Nickel-catalyzed trans-alkynylboration of alkynes via activation of a boron-chlorine bond

Article abstract of DOI:10.1021/ja055396z

Reaction of (diisopropylamino)chloroboryl ethers of alkynols with alkynylstannanes in the presence of nickel catalysts afforded formal trans-alkynylboration products in good yields. The products were isolated as pinacol boronates after treatment of the crude reaction mixture with acetic anhydride and pinacol in the presence of a base. The trans-alkynylboration products underwent Suzuki-Miyaura coupling reaction with organic halides. A 2-borylalkenylnickel(II) complex, in which the boryl and nickel groups are located in a trans fashion, was isolated in a reaction of the chloroboryl ether with a single equivalent of a nickel(0)-phosphine complex. Copyright

Full text of DOI:10.1021/ja055396z

Published on Web 10/22/2005
Nickel-Catalyzed trans-Alkynylboration of Alkynes via Activation of a
Boron-Chlorine Bond
Akihiko Yamamoto and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
Received August 8, 2005; E-mail: suginome@sbchem.kyoto-u.ac.jp
Scheme 1
Remarkable progress has been achieved during the past decade
in utilizing organoboron compounds in C-C bond forming reactions
with the aid of transition metal catalysts. In addition to the
significant improvement in the efficiency and scope of the Suzuki-
Miyaura coupling reaction,1 several new Rh-catalyzed reactions,
including enantioselective conjugate additions of organoboronic
acids, have appeared.2 To fully take advantage of the rapid
development of those boron-based reactions, exploration of efficient
synthetic routes to new functionalized organoboron compounds is
eagerly desired.
Addition of organoboron compounds to unsaturated organic
molecules is recognized as one of the most efficient methods to
Table 1. Ni-Catalyzed Reactions of Chloroboryl Ethers of Alkynols
with Alkynylstannanesa
produce functionalized organoboron compounds.1b Recent develop-
ments in this area mainly concern transition metal catalyzed
additions because of the high efficiency and regio- and stereo-
chemical controllability.3-5 We have been interested in carbobo-
ration reactions, which allow concomitant introduction of carbon
and boron substituents to organic frameworks, possibly in a regio-
and stereoselective fashion. As the first example of carboboration,
we have recently reported regio- and stereoselective cyanoboration,
which allowed convenient synthesis of â-boryl-R,â-unsaturated
nitriles.6 In this paper, we describe another example of carboboration
of alkynes. The reaction proceeds through the activation of the
boron-chlorine bond by a nickel catalyst,7,8 followed by delivery
of an organic group from organostannanes,9 affording formal trans-
carboboration products.
entry
n
R1
R2
R3
product
% yield of 3b
% yield of 4c
1d
2
3
4
5
1
1
1
1
1
1
1
2
H
Et
Et
Et
Ph
Ph
aa
ab
ac
ba
ca
da
ea
fa
93
98
78
99
76
89
nd
nd
85
81
70
85
67
61
90
71
H
H
H
H
Ph
n-Pr
Me3Si
Ph
CHdCMe2 Ph
Ph
6
Ph
Ph
Ph
7
Me Et
H Me
8e
a 1 (0.20 mmol) and 2 (0.20 mmol) in toluene (0.25 mL) were heated at
80 °C in the presence of Ni(cod)2 (0.0040 mmol) with PPh3 (0.016 mmol),
unless otherwise noted. After 3-24 h, yields of 3 were determined by NMR
using internal standard. Then, pinacol (0.40 mmol), Ac2O (50 µL), pyridine
(50 µL), and DMAP (3.0 mg) were added. The mixture was allowed to
react at 50 °C for 24 h. b NMR yield. c Isolated yield (PTLC) for two steps.
d On a 2.5 mmol scale. e At 110 °C.
