Insertion of reactive rhodium carbenes into boron-hydrogen bonds of stable N-heterocyclic carbene boranes

Article abstract of DOI:10.1021/ja4056245

Readily available rhodium(II) salts catalyze reactions between NHC-boranes (NHC-BH₃) and diazocarbonyl compounds (N₂CRCOR′). Stable α-NHC-boryl carbonyl compounds (NHC-BH₂-CHRCOR′) are isolated in good yields. The reaction is a reliable way to make boron-carbon bonds with good tolerance for variation in both the NHC-borane and diazocarbonyl components. It presumably occurs by insertion of a transient rhodium carbene into a boron-hydrogen bond of the NHC-borane. Competitive experiments show that a typical NHC-borane is highly reactive toward rhodium carbenes.

Full text of DOI:10.1021/ja4056245

Article
pubs.acs.org/JACS
Insertion of Reactive Rhodium Carbenes into Boron−Hydrogen
Bonds of Stable N‑Heterocyclic Carbene Boranes
Xiben Li and Dennis P. Curran*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
S
* Supporting Information
ABSTRACT: Readily available rhodium(II) salts catalyze
reactions between NHC-boranes (NHC-BH3) and diazocar-
bonyl compounds (N2CRCOR′). Stable α-NHC-boryl carbonyl
compounds (NHC-BH2−CHRCOR′) are isolated in good
yields. The reaction is a reliable way to make boron−carbon
bonds with good tolerance for variation in both the NHC-
borane and diazocarbonyl components. It presumably occurs by
insertion of a transient rhodium carbene into a boron−hydrogen bond of the NHC-borane. Competitive experiments show that a
typical NHC-borane is highly reactive toward rhodium carbenes.
boranes are Lewis acids that are not compatible with many
functional groups.
INTRODUCTION
■
N-heterocyclic carbene-boranes (NHC-boranes) differ in
fundamental respects from other classes of borane-Lewis base
complexes.1 Complexes of the parent borane (NHC-BH3) are
remarkably robust. They are typically white solids that are
stable to air and water, strong base, and mild acid. They resist
dissociation to release reactive BH3 even under relatively
forcing conditions.
NHC-boranes with additional boron-substituents (NHC-
BH2R, NHC-BHR1R2) can be made by direct complexation of
NHCs with substituted boranes whenever the needed boranes
(BH2R, BHR1R2) can be accessed by hydroboration or other
means (Figure 1a, left side).2 But when R is functionalized, the
complexation route is often not practical because trivalent
Various functionalized NHC-boranes can be made from the
parent NHC-BH3 complexes (Figure 1a, right side) by direct
substitution reactions with electrophiles and by reactions of
appropriately activated NHC-boranes with nucleophiles.3
Reactions of NHC-boryl radicals,4 and boryl anions5 also
offer opportunities to make functionalized NHC-boranes, as do
hydroboration reactions catalyzed by borenium ions or metals.6
Recently, we became interested in studying unknown α-
NHC-boryl carbonyl compounds, NHC-BHRCH2COR′.7
These might rearrange by 1,3-boryl shift to boron enolates8
or behave as nucleophiles themselves. They can also be
considered as unusual analogs of protonated α-amino ketones,
acids, and esters (+NH3CHRCOR′) where a tetravalent atom
with a negative formal charge (B) replaces one with a positive
formal charge (N).9 NHC-boranes are good hydride donors,10
so we hypothesized that they would also be good
carbenophiles, reacting especially with transient electrophilic11
carbenes (:CHRCOR′) or metal carbenes (MCHRCOR′) by
direct B−H insertion (Figure 1b).
Insertions into hydrogen-element bonds are defining
reactions of transient carbenes.12 Although bimolecular
reactions of carbenes and metal carbenes with C−H bonds
often occur with low selectivities and low yields, intramolecular
insertions are high value synthetic transformations.13 In
addition, Davies has discovered that selective bimolecular
insertions into activated C−H bonds are often possible with
rhodium carbenes bearing one acceptor and one donor
(typically aryl or alkenyl) substituent.14
In contrast, the literature on reactions of boron−hydrogen
bonds with carbenes is sparse. Carbethoxycarbene (generated
by irradiation of ethyl diazoacetate) inserts into the B−H bond
Figure 1. (a) B-functionalized NHC-boranes are made by complex-
ation of trivalent boranes or by functionalization of parent NHC-
boranes and (b) proposed B−H insertion reactions of carbenes or
metal carbenes (M = metal) to make α-NHC-boryl carbonyl
compounds.
