Alkynylboranes: A practical approach by zinc-catalyzed dehydrogenative coupling of terminal alkynes with 1,8-naphthalenediaminatoborane

Article abstract of DOI:10.1002/adsc.201400767

Under zinc Lewis acid catalysis, terminal alkynes coupled dehydrogenatively with 1,8-naphthalenediaminatoborane [HB(dan)]. It is important to note that the resulting alkynylboranes with an C(sp) - B(dan) bond are isolable by column chromatography on silica gel (SiO₂) and are usable as coupling partners for palladium- and copper-catalyzed cross-coupling reactions with (hetero)aryl halides.

Full text of DOI:10.1002/adsc.201400767

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DOI: 10.1002/adsc.201400767
Alkynylboranes: A Practical Approach by Zinc-Catalyzed
Dehydrogenative Coupling of Terminal Alkynes with
1,8-Naphthalenediaminatoborane
a,
a
a
a
Teruhisa Tsuchimoto, * Hirokazu Utsugi, Tetsuya Sugiura, and Shin Horio
a
Department of Applied Chemistry, School of Science and Technology, Meiji University, 1-1-1 Higashimita, Tama-ku,
Kawasaki 214-8571, Japan
Fax : (+81)-44-934-7228; e-mail: tsuchimo@meiji.ac.jp
Received: August 4, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400767.
Abstract: Under zinc Lewis acid catalysis, terminal
alkynes coupled dehydrogenatively with 1,8-naph-
thalenediaminatoborane [HB(dan)]. It is important
to note that the resulting alkynylboranes with an
C(sp)ÀB(dan) bond are isolable by column chroma-
transition metals should be due to their ability to acti-
vate the inert CÀH bond efficiently. Distinctly differ-
ent from the preceding studies, we have found that
a Lewis acid is able to participate as a catalyst in this
category. We thus report on the Lewis acid-catalyzed
dehydrogenative borylation, where the zinc–pyridine
system exhibits remarkable catalytic performance to
promote the coupling of terminal alkynes with 1,8-
tography on silica gel (SiO ) and are usable as cou-
2
pling partners for palladium- and copper-catalyzed
cross-coupling reactions with (hetero)aryl halides.
[
10,11]
naphthalenediaminatoborane [HB(dan)].
Keywords: boron; catalysis; dehydrogenation;
Lewis acids; multiple bonds
Upon preparing alkynylboranes, despite a low func-
tional group tolerance, chemists have mainly relied on
Brownꢀs method by treating alkynyllithiums with tri-
alkyl borates, followed by the addition of HCl/
[
5b,12]
Et O.
The lately reported iridium system seems to
have made significant progress on the functional
Due to their broad spectrum of applications in syn- group compatibility, while requiring multiple steps to
2
[
9]
thetic, material, biological, and medicinal chemistry, prepare an iridium pre-catalyst. We therefore envi-
organoboron compounds have permeated deeply into sioned that the development of a novel C(sp)ÀH
[
1]
the field of chemical science. Such a great impor- bond dehydrogenative borylation with simplicity and
tance has consistently stimulated the development of practicality would satisfy the needs of a variety of sit-
new synthetic routes to obtain target organoboron uations requiring alkynylboranes. On the basis of this
compounds more easily and efficiently. Among them, notion, we examined the dehydrogenative borylation
the dehydrogenative coupling of BÀH and HÀC of 1-octyne (1a) with a series of hydroboranes 2
bonds has seen considerable success in the past 15 (Scheme 1). Treating 1a and pinacolborane [2a,
[2,3,4]
years.
