Structures of Reactive Donor/Acceptor and Donor/Donor Rhodium Carbenes in the Solid State and Their Implications for Catalysis

Article abstract of DOI:10.1021/jacs.5b13321

Owing to its tremendous preparative importance, rhodium carbene chemistry has been studied extensively during past decades. The invoked intermediates have, however, so far proved too reactive for direct inspection, and reliable experimental information has been extremely limited. A series of X-ray structures of pertinent intermediates of this type, together with supporting spectroscopic data, now closes this gap and provides a detailed picture of the constitution and conformation of such species. All complexes were prepared by decomposition of a diazoalkane precursor with an appropriate rhodium source; they belong to either the dirhodium(II) tetracarboxylate carbene series that enjoys widespread preparative use, or to the class of mononuclear half-sandwich carbenes of Rh(III), which show considerable potential. The experimental data correct or refine previous computational studies but corroborate the currently favored model for the prediction of the stereochemical course of rhodium catalyzed cyclopropanations, which is likely also applicable to other reactions. Emphasis is put on stereoelectronic rather than steric arguments, with the dipole of the acceptor substituent flanking the carbene center being the major selectivity determining factor. Moreover, the very subtle influence exerted by the anionic ligands on a Rh(III) center on the chemical character of the resulting carbenes species is documented by the structures of a homologous series of halide complexes. Finally, the isolation of a N-bonded Rh(II) diazoalkane complex showcases that steric hindrance represents an inherent limitation of the chosen methodology.

Full text of DOI:10.1021/jacs.5b13321

Subscriber access provided by KUNGL TEKNISKA HOGSKOLAN
Article
The Structures of Reactive Donor/Acceptor and Donor/Donor Rhodium
Carbenoids in the Solid State and their Implications for Catalysis
Christophe Werlé, Richard Goddard, Petra Philipps, Christophe Farès, and Alois Fürstner
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b13321 • Publication Date (Web): 24 Feb 2016
Downloaded from http://pubs.acs.org on February 25, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 10
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
The Structures of Reactive Donor/Acceptor and Donor/Donor Rhodi-
um Carbenes in the Solid State and their Implications for Catalysis
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Christophe Werlé, Richard Goddard, Petra Philipps, Christophe Farès, and Alois Fürstner*
MaxꢀPlanckꢀInstitut für Kohlenforschung, Dꢀ45470 Mülheim/Ruhr, Germany
ABSTRACT: Owing to its tremendous preparative importance, rhodium carbene chemistry has been studied extensively during
past decades. The invoked intermediates have, however, so far proved too reactive for direct inspection and reliable experimental
information has been extremely limited. A series of Xꢀray structures of pertinent intermediates of this type, together with supportꢀ
ing spectroscopic data, now closes this gap and provides a detailed picture of the constitution and conformation of such species. All
complexes were prepared by decomposition of a diazoalkane precursor with an appropriate rhodium source; they belong either to
the dirhodium(II) tetracarboxylate carbene series that enjoys widespread preparative use,
or to the class of mononuclear halfꢀsandwich carbenes of Rh(III), which show considerꢀ
able potential. The experimental data correct or refine previous computational studies,
but corroborate the currently favored model for the prediction of the stereochemical
course of rhodium catalyzed cyclopropanations (which is likely applicable to other reacꢀ
tions too). Emphasis is put on stereoelectronic rather than steric arguments, with the
dipole of the acceptor substituent flanking the carbene center being the major selectivity
determining factor. Moreover, the very subtle influence exerted by the anionic ligands
on a Rh(III) center on the chemical character of the resulting carbenes species is docuꢀ
mented by the structures of a homologous series of halide complexes. Finally, the isolaꢀ
tion of an Nꢀbonded Rh(II) diazoalkane complex showcases that steric hindrance repreꢀ
sents an inherent limitation of the chosen methodology.
INTRODUCTION
tory is considered more plausible, invoking an “endꢀon” oriꢀ
ented olefin traversing alongside the donor (e. g. arene) subꢀ
stituent (Autschbach/Davies model).12 Moreover, the computꢀ
ed barriers along the reaction coordinate have been consideraꢀ
bly revised over the years.16
Since the first controlled decomposition of a diazo comꢀ
pound with catalytic amounts of Rh2(OAc)4,1,2 this methodolꢀ
ogy has gained considerable importance. The resulting carꢀ
benes power a host of intraꢀ and intermolecular bond forming
reactions or reaction cascades: For example, they engage in
cyclopropanations, cyclopropenations, [4+3] cycloadditions,
insertions into E−H (E = C, Si, O, NR, S) and carbonꢀ
R
R
R
O
Rh
O
O
Rh
O
O
Rh
O
R
R
R
O
O
O
O
O
O
X
O
O
O
O
Rh
O
Rh
O
O
O
O
Rh
O
H
OMe
O
R
O
R
O
R
ꢀ
R
R
R
heteroatom bonds, as well as in a myriad of ylide chemistry.3
O
OMe
O
OMe
O
OMe
10
C
A
Many of these transformations are distinguished by impresꢀ
B
sively high turnꢀover numbers and ꢀfrequencies as well as
truly remarkable levels of chemoꢀ, regioꢀ, diastereoꢀ and enanꢀ
tioselectivity.3ꢀ10 This is particularly true for reactions of carꢀ
benes of type C with a donor/acceptor substitution pattern that
slightly tempers the “superelectrophilic” character of the pure
acceptor analogues A and B.
R1
R2
R1
R2
LnRh
LnRh
E
D
The extensive body of preparative work on rhodium carꢀ
benes in general has led to a good qualitative understanding of
their reactivity profiles. Based on this knowledge it was possiꢀ
ble to conclude, for example, that olefin cyclopropanation
proceeds via a concerted but highly asynchronous mechaꢀ
nism.11,12 This scenario puts emphasis on the resonance exꢀ
treme D in which the reactive species is represented as a metꢀ
alꢀstabilized carbocation.13 Yet, even this consensus model has
witnessed a striking evolution: Specifically, it had originally
been assumed that an alkene approaches a donor/acceptor
carbene C in a “sideꢀon” manner while passing over the accepꢀ
tor (e. g. ester) group.14,15 Today, however, the opposite trajecꢀ
While these amendments certainly reflect advances in comꢀ
putational chemistry,11,12,17 they also emphasize the lack of
experimental calibration points. Relevant dirhodium carbenes
largely defied direct inspection. Only the Xꢀray structures of
various NHC adducts such as 1 are known.18,19 Although 1
misses the prototypical reactivity of electrophilic carbenes, the
lengthening of the Rh−Rh bond concurs with a three cenꢀ
ter/four electron bonding mode that is currently thought to be
the best description of the dimetallic core.20,21 Only very reꢀ
cently have Davies, Berry and coworkers managed to observe
the prototype push/pull dirhodium carbene 2 in solution.22 The
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 10
characteristic signal at δC = 242 ppm showcases the electroꢀ
Scheme 1. Preparation of dirhodium tetracarboxylate
complexes and their use in olefin cyclopropanation.a
1
2
3
4
5
6
7
philic nature of this species, whereas the recorded EXAFS
data suggest a linear Rh−Rh−C array and an elongated Rh−Rh
bond (2.43 Å). The averaged Rh−C and Rh−O distances deꢀ
rived from the EXAFS spectra were deconvoluted by DFT to
individual bond lengths of 1.972 Å and 2.041 Å, respectiveꢀ
ly.22 Relevant conformational details, however, could not be
deduced.
