Study of the ESI-mass spectrometry ionization mechanism of Fischer carbene complexes

Article abstract of DOI:10.1021/jo050553y

By means of deuterium-labeling experiments, we have carried out a systematic ESI-MS study to determine the mechanism of ESI ionization of alkenyl and alkynyl group 6 Fischer carbene complexes. These compounds can be ionized under ESI conditions only in the presence of additives such as hydroquinone (HQ) or tetrathiafulvalene (TTF). Our results demonstrate that in the ESI source an anion-radical is formed after the initial HQ- or TTF-mediated electron transfer to the metallic carbene complex. For alkenyl carbene complexes, this species evolves by extrusion of a hydrogen radical to form an allenylchromium anion that is detected as the [M - H]^- ion in the mass spectrum. The preference for this mechanistic pathway could be rationalized by DFT calculations. In the case of alkynyl carbene complexes, experiments combining deuterated substrate, additive, and solvent demonstrate that the previously proposed allene-anion carbene complex is not formed. Instead, the H transfer from the ethoxy group in the anion radical, followed by extrusion of a hydrogen radical, leads to an allenyl anion that is detected in the ESI-MS as [M - H - CO]^-.

Full text of DOI:10.1021/jo050553y

Study of the ESI-Mass Spectrometry Ionization Mechanism of
Fischer Carbene Complexes
William D. Wulff,*,† Keith A. Korthals,† Roberto Mart´ınez-AÄ lvarez,‡ Mar Go´mez-Gallego,‡
Israel Ferna´ndez,‡ and Miguel A. Sierra*,‡
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, and Departamento
de Quı´mica Orga´nica, Facultad de Ciencias Quı´micas, Universidad Complutense, E-28040 Madrid, Spain
wulff@chemistry.msu.edu; sierraor@quim.ucm.es
Received March 18, 2005
By means of deuterium-labeling experiments, we have carried out a systematic ESI-MS study to
determine the mechanism of ESI ionization of alkenyl and alkynyl group 6 Fischer carbene
complexes. These compounds can be ionized under ESI conditions only in the presence of additives
such as hydroquinone (HQ) or tetrathiafulvalene (TTF). Our results demonstrate that in the ESI
source an anion-radical is formed after the initial HQ- or TTF-mediated electron transfer to the
metallic carbene complex. For alkenyl carbene complexes, this species evolves by extrusion of a
hydrogen radical to form an allenylchromium anion that is detected as the [M - H]- ion in the
mass spectrum. The preference for this mechanistic pathway could be rationalized by DFT
calculations. In the case of alkynyl carbene complexes, experiments combining deuterated substrate,
additive, and solvent demonstrate that the previously proposed allene-anion carbene complex is
not formed. Instead, the H transfer from the ethoxy group in the anion radical, followed by extrusion
of a hydrogen radical, leads to an allenyl anion that is detected in the ESI-MS as [M - H - CO]-.
Introduction
ions can be subjected to mass spectrometric analysis. The
production of ESI ions involves three key steps. The first
event is the production of charged droplets at the ES
capillary, which is followed by the shrinkage of the
charged droplet due to the high voltage applied. These
two sequential steps lead to very small, highly charged
droplets in which the gas-phase ions are formed. This
last step has proven to be very difficult to establish.5 We
have recently become aware of the potential of the
electrospray ionization source to study electron-transfer
processes in nonconventional media.6 In fact, the behavior
of mono- and polymetallic group 6 (Fischer) carbene
complexes under ESI conditions clearly differs from that
observed with conventional electron-transfer (ET) re-
Electron spray ionization mass spectrometry (ESI-MS)
is a standard tool for the analysis of the composition of
solutions of organic, inorganic, and bioorganic materials.1
Different areas of chemistry ranging from host-guest
chemistry2 to structural biology3 benefit from the advan-
tages of this powerful analytical technique that is the
method of choice in the case of highly polar or labile
compounds. The ESI-MS is also a useful tool in the
detection of reactive intermediates of chemical reactions.4
ESI is a technique that allows the transfer of ions from
solution to the gas phase as isolated entities, and these
† Michigan State University.
‡ Universidad Complutense.
(1) Gaskell, S. J. Mass Spectrom. 1997, 32, 677.
(2) Vincenti, M. Mass Spectrom. 1995, 30, 925.
