Reactions of 1,3-diethyl-2-thiobarbituric acid with aldehydes: formation of arylbis(1,3-diethyl-2-thiobarbitur-5-yl)methanes† and crystallographic evidence for ground state polarisation in 1,3-diethyl-5-[4-(dimethylamino)benzylidene]-2-thiobarbituric acid

Abstract

The reactions of 1,3-diethyl-2-thiobarbituric acid (detba) in ethanol at room temperature with 4-(dimethylamino)benzaldehyde (dmabza) or cinnamaldehyde afford the Knoevenagel products 3 and 4, respectively. Under identical conditions, pyridine-4-carbaldehyde, 4-methoxybenzaldehyde, benzaldehyde, 4-cyanobenzaldehyde, 4-nitrobenzaldehyde and 4-ferrocenylbenzaldehyde yield exclusively the arylbis(1,3-diethyl-2-thiobarbitur-5-yl)methane Michael adducts 2, 5, 6, 7, 8 and 9, respectively. Although 3 can be forced to react further by treatment with excess detba in refluxing ethanol, the product 10 is unstable in solution and readily reverts to 3. The stability of 4 is attributed to extended conjugation, and it is likely that the pronounced difference in reactivity between dmabza and the other arylaldehydes arises primarily from electronic factors, i.e. the strongly electron donating effect of the -NMe₂ substituent. Single crystal X-ray structures have been determined for the products 3 and 6. The structure of 6 confirms the formation of the Michael adduct and shows that both of the detba rings are present in mixed keto–enol forms, although the oxygens differ slightly in their degree of enolic character. The bond distances in 3 provide clear evidence for extensive ground state polarisation, in accord with the marked molecular nonlinear optical properties of the analogous -N(ⁿBu)₂ compound.