Ruthenium-catalyzed oxidation of a carbon-carbon triple bond: Facile syntheses of alkenyl 1,2-diketones from alkynes

Article abstract of DOI:10.1039/c1dt11642h

A new oxidation procedure of alkynes catalyzed by Tp(PPh ₃)(CH₃CN)Ru-Cl is presented, which provides an efficient way to obtain alkenyl 1,2-diketones via ruthenium alkenyl 1,2-diketone intermediates. In contrast, the analogous reacti

Full text of DOI:10.1039/c1dt11642h

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Ruthenium-catalyzed oxidation of a carbon–carbon triple bond: facile
syntheses of alkenyl 1,2-diketones from alkynes†
Ting-Chuan Hu, Pei-I Hsiao, Tsang-Hsiu Wang, Yu-Wen Yang, Chih-Yun Chang, Yuan-Hsun Wu,
Wei-Chen Sun, Ming-Shiuan Yu, Ching-Yu Lee and Yih-Hsing Lo*
Received 31st August 2011, Accepted 11th October 2011
DOI: 10.1039/c1dt11642h
Table 1 Catalytic transformation under various conditionsa
A
new oxidation procedure of alkynes catalyzed by
Tp(PPh3)(CH3CN)Ru–Cl is presented, which provides an
efficient way to obtain alkenyl 1,2-diketones via ruthenium
alkenyl 1,2-diketone intermediates. In contrast, the analo-
gous reactions with Tp(PPh3)(PhCN)Ru–Cl gave rise to the
ruthenium metallacycle complexes.
1,2-Diketones are very important structural moieties in many
biologically interesting compounds1 and are versatile building
blocks in organic synthesis.2 The synthesis of simple alkyl- and
aryl-substituted 1,2-diketones has been extensively investigated,
and most popular methods are the oxidations of alkynes.3 By
contrast, there are relatively few reports on the synthesis of alkenyl
1,2-diketones. A fairly general method of alkenyl 1,2-diketone is
condensation of 2,3-butanedione with aldehydes;4 nevertheless,
it only provides acceptable results for the condensation of 2,3-
butanedione with activated aldehydes such as enals. Starting from
benzotriazole derivatives, some alkenyl 1,2-diketones have been
obtained in a few steps.5 So far, there is no general way to synthesise
alkenyl 1,2-diketones, which start from simple precursors, and is
generally useful for preparative purpose.
Ruthenium complexes play significant roles in many catalytic
reactions, such as olefin metathesis,6 polymerization7 and asym-
metric hydrogenation.8 A better understanding of the mechanism
of these reactions revealed the role of the metal and led to
extensive applications of ruthenium in organic synthesis. To
further expand the scope of these applications, it is important
to explore new reactivity of various complexes of ruthenium.
Recently, we have reported the ruthenium-catalyzed dimerization
of some terminal alkynes HC CR in organic and aqueous
media.9 As a continuation of this previous work, we report a new
catalytic transformation of alkynes into alkenyl 1,2-diketones by
using Tp(CH3CN)(PPh3)Ru–Cl10 {Tp = HB(pz)3, pz = pyrazolyl}
catalyst.
Entry
Solvent
Conditions
Yield (%)b
1
2
3
4
5
CH2Cl2
Toluene
CH3OH
CH2Cl2
CH2Cl2
Air
Air
Air
N2
99
81
32
Dimerc
99
Air, in the dark
a A mixture of Tp(PPh3)(CH3CN)Ru–Cl (1a) (2.0 mol %) and alkyne (0.06
M) in the selected boiling solvent was stirred for 4 h. b Yields are for isolated
products. Product has been determined by NMR spectroscopy. c The head
to head dimmer: (E)-1,4-diphenyl-1-buten-3-yne.
First, we examined the effect of solvents and gases on cat-
alytic reactions. Treatment of the ruthenium chloride complex
Tp(PPh3)(CH3CN)Ru–Cl (1a) with an excess of HC CPh in
CH2Cl2 at reflux under atmosphere for 4 h exclusively produced
the alkenyl 1,2-diketone product (E)-1,4-diphenylbut-3-ene-1,2-
dione (2a) (Table 1). Under similar conditions, the conversion
decreases from 99% in CH2Cl2 to 81% in toluene and 32%
in CH3OH (entries 1–3) respectively. The high conversion in
CH2Cl2, could be due to the high solubility of the catalyst.
Significantly, a similar reaction was performed under nitrogen
instead of atmosphere (entry 4) revealing oxygen as an oxidant
and gave the head to head dimmer (E)-1,4-diphenyl-1-buten-3-
yne11 Commonly, hydrocarbons reacting with O2 are required for
the photo-activation of O2 in free radical oxidation, and a number
of ruthenium complexes have been used as sensitizers in photo
reactions involving O2.12 However, we carried out the reaction in
the dark and found no effect on the formation of 2a (entry 5). Thus,
this oxidation was not a free radical oxidation. The ruthenium
metal center may assist this oxidation by possibly providing a
coordination site for O2.
