Ruthenium(II)-catalyzed regioselective synthesis of allyl ketones from alkynes and their silver (I)-catalyzed hydroarylation into γ- functionalized ketones

Article abstract of DOI:10.1002/adsc.200900236

The regioselective synthesis of β,γ-unsaturated ketones from terminal alkynes is achieved by cooperative action of tris(acetonitrile) pentamethylcyclopentadieneruthenium hexafluorophosphate [Cp*Ru (NCMe) ₃⁺ PF^-₆

Full text of DOI:10.1002/adsc.200900236

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DOI: 10.1002/adsc.200900236
Ruthenium(II)-Catalyzed Regioselective Synthesis of Allyl
Ketones from Alkynes and their Silver(I)-Catalyzed
Hydroarylation into g-Functionalized Ketones
Min Zhang,a,b Huanfeng Jiang,b,* and Pierre H. Dixneufa,*
a
Laboratoire Catalyse et Organomꢀtalliques, Institut Sciences Chimiques de Rennes, UMR 6226 CNRS – Universitꢀ de
Rennes, Campus de Beaulieu, 35042 Rennes, France
Fax : (+33)-22-2323-6939; e-mail: pierre.dixneuf@univ-rennes1.fr
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Peopleꢁs
b
Republic of China
E-mail: jhf@mail.gic.ac.cn
Received: April 7, 2009; Published online: June 24, 2006
Dedicated to Armin de Meijere for his tremendous scientific contributions and his research enthusiasm.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900236.
Abstract: The regioselective synthesis of b,g-unsatu-
rated ketones from terminal alkynes is achieved by
cooperative action of tris(acetonitrile)pentamethyl-
ed alkylation of aromatics and heterocycles consists in
their metal-catalyzed Friedel–Crafts allylation,[5,6] but
these reactions take place with the loss of an allyl
leaving group. Among the intermolecular catalytic
Friedel–Crafts reactions with C=C bonds, most of
them involve activated double bonds conjugated with
a functional group,[4] such as nitroalkenes[4a–c] or a,b-
unsaturated carbonyl derivatives.[4d,e] It is noteworthy
that hydroarylation of non-activated alkenes was re-
cently reported to be promoted with a Pt(II)-Ag(I)
catalyst,[4f] an Au(I)/Ag(I) catalyst assisted by micro-
wave irradiation,[4g] or with an Ag(I) salt for diene ad-
dition.[4h]
cyclopentadienerutheniÀum
hexafluorophosphate
(NCMe)3 PF6 ] and para-toluenesulfonic
+
[Cp*Ru
N
acid catalysts. These allyl ketones undergo direct re-
gioselective hydroarylation/Friedel–Crafts reaction
to introduce an electron-rich aryl group at the g-po-
sition in the presence of ligand-free silver triflate
(AgOTf) catalyst. Both catalytic reactions take
place with atom economy and provide an alterna-
tive to the synthesis of a variety of allyl ketones
and g-arylated ketones.
Allyl ketones have also the potential to be used as
alkylating reagents for aromatic compounds. Several
convenient routes already have led to allylic ke-
tones,[7] such as from allylmetal[7a,b] and alkenylmetal
derivatives,[7c,d] by the mild oxidation of homoallylic
alcohols,[7e,8] and by the ruthenium-catalyzed hydroa-
cylation of dienes.[7f–h] However, to the best of our
knowledge, the hydroarylation of b,g-unsaturated
Keywords: allyl ketones; atom economy; g-func-
tionalized ketones; ruthenium catalysts; sequential
catalysis; silver catalysts
The regioselective hydroarylation of functional alkene ketone C=C bonds has not been performed yet, likely
C=C bonds constitutes an important synthetic chal- due to the possible lack of regioselectivity in the ab-
lenge for the functionalization of aromatic com- sence of a conjugated carbonyl group, and their easy
pounds, especially when the reaction tolerates func- isomerization into a,b-unsaturated ketones under
tional groups and takes place with atom economy. In electrophilic reaction conditions.
this direction an emerging field deals with the metal-
Pursuing our efforts to generate functional dienes,
2
À
catalyzed activation of sp C H bonds with formal in- via the ruthenium-catalyzed dimerization of terminal
sertion of functional alkenes.[1,2] However, the most alkynes, such as 1,3-dienyl esters[9] or ethers,[10] we
general C=C bond hydroarylation reactions involve a have explored the possibility to directly generate al-
Friedel–Crafts process, corresponding to metal-cata- lylic ketones by hydrolysis of alkynes, via 1,3-dienol
lyzed alkylation of aromatic rings, frequently suitable generation, and to use them for the Friedel–Crafts al-
[3,4]
À
for enantioselective C C bond formation.
