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Synthetic Endeavors toward Titanium Based Frustrated Lewis Pairs with Controlled Electronic and Steric Properties

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posted on 26.05.2015 by Adrien T. Normand, Philippe Richard, Cédric Balan, Constantin G. Daniliuc, Gerald Kehr, Gerhard Erker, Pierre Le Gendre
A new family of cationic Ti complexes 4′ with a pendant phosphine of general formula [CpCpPTiOAr]­[BPh4] (Cp = η5-C5H5; CpP = η5-C5H4(CMe2)­PR2) has been prepared in four steps from 6,6-dimethylfulvene. These complexes were designed to behave as Ti based frustrated Lewis pairs (FLPs). The key synthetic step is a reduction–oxidation sequence from [CpCpPTiClOAr] complexes 3 using lithium phosphide salts as the reductants and ferricinium tetraphenylborate as the oxidant. Four complexes have been structurally characterized by X-ray diffraction and show elongated Ti–P bonds, above 2.60 Å. One complex (4b′: OAr = 2,6-Me2C6H3; PR2 = PCy2) reacted with benzaldehyde to form a typical FLP activation product. Complex 4b′ also reacted with 2 equiv of trans-chalcone to form a 10-membered Ti phosphonium macrocycle (6b′) by extrusion of 6,6-dimethylfulvene.