Imidazolidene carboxylate bound MBPh4 complexes (M = Li, Na) and their relevance in transcarboxylation reactions

Article abstract of DOI:10.1021/jo201647b

Combination of 1,3-bis(2,6-diisopropylphenyl)imidazolum-2-carboxylate (IPrCO₂) with the Lewis acids MBPh₄, where M = Li or Na, provided two separate complexes. The crystal structures of these complexes revealed that coordination to NaBPh₄ yielded a dimeric species, yet coordination of IPrCO₂ with LiBPh₄ yielded a monomeric species. Combination of 1,3-bis(2,4,6-trimethylphenyl)imidazolum-2-carboxylate (IMesCO₂) with LiBPh₄ also afforded a dimeric species that was similar in global structure to that of the IPrCO₂+NaBPh ₄ dimer. In all three cases, the cation of the organic salt was coordinated to the oxyanion of the zwitterionic carboxylate. Thermogravimetric analysis of the crystals demonstrated that decarboxylation occurred at lower temperatures than the decarboxylation temperature of the parent NHC·CO₂ (NHC = N-heterocyclic carbene). Kinetic analysis of the transcarboxylation of IPrCO₂ to acetophenone with NaBPh ₄ to yield sodium benzoylacetate was performed. First-order dependences were observed for IPrCO₂ and acetophenone, whereas zero -order dependence was observed for NaBPh₄. Direct dicarboxylation was observed when I^tBuCO₂ was added to MeCN in the absence of added MBPh₄.

Full text of DOI:10.1021/jo201647b

ARTICLE
pubs.acs.org/joc
Imidazolidene Carboxylate Bound MBPh4 Complexes (M = Li, Na) and
Their Relevance in Transcarboxylation Reactions
Bret R. Van Ausdall, Nils F. Poth, Virginia A. Kincaid, Atta M. Arif, and Janis Louie*
Department of Chemistry, Henry Eyring Building, University of Utah, 315 S. 1400 E, Salt Lake City, Utah 84112-0850, United States
S Supporting Information
b
ABSTRACT: Combination of 1,3-bis(2,6-diisopropylphenyl)-
imidazolum-2-carboxylate (IPrCO2) with the Lewis acids
MBPh4, where M = Li or Na, provided two separate complexes.
The crystal structures of these complexes revealed that coordi-
nation to NaBPh4 yielded a dimeric species, yet coordination of IPrCO2 with LiBPh4 yielded a monomeric species. Combination of
1,3-bis(2,4,6-trimethylphenyl)imidazolum-2-carboxylate (IMesCO2) with LiBPh4 also afforded a dimeric species that was similar in
global structure to that of the IPrCO2+NaBPh4 dimer. In all three cases, the cation of the organic salt was coordinated to the
oxyanion of the zwitterionic carboxylate. Thermogravimetric analysis of the crystals demonstrated that decarboxylation occurred at
lower temperatures than the decarboxylation temperature of the parent NHC CO2 (NHC = N-heterocyclic carbene). Kinetic
3
analysis of the transcarboxylation of IPrCO2 to acetophenone with NaBPh4 to yield sodium benzoylacetate was performed. First-
order dependences were observed for IPrCO2 and acetophenone, whereas zero -order dependence was observed for NaBPh4. Direct
dicarboxylation was observed when ItBuCO2 was added to MeCN in the absence of added MBPh4.
’ INTRODUCTION
by employing NHC CO2's as a trans-carboxylating reagent
3
(NHC = N-heterocyclic carbene, Scheme 1).40À42 The scope
was further expanded as other compounds containing acidic
α-protons, such as acetone, cyclohexanone, and benzylcyanide,
could be carboxylated. Although DBU can be used to faciliate
similar carboxylation reactions, the ambiguity by which DBU
interacts with CO2 has hampered the development of further
carboxylation chemistry.47,48 In contrast, the factors that influ-
ence binding of CO2 to NHCs are better understood.49À51 This,
in conjunction with the facile tunability of both the sterics and
electronics of NHCs,52 led us to believe that more effective
The ability to utilize carbon dioxide as a chemical feedstock
has been a long desired goal in synthetic chemistry.1À7 Every
year, billions of tons of CO2 are released into the atmosphere as
waste.8 The ability to harness CO2 from the point of origin into
high-yielding fine chemical processes would likely be a lucrative
one as the souce of carbon could be collected from point-source
waste streams. Yet, only a handful of solutions exist. For example,
Darensbourg9À11 and Coates12À14 independently developed
unique systems that copolymerize CO2 with epoxides that afford
biodegradable polycarbonate polymers that are adequate sub-
stitutes for bisphenol A based polymers. The synthesis of cyclic
carbonates from the reaction of epoxides and CO2 using various
catalysts has also received a large amount of attention in recent
years due to the industrial significance of cyclic carbonates.15À20
The Kolbe-Schmitt and Grignard reactions are relevant reactions
when discussing the topic of CO2 incorporation into fine
chemicals. However, both reactions are largely limited to phe-
nolic and halogenated substrates, respectively.21À23
Transition metal catalysts have found a home in CO2-
incorportation chemistry.24À28 A protocol utilizing a gold-
carbene species to carboxylate heteroaromatic and activated
nonphenolic aryl species has been developed.29 Ni30,31 catalysts
mediate the cycloaddition of CO2 with diynes32À34 as well as
the carboxylation of styrenes35,36 and alkylzinc compounds.37,38
Similarly, the combination of Cu and 1,10-phen catalyzes the
carboxylation of alkylboranes.39 In addition, a handful of non-
metal mediated incorporation of CO2 reactions that afford
noncyclic carbonate products have been developed.40À46
NHC CO2 mediated carboxylation reactions could be devel-
3
oped. As such, we embarked on an investigation of the mechan-
ism of the transcarboxylation reaction.
’ RESULTS AND DISCUSSION
Syntheses and Characterization of (NHC CO2) MBPh4
Complexes. Carboxylation of acetophenone requires NHC
3
3
3
CO2 as well as 1 equiv of MBPh4 (where M = Li, Na, or K).
Thus, precomplexation of the NHC CO2 with MBPh4 could
3
play an important role in carbon dioxide transfer. Although the
original transcarboxylation reactions reported by Tomassi uti-
lized 1,3-dimethylimidazolium-2-carboxylate (IMeCO2), the in-
solubitily of IMeCO2 led us to redirect our focus to more soluble
NHC CO2 adducts such as IPrCO2 and IMesCO2 (IPrCO2 =
3
1,3-bis(2,6-diisopropylphenyl)imidazolum-2-carboxylate; IMes-
CO2 = 1,3-bis(2,4,6-trimethylphenyl)imidazolium-2-carboxylate).
