Low-valent zirconocene

The first reported zirconacydopentadiene was 2,3,4,5-tetraphenyl-l-zirconacyclopenta-diene (la), prepared from two molecules of diphenylacetylene and a low-valent zircono-cene [3]. The low-valent zirconocene spedes was produced by the reaction of Cp2ZrCl2 with sodium/naphthalene (Eq. 2.1). 

There are several methods for generating low-valent zirconocene species in situ. Although the conventional method has been the reduction of Cp2ZrCl2 with Na/Hg or Mg/Hg, more convenient methods have since been developed for the preparation of symmetrical zirconacyclopentadienes such as Cp2ZrBu2 2 [the so-called Negishi reagent (Eq. 2.2)] [12]. 

Treatment of a stannylenyne/enene compound with a low-valent zirconocene derivative results in a bicydization reaction leading to the 1,1-stanniozirconocene complex 99 (Scheme 7.29) in moderate yield [168], The product of carbonylation of 99 can be readily... 

An intermolecular coupling reaction between three organonitrile molecules and a silicon-tethered diyne is reported to give good yields of pyrrolo[3,2-f]pyridine derivatives <2004JA7172>. The reaction is promoted by a low-valent zirconocene species (Equation 13). 

The addition of Grignard reagents to secondary silanes has been catalyzed by low-valent zirconocene compounds (Scheme 23).69 Oxidative addition of an organic halide to the divalent zirconocene occurs following reductive elimination of silane. Using this procedure, silanes can be alkylated in excellent yield. 

The structural characterization of (775-C5H5)2Zr(772-C6H4)(PMe3) 255 in 1986 established the utility of formally divalent zirconocene complexes to stabilize otherwise reactive and transient organic molecules.131 The synthesis and reactivity of these species has been the subject of two recent reviews.74,132 Building on these seminal discoveries, formally low-valent zirconocene fragments have been used to stabilize other aryne species. Typically, these syntheses are achieved by... 

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]. 

Independently generated low valent zirconocene and titanocene were demonstrated to be competent reagents for defluorination in the absence of terminal reductant. The critical role played by the metallocenes is best illustrated by noting that, in the absence of metallocene, Mg/HgCl2 causes exothermic reduction of perfluorodecalin to carbon. The milder aluminum system only functions in the presence of metallocene. Additional insight into the nature of this multistep process was obtained by showing that plausible flurocarbon intermediates react to afford perfluorodecalin [79]. 

Through a net [2 + 2+1] cycloaddition, iminocyclopentadi-enes can be obtained through the initial formation of a zircona-cyclopentadiene derivative (from two alkynes and a low valent zirconocene component), and subsequent reaction with an aryl isocyanate and BF3-Et20 (eq 65). Quite remarkably, no other Lewis acid investigated provided this cycloaddition only EtAlCL led to a productive reaction, although different adducts were obtained. 

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. 

On the other hand, the transition metal-catalyzed isomerization of terminal olefins into internal olefins has been extensively studied, and in general a mixture of 1-alkenes and E- and Z-2-alkenes, reflecting the thermodynamic equilibrium, is obtained [67]. Some low-valent titanocene derivatives are highly effective and stereoselective in favor of the E-2 isomer [68]. When non-conjugated dienes such as 92, containing one or two substituted vinyl groups, are treated with the zirconocene 21, a regioisomerization of the less-substituted... 

Cleavage of a C(sp)-C(sp) single bond by low-valent titanocene and zirconocene has been reported [36]. Treatment of a diyne with those metallocenes in tetrahydrofuran (THF) produced the dimer of the alkynylmetallocene complex, in which the alkyne moiety was bonded to the metal center in both a-and it-fashions (Scheme 1.27). 

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