Oxidative decomplexation of chromium Fischer carbene complexes induced by dioxiranes

Article abstract of DOI:10.1021/jo982065e

The reaction course of the oxidative decomplexation of Fischer carbene complexes with dioxiranes was examined. The portionwise addition of 2.2 equiv of dimethyldioxirane (DMD) to Fischer carbene complex 1 afforded ethyl phenylpropiolate in 90% yield. When the reaction was carried out using a CO₂-free DMD solution in a N₂ atmosphere ester 2 was formed in 40% yield, whereas in the presence of an O₂ atmosphere the yield increased to 70%. This same assay performed in the presence of ¹⁸O₂ atmosphere afforded the ester 2 partially labeled at the C=O moiety (approximately 50% GC-MS) with ¹⁸O. On the other hand, treatment of Fischer carbene complex 1 with a [¹⁸O₂]di- mesityldioxirane solution led to the formation of ¹⁸O-labeled CO₂ (trapped as BaCO₃ and detected by IRMS). From these results it can be suggested that the oxidative decomplexation of Fischer carbene complexes by dioxiranes involves an initial attack of the dioxirane to the metal coordination sphere. In this step a CO ligand is oxidized to CO₂ thus leaving an unstable chromium tetracarbonyl intermediate which would react with O₂ to give the final ester product and chromium(III) oxide.

Full text of DOI:10.1021/jo982065e

J . Org. Chem. 1999, 64, 1591-1595
1591
Oxid a tive Decom p lexa tion of Ch r om iu m F isch er Ca r ben e
Com p lexes In d u ced by Dioxir a n es
Mariona Gibert, Marta Ferrer, Anna-Maria Lluch, Francisco Sa´nchez-Baeza, and
Angel Messeguer*
Department of Biological Organic Chemistry, I.I.Q.A.B. (C.S.I.C.). J . Girona, 18. 08034 Barcelona, Spain
Received October 14, 1998
The reaction course of the oxidative decomplexation of Fischer carbene complexes with dioxiranes
was examined. The portionwise addition of 2.2 equiv of dimethyldioxirane (DMD) to Fischer carbene
complex 1 afforded ethyl phenylpropiolate in 90% yield. When the reaction was carried out using
a CO2-free DMD solution in a N2 atmosphere ester 2 was formed in 40% yield, whereas in the
presence of an O2 atmosphere the yield increased to 70%. This same assay performed in the presence
of 18O2 atmosphere afforded the ester 2 partially labeled at the CdO moiety (approximately 50%,
GC-MS) with 18O. On the other hand, treatment of Fischer carbene complex 1 with a [18O2]di-
mesityldioxirane solution led to the formation of 18O-labeled CO2 (trapped as BaCO3 and detected
by IRMS). From these results it can be suggested that the oxidative decomplexation of Fischer
carbene complexes by dioxiranes involves an initial attack of the dioxirane to the metal coordination
sphere. In this step a CO ligand is oxidized to CO2 thus leaving an unstable chromium tetracarbonyl
intermediate which would react with O2 to give the final ester product and chromium(III) oxide.
In tr od u ction
conversions to the arene derivative were obtained, al-
though the scope of the decomplexation was limited by
the presence of organic ligands more sensitive to the
dioxirane action, inter alia, the thioether moiety. Con-
versely, the slow reaction rates observed for the reaction
of DMD with tricarbonyl(η4-diene)iron and µ-(η2-alkyne)-
hexacarbonyldicobalt complexes and the chemoselectivity
problems encountered on the models studied raised
serious doubts on the efficiency of dioxiranes for the
decomplexation of these iron and cobalt compounds in
comparison with the use of more conventional reagents,
i.e., amine oxides.7
By contrast, dioxiranes appear to be valuable reagents
for the oxidative decomplexation of Fischer carbene
complexes. In a preliminary communication we described
the use of DMD for the decomplexation of selected
chromium carbene compounds.8 Later, Barluenga et al.