Importantly, transcarboxylation reactions employing either IPrCO2
Tommasi et al. demonstrated the fixing of carbon dioxide
onto acetophenone to form benzoyl acetate (1), methanol to
form monomethyl carbonate (2), and benzaldehyde to form 3
Received: August 9, 2011
Published: September 12, 2011
r
2011 American Chemical Society
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|

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ARTICLE
Scheme 1. Transcarboxylation Reactions Performed by Tommasi with NHC CO2's
3
Scheme 2. Formation of IPrCO2 MBPh4 Complexes
obtained in 95% yield (eq 1). Structural analysis of 6 revealed
that, in contrast to the reaction of IPrCO2 with LiBPh4, the
reaction with IMesCO2 with LiBPh4 afforded a dimeric complex.
3
Thus, although coordination of the NHC CO2 to the cation of
3
MBPh4 appears to be general, formation of either a monomer or
a dimer can only be determined a posteriori.
A variety of salts were added to a series of carboxylates to
examine possible trends in solubility (Table 1). All NHC CO2's
3
alone were insoluble in THF (entries 1À4) and were either
sparingly soluble in MeCN (entries 5, 6, 8) or reacted with
MeCN (entry 7, vide infra). When MX (where M+ = Li, Na, or K
and XÀ = BPh4) was added to a suspension of IMeCO2 in
MeCN, no reaction occurred and only a suspended solid
remained (entry 9). Addition of variety of salts to IPrCO2
resulted in homogeneous solutions (entries 10À13, 15). As
noted, addition of either LiBPh4 or NaBPh4 to IPrCO2 led to
the formation of isolable compounds (4 and 5, respectively) that
were amenable to crystallographic analysis (vide supra). The
addition of LiBF4, LiI, or NaI to IPrCO2 in MeCN also led to
homogeneous solutions (entries 13 and 15). Unfortunately, all
attempts in isolating compounds suitable for X-ray analysis were
unsuccessful. Interestingly, soluble complexes were not obtained
upon the addition of NaBF4, KBF4, or KI salts to IPrCO2 (entries
14 and 16). Homogeneous solutions were observed upon the
addition of LiBPh4 and LiI salts to IMesCO2 (entries 17 and 18).
Similarly, addition of LiBPh4 or NaBPh4 to ItBuCO2 also led to
homogeneous solutions (entry 19). However, a complex with
limited solubility formed when KBPh4 was added (entry 20).
or IMesCO2 afford product 1 in comparable yields to those
obtained with IMeCO2 (vide infra). Addition of NaBPh4 to a
suspension of IPrCO2 in THF53 led to a homogeneous solution.
A layer of ether was added and allowed slow diffusion into the
solution ultimately giving compound 4 as a crystalline solid in
96% yield (Scheme 2). Interestingly, compound 4 is a dimer
where the carboxylate acts as a bridging ligand between two Na
atoms. A similar carboxylate complex (5) was formed when
IPrCO2 was added to LiBPh4 in lieu of NaBPh4. However, single
crystal X-ray structural analysis showed that this complex was a
monomer, rather than a dimer. Again, the main group metal (i.e.,
Li) binds, in this case, to only one oxygen of a single carboxylate
1
group. Both complexes had distinctly different H NMR shifts
from noncomplexed IPrCO2.
In an effort to determine whether the formation of monomeric
or dimeric complexes was a general phenomenon for NHC
3
CO2+MBPh4 compounds, reactions with IMesCO2 were also
evaluated. When IMesCO2 was added to LiBPh4 in MeCN and
then crystallized via slow diffusion with ether, complex 6 was
Selected bond lengths and the NHC CO2 torsional angles for
3
compounds 4À6 as well as the parent IPrCO2 are listed in
Table 2. No structural data exists for IMesCO2. However, given
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ARTICLE
Table 1. Solubility of Salts and Carboxylates in Various Conditions
entry
NHC CO2
salt
solventa
THF
solubility
insolubleb
insolubleb
insolubleb
insolubleb
limitedc
3
1
IPrCO2
IMesCO2
ItBuCO2
IMeCO2
IPrCO2
IMesCO2
ItBuCO2
IMeCO2
IMeCO2
IPrCO2
IPrCO2
IPrCO2
IPrCO2
IPrCO2
IPrCO2
IPrCO2
IMesCO2
IMesCO2
ItBuCO2
ItBuCO2
none
none
none
none
none
none
none
none
2
THF
3
THF
4
THF
5
MeCN
6
MeCN
limited c
7
MeCN
reaction with MeCN
limitedc
8
MeCN
9
MBPh4 (M = Li, Na, K)
LiBPh4 (4)
NaBPh4 (5)
KBPh4
THF or MeCN
THF or MeCN
THF or MeCN
THF or MeCN
MeCN
insoluble
10
11
12
13
14
15
16
17
18
19
20
homogeneous soln
homogeneous soln
homogeneous solnd
homogeneous solnd
insoluble
LiBF4
MBF4 (M = Na, K)
Ml (M = Li, Na)
KI
MeCN
MeCN
homogeneous solnd
MeCN
insoluble
LiBPh4 (6)
LiI
THF or MeCN
MeCN
homogeneous soln
homogeneous solnd
homogeneous solnd
limitedc
MBPh4 (M = Li, Na)
KBPh4
THF
THF
a These solvents solvated either the NHCCO2 or the salt. b No proton signals were observed in THF-d8. c Proton signals were observed although the
sample did dissolve completely. d All attempts to grow crystals provided crystals of unsuitable quality for single crystal analysis.
Table 2. Bond Lengths and Torsional Angles of IPrCO2 and
Compounds 4, 5, and 6
IPrCO2+
IPrCO2+ IMesCO2+
bond lengths (Å)
C2ÀC6
IPrCO2 LiBPh4 (5) NaBPh4 (4) LiBPh4 (6)
1.510
1.222
1.225
NA
1.511
1.221
1.254
1.951
NA
1.525
1.239
1.233
2.244
2.236
1.344
1.333
27.66
1.515
1.232
1.235
1.906
1.887
1.342
1.338
36.47
C6ÀO2
C6ÀO1
O1ÀM1
O2ÀM2
N1ÀC2
N3ÀC2
NA
1.335
1.332
1.345
1.336
78.61
carboxylate torsional angle 89.75
N1C2ÀC6ÀO1 (deg)
Figure 1. TGA curves of IPrCO2, IPrCO2+LiBPh4 (4), and
IPrCO2+NaBPh4 (5).
the electronic and overall steric similarities between IMesCO2
and IPrCO2, the structure of dimer 6 was compared to that of
IPrCO2. In all cases, complexation to either Li or Na affects the
C6ÀO bond length and, to a lesser extent, the C2ÀC6 bond
length. In IPrCO2, the C6ÀO1 and C6ÀO2 bond lengths are
equivalent, which reflects the contributions of two equivalent
resonance structures. For dimers 4 and 6, the C6ÀO1 and
C6ÀO2 bond lengths are again equivalent, as expected, yet are
elongated with respect to uncomplexed IPrCO2. In general,
compounds 4À6 display both shortened CÀO bond lengths
LiÀO and NaÀO bond lengths of 1.950 and 2.451 Å, respec-
tively.
