The study was initiated based on the finding that a
rhodium complex catalyzes the acyl-transfer reaction of a
symmetric dibenzyl ketone. When 1-(p-chlorophenyl)-3-(p-
cyanophenyl)propane-2-one 1 was reacted in N,N0-dimethy-
limidazolidinone (DMI) at 150 °C for 12 h in the presence
of RhH(CO)(PPh3)3 (5 mol %) and 1,2-bis(diphenylphos-
phino)ethane (10 mol %), 1,3-bis(p-chlorophenyl)propane-
2-one 2 (24%) and 1,3-bis(p-cyanophenyl)propane-2-one
3 (25%) were obtained with the recovery of 1 (50%)
(Scheme 2). No reaction occurred in the absence of the
rhodium complex. The rhodium complex cleaved the COꢀC
bond of one molecule of 1 and transferred the (p-chloro-
phenyl)acetyl group to another molecule of 1 in an inter-
molecular manner, likely via acylrhodium and/or benzylrho-
dium intermediates.
molar ratio of 5 to 4 was changed from 1 to 3, the yields of 6
and 7 increased to 75 and 67%, respectively.7 These results
suggested the equilibrium nature of the reaction, because the
statistical yields were calculated as 50% for a 1:1 ratio of
substrates and 75% for a 3:1 ratio. Accordingly, the reverse
reaction of 6 (1 equiv) and 7 (1 equiv) with the same catalyst
in DMI at 150 °C for 12 h gave 4 and 5 in 32 and 32% yields,
respectively.
Scheme 3
Scheme 2
2-Aryl-1-phenyl-1-ethanones reacted with various
S-methyl thioesters (3 equiv) giving acyl-transferred ke-
tones and 7 in about 75% yields (Table 1). Aroyl groups
were effectively transferred from aromatic thioesters to the
benzyl carbon of 2-aryl-1-phenyl-1-ethanones, which pos-
sessed either p-electron-donating groups or p-electron-
withdrawing groups on the aryl moiety (Table 1, entries
1, 4ꢀ7). In the reaction of aliphatic thioesters, alkanoyl
groups were transferred in higher yields to the substrates
possessing p-cyano-, p-(ethoxycarbonyl)-, and p-chloro-
phenyl groups (entries 13ꢀ15), and the yield decreased for
a substrate with a phenyl group (entry 16). The reaction
could be applied to a thioester having the S-(p-tolyl) group
(entry 3). Chlorobenzene could be used as solvent in place
of DMI (entry 2). These reactions replaced the benzoyl
group of 1-aryl-2-phenyl-2-ethanones with various acyl
groups.
The alkanoyl group of 1-aryl-2-decanones could be
replaced with acyl groups as well as with the benzoyl
group in the above reactions (Table 2). The reaction
of 1-(p-cyanophenyl)-2-decanone and S-methyl p-(tert-
butyl)benzothioate (3 equiv) in the presence of RhH(CO)-
(PPh3)3 (8 mol %) and dppBz (16 mol %) in DMI at
150 °C for 12 h gave 4-[2-[p-(1,1-dimethylethyl)phenyl]-2-
oxoethyl]benzonitrile (80%) and S-methyl nonanethioate
8 (72%) (entry 1). The reaction of 1-aryl-2-decanones
showed a similar tendency to 1-aryl-2-phenyl-1-ethanones
in regard to the substituent effect on the aryl group (entries
1ꢀ11). Even the reaction of 1-aryl-2-decanones and ali-
phatic thioesters proceeded, which is an exchange reac-
tion of alkanoyl groups (entries 12ꢀ15). This method
has the advantage that benzyl ketones having functional
groups can be synthesized from other benzyl ketones and
thioesters.
We then considered that the COꢀC bond of benzyl
ketones and the CꢀS bond of organosulfur compounds
could be cleaved and exchanged.6 It was expected that such
rhodium-catalyzed reactions could be used to form new
COꢀC bonds in unsymmetric ketones without using stoi-
chiometric amounts of acid, base, or organometallic reagent.
When 1,2-diphenyl-1-ethanone 4 (1 equiv) was reacted with
S-methyl 4-(tert-butyl)benzothioate 5 (1 equiv) in the pre-
sence of RhH(CO)(PPh3)3 (5 mol %) and 1,2-bis(diphenyl-
phosphino)benzene (dppBz, 10 mol %) in DMI at 150 °Cfor
12 h, 1-[4-(1,1-dimethylethyl)phenyl]-2-phenylethanone 6
(51%) and S-methyl benzothioate 7 (42%) were obtained
(Scheme 3). No reaction occurred in the absence of the
rhodium complex or dppBz. Other metal complexes exhibit-
ing activity in the presence of dppBz were RhH(PPh3)4
(49%), Rh(acac)(CO)2 (43%), and Rh(acac)(CH2dCH2)
(20%). The bidentate ligands with phosphino groups sepa-
rated by two carbon atoms were also essential. When the
(5) Review: (a) Park, Y. J.; Park, J.-W.; Jun, C.-H. Acc. Chem. Res.
2008, 41, 222. (b) Jun, C.-H. Chem. Soc. Rev. 2004, 33, 610. For example,
see: (c) Kuninobu, Y.; Uesugi, T.; Kawata, A.; Takai, K. Angew. Chem.,
Int. Ed. 2011, 50, 10406. (d) Dreis, A. M.; Douglas, C. J. J. Am. Chem.
Soc. 2009, 131, 412. (e) Jun, C.-H.; Lee, H. J. Am. Chem. Soc. 1999, 121,
880.
(6) Arisawa, M.; Igarashi, Y.; Kobayashi, H.; Yamada, T.; Bando,
K.; Ichikawa, T.; Yamaguchi, M. Tetrahedron 2011, 67, 7846.
(7) Typical Experimental Procedures. In a two-necked flask were
placed 1,2-diphenyl-1-ethanone4 (0.25 mmol, 49.1 mg), S-methyl 4-(1,1-
dimethylethyl)benzothioate
5 (0.75 mmol, 152.2 mg), RhH(CO)-
(PPh3)3 (5 mol %, 11.5 mg), and dppBz (10 mol %, 11.2 mg) in DMI
(0.25 mL) under an argon atmosphere, and the solution was stirred at
150 °C for 12 h. The mixture was purified by flash column chromato-
graphy on silica gel, giving 1-[4-(1,1-dimethylethyl)phenyl]-2-phenyl-
ethanone 6 (47.5 mg, 75%) and S-methyl benzothioate 7 (25.5 mg,
67%).
Org. Lett., Vol. 14, No. 14, 2012
3805