A new dirhodium catalyst with hemilabile tropolonato ligands for C-H bond functionalization

Article abstract of DOI:10.1002/chem.201102427

Don't hold on too tightly! In a new dirhodium catalyst for C-H functionalization reactions, two tropolonato ligands are introduced as hemilabile chelating ligands (see scheme). Only two bridges hold the Rh-Rh core together. The tropolonato ligands can liberate a binding site in the equatorial coordination sphere of the catalyst. This opens a doorway to new mechanistic channels in C-H functionalization. Copyright

Full text of DOI:10.1002/chem.201102427

DOI: 10.1002/chem.201102427
ꢀ
A New Dirhodium Catalyst with Hemilabile Tropolonato Ligands for C H
Bond Functionalization
Frauke Thrun, Burkhard Butschke, and Timm Graening*[a]
Most organic synthetic methods are based on functional
groups and involve the transformation of a bond either at
the groups themselves or in their vicinity. Methods for the
ꢀ
functionalization of unactivated C H bonds render comple-
mentary synthetic strategies possible, since these bonds are
inert to the reaction conditions of standard transforma-
tions.[1] Metal-bound carbenes,[2] nitrenes,[3] or oxygen[4] have
been shown to be efficient catalytically generated species
Scheme 1. Paddle-wheel structure of the parent compound dirhodium tet-
raacetate (1). Structure of our novel dirhodium catalyst (2) with chelating
tropolonato ligands.
ꢀ
for the functionalization of C H bonds. Catalysts derived
from dirhodium tetraacetate (1, [Rh
2A
ly effective in the decomposition of diazo compounds to
form carbenes.[5] The most successful catalysts comprise pro-
line-derived systems from the groups of McKervey[6] and
Davies,[7] tert-leucine-derived systems of Hashimoto and Ike-
gami,[8] carboxamidates of Doyle[9] and tethered carboxy-
lates of DuBois et al.[10] Yet many problems regarding the
inherent reactivity of Rh carbenoids, particularly in syntheti-
Our approach to expand the chemical space of dirhodium
catalysts was guided by the idea that four bridging ligands,
as in the currently applied dirhodium catalysts, might not be
necessary to ensure the stability and catalytic activity of the
4+
Rh2
core. Dirhodium compounds containing only two
cally most relevant intermolecular C H insertion reactions,
bridging ligands have been described in the literature.[12] We
wanted to use this concept to alter the reactivity of dirhodi-
um catalysts more fundamentally than through exploiting
substituent effects on acetate derivatives and to put the cur-
rently accepted mechanisms of rhodium carbenoid chemistry
to a test. In particular, we wanted to scrutinize the mecha-
nistic paradigm that carbenoid transformations with dimeric
rhodium catalysts exclusively involve substrate binding to
the axial positions.[13] These positions can be readily occu-
pied by weak donor species, for example, solvent mole-
cules,[14] whereas the stronger equatorial rhodium spectator-
ligand bonds are considered to remain intact. This implies
that equatorial sites are inaccessible for substrate binding
throughout the catalytic cycle. In 2005, Corey and co-work-
ers put forward the hypothesis that the mechanism of cyclo-
ꢀ
remain unsolved: reactions are often found to be highly sub-
strate specific leading to low yields in unfavorable cases,
high catalyst loadings of 6 mol% rhodium (i.e., 3 mol%
[Rh2(L)4]) are typically required and high enantiomeric ex-
cesses can be accompanied by poor or no diastereoselectiv-
ity at all. These limitations can be attributed to a lack of
structural diversity in the currently applied catalysts. With
only a few exceptions,[11] the development of new dimeric
Rh-catalysts focuses on structures bearing four bridging li-
gands.
We report here on 1) the preparation of a novel dirhodi-
um catalyst (2 Scheme 1), in which two bridging carboxy-
lates are replaced by chelating tropolonato ligands, and its
X-ray crystal structure; 2) investigations on its solution be-
4+
havior that demonstrate the stability of the Rh2 core and
propenation of alkynes with [Rh2ACTHNGUTREN(UNG OAc)ACHTUNGTERNN(UNG dpti)3] (dpti=di-
the hemilabile nature of the tropolonato ligands, and 3) ex-
phenyltriflylimidazolidinone) involves a vacancy in an equa-
torial position on the rhodium dimer.[15] This suggestion was
later questioned by Nowlan and Singleton on the basis of ki-
netic isotope effects and theoretical calculations.[16]
ꢀ
amples of intra- and intermolecular C H insertion reactions,
which document the good catalytic activity of our new cata-
lyst system.
