Ruthenium-Catalyzed Hydroacylation of Internal Alkynes
COMMUNICATION
with methoxy or CF3 substituent and dithienylacetylene oc-
curred and the corresponding conjugated enones (3qb–d)
were obtained in high yields. For alkyl-substituted alkynes,
the reactions proceeded smoothly by using an increased
amount of HCO2Na to give the corresponding products
(3oe–g) in good-to-high yields with high regioselectivity.
The reaction of an aliphatic alkyne resulted in a low yield of
the product 3qh.
One of the most advantageous features of solid catalysts
is their high recyclability. After the reaction of 1a or 1o
with 2a, the solid ruthenium catalyst was separated from the
reaction mixture by centrifugation and washed with diethyl
ether and water/methanol. The resulting powder was cal-
cined in air at 4008C for 30 min to recover Ru/CeO2 for
subsequent catalytic runs. As shown in Scheme 2, the solid
Scheme 3. Possible reaction mechanism.
alkenes.[13] After the reaction of 2a with Ru/CeO2 catalyst, a
catalytic amount of stilbene was detected by GC-MS analy-
sis, indicating such RuII species should be reduced to Ru0
species.[14] The in situ generated Ru0 species cleaves the
Scheme 2. Recycling of the Ru/CeO2 catalyst.
À
formyl C H bonds to give acyl-Ru intermediate A. Alkynes
À
À
then inserted into Ru C or Ru H bonds and reductive
elimination gives conjugated enones 3 with the regeneration
of Ru0 species. The decarbonylation from intermediate A
may be suppressed by the steric hindrance of substituents
on the ortho-positions of aromatic aldehydes.[15]
Ru/CeO2 continued to show catalytic activity and gave 3aa
and 3oa without significant decreases in yields for at least
three times. Removal of the solid ruthenium catalyst by the
hot filtration through a polytetrafluoroethylene (PTFE)
filter (pore size 0.45 mm) after the reaction for 30 min
partly retarded further progress of reaction of 1i and 2a
(Figure S1 in the Supporting Information), which indicates a
significant contribution of soluble ruthenium species to the
catalysis. On the other hand, the amount of ruthenium spe-
cies leached into the solution after the reaction of 1o with
2a by the fresh Ru/CeO2 catalyst was 2.0% of the rutheni-
um species in the fresh catalyst (0.00050 mmol as Ru), and
In summary, intermolecular hydroacylation of internal al-
kynes in the presence of Ru catalysts, namely Ru/CeO2 and
Ru complex catalysts, together with HCO2Na and Xantphos
was described. Even the reaction of aromatic aldehydes
without coordinating groups gives the corresponding conju-
gated enones in good yields. In particular, various aromatic
aldehydes with substituents at the ortho-positions of aromat-
ic rings are transformed to the products in high yields. The
present catalytic system by using solid Ru/CeO2 shows high
environmental compatibility because of their high recyclabil-
ity and the very low contamination of the products by met-
allic species. Detailed mechanistic studies and expansion of
the reaction scope are currently underway in our laboratory.
the same amount of [RuCl2ACTHNUTRGNE(UNG p-cymene)]2 catalyst did not
show significant activity (Table S3 in the Supporting Infor-
mation).
Although the reaction mechanisms in the present Ru/
CeO2 catalytic system are not yet fully understood, a possi-
ble mechanism is shown in Scheme 3.[2,4–7,13] As was descri-
bed in our previous paper, a RuIV–oxo species is formed on
the surface of CeO2 or ZrO2 before the catalytic reac-
tions.[10e] An X-ray absorption fine structure (XAFS) study
revealed that this RuIV species on CeO2 is transformed into
RuII species by treatment with HCO2Na and Xantphos in
DMA at 1508C (Figures S3–S5 in the Supporting Informa-
tion). Darses and co-workers reported that the treatment of
Experimental Section
2,6-Dimethylbenzaldehyde (1o; 67 mg, 0.50 mmol), diphenylacetylene 2a
(267 mg, 1.5 mmol), sodium formate (10.2 mg, 0.15 mmol), Xantphos
(28.9 mg, 0.050 mmol), and N,N-dimethylacetamide (2.0 mL) were placed
in a glass Schlenk tube under an argon atmosphere together with Ru/
CeO2 (125 mg, 0.025 mmol as Ru). The reaction mixture was stirred at
1508C for 5 h on a hot stirrer equipped with a cooling block. After the
reaction, the solution was separated from the reaction mixture by centri-
[RuCl
2ACHTUNGTRENNUNG
(p-cymene)]2 with formate salts gave RuIIH2 species,
which are easily reduced to Ru0 species by the reaction with
Chem. Eur. J. 2013, 19, 861 – 864
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