Organosulfide-Catalyzed Diboration of Terminal Alkynes under Light

Article abstract of DOI:10.1002/chem.201502425

An efficient metal-free diboration of terminal alkynes is reported. In the presence of a catalytic amount of organosulfides under light, the addition of bis(pinacolato)diboron (B₂pin₂) to terminal alkynes takes place efficiently to produce the corresponding double borylation products in good yields. Mechanistic studies indicate that this metal-free sulfide-catalyzed diboration of alkynes likely occurs by generation of a boryl-centered radical with the aid of light and a sulfide, since such a radical was detected in the reaction mixture by electron spin resonance (ESR) spectroscopy. The present form of catalysis (sulfide/light) is thought to be unprecedented and provides a new means of preparation for organoboranes without heavy metal contamination in the products, which is highly desired in the preparation of drugs and electronic materials.

Full text of DOI:10.1002/chem.201502425

DOI: 10.1002/chem.201502425
Communication
&
Borylation
Organosulfide-Catalyzed Diboration of Terminal Alkynes under
Light
Aya Yoshimura,[a, b] Yuki Takamachi,[a] Li-Biao Han,[b] and Akiya Ogawa*[a]
Abstract: An efficient metal-free diboration of terminal al-
kynes is reported. In the presence of a catalytic amount of
organosulfides under light, the addition of bis(pinacolato)-
diboron (B2pin2) to terminal alkynes takes place efficiently
to produce the corresponding double borylation products
in good yields. Mechanistic studies indicate that this
metal-free sulfide-catalyzed diboration of alkynes likely
problem.[4] However, to our knowledge, metal-free diboration
occurs by generation of a boryl-centered radical with the
of alkynes has not, to date, been realized,[4] except for the
aid of light and a sulfide, since such a radical was detect-
diboration of propargylic alcohols by using equivalent
ed in the reaction mixture by electron spin resonance
amounts of base.[4z]
(ESR) spectroscopy. The present form of catalysis (sulfide/
Herein, we report a novel (PhS)2-catalyzed diboration of
light) is thought to be unprecedented and provides a new
alkynes under photoirradiation [Eq (1)]. Thus, under light
means of preparation for organoboranes without heavy
(tungsten, xenon, or mercury lamp), B2pin2 adds to terminal al-
metal contamination in the products, which is highly de-
kynes to produce the corresponding bisborylalkenes in good
sired in the preparation of drugs and electronic materials.
yields. To our knowledge, this is the first metal-free catalytic
diboration of alkynes.[4]
This catalytic reaction was discovered accidentally during
Organoboron compounds are important chemicals that have
wide applications in organic synthesis.[1] Alkenylboronic acids
are particularly useful precursors for the formation of new
carbon–carbon bonds by Suzuki–Miyaura cross-coupling reac-
tions and conjugated addition reactions.[1a] The addition of
easily handled bis(pinacolato)diboron (B2pin2) to alkynes
(diboration) is among the most straightforward ways for their
preparation and has been extensively studied.[2] However, since
B2pin2 alone is generally inactive, the aid of a transition metal
catalyst is usually required for these additions [Eq (1)]; in 1993,
Suzuki and co-workers reported the first addition of B2pin2 to
alkynes to generate alkenylboronic acids by using [Pt(PPh3)4] as
a catalyst.[3] Since then, this field has grown rapidly and the di-
boration of alkynes and analogues with metal catalysts based
on Pt, Pd, Ni, Rh, and Ir have been reported.[2] Although these
metal-catalyzed reactions proceed successfully, a metal-free
process has long been desired, partly because these boron
compounds are intermediates for the preparation of drugs and
electronic materials where a heavy metal can be a vital
a study on the photoinduced simultaneous addition of (PhS)2
to alkynes.[5] Two different chalcogen groups (for example, PhS
and PhY) could be simply introduced to alkynes selectively
from a mixture of (PhS)2, (PhY)2 (Y=Se, Te) and an alkyne.[5a,b]
An extension of this strategy was consequently carried out to
add B2pin2 [Eq (2)]. Thus, a mixture of 1-octyne (0.60 mmol),
bis(pinacolato)diboron (0.50 mmol), and diphenyl disulfide
(0.45 mmol) in CDCl3 (0.3 mL) was irradiated with a xenon
lamp through Pyrex for 3 h at ambient temperature. However,
the expected addition products 3 were not detected at all.
Instead, an unexpected diboration of 1-octyne took place to
give the corresponding bisboryl adduct 1a in 20% yield
(E/Z=35:65), together with 2a resulting from the normal
photoinduced addition of (PhS)2 (27% yield, E/Z=56:44).
The formation of 1a is surprising because B2pin2 cannot
simply add to an alkyne, even under light irradiation (see
below).[2,4] In addition, as described below, subsequent optimi-
zation of the reaction conditions successfully suppresses the
formation of 2a, leading to the discovery of a selective
diboration with a catalytic amount of (PhS)2.
