Rhodium-catalyzed cross-coupling of organoboron compounds with vinyl acetate

Full text of DOI:10.1002/anie.200903146

Angewandte
Chemie
DOI: 10.1002/anie.200903146
Cross-Coupling
Rhodium-Catalyzed Cross-Coupling of Organoboron Compounds with
Vinyl Acetate**
Jung-Yi Yu and Ryoichi Kuwano*
Transition-metal-catalyzed cross-coupling of organometallics
with organohalogen compounds is a powerful approach for
connecting two molecules by a carbon–carbon bond.[1] In
organic synthesis, either organobromine or organoiodine
compounds are typically used as the coupling partners with
an organometallic compound. The chloro[2] or sulfonate[3]
group is occasionally chosen as the leaving group of the
electrophilic substrate. Furthermore, the organometallic
compounds can couple with an electrophilic substrate bearing
a phosphate,[4] alkoxy,[5] thio,[6] siloxy,[7] diazonium,[8] ammo-
nium,[9] sulfonium,[10] chlorosulfonyl,[11] triazene,[12] azole,[13]
or phosphonium group.[14]
The usability and environmental friendliness of the cross-
coupling may be enhanced if an acetoxy group functions as
the leaving group of the electrophilic substrate. The acetate
substrates are readily available from commercial sources, or
the esterification of alcohols with acetic anhydride in general.
The acetate compounds are easier to handle in air than the
corresponding sulfonates or phosphates. The acetoxy group
itself does not have a serious impact on animals. Furthermore,
cross-coupling using acetate electrophiles will release an
acetate salt as the sole stoichiometric by-product, and this salt
is readily metabolized by microbes in the natural environ-
ment. However, the acetoxy group is an unusual leaving
group for metal-catalyzed cross-couplings, because the cata-
experiments. Vinyl tosylate can also be used for vinylation,
although it is inferior to halides in terms of availability.[21]
Therefore, vinyl acetate has emerged as an ideal coupling
partner with organometallic compounds in vinylation[22]
because it is easy to handle owing to its moderate boiling
point. Moreover, the vinylic electrophile is comparable in cost
to vinyl chloride. Herein, we report the cross-coupling of
organoboron compounds with vinyl acetate using a rhodium
complex as a catalyst.
We attempted the reaction of 4-tert-butylphenylboronic
acid (1a) with vinyl acetate (2) in the presence of various
metal complexes (Table 1). The desired cross-coupling
scarcely occurred in the presence of palladium or nickel
complexes that are commonly used for catalytic cross-
Table 1: Cross-coupling of 4-tert-butylphenylboronic acid (1a) with vinyl
acetate (2): Effect of catalyst or reaction conditions.[a]
À
lyst generally cleaves the acyl C O bond in preference to
Entry
[M]
Ligand
Additive
Yield [%][b]
[15]
À
another C O bond. Use of the acetate leaving group had
been limited to the reaction of allylic[16] or benzylic sub-
strates.[17] Very recently, the groups of Shi and Garg inde-
pendently developed the Suzuki–Miyaura[18] or Negishi
coupling[19] of aryl or alkenyl pivalates using a [NiCl2{P-
(cC6H11)3}2] catalyst.