(3) Schiller, J.; Arnold, K. Encyclopedia of Analytical Chemistry;
Meyers, R. A., Ed.; Wiley: Chichester, 2000.
(4) (a) Meyer, S.; Koch, R.; Metzger, J. O. Angew. Chem., Int. Ed.
2003, 42, 4700. (b) Traeger, J. C. Int. J. Mass Spectrom. 2000, 200,
387.
(5) Van Berkel, G. J., Electrospray Ionization Mass Spectrometry,
Wiley: New York, 1997.
(6) (a) Mart´ınez-AÄ lvarez, R.; Go´mez-Gallego, M.; Ferna´ndez, I.;
Manchen˜o, M. J.; Sierra, M. A. Organometallics 2004, 26, 4647. (b)
Sierra, M. A.; Go´mez-Gallego, M.; Manchen˜o, M. J.; Mart´ınez-Alvarez,
R.; Ram´ırez-Lo´pez, P.; Kayali, N.; Gonza´lez, A. J. Mass. Spectrom.
2003, 38, 151.
10.1021/jo050553y CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/25/2005
J. Org. Chem. 2005, 70, 5269-5277
5269

Wulff et al.
FIGURE 1. ESI mass spectrum of complex 5 showing the pseudomolecular ion [M - H]- at m/z 358. The magnified area
corresponds to the isotopic cluster of the [M - H]- ion.
The aim of this work is to understand the mechanism
agents such as Na/K alloy,7 potassium 1-methylnaph-
of ionization of group 6 Fischer carbene complexes
thalenide,8 SmI2,9 or potassium graphite (C8K).10 Our
mediated by electron carriers under ET-ESI (electron
results have shown that Fischer carbene complexes such
transfer-electrospray ionization) conditions. In this study,
as 1 can be ionized under ESI conditions in the pres-
we will use labeling experiments combined with DFT
ence of additives that act as electron carriers.6 The
calculations to unambiguously determine which of the
process could be interpreted by initial capture of one
two proposed mechanistic pathways in Scheme 1 is al-
electron to form intermediate 2 which would evolve to
ready occurring during the ESI process. An isotope tracer
is one of the traditional mechanistic tools of physical
the [M - H]- detected ion by loss of a hydrogen radical.
The nature of the [M - H]- ion is unknown, and although
organic chemistry that can also be applied to the inves-
it could have the structure of a carbene anion 36,11
tigation of organometallic reaction mechanisms.12 This
obtained by breakage of Hâ, the formation of allenyl
anion 4 by the alternate HR breakage should not be
discarded (Scheme 1).
is an area of research of growing importance. On the
other hand, the use of ESI combined with deuterium-
labeling experiments has been employed in the elucida-
tion of fragmentation mechanisms13 and in the investi-
SCHEME 1
gation of reactive intermediates in organometallic reac-
tions.14
SCHEME 2
(7) Krusic, P. J.; Klabunde, U.; Casey, C. P.; Block, T. F. J. Am.
Chem. Soc. 1976, 98, 2015.
(8) Lee, S.; Cooper, N. J. J. Am. Chem. Soc. 1990, 112, 9419.
Results and Discussion
(9) (a) Fuchibe, K.; Iwasawa, N. Org. Lett. 2000, 2, 3297. (b) Fuchibe,
K.; Iwasawa, N. Chem. Eur. J. 2003, 9, 905.
Monodeuterated complex 5 was prepared from 2-thio-
phenecarbonyl chloride by reduction with superdeuteride
(10) Sierra, M. A.; Ram´ırez-Lo´pez, P.; Go´mez-Gallego, M.; Lejon, T.;
Manchen˜o, M. J. Angew. Chem., Int. Ed. 2002, 41, 3442.
(11) (a) Casey, C. P.; Kraft, S.; Powell, D. R. J. Am. Chem. Soc. 2000,
122, 3771. (b) Casey, C. P.; Kraft, S.; Powell, D. R. J. Am. Chem. Soc.
2002, 124, 2584.
(12) See: Blum, S. A.; Tan, K. L.; Bergman, R. G. J. Org. Chem.
2003, 68, 4127 and references therein.
