T. Suzuki et al. / Tetrahedron Letters 43 (2002) 7531–7533
7533
formed from 3-phenyl-prop-1-yn-3-ol (1f), again with
the E isomer as the predominant form, but the reaction
was slow and a longer reaction time (60 h) was required
(entry 8). Tertiary propargylic alcohols, 3-methyl-but-1-
yn-3-ol, and 3-phenyl-but-1-yn-3-ol, were scarcely
transformed to a,b-unsaturated aldehydes, and
remained unreacted under these conditions: elevated
reaction temperatures (e.g. 140°C) caused formation of
side products.
hydes in high yields in the presence of water and a
catalytic amount of ruthenium complex. This reaction
is remarkably clean and proceeds under neutral condi-
tions. In general, the reaction mixture simply consists of
solvent, water, unreacted reactants, and the a,b-unsatu-
rated aldehydes formed regardless of the conversion,
while few if any by-products are detected throughout
the reaction. Further applications of this reaction, as
well as detailed mechanistic investigations, are now in
progress.
The reaction is formally a 1,3-shift isomerization of the
hydroxy group, but does not proceed without the pres-
ence of adequate water. To confirm that the present
process does not involve the concerted 1,3-migration of
the alcoholic OH group, the reaction of 1c was carried
out in a mixture of H218O and 2-propanol. The
obtained aldehyde exclusively contained 18O, n-
C5H11CHꢁCHC(ꢁ18O)H (2c−18O), as was obvious by
the red-shift of the stretching vibration of the carbonyl
group compared to that of 2c [n(CO): 1695 cm−1 in 2c,
1662 cm−1 in 2c−18O]. Therefore, it is obvious that the
net reaction is anti-Markovnikov hydration at the ter-
minal alkyne carbon with concomitant dehydration of
the original OH group. The established reaction mecha-
nism for the previously reported anti-Markovnikov
hydration of 1-alkynes12 may be applied to the present
hydration of propargylic alcohols, although it is not yet
clear when the dehydration takes place. Early-stage
dehydration should give Ru–allenylidene intermediate
(A) followed by nucleophilic attack of the a-carbon of
the allenylidene unit by OH(−): the ability of Ru–
allenylidene complexes to react with weak nucleophiles,
such as alcohol and amine, at C(a) has been reported.13
Alternatively, the dehydration could occur after the
attack of C(a) of g-hydroxyvinylidene intermediate (B)
by water and successive formation of a g-hydroxyacyl
complex. In any case, the key-intermediate should have
a Ru(IV) metal center which bears a hydride and an
allenylidene (or (hydroxy)vinylidene) units. Interest-
ingly, Bassetti and co-workers very recently applied the
anti-Markovnikov hydration of 1-alkynes to propar-
gylic alcohols in micellar solutions at 60°C and
obtained saturated b-hydroxyaldehydes.14
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In summary, we have found a highly selective transfor-
mation of propargylic alcohols to a,b-unsaturated alde-