The concept, which we followed in designing new catalyst
structures, is based on the notion that an equatorial coordi-
nation site could be deliberately offered, if a chelating non-
bridging ligand was introduced. Bipyridyl (bpy) and related
ligands are known to chelate dinuclear RhII-complexes
either in an equatorial–equatorial or an equatorial–axial
manner and have been tested for their antitumor activity.[17]
We sought for a less basic chelating O,O-ligand that would
behave similarly and thus potentially provide an equatorial
[a] Dr. F. Thrun, Dipl.-Chem. B. Butschke, Dr. T. Graening
Institut fꢀr Chemie
TU Berlin–Berlin University of Technology
Strasse des 17. Juni 135
10623 Berlin (Germany)
Fax : (+49)30-314-29745
E-mail: timm.graening@tu-berlin.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102427.
4854
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Chem. Eur. J. 2012, 18, 4854 – 4858

COMMUNICATION
Scheme 2. Two alternative binding modes for an O,O-chelating ligand on
4+
a Rh2 core. A vacant coordination site is indicated by a square. ax=
axial, eq=equatorial.
coordination site for substrate binding (4 Scheme 2), but
would resemble carboxylates more closely in terms of its
pKa value and electron density. We turned to the tropolona-
to ligand as a vinylogous carboxylic acid, bearing in mind
that it forms more stable complexes than for example, the
acetylacetonato, maltol or quinolinol ligands.[18] Whereas
mononuclear RhI-tropolonato complexes are known,[19] a di-
nuclear complex with tropolonato ligands has not yet been
described.
Figure 1. ORTEP representation of complex 2. Two molecules of CH2Cl2
within the unit cell and hydrogen atoms are omitted for clarity. Selected
bond lengths [ꢂ] and angles [8]: Rh–Rh 2.4963(8), Rh–O1 2.054(3), Rh–
O2 2.032(3), Rh–O3 1.997(3), Rh–O4 1.995(3); O1-Rh-O2 86.9(9), O3-
Rh-O4 81.2(2), O1-Rh-O3 95.9(3), O2-Rh-O4 95.7(9).
Initial attempts to prepare a dirhodium tropolonato com-
plex using the standard procedure, heating dirhodium tet-
raacetate in chlorobenzene in the presence of tropolone in a
Soxhlet apparatus equipped with sodium carbonate, were
not successful. By using the potassium salt of tropolone and
through their axial sites. The Rh–Rh distance of 2.4963(8) ꢂ
is consistent with the expected bond order of one (Figure 1).
The tropolone moieties are held in an eclipsed conformation
about the Rh–Rh single bond due to the two bridging ace-
tate ligands. The O3-Rh-Rh-O4 torsion angle is negligibly
small (0.18) and the bite angle of the tropolonato ligands of
81.2(2)8 is comparable to that of a mononuclear RhI tropo-
lonato complex (79.4(2)8).[19b] Another noteworthy feature
of the complex structure is the observation that contacts
considerably closer than acceptable for p-stacking (ca. 3.3–
3.5 ꢂ) between the tropolone moieties are avoided by a
15.48 divergence of the O(1), O(2), O(3), O(4) least-squares
turning to the cis-[Rh
complex 5 as a starting material, we were finally able to pre-
pare the complex [Rh2A(OAc)2A(trop)2] (2) in good yield
2ACHUTGTNRENNUG(OAc)2ACHTUNTGERN(NUGN tfa)2] (tfa=trifluoroacetate)
C
CHTUNGTRENNUNG
(79%, Scheme 3). Interestingly, the same product was ob-
planes. This is consistent with data of a [Rh
G
N
bis-chelate complex.[17b]
1
We recorded the H NMR spectra of [Rh
2A(OAc)
(2) in solvents with increasing donor capacity (CDCl3,
[D8]THF, [D6]acetone, CD3OD and CD3CN) to investigate
Scheme 3. Preparation of complex 2 by selective substitution of tfa li-
gands.