[a] Dr. A. Yoshimura, Y. Takamachi, Prof. Dr. A. Ogawa
Department of Applied Chemistry, Graduate School of Engineering
Osaka Prefecture University
1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531 (Japan)
E-mail: ogawa@chem.osakafu-u.ac.jp
[b] Dr. A. Yoshimura, Prof. Dr. L.-B. Han
National Institute of Advanced Industrial Science and Technology (AIST)
1-1-1, Higashi, Tsukuba, Ibaraki, 305-8565 (Japan).
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502425.
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the adduct. Dialkyl disulfides also promoted this reaction
(Table 2, entries 6 and 7), whereas PhSH produced 1a in 37%
NMR yield (entry 8). Quite surprisingly, even Ph2S could give
22% yield of the product (Table 2, entry 9). Equally surprisingly,
the heavier chalcogenides (PhSe)2, Ph2Se, PhSeH, and (PhTe)2,
which generate radicals more easily than sulfide under light,
could not promote this diboration at all (Table 2, entries
10–13), and Ph2O did not work either (entry 14).
Table 1. Reaction of 1-octyne and bis(pinacolato)diboron with (PhS)2
under different light sources
Entry
Light source
Yield [%][a]
1a (E/Z)
2a (E/Z)
1
2
3
4[b]
xenon lamp
tungsten lamp
high-pressure mercury lamp
high-pressure mercury lamp
24 (35:65)
29 (34:66)
36 (37:63)
50 (33:67)
21 (62:38)
26 (81:19)
12 (60:40)
trace
Further details of the reaction optimization are summarized
in Table 3. Although 1a could be generated when 1.5 equiva-
[a] Unless otherwise stated, a mixture of 1-octyne (0.30 mmol), B2pin2
(0.25 mmol) and (PhS)2 (0.32 mmol) in CDCl3 (0.15 mL) was irradiated for
12 h at ambient temperature. Yields refer to 1H NMR yields; [b] 1-octyne
(0.30 mmol), B2pin2 (0.74 mmol), and (PhS)2 (0.22 mmol) were used.
Table 3. (PhS)2-catalyzed diboration of 1-octyne under light
First, the effect of the light source was investigated (Table 1).
Although a large amount of the dithiolation product 2a was
generated with a xenon lamp or a tungsten lamp (Table 1, en-
tries 1 and 2), its formation could be considerably suppressed
by using a high pressure mercury lamp (entry 3), and it was
found that only a trace amount of 2a was detected with an
excess amount of B2pin2 (entry 4).
Entry
B2pin2 [mmol]
(PhS)2 [mmol]
Time
Yield [%] (E/Z)[a]
1
2
3
4
5
6
7
8
0.15
0.30
0.30
0.30
0.30
0.30
0.30
0.30
0.30
0.15
0.020
0.020
0.020
0.020
0.020
0.020
0.010
0.005
0.020
0
6 h
6 h
3 h
30 (38:62)
56 (34:66)
58 (33:67)
52 (31:69)[b]
54 (31:69)[b]
34 (28:72)
50 (38:62)
32 (41:59)
0
1 h
30 min
15 min
6 h
6 h
6 h
As shown in Table 2, this diboration even took place
smoothly in the presence of a catalytic amount of (PhS)2.
9[c]
10
12 h
0
[a] Yields refer to 1H NMR yields; [b] 1-octyne was not consumed
completely; [c] without light.
Table 2. Photoinduced diboration of 1-octyne with heterocompounds.
lents of B2pin2 were used (Table 3, entry 1), the formation of
side products could be suppressed to give a high yield of 1a
with an excess amount of B2pin2.[7] The reaction progressed
very quickly, proceeding almost to completion within half an
hour (Table 3, entries 3–6). Remarkably, even lower amounts of
(PhS)2 could also catalyze this diboration efficiently (Table 3,
entries 7 and 8). However, no 1a was formed in the absence of
the light (Table 3, entry 9) or in the absence of (PhS)2
(entry 10). The reaction time was optimized by varying the
times from 15 min to 6 h (Table 3, entries 2–6) and 1-octyne
was consumed completely after photoirradiation for 3 h
(entry 3).
