Alkyne oxidations by cis-dioxoruthenium(VI) complexes. A formal [3 + 2] cycloaddition reaction of alkynes with cis-[(Cn*)(CF3CO2)Ru(VI)O2]ClO4 (Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane)

Article abstract of DOI:10.1021/ja001707o

cis-Dioxoruthenium(VI) complexes, [Cn*(CF₃CO₂)Ru(VI)O₂]ClO₄ (1) (Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane) and cis-[(Tet-Me₆)Ru(VI)O₂](ClO₄)₂ (2) (Tet-Me₆ = N,N,N',N'-tetramethyl-3,6-dimethyl-3,6-diazaoctane-1,8-diamine), oxidize disubstituted alkynes to 1,2-diketones selectively in good to excellent yields under ambient conditions. The reactions proceed via the formation of dark blue [(Cn*)(CF₃CO₂)Ru(IV)OC₂R¹R²O]⁺ intermediates, which display a characteristic UV-visible absorption band at 550-680 nm. With bis(trimethylsilyl)acetylene as substrate and 1 as the oxidant, the intermediate was isolated and structurally characterized by X-ray crystallography as a [3 + 2] cycloadduct. The kinetics of the cycloaddition of 1 with various substituted trimethylsilylacetylenes has been studied by stopped-flow spectrophotometry. With the exception of bis(trimethylsilyl)acetylene, the second-order rate constants were found to vary over a range of less than an order of magnitude irrespective of a 2.3 eV change of the calculated I(p) of the alkynes; therefore, a rate-limiting single electron-transfer mechanism is unlikely. The participation of oxirene (oxene insertion) and metallaoxetene ([2 + 2] cycloaddition) intermediates appears to be implausible based on product analysis. A linear Hammett correlation was established using σ⁺ and σ(jj)(G) parameters for the cycloaddition of 1 with para-substituted aryl trimethylsilylacetylenes, and the rate-limiting vinyl radical intermediate formation is proposed.

Full text of DOI:10.1021/ja001707o

11380
J. Am. Chem. Soc. 2000, 122, 11380-11392
Alkyne Oxidations by cis-Dioxoruthenium(VI) Complexes. A Formal
[3 + 2] Cycloaddition Reaction of Alkynes with
cis-[(Cn*)(CF3CO2)RuVIO2]ClO4 (Cn* )
1,4,7-Trimethyl-1,4,7-triazacyclononane)
Chi-Ming Che,*,† Wing-Yiu Yu,*,† Pui-Ming Chan,† Wing-Chi Cheng,† Shie-Ming Peng,‡
Kai-Chung Lau,§ and Wai-Kee Li§
Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong,
Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan, and Department of Chemistry,
The Chinese UniVersity of Hong Kong, Shatin, Hong Kong
ReceiVed May 17, 2000
Abstract: cis-Dioxoruthenium(VI) complexes, [Cn*(CF3CO2)RuVIO2]ClO4 (1) (Cn* ) 1,4,7-trimethyl-1,4,7-
triazacyclononane) and cis-[(Tet-Me6)RuVIO2](ClO4)2 (2) (Tet-Me6 ) N,N,N′,N′-tetramethyl-3,6-dimethyl-3,6-
diazaoctane-1,8-diamine), oxidize disubstituted alkynes to 1,2-diketones selectively in good to excellent yields
under ambient conditions. The reactions proceed via the formation of dark blue [(Cn*)(CF3CO2)RuIV-
OC2R1R2O]+ intermediates, which display a characteristic UV-visible absorption band at 550-680 nm. With
bis(trimethylsilyl)acetylene as substrate and 1 as the oxidant, the intermediate was isolated and structurally
characterized by X-ray crystallography as a [3 + 2] cycloadduct. The kinetics of the cycloaddition of 1 with
various substituted trimethylsilylacetylenes has been studied by stopped-flow spectrophotometry. With the
exception of bis(trimethylsilyl)acetylene, the second-order rate constants were found to vary over a range of
less than an order of magnitude irrespective of a 2.3 eV change of the calculated Ip of the alkynes; therefore,
a rate-limiting single electron-transfer mechanism is unlikely. The participation of oxirene (oxene insertion)
and metallaoxetene ([2 + 2] cycloaddition) intermediates appears to be implausible based on product analysis.
A linear Hammett correlation was established using σ+ and σJJ parameters for the cycloaddition of 1 with
•
para-substituted aryl trimethylsilylacetylenes, and the rate-limiting vinyl radical intermediate formation is
proposed.
Introduction
oxidations.3 Because of their importance, much effort has been
directed to study their reactivities toward organic substrates.1,3,4
Previously we and others also reported several studies on the
mechanism of organic oxidations using well-characterized MdO
complexes under stoichiometric conditions.3c,5-9 Compared to
trans-dioxometal complexes, the oxidation chemistry of cis-
dioxometal complexes is less developed. Indeed, apart from
Highly oxidizing metal-oxo (MdO) complexes are widely
invoked to play a key role in many catalytic and enzymatic
oxidations1 such as the Mn(III)-catalyzed asymmetric organic
oxidations2 and the cytochrome P-450 mediated biological
† The University of Hong Kong.
RuO4,10-11 OsO4,12 KMnO4,8,13 and CrO2Cl2 there are few
14
‡ National Taiwan University.
§ The Chinese University of Hong Kong.
structurally characterized oxidizing cis-dioxo metal
(1) (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic
Compounds; Academic Press: New York, 1981. (b) Mimoun, H. In
ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard, R. D.,
McCleverty, J. A., Eds.; Pergamon: New York, 1986; Vol. 6, Chapter 61.3.
(c) Holm, R. H. Chem. ReV. 1987, 87, 1401. (d) Jorgensen, K. A. Chem.
ReV. 1989, 89, 431. (e) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple
Bonds; John Wiley: New York, 1988.
(2) Asymmetric epoxidation: (a) Collman, J. P.; Zhang, X.; Lee, V. J.;
Uffelman, E. S.; Brauman, J. I. Science 1993, 261, 1404. (b) Jacobsen, E.
N. In ComprehensiVe Organometallic Chemistry II; Wilkinson, G., Stone,
F. G. A., Abel, E. W., Hegedus, L. S., Eds.; Pergamon: New York, 1995;
Vol. 12, Chapter 11.1. (c) Katsuki, T. Coord. Chem. ReV. 1995, 140, 189.
Asymmetric hydroxylation: (d) Groves, J. T.; Viski, P. J. Am. Chem. Soc.
1989, 111, 8537. (e) Groves, J. T.; Viski, P. J. Org. Chem. 1990, 55, 3628.
(f) Larrow, J. F.; Jacobsen, E. N. J. Am. Chem. Soc. 1994, 116, 12129. (g)
Hamadii, K.; Irie, R.; Katsuki, T. Tetrahedron Lett. 1996, 37, 4979. (h)
Miyafuji, A.; Katsuki, T. Synlett 1997, 836. (i) Komiya, N.; Noji, S.;
Murahashi, S.-I. Tetrahedron Lett. 1998, 39, 7921. Asymmetric aziridina-
tion: (j) Noda, K.; Hosoya, N.; Irie, R.; Ito, Y.; Katsuki, T. Synlett 1993,
469. (k) Noshikori, H.; Katsuki, T. Tetrahedron Lett. 1996, 37, 9245. (l)
Lai, T.-S.; Kwong, H.-L.; Che, C.-M.; Peng, S.-M. Chem. Commun. 1997,
2373.
(3) (a) Meunier, B. Chem. ReV. 1992, 92, 1411. (b) Metalloporphyrins.
In Catalytic Oxidations; Sheldon, R. A., Ed.; Marcel Dekker: New York,
1994. (c) Ostovic, D.; He, G.-X.; Bruice, T. C. In Metalloporphyrins in
Catalytic Oxidations; Sheldon, R. A., Ed.; Marcel Dekker: New York, 1994;
Chapter 2, p 29. (d) Cytochrome P-450: Structure, Mechanism and
Biochemistry, 2nd ed.; Oritz de Montellano, P. R., Ed.; Plenum Press: New
York, 1995.
(4) Selected examples for the isolation and/or characterization of MdO
(M ) Fe, Cr, Mn, and Ru) complexes containing porphyrin and Schiff base
ligands. For [FeIVO(porphyrin)]: (a) Chin, D.-H.; Balch, A. L.; LaMar, G.
N. J. Am. Chem. Soc. 1980, 102, 1446, 5945. (b) Balch, A. L.; Chan, Y.-
W.; Cheng, R.-J.; LaMar, G. N.; Latos-Grazynsky, L.; Renner, M. W. J.
Am. Chem. Soc. 1984, 106, 7779. (c) Groves, J. T.; Gross, Z.; Stern, M. K.
Inorg. Chem. 1994, 33, 5065. For [CrIVO(porphyrin)]: (d) Groves, J. T.;
Krupper, W. J.; Haushalter, R. C.; Butler, W. M. Inorg. Chem. 1982, 21,
1363. For [CrVO(salen)]: (e) Srinivasan, K.; Kochi, J. K. Inorg. Chem.
1985, 24, 4671. For [MnIVO(porphyrin)]: (f) Groves, J. T.; Stern, M. K. J.
Am. Chem. Soc. 1988, 110, 8628. For [MnVO(salen)]: (g) Samsel, E. G.;
Srinivasan, K.; Kochi, J. K. J. Am. Chem. Soc. 1985, 107, 7606. For [RuVIO2-
(porphyrin)]: (h) Groves, J. T.; Quinn, R. Inorg. Chem. 1986, 25, 3845. (i)
Leung, W.-H.; Che, C.-M. J. Am. Chem. Soc. 1989, 111, 8812.
10.1021/ja001707o CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/04/2000

