DOI: 10.1002/chem.201406318

Concept

& Gold Chemistry

Gold Carbene or Carbenoid: Is There a Difference?
Yahui Wang,[a] Michael E. Muratore,[a] and Antonio M. Echavarren*[a, b]

Chem. Eur. J. 2015, 21, 7332 – 7339

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Abstract: By reviewing the recent progress on the elucidation of the structure of gold carbenes and the definitions of metal carbenes and carbenoids, we recommend
to use the term gold carbene to describe gold carbene-like
intermediates, regardless of whether the carbene or carbocation extreme resonance dominates. Gold carbenes,
because of the weak metal-to-carbene p-back-donation
and their strongly electrophilic reactivity, could be classified into the broader family of Fischer carbenes, although
their behavior and properties are very specific.

Introduction
Over the last decade, the use of gold complexes as carbophilic
p-acids has become a powerful tool for building molecular
complexity in an atom-economical fashion.[1] Gold carbenes
have very often been proposed as the key intermediates in
many gold-catalyzed transformations (Scheme 1).[1–3]

Scheme 1. Carbene and carbocation resonance forms of gold carbenes.

They are structurally related to singlet carbenes and possess
somewhat related reactivity. Most general methods for the
generation of gold carbene intermediates are summarized in
Scheme 2 and include the 1,2-acyloxy migration of propargylic
carboxylates (Scheme 2a),[4] cycloisomerization of 1,6-enynes
(Scheme 2b),[1b,k, 5] decomposition of diazo compounds
(Scheme 2c)[6, 7] and ring cleavage of cyclopropenes
(Scheme 2d).[8] Other gold(I)-catalyzed transformations based
on the oxidation of alkynes with pyridine N-oxides or
sulfoxides (Scheme 2e),[1i, 9, 10] acetylenic Schmidt reactions
(Scheme 2f),[11] the formation of gold vinylidene intermediates
from (2-ethynylphenyl)alkynes (Scheme 2g),[1h, 12] and the retroBuchner reaction of cycloheptatrienes (Scheme 2h)[13] have also
been proposed to generate gold(I) carbene-type intermediates.
Under stoichiometric conditions, the a-hydride abstraction of
alkylgold(I) complexes also gives gold(I) carbenes.[14]

[b] Prof. A. M. Echavarren
Departament de Química Analítica i Química Orgànica,
Universitat Rovira i Virgili, C/Marcel·lí Domingo s/n,
43007 Tarragona (Spain)
 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution Non-Commercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited and is not used for commercial purposes.

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Despite their central role in gold(I) catalysis, the nature of
gold carbenes has remained uncertain and even this very term
has been questioned.[15] The term “gold carbenoid” has even
been recommended to stress the carbocationic nature of these
intermediates.[16] Beyond the terminological debate,[17] the isolation and structural characterization of several well-defined
[LAuCR2] + complexes has provided new insight into the nature
of these key gold(I) species. Here we review these findings and
discuss the advantages of distinguishing gold carbenes and
carbenoids as different species.

The Structure of Gold Carbenes

[a] Y. Wang, Dr. M. E. Muratore, Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
¨
Av. Paısos Catalans 16, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es

Chem. Eur. J. 2015, 21, 7332 – 7339

Scheme 2. Most general methods for the generation of gold carbenes.

A fundamental description of the bonding mode of gold carbene was proposed by Toste and Goddard in 2009.[18] Accordingly, the ligand L and carbene can both donate their paired
electrons to gold, forming a three-center four-electron s-hyperbond. The gold center can also form two p-bonds by backdonation of its electrons from two filled 5d orbitals to empty
p-acceptors on the ligand and carbene (Scheme 3).
Considering the competition between ligand and carbene
for the electron density from gold, the ligand and substituent
have a significant influence on the bonding and reactivity of

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Concept

Scheme 3. Schematic representation of the bonding of gold carbenes.

a given gold carbene. An increase in gold-to-carbon p-bonding
and a decrease in the s-bonding leads to structures with more
carbene-like character (Scheme 4). Thus, strongly s-donating
and weakly p-acidic ligands, such as NHCs (N-heterocyclic carbenes), MICs (mesoionic carbenes),[19] or CAACs (cyclic (alkyl)(amino)carbenes)[20] are expected to increase the carbene-like
reactivity (Scheme 5).

emblematic examples involves the stereospecific trapping of
cyclopropyl gold(I) carbenes by alkenes.[5b, 21, 22] However, cyclopropanation of polarized alkenes, such as styrenes[21] or enol
ethers,[3a, b, 23] occur stepwise, although the overall process is
still stereospecific, since the second carbon–carbon bond formation occurs through a rather low energy barrier.[21]
Another illustrative example of a ligand-controlled gold(I)catalyzed reaction involving a gold(I) carbene-like intermediate
is the addition of indole to a 1,6-enyne to give two products
(Scheme 7).[24] A gold(I) complex with a strongly electron-do-