8
9
CPh3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
Rh
O
O
Rh
O
CPh3
O
O
O
O
O
O
OMe
O
O
Rh
O
O
Rh
O
O
N
Ph3C
N
O
OMe
CPh3
1
2
Outlined below is a series of crystal structures of reactive
carbenes of the Rh(II) tetracarboxylate as well as of the
Rh(III) halfꢀsandwich type with different substitution patterns.
This includes the donor/acceptor complex 10, which differs
from 2 only in that it carries a α,α,α′,α′ꢀtetramethylꢀ1,3ꢀ
benzenedipropionate (esp)23 rather than triphenylacetate (tpa)24
ligand set; esp ligands have an impressive track record in
rhodium catalysis.23,25 Collectively, our data provide a detailed
picture of the constitution, conformation and bonding of these
important reactive intermediates; they allow the published
computational data to be validated and the predictive models
to be checked.26
a
Reagents and conditions: (a) [Rh2(tpa)4]⋅CH2Cl2, CH2Cl2,
−10°C; b) pꢀmethoxystyrene, pentane, see Text; c) [Rh2(esp)2],
CH2Cl2, −10°C
After numerous attempts, we managed to grow crystals of
4a suitable for Xꢀray diffraction. Major difficulties arose from
the fact that even the pure crystalline material decomposes in
less than 12 h at −20°C; solute CH2Cl2 and toluene are necesꢀ
sary to ensure metaꢀstability, yet tend to be highly disordered
within the unit cell. Considerable experimentation was necesꢀ
sary to find that crystals grown from the ternary mixture
CH2Cl2/toluene/fluorobenzene were of better quality, allowing
all atoms of the complex to be refined anisotropically (R =
5.9%).
RESULTS AND DISCUSSION
The Donor/Donor Series. Since donor substituents are
known to tame an adjacent electrophilic carbene center, it
seemed reasonable to start our structural investigations by
targeting dirhodium carbenes bearing two donor groups.27 The
bis(pꢀmethoxyphenyl)carbene backbone was deemed a good
candidate, not least because this particular motif had allowed
us to isolate the first reactive gold carbenes in crystalline
form.28,29 Our efforts were invigorated by a control experiꢀ
ment, which showed that the reaction of the readily prepared
and safeꢀtoꢀhandle bis(4ꢀmethoxyphenyl)ꢀdiazomethane 3a30,31
with 4ꢀmethoxystyrene in the presence of [Rh2(tpa)4]⋅CH2Cl2
(0.5 mol%) cleanly furnished the desired cyclopropane derivaꢀ
tive 5a (Scheme 1). This result proves that the putative carꢀ
bene 4a generated in situ shows the prototype chemical behavꢀ
ior of this class of reactive intermediates;32,33 its structure is
therefore undoubtedly relevant for mechanistic discussions.
Treatment of a cold solution of [Rh2(tpa)4]⋅CH2Cl2 in
CH2Cl2 with a solution of 3a in the same solvent resulted in an
instantaneous color change from purple to dark turquoise and
the vigorous evolution of N2. The resulting species 4a turned
out to be sufficiently stable at −20°C for characterization by
NMR, UV, IR and even HRMS (ESI+). The resonance at δC =
268.9 ppm bears witness for its carbene character; somewhat
counterintuitively though, this signal is downfield from that of
the push/pull complex 2 (δC = 242 ppm) observed by Davies,
Berry and coworkers.22
Figure 1. Comparison of important structural features of comꢀ
plex 4a with in silico data
As the structure of 4a in the solid state has been published
in a recent Communication,26 it suffices to briefly summarize
the relevant attributes (Figure 1). As expected, the carbene
ligand occupies an axial coordination site on the dirhodium
cage and the Rh2ꢀRh1−C1 axis is almost linear (176.9°). The
ACS Paragon Plus Environment

Page 3 of 10
Journal of the American Chemical Society
Me2NC6H4ꢀ ring, the characteristic 15N NMR shift of the
Rh1−C1 bond distance (2.061(6) Å) is substantially longer
than that computed for the model compounds 7 (1.906 Å)20
and 8 (1.939 Å)20 and even the hemiꢀstabilized carbene 2
(1.972 Å).22 This fact suggests that backꢀdonation of electron
density from the metal into the carbene center is low. To comꢀ
pensate, C1 strongly engages with the flanking arenes, one of
which is coplanar to ensure maximum orbital overlap. This
tight interaction leads to a significant contraction of the bonds
between C1 and the Cipso atoms (1.426(8)/1.438(8) Å). The
entire carbene substituent adopts a staggered conformation
relative to the O−Rh−O unit, an aspect to be discussed in more
detail below, whereas 7 and 8 were computed to prefer the
eclipsing orientation.20
Me2Nꢀ group (δN = −282.6 ppm) is in excellent agreement
with this description.34 On the other hand, the rotation about
the Rh1−C1 bond is unrestricted on the NMR time scale,
which confirms the low bond order of the “carbene” unit deꢀ
duced from the crystallographic data.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The analogous complexes 4b and 6b have also been obꢀ
tained in crystalline form, which carry −NMe2 rather than
−OMe groups on the arene rings and differ from each other in
the ancillary tetracarboxylate ligand set (tpa in 4 versus esp in
6). Their basic structural attributes are largely similar to those
of 4a, with a staggered conformation of the carbene entities
relative to the dimetallic core and a strong participation of the
electron rich arenes in the stabilization of the carbene center
(Xꢀray structures of 4b and 6b are displayed in the Supporting
Information). A subtle and as yet unexplained difference,
however, relates to the fact that 4a and 6b crystallize as coorꢀ
dination polymers, in which the –OMe (−NMe2) group of one
monomer unit ligates Rh2 of the next unit, whereas 4b is
monomeric in the unit cell.
Figure 2. Structure of the donor/acceptor dirhodium carbene
4c in the solid state; the lateral phenyl rings of the tpa ligands
on the dimetallic cage are removed for clarity (for the comꢀ
plete structure and further details, see the Supporting Inforꢀ
mation).
The Donor/Acceptor Series. Next, we prepared complex
4c in which one electronꢀrich and one electronꢀdeficient pheꢀ
nyl ring flank the carbene center. The structure of this particuꢀ
lar push/pull dirhodium carbene in the solid state is informaꢀ
tive (Figure 2). As one might expect, the phenyl ring with the
−NMe2 substituent carries the burden of stabilizing the carꢀ
bene center; it adopts an almost perfectly coplanar orientation
(Θ = 0.9°) to maximize overlap of the πꢀcloud of the phenyl
ring with the (empty) carbene pꢀorbital representing the major
lobe of the LUMO. With only 1.389(3) Å, the C1ꢀC2 distance
is even shorter than the already very tight C1ꢀCipso bonds in 4a
(Figure 1). The F3CC6H4ꢀ ring as the acceptor substituent, in
contrast, lies orthogonal to the carbene center (Θ = −94.7°) to
largely cutꢀoff any destabilizing electronic communication. As
a consequence, the C1ꢀC10 bond (1.473(4) Å) is significantly
longer than C1ꢀC2. Once again, the carbene entity adopts a
staggered orientation relative to the dinuclear core. The strucꢀ
tural features of the analogous complex 6c endowed with esp
rather than tpa ligands are largely similar (for details, see the
Supporting Information). This comparison confirms that the
choice of ancillary ligands exerts only a small effect on the
structure of such dirhodium species.
