Synthesis of cyclic amine boranes through triazole-Gold(I)-catalyzed alkyne hydroboration

Article abstract of DOI:10.1002/anie.201402614

The first catalytic alkyne hydroboration of propargyl amine boranecarbonitriles is accomplished with triazole-Au^I complexes. While the typical [L-Au]⁺ species decomposes within minutes upon addition of amine boranecarbonitriles, the

Full text of DOI:10.1002/anie.201402614

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Angewandte
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DOI: 10.1002/anie.201402614
Homogeneous Catalysis
Synthesis of Cyclic Amine Boranes through Triazole-Gold(I)-
Catalyzed Alkyne Hydroboration**
Qiaoyi Wang, Stephen E. Motika, Novruz G. Akhmedov, Jeffrey L. Petersen, and Xiaodong Shi*
Abstract: The first catalytic alkyne hydroboration of prop-
argyl amine boranecarbonitriles is accomplished with triazole-
AuI complexes. While the typical [L-Au]+ species decomposes
within minutes upon addition of amine boranecarbonitriles, the
triazole-modified gold catalysts (TA-Au) remained active, and
allowed the synthesis of 1,2-BN-cyclopentenes in one step with
good to excellent yields. With good substrate tolerability and
mild reaction conditions (open-flask), this new method
provides an alternative route to reach the interesting cyclic
amine borane with high efficiency.
À
T
he isoelectronic and isosteric relationship between B N
=
and C C units initiated great interest in 1,2-azaborine
research during the last several decades.[1] Despite the
apparent similarity of these two units, there is relatively
little known about the BN-containing heterocycles, which
play a pivotal role for this class of BN-moiety-containing
molecules.[2] Because of their broad applications in chemical,
material, and energy storage research,[3] development of
efficient methodologies for the synthesis of compounds or
structures with BN moieties has become a topic of interest.[4]
The groups of Ashe,[5] Liu,[6] and Piers[7] have recently
contributed to the preparation of the novel BN heterocycles
based on a breakthrough strategy involving ring-closing-
metathesis (Scheme 1A). In addition, Vedejs and co-workers
Scheme 1. Synthesis of amine borane heterocycle.
À
reported the formation of a labile B I bond or a trivalent
borenium ion as a superelectrophile[8] to facilitate the desired
carbon substitution to access the BN heterocycle structure
(Scheme 1B). Despite these advances, most of the reported
methods involved air- and moisture-sensitive starting materi-
als or intermediates, which limited the synthetic applications.
Thus, new strategies with high efficiency and good substrate
tolerance are highly desirable. Herein, we report the triazole-
gold complex catalyzed intramolecular hydroboration of
alkynes (Scheme 1C) as a new strategy for the preparation
of cyclic amine boranes, in particular, 1,2-BN-cyclopentenes.
This new transformation occurs under open-flask conditions
with no requirement for solvent pre-treatment, and thus
Scheme 2. Amine-directed alkyne hydroboration.
presents a promising future as a general protocol in cyclic
amine borane synthesis.
The amine-directed alkene/alkyne hydroboration could
be a highly efficient approach for the preparation of cyclic
amine borane derivatives (Scheme 2). Alkene hydroboration
was used in cyclization transformations nearly a half century
before, whereas, an alkyne hydroboration strategy has never
been previously reported for cyclic amine borane synthesis.[9]
In general, amine-directed alkene hydroboration requires
harsh conditions (high temperature) because of the reduced
reactivity of boranes. With the application of I2 as a catalyst/
promoter (10% to 1 equiv), Vedejs and co-workers reported
a successful alkene hydroboration performed at room tem-
[*] Q. Wang, S. E. Motika, Dr. N. G. Akhmedov, Prof. J. L. Petersen,
Prof. X. Shi
C. Eugene Bennett Department of Chemistry
West Virginia University
Morgantown, WV 26506 (USA)
E-mail: xiaodong.shi@mail.wvu.edu
[**] We thank NSF (CHE-0844602, CHE-1228336) and NSFC (21228204)
for the financial supports.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402614.
