Relative kinetic basicities of organogold compounds

Article abstract of DOI:10.1021/om901101f

The relative kinetic basicities of a series of differentially substituted and hybridized neutral organogold compounds were examined through competitive protodeauration experiments and were found to span 2.2 orders of magnitude. The effect of electron-withdrawing and electron-donating substituents on the rate of protodeauration of alkenylgold and arylgold compounds was explored. An acid counterion effect indicated the presence of a gold-mediated substrate preequilibrium before protodemetalation, and hybridization effects and a Hammett correlation with -⁺ = -0.41 indicated the involvement of the C-C π system in the protodeauration of vinylgold, alkynylgold, and arylgold complexes.

Full text of DOI:10.1021/om901101f

1712 Organometallics 2010, 29, 1712–1716
DOI: 10.1021/om901101f
Relative Kinetic Basicities of Organogold Compounds
Katrina E. Roth and Suzanne A. Blum*
Department of Chemistry, University of California, Irvine, California 92697-2025
Received December 21, 2009
The relative kinetic basicities of a series of differentially substituted and hybridized neutral
organogold compounds were examined through competitive protodeauration experiments and were
found to span 2.2 orders of magnitude. The effect of electron-withdrawing and electron-donating
substituents on the rate of protodeauration of alkenylgold and arylgold compounds was explored.
An acid counterion effect indicated the presence of a gold-mediated substrate preequilibrium before
protodemetalation, and hybridization effects and a Hammett correlation with Fþ = -0.41 indicated
the involvement of the C-C π system in the protodeauration of vinylgold, alkynylgold, and arylgold
complexes.
Introduction
Results and Discussion
In the last seven years, the gold-catalyzed rearrangement
of alkenes, allenes, and alkynes has become a broadly
applicable method in organic synthesis.1-3 The majority
of these reactions feature protodeauration of an organo-
gold intermediate as the key step for catalyst regeneration.
Recently, strategies to outcompete protodeauration by trap-
ping the organogold intermediate with carbon,4-6 silicon,7
and sulfonyl8 electrophiles or via palladium-catalyzed cross-
coupling9 have been developed. Despite the key role of
protodeauration in competitive catalyst regeneration, to
date there has not been a comprehensive and quantitative
study of the relative basicities of organogold compounds. An
understanding of the relative basicites of organogold com-
pounds would be useful for expanding the substrate scope of
reactions that proceed through turnover-limiting protode-
metalation and for the design of new reactions that out-
compete protodemetalation with alternative functionali-
zation pathways.4-9 Herein, we quantify the effects of
hybridization and substitution on the kinetic basicities of a
series of organogold compounds, selected to represent a
range of the organogold intermediates recently characterized
or proposed in the literature.1-3 In addition, we identify a
dramatic acid counterion effect in a tandem gold-mediated
rearrangement/protodeauration reaction.
In order to probe the relative kinetic basicities of the
selected organogold compounds, a series of competitive
protodemetalation experiments were performed (Table 1).
The strong acid HCl initially was chosen as the proton source
to ensure full protodemetalation of the examined com-
pounds. Indeed, addition of 1 equiv of HCl Et2O to phe-
3
nylgold 1h in CD2Cl2 produced 1 equiv of benzene in less
than 10 min, as observed by 1H NMR spectroscopy. Addi-
tion of HCl Et2O (1.0 equiv) to a rapidly stirring mixture of
3
two organogold complexes (3.0 equiv each) yielded a mixture
of protodemetalation products (benzene and 2) (eq 1). The
relative rates of protodemetalation were determined on the
basis of the ratios of protodemetalated products 2 and
1
benzene, as observed by H NMR spectroscopy. Relative
basicities are listed in Table 1 and are normalized to the
kinetic basicity of PPh3AuPh, which was defined as 1.0.
These studies revealed a striking correlation between
hybridization and kinetic basicity. The basicities of organo-
gold compounds conformed to the following trend: sp3 < sp
< sp2, as observed with compounds 1c < 1d < 1h < 1m.
