SYNTHESIS, CHARACTERIZATION AND CATALYTIC ACTIVITY OF CHIRAL TETRAAZAMACROCYCLE - PC-L* -CU(I) AND AG (I) COMPLEXES
Tesi di Dottorato
Data di Pubblicazione:
2014
Citazione:
SYNTHESIS, CHARACTERIZATION AND CATALYTIC ACTIVITY OF CHIRAL TETRAAZAMACROCYCLE - PC-L* -CU(I) AND AG (I) COMPLEXES / B. Castano ; tutor: A. Caselli ; co-tutor: E. Gallo ; co-ordinator: E. Licandro. Università degli Studi di Milano, 2014 Jan 16. 26. ciclo, Anno Accademico 2013. [10.13130/castano-brunilde_phd2014-01-16].
Abstract:
Homogeneous catalysis is a constantly growing and evolving area of study: in the last forty years many discoveries have been reported in both industrial and academic fields. Thanks to the use of organometallic catalysts, many inorganic and organic reactions of great importance have been revolutionized, so that homogeneous catalysis is now essential in many aspects of organic chemistry and for several industrial processes. On the other hand, heterogeneous catalysis is important because of its easier application in industry. Heterogeneous single site catalysts in many cases show superior performances in terms of activity, selectivity and recyclability coupling together the advantages of heterogeneous and homogeneous systems. It is important to note that the most commonly used ligands to form catalytic complexes often need really complicated synthesis so that the cost of the catalyst becomes excessive. In our research group we are focusing on the synthesis of molecules obtainable in few synthetic steps and easy to modify in order to be used as asymmetric ligands for transition metals.
In particular we have prepared and fully characterised a small library of non-chiral and chiral pyridine containing macrocyclic ligands (Pc-L) possessing the same donor properties but with either C1, C2 or C2v symmetry. The strategy adopted for the synthesis of these macrocyclic ligands is very flexible and allows for structural modifications. Relative basicity of the four nitrogens atoms and metal complexation of Pc-L ligands with different copper(I) salts were studied.
The use of [Cu(I)(Pc-L)] complexes was investigated at first in Henry catalytic reaction under mild conditions. The remarkable diastereoselectivity observed when isatine was reacted with nitroethane under the optimised catalytic conditions is worth to note, considering the easy access to a highly functionalized isatine skeleton. However, further studies on the ligand steric requirements in order to achieve significant enantioselectivities are required.
Then [Cu(I)(Pc-L)] complexes were tested as competent catalysts in asymmetric cyclopropanation reactions, at first under homogenous conditions in 1,2-dichloroethane. Cyclopropanes were obtained in good to excellent yields and enantiomeric excesses up to 99%. Thanks to the collaboration with Dr. Dal Santo, we have developed supported hydrogen-bonded (SHB) chiral copper(I) complexes. These supported catalysts showed good performances in cyclopropanation reactions under heterogeneous conditions in batch. The heterogeneised systems showed higher or comparable activities than the homogeneous counterpart and a good recyclability, allowing the use of more environmentally friendly n-hexane as solvent in place of 1,2-dichloroethane. Cyclopropanes were obtained in good to excellent yields and enantiomeric excesses up to 67%. We did not observe any significant Cu leaching when employing n-hexane as a reaction medium and the catalytic system is of truly heterogeneous nature, since the filtered solution is not catalytically active. The observed confinement effects are more dependent on the employed solvent (non-polar vs halogenated) than to the kind of support (ordered or non-ordered). Worth to note is the fact that even commercial silica can be used as a support, without any need of structural modification of the ligand in order to strongly graft the complex.
Thanks to the three month collaboration with Prof. David J. Cole-Hamilton from the University of St. Andrews (UK), these catalytic system based on SHB chiral copper(I) complexes were also tested as competent catalysts for asymmetric cyclopropanation reactions under flow conditions allowing the use of more eco-sustainable CO2 as a vector instead of solvents normally used for these reactio
Tipologia IRIS:
Tesi di dottorato
Keywords:
tetraazamacrocyclic ligands ; homogeneous catalysis ; heterogeneous catalysis ; cyclopropanation reactions ; Henry reaction ; copper(I) complexes ; assymetric catalysis ; silver(I) complexes ; supported catalysts
Elenco autori:
B. Castano
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