reported the successful use of TFMD for the release of
the organic ligand present in Diels-Alder products from
reactions of -BF2- adducts of functionalized Fischer
vinylcarbene complexes with chiral 2-amino-1,3-dienes.9
More recently, we carried out a study on the decomplex-
ation of a Fischer carbene complex bearing a conjugated
enamino moiety.10
The capability of dioxiranes, in particular of dimethyl-
dioxirane (DMD) and trifluoromethyl(methyl)dioxirane
(TFMD), to transfer an oxygen atom to a wide variety of
substrates under mild, controlled conditions, accounts for
the numerous applications that these reagents have
found in organic synthesis.1 These oxidation reagents
have exhibited also interesting applications in the organo-
metallic field. In a pioneering work, Wolowiec and Kochi
used DMD for the generation of highly unstable oxometal
species from Mn(II) and Fe(II) porphyrins2 and for the
oxidative decarbonylation of tricarbonylrhenium and
molybdenum complexes.3 However, is in the release of
organic ligands from metal transition complexes and on
the selective oxidation of ligands present in these com-
plexes where the reactivity of dioxiranes has been most
widely explored.4,5
Concerning the release of organic ligands, we reported
the use of DMD for the decomplexation of arene tricar-
bonylchromium(0) compounds.6 In this case, excellent
(1) For reviews on dioxiranes, see: (a) Adam, W.; Curci, R.; Edwards,
J . Acc. Chem. Res. 1989, 22, 205-211. (b) Murray, R. W., Chem. Rev.
1989, 89, 1187-1201. (c) Curci, R.; Dinoi, A.; Rubino, M. F. Pure Appl.
Chem. 1995, 67, 811-822. (d) Adam, W.; Hadjiarapoglou, L. P.; Curci,
R.; Mello, R. In Organic Peroxides; Ando, W., Ed.; J ohn Wiley:
Chichester, 1992; pp 195-219. (e) Adam, W.; Smerz, A. K. Bull. Soc.
Chim. Belg. 1996, 105, 581599.
(2) Wolowiec, S.; Kochi, J . K. J . Chem. Soc., Chem. Commun 1990,
1782-1784.
(3) Wolowiec, S.; Kochi, J . K. Inorg. Chem. 1991, 30, 1215-1221.
(4) Messeguer, A. Acros Chim. Acta 1995, 1, 71-73.
Herein we report our results on the reaction course of
the oxidative decomplexation of Fischer carbene com-
plexes with dioxiranes. The studies on the decomplex-
ation mechanism of metal carbonyl complexes have been
mainly referred to metal carbonyls and metal carbonyl
clusters.11,12 In these studies, amine N-oxides were used
(5) For recent references on the oxidation of organic ligands, see:
(a) Adam, W.; Schuhmann, R. M. J . Organomet. Chem. 1995, 487, 273-
277. (b) Malisch, W.; Schmitzer, S.; Lankat, R.; Neumayer, M.; Prechtl,
F.; Adam, W. Chem. Ber. 1995, 128, 1251-1255. (c) Schulz, M.; Kluge,
R.; Schu¨bler, M.; Hoffmann, G. Tetrahedron 1995, 51, 3175-3180. (d)
Suns, S.; Edwards, J . O.; Sweigart, D. A.; D’Accolti, L.; Curci, R.
Organometallics 1995, 14, 4, 1545-1547. (e) Adam, W.; Smerz, A. K.
Tetrahedron 1996, 52, 5799-5804. (f) Adam, W.; Schuhmann, R. M.
J . Org. Chem. 1996, 61, 874-878. (g) Malisch, W.; Hindahl, K.; Gru¨n,
K.; Adam, W.; Prechtl, F.; Sheldrick, W. S. J . Organomet. Chem. 1996,
509, 209-14. (h) Mo¨ller, S.; Fey, O.; Malisch, W.; Seelbach, W. J .
Organomet. Chem. 1996, 507, 239-244.
(6) Lluch, A. M.; Sanchez-Baeza, F.; Camps, F.; Messeguer, A.
Tetrahedron Lett. 1991, 32, 5629-5630.
(7) Lluch, A. M.; J ordi, L.; Sanchez-Baeza, F.; Ricart, S.; Camps,
F.; Moreto´, J . M.; Messeguer, A. Anal. Quim. 1994, 90, 407-411.
(8) Lluch, A. M.; J ordi, L.; Sanchez, B. F.; Ricart, S.; Camps, F.;
Messeguer, A.; Moreto, J . M. Tetrahedron Lett. 1992, 33, 3021-3022.
