
6
F
5
)
2
BCl (4a) with cyclopropylacetylene in a 1:2
7b. During this process the color of the reaction mixture changed
from yellow to orange and then to dark red; addition of more
acetylene equivalents gave a dark red-brown solution. The
generated polyene products were characterized from the mixture
by NMR spectroscopy (see the Supporting Information for details). Addition of one molar equiv. of cyclopropylacetylene to a solution
of 7a in d -benzene (r.t., 1h) resulted in building up the linear
6
alkyne oligomer chain by further 1,2-carboboration. Under our
typical reaction conditions, we observed the formation of the
substituted linear triene 9a as the major product admixed with the
tetraene 10a and some residual diene 7a. The characteristic
olefinic section of the H NMR spectra of the oligomers is depicted
in Figure 3.
Table 2. Selected structural parametersa of the pyridine adducts 8a,b,c,f of
the acetylene coupling products 7a,b,c,f.
a (X: Cl)b
8b (X: Br)b
8
8c (X: Cl)
1.626(2)c
1.618(3)
1.347(3)
1.512(3)
1.313(4)
1.740(3)
11.5(3)
8f (X: Br)
1.641(5)c
1.612(5)
1.351(5)
1.496(5)
1.331(5)
1.921(3)
4.9(5)
1
B1N31
B1C1
1.637(3)
1.607(4)
1.338(3)
1.498(4)
1.320(4)
1.752(2)
6.5(4)
1.638(2)
1.615(3)
1.345(3)
1.491(3)
1.325(3)
1.905(2)
6.0(3)
The
(C
6
F
5
)
2
Bhalide
induced
cyclopropylacetylene
oligomerization reactions are not the only examples of this
reaction type. We found a similar behaviour upon treatment of the
compounds 4a and 4b with the conjugated enyne 2-
C1C2
C2C3
methylbutenyne 5b, as well. The reaction of 5b with (C
6 5 2
F ) BCl
C3C4
4a in a 2:1 molar ratio followed by addition of the pyridine trapping
C4X1
reagent (one molar equiv.) gave the acetylene coupling product
B1-C1-C2-
C2-C3-C4-
C1-C2-C3-
8c (Scheme 3), which we isolated as a white solid in 43% yield.
1
The H NMR spectrum of compound 8c shows a pair of methyl
2.8(4)
2.7(3)
-3.1(4)
-1.7(5)
group singlets, four olefinic exo-methylene resonances and two
singlets of the olefinic =CH- units within the main carbon chain.
-108.1(3)
-109.2(2)
-97.5(3)
-107.2(4)
a
bond length in Å, angles in deg, b values of molecule A. c B1-N1
The chloroborane 4a reacted in
cyclopropylacetylene (5a). When we reacted (C
a in a 1:1 molar ratio in deuterated benzene at r.t. we monitored
a
similar way with
6 5 2
F )
BCl (4a) with
5
the formation of a ca. 1:1 mixture of the conventional cis-1,2-
chloroboration product 6a with the extended product 7a (see
Scheme 2 and Figure 3). The 19F NMR spectra indicated the
presence of some residual chloroborane 4a in this experiment.
Scheme 3. (C
methylbutenyne units.
6 5 2
F ) B-halide induced carbon-carbon coupling of two 2-
1
Figure 3. Olefinic section of the H NMR (600 MHz, d
6
-benzene, 299 K) spectra
Figure 4. A view of the molecular structure of the acetylene coupling product 8c
thermal ellipsoids are shown at the 30% probability level). Selected bond
lengths (Å) and angles (°) are described in Table 2.
6 5 2
of the reaction of ClB(C F ) (4a) with (1) 1 equiv. cyclopropylacetylene (5a), (2)
a second equiv. 5a, and (3) a third equiv. 5a (for details see the text and the
Supporting Information).
(
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COMMUNICATION
Compound 8c was characterized by X-ray diffraction (single
crystals were obtained from pentane/dichloromethane by the
diffusion method). The solid state structure (see Fig 4) shows the
this has not yet been strictly proven. Hydride abstraction by the
Ph
(C
triphenylmethane co-product.
