Synthesis and Study of Oxazolium and Thiazolium Salts as Organocatalysts

Abstract

This thesis mainly focused on two projects. In the first project, electronically and sterically different N-alkyl and N-aryl oxazolium salts were prepared in three steps and explored as organocatalysts. These precatalysts were explored in the homo-benzoin reaction of benzaldehyde in basic reaction conditions under inert atmosphere. Among them, N-mesityl oxazolium salt was found to be a superior precatalyst for the homo-benzoin reaction. Aromatic, heteroaromatic, and aliphatic aldehydes formed the homo-benzoin products in excellent yields in a short reaction time, while electron-rich aldehyde (4-methoxybenzaldehyde) required double the amount of the precatalyst. Again, N-mesityl oxazolium salt was found to be the most effective precatalyst for the aza-benzoin reaction of aldehydes and various imine derived from α-amidoaryl sulfones and phenyl azirine. Aromatic and heteroaromatic aldehydes produced the amino ketones with imines in good to excellent yields whereas aliphatic aldehydes provided moderate yields. Additionally, the aza-benzoin reaction between azirine and aromatic aldehydes gave the coupled products in good yields. N-mesityl oxazolium salt was found to be an excellent precatalyst in Stetter reactions between various aldehydes and acceptors, such as chalcone and unreactive β-alkyl-α,β-unsaturated ketones. The comparison study of the N-mesityl oxazolium salt with thiazolium and triazolium salts as precatalysts in the Stetter reactions between electron-rich aromatic aldehyde and chalcone as well as β-alkyl-α,β-unsaturated ketone and methyl 4-formylbenzoate revealed that N-mesityl oxazolium salt was found to be the best precatalyst and gave the highest yield of 1,4-diketones. In the second project, chiral thiazolium and chiral oxazolium salts were synthesized from the L-valine with great effort. The amino group was blocked with various protecting groups to facilitate alkylation on the tertiary alcohol for generating an ether. All protected amines failed to form an ether. The amine was converted to an azide using imidazole sulfuryl azide and used as a protecting group to synthesize chiral precatalysts. These chiral azolium salts were tested in an enantioselective Stetter reaction obtaining the products in moderate to good yields with good ee’s.