Activation of mononuclear arene ruthenium complexes for catalytic propargylation directly with propargyl alcohols

Article abstract of DOI:10.1002/adsc.200600512

Mononuclear complexes of the type [(p-cymene)RuX(CO)(PR₃)][OTf] (R = Ph, Cy; X = Cl, OTf) promote the direct catalytic propargylation of furan with propargyl alcohols. These precursors are generated in situ from [(p-cymene)RuCl(OTf)(PR₃)] by activation of the propargylic alcohol, leading to the carbonyl ligand formation via allenylidene and alkenyl-hydroxycarbene intermediates. The generation of the catalytically active species requires a short initial thermal activation to induce decoordination of the p-cymene ligand. The in situ generated catalyst has been applied to catalytic transformations of alkynes and propargylic alcohols: propargylation of furans, propargyl ether synthesis from internal and terminal propargylic alcohols with propargyl, homopropargyl and allyl alcohols, selective dimerization of phenylacetylene into E-enyne, and propargyl alcohol rearrangement into α,β-unsaturated aldehydes and ketones via the Meyer-Schuster rearrangement. The propargylation of propargylic alcohols containing internal C≡C bonds suggests an activation via the Nicholas-type intermediate, the metalstabilized propargyl cation.

Full text of DOI:10.1002/adsc.200600512

FULL PAPERS
DOI: 10.1002/adsc.200600512
Activation of Mononuclear Arene Ruthenium Complexes for
Catalytic Propargylation Directly with Propargyl Alcohols
Emilio Busteloa,b and Pierre H. Dixneufa,*
a
UniversitØ de Rennes 1, CNRS, Institut de Chimie de Rennes, UMR 6509, OrganomØtalliques et Catalyse, Campus de
Beaulieu, 35042 Rennes, France
Fax : (+33)-2-2323-6939; e-mail: pierre.dixneuf@univ-rennes1.fr
Current address: Universidad de Cµdiz, Departamento de Ciencia de los Materiales, Ingeniería Metalfflrgica y Química
b
Inorgµnica, Apartado 40, 11510 Puerto Real (Cµdiz), Spain
Received: October 6, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Mononuclear complexes of the type [(p- of furans, propargyl ether synthesis from internal
cymene)RuX(CO)
N
N
OTf) promote the direct catalytic propargylation of mopropargyl and allyl alcohols, selective dimeriza-
furan with propargyl alcohols. These precursors are tion of phenylacetylene into E-enyne, and propargyl
generated in situ from [(p-cymene)RuCl
N
N
by activation of the propargylic alcohol, leading to hydes and ketones via the Meyer–Schuster rear-
the carbonyl ligand formation via allenylidene and rangement. The propargylation of propargylic alco-
ꢀ
alkenyl-hydroxycarbene intermediates. The genera- hols containing internal C C bonds suggests an acti-
tion of the catalytically active species requires a vation via the Nicholas-type intermediate, the metal-
short initial thermal activation to induce decoordina- stabilized propargyl cation.
tion of the p-cymene ligand. The in situ generated
catalyst has been applied to catalytic transformations Keywords: arene complexes; diynes; furans; propar-
of alkynes and propargylic alcohols: propargylation gylation; propargylic alcohols; ruthenium catalyst
Introduction
hols, alkenes by alkynol carbon-carbon triple bond
cleavage, alkylidenecyclobutenes through head-to-
Modern catalysis aims at the discovery of new reac- head propargyl alcohol dimerization, and others.[3]
tions to selectively produce useful added-value prod- The direct propargylic substitution of propargyl alco-
ucts from simple and cheap materials.[1] The under- hols would constitute a straightforward method for
standing of the catalytic cycle, based on the identifica- the synthesis of functional alkynes. In spite of its po-
tion of the catalytically active species, should lead to tential, the catalytic version of this reaction has been
the design of catalysts with an increased efficiency much less studied than the related allylic substitution.
and selectivity from the most appropriate precursors.
Of particular relevance is the ruthenium-catalyzed
As an example, the sequential use of allenylidene, al- propargylic substitution of propargyl alcohols with a
kenylcarbyne and indenylidene derivatives of the [(p- variety of heteroatom- and carbon-centered nucleo-
cymene)RuCl
the generation of a series of olefin metathesis cata- (Scheme 1).[4–7] A nucleophilic addition on the elec-
lysts with successively increased activity.[2]
trophilic g-carbon in allenylidene metal intermediates
(PCy3)]+ fragment, has recently allowed philes, recently developed by Nishibayashi et al.