Alkyne Oxidations
J. Am. Chem. Soc., Vol. 122, No. 46, 2000 11381
complexes.15-19 Holm and co-workers1c,20 had reported the
oxygen atom transfer reactions of cis-dioxomolydenum(VI)
complexes to phosphines and related substrates; however, this
class of complexes cannot oxidize hydrocarbons such as alkenes
and alkynes. In this connection, it is conceivable that cis-
dioxometal complexes having a high E° value would be effective
for organic oxidations. Of particular interest is that cis-dioxo
complexes may react with substrates by transferring two oxygen
atoms analogous to the alkene cis-dihydroxylations mediated
by OsO4 and KMnO4 (Scheme 1). In 1988 Davison and co-
workers reported a [3 + 2] cycloaddition reaction between a
[(L)(Cl)TcVIIO3] complex (L ) 1,10-phenanthroline or 2,2′-
biyridine) with alkenes under ambient conditions. A technetium-
(V)-oxo complex, formulated as a [(L)(Cl)TcV(O)(O2C2H2R1R2)]
(R1 * R2 ) alkyl, aryl or H), was isolated and spectroscopically
characterized.16a Later Gable and co-workers also reported that
Figure 1.
Scheme 1
(5) Alkene epoxidation: (a) Ho, C.; Lau, T.-C.; Che, C.-M. J. Chem.
Soc., Dalton Trans. 1991, 1901. (b) Ho, C.; Leung, W.-H.; Che, C.-M. J.
Chem. Soc., Dalton Trans. 1991, 2933. (c) Che, C.-M.; Li, C.-K.; Tang,
W.-T.; Yu, W.-Y. J. Chem. Soc., Dalton Trans. 1992, 3153. (d) Cheng,
W.-C.; Yu, W.-Y.; Li, C.-K.; Che, C.-M. J. Org. Chem. 1995, 60, 6840.
(e) Fung, W.-H.; Yu, W.-Y.; Che, C.-M. J. Org. Chem. 1998, 63, 7715. (f)
Liu, C.-J.; Yu, W.-Y.; Che, C.-M.; Yeung, C.-H. J. Org. Chem. 1999, 64,
7365. Alkane hydroxylation: (g) Che, C.-M.; Yam, V. W.-W.; Mak, T.
C.-W. J. Am. Chem. Soc. 1990, 112, 2284. (h) Che, C.-M.; Ho, C.; Lau,
T.-C. J. Chem. Soc., Dalton Trans. 1991, 1259. (i) Che, C.-M.; Tang, W.-
T.; Wong, K.-Y.; Li, C.-K. J. Chem. Soc., Dalton Trans. 1991, 3277. (j)
Che, C.-M.; Tang, W.-T.; Lee, W.-O.; Wong, K.-Y.; Lau, T.-C. J. Chem.
Soc., Dalton Trans. 1992, 1551. (k) Zhang, R.; Yu, W.-Y.; Lai, T.-S.; Che,
C.-M. Chem. Commun. 1999, 1791.
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1325. (b) Roecker, L.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 746. (c)
Stultz, L. K.; Binstead, R. A.; Reynolds, M. S.; Meyer, T. J. J. Am. Chem.
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38, 2174.
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(8) Mayer, J. M. Acc. Chem. Res. 1998, 31, 441.
(9) Marimion, M. E.; Leising, R. A.; Takeuchi, K. J. J. Coord. Chem.
1988, 19, 1.
(10) For reviews, see: (a) Organic Synthesis by Oxidation with Metal
Compounds; Mijs, W. J.; De Jonge, C. R. H. I., Eds.; Plenum Press: New
York, 1986. (b) ComprehensiVe Organic Synthesis; Trost, B. M.; Fleming,
I. Eds.; Pergamon: Oxford, 1991; Vol. 7.
(11) Lee, D. G.; van den Engh, M. In Oxidation in Organic Chemistry;
Trahanovsky, W. S., Ed.; Academic Press: New York, 1973; Part B, Chapter
4.
(12) (a) Schro¨der, M. Chem. ReV. 1980, 80, 187. (b) Kolb, H. C.;
VanNieuwenhze, M. S.; Sharpless, K. B. Chem. ReV. 1994, 94, 2483.
(13) (a) Stewart, R. In Oxidation in Organic Chemistry; Wiberg, K. B.,
Ed.; Academic Press: New York, 1971; Part A, Chapter 1. (b) Gardner, K.
A.; Mayer, J. M. Science 1995, 269, 1849.
(14) (a) Ley, S. V.; Madin, A. In ComprehensiVe Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 7, p 251.
(b) Muzart, J. Chem. ReV. 1992, 92, 113 and references therein. (c) Cook,
G. K.; Mayer, J. M. J. Am. Chem. Soc. 1994, 116, 1855. (d) Cook, G. K.;
Mayer, J. M. J. Am. Chem. Soc. 1995, 117, 7139.
(15) (a) Behling, T.; Caparcilli, M. V.; Skapski, A. C.; Wilkinson, G.
Polyhedron 1982, 1, 840. (b) Che, C.-M.; Leung, W.-H. J. Chem. Soc.,
Chem. Commun. 1987, 1376. (c) Perrier, S.; Kochi, J. K. Inorg. Chem.
1988, 27, 4165. (d) Griffiths, W. P.; Jolliffe, J. M.; Ley, S. V.; William, D.
J. J. Chem. Soc., Chem. Commun. 1990, 1219. (e) Blackbourn, R. L.; Jones,
L. M.; Ram, M. S.; Sabat, M.; Hupp, J. T. Inorg. Chem. 1990, 29, 179. (f)
Brown, S. N.; Mayer, J. M. J. Am. Chem. Chem. 1996, 118, 12 119. (g)
DuMez, D. D.; Mayer, J. M. Inorg. Chem. 1998, 37, 445.
(16) (a) Pearlstein, R. M.; Davison, A. Polyhedron 1988, 7, 1981. (b)
Gable, K. P.; Phan, T. N. J. Am. Chem. Soc. 1994, 116, 833. (c) For a
review of Re(VII) oxo compounds, see: Roma˜o, C. C.; Ku¨hn, F. E.
Herrman, W. A. Chem. ReV. 1997, 97, 3197.
(17) (a) Bailey, C. L.; Drago, R. S. J. Chem. Soc., Chem. Commun. 1987,
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Commun. 1991, 21.
Cp*ReO3 can transfer two oxygen atoms to a CdC bond of
alkenes in a cis manner.16b
The oxidation chemistry of several classes of RudO com-
pounds has been extensively investigated in the past decades.5,6,21
Important features of these compounds are that they possess
very high E° values yet can be readily isolated and structurally
characterized. Several years ago, we successfully isolated and
obtained the X-ray crystal structures of two cis-dioxoruthenium-
(VI) complexes, [Cn*(CF3CO2)RuVIO2]ClO4 (1) (Cn* ) 1,4,7-
trimethyl-1,4,7-triazacyclononane)19b and cis-[(Tet-Me6)RuVIO2]-
(ClO4)2 (2) (Tet-Me6 ) N,N,N′,N′-tetramethyl-3,6-dimethyl-3,6-
diazaoctane-1,8-diamine)19a (Figure 1). These complexes are
competent oxidants for organic oxidations, and their reactions
with the alkenes under stoichiometric conditions have been
reported.5d,22 Here, we demonstrate that the cis-dioxoruthenium-
(VI) complexes 1 and 2 react with alkynes by transferring two
oxygen atoms to a CtC bond, which we formally describe as
a [3 + 2] cycloaddition.23 Although it is generally accepted that
the oxidation of alkynes to 1,2-diketones by M)O reagents
involves the [3 + 2] cycloaddition reaction as the principal step,
cycloadduct intermediates haVe neVer been hitherto isolated or
characterized.10 To our knowledge, the present report features
the first evidence for [3 + 2] cycloaddition in the alkyne
oxidation by M)O reagents. In this work, we have also
examined the mechanism of the cycloaddition by kinetic studies
and product analysis.
Results
Synthesis of cis-[(Cn*)(CF3CO2)RuVIO2]ClO4. Monooxo-
and trans-dioxoruthenium complexes are usually prepared by
sequential oxidation/deprotonation reactions of the aqua-
(21) For reviews, see: (a) Che, C.-M.; Yam, V. W.-W. AdV. Inorg. Chem.
1992, 39, 233. (b) Che, C.-M.; Yam, V. W.-W. AdV. Transition Metal Coord.
Chem. 1996, 1, 209. (c) Che, C.-M.; Yu, W.-Y. Pure Appl. Chem. 1999,
71, 281.
(22) (a) Cheng, W.-C.; Fung, W.-H.; Che, C.-M. J. Mol. Cata. (A) 1996,
113, 311. (b) Fung, W.-H.; Yu, W.-Y.; Che, C.-M. J. Org. Chem. 1998,
63, 2873.
(23) A [3 + 2] cycloaddition mechanism has been proposed for the OsO4-
mediated alkene dihydroxylation, see ref 12a and: (a) Corey, E. J.; Noe,
M. C.; Sarshar, S. J. Am. Chem. Soc. 1993, 115, 3828. (b) Corey, E. J.;
Noe, M. C. J. Am. Chem. Soc. 1993, 115, 12579. (c) Corey, E. J.; Noe, M.
C.; Sarshar, S. Tetrahedron Lett. 1994, 35, 2861.
(18) (a) Dobson, J. C.; Meyer, T. J. Inorg. Chem. 1988, 27, 3283. (b)
Llobet. A.; Hogson, D. J.; Meyer. T. J. Inorg. Chem. 1990, 29, 3760.
(19) (a) Li, C.-K.; Che, C.-M.; Tong, W.-F.; Lai, T.-F. J. Chem. Soc.,
Dalton Trans. 1988, 1406. (b) Cheng, W.-C.; Yu, W.-Y.; Cheung, K.-K.;
Che, C.-M. J. Chem. Soc., Chem. Commun. 1994, 1063.
(20) Harlan, E. W.; Berg, J. M.; Holm, R. H. J. Am. Chem. Soc. 1986,
108, 6992.