the feasibility of displacing a tropolonato ligand from its
1
equatorial position. The H NMR spectrum of [Rh
2A
AHCTUNGRTENGUN(N trop)2] (2) in [D8]THF solution shows a typical AA’BB’C
tained using the trans-[Rh
G
G
spin system for two identical tropolone rings located in a
symmetric surrounding as it is found in the solid state (Fig-
ure 2a). Similar spectra are obtained in CDCl3, [D6]acetone,
and CD3OD. In CD3CN, which is a more strongly coordinat-
ing solvent, the formation of a new tropolone species is ob-
served at room temperature and more complex NMR spec-
ing material, even though with other ligands, substitution of
the tfa ligands occurs with retention of stereochemistry.[20]
The formation of a tropolonato complex containing trifluor-
oacetate ligands, for example [Rh
(OAc)(tfa)(trop)2], is not observed.
Since the NMR spectra of the title compound in standard
A
R
ACHTUNGTRENNUNG
1
tra are obtained (Figure 2b). In the H NMR spectrum, sig-
NMR solvents contained limited information due to the
high symmetry of the complex (cf. Figure 2a), we turned to
X-ray crystallography to establish the structure. Suitable
single crystals were grown by diffusion of n-hexane vapor
into a saturated solution of the complex in CH2Cl2 at 48C.
The complex crystallizes in the monoclinic space group C2/c
and exhibits a ribbon superstructure with an alternating ori-
entation of the molecules in which the rhodium atoms coor-
dinate to an oxygen atom of a neighboring molecule
nals of two different tropolone moieties appear: one in sym-
metrical surrounding giving rise to the familiar AA’BB’C
spin system, the other giving rise to a subspectrum with five
well separated resonances, indicated by arrows in Figure 2b.
This observation might be interpreted in terms of a struc-
ture comprising a monodentate tropolonato ligand as depict-
ed in Figure 3. The liberated equatorial position is most
likely to be occupied by a CH3CN molecule.[21] The NMR
resonances of the monodentate tropolonato ligand resemble
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4855

T. Graening et al.
the tropolonato ligands, that is,
complex 2 is more easily oxi-
dized. Interestingly, in THF so-
lution a significantly lower po-
tential of E1/2 =0.164 V was ob-
served for the title compound
(Figure 4c). For comparison, a
related [Rh2ACTHUNTRGENN(UG OAc)2ACHTUNGTNER(NUGN bpy)2] com-
plex with two chelating bpy li-
gands, exhibits a reversible one-
electron
reduction
at
ꢀ0.89 V.[17b]
We were pleased to find that
[Rh2ACHTNUGTRENNUG(OAc)2HCATUNGTREN(NNUG trop)2] (2), as a
Figure 2. 1H NMR resonances of complex 2 in different solvents: a) [D8]THF, RT, 400 MHz; b) CD3CN, RT,
500 MHz.
new type of dirhodium com-
plex, is catalytically active in
ꢀ
C H bond insertion reactions.
We investigated the chemose-
lectivity of 2 as a catalyst by performing intramolecular
competition experiments (Tables 1 and 2). Due to the irre-
ꢀ
versibility of the C H insertion reactions, the product ratios
of the competition experiments are characteristic for the re-
active intermediates involved. Applying diazoketone (rac)-6
ꢀ
as a starting material for an aliphatic C H bond insertion
furnished a good combined yield of 83% with this unopti-
mized catalyst (Table 1, entry 1). The observed chemoselec-
ꢀ
tivity, reflecting a preference for insertion into a tertiary C
ꢀ
H bond (! (rac)-7) over insertion into a secondary C H
bond (! (rac)-8) is slightly higher than the selectivities re-
ported by Lahuerta and Pꢃrez-Prieto for dirhodium catalysts
bearing chiral phosphine ligands (Table 1, entries 2–4)[23] and
dirhodium catalysts containing electron-withdrawing tfa li-
gands (Table 1, entries 5 and 6).
