The potassium hydride mediated trimerization of imines

Article abstract of DOI:10.1039/c0cc04565a

A novel reaction, the potassium hydride mediated synthesis of fulvenes, is described. The synthesis utilizes N-aryl imines as an inexpensive starting material affording novel substituted aminofulvenes. It is proposed that the presence of the metalated enamine as well as the imine (ratio 21) leads to the formation of an initial dimerization and a transient trimerization product, which cyclizes, giving rise to the aminofulvene.

Full text of DOI:10.1039/c0cc04565a

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COMMUNICATION
The potassium hydride mediated trimerization of iminesw
Kathrin Kutlescha, Gopaladasu T. Venkanna and Rhett Kempe*
Received 22nd October 2010, Accepted 11th February 2011
DOI: 10.1039/c0cc04565a
A novel reaction, the potassium hydride mediated synthesis of
fulvenes, is described. The synthesis utilizes N-aryl imines as an
inexpensive starting material affording novel substituted amino-
fulvenes. It is proposed that the presence of the metalated
enamine as well as the imine (ratio 2 : 1) leads to the formation
of an initial dimerization and a transient trimerization product,
which cyclizes, giving rise to the aminofulvene.
Fig. 2 Synthesis of fulvenes from N-aryl imines.
Pentafulvenes, first described by Thiele at the beginning of this
century,1 attracted much interest due to their color,2 reactivity
(especially cycloadditions),3 dipole moment4 and questions
regarding their aromatic or anti-aromatic5 character. Further-
more they represent a class of very interesting organic ligands.
Various organometallic compounds, being applied for instance
as polymerization catalysts6 or anticancer agents,7 have been
synthesized via fulvene routes.8 Fulvenes can be obtained by
condensation reaction9 of aldehydes or ketones with cyclo-
pentadienyl. Additionally, a few other methods can be utilized.10
Herein we report a novel potassium hydride mediated approach
towards 1,3,6-substituted 6-aminofulvenes. The imine trimeri-
zation reaction is based on tautomerization into metalated
enamines and proceeds via C–H activation and multiple C–C
bond formation steps.
Upon this discovery, we were interested in understanding
and using the side reaction. A precise reaction stoichiometry
(imine : KH ratio) is crucial to yield fulvenes as the main
product. Addition of a large excess of potassium hydride (3 eq.)
led to amine formation, whilst using one equivalent mainly
yielded the imine starting material (after workup). Upon
utilization of 0.7 equivalents of potassium hydride, complete
conversion of the imine to the corresponding 6-aminofulvene
as the main product was observed (Fig. 2). The addition of
several metal bases was investigated. Only the utilization of
potassium hydride gave rise to fulvene 2a with complete
conversion of the starting material.
Upon the addition of potassium hydride to imines 1a–i the
color of the reaction solution changed quickly to green and
then dark red, accompanied by hydrogen evolution. The
corresponding fulvenes 2a–i were obtained as dark red materials
in moderate yields (Fig. 3).
Within experiments regarding the asymmetric hydrogenation11
of 1a (Fig. 1), the formation of a by-product was observed if
KH was utilized as a base. A dark red material crystallized in one
of the catalysis samples. It was identified as [(2,4-diphenyl-cyclo-
penta-2,4-dienylidene)-phenyl-methyl]-phenyl-amine 2a (Fig. 1).
A time-conversion plot was generated to gain additional
insight into mechanistic details of this novel reaction (Fig. 4).
The isolated yield of the fulvene is consistent with the
GC-yield, which was obtained in the kinetic experiment.
Additionally, the formation of an intermediate (3a) and a
by-product (4a) could be observed as well as the formation of
aniline. The key intermediate (1,3-diphenyl-but-3-enylidene)-
phenyl-amine (3a) was independently synthesized. Reduction
of 3a gives rise (after aqueous workup) to the by-product
(1,3-diphenyl-butylidene)-phenyl-amine (4a), which could be
isolated from the reaction mixture and was characterized by
NMR spectroscopy and EA.
Fig. 1 Retrosynthetic approach towards novel fulvenes.
The potassium cyclopentadienylimine complex 5a was
crystallized from the reaction mixture and was analyzed via
X-ray crystal structure analysis to determine the molecular
structure (Fig. 5).
Lehrstuhl fur Anorganische Chemie II, Universitat Bayreuth,
¨
¨
Universitatsstraße 30, 95440 Bayreuth, Germany.
¨
E-mail: kempe@uni-bayreuth.de; Fax: +49 (0)921 552157;
Tel: +49 (0)921 552540
w Electronic supplementary information (ESI) available: Crystallographic
data, characterization data, and detailed experimental procedures. CCDC
782978–782980. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0cc04565a
In the dimeric complex 5a, the potassium is coordinated by
the N-atom and further stabilized by p-coordination of the
electron rich phenyl substituents of 5a and the cyclopentadienyl
moiety of a second ligand molecule. The bond lengths of 5a
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Chem. Commun., 2011, 47, 4183–4185 4183