Propargyl alcohols are simple organic species easily seems to be the key step. This innovative transforma-
available by addition of acetylenes to aldehydes and tion represents a catalytic alternative to the Nicholas
ketones. They represent an important group of chemi- reaction, which requires a stoichiometric amount of
cal intermediates for organic synthesis. Their selective cobalt complex to stabilize the propargyl cation inter-
catalytic transformations with ruthenium catalysts mediate.[8] However, the Nicholas reaction has the ad-
have recently led to the production of, for example, vantage of being applicable to propargyl alcohols
tetrahydrofuran and tetrahydropyrans by tandem cyc- bearing internal alkynes, unable to give allenylidene.
lization-reconstitutive addition of propargyl and allyl
Nishibayashiꢀs catalysis requires a thiolate-bridged
alcohols, aldehydes by hydration of propargyl alco- diruthenium complex. The search for alternative mon-
Adv. Synth. Catal. 2007, 349, 933 – 942
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Emilio Bustelo and Pierre H. Dixneuf
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extended to propargyl ether synthesis from propargyl
alcohols.
Results and Discussion
After Selegueꢀs discovery of a general method for the
synthesis of allenylidene metal complexes from prop-
argylic alcohols,[15] semi-sandwich arene ruthenium
compounds of the type [(arene)RuCl2(PR3)] are
A
among the first reported complexes able to activate
propargylic alcohols to give allenylidene species.[16]
Therefore, [(p-cymene)RuCl2(PR3)] complexes have
G
first been evaluated as mononuclear catalyst precur-
sors to perform the catalytic propargylation of differ-
ent substrates. 1-Phenyl-2-propynol (1a) has been
treated with a large excess (20 equivalents) of furan
and 2-methylfuran in the presence of catalytic
Scheme 1.
amounts of [(p-cymene)RuCl2(PR3)] [PR3 =PCy3
N
(2a), PPh3 (2b)] in 1,2-dichloroethane. A low conver-
sion took place and small amounts of the propargylat-
onuclear ruthenium catalysts able to promote this in- ed product were detected (yield <5%) (Scheme 2).
novative process has been a challenge. However, most
mononuclear ruthenium complexes are not active cat-
alysts for this reaction, and particularly ruthenium
semi-sandwich complexes, in spite of their well-known
reactivity with propargyl alcohols to give allenylidene
species, which are proposed as key intermediates for
this reaction.[4]
The first examples of mononuclear ruthenium cata-
lysts were simultaneously reported by Gimeno et al.[9]
and the Rennes group.[10,11] The catalyst precursors
Scheme 2.
[Ru(h3-2-C3H4Me)(CO)
(dppf)] and [(p-cymene)RuCl-
G
A
N
stitution reaction with alcohols and heterocycles, re-
spectively. Several recent reports have showed the
In order to enhance the interaction between the al-
rising interest in this propargylation reaction. Rheni- kynol and the ruthenium complex, [(p-cymene)RuCl-
um complexes (dppm)ReOCl3),[12] gold salts
N
N
(NaAuCl4·2H2O),[13] and organic acids (i.e., p-toluene- were prepared from 2a and 2b by chloride abstraction
sulfonic acid),[14] have been employed as catalysts with AgOTf in CH2Cl2. The reaction of alkynol 1a
with variable results depending on the use of terminal and furan (or 2-methylfuran) in the presence of 5%
or internal alkynols, and on the particular nucleo- of 3a/3b leads to the complete conversion of the prop-
philes (alcohols, thiols, heterocyclic and aromatic argyl alcohol, but the propargylated heterocycles
ꢀ
compounds, etc). Rhenium and gold catalysts are re- HC CCHPh(Nu) [Nu=2-furanyl (4a), 5-methyl-2-
stricted to internal propargyl alcohols and they seem furanyl (4b)] were obtained in low yields (yield<
to proceed through stabilized carbonium intermedi- 10%).[17]
ates.