Figure 3. Suggested structure of the tropolonato dirhodium complex in
acetonitrile solution based on DFT calculations using the B3LYP func-
tional.
those of a tropolone ether with more localized double
bonds, rather than a symmetrical bidentate tropolonato
ligand with highly delocalized p-bonds as in free tropolone.
The fact that no free tropolone is observed even after sever-
al hours at room temperature indicates that the monoden-
tate tropolonato ligand is not bound to an easily substituted
axial coordination site. After evaporation of the solvent
(CD3CN), the starting material is retrieved as determined
by NMR spectroscopy in CDCl3. Thus, the tropolonato
ligand behaves like a hemilabile ligand under these condi-
tions.[22]
By using (rac)-9 as a starting material, we wanted to
ꢀ
probe the selectivity of our catalyst for aliphatic C H bond
To substantiate our findings from NMR spectroscopy and
to prove the persistency of the Rh–Rh core in solution when
it is held together by only two bridging acetate ligands, we
used cyclic voltammetry as a sensitive tool to characterize
redox-active intermediates. Indeed, the cyclic voltammo-
gram of [Rh2ACHTUNGTRENNUNG(OAc)2ACHTUTGNREN(NGUN trop)2] (2) displays a single redox
couple even in the strongly coordinating solvent acetonitrile,
which is interpreted as a quasireversible one-electron oxida-
4+
5+
tion of Rh2 to Rh2 (Figure 4b). The potential of E1/2
0.471 V of complex 2 is shifted to lower potential by 336 mV
relative to the parent compound [Rh2A(OAc)4] (5) in acetoni-
=
Figure 4. Cyclic voltammograms of a) 5 in MeCN, scan rate 20 mVsꢀ1
;
CTHUNGTRENNUNG
b) 2 in MeCN, scan rate 10 mVsꢀ1 c) 2 in THF, scan rate 400 mVsꢀ1. Tet-
rabutylammonium hexafluorophosphate (0.01 m) was used as supporting
electrolyte.
trile as a solvent (Figure 4a). As expected, this fact can be
explained by a more pronounced electron-donor effect of
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Chem. Eur. J. 2012, 18, 4854 – 4858

A New Dirhodium Catalyst with Hemilabile Tropolonato Ligands
COMMUNICATION
ꢀ
Table 1. Intramolecular competition between tertiary and secondary C H bond inser-
tion.
isomer (rac)-13 can be achieved (Table 3, entry 1).
Rhodium tetraacetate and cis-[Rh2ACHTUNTRGENNUG(OAc)2ACHTUNGTRENNUNG(tfa)2]
(Table 3, entries 2 and 3) give similar yields but
slightly lower diastereoselectivities, whereas [Rh2-
A
2ACHTUNGTNER(NUNG piv)4] give better yields but a signif-
CTHUNGTRENNUNG
Entry Catalyst [(mol%)]
T
t
7/8
Yield
[%]
catalyst leads primarily to the formation of the
(2R*,aR*) diastereoisomer (rac)-14 (Table 3,
entry 6).
In summary, we report on the first example of a
dirhodium catalyst for C H bond functionalization
[8C] [h]
1
N
G
E
25
100
1
–
–
–
1
1
88:12 83
79:21 84
73:27 85
76:24 80
74:26 60
71:29 74
2[a]
3[a]
4[a]
5
[b]
[b]
[b]
T
(CH-R*-OH)][c] (1) 100
2ACHTUNGTRENNUNG
N
(CH-R*-OH)][c] (1)
100
25
25
ꢀ
with chelating tropolonato ligands containing only
two carboxylate bridges. Whereas other dirhodium
compounds containing only two carboxylate bridges
are known,[12,17] the incorporation of hemilabile tro-
polonato ligands broadens the chemical space of
available dirhodium catalysts and offers opportuni-
ties for further improvements in catalyst design. We have
6
G
U
[a] Data taken from ref. [23]. [b] Reaction times not indicated. [c] PPh2ACTHNUTRGNEU(GN CH-R*-
OH)=(1S,2S,5R)-(2-hydroxy-5-isopropenyl-2-methylcyclohexanyl)-
ACHTUNGTRENNUNG(diphenyl)phosphane.
