Mechanism and kinetics of HI addition to unsaturated carbon-carbon bonds in organic solvents: Addition of HI to methyl propiolate in chlorobenzene solution

Abstract

New dienamino esters (3b–k) were obtained by addition of enamino esters (1b–g) to methyl and ethyl propiolate (2a–b). Z,E-Configuration and a transoid conformation were assigned on the basis of spectral data which indicate also noncoplanarity of phenyl groups whenever present. The corresponding adducts with acetylenedicarboxylic ester (18 and 19) have a cisoid conformation and it was possible to differentiate between thermodynamically and kinetically controlled products. Deuteration experiments showed the existence of a 1,5-proton transfer while comparative examination of a whole series of NMR spectra furnished evidence for a head to tail attachment. Attempts to trap the intermediate zwitterion 10 resulted in the formation of 15a–b corresponding to a cyclobutene intermediate. The reaction repiesents a new synthesis of the benzene nucleus and a practical method to obtain the methyltrimesic and 2,4,6-biphenyltricarboxylic acids. Additions of enamino esters to the triple bond are best interpreted as occurring through a common key intermediate, a zwitterion of type 5 or 10. The former collapses by proton transfer and the resulting imino-derivative 6 tautomerizes to 3. The latter cyclizes to a non-isolable cyclo-butene 12 which by opening of the ring produces the dipolar species 13 which further reacts with propiolic ester. By cyclization of the dienamino esters 3a–j but not of 18 and 19 in dipolar aprotic solvents at 160–190° the corresponding a-pyridones 4a–t were obtained in good yields.