Bis(cyclopentadienyl)titanium(III) Chloride

Abstract

[60955-54-6] C10H10ClTi (MW 213.55) InChI = 1S/2C5H5.ClH.Ti/c2*1-2-4-5-3-1;;/h2*1-5H;1H;/q;;;+1/p-1 InChIKey = GMIFFCUUBDCNMU-UHFFFAOYSA-M (selective one-electron reductant for heteroatom abstraction and pinacol coupling) Physical Data: yellow-green paramagnetic solid, mp 282–283 °C. Solubility: typically generated and used as THF solution. Strong donor solvents suppress reactivity. Insoluble in ether. Form Supplied in: although pure Cp2TiCl (solid state) can be prepared and isolated, for synthetic applications the reagent is invariably prepared by in situ reduction of commercial bis(cyclopentadienyl)titanium(IV) dichloride, typically with a metal powder (see below). Handling, Storage, and Precautions: the reagent reacts rapidly with oxygen, so that reactions must be conducted with rigorous exclusion of air. Oxidizing functionalities such as nitro groups are likewise incompatible with Cp2TiCl while protic functionalities such as alcohols are well tolerated. Cp2TiCl is slowly hydrolyzed in deoxygenated water but can be stabilized in aqueous solution with excess chloride ion. Because of its air-sensitivity, for synthetic applications, Cp2TiCl is invariably generated in situ by reduction of Cp2TiCl2 either stoichiometrically or using a catalytic protocol. Metals that have been used for the generation of Cp2TiCl from Cp2TiCl2 include zinc powder, manganese powder, and aluminum foil. Stoichiometric reduction has also been accomplished with indium metal or Grignard reagents. When a red THF solution of Cp2TiCl2 is stirred with a slight excess of zinc powder for 15 min at room temperature, the color discharges to green, and Cp2TiCl is formed quantitatively. In some cases the metal halide co-product in this redox process contributes to the reaction; consequently, it is prudent to screen several reductants to optimize a particular transformation. Catalytic reactions are carried out using a stoichiometric amount of reductant and a catalytic quantity (typically 1–10%) of Cp2TiCl2 which is repeatedly reduced to the Ti(III) oxidation state. The issue of relative kinetics is often critical in Cp2TiCl chemistry. ‘Normal’ addition of substrate to reagent versus ‘inverse’ addition of reagent to substrate can lead to different organic products. In this context, the catalytic reactions more closely resemble the inverse addition protocol in that a low steady-state concentration of Ti(III) is maintained.