Nucleophilic β-Oniovinylation:  Concept, Mechanism, Scope, and Applications

Insertion of an electron-deficient alkyne A−C⋮C−A (A = CO₂Me) into the C−L⁺ bond of an acyl-onio salt R−C(O)−L⁺ (R = Ar, OAlk; L = 4-dimethylaminopyridine, PPh₃) has for the first time been achieved in the presence of catalytic amounts of the nucleophile L. For R = OMe, a second insertion of the alkyne was observed. X-ray structures were obtained for a number of such β-oniovinylation products. Depending on reaction conditions, preferentially <i>E</i>- or <i>Z</i>-stereochemistry was observed, the <i>Z</i>-isomer being the thermodynamically more stable. A mechanism for this novel insertion reaction is presented which accounts for the topology of the products and rationalizes the observed stereochemistry. The β-onio-activated Michael systems thus generated represent a virtually unexplored class of compounds. The onio substituent in such compounds can be selectively replaced by a number of nucleophiles. Thus a series of Michael systems with donor functions in the β-position is easily synthesized. These compounds represent a source for useful further transformations, for example, cyclizations to quinolones, thiochromones, and pyrazoles.