The most marked structural change from complexes with
either Na or Li is in the different torsional angles. That is, bind-
ing markedly lowers the N1ÀC2ÀC6ÀO1 torsional angle. For
monomer 5, the carboxylate moiety moved approximately 11°
toward planarity with the imidazolium ring (i.e., 89.75° in
IPrCO2 vs 78.61° in 5). An even more striking move toward
planarity was observed in dimers 4 and 6. Specifically, the
torsional angles decreased over 50° to 27.66° and 36.47°,
respectively.
ꢀ
(average =1.236 Å) as well as MÀO bond lengths [average =
ꢀ
ꢀ
1.915 Å for Li (5 and 6) and average =2.240 Å for Na (4)] relative
to known M-carboxylates.54À56 For example, the CÀO and
NaÀO bond lengths of a similar Na meta-iodobenzoate complex
We recently evaluated a series of NHC CO2 complexes and
3
found that decarboxylation correlated closely to torsional
angles.50 Carboxylates possessing a larger torsional angle under-
went decarboxylation at a lower temperature. As a consequence,
binding to Li or Na in compounds 4À6 could stabilize the
ꢀ
are 1.260/1.264 and 2.476 Å, respectively. Furthermore, anhy-
drous lithium and sodium formate crystals possess carboxylate
CÀO bond lengths of 1.242 and 1.246 Å, respectively, and the
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The Journal of Organic Chemistry
ARTICLE
carboxylate, thereby inhibiting decarboxylation. Surprisingly,
TGA analyses of the IPrCO2 MBPh4 and IMesCO2 LiBPh4
carboxylation reactions were first-order in acetophenone (Figure 3).
However, our investigations revealed that the reaction was first-
order in IPrCO2 yet independent of NaBPh4 concentration
(Figure 3). These results were particularly surprising given the
dimer coordination mode we obtained from the individual reaction
3
3
complexes revealed decarboxylation actually occurred at lower
temperatures than for the parent IPrCO2 or IMesCO2 (Figures 1
and 2). Decarboxylation of IPrCO2 occurs at 108 °C.49 In
contrast, both IPrCO2+LiBPh4 (5) and IPrCO2+NaBPh4 (4)
complexes lose CO2 at temperatures below 100 °C (76 and 81 °C,
respectively, Figure 1). In conjunction with the loss of CO2, loss of
coordinated solventmolecules (THF and ether) was also observed
at these temperatures. Thus, coordination to Li or Na significantly
lowers the temperature required for decarboxylation.
between IPrCO2 and NaBPh4 and the activation of the NHC CO2
3
with salts observed via TGA. Interestingly, an equilibrium kinetic
isotope effect of 1.57 ( 0.14 was observed when acetophenone was
replaced with acetophenone-d3 in a carboxylation reaction (entry 1
vs entry 9). In addition, a carboxylation reaction run under an
atmosphere of CO2 was an order of magnitude slower (entry 10).
The activation of IMesCO2 was also displayed in the IM-
esCO2+LiBPh4 complex 6 (Figure 2). The IMesCO2+LiBPh4
complex had three stages of weight loss, the first one occurring at
71 °C where CO2, ether, and THF were detected on the mass
spectrometer. This first decomposition is 84 °C lower than that
of IMesCO2. The TGA analysis indicates that there is activation
Deuterium Exchange Reactions. A series of exchange reac-
tions involving a 1:1 mixture of acetophenone and acetophe-
none-d3 were evaluated (eq 4, Table 5). The control reaction of
mixing acetophenone with acetophenone-d3 showed no ex-
change of deuterium, even at prolonged reaction times of 1 week
(entry 1). When 1 equiv of 1,3,4,5-tetramethylimidazolylidene
(IMeMe) was added to a C6D6 solution containing the control
mixture of acetophenone and acetophenone-d3, complete scram-
bling of the enol proton occurred within minutes (entry 2).
When IMeMe was substituted with the less basic IPr (calculated
pKa of 1,3-bis(2,6-dimethylphenyl)imidazolylidene = 17.0 (
0.11 in DMSO vs 23.9 ( 0.28 of IMeMe),57 facile scrambling
was still observed (entry 2), indicating that facile deprotonation/
reprotonation occurs regardless of which NHC catalyst is added.
of the NHC CO2 complexes where thermal decarboxylation is
3
facilitated relative to that of the parent NHC CO2. The tem-
3
perature at which each decomposition stage and the % of mass
lost at each stage of weight loss are listed in Table 3.
Carboxylation Reactions with (NHC CO2) MBPh4 Com-
plexes. Complex 4 was evaluated as a potential “all-in-one”
carboxylating agent. When stoichiometric amounts of 4 were
added to acetophenone in THF at 50 °C for 4 h, sodium
benzoylacetate was formed in 71% yield (eq 2).
3
3
Kinetic Analysis. Kinetic analysis of the IPrCO2/NaBPh4-
mediated carboxylation reaction of acetophenone at 50 °C in
THF-d8 was performed (eq 3, Table 4). Not surprisingly,
Reaction of IPr CO2 and TMS Enol. Given the lack of
3
NaBPh4 dependence determined from kinetic analysis, carboxyl-
ation reactions between a preformed enolate and IPrCO2 were
evaulated. Specifically, IPrCO2, TMS enol (7), and CsF were
combined in MeCN-d3 and heated to 50 °C (eq 5). After 24 h, no
reaction of the IPrCO2 was observed although complete con-
sumption of the TMS enol occurred. Similar results were
obtained when TBAF was used in lieu of CsF. In contrast, direct
carboxylation of acetophenone was achieved through the reac-
tion between the potassium enolate and dry ice at À78 °C to give
benzoyl acetic acid in 85% after acidic workup.58
Figure 2. TGA plots of IMesCO2 and IMesCO2+LiBPh4 (6).