This trend and the similarity in basicity between sp3- (0.35)
and sp- (0.46) hybridized complexes was particularly of
note, because the pKa values of the conjugate acids follow
a different trend: sp , sp2<sp3 (e.g., acetylene pKa 25,
ethylene pKa 44, methane pKa 48).10 The basicity of vinyl-
gold, phenylgold, and alkynlgold relative to methylgold
suggested a stabilizing involvement of the carbon-carbon
π system in the rate-limiting transition state.11 Berwin re-
ported a correlation between increased hyperconjugation
ability and increased rate of protodemetalation in the series
R3MPh, where M = C, Si, Ge, Sn, and Pb.12 On the basis
of these observations, a transition state involving overlap
of the Au-C σ bond with the carbon π system was
*Corresponding author. E-mail: blums@uci.edu.
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(2) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239.
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pubs.acs.org/Organometallics
Published on Web 03/05/2010
2010 American Chemical Society

Article
Table 1. Relative Rates of Protodemetalation of Organogold
Compounds
Organometallics, Vol. 29, No. 7, 2010 1713
HOMO energy of the acetylene π system compared to the
ethylene π system,14,15 alkynylgold 1d would be expected to
have a lower energy π system than alkenylgold 1m, and thus
the π system of alkynylgold 1d would be less accessible for
protonation. In this transition-state model, the lower energy
π HOMO contributes to the reduced kinetic basicity of
complex 1d.
A Hammett study was conducted to further determine the
electronic character of the rate-determining transition state.
A significantly better fit was obtained when the relative
reaction rates were plotted against σþ than against σp,
consistent with a direct resonance interaction between the
para position on the aromatic ring and the reacting center, as
suggested in the proposed transition-state model (Figure 1b
σþ fit; see Figure S1 for σp fit). When the series p-OMe, p-Me,
16
p-H, and p-CF3 ( 1n, 1i, 1h, and 1e) was examined, an
excellent fit leading to a Fþ value of -0.41 was obtained (R2
= 0.995). This small absolute value of Fþ contrasts with the
higher absolute values of Fþ observed for the Friedel-
Crafts alkylation of aromatic compounds in nitrobenzene
(Fþ=-4.9)17 and the formation of benzylic cations in ben-
zene (Fþ=-2.2).18 Therefore, protodeauration in methylene
chloride is less sensitive to substituent effects. The reduced
sensitivity is consistent with the proposed transition state
where hyperconjugation of the Au-C bond stabilizes the
developing cationic charge. The lower absolute value of Fþ
observed for protodeauration in comparison to protodesily-
lation of aryltrimethylsilane by sulfuric acid in acetic acid
(Fþ=-4.6)19,20 implies an increased hyperconjugation abil-
ity of the Au-C bond in comparison to the Si-C bond, as
would be expected from Berwin’s work in the series R3MPh,
where M = C, Si, Ge, Sn, and Pb.12 Protodestannylation and
deplumbylation are not linearly related to either σ or σþ
constants.19,21
a Ratio of 2:benzene using 1H NMR spectroscopy. b Measured by a
competition experiment between 1a and 1b. c Measured by a competi-
tion experiment between 1b and 1d. d Measured by the ratio of 1h: 1c
due to product volatility. e Measured by the ratio of 1a: 1e using proton-
coupled 31P NMR spectroscopy due to peak overlap in the 1H NMR
spectrum.
The effect of increasing substitution near the Au-C bond
was examined next. Comparison of methyl-substituted aryl
rings 1i, 1j, 1k, and 1l did not reveal a steric effect in this
series, possibly because steric effects were balanced by
electronic effects (e.g., by increased electron donation from
the methyl groups at the 2-position or by lengthening of the
arylgold bond upon increased substitution22); however alter-
native explanations cannot be ruled out at this time.
