Free titanocene

Aluminum-free titanocene-methylidene can be generated by thermolysis of titana-cyclobutanes 6, which are prepared by reaction of the Tebbe reagent with appropriate olefins in the presence of pyridine bases [9]. Alternatively, the titanacyclobutanes are accessible from titanocene dichloride and bis-Grignard reagents [10] or from 71-allyl titanocene precursors [11]. The a-elimination of methane from dimethyltitanocene 7 provides a convenient means of preparing titanocene-methylidene under almost neutral conditions [12] (Scheme 14.5). 

TITANOCENE-CATALYZED REDUCTION OF KETONES IN THE PRESENCE OF WATER. A CONVENIENT PROCEDURE FOR THE SYNTHESIS OF ALCOHOLS VIA FREE-RADICAL CHEMISTRY... 

To explain the unprecedented HATs from water it was proposed that the co-ordination of water to Ti(III) might weaken the strength of the O-H bond. In this manner a single-electron transfer from titanium to oxygen might facilitate the HAT from the titanocene aqua-complex 42 to the free radicals (Scheme 29). 

DNA is believed to be the most important cellular target of titanocene dichloride anticancer drugs when 3 is reacted with a diorganophosphate a chloride-free 1 2 (Ti P) adduct can be isolated, which shows coordination between titanium and the phosphate groups [44]. Recently, a molecular modelling study [45] was able to... 

To study the effects of water and other solvents on titanocene(III)-mediated processes we used the transannular cychzation of epoxygerma-crolides as a model reaction [47]. Thus, we found that in anhydrous, non-halogenated solvents such as THF the reaction led selectively to decalins with an exocyclic double bond (Scheme 5). In an aqueous medium (THF/H2O), however, the characteristic lime green color of Cp2TiCl turned deep blue and the main product was a reduced decalin (Scheme 5). Under these conditions, water (either H2O or D2O) proved to be more effective than the toxic and expensive hydrogen-atom donor 1,4-cyclohexadiene for the reduction of tertiary radicals [47]. This is an unusual phenomenon in free-radical chemistry [48-50], subsequently exploited by us for the selective reduction of aromatic ketones as we shall see later [51,52]. 

Finally, it can be stated that many reactivity patterns of free radical ions are equally found in oxidative and reductive transformations involving initial inner-sphere ET, such as in reactions with samarium iodide [389], low valent titanium [390] and titanocene complexes [391], manganese(III) [392], and CAN [393]. 

Alcohols from epoxides. Free radical reaction using titanocene dichloride as an... 

Scheme 10.6 Photoinitiation of the free radical polymerization of an alkyl acrylate with the aid of a fluorinated titanocene [47].

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