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-R-1-4-R-2}(2)S2C6H10] ([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 TiX4 (X=Cl, OiPr) proceeds in a diastereoselective fashion to give only the Lambda,RR and Delta,SS enantiomers. Racemic [Ti{(OC6H2-6-tBu-4-Me)(2)S2C6H10} Cl-2] 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 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 approximate to 5000 these oligostyrenes show optical activity.