Cationic adducts carbonyl bases

These compounds form an interesting contrast to the Lewis base adducts of silyl-metal carbonyls (Section II,C,4), where silicon is thought to be present in a cation. 

A Piperidine acts both as a nucleophile and as a base. First it combines with the pyrylium cation at C-2, forming an adduct which then ring opens, deprolonates and tautomerizes to an enamino ketone (Schema 4.3), This product cyclizes through an intramolecular reaction between the enamine unit and the carbonyl group, followed by dehydration to form the phenylpiperidine. 

In a similar context Amdtsen developed a new pyrrole synthesis from alkynes, acid chlorides either imines or isoquinolines, based on the reactivity of isocyanides (Scheme 35a) [197]. Although all atoms from the isocyanide are excluded from the final structure, its role in the reaction mechanism is crucial. The process takes place through the activation of the imine (isoquinoline) by the acid chloride to generate the reactive M-acyliminium salt, which is then attacked by the isocyanide to furnish a nitrilium ion. This cationic intermediate coordinates with the neighboring carbonyl group to form a miinchnone derivative, which undergoes a [3+2] cycloaddition followed by subsequent cycloelimination of the isocyanate unit, to afford the pentasubstituted pyrrole adducts 243 and 244 (Scheme 35a, b). 

Several mechanisms were offered to explain steric control in polymerizations of polar monomers. Furukawa and co-workers based their mechanism on infrared spectroscopy data of interactions between the cations and the growing polymeric chains in polymerizations of methyl methacrylate and methacrylonitrile. They observed a correlation between the tacticities of the growing molecules and the carbonyl stretching frequencies. The higher the frequency, the higher is the amount of isotactic placement in the resultant chains. The adducts, as in the initiation reactions, are resonance hybrids of two structures, A and B ... 

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