Synthesis of 1-alkenylboronic esters via palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron with 1-alkenyl halides and triflates

Article abstract of DOI:10.1246/cl.2000.126

The synthesis of 1-alkenylboronic acid pinacol esters via the palladium-catalyzed cross-coupling reaction of 1-alkenyl halides or triflates with bis(pinacolato)diboron [(Me₄C₂O₂)B-B(O₂C₂Me ₄

Full text of DOI:10.1246/cl.2000.126

126
Chemistry Letters 2000
Synthesis of 1-Alkenylboronic Esters via Palladium-Catalyzed Cross-Coupling Reaction of
Bis(pinacolato)diboron with 1-Alkenyl Halides and Triflates
Kou Takahashi, Jun Takagi, Tatsuo Ishiyama, and Norio Miyaura*
Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628
(Received November 2, 1999; CL-990936)
The synthesis of 1-alkenylboronic acid pinacol esters via
the palladium-catalyzed cross-coupling reaction of 1-alkenyl
halides or triflates with bis(pinacolato)diboron [(Me4C2O2)B-
B(O2C2 Me4)] was carried out in toluene at 50 °C in the pres-
ence of potassium phenoxide (1.5 equivalents) and
PdCl2(PPh3)2•2PPh3 (3 mol%).
The transition metal-catalyzed cross-coupling reaction of
disilanes and distannanes is an elegant method for the synthses
of organosilicone1 and -tin2 compounds directly from organic
electrophiles, but the lack of suitable boron nucleophiles has
limited this protocol. We have recently demonstrated the syn-
thesis of arylboronates from aryl halides3 or triflates,4 and
allylboronates from allyl acetates5 via a palladium-catalyzed
cross-coupling reaction of tetra(alkoxo)diborons. Very recent-
ly, Masuda and Murata6 found that pinacolborane
(Me4C2O2)BH is an excellent boron-nucleophile for the palla-
dium-catalyzed coupling reaction in the presence of triethy-
lamine. Thus, tetra(alkoxo)diboron and pinacolborane are two
nucleophiles now available for the boron cross-coupling reac-
tion.
In this paper, we report the palladium-catalyzed coupling
reaction of bis(pinacolato)diboron (1) and 1-alkenyl halides or
triflates, which provides a one-step procedure for the synthesis
of 1-alkenylboronic esters from vinyl electrophiles (Eq. 1).7
amount of 5, the homocoupling between 2a and 3 giving
dimer 6,9 and an unusual coupling giving a positional isomer 4
together with the desired coupling to afford 3. Although the
formation of positional isomer 4 is not well understood, the
Heck product and the dimer (5 and 6) can be eliminated by
changing KOAc or K2CO3 (entries 1 and 2) to a strong base
because those side reactions are due to the slow transmetala-
tion of 1 to the vinyl-Pd-Br intermediate. Thus, the selective
coupling to give 3 was finally achieved by carrying out the
reaction at 50 °C in the presence of a solid PhOK suspended in
toluene (entries 10 and 11), whereas other solid bases such as
K3PO4•nH2O and NaOEt were not effective. The palladium-
triphenylphosphine catalyst gave adequately better results than
the palladium-dppf complex (entries 3-11).
The synthesis of 1-alkenylboronates from the representa-
tive haloalkenes is summarized in Table 2. The reaction with
2-iododecene 2b and t-butyl derivative 2c still suffered from
the formation of a positional isomer (entries 1 and 2), but other
2-bromoalkenes (2d-2g) selectively gave alkenylboronates in
high yields, which are not available by conventional hydrobo-
ration of terminal alkynes10 (entries 2-6). The use of an insolu-
ble base suspended in toluene can tolerate various functional
groups in haloalkenes (entries 4-6). Although (E)-1-alkenyl-
boronates have been synthesized by hydroboration of terminal
alkynes and (Z)-derivatives by a two-step procedure from 1-
halo-1-alkynes,11 the coupling reaction of diboron provided an
alternative and stereoselective method for synthesizing both
isomers with high stereoselectivity over 99% (entries 7 and 8).
Our initial studies (Eq. 2 and Table 1) were focused on the
reaction conditions selectively yielding a vinyl boronate (3)
because the previous reaction reported for the coupling with
aryl halides3 resulted in the formation of an inseparable mix-
ture of several products (3-6) (entry 1). Thus, the reaction
involved the Heck coupling8 between 2a and 3 yielding a small
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
127
References and Notes
1
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3
T. Ishiyama, M. Murata, and N. Miyaura, J. Org. Chem.,
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4
5
T. Ishiyama, Y. Itoh, T. Kitano, and N. Miyaura,
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6
7
M. Murata, S. Watanabe, and Y. Masuda, J. Org. Chem.,
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During the course of our study, a similar cross-coupling
reaction of haloalkenes with diboron 1 or pinacolborane
was reported: S. M. Marcuccio, M. Rodopoulos, H.
Weigold, 10th International Conference on Boron
Chemistry, Durham, England, July 1999, PB-35; M.
Murata, T. Oyama, S. Watanabe, and Y. Masuda, 76th
Annual Meeting of Chemical Society of Japan, March
1999, 2B712.
8
9
R. F. Heck, in “Palladium Reagents in Organic
Syntheses,” Academic Press, New York (1985), pp. 214-
242.
Dimers included 2-octyl-1,3-dodecadiene and 2,3-dioctyl-
1,3-butadiene, but the reaction shown in entry 1 selective-
ly gave the former diene via the head to tail coupling
between 2a and 3. N. Miyaura and A. Suzuki, J.
Organomet. Chem., 213, C53 (1981).
2,2-Disubstituted 1-alkenylboronate and cyclic 1-alkenyl-
boronate were obtained from the corresponding bromides
without any difficulty because the side-reactions observed in
Eq. 2 were very slow for these substrates (entries 9 and 10).
However, all attempts at the couplings with 4-iodo-3-penten-2-
one and methyl 3-bromo-2-methylpropenoate were unsuccess-
ful.
10 I. Rivera and J. A. Soderquist, Tetrahedron Lett., 32, 2311
(1991); Y. Yamamoto, R. Fujikawa, A. Yamada, and N.
Miyaura, Chem. Lett., 1999, 1069.
Preliminary results for the cross-coupling of diboron 1
with 1-alkenyl triflate are shown in Eq. 3. Under similar reac-
tion conditions to those used for halides, various triflates pro-
vided the corresponding 1-alkenylboronates, which results will
be reported elsewhere.
11 D. S. Matteson, in “Stereodirected Synthesis with Organo-
boranes,” Springer, New York (1995), pp. 31-41.