Activation of a water molecule under mild conditions by ruthenacyclopentatriene: Mechanism of hydrative cyclization of diynes

Article abstract of DOI:10.1039/c0cc04345a

A ruthenium cyclic biscarbene complex reacted with a H₂O molecule under mild conditions to produce η⁵-oxapentadienyl complex, that proved to be the intermediate in the catalytic hydrative cyclization of a diyne.

Full text of DOI:10.1039/c0cc04345a

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Activation of a water molecule under mild conditions by
ruthenacyclopentatriene: mechanism of hydrative cyclization of diynesw
Yoshihiko Yamamoto,* Ken Yamashita and Hisao Nishiyama
Received 11th October 2010, Accepted 11th November 2010
DOI: 10.1039/c0cc04345a
A ruthenium cyclic biscarbene complex reacted with a H2O
molecule under mild conditions to produce g5-oxapentadienyl
complex, that proved to be the intermediate in the catalytic
hydrative cyclization of a diyne.
new complex 4 was obtained (Fig. 1). The single-crystal
X-ray analysis unambiguously showed that 4 is a half-open
oxaruthenocene possessing a five-membered oxacycle-fused
Z5-oxapentadienyl ligand. The Ru–O and Ru–C bond lengths
(2.167–2.235 A) are similar to those in previously characterized
examples.6 Although the relevant half-open oxaruthenocenes
have already been reported by several groups, this is the first
example of the formation of an Z5-oxapentadienyl ligand from
a cyclic biscarbene complex involving a novel activation mode
of a water molecule (vide infra).
Ruthenacyclopentatrienes, which are cyclic biscarbenes produced
by the oxidative cyclization of two alkyne components in a low-
valent ruthenium complex (Fig. 1),1 have attracted considerable
attention as key intermediates in transition-metal-catalyzed multi-
component coupling processes. Kirchner, Saa, and our group
have independently reported that the [2+2+2] cycloadditions of
alkynes with other unsaturated molecules catalyzed by
Cp*RuCl(cod) (1: Cp* = Z5-C5Me5, cod = 1,5-cyclooctadiene)
proceed via a ruthenacyclopentatriene.2,3 Recently, Dixneuf,
Trost et al. have reported interesting coupling processes
between alkynes with protic oxygen nucleophiles such as
carboxylic acids, water, and alcohols.4,5 Although the involve-
ment of ruthenacyclopentatriene intermediates was proposed
for these novel catalytic processes, no direct evidence on the
reaction of ruthenacyclopentatrienes with non-acidic oxygen
nucleophiles such as water has been provided thus far.
We wish to report a concrete evidence in which a fused
ruthenacyclopentatriene reacted with a water molecule via a
novel activation mode, giving rise to an Z5-oxapentadienyl
ruthenium complex and/or a bicyclic furan.
In order to obtain further insights into the formation of 4,
we examined the reaction of 3 with water under various
conditions (Table 1). It was observed that 3 slowly reacted
with water in a tetrahydrofuran (THF) solution at 25 1C to
afford 4 along with small amounts of dihydrofuryl ketone 5
and bicyclic furan 6 (entry 1). Obviously, 5 was produced by
the protonolysis of 4, whereas the formation of 6 cannot be
clearly explained. To the best of our knowledge, this is the first
example of the formation of a furan from a metallacycle by
utilizing a water molecule as an oxygen source.7 Because the
chloride ligand was observed to be lost during the transfor-
mation of 3 into 4, we examined the chlorine abstraction
from 3 using various silver salts. The addition of a silver salt
significantly accelerated the reaction: 3 was completely con-
sumed within a few hours. When AgBF4 was added, the yields
of 4 reached 44% (entry 2). It should be noted that the
reactions in the presence of the silver salt produced considerable
amounts of 6. Moreover, the reaction in the presence of
AgNO3 resulted in an almost exclusive formation of 6
(entry 3). It is assumed that the concomitant formations of
Brønsted acids such as HBF4 or HNO3 play an important role
in the formation of 6 when the corresponding silver salts are
added. In fact, the reaction in the presence of Ag2O selectively
afforded 4 in 71% yield (entry 4). The selective formation of 4
In an attempt to purify crude bicyclic ruthenacyclo-
pentatriene 3 formed from 1 and 1,7-diphenyl-4-oxahepta-
1,6-diyne (2) by neutral alumina column chromatography,
Table 1 Reaction of 3 with water in the presence of additives
Fig. 1 Ruthenium complexes 3 and 4. ORTEP drawing of 4 is shown
with 50% probability ellipsoids. All hydrogen atoms are omitted for
clarity.
