Zirconium stereochemistry

Alkenyl zirconium complexes derived from alkynes form C—C bonds when added to aHyUc palladium complexes. The stereochemistry differs from that found in reactions of corresponding carbanions with aHyl—Pd in a way that suggests the Cp2ZrRCl alkylates first at Pd, rather than by direct attack on the aUyl group (259). 

Table 21.2 Oxidation states and stereochemistries of titanium, zirconium and hafnium...

Zirconium, tetrakis(acetylacetonate)-stereochemistry, 1, 32, 94 Zi rconium, tris(phenylenedithio)-structure, 1, 63 Zirconium alkoxides oligomeric structure, 2,346 Zirconium chloride... 

Note also the stereochemistry. In some cases, two new stereogenic centers are formed. The hydroxyl group and any C(2) substituent on the enolate can be in a syn or anti relationship. For many aldol addition reactions, the stereochemical outcome of the reaction can be predicted and analyzed on the basis of the detailed mechanism of the reaction. Entry 1 is a mixed ketone-aldehyde aldol addition carried out by kinetic formation of the less-substituted ketone enolate. Entries 2 to 4 are similar reactions but with more highly substituted reactants. Entries 5 and 6 involve boron enolates, which are discussed in Section 2.1.2.2. Entry 7 shows the formation of a boron enolate of an amide reactions of this type are considered in Section 2.1.3. Entries 8 to 10 show titanium, tin, and zirconium enolates and are discussed in Section 2.1.2.3. 

A more complex cumulenyl carbenoid 80 may be generated in situ from 1,4-dihalobut-2-ynes and two equivalents of base (Scheme 3.21). Insertion into organozirconocene chlorides gives allenyl zirconium species 81, which are regioselectively protonated to afford enyne products 82 [38], The stereochemistry of the alkene in 82 stems from the initial elimination of hydrogen chloride to form 80. 

Nine-coordinate zirconium occurs in ZrC5Hs(CF3COCHCOCF3)3 using the formalism according to which an % -cyclopentadienyl ligand occupies three coordination sites but the stereochemistry is not readily described in terms of idealized nine-vertex polyhedra.312,313 Compounds of the type... 

Zirconium imido complexes have been used to carry out S 2 reactions of allylic chloride, bromide, iodide, and alkyl, aryl, and trimethylsilyl ethers in high yields at room temperature.12 The syn stereochemistry, an inverse secondary (k /k Oy = 0.88 obtained using the ( )-l-(r-butyldimethylsilyloxy)-3-deuterioprop-2-ene and the rate expression led the authors to suggest the reactions occurred via the mechanism in Scheme 4 with transition state (9). 

The hydrozirconation of alkynes is a well-established reaction, giving vinylic zirconium species of known regio-and stereochemistry.176 These species react with aryltellurium halides leading to vinylic tellurides with the ( )-stereochemistry 98 (Scheme 61),177,178 so complementing the other general routes to these compounds which give preferentially the (Z)-products (Sections 9.13.5.2.3, 9.13.5.2.5). 

Furthermore, if we consider the carbometalative ring expansion to produce the corresponding five-membered ring zirconacycle 86, the carbon-heteroatom bond of the sp3 metalated center Cx should isomerize to produce the most stable intermediate. Such isomerization could be due to an interaction between the heteroatom moiety XR and the zirconium atom [65], which would produce a weakness of the Cj-Zr bond and would facilitate the isomerization. Thus, whatever the stereochemistry of the starting material, a conformation is always possible in which Cj-SR is antiperiplanar to C2-C3 in 86 with a trans relationship between R and the ZrCp2 fragment. The elimination reaction, or decarbo-zirconation, occurs in a concerted way to give the E-vinyl zirconium 83. Unfortunately, neither the zirconacyclopentane nor the zirconacyclopropane have been trapped as intermediates. 

Table 18-A-l Oxidation States and Stereochemistry of Zirconium and Hafnium...

The oxidation states and stereochemistries of zirconium and hafnium are summarized in Table 18-A-l. These elements, because of the larger atoms and ions, differ from Ti in having more basic oxides, having somewhat more extensive aqueous chemistry, and more commonly attaining higher coordination numbers, 7 and 8. They have a more limited chemistry of the III oxidation state. 

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

Dibutylbis(cyclopentadienyl)zirconium was added to A-allyl-Ai-(benzyl)cyclo-hex-2-enylamine (282) and the generated zirkonacycle was treated with carbon monoxide (Scheme 32). The metal was removed with acid and the tricyclic ketone 285 was isolated in excellent yield. The relative configuration of the newly formed stereogenic centers could not be determined at this point, which turned out to be advantageous. The authors assumed according to kinetic preference that the tricycle 286 with all cis stereochemistry had been formed and started the... 

The most common coordination number of titanium is six, although four-, five-, seven-, and eight-coordinate compounds are known (Table 2). Table 3 summarizes the common oxidation states of titanium with the associated coordination numbers and stereochemistries. Zirconium shows a similar range of oxidation states (see Zirconium Hafnium Inorganic Coordination Chemistry), however, Zr and Flfr are much less stable, relative to Zr and Hf, than is the case for titanium. 

Coordination numbers (CN) of zirconium and hafnium range from 4 to 12, but because of the large values of their ionic and covalent radii (see Covalent Radii) (Table 1), their complexes typically have CNs of 6-8 (see Coordination Numbers Geometries). They have a varied stereochemistry... 

Aldol Reactions. Pseudoephedrine amide enolates have been shown to undergo highly diastereoselective aldol addition reactions, providing enantiomerically enriched p-hydroxy acids, esters, ketones, and their derivatives (Table 11). The optimized procedure for the reaction requires enolization of the pseudoephedrine amide substrate with LDA followed by transmeta-lation with 2 equiv of ZrCp2Cl2 at —78°C and addition of the aldehyde electrophile at — 105°C. It is noteworthy that the reaction did not require the addition of lithium chloride to favor product formation as is necessary in many other pseudoephedrine amide enolate alkylation reactions. The stereochemistry of the alkylation is the same as that observed with alkyl halides and the formation of the 2, i-syn aldol adduct is favored. The tendency of zirconium enolates to form syn aldol products has been previously reported. The p-hydroxy amide products obtained can be readily transformed into the corresponding acids, esters, and ketones as reported with other alkylated pseudoephedrine amides. An asymmetric aldol reaction between an (S,S)-(+)-pseudoephe-drine-based arylacetamide and paraformaldehyde has been used to prepare enantiomerically pure isoflavanones. ... 

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