With the role of substitution on the organic fragment
identified, the influence of the gold’s spectator ligand on
the rate of protodemetalation was examined. Gold catalysts
bearing N-heterocyclic carbene (NHC) ligands have been
explored less than their phosphine counterparts,1-3 and the
effect of the NHC ligand was examined with the goal of
providing predictive power for the selection of ligands during
reaction design.3,23 The presence of the NHC ligand IPr on
1g led to a 1.8ꢀ increase in the rate of protodemetalation
Figure 1. (a) Transition-state model for protodeauration,
showing partial contribution of the π system in the protonation
event. (b) Log of relative protodeauration rates vs σþ, R2 =
0.995 and Fþ = -0.41.
proposed (Figure 1a).13 This hyperconjugative interaction
would provide stabilization of the developing cationic charge
and afford an increased rate of protodeauration in 1h and
1m compared to methylgold 1c, which lacks this mechanism
for stabilization, and compared to alkynylgold 1d, which has
reduced potential for hyperconjugative stabilization in the
transition state.13 Additionally, on the basis of the lower
(16) Takeuchi, K.; Takasuka, M.; Ohga, Y.; Okazaki, T. J. Org.
Chem. 1999, 64, 2375.
(17) Brown, H. C.; Grayson, M. J. Am. Chem. Soc. 1953, 75, 6285.
(18) Blum, S. A.; Rivera, V. A.; Ruck, R. T.; Michael, F. E.;
Bergman, R. G. Organometallics 2005, 24, 1647.
(19) Wells, P. R. Chem. Rev. 1963, 63, 171.
(20) Brown, H. C.; Okamoto, Y. J. Am. Chem. Soc. 1958, 80, 4979.
(21) Eaborn, C.; Pande, K. C. J. Chem. Soc. 1961, 3715.
(22) For the bond length of 1h (2.05 A) see: Hashmi, A. S. K.;
ꢀ
Ramamurti, T. D.; Rominger, F. J. Organomet. Chem. 2009, 694, 592.
ꢀ
For the bond length of 1l (2.06 A) see: Croix, C.; Balland-Longeau, A.;
^
Allouchi, H.; Giorgi, M.; Duchene, A.; Thibonnet, J. J. Organomet.
Chem. 2005, 690, 4835.
(13) (a) Gabelica, V.; Kresge, A. J. J. Am. Chem. Soc. 1996, 118, 3838.
(b) Kresge, A. J.; Tobin, J. B. Angew. Chem., Int. Ed. Engl. 1993, 4, 413.
(14) Gorin, D. J.; Toste, F. D. Nature 2007, 446, 395.
(15) Fleming, I. Frontier Orbitals and Organic Chemical Reactions;
Wiley: Chichester, 1976; p 22.
ꢀ
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(23) Dıez-Gonzalez, S.; Marion, N.; Nolan, S. Chem. Rev. 2009, 109,
3612.

1714 Organometallics, Vol. 29, No. 7, 2010
Roth and Blum
compared to the analogous triphenylphosphine complex,
1d. This result suggests that NHC ligands may confer an
advantage for expanding the substrate scope of gold-cata-
lyzed rearrangements that are terminated by protodemetala-
tion. Conversely, the slower protodemetalation of phosphine
gold catalysts suggests that phosphine catalysts may impart
advantages toward outcompeting protodemetelation with
other functionalization reactions, such as cross-coupling.24
Thus far in our investigation, the basicity of neutral
organogold complexes had been explored. In order to gain
insight into the basicity of a class of cationic oxocarbenium
ions recently reported as intermediates in gold-catalyzed
rearrangements25 and cross-coupling reactions,9 intermedi-
ate 6 was synthesized by stoichiometric addition of PPh3-
AuOTf to allene 5 (>95% 1H NMR yield of 6). In contrast
to the facile protodemetalation of neutral compounds 1a- n
by HCl Et2O, treatment of cationic 6 with HCl Et2O did
the reactions were monitored by 1H NMR spectroscopy over
48 h. Rates of protodemetalation in CD2Cl2 correlated well
with the acids’ reported strengths in DMSO (Table 2).29
Negligible protodemetalation occurred with MeOH, while
PhOH reacted sluggishly with only the most basic of the
examined organogold compounds, 1n. Therefore, stoichio-
metric protodeauration reactivity corresponded approxi-
mately to acids with pKa<18 in DMSO over the time
scale 2-48 h.30
