Cyclic (Aryl)(Amido)Carbenes: NHCs with Triplet-like Reactivity

Article abstract of DOI:10.1002/anie.201910350

The synthesis and study of a library of cyclic (aryl)(amido)carbenes (CArAmCs), which represent a class of electrophilic NHCs that feature low calculated singlet-triplet gaps (ΔE_ST=19.9 kcal mol^?1; B3LYP/def2-TZVP) and exhibit reactivity profiles expected from triplet carbenes, are described. The electrophilic properties of the CArAmCs were quantified by analyzing their respective selenium adducts, which exhibited the largest downfield ⁷⁷Se NMR chemical shifts (up to 1645 ppm) measured for any NHC derivative known to date, as well as their Ir carbonyl complexes, from which large Tolman electronic parameter (TEP) values (up to 2064 cm^?1) were ascertained. The CArAmCs were found to engage in reactions that are typically observed with triplet carbenes, including C?H insertions, [2+1] cycloadditions with alkenes as well as alkynes, and spontaneous oxidation upon exposure to oxygen.

Full text of DOI:10.1002/anie.201910350

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International Edition: DOI: 10.1002/anie.201910350
German Edition: DOI: 10.1002/ange.201910350
N-Heterocyclic Carbenes
Cyclic (Aryl)(Amido)Carbenes: NHCs with Triplet-like Reactivity
Prakash R. Sultane, Guillermo Ahumada, Daniel Janssen-Mꢀller, and
Christopher W. Bielawski*
Abstract: The synthesis and study of a library of cyclic
(aryl)(amido)carbenes (CArAmCs), which represent a class of
electrophilic NHCs that feature low calculated singlet-triplet
gaps (DEST = 19.9 kcalmolÀ1; B3LYP/def2-TZVP) and exhibit
reactivity profiles expected from triplet carbenes, are described.
The electrophilic properties of the CArAmCs were quantified
by analyzing their respective selenium adducts, which exhibited
the largest downfield 77Se NMR chemical shifts (up to
1645 ppm) measured for any NHC derivative known to date,
as well as their Ir carbonyl complexes, from which large
from the carbene nucleus.[8] For example, the N,N’-diamido-
carbenes (DACs) (e.g., 1; Figure 1),[8a–h] which feature amides
N-bonded to the carbene nucleus, were found to function as
electrophiles in a broad range of chemical reactions.[8b,9] The
DACs were also found to retain some of the nucleophilic
characteristics displayed NHCs, and thus may be best
described as ambiphilic.[8a,b,f,9,10]
Tolman electronic parameter (TEP) values (up to 2064 cmÀ1
)
were ascertained. The CArAmCs were found to engage in
reactions that are typically observed with triplet carbenes,
À
including C H insertions, [2+1] cycloadditions with alkenes as
well as alkynes, and spontaneous oxidation upon exposure to
oxygen.
Figure 1. Generalized structures of selected electrophilic carbenes.
To realize a carbene that displayed an even higher degree
of electrophilic properties than the DACs, we considered
known triplet carbenes, particularly fluorenylidene in part
because it has been reported to feature a singlet-triplet gap
(DEST) of approximately 1 kcalmolÀ1.[11] Although fluoreny-
lidene is not isolable, it was hypothesized that the attachment
of a functional group to the carbene nucleus would enhance
stability while retaining a relatively high degree of electro-
Introduction
Although carbenes have served as reactive intermediates
for the past two centuries,[1] remarkable growth in the field
was triggered by the landmark reports of stable phosphino-
silyl carbenes[2] (Bertrand, 1989) and a crystalline imidazol-2-
ylidene[3] (Arduengo, 1991). Derivatives of the latter, which
are typically described as N-heterocyclic carbenes (NHCs),[4]
have been extensively developed over the past few decades
and are frequently utilized as ligands in transition metal
complexes,[4b,5] as reagents in chemical transformations,[6] and
as catalysts.[4g,7] The broad utility stems in part from their high
intrinsic nucleophilicities. Indeed, NHCs feature a bona fide
resonance structure wherein a formal negative charge is
assigned to the carbene nucleus. While many efforts have
been directed toward enhancing and exploiting such nucleo-
philic properties, our group and others have been developing
electrophilic analogues, in part by installing functional groups
(e.g., carbonyls) that effectively siphon electron density away
philicity.