Department of Applied Physics and Chemistry, Taipei Municipal University
of Education, Taipei, 10610, Republic of China. E-mail: yhlo@tmue.edu.tw;
Fax: (886)-2-2389-7641
† Electronic supplementary information (ESI) available: Experimental
details and characterization data, including crystallographic data for 4b,
5 and 6. CCDC reference numbers 824345, 824346 and 830357. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt11642h
Moreover, we prepared various alkynes to examine the scope
of catalytic reactions. These catalytic reactions are applicable to
the replacement of the substrates of R groups including electron-
withdrawing groups (R = 4-CF3Ph and 4-FPh). The corresponding
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alkenyl 1,2-diketones 2b–c were obtained in good yields; how-
ever, possessing electron-donating groups (R = 4-CH3Ph and 4-
CH3OPh) were poor substitutes to give the corresponding alkenyl
1,2-diketones 2d–e in low yields (Scheme 1).
resonance at d 58.3 and the 1H NMR spectrum displays a singlet
resonance at d 6.71 respectively. In the 13C NMR spectrum, the
doublet resonance at d 167.4 with JP-C = 12.3 Hz is assigned
to the vinyl Ca. Similarly, the p-tolyl derivative Tp(PPh3)Ru-
{C(C6H4Me) CHC(O)}C(O)(C6H4Me) (4b) was also isolated,
and the single crystal was obtained from a mixture of solvent
(CH2Cl2/MeOH). X-Ray analysis revealed that the ruthenium
atom connected with the triphenylphosphine, and the carbon and
oxygen atoms in the alkenyl diketone were moiety (Fig. 1).‡ The
2
˚
bond lengths of C(37)–C(38) (1.400(5) A), C(28)–O(1) (1.285(4)
˚
˚
A) and C(29)–O(2) (1.215(5) A) are typical of double bonds.
Scheme 1
We also carried out the reaction at room temperature for 1 h
to give a coordinatively unsaturated complex Tp(PPh3)Ru–Cl (3)
1
as monitored by 31P{ H} NMR spectroscopy (Scheme 2), and
a resonance was at d 34.5 (cf . 1a exhibits a signal at d 51.1).13
Unfortunately, complex 3 could not be isolated from the mixture
of the product due to its thermal instability. Furthermore, we
carried out the catalytic reactions for 3 h at room temperature,
and then it afforded the ruthenium alkenyl 1,2-diketone complex
Tp(PPh3)Ru-{C(Ph) CHC(O)}C(O)(Ph) (4a) (Scheme 2). The
reaction of 4a with protic acid breaks the Ru–C bond and
yields compound 2a, revealing that 4a is an intermediate in this
formation.
Fig. 1 An ORTEP drawing of 4b with thermal ellipsoids shown at the
◦
˚
50% probability level. Selected distances (A) and angles ( ): Ru(1)–O(1)
2.060(2), O(1)–C(28) 1.285(4), C(28)–C(37) 1.385(5), C(37)–C(38)
1.400(5), C(38)–Ru(1) 1.986(3), O(2)–C(29) 1.215(5); Ru(1)–O(1)–C(28)
112.6(2); O(1)–C(28)–C(37) 119.4(3), C(28)–C(37)–C(38) 115.3(3),
C(37)–C(3)–Ru(1) 112.5(2), C(38)–Ru(1)–O(1) 80.35(11).
A rational and simplified mechanism for the formation of 2a
is proposed in Scheme 2. The catalytic cycle begins by losing
the CH3CN ligand and then forms a coordinatively unsaturated
species 3. Subsequent a coordinatively unsaturated alkyne com-
plex A is formed by the liberation of the HCl. The next process
proceeds with A that is transformed into an intermediate B [cis-
alkynyl(p-alkyne)] via p-coordination of a second terminal alkyne
to the Ru. Intramolecular migration of the alkynyl ligand to the
p-alkyne in B forms an enyne (intermediate C). Followed by
oxidation of C, the ruthenium metal center may serve to assist
this oxidation by possibly providing a coordination site leading to
D. Then the intermediate D transforms to the ruthenium alkenyl
1,2-diketone product 4a. Protonation of 4a with HCl causes the
Ru–C bond cleavage and then affords complex 3 and alkenyl 1,2-
diketone 2a, respectively. The detailed mechanism is currently
under investigation.