Particularly noteworthy is the structure of complex 10 in the
solid state, which represents a prototype donor/acceptor carꢀ
bene of the type that has been most widely studied in the past
(Scheme 2).6,7,35 10 is the espꢀanalogue of the tpa complex 2
which could previously only be characterized by spectroscopic
means.22 As the data outlined above suggest that the change of
the ancillary ligand set has little structural bearing, the highꢀ
quality Xꢀray data (R = 3.18%) of this very labile intermediate
are considered an important reference point (Figure 3). This
notion is supported by the fact that the carbene centers of 10
(δC = 237.1 ppm) and of 2 (δC = 242 ppm)22 resonate at very
similar field.
Scheme 2. Preparation of the prototypical donor/acceptor
dirhodium tetracarboxylate complex 10 and summary of
relevant structural data.a
staggered conformation
[Rh]
MeO
2.423(3) Å
O
O
O
CF3
N
2.001(2) Å
COOMe
Rh
F
O
Rh
a)
O
N2
O
MeO
1.477(3) Å
1.406(3) Å
O
O
O
The NMR data suggest that the structure of these complexes
in solution must be very similar. Most notably, the rotation
about the C1−C2 bond in 4c and 6c is largely frozen out at
−10°C and the species are best depicted by the quinoid resoꢀ
nance extreme F. In addition to the inequivalent protons of the
9
10
OMe
a Reagents and conditions: [Rh2(esp)2], FC6H5/CH2Cl2, 0°C →
−20°C
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 10
by the ancillary ligands (Figure 3, bottom). Provided this
1
orientation is retained in solution, it is safe to predict that an
olefin (or other nucleophilic reaction partner) will hardly be
able to approach the (empty) carbene lobe by passing over the
ester (Figure 4). Though sterically unhindered, trajectory (a) is
stereoelectronically unfavorable and does not lead to efficient
orbital overlap. The seemingly most open trajectory (b) is
equally unlikely on stereoelectronic grounds because of masꢀ
sive dipoleꢀdipole repulsion between the ester carbonyl and
the incoming nucleophile. Therefore it is probable that the
reaction partner approaches the carbene center in a Bürgiꢀ
Dunitzꢀtype trajectory by gliding alongside the arene, which
itself is no obstacle in the orientation that it adopts. The strucꢀ
ture of 10 in the solid state therefore lends credence to the
AutschbachꢀDavies model currently favored in predicting the
stereochemical course of reactions involving dirhodium tetraꢀ
carboxylate carbenes derived from aryldiazoacetates, which
generally provide excellent diasteroselectivities in reactions
with olefins or other partners.12,36
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(b)
(c)
O
O
O
O
O
(a)
OMe
MeO
Figure 3. Structure of the donor/acceptor dirhodium carbene
10 in the solid state in two different orientations; for details,
see the Supporting Information
(c)
Figure 4. Newmanꢀtype projection of complex 10 along the
C1−Rh1−Rh2 bond; the blue arrows show possible trajectories
for an incoming nucleophile, the straight red arrow indicates
the dipole of the acceptor group. For the unhindered rotation
of the ester (curved red arrow), any backside attack will face
the same stereoelectronic demands.
The donor substituent is perfectly coplanar with the carbene
center (Θ = 0°); accordingly, the C1ꢀC4 bond is very short
(1.406(3) Å). The fact that all protons on the arene ring are
1
inequivalent in the H NMR spectrum recorded at −50°C
suggests that this structureꢀdetermining motif is retained in
solution (whereas the rotation about the Rh1ꢀC1 “carbene”
bond is unrestricted at this temperature). In contrast, the ester
is orthogonally oriented, such that the C1ꢀC2 distance is not
contracted (1.477(3) Å).
As outlined above, the structure of the donor/acceptor carꢀ
benes 4c and 6c bearing two aryl groups of different electronic
character are largely similar to that of 10. Even though the
dipole of the F3CC6H4ꢀ ring acting as the acceptor substituent
in lieu of the ester is orthogonal rather than parallel to the
carbene pꢀorbital, trajectories (a) and (b) remain stereoelecꢀ
tronically disfavored. Therefore one can expect the reaction of
pꢀmethoxystyrene with the diaryl diazomethane 3c catalyzed
by Rh2[(esp)]2 to provide cyclopropane 5c with excellent
selectivity (Scheme 1); this is indeed the case (dr > 98:2,
NMR).37
With regard to the carbene ligand itself, it is noteworthy that
the Rh1ꢀC1 bond in 10 (2.001(2) Å) clearly exceeds the 1.972
Å computed for 2, whereas the Rh2−Rh1 distance (2.4226(3)
Å) is somewhat shorter than the 2.43 Å proposed for 2 based
on EXAFS/DFT.22 The mutual compensation of the bond
lengths within the core is readily understood on the basis of a
3ꢀcenter/4ꢀelectron for the Rh−Rh−C unit.20,21 Another interꢀ
esting and as yet undescribed aspect concerns the asymmetry
of the bond angles about the carbene center: While the arene
ring is visibly bent away from the cage (Rh1ꢀC1ꢀC4,
127.6(1)°), likely to avoid a clash with the tetracarboxylate
ligand framework, the orthogonally disposed ester group is
sterically hardly inflicting as evident from the small Rh1ꢀC1ꢀ
C2 angle (113.7(2)°).
Limitation. The diazomethane derivative 11 was conceived
as an appropriate substrate in an attempt to probe the chemical
character of the resulting rhodium carbene. Any strong contriꢀ
bution of a bipolar resonance extreme D would impart “nonꢀ
classical” carbocation character onto the derived carbene.38,39
If so, ring opening of the cyclopropyl unit or, more likely, ring
expansion is expected to occur, because the cyclobutyl cation
resonance form 13 seems most favored for the extra benzylic
stabilization (Scheme 3).
The two esp ligands orient their metaꢀdisubstituted benzene
rings in a synꢀfashion, such that the dimetallic core adopts Cs
symmetry; it is noteworthy that this conformation is different
from that of [Rh2(esp)2](acetone)2, which is C2 symmetric.23
Once again, the carbene entity is staggered relative to the
O−Rh−O unit, formally bisecting the binding pocket created
ACS Paragon Plus Environment

Page 5 of 10
Journal of the American Chemical Society
folds.45ꢀ47 They are usually prepared from [Cp*RhX2]2 and an
Scheme 3. Attempted formation of a rhodium carbene with
“non-classical” carbocation character led to the isolation
of a diazo adduct.