Received: June 5, 2013
Published: July 18, 2013
© 2013 American Chemical Society
12076
dx.doi.org/10.1021/ja4056245 | J. Am. Chem. Soc. 2013, 135, 12076−12081

Journal of the American Chemical Society
Article
of an o-carborane in low yield,15 while Fisher alkynylcarbene
complexes react with NaCNBH3 by insertion.16 And dichlor-
ocarbene (generated by α-elimination reactions) inserts into
B−H bonds of amine- and phosphine-boranes.17 However, the
resulting products L-BH2CHCl2 (L = amine or phosphine) are
subject to decomplexation and internal redox reactions and are
not especially stable. The samarium carbenes derived from
CH2I2 and CH3CHI2 insert into B−H bonds of phosphine-
boranes although P−H insertions are faster.18 A recurring
theme in this reactive carbene chemistry is the use of large
excesses (7−40 equiv) of carbene-generating reagents.17,18
Here we report that rhodium-catalyzed reactions of
diazocarbonyl compounds,13,14,19 with readily available NHC-
boranes give rise to diverse α-NHC-boryl carbonyl compounds.
These transformations presumably occur by B−H insertion
reactions of transient rhodium carbenes, and the B−H bonds
are far more reactive than common activated C−H bonds. The
α-NHC-boryl carbonyl products are stable, and so far they have
exhibited no tendency to rearrange to NHC-boryl enolates.
borane component and are not corrected for recovered starting
material unless indicated.) That 3a is an α-NHC-boryl acetate
(C−B bond) and not an NHC-boryl enol (O−B bond) was
clear from the chemical shifts in the NMR spectra. The
resonances for the CH2 group adjacent to boron were shielded
in both the 1H (1.62 ppm, broad) and 13C (24.4 ppm, q, JCB
=
32 Hz) NMR spectra. And the ester carbonyl carbon of 3a
resonated at 181.1 ppm in 13C NMR spectrum.
In contrast to the convenient isolation of monoinsertion
product 3a, the very polar double insertion product 4a did not
emerge from the column even when 100% ethyl acetate was
used as eluent.22 Its yield was estimated at 14% based on the
11B NMR spectrum of the crude product. A similar reaction
with the more reactive Rh2(esp)2 catalyst (entry 2) gave
generally similar results to Rh2(OAc)4. Here the target product
3a was also isolated in 62% yield and the estimated yield of 4a
was 12%.
Next we varied the amount of diazoester 2a while keeping
the catalyst as Rh2(OAc)4. A reaction with a slight deficiency of
ethyl diazoacetate 2a (0.8 equiv) gave 53% monoinsertion
product 3a and 7% di-insertion product 4a along with 25%
recovered 1 (entry 3). With excess 2a (3 equiv, entry 4), there
was less 3a (40%), more 4a (22%), and only a trace of
unreacted 1 (4%).
Scope Studies: Diazo Partner. On the basis of the results
in Table 1 along with other test reactions (see Supporting
Information), we settled on the use of 1.2 equiv of diazo
partner to test the scope of the reactions of 1 with various
diazocarbonyl compounds. The results of representative
reactions in this series are summarized in Table 2. The catalyst
Rh2(esp)2 typically gave similar or better yields compared to
Rh2(OAc)2, so only the Rh2(esp)2 results are shown.
tert-Butyl diazoacetate 2b, 1-diazobutan-2-one 2c, and 2-
diazo-N,N-dimethylacetamide 2d all gave similar results to ethyl
diazoacetate 2a (compare Table 2, entries 1−3 with Table 1,
entry 2). Conversions of 1 ranged from 66−80%, and isolated
yields of stable insertion products 3b−d were 58%, 49%, and
55%, respectively. As with substrate 2a, small amounts of
double insertion products were formed in these experiments
(estimated 11%, 10% and 19% by 11B NMR spectroscopy), but
these products were not isolated. Instead, we targeted several
unsymmetrical double insertion products (see Scheme 1
below).
Diazoesters bearing additional conjugating substituents gave
better conversions and yields.23 Dimethyl diazomalonate 2e
produced α-NHC-boryl malonate 3e in 73% yield (entry 4),
while diazodimedone 2f provided 3f in 60% yield (entry 5).
