Der Mechanismus der Umlagerung substituierter Aminoacrylderivate

Thioacetic acid and dithioacetic acid react with alkynederivatives of the type (CH3)2NCCCOR (1) in the same way as other carboxylic acids: The addition to dimethylaminopropinal (1a) at low temperatures yields, after rearrangement of the very instable primary adducts,Z‐3‐acetoxy‐N,N‐dimethyl‐thioacrylamide (Z‐16) andZ‐3‐thioacetoxy‐N,N‐dimethylthioacrylamide (Z‐17) respectively. The structure of the two compounds can be proved by spectroscopic evidence of16and18, the latter being formed by elimination of thioketene from17. According to the distribution of S‐atoms in16and17, two reaction pathways including 4‐membered rings can be ruled out. Thus the rearrangement of 3‐acyloxy‐N,N‐dimethyl‐acrylamides most probably proceeds by a mechanism including a dipolar six‐membered intermediate. This mechanism cannot be valid for the rearrangement of the adducts2of hydrohalogen acids, alcohols and amines to the alkyne‐derivatives1. The acid‐catalysed reaction of 3‐chloro‐3‐dimethylamino‐propenal (2, XCl), labelled at position 1 with13C, yields 3‐chloro‐N,N‐dimethyl‐acrylamide (3, XCl), containing the label exclusively at position3. This result supports a mechanism including an immonium‐oxetene21(XCl) as intermediate. ‐ The experiments are in accord with kinetic investigations.