We considered the possibility of a “sulfur acid effect”,
whereby partial formation of a soft-soft32 gold-sulfur
bond in the transition state would lower the enthalpic barrier
to protodeauration with sulfur acids. If this effect resulted in
a significant lowering of the barrier to protodeauration, then
treatment of the series of arylgold compounds with thiophe-
nol would result in more rapid protodemetalation than the
pKa values would suggest, when compared to protodeaura-
tion using benzoic acid, a compound with a similar pKa value
in DMSO (Table 2). Monitoring the protodemetalation of
1e, 1h, and 1n upon treatment with thiophenol and with
benzoic acid in two separate reaction vessels established that
thiophenol produced similar yields of protodeaurated pro-
duct at each time point (Table 2). While this result was not
rigorously quantitative and did not rule out sulfur participa-
tion in the rate-determining transition state, it indicated that
rates of protodemetalation could be explained without in-
voking a substantial stabilizing gold-sulfur effect.
3
3
not result in protodemetalation. Instead, allene 5 was
rapidly and quantitatively regenerated, with concurrent for-
mation of PPh3AuCl (<10 min, observed by 1H NMR and
proton-coupled 31P NMR spectroscopy). The regeneration
of 5 was consistent with the previously reported equilibrium
between allenoates and auroxonium ions,9 with formation of
PPh3AuCl providing a thermodynamic trap for the equili-
brating triphenylphosphinegold cation. Treatment of catio-
nic 6 with the stronger acid HOTf, which also yielded the
more weakly coordinating triflate counterion,26,27 resulted in
formation of protodeaurated oxonium 7 with full con-
Gold-catalyzed rearrangements of allenoates containing
tert-butyl substitution of the ester have been reported to be
terminated by protodeauration, where liberated tert-butyl
cation is the acid source (Scheme 1, conditions A).33 Indeed,
when 1.0 equiv of tert-butyl allenoate 8 was treated with 0.05
equiv of PPh3AuOTf, reaction intermediate 1a24,34 under-
went rapid protodemetalation to produce isobutylene,
1
sumption of 6, as determined by H NMR spectroscopic
comparison with an authentic sample of 7 (eq 2). Addition
of HOTf to 5 in the absence of gold resulted in only trace
formation of 7 (<5%); therefore, formation of 7 was attri-
buted to protodeauration of 6 rather than proton-mediated
rearrangement of 5.
1
methane, and 9, as observed by H NMR spectroscopy
1
(92% H NMR yield of 9). We envisioned that the values
of relative basicity determined by this study could be em-
ployed to develop a reaction that would circumvent proto-
demetalation of 1a and regenerate a catalytic quantity of
gold cation. When 1.0 equiv of tert-butyl allenoate 8 was
treated with 0.05 equiv of PPh3AuOTf in the presence of 1.0
equiv of CH3AuPPh3, 1c, the more basic methylgold com-
plex successfully scavenged the acid, generating methane
and isobutylene and regenerating PPh3AuOTf (Scheme 1,
ꢀ
Gagne recently reported the minimum acid strength re-
quired to protodeaurate an alkyl-substituted alkenylgold
complex;28 however, a comprehensive study that includes
several acids and a range of basicities of organogold com-
pounds has not been reported. To investigate this point,
organogold compounds 1e, 1h, and 1n were treated with 1
equiv of the acids PhCOOH, PhSH, PhOH, and MeOH, and
(30) Organogold compounds may tolerate stronger acid sources
when those acids are kinetically inaccessible. For an example of a
vinylgold complex that does not react rapidly with 2,6-di-tert-butylpyri-
dinium (pKa ≈ 3.4 in DMSO, assigned on analogy to pyridinium), see:
ꢀ
Weber, D.; Tarselli, M. A.; Gagne, M. R. Angew. Chem., Int. Ed. 2009,
48, 5733. We confirmed that addition of 2,6-di-tert-butylpyridine (5.0 equiv)
inhibits protodeauration of 1a (eq 3).