The analysis of the by-products shows the presence
In this paper we report the enhanced catalytic ac- of aldehydes and polymeric materials. It is noteworthy
tivity of the mononuclear ruthenium precatalyst [(p- that a,b-unsaturated aldehydes and ketones can be
cymene)Ru
of its formation from [(p-cymene)Ru
[OTf] and propargyl alcohol, for the catalytic propar- ally catalyzed by strong acids, but also ruthenium
A
G
ACHTREUNG
A
ACHTREUNG
ACHTREUNG
gylic substitution of propargylic alcohols. We show complexes are able to catalyze the isomerization of
how the catalyst activity can be further improved by propargyl alcohols to a,b-unsaturated aldehydes
in situ thermal preactivation, and orientated towards through vinylidene and allenylidene intermediates.[19]
the Meyer–Schuster selective catalytic reaction, the More recently, Gimeno and co-workers have reported
catalytic coupling of heterocycles with ketones, and the Ru-catalyzed propargylic substitution of 1,1-di-
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Activation of Mononuclear Arene Ruthenium Complexes for Catalytic Propargylation
FULL PAPERS
while the other completely disappears (d=58.1 ppm).
(Scheme 4). The remaining species (d=39.0 ppm) was
identified by NMR as a hydroxycarbene complex. Its
main spectroscopic data are signals at d=14.18 ppm
1
[=C(OH)] and 287.5 ppm [Ru=C(OH)] in the H and
13C{1H} NMR spectra respectively, which were ana-
lyzed by COSY, HMBC and HMQC bidimensional
experiments. The low-field signal at d=14.18 ppm
does not correlate with any carbon atom in the
ꢁ
HMQC bidimensional experiment (one-bond C H
couplings). However, the signal shows a correlation
with the carbon atom appearing at d=287.5 ppm in
the HMBC experiment (two- and three-bond cou-
plings). Both low-field signals are characteristic of
metal-carbene species and consistent with the pro-
posed hydroxycarbene complex 5a/5b, thus arising by
addition of the released H2O to the allenylidene-
ruthenium intermediate (Scheme 4).
The addition of 100 mL of water to a CD2Cl2 solu-
tion of complex 3a/3b and the alkynol 1a in an NMR
tube at ꢁ608C, gave directly the hydroxycarbene
complex 5a/5b as the only observed reaction product.
At above 508C this complex slowly disappears, giving
Scheme 3.
phenyl-2-propynol with alcohols[9] and the alternative rise to
formation of a,b-unsaturated aldehydes via isomeriza- cymene)RuCl
tion in the absence of alcohol.[20]
of free styrene, observed by NMR and GC-MS
This isomerization process is expected to be respon- (Scheme 4).
a
new derivative identified as [(p-
AHCTREUNG
sible for the low yields observed in the first tests.
The presence of additional water accelerates the
Complexes 3a and 3b behave as efficient catalysts for formation of the hydroxycarbene complex [(p-
Meyer–Schuster rearrangement and the subsequent cymene)RuCl
A
(5a).
heterocyclic addition to the resulting a,b-unsaturated Compound 5a slowly disappears at above 508C,
ketones. This behaviour is revealed when the catalytic giving rise to the carbonyl complex [(p-cymene)RuCl-
reaction is carried out in acetone instead of 1,2-di-
chloroethane. The double addition of 2-methylfuran NMR and GC-MS (Scheme 4). The analogous PPh3
to acetone occurs in almost quantitative yield complexes [(p-cymene)RuCl(PPh3){=C(OH)CH=
(Scheme 3). Furthermore, substituted dipyrans are ob- CHPh}]+ (5b) and [(p-cymene)RuCl(PPh3)(CO)]+
tained by reaction of acetone or benzaldehyde with 2- (6b) are also observed starting from complex 3b.
methylpyrrole in the presence of a catalytic amount Werner et al. have reported similar mixed phos-
of the arene ruthenium complexes 3a or 3b phine-carbonyl arene ruthenium complexes.[21] In fact,
(Scheme 3). 6a/6b are more readily prepared from the parent com-
(PCy3)(CO)]+ (6a) and free styrene, as detected by
AHCTREUNG
AHCTREUNG
In view of this, substantial modifications of the cat- plexes 3a/3b on addition of carbon monoxide. The
alyst to avoid as much as possible undesirable side-re- question is whether these saturated complexes could
actions seemed mandatory and among a variety of ad- be related or not to the catalytically active species.
ditives it was observed that the addition of small Surprisingly, 6a and 6b also catalyze the propargyla-
amounts of water to the reaction mixture afforded an tion of furan and 2-methylfuran by alkynol 1a with a
increased of yield of 25–30% for the products 4a/4b remarkably shorter reaction time than 3a and 3b.
