Zirconocene alkyne complexes

Homoallylic ethers appeared much less reactive towards the f-elimination reaction with the zirconocene-alkyne complexes or zirconacyclopentenes [21]. No -elimination products were observed in these cases. In contrast, Szymo-... 

Typically, zirconocene alkyne complexes are isolated as ligand adducts, (GpR )2Zr(772-alkyne)(L), prepared from reduction of the corresponding dichloride complex with magnesium in the presence of a donor solvent such as THF (Scheme 28).89,90 This synthetic procedure has been extended to metallocene derivatives such as (OEP)Zr( 72-PhC=CPh) (OEP = octaethylpor-phyrinato) 170.91-93 In some instances, exchange of the THF ligand is readily achieved by addition of pyridine.94... 

Electrophilic attack on olefin ligands coordinated to electron-rich, strongly backbonding metals is illustrated by the reactions of (P group 4 olefin and alkyne complexes, as well as some electron-rich olefin complexes. Zirconocene- and and hafnocene-olefin complexes generated by reaction of zirconocene dichloride with two equivalents of alkyl lithium and isolated upon addition of a phosphine ligand react with carbonyl compounds and weak protic acids to form insertion products and alkyl complexes. Several examples of the reactions of these complexes with electrophiles are shown in Equations 12.65-12.66. Zirconocene-alkyne complexes prepared by thermolysis of vinyl alkyl complexes and titanium-alkyne complexes generated by the reduction of Ti(OPr ) also react with electrophiles, such as aldehydes and acid, to form products from insertion into the M-C bond and protonation of the M-C bond respectively. 

Rosenthal U, Ohff A, Michalik M et al (1993) Transformation of the first zirconocene alkyne complex without an additional phosphane ligand into a dinuclear o-alkenyl complex by hydrogen transfer from ti -C5H5 to the alkyne ligand. Angew Chem Int Ed Engl 32 1193-1195... 

Takahashi T, Suzuki N, Kageyama M et al (1993) Allylzirconation of alkynes by the reactions of zirconocene-alkyne complexes with allylic ethers. Tetrahedron Lett 34 4811-4814... 

Compared with symmetrical metallacyclopentadienes, unsymmetrical met-allacyclopentadienes from two different alkynes are not always easily prepared. Several methods have been reported for preparation of unsymmetrical zir-conacyclopentadienes in which the key step is the addition of a second alkyne to zirconocene-alkyne complexes generated in situ [20]. Formation of homocoupling products from the same alkyne is often the biggest problem (Eq. 21). 

The coupling reaction of zirconocene alkyne complexes with a second alkyne provides a general method for the preparation of asymmetrically substituted zircona-cyclopentadienes. The overall transformation is the chemoselective and regioselec-tive intermolecular cross-coupling reaction of two alkynes (Scheme 12.33) [44]. 

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