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N-Heterocyclic Carbenes Reagents Not Just Ligands!.

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V. Nair et al.
Carbene Reagents
N-Heterocyclic Carbenes: Reagents, Not Just Ligands!
Vijay Nair,* Santhamma Bindu, and Vellalath Sreekumar
Keywords:
carbenes · cycloadditions · Michael–Stetter reaction ·
multicomponent reactions · transesterification
Dedicated to Professor Gilbert Stork
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
relative to that of the carbenes derived from other nitrogen
heterocycles.
1. Introduction
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
Arduengo et al. in 1991.[1] The close analogy of NHCs to
trialkylphosphanes and their excellent s-donating properties
make them ligands of choice for transition metals, thus
leading to the preparation of organometallic catalysts of
enormous utility in organic synthesis.[2] NHC-containing
organometallic catalysts are found to be much more effective
than conventional catalysts in a number of reactions, for
example, the Heck reaction and olefin metathesis. Although
the use of NHCs in coordination chemistry and organometallic reactions has been studied extensively, very little is
known about the fundamental chemistry of these species. The
purpose of this review is to cast some light on the chemistry of
NHCs and to underscore the fact that, apart from being
excellent ligands for palladium[16] and related metals, they
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)
Fax: (+ 91) 471-249-1712
E-mail: vijaynair_2001@yahoo.com
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2. History of N-Heterocyclic Carbenes
Studies on NHCs date back to the work of Wanzlick in the
1960s.[3] Although Wanzlick was unsuccessful in isolating any
carbenes at that time, his recognition that a carbene center at
the 2-position of the imidazole ring would be stable due to the
electron-donating effects of adjacent nitrogen atoms provided
the conceptual framework for the development of the
chemistry of these species. As mentioned earlier, the current
growth in the chemistry of NHCs is mainly ascribed to the
pioneering work of Arduengo and co-workers, who isolated a
stable crystalline N-heterocyclic carbene by the deprotonation of bis(1-adamantyl)imidazolium chloride with sodium
hydride in tetrahydrofuran in the presence of a catalytic
amount of dimethyl sulfoxide [Eq. (1)].[1]
The carbene isolated was found to be a thermally stable
crystalline compound whose structure was unequivocally
established by single-crystal X-ray analysis. The unusual
DOI: 10.1002/anie.200301714
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Carbene Reagents
stability of 2 was explained by a number of factors, for
example, the large singlet–triplet energy gap in imidazol-2ylidene (~ 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 decomposition 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 singlecrystal X-ray analysis and found to be indefinitely stable in
air.[6]
The commonly used Arduengo prescription for the
generation of N-heterocyclic carbenes involves the deprotonation of azolium salts with NaH, KH, or KOtBu[4] in THF.
Enhanced rates of deprotonation with NaH or KH have been
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]
may also be made of an earlier method for the synthesis of
alkyl-substituted N-heterocyclic carbenes by the reaction of
potassium with cyclic thiones in THF.[12]
FBrstner and co-workers reported the synthesis and
isolation of N-heterocyclic carbenes consisting of pendant
alkenes and C H acidic sites; the structures of these have
been established by single-crystal X-ray analysis [Eq. (6)].[13]
Vijay Nair has PhD degrees from the Banaras Hindu University (1967, with Professor
R. H. Sahasrabudhey) and the University of
British Columbia (1969, with Jim Kutney),
and he was a postdoctoral fellow with Gilbert Stork at Columbia University. After a
16-year career (Outstanding Scientist
Award, 1981) with Lederle Laboratories
(American Cyanamid Company) in Pearl
River, NY, he returned to India and joined
the Regional Research Laboratory (CSIR) in
1990. From 1997 to 2001 he was the Director of the Institute. Presently he is continuing as a Director-Grade Scientist. In addition, he is an Honorary Professor in the Cochin University of
Science and Technology.
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Santhamma Bindu obtained her MSc degree in chemistry (first rank) from Mahatma Gandhi University. She completed her
PhD thesis (2003) under the supervision of
Dr. Vijay Nair at the Regional Research
Laboratory (CSIR). Subsequently she joined
the group of Professor Robert Coates at the
University of Illinois in Urbana-Champaign
as a postdoctoral fellow.
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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 sixmemebered 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
worthy of note that the aminocarbene-incorporated ruthenium alkylidene catalysts were found to be more versatile
than the conventional Grubbs catalyst in olefin metathesis
reactions.[15d] Although we will not address the application of
NHCs in organometallic chemistry here, since it is not directly
relevant to the present account, we note that important
contributions to this area have been made by the groups led
by Herrmann,[2c, 15a] Grubbs,[15b,c] Cavell,[16] and Nolan.[2d,c]
Excellent reviews on the subject are available in the
literature.[2a,b]
Alder and co-workers determined the nucleophilicity and
basicity of various aminocarbenes based on the Brønsted–
Lowry concept. They reported the pKa of 1,3-diisopropyl-4,5dimethyl-imidazol-2-ylidene as 24 in [D6]DMSO and found
that it is a much stronger base than 1,5-diazabicyclo[3.4.0]non-5-ene
(DBN),
1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), and proton sponge (1,8-bis(dimethylamino)naphthalene) but weaker than phosphazene bases.[17] Recently
Streitweiser and Kim calculated the pKa of 1,3-di(tertbutyl)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 degree in chemistry from the University of Calicut (2001). Currently he is a research
fellow working towards his PhD in the group
of Dr. Vijay Nair at the Regional Research
Laboratory (CSIR).
