of 1-octyne at 100 °C in 2-propanol. Since the activity of
this complex turned out to be quite low, it had to be used in
Table 1. Ruthenium Complex-Catalyzed anti-Markovnikov
Hydration of 1-Hexynea
30 mol % to generate octanal in 35% yield. Hence, the
reaction was only slightly catalytic. Importantly, however,
the selectivity of the reaction was excellent, since 2-octanone
was not detected at all. The major byproduct was n-heptane,
which was obtained in 18% yield, while the lost carbon was
trapped as the CO ligand on ruthenium, with RuCpCl(CO)-
entry
catalystb
mol %
time, h
yield, %c
1
2
3
4
5
6
7
8
9
RuCpCl(dppm)
RuCpCl(dppe)
RuCpCl(dppp)
RuCpCl(dppb)
RuCpCl(dppbts)
RuCpCl(dmpm)
RuCpCl(depe)
RuCpCl(PMePh2)2
RuCpCl(PMe2Ph)2
RuCpCl(PMe3)2
1.0
4.0
5.0
5.0
5.0
5.0
5.0
5.0
1.5
3.0
12
12
36
36
12
36
36
12
12
12
>99 (95)
>99
73
97
84
38
47
>99
97
97
3
(PPh ) isolated in 65% yield from the reaction mixture. Taken
together, these observations are fully consistent with the
presence of an intermediate complex with a Ru-C(O)-(n-
C
7
H
15) linkage.
In the reaction of (PNP)RuCl
CH PPh ) with excess phenylacetylene and water, Bianchini
et al. found that quantitative (based on Ru) amounts of
2 3 2
(PPh ) (PNP ) n-PrN(CH -
1
0
2
2 2
)
a
1-Hexyne, 1.0 mmol; Ru catalyst, 1-5 mol %; H2O, 0.75 mL;
(CO) and toluene are formed.10 According to
2-propanol, 2.5 mL in a sealed tube under argon at 100 °C (bath
temperature). b dppm, bis(diphenylphosphino)methane; dppe, 1,2-bis(diphen-
ylphosphino)ethane; dppp, 1,3-bis(diphenylphosphino)propane; dppb, 1,4-
bis(diphenylphosphino)butane; dppbts, 1,2-bis(di-4-sulfonatophenylphos-
phino)benzene tetrasodium; dmpm, bis(dimethylphosphino)methane; depe,
(
PNP)RuCl
2
their detailed investigation, the reaction involves the tau-
tomerization of Ru(1-alkyne) to Ru(vinylidene), attack by
2
water of the vinylidene to form Ru{C(O)CH Ph}, and its
,2-bis(diethylphosphino)ethane. c GC yield, isolated yield in parentheses.
1
further transformation to the decarbonylated intermediate
(
CO)Ru(CH
2
Ph). Similarly, the Cp-ligated cationic (vi-
+
nylidene)ruthenium complex, [RuCp(CdCHPh)(PPh
3
)(L)] ,
catalysts with a monodentate phosphine ligand, PMePh
PMe Ph, and PMe , were somewhat unexpected (entries
-10). These, and in particular RuCpCl(PMe Ph) , also
showed high activities for the hydration of 1-hexyne to
exclusively give hexanal, perhaps due to the higher basicity
and stronger coordination ability of these phosphines com-
2
,
has been reported to react with water to afford the phenyl-
acetyl complex, [RuCp(COCH Ph)(PPh )(L)] (L ) CO), or
the benzyl carbonyl complex, [RuCp(CH Ph)(CO)(L)] (L )
). Therefore, it is very likely that the hydration of
-octyne to octanal, which is promoted by RuCpCl(PPh
takes place via a vinylidene intermediate. Since RuCpCl-
CO)(PPh ) is too stable to show catalytic activity, the key
2
3
2
3
8
2
2
2
1
1
PPh
1
3
3 2
) ,
3
pared to PPh .
Using RuCpCl(dppm) as a catalyst, the scope of the anti-
(
3
to better catalytic efficiency is obviously suppression of
decarbonylation from the metal-acyl intermediate so that
hydrogen uptake by the acyl group can lead to the liberation
of aldehyde and re-formation of active Ru species.
Markovnikov hydration of various 1-alkynes was examined
(Table 2). With the exception of entries 4 and 9, reactions
using 2-10 mol % of catalyst in 2-propanol at 100 °C gave
the desired aldehydes in good to excellent yields after 12 h.
Indeed, bidentate phosphines have been found to have an
excellent effect. The reaction of 1-hexyne with water, carried
out in the presence of 1 mol % of RuCpCl(dppm) in
Table 2. RuCpCl(dppm)-Catalyzed anti-Markovnikov
Hydration of Terminal Alkynesa
2-propanol at 100 °C for 12 h, resulted in the complete
conversion of 1-hexyne and gave hexanal quantitatively,
while neither 2-hexanone nor pentane was observed (Table
1
, entry 1). 2-Propanol has been found to be a far better
solvent than other alcohols or organic solvents.
The addition of NH PF , HPF , or NaOH did not accelerate
4
6
6
the reaction. As indicated in Table 1, the dppm ligand gives
the best results among the bidentate phosphines. Although
RuCpCl(dppe), RuCpCl(dppp), and RuCpCl(dppb) also gave
aldehyde selectively without forming ketones, more than 4-5
mol % of ruthenium catalyst and/or prolonged heating were
required to give yields of 70-90% (entries 2-4). The water-
6 4 6 4 3 2 2
soluble bidentate phosphine, o-C H {P(p-C H SO Na) }
(dppbts), is also a good ligand (entry 5), but Ru complexes
with the bidentate trialkyl phosphine, dmpm and depe, show
lower activity (entries 6 and 7). The activities of ruthenium
(
8) (a) March, J. AdVanced Organic Chemistry; Wiley: New York, 1992;
p 787. (b) Pine, S. H.; Hendrickson, J. B.; Cram, D. J.; Hammond, G. S.
a
Organic Chemistry; McGraw-Hill Book Co: Singapore, 1981; p 523.
Alkyne or dialkyne, 1.0 mmol; RuCpCl(dppm), 2-10 mol %; H2O,
(
9) Tokunaga, M.; Wakatsuki, Y. Angew. Chem., Int. Ed. 1998, 37, 2867.
0.75 mL; 2-propanol, 2.5 mL in a sealed tube under argon at 100 °C for 12
h. Isolated yield, except entry 4 which shows GC yield. c Heated for 24
b
(10) Bianchini, C.; Casares, J. A.; Peruzzini, M.; Romerosa, A.; Zanobini,
d
F. J. Am. Chem. Soc. 1996, 118, 4585.
11) Bruce, M. I.; Swincer, A. G. Aust. J. Chem. 1980, 33, 1471.
h. At 130 °C in 2-methoxyethanol.
(
736
Org. Lett., Vol. 3, No. 5, 2001