ALKENE AND ALKYNE INSERTION REACTIONS OF CATIONIC CP2ZR(ETA-2-PYRIDYL)(L)+ COMPLEXES - ZIRCONIUM-MEDIATED FUNCTIONALIZATION OF PYRIDINES

Cationic Cp2Zr(eta-2-pyridyl)(L)+ complexes (L = THF, py), which are available via ortho C-H activation reactions of pyridines with Cp2Zr(Me)(THF)+, undergo insertion of alkenes and alkynes to yield new five-membered azazirconacycles. Normal 1,2-insertion leading to beta-substituted azametallacycles is observed for alpha-olefins H2C = CHR containing electron-donating substituents (R = alkyl, CH2SiMe3, CH2OR), while 2,1-insertion leading to alpha-substituted products is observed for The regioselectivity of these reactions is rationalized on the basis of electronic effects in the insertion transition states and products. Terminal alkynes insert regioselectivity to yield alpha-substituted unsaturated five-membered metallacycles, but the unsymmetrical internal alkyne 2-hexyne affords a mixture of regioisomers. Steric effects appear to determine the regioselectivity in these cases. The silylacetylene MeC=CSiMe3 inserts selectivity, yielding an alpha-SiMe3 substituted metallacycle as a result of steric and Si electronic effects. The overall sequence of pyridine ortho C-H activation and alkene/alkyne insertion provides a powerful zirconium-mediated approach to substituted/functionalized pyridines.