Zirconium insertion reactions

Bis(cyclopentadienyl)zirconium 1,3-alkadiene complexes19-20 show interesting stepwise double insertion reactions to carbonyl compounds, exploration with respect to their stereochemical features has only just begun21-23. 

Many of the older bis-indenyl catalysts are less selective at higher temperatures, which was ascribed initially to a lower selectivity of the insertion reaction itself. More recent work by Busico, based on deuteration studies and again based on very detailed and elegant analysis of 13C NMR spectra of the polymers, has shown that in fact epimerisation of the growing alkyl chain occurs via a series of (3-hydride eliminations and re-insertion reactions [36] involving even tertiary alkyl zirconium species. 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

Zirconium and hafnium dialkylamides are highly reactive compounds. They undergo (i) protolytic substitution reactions with reagents such as alcohols, cyclopentadiene and bisftrimethylsilyOamine 63 64 (ii) insertion reactions with C02, CS2, COS, nitriles, phenyl isocyanate, methyl isothiocyanate, carbodiimides and dimethyl acetylenedicarboxylate 69-72 and (iii) addition reactions with metal carbonyls.73 These reactions are summarized with reference to Zr(NMe2)4 in Scheme 1. 

Facile isocyanide insertion reactions into metal-carbon, -nitrogen, -sulfur, -oxygen, - hydride, and - halide bonds have been found to readily occur. The insertion into metal-hydrides to give stable formimidines is particularly noteworthy since corresponding formyls (—CHO) are exceptionally difficult to synthesize and tend to be very unstable. There is a great deal of interest in carbon monoxide reductions, and the instability of the intermediate reduction products has made a study of the reduction process extremely difficult. Recently, however, the interaction of isocyanides with zirconium hydrides has allowed the isolation of the individual reduction steps of the isocyanide which has provided a model study for carbon monoxide reduction (39). 

Zr(acac) OH) x] + ion (38) which it is proposed269 is formed in the reaction between Zr(acac)4 and Na2(3)-1,2-B9C2H in THF. The nB n.m.r. spectrum of the product is congruent with that of Me4N+[(3)-l,2-B9C2H12] and thus the product would appear to be [Zr4(acac)4(OH)11][(3)-l,2-B9C2H12] rather than the expected zirconium insertion product. 

The reasons why alkynes 14 and 19 having aryl or vinyl groups, respectively, insert into the zirconium-silicon bond of 3, whereas the alkyne 17 having alkyl groups inserts into the carbon-zirconium bond of 3, are still not clear. Presumably, electronic factors are important for the insertion reaction. 

A type of SO2 insertion reaction which has been reported among transition metal complexes exclusively with certain zirconium(IV) cyclopentadienyls is insertion into the M-Cp bond to give polymeric products characterized as O- and 0,0 -sulfinates on the basis of r(SO) values. An example is the reaction... 

The pincer like diamino functionalised carbene ligand stabilises the group 4 metal sufficiently to perform a few very interesting insertion reactions with the M-Me bond. The reactions were performed with the hafnium rather than the zirconium complexes [113]. Small molecules used for these insertion reactions include isonitriles (aryl and alkyl) and CO (see Figure 4.36). Reaction of the hafnium carbene dimethyl complex with xylyl-isonitrile results in addition of this excellent donor ligand and subsequent insertion into the Hf-Me... 

Transition metal-carbon a bonds are notoriously labile, although stable cr-organo-titanium or -zirconium compounds have been isolated from insertion reactions". ... 

A clean insertion reaction of the 2,3-dihydrophosphole 4 into the zirconium-carbon bond of the transient benzyne-zirconocene 1 occurs when 4 is heated with Cp2ZrPh2 at 80°C for 6 h, leading to the tricyclic compound 5 (Scheme 1). The regioselective formation of 5 can be regarded as the result of a strong interaction between the phosphorus lone pair and zirconium. It can be noted that under the same experimental conditions, the corresponding oxide 4 or the 3,4-di-hydrophosphole 4 do not react with 1. 

Insertion reaction of alkyne and zirconium-silene complexes 883, which are generated from disilylzirconocenes 882 prepared from the reaction of zirconocene dichloride with 2equiv. of Me2PhSiLi, affords... 

Aryne complexes of late transition metals are very reactive towards both nucleophiles (amines, alcohols, water) and electrophiles (iodine). They also undergo insertion reactions with CO, alkenes and alkynes,but while the behaviour of ruthenium complexes is somewhat similar to that of titanium or zirconium complexes, the reactivity of nickel complexes is rather different [6,8]. Examples of these reactions that are particularly interesting for the purposes of this chapter are shown in Schemes 8 and 9. Ruthenium complex 33 undergoes insertion of a molecule of benzonitrile,benzaldehyde or di(p-tolyl)acetylene to yield met-allacycles 40,41 and 42, respectively (Scheme 8). Further insertion of a second unsaturated molecule into these metallacycles has not been observed [25,27]. 

Many additional routes to metal-alkyl complexes other than transmetallation and alkylation are discussed in later chapters of this text. For example, metal-alkyl complexes are generated by insertion of an olefin into a metal-hydride or or metal-hydrocarbyl species. Such insertion reactions are discussed in Chapter 9, but an example of the synthesis of a zirconium alkyl by olefin insertion into a zirconium hydride is shown in Equation 3.9. Metal-alkyl complexes are also generated by nucleophilic attack on coordinated olefins (Equation 3.10) or carbene ligands (Equation 3.11). These reactions are presented in detail in Chapter 11. 

The next step is the migratory insertion reaction. This leads to the formation of a metal-alkyl complex. Note that we have ended up again with a three-coordinate zirconium cation, which has a metal-alkyl bond. In effect we have lengthened the alkyl chain by the process of olefin coordination followed by migratory insertion. 

Insertion Reactions Involving Zirconium and Titanium 5.2.1 Hydrozirconation and Carbozirconation... 

There have been three reports of cr-bond insertion reactions developed for use as tools for synthesis. In general these involve carbenoids rather than carbenes, but they are included here because of their interest as selective reagents. In the first a lithiated gem-dihalide (68) effectively inserted into carbon-zirconium bonds, for example (69), to give ultimately the corresponding carbocycles (70) in moderate-to-good yields. This process is arguably stepwise involving a carbenoid species rather than a carbene. ... 

The insertion reaction of an olefin into the Zr-H bond is immediately followed by the complete isomerization of the branched alkyl chain to the linear chain on zirconium, then bromation gives [ZrCp2(Cl)Br] and 1-bromooctane. 

Guram AS, Jordan RF. Alkene and alkyne insertion reactions of cationic Cp2Zr (l] -pyridy])(L) complexes. Zirconium-mediated functionalization of pyridines. Organometallics. 1991 10 3470-3479. 

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