A chloroboryl ether 1a (R1 ) H, R2 ) Et) was prepared from
3-hexyn-1-ol and reacted with alkynylstannane 2a (R3 ) Ph) in
the presence of a catalyst prepared from Ni(cod)2 (2 mol %) and
triphenylphosphine (8 mol %) at 80 °C (Scheme 1; Table 1, entry
1). We observed formation of the alkynylboration product 3aa in
93% yield, which was derived from 5-exo cyclization of 1a with
incorporation of the alkynyl unit of 2a. It became clear by the
subsequent screening of the phosphine ligands that triphenylphos-
phine was superior to other phosphine ligands, such as trimeth-
ylphosphine and tricyclohexylphosphine, with respect to the reaction
yield. The ligand screening also suggested that the Ni/P ratio of
1/4 showed the highest catalyst efficiency in the reaction. Our
interest then focused on the geometry of the CdC bond. Careful
inspection of the stereochemistry of the CdC bond by nOe
experiments for 3 and other related products shown below suggested
that the alkynylboration took place in a trans fashion.
Under the optimized reaction conditions, the chloroboryl ho-
mopropargylic ethers 1 underwent trans-alkynylboration (Table 1).
Because of the instability of the primary product 3 toward moisture,
the crude reaction mixtures were treated with pinacol and acetic
anhydride in the presence of a base to produce pinacolborane
derivatives 4, which could be isolated by silica gel chromatography.
As the alkynylstannane components, those derived from phenyl-
acetylene, 1-pentyne, and trimethylsilylacetylene were found to be
equally reactive (entries 1-3). The alkynyl carbon atoms of the
homopropargyl moieties of 1 could bear phenyl and even alkenyl
groups (entries 4-6). It is noteworthy that the alkynylboration of
1 having phenyl groups at the alkynyl carbons was remarkably
facilitated (entries 4 and 6), completing within 3 h, while the alkyl-
substituted derivatives required around 24 h to reach completion
(entries 1-3 and 7). We were pleased to find successful six-
membered ring formation by using a homologous alkynol (n ) 2)
(entry 8). Although higher temperature was required, the reaction
provided trans-alkynylboration product 4fa in good yield.
Besides alkynylstannanes, vinylstannane was found to take part
in the reaction. Using 5 mol % of Ni(cod)2 at 100 °C, tributylvi-
nylstannane afforded the corresponding trans-addition product 5
as the major isomer (83:17) in moderate yield after the pinacol/
Ac2O treatment (eq 1).
9
15706
J. AM. CHEM. SOC. 2005, 127, 15706-15707
10.1021/ja055396z CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
unclear, the reaction provides a new, stereoselective access to highly
functionalized organoboron compounds, which are otherwise dif-
ficult to synthesize.
Figure 1. Single-crystal X-ray structure of trans-8.
Scheme 2. Proposed Mechanism for the trans-Alkynylborationa
Acknowledgment. A.Y. acknowledges the Japan Society of
Promotion of Science for the fellowship support. The authors thank
Mr. Masahiro Horiguchi for his assistance in the single-crystal X-ray
analysis. This work is supported in part by Tokuyama Science
Foundation.
Supporting Information Available: Experimental procedures,
details of the X-ray analysis of trans-8, and spectral data for the new
compounds (29 pages, print/PDF). This material is available free of
charge via Internet at http://pubs.acs.org.
References
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(3) For B-H bond additions, see: (a) No¨th, H.; Ma¨nnig, D. Angew. Chem.,
Int. Ed. Engl. 1985, 24, 878. (b) Hayashi, T.; Matsumoto, Y.; Ito, Y. J.
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N.; Suzuki, A. J. Am. Chem. Soc. 1993, 115, 7219. (b) Review: Ishiyama,
T.; Miyaura, N. J. Organomet. Chem. 2000, 611, 392.
(5) For B-Si bond additions, see: (a) Suginome, M.; Nakamura, H.; Ito, Y.
J. Chem. Soc., Chem. Commun. 1996, 2777. (b) Suginome, M.; Ohmura,
T.; Miyake, Y.; Mitani, S.; Ito, Y.; Murakami, M. J. Am. Chem. Soc.
2003, 125, 11174. (c) Suginome, M.; Ito, Y. J. Organomet. Chem. 2003,
680, 43 and references therein.