1
2
3
appropriate diazo precursor (frequently in combination with a
soluble Ag(I) salt to abstract the chloride ligands from the
active species). It is believed, however, that the highly electroꢀ
philc metal fragment (cyclo)metalates the chosen arene subꢀ
strate before it actually reacts with the diazo derivative.45ꢀ47
This very large and rapidly growing body of work contrast to
the lack of information about proper pianoꢀstool Rh(III) carꢀ
benes devoid of a cyclometalated backbone. Yet, such species
exhibit excellent reactivity (Scheme 4). An unoptimized loadꢀ
ing of 1 mol% sufficed for typical O−H or Si−H insertions to
proceed in excellent yield. The cyclopropanation of 4ꢀ
methoxystyrene also worked well, provided that [Cp*RhI2]2
was chosen as the precatalyst, whereas [Cp*RhCl2]2 proved
markedly less efficient (see below for a possible explanation
of this different behavior). Particularly noteworthy is the direct
formation of the alkoxyfuran 18 on reaction of 9 with (4ꢀ
methoxyphenyl)acetylene; in judging the outcome, one has to
keep in mind that this particular product is prone to hydrolysis
on work up. In any case, terminal alkynes usually afford cyꢀ
clopropenes in the first place, which can be rearranged to
(alkoxy)furan derivatives in a separate step.4ꢀ9,48 While this
reactivity pattern has ample precedent in the acceptor/acceptor
series, the few recorded examples for donor/acceptor carbenes
tend to be lowꢀyielding.49 Therefore the direct formation of 18
is an interesting lead, not least because such electron rich
furan derivatives have promising followꢀup chemistry.50
N
4
5
6
7
8
9
N
[Rh2(esp)2]
Ar
(esp)2Rh2
Ar
Me2N
Me2N
12
13
N2
O
O
Rh
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
O
Ar
O
Me2N
11
Rh
O
O
(Ar = p-MeC6H4-)
[Rh2(esp)2]
Ar
N
N
14
CH2Cl2, -20°C
Me2N
Scheme 4.
Si(OEt)3
COOMe
OMe
COOMe
b) MeO
a)
MeO
MeO
16
15
MeO
N2
MeO
MeO
O
Figure 5. Structure of the diazo adduct 14 in the solid state;
for details, see the Supporting Information
OMe
O
O
c)
OMe
d)
18
9
17
Yet, the attempted formation of a carbene complex failed;
rather, adduct 14 was isolated (as a coordination polymer in
the solid state), in which the intact diazo group is ligated endꢀ
on to the dirhodium fragment (Figure 5).40 The doublyꢀbent
coordination geometry positions the substituents on the Nꢀ
atoms quasi orthogonal to each other (Rh1ꢀN1ꢀN2ꢀC1
108.2°).41 The failure to form a rhodium carbene is ascribed to
steric hindrance, which prevents bulky [Rh2(esp)2] from reachꢀ
ing the negatively polarized Cꢀatom of 11 with formation of
12 as the necessary first step en route to a metallocarbene.42
Under more forcing conditions, substrate 11 decomposed;
therefore the envisaged chemical interrogation of rhodium
carbenes with the help of suitable reporter groups must await
future studies.
OMe
OMe
a
Reagents and conditions: a) [Cp*RhI2]2 (1 mol%), CH2Cl2,
MeOH, 88%; b) [Cp*RhI2]2 (1 mol%), (EtO)3SiH, CH2Cl2,
81%; c) pꢀmethoxystyrene, pentane, 69% (with [Cp*RhI2]2 (1
mol%)); 46% (with [Cp*RhCl2]2 (1 mol%)); d) [Cp*RhI2]2 (1
mol%), (pꢀmethoxyphenyl)acetylene, CH2Cl2, 65%
Secured structural information about any type of halfꢀ
sandwich Rh(III) carbene is missing. The higher oxidation
state of the central rhodium atom and the lack of an adjuvant
second metal center, as present in AꢀC, suggest that the resultꢀ
ing mononuclear carbenes might be even less stable. In line
with this notion, no carbene signal was detected by 13C NMR
in the reaction of [Cp*RhCl2]2 with the aryldiazoester 9
(Scheme 5), but a new resonance appeared at 70.6 ppm. It was
not clear at the outset whether this observation reflected inherꢀ
ent chemical issues or was simply caused by the instability of
the complex, which decomposes in solution in < 1.5 h at
−50°C. An Xꢀray structure clarified the point: As can be seen
from Figure 6, the push/pull carbene 19a generated in situ has
inserted into one of the Rh−Cl bonds, resulting in the forꢀ
mation of the chloroalkyl species 20. This species can be
Rhodium(III) Half-Sandwich Carbene Complexes. Altꢀ
hough the use of the dinuclear Rh(II) tetracarboxylate carꢀ
benes AꢀC dominates the field, not least because they are
easily rendered chiral, it is well established that various Rh(I)
as well as Rh(III) sources also lead to carbenes with useful
reactivity profiles.1,2,43,44 A potentially interesting class are
Rh(III) carbenes of the halfꢀsandwich type, which are invoked
in a large body of (directed) C−H activation chemistry for the
(lateꢀstage) functionalization of various heterocyclic scafꢀ
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 10
viewed as a functionalized Cꢀmetalated rhodium enolate, assume that the reaction of 9 with [Cp*RhX2]2 (X = Br, I) in
1
2
3
4
5
6
7
8
chiral at metal and at the metalated C atom; it is formed as a
single diastereomer.51,52 To compensate for the loss of an
anionic ligand, the now formally 16e Rh(III) center engages
with the aromatic ring, as evident from the short contacts to
the ipsoꢀ and one of the orthoꢀC atoms. This interaction likely
persists in solution as suggested by the highꢀfield shift of the
corresponding Cꢀatoms (broad signals at ≈ 94 and ≈116 ppm).
The comparatively long Rh1−C1 (2.1226(12) Å) and C1−C4
(1.4666(17) Å) bonds clearly reflect the loss of true “carbene”
character (note that 20 is still “carbenoid” in the sense that it
contains a carbon atom carrying at the same time a metal and a
leaving group).
lieu of [Cp*RhCl2]2 should also result in halide migration with
formation of the corresponding haloalkyl species. Surprisingꢀ
ly, however, the corresponding carbene complexes 19b,c
were the major species (Scheme 5). Of note it their highly
deshielded carbene center (δC = 314.2 ppm, X = Br; 316.4
ppm, X = I), which resonates downfield from that in the dirꢀ
hodium(II) donor/acceptor carbene 10 (δC = 237.1 ppm).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 5. Preparation of Rh(III) half-sandwich carbene
complexes.a
2 SbF6
2
a)
b)
[Cp*RhX2]2
X
X
Rh
Rh
OMe
N
N
MeO
O
21
19a
[
, X = Cl]
c)
Figure 6. Structure of the chloroalkyl complex 20 in the solid
state
19b, X = Br
19c
X = Cl
, X = I
SbF6
OMe
Rh
Rh
N
Cl
Cl
MeOOC
MeOOC
22
20
OMe
OMe
a Reagents and conditions: a) 9, FC6H5/CH2Cl2, 0°C → −20°C;
b) 2 AgSbF6, MeCN; c) 9, MeOH, 0°C → −20°C
The formation of 20 is thought to illustrate the exceptional
electron demand of the transient Rh(III) carbene 19a, which
engenders the halide shift at low temperature. This “internal
quenching” mechanism might well explain why the majority
of catalytic transformations based on the decomposition of a
diazoester derivative with [Cp*RhCl2]2 uses silver salts as
additives, which likely prevent such halide shifts from occurꢀ
ring and allows nucleophilic partners to react with the carbene
center.45ꢀ47 This hypothesis is corroborated by the outcome of
the reaction of aryldiazoacetate 9 with the cationic rhodium
precursor 21 (Scheme 5). In line with our expectations, the
clean formation of the corresponding cationic methyl ether
derivative 22 was observed, which proves that the transient
rhodium carbene devoid of the potentially interfering chloride
ligands is trapped by the external nucleophile MeOH chosen
as the reaction medium. Complex 22 also represents a funcꢀ
tionalized, Cꢀmetalated and chiral Rh(III) enolate; its structure
is similar to the chloroalkyl species 20 even with regard to the
interaction of the metal center with the electron rich arene
(Figure 7).