(Table 1, entries 1 and 2).
Under similar conditions (1,2-dichloroethane, 608C)
Stoichiometric low temperature NMR experiments the catalytic reaction was completed in 2 h. Therefore,
provided valuable information and an accurate ex- the addition of water to the allenylidene intermediate
planation of the water effect: Two complexes result- favours the hydroxycarbene and thus the carbonyl
ing from the activation of alkynol 1a by [(p- complex formation, which acts as an improved cata-
cymene)RuCl
C
G
Complexes 6a and 6b provide similar catalytic re-
allenylidene intermediate and alkenyl-hydroxycar- sults. The catalyst of choice was the complex [(p-
bene species, showing singlets at d=58.1 and cymene)RuCl(CO)
39.0 ppm respectively. When the temperature is raised prepared with high yield in a three-step synthesis
to 08C, the second signal (d=39.0 ppm) increases from readily available reagents [RuCl2(p-cymene)]2
A
G
AHCTREUNG
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Table 1. Ruthenium-catalyzed propargylation of furans by propargyl alcohols.
Entry
Catalyst
A
B
Time[a]
Product
Yield [%][b]
25
1
3a/3b + 100 mL H2O
4a
4b
4a
4b
1a
15 h
2
3
4
3a/3b + 100 mL H2O
30
25
30
6b
6b
1a
2 h
5
6b
1d
1a
4c
45
6
7
7a
7a*
27
42
2 h
2 h
4a
4d
8
7a*
7a*
31
25
1e
9
4e
[a]
Reaction conditions: 1,2-dichloroethane, 608C, 5% catalyst loading.
Isolated yield.
[b]
Scheme 4.
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Activation of Mononuclear Arene Ruthenium Complexes for Catalytic Propargylation
FULL PAPERS
and PPh3. A catalyst loading of 5% of 6b affords 693.1936 (calcd. for [C30H47O4F3PS102Ru]+: 693.1928),
propargylated furan and 2-methylfuran in moderate corresponding to a mononuclear monocationic com-
yields (Table 2, entries 3 and 4).
plex with one coordinated triflate ligand and one tri-
Since the catalyst precursors 6a and 6b are saturat- flate counterion.
ed 18-electron complexes, the release of one ligand is
An acetone-d6 solution of complex 7a has been
1
required to allow alkyne coordination prior to alleny- monitored by H and 31P{1H} NMR. The release of p-
lidene or propargyl cation formation. As a working cymene at room temperature is slow, but at 608C all
hypothesis, the release of p-cymene during the initial the arene ligand is completely removed after 20 min,
catalytic step would generate a coordinatively unsatu- as shown by the characteristic free p-cymene signals
rated, and thus very reactive, electrophilic species in the H NMR spectrum. The 31P{1H} NMR signal
1
“
A
G
N
besides the catalysis product.
unsaturated complex such as “[Ru
A
As an attempt to provide a higher stability to the
ACHTREUNG
catalytic intermediates, the complex [(p-cymene)Ru- would exhibit two bidentate triflate ligands to com-
(OTf)(CO)(PCy3)][OTf] (7a) has been prepared by plete 18 valence electrons, although the coordination
abstraction of the two chlorides with 2 equivalents of of an acetone molecule can also be considered. At-
AgOTf from [(p-cymene)RuCl2(PCy3)] (2a), followed tempts to isolate [8a] as a solid were unsuccessful,
by CO bubbling. This complex should benefit from showing the low stability of this reactive species.
the different coordination modes of the triflate
Arene decoordination by treatment of [(h6-
ligand, which can act as a counterion or be bonded in toluene)Ru{k2
(Si,Si)-xantsil}(CO)] [xantsil=bis(dime-
A
N
ACHTREUNG
AHCTREUNG
AHCTREUNG
mono- or bidentate modes. High-resolution mass thylsilyl) chelating ligand] with PCy3 has been recent-
spectrometry of 7a confirms the expected m/z= ly reported to generate a highly unsaturated rutheni-
Table 2. Ruthenium-catalyzed propargyl ether synthesis.