(6) (a) Suginome, M.; Yamamoto, A.; Murakami, M. J. Am. Chem. Soc. 2003,
125, 6358. (b) Suginome, M.; Yamamoto, A.; Murakami, M. Angew.
Chem., Int. Ed. 2005, 44, 2380.
(7) For Ni-catalyzed reactions of alkynes, see: Ikeda, S. In Modern Orga-
nonickel Chemistry; Tamaru, Y., Ed.; Wiley: Weinheim, Germany, 2005;
p 102.
(8) As far as we are aware, Pd-catalyzed diboration and silaboration in the
presence of alkenyl iodide are the only examples of catalytic reactions in
which involvement of oxidative addition of B-X bonds is proposed,
although the boron iodide is generated in situ in those reactions. (a) Yang,
F.-Y.; Cheng, C.-H. J. Am. Chem. Soc. 2001, 123, 761. (b) Chang, K.-J.;
Rayabarapu, D. K.; Yang, F.-Y.; Cheng, C.-H. J. Am. Chem. Soc. 2005,
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The origin of the observed trans-addition mode of the present
reaction seems interesting from the mechanistic point of view. In
an attempted stoichiometric reaction of 1g with a Ni(0)-PMe3
complex, we could isolate alkenylnickel(II) chloride complex trans-
8, in which the boron and nickel are located in a trans fashion (eq
2). The structure of the nickel complex was determined by a single-
crystal X-ray analysis (Figure 1). On the basis of this result, the
catalytic cycle of the trans-alkynylboration may be presumed as
shown in Scheme 2. The proposed mechanism involves an oxidative
addition of the B-Cl bond to nickel,8,10 followed by insertion of
the triple bond to the B-Ni bond in a cis-addition manner,
producing cis-B. The transmetalation step may be slow, allowing
isomerization of cis-B to trans-B (corresponding to trans-8).11,12
The trans-B may be more favorable than cis-B due to the
considerable steric repulsion in cis-B between the chlorobis-
(triphenylphosphine)nickel moiety and the diisopropylamino group.
(9) For the related alkynylation of alkynes using alkynylstannanes, see: (a)
(silylalkynylation, Pd) Chatani, N.; Amishiro, N.; Murai, S. J. Am. Chem.
Soc. 1991, 113, 7778. (b) (stannylalkynylation, Pd) Shirakawa, E.; Yoshida,
H.; Kurahashi, T.; Nakao, Y.; Hiyama, T. J. Am. Chem. Soc. 1998, 120,
2975. (c) (allylalkynylation, Ni) Cui, D.-M.; Tsuzuki, T.; Miyake, K.;
Ikeda, S.; Sato, Y. Tetrahedron 1998, 54, 1063.
(10) Formation of borylpalladium(II) chloride complexes by oxidative addition
of the B-Cl bond to a Pd(0) complex has been reported. See: Onozawa,
S.-y.; Tanaka, M. Organometallics 2001, 20, 2956. The boryl complex
undergoes insertion of alkynes into B-Pd, giving a cis-addition product.
The synthetic utility of the alkynylboration has been demonstrated
by the one-pot alkynylboration/Suzuki-Miyaura cross-coupling
sequence (Scheme 3). The highly substituted conjugated enyne 6
and dienyne 7 were isolated in high yields on treating the crude
alkynylboration mixture containing 3ea with the corresponding
organic halides, base, and Pd(PPh3)4.
(11) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics 1993, 12, 988.
(12) Carbene-like zwitterionic resonance structures have been proposed for
the cis-trans isomerization of the alkenyl transition metal complexes.
(a) Brady, K. A.; Nile, T. A. J. Organomet. Chem. 1981, 206, 299. (b)
Ojima, I.; Clos, N.; Donovan, R. J.; Ingallina, P. Organometallics 1990,
9, 3127. (c) Murakami, M.; Yoshida, T.; Kawanami, S.; Ito, Y. J. Am.
Chem. Soc. 1995, 117, 6408.
In summary, we report new trans-carboboration reactions
catalyzed by nickel complexes. Although mechanistically still
JA055396Z
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