A. Nakamura, M. Tokunaga / Tetrahedron Letters 49 (2008) 3729–3732
3731
reactivity so that required longer times compared to
primary alcohols to give corresponding vinyl ethers in
64–90% yield (entries 1 and 6–9). In entry 9, the substrate
alcohol was diastereomer mixture, and the produced vinyl
ether found to have a similar diastereomer ratio (ꢀ1:1) to
the substrate. Further lower reactivities were observed with
tertiary alcohols (entries 10 and 11). In entry 10, the cata-
lyst system of entry 8 in Table 1 (AuClPPh3/AgTFA) with
20 equiv of ethyl vinyl ether exhibited better yield but
resulted in 26% at 40 h. Linalool remained intact under
the condition of entry 11. Actually, this compound is
known as one of the most difficult substrate for vinylation.
Both Pd and Ir catalysts failed in obtaining the product.11d
Recently, Ishii et al. reported that silanol derivatives were
vinylated using Ir catalysts with vinyl acetate.16 However,
our catalyst did not afford desired vinylated product of tri-
phenylsilanol (entry 12). GC and GC–MS analysis sug-
gested the formation of dehydrated dimer of silanol
(Ph3SiOSiPh3). In entry 13, transfer vinylation of phenol
was tested. When ethyl vinyl ether was used as vinylating
reagent, no product formation was observed but vinyl ace-
tate as vinylating reagent gave the product in 20% yield.
Addition of NEt3 caused the formation of phenyl acetate.
Table 3 shows the result of transfer vinylation of carb-
oxylic acids. The optimized catalyst system (AuClPPh3/
AgOAc) with vinyl acetate as vinylating reagent was found
to be effective for this transformation. The product vinyl
esters of both aliphatic and aromatic carboxylic acids were
attained in 78–96% yield (entries 1–5).
Although detailed mechanism of the present Au(I)-cata-
lyzed vinyl transfer reaction is unclear, a similar function of
Au(I) and Pd(II) metal centers is expected. Thus, oxygen
nucleophiles attack the a-position of the coordinated vinyl
group, which is followed by rotation and b-alkoxy elimina-
tion. Since the known Pd(II)-catalysts for this transforma-
tion employ nitrogen ligands rather than phosphine
ligands, the Au(I) catalysts may present an opportunity
for further optimization by modifying the phosphine ligand
used. It is reported that Pd(II) was deactivated to a consid-
erable extent when phosphines were used as a ligand.11b
Other features of the present reaction compared to the
Ir(I)-catalyzed reactions are a milder condition (50 °C cf.
100 °C for Ir) and nonnecessity of an inert-gas atmosphere.
In conclusion, we have developed the first example of Au-
catalyzed transfer vinylation of alcohols and carboxylic
acids.
References and notes
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´
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Table 3
AuClPPh3/AgOAc-catalyzed vinylation of carboxylic acidsa
O
O
2 mol%
cat
+
OH
+
AcO
AcOH
Yieldb (%)
R
O
R
50 ˚C
Entry
1
Substrate
Time (h)
22
COOH
89
78
96
81
83
COOH
2
3
4
44
52
40
51
COOH
COOH
COOH
5
a
14. Typical procedure forAu-catalyzed transfer vinylation of alcohols: To a
screw cap tube containing AuClPPh3 (9.9 mg, 2 mol %) and silver salt
(2 mol %), 1 ml of ethyl vinyl ether (10 equiv) was added under air.
A mixture of AuClPPh3 (2 mol %), AgOAc (2 mol %), vinyl acetate
(1 ml), and carboxylic acids (1 mmol) was stirred at 50 °C.
b
GC yield.