The cross-coupling of organometallic species with vinyl
chloride or bromide allows access to terminal alkenes,[2a,20]
which are widely used as substrates in many organic reactions
and also as monomers in polymer synthesis. However, the
boiling points of the vinyl halides are lower than ambient
temperature, which impairs their use in laboratory-scale
1
[Ni(cod)2]
[NiCl2{P(c-C6H11)3}2]
[Pd(dba)2]
[{RuCl2(p-cymene)}2]
[{IrCl(cod)}2]
[{RhCl(cod)}2]
[{IrCl(cod)}2]
[{RhCl(cod)}2]
[{RhCl(cod)}2]
[{RhCl(cod)}2]
[{RhCl(cod)}2]
[{RhCl(cod)}2]
[{RhCl(cod)}2]
[{RhCl(cod)}2]
[{RhCl(cod)}2]
[{RhCl(cod)}2]
–
–
–
–
–
–
–
–
–
–
–
–
–
–
8
17
0
16
36
26
0
34
75[d]
63
54
55
21
54
0
2[c]
3
4
5
6
7
8
9
10
11
12
13
14
15
16
DPPB
DPPB
DPPB
DPPB
DPPB
DPPB
P(cC6H11)3
DPPP
DPPPent
DPEphos
tAmOH
iPrOH
EtOH
Et2NH
tAmOH
tAmOH
tAmOH
tAmOH
[*] J.-Y. Yu, Prof. Dr. R. Kuwano
36
Department of Chemistry, Graduate School of Sciences, Kyushu
University
[a] Reactions were conducted in toluene (1.0 mL). 1a (0.20 mmol)/
2 K3PO4/additive/[M]/ligand 100:1000:300:150:5.0:5.5. [b] GC yield
(average of two runs). [c] The reaction was conducted in dioxane at
1108C. [d] 3a was isolated in 75% yield when the reaction was
conducted in 0.5 mmol scale for 3 h. See the Supporting Information
for details. cod=cycloocta-1,5-diene, dba=dibenzylideneacetone,
DPPB=1,4-bis(diphenylphosphino)butane, DPPP=1,3-bis(diphosphi-
no)propane, DPPPent=1,5-bis(diphenylphosphino)pentane, DPE-
phos=2,2’-bis(diphenylphosphino)diphenyl ether, tAm=1,1-dimethyl-
propyl.
6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 (Japan)
Fax: (+81)92-642-2572
E-mail: rkuwano@chem.kyushu-univ.jp
Homepage: http://www.scc.kyushu-u.ac.jp/Yuki/main.html
[**] This work was supported by Tosoh Organic Chemistry, Co. Ltd.,
KAKENHI (No. 19020051), and a Grant-in-Aid for the Global COE
Program, “Science for Future Molecular Systems” from MEXT.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903146.
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Table 2: Cross-coupling of arylboronates 1 with 2.[a]
coupling reactions (Table 1, entries 1–3). Even [NiCl2{P-
(cC6H11)3}2], which is the most effective catalyst for the
cross-coupling of aryl pivalates,[18,19] failed to produce the
desired vinylarene 3a in high yield. A certain amount of 3a
was obtained from the reaction with [{IrCl(cod)}2] or [{RhCl-
(cod)}2] (cod = cycloocta-1,5-diene; Table 1, entries 5 and 6);
however, the cross-coupling was accompanied by the forma-
tion of tert-butylbenzene (25–40%). This undesirable side-
reaction was suppressed by using a bisphosphine ligand,
DPPB (Table 1, entry 8). Furthermore, the addition of a
stoichiometric protic compound improved the rhodium-
catalyzed carbon–carbon bond formation remarkably
(Table 1, entries 9–12). In particular, tert-amyl alcohol is
effective for the production of 3a; the vinylated product was
isolated in 75% yield. However, the yield of 3a significantly
decreased when the alcohol was used as a reaction solvent.
DPPB is the ligand of choice: the use of other phosphine
ligands caused a decrease in the yield of 3a (Table 1,
entries 13–16).
Vinylations of arylboronic acids other than 1a with vinyl
acetate were conducted under the optimized conditions, but
owing to the protodeboration of the boronic acids, in most
cases the yields of the coupling products were low or
moderate. The reaction of 3-butoxyphenylboronic acid
afforded 3-butoxystyrene (3b) in 48% yield of isolated
product along with a significant amount of butoxybenzene
(ca. 2:1 ratio determined by GC). The side reaction was
successfully avoided by using ethylene glycol ester 1b in place
of the arylboronic acid. The vinylation of 1b gave the desired
product 3b in 88% yield (Table 2, entry 1). A range of aryl
boronates were coupled with 2 in the presence of the DPPB–
rhodium catalyst and were transformed into the correspond-
ing substituted styrenes. Neither an electron-donating nor an
electron-withdrawing group on the aromatic ring of 1 caused
significant inhibition of the rhodium catalysis (Table 2,
entries 1–7). The catalytic vinylation was compatible with
Boc-protected amino or alkoxycarbonyl groups (Table 2,
entries 4 and 7). Ortho-substituted styrene 3i was obtained
from 1i in good yield (Table 2, entry 8). As with monocyclic
aryl boronates, polycyclic or heterocyclic aryl boronates acted
as coupling partners with 2 (Table 2, entries 9 and 10).
