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

• Published in: Chemistry, an Asian journal an Asian journal, 2008-09, Vol.3 (8-9), p.1312-1323
• Main Authors: Meppelder, Geert-Jan M., Beckerle, Klaus, Manivannan, Ramanujachary, Lian, Bing, Raabe, Gerhard, Spaniol, Thomas P., Okuda, Jun
• Format: Article
• Language: English
• Subjects: catalysis; chirality; polymerization; styrene; titanium
• ISSN: 1861-4728; 1861-471X
• DOI: 10.1002/asia.200800064

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. The configurational stability of chiral bis(phenolate) titanium complexes results from the diastereoselective chirality transfer from the ligand to the metal center. When activated with MAO using 1‐hexene as chain‐transfer agent, this chirality can be catalytically transferred from the enantiopure catalyst to the incoming styrene monomer to give homochiral isotactic oligostyrenes.