On the other hand, we examined two other catalysts to
assess the effect of the ligands on the catalysts. Intriguingly, the
reaction of Tp(PPh3)(PhCN)RuCl (1b)14 with excess HC CPh
in CH3OH at reflux for 4 h did not yield the expected com-
pound 2a but gave the yellow ruthenium alkenyl imine com-
plex Tp(PPh3)Ru{C(OCH3) C(Ph)C(Ph) NH} (5) in 46% yield
(Scheme 3) instead. Furthermore, no reaction was observed
in CH2Cl2, THF and CHCl3. The IR spectrum of 5 shows
Scheme 2
The spectroscopy data was sufficient to unequivocally assign
the structures of 2a and 4a. The 1H NMR spectrum of 2a displays
well-resolved doublets at d 7.82 with JH-H = 16.2 Hz and d 6.87
1
with JH-H = 16.2 Hz. The 13C { H} NMR exhibits two carbonyl
carbons at d 194.2 and 193.8 and exhibits two olefinic carbons at d
151.3 and 149.5. In the EI-MS mass spectrum, the parent peak is
observed at m/z = 236.1 indicating the molecular weight of 2a. In
addition, the IR spectrum of 4a shows two strong bands at 1642
and 1612 cm-1 due to two C O groups, and a medium intensity
absorption is assigned at 1537 cm-l resulting from the C C. Owing
1
to the vinylic proton, the 31P{ H} NMR spectrum shows a singlet
12664 | Dalton Trans., 2011, 40, 12663–12666
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Scheme 3
the disappearance of a strong CN (2214 cm-1) band and the
appearance of the characteristic bands at 3252 cm-1, assigning
to n(NH). The 1H NMR spectrum displays a characteristic singlet
resonance at d 12.68 and assigns to the NH proton of the imine
group; in addition, a singlet resonance at d 3.77 is attributed to the
three methoxyl protons. The 31P NMR spectrum of the complex
5 displays a singlet at d 57.7; in the EI-MS mass spectrum, the
parent peak is observed at m/z = 812.2 indicating an additional
CH3OH and an eliminated HCl from 1b.
The structure of 5 was further confirmed by a solid-state single-
crystal X-ray diffraction analysis (Fig. 2).‡ It is apparent that the
C–C bond formation between Cb of vinylidene moiety and the
carbon of benzylnitrile has occurred. The N(7)–Ru(1)–C(42) bite
angle is 76.79(8)◦. The Ru(1)–N(7) and Ru(1)–C(42) bond lengths
Scheme 4
by a nitrogen or an oxygen donor and then gives a Fischer type
carbene complex. The nucleophilic attack of MeOH is followed
by formation of a Cb–C bond, and then an intramolecular
coordination of the iminyl nitrogen to the Ru generates the carbene
intermediate E. Finally, the HCl is eliminated form E and affords
the complex 5. Compared to CH3CN, PhCN ligand is apt to expose
the Ca atom of the ruthenium vinylidene complex that leads to
much easier attack at Ca by other nucleophiles.
˚
are 2.0334(18) and 2.032(2) A, respectively. The C(42)–C(35) and
Moreover, the reaction of 1b with excess of HC C(p-
MeC6H4) in MeOH at reflux for 4 h did not yield the
expected alkenyl imine complex Tp(PPh3)Ru{C(OCH3) C(p-
MeC6H4)C(Ph) NH} but gave the alkenyl ketone
˚
C(28)–N(7) bond lengths are 1.404(3) and 1.307(3) A, respectively,
compared with those found for the C–C and C–N double bond
lengths.
Tp(PPh3)Ru{C(p-MeC6H4) CHC(O)CH2(p-MeC6H4)}
(6)
in high yields (Scheme 3) instead. Characterization of 6 was
1
1
performed on the basis of H and 13C { H} NMR, IR, FAB-
MS, elemental analysis and X-ray diffraction analysis. The IR
spectrum shows the presence of a C C and C O group at
ca. 1567 and 1618 cm-1, respectively. The 31P NMR spectrum
displays a singlet at d 57.1. The 1H NMR spectrum displays two
doublet resonances at d 3.45 and 3.73 with a coupling constant
of JH-H = 16.2 Hz, it is assigned to the CH2Ph group. Another
singlet resonance is at d 6.72 and is assigned to the CH group.
In the 13C NMR spectrum, the doublet resonance at d 168.1
with 2JP-C = 14.0 Hz is assigned to the vinyl Ca. A similar alkenyl
ketone structure with a five-membered ring has also been reported
recently.16 Recently, there have been considerable efforts devoted
to the understanding of the origins of the selectivity and the
development of alkynes coupling.17 The solid state structure of
6 contains two crystallographically distinct molecules, although
there is no essential structural difference between them (Fig. 3).‡
˚
The bond lengths of C(37)–C(38) (1.397(5) A) and C(28)–O(1)
Fig.
2
An ORTEP drawing of 5 with thermal ellipsoids shown
˚
(1.278(4) A) are typical double bonds.