Scheme 4. Influence of the ligand on the carbocation-like or carbene-like
properties of gold carbenes.[18]
Scheme 7. Site-selectivity in the gold(I)-catalyzed nucleophilic attack of
indole on a 1,6-enyne.[24b]

Scheme 5. NHC ligands increase the back-donation from gold to carbene.[18]

For example, it was shown that a gold complex with IPr (1,3bis(2,6-diisopropylphenyl)imidazolidene) as the ligand was the
best catalyst for the cyclopropanation of cis-stilbene,[18] which
is a typical carbene-type transformation (Scheme 6). In contrast, poor s-donating and p-acidic ligands, such as phosphites, increase the carbocation-like character[7] of the intermediates by decreasing the electron density around gold and
thus the gold-to-carbene p-donation.
Gold(I) carbenes have been proposed as intermediates in
other types of cyclopropanation reactions.[1] One of the most

Scheme 6. NHC ligands favor carbene-type transformations.[18]
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nating NHC ligand favors attack at the carbene center of the
intermediate, whereas reaction at the cyclopropyl moiety is favored using a phosphine–gold(I) complex. This result can
again be explained by the enhancement of the carbene-like
character of the cyclopropyl gold(I) carbene intermediate by
the strongly donating carbene ligand. Regarding the nature of
the intermediates formed in the reactions of 1,6-enynes, according to theoretical calculations, their structures are intermediate between cyclopropyl gold(I) carbenes and gold(I)-stabilized cyclopropylmethyl/cyclobutyl/homoallyl carbocations.[25]
The reactivity displayed by intermediates formed in the
gold(I)-catalyzed retro-Buchner reaction (Scheme 2h) has been
shown to be more similar to that of metal carbenes of rhodium or copper, or even free carbenes, than carbocations.[13b]
Thus, like free o-biphenylcarbene, 2-cycloheptatrienyl biphenyls give rise to fluorenes,[13b] whereas an o-phenylbenzyl carbocation failed to undergo a similar cyclization (Scheme 8).[26]
Benzylidene [(NHC)AuCHPh] + and methoxymethylidene
[(NHC)AuCHOMe] + gold(I) carbenes have been generated in
the gas phase by the collision-induced dissociation of ylide
precursors and undergo cyclopropanation reactions with electron-rich alkenes.[3, 23] A related study has been carried out in
the gas phase on related phosphine complexes of formal
[LAuCHR] + .[27] Interestingly, for the series of [(NHC)MCHOMe] +
coinage metal cations, gold(I) and copper(I) stabilize the carbene more significantly than silver(I), which binds weakly to
the methoxymethylidene moiety due to a weak p-back-donation.[3d]

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Table 1. Bond lengths of several gold carbene complexes.
Structure

CÀAu
bond
length [Š]

Ref.

1

2.02(3)

[28]

2

2.010(10)

[29]

Scheme 8. Formation of fluorenes via retro-Buchner reaction.[13b]
X=H
2.046(5)
X = OMe
2.0398(15)

3

A comparison of bond lengths should provide information
about the bonding mode of a given metal carbene. Representative bond lengths for the heteroatom-stabilized gold(I) complexes (Table 1, entries 1–5),[16, 28–31] are rather similar with that
of a C(sp2)ÀAu single bond (e.g., the AuÀC bond length of
gold complex [Ph3PAuC6H5] is 2.045(6) Š[32]). The observation of
shortened CÀO or CÀN bonds demonstrates that the heteroatoms strongly stabilize the carbene center by sharing the electrons of one of their lone pairs, which is characteristic of Fischer carbenes. A similar observation was made for gold–allenylidene species[12e] and for complexes formed in the cyclization of
allenoates.[33]
More recently, Fürstner and co-workers isolated the first
gold carbene complex without heteroatom stabilization, which
is more closely related to the gold(I) carbene intermediates
proposed in many gold(I)-catalyzed reactions (Table 1,
entry 6).[16] In this case, the bond between the carbene center
and the aryl group is shortened appreciably. The more contracted CÀC bond corresponds to an aryl ring that is nearly coplanar with the carbene center to enable efficient orbital overlap, which stabilizes the electron-deficient center. However,
since the 4-methoxylphenyl substituent is a very strong electron-donating group, its strong ability to stabilize the carbene
center gives gold little opportunity to back-donate electrons
to the empty p orbital of carbene.
Predictably, in cases in which the carbene center cannot
obtain sufficient stabilization from substituents, the back-donation from gold is a source of stabilization, and the gold carbene bonding model is still a pertinent representation. This
prediction was proved by the group of Straub using a sterically
demanding NHC ligand (Scheme 9) to form a gold carbene
complex (Table 1, entry 7).[34] The AuÀCMes2 bond (2.014(6) Š)
is slightly shorter than the AuÀC(IPr**) bond (2.030(6) Š). According to Bent’s rule,[35] the more electronegative nitrogen
atoms in the IPr** ancillary ligand and the larger Au-C-N angles
lead to a greater s character in the AuÀC(IPr**) s-bond than in
the AuÀCMes2 a-bond. Since NHCs are strong s-donating and
very weak p-acidic ligands, the p-back-bonding in AuÀC(IPr**)
is negligible. Thus, without p-back-bonding, the expected AuÀ
CMes2 bond should have been longer than the AuÀC(IPr**)
bond; however, the opposite was observed, which was attributed to a significant, although not predominant, double bond
character in Au=CMes2. Furthermore, this research group
found that the activation energy for the rotation of the mesityl
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4