Furthermore, the rhodium-catalyzed reaction is applicable
to the synthesis of 1,3-dienes. (E)-1-Aryl-1,3-butadiene 3l was
obtained from the reaction of alkenylboronate 1l with 2 in
moderate yield (Table 2, entry 11). No formation of the
Z isomer was detected during the course of the rhodium-
catalyzed carbon–carbon bond formation.
Entry
1[c]
1
Product (3)
Yield [%][b]
88
2
3
91
77
4[c]
51
5
77
68
57
6
7[d]
8[e]
79
85
9
10
43
49
11[c,f]
[a] Reactions were conducted in toluene (2.0 mL). The ratio
of (0.50 mmol)/2 K3PO4/tAmOH/[{RhCl(cod)}2]/DPPB was
100:500:300:300:2.5:5.5. [b] Yield of isolated product. [c] The ratio of
1/tAmOH was 1:2. [d] The ratio of 1/tAmOH was 1:1.1. [e] The reaction
was conducted for 36 h. [f] The reaction was conducted for 48 h. Boc=
tert-butoxycarbonyl.
1
Reactions of some substituted vinyl acetates were
attempted with the present rhodium catalyst system. a-
Acetoxystyrene 4 reacted with 1a, affording 1,1-diarylethene
5 in moderate yield [Eq. (1)]. Furthermore, (E)-b-acetoxy-
acrylate 6 was converted into (E)-cinnamate 7 with no
formation of its Z isomer [Eq. (2)]. The absence of tert-amyl
alcohol improved the yield of 7 because the alcohol additive
caused the solvolysis of the acetate. However, the DPPB–
rhodium catalyst failed to effectively couple arylboron
compounds to other substituted vinyl acetates, such as b-
acetoxystyrene and 2-propenyl acetate (16% and 0% yield,
respectively).
We consider that the rhodium-catalyzed vinylation pro-
ceeds through a pathway similar to a typical mechanism of
palladium-catalyzed Suzuki–Miyaura reaction (path a in
Scheme 1).[23] The DPPB-ligated rhodium(I) species 8 under-
7218
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13 mmol), DPPB (11.7 mg, 28 mmol), and K3PO4 (320 mg, 1.5 mmol)
was diluted with toluene (2.0 mL). Alkenyl acetate (1.1 or 2.5 mmol)
and tert-amyl alcohol (0.55–1.5 mmol) were added into the resulting
suspension at room temperature. The mixture was stirred at 1008C for
24 h and then diluted with hexane (2.0 mL) or EtOAc (2.0 mL). After
the filtration through a Celite pad, solvent was removed from the
filtrate under reduced pressure. The residue was purified with a flash
column chromatography (EtOAc/hexane) to give the desired product
3.
Received: June 11, 2009
Published online: August 27, 2009
Keywords: acetates · boranes · cross-coupling ·
.
homogeneous catalysis · rhodium
Scheme 1. Two possible reaction pathways of the rhodium-catalyzed
cross-coupling of alkenyl acetates with organoboron compounds 1.
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À
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alkenyl acetate. The stereochemistry of the reaction in
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catalyzed cross-coupling proceeds through path b.[27]
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usable as an electrophilic substrate for the catalytic cross-
coupling with organoboron compounds. The substitution of
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by using a bisphosphine-ligated rhodium catalyst, whereas
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benign methodology for connecting two molecules through a
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Experimental Section
General procedure: Under nitrogen atmosphere, a mixture of an
arylboron compound
1
(0.50 mmol), [{RhCl(cod)}2] (6.2 mg,
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7219

Communications
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[27] (Z)-6 and (Z)-7 were not detected in the GC analyses of the
reaction mixture given in Eq. (2). (Z)-7 isomerized sluggishly
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catalyst and K3PO4 (9% at 40 min, 100% at 8 h). (Z)-7 would
have been detected in the GC analyses if the reaction of (E)-6
produced (Z)-7 directly.
[28] During preparation of this manuscript, Larhed and co-workers
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À
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