Racemo-Selective Synthesis of <i>a</i><i>nsa</i>-Zirconocene Derivates from Zirconium Biphenolate Complexes

Abstract

Stereoselective reactions of zirconium biphenolate complexes of the general formula (BIP)ZrCl 2 (THF) 2 ( 1a − c ), where BIP = 3,3‘- i Pr 2 -6,6‘-Me 2 -1,1‘-bi-2-phenolate ( a ), 3,3‘- t Bu 2 -5,5‘-Me 2 -1,1‘-bi-2-phenolate ( b ), or 3,3‘,5,5‘- t Bu 4 -1,1‘-bi-2-phenolate ( c ), with 1 equiv of the lithium or magnesium salts of various ansa -bis(indenyl) ligands give the ansa -zirconocenes rac -C 2 H 4 (Ind) 2 Zr(BIP) ( 2b ), rac -Me 2 Si(Ind) 2 Zr(BIP) ( 3c ), rac -Me 2 Si(2-Me-Ind) 2 Zr(BIP) ( 4c ), rac- Me 2 Si(2-Me-4-Ph-Ind) 2 Zr(BIP) ( 5c ), and rac -Me 2 Si(2-Me-Benz[ e ]Ind) 2 Zr(BIP) ( 6c ) in isolated yields of 60−75%. Reactions of C 2 H 4 (Ind) 2 Mg(THF) 2 with 1a and 1b and of Me 2 Si(Ind) 2 Li 2 with 1c give directly the pure diastereomers rac - 2a, rac - 2b, and rac - 3c, respectively. More highly substituted indenyl ligands, on the other hand, give initially kinetically controlled mixtures of the rac- and meso- zirconocene BIP derivatives 4c, 5c, and 6c, from which the pure rac -derivatives are obtained as thermodynamically favored products by heating the reaction mixtures to 75 °C for 5−7 h. The meso -to- rac isomerization of 6c is catalyzed by addition of TEMPO and is thus likely to occur by way of radical intermediates. Crystal and molecular structures of 1b and 2b and of 1c and 6c indicate that formation of the racemo diastereomers is favored by repulsive interactions between biphenolate and ansa -bis(indenyl) ligands.