Figure 7. Structure of the methanol adduct 22 in the solid
state; only the complex cation is shown for clarity
In this context it is also noteworthy that a complex related to
the chloroalkyl species 20 has previously been inferred from
spectroscopic data. Thus, reaction of iodo-rhodium(III) tetraꢀ
pꢀtolylporphyrin with ethyl diazoacetate was proposed to
afford an iodoalkyl complex that failed to cyclopropanate
styrene.53,54 In view of this precedent, it seemed reasonable to
Figure 8. Structure of the halfꢀsandwich donor/acceptor
Rh(III) carbene 19c in the solid state
The constitution of 19b,c was unambiguously established by
Xꢀray diffraction (Figure 8; for the dibromo complex 19b, see
ACS Paragon Plus Environment

Page 7 of 10
Journal of the American Chemical Society
* fuerstner@kofo.mpg.de
the Supporting Information). As expected, the arene as the
1
2
3
4
5
6
7
8
donor substituent is coplanar with the carbene center (Θ =
0.5°) to maximize orbital overlap, whereas the plane of the
ester is essentially perpendicular to it (Θ = 104.5°). Along with
this come a contracted C1ꢀC4 (1.414(2) Å) but a long C1ꢀC2
(1.497(2) Å) bond; with only 1.9671(1) Å, the Rh1ꢀC1 bond is
visibly shorter than the analogous carbene distances in the
dirhodium(II) tetracarboxylate complexes outlined above. The
fact that the nature of the anionic ligand determines whether a
true carbene (19b,c) or a “carbenoid” (20) is passed through
illustrates an as yet hardly recognized way to control rhodium
catalyzed reactions of diazo compounds. The better perforꢀ
mance of [Cp*RhI2]2 as compared to [Cp*RhCl2]2 in the cyꢀ
clopropanation shown in Scheme 4 likely reflects this aspect.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
Generous financial support by the MPG is gratefully acknowlꢀ
edged. We thank all analytical departments of our Institute for
excellent support, as well as Umicore AG & Co KG, Hanau, for a
generous gift of noble metal salts.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
REFERENCES
(1) Paulissen, R.; Reimlinger, H.; Hayez,, E.; Hubert, A. J.;
Teyssié, P. Tetrahedron Lett. 1973, 14, 2233ꢀ2236.
(2) (a) Hubert, A. J.; Noels, A. F.; Anciaux, A. J.; Teyssié, P.
Synthesis 1976, 600ꢀ602. (b) Demonceau, A.; Noels, A. F.; Hubert, A.
J.; Teyssié, P. J. Chem. Soc. Chem. Commun. 1981, 688ꢀ689.
(3) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic
Methods for Organic Synthesis with Diazo Compounds: From Cyclo-
propanes to Ylides; Wiley: New York, 1998.
(4) Modern Rhodium Catalyzed Organic Reactions; Evans, P. A.,
Ed.; WileyꢀVCH: Weinheim, 2005.
(5) (a) Padwa, A.; Weingarten, M. D. Chem. Rev. 1996, 96, 223ꢀ
269. (b) Padwa, A. Chem. Soc. Rev. 2009, 38, 3072ꢀ3081.
(6) Davies, H. M. L.; Antoulinakis, E. G. Org. React. 2001, 57, 1ꢀ
326.
(7) (a) Davies, H. M. L.; Beckwith, R. E. J. Chem. Rev. 2003, 103,
2861ꢀ2904. (b) Davies, H. M. L.; Manning, J. R. Nature 2008, 451,
417ꢀ424. (c) Davies, H. M. L. ; Morton, D. Chem. Soc. Rev. 2011, 40,
1857ꢀ1869. (d) Davies, H. M. L.; Lian, Y. Acc. Chem. Res. 2012, 45,
923ꢀ935.
(8) (a) Doyle, M. P. Chem. Rev. 1986, 86, 919ꢀ939. (b) Doyle, M.
P.; Forbes, D. C. Chem. Rev. 1998, 98, 911ꢀ935. (c) Doyle, M. P.;
Duffy, R.; Ratnikov, M.; Zhou, L. Chem. Rev. 2010, 110, 704ꢀ724.
(9) (a) Lebel, H.; Marcoux, J.ꢀF.; Molinaro, C.; Charette, A. B.
Chem. Rev. 2003, 103, 977ꢀ1050. (b) Zhang, Y.; Wang, J. Coord.
Chem. Rev. 2010, 254, 941ꢀ953. (c) Ye, T.; McKervey, M. A. Chem.
Rev. 1994, 94, 1091ꢀ1160. (d) Gois, P. M. P.; Afonso, C. A. M. Eur.
J. Org. Chem. 2004, 3773ꢀ3788. (e) Zhang, Z.; Wang, J. Tetrahedron
2008, 64, 6577ꢀ6605. (f) Ford, A.; Miel, H.; Ring, A.; Slattery, C. N.;
Maguire, A. R.; McKervey, M. A. Chem. Rev. 2015, 115, 9981ꢀ
10080. (g) DeAngelis, A.; Panish, R.; Fox, J. M. Acc. Chem. Res.
2016, 49, 115ꢀ127. (h) Gillingham, D.; Fei, N. Chem. Soc. Rev. 2013,
42, 4918ꢀ4931. (i) Zhu, S.ꢀF.; Zhou, Q.ꢀL. Acc. Chem. Res. 2012, 45,
1365ꢀ1377. (j) Guo, X.; Hu, W. Acc. Chem. Res. 2013, 46, 2427ꢀ
2440. (k) Murphy, G. K.; Stewart, C.; West, F. G. Tetrahedron 2013,
69, 2667ꢀ2686.
(10) For the preparation of metal carbenes by the decomposition of
precursors other than diazoalkanes, see: (a) Gulevich, A. V.; Geꢀ
vorgyan, V. Angew. Chem., Int. Ed. 2013, 52, 1371ꢀ1373. (b) Davies,
H. M. L.; Alford, J. S. Chem. Soc. Rev. 2014, 43, 5151ꢀ5162. (c)
Anbarasan, P.; Yadagiri, D. Rajasekar, S. Synthesis 2014, 46, 3004ꢀ
3023. (d) Wang, Y.; Lei, X.; Tang, Y. Synlett 2015, 26, 2051ꢀ2059.
(e) Barluenga, J.; Valdés, C. Angew. Chem., Int. Ed. 2011, 50, 7486ꢀ
7500. (f) Shao, Z.; Zhang, H. Chem. Soc. Rev. 2012, 41, 560ꢀ572.
