Mononuclear biscarbene complexes by direct nucleophile addition to a CO ligand of fischer arylcarbene complexes

Article abstract of DOI:10.1002/chem.200800452

A study was conducted to demonstrate the synthesis of a mononuclear biscarbene complex by direct reaction of a Fischer monocarbene complex with a nucleophile addition to a CO ligand. The study proposed mononuclear biscarbene complexes of Group 6, as intermediates in the thermal carbene-carbene coupling reactions of Fischer mononuclear monocarbene complexes. These carbene ligand coupling processes that produced alkenes were reported to occur at room temperature in the presence of palladium(0), or a copper(I) catalyst. It was found that in case of (methyloxy)(aryl)monocarbene complexes, the site of nucleophilic addition is strongly depending on the nature of the organolithium compound. It was found that heteronucleophiles, as lithium amides or potassium alkoxides are also to react with a CO ligand of complexes.

Full text of DOI:10.1002/chem.200800452

COMMUNICATION
DOI: 10.1002/chem.200800452
Mononuclear Biscarbene Complexes by DirectNucleophile Addiiton
to a CO Ligand of Fischer Arylcarbene Complexes
JosØ Barluenga,*[a] AndrØs A. Trabanco,[a] Ivµn PØrez-Sµnchez,[a] Raquel De la Campa,[a]
Josefa Flórez,[a] Santiago García-Granda,[b] and ngel Aguirre[b]
Mononuclear Group 6 Fischer biscarbene complexes I,
which are organometallic compounds that have two carbene
ligands bonded to the same transition-metal atom, have
been much less investigated than the corresponding Fischer
monocarbene complexes II (Figure 1).[1] Only very few ex-
amples of this class of compounds I have been reported and
characterized by reactivity and/or X-ray studies, and their
synthetic potential remains to be explored. In addition,
there is a lack of a general method to produce such com-
plexes.
In addition, these complexes have been prepared by thermal
reaction of tetraaminoalkenes with the corresponding
Group 6 M(CO)6 complex,[4,5] and a single tungsten biscar-
bene derivative was synthesized from an (amino)-
A
under photochemical conditions.[6]
The synthesis of a mononuclear biscarbene complex by
direct reaction of a Fischer monocarbene complex with a
nucleophile has been reported to succeed only in the case of
cyclic diaminocarbene complexes which after successive
treatment with an organolithium (MeLi or PhLi) and a
strong electrophile (MeSO3F or Et3OBF4) provided mixed
cis-biscarbene complexes.[4] The addition of a nucleophile to
a pentacarbonylcarbene complex such as II usually takes
place at the carbene carbon atom, which has a strong elec-
trophilic character, and not at one of the CO ligands.[7] Nev-
ertheless, organolithium addition at a coordinated CO oc-
curred when nucleophilic attack at the carbene carbon has
been suppressed by a previous enolization-type reaction;[8]
and an intramolecular addition of a heteronucleophile to a
CO ligand of a neutral alkoxymonocarbene complex has
been referred to explain the formation of a reaction prod-
uct.[9]
On the other hand, mononuclear biscarbene complexes of
Group 6 have been proposed as intermediates in the ther-
mal carbene–carbene coupling reactions of either Fischer
mononuclear monocarbene complexes[10] or dinuclear biscar-
bene complexes.[11] These carbene ligand coupling processes,
that produced alkenes, have also been reported to occur at
room temperature in the presence of a palladium(0)[12] or a
copper(I)[13] catalyst.
Figure 1. General structure of mononuclear Fischer biscarbene complexes
I and monocarbene complexes II.
The classical Fischer synthesis (successive treatment of a
Group 6 hexacarbonylmetal with an organolithium (2 equiv)
and then a strong electrophile) is limited to only particular
types of both cyclic[2] and acyclic mononuclear cis-biscar-
bene complexes,[3] and typically affords low product yields.
[a]Prof. Dr. J. Barluenga, Dr. A. A. Trabanco, Dr. I. PØrez-Sµnchez,
R. De la Campa, Dr. J. Flórez
Instituto Universitario de Química Organometµlica “Enrique Moles”
Unidad Asociada al CSIC, Universidad de Oviedo
Juliµn Clavería 8, 33006 Oviedo (Spain)
In the context of our ongoing studies of the reactivity of
Fax : (+34)98-510-3450
E-mail: barluenga@uniovi.es
Fischer carbene complexes with nucleophiles,[14] we have
[b]Prof. Dr. S. García-Granda, Dr. . Aguirre
Departamento de Química Física y Analítica
(X-ray structure analysis)
found that in the case of (menthyloxy)(aryl)monocarbene
A
complexes the site of nucleophile addition is strongly de-
pending on the nature of the organolithium compound.
While butyllithium and alkynyllithiums react with the car-
bene carbon atom,[15a,16] and more substituted alkyllithiums
(such as sBuLi or tBuLi) and phenyllithium undergo conju-
Facultad de Química, Universidad de Oviedo, Juliµn Clavería 8
33006 Oviedo (Spain)
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
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5401