(9) Barluenga, J .; Canteli, R.-M.; Florez, J .; Garcia-Granda, S.;
Gutierrez-Rodriguez, A. J . Am. Chem. Soc. 1994, 116, 6949-6950.
(10) Lluch, A.-M.; Gibert, M.; Sa´nchez-Baeza, F.; Messeguer, A.
Tetrahedron 1996, 52, 3973-3982.
10.1021/jo982065e CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/17/1999

1592 J . Org. Chem., Vol. 64, No. 5, 1999
Gibert et al.
Sch em e 1
as oxidation reagents.13 However, to our knowledge, the
case of Fischer carbene complexes had not been examined
in detail. Moreover, the consensus reached on the elec-
trophilic character of dioxirane reactivity14-17 makes
difficult to extrapolate the results found for the reactivity
of amine N-oxides in the above studies to the decomplex-
ation reactions promoted by dioxiranes on Fisher carbene
derivatives. Thus, it was interesting to ascertain whether
the attack of the dioxirane would involve the reaction of
an oxygen atom with the carbene moiety or it would
create a vacant on the metal coordination sphere by the
oxidative removal of a CO ligand, similarly to which
occurs in the N-oxides mechanism. Furthermore, the
roles of the dioxirane and that of O2 (a molecule always
present in the solutions of these oxidation reagents) in
the subsequent steps of the decomplexation process would
also be interesting to clarify. In this regard, the decom-
plexation experiments carried out in 18O2 atmosphere or
by using for the first time an 18O-labeled dioxirane, i.e.,
[18O]dimesityldioxirane, were crucial for supporting our
conclusions.
F igu r e 1. Mass spectrum (GC-MS, 70 eV) of ethyl phenyl-
propiolate (2) partially labeled with 18O.
On the other hand, 2.2 equiv of the oxidant were also
consumed in the reaction of carbene complex 1 with a
CO2-free DMD solution performed under the above
conditions.19 When this reaction was carried out in N2
atmosphere, only a 40% conversion of 1 into 2 was
observed. However, a 70% yield (NMR analysis) of ester
2 was formed when N2 was replaced by O2. In this case,
an equimolecular amount of CO2 (determined by absorp-
tion into a Ba(OH)2 solution) with respect to the starting
carbene 1 was trapped.20 Moreover, the Cr(III) present
in the insoluble part of the crude reaction mixture
accounted for 67% of the chromium present in 1, which
is a percentage comparable to the yield in ester 2. These
results indicated that (i) the dioxirane probably induced
the oxidation of a CO ligand, (ii) part of the dioxirane
could be involved in the further oxidation of the inter-
mediate chromium carbonyl species, and (iii) yields in
the reaction product depended also on the amount of O2
present in the reaction medium.
Resu lts a n d Discu ssion
The Fischer carbene complex 1 was selected for the
present study (Scheme 1). The preliminary assays carried
out to determine the stoichiometry of the decomplexation
reaction showed that, in contrast to the case of the
arenetricarbonylchromium(0) complexes where 3 equiv
of the dioxirane were needed to release the arene (eq 1),6
the amount of DMD required for the complete conversion
of the carbene complex 1 was highly dependent on the
temperature and on the addition procedure of the diox-
irane to the reaction mixture. Thus, 6-7 equiv of DMD
was needed if the reagent was added all at once to a
solution of 1. This amount was reduced to 2.2 equiv when
the addition was carried out portionwise and at -20 °C.
Under these conditions, ethyl phenylpropiolate (2) was
isolated in 90% yield. Likewise, the addition of 2 molar
equiv of TFMD (0.5 equiv added at 30 min intervals at
-20 °C) to a solution of 1 afforded ester 2 in 91% yield.
In both cases, the analysis of the insoluble material
formed in the crude reaction mixture revealed that the
chromium was present as Cr(III).18
Then, a series of experiments were carried out in the
presence of labeled O2 atmosphere to elucidate the role
of O2 in this decomplexation reaction. When an acetone
solution of carbene complex 1 was allowed to stand in
an 18O2 atmosphere for 7 h at -20 °C, no decomplexation
was observed (GC analysis). However, when this assay
was performed in the presence of the DMD solution in a
solvent system which was previously degassed to ensure
the absence of dissolved O2, a 25:75 mixture of com-
pounds 1:2 was obtained and the GC-MS analysis of this
mixture revealed the partial incorporation (approxi-
mately 50%) of 18O in the carbonyl moiety of ester 2
(Figure 1). The presence of peaks at m/z 176 (M+ + 2)
and 131 (M+ + 2 - OEt), accompanied by their non-
labeled counterparts at m/z 174 and 129, supported this
identification. This result confirmed the intervention of
O2 in the decomplexation process.
