1
44
LETTER
Low-Coordinated Diphosphinidenecyclobutene Ligands: A New Entry for
Stille Cross-Coupling of Aryl Bromides
a
a
a
a
b
L
A
ow-Coordinated
n
D
iphosphini
i
denecyc
l
lobutene
Lig
S
ands . Gajare, Rader S. Jensen, Kozo Toyota, Masaaki Yoshifuji,* Fumiyuki Ozawa
a
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
Fax +81(22)2176562; E-mail: yoshifj@mail.tains.tohoku.ac.jp
b
Received 7 September 2004
This paper is dedicated with best wishes to Prof. Iwao Ojima on the occasion of his 60th birthday.
transition metal complexes of 3,4-diphosphinidenecyclo
Abstract: A sterically protected diphosphinidenecyclobutene/pal-
ladium system has been used to mediate the catalytic cross-coupling
of aryl bromides with organostannanes. An air- and moisture-stable
butene ligands (hereafter abbreviated as DPCB)10 and
have applied these complexes to several catalytic reac-
diphosphinidenecyclobutene ligand proved the efficiency and gen- tions,8 because the DPCB ligands are air- and moisture-
,11
erality of this class of ligand for cross-coupling reactions.
stable and have turned out to act as efficient p-acceptors.
Key words: Stille cross-coupling, diphosphinidenecyclobutene,
palladium, aryl bromides
Herein we report that DPCB/Pd(OAc)2 and DPCB/
Pd(OCOCF ) complexes catalyze Stille cross-coupling
3
2
reactions of aryl bromides with organotin reagents
Scheme 1). These reactions proceed under relatively mild
(
1
0a,d
Transition metal-catalyzed Stille cross-coupling reactions conditions, with 2 mol% 1
at 100 °C for 12 hours.
play a major role in the formation of C-C bonds. As a
result, the cross-coupling of aryl halides with organotin
reagents has become a steadfast method in organic synthe-
sis. This methodology has been used to prepare biaryl
R1
Br
2
mol% 1/Pd(OAc)2
Bu SnR1
R
+
3
R
1
2 equiv CsF,
dioxane, 100 °C
1
.1 equiv
compounds and allylation products, which are prevalent
2
in both natural products and drug compounds. Stille
Mes*
3
cross-couplings have proven to be an especially popular
R2
R2
1
P
4
S
S
tool for synthetic organic chemists, due in part to the air
and moisture stability of organotin reagents, as well as the
excellent functional group compatibility of the process.
To date very few reports have appeared on Stille cross-
R2
=
P
Mes*
A
B
C
5
6
coupling reactions with aryl chloride and aryl bromide.
However, significant progress has recently been achieved.
(DPCB)
1
5
b,c
R = vinyl, allyl, Ph; Mes* = 2,4,6-t-Bu C H
3
6
2
For example, Fu in his pioneering studies, disclosed a
palladium catalyst based on sterically hindered electron-
Scheme 1
rich P(t-Bu) for the Stille cross-coupling of aryl chlorides
3
and aryl bromides. This method, although general, re-
quires a highly air-sensitive ligand that requires special
handling techniques. Fu reported an air-stable [(t-
For optimization of reaction conditions, Stille coupling of
electron-rich 4-bromoanisole with tributyl(vinyl)tin in di-
oxane at 100 °C was chosen as a model reaction. We ini-
tially screened base and solvent as shown in Table 1. The
Stille coupling of 4-bromoanisole with tributyl(vinyl)tin
in the presence of 2 equivalents of CsF, 2 mol% of 1A/
1
d
6a
Bu) PH]BF precursor of P(t-Bu) and Nolan reported
3
4
3
a Pd/N-heterocyclic catalyst for the Stille reaction. Re-
cently, Baldwin reported that CuI is required as co-cata-
7
lyst for Stille coupling using the PdCl /P(t-Bu) system.
2
3
Pd(OAc) and 2 mL dioxane at 100 °C gave the best re-
2
We have been interested in developing low-coordinate sults (entry 1). In entry 2, the reaction conducted with 2
phosphorus compounds bearing a low-lying p* orbital, equivalents of TBAF (1 mol solution in THF) gave the
2
8
mainly located around the sp phosphorus atoms. The co- product in slightly lower yield. The bases CsCO , Et N
3
3
ordination chemistry of phosphaalkenes has attracted a and NaOH proved unsuitable for this reaction (entries 3, 4
great deal of recent interest because of the unique elec- and 5). Fu’s5 and Nolan’s studies on the Stille reaction
b,c
6a
9
tronic structure of C=P bonds. In the course of our studies have revealed the importance of a fluoride source in acti-
on low coordinated phosphorus compounds, we have in- vating organotin compounds for the transmetalation step.
terests in the identification and development of various Thus, we chose CsF as the most efficient among those that
we have examined. In entry 6 the reaction conducted with-
out solvent afforded the corresponding product in 34%
yield. Toluene proved to be less suitable than dioxane as
a solvent providing 80% yield after 12 hours at 100 °C
SYNLETT 2005, No. 1, pp 0144–0148
Advanced online publication: 08.11.2004
DOI: 10.1055/s-2004-836023; Art ID: U25204ST
0
5.
0
1.
2
0
0
5
(
entry 7). Dichloromethane, N-methyl-2-pyrrolidinone
©
Georg Thieme Verlag Stuttgart · New York