Enantiomerically Pure Titanium Complexes Containing an [OSSO]‐Type Bis(phenolate) Ligand: Synthesis, Structure, and Formation of Optically Active Oligostyrenes

Abstract

Chiral 1,2-trans-dithiocyclohexanediyl-bridged bis(phenols) of the type [2,2'-{HOC(6)H(2)-6-R(1)-4-R(2)}(2)S(2)C(6)H(10)] ([OSSO]H(2), R(1)=tBu, iPr, H; R(2)=tBu, iPr, Me) could be conveniently and selectively synthesized in three steps, starting from cyclohexene oxide and arene thiolate. The racemic bis(phenols) could be resolved using an enantiopure (S)-camphorsulfonic ester auxiliary or by (chiral) HPLC. Complexation of the racemic bis(phenols) to TiX(4) (X=Cl, OiPr) proceeds in a diastereoselective fashion to give only the Lambda,R,R and Delta,S,S enantiomers. Racemic [Ti{(OC(6)H(2)-6-tBu-4-Me)(2)S(2)C(6)H(10)}Cl(2)] reacts with benzyl magnesium bromide to afford the crystallographically characterized dibenzyl complex. The benzyl cation formed using B(C(6)F(5))(3) in C(6)D(5)Br slowly decomposes at temperatures above +10 degrees C. When treated with methylaluminoxane, the dichloro complexes [Ti{OSSO}Cl(2)] polymerize styrene with activities up to 146 kg (mol catalyst)(-1) [styrene (mol L(-1))](-1) h(-1); diisopropoxy complexes [Ti{OSSO}(OiPr)(2)] show mere trace activity. With 1-hexene as a chain-transfer agent, activated enantiopure titanium complexes give low-molecular-weight homochiral isotactic oligostyrenes, terminated by one to five 1-hexene units with M(n) values as low as 750 g mol(-1) for R=tBu and 1290 g mol(-1) for R=Me. Below M(n) approximately 5000 these oligostyrenes show optical activity.