The unequivocal confirmation that the first step of the
decomplexation process involved the dioxirane promoted
oxidation of a CO ligand was obtained from the assays
carried out using an 18O-labeled reagent. The reports on
(11) Shen, J . K.; Shi, Y. L.; Gao, Y. C.; Shi, Q. Z.; Basolo, F. J . Am.
Chem. Soc. 1988, 110, 2414-2418.
(12) Shen, J . K.; Gao, Y. C.; Shi, Q. Z.; Basolo, F. Organometallics
1989, 8, 2144-2147.
(13) For a review on the application of amine N-oxides to transition
group organometallics, see: Albini, A. Synthesis 1993, 263-277.
(14) Adam, W.; Haas, W.; Lohray, B. B. J . Am. Chem. Soc. 1991,
113, 6202-6208.
(18) In this context, it had been previously checked that the DMD
solutions are stable in the presence of Cr(III) salts under the
experimental conditions used.
(19) CO2-free DMD solutions are prepared by employing a phosphate
solution instead of the sodium bicarbonate solution used to buffer the
acetone-water-oxone reaction mixture (cf. ref 33).
(20) A blank assay carried out in the absence of carbene 1 showed
that the DMD concentration remained stable during 4 h.
(15) Baumstark, A. L.; Harden, D. B. J . J . Org. Chem. 1993, 58,
7615-7618.
(16) Murray, R. W.; Gu, D. J . Chem. Soc., Perkin Trans 2 1993,
2203-2207.
(17) Adam, W.; Curci, R.; D’Acccolti, L.; Dinoi, A.; Fusco, C.;
Gasparrini, F.; Kluge, R.; Paredes, R.; Schulz, M.; Smerz, A. K.; Angela
Velosa, L.; Weinko¨tz, S.; Winde, R. Chem. Eur. J . 1997, 3, 105-109.

Decomplexation of Cr Fischer Carbene Complexes
J . Org. Chem., Vol. 64, No. 5, 1999 1593
Sch em e 2
Sch em e 3
the generation of 18O-labeled dioxiranes are scarce.21-25
In a pioneering work, Edwards et al. used doubly labeled
caroate -OSO218O18OH to provide evidence for the in-
volvement of dioxirane intermediates in the ketone-
catalyzed decomposition of peroxomonosulfate.21 How-
ever, to our knowledge there is no report on the isolation
of an 18O-labeled dioxirane. In this context, the Sander
group described the preparation of the first dioxirane
stable in the solid state: dimesityldioxirane (3). This
compound was obtained by photochemical reaction of the
appropriate diazo precursor with oxygen to give the
corresponding carbonyl oxide which upon irradiation
rearranged into the dioxirane.26,27 These authors de-
scribed the generation of [18O]dimesityldioxirane in argon
matrixes and the registration of its IR spectrum, but
there are no data on its isolation and reactivity applica-
tions. We deemed that if the above synthetic pathway
could be conducted in solution in the presence of 18O2,
the expected [18O]dimesityldioxirane could be isolated.
The ability of this less reactive dioxirane to attack
carbene complex 1 as DMD does was, however, unpre-
dictable.
spectra of this compound agreed with those reported by
Sander et al. for the non-labeled derivative. Moreover,
the GC-MS analysis of the crude reaction mixture showed
the presence of dimesityl ketone and mesityl mesitoate,
both labeled with 18O. The presence of this ester, a
product from the rearrangement of the dioxirane under
the analytical conditions used, confirmed the formation
of the oxidation reagent.