(24) Provided that other functionalization reactions are not equally
slowed.
(25) Lui, L.-P.; Xu, B.; Mashuta, M. S.; Hammond, G. B. J. Am.
Chem. Soc. 2008, 130, 17642.
(26) Bardajı, M.; Laguna, A. Inorg. Chem. 2000, 39, 3560.
´
(27) Annibale, G.; Bergamini, P.; Bertolasi, V.; Bortoluzzi, M.;
Cattabriga, M.; Pitteri, B. Eur. J. Inorg. Chem. 2007, 5743.
ꢀ
(28) Weber, D.; Tarselli, M. A.; Gagne, M. R. Angew. Chem., Int. Ed.
(31) To our knowledge, the estimated pKa value of protonated Et2O
in DMSO has not been reported; see: Fu, Y.; Liu, L.; Li, R.-Q.; Liu, R.;
Guo, Q. J. Am. Chem. Soc. 2004, 126, 814.
(32) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry Part
A; Structure and Mechanisms; Springer: New York, 2007, pp 14-16.
(33) Kang, J.-E.; Lee, E.-S.; Park, S.-I.; Shin, S. Tetrahedron Lett.
2005, 46, 7431.
2009, 48, 5733.
(29) (a) Bordwell, F. G.; Hughes, D. L. J. Org. Chem. 1982, 47, 3224.
(b) Bordwell, F. G.; Algrim, D. J. Org. Chem. 1976, 41, 2507. (c) Bordwell,
F. G.; McCallum, R. J.; Olmstead, W. N. J. Org. Chem. 1984, 49, 1424.
(d) Olmstead, W. N.; Margolin, Z.; Bordwell, F. G. J. Org. Chem. 1980, 45,
3295. (e) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456. (f) Bordwell, F. G.
Acc. Chem. Res. 1988, 21, 463.
(34) Lui, L.-P.; Hammond, G. B. Chem. Asian J. 2009, 4, 1230.

Article
Organometallics, Vol. 29, No. 7, 2010 1715
Table 2. 1H NMR Yields of Protonated Products Using Different Proton Sources
% 1H NMR yield of protonated product (2 h, 48 h)
HCl
PhSH
PhCOOH
PhOH
MeOH
entry
R
compound
pKa 1.8a
pKa 10
pKa 11
pKa 18
pKa 29
1
2
3
CF3
H
OMe
1e
1h
1n
<5, 81
74, 100
72, 100
<5, 34
27, 80
0, 0
0, <5
6, 22
0, 0
0, 0
0, <5
100c
92, 95d
a HCl in DMSO.31 b 1.25 h. c 10 min. d 24 h.
Scheme 1. Reagent-Based Selectivity Allows Stochiometric Formation of Either 1a or 9 in the Presence of the Acid tert-Butyl Cation, via
Reactions Catalytic in PPh3AuOTf
conditions B). These conditions allowed stoichiometric for-
Methylene chloride-d2 was dried over CaH2, degassed using
three freeze-pump-thaw cycles, and vacuum transferred prior
to use. Analytical and preparatory TLC was performed on
Merck F250 TLC plates. Flash chromatography was performed
on Dynamic Absorbents 43-60 μm silica gel or Acros 50-
200 μm basic aluminum oxide (activity I). 1H NMR spectra
were acquired on either the GN-500 or Bruker CRYO-500
spectrometer. 13C NMR spectra were acquired on the Bruker
CRYO-500 spectrometer. 31P NMR spectra were acquired
on the Bruker DRX-400. 19F NMR spectra were acquired
on the Bruker DRX-400. Chemical shifts (δ) are reported in
parts per million (ppm) and referenced to either the resi-
dual protiated solvent peak (1H NMR) or an external stan-
dard of 85% H3PO4 (31P NMR) or 80% CClF3 in CDCl3
(19F NMR). High-resolution mass spectrometry (HRMS)
was performed at the facility operated by the University of
California, Irvine.
