Titanocene species

To improve the utility of Nugent s and RajanBabu s conditions even further, catalytic conditions for cyclizations have been developed. They address the issue of reagent control of the cyclization and the mode of its termination. The formation of an alkyl titanocene species after reductive trapping allows two distinctive pathways for the regeneration of the catalyst. 

Perhaps the most exciting recent advances in the organometallic chemistry of fluorocarbons have been the contemporaneous and complementary discoveries of examples of catalytic activation and functionalization of perfluorocarbons in laboratories led by Crabtree and Richmond [23]. Kiplinger and Richmond [64] showed that Group 4 metallocenes function as catalysts in the synthesis of per-fluoronapthalene from perfluorodecalin using activated Mg or Al as the terminal reductant. Low valent zirconocene or titanocene species were postulated as intermediates in the catalytic cycle and control experiments showed the central role played by the metallocene in mediating electron transfer in these systems. Turnover numbers up to 12 (net removal of 120 fluorines/metallocene) were noted [64]. 

Electroreduction titanium (IV) iso-propoxide under molecular nitrogen at 1 atm in diglyme led to the generation of titanium nitride species that could be hydrolyzed to yield ammonia [7]. The electrochemical reduction of dinitrogen to ammonia in aqueous electrolytes using vanadium (IV) catecholate mediators and other transition metal systems has been reported, including the titanocene species [8-10]. 

The stoichiometric reaction of 1,6-enone and 35 gives bicyclic oxatitanacycle whose carbonylation resulted in the formation of bicyclic y-butyrolactone (Scheme 6.7) [112,113]. Addition of H2SiPh2 and HSi(OEt)3 to the oxatitanacycle cleaves Ti—O bond to generate titanocene species together with silyl ether product, and thus this process goes catalytically [114,115]. 

Manners and coworkers described dehydrogenation of dimethylamine borane with titanocene complexes prepared in situ by reaction between the titanocene species and bulyllithium. The catalysts displayed good catalytic activity giving good yields. ... 

The mode of reaction of the reduced titanocene species Cp2Ti with disubstituted butadiynes RCC-CCR was reported to depend strongly on the nature of the substituents R and R . For R = R = SiMea the starting butadiyne is split by titanocene to yield the dinuclear complex [ Cp2Ti(CCSiMea) 2]> (43). For other symmetrically substituted (R = R = Ki or Bu ) or unsymmetrically substituted butadiynes (R = SiMea, R = Ph or R = SiMea, R = Bu ) the reaction was reported to proceed to give dinuclear complexes with a central 1,4-disubstituted ii-T (1-3),ti(2-4)-trans, trans-butadiene unit ( zig-zag-butadiyne ) between the two titanium centres. The latter compounds with (R = R = Bu ) and (R = SiMea, R = Ph) were characterised by X-ray diffraction. 

Polystyrene-Attached Titanocene Species Preparation and Reactions [89]... 

Table 7 Hydrogenation Rates with Polymer-Attached Titanocene Species...

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