Table 3. Mass Percent Lost and Temperatures at Each Stage of Decomposition
mass lost (%), temperature (°C)
IPrCO2+NaBPh4 (5)
decomposition stage
IPrCO2
IPrCO2+LiBPh4 (4)
IMesCO2+LiBPh4 (6)
IMesCO2
1
2
3
4
9.8, 108
90.9, 198
NA
6.2, 81
93.0, 339
NA
22.1, 76
17.9, 71
11.5, 115
52.1, 277
NA
9.6, 155
78.0, 216
NA
10.3, 239
54.5, 329
13.9, 430
NA
NA
NA
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ARTICLE
Table 4. Kinetic Analysis of the Carboxylation of
Acetophenonea
Table 5. NHC-Catalyzed H/D Exchange Reaction
conditions
[acetophenone],
equiv
[NaBPh4],
equiv
kobs rate constant
(Â 10À3 M sÀ1
b
entry catalyst
additives
solvent temp H/D scrambling (t1/2)
entry
)
3
1
2
3
4
5
6
7
none
IMeMe none
IPr none
IPrCO2 none
none
C6D6
C6D6
C6D6
rt
rt
rt
not observed
>5 min
1
2
0.22 M, 5
0.22 M, 5
0.22 M, 5
0.22 M, 5
0.22 M, 5
0.33 M, 7.5
0.44 M, 10
0.66 M, 15
0.22 M, 5c
0.22 M, 5
0.23 M, 5.3
0.35 M, 7.9
0.46 M, 10.5
0.69 M, 15.7
0.46 M, 10.5
0.46 M, 10.5
0.46 M, 10.5
0.46 M, 10.5
0.23 M, 5.3
0.23 M, 5.3
À7.8 ( 0.5
À7.7 ( 0.8
À7.8 ( 0.01
À8.0 ( 0.1
À7.8 ( 0.5
À10.5 ( 0.6
À13.7 ( 1.3
À23.9 ( 0.3
À4.9 ( 0.3
3 h
3
CD3CN 50 °C
CD3CN 50 °C
CD3CN 50 °C
50 min
NAa
4
IPrCO2 NaBPh4
IPrCO2 CO2 (g)
5
not observed
6
IPrCO2 NaBPh4, CO2 (g) CD3CN 50 °C
not observed
7
a Scrambling occurred, but slowly. Integration was not possible due to
overlap of the IPr septet in the acetophenone methyl region. The point
at which scrambling was detected was at 3 h.
8
9
10
À0.43 ( 0.06d
a Reaction conditions: [IPrCO2] = 0.043 M (1 equiv) in THF-d8, 50 °C.
b All runs were performed at least twice. Acetophenone-d3 was used.
c
’ CONCLUSION
d The reaction solution was sparged with CO2, and the reaction was ran
with a CO2 atmosphere.
Despite the isolation of a variety of interesting NHC
3
CO2+MBPh4 complexes, our data suggest that the complexes
do not remain aggregated during carboxylation. That is, carbox-
ylation reactions rates were independent NaBPh4 concentration.
Our evidence also points away from a mechanism that involves a
salt-assisted deprotonation of acetophenone (i.e., enhanced
acidity of the α-proton through precoordination of the Na+ to
either the carbonyl or the arene).59,60 In addition, facile carbox-
ylation of MeCN occurred in the absence of added salt. Thus, the
role of NaBPh4 may be to help bring the NHC CO2 into solution
3
such that it is available to react. Indeed, our investigations indicate
that the addition of salts to otherwise insoluble NHC CO2
3
compounds led to homogeneous solutions. Given the propensity
of the carboxylated product to undergo spontaneous decarbox-
ylation, the role of the NaBPh4 may also serve to stabilize the
product through ion-pairing. A proposed mechanism that is
analogous to carboxylation of MeCN is shown in Scheme 4.
Figure 3. Plots of Àln[acetophenone] vs Àln[NaBPh4] vs Àln(kobs
for the carboxylation of acetophenone at 50 °C.
)
Reaction of ItBuCO2 with Acetonitrile. Although no car-
boxylation products were detected between the reaction of
IPrCO2 and TMS enol 7, direct carboxylation of MeCN does
occur in the absence of NaBPh4. During our attempts to
recrystallize ItBuCO2 in MeCN and ether, single crystals of
a dianionic, dicarboxylated ketenimide product were obtained
(eq 6). A proton from the acetonitrile is covalently bound to
one oxygen (1.113 Å) and forms a strong hydrogen bond to
the other oxyanion (1.323 Å) between the planar dicarbox-
ylate moieties. A reaction mechanism for the formation of the
ketenimide is proposed in Scheme 3. Carbon dioxide dissocia-
tion from ItBuCO2 affords the free carbene, ItBu, which then
deprotonates the α-proton of MeCN to afford a ketenimide.
Nucleophilic attack of either free carbon dioxide (shown) or of
ItBuCO2 gives rise to the initial carboxylation observed. The
process is then repeated to ultimately afford dicarboxylated
product 8.
’ EXPERIMENTAL SECTION
General Procedures. All reactions and procedures were con-
ducted under an atmosphere of N2 using standard Schlenk techniques
or in a N2-filled glovebox unless otherwise noted. 1H and 13C nuclear
magnetic resonance spectra of pure compounds were acquired at 500
and 125 MHz, respectively, unless otherwise noted. All spectra are
referenced to residual solvent peaks. The abbreviations s, d, dd, dt, dq, t,
q, quint, sept, m stand for singlet, doublet, doublet of doublets, doublet
of triplets, doublet of quartets, triplet, quartet, quintet, septet, and
multiplet, respectively. All coupling constants, J, are reported in Hz.
All 13C NMR spectra were proton decoupled. All TGA analyses were
performed in a N2 atmosphere at a heating rate of 5 °C/min.
Nondeuterated solvents were purified and deoxygenated by passing
through packed silica columns. All oil from NaH was removed by
thorough washing with hexanes. LiBPh4(DME)3, KBPh4, LiI, NaI, KI,
LiBF4, NaBF4, and KBF4 were dried by placing in a 130 °C oven for
several days, further dried and cooled under a high vacuum over the solid
for 30 min, and stored in a N2-filled glovebox. NaBPh4 was dried by
dissolving in a minimal amount of THF and stirring with NaH for
30 min, filtering through Celite, and removing solvent in vacuo and stored
under nitrogen. All other reagents were purchased from the chem-
ical provider without further purification, unless specified. All NMR
solvents were thoroughly dried using standard procedures prior to use.