insertion versus aromatic substitution by means of an elec-
trophilic addition/hydride migration process. In this reaction,
the catalyst performed well, too, giving a combined yield of
72% with a selectivity of 87:13 ((rac)-10/ACHTNUTRGNEUNG(rac)-11) in favor
of the aromatic substitution product (Table 2, entry 1). Al-
demonstrated the catalytic activity of [Rh2ACTHNUTRGENN(UG OAc)2CAHTUNGTRNE(NGUN trop)2] (2)
ꢀ
in two competition experiments and an intermolecular C H
bond insertion reaction. By using NMR spectroscopy, we
were able to show that the chelating tropolonato ligands are
hemilabile in a coordinating solvent (acetonitrile). This ex-
periment serves as proof of our design concept, which was
to make an equatorial coordination site accessible for sub-
strate binding. Whereas we are not certain whether an equa-
torial coordination site is occupied by a substrate in the cat-
alytic reactions we examined, such a mechanism would be
fully plausible with our catalyst. The new catalyst we de-
scribe could be valuable for improving the mechanistic un-
Table 2. Intramolecular competition between insertion into a secondary
ꢀ
C H bond and aromatic substitution.
Entry
Catalyst [(mol%)]
t
10/11
87:13
78:22
>99:1
96:4
Yield
[%]
ꢀ
derstanding of C H functionalization reactions with dimeric
[h]
rhodium catalysts.
1
N
G
E
1
72
83
80
84
88
2[a]
3[c]
4
–
[b]
ꢀ
0.25
1
1
Table 3. C H insertion reaction of methyl phenyldiazoacetate and tetra-
hydrofuran.
5
G
U
>99:1
[a] Data taken from ref. [24]. [b] Reaction times not indicated. [c] Data
taken from ref. [25].
though this selectivity is higher than what has been reported
for dirhodium tetraacetate (Table 2, entry 2),[24] with [Rh2-
Entry
Catalyst
t
13/14[a]
Yield
[%]
[h]
ACHTUNGTRENNUNG
(tpa)4][25] (tpa=triphenylacetate) and catalysts containing tfa
1
2
3
4
5
6
A
T
E
12
3
7:1
4:1
4:1
3:1
1:1
1:2
45[b]
46[b]
46[b]
69[b]
67[c]
71[c]
ligands (Table 2, entries 3–5), almost complete selectivity for
the formation of the indanone (rac)-10 is observed.
N
A
U
3
1
1
1
ꢀ
Intermolecular C H insertions into cycloalkenes and satu-
A
U
A
U
rated heterocycles are more challenging than the corre-
sponding intramolecular reactions, but have been shown to
be feasible with aryldiazoacetates.[2c–e] Tetrahydrofurans are
synthetically most relevant, but diastereoselectivities are low
in these reactions.[26] A highly effective catalyst for enantio-
A
U
[a] Diastereoselectivity was determined by 1H NMR spectroscopy.
[b] Yields were determined by GC-MS with n-dodecane as internal stan-
dard. [c] Isolated product yields.
ꢀ
selective C H bond insertions, [Rh2((s)-DOSP)4] (DOSP=
N(p-dodecylphenylsulfonyl)prolinato), for example, fur-
nishes a diastereoselectivity of 2.5:1 to 2.8:1.[27] In the reac-
tion of methyl phenyldiazoacetate (12) using 2 mol% of our
catalyst (2) in tetrahydrofuran at ambient temperature a dia-
stereoselectivity of 7:1 in favor of the (2R*,aS*) diastereo-
Acknowledgements
The authors wish to thank Dr. Elisabeth Irran for X-ray structural analy-
sis. Financial support from the Fonds der Chemischen Industrie and gen-
Chem. Eur. J. 2012, 18, 4854 – 4858
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4857

T. Graening et al.
erous gifts of chemicals from Umicore and BASF are gratefully acknowl-
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ꢀ
Keywords: carbenes
homogeneous catalysis · rhodium
·
C H activation
·
chelates
·
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Received: August 4, 2011
Published online: March 19, 2012
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