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perature.[10] Although cyclic B N compounds have been
which is clearly formed during the reaction process). To
À
isolated in some cases, the yield (< 50%) is moderate, and is
overcome this problem, we put our efforts into the develop-
ment of less reactive amine boranes with the aim of slowing
down the decomposition process of the gold catalysts. After
extensive screening of various substrates, the cyano-substi-
tuted amine borane 3 was identified and accessible through
the general synthetic route summarized in Scheme 3.
3
[11]
À
likely due to the poor stability of the C(sp ) B bond. This
limitation decreased the potential of using this method for the
synthesis of cyclic amine boranes.[10d] An amine-directed
alkyne hydroboration will result in the formation of more
2
[11]
À
stable vinyl boranes [C(sp ) B],
and could be more
efficient for cyclic amine borane synthesis. However, to the
best of our knowledge, this transformation has never been
reported. In fact, because of the reduced reactivity of the
ꢀ
alkyne, treatment of the propargyl amine borane 1 (HC
CCH2NH2-BH3) with I2 (2 equiv, Vedejs protocol) gave no
alkyne addition product after 24 hours.[12] The major chal-
lenges associated with this transformation are: 1) low reac-
tivity of the alkyne; 2) the possibility of double addition
rather than desired monohydroboration.
Although transition metal catalyzed hydroboration suf-
fers from catalyst decomposition, successful examples have
been reported.[13] To avoid the undesired catalyst decompo-
sition (caused by borane reduction), almost all of the reported
cases used metal cations which were able to form a metal
hydride (M-H) bond. Meanwhile, the M-H cis addition to the
alkyne led to the formation of the trans isomer, which is not
Scheme 3. Synthesis of the amine boranecarbonitrile 3.
Under the optimal reaction conditions, 3 was prepared in
good to excellent yields (generally > 85%; see the Supporting
Information). This method tolerated various substituents at
the alkyne terminus. Both primary and aliphatic-substituted
À
suitable for the C B bond cyclization (through M-B trans-
À
metallation, Scheme 2B). Thus, transition metal catalyzed
alkyne hydroboration for the synthesis of cyclic amine borane
has not been achieved in the past.
secondary propargyl amines worked well for the B N
formation. The aromatic-substituted propargyl amine (R2 =
Ar) could not give the corresponding complex 3 under the
optimal reaction conditions, which is likely a consequence of
the decreased basicity of the nitrogen atom. Nevertheless, this
method allowed general access to the cyano-stabilized amine
During the past decade, gold cations have been identified
as one of the most effective p acids for alkyne activation.[14]
Based on this activation mode, we hypothesized that cyclic
amine boranes could be synthesized through the intramolec-
ular alkyne hydroboration as shown in Scheme 2C, if an
appropriate gold catalyst (that tolerates borane substrates)
and amine borane precursor could be identified. Two practical
concerns are: 1) the stability of active gold catalysts under the
reductive conditions; 2) the feasibility of proposed Au–B
transmetallation.
Our group has focused on the development of 1,2,3-
triazole-coordinated metal complexes as new catalysts for
challenging chemical transformations.[15] While interesting
reactivity has been observed with the triazole/gold (TA-Au)
complexes, the main advantage of this catalyst is the
significantly improved stability.[16] Thus, we postulated that
the TA-Au catalysts would provide the required p-acid
reactivity toward alkyne activation while maintaining stability
towards the borane, and thus afford a new and efficient
synthesis of cyclic amine boranes under mild reaction
conditions.
To test our hypothesis, various gold and TA-Au catalysts,
[L-Au/TA]+XÀ (L = PPh3, IPr, XPhos; XÀ = TfOÀ, F6SbÀ),
were tested in the hydroboration of 1 (see structure of 1 in
Ref. [12]). Unfortunately, significant gold decomposition
(formation of gold nanoparticle or gold mirror) occurred,
even with the TA-Au catalysts. Corma and co-workers have
recently reported the highly efficient alkyne hydration with
small gold clusters.[17] Their work suggested the in situ formed
Au0 cluster was the active catalyst. Unfortunately, no reaction
occurred after this reduction process by 1 (presumably
because of the formation of catalytically inactive Au0 species,
À
borane containing a less reactive borohydride (B H).
