1 monoxide Titanocene

Bis(i -cyclopentadienyl)titanium or titanocene, (Tj-C5H5)2Ti (1), and bis(i7-cyclopentadienyl)zirconium or zirconocene, (i7-C5H5)2Zr (2), although frequently referred to in the literature, have never actually been isolated as discrete chemical compounds. However, these molecules have been implicated as highly reactive intermediates in a wide variety of chemical reactions with olefins, hydrogen, carbon monoxide, and dinitrogen. In recent years some discrete, well-characterized bis(7j-cyclopenta-dienyl) and bis(Tj-pentamethylcyclopentadienyl) complexes of low-valent titanium and zirconium have been isolated and studied, and it has become possible to understand some of the reasons for the remarkable reactivity of titanocene- and zirconocene-related organometallics toward small unsaturated molecules. 

The synthesis and spectroscopic features of a number of titanocene dicarbonyl derivatives have been reviewed previously.41 Typically, these complexes are prepared via magnesium reduction of the corresponding dihalide derivative under 1 atm of carbon monoxide. New compounds prepared since COMC(1995) are presented in Scheme 10. Mixed cyclopentadienyl phosphinimide dicarbonyl compounds such as (if-CsMcs)-(Buc3PN)Ti(CO)2 61 have also been isolated.46... 

Substituted-titanocene dicarbonyl complexes, ( ]S-CsRBHs B)2Ti(CO)2, have also been used as isolable precursors to highly reactive titanium sandwiches, ( ]S-CsRBHs B)2Ti. For example, reaction of a,/3-unsaturated ketones with ( -CsHs TFCO 68 results in reductive coupling of the organic substrate to form (77S-CsHs)2Ti(0C(R1)CHCH(R2)CH(R2)CHC(R1)0) 69 with liberation of carbon monoxide (Scheme 12).47 48... 

The catalytic intramolecular hetero-Pauson—Khand reaction has been extensively studied by several groups.184 Kablaoui et al. reported the titanocene-catalyzed hetero-Pauson—Khand reaction of the 2-ally lphenyl ketones 382.184ab In the presence of Cp2Ti-(PMe3)2 and trimethylphosphine, the reaction of 382 with carbon monoxide gave the polycyclic lactones 383 in high yields (Scheme 121). 

Transfer of the organic fragment from a titanocene intermediate to carbon monoxide appears to lead to the corresponding 2-one (Equation (18)) <90MI 922-02). 

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