The Davies-type14 donor−acceptor precursor methyl 2-phenyl-
diazoacetate 2g gave 3g in 74% yield (entry 6). Reaction of 1
with (−)-menthyl diazoacetate 2h and the standard Rh2(esp)2
catalyst provide 3h as an 81/19 mixture of diastereomers in
60% yield (entry 7). We have not yet succeeded in separating
these diastereomers, so their configurations are unassigned.
In contrast to the successes with additional conjugating
substituents, methyl 2-benzyldiazoacetate 2i24 gave 3i in only
26% yield (entry 8). The main problem here is not the
formation of di-insertion product but the low conversion; the
yield of 3i based on recovered starting material is actually 93%
because only 28% of 1 was consumed. A similar result was
obtained in a reaction of 1 and 2i conducted at 84 °C in 1,2-
dichloroethane while no product was formed in acetonitrile
(see Supporting Information, SI).
RESULTS AND DISCUSSION
Survey of Reaction Conditions. Table 1 shows key test
reactions of readily available 1,3-dimethylimidazol-2-ylidene
■
Table 1. Results of Representative Initial Experiments with
NHC-Borane 1 and Ethyl Diazoacetate 2a
a
b
a
entry equiv 2a catalyst (1%) conv (%) yield 3a (%) yield 4a (%)
1
2
3
4
1.2
1.2
0.8
3.0
Rh2(OAc)4
Rh2(esp)2
Rh2(OAc)4
Rh2(OAc)4
94
92
75
96
62
62
53
40
14
12
7
22
a
Estimated from the 11B NMR spectrum of the crude product;
Isolated by automated flash chromatography.
b
borane 1 and ethyl 2-diazoacetate 2a with two common
catalysts for C−H insertions: (1) dirhodium tetraacetate20
[Rh2(OAc)4], and (2) dirhodium bis-3,3′-(1,3-phenylene)-
bis(2,2-dimethylpropanoate) [Rh2(esp)2].21 In a typical experi-
ment, a slight excess of diazoacetate 2a (1.2 equiv) was added
by syringe pump over 4 h to a solution of NHC-borane 1 (1
equiv) and Rh2(OAc)4 (1 mol %) in dichloromethane at 40 °C
(entry 1). The 11B NMR spectrum of the crude product
showed a large triplet −28.3 ppm, which we assigned to the
insertion product 3a. There was also a smaller doublet at −20.3
ppm, which we assigned to the double insertion product 4a,
along with an even smaller quartet from remaining 1 (−37.5
ppm, 6%).
Purification of the crude product by flash chromatography
with 1/1 hexane/ethyl acetate provided pure insertion product
3a in 62% yield. (All reported yields are based on the NHC-
12077
dx.doi.org/10.1021/ja4056245 | J. Am. Chem. Soc. 2013, 135, 12076−12081

Journal of the American Chemical Society
Article
Table 2. Scope of the B−H Insertion with 1% Rh2(esp)2:
Table 3. Scope of the B−H Insertion with 1% Rh2(esp)2:
a
a
Variation of the Diazo Partner
Variation of the NHC-Borane Partner
a
Same conditions as Table 1, entry 2 with 1.2 equiv of diazoester 2a.
3 equiv of diazoester 2a
b
With bulkier substrates dimesitylimidazolylidene borane 9,
tricyclic boranes 11 and 13 (enantiopure), and dipp-
imidazolylidene borane 15 (dipp is 2,6-diisopropylphenyl),
the conversions of NHC-boranes were a bit lower but there was
not much double insertion product formed (entries 4−6a).
Indeed, the isolated yields of monoinsertion products 10, 12,
14 (enantiopure), and 16 (59−63%) approached the
conversions of the precursors (65−66%). In the first three
cases (9, 11, 13), only 5−6% of the di-insertion product was
detected. And in the case of the very hindered dipp borane 15,
there was no resonance at all for the di-insertion product in the
11B NMR spectrum of the crude product (entry 6a).
These results suggest that the only significant competing
pathway for B−H insertion to 15 is reaction of the rhodium
carbene with itself or its precursor. This in turn suggests that
higher yields might be obtained by increasing the amount of the
diazo precursor. This approach failed with the smaller NHC-
borane 1 because the di-insertion product was formed
competitively (see Table 1, entry 4). In contrast, a reaction
a
b
Same conditions as Table 1 entry 2. 81/19 mixture of diastereomers.