All NHC CO2's used were synthesized using previously reported
3
procedures.49 Carboxylation of acetophenone was performed using a
modification of an existing procedure.52 Deuterated solvents were
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ARTICLE
Scheme 3. Proposed Mechanism for the Formation of 8
Scheme 4. Proposed Reaction Mechanism for the Carboxylation of Acetophenone
purchased from Cambridge. CD3CN was dried and distilled from CaH2,
and THF-d8 was distilled from benzophenone-Na.
6.64 (t, 2H, J = 7.1), 3.57 (m, 3.4H), 3.35 (q, 0.6H, J = 7.0), 2.48 (sept.,
4H, J = 6.9), 1.73 (m, 3.4H), 1.22 (d, 12H, J = 6.8), 1.17 (d, 12H,
J = 7.0), 1.08 (t, 1.1H, J = 7.0) 13C NMR (THF-d8, ppm) δ 165.1, 164.8,
164.4, 155.8, 146.2, 145.8, 136.9, 132.7, 131.6, 124.5, 125.46, 125.44,
125.42, 125.40, 124.8, 124.7, 124.5, 68.0, 29.8, 26.1, 24.3, 23.3, 15.4.
Preparation of [(IMesCO2Li)2]2+2[BPh4]2À (6). In a 5 mL dram vial,
IMesCO2 (0.020 g, 57 μmol, 1 equiv) was weighed and dissolved in a
minimal amount of dry THF (or MeCN). In another 5 mL dram vial,
LiBPh4(DME)3 (0.036 g, 59 μmol, 1.03 equiv) was weighed and
dissolved in a minimal amount of THF (or MeCN). The saturated
solution of IPrCO2 was added to the saturated solution of LiBPh4-
(DME)3, and the vial was placed into an empty 25 mL dram vial. Ether
was added to the 25 mL vial, and the vial was capped. Within 12 h, slow
diffusion of ether into the THF solution containing IPrCO2 and NaBPh4
afforded 6 (33 mg, 85% yield) as colorless crystals. 1H NMR (THF-d8,
ppm) δ 7.26 (m, 8H), 7.07 (s, 2H), 7.02 (s, 4H), 6.81 (t, 8H, J = 7.1),
6.67 (t, 4H, J = 7.1), 3.61 (m, 6H), 3.39 (q, 0.8H, J = 7.0), 2.32 (s, 6H),
2.09 (s, 12H), 1.77 (m, 6H), 1.18 (t, 1.2H, J = 7.0). 13C NMR (THF-d8,
ppm) δ 165.5, 165.1, 164.7, 164.4, 156.2, 145.9, 142.1, 141.2, 136.9,
135.7, 134.9, 132.6, 131.6, 130.4, 129.7, 169.6, 125.5, 125.4, 126.04,
122.9, 121.7, 121.5, 68.0, 26.2, 20.9, 20.8, 17.4, 17.2.
Combination of IPrCO2+KBPh4. IPrCO2 (0.020 g, 46 μmol, 1equiv)
and KBPh4 (0.018 g, 48 μmol, 1.05 equiv) were weighed out into
separate 5 dram vials and then mixed using a minimal amount of dry
THF-d8 until a homogeneous solution was obtained. 1H NMR (THF-d8,
ppm) δ 7.46 (t, 2H, J = 7.8), 7.44 (s, 2H), 7.32 (d, 4H, J = 7.8), 7.28 (m,
8H), 6.81 (t, 8H, J = 7.5), 6.67 (t, 4H, J = 7.2), 2.54 (sept, 4H, J = 6.8),
1.25 (d, 12H, J = 6.8), 1.20 (d, 12H, J = 6.8). 13C NMR (THF-d8, ppm) δ
163.9, 163.5, 163.2, 162.8, 144.4, 135.3, 131.4, 129.8, 132.9, 123.8, 123.0,
122.5, 119.9, 28.2, 22.7, 21.9.
Syntheses of NHC CO2+MX complexes. Preparation of
2À 3
[(IPrCO2Na)2]2+2[BPh4] (4). In a 5 mL dram vial, IPrCO2 (0.025 g,
57 μmol, 1 equiv) was weighed and dissolved in a minimal amount of dry
THF (or MeCN). In another 5 mL dram vial, NaBPh4 (0.020 g,
59 μmol, 1.03 equiv) was weighed and dissolved in a minimal amount
of THF (or MeCN). The saturated solution of IPrCO2 was added to the
saturated solution of NaBPh4, and the vial was placed into an empty
25 mL dram vial. Ether was added to the 25 mL vial, and the vial was
capped. Within 12 h, slow diffusion of ether into the THF solution
containing IPrCO2 and NaBPh4 afforded 4 (40 mg, 88% yield) as
colorless crystals. 1H NMR (THF-d8, ppm) δ 7.43 (t, 2H, J = 7.8), 7.41
(s, 2H), 7.27 (d, 4H, J = 7.8), 7.23 (m, 8H), 6.77 (t, 8H, J = 7.5), 6.62 (t,
2H, J = 7.2), 3.36 (q, 3H, J = 7.0), 2.49 (sept, 4H, J = 6.9), 1.19 (d, 12H,
J = 6.8), 1.16 (d, 12H, J = 7.0), 1.08 (t, 5H, J = 7.1). 13C NMR (THF-d8,
ppm) δ 165.6, 165.2, 164.8, 164.4, 147.3, 146.2, 145.8, 137.4, 137.1, 136.9,
136.6, 132.5, 131.6, 130.97, 125.7, 125.2, 124.6, 124.1, 123.8, 122.2, 121.7,
121.4, 121.0, 68.0, 66.11, 30.1, 29.85, 26.1, 24.4, 24.3, 23.5, 23.41, 15.53, 15.44.
Preparation of [IPrCO2Li]+[BPh4]À (5). In a 5 mL dram vial, IPrCO2
(0.025 g, 57 μmol, 1 equiv) was weighed and dissolved in a minimal
amount of dry THF (or MeCN). In another 5 mL dram vial, LiBPh4-
(DME)3 (0.036 g, 59 μmol, 1.03 equiv) was weighed and dissolved in a
minimal amount of THF (or MeCN). The saturated solution of IPrCO2
was added to the saturated solution of LiBPh4(DME)3, and the vial was
placed into an empty 25 mL dram vial. Ether was added to the 25 mL vial,
and the vial was capped. Within 12 h, slow diffusion of ether into the THF
solution containing IPrCO2 and NaBPh4 afforded 5 (39 mg, 95% yield)
as colorless crystals. 1H NMR (THF-d8, ppm) δ 7.46 (t, 2H, J = 7.8),
7.42 (s, 2H), 7.31 (d, 4H, J = 7.8), 7.23 (m, 8H), 6.79 (t, 8H, J = 7.3),
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The Journal of Organic Chemistry
ARTICLE
Combination of IPrCO2+LiBF4. IPrCO2 (0.020 g, 46 μmol, 1 equiv)
and LiBF4 (0.005 g, 48 μmol, 1.05 equiv) were weighed out into separate
5 dram vials and then mixed using a minimal amount of dry CD3CN
until a homogeneous solution was obtained. 1H NMR (CD3CN, ppm) δ
7.54 (s, 2H), 7.52 (t, 2H, J = 7.8), 7.34 (d, 4H, J = 7.8), 2.38 (sept, 4H, J =
6.8), 1.18 (d, 12H, J = 6.9), 1.20 (d, 12H, J = 6.7). 13C NMR (THF-d8,
ppm) δ 155.0, 146.4, 145.6, 144.8, 133.3, 131.7, 131.5, 130.8, 125.6,
125.4, 125.1, 124.8, 30.1, 29.9, 24.5, 24.5, 23.6, 23.5.