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The reactivity of stable diaminocarbenes towards water,
oxygen, and hydrogen has also been investigated. Imidazolin2-ylidenes underwent instant hydrolysis on exposure to moist
THF, whereas hydrolysis of the aromatic congeners such as
1,3-di(tert-butyl)imidazol-2-ylidene to the corresponding aldehydes took days.[21] These carbenes were found to be inert
towards oxygen and hydrogen, but in the presence of
hydrogen and a platinum or palladium catalyst, they underwent slow hydrogenation.
Aminocarbenes such as triazolylidene have been shown to
insert into strongly polar X H bonds (X = OR1, NR2) to
afford the corresponding 1,1-addition product. The insertion
of such species into unpolarized C H bonds, however, has not
been reported so far.[11]
Imidazolin-2-ylidene 23 was shown to react with methyl
iodide and dichloromethane to afford the olefins 25 and 27
along with the corresponding imidazolinium salts [Eq. (10)].[5]
Similar addition products were obtained from the reaction of
imidazol-2-ylidenes with trimethylsilyl iodide.[22]
The reaction of 1,3-dimesitylimidazol-2-ylidene with diazo compounds such as diazofluorene and diphenyldiazomethane afforded the corresponding azines as the addition
products [Eq. (11)], whereas the reaction of the carbene with
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azidotrimethylsilane furnished the imine by Staudinger
ligation of the azide mediated by phosphanes.[23]
As early as 1958, Breslow recognized the role of Nheterocyclic carbenes as nucleophilic catalysts in enzymatic
reactions. His seminal work showed that the vitamin B1
enzyme cofactor thiamine (29), a naturally occurring thiazolium salt, plays a crucial role in biochemical transformations
(Figure 1).[24] As thiamine diphosphate, it catalyzes the
by Enders et al. [Eq. (13)].[27a,b] Independently, Kerr and
Alaniz showed that the use of a fused chiral triazolium salt
leads to the product in higher yield and enantioselectivity.[27c]
The potential utility of NHCs as catalysts in nucleophilic
aromatic substitution reactions was demonstrated by Miyashita 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 thiamine.
Aliphatic aldehydes are also reported to undergo benzointype condensation called the Stetter reaction under ylidene
catalysis. Several azolium salts such as imidazolium, thiazolium, and triazolium salts were found to be effective as
catalysts for this transformation. When a,b-unsaturated
ketones are employed, the reaction is called the Michael–
Stetter reaction; a typical reaction involving an imidazolium
salt, an a,b-unsaturated ketone, and an aldehyde is shown in
Equation (12).[25]
A number of imidazolium salts have found widespread
use as ionic liquids. Afonso and co-workers reported on the
beneficial properties of imidazolium salts as ionic liquids in
the Baylis–Hillman reaction.[30] However, in a more recent
study, it was shown that the use of imidazolium salts as ionic
liquids in the Baylis–Hillman reaction results in low yields of
products due to side reactions of the carbene generated with
aldehydes.[31] Importantly, the study demonstrated that the
deprotonation of imidazolium salts requires only mild bases
such as 1,4-diazabicyclo[2.2.2]octane (DABCO) and 3-hydroxyquinuclidine.
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 triazolylidene 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 ylidene catalysis leads to the formation of carbohydrates.[26] An asymmetric version of the Michael–Stetter
reaction employing a chiral azolium salt for the synthesis of the benzopyran derivative 39 was first reported
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V. Nair et al.
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]
A similar reaction of triazolylidene 13 with dimethyl
acetylenedicarboxylate(DMAD) afforded the spiro compound 54, which rearranged to the more stable bicyclic
compound 55 on heating [Eq. (18)].[33] Presumably, 54 arises
by the addition of a second molecule of DMAD to the initially
formed zwitterion.
A mechanistically related reaction involving 13 and excess
phenyl isocyanate led to the formation of the spiro compound
58, presumably by means of a [3+2] cycloaddition with the
intermediate betaine 57 [Eq. (19)].[33]
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 hydroindolone [Eq. (20)].[34a] and cyclopentenone derivatives
[Eq. (21)].[34b]
The [4+1] cycloaddition of 64 with diphenyl tetrazine has
been shown to afford the spiro compound 66 [Eq. (22)].[35]
As a part of our continued interest in devising novel
multicomponent reactions based on nucleophilic carbenes,[36]
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
we embarked on a systematic investigation of the reactivity of
various N-heterocyclic carbenes towards activated acetylenes
and aldehydes. Our studies started with the reaction of an
aminocarbene, which was generated in situ from 67, with
DMAD and aldehyde; the corresponding 2-oxymaleate
derivative 69 was obtained in good yield [Eq. (23)].[37]
The reaction was found to be sensitive to the nature of the
carbene employed. When the less nucleophilic 1,3-dimesitylimidazol-2-ylidene was employed, the reaction followed a
different but interesting pathway leading to the furanone
derivative 70 in good yield [Eq. (24)]. Although the mechanistic underpinnings are yet to be investigated, these reactions
point to a rich and fascinating area of research that lies ahead.
4. Conclusion
In conclusion, the chemistry of NHCs reviewed in this
article underlines the enormous potential of these species in
effecting a wide range of synthetic applications. In particular,
the close analogy of NHCs to phosphanes makes them
valuable as powerful nucleophiles in many synthetically
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useful reactions. Beyond this, the ability of NHCs to
participate as reactants towards a variety of electrophilic
species, leading to novel molecular frameworks offers promise for their use in organic synthesis. It is conceivable that the
exploration of NHCs will uncover much fascinating and
useful chemistry.
Received: September 26, 2003
Revised: March 9, 2004
Published Online: August 19, 2004
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