A relat- kylation of aromatic compounds.
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Ruthenium(II)-Catalyzed Regioselective Synthesis of Allyl Ketones from Alkynes
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We now wish to report (i) the catalytic synthesis of lysts[10], respectively. However, the direct hydrolysis
allylic ketones from terminal alkynes and water with of the bis-carbene-ruthenium intermediate did not
the cooperative actions of both [RuCp*ACHTUNRGTNEUNG(NCMe)3]PF6 lead to the expected formation of allyl ketones or to
and para-toluenesulfonic acid (p-TSA) catalysts and their isomerization into a,b-unsaturated ketones,
(ii) the AgOTf-catalyzed regioselective Friedel– whereas the intramolecular oxidative coupling of non-
+
À
Crafts/hydroarylation of b,g-unsaturated ketones with conjugated diynes with RuCpACHTNUGTRNEUNG(NCMe)3 PF6 in the
electron-rich arenes. These sequential catalytic reac- presence of water led by contrast to the formation of
tions allow the two-step transformation of terminal al- cyclic a,b-unsaturated ketones under mild condi-
kynes into g-arylated ketones [Eq. (1)].
tions.[12]
We have thus first investigated the direct access to
allyl ketones from terminal alkynes in the presence of
+
À
RuCp*ACTHNUGRTNEUNG(NCMe)3 PF6 catalyst A. Whereas the reac-
tion of phenylacetylene with water alone in dioxane
and catalyst A led to a mixture of compounds, we ex-
plored the influence of acids as the bis-carbene inter-
mediate needs to be protonated before nucleophilic
addition to give a dienyl product.[9] The reaction of
phenylacetylene 1a (1 mmol) with 3 equiv. of water
The stoichiometric head-to-head dimerization of al- and 0.15 mmol of para-toluenesulfonic acid monohy-
kynes with Cp*Ru complexes leading to a bis-carbene drate (p-TSA) in dioxane with 4 mol% of
+
À
intermediate[11] was recently used for the catalytic for- RuCp*
ACHTUNGTRENNUNG
+
À
RuClACHTUNGTRENNUNG(Cp*)ACHTUNGTRENNUNG ACHTNUGTNERN(UNG NCMe)3 PF6 cata- of allyl ketone 2a, isolated in 96% yield [Eq. (2),
(COD)[9] and RuCp*
Table 1. Ruthenium-catalyzed synthesis of b,g-unsaturated ketones directly from alkynes.[a]
Entry
1
Alkyne
Reaction time [h]
0.5
Ketone 2
Yield [%][b]
96
1a
2a
2
3
1b
1c
0.5
3 h
2b
2c
88
85[c]
4
5
1d
1
2d
88
1e
1f
0.5
1
2e
2f
78
93
6
[a]
+
À
Reaction conditions: 1 mmol of alkyne, p-TSA.H2O (28.5 mg, 0.15 mmol), H2O (3 equiv.) and Cp*Ru
(0.04 mmol) in 1 mL dioxane at room temperature for 0.5 to 3 h.
Isolated yields.
ACHTUNGRTEN(NUNG MeCN)3 PF6
[b]
[c]
Obtained yield in the presence of 15 mol% of ruthenium catalyst A after 3 h of reaction.
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Table 1, entry 1). The formation of the a,b-unsaturat-
As the allylic ketones 2 are formed in the presence
ed ketone was not observed. This atom-economical of a strong acid without their isomerization, we have
synthetic method to form b,g-unsaturated ketone 2a explored their use for the hydroarylation of electron-
under mild contitions was studied for the catalytic rich arenes via a Friedel–Crafts process.
transformation of a variety arylacetylenes 1b–1f. All
The ketone 2a and anisole 3a were first reacted
reactions proceeded smoothly and afforded the ke- with various Brønsted acid and Lewis acid catalysts to
tones 2a–2f in good to excellent isolated yields of 78– favour the hydroarylation reaction (Table 2). Cata-
96% (Table 1, entries 2–6). However, the reaction is
slower with electron-releasing substituent such as for
Table 2. Hydroarylation reaction of 2a with anisole into
ketone 4a.[a]
1c, for which an increase of the catalyst loading was
required to reach a decent yield (entry 3). The reac-
tion tolerates a para-bromo substituent (entry 6) thus
making possible further modifications of the ketone
and derivatives via classical catalytic cross-coupling
À
À
C C or C N bond formation. All ketones 2a–2f pos-
sess the E-configuration of their CH=CH bond as
1
shown by H NMR spectroscopy.