Combination of IPrCO2+LiI. IPrCO2 (0.020 g, 46 μmol, 1 equiv) and
LiI (0.007 g, 48 μmol, 1.05 equiv) were weighed out into separate 5 dram
vials and then mixed using a minimal amount of dry CD3CN until a
homogeneous solution was obtained. 1H NMR (CD3CN, ppm) δ 7.60
(s, 2H), 7.52 (t, 2H, J = 7.8), 7.34 (d, 4H, J = 7.8), 2.36 (sept, 4H, J = 6.8),
1.17 (d, 12H, J = 6.5), 1.13 (d, 12H, J = 6.7). 13C NMR (THF-d8, ppm) δ
155.5, 146.9, 146.1, 133.7, 132.2, 132.0, 131.3, 127.7, 127.5, 126.2, 126.0,
125.6, 125.3, 30.6, 30.4, 25.2, 25.1, 25.0, 24.4, 24.3, 24.1, 24.0.
Combination of IPrCO2+NaI. IPrCO2 (0.020 g, 46 μmol, 1 equiv)
and NaI (0.008 g, 48 μmol, 1.05 equiv) were weighed out into separate
5 dram vials and then mixed using a minimal amount of dry CD3CN
until a homogeneous solution was obtained. 1H NMR (CD3CN, ppm) δ
7.53 (s, 2H), 7.53 (t, 2H, J = 7.8), 7.37 (d, 4H, J = 7.8), 2.48 (sept, 4H, J =
6.9), 1.22 (d, 12H, J = 6.9), 1.20 (d, 12H, J = 6.9). 13C NMR (THF-d8,
ppm) δ 155.5, 147.3, 146.9, 146.1, 133.9, 132.4, 132.2, 131.4, 125.0,
124.8, 30.8, 30.7, 30.6, 30.5, 30.2, 25, 0.9, 25.1, 25.0, 24.9, 25.2, 24.1, 23.2.
Combination of IMesCO2+LiI. IMesCO2 (0.030 g, 86 μmol, 1 equiv)
and LiI (0.012 g, 90 μmol, 1.05 equiv) were weighed out into separate 5
dram vials and then mixed using a minimal amount of dry CD3CN until a
homogeneous solution was obtained. 1H NMR (CD3CN, ppm) δ 7.51
(s, 2H), 7.08 (s, 4H), 7.37 (d, 4H, J = 7.8), 2.34 (sept, 6H), 2.05 (s, 12H).
13C NMR (THF-d8, ppm) δ 154.9, 143.2, 141.5, 140.6, 137.5, 134.8,
134.6, 131.8, 130.8, 130.0, 129.8, 129.6, 129.4, 129.1, 128.8, 128.6, 125.0,
124.8, 123.6, 123.4, 123.2, 122.9, 20.40, 20.32, 20.26, 16.9, 16.8, 16.6, 16.4.
Combination of ItBuCO2+LiBPh4. ItBuCO2 (0.030 g, 130 μmol,
1 equiv) and LiBPh4(DME)3 (0.083 g, 140 μmol, 1.05 equiv) were
weighed out into separate 5 dram vials and then mixed using a minimal
amount of dry THF-d8 until a homogeneous solution was obtained. 1H
NMR (THF-d8, ppm) δ 7.29 (m, 8H), 7.01 (s, 2H), 6.86 (s, 8H), 6.72 (t,
4H, J = 7.2), 3.42 (s, 4H), 3.26 (s, 6H), 1.58 (s, 18H). 13C NMR (THF-
d8, ppm) δ 165.4, 164.2, 164.7, 164.5, 161.5, 144.5, 137.0, 136.9, 125.5,
121.7, 117.8, 117.6, 72.5, 62.4, 60.8, 58.7, 29.7, 29.6.
46 μmol, 1 equiv), NaBPh4 (0.166 g, 485 μmol, 10.5 equiv), and
trimethoxybenzene (0.035 g, 21μmol, 5 equiv) were weighedinto separate
5 dram vials. Dry THF-d8 was then used to dissolve the all three solids. The
solution was transferred to a NMR tube, and the vials were rinsed
thoroughly to ensure complete transfer of the IPrCO2, NaBPh4, and
TMB. The final volume of this solution was 1.00 mL. To the 1.0 mL of
solution was added 54 μL of acetophenone to make a 1.054 mL solution.
The solution was sealed with parafilm, mixed thoroughly, and placed into
an ice bath. After heating the NMR spectrometer to 50.1 °C, the sample
was inserted and initial rates of the reaction were measured.
Order in NaBPh4. Three additional samples were prepared and
evaluated in an analogous method as described above (i.e., Order in
IPrCO2). The concentration of IPrCO2 (0.020 g, 46 μmol, 1 equiv),
acetophenone (27 μL, 231 μmol, 5 equiv), and trimethoxybenzene
(0.035 g, 21 μmol, 5 equiv) were kept constant for each sample. The
concentration NaBPh4 varied as follows: Sample 1 = 0.083 g, 243 μmol,
5.3 equiv; Sample 2 = 0.124 g, 364 μmol, 7.9 equiv; Sample 3 = 0.249 g,
728 μmol, 15.8 equiv. The following pseudo-first-order rate constants
were obtained at different concentrations of acetophenone (Àk, [NaBPh4]),
respectively: 7.8 Â 10À3 M sÀ1, 0.23 M; 7.7 Â 10À3 M sÀ1, 0.35 M;
3
3
8.1 Â 10À3 M sÀ1, 0.69 M.