Bertrandꢀs
cyclic
(amino)(aryl)carbenes
(CAArCs)[12] (2) serve as examples of such systems. While
a CAArC was calculated to exhibit a lower DEST value when
compared to its analogous cyclic (amino)(alkyl)carbene
(CAAC) (39.5 kcalmolÀ1 vs. 47.5 kcalmolÀ1, respectively),
a similar value was calculated for a DAC (42.3 kcalmolÀ1).[8i]
More recently, a coumaraz-2-on-4-ylidene (3),[8i] which fea-
tures a carbene nucleus linked to a carbamoyl group, was
reported by Song and Lee to exhibit a relatively low DEST
value (26.7 kcalmolÀ1).[8i] Using a similar level of theory
(B3LYP/def2-TZVP), we calculated a DEST of 19.9 kcalmolÀ1
for a cyclic (aryl)(amido)carbenes (CArAmC) 4 (R = 2,6-
diisopropylphenyl). Moreover, the LUMO for 4 was calcu-
lated (À3.02 eV) to be significantly lower in energy than the
LUMOs calculated for DAC 1 (À2.20 eV), CAArC 2
(À1.47 eV), and coumaraz-2-on-4-ylidene 3 (À2.43 eV).[8i]
Motivated by these results, we hypothesized that the CAr-
AmCs should exhibit relatively strong electrophilic character-
istics and may potentially react in a manner similar to that
expected from a triplet carbene. Herein, we describe the
synthesis of a library of CArAmCs that feature different N-
substituents in order to facilitate isolation as well as deriva-
tives with halogen groups to pronounce electrophilicity. The
electronic properties as well as the chemistry of the CAr-
AmCs are also presented.
[*] P. R. Sultane, G. Ahumada, C. W. Bielawski
Center for Multidimensional Carbon Materials (CMCM), Institute for
Basic Science (IBS), Ulsan 44919 (Republic of Korea)
E-mail: bielawski@unist.ac.kr
D. Janssen-Mꢀller
Organisch-Chemisches Institut, Westfꢁlische Wilhelms-Universitꢁt
Mꢀnster, Mꢀnster 48149 (Germany)
C. W. Bielawski
Department Chemistry and Department of Energy Engineering,
Ulsan National Institute of Science and Technology (UNIST), Ulsan
44919 (Republic of Korea)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.201910350.
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Results and Discussion
HCl adducts of the targeted carbenes (8a–8e) in yields that
exceeded 90%.
The syntheses of the precursors to various CArAmCs are
summarized in Scheme 1. Commercially-available phthalic
anhydrides (5a–5c) were first converted to their respective
imides (6a–6e) by condensing the corresponding amines in
acetic acid at 1408C for 12 h. Pure imides were obtained in 84
With 8 in hand, subsequent efforts were directed toward
the generation of the corresponding carbenes (4). As
summarized in Scheme 2, treating 8a with lithium hexame-
thyldisilazide (LiHMDS) (1.1 equiv) at À788C in THF
followed by warming the reaction mixture to room temper-
ature afforded a yellow product, which
was subsequently isolated by purifica-
tion using silica gel column chroma-
tography. NMR and IR spectroscopy
along with a single-crystal X-ray dif-
fraction analysis (see the Supporting
Information) revealed the structure to
be the carbene dimer 9a. In contrast,
À
deprotonation of 8c afforded the C H
insertion product 10, as determined in
part by the observation of benzylic
(d = 5.18 ppm,
CDCl3),
methyl
(1.85 ppm and 0.79 ppm), and other
1
key H NMR signals that were consis-
tent with the cyclic product. The struc-
ture of 10 was unambiguously con-
firmed by a single crystal X-ray dif-
fraction analysis (Figure 2). Similarly,
deprotonation of the carbene precur-
sor 8e afforded carbene dimer 9b.[13]
Attempts to deprotonate the other
precursors (e.g., 8b and 8d) afforded
mixtures of unidentified products.
