Gas-phase synthesis and reactivity of a gold carbene complex

Article abstract of DOI:10.1021/ja802060t

Electrospray ionization tandem mass spectrometry of a phosphonium ylid complex of gold produces an ion whose mass and gas-phase chemical reactivity indicate that it is a gold benzylidene complex. The complex, with a supporting NHC ligand, corresponds to a

Full text of DOI:10.1021/ja802060t

Published on Web 06/19/2008
Gas-Phase Synthesis and Reactivity of a Gold Carbene Complex
Alexey Fedorov, Marc-Etienne Moret, and Peter Chen*
Laboratorium fu¨r Organische Chemie, ETH Zu¨rich, Wolfgang-Pauli-Strasse 10, CH-8093, Switzerland
Received March 20, 2008; E-mail: peter.chen@org.chem.ethz.ch
We report quantitative data for a gas phase synthesis of a NHC-
supported benzylidene gold carbene and its characteristic reactions.
Given the remarkable progress in the catalytic transformations
presumably involving gold carbenes and carbenoids,1 demand for
spectroscopic evidence of such species is very high. Indeed, besides
a rather special case of the “bare” methylidene gold carbene
Au(CH2+)2 to the best of our knowledge there is no experimental
data on the compounds of the general type LnAu)CHR+. We
address this issue by the synthesis of a gold carbene directly in the
mass-spectrometer from a labile precursor, where the reactive
intermediate is stabilized by the interaction with a basic leaving
group. Recently, Milstein and co-workers3 have demonstrated the
Figure 2. Energy resolved reactive cross-section measured for the loss of
practical significance of such a transformation for the synthesis of
late transition metal carbenes and this chemistry has been exploited
for the early transition metals as well.4 During mechanistic studies
on the intermediates operating in the rhenium-catalyzed aldehyde
olefination, we also discovered interconversion of the carbene and
ylid complexes.5
triphenylphosphine from 3: extrapolation to zero pressure (red circles) and
L-CID-fitted curve (black line).
We conducted quantitative determination of PPh3 dissociation
energy from collision-induced dissociation (CID) energy-resolved
cross sections measurements under near single-collision conditions
and compared the experimental value with calculated results (Figure
2). Extrapolation to zero-pressure and subsequent analysis with the
L-CID program8 afforded the value for E0 ) 51.7 ( 3.3 kcal/mol
assuming that the loss of the PPh3 from 4 occurs via a loose
transition state.9
Scheme 1. Synthesis of the Gold-Ylide Adduct 3
Density functional theory (DFT) calculations at the B3LYP level
yielded a binding energy of 30.5 kcal/mol, that is, about 20 kcal/
mol lower than the experimental value.
This discrepancy could be explained by the inaccurate treatment
of the dispersive interaction contribution to the binding energy by
DFT. To check this hypothesis, we performed the same calculations
using the recently developed M06-L functional, which has been
shown to perform well for transition metal thermochemistry and
noncovalent interactions.10 The M06-L calculations gave a binding
energy of 58.8 kcal/mol, in acceptable agreement with the experi-
ment, thus confirming the importance of dispersion forces for
fragments of this size.
After some screening we found that the phosphonium ylid
complex 3 (Scheme 1) could provide access to the desired carbene
4 (Figure 1). Adduct 3 was prepared in situ adopting an established
procedure6 and subjected to electrospray ionization tandem mass
spectrometric (ESI-MS/MS) experiments using a modified TSQ-
700 mass spectrometer (Finnigan MAT).7
Taking into account that mass alone does not provide a definitive
structural assignment, we investigated whether 4 reacts like a
carbene complex. Carbene 4 was generated during the spray process
by applying a hard tube lens voltage. Introduction of olefins into
the 24-pole rf ion guide led to the formation of new signals in the
spectrum.7 An ion with m/z 679 (5 or 6) that arises when
cis-dimethoxyethylene is introduced into the ion guide region gives
three further signals upon mass-selection and collision with xenon
(Figure 3). Importantly, the signal with m/z 545 is assigned7 to the
Au carbene 9 thus demonstrating metathesis-type reactivity un-
precedented for gold. The pathway leading to the methoxy-group-
stabilized carbene 9 is the preferred reaction manifold. Interestingly,
the cyclopropanation reaction, which is often observed in the
solution for gold carbenoids,11 also operates in the gas phase: the
signal with m/z 501 corresponding to the monocoordinated species
7 is consistent with benzylidene transfer to the CdC double bond.
This pathway becomes preferred upon changing the substituents at
Figure 1. Daughter-ion spectrum obtained after the mass-selection of 3 at
m/z 853 and collision-induced dissociation with xenon in the octopole
collision cell exhibits the benzylidene complex 4 as the sole product.
Under a broad range of collision offsets and target gas (Xe)
pressures in the collision chamber, the fragmentation of the cation
3 furnishes mainly a signal with m/z 591 that corresponds to the
PPh3 loss from the parent complex (Figure 1).
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8880 J. AM. CHEM. SOC. 2008, 130, 8880–8881
10.1021/ja802060t CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Competing Cyclopropanation and Metathesis Channels
Supporting Information Available: Experimental procedures,
computational details, energies, reference mass spectra, as well the
energy-resolved collision-induced cross-section spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 3. Daughter-ion spectrums produced by mass selection of adducts
5/6 and 11/12 formed by gas phase reaction of 3 with cis-dimethoxyethylene
and cis-3-hexene, respectively. The observed fragments indicate that both
olefin metathesis and cyclopropanation take place.
References
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It is highly desirable to develop a convenient cyclopropanation
12
strategy based on the easily available phosphonium ylids.
The current study sheds light on the possibility to use gold ylid
complexes for such a purpose. Solution-phase results as well as
dependence of the cyclopropanation versus metathesis ratio on the
electronic and steric effects of the substituents (Scheme 2) explored
by gas-phase methods will be reported in due course.
(7) For details see Supporting Information.
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In conclusion, we report a first example of the mass spectrometric
characterization of the benzylidene gold carbene. The measured
activation energy for its formation is in agreement with the value
predicted by DFT calculations using the M06-L functional. The
chemical nature of the synthesized carbene is clearly demonstrated
by its characteristic reactivity with olefins, including cross-
metathesis and cyclopropanation.
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Acknowledgment. The support from the ETH Zu¨rich and Swiss
Nationalfonds is gratefully acknowledged. Authors also thank Dr.
Andreas Bach for computations with M06-L functional and Dr.
Bernd Schweizer for crystal structure determinations.
(12) For application of ylid complexes in stoichiometric cyclopropanation see:
(a) Brookhart, M.; Studabaker, W. B. Chem. ReV. 1987, 87, 411–432.
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