Enantiomerically pure titanium complexes containing an [OSSO]-type bis(phenolate) ligand: synthesis, structure, and formation of optically active oligostyrenes.

Chiral 1,2-trans-dithiocyclohexanediyl-bridged bis(phenols) of the type [2,2′-{HOC6H2-6-R1-4-R2}2S2C6H10] ([OSSO]H2, R1=tBu, iPr, H; R2=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 TiX4 (X=Cl, OiPr) proceeds in a diastereoselective fashion to give only the Λ,R,R and Δ,S,S enantiomers. Racemic [Ti{(OC6H2-6-tBu-4-Me)2S2C6H10}Cl2] reacts with benzyl magnesium bromide to afford the crystallographically characterized dibenzyl complex. The benzyl cation formed using B(C6F5)3 in C6D5Br slowly decomposes at temperatures above +10 °C. When treated with methylaluminoxane, the dichloro complexes [Ti{OSSO}Cl2] 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 Mn values as low as 750 g mol−1 for R=tBu and 1290 g mol−1 for R=Me. Below Mn≈5000 these oligostyrenes show optical activity.