5270 J. Org. Chem., Vol. 70, No. 13, 2005

ESI-MS Ionization Mechanism of Fischer Carbene Complexes
FIGURE 2. ESI mass spectrum of complex 11 showing the pseudomolecular ion [M - D]- at m/z 357. The magnified area
corresponds to the isotopic cluster of the [M - D]- ion.
to yield dideuterated alcohol 6, Swern oxidation to alde-
hyde 7, and reaction of this compound with [ethoxymethyl-
pentacarbonyl]chromium(0)carbene under Aumann con-
ditions (Et3N/TMSCl).15 Complex 5 was determined to be
97% deuterated at the â-position (Scheme 2).
SCHEME 4
SCHEME 3
Thus, the loss of two CO molecules affords a peak at m/z
302 [M - H - (2CO)]- (C12H8DO4SCr) corresponding to
MS2, while MS3 yields a peak at m/z 218 which corre-
sponds to the elimination of another three CO mole-
cules. This fragmentation pattern was identical to that
observed for the nondeuterated compound 1 (M )
Cr).6b These results suggested that the evolution of the
initially formed radical-anion 8 occurs as proposed in
path B in Scheme 1, by loss of the HR and formation of
the allenyl anion 9 detected as [M - H]-. Further
extrusion of two CO ligands should provide the ion 10
(m/z 302) (Scheme 3).
To discard the possibility of isotopic scrambling with
the electron-transfer reagent or the solvent within the
SCHEME 5
ESI-MS spectra of complex 5 were recorded using an
ESQUIRE-LC ion-trap spectrometer in negative mode of
detection. To a sample of 5 in chloroform (1.5 × 10-5 mol
L-1) a methanolic solution of hydroquinone (HQ, 25 mM)
was added following our previously reported methodol-
ogy.6 A pseudomolecular ion at m/z 358 corresponding to
[M - H]- was observed (Figure 1). The isotopic cluster
of this ion (Figure 1) reveals a composition related to
C12H8DO6SCr. Tandem mass spectrometry (MSn) was
used to determine the fragmentation pathway of this ion.
(13) See, for example: (a) Gatlin, C. J.; Turecek, F.; Vaisar, T. J.
Mass Spectrom. 1995, 30, 1617, 1636. (b) Goodwin, L.; Startin, J. R.;
Goodall, D. M.; Keely, B. J. Rapid Commun. Mass Spectrom. 2004,
18, 37. (c) Shoeib, T.; Hopkinson, A. C.; Siu, K. W. M. J. Phys. Chem.
B 2001, 105, 12399. (d) Shoeib, T.; Cunje, A.; Hopkinson, A. C.; Siu,
K. W. M. J. Am. Chem. Soc., Mass Spectrom. 2002, 13, 408.
(14) Plattner, D. A. Int. J. Mass Spectrom. 2001, 207, 125.
(15) Aumann, R.; Heinen, H. Chem. Ber. 1987, 120, 537.
J. Org. Chem, Vol. 70, No. 13, 2005 5271

Wulff et al.
FIGURE 3. Pseudomolecular ion region of the ESI mass spectrum of the mixture of complexes 11 and 16. The ion at m/z 351
corresponds to the [M - H]- of 16 while the ion at m/z 357 corresponds to the [M - D]- of 11.
droplet during the ionization process, an experiment with
complex 5, perdeuterated hydroquinone (HQ-d6), and
CD3OD was carried out. Under these conditions, only the
pseudomolecular [M - H]- ion 9 at m/z 358 was observed,
which indicates that the possible H/D interchange does
not take place in any position of the complex (ESI-MS
included as Supporting Information).
To unambiguously confirm the proposed ionization
pattern, R-monodeuterated complex 11 was prepared. In
this case, perdeuterated methyl chromium(0)carbene
FIGURE 4. Optimized S0 geometries (B3LYP/LANL2DZ&6-31+G(d)) of radical anions 18 and 21, carbene anions 19 and 22,
and allenylchromium anions 20 and 23.