This would be due to the high availability HB(pin)] with 5 mol% of Zn(OTf)2 (Tf=SO CF )
2 3
of the two substrates. The high atom-economy of the with the aid of pyridine (20 mol%) in EtCN at 1008C
process is also an important advantage in terms of re- for 20 h, however, gave no coupling product 3aa. Re-
quiring no pre-activation of the CÀH bond, and thus placing 2a with 2b and 2c provided no improvements.
producing no particular waste, in contrast to the two To address why, for instance, 2a is useless, a control
common methods. One of these is the reaction of experiment was conducted (Scheme 1). When 3aa
[
12b]
main group organometallic reagents with trialkyl bo- prepared by the authentic method
was thus sub-
[5]
rates; the other should be the coupling of organic jected to the reaction conditions, 1a was formed in
halides with mono- or diboron reagents under palladi- 96% GC yield, and no 3aa was recovered, showing
[6,7]
um catalysis. To date, transition metal complexes of that 3aa is unable to survive under the reaction condi-
Co, Ru, Rh, Pd, and Ir have had a monopoly on cata- tions. The protodeborylation of 3aa might be ascribed
2
lyzing the dehydrogenative borylation of C(sp )ÀH to the influence of water, possibly present in a small
3
[2,8]
and C(sp )ÀH bonds.
This holds true for the dehy- amount in the reaction solution, whereas EtCN dis-
drogenative borylation of an alkynyl CÀH bond, final- tilled from P O was used. The incompatibility of 2b
2
5
[
9]
ly achieved just recently. Such a dependence on the and 2c might be due to the same reason. In order to
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Table 1. Lewis acid-catalyzed dehydrogenative borylation of
[a]
1
-octyne with HB(dan).
[b]
Entry Lewis acid Organic base Solvent
Yield [%]
1
2
3
4
5
6
7
8
Cu(OTf)2
AgOTf
In(OTf)3
Bi(OTf)3
Sc(OTf)3
pyridine
pyridine
pyridine
pyridine
pyridine
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
<1
<1
<1
<1
<1
76
80
18
63
29
Zn(NTf2)2[ pyridine
c]
Zn(ONf)2 pyridine
Zn(OAc)2 pyridine
9
ZnF2
ZnCl
pyridine
pyridine
quinoline
Scheme 1. Zinc-catalyzed dehydrogenative borylation of 1-
10
11
12
1
1
1
1
1
1
1
2
octyne, and the control experiment to ascertain the stability
Zn(OTf)
Zn(OTf)
2
2
45
61
55
27
1
[d]
of 3aa. Yields determined by H NMR are shown here.
DMAP
3
4
5
6
7
8
9
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
none
Et N
DBU
none
3
[e]
18
remedy this situation, we focused on the B(dan)
group that was first introduced by Suginomeꢀs team
as a masked boryl group, capable of offering a stable
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
1,4-dioxane 33
Bu O
2
2
PhCl
68
58
40
<1
2
C(sp )ÀB bond, even under the conditions for the
PhMe
EtCN
EtCN
[
13]
[f]
Suzuki–Miyaura cross-coupling. To our delight, use 20
of HB(dan) (2d) as the boron source provided the de- 21
sired octynylborane (3ad) in 87% yield. No alkenyl-
[a]
Reagents: 1a (0.40 mmol), 2d (0.60 mmol), Lewis acid
20 mmol), base (80 mmol), solvent (0.40 mL).
Determined by H NMR.
and/or alkylboranes derived from hydroboration of 1a
with 2d were present as contaminants in 3ad. Of im-
portance to note here is that inexpensive Zn(OTf)2
and pyridine are available as the catalyst system.
Inspired by the above results, we studied the effects
of changing the reaction conditions (Table 1). In stark
contrast to the outstanding performance of Zn(OTf)2,
other triflate salts were totally inactive (entries 1–5).
(
[b]
[c]
1
Nf=SO C F .
2
4 9
[
[
[
d]
e]
f]
DMAP=4-(dimethylamino)pyridine.
DBU=1,8-diazabicyclo[5.4.0]undec-7-ene.
Zn(OTf) (8.0 mmol) was used.
