Efficient access to conjugated dienones and diene-diones from propargylic alcohols and enolizable ketones: A tandem isomerization/condensation process catalyzed by the sixteen-electron allyl-ruthenium(II) complex [Ru(η3-2-C3H4Me)(CO)(dppf)] [SbF 6]

Article abstract of DOI:10.1002/adsc.200606162

A large variety of conjugated dienones R¹R²C=CHCH= C(R³)C(=O)R⁴ and diene-diones R¹R ²C=CHCH=C{C(=O)R³}C(=O)R⁴ have been synthesized in high yields by reacting terminal propargylic alcohols HO=CCR ¹R²COH) with enolizable ketones R³CH ₂C(=O)R⁴ and β-dicarbonyl compounds R ³C(=O)CH₂C(=O)R⁴, respectively. The process, which is catalyzed by the 16e^- (η³-allyl)- ruthenium(II) complex [Ru(η³-2-C₃H₄Me)(CO) (dppf)] [SbF₆] associated with CF₃CO₂H, involves the initial isomerization of the propargylic alcohol into the corresponding α,β-unsaturated aldehyde R¹R ²C=CHCHO (Meyer-Schuster rearrangement) and subsequent aldol-type condensation.

Full text of DOI:10.1002/adsc.200606162

FULL PAPERS
DOI: 10.1002/adsc.200606162
Efficient Access to Conjugated Dienones and Diene-diones from
Propargylic Alcohols and Enolizable Ketones: A Tandem Isomeri-
zation/Condensation Process Catalyzed by the Sixteen-Electron
Allyl-Ruthenium(II) Complex [Ru(h3-2-C3H4Me)(CO)
(dppf)]
G
[SbF6]
Victorio Cadierno,a,* Josefina Díez,a Sergio E. García-Garrido,a JosØ Gimeno,a,*
and Noel Nebraa
a
Departamento de Química Orgµnica e Inorgµnica, Instituto Universitario de Química Organometµlica “Enrique Moles”
(Unidad Asociada al CSIC), Facultad de Química, Universidad de Oviedo, 33071 Oviedo, Spain
Fax : +(34)-985-103446; e-mail: vcm@uniovi.es or jgh@uniovi.es
Received: April 5, 2006; Accepted: July 11, 2006
In memoriam of our friend and colleague Prof. Dr. Marcial Moreno-MaÇas.
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A large variety of conjugated dienones [SbF6] associated with CF3CO2H, involves the initial
R1R2C=CHCH=C(R3)C(=O)R4 and diene-diones isomerization of the propargylic alcohol into the cor-
R1R2C=CHCH=C{C(=O)R3}C(=O)R4 have been responding a,b-unsaturated aldehyde R1R2C=
synthesized in high yields by reacting terminal prop- CHCHO (Meyer–Schuster rearrangement) and sub-
1
2
ꢀ
argylic alcohols HC CCR R (OH) with enolizable sequent aldol-type condensation.
ketones R3CH2C(=O)R4 and b-dicarbonyl com-
pounds R3C(=O)CH2C(=O)R4, respectively. The pro-
cess, which is catalyzed by the 16e- (h3-allyl)-ruthe- Keywords: aldol condensation; dienones; enals; iso-
nium(II) complex [Ru(h3-2-C3H4Me)(CO)
(dppf)] merization; propargylic alcohols; ruthenium
A
Introduction
proven to be highly chemoselective since it leads to
the exclusive formation of enals A (Meyer–Schuster-
Over the last decade the interest in ruthenium-cata- type rearrangement) or enones B (Rupe-type rear-
lyzed reactions directed to organic synthesis has spec- rangement) depending on the nature of the propargyl-
tacularly increased and a large number of new highly ic alcohol substituents.[7]
efficient synthetic approaches are nowadays well
Quite recently, Nishibayashi and co-workers have
documented.[1–3] Among them, those proceeding in an found that the treatment of terminal propargylic alco-
atom economical manner, that is, all atoms of the re- hols with acetone in the presence of a catalytic
actants end up in the final product,[4] are probably the amount of the dinuclear species [Cp*Ru
ACHTREUNG
most promising.[1f] Isomerization reactions are typical XMe)2RuCp*
A
ACHTREUNG
examples of atom economical transformations since result in the expected propargylic substitution reac-
no by-products are generated. In this context, we tion,[8] leading instead to the formation of conjugated
have reported that the 16e- (h3-allyl)-ruthenium(II) dienones C (see Scheme 2).[9] These highly unsaturat-
complex [Ru(h3-2-C3H4Me)(CO)
N
N
1,1’-bis(diphenylphosphino)ferrocene] (1),[5] associat- propargylic alcohol into the corresponding a,b-unsa-
ed with CF3CO2H, is a highly efficient catalyst for the turated aldehyde (A; Meyer–Schuster rearrange-
isomerization of readily available terminal propargylic ment), followed by aldol condensation between A
1
2
ꢀ
alcohols HC CCR R (OH) into synthetically useful and acetone. Nevertheless, it should be noted that, de-
a,b-unsaturated carbonyl compounds (see spite the great interest of catalytic tandem processes
Scheme 1).[6] Moreover, this catalytic system has in synthesis,[10] the practical application of this unpre-
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Victorio Cadierno et al.