3
Order in Acetophenone. Three additional samples were prepared and
evaluated in an analogous method as described above (i.e., Order in
IPrCO2). The concentration of IPrCO2 (0.020 g, 46 μmol, 1 equiv),
NaBPh4 (0.166 g, 485 μmol, 10.5 equiv), and trimethoxybenzene (0.035 g,
21 μmol, 5 equiv) were kept constant for each sample. The concentration
acetophenone varied as follows: Sample 1 = 40 μL, 346 μmol, 7.5 equiv;
Sample 2 = 54 μL, 460 μmol, 10 equiv; Sample 3 = 81 μL, 693 μmol,
15 equiv. The following pseudo-first-order rate constants were obtained at
different concentrations of acetophenone (Àk, [acetophenone]), respec-
tively: 7.8 Â 10À3 M sÀ1, 0.22 M; 14.7 Â 10À3 M sÀ1, 0.44 M; 24.1 Â
3
3
10À3 M sÀ1, 0.66 M.
3
Effect of CO2 (g) on Reaction. An NMR sample containing IPrCO2
(0.020 g, 46 μmol, 1 equiv), NaBPh4 (0.083 g, 243 μmol, 5.3 equiv),
acetophenone (27 μL, 231 μmol, 5 equiv), and trimethoxybenzene
(0.035 g, 21 μmol, 5 equiv) in 1 mL dry THF-d8 (Total volume is
1.054 mL) was prepared analogously to the method described above.
The solution was cooled and the N2 atmosphere was removed and
replaced with CO2 three times. The sample was inserted into a
preheated NMR spectrometer at 50.1 °C and the initial loss of IPrCO2
was monitored to give the following pseudo-first-order rate constants
Combination of ItBuCO2+NaBPh4. ItBuCO2 (0.030 g, 130 μmol,
1 equiv) and NaBPh4 (0.048 g, 140 μmol, 1.05 equiv) were weighed out
into separate 5 dram vials and then mixed using a minimal amount of dry
THF-d8 until a homogeneous solution was obtained. 1H NMR (THF-d8,
ppm) δ 7.25 (m, 8H), 7.08 (s, 2H), 6.81 (t, 8H, J = 7.4), 6.67 (t, 4H, J =
7.2), 1.63 (s, 18H). 13C NMR (THF-d8, ppm) δ 165.6, 165.2, 164.8, 164.4,
161.9, 145.6, 137.1, 136.9, 125.5, 121.7, 117.3, 117.2, 68.0, 62.2, 60.8, 29.7.
Combination of ItBuCO2+KBPh4. ItBuCO2 (0.030 g, 130 μmol,
1 equiv) and KBPh4 (0.050 g, 140 μmol, 1.05 equiv) were weighed out
into separate 5 dram vials and then mixed using a minimal amount of dry
THF-d8 until a homogeneous solution was obtained. 1H NMR (THF-d8,
ppm) δ 7.29 (m, 8H), 7.13 (s, 2H), 6.86 (t, 8H, J = 7.4), 6.71 (t, 4H, J =
7.2), 1.52 (s, 18H). 13C NMR (THF-d8, ppm) δ 165.7, 165.3, 164.9, 164.5,
161.6, 146.8, 137.1, 137.0, 125.7, 121.8, 63.1, 62.1, 60.9, 60.3, 29.7, 29.5.
Preparation of 2[ItBuH]2+[dicarboxylatoketenimide]2À (8). In a
5 mL dram vial, ItBuCO2 (0.050 g, 0.222 mmol, 1 equiv) was weighed
and dissolved in a minimal amount of dry MeCN. The vial was placed
into an empty 25 mL dram vial. Ether was added to the 25 mL vial, and
the vial was capped. Within 12 h, slow diffusion of ether into the THF
were: 0.39 Â 10À3 M sÀ1 and 0.43 Â 10À3 M sÀ1
.
3
3
’ ASSOCIATED CONTENT
Supporting Information. X-ray, 1H NMR, and 13C
S
b
NMR data for all compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: louie@chem.utah.edu.
’ ACKNOWLEDGMENT
We gratefully acknowledge the Department of Energy and the
NSF for supporting this research.
1
solution afforded 8 (40 mg, 37% yield) as colorless crystals. H NMR
’ REFERENCES
(1) Carbon Dioxide as Chemical Feedstock; Aresta, M., Ed.; Wiley-
VCH: Weinheim, Germany, 2010.
(2) Patil, Y. P.; Tambade, P. J.; Jagtap, S. R.; Bhanage, B. M. Front.
Chem. Eng. China 2010, 4, 213.
(CD3CN, ppm) δ 9.73 (s, 1H), 7.73 (s, 2H), 1.67 (s, 18H). 13C NMR
(MeCN-d3, ppm) δ 164.5, 133.6, 120.9, 120.3, 61.1, 29.9, 27.7.
Pseudo-First -Order Kinetic Studies with IPrCO2+NaB-
Ph4+Acetophenone in THF-d8. Order in IPrCO2. IPrCO2 (0.020 g,
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The Journal of Organic Chemistry
ARTICLE
(3) Miller, J. E.; Diver, R. B.; , N. P.; Coker, E. N.; Ambrosini, A.;
Dedrick, D. E.; Allendorf, M. D.; McDaniel, A. H.; Kellogg, G. L.; Hogan,
R. E.; Chen, K. S.; Stechel, E. B. In Energy Technology 2010: Conservation,
Greenhouse Gas Reduction and Management, Alternative Energy Sourcesy;
Wiley: New York, 2010; p 27.
(41) Tommasi, I.; Sorrentino, F. Tetrahedron Lett. 2006, 47, 6453.
(42) Tommasi, I.; Sorrentino, F. Tetrahedron Lett. 2009, 50, 104.
(43) Kayaki, Y.; Yamamoto, M.; Ikariya, T. Angew. Chem., Int. Ed.
2009, 48, 4194.
(44) Paddock, R. L.; Nguyen, S. T. J. Am. Chem. Soc. 2001, 123,
(4) Mikkelsen, M.; Jorgensen, M.; Krebs, F. C. Energy Environ. Sci.
2010, 3, 43.
(5) Sakakura, T.; Choi, J.-C.; Yasuda, H. Chem. Rev. 2007, 107, 2365.
(6) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388.
(7) Coates, G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43,
6618.
11498.
(45) Haruki, E.; Arakawa, M.; Matsumura, N.; Otsuji, Y.; Imoto, E.
Chem. Lett. 1974, 427.
(46) Mizuno, T.; Ishino, Y. Tetrahedron 2002, 58, 3155.
(47) Heldebrant, D. J.; Jessop, P. G.; Thomas, C. A.; Eckert, C. A.;
Liotta, C. L. J. Org. Chem. 2005, 70, 5335.