Our study shows that the (C
reagents, which are now easily available by a reaction scheme
3
C+ cation would then directly lead to the (C
6
5
F )
2
BCl or
6 5 2
F ) BBr products with formation of the stoichiometric
central C --system constructed by C-C coupling of the two
4
substituted acetylene monomer units. We note that the two
conjugated four-electron -systems originating from the two
enyne monomers are oriented close to orthogonal to each other
6 5 2
F ) Bhalides may serve as useful
that avoids the hazardous C
6
F
5
Li reagent and potentially
problematic organotin or organomercury reagents.[ The
9]

C1-C2-C3-C4 = -97.5°). Carbon atom C1 bears the –B(C
6
F
5
)
2
group (with pyridine attached to the boron Lewis acid), and
6 5 2
(C F ) Bhalides 4a and 4b readily undergo the cis-1,2-
chloride originating from the 4a reagent is found bonded to C4.
The reaction of compound 5b with (C F ) BBr (4b) proceeded
6 5 2
analogously and furnished the product 8d (for details see the
Supporting Information). We reacted the conjugated enyne
halogenoboration reaction with cyclopropylacetylene to give the
respective -chloro- or bromoalkenylboranes 7a,b. Their
subsequent reactions with additional equivalents of the alkyne are
unusual. They form series of linear alkyne oligomers that are
apparently formed by successive 1,2-carboboration reactions with
the added alkyne. Similar reaction sequences were observed with
the conjugated enynes 5b and 5c. 1,2-Carboboration reactions in
general are not too frequently observed; they seem to require
rather electrophilic boranes in order to proceed with acceptable
rates.[11] In addition there seems to be a steric component.
6 5 2
ethynylcyclohexene (5c) with each of the (C F ) B-halide
reagents 4a/4b. In reactions under direct NMR control using
increasing enyne to halogenoborane ratios we could observe the
initial 1,2-halogenoboration products 6e,f admixed with the
“
“
dimers” 7e,f, followed by the formation of a mixture of higher
oligomers” at higher 5c/4 ratios (for details including the depicted
NMR spectra see the Supporting Information). On a preparative
scale we synthesized the pyridine adducts of the “dimeric”
products and isolated them as white solids in 75% (8e, X = Cl)
and 92% (8f, X = Br) yield, respectively. Both the products 8e and
Alkyne oligomerization and polymerization is usually a domain of
transition metal catalysis. Many transition metal systems have
been shown to induce the thermodynamically favourable
cyclotrimerization of alkynes to give benzene derivatives.[
Acetylene polymerization and linear oligomerization of substituted
alkynes is mostly carried out with specific Ziegler-Natta
12]
8f were characterized by X-ray crystal structure analyses. The
structure of compound 8f is shown in Figure 5; the structure of
compound 8e is depicted in the Supporting Information). Each of
catalysts[ or by metathesis reactions. In our case this can be
13]
[14]
1
these compounds shows a set of four olefinic H NMR signals in
achieved in a stoichiometric way simply by the action of the
solution at 299 K (for further details see the Supporting
Information).
(C
The sequence of 1,2-halogenoboration combined with multiple
,2-carboboration results in the formation of series of linear alkyne
6 5 2
F ) Bhalide reagents with the here chosen alkyne derivatives.
1
oligomers that feature pairs of halide and –boryl functional groups
at the terminal carbon atoms of the oligo-ene -systems. We will
see if this new reaction scheme will lead to interesting applications
of the resulting conjugated functionalized olefinic systems.
Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. A.U. thanks the JSPS for a postdoctoral
stipend.
Keywords: 1,2-halogenoboration • carboboration • metal-free
oligomerization • oligo-acetylenes • boron halide synthesis
[
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C
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F
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)
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Bhalide reagents 4a and 4b by treatment of Piers’ borane
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CX with the trityl cation and halide anion
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F
5
)
2
BH Lewis acid to generate [(C
6
F
5
)
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B(H)X] , although
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Chemistry - A European Journal
10.1002/chem.201802084
COMMUNICATION
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Chemistry - A European Journal
10.1002/chem.201802084
COMMUNICATION
COMMUNICATION
Treatment of Piers’ borane
Atsushi Ueno, Jun Li, Constantin G.
Daniliuc, Gerald Kehr, Gerhard Erker*
[
HB(C
X-B(C
6
F
F
5
)
)
2
] with trityl halides gave the
reagents (X: Cl or Br).
6
5
2
They reacted with terminal acetylenes
under mild conditions by sequential
Page No. – Page No.
Metal-free Acetylene Coupling by
1,2-halogenoboration followed by
Means of the (C
6 5 2
F ) B-X 1,2-
multiple alkyne addition to give series
of the linear conjugated alkyne
Halogenoboration Reaction
oligomers in a metal-free synthesis.
This article is protected by copyright. All rights reserved.