Organometallics
Article
obtain a mass spectrum. Ten different time delays were used in each
reaction. Reagent flows and time delays were varied to obtain plots
that cover 2−3 half-lives of the ionic reagent. Data were obtained over
at least 2 days with multiple neutral and ionic solution preparations.
Kinetic plots showed sufficient linearity with correlation coefficients
(r2) of 0.98 or greater.
Scheme 11. CH2 Carbene Precursor using 1,3-
Diisopropylimidizolium as the Ligand on Gold
All neutral reagents were obtained from commercial sources in the
highest purity available and used without further purification. As
needed, neutral reagents were diluted in cyclohexane. Gold salts,
triphenylphosphine gold(I) chloride, and dimethyl sulfide gold(I)
chloride were obtained from commercial sources and used without
further purification. 1,3-Diisopropyimidizolium gold(I) chloride was
synthesized following a previously reported synthesis by Taton et al.35
Gold ylide compounds of the form PPh3RCHAu+L (where R = H, Ph,
OMe, COOMe and L = PPh3, 1,3-diisopropylimidizolium) were
synthesized by a ligand exchange procedure that has been reported by
Chen et al.17
Computational data were obtained using the Gaussian09 suite of
quantum mechanical codes.36 A density functional approach was taken
using the M06 functional and the QZVP basis set. The energetic data
are estimated enthalpies at 298 K, with no corrections to the zero-
point energies or harmonic frequencies. The data are consistent with
preliminary results using the LANL2DZ basis set on gold and 6-
311+G** basis set on second-row atoms.
undergoes fragmentations related to the 1,3-diisopropylimidi-
zolium ligand. Specifically, propyl loss and ligand loss are
observed. This behavior was not seen in the CID reactions of
the precursors to the NHC gold(I) benzylidenes. These results
again suggest that the ylide dissociation pathway is best suited
for the formation of stabilized carbenes. In the absence of
stabilization, other pathways dominate and useful yields of
carbenes are not possible. This is consistent with the
conclusions from Roithova’s recent study.31
CONCLUSIONS
■
Ligated gold(I) carbene complexes can be formed in the gas
phase by several pathways, including reactions with diazo
species and dissociations of phosphorus ylide precursors.
Gold(I) benzylidenes react with olefins to give addition
products, with rates that are sensitive to the electron density
of the olefin’s π-system and that increase to near the collision-
controlled limit for olefins with electron-donating groups (e.g.,
ethyl vinyl ether). Chen has previously shown that the addition
products of the gold(I) benzylidenes complete what appear to
be cyclopropanation and metathesis reactions during CID.15
Here, we show that they have bimolecular reactivity with olefins
and secondary substitution reactions occur (olefin displacing
cyclopropane). These substitution reactions are slower than the
addition reactions of the gold(I) benzylidenes. A Hammett plot
analysis shows that the ligand-swapping process is more
sensitive to the electron density of the olefin’s π system and
is consistent with a later transition state and greater positive
charge on the olefin unit.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and an xyz file giving experimental
procedures, computational data and the complete citation for
ref 36. This material is available free of charge via the Internet
AUTHOR INFORMATION
Corresponding Author
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Professor Donald Aue for helpful
suggestions with respect to the computational methodology.
Support from the National Science Foundation (CHE-1011771
and CHE-1300886) is acknowledged.
The gas-phase syntheses of gold(I) carbenes tend to have
limited scopes. When the carbene is not stabilized, the ylide
dissociation pathway produces other decomposition products.
This limitation can be overcome by employing diazo precursors
in bimolecular reactions, but in the case of ester-substituted
carbenes, the desired carbene undergoes a spontaneous Wolff
rearrangement followed by CO loss. The product is a gold(I)
complex of an alkoxy carbene, which is highly stabilized and
does not give reactivity with olefins. Future work will focus on
alternative, more general avenues to gold(I) carbene formation.
REFERENCES
■
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EXPERIMENTAL SECTION
■
All experiments were conducted in a modified ThermoFinnigan LCQ
Deca XP Plus quadrupole ion trap mass spectrometer equipped with
electrospray ionization (ESI). Gold(I) salts and carbene precursors
were dissolved in methanol at 10−4−10−5 M. Typical ESI conditions
involved flow rates of 3−5 μL/min with needle potentials between 3.5
and 6 kV and heated capillary temperatures from 125 to 200 °C. A
notched waveform is used for isolating the cationic gold(I) species at
room temperature.33 When a stable signal is obtained, neutral reagents
can be spiked into the helium via a custom gas-handling system which
has been previously described.34
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dx.doi.org/10.1021/om500926v | Organometallics XXXX, XXX, XXX−XXX