Synthesis of (IPr)gold(3,3-dimethyl-1-butynyl), 1g. A dram
vial was charged with 1,3-bis(2,6-di-isopropylphenyl)imidazol-
2-ylidene gold(I) chloride (70.2 mg, 0.113 mmol). Diethylamine
(1.5 mL, 15 mmol), 3,3-dimethyl-1-butyne (19.5 μL, 0.158
mmol), and a stir bar were added. The dram vial was capped,
and the resulting suspension was stirred for 24 h at room
temperature. The suspension was then filtered to remove
NH2Et2Cl, and the filtrate was concentrated in vacuo to yield
the crude product as a yellow oily solid. The crude product
was purified by flash chromatography on a basic alumina
column (5:1 hexanes/ethyl acetate). Compound 1g (20 mg,
28% yield) was obtained as a white solid. 1H NMR (CD2-
Cl2, 500 MHz): δ 1.07 (s, 9H), 1.20 (d, J = 6.9 Hz, 12H), 1.34
(d, J = 6.9 Hz, 12H), 2.57 (septet, J = 6.9 Hz, 4H), 7.16 (s,
2H), 7.36 (d, J = 7.8 Hz, 4H), 7.58 (t, J = 7.7 Hz, 2H). 13C NMR
(CD2Cl2, 125 MHz): δ 24.2, 24.9, 28.4, 29.3, 32.5, 112.9,
113.8, 123.9, 124.7, 131.0, 135.0, 146.4, 191.8. HRMS (ESI):
[M þ Na]þ calcd for C33H45AuN2Na, 689.3146; found,
689.3132.
1
mation of organogold 1a (98% H NMR yield) under the
acidic conditions that previously formed 9.
Conclusion
In conclusion, the global range of kinetic basicities for the
studied neutral organogold complexes spanned approxi-
mately 2.2 orders of magnitude. This relatively narrow range
of basicities suggests that conditions may be identified that
permit access of similar catalytic reaction chemistry and
catalyst regeneration pathways for organogold intermedi-
ates with significantly different substitutions and hybridiza-
tions. Spectator ligand effects, effective acid donor strengths,
and substituent effects have been quantified with respect to
rates of protodeauration, providing predictive power for
future catalytic reaction design.
Experimental Section
The following section contains key general procedures and
synthetic procedures for the preparation of new compounds.
Spectroscopic characterization, synthetic procedures for pre-
viously reported compounds, and additional experimental
details can be found in the Supporting Information.
General Procedures. Chemicals were used as received from
commercial sources unless otherwise noted. The complexes
PPh3AuCl, PPh3AuMe, and PPh3AuOTf were purchased from
Strem Chemical Co. Vinylmagnesium bromide (0.7 M in THF)
was purchased from Acros. Anhydrous HCl in diethyl ether
(1.0 M) was purchased from Acros and used as purchased in an
inert-atmosphere glovebox. Diethyl ether (Et2O), methylene chlo-
ride(CH2Cl2), tetrahydrofuran (THF), and pentanes were purified
by passage through an alumina column under argon pressure.

