A R T I C L E S
Takagi et al.
transmetalation protocol requires a tedious protection-depro-
tection process for preparing functionalized 1-alkenylboron
compounds.
Transition metal-catalyzed cross-coupling reactions of metal
nucleophiles with organic electrophiles have emerged as an
efficient method for the synthesis of organometallic compounds
because of the ready availability of various organic electrophiles.
Extensive studies have been made on the silylation, stannylation,
and germylation by using disilanes,7 distannanes,8 and diger-
manes7c,9 as metal-anion equivalents; however, the correspond-
ing reaction of boron has not been well developed. Recently,
we have demonstrated that tetra(alkoxo)diborons such as bis-
(pinacolato)diboron are excellent boron nucleophiles which
allow the palladium-catalyzed borylation of organic halides or
triflates in the presence of a base.10 Since the tetra(alkoxo)-
diborons highly tolerate various functional groups, the reaction
provided variously functionalized arylboronates,10,11 allylbor-
onates,10,12 and benzylboronates10 from the corresponding
organic electrophiles. Very recently, Masuda and Murata found
that pinacolborane is an economical alternative for analogous
palladium-catalyzed borylation of organic halides or triflates.13
However, their method suffered from low yields, resulting in a
complex mixture of several byproducts for the synthesis of
1-alkenylboronates.13c
coupling sequence. Although similar borylation of 2 with 1
catalyzed by PdCl2(PhCN)2 or PdCl2(dppf) was reported by
Marcuccio15 and Eastwood,16 respectively, the present method
may have broader generality with respect to the range of
applicable 2.
Results and Discussion
Palladium-Catalyzed Cross-Coupling Reaction of Bis-
(pinacolato)diboron with 1-Alkenyl Halides or Triflates.
Because the cross-coupling of the diboron 1 with 1-decen-2-yl
bromide (4a) under the previous reaction conditions reported
for the borylation of aryl,10,11 allyl,10 or benzyl10 halides only
gave a small amount of the expected 1-decen-2-ylboronate (5)
together with a large amount of several byproducts (6-8) (eq
2), our initial efforts were focused on the reaction conditions
selectively giving 5.
In this paper, we report a palladium-catalyzed coupling
reaction of bis(pinacolato)diboron (1, pin2B2, pin ) Me4C2O2)
with 1-alkenyl halides or triflates (2), which provides a one-
step procedure for the synthesis of pinacol 1-alkenylboronic
esters (3) from 1-alkenyl electrophiles (eq 1).14 The utility of
the method was also demonstrated by a one-pot, two-step
synthesis of unsymmetrical 1,3-dienes via the borylation-
A combination of PdCl2(dppf), KOAc, and DMSO used for
the borylation of haloarenes11a resulted in a mixture of a desired
boronate (5, 27%), a Heck coupling product between 4a and 5
(6, <1%), a homo-coupling product (7, 20%), and a regioisomer
of 5 (8, 6%). Probable processes for the formation of these
products are shown in Scheme 1. The desired reaction to form
5 proceeds by a process involving oxidative addition of 4a to
Pd(0) to generate a 1-alkenylpalladium(II) bromide (9), trans-
metalation between 1 and 9 with the aid of the base to give a
1-alkenyl(boryl)palladium(II) species (10), and reductive elimi-
nation of 5 to regenerate Pd(0).11a The Heck coupling product
6 can be provided by insertion of the double bond of 5 into the
C-Pd bond of 9 followed by â-hydride elimination.17 On the
other hand, two different processes can be operative to yield
the homo-coupling products (7a and 7b). The head-to-tail
coupling product 7a is obtained by an addition-elimination
sequence involving formation and conversion of 11, 6, and 12,
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8002 J. AM. CHEM. SOC. VOL. 124, NO. 27, 2002