Alkynyl-aryl bridging reactions

The above intramolecular reactions involve dinuclear intermediate with a symmetrically bridging alkyl, aryl, and alkynyl ligands. 

The aryl- and alkyl-transition metal bonds are thermodynamically less stable than the alkynyl-metal bond [223]. The number of intermolecular transmetala-tion reactions of alkynyl transition metal complexes, however, is much larger than those of alkyl and aryl complexes as shown above. The stable bridging coordination of alkynyl ligands of the intermediate dinuclear complexes with a 0,7t-coordination probably makes the transmetalation, which involves activation of the thermodynamically stable metal-alkynyl bond, kinetically favorable. 

Aryl-substituted pyridines and pyrimidines have been prepared by 2 - - 2-cycloadditions of alkynyl-substituted pyridines and pyrimidines with electron-rich dienes." The reactions proceed by formation of a bridged cycloadduct and subsequent thermal extrusion of ethylene. The pyridine moiety plays a crucial role for the success of the reaction. 2-Benzoylbenzoyl azides undergo facile cycUzation under acidic conditions to give substituted dibenzo[ / [l,5]-diazocines in good yields. The mechanism of the diazocine synthesis was assumed to proceed by an unprecedented intermolecular 2 - - 2-cyclization. 

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