Scheme 1. Synthesis of various carbene precursors. Mes=2,4,6-trimethylphenyl. DIPP=2,6-
diisopropylphenyl. Ad=1-adamantyl.
to 95% yield after sequentially washing the crude products
with aqueous HCl (1n), an aqueous solution saturated with
NaHCO3, n-hexane, and Et2O. Independently treating the
imides with 1.2 equiv of NaBH4 in a mixture of MeOH and
THF (2:8, v/v) afforded the corresponding alcohols (7a–7e)
in a 65 to 93% yield after purification by washing with diethyl
ether or silica gel column chromatography. Finally, chlorina-
tion of the alcohols with thionyl chloride (SOCl2) afforded the
As isolation of the free carbenes proved to be challenging,
in situ formation was probed using a trapping experiment.[14]
As summarized in Scheme 3, the addition of LiHMDS
(1.1 equiv) to a stirred solution of 8b and elemental sulfur
(2 equiv) in THF at À788C afforded the corresponding thione
11. The structure of the compound was confirmed using NMR
spectroscopy as well as mass spectrometry. The salient
13
=
C NMR signal assigned to the C S bond (d = 198 ppm,
Figure 2. ORTEP diagram of 10 with thermal displacement ellipsoids
drawn at the 50% probability level.[29] Selected distances (ꢂ): N1-C8,
1.472(2); N1-C1, 1.383(2); C1-O1, 1.2121(19); N1-C17, 1.4284(19);
C3-Cl1, 1.7205(19). Selected angles (8): C8-N1-C1, 113.38(12); O1-C1-
N1, 126.62(14); C1-N1-C17, 127.08(12).
Scheme 2. Products obtained upon deprotonating carbene precursors
8a, 8c, and 8e.
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Figure 3. ORTEP structure of 12b with thermal displacement ellipsoids
drawn at the 50% probability level.[29] Note: although two crystallo-
graphically independent molecules were identified in the unit cell, only
one is shown for clarity. Selected distances (ꢂ): Se1-C8, 1.782(3); C1-
O1, 1.245(5); N1-C5, 1.377(5); N1-C1, 1.402(5); C3-Cl1, 1.719(3).
Selected angles (8): N1-C8-Se1, 123.50(3); O1-C1-N1, 124.90(3); C8-
N1-C1, 112.10(3).
Scheme 3. Generation and trapping of the carbenes by elemental S
and Se.
CDCl3) was downfield when compared to the values reported
for the sulfur adducts of DAC 1 (d = 177 ppm, CDCl3)[15] as
well as the sulfur adducts of 2 and 3 (d = 191–197 ppm,
CDCl3).[8i,12]
that are comparatively downfield. As shown in Scheme 4,
treating a mixture of 8b and [Ir(COD)Cl]2 (COD = 1,5-
cyclooctadiene) (0.5 equiv) with LiHMDS (1.1 equiv) in THF
at À788C afforded the Ir complex 13. 1H NMR analysis of 13
revealed that the signals assigned to the olefins in the COD
ligand (d = 5.52 and 5.22 ppm, CDCl3) were the most down-
field for any NHC-Ir complex known to date. Moreover, the
13C NMR resonance of the carbene nucleus in 13 was
recorded at d = 249.3 ppm (CDCl3), which is significantly
downfield when compared to the values reported for DAC
1 (231.3 ppm; C6D6)[8b] and other NHC complexes (179.6–
208.2 ppm; CDCl3)[20] reported in the literature but upfield
Next, efforts were directed toward studying the electronic
properties displayed by the carbenes. Ganter reported that
the chemical shifts exhibited by the selenium nuclei of NHC-
Se adducts reflect the p-accepting properties of the carbene
nuclei and can be conveniently measured using 77Se NMR
spectroscopy.[16] The use of elemental Se instead of S in the
aforementioned trapping experiment afforded carbene–sele-
nium adduct 12a. The 77Se NMR signal recorded for 12a (d =
1204.4 ppm, [D6]acetone; 1267.9 ppm, CDCl3) was found to
be downfield when compared to the values reported for the
Se adducts of a cyclic (alkyl)(amido)carbene (1179.71 ppm,
[D6]acetone), DAC 1 (d = 847 ppm, [D6]acetone),[17] CAArC
2 (d = 616 ppm, [D6]acetone),[12] and coumaraz-2-on-4-yli-
when compared to
a
cyclic (alkyl)(amido)carbene
(287.6 ppm; CDCl3).[8f] Collectively, the downfield NMR
signals indicated that 4b is a strong p-acceptor. Further
support for this conclusion was achieved through a single-
crystal X-ray diffraction analysis (Figure 4). Reflective of
their relative p-back-bonding interactions, the distance be-
tween the Ir and Ccarbene nuclei in the solid-state structure of 13
(1.937(3) ꢁ) was measured to be shorter than the corre-
sponding value reported for an analogous complex supported
dene
3
(d = 1103 ppm, [D6]acetone; d = 1143.3 ppm,
CDCl3).[8i] Likewise, the 13C NMR signal assigned to carbene
nucleus in 12a was found to resonate at a relatively downfield
value (d = 202.8 ppm, CDCl3) when compared to the analo-
gous signals recorded for the Se adducts of DAC 1 (d =
182.7 ppm, CDCl3)[17] or coumaraz-2-on-4-ylidene 3 (d =
197 ppm, CDCl3).[8i] The carbene-selenium adducts 12b and
12c, which were prepared in a similar manner as 12a,
exhibited the largest downfield 77Se NMR signals reported
to date (12b: d = 1645.5 ppm, [D6]acetone; 1688.4 ppm,
by 1 (1.998 ꢁ).[21] Likewise, the distances of the C C bonds
=
within the coordinated COD ligand were measured to be
shorter (1.379(6) and 1.408(7) ꢁ) as well (c.f., 1.387 and
1.412 ꢁ, respectively). Collectively, these structural features
reflect the relatively strong p-acceptor characteristics of 4b.
