Minireviews V. Nair et al.

Carbene Reagents

N-Heterocyclic Carbenes: Reagents, Not Just Ligands!

Vijay Nair,* Santhamma Bindu, and Vellalath Sreekumar
Keywords: Dedicated to Professor Gilbert Stork
carbenes · cycloadditions · Michael–Stetter reaction ·
multicomponent reactions · transesterification

The unique properties of N-heterocyclic carbenes (NHCs) have

attracted much attention, mainly from theorists and organometallic
chemists, the latter using them impressively as ligands for metals. Less
well known, however, has been their suitability as excellent catalysts
and nucleophilic reagents. Transesterification, nucleophilic aromatic
substitution, and cycloaddition reactions are examples in which NHCs
can play an important role. Asymmetric reactions using catalytic
amounts of chiral NHCs are an efficient approach to optically active
compounds. This minireview focuses on this aspect of the chemistry of
NHCs. carbenes derived from five-membered
heterocycles in which the carbene
center is flanked by two nitrogen
atoms. The choice is based on the
stability and versatility of these species
1. Introduction relative to that of the carbenes derived from other nitrogen
heterocycles.
In recent years N-heterocyclic carbenes (NHCs) have
evoked considerable interest, and this is attributed in large
measure to the isolation of a stable imidazol-2-ylidene by 2. History of N-Heterocyclic Carbenes
Arduengo et al. in 1991.[1] The close analogy of NHCs to
trialkylphosphanes and their excellent s-donating properties Studies on NHCs date back to the work of Wanzlick in the
make them ligands of choice for transition metals, thus 1960s.[3] Although Wanzlick was unsuccessful in isolating any
leading to the preparation of organometallic catalysts of carbenes at that time, his recognition that a carbene center at
enormous utility in organic synthesis.[2] NHC-containing the 2-position of the imidazole ring would be stable due to the
organometallic catalysts are found to be much more effective electron-donating effects of adjacent nitrogen atoms provided
than conventional catalysts in a number of reactions, for the conceptual framework for the development of the
example, the Heck reaction and olefin metathesis. Although chemistry of these species. As mentioned earlier, the current
the use of NHCs in coordination chemistry and organo- growth in the chemistry of NHCs is mainly ascribed to the
metallic reactions has been studied extensively, very little is pioneering work of Arduengo and co-workers, who isolated a
known about the fundamental chemistry of these species. The stable crystalline N-heterocyclic carbene by the deprotona-
purpose of this review is to cast some light on the chemistry of tion of bis(1-adamantyl)imidazolium chloride with sodium
NHCs and to underscore the fact that, apart from being hydride in tetrahydrofuran in the presence of a catalytic
excellent ligands for palladium[16] and related metals, they amount of dimethyl sulfoxide [Eq. (1)].[1]
have a place of their own as reagents in organic synthesis. Our
objective is to inspire organic chemists to explore the
seemingly vast and untapped potential of NHCs. As a prelude
to this, a brief history of N-heterocyclic carbenes is also
included. By design, this review is primarily focused on the

[*] Dr. V. Nair, Dr. S. Bindu, V. Sreekumar

Organic Chemistry Division
Regional Research Laboratory (CSIR)
Trivandrum 695 019 (India) The carbene isolated was found to be a thermally stable
Fax: (+ 91) 471-249-1712 crystalline compound whose structure was unequivocally
E-mail: vijaynair_2001@yahoo.com established by single-crystal X-ray analysis. The unusual

5130 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200301714 Angew. Chem. Int. Ed. 2004, 43, 5130 –5135
 Angewandte
Carbene Reagents Chemie

stability of 2 was explained by a number of factors, for

example, the large singlet–triplet energy gap in imidazol-2-
ylidene (~ 336 kJ mol 1), p interactions in the imidazole ring,
and the electronegativity of the nitrogen atoms. In addition to
electronic factors, steric effects were also believed initially to
play a major role in stabilizing the carbene 2. Later Arduengo
et al. demonstrated that stable carbenes can be prepared by
the deprotonation of imidazolium salts bearing less bulky
substituents in the 1- and 3-positions.[4]
Since then, a wide variety of aminocarbenes have been
synthesized, including the first air-stable carbene in 1997
[Eq. (2)]. A solid sample of the stable carbene 5 exposed to

