Frustrated Lewis pair induced boroauration of terminal alkynes

Article abstract of DOI:10.1002/anie.201206927

Golden frustration: The facile activation of terminal alkynes with a frustrated Lewis pair (1 and 1,4-diazabicyclo[2.2.2]octane (DABCO)) enables the development of an efficient boroauration reaction. This process can be achieved directly from alkynes, a h

Full text of DOI:10.1002/anie.201206927

Angewandte
Chemie
DOI: 10.1002/anie.201206927
Borometalation
Frustrated Lewis Pair Induced Boroauration of Terminal Alkynes**
Hongyan Ye, Zhenpin Lu, Di You, Zhenxia Chen, Zhen Hua Li,* and Huadong Wang*
Organoborane compounds are versatile reagents for organic
synthesis. Various methods have been developed to incorpo-
rate boryl groups into organic frameworks. Among others,
borometallation of alkynes, such as silaboration and stanna-
boration, has received substantial attention in recent years.[1]
These reactions can introduce multiple functionalities in one
step, providing an efficient way for the stereoselective
synthesis of highly substituted alkenes. Whereas silaboration
and stannaboration of alkynes often require transition-metal
complexes as catalysts (Scheme 1a),[1] the alkynes were
Particularly, Stephan[9] and Berke[10] et al. discovered that
FLPs can activate terminal alkynes, which can lead to the
formation of alkynylborates. Inspired by their work, we
became interested in applying such a concept to the borome-
tallation of alkynes. Herein, we describe the reaction of
terminal alkynes with a FLP comprised of HBArF (1, ArF =
2
2,4,6-tris(trifluoromethyl)phenyl)) and 1,4-diazabicyclo-
[2.2.2]octane (DABCO)[11] and its application in catalyst-
free boroauration of terminal alkynes.
When 1 was mixed with phenylacetylene in hexane
solution at room temperature, the hydroboration reaction
took place in a very slow manner, and only 80% conversion
was obtained after 5 h, which is possibly due to the large steric
bulk around the boron center in 1. When DBArF2 was applied
F
2
=
to phenylacetylene, the formation of E-PhC(D) C(H)BAr
was observed, confirming that the resulting alkenylborane
was obtained through 1,2-hydroboration (Scheme 2).
Scheme 1. Different approaches to the borometalation of alkynes.
À
reported to be able to directly insert into the M B bond of
boryl transition-metal complexes, resulting in the formation
of (2-borylalkenyl)metal complexes (Scheme 1b).[2–6] One
drawback of the abovementioned approaches is the require-
ment of boryl metal species, some of which are not easily
accessible. Thus it will be highly interesting to develop
a method through which the borometalation reaction can be
directly achieved with hydroborane and metal complexes
(Scheme 1c).
Scheme 2. Synthesis and reactivity of complexes 3 and 4.
The slowness of this hydroboration reaction allowed us to
investigate the FLP reactivity of 1 and DABCO against
terminal alkynes. When 1 was added to the mixture of the
alkyne 2a or 2b and DABCO in the hexane solution at 08C,
a white precipitate was immediately observed. After 3 h, the
precipitate was isolated in 75% (from 2a) or 72% (from 2b)
yield, and subsequently identified as ammonium alkynylhy-
dridoborate salts 4a and 4b resulting from the deprotonation
of the corresponding alkynes (Scheme 2).[9,10] In the 11B NMR
spectra, a doublet (À21.6 ppm with 85 Hz for 4a and
À22.1 ppm with 82 Hz for 4b) was observed, indicating the
existence of the hydridoborate moiety. Both elemental
analysis and 1H NMR spectrum suggest that each ammonium
alkynylhydridoborate salt molecule contains one half of
a neutral DABCO molecule. The salt 4b was further
characterized by X-ray diffraction. In each unit cell, there
are two ammonium alkynylhydridoborate salt molecules and
Recently, the chemistry of frustrated Lewis pairs (FLP)
has provided a new avenue for small-molecule activation.[7,8]
[*] H. Ye, Z. Lu, D. You, Dr. Z. Chen, Prof. Z. H. Li, Prof. H. Wang
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Material
Department of Chemistry, Fudan University
Shanghai, 200433 (China)
E-mail: huadongwang@fudan.edu.cn
[**] Financial support from Shanghai Science and Technology Com-
mittee (10ZR1404200), Shanghai Leading Academic Discipline
Project (B108), the National Nature Science Foundation of China
(21102018, 20973041) and National Basic Research Program of
China (2011CB808505) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206927.