[30]
[16]

5[a]

2.032(4)

[31]

6

2.039(5)

[16]

7[b]

2.014(6)

[34]

8

2.035(6)

[40]

9

1.984(4)

[41]

[a] L = JohnPhos. [b] For IPr** see Scheme 9.

Scheme 9. Rotation of the mesityl substituents in [IPr**Au=CMes2].[34]

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Concept
group is approximately 9 kcal molÀ1, which also supports the
partial double bond character of the Au=CMes2 bond
(Scheme 9).[34] It is important to note that, although this barrier
is low, a 12.4 kcal molÀ1 barrier of rotation was determined for
the first Fischer carbene complex [(CO)5Cr=C(OMe)Me].[36]
Indeed, typical rotational barriers in a wide variety of alkylidene metal complexes are in the range of 10 to 18 kcal
molÀ1.[37, 38]
As noted by Toste and Goddard,[18] the overall bond order of
gold(I) carbenes is generally less than or equal to one, much
like the double “half-bond” model proposed for rhodium carbenes,[39] which is not well represented either by the carbene
resonance (bond order = 2) or the carbocationic resonance
(bond order = 1). Straub attributed this phenomenon to the
antibonding interaction of gold 5d shell with the sp2-hybridized electron pair of the carbene ligand, which weakens the sbond of gold carbene and therefore decreases the observed
bond order of gold carbenes.[34]
Back-donation from gold to carbene was also found by the
group of Widenhoefer when they compared the bond lengths
difference of two distinct CÀC bonds of a cyclopropyl group
with that of some known cyclopropyl cationic compounds
(Table 1, entry 5).[31] These authors concluded that the carbocation stabilizing ability of LAu fragment is similar to that of
a cyclopropyl group, and it exceeds that of a methyl or phenyl
group. Widenhoefer also synthesized a cycloheptatrienylidene
gold(I) complex bearing the bulky phosphine (o-biphenyl)P(tBu)2 (Johnphos) ligand (Table 1, entry 8).[40] Although, as expected, the cycloheptatrienylidene protons were characteristic
of a tropylium cation, shifted downfield in the 1H NMR spectrum with respect to the protons of neutral (Johnphos)Au–cycloheptatrienyl complex, the relatively compressed C7-C1-C2
bond angle (123.388) of the cycloheptatrienylidene ligand suggested a contribution of a gold carbene resonance form. For
related gold–allenylidene complexes, DFT calculations supported a higher allenylidene character for species that do not possess other heteroatom stabilization.[12e]
Very recently, the group of Bourissou reported the synthesis
and structural characterization of a remarkable diphenylcarbene complex [L2Au=CPh2] + with an o-carborane diphosphine,
which shows the shortest AuÀC bond (1.98 Š) to date (Table 1,
entry 9).[41] The bending induced by the o-carborane diphosphine ligand substantially raises the energy of the dxz(Au) orbital, increasing significantly the p-back-donation to the carbene
ligand. Interestingly, the bonds between the carbene center
and the Cipso(Ph) carbon atoms are not contracted and the two
phenyl rings are significantly twisted from the carbene plane,
implying a very weak p-stabilization of the carbene by these
substituents. The related carbonyl complex [L2AuCO] + was also
prepared as the first carbonyl complex of gold with a CO
stretching frequency lower than that of free CO (2113 vs.
2143 cmÀ1), which again is indicative of a substantial Au to CO
p-back-donation.[41]

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Carbenoids and Carbenes
The IUPAC definition of carbenoids as “complexed carbene-like
entities that display the reactivity characteristics of carbenes,
either directly or by acting as sources of carbenes”[42] is rather
vague and would lead, for example, to classify Fischer carbenes as carbenoids, because of their known involvement in
cyclopropanation reactions with electron-rich alkenes.[43] Often,
the term carbenoid is used to describe organometallic species
[LnMCH2X] that contain a good leaving group and a carbon–
metal bond at the same site, such as the Simmons–Smith reagent (iodomethylzinc iodide) and related species.[44]
More general, and didactically useful, is the distinction between carbenoids and carbenes as different species that are,
however, related by the addition/elimination of an XÀ group,
regardless of the double bond character of the metal–carbon
bond in the carbene (Scheme 10).[45] Thus, [(Ph3P)2M(CH2Cl)Cl]