2
Zinc salts other than Zn(OTf) also catalyzed the de- ture (see Scheme 3). A continuous survey of the sol-
2
hydrogenative borylation, but were less effective (en- vent effect showed that the choice of EtCN is suitable
tries 6–10). These results indicate that the use of zinc (entries 16–19). Lowering the loading of Zn(OTf) to
2
Lewis acids is crucial for the progress of the dehydro- 2 mol% made the dehydrogenative borylation much
genative borylation, and an anionic ligand with strong slower, and no dehydrogenative borylation was ob-
electron-withdrawing character is likely to be necessa- served without Zn(OTf) . (entries 20 and 21).
2
ry on zinc(II), possibly to impart strong Lewis acidity
With the promising reaction conditions in hand, we
to the zinc salt. We then found that adding an organic explored the substrate scope of the dehydrogenative
base is important because the absence of a base led to borylation (Table 2). In addition to 1a, a range of ali-
the much lower conversion of 1a, and that pyridine is phatic terminal alkynes 1 with branched or functional-
the most reliable (entries 11–15). These observations ized alkyl groups were successfully borylated in a de-
may suggest that the electronic and steric properties hydrogenative fashion (3ad–3id). Importantly, where-
of the organic bases have a significant influence on as both of the C(sp)H and OH groups underwent bor-
the reaction rate [the pK value of each base: quino- ylation when using 3-butyn-1-ol (1g), silica gel column
a
line (4.9), pyridine (5.3), DMAP (9.7), Et N (11.0), chromatography for purification caused protodebory-
3
[14]
DBU (11.9)]. Thus, a stronger base than pyridine lation of only the OB(dan) part to give 3gd in 86%
might impair the catalytic activity of the zinc Lewis yield. A series of aryl- and heteroarylacetylenes with
acid, possibly by strong coordination to the zinc different electronic and steric natures participated
center. Quinoline, which is slightly less basic and, in well in this strategy (3jd–3td); for instance, all of the
addition, is more bulky compared to pyridine, could regioisomeric ethynyltoluenes coupled with 2d, pro-
be disadvantageous in forming a transition state struc- viding 3kd–3md in high yields. The C=C moiety of 1-
2
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Alkynylboranes: A Practical Approach
Table 2. Zinc–pyridine-catalyzed dehydrogenative borylation
[a]
of terminal alkynes with HB(dan).
Scheme 2. Synthetic applications of RCꢀCB(dan). Isolated
yields of products are shown here. HetAr=heteroaryl. Ac=
acetyl. SPhos=2-dicyclohexylphosphino-2’,6’-dimethoxybi-
phenyl. Ar=aryl. DMI=1,3-dimethyl-2-imidazolidinone.
Reaction conditions: [a] 10% Pd/C (0.7 mol% on Pd), H2
(
balloon), 1,4-dioxane, room temperature, 80 min; [b] pina-
col (3.0 equiv.), 6M aqueous HCl (6.0 equiv.), 1,4-dioxane,
room temperature, 12 h; [c] BrÀHetAr (0.67 equiv.),
Pd(OAc) (5 mol%), SPhos (10 mol%), K CO (3.0 equiv.),
2
2
3
toluene, 1008C, 5 h for 5ja or 7 h for 5jb; [d] IÀAr
(
0.83 equiv.), CuCl (10 mol%), PPh (10 mol%), K CO
3 2 3
(
1.0 equiv.), DMI, 1208C, 12 h.
ment of 5 mmol of 1a or phenylacetylene (1j) with 2d
1.2 equiv.) under the standard reaction conditions
furnished 1.18 g (84% yield) of 3ad or 1.19 g (86%
(
[
16]
yield) of 3jd, respectively. The achievement of scal-
able synthesis, which will be important as a synthetic
reaction, would enhance the practicality of our
method. An additional feature of this reaction is that
products, RCꢀCB(dan) 3, can be safely purified on
silica gel columns without any special precautions.