5272 J. Org. Chem., Vol. 70, No. 13, 2005

ESI-MS Ionization Mechanism of Fischer Carbene Complexes
FIGURE 5. Pseudomolecular peaks region of the ESI-MS of complex 24 recorded with HQ-d6 and CD3OD.
complex 12 (obtained from [ethoxymethylpentacarbonyl]-
chromium(0)carbene by treatment with NaOEt/EtOD,
followed by quenching with HCl(gas)/EtOD) was deproto-
nated with BuLi and condensed with 2-thiophene car-
boxaldehyde to yield 11 in 17% yield. The degree of
deuteration of this carbene complex in the R-position was
95% (Scheme 4).
ionization of deuterated carbene complexes 5 and 11
indicate that the process occurs by the transfer of 1e- to
the organometallic compound, followed by extrusion of
the R-hydrogen to form an allenyl anion which is the [M
- H]- species detected in the MS spectra.
SCHEME 6
Submission of complex 11 to the ESI-MS conditions
used above resulted in a pseudomolecular [M - D]- ion
14 at m/z 357 (Figure 2). No pseudomolecular [M - H]-
ion (m/z 358) was observed in this case. The fragmenta-
tion of the [M - D]- ion (m/z 357) shows again the double
simultaneous decarbonylation process characteristic of
these compounds, affording exclusively an ion 15 at m/z
301 (see the Supporting Information). By analogy with
the reaction pattern depicted in Scheme 3, after the
initial ET to 11, the formation of radical anion 13 should
lead to allenyl anion 14 by C-D bond breakage. Finally,
the double elimination of CO would form 15 (Scheme 5).
The possibility of deuterium scrambling with the HQ
and/or with the solvent was rejected again by recording
the ESI-MS of complex 11 with HQ-d6 and CD3OD. Only
the pseudomolecular peak at m/z 357 was detected in the
experiment (ESI-MS included as Supporting Informa-
tion).
As an additional confirmation of these results, and to
discard any possibility of intermolecular H/D exchange
among carbene complex molecules during the experi-
ment, the ESI-MS of an equimolar mixture of labeled
complex 11 and compound 16 was recorded under the
conditions used through this work. The resulting mass
spectrum showed the expected peak [M - D]- 14 (m/z
357) of complex 11, together with a [M - H]- pseudo-
molecular ion (m/z 351) corresponding to complex 16
(Figure 3). In both cases, the intensities and the isotopic
distribution of the peaks were identical to those observed
for the single complexes, which discards any significant
H/D scrambling in the process. Therefore, it can be
concluded that the results obtained in the ESI-MS
It has been well stated that in kinetically controlled
reactions the most exothermic pathway requires a lower
energy of activation.16 Thus, we have rationalized the
preference for the HR breakage in the ionization process
(path B, Scheme 1) by comparing the stability of allenyl
anion 4 and carbene anion 3, the product that would
result from the alternative Hâ fragmentation pathway
(path A, Scheme 1). DFT calculations were carried out
in the model complex [(CO)5Cr(OMe)CHdCH2], 17. The
S0 geometries of radical anion 18, carbene anion 19, and
(16) The Bell-Evans-Polanyi principle states the linear relation
observed between energy of activation (Ea) and enthalpy of reaction
(∆Hr) within a series of closely related reactions. IUPAC Compendium
of Chemical Terminology, 2nd ed., Pergamon Press: New York, 1997.
See also: Dewar, M. J. S.; Dougherty, R. C. The PMO Theory of Organic
Chemistry; Plenum: New York, 1975.
J. Org. Chem, Vol. 70, No. 13, 2005 5273

Wulff et al.
FIGURE 6. (a) ESI mass spectrum of complex 24 with TTF and MeOH. (b) ESI mass spectrum of complex 24 with TTF and
CD3OD.
allenylchromium anion 20 were optimized at the B3LYP/
LANL2DZ&6-31+G(d) level and are represented in Fig-
ure 4. Our results showed that allenyl anion 20 was
considerably more stable (42.02 kcal mol-1) than carbene
anion 19. Being aware that these results could be
oversimplified by the fact that model complex 17 lacked
an aromatic ring in the â-position, additional calculations
were carried out in phenyl-substituted radical anion 21
(formed from carbene complex [(CO)5Cr(OMe)CHdCHPh]),
carbene anion 22 and allenylchromium anion 23. Again,
the allenyl anion 23 resulted to be more stable than the
carbene anion 22 (34.05 kcal mol-1), even though this
latter species benefits from the additional conjugative
stabilization caused by the phenyl group. Nevertheless,
it should be noticed that the difference in energies
between these species is very high. As a matter of fact,
the value of 34.05 kcal mol-1 is an estimation of the
stabilization of an allenyl-metal species compared to a
vinylmetal species (Figure 4). Furthermore, the charge
on the chromium atom is nearly identical in species 21-
23, which points to the small effect of the metal in the
evolution of the radical anion formed after the initial
electron-transfer reaction.
