OXIDATIVE COUPLING OF PHOSPHINOMETHANIDE LIGANDS ON TITANOCENE AND BISMUTH CENTERS - SELECTIVE C-C, P-C AND P-P BOND FORMATION

The reaction of Cp2TiCl2 with two equivalents of Lithium-phosphinomethanides is described. The reaction with LiCH2PMe2 leads to a C-C coupling product Cp2Ti[Me2PCH2CH2PMe2], whereas with {(TMEDA)Li[CH(PMe2)(SiMe3)]}2, the P-C coupling product Me3SiCH2PMe2=C(PMe2)(SiMe3), a P-ylide, is formed. An anlogous P-C coupling is observed with Li[CH(PMe2)2], whereby a novel titanocene heterocycle (Cp2Ti(H)[(Me2P)2C-PMe2-CH2-PMe2)]}, with an agostic Ti ... H ... C interaction, may be characterized as an intermediate. In contrast, with Li[CH(PMe2)(SiMe3)] (without TMEDA) P-P coupling, yielding [(Me3Si)-(H)C=PMe2]2, is observed exclusively. An analogous result is obtained with Li[C(PMe2)(SiMe3)2], leading to [(Me3Si)2C=PMe2]2, and, likewise, in the similar reaction of Cp2TiCl2 with Li[C(PMe2)2(SiMe3)], P-P Coupling is observed leading to [(Me2P)(Me3Si)C=PMe2]2 and (CP2Ti(III)[(Me2P)2C(SiMe3)]}. With (TMEDA)Li[HC(PPh2)2], both P-P (34%) and P-C (66%) coupling are observed, yielding the P-ylides [(Ph2P)HC=PPh2]2 and Ph2PCH2PPh2=C(PPh2)2, respectively. The results are compared with the respective coupling reactions at bismuth centers: BiCl3 reacts with LiCH2PMe2 to give Bi(CH2PMe2)3; with {(TMEDA)Li[CH(PMe2)(SiMe3)]}2 only 5% of the P-C coupling product Me3SiCH2PMe2=C(PMe2)(SiMe3) are found, whereas 95% of the P-P coupling product [(Me3Si)(H)C-PMe2]2 are obtained. Exclusive P-P coupling is also observed from BiCl3 and (TMEDA)Li[C(PMe2)(SiMe3)2] and Li[C(PMe2)2(SiMe3)], respectively. The results clearly indicate that a metal-centered process, sensitive to steric and coordination effects, is active. Selective C-C, P-C and P-P coupling reactions are possible by selecting the appropriate combinations of reagents. The novel P-ylides are characterized by use of NMR spectroscopy; (Cp2Ti(III)[(Me2P)2C(Si-Me3)]} and [(Me3Si)2C=PMe2]2 are further characterized by X-ray structure determinations.