Stereospecific palladium-catalyzed borylation reaction of 1-alkenyl halides with diispropylaminoborane

Article abstract of DOI:10.1016/j.jorganchem.2005.01.054

The palladium catalysed borylation reaction of 1-alkenyl halides with diisopropylaminoborane was investigated. The reaction was conducted in dioxane at 80°C for 15h, in the presence of palladium catalyst, and triethylamine as base. The compounds produced

Full text of DOI:10.1016/j.jorganchem.2005.01.054

Journal of Organometallic Chemistry 690 (2005) 2721–2724
www.elsevier.com/locate/jorganchem
Stereospecific palladium-catalyzed borylation reaction
of 1-alkenyl halides with diispropylaminoborane
*
Lisenn Euzenat, David Horhant, Cedric Brielles, Gilles Alcaraz , Michel Vaultier
´
´
´
Laboratoire SESO,UMR CNRS 6510, Universite de Rennes1, 35042 Rennes1, France
Received 13 September 2004; revised 11 January 2005; accepted 11 January 2005
Available online 23 May 2005
Abstract
The palladium-catalyzed borylation reaction of 1-alkenyl halides with diisopropylaminoborane leads efficiently and stereospecif-
ically to the corresponding trans-1-alkenyl(diisopropylamino)boranes in good yields.
Ó 2005 Published by Elsevier B.V.
Keywords: Palladium; Borylation; Alkenes; Aminoboranes
1. Introduction
(Pin2B2) in the presence of alkenes [7], alkynes [8] or
1-alkenyl electrophiles [9] exclusively afford pinacol or
catechol 1-alkenyl boronates (Fig. 1).
1-Alkenylboron derivatives have attracted consider-
able attention as versatile intermediates in synthetic or-
ganic chemistry [1–4]. They are involved in the synthesis
of many natural products, bioactive compounds, and
are often the milestone for novel p-conjugated organic
photoactive materials and electronic devices [5].
These compounds have been mainly prepared by
using the well-known alkyne hydroboration reaction
[2] or by transmetalation of 1-alkenylmetal reagents by
organoboranes [3]. Although efficient, these methods
suffered from severe restrictions. Indeed (1) the trans-
metalation protocols require preliminary laborious
peripheral functions protection steps, (2) hydroboration
of internal alkynes takes place in number of cases only
regioselectively [4] and (3) terminal alkynes often afford
undesired gem-dihydroboration side-products [6]. The
most recent syntheses of vinylboron derivatives by cata-
lytic borylation either with pinacolborane (PinBH),
catecholborane (CatBH) or bis(pinacolato)diboron
However, 1-alkenylboron compounds including in
their framework a Bsp2 –H moiety have to the best of
our knowledge never been isolated nor spectroscopically
characterized. They are only known as intermediates in
situ generated by hydroboration reaction of alkynes
with thexylborane [10] and monochloroborane [11],
respectively (Fig. 1). In this paper, we wish to report
an unprecedented one-step procedure synthesis of
Bsp2 –H containing 1-alkenyl boron derivatives as stable
and isolable species.
2. Results and discussion
Recently, we demonstrated the excellent ability of
monomeric aminoboranes to react as BH sources for
Csp2 –Bsp2 bond formation in a metal-catalyzed process
[12]. For our purpose, we naturally envisaged applying
our palladium-catalyzed borylation reaction to 1-alke-
nyl halides.
*
Corresponding author.
E-mail address: gilles.alcaraz@univ-rennes1.fr (G. Alcaraz).
0022-328X/$ - see front matter Ó 2005 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2005.01.054

