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
References
(1) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Suzuki, A.;
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(5) For B-Si bond additions, see: (a) Suginome, M.; Nakamura, H.; Ito, Y.
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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.;
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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.
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(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
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