Scopes Studies: NHC-Borane Partner. We selected
NHC-borane 1 for the initial survey in part because it offered
little opportunity for competing bimolecular reactions of the
rhodium carbenes with the NHC ring or its substituents. We
next varied the structure of the NHC-carbene partner while
holding the diazo partner as ethyl diazoacetate 2a. The standard
conditions were used with the Rh2(esp)2 catalyst, and the
results are summarized in Table 3. Results with triazolylidene
borane 5, benzimidazolylidene borane 7 were similar to
dimethylimidazolylidene borane 1. Conversions were 65−80%
and isolated yields of monoinsertion products 6 and 8 were
52% and 60% (entries 1, 2). Minor amounts of di-insertion
products were formed (11% estimated yield from 5 and 19%
isolated yield from 7, see SI).
12078
dx.doi.org/10.1021/ja4056245 | J. Am. Chem. Soc. 2013, 135, 12076−12081

Journal of the American Chemical Society
Article
of dipp-Imd-BH3 15 with 3 equiv of ethyl diazoacetate 2a gave
complete conversion of 15, and isolated yield of insertion
product 16 increased from 63% (entry 6a) to 96% (entry 6b).
Unsymmetrical Double Insertion Products. We also
briefly investigated the formation of unsymmetrical double
insertion products by treatment of the monoinsertion products
from one diazo compound with a second, different diazo
compound, as shown in Scheme 1. Reaction of insertion
product 3a with methyl 2-phenyldiazoacetate 2g (2 equiv)
provided separable diastereomers of the double insertion
product 17 in 70% combined yield (56/44 diastereomer
ratio). These diastereomers arise because the boron atom and
one of the adjacent carbon atoms in 17 are both stereogenic
centers. The phenyl substituent in this product is important for
purification since related diesters lacking it (for example, 4a)
did not reliably come off the column.
started and t-butyl diazoacetate 2b (2 equiv) was added, again
over 4 h. Although the mixture remained orange, TLC analysis
showed that the second insertion was progressing during the
second syringe pump period. Standard workup and chromatog-
raphy gave diester 19 in 55% isolated yield.
Competitive Rate Experiments. The NHC-rings and N-
substituents of the precursors in Table 3 and Scheme 1 contain
various potential sites for reactions of rhodium carbenes,
including aromatic rings and activated C−H bonds. However,
side products from reaction at sites other than boron have so
far not been isolated. This implies that the B−H bonds of these
NHC-boranes are very reactive toward rhodium carbenes. This
notion is also supported by the good yields obtained without
large excesses of reagents and by the scope of the different
diazo compounds that can be used.
To better address the reactivity question, we tapped into a
scale put forth by Davies and co-workers.25 The scale is for
reactions between various carbenophiles and methyl 2-phenyl-
diazoacetate 2g catalyzed by Rh2(S-DOSP)4 (DOSP is 4-
dodecabenzenesulfonyl prolinate). We used the Rh2(esp)2
catalyst for internal consistency, so we cannot place our results
quantitatively on the Davies scale. Still, the key conclusion
that boron−hydrogen bonds are highly reactive carbeno-
philesis clear-cut.
Scheme 1. Stepwise and One-Pot Formation of Double
Insertion Products
We started competition experiments with 1 and THF, whose
CH2O groups are considered to be activated toward C−H
insertion of Rh-carbenes.25 But in the preliminary reactions of
2g, 1 and THF, the THF insertion product was hardly formed
at all even when THF was used in 50-fold excess over 1. The
THF did not interfere with the B−H insertion reaction, and 3g
was the major product as usual.
We then moved directly to the top of the Davies reactivity
scale with 1,4-cyclohexadiene and styrene (Scheme 2).
Scheme 2. Competition Experiments of NHC-Borane 1 with
1,4-Cyclohexadiene (1,4-CHD) and Styrene (STY)
Back-to-back reaction of benzimidazolylidene borane 7 with
two different diazo compounds gave a chiral product 18 whose
only stereocenter is at boron. A first reaction with diazoamide
2d and Rh2(esp)2 was followed by a second, similar reaction
with tert-butyl diazoacetate 2b and the same catalyst to give 18.