Having established the viability of the use of labeling
experiments in the study of the behavior of Fischer
carbene complexes under ESI conditions, we decided to
employ this methodology to confirm the ionization pat-
tern previously proposed by us for alkynyl carbene
complexes 24.6b In this case, the anion radical 25 is
formed after the capture of an electron, and this initial
ionization is followed by the loss of a hydrogen radical
to form the [M - H]- ion (m/z 349) detected in the mass
spectrum. We postulated6b that the cleavage of the
aromatic C-HA bond to yield allene anion carbene
complex 26 was the most likely fragmentation pathway
(path A, Scheme 6). However, the intramolecular HB
5274 J. Org. Chem., Vol. 70, No. 13, 2005

ESI-MS Ionization Mechanism of Fischer Carbene Complexes
FIGURE 7. Tandem mass spectrum of the pseudomolecular ion of 24 at m/z 321 showing the characteristic loss of CO molecules.
transfer from the ethoxy group in 25, followed by the loss
of a hydrogen radical to form allenyl anion 27, cannot be
discarded (path B, Scheme 6). If this species was formed,
it will be also detected as the [M - H]- ion (m/z 349) in
the ESI-MS spectrum.
ethynylbenzene 29 following the standard procedure for
the synthesis of Fischer carbene complexes (Cr(CO)6/
M-CtC-Ph/EtOTf). p-D-Ethynylbenzene 29 was made
from 30 by Sonogashira coupling, D-halogen inter-
change, and TMS group removal. The desired complex
28 was prepared in 55% yield and g98 D-incorporation
(Scheme 7).
Prior to working with a labeled substrate, we checked
the possible proton interchange between the aromatic
hydrogens of 24 and the HQ or the solvent (MeOH)
during the experiment. The ESI-MS spectrum of 24
was recorded in the presence of HQ-d6 and CD3OD.
The isotopic cluster of the pseudomolecular ion is dis-
played in Figure 5. The presence of peaks at m/z 351,
352, and 353 only can be explained assuming H/D
scrambling in the aromatic ring. The most intense peak
(m/z 351) could correspond to both the [M - D]- ion
obtained from a trideuterated complex 24 and the [M -
H]- ion obtained from this complex bisdeuraterated in
the aromatic ring.
SCHEME 7
Assuming the proton exchange between alkynyl car-
bene complex 24 and the HQ/MeOH employed in the ESI-
MS experiments, we decided to change the additive, using
tetrathiafulvalene (TTF) instead of hydroquinone. Previ-
ous work by our research group has demonstrated that
TTF can be also used as an efficient electron carrier in
ESI-MS experiments with Fischer carbene complexes.6
The ESI-MS spectra of 24 with TTF (25 mM) in both
MeOH and CD3OD are displayed in Figure 6. The mass
spectra show identical pseudomolecular ions correspond-
ing to [M - H - CO]- (m/z 321), which indicate that the
exchange H/D of 24 with TTF or solvent does not take
place. The TTF appears in both cases at m/z 204 as TTF•-.
Tandem mass spectrometry of the [M - H - CO]- ion
showed the successive loss of CO molecules displayed in
Figure 7.
The ionization of complex 28 was carried with TTF in
either in MeOH or in CD3OD (ESI-MS spectra are shown
in the Supporting Information). A pseudomolecular [M
- H - CO]- ion (m/z 322, C15H8DCrO5) was observed in
both cases. The possibility of scrambling was excluded
in the previous experiments and no other peaks due to
the fragmentation of the C-D bond could be detected.
