Racemo-selective synthesis of ansa-zirconocene derivates from zirconium biphenolate complexes

Stereoselective reactions of zirconium biphenolate complexes of the general formula (BIP)ZrCl2(THF)(2) (la-c), where BIP = 3,3'-(i)Pr2-6,6'-Me-2-1,1'-bi-2-phenolate (a), 3,3-Bu-t(2)-5,5'-Me-2-1, 1'-bi-2-phenolate (b), or 3,3',5,5'-Bu-t(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-C2H4(Ind)(2)Zr(BIP) (2b), rac-Me2Si(Ind)(2)Zr(BIP) (3c), rac-Me2Si(2-Me-Ind)(2)Zr(BIP) (4c), rac-Me2Si(2-Me-4-Ph-Ind)(2)Zr(BIP) (5c), and rac-Me2Si(2-Me-Benz[e]Ind)(2)Zr(BIP) (6c) in isolated yields of 60-75%. Reactions Of C2H4(Ind)(2)Mg(THF)(2) with 1a and 1b and of Me2Si(Ind)(2)Li-2 with le 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 degreesC 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 lb and 2b and of le and 6c indicate that formation of the racemo diastereomers is favored by repulsive interactions between biphenolate and ansa-bis(indenyl) ligands.