(11) This proposal is in accord with the observed kinetic isotope
effects and supported by computational studies, see: Nowlan, D. T.;
Gregg, T. M.; Davies, H. M. L.; Singleton, D. A. J. Am. Chem. Soc.
2003, 125, 15902ꢀ15911.
CONCLUSION
The tremendous importance of rhodium carbene chemistry
in general stands in striking contrast to the lack of experiꢀ
mental data concerning the structure of the relevant intermediꢀ
ates. For their pronounced electrophilicity, such species are
highly unstable and therefore largely defied direct inspection
by spectroscopic or crystallographic means. This difficulty
notwithstanding, we are able to present a series of crystal
structures of dinuclear tetracarboxylate complexes of Rh(II) as
well as mononuclear halfꢀsandwich complexes of Rh(III),
which are by far the most commonly used intermediates in this
field. They provide an unprecedentedly close look at repreꢀ
sentative members of these families and reveals many conforꢀ
mational details which could hitherto only be inferred from
indirect evidence and/or in silico data. The experimental reꢀ
sults refine the understanding of their gross structural attribꢀ
utes and correct pertinent bond lengths, which had previously
been deduced from computational studies at different levels of
theory. A recurrent theme in these Xꢀray structures is the
staggered orientation of the carbene unit relative to the dirhoꢀ
dium tetracarboxlyate core carrying the ancillary ligands. This
trait is critically important for the understanding of the diaꢀ
stereoꢀ and enantioselective course of rhodium catalyzed reacꢀ
tions of diazoalkanes. While our data suggest that stereoelecꢀ
tronic (rather than purely steric) factors play a decisive role,
the basic concept of the currently favored predictive model is
validated.12 Finally, it is shown how the anionic ligands preꢀ
sent in halfꢀsandwich Rh(III) carbenes regulate the electroꢀ
philicity of reactive intermediates of this type. Thus, the seemꢀ
ingly subtle change from bromide (or iodide) to chloride draꢀ
matically alters the chemical nature of the resulting complexes
from a prototype carbene to a chloroalkyl species. This insight
allows the common use of silver additives in reactions of
diazoesters catalyzed by [Cp*RhCl2]2 to be rationalized.
ASSOCIATED CONTENT
Supporting Information
(12) Hansen, J.; Autschbach, J.; Davies, H. M. L. J. Org. Chem.
2009, 74, 6555ꢀ6563.
(13) The conceivable alternative pathway invoking the true carbene
resonance form E that undergoes a [2+2] cycloaddition with forꢀ
mation of a metallacyclobutane followed by reductive elimination is
unlikely in the case of rhodium; for a general discussion of this mechꢀ
anistic scenario, see: Brookhart, M.; Studabaker, W. B. Chem. Rev.
1987, 87, 411ꢀ432.
Experimental part including characterization data and copies of
NMR spectra of new compounds, as well as supporting crystalloꢀ
graphic information concerning the structures of complexes 4b,
4c, 6b, 6c, 10, 14, 19b, 19c, 20 and 22 in the solid state.
AUTHOR INFORMATION
Corresponding Author
(14) Davies, H. M. L.; Bruzinski, P. R.; Lake, D. H.; Kong, N.;
Fall, M. J. J. Am. Chem. Soc. 1996, 118, 6897ꢀ6907.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 10
(15) See also: (a) Doyle, M. P.; Griffin, J. H.; Bagheri, V.; Dorow,
Squitieri, R. A.; GonzálezꢀLópez, M.; Di Maso, M. J.; Shaw, J. T. J.
R. L. Organometallics 1984, 3, 53ꢀ61. (b) O’Bannon, P. E.; Dailey,
W. P. J. Org. Chem. 1989, 54, 3096ꢀ3101. (c) Brown, K. C.; Kodadꢀ
ek, T. J. Am. Chem. Soc. 1992, 114, 8336ꢀ8338.
(16) The barrier for the cyclopropanation of styrene with a vinylꢀ
carbene was originally computed as 0.5 kcal/mol (see ref. 11), which
was later deemed inconsistent with experimental data; a subsequent
study found a barrier of no less than 4.5 kcal/mol for the analogous
reaction of a closely related phenylcarbene, cf. ref. 12.
Am. Chem. Soc. 2014, 136, 15142ꢀ15145.
(34) Schwotzer, W.; von Philipsborn, W. Helv. Chim. Acta 1977,
60, 1501ꢀ1509.
1
2
3
4
5
6
7
8
(35) In this context, it is noteworthy that the first free carbene to be
characterized by Xꢀray diffraction also belonged to the donor/acceptor
type, see: Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G.
Science 2000, 288, 834ꢀ836.
(36) For recent advances, see: (a) Qin, C.; Boyarskikh, V.; Hansen,
J. H.; Hardcastle, K. I.; Musaev, D. G.; Davies, H. M. L. J. Am.
Chem. Soc. 2011, 133, 19198ꢀ19204. (b) Negretti, S.; Cohen, C. M.;
Chang, J. J.; Guptill, D. M.; Davies, H. M. L. Tetrahedron 2015, 71,
7415ꢀ7420.
(17) For further pertinent in silico studies into various types of rhoꢀ
dium carbenes and their reactivity at different levels of theory, see: (a)
Doyle, M. P.; Winchester, W. R.; Hoorn, J. A. A.; Lynch, V.; Simonꢀ
sen, S. H.; Ghosh, R. J. Am. Chem. Soc. 1993, 115, 9968ꢀ9978. (b)
Pirrung, M. C.; Morehead, A. T., Jr. J. Am. Chem. Soc. 1994, 116,
8991ꢀ9000. (c) Sheehan, S. M.; Padwa, A.; Snyder, J. P. Tetrahedron
Lett. 1998, 39, 949ꢀ952. (d) Padwa, A.; Snyder, J. P.; Curtis, E. A.;
Sheehan, S. M.; Worsencroft, K. J.; Kappe, C. O. J. Am. Chem. Soc.
2000, 122, 8155ꢀ8167. (e) Yoshikai, N.; Nakamura, E. Adv. Synth.
Catal. 2003, 345, 1159ꢀ1171. (f) Nowlan, D.T.; Singleton, D. A. J.
Am. Chem. Soc. 2005, 127, 6190ꢀ6191. (g) Liang, Y.; Zhou, H.; Yu,
Z.ꢀX. J. Am. Chem. Soc. 2009, 131, 17783ꢀ17785. (h) Hansen, J. H.;
Gregg, T. M.; Ovalles, S. R.; Lian, Y.; Autschbach, J.; Davies, H. M.
L. J. Am. Chem. Soc. 2011, 133, 5076ꢀ5085. (i) Hansen, J. H.; Parr, B.
T.; Pelphrey, P.; Jin, Q.; Autschbach, J.; Davies, H. M. L. Angew.
Chem., Int. Ed. 2011, 50, 2544ꢀ2548.
(18) (a) Snyder, J. P.; Padwa, A.; Stengel, T.; Arduengo, A. J.;
Jockisch, A.; Kim, H.ꢀJ. J. Am. Chem. Soc. 2001, 123, 11318ꢀ11319.