11382 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Che et al.
mmol) with complex 1 (0.11 µmol) in a degassed 0.2 M CF3-
CO2H/CH3CN solution (10 mL) at room temperature afforded
instantaneously a dark blue solution. The reaction mixture
gradually decolorized within 15 min to give a pale yellow
solution. After workup, the organic product benzil was obtained
in 98% yield (Table 1, entry 1). No other organic side-products
such as benzoic acid (CtC cleavage) or diphenylacetic acid
(rearrangement) were detected. The percentage yield of benzil
was calculated based on the stoichiometry of Ru:PhCtCPh )
1:1. The ruthenium(VI) oxidant was quantitatively reduced to
cis-[(Cn*)(CF3CO2)RuII(CH3CN)2]ClO4 (93% isolated yield, see
Experimental Section).
The presence of trifluoroacetic acid is essential for obtaining
benzil in good yield. In neutral acetonitrile the diphenylacetylene
oxidation also proceeds through a dark blue intermediate, and
benzil and cis-[(Cn*)(CF3CO2)RuII(CH3CN)2]ClO4 are isolated
in 45% and 97% yields, respectively. Just as for acidified
conditions, no side-products from CtC bond cleavage and
rearrangement reactions could be detected. The low yield of
benzil under these conditions remains ill-understood at present
because we could not establish other organic products.
Oxidation of other disubstituted alkynes including 1-phenyl-
1-propyne, 4-octyne, 2-butyne, and 1-trimethylsilyl-1-propyne
by 1 was equally effective under the typical reaction conditions:
i.e., 1.1 mmol of alkyne, 0.11 µmol of 1, and 0.2 M CF3CO2H
in CH3CN, and 1,2-diketones were produced in 94-98% yields
(Table 1, entries 2-5). In all cases, the 1,2-diketone formations
were preceded by transient appearance of a dark blue solution.
It has been reported that RuVIIIO4, generated in situ by the action
of NaOCl on ruthenium dioxide,26 readily oxidized internal
alkynes to give a substantial amount of carboxylic acids arising
from oxidative C-C bond cleavage of the 1,2-diketones.26a,b
For example, 4-octyne reacted with RuO4 to give 4,5-octanedi-
one and butyric acid in 60 and 40% yields,26b respectively. In
this work, when subjecting 4-octyne (1.1 mmol) to the oxidizing
conditions, i.e., 1 (0.11 µmol) in a degassed 0.2 M CF3CO2H/
CH3CN solution, 4,5-octanedione was obtained in 94% yield
(Table 1, entry 3) without any detectable formation of butyric
acid.
Figure 2. UV-vis spectrum of 1 in 0.1 M CF3CO2H.
ruthenium (Ru-H2O) precursors in aqueous solution.21b,c In an
attempt to synthesize cis-dioxoruthenium(VI) complexes, Meyer
and co-workers noted that oxidative deprotonation reactions of
cis-[(bpy)2RuVI(H2O)2]2+ (bpy ) 2,2′-bipyridine)18a and [(tpm)-
RuII(H2O)3]2+ [tpm ) tris(1-pyrazolyl)methane]18b complexes
had resulted in partial ligand loss and extensive formation of
trans-dioxoruthenium(VI). The trans-dioxo formation can be
avoided by employing bipyridine ligands with sterically bulky
substituents at the 6,6′-positions as in cis-[(L)2RuVIO2](ClO4)2
(where L ) 6,6′-dichloro-2,2′-bipyridine24 and 2,9-dimethyl-
1,10-phenathroline17). Previously we also demonstrated the use
of macrocyclic tertiary amine ligands with small bite angle in
the preparation of cis-dioxo complexes such as cis-[(Cn*)(CF3-
CO2)RuVIO2]ClO4 (1)19b and cis-[(Tet-Me6)RuVIO2](ClO4)2 (2).19a
Treatment of [(Cn*)RuIII(CF3CO2)3]‚H2O22b by (NH3)2CeIV-
(NO3)6 in 0.2 M CF3CO2H afforded cis-[(Cn*)(CF3CO2)RuVIO2]-
ClO4 (1) in 55% yield. Complex 1 is a diamagnetic light-green
microcrystalline solid and is stable as a solid for several hours
at room temperature. Its infrared spectrum shows two absorp-
tions at 842 and 856 cm-1 that we assign to the asymmetric
and symmetric RuO2 stretches, respectively. Comparable spec-
tral feature was obtained for the structurally related cis-[(Tet-
Me6)RuVIO2](ClO4)2 (2), in which case the asymmetric and
Common organic oxidants such peracids27 and dioxiranes28
are known to oxidize alkynes to produce 1,2-diketones but in
poor yields. For example, 2-butyne and 1-trimethylsilyl-1-
propyne reacted with dioxiranes to afford 2-hydroxy-2-meth-
ylpropanoic acid and 2-hydroxy-2-(trimethylsilyl)propanoic acid
as the major products, respectively.28b In this work, we found
that complex 1 oxidized 2-butyne and 1-trimethylsilyl-1-propyne
to their 1,2-diketone derivatives in 97-98% yields (see Table
1, entries 4 and 5). As has been noted in the literature, 1-oxo-
1-(trimethylsilyl)-2-propanone readily undergoes photolysis
under ambient lighting to afford 2-hydroxy-2-(trimethylsilyl)-
propanoic acid;29 therefore, the 1-trimethylsilyl-1-propyne oxi-
dation by 1 and its subsequent workup were carried out with
protection from light.
(26) (a) Wolfe, S.; Hasan, S. K.; Campbell, J. R. J. Chem. Soc., Chem.
Commun. 1970, 1420. (b) Gopal, H.; Gordon, A. J. Tetrahedron Lett. 1971,
2941. (c) Gopal, H.; Adam, T.; Moriarty, R. M. Tetrahedron 1972, 28,
4259. (d) Carlsen, P. H.; Katsuki, T.; Martin, V. S.; Sharpless, K. B. J.
Org. Chem. 1981, 46, 3936.
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4871. (b) McDonald, R. N.; Schwab, P. A. J. Am. Chem. Soc. 1964, 86,
4866. (c) Ciabattoni, J.; Campbell, R. A.; Renner, C. A.; Concannon, P.
W. J. Am. Chem. Soc. 1970, 92, 3826.
symmetric RuO2 stretches are located at 855 and 874 cm-1 19a
.
The UV-visible spectrum of a freshly prepared sample of 1 in
0.2 M CF3CO2H (Figure 2) exhibits an intense charge-transfer
absorption band at 329 nm (ꢀmax ) 2400 dm3 mol-1 cm-1) and
a weak d-d absorption at 695 nm (ꢀmax ) 50 dm3 mol-1 cm-1).
The solution is stable for more than 1 h at room temperature.
The UV-vis spectra of the trans-dioxoruthenium(VI) analogues
usually display a vibronic-structured absorption band at 380-
400 nm arising from the spin-allowed pπ(O2-)fRu(VI) charge-
transfer transition.25 However, a similar vibronic-structured
UV-vis band is not observed for the cis complexes 1 and 2 at
room temperature.19
Stoichiometric Oxidation of Alkynes by cis-[(Cn*)(CF3CO2)-
RuVIO2]ClO4. Treating excess diphenylacetylene (0.2 g, 1.1
(24) Che, C.-M.; Leung, W.-H. J. Chem. Soc., Chem. Commun. 1987,
1376.
(25) (a) Che, C.-M.; Wong, K.-Y.; Leung, W.-H.; Poon, C.-K. Inorg.
Chem. 1986, 25, 345. (b) Che, C.-M.; Lai, T.-F.; Wong, K.-Y. Inorg. Chem.
1987, 26, 2289. (c) Che, C.-M.; Tang, W.-T.; Li, C.-K. J. Chem. Soc., Dalton
Trans. 1990, 3735.
(28) (a) Curci, R.; Fiorentino, M.; Fusco, C.; Mello, R.; Ballistreri, F.
P.; Failla, S.; Tomaselli, G. A. Tetrahedron Lett. 1992, 33, 7929. (b) Murray,
R. W.; Singh, M. J. Org. Chem. 1993, 58, 5076.
(29) (a) Reich, H. J.; Kelly, M. J. J. Am. Chem. Soc. 1982, 104, 1119.
(b) Reich, H. J.; Kelley, M. J.; Olson, R. E.; Holtan, R. C. Tetrahedron
1983, 39, 960.

Alkyne Oxidations
J. Am. Chem. Soc., Vol. 122, No. 46, 2000 11383
Table 1. Stoichiometric Oxidation of Alkynes by cis-[(Cn*)(CF3CO2)RuVIO2]ClO4 (1)
Scheme 2. Reaction of Bis(trimethylsilyl)acetylene with 1 in
Neutral MeCN
We found that the oxidation of phenylacetylene by 1 gave
benzoic acid as the only isolable product, in ∼30% yield.
Presumably, the phenylglyoxylic acid (PhCOCO2H) product
decomposed to benzoic acid during the reaction.
Unlike 1, cis-[(Tet-Me6)RuVIO2](ClO4)2 (2) failed to oxidize
diphenylacetylene to benzil in CF3CO2H/CH3CN, and the
starting alkyne was recovered. But it is noteworthy that 2 can
oxidize the less bulky 2-butyne and 1-trimethylsilyl-1-propyne
to give the corresponding 1,2-diketones in 87 and 85% yields,
respectively (Table 1, entries 4 and 5). Similar to the alkyne
oxidation by 1, these oxidations were accompanied by transient
formation of a dark blue solution. The lack of reactivity of 2
toward diphenylacetylene is probably due to steric effects
imposed by the N-methyl groups of the Tet-Me6 ligand.
It has been reported that two-electron oxidants such as
oxochromium(V) salen complexes can effect diphenylacetylene
oxidation to benzil at ambient conditions, albeit with very small
reaction rate constants (ca. 10-5 dm3 mol-1 s-1).30 However,
neutral CH3CN, a dark blue solution with a strong absorption
band at λmax ) 658 nm develops,19b and the color gradually
fades out within 30 min. This absorption band does not originate
from diphenylacetylene or cis-[(Cn*)(CF3CO2)RuVIO2]ClO4,
since neither of these species have comparable absorptions in
the 500-700 nm spectral region.
Likewise, the reactions of 4-octyne, 1-phenyl-1-propyne, and
2-butyne gave rise to dark colored intermediates (λmax ) 639,
647, and 649 nm, respectively), which converted spontaneously
to [(Cn*)(CF3CO2)RuII(CH3CN)2]ClO4 in CH3CN at room
temperature. Unfortunately, attempts to isolate these species
were futile.
in this work, we found that strong two-electron oxidants such
31
as trans-[(N2O2)RuVIO2](ClO4)2
and trans-[(pytn)RuVIO2]-
(ClO4)2 31 complexes [N2O2 ) 1,12-dimethyl-3,4:9,10-dibenzo-
1,12-diaza-5,8-dioxocyclopentadecane; pytn ) N,N′-dimethylbis-
(2-pyridylmethyl)propylenediamine] (respective E° [Ru(VI)/
(IV)] ) 0.92 and 0.89 V vs SCE at pH 1.0) do not exhibit any
appreciable reactivity toward diphenylacetylene, 2-butyne, and
1-trimethylsilyl-1-propyne under similar reaction conditions. In
all cases, no 1,2-diketones were detected, and all the starting
alkynes were recovered. We suggest that the reactions of these
complexes with alkynes are slow so that reduction to trans-
[Ru(L)(CH3CN)2]2+ complexes (L ) N2O2 and pytn) by
acetonitrile solvent prevails.
When 1 was treated with excess bis(trimethylsilyl)acetylene
in neutral CH3CN, complex 3a showing an intense absorption
band at λmax ) 566 nm was immediately formed (Scheme 2).
Unlike the other acetylenes we examined, this complex was
found to be stable at ambient conditions over 24 h in solution
and could be isolated by slow solvent evaporation from aqueous
acetone.
In CH3CN, a purified sample of 3a exhibits an identical UV-
vis spectral feature to the initial reaction mixture [λmax ) 566
nm (ꢀmax ) 5100)] (see Experimental Section). Its infrared
spectrum shows strong absorption peaks at 1698 and 1091 cm-1
,
The [3 + 2] Cycloaddition of cis-Dioxoruthenium(VI) and
Disubstituted Acetylenes. Pertinent to the mechanism of alkyne
oxidation, we have sought to characterize the transient dark blue
intermediate. Upon mixing excess diphenylacetylene with 1 in
which we assign to the νCdO stretch of the trifluoroacetate ligand
and the νCldO stretch of the ClO4- anion, respectively. Notably
the RuO2 stretches at 842 and 856 cm-1 of the starting complex
1 are absent. Complex 3a gave well-resolved 1H and 13C NMR
spectra, indicative of its diamagnetic nature. All the 1H and 13
C
(30) Rihter, B.; SriHari, S.; Hunter, S.; Masnovi, J. J. Am. Chem. Soc.
1993, 115, 3918.
resonances of the Cn* ligand are at their normal positions, but
the 13C signal(s) of the acetylenic carbon atoms (ca. δ 115 ppm)
corresponding to free bis(trimethylsilyl)acetylene are not ob-
served. A striking characteristic of 3a is that the compound
shows a very downfield 13C signal at δ 239.1 ppm, indicative
of a very deshielded carbon atom (see Supporting Information).
On the basis of FAB-MS (m/z ) 588 M+) and elemental
(31) (a) Che, C.-M.; Tang, W.-T.; Wong, W.-T.; Lai, T.-F. J. Am. Chem.
Soc. 1989, 111, 9048. (b) Che, C.-M.; Tang, W.-T.; Li, C.-K. J. Chem.
Soc., Dalton Trans. 1990, 3735. (c) Che, C.-M.; Tang, W.-T.; Lee, W.-O.;
Wong, K.-Y.; Lau, T.-C. J. Chem. Soc., Dalton Trans. 1992, 1551.
(32) (a) Collin, R. J.; Jones, J.; Griffith, W. P. J. Chem. Soc., Dalton
Trans. 1974, 1094. (b) Schro¨der, M.; Griffith, W. P. J. Chem. Soc., Dalton
Trans. 1978, 1599. (c) Bassignani, L.; Brandt, A.; Caciagli, V.; Re, L. J.
Org. Chem. 1978, 21, 4245.