Enders and co-workers reported the first synthesis of the

crystalline triazole-derived carbene 13 by the thermal decom-
position of the 5-methoxytriazole 12 [Eq. (5)].[11] Mention

air did not show any decomposition even after two days.[5] The
remarkable stability of 5 was explained on the basis of the
electronegative effect of the chlorine atoms, which reduce the
reactivity and make the carbene air stable. Very recently, Cole
and co-workers have reported the synthesis of bromo
analogue of 5 from 4 by treatment with carbon tetrabromide.
The carbene 6 thus generated was characterized by single- may also be made of an earlier method for the synthesis of
crystal X-ray analysis and found to be indefinitely stable in alkyl-substituted N-heterocyclic carbenes by the reaction of
air.[6] potassium with cyclic thiones in THF.[12]
The commonly used Arduengo prescription for the FBrstner and co-workers reported the synthesis and
generation of N-heterocyclic carbenes involves the deproto- isolation of N-heterocyclic carbenes consisting of pendant
nation of azolium salts with NaH, KH, or KOtBu[4] in THF. alkenes and C H acidic sites; the structures of these have
Enhanced rates of deprotonation with NaH or KH have been been established by single-crystal X-ray analysis [Eq. (6)].[13]
observed with the addition of catalytic amounts of DMSO or
KOtBu.[1, 7] More recently, Herrmann and co-workers have
developed a more general and efficient route. NaH or KH in
liquid ammonia was used to rapidly convert the azolium salts
to their corresponding carbenes in a homogeneous phase.[8]
An illustration of this strategy is the synthesis of the stable
biscarbene 8 [Eq. (3)] and the triscarbene 10 [Eq. (4)].[9]
Stable N-functionalized “pincer” biscarbenes have also been
reported and used in the synthesis of various organometallic
catalysts.[10]

Vijay Nair has PhD degrees from the Bana- Santhamma Bindu obtained her MSc de-
ras Hindu University (1967, with Professor gree in chemistry (first rank) from Mahat-
R. H. Sahasrabudhey) and the University of ma Gandhi University. She completed her
British Columbia (1969, with Jim Kutney), PhD thesis (2003) under the supervision of
and he was a postdoctoral fellow with Gil- Dr. Vijay Nair at the Regional Research
bert Stork at Columbia University. After a Laboratory (CSIR). Subsequently she joined
16-year career (Outstanding Scientist the group of Professor Robert Coates at the
Award, 1981) with Lederle Laboratories University of Illinois in Urbana-Champaign
(American Cyanamid Company) in Pearl as a postdoctoral fellow.
River, NY, he returned to India and joined
the Regional Research Laboratory (CSIR) in
1990. From 1997 to 2001 he was the Direc-
tor of the Institute. Presently he is continuing as a Director-Grade Scien-
tist. In addition, he is an Honorary Professor in the Cochin University of
Science and Technology.

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Angewandte
Chemie V. Nair et al.

Very recently Bertrand reported a novel procedure using

bis(trimethylsilyl)mercury for generating diaminocarbenes,
and it has been applied to the synthesis of the stable six-
memebered NHC 18 [Eq. (7)].[14] The generality of this
reaction remains to be established.

of carbene 4 with its azolium salt [Eq. (9)].[20a] Stable carbene

4 also forms C H···C (p) complexes with hydrocarbons such
as indene and fluorene.[20b]