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Fig. 5 Molecular structure of 5a; selected bond lengths [A] and angles [1]:
C1–C2, 1.404(5); C1–C24, 1.416(6); C2–C3, 1.409(6); C3–C4, 1.386(6);
C4–C24, 1.425(6); C17–K1, 3.208(4); C22–K1, 3.229; C23–N1, 1.304(5);
C23–C24, 1.458(6); N1–K1, 2.814(4); N1–C22–K1, 60.4(2); N1–C23–C24,
120.1(4); N1–C23–C25, 122.5(4); C23–N1–C22, 121.4(4).
Since Knorr et al.12 reported the formation of metastable
secondary enamines via lithiation of imines with lithium
diisopropylamide, we assumed that enamine-formation upon
potassium hydride addition is a crucial reaction step in fulvene
formation. The two olefinic hydrogen-atoms of the enamine 6a
were detected as doublets (J = 1.4 Hz) at 4.29 and 4.12 ppm
(solvent C6D6 : THF-d8 10 : 1). The presence of the imine as
well as the enamine is necessary for the reaction to take place,
which is supported by the results of the KH : imine ratio
screening. As indicated in Fig. 6, the enamine species 6a
Fig. 3 6-Aminofulvenes 2a–i.
Fig. 4 Time-conversion plot; determined via GC with dodecane as an
internal standard.
differ significantly from the bond lengths of the isolated fulvene
2a. Whereas in 2a the three double bonds (1.36–1.38 A) are
notably shorter than the sigma-bonds (1.45–1.47 A), in 5a only
the C3–C4 bond length (1.39 A) is in the supposed range. The
other C–C bond lengths vary between 1.40 and 1.46 A. The
C–N bond length of only 1.30 A indicates a C–N double bond.
These data provide a consistent picture of the coordinated
ligand as a cyclopentadienylimine rather than an amidofulvene.
The deviation of the cyclopentadienyl plane is 0.005 A. The
nitrogen atom is out of this plane (distance 0.41 A), because it
coordinates the potassium atom.
Fig. 6 Proposed reaction mechanism.
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attacks the C-atom of the CQN bond13 of 1a to yield
(1,3-diphenyl-but-3-enylidene)-phenyl-amine 3a, thereby potassium
anilide is eliminated. A second attack of 6a at the imino-group
of 3a occurs and subsequently the trimerization product 7a
cyclizes to 5a. The proposed mechanism is summarized in
Fig. 6.
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idene)-amine or N-phenyl-(thiophen-2-yl-ethylidene)-amine,
do neither convert to fulvenes under the general conditions
nor under harsh conditions (110 1C, diglyme or 1,4-dioxane).
If higher temperatures are applied, an additional by-product
was observed. This N-substituted 2,4-aryl-pyrrole is formed
due to cyclization of the intermediate 3.
The reaction of 3- or 4-substituted heterocyclic imines with
potassium hydride under harsh conditions (110 1C, diglyme) was
rather unselective leading to a mixture of various compounds.
In conclusion, a novel reaction was discovered. A series of
novel 1,3,6-substituted 6-aminofulvenes was synthesized
by a facile approach, which utilizes inexpensive and readily
available imines as starting material. Furthermore, the mecha-
nism of the reaction was investigated. We propose that the
potassium-mediated trimerization reaction of N-aryl imines
proceeds via an observed dimerization and a transient trimeriza-
tion product, which subsequently cyclizes, thereby giving rise
to novel fulvenes. In terms of organic synthesis a variety of
fulvenes suited to stabilize constrained geometry type olefin
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Financial support by NanoCat, an International Graduate
Program within the Elitenetzwerk Bayern, is gratefully acknow-
ledged. We thank Dr G. Glatz for his support in the X-ray labs.
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Chem. Commun., 2011, 47, 4183–4185 4185