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L. Euzenat et al. / Journal of Organometallic Chemistry 690 (2005) 2721–2724
2722
R3
H
B
H
R3
BH2
R1
R2
R1
R2
R3 = Thex, Cl
O
O
O
O
B
R2
R1
B
R2
R1
R3
R3
pinacol alkenylboronate
catechol alkenylboronate
Fig. 1. Vinylboron derivatives.
The reaction was conducted in dioxane at 80°C for
15 h, in the presence of a catalytic amount (5 mol%) of
(Ph3P)2PdCl2, triethylamine (5 eq.) as the base,
diisopropylaminoborane (iPr2N–BH2, 2 eq.) and 1-
alkenyl halides (1 eq.) 1a–f [13]. In all cases, the 11B
NMR spectroscopy revealed for isolated 2 a large sin-
glet-like signal (68 < m1/2 < 90 Hz) contrarily to
aryl(diisopropylamino)boranes [14], in a range of 34–
37 ppm which is in agreement with the presence of
the iPr2NBðHÞ–Csp2 moiety. Notwithstanding the
1
non-observable JBH, the presence of the B–H bond
1
in 2 was surprisingly confirmed by H NMR spectros-
copy. The vinylic proton of the CH group adjacent to
the boron atom exhibited an unexpected well-resolved
doublet of doublet pattern for 2a and 2d and a dou-
blet in the case of 2d. The hypothesis of a through
3
the boron JHH-coupling was then investigated by
homonuclear decoupling experiments in the case of
2a and 2b. When irradiating in the zone corresponding
to the more shielded but barely visible BH signal at
4.72 ppm (1416.6 Hz/300 MHz), the initially observed
vinylic doublet of doublet collapsed to a doublet
Fig. 2. Homodecoupling experiment: aromatic and vinylic protons
zone of 2a before (A) and after (B) irradiation. 11B{1H} NMR
spectruma of 2a (C). a2a is contaminated by traces of [iPr2NB(H)]2O
giving a more shielded signal at 29.3 ppm.
revealing
a
vinylic trans-coupling constant of
prior attack of the zerovalent palladium to the halogen
substituted carbon of the 1-alkenyl halides in the oxida-
tive addition step of the catalytic cycle [17]. The forma-
tion in all cases of trans adducts 2 would be dependent
of this elementary step which generally takes place with
retention of stereochemistry in the case of palladium,
platinum or cobalt [18]. Now, cis-r-vinylpalladium(II)
complexes have been also reported to isomerize easily
to the corresponding thermodynamically favoured
trans-r-vinylpalladium(II) specie [19]. Assuming the
subsequent boron transfer step takes place as well with
retention of stereochemistry, the hypothesis of the cis
to trans-isomerization of the r-vinylpalladium(II) inter-
mediate vinylic bond under the reaction conditions
17.40 Hz in both cases (Fig. 2) [15]. Compounds 2
were obtained in medium to high yields (Table 1)
and their spectroscopic data are fully in agreement
with the proposed structures [16]. These features
clearly indicates that whatever the stereochemistry of
the starting styryl halide, the borylation reaction leads
stereospecifically to the corresponding trans-sty-
ryl(diisopropylamino)boranes. These results are fully
in accordance with MasudaÕs observations for the cou-
pling of cis-1-iodo-3,3-dimethyl-but-1-ene with pin-
acolborane [9a].
trans Adducts 2 derive from the corresponding r-
vinylpalladium(II) intermediates resulting from the

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2723
Table 1
Synthesis of 1-alkenyl(diisopropylamino)hydridoboranes
R1
R2
R3
B
H
H
R1
R3
(Ph3P)2PdCl2 cat./ Et3N
dioxane, 80˚C
iPr2N
B
NiPr2
X = I, Br
R2
X
1a-f
2a,d,f
H
1
R1
R2
R3
X
Yielda(%)
d11B/ppm (CDCl3)
1a
1b
1c
1d
1e
1f
a
Ph
Ph
H
H
H
H
H
H
H
I
2a: 33
2a: 94
2a: 72
2d: 34
2d: 84
2f: 81
36.7 (m1/2 = 88 Hz)
36.7 (m1/2 = 88 Hz)
36.7 (m1/2 = 88 Hz)
36.9 (m1/2 = 68 Hz)
36.9 (m1/2 = 68 Hz)
33.9 (m1/2 = 61 Hz)
H
Br
Br
Br
Br
Br
H
p-MeOC6H4
Ph
H
H
–CH2–(CH2)3–CH2–
p-MeOC6H4
Isolated yields.
not perfectly rationalized and is currently under
investigation.
PdL2
R
PdL2X
cis
R
X
cis-1
Acknowledgements
We thank D.S. Sinbandhit for homodecoupling
experiments, the French ministry of research and the
CNRS for their financial support.
X
PdL2X
PdL2
trans
R
R
trans-1
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