With this new amine borane 3 in hand, we investigated its
reactivity toward gold catalysts. Notably, compared to the
terminal alkyne, the internal alkyne is usually much less
reactive toward p-acid catalysts. Therefore, to better validate
the feasibility of this gold catalyzed intramolecular hydro-
boration, 3b was selected for our initial screening. The results
are summarized in Table 1. Unlike 1 (which caused rapid gold
decomposition), the 3b gave the desired cyclic amine borane
4b using the PPh3 AuCl/AgOTf catalyst, albeit with very low
yield (17%; entry 2). The structure of 4b was determined by
X-ray crystallography. Notably, the reaction gave 3:1 (cis/
trans) isomer mixtures in solution. However, upon crystal-
lization, all of the trans isomer was converted into the
cis isomer. This process has been confirmed by NMR
spectroscopy: the cis isomer was dissolved and a slow equi-
librium was observed (up to 3 h), thus giving a 3:1 cis/trans
mixture. Slowly evaporation of the solvent gave the pure
cis isomer again (structure confirmed by X-ray). Overall, the
success in obtaining 4b greatly supported our hypothesis that
the gold-catalyzed (p-acid) alkyne activation is a feasible
approach for the preparation of cyclic amine boranes. To
improve the reaction performance, we screened other gold
catalysts. As shown in entries 1–4, although 3b was a weaker
reductant, the reaction still suffered from gold decomposition.
Interestingly, the TA/Au catalysts, though giving no reaction,
indicated better tolerance of 3b given the decreased rate of
gold mirror formation. To verify this result, we monitored the
catalyst decomposition (upon treatment with 3b) using
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Table 1: Reaction condition screening.[a]
was isolated in excellent yield (93%; Table 1, entry 9).
Solvent screening revealed DCE as the optimal choice.
Other factors, such as counter anions (SbF6 or Tf2NÀ) and
À
MeCN-stabilized [L-Au]+,[18] have also been screened. The
triazole/gold complex gave the best result because of the
superior stability under the reductive conditions (see the
Supporting Information).
Entry Catalyst
Solvent
t
T
Conv.
Yield
[%]
[h] [8C] [%]
With this optimized reaction conditions in hand, we
explored the reaction substrate scope. The results are
summarized in Figure 1. As shown, this method tolerates
a large substrate scope. First, both terminal and internal
alkynes were suitable for this transformation. The more
reactive terminal alkynes gave excellent yields (4h and 4i).
Most of the tested internal alkynes also worked well under
these reaction conditions. The alkyne terminal (R1) can be
alkyl, aryl, cyclopropanyl, and even alkenyl (from enyne). The
aliphatic internal alkynes were less reactive, thus giving
slightly lower yields (4e and 4 f) compared to the aromatic
internal alkynes. Alkynes substituted with electron-donating
groups (3j to 4j) reacted slowly under the reaction conditions
(even with an extended reaction period, 4 days). The reaction
was quenched upon the observed decomposition of the
starting material 3j. The overall yield was moderate when
compared with the obtained with the electron-deficient
substrate 3k. No ring-opening product was observed in the
cyclopropane-modified alkyne (4g), thus excluding single-
electron process mechanism in this transformation. Both
primary and secondary propargyl amines were tolerated, thus
giving cyclic amine borane products as a mixture of cis and
trans isomers with different ratios (4b, 4 f, and 4i). The overall
reactivity of these two types of amines was similar. Substitu-
tion at the propargyl position (R3) had little influence on the
reactivity, thus giving 4c in excellent yield. Overall, the good
functional-group tolerance at alkyne terminus (R1), the
propargyl position (R3), and amine (R2) highlights the
potential of this method for the preparation of cyclic amine
boranes with broad substrate scope.
1
2
3
4
5
6
7
8
PPh3AuCl
PPh3AuCl/AgOTf
IPrAuCl/AgOTf
XPhosAuCl/AgOTf CH2Cl2 24 RT <5
[PPh3Au-TA]+TfOÀ
CH2Cl2 72 RT <5
[XPhosAu-TA]+TfOÀ CH2Cl2 72 RT <5
CH2Cl2 72 RT <5
CH2Cl2 0.5 RT 19
CH2Cl2 0.5 RT 15
n.d.
17
11
n.d.
n.d.
n.d.
n.d.