11384 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Che et al.
Table 2. Crystal Data and Structure Refinement for Complex 3a
compound
3a
empirical formula
formula wt
temp
C
19.67H40ClF3N3.33O8RuSi2
700.92
295(2) K
wavelength
crystal system
space group
unit cell dimens
0.710 73 Å
monoclinic
P21/n (No. 14)
a ) 23.4294(2) Å; R ) 90°
b ) 8.5762(2) Å; â ) 90.960(1)°
c ) 48.8457(9); γ ) 90°
9813.2(3) Å3, 12
1.423 mg/m3
vol, Z
density (calcd)
F(000)
4336
40 484
17 207 (Rint ) 0.0488)
16 231
1047
1.105
reflectns collected
independent reflectns
no. of data used
no. of parameters
goodness-of-fit on F 2
final R indices [I > 2σ(I)]a
R indices (all data)a
Figure 3. Molecular structure of the [(Cn*)(CF3CO2)RuIVOC2(SiMe3)2-
O]+cation complex and atom labeling scheme. Selected bond distances
(Å): Ru-O(1) 1.977(5), Ru-O(2) 1.979(6), Ru-O(3) 2.075(6), Ru-
N(1) 2.089(6), Ru-N(2) 2.109(7), Ru-N(3) 2.095(6). Selected bond
angles (°): O(1)-Ru-O(2) 78.1(2), O(1)-Ru-O(3) 89.4(2), O(2)-
Ru-O(3) 87.0(2), O(1)-Ru-N(1) 95.9(2), O(2)-Ru-N(1) 95.5(3),
O(3)-Ru-N(1) 174.5(2), O(1)-Ru-N(3) 176.7(3), O(2)-Ru-N(3)
98.7(3), O(3)-Ru-N(3) 91.0(3), N(1)-Ru-N(3) 83.9(3), O(1)-Ru-
N(2) 99.4(2), O(2)-Ru-N(2) 117.1(2), O(3)-Ru-N(2) 94.5(3), N(1)-
Ru-N(2) 83.2(3), C(11)-O(2) 1.305(10), C(10)-O(1) 1.301(9),
C(11)-C(10) 1.439(12).
R1 ) 0.085, wR2 ) 0.20
R1 ) 0.12, wR2 ) 0.22
a R ) ∑||F0| - |Fc||/∑|F0|; Rw ) [∑w(|F0| - |Fc|)2/∑wF0 ]1/2 where
2
w ) 4F0 /σ2(F0 ).
2
2
lacetonate).35 These findings suggest that the ligand π-electrons
are in fact delocalized among the O(1), C(10), C(11), and O(2)
atoms, and we propose that the metallacycle is a resonance
hybrid between a dioxolene and a diketone.
As expected, the Cn* ligand is facially coordinated to the
Ru atom. The respective Ru(1)-N(1), Ru(1)-N(2), and Ru-
(1)-N(3) distances of 2.089(6), 2.109(7), and 2.095(6) Å are
comparable to the corresponding Ru-N distances in cis-[(Cn*)-
(CF3CO2) RuVIO2]ClO4 (1) [2.097(9), 2.23(2), and 2.197(9)
Å].19b
Aryl- and alkyl-substituted trimethylsilylacetylenes and dim-
ethyl acetylenedicarboxylate reacted with complex 1 in a similar
manner to afford dark blue to purple solutions with λmax ranging
from 553 to 626 nm. The spectral data are summarized in Table
3. Unlike the bis(trimethylsilyl)acetylene derivative, these
intermediates are unstable in solution and readily converted to
[(Cn*)(CF3CO2)RuII(MeCN)2]ClO4 at room temperature. Their
instabilities were manifested by the decay of the 523-626 nm
band (ca. 55% from its initial absorbance) after standing the
solutions for 24 h at room temperature. Although dark blue
solids could be isolated by precipitation with diethyl ether, they
deteriorated rapidly and could not be purified in our hands.
Characterization of the Cycloadducts by Electrospray
Ionization Mass Spectroscopy. Electrospray ionization mass
spectroscopy (ESI-MS) has been useful for characterization of
large biomolecules and multiply charged metal complexes in
solution.36 Because of the mildness of this ionization method,
it has recently been applied to detect the formation of reactive
transient intermediates such as [OdMnV(salen)]+ (salen )
Schiff bases)37 and (hydroxperoxy)iron(III) (HOOFeIII) species.38
analyses, we formulate complex 3a to be an adduct of 1 and
bis(trimethylsilyl)acetylene.
The molecular structure of 3a has been established by X-ray
crystallography (Figure 3). Figure 3 shows that the cis-
configured O(1)-Ru-O(2) moiety is directly bound to the
acetylenic carbon atoms C(10) and C(11) to form a five-
membered planar metallacycle, and the compound can be
regarded as a [3 + 2] cycloadduct of complex 1 and bis-
(trimethylsilyl)acetylene. Although similar cyclic intermediates
have been proposed in the oxidation of acetylenes by OsO4,
such complexes have never been isolated or well characterized
(see later sections).12a,32 In complex 3a, the ruthenium atom
adopts a distorted octahedral geometry coordinating to the Cn*
ligand and a η1-bound trifluoroacetate. The bond angles around
C(10) and C(11) atoms are O(1)-C(10) -Si(1) ) 112.9(6)°,
O(1)-C(10)-C(11) ) 113.6(7)°, and C(11)-C(10)-Si(1) )
133.4(7)°; O(2)-C(11) -Si(2) ) 112.8(6)°, O(2)-C(11)-C(10)
) 114.0(7)°, and C(10)-C(11)-Si(2) ) 133.0(7)°. The sizable
deviation from an ideal bond angle of 120° for a sp2 hybridized
carbon atom is ascribed to steric repulsion between the two cis-
SiMe3 groups. The Ru(1)-O(1) and Ru(1)-O(2) distances are
1.977(5) and 1.979(6) Å, respectively, comparable to the related
Ru-O distances found in [nBu4N][RuVI(N)(L)] [1.956(3) and
1.957(3) Å; L ) 1,2-bis(2-hydroxyphenyl-1-carboxamido)-
benzene tetraanion]33 and [Ru2(Cl4Cat)4]3- complexes [1.998-
(2), 1.965(2), 1.990(2), and 1.973(2) Å] (Cl4Cat ) tetrachlo-
rocatecholate) (see Table 2).34
(35) Schneider, R.; Weyhermu¨ller, T.; Wieghardt, K.; Nuber, B. Inorg.
Chem. 1993, 32, 4925.
(36) (a) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse,
C. M. Science 1989, 246, 64. (b) Fenn, J. B.; Mann, M.; Meng, C. K.;
Wong, S. F.; Whitehouse, C. M. Mass Spectrum. ReV. 1990, 9, 37. (c) Smith,
R. D.; Loo, J. A.; Edwards, C. G.; Barinaga, C. J.; Udseth, H. D. Anal.
Chem. 1990, 62, 882.
The respective C(11)-O(2) and C(10)-O(1) bond distances
are 1.305(10) and 1.301(9) Å, which are slightly shorter than
the related C(sp2)-O single bond distances (1.32-1.35 Å) found
in [Ru2(Cl4Cat)4]3- 34 and [nBu4N][RuVI(N)(L)].33 On the other
hand, the measured C(11)-C(10) bond distance [1.439(12) Å]
is significantly elongated from a typical CdC double (1.33 Å);
this value is comparable to that of a CH-C(O) linkage [1.382-
(11) Å] found in [Cn*RuIII(acac)(OH)]PF6‚H2O (acac ) acety-
(37) Feichtinger, D.; Plattner, D. A. Angew. Chem., Int. Ed. Engl. 1997,
36, 1718.
(38) (a) Sam, J. W.; Tang, X.-J.; Peisach, J. J. Am. Chem. Soc. 1994,
116, 5250. (b) Sam, J. W.; Tang, X.-J.; Magliozzo, R. S.; Peisach, J. J.
Am. Chem. Soc. 1995, 117, 1012. (c) Westre, T. E.; Loeb, K. E.; Zaleski,
J. M.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc.
1995, 117, 1309. (c) Kim, J.; Dong, Y.; Larka, E.; Que, L., Jr. Inorg. Chem.
1996, 35, 2369. (d) Jensen, K. B.; McKenzie, C. J.; Nielsen, L. P.; Pedersen,
J. Z.; Svendsen, H. M. Chem. Commun. 1999, 1313.
(33) Chan, P.-M.; Yu, W.-Y.; Che, C.-M.; Cheung, K.-K. J. Chem. Soc.,
Dalton Trans. 1998, 3183
(34) Kondo, M.; Hamatani, M.; Kitagawa, S.; Pierpont, C. G.; Unoura,
K. J. Am. Chem. Soc. 1998, 120, 455.

Table 3. UV-Visible and Electrospray Spectral Data for the Ruthenium(IV) Cycloadduct Complexes
λ
max/nm
λmax/nm
(ꢀmax/dm3
(ꢀmax/dm3
entry
1
alkynes
mol-1 cm-1
)
ions (m/z and relative abundance)
entry
11
alkynes
mol-1 cm-1
)
ions (m/z and relative abundance)
Me3SiCtCSiMe3
566 (5100) [Cn*(CF3CO2)RuIVOC2(SiMe3)2O]+
(3a) (588, 100%)
p-CH3O-
C6H4CtCSiMe3
nC3H7CtC-nC3H7
626 (5000) [Cn*(CF3CO2)RuIVOC2(SiMe3)(CH3O-C6H4)O]+
(3k) (621.9, 100%)
2
3
4
5
6
Me3SiCtCCO2Et
Me3SiCtCC(CH3)3
Me3SiCtCCH3
553 (5400) [Cn*(CF3CO2)RuIVOC2(SiMe3)(CO2Et)O]+
(3b) (588.1, 100%)
12
13
14
15
16
639
647
649
658
[Cn*(CF3CO2)RuIVOC2(C3H7)2O]+ (3l) (528.2, 20%),
[(Cn*)(CF3CO2) RuII(MeCN)2]+ (468.0, 35%),
[(Cn*)(CF3CO2)RuII(MeCN)]+ (427.1, 100%)
[Cn*(CF3CO2)RuIVOC2(CH3)(C6H5)O]+ (3m) (533.9, 80%),
[(Cn*)(CF3CO2)RuII (MeCN)2]+ (468.0, 45%),
[(Cn*)(CF3CO2)RuII(MeCN)]+ (427.1, 100%)
[Cn*(CF3CO2)RuIVOC2(CH3)2O]+ (3n) (471, 15%),
[(Cn*)(CF3CO2)RuII(MeCN)2]+ (468.0, 60%),
[(Cn*)(CF3CO2) RuII(MeCN)]+ (427.1, 100%)
[Cn*(CF3CO2)RuIVOC2(C6H5)2O]+ (3o) (596.1, 100%),
[(Cn*)(CF3CO2) RuII(MeCN)2]+ (468.0, 90%),
[(Cn*)(CF3CO2)RuII(MeCN]+ (427.1, 35%)
581 (5600) [Cn*(CF3CO2)RuIVOC3(SiMe3)(CH3)3O]+
C6H5CtCCH3
CH3CtCCH3
C6H5CtCC6H5
(3c) (571.9, 100%)
588 (5200) [Cn*(CF3CO2)RuIVOC2(SiMe3)(CH3)O]+
(3d) (529.9, 100%)
Me3SiCtCC4H9
604 (5300) [Cn*(CF3CO2)RuIVOC2(SiMe3)(C4H9)O]+
(3e) (571.9, 100%)
p-ClC6H4CtCSiMe3
605 (5200) [Cn*(CF3CO2)RuIVOC2(SiMe3)(ClC6H4)O]+
MeO2CCtCCO2Me 535 (4900) [Cn*(CF3CO2)RuIVOC2(CO2Me)2O]+ (3p) (559.9, 100%),
[(Cn*)(CF3CO2)RuII (MeCN)2]+ (468.0, 35%),
(3f) (625.9, 100%)
[(Cn*)(CF3CO2)RuII (MeCN)]+ (427.1, 60%)
7
8
C6H5CtCSiMe3
606 (5200) [Cn*(CF3CO2)RuIVOC2(SiMe3)(C6H5)O]+
(3g) (591.9, 100%)
p-FC6H4CtCSiMe3
p-CF3C6H4CtCSiMe3
606 (5200) [Cn*(CF3CO2)RuIVOC2(SiMe3)(FC6H4)O]+
(3h) (609.9, 100%)
9
609 (5000) [Cn*(CF3CO2)RuIVOC2(SiMe3)(CF3-C6H4)O]+
(3i) (659.9, 100%)
10
p-CH3-C6H4CtCSiMe3 613 (5400) [Cn*(CF3CO2)RuIVOC2(SiMe3)(CH3-C6H4)O]+
(3j) (605.9, 100%)