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Scheme 1. Ru-catalyzed isomerization of propargylic alcohols.
Scheme 2. Coupling reaction of propargylic alcohols with acetone catalyzed by ruthenium.
ꢀ
cedented coupling process shows serious drawbacks readily accessible terminal propargylic alcohols HC
mainly due to the low yields (<54%), the very limit- CCR1R2(OH) and commercially available enolizable
ed scope of substrates, and the extremely long reac- ketones R3CH2C(=O)R4 and b-dicarbonyl compounds
tion times (ca. 70 h).[9] Apparently, all these limita- R3C(=O)CH2C(=O)R4, respectively.
tions seem to arise from the low activity of the cata-
lysts in the initial isomerization step, i.e., using 5
mol% of [Cp*Ru
mol% in Ru) HC CCHPh(OH) is isomerized into
A
(OH2)]
U
ꢀ
PhHC=CHCHO in only 15% yield after 20 h (1,2-di- Tandem Isomerization/Aldol Condensation of
chloroethane, 608C).[9] This result contrasts with the Propargylic Alcohols with Enolizable Ketones:
behaviour shown by the 16e- complex, [Ru(h3-2- Synthesis of Conjugated Dienones
C3H4Me)(CO)
alyzes the quantitative isomerization of HC
CCHPh(OH) into PhHC=CHCHO after only 3.5 h in mote the catalytic Meyer–Schuster isomerization of
A
refluxing THF.[6]
With all these precedents in mind, and as part of enals R1R2C=CHCHO by [Ru(h3-2-C3H4Me)(CO)-
our ongoing interest in the catalytic applications of (dppf)][SbF6] (1) (i.e., 1.0M solutions of the substrate
the allyl-ruthenium(II) complex
[Ru(h3-2- in refluxing THF (undistilled) and a [substrate]:[1]:-
C3H4Me)(CO)(dppf)][SbF6], we have investigated the
[CF3CO2H] ratio=20:1:2)[6] we have studied the reac-
1
2
ꢀ
C
potential of our catalytic system for the tandem iso- tion in the presence of 10 equivalents of acetone.
merization/aldol condensation coupling processes. Thus, in a preliminary experiment it was found that
Thus, herein we describe a general and highly effi- 1,1-diphenyl-2-propyn-1-ol (2a) is converted into (E)-
cient synthetic approach to conjugated dienones 6,6-diphenyl-3,5-hexadien-2-one (3a) in ca. 70% yield
R1R2C=CHCH=C(R3)C(=O)R4 and diene-diones after 50 h. A significant rate enhancement was ob-
R1R2C=CHCH=C{C(=O)R3}C(=O)R4 starting from served working without solvent (THF) and raising the
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Conjugated Dienones and Diene-diones from Propargylic Alcohols and Enolizable Ketones
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temperature to 758C (sealed tube). Thus, under these hexadien-2-one (3a) in much lower yield after 24 h,
optimized reaction conditions ([2a]:[acetone]:[1]: i.e., 20% vs. 91% (entry 1 in Table 1). This fact clear-
AHCTREUNG
[CF3CO2H] ratio=20:200:1:2), an almost quantitative ly indicates that Lewis acid-ruthenium species, formed
transformation of 2a into dienone 3a was achieved in situ from 1 and CF3CO2H, are the actual catalytic
after 24 h (91% isolated yield; see entry 1 in Table 1). active species in the aldol condensation step.[11]
The high catalytic activity shown by [Ru(h3-2-
Other 1,1-diaryl (2b–d) and 1-aryl (2e, f) substitut-
[SbF6]/CF3CO2H (5 mol% in ed propargylic alcohols also reacted with acetone, in
Ru) contrasts with that of the dinuclear species the presence of 1/CF3CO2H, to afford the correspond-
[Cp*Ru (OH2)][OTf]2 (X=S, Se, Te), ing conjugated (E)-dienones 3b–f in moderate to
