1,4-Addition of Arylsiloxanes to Enones Catalyzed by Dicationic Palladium(II) Complexes in Aqueous Media

Article abstract of DOI:10.1246/cl.2003.752

Catalytic 1,4-addition of arylsiloxanes to enones was carried out at 75°C in the presence of a dicationic palladium(II) catalyst in aqueous 1,4-dioxane. A nitrile-free complex generated in situ from Pd(dba)₂ and Cu(BF₄)₂ in the presence of dppe or dppben was recognized to be the best catalyst to achieve high yields for the representative enones and enals.

Full text of DOI:10.1246/cl.2003.752

7
52
Chemistry Letters Vol.32, No.8 (2003)
1,4-Addition of Arylsiloxanes to Enones Catalyzed by Dicationic Palladium(II) Complexes
in Aqueous Media
Ã
Takashi Nishikata, Yasunori Yamamoto, and Norio Miyaura
Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628
(Received June 9, 2003; CL-030518)
Catalytic 1,4-addition of arylsiloxanes to enones was car-
from Pd(dppe)Cl2 and AgBF4 gave an addition product in
38% yield (Entry 2). Addition of HBF4 (20 mol%) was also ef-
fective, presumably for accelerating the dissociation of benzoni-
trile (Entry 3). Although cationic Pd(II) complexes have been
synthesized from Pd(II) halide complexes and silver salts (Entry
ꢀ
ried out at 75 C in the presence of a dicationic palladium(II)
catalyst in aqueous 1,4-dioxane. A nitrile-free complex generat-
ed in situ from Pd(dba)2 and Cu(BF4)2 in the presence of dppe
or dppben was recognized to be the best catalyst to achieve high
yields for the representative enones and enals.
1
2
2) or by electrochemical oxidation of Pd(PPh3)4, Cu(BF4)2
was found to be a mild oxidizing reagent of palladium(0) spe-
cies for in situ preparation of nitrile-free catalysts. The catalyst
synthesized from Cu(BF4)2Á6H2O (20 mol%) and Pd(dba)2
(5 mol%) in the presence of dppe (5 mol%) provided the product
in 80% yield (Entry 5), whereas other copper(II) salts such as
Cu(ClO4)2 and Cu(OTf)2 were not effective (Entries 6 and 7).
The presence of an oxidant may also serve to recycle Pd(0) spe-
cies that are generated by double transmetalation giving a ho-
mocoupling product or by other side reactions, since 3a and
3b resulted in much higher yields than did the catalyst generat-
ed from Pd(dppe)Cl2 and AgBF4 (Entries 2, 5, and 8). Although
the reaction provided a mixture of 4 and 5, the formation of al-
kene product (5) derived from b-hydride elimination decreased
Metal-catalyzed 1,4-addition of organometallic reagents to
a,b-unsaturated carbonyl compounds is a widely used process
in organic chemistry. Since the reaction yields a stereogenic
center at the b-carbon, considerable efforts have been devoted
to the development to asymmetric syntheses. Among these stud-
ies, rhodium complexes have recently been found to be excel-
1
lent catalysts for 1,4-addition of aryl- or alkenylboron, -sili-
2
3
4
5
6
con, -bismuth, -tin, -titanium, and zirconium compounds.
Although divalent palladium complexes have rarely been used
for such a catalytic cycle starting from transmetalation, Pd(II)
complexes in acidic media have been successfully used for
7
8
ꢀ
1,4-addition of aryltin and -boron compounds to enones. Very
recently, we also reported 1,4-addition of arylboronic acids cat-
when the reaction was carried out at 50 C (48%, 4/5=96/4).
Dppben was a better ligand than dppe to suppress the formation
ꢀ
9
alyzed by [Pd(dppe)(PhCN)2](BF4)2 (3d). Since arylsiloxanes
are convenient reagents that are easily available via transmeta-
of 5 at 75 C (Entry 8). On the other hand, other bidentate li-
gands such as dppm, dppp, dppb and dppf were not effective,
and the reaction was strongly retarded by addition of a silicone
activator such as CsF and Bu4NFnH2O.
10
11
lation or cross-coupling reaction of HSi(OR)3, the protocol
was extended to an analogous reaction of silicone compounds
(
Scheme 1).
Reaction between trimethoxyphenylsilane and 2-cyclohexe-
Results of 1,4-addition of representative arylsiloxanes to
ꢀ
enones or enals at 75 C with a catalyst system (3a, 3b, or 3c)
are summarized in Table 2. Among the substrates screened, 2-
ꢀ
none was carried out at 75 C in dioxane/water (2/1) (Table 1).