A
B
Product[a]
Yield [%][b]
A
B
Product[a]
Yield [%][b]
1a
1a
1a
1a
10 61
1d
1e
1e
1e
11b 31
11a 35
12a 51
13a 15
11c 41
12b 43
13b 28
[a]
[b]
Reaction conditions: acetone, 608C, 5% catalyst loading.
Isolated yield.
Scheme 5. Generation and possible structure of catalyst 8a.
Adv. Synth. Catal. 2007, 349, 933 – 942
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937

Emilio Bustelo and Pierre H. Dixneuf
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um complex [Ru{k3
N
N
considered as a 14-electron synthetic equivalent. The
release of the arene ligand is also described in analo-
gous ruthenium complexes of type [(h6-p-cymene)-
RuX2L] (L=PR3, NHC),[23] and it is proposed as the
initial step towards the generation of the olefin meta-
thesis catalytically active species from [(h6-p-
cymene)RuCl
A
ACHTREUNG
The analogous complex [(p-cymene)Ru
AHCTREUNG
A
[OTf] (7b) exhibits
a
[31P{1H} NMR changes from 38.6 ppm to 47.8 ppm
when heated] but the corresponding activated species
[8b] shows a lower stability.
N
A
ACHTREUNG
The best yield for the furan propargylation with cata-
lyst 6b was 25% (Table 1, entry 3). The yield is analo-
gous with that of the precatalyst 7a (27%, entry 6).
However, it is noteworthy that a preactivation of the
catalyst by heating 7a at 608C for 30 min before the
addition of the catalysis substrates, significantly in-
creases the yield to 42% (Scheme 6, Table 1, entry 7).
These results clearly indicate that the actual catalytic
active species is related to the complex [8a], generat-
ed in situ from 7a.
Scheme 7.
ing product (40%). The propargylation of furan with
1c always provides yields lower than 10%. As a con-
clusion, the fast Meyer–Schuster rearrangement pre-
vents the propargylic substitution on tertiary alkynols.
It should be noted that the Meyer–Schuster rear-
rangement usually requires strong acids and high tem-
peratures in absence of metal catalysts.[24]
An unforeseen behaviour is observed for alkynol
1a when treated with 5% of complex 7a previously
activated in acetone at 608C to give [8a]. Whereas
the expected (E)-cinnamaldehyde (9c) is only ob-
tained in 8% yield, the alkynol undergoes self-con-
Scheme 6.
ꢀ
densation and the dipropargyl ether (HC CCHPh)2O
The aforementioned preactivation of the catalyst (10) is isolated as a mixture of diasteroisomers in
has also been employed to extend the propargylic 61% yield (Scheme 7).
substitution reaction to other propargyl alcohols.
The self-condensation reaction of 1a has not been
Nishibayashiꢀs catalyst shows a marked preference for reported for other ruthenium catalysts. Small amounts
ꢀ
secondary propargyl alcohols [HC CCH(OH)Ar] as of 10 (<15%) were found as a by-product of the
substrate for the propargylic substitution. Gimenoꢀs etherification of 1a with EtOH catalyzed by organic
catalyst is limited to the tertiary 1,1-diphenyl-2-propy- acid.[14] A direct access to dipropargyl ethers is inter-
[9]
ꢀ
nol, HC CC(OH)Ph2 (1b). In our case, the preacti- esting, since a,w-diynes are useful building blocks fre-
vated catalyst appears as an excellent Meyer–Schuster quently employed for [2+2+2] cyclization reactions
catalyst with tertiary alkynols. When 1b is treated catalyzed by transition metal complexes,[25,26] and also
with 5% of 7a (preactivated at 608C, thus 7a![8a]), with potential interest as monomers for the prepara-
the Meyer–Schuster rearrangement takes place in just tion of conducting polymers.[27] Propargyl ethers are
5 min to give 3,3-diphenylacrylaldehyde (9a) in 89% also key substrates for the ruthenium-assisted retro-
isolated yield (Scheme 7).
ene reaction and generation of alkene metathesis cat-
A similar behaviour is observed for 2-phenyl-3- alysts.[2d]
ꢀ
butyn-2-ol, HC CC(OH)MePh (1c). After 10 min at
The synthesis of propargyl ethers has been extend-
608C a mixture of (E)- and (Z)-3-phenyl-2-butenal ed to the condensation of propargyl alcohol 1a with
ꢀ
(9b) is obtained in 25% yield, besides unreacted start- propargyl alcohol itself (HC CCH2OH), homopro-
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Activation of Mononuclear Arene Ruthenium Complexes for Catalytic Propargylation
FULL PAPERS
ꢀ
pargyl alcohol (HC CCH2CH2OH) and allyl alcohol nal alkynols. In our case, experiments with internal al-
(H2C=CHCH2OH) (Table 2).