The yield of the first insertion was 62% while the second
insertion occurred in 71% yield.
The two enantiomers of 18 were stable on both silica gel and
on an (S,S)-Whelk-O1 column with a chiral stationary phase. A
small amount of the racemate (22 mg) was preparatively
resolved on the chiral column to give about 10 mg each of the
two component enantiomers in high enantiopurity. Optical
rotations were −34 (first eluting enantiomer) and +33.8 (both c
= 1, CHCl3). To the best of our knowledge, this is the first
example of isolation and characterization of stable enantiomers
of a chiral carbene-borane whose only stereocenter is on boron.
Finally, a sequential one-pot reaction at 40 °C was also
successful. First, ethyl diazoacetate 2a (1.2 equiv) was added by
syringe pump over 4 h to benzimidazolylidine borane 7 (1
equiv) and 1% Rh2(esp)2. Like some of the other reactions (see
SI), the color of the mixture gradually changed from green to
orange.21 Just as the first syringe pump stopped, a second was
Competition of 1 equiv of 1,4-cyclohexadiene (1,4-CHD) and
1 equiv 1 again provided 3g, this time with a little of the
cyclohexadiene C−H insertion product. Finally, competition of
a 10-fold excess of 1,4-CHD with 1 provided the B−H
insertion product 3g and the C−H insertion product 20 in a
ratio of about 1.2/1.
Likewise, reaction of 1 and styrene (STY) in a 1/1 ratio gave
3g with only a small amount of the styrene cyclopropanation
product. However, competition of 1 equiv of 1 with 10 equiv of
styrene gave 3g and 21 in about a 1/1 ratio.
These results show that NHC-borane 1 is roughly 10−12
times more reactive than 1,4-cyclohexadiene and styrene, and it
12079
dx.doi.org/10.1021/ja4056245 | J. Am. Chem. Soc. 2013, 135, 12076−12081

Journal of the American Chemical Society
Article
is over 100 times more reactive than THF. Accordingly, the B−
H bonds in 1 are much more reactive than typical “activated”
C−H bonds toward rhodium carbenes. Further, a better
comparable compound to NHC-BH3 would contain a CH3
group rather than a CH2 group. However, methyl groups rarely
participate in selective bimolecular insertion reactions.13,14
Clearly, NHC-boranes are potent carbenophiles toward
electrophilic rhodium carbenes and, by extension, other classes
of electrophilic carbenes or metal carbenes.
Isotope Effect Experiments. Finally, we conducted simple
experiments to estimate the bimolecular isotope effect in a
typical B−H insertion reactions. Here again, comparables are
provided by Davies in Rh2(S-DOSP)4-catalyzed reactions of
labeled and unlabeled substrates with methyl 2-phenyl-
diazoacetate 2g. The isotope effect was about 2 in a
competition of cyclohexane with cyclohexane-d12 and about 3
with THF and THF-d8.25
Figure 2. The B−H insertion reaction is probably concerted. The TS
shares characteristics of a borenium ion and a rhodium-enolate
because hydride abstraction is leading and C−B bond formation is
lagging.
NHC-boranes are excellent hydride donors,10 and have
relatively weak B−H bonds (<85 kcal/mol).27 These electronic
effects explain why they are such reactive carbenophiles toward
electrophilic rhodium carbenes. However, steric effects must
also be important because the second insertion to NHC-BH2R
is slower than the first to NHC-BH3, especially when the NHC
ring has large N-substituents.
The key competition experiment in the NHC-borane series
required care because a control experiment with 1 (diMe-Imd-
BH3), 1-d3 (diMe-Imd-BD3, >95% D) and Rh2(esp)2 but
lacking the diazoester showed that H/D exchange occurred
over some hours at 40 °C. No exchange occurred without the
catalyst. The exchange was easy to follow approximately by 11
B
CONCLUSIONS
NMR spectroscopy because the multiplicities changed due to
the differences in spin and coupling constant (J) between H
and D; however, it was hard to quantitate accurately because of
overlapping. We suspect that the catalyst and the NHC-borane
react, perhaps by direct hydride transfer, to make a small
■
Readily available rhodium(II) salts catalyze reactions between
NHC-boranes and diazocarbonyl compounds. Stable α-NHC-
boryl carbonyl compounds are isolated in good yields, so the
reaction is a reliable way to make boron−carbon bonds. There
is broad tolerance for variation in both the NHC-borane and
diazo carbonyl component. More hindered NHC-boranes
actually give better yields of monoinsertion products because
the primary products are not subject to a competing second
insertion.
