Ashis Kumar Das
Transition metal-catalyzed C-H activation is a powerful tool for the synthesis of organic building blocks, materials, and natural products. Over the last decade, directing-group-assisted metal-catalyzed C-H functionalization has attracted increasing interest as a means to target specific C-H bonds. Moreover, a large variety of electrophiles has been employed in this kind of processes. In this thesis, we describe a novel C-6 selective C-H heteroarylation of pyridine-2-ones using indoleBX reagents as coupling partners under Rh(I)-catalysis. The reaction worked efficiently also with other analogous heterocyclic substrates and with a quinoline-N-oxide. We were able to obtain 6-(indol-3-yl)pyridinone and 8-(indol-3-yl)quinolone after cleavage of the directing group or rearrangement of the N-oxide moiety. 1,2-Amino alcohols are important structural motifs in organic chemistry that can be observed in natural products, pharmaceutically and biologically active molecules. The palladium-catalyzed difunctionalization of C-C multiple bonds has been broadly investigated as an efficient strategy to access such substructures. This transformation has been reported using either Pd(0)/Pd(II) or Pd(II)/Pd(IV) catalytic cycles, in which the nucleophilic functionalization of a putative Pd(II)-alkyl intermediate is achieved prior to beta-hydride elimination. As a further topic of this thesis, we developed new methods for the synthesis of amino alcohols whereby the Pd-catalyzed difunctionalization of alkynes and alkenes was combined with the tethering approach previously established in our laboratories. We first investigated the Pd-catalyzed enantioselective hydroalkoxylation of propargylic amines. The latter were tethered in situ with a trifluoroacetaldehyde hemiacetal. Best results were obtained with commercially available DACH-Phenyl Trost ligand. Aryl iodides were required as an additive in catalytic amount for successful outcome. A large variety of chiral oxazolidines were obtained in good to excellent yields and ee, followed by a diastereoselective reduction to give protected enantioenriched amino alcohols. A possible mechanism for this transformation relies on a Pd(0)/Pd(II) catalytic cycle involving oxidative addition of aryl iodide with a subsequent oxypalladation/protodemetallation (or reductive elimination) reaction. This is necessary to generate the active complex, not as the start of a cross-coupling event. The use of a commercially available DACH-Ph Trost ligand to produce high enantioselectivity in an unprecedented DYKAT process was critical to success.The Pd-catalyzed amino acetoxylation of cinnamyl alcohols was next taken into consideration towards the synthesis of valuable amino diol derivatives. In this case, preformed substrates were used, where the alcohol moiety was tethered to N-nucleophilic groups. Conditions involving Pd(II)/Pd(IV) catalysis and employing PIDA as a strong oxidant were employed. The use of this hypervalent iodine reagent was key to the success of the transformation. Amino acetoxylated products were obtained in moderate to good yields and dr. The scope of this reaction remains mostly limited to cinnamyl derived substrates. The reaction conditions did not tolerate substituents on the alpha, beta, and gamma positions of the allylic chains. Alkyl chain containing unsaturated alcohols delivered Aza-Wacker-type products.
EPFL2022