˚
at the 50% probability level. Selected distances (A) and an-
gles (◦): Ru(1)–N(7) 2.0334(18), N(7)–C(28) 1.307(3), C(28)–C(35)
1.422(3), C(35)–C(42) 1.404(3), C(42)–Ru(1) 2.032(2); Ru(1)–N(7)–C(28)
118.79(13); N(7)–C(28)–C(35) 115.3(2), C(28)–C(35)–C(42) 112.68(19),
C(35)–C(42)–Ru(1) 116.32(16), C(42)–Ru(1)–N(7) 76.79(8).
In conclusion, the treatment of Tp(PPh3)(CH3CN)Ru–Cl with
terminal alkynes afforded the alkenyl 1,2-diketone products
via ruthenium alkenyl 1,2-diketone intermediates, whereas the
analogous reactions with Tp(PPh3)(PhCN)Ru–Cl gave rise to the
ruthenium metallacycle complexes. To our knowledge, this is the
first example that the ruthenium chloride complex with terminal
alkynes yields an alkenyl 1,2-diketone product. A new oxidation
of alkynes has been developed. This transformation showed high
efficiency. Further investigations on mechanistic understanding
and substrate scope expanding are ongoing
The formation of 5 can be accounted for by the mechanism
depicted in Scheme 4. The reaction of 1b with phenylacetylene
gives the cationic vinylidene complex D, and chloride is the
counteranion of the complex D. It is well-known that Ca of a
vinylidene ligand is susceptible to nucleophilic attack15 particularly
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Fig.
3
An ORTEP drawing of 6 with thermal ellipsoids shown
◦
˚
at the 50% probability level. Selected distances (A) and angles ( ):
Ru(1)–O(1) 2.086(3), O(1)–C(28) 1.278(4), C(28)–C(37) 1.404(5),
C(37)–C(38) 1.397(5), C(38)–Ru(1) 1.999(3); Ru(1)–O(1)–C(28)
113.5(2); O(1)–C(28)–C(37) 118(3), C(28)–C(37)–C(38) 115.3(3),
C(37)–C(38)–Ru(1) 113.4(3), C(38)–Ru(1)–O(1) 79.4(13).
We gratefully acknowledge the financial support from the
National Science Council of Taiwan (NSC 99-2113-M-133-001-
MY3). We also thank Mr Ting Shen Kuo (Department of
Chemistry, National Taiwan Normal University, Taiwan) for his
assistance in X-ray single crystal structure analysis.
Notes and references
10 Y. Z. Chen, W. C. Chan, C. P. Lau, H. S. Chu, H. L. Lee and G. Jia,
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‡ Data for 4b: C45H40BN6O2PRu·0.79(C7H8), M = 912.29, triclinic, space
11 The dimer is characterized by well-resolved doublets at d 6.30 with
¯
˚
˚
˚
group P1, a = 10.1964(4)◦A, b = 12.4327(◦4) A, c = 18.4795(7) A, a =
JH-H = 16.2 Hz and d 7.04 with JH-H = 16.2 Hz.
◦
3
˚
89.086(2) , b = 77.852(2) , g = 83.264(2) , V = 2274.27(14) A , T =
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200(2)K, Z = 2, 15791 reflections collected, 7759 unique (Rint = 0.0365)
which were used in all calculations. R1 = 0.0392 for I > 2s. The final
wR2 was 0.0915 (all data). Data for 5: C43H39BN7OPRu, M = 812.66,
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X. Zhou,W.-H. Lei, J.-R. Chen, C. Li, Y.-J. Hou, X.-S. Wang and B.-W.
Zhang, Chem.–Eur. J., 2010, 16, 3157.
˚
˚
˚
monoclinic, space group P21/◦n, a = 11.7013(7) A, b = 14.1739(9) A, c =
3
˚
23.4437(13) A, b = 104.301(2) , V = 3767.7(4) A , T = 200(2)K, Z = 4,
21592 reflections collected, 6479 unique (Rint = 0.0310) which were used in
all calculations. R1 = 0.0285 for I > 2s. The final wR2 was 0.0737 (all data).
Data for 6: C45H42BN6OPRu, M = 825.70, monoclinic, space group P21/n,
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◦
˚
˚
˚
a = 24.6370(10) A, b = 10.0656(4) A, c = 32.0278(13) A, b = 91.513(2) ,
3
˚
V = 7939.7(6) A , T = 200(2)K, Z = 8, 53461 reflections collected, 13970
unique (Rint = 0.0574) which were used in all calculations. R1 = 0.0457 for
I > 2s. The final wR2 was 0.1148 (all data).
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12666 | Dalton Trans., 2011, 40, 12663–12666
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The Royal Society of Chemistry 2011
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