+
amount of a borenium ion (NHC-BH2 ) or its reactive
equivalent.26 This could catalyze the H/D exchange by
reversible hydride/deuteride transfer.6a
Scheme 3. Competition Experiment to Estimate the Isotope
Effect
These first reactions of stable carbene-boranes with transient
metal carbenes suggest that NHC-boranes are superb
carbenophiles and encourage further study of their reactions
with rhodium carbenes and with other types of metal carbenes.
ASSOCIATED CONTENT
* Supporting Information
Experimental details, compound characterization data, and
copies of spectra of all products. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
We discovered that the H/D exchange could be superseded
at rt by rapidly adding a solution of 0.5 equiv of the methyl 2-
phenyldiazoacetate 2g to a CD2Cl2 solution of the catalyst and
1 equiv each of 1 and 1-d3. The expected insertion products 3g
and 3g-d3 were formed after 5 min in a ratio of about 4.5/1.
The unreacted boranes at this point were mostly (>90%) 1 and
1-d3, and the exchanged products 1-d1 or 1-d2 were not
prevalent in the 11B NMR spectrum (<10%).
AUTHOR INFORMATION
Corresponding Author
curran@pitt.edu
Notes
■
The authors declare no competing financial interest.
Mechanism. All these data combine to form a consistent
picture of the B−H insertion reactions of rhodium carbenes,
and Figure 2 shows a simple transition state model. The
rhodium-carbene C−H insertion reactions are thought to be
concerted but not synchronous,13,14 so the B−H insertions may
be likewise. In the transition state, the hydride abstraction by
the rhodium carbene to form the new C−H bond is advanced
(evidenced by the large isotope effect) while the accompanying
C−B bond formation lags. In this view, it is productive to
model the transition state as having the character of an NHC-
borenium ion associated to a rhodium enolate. As the transition
state is passed, these two components collapse by B-alkylation
to form the product and return the catalyst.
ACKNOWLEDGMENTS
We thank the National Science Foundation for funding. We
also thank Mr. Everett Merling, University of Pittsburgh, for
preparing the sample of 1-d3 and Dr. Emmanuel Lacote, CNRS
̂
(University of Lyon 1), for helpful discussions.
■
REFERENCES
■
(1) Curran, D. P.; Solovyev, A.; Makhlouf Brahmi, M.; Fensterbank,
L.; Malacria, M.; Laco
10317.
(2) Makhlouf Brahmi, M.; Monot, J.; Desage-El Murr, M.; Curran, D.
̂
te, E. Angew. Chem., Int. Ed. 2011, 50, 10294−
P.; Fensterbank, L.; Laco
6983−6985.
̂
te, E.; Malacria, M. J. Org. Chem. 2010, 75,
12080
dx.doi.org/10.1021/ja4056245 | J. Am. Chem. Soc. 2013, 135, 12076−12081

Journal of the American Chemical Society
Article
(3) (a) Solovyev, A.; Laco
42, 695−700. (b) Solovyev, A.; Chu, Q.; Geib, S. J.; Fensterbank, L.;
Malacria, M.; Lacote, E.; Curran, D. P. J. Am. Chem. Soc. 2010, 132,
15072−15080.
(4) (a) Solovyev, A.; Ueng, S.-H.; Monot, J.; Fensterbank, L.;
Malacria, M.; Lacote, E.; Curran, D. P. Org. Lett. 2010, 12, 2998−3001.
(b) Chu, Q.; Makhlouf Brahmi, M.; Solovyev, A.; Ueng, S.-H.; Curran,
D.; Malacria, M.; Fensterbank, L.; Laco
te, E. Chem.Eur. J. 2009, 15,
12937−12940.
(5) Monot, J.; Solovyev, A.; Bonin-Dubarle, H.; Derat, E.; Curran, D.
P.; Robert, M.; Fensterbank, L.; Malacria, M.; Lacote, E. Angew. Chem.,
Int. Ed. 2010, 49, 9166−9169.
(6) (a) Prokofjevs, A.; Boussonnier
̂
te, E.; Curran, D. P. Dalton Trans. 2013,
̂
̂
̂
́
̂
̀
̂
e, A.; Li, L.; Bonin, H.; Lacote, E.;
Curran, D. P.; Vedejs, E. J. Am. Chem. Soc. 2012, 134, 12281−12288.
