Pyridines ring-closing metathesis reaction

Olefin-containing esters of pyridine-3,5-dicarboxylic acid are able to form macrocycles using a ring-closing metathesis reaction if the pyridine was protected with complexing Pt. The method was applied to 69- and 75-membered macrocycles <03ZOR449>. An example of a 17-membered ring formation is shown below. 

Recent advances on the synthesis of piperidines through ruthenium-catalyzed ring-closing metathesis reactions 12H(84)75. Rhodium-catalyzed [2+2+2] cycloaddition for the synthesis of substituted pyridines, pyridones, and thiopyranimines 12H(85)1017. Synthesis of 2,2 -bipyridines by transition metal-catalyzed alkyne/nitrile [2+2+2] cycloaddition reactions 12H(85)1579. 

Abstract The reactions of Cl2-Ru-(3-phenylindenylidene) eomplexes with excess pyridine lead to new ruthenium (Ru)-based bis(pyridine) adducts in good yield. These thermally robust catalysts have been tested in ring-closing metathesis (RCM). In spite of promising initiation rates, only moderate to good activities have been observed in kinetic studies on several substrates. 

A recent noteworthy example is the 14-electron metathesis catalyst 72 reported by Piers, which have no ancillary ligand to dissociate. " As expected, this catalyst initiates extremely rapidly, displaying initial rates of ring-closing metathesis (RCM) better than the bis-pyridine substituted Grubbs catalysts. Furthermore, reaction of the catalyst with 2.2 equivalents of ethylene at — 50°C in CD2CI2 allowed the observation and characterization of the metallocyclobutane 73 formed on expulsion of the vinyl phosphonimn salt (Equation (3.13)). " This represents the first observation of the generally accepted intermediate in the catalytic cycle. 

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