Acyl pyridinium complex

A related procedure involves the diacylation of water by the pyridinium salts of acyl halides to afford anhydrides (Eq. 4). The reaction was earlier described by Minunni to involve an acyl pyridinium complex. 

An extended application of the resin-capture-release technique is depicted in Scheme 13. With the help of reagent 31, a functionalized pyridine was captured as an acyl pyridinium cation 32 on a solid support which was followed by Grignard addition and hydrolysis under acidic conditions to afford polymer-supported N-acylated dehydropyridinones 33 [39]. Advantageously, any unreacted acylium complex collapses to the parent resin upon workup. These heterocycles, which ideally can serve as scaffolds, are then released under basic conditions. 

The presence of the Ru(III)-complex [Ru-H]+ that is formed from 12 through electron transfer to the acyl pyridinium has been demonstrated by EPR spectroscopy. The reaction can be rendered catalytic in Ru by the addition of a base (Scheme 12). 

Ketenes generated by dehydrobromination of acyl bromides undergo trans-selective [2-f2] cycloaddition with carbonyl groups, using the chiral catalyst bis-pyridinium aluminum-salen complex, to form p-lactones (Scheme 7.51). 

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