relevant cationic Pt(II) species are inhibited by coordination
with the N atoms of these substrates. Carboxylic acid
anhydrides other than Ac2O can be used with similar
efficiency (entries 4, 5, 10, 11). It is also worth mentioning
that phenol is O- rather than C-acylated under these condi-
tions (entry 12).
however, it cannot be excluded that traces of HX (X ) OTf,
SbF6) are formed under these conditions which may them-
selves be catalytically relevant.12 Despite this ambiguity,
RuCl3‚(H2O)n is an attractive pre-catalyst due to its com-
paratively low cost and because the solvents need not be
rigorously dried.
To study the scope of the proposed “hard/soft-mismatch”
principle, a variety of other transition metal salts have also
been screened (Table 3). Among them, the combination of
Spectroscopic investigations provide some insights into
the nature of the catalytically relevant species as well as into
their interactions with the substrates. Whereas addition of
(PhCN)2PtCl2 (1 equiv) to either Ac2O or anisole causes only
marginal changes in the IR and 13C NMR spectra of these
reagents, exposure to the cationic species generated from
(PhCN)2PtCl2/2AgSbF6 leads to strong effects.13 The ob-
served shifts for Ac2O (Table 4) are in accordance with the
Table 3. Acylation of Anisole by Ac2O: Screening of the
Catalytic Activity of Various Transition Metal Salts (2.5 mol %)
in the Presence of AgX (5 mol %). All Reactions Were Carried
out in Refluxing CH2Cl2 unless Stated Otherwise
no.
metal salt
AgX
t (h)
yield (%)
Table 4. Characteristic Changes in the IR (∆ν, cm-1) and 13C
NMR Spectra (∆δ, ppm) of Acetic Anhydride upon Addition of
a Neutral or a Cationic Pt(II) Complex (1 equiv of each) to a
Solution in CD2Cl2
1
2
3
4
5
6
7
8
9
(COD)CuCl
CuCl2‚2[P(C6F5)3]
(MeCN)2PdCl2
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
20
20
72
20
72
21
21
48
20
21
21
58
78
75
2
79
31
80
45
81
70
73
RhCl(CO)(PPh3)2
a
[RuCl2(CO)3]2
RuCl3‚(H2O)n
RuCl3‚(H2O)n
RuCl3‚(H2O)n
(MeCN)2PtCl2
(MeCN)2PtCl2/2AgSbF6
b
∆δ(CdO)
∆ν(CdO)
-0.03
(0
+2.4
-31/-50
c
c
AgBF4
AgOTf
AgSbF6
d
RuCl3‚(H2O)n
RhCl3‚(H2O)n
e
e
10
11
AgBF4
f
Hg(ClO4)2‚(H2O)n
expected complexation of its carbonyl function onto a Lewis
acidic agent (∆δ ) +2.4 ppm; ∆ν ) -31/-50 cm-1),
whereas a significant upfield shift is noticed for C(para) of
anisole (∆δ ) -5.9 ppm), indicating a higher electron
density at the position undergoing FC-acylation (Table 5).
a Using only 1.25% of the dimeric complex. b 8 mol % in CH3NO2 at
50 °C. c 8 mol % (each metal) in CH3NO2 at 50 °C. d 5 mol %. e 10 mol %
(each metal) in CH3NO2 at 50 °C. f 5 mol % in CH3NO2 at 50 °C.
(MeCN)2PdCl2 and AgSbF6 shows appreciable activity,
although the reaction rate is significantly lower than with
the corresponding platinum system. Good results are obtained
with [RuCl2(CO)3]2 or CuCl2‚2[P(C6F5)3] in combintion with
AgSbF6. Even better is a mixture of RuCl3 and AgX,
affording the desired product in up to 81% yield. Since the
commercial hydrate of RuCl3 is used for solubility reasons,
Table 5. Changes (∆δ, ppm) in the 13C Chemical Shifts of
Anisole upon Addition of a Neutral or a Cationic Pt(II)
Complex (1 equiv. each) to a Solution in CD2Cl2
(MeCN)2PtCl2
(MeCN)2PtCl2/2AgSbF6
C(quart.)