Ir carbonyl complexes also enable the comparison of the
donating properties of NHCs, primarily through the calcu-
CDCl3; 12c: d = 1690.7 ppm, CDCl3),[18] even though the C
=
Se bond distance measured (1.782(3) ꢁ) in the solid state
structure of 12b (Figure 3) was within the range reported for
other selenium adducts (1.765 ꢁ).[8i]
Iridium olefin complexes are
commonly used to quantify the
electron-donating or accepting
abilities of NHCs and other li-
gands.[19] NMR analyses of the
former provide a means to mea-
sure the p-accepting abilities of the
corresponding carbene nuclei.
NHCs that are relatively strong p-
acceptors often give NMR signals
Scheme 4. Structures of the Ir complexes that were used to evaluate electronic properties of the
cyclic (aryl)(amido)carbene 4b.
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Figure 4. ORTEP diagram of 13 with thermal displacement ellipsoids
drawn at the 50% probability level.[29] Selected distances (ꢂ): Ir1-Cl1,
2.3598(10); Ir1-C8, 1.937(3); C21-C22, 1.408(7); C26-C27, 1.379(6).
Selected angles (8): C8-Ir1-Cl1, 92.40(9); N1-C8-Ir1, 128.00(2); O1-C1-
N1, 124.00(3).
Scheme 5. Carbene generation from 8b followed by addition with
styrene, 2-butyne or p-anisaldehyde.
lation of a Tolman electronic parameter (TEP), a comparative
value that is derived from the observed carbonyl stretching
frequencies (nCO) displayed by the corresponding com-
plex.[19,22] Carbonyl complexes supported by NHCs with
relatively strong p-acceptor characteristics generally exhibit
relatively high TEP values. Conversion of 13 to dicarbonyl
complex 14 was achieved by bubbling a CD2Cl2 solution of 13
with carbon monoxide. The IR spectrum recorded for 14
exhibited two prominent nCO absorption bands at 2078.8 and
1998.9 cmÀ1 (CD2Cl2). Converting the IR data using Nolanꢀs
method[16b,20a,23] afforded a TEP value (2064 cmÀ1) that was
significantly higher than that reported for DAC
1 (2057 cmÀ1)[21] as well as carbenes 2 and 3 (2050 cmÀ1).[8i,12]
Moreover, a 13C NMR signal assigned to the carbene nucleus
in 14 was recorded at d = 247.9 ppm, which is significantly
downfield when compared to DAC 1 (d = 224.1 ppm, C6D6)
and other NHC-Ir complexes (179.9–209.5, CDCl3).[8b]
Based on the aforementioned results, we concluded that 4
was relatively electron deficient and hypothesized that it
should react as an electrophile. Since nucleophilic NHCs
generally do not undergo cycloaddition reactions with
alkenes, whereas electrophilic derivatives engage in such
chemistry,[9] a series of cycloaddition reactions were explored
(Scheme 5). Deprotonation of 8b using LiHMDS (1.1 equiv)
in THF at À788C followed by the addition of excess (5 equiv)
styrene or 2-butyne afforded the corresponding cycloadducts
15 or 16, respectively, as determined by NMR spectroscopy
and mass spectrometry. For example, 1H NMR analysis of 15
revealed diagnostic signals at d = 2.69, 1.75 and 1.67 ppm
(CDCl3), which were consistent with the formation of a cyclo-
propyl system. Likewise, a singlet was observed at d =
2.01 ppm in the 1H NMR spectrum recorded for 16 and
attributed to six equivalent methyl protons. The structures of
15 (see Supporting Information) and 16 (Figure 5) were
independently elucidated by single-crystal X-ray diffraction
analyses.