(b) Trindade, A. F.; Gois, P. M. P.; Veiros, L. F.; André, V.; Duarte,
M. T.; Afonso, C. A. M.; Caddick, S.; Cloke, F. G. N. J. Org. Chem.
2008, 73, 4076ꢀ4086. (c) Kwak, J.; Kim, M.; Chang, S. J. Am. Chem.
Soc. 2011, 133, 3780ꢀ3783. (d) Na, S. J.; Lee, B. Y.; Bui, N.ꢀN.; Mho,
S.; Jang, H.ꢀY. J. Organomet. Chem. 2007, 692, 5523ꢀ5527.
(19) See also: (a) Gois, P. M. P.; Trindade, A. F.; Veiros, L. F.;
André, V.; Duarte, M. T.; Afonso, C. A. M.; Caddick, S.; Cloke, F. G.
N. Angew. Chem., Int. Ed. 2007, 46, 5750ꢀ5753. (b) Trindade, A. F.;
André, V.; Duarte, M. T.; Veiros, L. F.; Gois, P. M. P.; Afonso, C. A.
M. Tetrahedron 2010, 66, 8494ꢀ8502.
(20) Nakamura, E.; Yoshikai, N.; Yamanaka, M. J. Am. Chem. Soc.
2002, 124, 7181ꢀ7192.
(21) Berry, J. F. Dalton Trans. 2012, 41, 700ꢀ713.
(22) Kornecki, K. P.; Briones, J. F.; Boyarskikh, Y.; Fullilove, F.;
Autschbach, J.; Schrote, K. E.; Lancaster, K. M.; Davies, H. M. L.;
Berry, J. F. Science 2013, 342, 351ꢀ354.
(23) Espino, C. G.; Fiori, K. W.; Kim, M.; Du Bois, J. J. Am.
Chem. Soc. 2004, 126, 15378ꢀ15379.
(24) Hashimoto, S.ꢀI.; Watanabe, N.; Ikegami, S. Tetrahedron Lett.
1992, 33, 2709ꢀ2712.
(25) (a) For a pertinent discussion and comparison with other rhoꢀ
dium catalysts, see: Roizen, J. L.; Harvey, M. E.; Du Bois, J. Acc.
Chem. Res. 2012, 45, 911ꢀ922. (b) See also: Warzecha, E.; Berto, T.
C.; Berry, J. F. Inorg. Chem. 2015, 54, 8817ꢀ8824.
(26) For a preliminary communication, see: Werlé, C.; Goddard,
R.; Fürstner, A. Angew. Chem., Int. Ed. 2015, 54, 15452ꢀ15456.
(27) For pioneering studies on the formation of (fairly stable) manꢀ
ganese carbenes by decomposition of aryl diazo derivatives, see: (a)
Herrmann, W. A. Chem. Ber. 1975, 108, 486ꢀ499. (b) Herrmann, W.
A. Angew. Chem., Int. Ed. Engl. 1978, 17, 800ꢀ812.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(37) Unfortunately, however, this product is prone to rapid epimerꢀ
ization during flash chromatography.
(38) Related electrophilic gold and platinum carbenes exhibit conꢀ
siderable nonꢀclassical carbocation character, see: (a) Fürstner A.;
Szillat, H.; Gabor, B.; Mynott, R. J. Am. Chem. Soc. 1998, 120, 8305ꢀ
8314. (b) Fürstner, A.; Stelzer, F.; Szillat, H. J. Am. Chem. Soc. 2001,
123, 11863ꢀ11869. (c) Fürstner, A.; Davies, P. W.; Gress, T. J. Am.
Chem. Soc. 2005, 127, 8244ꢀ8245. (d) Fürstner, A.; Aïssa, C. J. Am.
Chem. Soc. 2006, 128, 6306ꢀ6307. (e) Fürstner, A.; Morency, L.
Angew. Chem., Int. Ed. 2008, 47, 5030ꢀ5033. (f) Seidel, G.; Mynott,
R.; Fürstner, A. Angew. Chem., Int. Ed. 2009, 48, 2510ꢀ2513.
(39) (a) Fürstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007,
46, 3410ꢀ3449. (b) Fürstner, A. Chem. Soc. Rev. 2009, 38, 3208ꢀ3221.
(40) For reviews on diazoalkane metal complexes, see: (a) Sutton,
D. Chem. Rev. 1993, 93, 995ꢀ1022. (b) Mizobe, Y.; Ishii, Y.; Hidai,
M. Coord. Chem. Rev. 1995, 139, 281ꢀ311. (c) Dartiguenave, M.;
Menu, M. J.; Deydier, E.; Dartiguenave, Y.; Siebald, H. Coord.
Chem. Rev. 1998, 178ꢀ180, 623ꢀ663.
(41) It is also noteworthy that the conformation of the esp ligands
is different from that in 10 but similar to that of the known adduct
[Rh2(esp)2](acetone)2 (cf. ref. 23).
(42) For an interesting example in which a diazo compound gets
ligated endꢀon to copper but the resulting complex eventually transꢀ
forms into a true copper donor/acceptor carbene upon raising the
temperature, see: Pereira, A.; Champouret, Y.; Martín, C.; Álvarez,
E.; Etienne, M.; Belderraín, T. R.; Pérez, P. J. Chem.
21, 9769ꢀ9775.
(43) For representative catalytic reactions involving Rh(I) carꢀ
benes, see the following and literature cited therein: (a) Werner, H.;
Schneider, M. E.; Bosch, M.; Wolf, J.; Teuben, J. H.; Meetsma, A.;
−Eur. J. 2015,
Troyanov, S. Chem. Eur.−J. 2000, 6, 3052ꢀ3059. (b) Tsoi, Y.ꢀT.;
Zhou, Z.; Yu, W.ꢀY. Org. Lett. 2011, 13, 5370ꢀ5373. (c) Demonceau,
A.; Simal, F.; Noels, A. F.; Vinas, C.; Nunez, R. ; Teixidor, F. Tetra-
hedron Lett. 1997, 38, 7879ꢀ7882. (d) Chen, D.; Zhang, X.; Qi, W.ꢀ
Y.; Xu, B.; Xu, M.ꢀH. J. Am. Chem. Soc. 2015, 137, 5268ꢀ5271. (e)
Xia, Y.; Liu, Z.; Feng, S.; Ye, F.; Zhang, Y.; Wang, J. Org. Lett.
2015, 17, 956ꢀ959. (f) Yada, A.; Fujita, S.; Murakami, M. J. Am.
Chem. Soc. 2014, 136, 7217ꢀ7220. (g) Nishimura, T.; Maeda, Y.;
Hayashi, T. Angew. Chem. Int. Ed. 2010, 49, 7324ꢀ7327. (g) Chen,
D.; Zhu, D.ꢀX.; Hu, M.ꢀH. J. Am. Chem. Soc. 2016, 138, 1498ꢀ1501.
(44) For representative catalytic reactions involving Rh(III) carꢀ
benes, see the following and literature cited therein: (a) Callot, H. J.;
Piechocki, Tetrahedron Lett. 1980, 21, 3489ꢀ3492. (b) Callot, H. J.;
Metz, F.; Piechocki, C. Tetrahedron 1982, 38, 2365ꢀ2369. (c)
O’Malley, S.; Kodadek, T. Tetrahedron Lett. 1991, 32, 2445ꢀ2448.