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ꢀ
one neutral DABCO molecule, which was linked to an
ammonium cation through N···H···N hydrogen bonds.
order of C C bond decreases to 2.34. We also calculated the
free-energy barrier for the direct 1,2-hydroboration reaction
between 1 and phenylacetylene (DG° = 18.2 kcalmolÀ1),
which is 2.5 kcalmolÀ1 higher than that of the FLP-induced
deprotonation process, which is consistent with the exper-
imental observation that 4a is the major product in the
presence of DABCO. The transformation of 4a to 3a is via
transition state TS3, and the calculated activation enthalpy
(DH° = 25.2 kcalmolÀ1) is very close to the experimental
value. The large deviation of the calculated activation entropy
(DS° = À2.3 calKÀ1 molÀ1) from the experimental value is
possibly due to the fact that the half neutral DABCO
molecule in 4a was omitted in the calculation process to
simplify the calculation. Examining the transition state TS3
reveals that the proton transfer and hydride migration take
place in a concerted manner. The computed KIE value of 1.60
is in good agreement with the measured KIE value.
Heating the solution of 4a or 4b in C6D6 to 558C
prompted the conversion of these ammonium alkynylhydri-
doborate salts into alkenylborane 3a or 3b and DABCO
(Scheme 2). The thermolysis of the ammonium alkynyldeu-
terioborate salt, prepared from DBArF /DABCO and phenyl-
2
F
2
=
acetylene, led to the formation of E-Ph(H)C C(D)BAr ,
which suggests that the thermolysis process occurs through
1,2-hydride migration and proton transfer, thus resulting in
a formal 1,1-hydroboration.[12] While Wrackmeyer et al. have
shown that 1,1-hydroboration can take place for activated
alkynes (EC CR’, E = SiR3, SnR3),[13,14] 1,1-hydroboration for
ꢀ
terminal alkynes is extremely rare. The only other case we
noticed is reported by Paetzold et al,[15] who claimed the
observation of 1,1-hydroboration of terminal alkynes with 6-
aza-nido-decarboranes.[16]
Kinetic studies of the thermolysis of 4a in C6D6 by
monitoring 19F NMR revealed that this reaction is a first-
order reaction. The activation parameters DH° and DS° are
estimated to be 25(1) kcalmolÀ1 and 14(1) calKÀ1 molÀ1. The
positive value of DS° is consistent with a dissociation process.
To further characterize the reaction mechanism of this
process, a kinetic isotope effect (KIE) was obtained by
Noticing that AuI species are often considered to be
isolobal to a proton,[21] we wondered whether AuI could be
applied as an electrophile to attack the triple bond of 4a or
4b, which would lead to the formation of (2-borylalkenyl)gold
complexes.[22,23] When one equivalent of PPh3AuCl was added
to 4a in CD2Cl2 solution at room temperature, the reaction
mixture turned to bright yellow instantly and both starting
materials were consumed within 30 min, affording the (Z)-(2-
borylalkenyl)gold complex 5a as the only isomer in nearly
quantitative yield, as indicated by NMR spectroscopy. Com-
plex 5a was fully characterized by NMR spectroscopy and
elemental analysis. In the 1H NMR spectrum of 5a, the signals
for ArF appeared as three broad singlets with an integral ratio
of 1:2:1, indicating hindered rotation of ArF substituents at
room temperature. The 11B NMR resonance appeared at
60.4 ppm for 5a, which is comparable to the alkenylborane
measuring the rate constants of the thermolysis of 4a and
F
ꢀ
[PhC CB(D)Ar ][HDABCO][0.5DABCO]. The KIE value
2
at 558C is 1.49(7), which falls within the range for a secondary
kinetic isotope effect,[17] and rules out the hydride migration
as the rate limiting step.