11386 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Che et al.
pattern, the molecular composition of the remaining ion cluster
is best formulated as the complex [(Cn*)(CF3CO2)RuIV-
OC2R1R2O]+ (R1 ) R2 ) n-C3H7, m/z ) 527; R1 ) R2 ) Ph,
m/z ) 596; R1 ) Ph, R2 ) Me, m/z ) 532; R1 ) R2 ) Me, m/z
) 470, see Table 3). The identities of the two cluster peaks in
the region of m/z ) 500-560 presumably due to fragmentation
from the cycloadduct complex remains unknown in this work.
Similar mass losses are not observed for the reactions with
4-octyne, 1-phenyl-1-propyne, and 2-butyne.
Kinetic Studies on the [3 + 2] Cycloaddition of cis-[(Cn*)-
(CF3CO2)RuVIO2]ClO4 with Some Disubstituted Alkynes.
The kinetics of the cycloaddition of 1 with a series of substituted
trimethylsilylacetylenes and dimethyl acetylenecarboxylate has
been studied by stopped-flow spectrophotometry. These alkynes
were chosen for the kinetic studies because the deterioration of
their cycloadducts occurred on a time scale much slower than
their formation so that the kinetic analysis was feasible.
Monitoring the growth of the 535-626 nm absorption band
allowed us to establish the rate law k2[RuVI][alkyne], where k2
) second-order rate constant. In the presence of 10- to 50-fold
excess alkyne, we observed clean first-order kinetics (Figure
6). The pseudo-first-order rate constants (kobs) were obtained
by the nonlinear least-squares fits of the growth curves, and
the slopes of the kobs vs [alkyne] plots gave the k2 values. The
results are shown in Table 4. Under the concentrations employed
in this work, no rate saturation was observed. For comparison,
Table 5 lists some rate constants for the oxidation of alkynes
by some other oxidants reported in the literature.
Figure 4. Experimental (top) and calculated (bottom) isotopic distribu-
tion pattern of [(Cn*)(CF3CO2)RuIVOC2(SiMe3)2O]+ (3a).
When the diphenylacetylene, 2-octyne and 1-phenyl-1-
propyne oxidations were examined by stopped-flow spectro-
photometry, the kinetic profiles of the initial cycloadduct
formations were kinetically inseparable from a secondary
chemical reaction, presumably the decomposition of the initial
cycloadducts. The spectral development at ca. 660 nm of these
reactions did not conform to first-order kinetics even in the
presence of 40-fold excess of alkyne. For this reason these
reactions had not been studied in detail.
In this work, we employed ESI-MS to determine the molecular
composition of the metastable dark colored intermediates
observed in the stoichiometric alkyne oxidations by 1.
As shown in Figure 4, the mass spectrum obtained by mixing
1 with excess bis(trimethylsilyl)acetylene in CH3CN revealed
a single prominent ion cluster at m/z ) 588, which we assign
to the [(Cn*)(CF3CO2)RuIVOC2(SiMe3)2O]+ complex cation
based on excellent agreement between the experimental and
calculated isotopic distribution pattern. Also, the spectral pattern
is identical to that obtained with an authentic complex 3a. The
molecular cluster corresponding to the cis-[(Cn*)(CF3CO2)-
RuVIO2]+ cation (m/z ) 418) was not observed, indicating that
the ruthenium oxidant had reacted completely. Complex 2 was
also found to react with an excess of bis(trimethylsilyl)acetylene
in CH3CN at room temperature to afford a similar cycloadduct
that was characterized by UV-vis spectroscopy (λmax ) 592
nm) and ESI-MS [m/z ) 534 (M+), 100%].
Similarly, the ESI-MS spectra obtained by reacting 1 with
other silylated acetylenes and dimethyl acetylenedicarboxylate
in CH3CN showed a single prominent molecular ion species
that is best formulated as a cycloadduct between the alkyne and
the ruthenium oxidant. The mass spectral data are listed in Table
3.
For the oxidation of 4-octyne, diphenylacetylene, 1-phenyl-
1-propyne, and 2-butyne by 1, the ESI-MS analyses of the
reaction mixtures [conditions: 1 (2 µmol) + alkynes (10 mmol)
in CH3CN (3 mL)] gave three sets of ion cluster peaks. The
ESI mass spectrum for the reaction of 1 with diphenylacetylene
is shown in Figure 5. The two cluster peaks at m/z ) 468 (M+);
427(M+ - CH3CN) are assigned to the [(Cn*)(CF3CO2)RuII(CH3-
CN)2]+ cation, since the same ESI-MS spectrum has been
obtained with an authentic sample of [(Cn*)(CF3CO2)RuII(CH3-
CN)2]ClO4. On the basis of the mass value and the isotope
The temperature dependence of the k2 values for the reactions
of bis(trimethylsilyl)acetylene, 1-trimethylsilylhexan-1-yne, and
1-(4-chlorophenyl)-2-trimethylsilylacetylene with 1 has been
examined. The activation parameters were determined from the
ln(k2/T) vs 1/T plots, which are linear over the temperature range
studied (286-315 K). The entropies of activation (∆Sq) are -18
( 1.7, -17 ( 1.9, and -15 ( 1.4 cal mol-1 K-1 and enthalpies
of activation (∆Hq) are 9.3 ( 0.9, 10.9 ( 1.2, and 11.9 ( 1.4
kcal mol-1 for the bis(trimethylsilyl)acetylene, 1-trimethylsilyl-
1-hexyne, and 1-(4-chlorophenyl)-2-trimethylsilylacetylene oxi-
dations, respectively. Attempts to determine the k2 values at T
> 328 K met with significant decomposition of the cycloadduct
complexes, and clean pseudo-first-order kinetic profiles were
not obtained. The ∆Hq values for the three reactions are
comparable to some reported values for the oxidation of
propiolic acid (6 kcal mol-1) and phenylpropiolic acid (7 kcal
mol-1) by KMnO4.41 The ∆Sq values are large and negative in
all cases, consistent with the rate-determining association of the
cis-dioxoruthenium(VI) oxidant and alkynes. However, these
∆Sq values are somewhat smaller than that obtained in the
oxidation of propiolic and phenylpropiolic acids (-31 cal mol-1
K-1) by KMnO4.41
(39) Sima´ndi, L. I.; Ja´ky, M. Tetrahedron Lett. 1970, 3489.
(40) Sima´ndi, L. I.; Ja´ky, M. J. Chem. Soc., Perkin Trans. 2 1972, 1481.
(41) Sima´ndi, L. I.; Ja´ky, M. J. Chem. Soc., Perkin Trans. 2 1977, 630.

Alkyne Oxidations
J. Am. Chem. Soc., Vol. 122, No. 46, 2000 11387
Figure 5. Electrospray ionization mass spectrum obtained from the reactions of diphenylacetylene with 1 in neutral CH3CN. Insets: (a) Calculated
and (b) experimental isotopic distribution pattern for the cycloadduct [(Cn*)(CF3CO2)RuIVOC2Ph2O]+ complex.
Table 4. Second-Order Rate Constants for the Cycloaddition of
Alkynes with cis-Dioxoruthenium(VI) Complex 1a
entry
R1
C4H9
CH3
EtO2C
(CH3)3C
Me3Si
MeO2C
p-MeOC6H4
p-MeC6H4
p-FC6H4
p-ClC6H4
C6H5
R2
k2/dm3 mol-1 s-1
Ip/eVb
1
2
3
4
5
6
7
8
9
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
CO2Me
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
10.7 ( 0.4d
5.7 ( 0.2
5.1 ( 0.2
3.6 ( 0.2
103 ( 4c
1.9 ( 0.1
18.8 ( 0.8
8.6 ( 0.4
5.8 ( 0.2
4.6 ( 0.2e
4.7 ( 0.2
2.1 ( 0.1
9.34
9.53
9.98
10.19
10.5
10.94
Figure 6. Experimental and calculated absorbance-time curves and
residue for the reaction of Me3SiCtCSiMe3 (0.1 M) with 1 (1 mM) in
CH3CN at 296 K.
8.63
8.87
8.93
9.06
10
11
12
Discussion
p-CF3C6H4
Internal alkynes react with M)O oxidants such as OsO4,
KMnO4, or RuO4 to afford 1,2-diketones, which are synthetically
useful intermediates.45 It has long been postulated that the alkyne
oxidations proceed via a putative cyclic diester intermediate
generated by a formal [3 + 2] cycloaddition between a MO2
moiety and CtC bond, analogous to alkene dihydroxylations.10
However, in all cases, structural evidence of this cyclic diester/
cycloadduct is lacking in the literature. Previously Griffith and
co-workers32b reported that the reaction of OsO4 with alkynes
in the presence of tertiary bases afforded trans-dioxoosmium-
(VI) ester complexes [Os2O4(O4C2RR′)L4] (L ) isoquinoline
a T ) 297 ( 0.1 K. b Calculated ionization potential. c ∆Hq ) 9.3
( 0.9 kcal mol-1; ∆Sq ) -18 (1.7 cal mol-1 K-1 d ∆Hq )10.9 (
1.2 kcal mol-1; ∆Sq ) -17 (1.9 cal mol-1 K-1 e ∆Hq ) 11.9 (1.4
kcal mol-1; ∆Sq ) -15 ( 1.4 cal mol-1 K-1
.
.
.
or pyridine) with a proposed structure depicted in Scheme 3
(entry 1). These dioxoosmium(VI) complexes were presumably
derived from a cyclic diester osmium(VI) intermediate via a [3
+ 2] cycloaddition pathway.12a,32
While the mechanism of the alkene oxidations by M)O
species has been extensively investigated, related studies on the
alkyne oxidations are sparse. In the 1970s Sima´ndi and Ja´ky
reported their mechanistic studies on the alkyne oxidation by
KMnO4.39-41 In 1985 Lee and co-workers reported similar
studies with nBu4NMnO4 as oxidant.42 In the study by Sima´ndi
and Ja´ky, the oxidation was proposed to be initiated by a
nucleophilic attack of MnO4- anion on the CtC bond leading
(42) Lee, D. G.; Lee, E. J.; Chandler, W. D. J. Org. Chem. 1985, 50,
4306.
(43) Ibne Rasa, K.; Pater, R. H.; Ciabatonni, J.; Edwards, J. O. J. Am.
Chem. Soc. 1973, 95, 7894.
(44) Ogata, Y.; Sawaki, Y.; Inoue, H. J. Org. Chem. 1973, 38, 1044.
(45) Vollhardt, K. P. C.; Schore, N. C. Organic Chemistry, 2nd ed.;
Freeman: New York, 1994; p 924.