These experimental results suggest that the ionization
of alkynyl carbene complexes 24 and 28 occurs as
suggested in path B of Scheme 6. Once the radical anion
Once the ESI ionization conditions were established,
monodeuterated complex 28 was prepared from p-D-
J. Org. Chem, Vol. 70, No. 13, 2005 5275

Wulff et al.
tion), along with 0.44 g (33%) of recovered [ethoxymethylpen-
tacarbonyl]chromium(0) carbene. The deuterium incorporation
was determined by 1H NMR by comparison of the integration
of the signals at 7.16 ppm (d) and 7.08 ppm (dd). Spectral data
for compound 5: 1H NMR (CDCl3) δ 1.67 (t, 3 H, J ) 7.1 Hz),
5.07 (q, 2 H, J ) 7.1 Hz), 7.08 (dd, 1 H, J ) 3.7, J ) 5.1 Hz),
7.38 (dd, 1 H, J ) 1.0, J ) 3.7 Hz), 7.47 (dd, J ) 1, J ) 4.9
25 is formed, H-transfer from the ethoxy group, and
sucessive loss of a hydrogen radical give allenyl anion
27. This species losses a CO molecule to form the [M -
H - CO]- ion detected in the ESI-MS spectra. Tandem
MS of complex 28 also affords the typical sequence of CO
loss (see the Supporting Information).
1
Hz); 13C NMR (CDCl3) δ: 15.1, 75.8, 123.5 (t, J ) 22.5 Hz,
In conclusion, through this study we have combined
deuterium labeling in substrate, additive, and solvent to
establish the mechanism of ionization of conjugated
Fisher carbene complexes under ESI conditions. The
results indicate that in the ESI source, in the presence
of HQ or TTF, an anion-radical is formed. For alkenyl
carbene complexes, this species evolves by extrusion of
a radical hydrogen to form an allenylchromium anion
that is detected in the mass spectrum as [M - H]- ion.
The rationalization based on the higher stability of the
allenylchromium anion species compared to the carbene-
anion species previously postulated by us in these
processes has been extracted from DFT calculations.
From the comparison of the energy values it can also be
concluded that the presence of the metal has little
influence in the evolution of the radical anion formed
after the initial electron transfer reaction. In the case of
alkynyl carbene complexes, our results demonstrate that
the previously proposed allene-anion carbene complex is
not formed. Instead, H-transfer from the ethoxy group
and successive loss of a hydrogen radical gives an allenyl
anion that is detected in the ESI-MS spectra as [M - H
- CO]- ion. To avoid undesirable proton interchange
during the experiments, TTF instead of HQ was used in
these cases.
very weak) 128.7, 130.4, 133.5, 138.3, 139.9, 216.8, 224.4,
328.6; IR (thin film) 3080w, 3026w, 2963m, 2901w, 2108s,
1480m cm-1. Anal. Calcd for C14H9DCrO6S: C, 46.80; H, 3.09.
Found; C, 46.72; H, 2.81.
Preparation of 12. To [ethoxymethylpentacarbonyl]chro-
mium(0) carbene19 (4.57 g, 17.3 mmol) was added 10 mL of
ethanol-d1 with predissolved sodium metal (<4 mg). The so-
lution was stirred for 15 min and acidified with concd HCl/
ethanol-d1 (1:3, v/v). The solution was then taken up in Et2O
and filtered through Celite 503. The organic solvent was
removed under reduced pressure. This procedure was repeated
four times, resulting in 2.94 g of 12 as orange oil at rt, yellow
solid at -20 °C, Rf ) 0.42 (9:1 hexane/EtOAc) (64% yield, 96%
deuterium incorporation). The spectral data were consistent
with the known literature compound.20 The deuterium incor-
poration was determined by 1H NMR by comparison of the
integration of the signals at 2.88 and 1.65 ppm. Spectral data
for 12: 1H NMR (CDCl3) δ 1.64 (t, 3 H, J ) 7.1 Hz), 2.88 (br
s, 0.15 H, from nondeuterated compound), 5.01 (bs, 2 H); 13C
NMR (CDCl3) δ 14.9, 49.3 (broad and weak), 216.5, 223.4, 357.8
(deuterated carbon missing due to the relaxation time of C-D);
IR (thin film) 2991.97m, 2945.68m, 2905.17w, 2064.10vs,
1917.49vs, 1469.94m, 1367.76s, 1250.03vs, 1128.50s, 1030.12s,
978.03s 812.13m, 640.45vs cm-1
.