(d) Hayashi, T.; Kato, T.; Kaneko, T.; Asai, T.; Ogoshi, H. J. Organ-
omet. Chem. 1994, 473, 323ꢀ327. (e) Lo, V. K.ꢀY.; Thu, H.ꢀY.; Chan,
Y.ꢀM.; Lam, T.ꢀL.; Yu, W.ꢀY. ; Che, C.ꢀM. Synlett 2012, 23, 2753ꢀ
2757. (f) Franssen, N. M. G.; Finger, M.; Reek, J. N. H.; de Bruin, B.
Dalton Trans. 2013, 42, 4139ꢀ4152.
(28) Seidel, G.; Fürstner, A. Angew. Chem., Int. Ed. 2014, 53,
4807ꢀ4811.
(29) (a) Seidel, G.; Gabor, B.; Goddard, R.; Heggen, B.; Thiel, W.;
Fürstner, A. Angew. Chem., Int. Ed. 2014, 53, 879ꢀ882. (b) G. Seidel,
R. Mynott, A. Fürstner, Angew. Chem., Int. Ed. 2009, 48, 2510ꢀ2513.
(c) Fürstner, A. Acc. Chem. Res. 2014, 47, 925ꢀ938.
(30) Staudinger, H.; Kupfer, O. Ber. Dtsch. Chem. Ges. 1911, 44,
2197ꢀ2212.
(31) For the Xꢀray structure of 3a, see: Davis, P. J.; Harris, L.; Kaꢀ
rim, A.; Thompson, A. L.; Gilpin, M.; Moloney, M. G.; Pound, M. J.;
Thompson, C. Tetrahedron Lett. 2011, 52, 1553ꢀ1556.
(45) Chen, W.ꢀW.; Lo, S.ꢀF.; Zhou, Z.; Yu, W.ꢀY. J. Am. Chem.
Soc. 2012, 134, 13565ꢀ13568.
(46) See the following for leading references and literature cited
therein: (a) Shi, Z.; Koester, D. C.; BoultadakisꢀArapinis, M.; Glorius,
F. J. Am. Chem. Soc. 2013, 135, 12204ꢀ12207. (b) Liang, Y.; Yu, K.;
Li, B.; Xu, S.; Song, H.; Wang, B. Chem. Commun. 2014, 50, 6130ꢀ
6133. (c) Jeong, J.; Patel, P.; Hwang, H.; Chang, S. Org. Lett. 2014,
16, 4598ꢀ4601. (d) Hu, F.; Xia, Y.; Ye, F.; Liu, Z.; Ma, C.; Zhang, Y.;
(32) Isolated 4a was shown to undergo the same reaction at −10°C.
(33) For a leading reference on the use of donor/donor rhodium
carbenes in asymmetric synthesis, see: Soldi, C.; Lamb, K. N.;
ACS Paragon Plus Environment

Page 9 of 10
Journal of the American Chemical Society
Wang, J. Angew. Chem., Int. Ed. 2014, 53, 1364ꢀ1367. (e) Ye, B.;
Cramer, N. Angew. Chem., Int. Ed. 2014, 53, 7896ꢀ7899. (f) Shi, J.;
Yan, Y.; Li, Q.; Xu, H. E.; Yi, W. Chem. Commun. 2014, 50, 6483ꢀ
6486. (g) Yang, Y.; Wang, X.; Li, Y.; Zhou, B. Angew. Chem., Int.
Ed. 2015, 54, 15400ꢀ15404. (h) Zhou, J.; Shi, J.; Liu, X.; Jia, J.; Xu,
H. E.; Yi, W. Chem. Commun. 2015, 51, 5868ꢀ5871. (i) Iagafarova, I.
E.; Vorobyeva, D. V.; Peregudov, A. S.; Osipov, S. N. Eur. J. Org.
Chem. 2015, 4950ꢀ4955. (j) Wang, L.; Li, Z.; Qu, X.; Peng, W.ꢀM.;
Hu, S.ꢀQ.; Wang, H.ꢀB. Tetrahedron Lett. 2015, 56, 6214ꢀ6218. (k)
Ai, W.; Yang, X.; Wu, Y.; Wang, X.; Li, Y.; Yang, Y.; Zhou, B.
1
2
3
4
5
6
7
8
Chem.−Eur. J. 2014, 20, 17653ꢀ17657. (l) Sharma, S.; Han. S. H.;
9
Han, S.; Ji, W.; Oh, J.; Lee, S.ꢀY.; Oh, J. S.; Jung, Y. H.; Kim, I. S.
Org. Lett. 2015, 17, 2852ꢀ2855. (m) Zhang, Y.; Zheng, J.; Cui, S. J.
Org. Chem. 2014, 79, 6490ꢀ6500. (n) Yu, S.; Liu, S.; Lan, Y.; Wan,
B.; Li, X. J. Am. Chem. Soc. 2015, 137, 1623ꢀ1631. (o) Cheng, Y.;
Bolm, C. Angew. Chem., Int. Ed. 2015, 54, 12349ꢀ12352. (p) Zhou,
B.; Chen, Z.; Yang, Y.; Ai, W.; Tang, H.; Wu, Y.; Zhu, W.; Li, Y.
Angew. Chem., Int. Ed. 2015, 54, 12121ꢀ12126.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(47) For a pertinent silverꢀfree case, see: Hyster, T. K.; Ruhl, K. E.;
Rovis, T. J. Am. Chem. Soc. 2013, 135, 5364ꢀ5367.
(48) See the following for leading references and the literature citꢀ
ed therein: (a) Padwa, A.; Kassir, J. M.; Xu, S. L. J. Org. Chem. 1991,
56, 6971ꢀ6972. (b) Chen, J.; Ma, S. Chem. Asian J. 2010, 5, 2415ꢀ
2421. (c) Cui, X.; Xu, X.; Wojtas, L.; Kim, M. M.; Zhang, X. P. J.
Am. Chem. Soc. 2012, 134, 19981ꢀ19984. (d) Breslow, R.; Winter, R.;
Battiste, M. J. Org. Chem. 1959, 24, 415ꢀ416.
(49) (a) Davies, H. M. L.; Romines, K. R. Tetrahedron 1988, 44,
3343ꢀ3348. (b) Müller, P.; Gränicher, C. Helv. Chim. Acta 1993, 76,
521ꢀ534.
(50) Ma, S.; Lu, L.; Lu, P. J. Org. Chem. 2005, 70, 1063ꢀ1065.
(51) For a Cꢀmetalated Rh(III) enolate prepared by an entirely difꢀ
ferent route, see: Wu, J.; Bergman, R. G. J. Am. Chem. Soc. 1989,
111, 7628ꢀ7630.
(52) Braun, M., Modern Enolate Chemistry. From Preparation to
Applications in Asymmetric Synthesis, WileyꢀVCH, Weinheim, 2016.
(53) Maxwell, J.; Kodadek, T. Organometallics 1991, 10, 4ꢀ6
(54) For a related chloride migration to the carbene center in the
platinum series, see: Bergamini, P.; Costa, E.; Sostero, S.; Orpen, A.
G.; Pringle, P. G. Organometallics 1991, 10, 2989ꢀ2990.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 10
1
2
3
4
5
6
For Table of Contents Only
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
10
ACS Paragon Plus Environment