C(ortho)
C(meta)
C(para)
OMe
-0.06
-0.05
+0.08
-0.04
-0.02
+1.7
+0.4
-1.2
-5.9
+0.5
(7) For previous applications of cationic Pd(II) or Pt(II) complexes in
synthesis, see: (a) Sodeoka, M.; Tokunoh, R.; Miyazaki, F.; Hagiwara, E.;
Shibasaki, M. Synlett 1997, 463. (b) Kataoka, Y.; Matsumoto, O.; Ohashi,
M.; Yamagata, T.; Tani, K. Chem. Lett. 1994, 1283. (c) Ghosh, A. K.;
Matsuda, H. Org. Lett. 1999, 1, 2157. (d) Oi, S.; Terada, E.; Ohuchi, K.;
Kato, T.; Tachibana, Y.; Inoue, Y. J. Org. Chem. 1999, 64, 8660. (e) Nieddu,
E.; Cataldo, M.; Pinna, F.; Strukul, G. Tetrahedron Lett. 1999, 40, 6987.
(f) Oi, S.; Kashiwagi, K.; Inoue, Y. Tetrahedron Lett. 1998, 39, 6253. (g)
Hagiwara, E.; Fujii, A.; Sodeoka, M. J. Am. Chem. Soc. 1998, 120, 2474.
(h) Ferraris, D.; Young, B.; Dudding, T.; Lectka, T. J. Am. Chem. Soc.
1998, 120, 4548. (i) Sodeoka, M.; Ohrai, K.; Shibasaki, M. J. Org. Chem.
1995, 60, 2648. (j) Cataldo, M.; Nieddu, E.; Gavagnin, R.; Pinna, F.; Strukul,
G. J. Mol. Catal. A: Chem. 1999, 142, 305. (k) Gorla, F.; Venanzi, L. M.
HelV. Chim. Acta 1990, 73, 690. (l) Hori, K.; Kodama, H.; Ohta, T.;
Furukawa, I. J. Org. Chem. 1999, 64, 5017. (m) Jia, C.; Piao, D.; Oyamada,
J.; Lu, W.; Kitamura, T.; Fujiwara, Y. Science 2000, 287, 1992.
(8) (a) Fu¨rstner, A.; Szillat, H.; Gabor, B.; Mynott, R. J. Am. Chem.
Soc. 1998, 120, 8305. (b) Fu¨rstner, A.; Szillat, H.; Stelzer, F. J. Am. Chem.
Soc. 2000, 122, 6785.
The signal of H-4 is fully preserved in the 1H NMR spectrum.
These results are best interpreted in terms of a preferred
coordination of anisole via the π-system rather than the ether
oxygen although the precise hapticity of binding cannot yet
be unambiguously deduced. It is conceivable that such an
intermediate undergoes reversible electrophilic substitution
with formation of a para-substituted Pt-arene complex.14-16
(12) Triflic acid is known to catalyze various FC acylations, cf. refs 5b,
5c, 5d, and literature cited therein.
(9) Fu¨rstner, A.; Voigtla¨nder, D. Synthesis 2000, 959.
(10) Several control experiments have shown that AgX (X ) SbF6, OTf,
ClO4, BF4, PF6) itself is not responsible for the observed catalytic activity
in any of these cases.
(11) A screening of different solvents in the acylation of anisole by Ac2O
catalyzed by PtCl2(PhCN)2 (2.5 mol %) and AgSbF6 (5 mol %) gave the
following results: CH2Cl2 (75%), CHCl3 (56%), CH3NO2 (63%), MeCN
(3%), THF (0%).
(13) Precipitated AgCl was filtered off under Ar prior to these spectro-
scopic investigations.
(14) Note, however, that neutral Pt(II) triflate (trifluoroacetate) salts
complexed by phosphines do not undergo electrophilic substitution of arenes,
cf: (a) Brainard, R. L.; Nutt, W. R.; Lee, T. R.; Whitesides, G. M.
Organometallics 1988, 7, 2379. (b) Peters, R. G.; White, S.; Roddick, D.
M. Organometallics 1998, 17, 4493.
Org. Lett., Vol. 3, No. 3, 2001
419