Preparation of 11. To a solution of 12 (1.0 g, 3.8 mmol) in
Et2O (25 mL) at -78 °C was added 2.4 mL of n-butyllithium
(1.6 M in hexanes, 3.8 mmol). The solution was stirred for 20
min, and then 2-thiophenecarbaldehyde (0.7 mL, 7.7 mmol)
was added at -78 °C. The solution was stirred for 45 min at
-78 °C, warmed to rt, and stirred for an additional 4 h. The
reaction was extracted with 10 mL of D2O, and the D2O layer
was adjusted to pH < 7 with acetic acid-d4. The D2O layer was
extracted with CH2Cl2 (3 × 50 mL), the organic extracts were
combined and dried over sodium sulfate, and the solvent was
removed under reduced pressure. Column chromatography (2.8
cm × 26 cm, silica gel) with 5% CH2Cl2 to 10% CH2Cl2/hexane
gave 0.24 g of 11 as red solid, mp 86-88 °C,13 Rf ) 0.30
(hexane) (17% yield, 95% deuterium incorporation). The
deuterium incorporation was determined by 1H NMR by
comparison of the integration of the signals at 7.70 ppm (d)
and 7.36 ppm (d). Spectral data for 11: 1H NMR (CDCl3) δ 1.66
(t, 3 H, J ) 7.0 Hz), 5.04 (q, 1 H, J ) 7.2 Hz), 7.08 (dd, 1 H, J
) 3.8, J ) 5.0 Hz), 7.11 (s, 1 H), 7.36 (d, 1 H, J ) 3.8 Hz), 7.47
(d, 1 H, J ) 5.0 Hz), 7.70 (d, 0.05 H, J ) 15.0 Hz, from
nondeuterated compound); 13C NMR (CDCl3) δ 15.2, 75.9,
123.0, 128.7, 130.5, 133.6, 137.9 (t, 1J ) 23.7 Hz, weak), 139.9,
216.8, 224.4, 328.3; IR (thin film) 3112w, 2994m, 2056s, 1916s,
1561s cm-1. Anal. Calcd for C14H9DCrO6S: C, 46.80; H, 3.09.
Found; C, 46.86; H, 2.82.
Preparation of 28. To a solution of 29 (0.37 g, 3.6 mmol)
at -78 °C in 20 mL of THF was added n-butyllithium (1.6 M
in pentane, 2.25 mL, 3.6 mmol). The solution was warmed to
0 °C and stirred for 45 min, which gave a yellow-brown
solution. At this point, chromium hexacarbonyl (0.80 g, 3.6
mmol) was added at 0 °C, and the orange solution was allowed
to warm to rt over 45 min. The solution was cooled to 0 °C,
and ethyl triflate (0.94 mL, 7.3 mmol) was added. The solution
was stirred for 35 min, which resulted in a blood-red solution.
The contents of the flask were poured onto brine (50 mL) and
extracted with three portions of Et2O (20 mL, 2 × 75 mL). The
organic layers were combined and dried with sodium sulfate.
Experimental Section
For General Procedures see the Supporting Information.
Preparation of Perdeuterated Hydroquinone (HQ-d6).
To a solution of hydroquinone (0.11 g, 1 mmol) in 5 mL of
diethyl ether was added sodium (0.07 g, 3 mmol). The reaction
mixture was stirred at rt for 2 h. The solution was carefully
quenched with 1 mL of DCl (95% deuteration). After addition
of 2 mL of D2O, the organic layer was separated and the
solvent removed under reduced pressure. The deuterium
incorporation was checked by mass spectrometry (see the
Supporting Information). ESI-MS (C6D6O2): m/z 114 [M - D]-
(negative mode) and m/z 139 [M + Na]+ (positive mode).
Preparation of 5. In a 50 mL round-bottom flask where
the 14/20 glass joint was replaced with a threaded high-
vacuum Teflon stopcock were added 30 mL of diethyl ether,
compound 7 (0.59 g, 5.22 mmol), [ethoxymethylpentacarbonyl]-
chromium(0)carbene19 (1.35 g, 5.11 mmol), triethylamine (3.0
mL, 21.5 mmol), and TMSCl (2.0 mL, 15.8 mmol).15 Three
freeze-thaw cycles were done to remove oxygen. The solution
was stirred for 23 h at rt and then stored 24 h at -20 °C. The
contents of the flask were poured onto silica gel and the solvent
removed under reduced pressure. Column chromatography
(24.5 cm × 4 cm, silica gel) with 30% dichloromethane/hexane
gave 0.89 g of 5 as a black solid, Rf ) 0.51 (30% CH2Cl2/
hexane), mp 86-87 °C (49% yield, 97% deuterium incorpora-
(17) (a) Fetizon, M.; Henry, Y.; Moreau, N.; Moreau, G.; Golfier, M.;
Prange, T. Tetrahedron 1973, 29, 1011. (b) Macco, A. A.; Brouwer, R.