3. Reactivity of N-Heterocyclic Carbenes

By virtue of their strong s-donating ability, N-heterocyclic

carbenes have found impressive use as ligands in the
preparation of catalysts in organometallic chemistry. It is The reactivity of stable diaminocarbenes towards water,
worthy of note that the aminocarbene-incorporated ruthe- oxygen, and hydrogen has also been investigated. Imidazolin-
nium alkylidene catalysts were found to be more versatile 2-ylidenes underwent instant hydrolysis on exposure to moist
than the conventional Grubbs catalyst in olefin metathesis THF, whereas hydrolysis of the aromatic congeners such as
reactions.[15d] Although we will not address the application of 1,3-di(tert-butyl)imidazol-2-ylidene to the corresponding al-
NHCs in organometallic chemistry here, since it is not directly dehydes took days.[21] These carbenes were found to be inert
relevant to the present account, we note that important towards oxygen and hydrogen, but in the presence of
contributions to this area have been made by the groups led hydrogen and a platinum or palladium catalyst, they under-
by Herrmann,[2c, 15a] Grubbs,[15b,c] Cavell,[16] and Nolan.[2d,c] went slow hydrogenation.
Excellent reviews on the subject are available in the Aminocarbenes such as triazolylidene have been shown to
literature.[2a,b] insert into strongly polar X H bonds (X = OR1, NR2) to
Alder and co-workers determined the nucleophilicity and afford the corresponding 1,1-addition product. The insertion
basicity of various aminocarbenes based on the Brønsted– of such species into unpolarized C H bonds, however, has not
Lowry concept. They reported the pKa of 1,3-diisopropyl-4,5- been reported so far.[11]
dimethyl-imidazol-2-ylidene as 24 in [D6]DMSO and found Imidazolin-2-ylidene 23 was shown to react with methyl
that it is a much stronger base than 1,5-diazabicyclo[3.4.0]- iodide and dichloromethane to afford the olefins 25 and 27
non-5-ene (DBN), 1,8-diazabicyclo[5.4.0]undec-7-ene along with the corresponding imidazolinium salts [Eq. (10)].[5]
(DBU), and proton sponge (1,8-bis(dimethylamino)naphtha- Similar addition products were obtained from the reaction of
lene) but weaker than phosphazene bases.[17] Recently imidazol-2-ylidenes with trimethylsilyl iodide.[22]
Streitweiser and Kim calculated the pKa of 1,3-di(tert-
butyl)imidazol-2-ylidene in THF as 20, which is much less
than that of the dimesityl derivative reported by Alder.[18] The
pronounced basicity of the N-heterocyclic carbene 4 is
attested by the isolation of crystalline compounds with
organic acids such as phenols [Eq. (8)].[19]
Similarly, Arduengo et al. reported the isolation of stable
hydrogen-bonded bis(carbene) complex 22 by the interaction

Vellalath Sreekumar obtained his MSc de-

gree in chemistry from the University of Cal-
icut (2001). Currently he is a research
fellow working towards his PhD in the group The reaction of 1,3-dimesitylimidazol-2-ylidene with di-
of Dr. Vijay Nair at the Regional Research azo compounds such as diazofluorene and diphenyldiazo-
Laboratory (CSIR).
methane afforded the corresponding azines as the addition
products [Eq. (11)], whereas the reaction of the carbene with

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 Angewandte
Carbene Reagents Chemie

azidotrimethylsilane furnished the imine by Staudinger by Enders et al. [Eq. (13)].[27a,b] Independently, Kerr and
ligation of the azide mediated by phosphanes.[23] Alaniz showed that the use of a fused chiral triazolium salt
As early as 1958, Breslow recognized the role of N- leads to the product in higher yield and enantioselectivity.[27c]
heterocyclic carbenes as nucleophilic catalysts in enzymatic
reactions. His seminal work showed that the vitamin B1
enzyme cofactor thiamine (29), a naturally occurring thiazo-
lium salt, plays a crucial role in biochemical transformations
(Figure 1).[24] As thiamine diphosphate, it catalyzes the

The potential utility of NHCs as catalysts in nucleophilic

aromatic substitution reactions was demonstrated by Miya-
shita et al. in their report on the acylation of aryl fluorides
catalyzed by imidazol-2-ylidene [Eq. (14)].[28]
Figure 1. Thiamine (29) and the derived carbene 30.

decarboxylation of pyruvic acid to active acetaldehyde as

well as the benzoin condensation of aromatic aldehydes. The
active species involved in this reaction was found to be the
thiazolylidene 30 (Scheme 1).