11388 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Che et al.
Table 5. Some Reported Kinetic Data for the Alkyne Oxidations by Other Oxidantsa
entry
1
alkynes
oxidants
KMnO4
solvent
1.5 M NaClO4
rate constants, dm3mol-1 s-1
ref
k1 ) 1230; k2 ) 520; k3 ) 35 (∆Hq ) 6.1 kcal mol-1
∆Sq) -32 eu)
;
39
2
3
KMnO4
KMnO4
0.3 M NaClO4
0.3 M NaClO4
10.0 (∆Hq ) 6.3 kcal mol-1; ∆Sq ) -33 eu)
40
12.5 (∆Hq ) 5.3 kcal mol-1; ∆Sq ) -36 eu)
40
4
5
6
7
8
KMnO4
pH 3.77, ionic strength ) 1.5 M k1 ) 183; k2 ) 43.2 (∆Hq ) 6.2 kcal mol-1
;
41
41
42
42
42
∆Sq ) -31 eu)
KMnO4
pH 3.77, ionic strength ) 1.5 M k1 ) 38.8; k2 ) 11.6 (∆Hq ) 7.2 kcal mol-1
∆Sq ) -31 eu)
;
Bu4NMnO4
Bu4NMnO4
Bu4NMnO4
CH2Cl2
CH2Cl2
CH2Cl2
7.3 ( 0.7 × 10-4
30 ( 5 × 10-4
60 ( 5 × 10-4
9
10
11
mClC6H4CO3H
mClC6H4CO3H
CH2Cl2
C6H6
21.2 × 10-4
1.65 × 10-5
4.7 ( 0.9 × 10-5
43
44
30
[OdCrV(salen)]OTf CH3CN
a k1, k2, and k3 are rate constants for undissociated (H2A), monodissociated (HA-), and double dissociated (A2-) species, respectively.
Scheme 3. Proposed Reaction Schemes for the Alkyne Oxidation by MdO Reagents
to a Mn(V) diester intermediate (Scheme 3, entry 2, pathway
a), which subsequently decomposed to 1,2-diketones and Mn-
(III).41 However, Lee and co-workers suggested an electrophilic
mechanism wherein the Mn(V) diester was derived from a
metallaoxetene intermediate (Scheme 3, entry 2, pathway b).42
A similar metallaoxetene intermediate had also been proposed
by Manosi and co-workers in 1993 on the oxidation of
diphenylacetylene by oxochromium(V) salen complexes to
benzil.30 Despite these considerable efforts in this field, the
mechanism of alkyne oxidations by M)O compounds has
remained quite speculative because of the near total absence of
experimental information on the nature of the MdO/alkyne
intermediate.
electrospray ionization mass spectroscopies, 1 was found to
undergo an initial [3 + 2] cycloaddition reaction with alkynes
[R-CtC-R′] to give the [(Cn*)(CF3CO2)RuIVOC2RR′O]+
intermediates. These Ru(IV) cycloadducts are featured by an
intense low energy absorption band with λmax in 550-660 nm.
One of the cycloadducts, [(Cn*)(CF3CO2)RuIVOC2(SiMe3)2O]-
ClO4 (3a), has been isolated and structurally characterized by
X-ray crystallography. These findings are the first experimental
evidence for a [3 + 2] cycloaddition between a cis-MO2 unit
with a CtC bond of alkyne in the MdO mediated alkyne
oxidations.
Regarding the intense low energy absorption of the cycload-
ducts, because of their large ꢀmax values (ca. 5000 dm3 mol-1
cm-1) the electronic transition involved can be assigned as
In this work, we have found that the cis-[(Cn*)(CF3CO2)-
RuVIO2]ClO4 complex (1) readily oxidized disubstituted alkynes
to give 1,2-diketones in good yields. Using UV-visible and

Alkyne Oxidations
J. Am. Chem. Soc., Vol. 122, No. 46, 2000 11389
charge transfer in nature, presumably π(-O-CRdCR′-O-)fRu-
(IV). In accordance with this assignment, the transition energy
as reflected by the λmax increases with electron-withdrawing
substituents (see Table 3, compare entries 2 and 5 and entries
9 and 11). We note that LMCT transitions in Ru(IV) complexes
with reducing auxiliary ligands such as phenoxides and amides,
such as [RuIV(chbae)(PPh3)(py)] (H4chbae ) 1,2-bis(3,5-dichloro-
2-hydroxybenzamido)ethane; py ) pyridine), usually occur in
the visible region.46
Nature of the Transition State of the [3 + 2] Cycloaddi-
tion. When the log k2 values of the cycloaddition reactions were
plotted against the calculated ionization potentials (Ip) of the
alkynes, no linear free energy relationship can be established
(see Supporting Information). The Ip values were calculated at
the MP2(FC)/6-31 + G(2df,p)//HF/6-31 level and have been
corrected for zero-point vibrational energies (see Experimental
Section for details).
Indeed, except for Me3SiCtCSiMe3, the rate constants vary
over a range of less than an order of magnitude, albeit with a
change of Ip by 2.3 eV. Apparently electron-rich alkynes (lower
Ip values) react faster with 1 than those bearing electron-
withdrawing substituents. This is best described by a mechanism
involving an electrophilic attack of the cis-dioxoruthenium(VI)
moiety to the CtC bond. Among the nine disubstituted
acetylenes studied, the aliphatic alkynes appear to be more
reactive than the aromatic analogues toward the cycloaddition
reaction, since some of the aliphatic alkynes exhibit comparable
reactivity to the aromatic analogues despite larger Ip values (see
Table 4, compare entries 1 and 8; 2 and 11).
Indeed, the bis(trimethylsilyl)acetylene oxidation is particu-
larly outstanding in the log k2 vs Ip plot. The k2 value for this
reaction is about 10 times faster than the similar reaction with
C3H7CtCSiMe3 (Ip ) 9.34 eV), even though Me3SiCtCSiMe3
has a calculated Ip value of 10.50 eV. As opposed to Me3SiCt
CSiMe3, the reaction of MeO2CCtCCO2Me (calculated Ip )
10.94 eV) with 1 is 5-fold slower than that of C3H7CtCSiMe3.
For a reaction involving rate-limiting single electron transfer,
the slope of the log k2 vs ∆G plot (∆G ) driving force of the
reaction) would equal to -16.8 eV-1; therefore, lowering the
ionization potential of the substrate by 1 eV should result in a
dramatic rate acceleration.47 In this work, however, with the
exception of Me3SiCtCSiMe3 the k2 values are rather insensi-
tive to the change of the calculated Ip values of the alkynes,
which is incompatible with a single electron-transfer mechanism.
Product Studies. Oxene Insertion. Analogous to the alkene
epoxidation by reactive RudO complexes,5a-f,6a,b the alkyne
oxidation by 1 may occur via an oxirene intermediate, i.e., an
“epoxide” of alkyne. Theoretical studies have suggested that
oxirene is a highly reactive species, energetically similar to a
ketocarbene.48 Oxirene intermediates have been proposed for
alkyne oxidations by peracids,25,43,44 dioxiranes,28,48 mi-
crosomes,49 and methylrhenium(VIII) trioxide.50 For example,
the oxidation of 2-butyne by dimethyldioxirane was found to
produce 2-hydroxy-2-methylpropanoic acid as the major
product.28b A proposed mechanism is that the reaction is initiated
by oxygen atom transfer reaction from the dioxirane to the
alkyne to give an oxirene intermediate. The oxirene subsequently
rearranges to a ketocarbene, which then converts to the product
propanoic acid. It should be noted that no diacetyl was formed
in this reaction; other products such as 1-oxiranylethanone and
2,2,5,5-tetramethyl-1,3-dioxolan-4-one were detected only in
low yields.
If the alkyne oxidation by 1 takes place via oxene insertion,
the corresponding 2-butyne oxidation should result in a similar
product profile as the dioxirane-mediated oxidation, i.e., a
predominant formation of 2-hydroxy-2-methylpropanoic acid.
However, in this work, diacetyl was formed exclusively in 97%
yield (see Table 1, entry 4). The absence of rearrangement
products does not rigorously exclude the oxirene intermediate
since it is possible that the formation of the second C-O bond
would precede the Wolff rearrangement within the coordination
sphere. Nonetheless, our findings do not warrant a rate-limiting
oxirene formation mechanism.
[2 + 2] Cycloaddition. It is well documented that d0
oxotitanium(IV) and zirconium(IV) complexes undergo [2 +
2] cycloaddition with CtC bond to furnish metallaoxetene
complexes.51 Some of them with phenylacetylene and diphe-
nylacetylene as substrates have been characterized by X-ray
crystal analysis. As noted earlier, metallaoxetene intermediates
have been postulated for the oxidation of alkynes by potassium
permanganate42 and oxochromium(V) Schiff-base complexes.30
For a [2 + 2] cycloaddition, the presence of a vacant
coordination site is desirable. If the metallaoxetene formation
is an obligatory pathway, then we anticipate that the reaction
of alkynes with coordinatively saturated cis-[(TetMe6)RuVIO2]-
(ClO4)2 (2) would not be facile because of the formation of a
highly energetic seven-coordinated ruthenium center. However,
we found that complex 2 oxidized 2-butyne and 1-trimethylsilyl-
1-propyne to their 1,2-diketones in 97% yields. Furthermore,
facile cycloaddition reaction of 2 with bis(trimethylsilyl)-
acetylene similar to complex 1 has been observed based on UV-
vis and ESI-MS studies.
Hammett Correlation Studies. Rate-Limiting Vinyl Radi-
cal Formation. The effect of para-substituents on the k2 values
of the reactions of ArCtCSiMe3 with 1 has been investigated,
and the reactivity of the substituted YC6H4CtCSiMe3 was
found to decrease in the order Y ) MeO > Me > F > Cl > H
> CF3. Fitting (by least-squares method) the log krel with
Hammett σ+ constants gave a straight line (R ) 0.97) with a
F+ value ) -0.61. This finding is contrary to the report by
Lee and co-workers on the alkyne oxidation using nBu4NMnO4
as oxidant, wherein a concave free energy plot was obtained.42
The small F+ value found in this work indicates little positive
charge development at the R-carbon atom consistent with a
weakly polar transition state; this also coincides with the small
dependence of the k2 values on the calculated ionization potential
of alkynes as noted earlier. By the same token, the participation
of carbocation radical is excluded. A vinyl cation intermediate
also appears unlikely because the rate-limiting formation of a
vinyl cation is always associated with a large F+ value,52 such
as F+ ) -3.84 (electrophilic addition to CtC bond)53a and F+
(51) (a) Carney, M. J.; Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J.
Am. Chem. Soc. 1989, 111, 8751. (b) Walsh, P. J.; Baranger, A. M.;
Bergman, R. G. J. Am. Chem. Soc. 1992, 114, 1708. (c) Baranger, A. M.;
Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 2753.
(52) (a) Modena, G.; Tonellato, U. In AdVances in Physical Organic
Chemistry; Gold, V., Ed.; Academic Press: New York, 1971; Vol. 9, p
185. (b) Drenth, W. In The Chemistry of Triple-Bonded Functional Groups,
Supplement C2; Patai, S., Ed.; John Wiley & Son: New York, 1994; Chapter
15, p 873.
(46) Che, C.-M.; Cheng, W.-K.; Leung, W.-H. J. Chem. Soc., Chem.
Commun. 1987, 418.
(47) (a) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259. (b) Andrieux,
C. P.; Blocman, C.; Dumas-Bouchiat, J.-M.; Saveant, J.-M. J. Am. Chem.
Soc. 1979, 101, 3431.
(48) Lewars, E. G. Chem. ReV. 1983, 83, 519.
(49) Oritz de Montellano, P. R.; Kunze, K. L. J. Am. Chem. Soc. 1980,
102, 7373.
(53) (a) Noyce, D. S.; Schiavelli, M. D. J. Am. Chem. Chem. Soc. 1968,
90, 1020. (b) Stang, P. J.; Hardgrove, R. J.; Bueber, T. E. J. Chem. Soc.,
Dalton Trans. 2 1977, 1486.
(50) Zhu, Z.; Espenson, J. H. J. Org. Chem. 1995, 60, 7728.