gate addition to the aromatic ring,[15] alkenyllithiums exhibit
clean addition to a CO ligand. Herein, we describe a general
method for the preparation of different types of mononu-
clear biscarbene complexes of chromium and tungsten
which involves the unusual attack of a nucleophile on a CO
ligand of the corresponding Fischer arylcarbene complex.
In the initial experiments we observed that the successive
furnished 3b as a roughly equimolecular mixture of isomers.
Accordingly, these reactions would involve formation of
(alkoxy)(hydroxy)biscarbene complexes C and, after intra-
G
molecular carbene–carbene coupling, 2-menthyloxy substi-
tuted enols D (Scheme 2).
treatment of tungsten (menthyloxy)(phenyl)carbene com-
U
plex (À)-1a[17] with an alkenyllithium [(E)-2-phenylethenyl-
lithium or 1-methylethenyllithium]and then with methyl tri-
flate under the reaction conditions shown in Scheme 1, pro-
vided, after silica gel column chromatography, cis-1,2-dia-
lkoxy-1,3-dienes 2a,b, each one as a single diastereoisomer.
The selective formation of compounds 2 can be understood
assuming an addition of the alkenyllithium to a CO ligand
of complex (À)-1a to give lithioxy intermediate A and after
treatment with MeOTf (alkoxyaryl)(methoxyalkenyl)biscar-
bene complexes B, which would undergo a spontaneous car-
bene–carbene coupling reaction. Presumably, biscarbene
complexes B have the two carbene ligands in the thermody-
namically more stable cis orientation[18] and with a disposi-
tion of the alkoxy groups to the same side of the molecule
what could be controlled by intramolecular coordination of
the lithium cation to the menthyloxy group oxygen as de-
picted in model Aꢀ (Scheme 1). This model accounts for the
selective formation of diastereoisomers 2 with a Z configu-
ration of the 1,2-dialkoxy substituted C=C bond. The stereo-
chemistry was ascertained on the bases of a NOE difference
experiment performed with compound 2a.
Scheme 2. Two-heteroatom-stabilized mononuclear (alkoxy)(hydroxy)-
AHCTREUNG
We found that heteronucleophiles as lithium amides or
potassium alkoxides are also able to react with a CO ligand
of complexes 1 (Scheme 3). Thus, the reaction of chromium
carbene complex (Æ)-1b with the lithium amide of morpho-
line, which requires the presence of TMEDA (N,N,N’,N’-tet-
ramethylethylenediamine),[20] furnished after treatment with
silica gel a-menthyloxyamide 4a as a single isomer.[21] In a
similar way, addition of potassium tert-butoxide to complex
(À)-1a led to a-alkoxyester 4b (1:1 mixture of diastereoiso-
mers).[22] These products 4a,b are likewise the stable tauto-
meric form of 2-alkoxyenols arising from intramolecular
Scheme 1. Two-heteroatom-stabilized
(alkoxy)biscarbene complexes B. a) THF, À78 to 208C; b) MeOTf, Et2O,
0 to 208C. R*OH=(À)-menthol.
mononuclear
(alkoxy)-
ACHTREUNG
Analogous reactions carried out with carbene complex 1a
and functionalized organolithium compounds but quenched
with silica gel instead of MeOTf afforded functionalized a-
alkoxyketones 3 (Scheme 2). The addition of 1-(dibenzyl-
aminomethyl)ethenyllithium to (À)-1a provided enone 3a[19]
as a 10:1 mixture of diastereoisomers while combination of
2-lithio-1,3-dithiane (a stabilized alkyllithium) with (Æ)-1a
Scheme 3. Three-heteroatom-stabilized mononuclear (alkoxy)(alkoxyhy-
droxy or aminohydroxy)biscarbene complexes E. R*OH=(À)-menthol
or (Æ)-menthol. [Cr]=(CO)5Cr. TMEDA=N,N,N’,N’-tetramethylethyl-
enediamine.
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Fischer Arylcarbene Complexes
COMMUNICATION
coupling of the two carbene ligands of (alkoxy)(alkoxyhy-
droxy
or
aminohydroxy)biscarbene
complexes
E
(Scheme 3).
To obtain a more direct evidence of the formation of
mononuclear biscarbene complexes we decided to test (ami-
no)arylcarbene complexes, which under the above reaction
sequence would produce more stable biscarbene complexes
given the greater stability of the aminocarbene complexes in
relation to the alkoxycarbene derivatives.[23] Gratifyingly, it
was observed that aminocarbene complex 1c reacted with
an alkenyllithium and MeOTf as summarized in Scheme 4
to afford the desired (aminoaryl)(alkoxyalkenyl)biscarbene
complexes 5a,b each one as a single diastereoisomer. These
mononuclear biscarbene complexes 5 are relatively stable
derivatives at room temperature but as a preventive mea-
sure they were stored refrigerated (À38C) and under N2. In