Run
Additive
None
AgBF4
Time/h
Yields of 4, 5, and 6a (%)
1
2
3
4
5
6b
1
1
2
54, 5, 3
44, —, 34
7, —, 52
71 (60)d, —, 1
79 (73)d, —, —
c
Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Chikusa, Nagoya 464-8603, Japan.
E-mail: yamamoto@apchem.nagoya-u.ac.jp; Fax: +81 52 789 3209;
Tel: +81 52 789 3337
w Electronic supplementary information (ESI) available: Experimental
procedures, spectroscopic data, and CIF file. CCDC 766961. For ESI
and crystallographic data in CIF or other electronic format see
DOI:10.1039/c0cc04345a
c
AgNO3
Ag2Oc
Al2O3
e
4
a
b
Yields were determined by 1H NMR analysis. 84% conversion.
1.1 equiv. of Ag+ per 3. Yields of isolated product. 3 : Al2O3 =
1 : 2.5 w/w.
c
d
e
c
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presumably requires neutral reaction conditions; hence, we
employed neutral alumina as an acid scavenger. As a result, 4
was exclusively produced in the highest yield of 79%, albeit
with a reaction rate slower than those observed in the presence
of silver additives (entry 5).
Subsequently, the b-H abstraction took place through
a late transition state, TS-CD, with an activation barrier of
18.8 kcal molÀ1. The Ru–H and O–H distances are 1.72 and
1.34 A, respectively. The formation of hydride complex D is
estimated as exothermic by 8.1 kcal molÀ1. The subsequent
1,2-H shift was found to be very facile. This process was
estimated to proceed via an early transition state (TS-DE): the
Ru–H bond was elongated by only 0.06 A when compared
with D. The activation energy was estimated to be very small
(2.4 kcal molÀ1). The formation of s-allyl complex E bearing
a side-on bound formyl ligand from D was found to be
Dixneuf et al. proposed that 2 : 1 coupling of aryl-
alkynes with carboxylic acids is initiated by protonation of
ruthenacycles.4 This assumption was supported by their DFT
calculations for protonated ruthenacycle intermediates.4b
However, the protonation of ruthenacycles by water seems
to be unrealistic, because water is a much weaker proton
donor (by 102–105 times) than carboxylic acids (pKa = 2–5).
Thus, we investigated on the protonation of a model
ruthenacycle composed of the [CpRu]+ fragment and two
acetylene molecules using the density functional theory (DFT)
calculations. The DFT calculations suggest that the protona-
tion of the cationic model complex is highly unlikely because
of the considerably large activation energy (27.6 kcal molÀ1) of
this process (ESIw Fig. S1). Hence, alternative mechanisms
should be operative during the reactions of cationic ruthenacycle
with water. A reasonable option involves a hydroxylated
ruthenacycle as proposed by Trost and coworkers.5 Such an
intermediate could be produced by the addition of a water
molecule to one of the carbene carbons, followed by deproto-
nation. However, no stationary point could be located for the
model water adduct A (Scheme 1). Thus, we investigated
the hydroxo ligand migration with model B, because the
deprotonation of an aquo precursor can afford such a
hydroxo complex and a similar migration of PR3 ligands has
previously been reported by Kirchner and others.2a DFT
calculations suggest that 1,2-migration of the hydroxo ligand
takes place with an activation energy of 16.9 kcal molÀ1
via TS-BC (Fig. 2). This activation barrier is much lower
than that of the protonation of the cationic ruthenacycle
shown in Fig. S1 (ESIw). The formation of the hydroxy-
lated ruthenacycle C is estimated to be endothermic by
exothermic by 6.6 kcal molÀ1
.