Figure 5. ORTEP diagram of compound 16 with thermal displacement
ellipsoids drawn at the 50% probability level.[29] Selected distances (ꢂ):
N1-C1, 1.3636(14); N1-C8, 1.4760(13); O1-C1, 1.2296(13); N1-C13,
1.4330(13). Selected angles (8): C1-N1-C8, 112.66(8); C13-N1-C8,
122.66(8); N1-C8-C9, 122.66(8).
anisaldehyde, ketone 17 was obtained as the major product, as
determined in part by NMR spectroscopy. For example, the
1H NMR spectrum recorded for 17 revealed a singlet at d =
6.29 ppm (CDCl3) which was assigned to the salient methine.
The carbonyl groups assigned to the ketone and amide groups
resonated at d = 191.9 and 169.6 ppm, respectively, upon
13C NMR analysis of the compound. A mechanism similar to
that deduced by Berkessel[24] for the formation of Breslowꢀs
intermediate is proposed in Scheme 6.[25] The carbene 4b,
generated in situ by treatment of 8b with LiHMDS, may
condense with p-anisaldehyde to form a zwitterionic adduct
that subsequently undergoes a hydrogen shift to afford the
enol tautomer of ketone 17. Such transformations can be
expected to be facilitated by electrophilic NHCs.[25,26]
Because of the unique reactivity profile observed by the
cyclic (aryl)(amido)carbene 4 as well as the low singlet-triplet
gap calculated for the compound, subsequent efforts were
directed toward determining if the carbene could react in
a manner similar to that of a triplet carbene. Cyclohepta-
trienylidene, a triplet carbene that must be generated in situ
via photolysis or thermolysis of the sodium salt of tropone
Electrophilic carbenes, such as the DACs, have also been
shown to undergo [2+1] cycloadditions with aldehydes.[9]
When cyclic (aryl)(amido)carbene 4b was treated with p-
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pane derivative 18 (54% yield). Treating 8b with
dimethyl maleate under similar conditions also
afforded 18 (74% yield). The structure of 18 was
confirmed by 1H and 13C NMR (CDCl3) spec-
troscopy, IR spectroscopy, mass spectrometry, as
well as X-ray crystallography (Figure 6). Forma-
tion of the cis isomer was not observed when the
crude reaction mixtures were monitored by
NMR spectroscopy. Although the mechanism
to obtain 18 from dimethyl fumarate is consistent
with a concerted process, isomerization and thus
a stepwise process is required to obtain the same
product from dimethyl maleate. Collectively,
these results suggested to us that 4 reacts as
a triplet carbene when treated with disubstituted
Scheme 6. Proposed mechanism for the formation of ketone 17.
olefins.[27]
To gain additional support for the notion that 4 can react
as a triplet carbene, other reactions were explored. For
example, triplet carbenes are known to react with molecular
oxygen to form corresponding carbonyl products.[11,28] When
the carbene precursor 8b was treated with LiHMDS
(1.1 equiv) in THF at À788C under an atmosphere of oxygen,
imide 6b was isolated as the major product. For comparison,
hydrolysis of the carbene precursor 8b formed the corre-
sponding alcohol 7b in quantitative yield; spontaneous
oxidation to form 6b was not observed.
Conclusion
Scheme 7. In situ generation of cyclic (aryl)(amido)carbene 4b fol-
lowed by cycloaddition with dimethyl maleate or fumarate or reaction
with oxygen.