24
93
73
63
36
PPh3AuCl
DCE
DCE
72 80 <5
24 80 35
72 80 >99
72 65 85
72 80 84
[PPh3Au-TA]+TfOÀ
9
[XPhosAu-TA]+TfOÀ DCE
[XPhosAu-TA]+TfOÀ THF
[XPhosAu-TA]+TfOÀ DMF
10
11
12
13
[XPhosAu-TA]+TfOÀ toluene 72 90 39
[XPhosAu-TA]+TfOÀ MeCN 72 80 88
65
[a] All conversions and yields determined by NMR spectroscopy with
p-xylene as an internal standard. n.d.=not determined. DMF=N,N-
dimethylformamide, THF=thetrahydrofuran.
The cyano group on boron was originally introduced to
overcome the undesired gold catalyst decomposition. Thus, to
further extend this methodology as a general protocol for
potential cyclic amine borane synthesis, we investigated the
post-cyclization derivatization with the expectation of intro-
ducing diverse functional groups on boron. Two representa-
tive transformations on the boron position are summarized in
Scheme 4. First, treatment of the cyclic amine borane 4a/4b
with LiAlH4 gave the corresponding reduction product 5a/5b
in good to excellent yields (Scheme 4A).[19] Notably, the
Figure 1. Reaction substrate scope: conversion, yield, and d.r. values
were determined by NMR spectroscopy. Gram-scale synthesis of 4a
was conducted and product was isolated in 85% yield.
31P NMR spectroscopy (see S-Figure 1 in the Supporting
Information).
As shown in the 31P NMR data, treating [L-Au]+ species
with 3b caused immediate decomposition, even with XPhos
as the primary ligand. Interestingly, with the 1,2,3-triazole-
modified gold complexes, much slower decomposition was
observed. The bulky XPhos ligand further improved the gold
cation stability, thus giving little decomposition even after
20 hours at room temperature. With this improved catalyst
stability, we further evaluated the reaction conditions at
elevated temperature. To our great delight, with the XPhos-
based TA-Au catalyst at 808C, all 3b was consumed and 4b
Scheme 4. Introducing functional groups at boron position.
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nonsubstituted amine borane 5a was not stable to air. Thus,
excellent yields (91%) were observed by NMR spectroscopy,
but only 73% product was isolated. This product loss was
probably because of the loss of H2 and sequential decom-
position. Nevertheless, considering the fact that the BH3-
based amine borane 1 (see Ref. [12]) caused rapid gold
decomposition and was not suitable for this transformation at
all, this cyano borane modification provided an alternative
solution to extend the reaction scope. Moreover, reaction of
4a with 3 equivalents of PhMgBr gave the corresponding
phenyl substituted cyclic amine borane 6, the structure of
which was confirmed by X-ray crystallography. The two
strategies in Scheme 4 resulted in ready functionalization of
boron, and highlights the scope of cyclic amine borane
synthesis facilitated by TA-Au catalysis.[20]
Overall, we have reported a novel strategy for highly
efficient preparation of cyclic amine boranes. With the newly
developed cyano-modified amine borane and stable XPhos-
based TA-Au catalyst, the challenging alkyne hydroboration
was achieved successfully. The overall transformation oper-
ates under open-flask conditions. The excellent substrate
tolerance and readily available derivatization methods make
this transformation an efficient approach for cyclic amine
borane synthesis. Preparation of other BN/CC isosteric
derivatives (such as benzene and indole) and use of the
resulting compounds in H2 storage and catalysis are currently
under investigation in our lab and will be reported in due
course.
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Experimental Section
4a: [XPhosAu(TA)]+TfOÀ (9.3 mg, 5 mol%) was added to a solution
of 3a (0.2 mmol, 1.0 equiv) in 1,2-dichloroethane (5 mL, 0.04m) at
room temperature. The reaction mixture was stirred at 808C and
monitored by TLC. Upon completion, solvent was removed under
reduced pressure and the residue was purified by flash chromatog-
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For explicit experimental data, including spectroscopic data, see
the Supporting Information.
Received: February 20, 2014
Published online: April 9, 2014
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Keywords: alkynes · cyclization · boranes · gold ·
synthetic methods
.
À
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À
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[20] The homopropargyl amine derivatives (for the synthesis of
azaborine) have been briefly tested and poor yield (< 20%) was
observed under identical reaction conditions. Further optimiza-
tion is currently under investigation and will be reported in due
course.
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