This means the following two things: (i) using
B(OH) -impregnated SiO to suppress confounded
3
2
[
a]
over-adsorption to silica gel in the purification process
20 mmol), pyridine (80 mmol), EtCN (0.40 mL). Yields of of common organoboron compounds is unnecessa-
Reagents:
(
1 (0.40 mmol), 2d (0.60 mmol), Zn(OTf)2
[
17,18]
isolated 3 based on 1 are shown here. TBDMS=t- ry;
(ii) RCꢀCB(dan) molecules are stable enough
BuMe Si.
to silica gel in contrast to likely the most commonly
2
[
[
b]
c]
2
d (1.2 mmol) was used.
used alkynylboranes, RCꢀCB(pin), the isolation of
4-Methoxypyridine instead of pyridine was used.
[19]
which has only been achieved by distillation.
Ac-
cordingly, our method can thus be applied to prepare
RCꢀCB(dan) with high molecular weight without
ethynylcyclohexene was also retained without suffer- regard to their boiling points.
ing hydroboration (3ud). Metalloborylacetylene 3vd,
which could be potentially useful as a key unit for op- been reported in the literature are restricted to the
Because RCꢀCB(dan) molecules that have ever
[
20]
toelectronic materials, was obtained in a high yield. case where R is hydrogen, there is little information
Borylsilylacetylene 3wd, which might be a platform to concerning their utility in organic synthesis. We there-
synthesize mutli-substituted alkenes, can be adopted fore examined potential applications of RCꢀCB(dan)
[
15]
as a target structure. As shown thus far, the com-
3
as
a
virtually new class of alkynylboranes
patibility of the functional groups, Cl, OCOMe, (Scheme 2). As the opening application, we prepared
phthalimidoyl, OH, OCH CꢀC, TBDMSO, CF , Br, alkylboranes, which are also useful in synthetic and
2
3
2
[21]
C(sp )ÀB(pin), pyridyl, C=C, ferrocenyl, is notewor- medicinal chemistry. Thus, the palladium-catalyzed
thy. Our unique system also allows us to perform hydrogenation of 3jd proceeded smoothly to provide
preparative scale syntheses; for example, the treat- alkylborane 4 in 95% yield. The ligand exchange of
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Scheme 4. Trapping experiment for evolved hydrogen gas.
ly, electrophilic boron species 7 should react with nu-
cleophilic 1, assisted by pyridine through possible
transition state 10 to make the BÀC bond and to re-
Scheme 3. A possible reaction mechanism.
II
generate Zn and pyridine as well as to release hydro-
3
ad with pinacol could also be performed easily, af- gen gas.
fording 3aa in 90% yield. The B(dan) group connect-
Finally, in order to confirm the evolution of the hy-
2
ed with a C(sp ) atom has now been recognized as drogen gas, a trapping experiment was conducted, as
being a masked boryl group, and thus does not partic- shown in Scheme 4. Thus, the palladium-catalyzed hy-
ipate in carbon–carbon bond-forming reactions under drogenation of trans-stilbene (11) was carried out
the palladium-catalyzed Suzuki–Miyaura cross-cou- with the intention of capturing the hydrogen gas that
[13]
pling conditions. Accordingly, the transformation of would be released from the dehydrogenative boryla-
[
25]
3ad to 3aa should be meaningful to offer active sub- tion of 1a with 2d. As a result, 1,2-diphenylethane
strates for such carbon–carbon bond elongation. (12) was obtained in 97% yield. This result should
However, it was simply surprising that the boryl support the possibility of the reaction mechanism pro-
group of 3jd is directly transformable to the heteroar- posed in Scheme 3.
yl unit under the Suzuki–Miyaura cross-coupling con-
In closing, we have disclosed that a catalyst system
ditions, thereby giving 5ja and 5jb in high yields. consisting of Zn(OTf) and pyridine can be used to
2
Other than the palladium catalysis, we presently find connect terminal alkynes with HB(dan) in a dehydro-
that a copper salt highly effectively catalyzes C(sp)– genative manner. This is the first example of the de-
aryl bond-forming reactions, the efficiency of which is hydrogenative borylation of terminal alkynes cata-
equal to that of the original using RCꢀCB(pin) in- lyzed by a Lewis acid. Our method features a broad
[
22]
stead under the same reaction conditions. The re- range of substrate coverage with a high functional
sults on the cross-coupling reactions show that the re- group tolerance. In terms of the promotion of sustain-
activity of the C(sp)ÀB(dan) unit is significantly dif- able chemistry, the use of a common metal, zinc, for
2
ferent from that of the C(sp )ÀB(dan), and could be the catalyst is also a distinct advantage of our strategy.