(m2-XMe)2RuCp*
i.e., using [Cp*Ru
(m2-SMe)2RuCp*
C3H4Me)(CO)
A
ACHTREUNG
A
N
ACHTREUNG
A
G
A
mol% in Ru) HC CCPh2(OH) (2a) was transformed in Table 1).[12] All the processes are chemo- and ste-
into (E)-Ph2C=CHCH=C(H)COMe (3a) in only 53% reoselective since no propargylic substitution by-prod-
ꢀ
1
2(CH2COMe),[13] or Z-ste-
ꢀ
yield after 70 h in refluxing acetone (to be compared ucts, namely HC CCR R
with entry 1).[9] It is also worthy of note that, in the reoisomers were observed by GC and NMR spectros-
absence of the ruthenium catalyst 1, the treatment of copy (see the Supporting Information). The stereose-
isolated Ph2C=CHCHO with acetone in the presence lectivity of this coupling process is in complete accord
of 10 mol% of CF3CO2H gives (E)-6,6-diphenyl-3,5- with the E-stereochemistry previously reported by
Table 1. Tandem isomerization/condensation of propargylic alcohols 2 with acetone catalyzed by [Ru]/CF3CO2H: Synthesis
of conjugated dienones 3.[a]
Entry
1
Substrate
Product
Time
24 h
Yield[b]
91%[c]
2a
2b
3a
3b
2
3
24 h
20 h
89%
86%
2c
3c
4
5
2d
2e
3d
3e
72 h
24 h
90%
64%
6
2f
3f
24 h
59%
[a]
Reactions performed under N2 atmosphere at 758C using 1 mmol of the corresponding propargylic alcohol and 10 mmol
of acetone. [substrate]:
Isolated yield.
A
G
[b]
[c]
Dienone 3a is obtained in 20% yield when a 10 mol% of CF3CO2H is used in the absence of catalyst 1.
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Victorio Cadierno et al.
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Nishibayashi,[9] and with the stereoselective Meyer– from propargylic alcohol HC CCPh2(OH) (2a) and
Schuster rearrangement of monosubstituted propar- MeC(=O)-i-Pr failed. Instead, following the same cat-
ꢀ
gylic alcohols observed by us.[6]
alytic protocol, diene Ph2C=CHCH=CMe2 (13) was
The following features are also worth noting: (i) As selectively formed (81% isolated yield; 12 h) as the
inferred by monitoring the reactions by GC, the rate- result of the formal olefination of the intermediate al-
limiting step of this tandem process is in all cases the dehyde Ph2C=CHCHO (see Scheme 3).[14]
aldol condensation between the intermediate enal
R1R2C=CHCHO and acetone (the initial isomeriza-
tion step proceeds quantitatively in 2–3 h). (ii) The Tandem Isomerization/Aldol Condensation of
nature of the aryl groups in the propargylic alcohol Propargylic Alcohols with b-Dicarbonyl Compounds:
has a marked influence in the reaction rate, the intro- Synthesis of Conjugated Diene-Diones
duction of an electron-releasing substituent in the ar-
omatic ring slowing down the rate considerably Aldol-type
condensations
between
aldehydes
(entry 4 vs. entries 1–3). And, (iii) although in all (R1CHO) and b-dicarbonyl compounds (1,3-dike-
cases the complete consumption of the starting prop- tones; R2C(=O)CH2C(=O)R3) are well-known pro-
argylic alcohol 2 as well as the intermediate enal has cesses, the resulting ene-diones R1CH=C{C(=O)
been observed by GC, some by-products are also R3}C(=O)R4 being important synthetic intermediates
formed starting from the monosubstituted alkynols for the construction of five- and six-membered heter-
2e, f decreasing the yields of dienones 3e, f (entries 5 ocyclic systems.[15] This fact prompted us to investigate
and 6 in Table 1).