kynols give better results, and support the existence
The access to a series of non-conjugated enynes of propargyl cation intermediates instead of allenyli-
and diynes is motivated by their use as substrates for denes, whose formation is restricted to terminal alky-
selective catalytic transformations into complex mole- nols.
cules especially promoted by platinum(II) or gold cat-
In an attempt with catalyst 6b, the internal proparg-
alysts.[28] The treatment of 1a and a slight excess of yl
alcohol
1-phenyl-2-heptyn-1-ol,
n-BuC
ꢀ
propargyl alcohol with 5 mol% of preactivated com- CCH(OH)Ph (1d) and 2-methylfuran afforded n-
ꢀ
plex 7a in acetone at 608C for 3 h, gives the dipro- BuC CCH(C4H2O-CH3)Ph (4c) in 45% yield
pargyl ether HC CCH(Ph)OCH2C CH (11a) in 24% (Table 1, entry 5).[10] Additionally, the internal prop-
isolated yield. Optimization of the catalytic conditions argyl
ꢀ
ꢀ
ꢀ
PhC
alcohol
1,3-diphenyl-2-propynol,
led to the use of an excess of the propargyl alcohol CCH(OH)Ph (1e), has been reacted with furan and 2-
(1 mL) and a longer reaction time (overnight), in- methylfuran at 608C in acetone in the presence of 5
creasing the yield up to 35%. It is noteworthy that a mol% of the preactivated complex 7a(![8a]), afford-
ꢀ
classical etherification to obtain 11a from alkynol 1a, ing the corresponding propargylated product PhC
propargyl bromide and a strong base provides yields CCH(R)Ph [R=C4H3O (4d), C4H2O-CH3 (4e)] in
around 25%.[26]
31% and 25% isolated yields, respectively (Table 1,
Homopropargyl alcohol condensation with 1a is entries 8 and 9).
more effective and the propargyl ether derivative The internal propargyl alcohols 1d and 1e are also
ꢀ
ꢀ
HC CCH(Ph)OCH2CH2C CH (12a) is isolated in useful for condensation reactions to give propargyl
51% yield under similar conditions. In sharp contract ethers in the same conditions as for alkynol 1a. Reac-
ꢀ
with Gimenoꢀs and Nishibayashiꢀs propargylation cat- tions with propargyl alcohol HC CCH2OH provide
alysts,[6,9] no reaction is observed with simple alcohols dipropargyl ethers RC CCH(Ph)OCH2C CH [R=n-
such as methanol, ethanol or 2-propanol. This sug- Bu (11b), Ph (11c)] in 31% and 41% yields, respec-
gests that the condensation process is promoted by tively (Table 2). In similar conditions, the internal al-
the prior coordination of the two acetylenic substrates kynol 1e reacts with homopropargyl alcohol to give
ꢀ
ꢀ
ꢀ
ꢀ
to the unsaturated active species [8a] (Scheme 8). The the corresponding ether PhC CCH(Ph)OCH2CH2C
condensation reaction between 1a and allyl alcohol CH (12b), and with allyl alcohol to obtain the allyl
ꢀ
takes places with a lower yield, furnishing the allyl propargyl ether PhC CCH(Ph)OCH2CH=CH2 (13b),
ꢀ
propargyl ether HC CCH(Ph)OCH2CH=CH2 (13a) in 43% and 28% isolated yields, respectively
in 15% isolated yield.
(Table 2). The better coordinative capability of al-
kynes with respect to alkenes might justify the higher
condensation yields for propargyl and homopropargyl
alcohols with regard to allyl alcohol (Scheme 8).
By contrast to 1a, the internal propargyl alcohol 1e
does not undergo the self-condensation reaction,
likely because of the higher steric requirements, hin-
dering the prior coordination of a second alkyne mol-
ecule before condensation. Instead, a mixture of (Z)-
and (E)-chalcone PhCOCH=CHPh (9d) is obtained
in 49% isolated yield, as a result of the Meyer–Schus-
ter rearrangement.