Pd(dppe)(PhCN)2](BF4)2 (3d) unexpectedly resulted in no re-
action (Entry 1), but a nitrile-free complex generated in situ
[
cyclopentenone (Entries
(Table 1) have a strong tendency to yield an alkene product
5), though such a side reaction was negligible for 2-cyclohep-
tenone (Entries 3 and 4), acyclic enones (Entries 5-20) and enals
(Entries 21-23). The reactions of substituted arylsiloxanes often
1 and 2) and 2-cyclohexenone
(
R1
Ar
R1
Ar
R1
ArSi(OMe) (2)
3
+
resulted in moderate yields due to formation of the correspond-
R2 Pd catalyst (3)
R2
R2
O
aq. 1,4-dioxane
O
O
Table 1. Addition of PhSi(OMe)3 to 2-cyclohexenonea
75 °C
1
4
5
Entry
Catalyst
Yield/% (4/5)
1
2
3
4
5
3d
Pd(dppe)Cl2/AgBF4
3c
Pd(dba)2/dppe
3a
0
38
Pd catalyst
3a: Pd(dba) /dppe/Cu(BF )
Si(OMe)3
2
4 2
64 (72/18)
0
80 (88/12)
3
3
b: Pd(dba) /dppben/Cu(BF )
2 4 2
FG
c: [Pd(dppe)(PhCN) ](BF ) (3d)/HBF
2
4 2
4
2
a: FG=H
6
7
8
Pd(dba)2/dppe/Cu(ClO4)2
Pd(dba)2/dppe/Cu(OTf)2
3b
0
Ph Ph
2b: FG=4-MeO
2c: FG=2-MeO
2d: FG=4-Me
2e: FG=4-MeCO
2f: FG=4-CF3
PPh2
PPh2
0
77 (92/8)
P
NCPh
Pd
NCPh
Ph
(BF4)2
P
a
ꢀ
All reactions were carried out at 75 C for 23 h in the pres-
ence of 2-cyclohexenone (1 mmol), PhSi(OMe) (2.5 mmol),
a palladium complex (5 mol%), a ligand (if used, 5 mol%)
and an additive (if used, 20 mol%) in 1,4-dioxane/water
(2/1, 9 mL).
Ph
3
dppben
Scheme 1.
3
d
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.8 (2003)
753
ing arenes (Entries 7, 10, 12), but they were improved signifi-
cantly when the water content in 1,4-dioxane was decreased
to 6:1 or 12:1 (Entries 8, 13, 14). Enals required a higher tem-
perature than that of enones because the reactions were retarded
by formation of hemiacetals in aqueous media (Entries 22 and
Ph
Ph
Ph
Pd(dba)2 + dppe + Cu(BF4)2
Ar
P
P
S
Pd
S
(BF4)2
R1
R2
Ph
23).
O
6 S= H O or solvent
2
A probable catalytic cycle is shown in Scheme 2. In con-
trast to a very smooth transmetalation of arylboronic acids to di-
cationic Pd(II)-nitrile complexes at room temperature, transme-
H O + HBF
Ar [Si] + H O
2
2
4
ꢀ
talation of arylsiloxanes require a nitrile-free complex at 75 C.
[Pd]+
HO-[Si] + HBF4
Ar
R1
Thus, a method for preparing nitrile-free catalysts (6) via oxida-
tion of Pd(dba)2 with Cu(BF4)2 was newly developed. The for-
mation of a dicationic species (6) was confirmed by 31P NMR,
which exhibited a single signal at 74.9 ppm. Arylsilanols pro-
duced by hydrolysis may participate in transmetalation, giving
enone (1)
Ar [Pd]+
R2
O
7
8
Scheme 2.
7
dition of arylsiloxanes to enones. Insertion of an enone into the
, as was demonstrated in the related rhodium-catalyzed 1,4-ad-
2
References
1
C-Pd bond yields a C-bound palladium enolate (8), which is
highly susceptible to hydrolytic C-Pd bond cleavage.
a) M. Sakai, H. Hayashi, and N. Miyaura, Organometallics,
6, 4229 (1997). b) Y. Takaya, M. Ogasawara, T. Hayashi,
1
3
1
M. Sakai, and N. Miyaura, J. Am. Chem. Soc., 120, 5579
(1998). c) T. Hayashi, M. Takahashi, Y. Takaya, and M.