In order to evaluate the scope of this catalytic and b-dicarbonyl compounds catalyzed by [Ru(h3-2-
tandem process, the behaviour of 1,1-diphenyl-2- C3H4Me)(CO)(dppf)][SbF6]/CF3CO2H. Representa-
the coupling processes between propargylic alcohols
A
ACHTREUNG
propyn-1-ol (2a) towards a variety of enolizable ke- tive results of this unprecedented synthetic approach
tones was explored under the standard reaction condi- for the preparation of conjugated diene-diones are
tions described above ([2a]:
N
N
ratio=20:200:1:2; 758C). Selected results are sum- ing our standard protocol, alkynols 2a–d smoothly
marized in Table 2. Thus, both aromatic, i.e., aceto- react with 2,4-pentanedione (entries 1–4), 3,5-hept-
phenones (entries 1–3) or propiophenones (entries 6– anedione (entries 5–7) or 1,1,1,5,5,5-hexafluoro-2,4-
8), and aliphatic ketones, i.e., 2-butanone (entry 4), 3- pentanedione (entry 8) to afford diene-diones 14–16
pentanone (entry 5) or cyclohexanone (entry 9), were in excellent yields (84–95%; quantitative GC yields).
found to be suitable substrates for this transforma- The structures of all these compounds were confirmed
tion, the resulting dienones 4–12 being isolated in 79– by IR and NMR spectroscopy, mass spectra and ele-
94% yield (quantitative conversions were in almost mental analyses (see the Supporting Information).
all cases observed by GC). As expected, when 2-buta- Remarkably, with the exception of the propargylic al-
none was used as substrate, the aldol condensation cohol 2d, which requires 11 h due to the presence of
process between the intermediate 3,3-diphenyl-2-pro- the electron-releasing methoxy substituents (entry 4),
penal and the ketone takes place selectively on the all these reactions proceed with a very high rate (1–
more activated CH2 vs. CH3 unit (entry 4). We also 3 h) conferring to this coupling process genuine po-
note that, as previously observed for 3a–f, dienones tential for practical application in synthetic organic
4–12 were in all cases stereoselectively obtained as chemistry.
the thermodynamically more stable E isomer.[12] The
Related coupling reactions also occur starting from
high yield formation of 4–12 clearly demonstrates the the highly activated methyl acetoacetate (Scheme 4).
generality of this isomerization/aldol condensation Nevertheless, the products 17 are in all cases obtained
tandem process. Nevertheless, it should be noted that as a non-separable mixture of the corresponding E/Z
attempts to generate Ph2C=CHCH=C(H)C(=O)-i-Pr stereoisomers. It should be also noted that no reaction
was observed between 2a and dimethyl malonate (the
isomerization product 3,3-diphenyl-2-propenal is ex-
clusively formed), pointing out that the presence of at
least one keto function is indispensable to promote
this coupling process.
Conclusions
In this work a new highly efficient catalytic synthetic
approach to conjugated dienones and diene-diones
starting from readily accessible propargylic alcohols
Scheme 3. Formation of diene 13 from propargylic alcohol
2a and 3-methyl-2-butanone.
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Table 2. Tandem isomerization/condensation of propargylic alcohol 2a with different ketones catalyzed by [Ru]/CF3CO2H:
Synthesis of conjugated dienones 4–12.[a]
Entry
1
Ketone
Product
Time
20 h
Yield[b]
91%
4
5
2
3
24 h
7 h
90%
85%
6
4
5
6
7
8
9
24 h
7 h
94%
93%
88%
3 h
7
8
10
12 h
86%
11
12
12 h
7 h
79%
83%
9
[a]
Reactions performed under a N2 atmosphere at 758C using 1 mmol of 2a and 10 mmol of the corresponding ketone. [2a]:
[ketone]:[Ru]:[CF3CO2H] ratio=20:200:1:2.
Isolated yield.
ACHTREUNG
[b]
and commercially available enolizable ketones is de- much more efficient than that using the dinuclear spe-
scribed. This coupling reaction, which consists of a cies [Cp*Ru (OH2)][OTf]2 (X=S, Se,
(m2-XMe)2RuCp*
tandem isomerization/aldol condensation process, is Te) recently reported.[9] In addition, we have also
catalyzed by the complex
16eÀ [Ru(h3-2- shown that this catalytic route is quite general, being
C3H4Me)(CO)(dppf)][SbF6] (1) in the presence of applicable to a variety of aryl and alkyl ketones as
CF3CO2H. The catalytic approach described herein is well as b-dicarbonyl compounds, in contrast to that
A
N
ACHTREUNG
A
ACHTREUNG
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Table 3. Tandem isomerization/condensation of propargylic alcohols 2 with b-dicarbonyl compounds catalyzed by [Ru]/
CF3CO2H: synthesis of conjugated diene-diones 14–16.[a]
Entry
1
Substrate
Diketone
Product
Time
2 h
Yield[b]
87%
2a
14a
14b
2
3
2b
2c
3 h
1 h
90%
95%
14c
4
5
6
2d
2a
2b
14d
15a
15b
11 h
1 h
93%
92%
86%
2 h
7
2c
2a
15c
16
1 h
3 h
84%
90%
8
[a]
Reactions performed under N2 atmosphere at 758C using 1 mmol of the corresponding propargylic alcohol and 10 mmol
of the appropriate diketone. [substrate]:
Isolated yield.