The propargylic substitution of internal propargylic
alcohols cannot involve allenylidene intermediates,
ꢁ
which require an initial acetylenic C H activation.
Although different pathways could be operative for
terminal or internal alkynols, it seems more plausible
to propose a common mechanism analogous to that
Scheme 8.
In general, ruthenium catalysts are more specific of Nicholas reaction, including temporary coordina-
for propargylic substitution of terminal propargyl al- tion of the triple bond and assistance to OH elimina-
cohols and allenylidene complexes have always been tion by stabilization of the resulting propargyl cation
proposed as key intermediates.[4,5,9] However, for (Scheme 1).
Nishibayashiꢀs catalyst very similar diruthenium com-
The proposed initial coordination of two alkyne
plexes have been reported to be active also for the molecules is further supported by the ability of the in
propargylic substitution of internal alkynols.[7] On the situ generated catalyst [8a] to promote the catalytic
other hand, catalysts with other metals (Re,[12] Au[13]
)
phenylacetylene dimerization, which mechanism
and organic acids,[14] offer their best results with inter- occurs either via alkynyl/vinylidene M
(C CR)
(=C=
ꢀ
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www.asc.wiley-vch.de
939

Emilio Bustelo and Pierre H. Dixneuf
FULL PAPERS
2
ꢀ
ꢀ
CHR) or alkynyl/p-alkyne M
G
(h -HC CR)
Despite the moderate yields obtained with respect
to the Nishibayashi catalyst, this study reveals a
[29]
ꢁ
coupling, by insertion into the s Ru C bond.
Due to the difficulty of isolating the catalytically family of readily available mononuclear ruthenium
active species, the phenylacetylene dimerization reac- precursors for selective catalytic propargylation with
tion is carried out by heating a solution of the catalyst propargylic alcohols. The Meyer–Schuster rearrange-
7a (2.5%) at 608C during 30 min (7a![8a]) and then ment constitutes a competitive process, which has
adding phenylacetylene. The mixture is heated over- been used to prepare a,b-unsaturated aldehydes and
night. Removal of the solvent gives a white solid cor- ketones in good yields. The alternative self-condensa-
responding exclusively to the (E)-1,3-enyne product tion of secondary propargyl alcohols has been applied
(14, Scheme 9).
to the formation of non-conjugated diynes and
enynes.
Experimental Section
General Remarks
All catalytic reactions were carried out under an inert at-
mosphere in Schlenk tubes. Chemicals were obtained com-
mercially and used as supplied, except for the alkynols 1d
and 1e.[30] Complexes 2a and 2b,[21] and 3a and 3b,[31] were
prepared according to reported methods. Complexes 6a and
6b were obtained as described by Werner et al.,[21] but using
AgOTf as chloride abstractor. The formation of compounds
Scheme 9.
1
5a, 5b, 6a and 6b, observed by low temperature H, 31P{1H}
NMR and bidimensional experiments (COSY, HMQC,
HMBC) was described in the Experimental Section of the
previous communication.[10] NMR spectra were recorded on
Bruker AM 3000 WB and DPX 200 spectrometers in deu-
terated solvents. Elemental analysis and high resolution
mass spectrometry were performed by the Centre RØgional
de Mesures Physiques de l’Ouest, UniversitØ de Rennes 1.
IR spectra were recorded on a Bruker IFS28 spectrometer.
Solvents were distilled before use from the appropriate
drying agents. A Büchi GKR-51 oven was used for distilla-
tion.
This reaction is very solvent-dependent, and where-
as THF and 1,2-dichloroethane give low yield, ace-
tone, DMF and toluene provide 52%, 50% and 66%
isolated yields, respectively. The presence of a base
such as pyrrolidine is also required (1:2 ratio with
regard to phenylacetylene), likely to generate the es-
sential alkynyl intermediate by deprotonation of vi-
nylidene or p-alkyne ligands.[29]
Conclusions
General Procedure for the Catalytic Propargylation
of Furan Derivatives
This study has revealed the existence of alternative
mononuclear ruthenium catalysts for the propargyl
substitution reaction, and how to find out the appro-
priate modifications of the initial arene ruthenium
Catalyst 7a (42 mg,0.05 mmol) was dissolved in 2 mL of 1,2-
dichloroethane (DCE) and heated at 608C for 30 min. Then
1 mmol of the corresponding propargyl alcohol (132 mg of
1a, 188 mg of 1d, 208 mg of 1e) and the corresponding het-
erocycle (10 mmol of furan, 1 mmol of 2-methylfuran) were
added. After heating for 2 h at 608C, the solvent was re-
moved under vacuum and the residue extracted with 2”
5 mL of Et2O. Purification of the product was carried out by
distillation under reduced pressure, giving a pale yellow oil.