roughly similar to that of the C(sp)ÀB(pin). RCꢀ Noteworthy is that the alkynyl moiety of RCꢀ
CB(dan) with sufficient reactivity, but with stability CB(dan) proved to have reactivity to transmetallate
isolable on SiO columns, would thus have a good po- onto Pd(II) and Cu(I) metals, in contrast to the
2
2
tential to be used in organic synthesis.
C(sp )-based organic unit bound to the B(dan) group.
We previously confirmed in the corresponding de- This indicates that RCꢀCB(dan) have a considerable
hydrogenative silylation that the zinc–pyridine system potential as substrates for organic synthesis. Further
in the nitrile medium has no ability to activate the CÀ studies on the mechanism and synthetic applications
H bond of terminal alkynes, but is able to activate the are in progress in our laboratory.
[
10]
HÀSi bond of hydrosilanes. Therefore, the present
dehydrogenative borylation operated by the same
system in the same medium seems likely to start with Experimental Section
activation of the HÀB bond. On the basis of our pre-
ceding study as well as other significant reports, a pos- General Procedure Exemplified by the Synthesis of
sible route is proposed in Scheme 3. The beginning 3ad on a 5-mmol Scale
should thus be activation of the HÀB bond of 2d by
II
Zn(OTf)
2
(90.9 mg, 0.250 mmol) was placed in a 50-mL
Zn(OTf) (Zn ) for enhancing the electrophilicity of
2
Schlenk tube, which was heated at 1508C under vacuum for
h. The tube was cooled down to room temperature and
the boron center (7), thereby possibly giving boron
cation 8 and zinc hydride 9.
pyridine, which remarkably accelerates the reaction
2
[
23,24]
On the other hand,
filled with argon. HB(dan) (2d) (1.01 g, 6.00 mmol) and
EtCN (5.0 mL) were added to the tube, and the resulting
rate, may support a process by enhancing the nucleo- mixture was stirred at room temperature for 3 min. To this
philicity of the terminal carbon atom of 1. According- were added 1-octyne (1a) (551 mg, 5.00 mmol) and pyridine
4
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Alkynylboranes: A Practical Approach
(
79.1 mg, 1.00 mmol) successively, and the resulting solution
r) I. A. Mkhalid, J. H. Barnard, T. B. Marder, J. M.
Murphy, J. F. Hartwig, Chem. Rev. 2010, 110, 890–931.
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also used for the borylation of the CÀH bond, see re-
was then stirred at 1008C for 20 h. A saturated NH Cl aque-
4
ous solution (2 mL) was added to the mixture, and the aque-
ous phase was extracted with EtOAc (20 mLꢂ3). The com-
bined organic layer was washed with brine (3 mL) and then
dried over anhydrous sodium sulfate. Filtration through
a pad of Celite and evaporation of the solvent followed by
column chromatography on silica gel (hexane/EtOAc=10:1)
gave 2,3-dihydro-2-(1-octyn-1-yl)-1H-naphtho[1,8-de]-1,3,2-
diazaborine (3ad); yield: 1.17 g (84%).
[2q,2r]
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given in the Supporting Information.
6
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ÝÝ
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Alkynylboranes: A Practical Approach by Zinc-Catalyzed
Dehydrogenative Coupling of Terminal Alkynes with 1,8-
Naphthalenediaminatoborane
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Adv. Synth. Catal. 2014, 356, 1 – 7
Teruhisa Tsuchimoto,* Hirokazu Utsugi, Tetsuya Sugiura,
Shin Horio
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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ÞÞ
These are not the final page numbers!