N
G
[b]
reported previously which is limited to the use of ace- chemists, may be including its use in their future re-
tone. In summary, selectivity, atom economy, time/ search programmes.
cost saving and experimental simplicity, concepts to
be assembled by modern academic and industrial syn-
thetic chemist to reach the maximum of efficiency,
are clearly represented in this one-pot and solvent-free
Experimental Section
catalytic transformation. In fact, we are confident that
All reagents were obtained from commercial suppliers and
the high yields and remarkable stereoselectivity found
used without further purification with the exception of com-
plex [Ru(h3-2-C3H4Me)(CO) [SbF6] (1)[5] and propar-
(dppf)]
will be of interest to a wide range of synthetic organic gylic alcohols 2c, d and f[17] which were prepared by follow-
using [Ru(h3-2-C3H4Me)(CO)
N
N
A
ACHTREUNG
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References
[1] For reviews covering this field see: a) C. Bruneau, P. H.
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[2] For books covering this field see: a) Ruthenium in Or-
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Berlin, 2004.
Scheme 4. Coupling of propargylic alcohols 2 with methyl
acetoacetate.
ing the methods reported in the literature. Gas chromato-
graphic (GC) measurements were made on a Hewlett–Pack-
ard HP6890 instrument using a HP-INNOWAX cross-linked
polyethylene glycol (30 m, 250 mm) or a Supelco Beta-
DexTM 120 (30 m, 250 mm) column. GC/MS measurements
were performed on a Agilent 6890N instrument coupled to
a 5973 mass detector (70 eV electron impact ionization)
using an HP-1MS column.
[3] Ruthenium Catalyzed Processes, (Guest Ed.: J.
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[4] For reviews on atom-economical processes see:
a) B. M. Trost, Science 1991, 254, 1471–1477; b) B. M.
Trost, Angew. Chem. Int. Ed. Engl. 1995, 34, 259–281;
c) B. M. Trost, Acc. Chem. Res. 2002, 35, 695–705.
[5] The synthesis of complex 1, as well as its application in
catalytic propargylic substitution processes, has been
reported in: V. Cadierno, J. Díez, S. E. García-Garrido,
J. Gimeno, Chem. Commun. 2004, 2716–2717.
General Procedure for the Catalytic Tandem
Isomerization/Aldol Condensation Reactions
[6] V. Cadierno, S. E. García-Garrido, J. Gimeno, Adv.
The catalyst [Ru(h3-2-C3H4Me)(CO)
N
N
Synth. Catal. 2006, 348, 101–110.
(0.049 g, 0.05 mmol), the corresponding propargylic alcohol
2a–f (1 mmol), the appropriate enolizable ketone or dike-
tone (10 mmol) and CF3CO2H (7.4 mL, 0.1 mmol) were in-
troduced into a sealed tube under a nitrogen atmosphere.
The reaction mixture was then heated at 758C for the indi-
cated time (see Tables 1–3 and Scheme 3-4; the course of
the reaction was monitored by regular sampling and analysis
by GC or GC/MS). After removal of volatiles under
vacuum, the residue was purified by column chromatogra-
phy (silica gel) using a mixture EtOAc/hexane (1:10) as
eluent (with the exception of compound 13 which was
eluted with pure hexane).
[7] Complex 1 represents the first example of a ruthenium
catalyst for the Rupe-type rearrangement of propargyl-
ic alcohols. In contrast, other ruthenium complexes,
such as [Ru(h3-2-C3H4Me)2
ACHTREUNG
A
ACHTREUNG
the Meyer–Schuster rearrangement: a) M. Picquet, C.
Bruneau, P. H. Dixneuf, Chem. Commun. 1997, 1201–
1202; b) M. Picquet, A. Fernµndez, C. Bruneau, P. H.
Dixneuf, Eur. J. Org. Chem. 2000, 2361–2366; c) T.