fact, when a solution of biscarbene complex 5a in hexane/
ethyl acetate (5:1) was heated at 358C for a short time
(15 min) the diastereoselective and almost quantitative for-
mation of (1Z,3E)-1-amino-2-alkoxy-1,3-diene (6) was ob-
served (Scheme 4). The stereochemistry of this carbene–car-
Scheme 5. Three-heteroatom-stabilized mononuclear (amino)(aminoalk-
oxy)biscarbene complex 7. [W]=(CO)5W. TMEDA=N,N,N’,N’-tetrame-
thylethylenediamine.
gands and allowed to ascertain a syn orientation of the pyr-
rolidinyl and methoxy groups (Figure 2). Biscarbene com-
plex 7 stabilized by three heteroatoms is stable at room tem-
Figure 2. ORTEP representation of (R)-7 in the crystal structure with
thermal ellipsoids set at 30% probability. The dihedral angle between the
least squares planes through the atoms W1-C1-N1-C2 and W1-C12-N2-
À
À
O1 is 69.4(1)8. Bond lengths W1 C1 2.260(6) and W1 C12 2.292(7) Š.
The W coordination angles vary from 83.4(3) to 98.6(3)8 indicating a dis-
torted octahedral environment.
Scheme 4. Two-heteroatom-stabilized
mononuclear
(amino)-
ACHTREUNG
bene coupling product 6 was ascertained from a 2D NOESY
experiment.[24] Biscarbene derivatives 5 represent the first
examples of mononuclear biscarbene complexes stabilized
by two heteroatoms and with an acyclic structure which
have been isolated and characterized. The 13C NMR spectra
of compounds 5 show four signals between 200–220 ppm as-
signed to four chemical inequivalent CO ligands what re-
veals a cis relative configuration of the two carbene ligands.
From the 1D and 2D NMR studies the relative arrangement
of the substituents of each carbene ligand could not be es-
tablished.
Although compounds 5 are oil materials, a solid derivative
7 was successfully synthesized using a heteronucleophile
(Scheme 5). The successive addition to complex 1c of mor-
pholine lithium amide and MeOTf provided diastereoselec-
tively (amino)(aminoalkoxy)biscarbene complex 7 (yellow
solid). An X-ray structure determination of a single crystal
of 7[25] confirmed the cis disposition of the two carbene li-
perature for long periods of time even in solution (at least
two days). However, heating (458C) solutions of this com-
plex 7 in THF, toluene or hexane/EtOAc did not lead to the
carbene–carbene coupling product, instead decomposition
materials were obtained. In addition, the X-ray analysis re-
vealed that the single crystal of 7 chosen for the study had a
chiral space group (P212121) while, as expected, a dichloro-
methane solution of compound 7 showed no optical activity.
This can be explained assuming a segregation of enantio-
mers upon crystallization which is known as spontaneous
resolution.[26] Compound 7 which is a chiral complex with
exclusively achiral ligands in an octahedral environment,
presumably exhibits atropisomerism (carbene ligands do not
have free rotation) and condensation into crystals occurred
with formation of a conglomerate[27] (each crystal is formed
by only one enantiomer but the sample as a whole is race-
mic because it contains equal amounts of enantiomorphic
Chem. Eur. J. 2008, 14, 5401 – 5404
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Acknowledgements
Financial support for this work from the Ministerio de Educación y Cien-
cia (MEC) of Spain (grants CTQ-2004-08077-C02-01/BQU and MEC-06-
MAT-01997 and predoctoral fellowship to R.D.C.) and from the Principa-
do de Asturias (FICYT) (project IB05-136 and predoctoral fellowship to
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Keywords: diene ligands
carbene coupling · carbenes · nucleophilic addition
· alkenyllithiums · carbene–
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[24]HMQC, HMBC, COSY and NOESY NMR spectra were measured.
[25]Crystal data for 7 [(R)-7] : C H24N2O6W Mr =584.27, orthorhombic,
21
space group P212121, a=8.9435(1), b=14.2153(2), c=16.8631(2) Š,
V=2143.88(5) Š3, Z=4, 1calcd =1.81 MgmÀ3, CuKa radiation (graph-
ite crystal monochromator, l=1.54184 Š), m=10.35 mmÀ1, F
(000)=
G
1144, T=120(2) K. Absolute configuration Flack parameter,
À0.03(2). Final conventional R=0.0405, wR2=0.1028 and S=1.033,
for 4006 “observed” reflections and 274 variables. CCDC 648590
(7) contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
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Received: March 12, 2008
Published online: April 30, 2008
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