Carbon-carbon bond activation by 1,1-carboboration of internal alkynes

Article abstract of DOI:10.1021/ja106365j

Internal alkynes undergo 1,1-carboboration reactions upon treatment with boranes RB(C₆F₅)₂ (R = C₆F ₅, CH₃) to yield trisubstituted alkenylboranes. These products can be used as substrates

Full text of DOI:10.1021/ja106365j

Published on Web 09/10/2010
Carbon-Carbon Bond Activation by 1,1-Carboboration of Internal Alkynes
Chao Chen, Gerald Kehr, Roland Fro¨hlich, and Gerhard Erker*
Organisch-Chemisches Institut, UniVersita¨t Mu¨nster, Corrensstrasse 40, D-48149 Mu¨nster, Germany
Received July 19, 2010; E-mail: erker@uni-muenster.de
Abstract: Internal alkynes undergo 1,1-carboboration reactions
upon treatment with boranes RB(C6F5)2 (R ) C6F5, CH3) to yield
trisubstituted alkenylboranes. These products can be used as
substrates in Pd-catalyzed cross-coupling reactions.
Developing new activation protocols for the chemical utilization
and modification of otherwise unactivated strong σ bonds is a central
task in experimental chemistry. In the past several years, a variety of
convincing solutions have been developed, for example, for C-H bond
activation1 and quite recently even for metal-free splitting of dihy-
drogen.2 In contrast, the chemical repertoire for selectively attacking
unactivated carbon-carbon single bonds is still small.3 We here report
on a surprisingly simple C-C bond-activation process that takes place
upon treatment of internal alkynes with a variety of strongly electro-
philic boranes, and we demonstrate that the resulting new borane-
substituted frameworks can be utilized for additional carbon-carbon
bond formation to eventually yield tetrasubstitued alkenes.
Figure 1. Molecular structure of compound 3 (hydrogens at the phenyl
groups have been omitted for clarity; thermal ellipsoids are shown at the
50% probability level).
We then reacted tolane with B(C6F5)3. Clean 1,1-carboboration
took place in a slow reaction in toluene at 110 °C with 1,2-phenyl
migration, giving 2b in 88% yield. The product shows a typical
pair of 19F NMR spectroscopic sets of resonances in a 2:1 ratio for
the -B(C6F5)2 moiety and the migrated -C6F5 substituent. This
product was directly characterized by X-ray crystal structure
analysis [B1-C2, 1.559(3) Å; B1-C2-C3, 122.0(2)°; C2-C3,
1.364(3) Å; see Figure 2 left]. Bis(p-tolyl)acetylene reacted
analogously with B(C6F5)3. We isolated the corresponding 1,1-
carboboration product 2c as a yellow solid in 63% yield.
We found that the borane B(C6F5)3 (1a)4 reacts cleanly with
4-octyne in toluene solution at elevated temperature. After 3 days
at reflux temperature, we isolated the 1,1-carboboration product
2a in 89% yield. NMR analysis revealed that one C(sp)-C(sp3)
carbon-carbon single bond had been broken in the course of the
reaction and that the corresponding n-propyl group had migrated
to the other alkyne terminus. Additionally, in the course of this
reaction, a C6F5 substituent was transferred from boron to carbon.5
1
The H/13C NMR spectra of 2a feature the typical signals of a
geminal pair of n-propyl groups. Compound 2a shows a typical
11B NMR signal (at δ 60.9) of a tricoordinated RB(C6F5)2 Lewis
acid. The structural characterization of 2a was achieved after
derivatization. Treatment of the 1,1-carboboration product 2a with
a 1:1 mixture of triphenylphosphine/p-tolylacetylene gave the
typical trans-1,2-addition product of the resulting P/B frustrated
Lewis pair to the terminal alkyne6 to yield 3 (see Scheme 1 and
Figure 1). Compound 3 was characterized by X-ray crystal
structure analysis. The structure contains the tetracoordinated
olefinic (n-C3H7)2CdC(C6F5)[B] unit that was typically obtained
by 1,2-alkyl migration proceeding in accord with B-C and
C-C6F5 bond formation in the 1,1-carboboration reaction
sequence.5
Scheme 1
Figure 2. Molecular structures of (left) 2b and (right) 4b (thermal ellipsoids
are shown at the 50% probability level).
The reaction is not limited to transfer of a C6F5 group from boron
to carbon. Treatment of readily available CH3B(C6F5)2 (1b)7 with
bis(p-tolyl)acetylene at 125 °C showed that the 1,1-carboboration
of the internal alkyne was achieved with selective migration of the
methyl group from boron to the former acetylene carbon in the
course of the slow C-C bond-activation process to yield 2d, which
was isolated in 49% yield (see Scheme 2).
Alkenylboron compounds are widely used in organic synthesis,8
photochemistry,9 and coordination chemistry.10 In particular, they
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13594 J. AM. CHEM. SOC. 2010, 132, 13594–13595
10.1021/ja106365j  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
these products are amenable to subsequent utilization of the boryl
functionality for carbon-carbon bond formation. Such new variants
of the 1,1-carboboration reaction14 are well-suited for opening new
general pathways in carbon-carbon bond-activation chemistry.
Acknowledgment. Financial support from the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen Industrie
is gratefully acknowledged. C.C. thanks the Alexander von Hum-
boldt Foundation for a stipend.
Supporting Information Available: Text and figures giving further
experimental and spectroscopic details and CIF files giving crystal-
lographic data for 2b, 3, and 4b. This material is available free of charge
via the Internet at http://pubs.acs.org.
are used as alkenyl reagents in coupling reactions for the preparation
of substituted alkenes.8a The products 2 all contain a reactive boryl
substituent. The newly formed alkenylborane moiety in these
compounds can be synthetically utilized for subsequent carbon-
carbon bond formation in a Pd-catalyzed reaction that is related to
Suzuki-Miyaura-type coupling.8c Thus, we treated the 1,1-car-
boboration product 2a with phenyl bromide at 70 °C in aqueous
THF in the presence of sodium hydroxide base and ∼10 mol %
Pd(PPh3)4 catalyst. This led to C-C coupling that resulted in the
formation of tetrasubstituted olefin 4a, which we isolated in 50%
yield. Similarly, the Pd-catalyzed arylation of the 1,1-carboboration
product 2d with phenyl iodide gave the cross-coupled product 4b
in good yield (see Scheme 3). X-ray crystal structure analysis of
4b showed the geminal pair of p-tolyl substituents at C1 of the
double bond, which originated from the 1,1-carboboration reaction
of bis(p-tolyl)acetylene with 1b, and the phenyl and methyl groups
on C2 [C1-C2, 1.349(2) Å; ∑deg(C1) ) 360°; ∑deg(C2) ) 360°;
see Figure 2 right].
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1,1-Carboboration reactions had been observed to rapidly proceed
upon treatment of various main-group- or transition-metal-
substituted alkynes with boranes,5,11 but they have rarely been
described in cases of simple organic acetylenes.12,13 However, this
unique topology of the 1,1-carboboration sequence is especially
attractive starting from simple internal alkynes that bear conven-
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observed here leads to trisubstituted alkenylboranes that, as we have
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