An unusual dimerization of primary unsaturated alcohols catalyzed by RuHCl(CO)(PPh3)3

Article abstract of DOI:10.1039/b602103d

When primary unsaturated alcohols were treated with a catalytic amount of RuHCl(CO)(PPh₃)₃ in benzene under reflux, dimerization reactions took place to give α-hydroxymethyl ketones as major product. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602103d

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An unusual dimerization of primary unsaturated alcohols catalyzed by
RuHCl(CO)(PPh3)3{
Takashi Doi, Takahide Fukuyama, Satoshi Minamino, Guillaume Husson and Ilhyong Ryu*
Received (in Cambridge, UK) 13th February 2006, Accepted 3rd March 2006
First published as an Advance Article on the web 21st March 2006
DOI: 10.1039/b602103d
When primary unsaturated alcohols were treated with a
catalytic amount of RuHCl(CO)(PPh3)3 in benzene under
reflux, dimerization reactions took place to give a-hydroxy-
methyl ketones as major product.
The isomerization of carbon–carbon double bonds, a process
Scheme 1 Ru–H catalyzed reaction of unsaturated alcohols.
catalyzed by metal hydride complexes via hydrometalation and
subsequent b-hydride elimination, is among the most familiar
reactions in transition metal chemistry.1 Ruthenium hydride
Table 1 summarizes results of RuH-catalyzed dimerization of
complexes are used in such alkene-isomerization reactions,2–4 in
which RuHCl(CO)(PPh3)3 is the most frequently used catalyst.3
unsaturated alcohols. Dimerization of the isomeric hexenyl
alcohols 1b, 1c, 1d, 1e and 1f under similar conditions (0.33 M,
5 mol% RuHCl(CO)(PPh3)3, benzene reflux, 13 h) also occurs to
While the isomerization of secondary unsaturated alcohols to the
give the same product 2a in similar yields (entries 1–5). Both cis
corresponding saturated ketones by HRu-catalysts is known
(Scheme 1),4 remarkably, to the best of our knowledge,
and trans isomers underwent a smooth dimerization. On the other
hand, no dimerization was observed with the secondary alcohol
1g, which underwent the isomerization to 3-hexanone (5) (entry 6).7
isomerization reactions of primary unsaturated alcohols have not
been investigated in any detail.4a,e,5 In this communication we
Thus, the dimerization of primary hexenols proceeds irrespective
report that primary unsaturated alcohols, when treated with a
of the position or the E- or Z-geometry of the double bonds.
catalytic amount of RuHCl(CO)(PPh3)3, undergo an unusual
The reaction of unsaturated alcohol 1i, which contains a longer
dimerization to give a-hydroxymethyl ketones as the major
substituent at the 3-position, gave the saturated dimer 2i in 33%
product.
yield together with a significant amount of unsaturated dimer 3i, in
Treatment of 5-hexen-1-ol (1a) with RuHCl(PPh3)3 and
which the position of the C–C double bond was dispersed into the
RuH2(CO)(PPh3)3, resulted in the isomerization of the C–C
longer carbon chain (entry 8). The subsequent treatment of the
double bond to give a mixture of hexenols, while hexanal was
mixture of 2i and 3i with hydrogen and a Pd/C catalyst gave 2i as
formed in only trace amounts with these catalysts. In contrast,
the sole dimerization product. The experimental result of 1i, which
when a 0.33 M benzene solution of 5-hexen-1-ol (1a) was treated
was obtained at a low concentration (0.017 M), which is 20 times
with 5 mol% of RuHCl(CO)(PPh3)3 for 1 h at reflux, a mixture of
lower than the standard concentration, is interesting, since decanal
5-hydroxymethyl-6-undecanone (2a) and the related unsaturated
(7) and an isomeric mixture of decenals 8 were obtained in
products 3a were obtained in 33% and 8% yields, respectively
significant amounts (entry 9). This result proposes that a double
(eqn (1)). The crude mixture also contained the saturated alcohol,
bond migration reaction of 1i would take place to give 7, whereas
1-hexanol and diol 4a, the reduced form of 2a.6 Interestingly,
allylic alcohol 1i would also serves as the hydrogen source of
however, when the reaction time was extended to 13 h, the yield of
transfer hydrogenation to be converted into 2-decenal.8,9 Cinnamyl
a-hydroxymethyl ketone 2a increased to 59%, whereas the yield of
alcohol (1j) also underwent dimerization to give a mixture of 2j
3a and 4a diminished to the level of trace amounts. It seems likely
and 3j (entry 10).
that 3a and 4a were converted to 2a under the reaction conditions.
Following up the above observation, we propose a rationale for
the dimerization of 1a to 2a based on an Ru-aldol reaction10 and
transfer hydrogenation (Scheme 2). Thus, as a consequence of
ruthenium hydride-mediated isomerization of a C–C double bond,
ð1Þ
aldehyde B would be formed via allylic alcohol A. Transfer
hydrogenation of B with A would produce a,b-unsaturated
aldehyde C and n-hexanol. Hydroruthenation of C would produce
ruthenium enolate D, which then undergoes aldol reaction with B
to give F via E. Transfer hydrogenation of F with A provides the
dimerization product 2a and a,b-unsaturated aldehyde C. A
similar scenario based on aldol reaction of D with C may account
for the formation of unsaturated by-product 3a.
Department of Chemistry, Graduate School of Science, Osaka
Prefecture University, Sakai, Osaka 599-8531, Japan.
E-mail: ryu@c.s.osakafu-u.ac.jp; Fax: +81 72 254 9695;
Tel: +81 72 254 9695
{ Electronic supplementary information (ESI) available: Experimental
procedure and NMR data. See DOI: 10.1039/b602103d
In summary, we have found that RuHCl(CO)(PPh3)3 catalyzes
the dimerization of unsaturated primary alcohols leading to the
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Table 1 Representative examples of Ru–H catalyzed dimerization of primary unsaturated alcoholsa
Entry
1
Unsaturated alcohol 1
Products (yieldb)
2
3
2a 56%
2a 52%
4
2a 58%
2a 58%
5
6
7
8
c
9d
1i
2i 19% 3i 4%
10
11e,,f
a
b
c
d
General Conditions: [1] = 0.33 M, RuHCl(CO)(PPh3)3 (5 mol%), C6H6, reflux, 13 h. Isolated yield. Determined by 1H NMR. [1i] =
0.017 M. 10 mol% of RuHCl(CO)(PPh3)3 was used. 2-Ethylhexane-1,3-diol was obtained in 13% yield as a by-product.
e
f
formation of a-hydroxymethyl ketones. Formation of ruthenium
enolates and aldol reaction with aldehydes was proposed as a
rationale to account for the unusual reaction. Further detailed
mechanistic studies as well as optimization of the dimerization
reaction are currently being pursued in our laboratory.
IR thanks a Grant-in-Aid for Scientific Research on Priority
Areas (A) ‘‘Reaction Control of Dynamic Complexes’’ from the
MEXT, Japan for the financial support.
Notes and references
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Scheme 2 A possible reaction mechanism for the dimerization of 1a.
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6 A mixture of unsaturated diols, reduced form of 3a, was also detected
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7 We also tested the reaction of hexanal under the same reaction
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reaction was very sluggish and unreacted hexanal was recovered, along
with a small amount of the Tischenko type ester (3%) and 1-hexanol
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8 For HRu-catalyzed transfer hydrogenation of enones, see: T. Doi,
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