Catalytic synthesis of dihydropyridines by domino reaction of terminal alkynes, carbodiimides, and acetylenic esters

Zinc amidinate-catalysed cyclization reaction of carbodiimides and alkynes. An insight into the mechanism

“…The structure of compound 6 contrasts with that found for the dinuclear complex [Zn{κ 2 -C(CCPh)(N i Pr) 2 }{μ-κ 1 -κ 1 -C(CCPh)(N i Pr) 2 }] 2 , 16 where the amidinato ligands are in both chelated and bridging form, which was in equilibrium with a mononuclear form similar to that found in compound 6 . This difference is probably explained by the larger steric bulk of the nitrogen atom substituents in compound 6 , which inhibits the formation of bridging ligands.…”

“…15 Recently, another route was proposed to obtain these compounds, which goes via a carbodiimide activated by tetrabutylphosphonium acetate employing CuI as catalyst. 16 This procedure goes through an amidinato intermediate and allows to combine terminal and internal alkynes as shown in Scheme 1b.…”