1716 Organometallics, Vol. 29, No. 7, 2010
Roth and Blum
Representative Procedure for the Preparation of Arylgold
Compounds, 1i-k.35 Synthesis of (4-methylphenyl)gold(triphe-
nylphosphine), 1i. In an inert-atmosphere glovebox, PPh3AuCl
(302 mg, 0.611 mmol) was dissolved in THF (12.5 mL), and the
resulting solution was drawn into a syringe and taken out of the
glovebox. To an oven-dried Schlenk tube under N2 was added a
stir bar, and the vessel was capped with a septum. To the Schlenk
tube was added a 0.75 M solution of 4-methylphenylmagnesium
bromide (4.4 mL, 3.3 mmol) via syringe, and the solution was
heated to 50 °C for 5 min. The solution of PPh3AuCl in THF was
added dropwise to the Schlenk tube over 1 min, and the resulting
solution was heated at 50 °C for 15 min. The Schlenk tube was
then placed in an ice bath, and the solution was allowed to cool
for 5 min. Excess 4-methylphenylmagnesium bromide in the
solution was quenched by adding small ice chunks to the
solution. The solution was then transferred to a separatory
funnel, diluted with Et2O (100 mL), and washed with H2O
(2 ꢀ 100 mL). The organic layer was collected, dried over
K2HCO3, filtered, and concentrated in vacuo. The resulting
white solid was loaded onto a basic alumina column (2 ꢀ 5 cm)
and eluted with THF. The product-containing fractions were
detected by TLC and concentrated in vacuo. The resulting white
solid was dissolved in a minimum of benzene and layered with
pentanes to initiate recrystallization. The recrystallization pro-
ceeded for 3 days at 4 °C. The resulting white crystals were
filtered, washed with pentanes, and subjected to high vacuum
(15 mTorr) to remove excess solvent. Compound 1i was
obtained as a white solid (180 mg, 54% yield). 1H NMR
(CD2Cl2, 500 MHz): δ 2.29 (s, 3H), 7.06 (d, J = 7.5 Hz, 2H),
7.38 (dd, J = 5.5, 7.5 Hz, 2H), 7.47-7.53 (m, 9H), 7.59-7.63 (m,
6H). 13C NMR (CD2Cl2, 125 MHz): δ 21.5, 128.6 (d, J = 6.3
Hz), 129.6 (d, J = 10.1 Hz), 131.6 (d, J = 20.0 Hz), 131.8 (d, J =
25.0 Hz), 134.9 (d, J = 13.8 Hz), 135.4, 139.7, 169.5 (d, J = 117
Hz). 31P NMR (CD2Cl2, 161.9 MHz): δ 44.1. HRMS (ESI):
[M þ Na]þ calcd for C25H22AuPNa, 573.1022; found, 573.1027.
Synthesis of (2,4-Dimethylphenyl)gold(triphenylphosphine), 1j.
Compound 1j was synthesized according to the above proce-
dure for 1i. Compound 1j was obtained as a white solid (110 mg,
34% yield). 1H NMR (CD2Cl2, 500 MHz): δ 6.88 (d, J = 7.1 Hz,
1H), 7.01 (s, 1H), 7.29 (dd, J = 5.4, 7.2 Hz, 1H), 7.46-7.54 (m,
9H), 7.60-7.64 (m, 6H). 13C NMR (CD2Cl2, 125 MHz): δ 21.5,
27.3, 125.2 (d, J = 6.3 Hz), 129.6 (d, J = 11.3 Hz), 131.7 (d, J =
2.5 Hz), 132.0, 134.9 (d, J = 13.8 Hz), 134.9, 139.2, 146.3, 170.3
(d, J = 114 Hz). 31P NMR (CD2Cl2, 161.9 MHz): δ 44.4. HRMS
(ESI): [M þ Na]þ calcd for C26H24AuPNa, 587.1179; found,
587.1171.
gold compound 1 (0.0210 mmol) and a stir bar. A separate
dram vial was charged with PhAuPPh3 (11.3 mg, 0.0210 mmol).
To the dram vial containing PhAuPPh3 was added dry CD2Cl2
(0.3 mL) via syringe, and the solution was transferred via
pipet to the dram vial containing organogold compound 1.
The vial previously containing PhAuPPh3 was rinsed with
additional CD2Cl2 (0.2 mL), and the rinse was added to the
vial containing the solution of organogold compound 1 and
PhAuPPh3. Using a gastight syringe, anhydrous 1.0 M HCl
3
Et2O (7.0 μL, 0.0070 mmol) was delivered to the vigorously
stirring solution, with the tip of the syringe below the solvent
level. The resulting solution was then transferred into an NMR
tube. The ratio of protodemetalated 2 to benzene, observed
1
by H NMR spectroscopy, was used to determine the relative
rates of protodemetalation. Relative rates of protodemetalation
for organogold compounds 1a- c and 1e were performed using
an analogous procedure (see Supporting Information for
details).
