Download Synthesis and Application of Functionalized NHC

Synthesis and Application of Functionalized NHC-Gold Complexes
Viola Breker and Norbert Krause*
Dortmund University of Technology, Organic Chemistry, 44227 Dortmund, Germany
Introduction
First investigated by Wanzlick and Öfele in the early 1960s,[1] N-heterocyclic
carbenes (NHC’s) are a topic of high current interest, in particular in transition
metal catalysis. They are an attractive alternative to phosphine ligands, thanks
to their strong σ-donating capability and low level of π-acidity. Moreover,
extensive variation of the steric and electronic features of NHC’s is possible.[2]
Still, the number of NHC´s bearing reactive functionalities, which might allow
their linkage to solid supports or surfaces, is limited. Based on precedent by
Grubbs and Nolan,[3] we developed a new route to functionalized, unsymmetrical
N-heterocyclic carbenes which were converted into the corresponding gold
complexes. Both complexes catalyze the cycloisomerization of functionalized
allenes.
Results
As known in literature, NHC-metal complexes show a broad range of
catalytic activities.[4] Here, we report the application to the
cycloisomerization of functionalized allenes.[5]
Synthesis of the NHC Precursors
O
4-(Chloromethylbenzoyl)chloride
R
NH2
R
DMAP, pyr
CH2Cl2, RT, 24 h
N
H
Cl
3 (R = (CH3CH2)2NH)
4 (R = COOCH2CH3)
O
O
R
N
H
Cycloisomerization of Allenes
Mesitylimidazole, NaI
acetone, 45°C, 24 h
R
N
H
N
Cl
N
I
Entry
5 (R = (CH3CH2)2NH2)
6 (R = COOCH2CH3)
Starting from 4-(Chloromethylbenzoyl)chloride, two new imidazolium salts
were built up in a two step synthesis. By changing the amine in the first step,
it is possible to obtain new functionalized imidazolium salts with different
chemical properties.
1
2
3
4
Catalyst
1
1[a]
2
2[a]
Yield
Conditions
--93%
98%
42%
CH2Cl2
CH2Cl2, 3 d
CH2Cl2, 45 min
CH2Cl2, 75 min
[a] Addition of 5 mol% AgBF4
We observed that catalyst 2 shows the best results without any additives.
In contrast to this, the cycloisomerization with catalyst 1 takes place only
in the presence of AgBF4.
Synthesis of the NHC-Gold Complexes
The formation of the corresponding gold complexes was carried out under
conditions developed by Nolan. He described the synthesis of gold
complexes via transmetalation from silver to gold. The application of this
procedure was successful with moderate yields.
Entry
Catalyst
Yield
Conditions
1
2
3
4
1[a]
2[a]
2[a]
2
55%
42%
63%
50%
CH2Cl2, 1 h
CH2Cl2, 2 h
Toluene, 2 h
Toluene, 5 h
[a] Addition of 5 mol% AgBF4
Changing the allene to a more bulky substrate gave different results. The
yields are lower and the presence of AgBF4 as additive is required for
both catalysts in order to achieve a satisfactory reactivity.
Conclusion
In the recent years transition metal catalysis has become a topic of
high interest. In comparison to phosphines, NHC´s are an attractive
alternative as ligands for transition metal catalysts. Here, we report
the development of two new catalytically active NHC-gold
complexes. Due to their reactive functionalities, it might be
possible to link them to solid supports or surfaces.
References
(1)
(2)
(3)
(4)
H. W. Wanzlick, Angew. Chem. 1962, 74, 129−134; K. Öfele,
(5)
J. Organomet. Chem. 1968, 12, 42-43.
R. H. Grubbs, A. W. Waltman, Organometallics 2004, 23, 3105-3107.
R. H. Grubbs et al., Tetrahedron Lett. 2005, 46, 2577-2580; P. de Frémont,
N. M. Scott, E. D. Stevens, S. P. Nolan, Organometallics 2005, 24, 2411-2418.
S. Díez-Gonzàlez, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109, 3612-3676.
A. Hoffmann-Röder, N. Krause, Org. Lett. 2001, 3, 2537-2538; N. Krause,
Ö. Aksin-Artok, V. Breker, C. Deutsch, B. Gockel, M. Poonoth, Y. Sawama,
Y. Sawama, T. Sun, C. Winter, Pure Appl. Chem. 2010, 82, 1529-1536.