11390 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Che et al.
ClO4 (1) involves a rate-limiting formation of a vinyl radical
intermediate (Scheme 4), which subsequently collapses at the
adjacent RudO moiety to form the ruthenium(IV) dioxolene
product. A linear (sp hybridized) configuration for the vinyl
radical is assumed in order to achieve maximum orbital overlap
for spin delocalization. Indeed extensive experimental and
theoretical investigations have suggested that R-phenyl alkenyl
radicals would favor a linear structure.56
To account for the extra reactivity of Me3SiCtCSiMe3
toward cycloaddition with 1, we suggest that the â-silyl group
can stabilize the vinyl radical intermediate by hyperconjugation
(â-silyl effect, Scheme 5). Although the stabilizing effect of
â-silyl substituent in promoting the reactivity of carbocations57
and vinyl cations58 has been widely studied, silicon effects on
vinyl radical stability are less established. According to a report
by Ingold and co-workers,59 Et3SiCH2CH3 was found to be about
5.5 times more reactive than n-C5H12 for â-H atom abstraction
t
by BuO‚ radical. The higher reactivity of Et3SiCH2CH3 was
Figure 7. Linear dual-parameter Hammett correlation (R ) 0.99, slope
) 1.00) using σJJ• and σ+ for the cycloaddition of 1 with p-Y-C6H4Ct
CSiMe3 (krel ) kY/kH; Y ) OCH3, CH3, F, H, Cl, and CF3).
explained by the ability of the â-silyl group to stabilize the
incipient radical. Since there is little C-H bond breaking at
the transition state, the modest substituent effects observed for
the H-atom abstraction is in accord with Hammond’s postulate.
Recently, Zhang and Bordwell60 showed that the â-Me3Si group
can exert substantial stabilization effects of 3-5 kcal mol-1 on
carboradicals based on the C-H bond dissociation energy
measurements of Me3SiCH2- and MeCH2-substituted thioesters.
The effect of the R-SiMe3 group on the reaction of Me3SiCt
CSiMe3 with 1 is less clear-cut. Davidson and co-workers have
suggested that R- and â-silyl groups can stabilize free radicals
to a similar extent (ca. 2.6 kcal mol-1) based on gas kinetic
experiments.61 On the contrary, recent work by Zhang and
Bordwell found that R-SiMe3 showed little or no stabilization
to carbon-centered radicals.60
) -5.1 (solvolysis of vinyl tosylates in ethanol).53b On the other
hand, rate-limiting vinyl radical formation54a is known to have
small F+ values as reported in the radical addition to substituted
phenylacetylenes: F+ ) -0.53 (for p-Br-C6H4S‚)54b and -0.64
(for p-Cl-C6H4SO2‚).54c Therefore, the small charge dependence
observed in this work might be expected for a vinyl radical
intermediate formation.
Apart from polar substituent effect, radical reactivity is also
influenced by the substituent’s ability to delocalize the spin
density on the carbon radical center, namely, spin delocalization
effect. Previously, we applied a carboradical substituent constant
•
σJJ , developed by Jiang and Ji,55 to our kinetic studies on the
styrene epoxidation by RudO complexes.5e-f With this param-
eter the epoxidation reaction was rationalized by the rate-limiting
formation of benzylic radical intermediate, and both spin
delocalization and polar substituent effects would affect the rate
Experimental Section
Materials. All solvents and alkynes were purified by standard
procedures. Diphenylacetylene, 1-phenyl-1-propyne, bis(trimethylsilyl)-
acetylene, 1-trimethylsilyl-1-propyne, ethyl 3-(trimethylsilyl)propynoate,
4-octyne, 2-butyne, 1-trimethylsilyl-3,3-dimethylbutyne, 1-trimethyl-
silylhexayne, dimethyl acetylenedicarboxylate, trimethylsilylacetylene,
iodobenzene, 1-chloro-4-iodobenzene, 1-fluoro-4-iodobenzene, 4-io-
dotoluene, dichlorobis(triphenylphosphine)palladium(II), and copper-
(I) iodide were obtained commercially. Triethylamine was distilled over
CaH2 before use. Acetonitrile (AR, Lab-scan) for stoichiometric
reactions and kinetic studies was first distilled over KMnO4 and then
CaH2 before used. The alkyne substrates were purified by distillation
after passage through a neutral aluminum oxide column. [(Cn*)-
RuIII(CF3CO2)3]‚H2O was prepared by the previously reported
procedure.22b
•
of reaction.5e,f The σJJ scale was chosen because it measures
the spin delocalization effect of the substituent and is relatively
free from residual polar effect.55
If the cycloaddition reactions of alkynes with cis-dioxoru-
thenium(VI) involve a vinyl radical intermediate, we anticipate
similar effects to operate as spin density develops at the
R-carbon while advancing to the transition state. Considering
•
spin delocalization effect by employing the σJJ parameters, a
•
dual-parameter Hammett correlation (log krel ) FJJ σJJ• + F+σ+
) +0.21σJJ - 0.58σ+) afforded a better straight line (R )
•
0.99, slope ) 1.00) than the single parameter log krel vs σ+
regression (Figure 7). The linear free energy correlation with
the spin delocalization and polar effects is a good indication of
radical character developed at the R-carbon in the transition state.
The positive FJJ• value implies that the spin delocalization effect
stabilizes the radical center and promotes the cycloaddition
reaction. The negative F+ value is indicative of the electrophilic
(56) (a) Kasai, P. H.; Whipple, E. B. J. Am. Chem. Soc. 1967, 89, 1033.
(b) Bennett, J. E.; Howard, J. A. Chem. Phys. Lett. 1971, 9, 460. (c)
Bonazzola, L.; Feinstein, S.; Marx, R. Mol. Phys. 1971, 22, 689. (d) Neilson,
G. W.; Symons, M. C. R. J. Chem. Soc., Perkin Trans. 2 1973, 1405. (e)
Panek, E. J.; Kaiser, L. R.; Whiteside, G. M. J. Am. Chem. Soc. 1977, 99,
3708. (f) Rubin, H.; Fischer, H. HelV. Chim. Acta 1996, 79, 1670.
(57) Reviews for â-silicon effect on reactivity of organic molecules
involving carbocation intermediates: (a) Lambert, J. B. Tetrahedron, 1990,
46, 2677. (b) White, J. M. Aust. J. Chem. 1995, 48, 1227.
(58) (a) Siehl, H. U.; Kaufmann, F. P.; Apeloig, Y.; Danovich. D.; Berndt,
A.; Stamatis, N. Angew. Chem., Int. Ed. Engl. 1991, 30, 1479. (b) Siehl,
H. U.; Kaufmann, F. P. J. Am. Chem. Soc. 1992, 114, 4937. (c) Siehl, H.
U.; Kaufmann, F. P.; Hori, K. J. Am. Chem. Soc. 1992, 114, 9343. (d)
Siehl, H. U.; Muller, B.; Fu, M. In Organometallic Chemistry II, From
Molecules to Materials; Auner, N., Weis, J., Eds.; Weimheim: New York,
1996; p 361.
nature of the cycloaddition reaction. The magnitude of |F+/FJJ |
•
) 2.8 suggests that the polar substituent effect is more important
in the cycloaddition of the cis-dioxoruthenium(VI) to alkynes.55
Proposed Mechanism. We conclude that the cycloaddition
reaction of aromatic alkynes with cis-[(Cn*)(CF3CO2)RuVIO2]-
(54) (a) Chatgilialogu, C.; Ferreri, C. In The Chemistry of Triple-Bonded
Functional Groups, Supplement C2; Patai, S., Ed.; John Wiley & Son: New
York, 1994; Chapter 16, p 917. (b) Ito, O.; Fleming, M. D. C. M. J. Chem.
Soc., Perkin Trans. 2 1989, 689. (c) da Silva Correa, C. M. M.; Fleming,
M. D. C. M. J. Chem. Soc., Perkin Trans. 2 1987, 103.
(59) (a) Jackson, R. A.; Ingold, K. U.; Griller, D.; Nazran, A. S. J. Am.
Chem. Soc. 1985, 107, 208. (b) Wilt, J. W.; Lusztyk, J.; Peeran, M.; Ingold,
K. U. J. Am. Chem. Soc. 1988, 110, 281.
(60) Zhang, S.; Bordwell, F. G. J. Org. Chem. 1996, 61, 51.
(61) Davidson, I. M. T.; Barton, T. J.; Hughes, K. J.; Ijadi-Maghsoodi,
S.; Revis, A.; Paul, G. C. Organometallics 1987, 6, 644.
(55) Jiang, X.-K. Acc. Chem. Res. 1997, 30, 283 and references therein.