The haptotropic rearrangement from the g-formylallyl
complex to an Z5-oxapentadienyl one was investigated using
higher models including a 2,5-dihydrofuran moiety (Fig. 3).
This modification helps us to simplify further investigations by
avoiding the s-cis/s-trans conformational change of the diene
moiety at the expense of computational efficiency. As a result,
we could find that the g-formylallyl complex F was trans-
formed into the final Z5-oxapentadienyl complex H through
an intermediate, s-allyl complex G with an end-bound formyl
group (the Ru–O and Ru–C bond distances are 2.06 and
3.04 A, respectively). The initial side-on to end-on isomerization
of F was estimated to occur with almost no barrier via an
early transition state (TS-FG). The formation of G is thermo-
dynamically favorable because of relatively large exothermicity
6.2 kcal molÀ1
.
Scheme 1 Formation of hydroxo complex B.
Fig. 3 Reaction profile for haptotropic rearrangement.
Fig. 2 Reaction profile for transformation of hydroxo complex B into s-allyl complex E.
c
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of À21.0 kcal molÀ1. Subsequent rearrangement to H took
Under a neutral condition, bicyclic ruthenacyclopentatriene 3
reacts with H2O to afford new half-open oxaruthenocene
complex 4 possessing an Z5-oxapentadienyl ligand. On the
other hand, bicyclic furan 6 was predominantly formed in the
presence of silver salts such as AgNO3 via the incorporation of
one oxygen atom from a water molecule. The DFT calcula-
tions suggested that the cationic ruthenacycle undergoes
1,2-migration of the hydroxo ligand to form the hydroxylated
ruthenacycle. Subsequent b-H elimination followed by
1,2-hydride migration produces the g-formylallyl complex,
which finally evolves into the Z5-oxapentadienyl complex.
The isolated complex 4 proved to be an intermediate in the
catalytic hydrative cyclization of diynes.
place with the dissociation of the formyl group. This process
was estimated to require activation energy of 11.8 kcal molÀ1
.
The thermodynamically favorable formation of Z5-oxapentadienyl
complex (the Ru–O and Ru–C bond lengths are
H
between 2.22 A and 2.25 A) is estimated to proceed with an
exothermicity of 14.9 kcal molÀ1. The entire rearrangement
pathway was found to be a downhill process with a considerable
exothermicity of 35.9 kcal molÀ1
.
We anticipated that Z5-oxapentadienyl complexes are
possible intermediates in the hydrative cyclization that trans-
forms diynes into cycloalkenyl ketones. In fact, ketone
product 5 was obtained in the reaction of 3 with water, albeit
in a low yield (Table 1, entry 1). To investigate the role of the
oxapentadienyl complex in hydrative cyclization, the forma-
tion of 5 from 4 was examined. As is expected in the case of 18e
complexes, 4 was inert to nucleophilic reaction and it remained
intact after refluxing in an aqueous THF solution for 6 h. In
contrast, 1H NMR analysis indicated that 4 decomposed even
under weakly acidic conditions (4.2 mM HCl, 1 equiv.). In the
presence of diphenylphosphinoethane (dppe, 1.05 equiv.),
86% of 4 was consumed at 70 1C for 12 h to afford 5 and a
dppe complex 7 in 85% and 86% NMR yields, respectively
(Scheme 2). These results indicate that Brønsted acid plays an
indispensable role in the formation of 5 from 4. Because
the Cp*RuCl fragment was restored effectively, a series of
transformations were expected to proceed catalytically. In
fact, an aqueous THF solution of diyne 2 was heated in the
presence of 5 mol% 1 for 24 h to obtain 5 in 64% isolated yield
(Scheme 2). Interestingly, the combination of 5 mol% each of
4 and HCl effectively catalyzed the hydrative cyclization to
afford 5 in a higher isolated yield of 88%, although 4 itself
exhibited no catalytic activity.
This research was partially supported by the MEXT,
Grant-in-Aid for Scientific Research (B) (20350045), and the
Global COE program (Nagoya University).
Notes and references
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see: (a) E. Muller, Synthesis, 1974, 761; (b) K. Moseley and
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Scheme 2 Catalytic formation of ketone 5 from diyne 2.
c
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This journal is The Royal Society of Chemistry 2011