Zirconium-mediated carbon–carbon bond formation reactions of 1,3-enynes with esters and isocyanates

Take a "Snapshot" of New Syntheses, Reactions, and Characterizations from Unusual Unsaturated Ring Strained Group 4 Metallacycles

Recently published syntheses, reactions and characterizations of unusual unsaturated ring strained Group 4 metallocene metallacycles like metalla‐cyclocumulenes, ‐cycloallenes and ‐cycloalkynes with different ring size are updated for the last three years. There exist for some of these metallacycles, depending on the ring size, 7‐, 5‐ and 4‐membered compounds. The new results for these metallacycles are summarized here and considered in addition to the former published results. Additionally, several compounds of this type were now characterized by new reactions. For a better understanding of these compounds, some spectroscopical methods as well as theoretical calculations were published. Despite of these all‐C‐metallacycles, only in some cases the syntheses and reactions for the corresponding hetero‐metallacycles were published too. Examples for these metallaheterocyclic compounds will not be considered in this article. All these unusual ring strained compounds have a great potential for a lot of interesting synthetic applications in the future. Additionally, they are very interesting from the theoretical point of view.

Reactions of five-membered zirconacycloalkynes and zirconacycloallenes with Cp2Zr(H)Cl; formal hydrogenation by metal hydrides

Reactions of five-membered zirconacycloalkynes and zirconacyloallenes (1-zirconacyclopent-3-ynes and 1-zirconacyclopenta-2,3-dienes) with an excess of Cp₂Zr(H)Cl, known as the Schwartz reagent, were studied. Both reactions gave five-membered zirconacycloalkenes, 1-zirconacyclopent-3-enes without subsequent work-up such as protonolysis and hydrogenolysis. The product was identical to the zirconocene-diene complex that was prepared from Cp₂Zr(n-Bu)₂ (Negishi reagent) and the corresponding 1,4-disubstituted 1,3-dienes. These results indicate that formal hydrogenation by metal hydride took place. The use of diisobutylaluminum hydride or 9-borabicyclo[3.3.1]nonane also gave the same product, albeit in lower yields. The reactions starting from deuterated compounds suggested that double hydrozirconation followed by elimination of a dinuclear zirconium complex resulted in the hydrogenated products.