General Procedure for Protodemetalation Studies Using Dif-
ferent Acid Sources. In an inert-atmosphere glovebox, a dram
vial was charged with an organogold compound ( 1e, 1h, or 1n)
(0.0070 mmol) and dry CD2Cl2 (0.5 mL). Each organgold
compound in solution was subjected to one acid (PhCO2H,
PhSH, PhOH, or MeOH) in anhydrous ether (7.0 μL of 1.0 M
stock solution, 0.0070 mmol) delivered via gastight syringe. The
resulting solution was transferred into a J-Young tube, and
1
conversion to product ( 2e, 2h, or 2n) was monitored by H
NMR or proton-coupled 19F NMR spectroscopy at time inter-
vals of 10 min, 2 h, 24 h, and 48 h (results are tabulated in the
Supporting Information).
Competitive Trapping with MeAuPPh3. In an inert-atmo-
sphere glovebox, a dram vial was charged with 8 (3.5 mg,
0.021 mmol) and dissolved in CD2Cl2 (0.3 mL). The solution
was transferred to an NMR tube, the dram vial previously
containing 8 was rinsed with additional CD2Cl2 (0.2 mL), and
the rinse was transferred to the NMR tube. To the NMR tube
was added mesitylene (2.9 μL, 0.021 mmol) internal standard via
gastight syringe. An initial 1H NMR spectrum of 8 with
mesitylene was acquired in order to calibrate the ratio of starting
material 8 to mesitylene. In the glovebox, the solution of 8 and
mesitylene from the NMR tube was added to a vial containing
1c (10.0 mg, 0.0210 mmol), and the resulting solution was
transferred to a vial containing PPh3AuOTf (0.6 mg, 0.001
mmol). The solution of 8, 1c, PPh3AuOTf, and mesitylene
was transferred back to the NMR tube. The vials previously
containing the PPh3AuOTf and 1c were sequentially rinsed
with additional CD2Cl2 (0.1 mL), and the rinses were added
to the previous NMR tube solution. An 1H NMR spectrum
obtained after 10 min showed the presence of 1a (98%),
isobutylene, and methane. For procedure details on the con-
trol experiment without MeAuPPh3, see the Supporting Infor-
mation.
Synthesis of (2,6-Dimethylphenyl)gold(triphenylphosphine),
1k. Compound 1k was synthesized according to the above
procedure for 1i. Compound 1k was obtained as a white solid
(160 mg, 47% yield). 1H NMR (CD2Cl2, 500 MHz): δ 2.57 (s,
6H), 6.89 (t, J = 7.4 Hz, 1H), 7.01 (d, J = 8.0 Hz, 2H),
7.47-7.60 (m, 9H), 7.60-7.64 (m, 6H). 13C NMR (CD2Cl2,
125 MHz): δ 27.2 (d, J = 1.3 Hz), 125.6 (d, J = 6.3 Hz), 125.9,
129.6 (d, J = 11.3 Hz), 131.7 (d, J = 2.5 Hz), 132.0, 134.8 (d, J =
13.8 Hz), 146.1, 174.5 (d, J = 44.5 Hz). 31P NMR (CD2Cl2,
161.9 MHz): δ 44.7. HRMS (ESI): [M þ Na]þ calcd for
C26H24AuPNa, 587.1179; found, 587.1168.
Acknowledgment. We thank the NSF (CAREER
Award, CHE-0748312) and the University of California,
Irvine, for funding. We thank Mr. Yili Shi for helpful
discussions.
General Procedure for Competition Experiments. In an inert-
atmosphere glovebox, a dram vial was charged with an organo-
Supporting Information Available: Experimental procedures
and compound characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
(35) Shi, Y.; Ramgren, S. D.; Blum, S. A. Organometallics 2009, 28,
1275.