A new class of singlet carbene ligands

Article abstract of DOI:10.1002/chem.201001779

(Figure Presented) What are you? Deprotonation of fluorenylidene phosphonium salts generates reactive intermediates that can be described as push-pull cumulenes or singlet carbenes (see scheme). Whereas the NMR data are ambiguous, the coordination behavior shows the ready availability of a lone pair at the central C atom. The donor ability of singlet carbene ligands of this new class exceeds that of the commonly used N-heterocyclic carbenes (NHCs).

Full text of DOI:10.1002/chem.201001779

DOI: 10.1002/chem.201001779
A New Class of Singlet Carbene Ligands
[
a]
Manuel Alcarazo, Rosa M. Suꢀrez, Richard Goddard, and Alois Fꢁrstner*
Dedicated to Professor Josꢀ Barluenga on the occasion of his 70th birthday
[1]
Ever since the first isolation, stable carbenes have
gained tremendous importance as ancillary ligands for cata-
lytically active transition-metal complexes and as organoca-
[2]
talysts in their own right. To increase the structural range,
our group has recently reported the prototype of a new class
[3]
of singlet carbene ligands. Specifically, deprotonation of
the alkenyl phosphonium salt 3a with potassium hexa-
ACHTUNGTRENNUNGm ethyldisilazide (KHMDS) afforded compound 4a, which
can either be seen as a push–pull cumulene or be described
by a zwitterionic resonance extreme owing to the opposing
[4,5]
polarization of the two orthogonal double bonds;
in this
representation, the carbene nature of the central C atom be-
comes clear (Scheme 1).
As part of a program devoted to the synthesis and evalua-
[3,6–9]
tion of novel carbon-based ligands,
we are committed to
explore the scope of this new design principle in more
detail. Outlined below are the results of a first round of in-
vestigations in which the fluorenyl part of 4 was kept con-
stant but the cationic terminus of the zwitterion was modi-
fied. A set of possible precursors was readily attained start-
ing from 9-fluorenone (1), which was converted to alkenyl
Scheme 1. a) Ph
3
P=CHBr, THF, ꢀ608C!RT, 82%, Ref. [10]; b) R
3
P,
toluene, reflux; c) NaBF , MeOH/H O, 55 % (3a), 68% (3b), 77% (3c),
4
2
62% (3d); d) KHMDS, THF, ꢀ788C; e) [AuCl·SMe
5b), 19% (5c), 28% (5d); f) [{Rh(CO) Cl}
], ꢀ788C!08C, 14%;
g) [{Rh(cod)Cl} (cod)Cl} ], ꢀ788C!ꢀ208C,
], ꢀ788C!RT, 45%; h) [{Rh
then AgBF
, ꢀ208C!08C, 55%.
2
], 41% (5a), 31%
(
2
2
bromide
2
according to
Treatment with various phosphines or
a
literature procedure
[
10,11]
A
H
U
G
R
N
U
G
2
A
H
U
G
R
N
U
G
2
(
Scheme 1).
4
HMPA (HMPA=hexamethylphosphoric acid triamide) then
gave the corresponding alkenyl phosphonium salts 3a–d in
respectable yields after ion exchange for the less coordinat-
ꢀ
ing BF4 (for the structures of two such salts in the solid
[12]
state, see the Supporting Information). Yet another possi-
ble carbene precursor 10 was obtained by condensation of
[13]
the known ylide 9 with commercial 1-methyl-2-formylbenz-
imidazole and N-methylation of the resulting product with
Meerwein salt (Scheme 2).
[
a] Dr. M. Alcarazo, Dr. R. M. Suꢀrez, Dr. R. Goddard,
Prof. A. Fꢁrstner
Max-Planck-Institut fꢁr Kohlenforschung
4
5470 Mꢁlheim/Ruhr (Germany)
Fax : (+49)208 306 2994
E-mail: fuerstner@kofo.mpg.de
Scheme 2. a) 1-Methyl-2-formylbenzimidazole,
b) Me O·BF CH Cl
], ꢀ788C!08C, 45%; e) [{Rh
THF,
reflux, 98%; c) KHMDS, THF, ꢀ788C;
(cod)Cl}
], ꢀ788C!RT,
reflux,
80%;
3
4
,
2
2
,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001779.
d) [AuCl·SMe
2
A
H
U
G
R
N
U
G
2
28%.
9746
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Chem. Eur. J. 2010, 16, 9746 – 9749