When a solution of [18O2]dimesityldioxirane was re-
acted with Fischer carbene 1 under the conditions used
for the assay with the non-labeled dioxirane (Scheme 3),
the formation of BaCO3 was observed in the Ba(OH)2
solution trap. The IRMS analysis of the CO2 present in
this sample gave a δ18OPDB value of -15.04, which
compared to the value obtained from a blank BaCO3
sample (δ18OPDB ) -19.00),29 confirmed the incorporation
1
of 18O into the CO2. On the other hand, the H NMR of
the crude reaction mixture showed the presence of a 30%
of ester 2. Moreover, the GC-MS analysis of the fractions
obtained from the purification of this crude reaction
mixture by column chromatography showed that ethyl
phenylpropiolate was not labeled with 18O. These results
indicated that the dioxirane oxidized a CO ligand of the
Fischer carbene complex to give CO2 and that this
reaction would originate an unstable species which
reacted with O2 and not with labeled dioxirane 3 to give
the non-labeled ester 2.
Dimesityldioxirane was synthesized as described by
1
Sander et al. (58% conversion yield, H NMR) (Scheme
2).27 Minor modifications were then introduced to adapt
the reaction conditions to the use of an 18O2 atmosphere.
In a preliminary assay, a dichloromethane solution of the
nonlabeled dioxirane 3 was allowed to react with the
carbene complex 1 for 20 h at -20 °C. An O2 flow was
bubbled into the reaction medium and then through a
Ba(OH)2 solution to capture the CO2 eventually formed
At this point the detection of the intermediate chro-
mium tetracarbonyl species was attempted. Foley et al.
had described the characterization of tetracarbonyl spe-
cies derived from tungsten Fischer carbene complexes.
Thus, upon irradiation of [W(CO)5C(OMe)Ph] in CH3CN
solution, the formation of the corresponding [W(CO)4-
(CH3CN)C(OMe)Ph] complex was observed by IR, UV,
1
in the decomplexation reaction. The H NMR analysis of
the crude reaction mixture showed the presence of ester
2 (15% yield) and unreacted carbene complex 1 as sole
reaction products. Likewise, the formation of BaCO3 was
also observed. These results confirmed that the dioxirane
3 was capable of inducing the oxidative decomplexation
of the carbene complex, although the yield was consider-
ably lower than that for the case of DMD.
1
and H NMR. The data obtained supported that the CO
ligand replaced by the CH3CN moiety was in the cis
configuration.30 In our case, the irradiation of a solution
of the chromium complex 1 in CH3CN led to the forma-
tion of a species with IR absorptions at 2023, 1946, 1905,
and 1841 cm-1, consistent with the presence of the
expected tetracarbonyl intermediate. When the reaction
was assayed in THF solution, the above peaks appeared
at 2059, 1978, 1938, and 1895 cm-1. In both cases,
exposure of these solutions to the air atmosphere resulted
in the formation of ester 2.31 To our knowledge this is
the first detection of a tetracarbonyl species derived from
a chromium Fischer carbene complex. However, when a
solution of carbene complex 1 in CH3CN was allowed to
The preparation of [18O2]dimesityldioxirane took place
in 34% conversion yield (1H NMR).28 The 1H and 13C NMR
(21) Edwards, J . O.; Pater, R. H.; Curci, R.; Di-Furia, F. Photochem.
Photobiol. 1979, 30, 63-70.
(22) Armstrong, A.; Clarke, P. A.; Wood, A. J . Chem. Soc., Chem.
Commun. 1996, 849-850.
(23) Armstrong, A.; Barsanti, P. A.; Clarke, P. A.; Wood, A. J . Chem.
Soc., Perkin Trans. 1 1996, 1373-1380.
(24) Denmark, S. E.; Wu, Z. J . Org. Chem. 1997, 62, 8964-8965.
(25) Schulz, M.; Liebsch, S.; Kluge, R.; Adam, W. J . Org. Chem.
1997, 62, 188-193.
(26) Kirscheld, A.; Muthusamy, S.; Sander, W. Angew. Chem., Int.
Ed. Engl. 1994, 33, 2212-2214.
(29) The BaCO3 formed in the preliminary reaction carried out in
the presence of non-labeled dioxirane 3 was used as a blank probe for
the IRMS measurements.
(27) Sander, W.; Schroeder, K.; Muthusamy, S.; Kirschfekd, A.;
Kappert, W.; Boese, R.; Kraka, E.; Sosa, C.; Cremer, D. J . Am. Chem.
Soc. 1997, 119, 7265-7270.
