Planar Chiral Flavinium Salts: Synthesis and Evaluation of the Effect of Substituents on the Catalytic Efficiency in Enantioselective Sulfoxidation Reactions

A series of substituted planar chiral flavinium salts with a phenyl “cap” have been prepared as potential catalysts for enantioselective sulfoxidation reactions with hydrogen peroxide by using an approach based on the synthesis of (arylamino)uracils and their condensation with substituted nitrosobenzenes. The effect of substituents at various positions on the ability of the catalyst to promote enantioselective sulfoxidation recations was investigated. Introduction of the tyrosine group into the side-chain of the flavinium species or substitution of the nitrogen N-3 of the flavin unit by o-isopropylphenyl has a remarkably positive effect on the enantioselectivity of the sulfoxidation reactions of aromatic and aliphatic substrates, respectively. On the other hand, substitution of the phenyl “cap” led to a substantial decrease in the efficiency of the catalyst. In summary, optimisation of the structures of the planar chiral flavinium catalysts led to enantioselectivities of up to 61 % ee for aromatic sulfides and of up to 65 % ee for tert-butyl methyl sulfide.