On the basis of the mechanism for the catalytic for-
mation of dienyl esters from alkynes, and catalyzed
by Ru(Cl)ACHTUNGTRENNUNG
(COD)Cp*,[9] we can suggest that the for-
mation of allylic ketones 2 takes place as shown in
Scheme 1. The terminal alkynes react with catalyst A
Entry Cat.
Solvent
Temp. Yield [%][b]
[8C]
of 4a
1
2
3
4
5
6
7
H2SO4
p-TSA
1,2-dichloroethane 100
1,2-dichloroethane 100
76
22
79
56
91
–
CF3SO3H 1,2-dichloroethane 100
Cu(OTf)2 1,2-dichloroethane 100
AHCTUNGTRENNUNG
AgOTf
AgOTf
1,2-dichloroethane 100
dioxane 100
AgOTf[c] 1,2-dichloroethane 80
34
[a]
Reaction performed in a sealed Schlenk tube: 0.5 mmol
of 2a, 1.5 mmol of 3a and 0.025 mmol of AgOTf
(5 mol%) in 1 mL of solvent in a bath at 1008C for 1 h.
GC yield with tetradecane as the internal standard.
At 808C for 1 h.
[b]
[c]
lysts such as H2SO4, p-TSA, CF3SO3H, CuACTHNUGTRNENG(U OTf)2 can
promote the hydroarylation reaction (Table 2, en-
tries 1–4), but the desired product was obtained in rel-
atively low yields. The silver salt AgOTf without an
additional ligand led to the best result as 4a was
Scheme 1. Proposed mechanism for the catalytic formation
of allyl ketones 2 from terminal alkynes.
and lead to the head-to-head dimerization of alkynes formed in 91% GC yield (Table 2, entry 5). The reac-
and the formation of the bis-carbene intermediate I tion was found to be highly dependent on the nature
with both Schrock- and Fischer-type behaviour.[9] Pro- of the solvent. Among the various solvents, dichloro-
tonation with a strong acid is required for the hydrol- ethane (DCE) was the most effective one. The reac-
ysis to take place and thus the intermediate II should tion gave a mixture of products in coordinating sol-
be formed on protonation of the Schrock-type car- vents such as dioxane or dimethylformamide (Table 2,
bene carbon atom, thus increasing the electrophilic entries 6). However, only a 34% yield was found in
behaviour of the Fischer carbene carbon atom and al- the presence of AgOTf in DCE at 808C (Table 2,
lowing the addition of water to give the intermediate entry 7). Thus it can be concluded that 5 mol% of
III, that generates the 1,3-dienol intermediate leading AgOTf in dichloroethane is the best system for this
allyl ketone 2.
transformation performed at 1008C.
This bis-carbene intermediate I is expected to be
Having optimized the reaction conditions, the scope
stabilized by the conjugated aryl groups. This is likely of this catalytic reaction was investigated in the pres-
the reason why this dimerization reaction cannot be ence of 5 mol% of AgOTf with various allylic ketones
extended to alkylacetylenes such as 1-hexyne.
and excess of electron-rich arenes, such as methoxy-,
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Ruthenium(II)-Catalyzed Regioselective Synthesis of Allyl Ketones from Alkynes
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Table 3. Catalytic synthesis of g-diarylated ketones 4.[a]
Entry
2
3
Reaction time [h]
Ketone 4
Yield [%][b]
1
2a
3a
0.5
4a
81
2
3
2b
2c
3a
3a
0.6
0.5
4b
4c
65
85
4
2d
3a
0.5
4d
76
5
6
2e
2f
3a
3a
1
2
4e
4f
51
<10
7
2a
2b
2d
2e
3b
3b
3b
3b
2
2
2
2
4g
4h
4i
62
67
63
33
8
9
10
4j
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Min Zhang et al.
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Table 3. (Continued)
Entry
2
3
Reaction time [h]
Ketone 4
Yield [%][b]
11
2a
3c
2
4k
53
12
2d
2a
3c
0.5
10
4l
78
53
13
3d
4m
[a]
Reactions performed in a sealed Schlenk tube: 0.5 mmol of 2 and 1.5 mmol of 3 with 0.025 mmol of AgOTf catalyst in
2 mL of dichloroethane was stirred in a bath at 1008C for 0.5 to 10 h.
Isolated yields.