(30) Foley, H. C.; Strubinger, L. M.; Targos, T. S.; Geoffroy, G. L. J .
Am. Chem. Soc. 1983, 105, 3064-3073.
(28) Actually a 62% of the corresponding ketone derivative was
present in the crude reaction mixture. It has been described that this
ketone is mostly originated from the carbonyl oxide intermediate (cf.
ref 27).
(31) All attempts to identify the above species by 13C NMR were
unsuccessful and only the presence of ester 2 was detected in the NMR
tube. This result could be due to the long times needed for the
accumulation of the corresponding spectra.

1594 J . Org. Chem., Vol. 64, No. 5, 1999
Gibert et al.
Sch em e 4
neutralized CDCl3 solutions, and chemical shifts are given in
ppm downfield from Si(CH3)4 for 1H and CDCl3 for 13C. All
spectra were registered from solutions where Fischer carbene
complexes could be present that were previously filtered
through Millipore Millex-HV13 cartridges. For the quantitative
measures carried out by 1H NMR using 1,1,2,2-tetrachloro-
ethane as internal standard, 19 s per scan were fixed for
ensuring the same response for all hydrogen atoms. The gas
chromatography-mass spectrometry (GC-MS) analyses were
performed with the electron ionization mode using a 30 m
HP-5 capillary column (0.25 mm i.d.). The HPLC analyzes
were performed with a modular system formed by two Waters
510 pumps and an Model 783 Applied Biosystems detector-
gradient controller (UV detection). The HPLC column used was
a Spherisorb ODS-2 (5 µm) and CH3CN-H2O mixtures at 1
mL/min were employed as mobile phase. Unless indicated
otherwise, photochemical experiments were carried out by
employing a Rayonet photochemical reactor (Middletown, CT,
USA). The 18O2 cylinder (98.1 atom %) was from SDS, Peypin,
France. The Isotopic Ratio Mass Spectrometry (IRMS) deter-
minations were carried out with a Delta S (Finnigan Mat)
apparatus at the University of Barcelona.
The DMD solutions (80 mM in acetone) were prepared as
described elsewhere33 and dried over 4 Å molecular sieves. For
the experiments carried out to measure the evolution of CO2,
the DMD was generated in a phosphate buffer solution at pH
7.8. The TFMD solutions (0.5-0.6 M) were prepared as
described elsewhere.34 Unless otherwise stated, organic solu-
tions obtained from the workup of crude reaction mixtures
were dried over MgSO4 and the purification procedures were
carried out by flash chromatography on silica gel.
P en ta ca r bon yl((eth oxy)p h en yleth yn ylca r ben e)ch r o-
m iu m (0) (1a ). This complex was prepared according to the
procedure described by Fischer and Kreissl35 with slight
modifications. Butyllithium (12 mmol, 1.5 M in hexane) was
added to a solution of phenylacetylene (1.3 mL, 12 mmol) in
anhydrous THF (70 mL) at -78 °C under argon atmosphere.
The reaction mixture was stirred for 30 min at that temper-
ature and 1 h at -30 °C; then 100 mL of anhydrous THF and
hexacarbonylchromium(0) (2.6 g, 12 mmol) were added to the
crude reaction mixture, and the stirring was prolonged at the
same temperature for 2 h. Triethyloxonium tetrafluoroborate
(5 g, 26 mmol) was added to the mixture, and after 5 min the
reaction was quenched by addition of saturated NaHCO3
solution (25 mL). The crude reaction mixture was extracted
with hexane, and the organic fractions were washed with brine
and dried. The residue obtained from the elimination of
solvents was purified by chromatography on silica gel eluting
with a 95:5 hexane-CH2Cl2 mixture to give 3.50 g (83% yield)
react with 1 equiv of a CCl4 solution of DMD at 0 °C in
the absence of oxygen, the IR spectrum of the crude
reaction mixture showed the presence of ester 2 and no
evidence of the above tetracarbonyl species was observed.
This result suggested that DMD might oxidize a CO
ligand different from that removed under the above
photochemical conditions and that the generated chro-
mium tetracarbonyl intermediate would be highly un-
stable.
