Reactions of Potassium Alkenyltrifluoroborates
facilitate the cross-coupling and expand its scope.3d,g,h
Fewer endeavors have concentrated on expanding the
range of the organoboron coupling partner. With regard
to the use of alternative organoboron derivatives, it has
been revealed that potassium organotrifluoroborates offer
solutions to the problems that occur through the use of
other organoboron partners. These materials are readily
prepared by the addition of inexpensive KHF2 to a variety
of organoboron intermediates.7 The trifluoroborates are
monomeric, crystalline solids that are readily isolated
and indefinitely stable in the air.7a,8 Despite their ad-
vantages, in comparison to other organoboron species
organotrifluoroborates have seen minimal use in pal-
ladium-catalyzed cross-coupling reactions.8-14
Preliminary results of Suzuki-Miyaura cross-coupling
reactions between alkenyl trifluoroborates and aryl ha-
lides and triflates have been previously reported by our
research group.15 In that study, the coupled products
were obtained in good yields using PdCl2(dppf)‚CH2Cl2
as catalyst, in THF/water or n-PrOH, using Cs2CO3 or
Et3N as base (eq 1).
coupling reaction. This salt was prepared from 1-decyne
(eq 2) using known procedures. Hydroboration with
HBBr2‚SMe2 followed by treatment of the resulting
dibromoborane with KHF2 in the presence of water
16
afforded 1a in 71% yield for the two-step, one-pot process.
The cross-coupling of 1a was optimized by using 1-
bromonaphthalene (2a ) as the electrophile, PdCl2(dppf)‚
CH2Cl2 or PdCl2 as catalyst, a variety of different bases
(e.g., Et3N, Hunig’s base, t-BuNH2, K2CO3, Cs2CO3), and
different solvent systems (MeOH, EtOH, n-PrOH, i-
PrOH, THF, DME, dioxane) under both anhydrous and
aqueous conditions. Among the solvents examined, the
mixture of i-PrOH-H2O (2:1) provided the best results.
In this mixture, not all of the aryl halides and triflates
were completely soluble, but over time the reaction
mixtures turned more homogeneous. In the presence of
2 mol % of PdCl2(dppf)‚CH2Cl2 and 3 equiv of t-BuNH2
this solvent system proved to be the best (eq 3), furnish-
ing trans-1-decen-1-yl-naphthalene 3a in 78% yield.
Because the reaction demonstrated sensitivity to oxygen,
it was necessary to degas the solvent, base, and the
reagents before the reaction was performed.
Herein we provide a full account of our exploration of
these reactions, including the coupling of a variety of
alkenyltrifluoroborates with functionalized aryl halides
or triflates and with heteroaryl halides.
Resu lts a n d Discu ssion
We initially focused our attention on the use of potas-
sium trans-1-dec-1-enyl trifluoroborate (1a ) in the cross-
The use of Et3N as base and the use of anhydrous
n-PrOH also provided good results. Curiously, n-PrOH-
H2O mixtures did not provide the product in as satisfac-
tory yields as the i-PrOH-H2O mixture. Between these
two options, we decided to use i-PrOH-H2O, considering
it is less expensive and the most environmentally sound
among the other successful combinations tested.
The optimized conditions were subsequently applied
to the coupling reaction of 1a with different aryl halides
and -triflates. As outlined in Table 1, the reaction
proceeded with satisfactory yields in most cases. The
reaction was tolerant of a variety of functional groups
including ethers, ketones, nitriles and nitro groups
despite the aqueous, basic conditions. Moreover, electro-
philes bearing both electron-withdrawing and electron-
donating groups react with the trifluoroborates, affording
the desired products in good yields.
(6) (a) Uenishi, J .; Beau, J .-M.; Armstrong, R. W.; Kishi, Y. J . Am.
Chem. Soc. 1987, 109, 4756-4758. (b) Kobayashi, S.; Mori, K.;
Wakabayashi, T.; Yasuda, S.; Hanada, K. J . Org. Chem. 2001, 66,
5580-5584. (c) Scheidt, K. A.; Bannister, T. D.; Tasaka, A.; Wendt,
M. D.; Savall, B. M.; Fegley, G. J .; Roush, W. R. J . Am. Chem. Soc.
2002, 124, 6981-6990. (d) Pazos, Y.; Iglesias, B.; de Lera, A. R. J . Org.
Chem. 2001, 66, 8483-8489. (e) Mergott, D. J .; Frank, S. A.; Roush,
W. R. Org. Lett. 2002, 4, 3157-3160.
(7) (a) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.; Schrimpf,
M. R. J . Org. Chem. 1995, 60, 3020-3027. (b) Vedejs, E.; Fields, S. C.;
Hayashi, R.; Hitchock, S. R. Powell, D. R.; Schrimpf, M. R. J . Am.
Chem. Soc. 1999, 121, 2460-2470.
(8) Coupling of organotrifluoroborates with arenediazonium salts:
(a) Darses, S.; Michaud, G.; Geneˆt, J .-P. Eur. J . Org. Chem. 1999,
1875-1883. (b) Darses, S.; Michaud, G.; Geneˆt, J .-P. Tetrahedron Lett.
1998, 39, 5045-5084. (c) Darses, S.; Michaud, G.; Geneˆt, J .-P.; Brayer,
J .-L.; Demounte J . P. Tetrahedron Lett. 1997, 38, 4393-4396.
(9) Coupling of organotrifluoroborates with diaryliodonium salts:
Xia, M.; Chen, Z. C. Synth. Commun. 1999, 29, 2457-2465.
(10) Coupling of vinyltrifluoroborate with halopyrimidines: Puen-
tener, K.; Scalone, M. Eur. Pat. Appl. EP 1,057,831 A2, 2000.
(11) Use of tetraalkylammonium trifluoroborates in cross-coupling
reactions: Batey, R. A.; Quach, T. D. Tetrahedron Lett. 2001, 42, 9099-
9103.
ortho-Substituted substrates (entries 4 and 8) react to
provide the coupling products in satisfactory yields. Even
in the case of the highly hindered 2-bromomesitylene
(entry 10), a 38% yield of the desired product was
obtained.
(12) Cross-coupling of alkyltrifluoroborates: Molander, G. A.; Ito,
T. Org. Lett. 2001, 3, 393-396.
(13) Cross-coupling of aryltrifluoroborates: Molander, G. A.; Bi-
olatto, B. Org. Lett. 2002, 4, 1867-1870.
(14) Cross-coupling of alkynyltrifluoroborates: Molander, G. A.;
Machrouhi, F.; Katona, B. J . Org. Chem., 2002, 67, 8416-8423.
(15) Molander, G. A.; Rodriguez Rivero, M. Org. Lett. 2002, 4, 107-
109.
(16) Brown, H. C.; Bhat, N. G.; Somayaji, V. Organometallics 1983,
2, 1311-1316.
J . Org. Chem, Vol. 67, No. 24, 2002 8425