In conclusion, we disclose the synthesis and study of
a series of cyclic (aryl)(amido)carbenes. The carbenes were
generated in situ from their corresponding HCl adducts,
which were prepared in three steps from readily available
starting materials and in good overall yield. The electronic
properties of the carbenes were examined from their adducts
with selenium and iridium, which collectively indicated that
the carbenes are among the most highly p-accepting carbenes
to date. Such characteristic manifested in a range of reactions
that can be expected from electrophilic carbenes, including
tosylhydrazone, has been reported to form trans-cyclopropyl
products when reacted with dimethyl maleate or fumarate.[27]
Deprotonating 8b with LiHMDS (1.1 equiv) in THF at
À788C followed by the addition of excess of dimethyl
fumarate (2 equiv) afforded corresponding trans-cyclopro-
À
C H insertions, cycloadditions with alkenes and alkynes, and
formation of Breslow intermediates upon treatment with
aldehydes. In some cases, triplet-like reactivity was observed.
For example, [2+1] cycloaddition with dimethyl maleate or
fumarate afforded the trans-cyclopropane product and the
carbenes underwent spontaneous oxidation upon exposure to
oxygen. In a broader context, these results provide a new
direction toward achieving neoteric classes of electrophilic
carbenes and enrich methodology for probing the chemistry
displayed by such compounds.
Acknowledgements
The IBS (IBS-R019) and the BK21 Plus Program as funded by
the Ministry of Education and the National Research
Foundation of Korea are acknowledged for support. We
thank Geon-Hui Park for assistance with the HR-MS
measurements.
Figure 6. ORTEP diagram of 18 with thermal displacement ellipsoids
drawn at the 50% probability level.[29] Selected distances (ꢂ): O1-C1,
1.2195(13); N1-C8, 1.4457(13); N1-C1, 1.3866(13); C8-C9, 1.5059(15).
Selected angles (8): N1-C8-C9, 122.60(8); N1-C8-C12, 127.89(8); C1-
N1-C8, 111.41(8).
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l) V. Cꢆsar, N. Lugan, G. Lavigne, Chem. Eur. J. 2010, 16, 11432 –
Conflict of interest
11442; m) N. Vujkovic, V. Cꢆsar, N. Lugan, G. Lavigne, Chem.
Eur. J. 2011, 17, 13151 – 13155; n) A. Makhloufi, W. Frank, C.
Ganter, Organometallics 2012, 31, 2001 – 2008; o) T. Sato, Y.
Hirose, D. Yoshioka, S. Oi, Organometallics 2012, 31, 6995 –
7003; p) T. Sato, Y. Hirose, D. Yoshioka, T. Shimojo, S. Oi,
Chem. Eur. J. 2013, 19, 15710 – 15718; q) B. Hildebrandt, C.
Ganter, J. Organomet. Chem. 2012, 717, 83 – 87; r) B. Hilde-
brandt, W. Frank, C. Ganter, Organometallics 2011, 30, 3483 –
3486.
The authors declare no conflict of interest.
Keywords: N-heterocyclic carbenes ·
organic reaction mechanisms · singlet carbenes ·
synthetic methods · triplet carbenes
[9] J. P. Moerdyk, C. W. Bielawski, Nat. Chem. 2012, 4, 275 – 280.
[10] T. W. Hudnall, E. J. Moorhead, D. G. Gusev, C. W. Bielawski, J.
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[13] A low singlet-triplet barrier may facilitate the formation of
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[29] CCDC 1946786 (9), 1946787 (10), 1946788 (12b), 1946789 (13),
1946790 (15), 1946791 (16), and 1946792 (18) contain the
supplementary crystallographic data for this paper. These data
are provided free of charge by The Cambridge Crystallographic
Data Centre.
Manuscript received: August 13, 2019
Accepted manuscript online: August 28, 2019
Version of record online: && &&
,
&&&&
Angew. Chem. Int. Ed. 2019, 58, 2 – 8
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
N-Heterocyclic Carbenes
P. R. Sultane, G. Ahumada,
D. Janssen-Mꢁller,
C. W. Bielawski*
&&&& — &&&&
Cyclic (Aryl)(Amido)Carbenes: NHCs
with Triplet-like Reactivity
Triplet curious carbenes: The cyclic
(aryl)(amido)carbenes (CArAmCs) con-
stitute a class of NHCs that react in
a manner akin to triplet carbenes. Exam-
ining the electronic properties of the
carbenes revealed that they are among
the most highly p-accepting and electro-
philic carbenes to date. Such features
À
enabled the CArAmCs to undergo C H
insertions, engage in [2+1] cycloaddi-
tions, and oxidize spontaneously upon
exposure to oxygen.
&&&&
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 2 – 8
These are not the final page numbers!