NMR, IR and analysis data for 2-(1-phenyl-2-propynyl)furan
(4a), 2-methyl-5-(1-phenyl-2-propynyl)furan (4b), 2-methyl-
5-(1-phenyl-2-heptynyl)furan (4c), 2-(1,3-diphenylprop-2-
ynyl)furan (4d) and 2-(1,3-diphenylprop-2-ynyl)-5-methyl-
furan (4e) match well with those previously reported.[5] See
yields in Table 1.
complex [(p-cymene)RuCl2(PR3)] on the basis of stoi-
G
chiometric key experiments. Low temperature NMR
analysis allowed us to characterize a hydroxycarbene
intermediate, leading to [(p-cymene)RuCl(CO)
A
ACHTREUNG
proved catalytic efficiency and provide a better under-
standing of the catalytic steps.
The thermal preactivation of [(p-cymene)Ru-
A
G
N
species likely by loss of the p-cymene ligand. The cat-
alysis activity has been tested for the propargylation
of furan derivatives, the formation of propargyl ethers
and non-conjugated enynes, and the Meyer–Schuster
reaction, with internal and terminal propargyl alco-
hols, as well as the selective dimerization of phenyl-
acetylene.
General Procedure for Catalytic Propargylic Ether
Synthesis
Catalyst 7a (42 mg, 0.05 mmol) was dissolved in 5 mL of
acetone and heated at 608C for 30 min. Then 1 mmol of the
940
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Adv. Synth. Catal. 2007, 349, 933 – 942

Activation of Mononuclear Arene Ruthenium Complexes for Catalytic Propargylation
FULL PAPERS
corresponding propargyl alcohol (132 mg of 1a, 188 mg of
1d, 208 mg of 1e) and 1 mL of 2-propyn-1-ol, 3-butyn-1-ol or
2-propen-1-ol were added. After heating overnight at 608C,
the solvent was removed under vacuum and the residue ex-
tracted with 2”5 mL of Et2O. Purification of the product
was carried out by column chromatography (silica, pentane),
giving the corresponding propargyl ether as yellow oils.
Characterization data are provided as Supporting Material.
See yields in Table 2.
1974, 2032 cmꢁ1.; ESI-MS: m/z=693.1936 [C+], calcd. for
C30H47O4F3PS102Ru: 693.1928.
Supporting Information
Characterization data for compounds 10–14.
Acknowledgements
General Procedure for Meyer–Schuster
Rearrangement
The authors are grateful to the European Union (COST
D17) and Bretagne region (PRIR catalyse No. A2CBC4) for
support, and to the “Ministerio de Educación y Ciencia” of
Spain and “Marie Curie” Fellowship Programme for post-
doctoral grants to E.B.
Catalyst 7a (42 mg, 0.05 mmol) was dissolved in 5 mL of
acetone and heated at 608C for 30 min. Then 1 mmol of the
corresponding propargyl alcohol (132 mg of 1a, 208 mg of
1b, 146 mg of 1c, 188 mg of 1d, 208 mg of 1e) were added.
After heating at 608C overnight, the corresponding alde-
hyde or ketone was isolated by column chromatography
(silica, 5% Et2O/pentane). 3,3-Diphenylacrylaldehyde (9a)
was obtained as the major product (89% yield) in 5 min. 3-
Phenyl-2-butenal (9b) in 25% yield as a mixture of (E)- and
(Z)-isomers, besides unreacted alkynol were also obtained.
(E)-Cinnamaldehyde (9c) was isolated in 8% yield as a by-
product of dipropargyl ether 10. With alkynol 1e a mixture
of (Z)- and (E)-chalcone (9d) was isolated in 49% yield as
the main product. NMR data match well with those previ-
ously reported.[32]
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Adv. Synth. Catal. 2007, 349, 933 – 942
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www.asc.wiley-vch.de
941

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FULL PAPERS
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