Scheme 10. Carbenoid and carbene equilibrium and isolated gold(I) carbenoids.[47b]

(M = Pd, Pt) are two well-characterized examples of d8 square
planar Group 11 metal carbenoids.[46] Indeed, two authentic
gold(I) carbenoids have been prepared and structurally characterized.[47] As expected, the metal–carbon bond length in these
gold(I) carbenoids (2.09 Š) is longer than in gold(I) carbenes of
Table 1.
The terminological distinction between gold(I) carbenes and
carbenoids is also useful in the context of mechanistic discussions. Thus, a recent study in the gas-phase strongly suggests
that the gold(I)-catalyzed oxidation of alkynes by pyridine Noxides actually gives gold(I) carbenoids instead of a-oxo carbenes (Scheme 11).[10] a-Oxo carbenes might be formed by
elimination of pyridine from the initially adducts, but presumably react rapidly with pyridine to form the more stable gold(I)
carbenoids.
Metal-coordinated carbenes, [LnM=CR2], have traditionally
been divided into singlet-derived Fischer carbenes and tripletderived Schrock carbenes.[48] Fischer carbenes usually contain
a p-donating group, such as ÀOMe or ÀNMe2,[49] on the carbene carbon atom to stabilize the empty p orbital on the carbene carbon atom by p-donation from one of the heteroatom
lone pairs. This stability effect makes the metal-to-carbene pback-donation weak and the direct carbene-to-metal s-donation predominates. Therefore, the carbon atom is positively
charged, and the carbene behaves as an electrophilic species

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Scheme 11. Generation of gold(I) carbenoids in the oxidation of alkynes
with pyridine N-oxides.[10]

(Scheme 12).[50] Thus, typical Fischer carbenes can be viewed as
metal- and heteroatom-stabilized carbocations or more simply
as metalated carbocations.[48b] This is the case for metal carbenes of late transition metals, which show higher electronegativity and more stable metals (dp). In the case of Schrock carbenes, two covalent bonds between metal and triplet carbene
are formed, and each metal–carbon bond is polarized toward
the carbon atom because of its higher electronegativity. As
a result, the carbon atom tends to be more negatively
charged, and the carbene exhibits nucleophilic character.
Schrock carbenes can also be viewed as a Fischer carbenes
with very strong back-donation.

Although the term carbenoid for [LAuCR2] + species has been
preferred by many authors to underline their carbocationic
character and poor p-back-donation from gold(I), this would
be less appropriate for complexes such as those characterized
by Straub[34] and Bourissou.[41] In our opinion, the term carbenoid should be better applied to metal complexes such as
[LAuCH2Cl],[47] which bear a clear structural similarity to well-established zinc carbenoids involved in the Simmons–Smith cyclopropanation. Therefore, to conclude, we agree with the description of gold carbenes formulated by Toste and Goddard:[18] “The reactivity in gold(I)-coordinated carbenes is best
accounted for by a continuum ranging from a metal-stabilized
singlet carbene to a metal-coordinated carbocation. The position of a given gold species on this continuum is largely determined by the carbene substituents and the ancillary ligand”.

Acknowledgements
We thank MINECO (Severo Ochoa Excellence Accreditation
2014–2018 (SEV-2013-0319) and project CTQ2013-42106-P), the
European Research Council (Advanced Grant No. 321066), the
AGAUR (2014 SGR 818), and the ICIQ Foundation. M.E.M. acknowledges the receipt of a COFUND (Marie Curie program)
postdoctoral fellowship.
Keywords: carbenes · carbenoids · carbocations · Fischer
carbenes · gold

Scheme 12. The bonding structure of typical Fischer carbenes.

The back-donation ability is expected to vary depending on
the electronegativity of the metal complex, with the s-bond
being stronger and the p-bond becoming weaker as the metal
becomes more electrophilic. Therefore, it is hardly surprising
that in the case of gold, the most electronegative of all transition metals, the corresponding carbenes also exhibit special
characteristics. Thus, from the experiments and data analysis, it
is clear that the gold(I)-to-carbene back-donation does exist,
especially when the structure is not highly stabilized by strong
electron-donating groups (e.g., heteroatoms or electron-rich
arenes). Strongly electrophilic cationic gold(I) carbenes are
structurally related to the Fischer carbene family and are an extreme case in which the metal-to-carbene p-back-donation is
weak. It is important to note that although Fischer carbenes
usually have a heteroatom substituent at the carbene center,
this is not a necessary condition.[49b, 51]
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