Ogasawara, J. Am. Chem. Soc., 124, 5052 (2002). For a re-
view, d) K. Fagnou and M. Lautens, Chem. Rev., 103, 169
Table 2. Addition of ArSi(OMe)3 to enones (Scheme 1)a
Entry
2
Enone
Catalyst Yield/% (4/5)
1
2
3
4
5
6
7
2a 2-cyclopentenone
2a 2-cyclopentenone
2a 2-cycloheptenone
2a 2-cycloheptenone
3b
3a
3b
3c
3b
3c
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3b
3c
3c
3b
3c
3b
3b
60 (43/57)
58 (50/50)
86 (4>99)
87 (4>99)
84 (4>99)
82 (4>99)
64 (4>99)
77 (4>99)
74 (4>99)
61 (4>99)
81 (4>99)
62 (4>99)
85 (4>99)
90 (4>99)
90 (4>99)
96 (4>99)
60 (4>99)
75 (4>99)
80 (4>99)
95 (4>99)
56 (4>99)
72 (4>99)
74 (4>99)
(
2003).
a) T. Koike, X. Du, A. Mori, and K. Osakada, Synlett, 2002,
01. b) S. Oi, Y. Honma, and Y. Inoue, Org. Lett., 4, 667
2002).
2
3
3
(
2a n-C5H11CH=CHCOCH3
2a n-C5H11CH=CHCOCH3
2b n-C5H11CH=CHCOCH3
2b n-C5H11CH=CHCOCH3
2c n-C5H11CH=CHCOCH3
2d n-C5H11CH=CHCOCH3
2d n-C5H11CH=CHCOCH3
2e n-C5H11CH=CHCOCH3
2e n-C5H11CH=CHCOCH3
2f n-C5H11CH=CHCOCH3
2a PhCH=CHCOCH3
a) T.-S. Huang and C.-J. Li, Org. Lett., 3, 2037 (2001). b) S.
Venkatraman and C.-J. Li, Tetrahedron Lett., 42, 781
(
2001).
b
8
9
4
5
6
7
S. Oi, M. Moro, H. Ito, Y. Honma, S. Miyano, and Y. Inoue,
Tetrahedron, 58, 91 (2002).
T. Hayashi, N. Tokunaga, K. Yoshida, and J. W. Han, J. Am.
Chem. Soc., 124, 12102 (2002).
A. Kakuuchi, T. Taguchi, and Y. Hanzawa, Tetrahedron
Lett., 44, 923 (2003).
a) S. Cacchi, D. Misiti, and G. Palmieri, Tetrahedron, 37,
2941 (1981). b) T. Ohe and S. Uemura, Tetrahedron Lett.,
43, 1269 (2002).
C. S. Cho, S. Motofusa, K. Ohe, and S. Uemura, J. Org.
Chem., 60, 883 (1995).
T. Nishikata, Y. Yamamoto, and N. Miyaura, Angew.
Chem., Int. Ed. Engl., 42, 2768 (2003).
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
c
b
3
d
4
5
6
7
8
9
0
1
2
2a PhCH=CHCOCH3
2a i-C3H7CH=CHCOCH3
2a i-C3H7CH=CHCOCH3
2a PhCH=CHCOPh
2a PhCH=CHCOPh
2a CH3CH=CHCHO
8
9
10 W. P. Weber, in ‘‘Silicon Reagents for Organic Synthesis,’’
Springer-Verlag, Berlin (1983).
11 M. Murata, M. Ishikura, M. Nagata, S. Watanabe, and Y.
Masuda, Org. Lett., 4, 1843 (2002).
2 C. Amatore, A. Jutand, M. Mederious, and L. Mottier, J.
Electroanal. Chem., 422, 125 (1997).
13 A. C. Alb e´ niz, N. M. Catalina, P. Espinet, and R. Red o´ n,
Organometallics, 18, 5571 (1999).
c
2a n-C3H7CH=CHCHO
2a PhCH=CHCHO
c,e
2
3
aAll reactions were carried out for 23 h in the presence of enone
1
(
dioxane/water (2/1, 9 mL). E-isomers were used for all acyclic
1 mmol), ArSi(OMe)3 (2.5 mmol) and a catalyst (5 mol%) in
b
c
ꢀ
d
enones and enals. dioxane/water=6/1. At 95 C. dioxane/
water=12/1. For 9 h.
e
Published on the web (Advance View) July 21, 2003; DOI 10.1246/cl.2003.752