A Definitive Study of Aliphatic and Aromatic ?-ketosulfoxides: Promising Aromatic Chelates

摘要

This paper begins with a detailed description of the conditions necessary for the formation of chelates in general, and ?-ketosulfoxides in particular. While symmetric chelates such as 2,4-pentanedione have been known and studied for a long time, asymmetric chelate structures, such as the aliphatic ω-(methylsulfinyl)-acetophenone, Ph-CO-CH_(2)-SO-CH_(3), for example, have not been investigated. Our detailed studies of solubility, acidity, IR, UV, and NMR spectroscopies all indicate very low enolization and hence low chelation, if any. Nevertheless, using rather strong conditions – such as molten reagents – we experienced some evidence for reaction. We extended our studies to aromatic ketosulfoxides by fusing these into benzene, assuming that its resonating ring will not only withdraw electrons from the S=O group, but might also encourage conjugation. We focused on o-(methylsulfoxo)-acetophenone with its S=O group in one position on the benzene ring and the –OH group in an adjacent position. Three separate facts confirmed our belief that such a fused structure should undergo enolization and thus chelation. First, salicylaldehyde and 2-hydroxyacetophenone, both structurally analogous to the compound of interest o-(methylsulfoxo)-acetophenone, do show strong chelation; second, approximate linear combination of atomic orbitals-molecular orbital (LCAO-MO) calculations point to positive results; and third, a comparison of the acidity constants of six acids ranging from ω-(methylsulfinyl)-acetophenone (3 x 10~(-11)) to salicylic acid (1.06 x 10~(-3)) place the compound in question, o-(methylsulfoxo)-acetophenone, at the upper end of acidity. Indeed, metal derivatives of o-(methylsulfoxo)-acetophenone form rather easily as evidenced by O-H and S=O shifts in IR, UV, and NMR spectroscopies. Calcium forms a chelate, whereas iron and niobium form adducts.