Dottorato in SCIENZE CHIMICHE

Titolo della tesi: Photoexcitation of Aldehyde Derivatives to Access Chiral Added Value Products

A renewed interest is recently experienced in synthetic photochemical processes thanks to the availability of different light sources and, above all, for they extraordinary potential for the sustainable preparation of chiral molecules. The short-lived electronically excited states generated by photostimulation of organic compounds, indeed, can resort to a manifold variety of mechanism reactions that are not available to conventional thermal pathways. In this thesis work, photostimulation of aldehyde derivatives are used for diastereoselective synthesis of strain cyclobutane rings and for enantioselective remote perfluoroalkylation. The cyclobutanol motif is of interest in organic synthesis as a prochiral unit, it is present in natural products and has been used as an intermediate in the total synthesis of some sesterpenoids. In medicinal chemistry, the established benefit of fluorine-containing fragments relies on their ability to alter the physicochemical and pharmacokinetic properties of organic compounds. These aspects explain the importance of developing novel methods for the stereoselective incorporation of trifluoromethyl (CF3) and perfluoroalkyl (RF) units within organic molecules. However, only a few strategies are available for the installation of perfluoroalkyl-containing stereogenic centres. In the main part of the work, an efficient diastereoselective synthesis of cyclobutanol oximes (CBOs) has been achieved via Norrish-Yang photoisomerization of several 2-(hydroxyimino)-aldehydes (R1R2CHC(=NOH)CHO, HIAs) under LED UV light irradiation: The Norrish-Yang reaction is a photo-activated transformation of carbonyl compounds that leads to the formation of cyclobutanols, the only requirement being the presence of a -H in the substrate. The n-π* transition of a carbonyl group can lead, via intramolecular hydrogen abstraction, to the generation of an excited 1,4-biradical triplet state which then undergoes either a Norrish-type II fragmentation reaction or a Norrish-Yang cyclization with the formation of cyclobutanols. The synthetic methodology is generally applicable to a series of HIAs, affording the corresponding cyclobutanol oximes (CBOs) chemoselectively (i.e. without sizable fragmentation side-reactions), diastereoselectively (up to >99:1) and in good to excellent yields (up to 95%). CBO oxime ether derivatives can be purified and diastereomers isolated by standard column chromatography. Being the CBO moiety a confined small ring with many functional groups in proximal vicinity, the spectroscopic determination of the relative configuration of the obtained compounds was a key challenge. The assignment of cis/trans ring signals was determined by combination of spin-decoupled 1H, 13C, HSQC, and NOESY NMR spectroscopy. A detailed investigation combining experimental results, molecular dynamics calculations, spectroscopic data and DFT calculations was undertaken to complete the mechanistic and stereochemical picture of this photocyclization reaction. Moreover, molecular dynamics and NMR analyses were combined to study the observed post-cyclization E/Z isomerization of the CBO oxime double bond. In the second part of the work, carried out at Institut Català d'Investigació Química -ICIQ (Tarragona, Spain), a photochemical procedure for the asymmetric introduction of CF3 and perfluoroalkyl groups at the remote γ position of α-branched enals has been developed: Mechanistic investigations suggest that the stereocontrolled radical perfluoroalkylation of enals at the γ-carbon relies on the excitation of an electron donor acceptor EDA complex between the catalytic dienamine and the perfluoroalkyl iodide. Irradiation of the EDA complex by a blue light LED generates via a single electron transfer, the perfluoroalkyl radical. This radical is then intercepted by the chiral dienamine, in a regio- and stereoselective fashion affording the desired product. All perfluoroalkylated enals are obtained with complete γ-regioselectivity, good enantiomeric ratio (up to 93:7), and in good yield (up to 70%), depending on the structure of catalyst, enals and perfluoroalkyl iodides.