Suzuki, M. Tokunaga, Y. Wakatsuki, Tetrahedron Lett.
2002, 43, 7531–7533.
[8] Y. Nishibayashi, S. Uemura, Curr. Org. Chem. 2006, 10,
135–150, and references cited therein.
[9] G. Onodera, H. Matsumoto, Y. Nishibayashi, S.
Uemura, Organometallics 2005, 24, 5799–5801.
Supporting Information
Characterization data for compounds 3–17. Copy of the
13C{1H} NMR spectra of dienones 7 and 8. Details of the X-
ray diffraction analysis of compounds 5 and 9.
[10] For a recent review see: D. E. Fogg, E. N. dos Santos,
Coord. Chem. Rev. 2004, 248, 2365–2379.
[11] We also note that, while 1,1-diphenyl-2-propyn-1-ol
(2a) remains unreacted in the absence of 1, only the
isomerization of 2a into 3,3-diphenyl-2-propenal
(Ph2C=CHCHO) exclusively occurs when CF3CO2H is
not present in the reaction media.
[12] The stereochemistry of the C=C double bond in com-
1
pounds 1–6 was clearly assessed by H NMR spectros-
Acknowledgements
copy (see the Supporting Information). In the case of
3
dienones 7–12 it was elucidated by measuring the JCH
We are indebted to the Ministerio de Ciencia y Tecnología
(MCyT) of Spain (Project BQU2003–00255) and the Gobier-
no del Principado de Asturias (FICYT Project IB05–035) for
financial support. S.E.G.G and V.C. thank the MCyT and the
European Social Fund for the award of a Ph.D. grant and a
“Ramón y Cajal” contract, respectively.
coupling constants (1H-C=C-13C) of the carbonyl and
methyl (or methylenic) groups [3JCH =6–8 (trans) vs. 2–
4 Hz (cis)]; see: U. Vçgeli, W. V. Philipsborn, Org.
Magn. Reson. 1975, 7, 617–627). Moreover, the struc-
tures of dienones 5 and 9 were unambiguously con-
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Victorio Cadierno et al.
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firmed by single-crystal X-ray diffraction methods (de-
tails are given in the Supporting Information).
[13] Although complex 1 is known to be an active catalyst
quette, J. Org. Chem. 1994, 59, 528–532; c) M. Bao, Y.
Zhang, J. Wang, Synth. Commun. 1996, 26, 3025–3028;
d) V. Henryon, L. W. Liu, R. Lopez, J. Prunet, J. P. FØr-
Øzou, Synthesis 2001, 2401–2414; e) S. Onitsuka, H.
Nishino, K. Kurosawa, Tetrahedron Lett. 2000, 41,
3149–3152; f) R. Antonioletti, P. Bovicelli, S. Malanco-
na, Tetrahedron 2002, 58, 589–596; g) C. Simon, T.
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4957–4980, and references cited therein.
in propargylic substitution processes of alkynols with
[5]
ꢀ
alcohols (see ref. ), formation of 5-hexyn-2-ones HC
CCR1R2
(CH2COMe) was not observed (see ref.[8]). In
fact, we have found that, even in the absence of
CF3CO2H, complex 1 is not able to catalyze the propar-
gylic substitution of propargylic alcohols 2 with acetone
or related carbonyl compounds, the only products
formed in these reactions being the corresponding
enals derived from the Meyer–Schuster rearrangement
of the alkynols.
[16] The catalytic coupling of propargylic alcohols with
cyclic 1,3-diketones to yield chromen-5-ones and
pyran-5-ones has been recently reported. The first step
of this coupling, which is catalyzed by the dinuclear
[14] The mechanism, as well as the scope, of such an unusu-
al olefination reaction is still unknown, being actually
under active investigation.
[15] See, for example: a) M. Iwata, S. Emoto, Bull. Chem.
Soc. Jpn. 1976, 49, 1369–1374; b) K. Fuschs, L. A. Pa-
ruthenium species [Cp*RuCl
(m2-SR)2RuCp*Cl] (R=
A
Me, n-Pr, i-Pr), involves a propargylic substitution pro-
cess: Y. Nishibayashi, M. Yoshikawa, Y. Inada, M.
Hidai, S. Uemura, J. Org. Chem. 2004, 69, 3408–3412.
[17] M. M. Midland, J. Org. Chem. 1975, 40, 2250–2252.
2132
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Adv. Synth. Catal. 2006, 348, 2125 – 2132