(b) Toure, M.; Chuzel, O.; Parrain, J. L. J. Am. Chem. Soc. 2012, 134,
17892−17895.
(7) The closest known analogs are two α-NHC-boryl malononitrile
compounds made by hydride reduction of ylidene malanonitriles. See
Figure 3 in ref 10.
(8) Cowden, C. J.; Paterson, I. Org. React. 1997, 51, 1−200.
(9) Burnham, B. S. Current Med. Chem. 2005, 12, 1995−2010.
̂
(10) Horn, M.; Mayr, H.; Lacote, E.; Merling, E.; Deaner, J.; Wells,
S.; McFadden, T.; Curran, D. P. Org. Lett. 2012, 14, 82−85.
(11) Moss, R. A. Carbenic Philicity. In Carbene Chemistry; Bertrand,
G., Ed.; Marcel Dekker: New York, 2002; pp 57−101.
(12) Bertrand, G. Carbene Chemistry; Marcel Dekker: New York,
2002.
́
(13) (a) Díaz-Requejo, M. M.; Perez, P. J. Chem. Rev. 2008, 108,
3379−3394. (b) Doyle, M. P.; Duffy, R.; Ratnikov, M.; Zhou, L. Chem.
Rev. 2010, 110, 704−724.
(14) Davies, H. M. L.; Morton, D. Chem. Soc. Rev. 2011, 40, 1857−
1869.
(15) Zheng, G.; Jones, M. J. Am. Chem. Soc. 1983, 105, 6487−6488.
(16) Ramírez-Lo
́
pez, P.; Sierra, M. A.; Go
́
mez-Gallego, M.;
Mancheno, M. J.; Gornitzka, H. Organomet. 2003, 22, 5092−5099.
̃
(17) (a) Bedel, C.; Foucaud, A. Tetrahedron Lett. 1993, 34, 311−314.
́
́
́ ̋ ́ ̋
(b) Keglevich, G.; Ujszaszy, K.; Szollosy, A.; Ludanyi, K.; Toke, L. J.
̈
Organomet. Chem. 1996, 516, 139−145. (c) Monnier, L.; Delcros, J.-
G.; Carboni, B. Tetrahedron 2000, 56, 6039−6046.
(18) Imamoto, T.; Yamanoi, Y. Chem. Lett. 1996, 705−706.
(19) Davies, H. M. L.; Beckwith, R. E. J. Chem. Rev. 2003, 103,
2861−2903.
(20) Padwa, A.; Austin, D. J. Angew. Chem., Int. Ed. 1994, 33, 1797−
1815.
(21) Fiori, K. W.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 562−568.
(22) It is also possible that the double insertion product eventually
decomposes on the column. This would probably give the
corresponding imidizolium salt, which is also very polar.
(23) Estimated amounts of double insertion products in entries 4−8
ranged from 0−9%, see Supporting Information.
(24) Taber, D. F.; Sheth, R. B.; Joshi, P. V. J. Org. Chem. 2005, 70,
2851−2854.
(25) Davies, H. M. L.; Hansen, T.; Churchill, M. R. J. Am. Chem. Soc.
2000, 122, 3063−3070.
(26) De Vries, T. S.; Prokofjevs, A.; Vedejs, E. Chem. Rev. 2012, 112,
4246−4282.
(27) (a) Tehfe, M.-A.; Makhlouf Brahmi, M.; Fouassier, J.-P.; Curran,
̂ ́
D. P.; Malacria, M.; Fensterbank, L.; Lacote, E.; Lalevee, J.
Macromolecules 2010, 43, 2261−2267. (b) Tehfe, M.-A.; Monot, J.;
Makhlouf Brahmi, M.; Bonin-Dubarle, H.; Curran, D. P.; Malacria, M.;
̂ ́
Fensterbank, L.; Lacote, E.; Lalevee, J.; Fouassier, J.-P. Polym. Chem.
2011, 2, 625−631. (c) Hioe, J.; Karton, A.; Martin, J. M. L.; Zipse, H.
Chem.Eur. J. 2010, 16, 6861−6865.
12081
dx.doi.org/10.1021/ja4056245 | J. Am. Chem. Soc. 2013, 135, 12076−12081