[b]
dimethoxybenzene and toluene. The results are sum-
marized in Table 3. The reaction proceeded smoothly
and was completed within 0.5 to 2 h at 1008C in a
closed Schlenk tube to afford a variety of polysubsti-
tuted g-arylated ketones in moderate to excellent
yields (Table 3, entries 1–5 and 7–13), except for 4f
arising from the allyl ketone 2f with a strong elec-
tron-withdrawing group on the ketone aryl groups
(entry 6). However the bromo-substituted allyl ketone
2e was able to form the corresponding Friedel–Crafts
products 4e and 4j in moderate yields (entries 5 and
10). The reaction does not occur with N,N-dimethyl-
Scheme 2. Possible interaction of AgOTf with allyl ketones
favouring Friedel–Crafts reaction.
+
À
A
tion times.
formation[13] (Scheme 2).
This reaction leads to high regioselectivity of hydro-
In conclusion, a new catalytic and regioselective
arylation as the 1H NMR spectra showed the method to make b,g-unsaturated allylic ketones, with-
CHCH2CH2CO fragment. The structure of the repre- out isomerization, from terminal arylalkynes is pre-
sentative example 4d was confirmed by its HMBC sented by the cooperative action of ruthenium and p-
spectra: they showed the correlation of the carbonyl TSA catalysts. The potential of the resulting allyl ke-
carbon with a-methylene and b-methylene protons, tones has been shown in the direct regioselective hy-
with no correlation with the g-proton (see Supporting droarylation/Friedel–Crafts reaction to introduce an
Information for 4d).
electron-rich aryl group at the g-position in the pres-
This selective formation of ketones 4 indicates that ence of ligand-free AgOTf catalyst. Both catalytic re-
AgOTf does not isomerize the ketones into a,b-unsa- actions take place with high atom economy and pro-
turated ketones. The formation of g-functionalized vide an alternative to the synthesis of g-arylated ke-
ketones 4 can be rationalized by an interaction of the tones. Thus it appears that a Ag(I) catalyst promotes
allyl ketone C=C bond with a silver cation leading to the intermolecular hydroarylation of allylic ketones in
a more electrophilic g-benzylic carbon cation, as in the presence of an aromatic partner, whereas Cu(II)
the alkylation and allylation of aromatic compounds salts favour the intramolecular formation of furans
with silver salts.[6] The oxophilicity of the silver from allyl ketones arising from dienyl ethers.[9]
cation[13] may also allow its coordination with the
neighbouring ketone oxygen atom, as it was proposed
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Ruthenium(II)-Catalyzed Regioselective Synthesis of Allyl Ketones from Alkynes
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[3] For reviews, see: a) T. B. Poulsen, K. A. Jørgensen,
Chem. Rev. 2008, 108, 2903–2915; b) M. Bandini, E.
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Umani-Ronchi, Angew. Chem. 2004, 116, 560–566;
Angew. Chem. Int. Ed. 2004, 43, 550–556.
Experimental Section
Typical Procedure for Synthesis of the Allylic Ketone
2a
+
À
The catalyst RuCp*ACHTNUTRGNE(NUG MeCN)3 PF6 (20 mg, 0.04 mmol, 4
mol%) and para-toluenesulfonic acid monohydrate (p-TSA)
(0.15 mmol, 28.5 mg) were dissolved in 1 mL of 1,4-dioxane:
then phenylacetylene 1a (1 mmol, 102 mg), H2O (3 mmol)
were added, and the mixture was stirred under argon pro-
tection at room temperature for 0.5 hour. Then petroleum
ether (5 mL) was added, the resulting mixture was directly
filtered through a pad of silica and concentrated to afford
the crude product, which was purified by recrystallization in
pentane, to give the white solid 2a; yield: 107 mg (96%).
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General Procedure for Synthesis of Saturated Ketone
4a from 2a
A Schlenk tube equipped with a rubber septum and a mag-
netic stirring bar was charged with 2a (0.5 mmol, 111 mg)
and AgOTf (0.025 mmol, 6.4 mg, 5 mol%) under an argon
atmosphere. Anisole (1.5 mmol, 162 mg) and 1,2-dichloro-
ethane (1 mL) were added by syringe. The septum was then
replaced by a teflon-coated screw cap, and the reaction
vessel was placed in an oil bath at 1008C. After stirring for
0.5 h, it was cooled down to room temperature. The result-
ing mixture was directly filtered through a pad of silica and
concentrated to afford the crude product, which was puri-
fied by flash column chromatography on silica, eluting with
petroleum ether: ethyl acetate (25: 1) to give 4-(4-methoxy-
phenyl)-1,4-diphenylbutan-1-one 4a as a light yellow oil;
yield: 134 mg (81%).
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Acknowledgements
The authors are grateful to the European Network IDECAT
for support, to the China Scholarship Council for a grant to
Min Zhang, and to the Institut Universitaire de France for
membership.
[7] Allyl ketone preparations: a) via acylation: R. C.
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