In conclusion, we propose that the dioxirane promoted
decomplexation of Fischer carbene complexes follows the
reaction course shown in Scheme 4. Thus, the dioxirane
would oxidize a CO ligand to CO2 with the concomitant
formation of an unstable chromium tetracarbonyl species.
This species would react with O2 to give the final ester
product and chromium(III) oxide. Therefore, the metal
coordination sphere, and not the carbene moiety, is the
target for the initial attack of the dioxirane. On the other
hand, the generated chromium tetracarbonyl intermedi-
ate, a species with an electron configuration capable of
promoting monoelectron-transfer processes, would be the
responsible for the first step of the reaction with O2. This
reaction might involve a dioxetane-like intermediate
which would lead to the formation of ester 2 and a
chromium tetracarbonyloxo species, and ultimately to
chromium(III) oxide by further oxidation. The proposed
mechanism is also consistent with our observations about
the DMD excesses required for driving the decomplex-
ation of compound 1 to completion, the fact that only a
50% label was incorporated into ester 2 when the
decomplexation of 1 was carried out in 18O2 atmosphere
and that in the absence of oxygen atmosphere the
reaction does take place although with lower conversion
yields. It is known that reducing species such as super-
oxide or iodide anions are capable to catalytically induce
an efficient decomposition of DMD with the concomitant
release of oxygen.32 In the present case, it is conceivable
that the intermediate chromium species generated during
the decomplexation could act as electron-transfer agents
in front of DMD and promote its decomposition.
Exp er im en ta l Section
Gen er a l. The IR spectra were recorded in with a Bomen
model MB120 apparatus. The NMR spectra (1H NMR, 300
MHz; 13C NMR, 75 MHz) were recorded with a Varian model
Unity 300 apparatus: the spectra were performed in freshly
(33) Adam, W.; Bialas, J .; Hadjiarapoglou, L. Chem. Ber. 1991, 124,
2377.
(34) Mello, R.; Fiorentino, M.; Fusco, C.; Curci, R. J . Am. Chem. Soc.
1989, 111, 6749-6757.
(35) Fischer, E. O.; Kreissl, F. R. J . Organomet. Chem. 1972, 35,
C47-C51.
(32) Adam, W.; Asensio, G.; Curci, R.; Gonzalez, N. M. E.; Mello, R.
J . Am. Chem. Soc. 1992, 114, 8345-9.

Decomplexation of Cr Fischer Carbene Complexes
J . Org. Chem., Vol. 64, No. 5, 1999 1595
of the pure carbene complex. For 1a : IR (CH3CN) 2156, 2061,
labeled dimesityldioxirane (1 mL) was added to the reaction
flask in two portions at 30 min intervals, and the mixture was
allowed to react for 20 h maintaining the O2 flow to pull the
evolved CO2 into the Ba(OH)2 solution trap. The BaCO3
precipitate was isolated by centrifugation, dried (15 h, 60 °C),
and analyzed by isotopic ratio mass spectrometry (IRMS). For
this purpose, the precipitate was treated with H3PO4 for 2 h
at 50 °C and the CO2 released was extracted and purified in
a vacuum line by means of appropriate cryogenic traps. The
BaCO3 formed in the preliminary reaction carried out in the
presence of non labeled dioxirane 3 was used as a blank probe
1
1988, 1951; H NMR δ 7.45-7.60 (5 H), 4.76 (q, 2 H, J ) 7.1
Hz), 1.59 (t, 3 H, J ) 7.1 Hz); 13C NMR δ 313.8, 225.7, 216.3,
135.6, 132.7, 131.6, 128.9, 121.0, 91.8, 75.8, 15.0.
Decom p lexa tion of th e F isch er Ca r ben e Com p lex 1
w ith DMD. A solution of complex 1 (0.099 g, 0.28 mmol) in
acetone (10 mL), previously filtered over Celite and protected
from light, was treated with the DMD solution (two dropwise
additions of 6.8 mL, 0.56 mmol overall, during 4 h at -20 °C).
When the reaction was completed (TLC monitoring), the excess
of reagent was eliminated by evaporation under vacuum (25
°C, 20 Torr). The residue was resuspended in CH2Cl2 and
filtered through Celite to remove the chromium oxides. The
evaporation of the solvent afforded pure ethyl phenylpropiolate
(2) (0.044 g, 90% yield).