Alkyne Oxidations
J. Am. Chem. Soc., Vol. 122, No. 46, 2000 11391
Scheme 4. Proposed Mechanism for the Cycloaddition of Alkynes to cis-Dioxoruthenium(VI)
1
H, 8.23. IR (Nujol, cm-1): 3010, 2885, 2095, 1595. H NMR (300
MHz, 298 K, CDCl3): δ 0.25 (s, 9 H), 7.28 (m, 3 H), 745 (m, 2 H).
13C NMR (75.47 MHz, 298 K, CDCl3): δ 0.00, 95.29, 105.22, 115.77,
117.98, 119.37, 136.41, 143.45, 146.30. EI-MS: m/z 174 (M+).
Scheme 5. Resonance Structures Representing the â-Silyl
Effect
1-(4-Chlorophenyl)-2-trimethylsilylacetylene. Yield: 2.77 g, 83%.
Anal. Calcd for C11H13SiCl (Mr ) 208.76): C, 63.29; H, 6.28. Found:
C, 63.33; H, 6.32. IR (Nujol, cm-1): 3012, 2895, 2098, 1595. 1H NMR
(300 MHz, 298 K, CDCl3): δ 0.25 (s, 9 H), 7.26 (d, 2 H, J ) 6.8 Hz),
7.39 (d, 2 H, J ) 6.8 Hz). 13C NMR (75.47 MHz, 298 K, CDCl3): δ
0.00, 95.29, 105.22, 115.77, 117.98, 119.37, 136.41, 143.45, 146.30.
EI-MS: m/z 208 (M+).
Instrumentation. Electrospray ionization mass spectra were acquired
with a Finningan MAT LCQ spectrometer. The sheath (compressed
air) and auxiliary (nitrogen) gases were operated at 100 and 40 psi,
respectively. Typical operating voltages were 3.0 V capillary voltage
and 3.5 kV spray voltage. All the spectra were collected at 22 °C
capillary temperature.
UV-visible kinetic measurements for rapid reaction were performed
on an Applied Photophysics Ltd. SX.18MV sequential stopped-flow
spectrofluorimeter by using Applied Photophysics Ltd. SX.18 MV
kinetic spectrometer workstation software (version 4.24) on a Acorn
Risc PC computer. Kinetic runs were initiated by mixing an equivalent
volume of alkynes solution with the ruthenium solution. The temper-
ature of solutions was maintained to within (0.2 °C using a RET-100
(NESLAB Instruments Inc.) circulating water bath. Product analyses
were performed on a G1800C GCD Series II spectrometer equipped
with an electron ionization detector, connected with a HP Vectra XA
computer containing an 82341C HP-IB card.
General Procedure for the Preparation of Substituted 1-Phenyl-
2-trimethylsilylacetylenes. To a mixture of substituted iodobenzene
(3.55 g, 16 mmol) and trimethylsilylacetylene (1.57 g, 16 mmol) in
dry triethylamine (20 mL) was added with Pd(PPh3)2Cl2 (0.112 g, 1
mol %) and CuI (0.15 g, 5 mol %) under a nitrogen atmosphere. The
mixture was heated at 70 °C overnight. After cooling the mixture to
room temperature, diethyl ether (50 mL) was added, and the suspension
was filtered. The filtrate was collected and evaporated to dryness by
rotory evaporation. The liquid residue was then purified on an alumina
column using hexanes as the eluant. After solvent evaporation, the
product was further purified by vacuum distillation.
1-(4-Toluene)-2-trimethylsilylacetylene. Yield: 2.56 g, 85%. Anal.
Calcd for C12H16Si (Mr ) 188.34): C, 76.53; H, 8.56. Found: C, 76.33;
1
H, 8.58. IR (Nujol, cm-1): 3015, 2890, 2098, 1595. H NMR (300
MHz, 298 K, CDCl3): δ 0.24 (s, 9 H), 2.31 (s, 3 H), 7.07 (d, 2 H, J
) 7.9 Hz), 7.34 (d, 2 H, J ) 7.9 Hz). 13C NMR (75.47 MHz, 298 K,
CDCl3): δ 0.00, 20.98, 90.16, 93.17, 105.31, 119.98, 128.90, 131.15,
138.55. EI-MS: m/z 188 (M+).
1-(4-Anisole)-2-trimethylsilylacetylene. Yield: 2.58 g, 79%. Anal.
Calcd for C12H16OSi (Mr ) 204.34): C, 70.53; H, 7.89. Found: C,
1
70.54; H, 7.88. IR (Nujol, cm-1): 3013, 2990, 2100, 1595. H NMR
(300 MHz, 298 K, CDCl3): δ 0.24 (s, 9 H), 3.78 (s, 3 H), 6.80 (d, 2
H, J ) 8.9 Hz), 7.40 (d, 2 H, J ) 8.9 Hz). 13C NMR (75.47 MHz, 298
K, CDCl3): δ 0.00, 55.18, 82.58, 92.32, 105.11, 113.71, 115.14, 116.25,
133.37, 138.09. EI-MS: m/z 204 (M+).
Preparation of cis-[(Cn*)(CF3CO2)RuVIO2]ClO4 (1). To ice-cooled
0.2 M CF3CO2H (10 mL) containing [(Cn*)RuIII(CF3CO2)3]‚H2O (0.12
g) was added a saturated solution of (NH4)2CeIV(NO3)6 (1 g in 2 mL
of deionized water). An immediate color change from light yellow to
dark green was observed. Addition of NaClO4 to the green solution
induced precipitation of the desired complex. The solid was collected
on a frit and air-dried. Yield: 55%. UV-vis in 0.2 M CF3CO2H [λmax
/
nm (ꢀmax/dm3 mol-1 cm-1)]: 329 (2400), 675 (50). IR (Nujol, cm-1):
1647 (νCdO), 842 (νasym RuO2), 856 (νsym RuO2). ESI-MS (CH3CN)
calcd for C11H21N3O4F3Ru: m/z 418 (M+). Anal. Calcd For C11H21N3O8-
ClF3Ru: C, 25.56; H, 4.10; N, 8.13. Found: C, 25.86; H, 4.07; N,
8.21.
Caution! Transition metal perchlorates are potentially explosive,
particular care should be exercised in preparation and handling.
1-(4-Fluorophenyl)-2-trimethylsilylacetylene. Yield: 2.37 g, 77%.
Anal. Calcd for C11H13FSi (Mr ) 192.31): C, 68.07; H, 6.81. Found:
C, 68.11; H, 6.77. IR (Nujol, cm-1): 3025, 3010, 2890, 2100, 1595.
1H NMR (300 MHz, 298 K, CDCl3): δ 0.24 (s, 9 H), 6.97 (t, 2 H, J
) 6.8 Hz), 7.44 (dd, 2 H, J ) 8.85, 5.45 Hz). 13C NMR (75.47 MHz,
298 K, CDCl3): δ 0.00, 93.89, 104.07, 115.69, 117.96, 119.37, 134.01,
1390.1, 164.30. EI-MS: m/z 192 (M+).
General Procedure for the Stoichiometric Oxidation of Alkynes.
To a 0.2 M CF3CO2H/CH3CN (10 mL) solution of alkynes (30 mmol)
was added 1 (10 µmol) under a nitrogen atmosphere. The reaction
mixture was stirred for 5 h at room temperature. The solvent was
removed by vacuum, and the residue was extracted by diethyl ether (2
× 10 mL). The combined organic extracts were washed with 0.1 M
NaHCO3 (2 × 10 mL) and then concentrated to about 3 mL by reduced
pressure. Aliquots were taken for GC-MS analyses.
1-Phenyl-2-trimethylsilylacetylene. Yield: 2.45 g, 88%. Anal.
Calcd for C11H14Si (Mr ) 174.32): C, 75.79; H, 8.10. Found: C, 75.67;

11392 J. Am. Chem. Soc., Vol. 122, No. 46, 2000
Che et al.
For all the stoichiometric reactions, cis-[(Cn*)(CF3CO2)RuII(CH3-
CN)2]ClO4 complex was isolated and purified by recrystallization from
a CH3CN/diethyl ether mixture (92% yield). Anal. Calcd for C15H27F3N5-
ClO6Ru (Mr ) 566.93): C, 31.78; H, 4.80; N, 12.35. Found: C, 31.51;
H, 5.03; N, 12.07. IR (Nujol, cm-1): 2275, 2011 (νCtN), 1091 (νCldO).
UV-vis (CH3CN) [λmax (ꢀ/cm-1 mol-1 dm3)]: 328 nm (210). 1H NMR
(300 MHz, 298 K, CD3CN): 2.12 (s, 3 H), 2.25 (m, 6 H), 2.83 (s, 6
H), 2.94-3.21 (m, 6 H). 13C NMR (75.47 MHz, 298 K, CD3CN): δ
50.24, 55.28, 55.50, 62.95, 63.76, 64.33, 65.13, 239.10. ESI-MS (CH3-
CN) calcd as C15H27F3N5O2Ru cation: m/z 468 (M+), 427 (M+ - CH3-
CN).
order rate constants (k2) were obtained from linear fits of kobs to the
concentration of alkynes.
Activation enthalpy (∆Hq) and activation entropy (∆Sq) values were
obtained from the slope and intercept of plots of ln(k2/T) vs 1/T using
the following equation:
ln(k2/T) ) ln(R/Nh) + ∆Sq/R - ∆Hq/RT
where N ) Avogadro’s number, R ) universal gas constant, and h )
Planck’s constant.
Computational Methods and Results for the Calculation of the
Ionization Potential (Ip) of the Disubstituted Acetylenes. The
geometries of all the acetylenic molecules as well as their cations were
optimized at the levels of HF/6-31G(d) and MP2(Full)/6-31G(d). The
only exception was [MeO2CCtCCO2Me]+ cation, for which the MP2
geometry optimization was not successful. Hence the structure of this
cation was determined only at the HF/6-31G(d). In addition, to
characterize all the stationary points as minima and to include zero-
point vibrational energy (ZPVE) corrections in the calculations of the
ionization potentials, harmonic vibrational frequency calculations at
HF/6-31G(d) for all species were also carried out. Based on the MP2
optimized geometries, we performed single-point energy calculations
at the level of MP2(FC)/6-31+G(2df,p)//MP2(Full)/6-31G(d), plus a
ZPVE (scaled by 0.8929) correction. All the calculations were
performed with the Gaussian 98 package of programs.62
Reaction of Complex 1 with Bis(trimethylsilyl)acetylene. Isolation
of [(Cn*)(CF3CO2)RuIVOC2(SiMe3)2O]ClO4 (3a). To an excess of bis-
(trimethylsilyl)acetylene (2 mmol) in CH3CN (10 mL) cis-[(Cn*)(CF3-
CO2)RuVIO2]ClO4 (0.1 g, 0.2 mmol) was added under an argon
atmosphere. A deep purple coloration instantaneously developed. The
reaction mixture was vigorously stirred for 1 h. Solvent was then
removed under vacuum, and the residue was redissolved in 80%
aqueous acetone. Addition of LiClO4 (0.05 g, 0.47 mmol) and slow
evaporation of solvent afforded a purple microcrystalline solid. The
solid was collected on a frit, washed with water and then dried in vacuo.
Yield: 0.13 g, 98%. Anal. Calcd for C19H39Si2N3O8F3ClRu (Mr )
687.22): C, 33.21; H, 5.72; N, 6.11. Found: C, 33.23; H, 5.68; N,
6.22. IR (Nujol, cm-1): 3013, 1698 (νCdO), 1256 (νC-O), 1091. UV-
1
vis (CH3CN) [λmax (ꢀ/cm-1 mol-1 dm3)]: 566 nm (5100). H NMR
Acknowledgment. We acknowledge the support of The
University of Hong Kong, the Hong Kong Research Grant
Council, and The Hong Kong University Foundation. We are
also thankful to Dr. T.-C. Lau and Mr. Eddie Chow (City
University of Hong Kong) for their assistance in the stopped-
flow kinetic experiments.
(300 MHz, 298 K, CDCl3): δ 0.11 (s, 18 H), 2.12 (s, 3 H), 2.25 (m,
6 H), 2.83 (s, 6 H), 2.94-3.21 (m, 6 H). 13C NMR (75.47 MHz, 298
K, CDCl3): δ 3.11, 3.25, 3.41, 3.58, 3.74, 3.91, 50.24, 55.28, 55.50,
62.95, 63.76, 64.33, 65.13, 239.10. ESI-MS (CH3CN) calcd as
C19H39F3N3O4Si2Ru cation: m/z 588 (M+).
Electrospray Ionization Mass Spectroscopic Analyses of the
Reaction of R1sCtCsR2 and cis-[RuVI(Cn*)(CF3CO2)O2]ClO4.
Alkynes (R1 ) alkyl or aryl, R2 ) alkyl, aryl, or TMS) (10 mmol)
dissolved in distilled CH3CN (3 mL) were treated with cis-[(Cn*)(CF3-
CO2)RuVIO2]ClO4 (2 µmol) at room temperature. The mixtures were
stirred vigorously and a blue/purple color immediately developed. The
reaction mixture was then filtered through a 0.5 µm syringe filter before
analyses. An average of 20 scans of mass spectra was obtained. For
all reactions, the base peak can be formulated as the cycloadduct [Cn*-
Supporting Information Available: ORTEP diagram and
table of crystal data, atomic coordinates, calculated hydrogen
coordinates, anisotopic displacement parameters, and bond
distances and bond angles for 3a; the 1H and 13C NMR spectra
of 3a; UV-vis and ESI-MS spectra for the reactions of 1 with
selected disubstituted acetylenes, listing of energies and zero-
point vibrational energies (ZPVE) of all the acetylenic com-
pounds and their cations and ionization potentials of the neutrals,
and the free-energy plot of log(second-order rate constant, k2,
of the cycloaddition) vs the calculated ionization potentials of
alkynes. This material is available free of charge via the Internet
at http://pubs.acs.org.
(CF3CO2)RuIVOCR1CR2O]+ cation complexes. For the reactions of
diphenylacetylene, 4-octyne, and 1-phenyl-1-propyne, ion clusters [m/z
) 468 (M+) and 427 (M+ - CH3CN)] arising from [(Cn*)(CF3-
CO2)RuII(CH3CN)2]+ cation were detected along with the ion cluster
corresponding to the cycloadduct complexes.
Kinetic Measurements. The oxidations of alkynes by cis-[(Cn*)-
(CF3CO2)RuVIO2]ClO4 were monitored by the increase in absorbance
of the absorption band at 553-615 nm under pseudo-first-order
conditions where the alkyne concentration is at least 20-fold in excess
of the ruthenium oxidant. Pseudo-first-order rate constants, kobs, were
obtained by nonlinear least-squares fits of At to time t according to the
equation below:
JA001707O
(62) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scueseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski. V. G.; Montgomery, J. A.,
Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, B.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Oritz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C.-Y.; Nanayakkara, A.; Gonzala, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B.; Chen, W.; Wong, M.-W.; Andres, J. L.; Gonzalez, C.;
Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6,
Gaussian, Inc., Pittsburgh, PA, 1998.
At ) A∞ + (A0 - A∞) exp(-kobst)
where A0 and A∞ are the initial and final absorbances and At is the
absorbance at time t. The kobs is the observed first-order rate constant.
Kinetic data over four half-lives were used for the fits. The second-