J. D.; Nossin, P. M. M.; Godefroi, E. F.; Buck, H. M. J. Org. Chem.
1978, 43, 1591.
(18) (a) Hoskovec, M.; Luxova, A.; Svatos, A.; Boland, W. Tetrahe-
dron 2002, 58, 9193. (b) Chadwick, D. J.; Chambers, J.; Hargrave, He;
Meakins, G. D.; Snowden, R. L. J. Chem. Soc., Perkin Trans. 1 1973,
2327. (c) Chadwick, D. J.; Chambers, J.; Meakins, G. D.; Snowden, R.
L. J. Chem. Soc., Perkin Trans. 2 1975, 604.
(20) Chelain, E.; Parlier, A.; Audouin, M.; Rudler, H.; Daran, J. C.;
Vaissermann, J. J. Am. Chem. Soc. 1993, 115, 10568.
(19) Fischer, E. O.; Maasbo¨l, A. J. Organomet. Chem. 1968, 12, P15.
5276 J. Org. Chem., Vol. 70, No. 13, 2005

ESI-MS Ionization Mechanism of Fischer Carbene Complexes
The solvent was removed under reduced pressure with care
taken to maintain a temperature of 0-10 °C for the water
bath. Column chromatography (30 cm × 3.8 cm, silica gel) first
with hexane and then with 5% EtOAc/hexane resulted in 0.69
g of 28 as a black solid (hexane), mp 61-63 °C, Rf ) 0.18
(hexane), Rf ) 0.34 (5% EtOAc/hexane)21 (55% yield, g98%
deuterium incorporation). Again, care was take to maintain a
temperature of 0 to 10 °C of the water bath when the solvent
was removed. The deuterium incorporation was determined
Acknowledgment. Financial support by the Span-
ish Ministerio de Ciencia y Tecnolog´ıa (Grant Nos.
CTQ2004-06250-CO2-01 and BQU2002-00406) are grate-
fully acknowledged. I.F. thanks the Ministerio de Edu-
cacio´n y Ciencia for a predoctoral fellowship.
Supporting Information Available: General experimen-
tal procedures; synthesis and full characterization of com-
pounds 6, 7, 29, and 30; MS2 spectra of complexes 5, 11, and
28; ESI-MS spectra of 5 and 11 with HQ-d6 in CD3OD;
ESI-MS spectra of HQ-d6; ESI-MS spectra of 28 with TTF.
Cartesian coordinates (Å) and total energies (in au, noncor-
rected zero-point vibrational energies included) of all station-
ary points discussed in the text. This material is available free
of charge via the Internet at http://pubs.acs.org.
1
by H NMR by setting the region of integration of 7.58 ppm
(d) to 2 and noting the difference of the integration of the
region of 7.45 ppm (d). This difference was calibrated to the
undeuterated compound. Spectral data for 28: 1H NMR
(CDCl3) δ 1.58 (t, 3 H, J ) 7.1 Hz), 4.74 (q, 2 H, J ) 6.9 Hz),
7.45 (d, 2 H, J ) 7.8 Hz), 7.58 (d, 2 H, J ) 7.3 Hz); 13C NMR
(CDCl3) δ 15.0, 75.8, 91.8, 121.0, 128.8, 131.3 (t, 1J ) 22.8 Hz,
weak), 132.7, 135.7 (weak), 216.3, 225.7, 313.8; IR (thin film)
2999wb, 2155m, 2060s, 1935s, 1294m, 1196m, 1146w, 1109w,
1036m, 862m, 681m, 650m, 608m, 594m cm-1. Anal. Calcd for
C16H9DCrO6: C, 54.71; H, 3.16. Found: C, 54.40; H, 3.00.
Attempts to make compound 28 by treatment of 4-D-trimeth-
ylsilylethynylbenzene with MeLi-LiBr complex were un-
successful at producing yields greater than 0.1% of the carbene
complex.
JO050553Y
(21) Fischer, E. O.; Kreissl, F. R. J. Organomet. Chem. 1972, 35,
C47.
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