Entry
1
Catalyst
Yield [%] (E/Z)[a]
(PhS)2
0.02 mmol
55 (36:64)
73 (34:66)[b]
58 (33:67)
53 (34:66)
31 (26:74)
28 (29:71)
43 (30:70)
40 (33:67)
37 (37:63)
22 (41:59)
0
2
3
4
5
6
7
8
9
10
11
12
13
14
(p-ClC6H4S)2
(p-CH3C6H4S)2
(p-NO2C6H4S)2
0.02 mmol
0.02 mmol
0.02 mmol
0.02 mmol
0.02 mmol
0.02 mmol
0.02 mmol
0.03 mmol
0.02 mmol
0.06 mmol
0.05 mmol
0.02 mmol
0.03 mmol
(p-CH3OC6H4S)2
(nBuS)2
(MeS)2
PhSH
Ph2S
(PhSe)2
Ph2Se
PhSeH
(PhTe)2
Ph2O
0
0
0
0
Regarding optimization of the solvent, in addition to chloro-
form, the diboration also proceeded in THF (49% yield, E/Z=
20:80), benzene (54% yield, E/Z=22:78), toluene (53% yield,
E/Z=25:75), and acetone (53% yield, E/Z=30:70). However,
methanol and DMSO gave low yields of the products, and only
trace amount of 1a was generated in acetonitrile.[8]
[a] Unless otherwise stated, yields refer to 1H NMR yields; [b] yield of
isolated product.[6]
This novel (PhS)2-catalyzed diboration reaction could be ap-
plied to other alkynes, producing the corresponding diboration
products in good yields (Table 4). Thus, in addition to 1-octyne
(Table 4, entry 1), 5-methyl-1-hexyne (entry 2) and (5-phenyl)-
propylacetylene (entry 3) also produced the corresponding ad-
ducts in 59% and 66% yields, respectively. Benzylacetylene
(Table 4, entry 4) gave 70% yield of the diboration product. Al-
kynes with functionalities such as acetoxy (Table 4, entry 5) and
Compound 1a[6] was obtained in 73% yield from a mixture of
1-octyne (0.1 mmol) and B2pin2 (0.35 mmol) in the presence of
20 mol% (0.02 mmol) of (PhS)2 (Table 2, entry 1; for further
optimization of the catalyst, see Table 2). As expected, other
diaryl disulfides also promoted the reaction (Table 2, entries 2–
5). Whereas (p-ClC6H4S)2 and (p-CH3C6H4S)2 gave a similar result
to (PhS)2, (p-NO2C6H4S)2 and (p-CH3OC6H4S)2 gave low yields of
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Table 4. (PhS)2-catalyzed diboration of alkynes with bis(pinacolato)-
diboron under light
Entry
1
Alkyne
Product
Yield [%] (E/Z)[a]
Scheme 1. Proposed catalytic cycle for the sulfide-catalyzed diboration.
1a
73 (34:66)
2
1b
59 (38:62)
3
4
1c
66 (31:69)
70 (28:72)
1d
5
6
1e
1 f
75 (30:70)
72 (30:70)
7
8
1g
1h
43 (24:76)
trace[b]
[a] Yields of isolated product; [b] complex mixture including polymer was
obtained.
chloro (entry 6) also produced the corresponding adducts in
good yields. Surprisingly, even 5-hexynenitrile (Table 4, entry 7)
could give the diboration product in a moderate yield, despite
the deactivation effect of the CN group described above.[8]
However, terminal alkynes such as phenylacetylene (entry 8)
did not give satisfactory results due to severe polymerization
under the reaction conditions.
Figure 1. ESR spectrum of the reaction mixture of the (PhS)2-catalyzed
diboration.
Detailed mechanistic pathways for this sulfide-catalyzed di-
boration of alkynes under light are not understood at present.
However, it was confirmed that diboration did proceed, albeit
sluggishly, with the radical initiator 2,2’-azobis(isobutyronitrile)
(AIBN), in the presence or absence of (PhS)2 in the dark [no di-
boration took place under similar conditions in the absence of
AIBN; Eq (3)]. Therefore, it is assumed that a similar radical
path is applicable to this sulfide-catalyzed diboration. Although
B2pin2 alone could not be cleft by the light to generate a boryl
radical, with the help of a sulfide (which properly coordinates
to B2pin2) this cleavage becomes easier, leading to this novel
diboration reaction (Scheme 1).[9]
Experimental Section
The general procedure for the synthesis of alkenylboron derivatives
was as follows. In a Pyrex glass tube were placed an alkyne
(0.10 mmol), B2pin2 (88.88 mg, 0.35 mmol), and (PhS)2 (4.37 mg,
0.020 mmol) in CDCl3 (0.07 mL) under nitrogen. The mixture was
irradiated with a high-pressure mercury lamp (500 W) at ambient
temperature for 3 h. The diboration product was isolated by using
a preparative GPC (Japan Analytical Industry Co., Ltd., LC-908) with
chloroform as an eluent. Further details of the experimental
procedures and characterization data for the new compounds are
included in Supporting Information.
In support of this radical mechanism, ESR measurement of
the reaction mixture (Table 2, entry 1) clearly detected a radical
species with g=2.0035 (Figure 1).[10]
Acknowledgements
In conclusion, a novel sulfide-catalyzed diboration of alkynes
under light has been reported. This reaction not only repre-
sents the first metal-free diboration of unactivated alkynes, but
also disclosed a unique, unprecedented[11] form of catalysis
with a simple sulfide compound under light. This novel
phenomenon merits future full exploration because of its
easily expected great potential in organic synthesis. Studies
along this line are currently underway in our laboratory.
We thank Dr. Liang Chen of National Institute of Advanced In-
dustrial Science and Technology (AIST) for ESR measurement.
This research was supported by a Grant-in-Aid for Exploratory
Research (26620149), from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. The authors thank the
Nara Institute of Science and Technology of the Kyoto
Advanced Nanotechnology Network for their assistance with
the mass spectrometry measurements.
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Keywords: alkynes
photochemistry · sulfides
·
borylation
·
organocatalysis
·
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Received: June 23, 2015
Published online on August 13, 2015
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