COMMUNICATION
Deprotonation of 3a with KHMDS in [D ]THF at ꢀ788C
8
cleanly generated the reactive intermediate 4a within
1
1
0 min, the central C-atom of which resonates at d =
98.8 ppm. The fact that the corresponding signal of 4d is
C
even more deshielded (d =214.1 ppm) shows that the
C
nature of the P substituents is intimately correlated with the
[14]
electronic properties of this site.
However, attempts at
isolation of either of these species in pure form were unsuc-
cessful.
A low-field signal for the central C atom does not allow
the carbene and the cumulene resonance forms of such in-
termediates to be discriminated; rather, the coordination be-
havior is thought to be more indicative. Therefore, the cor-
responding gold and rhodium complexes were targeted,
which were obtained on treatment of the respective salt
with KHMDS at ꢀ788C followed by addition of either
[15]
4ꢀ
Figure 2. Structure of complex 8 in the solid state. The BF anion is
omitted for clarity.
AuCl·SMe , [{Rh(CO) Cl} ] or [{Rh ACHTNUGTERNN(UNG cod)Cl} ] (cod=1,5-cy-
2
2
2
2
clooctadiene). The greatly different stability of the resulting
complexes became apparent upon slow warming of the reac-
tion mixtures. Whereas the metal adducts derived from 3c
and 3d had to be handled with great care at temperatures
planarity with the fluorenyl unit, which in turn prevents op-
timal orbital overlap in the backbone (the angle between
the planes defined by the fluorenyl ring and the Au-C-P
atoms is 24.398 in 5c; Figures 1
and 3). A similar distortion
may explain why the free car-
ꢁ
ꢀ208C under Ar, all other complexes were stable at am-
bient temperature in air. The gold complex 11 as well as the
rhodium complexes 6, 7, 8 and 12 could even be purified by
flash chromatography over Florisil, although the need for
MeOH as a coeluent resulted in some loss of material. Rep-
resentative examples of the new complexes were character-
ized by X-ray diffraction (see Figure 1 and 2 and the Sup-
bene of this new push–pull type
are less stable than ordinary N-
heterocyclic carbenes (NHCs).
The IR data of the rhodium
Figure 3. Allylic strain inter-
feres with the optimal orbital
overlap within the carbene
[15]
porting Information).
complex 6 are also quite infor- backbone.
mative. Frequencies for the un-
symmetrical and symmetrical
stretching mode of the carbonyl
ꢀ
1
groups at 1979 and 2056 cm portray the new carbene
ligand 4a as a powerful donor. As can be seen from the ref-
erence data compiled in Table 1, 4a actually surpasses pro-
totype NHCs in its electron-releasing capacity.
Finally, an attempt was made to extend the concept to the
[21]
preparation of all-carbon-based singlet carbenes. To this
[22]
end, the precursor salt 14 and the known olefin 17 were
prepared as shown in Scheme 3. Whereas the putative inter-
mediate 15 generated upon deprotonation of 14 decom-
[
15]
Figure 1. Structure of complex 5c in the solid state.
In all cases investigated, the metal was bound to the cen-
tral C atom, which shows the availability of a lone pair at
this site and hence the carbene character of the new class of
ligands. The conceivable alternative bonding mode, that is,
the formation of a cumulene p adduct, makes no noticeable
contribution to the ground-state structure of the complexes.
The observed CꢀAu bond lengths (for example,
3 4 2 2
Scheme 3. a) 9, toluene, 1008C, 58%; b) [Ph C] ACHTUNGTNERNU[G BF ], CH Cl , 08C, 75%;
2
.0408(14) ꢃ in 5c) are similar to those of typical AuꢀNHC
c) KHMDS, THF, ꢀ788C; d) 9, toluene, reflux, 73%; e) DDQ, toluene,
[16]
complexes. Allylic strain forces the CꢀM vector out of co-
13%, 908C, see Ref. [22].
Chem. Eur. J. 2010, 16, 9746 – 9749
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9747

A. Fꢁrstner et al.
Table 1. Comparison of carbonyl stretching frequencies in complexes of
type [L!Rh(CO) Cl] in the solid state, which incorporate commonly
used singlet carbene donor ligands L [ n˜ in cm ].
catalysis. Investigations along these lines together with fur-
ther structural variations will be the subject of future reports
from this laboratory.
2
ꢀ
1
L
n˜ asymm
n˜ symm
Reference
1
1
965
976
2052
[6]
Experimental Section
2072
2056
[17]
The entire experimental section, including the characterization of all new
substances and the structure of compounds 3b, 3c, 5b, 7, and 14 in the
solid state can be found in the Supporting Information.
1
979
this work
Acknowledgements
1
1
2
984
990
009
2079
2070
2089
[18]
[19]
[20]
Generous financial support by the MPG and the Fonds der Chemischen
Industrie is gratefully acknowledged. We thank H. Bruns for the prepara-
tion of several starting materials and the analytical departments of our
institute for excellent support.
Keywords: allenes · carbenes · gold · rhodium · zwitterions
posed almost instantaneously even at ꢀ788C and could not
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 9746 – 9749

A New Class of Singlet Carbene Ligands
COMMUNICATION
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[
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Received: June 23, 2010
Published online: July 29, 2010
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www.chemeurj.org
9749