Stable NHCs have been found to be very efficient

catalysts for transesterification and acylation reactions
[Eq. (15)].[29]

Scheme 1. Mechanism of the benzoin condensation catalyzed by thia-

mine.
A number of imidazolium salts have found widespread
use as ionic liquids. Afonso and co-workers reported on the
Aliphatic aldehydes are also reported to undergo benzoin- beneficial properties of imidazolium salts as ionic liquids in
type condensation called the Stetter reaction under ylidene the Baylis–Hillman reaction.[30] However, in a more recent
catalysis. Several azolium salts such as imidazolium, thiazo- study, it was shown that the use of imidazolium salts as ionic
lium, and triazolium salts were found to be effective as liquids in the Baylis–Hillman reaction results in low yields of
catalysts for this transformation. When a,b-unsaturated products due to side reactions of the carbene generated with
ketones are employed, the reaction is called the Michael– aldehydes.[31] Importantly, the study demonstrated that the
Stetter reaction; a typical reaction involving an imidazolium deprotonation of imidazolium salts requires only mild bases
salt, an a,b-unsaturated ketone, and an aldehyde is shown in such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and 3-hy-
Equation (12).[25] droxyquinuclidine.
The reactivity of stable diaminocarbenes towards C C
multiple bonds has been studied by Enders and co-workers,
who showed that unlike other singlet carbenes, the triazolyl-
idene 13 does not furnish any cyclopropane derivative with
dimethyl fumarate; instead it affords the methylene triazoline
derivative 49 [Eq. (16)]. According to them, the initial event

The polycondensation of formaldehyde under yli-

dene catalysis leads to the formation of carbohy-
drates.[26] An asymmetric version of the Michael–Stetter
reaction employing a chiral azolium salt for the syn-
thesis of the benzopyran derivative 39 was first reported

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in this reaction is the [2+1] cycloaddition of the carbene with

the alkene to form the cyclopropane derivative 47, which
undergoes rapid ring opening to afford the zwitterionic
intermediate 48. Subsequent [1,2]-hydrogen shift affords the
methylene triazoline derivative 49.[11]
Similarly, the reaction of 13 with N-alkyl or N-aryl
maleimides afforded the corresponding 1:1 and 1:2 adducts
depending on the conditions employed [Eqs. (17a) and
(17b)].[32]

we embarked on a systematic investigation of the reactivity of

various N-heterocyclic carbenes towards activated acetylenes
A similar reaction of triazolylidene 13 with dimethyl and aldehydes. Our studies started with the reaction of an
acetylenedicarboxylate(DMAD) afforded the spiro com- aminocarbene, which was generated in situ from 67, with
pound 54, which rearranged to the more stable bicyclic DMAD and aldehyde; the corresponding 2-oxymaleate
compound 55 on heating [Eq. (18)].[33] Presumably, 54 arises derivative 69 was obtained in good yield [Eq. (23)].[37]

by the addition of a second molecule of DMAD to the initially The reaction was found to be sensitive to the nature of the
formed zwitterion. carbene employed. When the less nucleophilic 1,3-dimesityl-
A mechanistically related reaction involving 13 and excess imidazol-2-ylidene was employed, the reaction followed a
phenyl isocyanate led to the formation of the spiro compound different but interesting pathway leading to the furanone
58, presumably by means of a [3+2] cycloaddition with the derivative 70 in good yield [Eq. (24)]. Although the mecha-
intermediate betaine 57 [Eq. (19)].[33] nistic underpinnings are yet to be investigated, these reactions
point to a rich and fascinating area of research that lies ahead.

In a recent report, Rigby and Wang have shown that the

[4+1] cycloaddition of N-heterocyclic carbenes with vinyl
isocyanates and vinyl ketenes leads to functionalized hydro- 4. Conclusion
indolone [Eq. (20)].[34a] and cyclopentenone derivatives
[Eq. (21)].[34b] In conclusion, the chemistry of NHCs reviewed in this
The [4+1] cycloaddition of 64 with diphenyl tetrazine has article underlines the enormous potential of these species in
been shown to afford the spiro compound 66 [Eq. (22)].[35] effecting a wide range of synthetic applications. In particular,
As a part of our continued interest in devising novel the close analogy of NHCs to phosphanes makes them
multicomponent reactions based on nucleophilic carbenes,[36] valuable as powerful nucleophiles in many synthetically

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Carbene Reagents Chemie

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