(8) Tans, P. NOAA/ESRL; www.esrl.noaa.gov/gmd/ccgg/trends.
(9) Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995,
28, 7577.
(48) Villiers, C.; Dognon, J.-P.; Pollet, R.; Thuery, P.; Ephritikhine,
M. Angew. Chem., Int. Ed. 2010, 49, 3465.
(49) Duong, H. A.; Tekavec, T. N.; Arif, A. M.; Louie, J. Chem.
Commun. 2004, 112.
(10) Darensbourg, D. J.; Moncada, A. I.; Choi, W.; Reibenspies, J. H.
J. Am. Chem. Soc. 2008, 130, 6523.
(50) Van Ausdall, B. R.; Glass, J. L.; Wiggins, K. M.; Aarif, A. M.;
(11) Wu, G.-P.; Wei, S.-H.; Lu, X.-B.; Ren, W.-M.; Darensbourg,
D. J. Macromolecules 2010, 43, 9202.
(12) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc.
1998, 120, 11018.
(13) Cohen, C. T.; Chu, T.; Coates, G. W. J. Am. Chem. Soc. 2005,
127, 10869.
Louie, J. J. Org. Chem. 2009, 74, 7935.
(51) Zhou, H.; Zhang, W.-Z.; Liu, C.-H.; Qu, J.-P.; Lu, X.-B. J. Org.
Chem. 2008, 73, 8039.
(52) Benhamou, L.; Chardon, E.; Lavigne, G.; Bellemin-Laponnaz,
S. p.; Cꢀesar, V. Chem. Rev. 2011, 111, 2705.
(53) MeCN can be used in lieu of THF.
(14) Kim, J. G.; Cowman, C. D.; LaPointe, A. M.; Wiesner, U.;
Coates, G. W. Macromolecules 2011, 44, 1110.
(15) North, M.; Villuendas, P.; Young, C. Chem.—Eur. J. 2009,
15, 11454.
(54) Van, D. R.; Ramaekers, J.; Nockemann, P.; Van, H. K.; Van,
M. L.; Binnemans, K. Eur. J. Inorg. Chem. 2005, 563.
(55) Kansikas, J.; Hermansson, K. Acta Crystallogr., Sect. C: Cryst.
Struct. Commun. 1989, C45, 187.
(16) Sit, W. N.; Ng, S. M.; Kwong, K. Y.; Lau, C. P. J. Org. Chem.
2005, 70, 8583.
(17) Xie, Y.; Ding, K.; Liu, Z.; Li, J.; An, G.; Tao, R.; Sun, Z.; Yang, Z.
Chem.—Eur. J. 2010, 16, 6687.
(56) Markila, P. L.; Rettig, S. J.; Trotter, J. Acta Crystallogr., Sect. B:
Struct. Crystallogr. Cryst. Chem. 1975, B31, 2927.
(57) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004,
126, 8717.
(18) Huang, J.-W.; Shi, M. J. Org. Chem. 2003, 68, 6705.
(19) Lu, X.-B.; Liang, B.; Zhang, Y.-J.; Tian, Y.-Z.; Wang, Y.-M.; Bai,
C.-X.; Wang, H.; Zhang, R. J. Am. Chem. Soc. 2004, 126, 3732.
(20) Lichtenwalter, M.; Cooper, J. F. Jefferson Chemical Co., Inc.
1956.
(21) Gilman, H.; Kirby, R. H. Org. Synth. 1932, Collect. Vol. I, 353.
(22) Lindsey, A. S.; Jeskey, H. Chem. Rev. 1957, 57, 583.
(23) Snaddon, T. N.; Buchgraber, P.; Schulthoff, S.; Wirtz, C.;
Mynott, R.; Fuerstner, A. Chem.—Eur. J. 2010, 16, 12133.
(24) Zhang, Y.; Riduan, S. N. Angew. Chem., Int. Ed. 2011, 50, 6210.
(25) Ohishi, T.; Zhang, L.; Nishiura, M.; Hou, Z. Angew. Chem., Int.
Ed. 2011, 50, 8114.
(58) The preparation was modified from the following reference,
where KOtBu was used instead of LDA: Flowers, B. J.; Gautreau-Service,
R.; Jessop, P. G. Adv. Synth. Catal. 2008, 350, 2947.
(59) House, H. O.; Chu, C.-Y. J. Org. Chem. 1976, 41, 3083.
(60) Crist, D. R.; Hsieh, Z. H.; Quicksall, C. O.; Sun, M. K. J. Org.
Chem. 1984, 49, 2478.
(26) Huang, K.; Sun, C.-L.; Shi, Z.-J. Chem. Soc. Rev. 2011, 40, 2435.
(27) Fujihara, T.; Xu, T.; Semba, K.; Terao, J.; Tsuji, Y. Angew.
Chem., Int. Ed. 2011, 50, 523.
(28) Ohmiya, H.; Tanabe, M.; Sawamura, M. Org. Lett. 2011, 13,
1086.
(29) Boogaerts, I. I. F.; Nolan, S. P. J. Am. Chem. Soc. 2010, 132,
8858.
(30) Wu, J.; Hazari, N.; Incarvito, C. D. Organometallics 2011,
30, 3142.
(31) Lee, C. H.; Laitar, D. S.; Mueller, P.; Sadighi, J. P. J. Am. Chem.
Soc. 2007, 129, 13802.
(32) Tsuda, T.; Sumiya, R.; Saegusa, T. Synth. Commun. 1987, 17,
147.
(33) Louie, J.; Gibby, J. E.; Farnworth, M. V.; Tekavec, T. N. J. Am.
Chem. Soc. 2002, 124, 15188.
(34) Tekavec, T. N.; Arif, A. M.; Louie, J. Tetrahedron 2004, 60,
7431.
(35) Dalton, D. M.; Rovis, T. Nat. Chem. 2010, 2, 710.
(36) Williams, C. M.; Johnson, J. B.; Rovis, T. J. Am. Chem. Soc. 2008,
130, 14936.
(37) Ochiai, H.; Jang, M.; Hirano, K.; Yorimitsu, H.; Oshima, K. Org.
Lett. 2008, 10, 2681.
(38) Yeung, C. S.; Dong, V. M. J. Am. Chem. Soc. 2008, 130, 7826.
(39) Ohmiya, H.; Tanabe, M.; Sawamura, M. Org. Lett. 2011,
13, 1086.
(40) Tommasi, I.; Sorrentino, F. Tetrahedron Lett. 2005, 46, 2141.
8420
dx.doi.org/10.1021/jo201647b |J. Org. Chem. 2011, 76, 8413–8420