NEW CHIRAL AMINOPHOSPHINE AND DIAMINES LIGANDS, CORRESPONDING TRANSITION METAL COMPLEXES, THEIR APPLICATIONS IN ASYMMETRIC SYNTHESIS AND DEVELOPMENT OF HYBRID SYSTEM (ARTIFICIAL METALLOENZYMES)
Creators: name: PELLIZZONI, MICHELA
Publisher: name: Universit� degli Studi di Milano
Year: 2015

description: Transition metal complexes containing chiral bidentate ligands are robust catalytic system for the preparation of chiral compounds as fine chemicals, fragrances and insecticides. Bifunctional P-ligands, N-ligands or P-N ligands are preferentially used for the preparation of late transition metals complexes (Rh, Ru, Pd, Ir, Pt) to modify their catalytic performance. While the reduction of olefin and carbonyl groups has been widely investigated, the asymmetric reduction of imines is relatively underdeveloped, although enantiopure amines play an important role in the preparation of many products. The asymmetric hydrogenation of cyclic imines is more difficult in comparison to the acyclic analogue because aromatic compounds are more stable, the reaction requires harsher conditions and it suffers from easy deactivation of the catalysts due to the poisoning.
The interest of both academic and industrial research groups has increased in recent years in asymmetric hydrogenation of cyclic and acyclic imines, with a focus on the discovery of catalytic system with excellent enantioselectivity and activity under low hydrogen pressure or alternative source of hydrogen. Moreover, the development of ATH in aqueous media is emerged as a valid alternative to the use of organic solvents for its no toxic, economic and environmental compatible profile.
Since the pioneering work of Noyori and Ikariya groups in 1995, the catalysts of choice in ATH reductions of ketones have been established to be the ruthenium(II) complexes chelating different substituted 1,2-diamines such as DPEN and its derivatives. In particular the monotosylated compounds were revealed as the most efficient ones. All these types of catalysts were based on the presence of ligands forming a five membered ring in coordination to the metal centre. Some examples of symmetric 1,4 diamines and few examples of 1,3-diamines were reported in literature, mainly used as a typical ruthenium complex [(diphosphine)-RuCl2-(diamine)] for hydrogenation of simple aromatic and aliphatic ketones.
In this PhD thesis are reported the synthesis of simple asymmetric monotosylated 1,3-aminophosphine and monotosylated 1,3-diamines, up to now poorly investigated. The evaluation of their catalytic performances and their application for the preparation of artificial hydrogen transferases are also investigated.
Chiral benzyl alcohols, used as synthons for the preparation of bidentate ligands, were obtained by biotransformation. Screening results revealed Rhodotorula rubra MIM 147 as an efficient catalytic system for this purpose.
The first generation of this type of ligand was based on chiral 1,3 tosyl aminophosphine compounds able to use for the preparation of chelating six member ring complexes containing Ir(I) and Ru(II). Chelating aminophosphines have a combination of hard (N) and soft (P) Lewis base centers which make these ligands particularly useful in a variety of catalytic reactions. In fact these precatalysts, in hydrogenation reaction of several prochiral substrates such as ketones, imines and inactivated double bonds, showed a wide activity without stereoselectivity.
Second generation of ligands containing linear and branched 1,3 tosyl diamines in which the tosyl moiety was present in different position, were synthesised to improve stereoselectivity.
In respect to the tosyl aminophosphines: they are more easy to synthesise, to functionalize and are more easy to handle because are not air sensitive. Moreover these compounds are water soluble and act as suitable ligands for hydrogen transfer reaction conditions.
[Ru (p-Cymene) (Tsdiamine)] complex (S-9a) results the best catalyst for the reduction of acetophenone in water used under ATH conditions (e.e.=56%, Yield=97%), revealing the importance of stereogenic centre to be in proximity of the amine involved in the catalytic cycle. The Ts moiety contribute to increase both the reaction conversion and enantioselectivity through a steric and/or an electronic effects. Considering different hydrogen donors, the used of HCOONa was revealed the best choice.
Ligand (S-9) was used for the preparation of [IrCp*Tsdiamine] complex (S-9b) and applied in the reduction of cyclic imines in ATH reaction conditions.
Substrates were reduced in excellent yield, in aqueous medium even if the enantioselectivity was low. The conformation of chelating six member ring seems to play a primary role for the enantiodiscrimination of the substrate adopting a chair conformation in this condition. Starting from the assumption that the reduction of these substrates is not easy to obtain, the results using this catalyst, encouraged our research to improve the stereoselectivity of the system, and for this reason, transition metal catalysts was applied in the development of artificial metallo enzymes. This hybrid system results from the incorporation of a catalytically competent organometallic moiety within a macromolecule. For this work was exploited the biotin-streptavidine technology. In fact tethering a biotin anchor to a catalyst precursor ensures that, in presence of streptavidine (Sav), the metal moiety is quantitatively incorporated within the host protein.
Meta and para biotinylated aminosulfonamide iridium d5-pianostool complexes were prepared and their performances were evaluated in combination with Sav wild type and 10 different mutants in position 112. After this chemo-genetic optimization, the para-biotinylated aminosulfonamide iridium d5-pianostool complex (S-33a) embedded in combination with Sav S112C allowed to obtain an imine reductase able to reduce dihydroisoquinolines with good activity and enantioselectivity (e.e.=66%,Yield=90%), in aqueous solution using HCOONa as hydrogen donor.