1
for the IRMS measurements. On the other hand, the H NMR
analysis of the crude reaction mixture revealed the presence
of a 30% of ethyl phenylpropiolate, and the GC-MS analysis
of the collected fractions from the purification of the crude
reaction mixture by column chromatography (elution with 4:1
hexane-CH2Cl2), showed only the presence of non-labeled
ethyl phenylpropiolate.
Ir r a d ia tion of F isch er Ca r ben e Com p lex 1. Following
a procedure described by Foley et al.,30 a solution of 1 (25 mg,
0.07 mmol) in anhydrous AcCN (4 mL) was irradiated with
UV light at 350 nm under inert atmosphere until reaction was
completed (1 h, IR monitoring). The IR analysis (CH3CN) of
the crude reaction mixture showed the presence of absorption
peaks (2154, 2023, 1946, 1905, 1841) that were assigned to a
tetracarbonyl ((ethoxy)phenylethynylcarbene) chromium(0)
derivative.
Deter m in a tion of th e CO2 Gen er a ted d u r in g th e Re-
a ction of th e F isch er Ca r ben e Com p lex 1 w ith DMD. A
solution of 1 (0.140 g, 0.40 mmol) in acetone (20 mL) was
treated with DMD (19 mL, 0.9 mmol, generated in phosphate
buffer and added in two aliquots within 4 h). The volatile
compounds formed were flushed with an O2 stream and
bubbled through a known volume of a 0.9 M Ba(OH)2 solution.
Titration of the nonreacted Ba(OH)2 permitted the determi-
nation of the CO2 evolved (0.4 mmol). On the other hand, the
1H NMR analysis of the crude reaction mixture showed the
presence of ethyl phenylpropiolate (2) (70% conversion yield).
Dim esityld ioxir a n e. This compound was prepared accord-
ing to the procedure described previously by Kirschfeld et al.26
with slight modifications. An oxygen-saturated solution of
dimesityldiazomethane in CFCl3, maintained at -78 °C, was
irradiated with a white lamp (50 W, high-temperature halo-
gen), filtered through a solution of CuCl2 (7.2 g), and concen-
trated HCl (5 mL) in 100 mL of H2O (2 cm optical path). When
the reaction was completed (3 h, HPLC control), the analysis
of the crude reaction mixture by 1H NMR revealed the presence
of 58% of dimesityldioxirane and 32% of dimesityl ketone. For
dimesityldioxirane: 1H NMR δ 6.82 (s, 4 H), 2.26 (s, 6 H), 2.21
(s, 12 H); 13C NMR δ 138.9, 137.7, 130.8, 103.1, 21.5, 21.3.
When the decomposition of the diazo compound was carried
out in 18O2 atmosphere, a mixture of dimesityl ketone (62%)
and dimesityldioxirane (34%) was obtained. The GC-MS
analysis of this mixture revealed the presence of the ketone
and mesityl mesitoate both labeled with 18O.
Ack n ow led gm en t. Financial support from CICYT-
Generalitat de Catalunya (grant QFN 95-4717) and
fellowships from the Generalitat de Catalunya (M.G.)
and the Ministerio de Educacio´n y Ciencia (A.Ll.) are
acknowledged. We are indebted to Prof. W. Sander and
Mrs. Kerstin Schroeder (der Ruhr-Universita¨t, Ger-
many) for their valuable advise on the preparation of
dimesityldioxirane. We thank Prof. J . L. Burdelande
(Universitat Auto`noma de Barcelona) for helpful dis-
cussions and suggestions on the photochemical aspects
of this work and Dr. Pilar Teixido´ (University of
Barcelona) for the IRMS determinations.
Decom p lexa tion of Ca r ben e Com p lex 1b w ith Dim esi-
tyld ioxir a n e. A three-necked flask containing 5.3 mg (15.1
µmol) of complex 1 was purged with dry and CO2-free oxygen
gas; then CH2Cl2 (4 mL) was added to the flask, and the
solution was purged for an additional 20 min. The reaction
flask was immersed in an ice bath, and one outlet was
connected to a 30 mM Ba(OH)2 solution. A solution of the 18O-
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures, including synthesis and characterization data of all
new compounds, and decomplexation experiments with diox-
iranes. This material is available free of charge via the Internet
at http://pubs.acs.org.
J O982065E