The reaction mechanism was further investigated by DFT
(M06-2X) calculations (Figure 1).[18,19] The calculation of the
À
species 3a (d = 65.1 ppm), implying no significant Au B
interaction.[24] This was further confirmed by the X-ray
diffraction analysis of 5a (Figure 2). In the solid structure of
5a, the distance between Au and B atoms is 3.55 ꢀ, which is
close to the sum of van der Waals radii of gold (1.66 ꢀ) and
boron atoms (1.92 ꢀ). Owing to the considerable steric
congestion, both C2-C1-Au (126.9(4)8) and C1-C2-B
(127.3(5)8) angles are deviated from the ideal 1208. Our
preliminary theoretical studies suggested that upon mixing 4a
with PPh3AuCl, an intermediate gold–alkyne p adduct is
formed,[25] which then undergoes concerted 1,2-hydride
À
migration and Au C s bond formation to yield 5a
(Scheme 3). This concerted process could be responsible for
the observation that only the cis addition product exists in the
reaction mixture despite considerable steric repulsion
between the gold and boryl moieties. The free-energy barrier
of this auration process is 18.0 kcalmolÀ1 at 298 K, which is
7.9 kcalmolÀ1 lower than the thermolysis of 4a, in agreement
with our observation that the former process is much faster.
Figure 1. The Gibbs free energy profile at 328 K for the reaction
between 1/DABCO and phenylacetylene. The CF3 groups and the
À
ꢀ À
hydrogen atoms except B H and C C H are omitted for clarity.
reaction between phenylacetylene and 1/DABCO indicates
that 1 first coordinates to phenylacetylene to form the
intermediate IM1 through weak unsymmetrical p interaction,
which is then depronated by DABCO to yield 4a via
transition state TS2.[10] In the simulated structure of IM1,
the distances between the boron atom and two alkynyl
carbons are 1.80 ꢀ (for terminal carbon) and 2.41 ꢀ (for
internal carbon). The corresponding bond orders are 0.55 and
0.16 as suggested by natural bond orbital analysis.[20] The bond
Scheme 3. Synthesis of 5a from 4a.
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Scheme 5. One-pot synthesis of the trisubstituted alkene 6.
quent auration reaction. As these two reactions can occur in
a tandem fashion, (Z)-(2-borylalkenyl)gold complexes can be
straightforwardly prepared from alkynes, hydroborane,
PPh3AuCl, and organic base, thus circumventing the necessity
of transition-metal catalysts or boryl metal complexes. Our
preliminary tests show that these (Z)-(2-borylalkenyl)gold
complexes could be useful building blocks for the stereose-
lective synthesis of trisubstituted alkenes. Efforts to extend
the scope of metal complexes applied in this new type of
borylation reaction are currently underway.
Received: August 27, 2012
Published online: October 18, 2012
Figure 2. ORTEP of the molecular structure of 5a (ellipsoids set at the
30% probability). Hydrogen atoms are omitted for clarity.
Keywords: alkynes · boron · gold · metalation
.
The swiftness of this reaction prompted us to design
a tandem reaction through which boroauration of terminal
alkynes can be achieved directly with a hydroborane and
PPh3AuCl in the presence of DABCO. Addition of 1 into the
mixture of phenylacetylene, DABCO, and PPh3AuCl in
toluene/CH2Cl2 solution at 08C led to the formation of 5a,
which can be isolated with 74% yield (Scheme 4). Complex
5b can be prepared in an analogous fashion in 70% yield. To
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