Zirconium addition initiated

Whereas zirconium-catalyzed alkylalumination of alkynes is at present limited to some simple alkyl groups, it has been found that carbozirconation with Cp2Zr(R)Cl considerably expands the scope of the alkylmetalation of alkynes. The reactions are initiated by catalytic amounts of additives (typically Ph3C[B(C6F5)4]), which probably serve to generate cationic zirconocene species. Regioselectivity in favor of attack at the more highly substituted carbon is observed [70,71] (Scheme 8.35). 

Randon et al. reported on an alternative approach for the preparation of zirconia monoliths [96]. The sol-gel process is initiated by hydrolysis of an ethanolic zirconium alkoxide solution, on addition... 

However, in a comprehensive study, Seebach and coworkers have shown the usefulness of titanium and zirconium alkyls as selective nucleophilic reagents.106,107 Typically compounds of the type (RO)3MR (M = Ti, Zr) are reacted with aldehydes or ketones to yield asymmetric alkoxides (equation 32),107 The method has been extended to allow enantioselective addition using chiral organotitanium reagents where the chirality is imposed by the initial alkoxide substituents.107,108... 

The compound (XXII) is also obtained upon addition of 3 mol (i-Bu)2A1H to Cp2ZrCl2 in benzene. It is interesting that this compound absorbs 2 mol CO, and the resulting golden-yellow solution on hydrolysis yields a series of alcohols ROH (R = Me, Et, n-Pr, n-Bu). The mechanism has been postulated to involve the initial dissociation of 1 mol (i-Bu)2AlH, followed by coordination of CO to zirconium subsequent chain propagation occurs as shown below (118). 

The hydrozirconation of alkynes by Cp2ZrHCl has been studied in detail . The addition of Zr-H is uniquely cis, and in 1-alkynes the zirconium attaches to the terminal carbon atom with high regioselectivity ( 98%). The direction of cis-fi addition of Zr-H to internal alkynes is sensitive to the steric bulk of the two substituents of the alkyne and, in the absence of excess Zr-H, is subject to kinetic control. The presence of excess Zr-H results in rapid equilibration of the initial mixture. The results of some hydrozirconations of internal alkynes are shown in Table H. 

It is well known that transition metal complexes catalyze the decomposition of diazoacetates in the presence of acetylenes. Thus addition of 63 to the acetylenic species 76 and 77 resulted in loss of nitrogen and formation of the stable C-metallated cyclopropene compounds 78 and 79. For the first time loss of nitrogen in the formation of cyclopropene rings is initiated by the presence of the zirconium metal fragment. 

Analogous dienyl zirconium compounds can be prepared by addition of unconjugated dienes that contain enol ethers to 105 (Scheme 19).56-58 These reactions are believed to proceed by initial coordination of the terminal olefin followed by isomerization via zirconocene allyl hydride intermediates to eventually yield zirconacyclopentanes that... 

The zirconium-catalyzed carbometalation reaction has been developed from the initial observation by Dzhemilev in 1983 that Cp2ZrCl2 (1) catalyzes the addition of ethyl-magnesium halide to unactivated alkenes [104-106]. A plausible mechanism for this carbometalation involves zirconocene and zirconacyclopentane species (Scheme 6.5) [107-109]. Diastereo- and enantioselective carbomagnesation is accomplished by use of chiral fl .sa-zirconocene derivative [110]. 

As it can be observed in Fig. 4, when only methane is added to the feed, catalyst A shows the best behaviour, followed by catalysts B and U. According to this, the addition of cerium to the zirconium hydroxide increases the activity of the zirconia-supported palladium catafysts. Comparing the performance of the cerium-containing catalysts, it is remarkable that catalyst B presents a poorer performance, than catalyst A (slightly lower initial conversion and foster deactivation). This result suggests that the interaction between Pd and Ce, revealed in the TPR experiments, does not enhance the activity of the active phase (Pd). In contrast, the interaction Ce-Zr in catalysts A increases the thermal stability, considered as the main foctor for preventing catalyst deactivation in these reactions [9]. 

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