Zirconium complexes with alkenes

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. 

A zirconium complex of tropyne has been prepared by hydride abstraction from a mixture of 231a-c (Scheme 28).87 This compound is extremely thermally unstable, decomposing to a complex mixture of products above —50°C. Identification of both 230 and 232 rests exclusively on low-tempera-ture, multinuclear NMR spectra. Attempts to couple 232 with reagents such as alkenes, alkynes, nitriles, or ketones gave only complex mixtures of products. 

Fig. 6.19. Isotactic polymerization of propene with rac-l,2-ethanediyl-bis(indenyl)zirconium complexes. The plane of the drawing coincides with the plane bisecting the two planes of the indenyl ligands. Propene coordination takes place with the alkene Tt-orbitals in the plane of the drawing. The carbon atoms of the growing chain are depicted in the plane of the figure in between insertions no rearrangements to more stable conformations have been drawn. Indenyl ligand above the plane drawn in full.

Zirconocene dichloride reacts with dibromomethane in the presence of Zn to yield a methylene-zirconium complex 4.105, which is used for the methylenation of carbon compounds to produce the terminal alkenes h... 

In general, isolable zirconium and hafnium alkene complexes are rare, as they typically undergo carbon-carbon coupling reactions with additional olefin to yield metallacyclopentanes. Addition of an exogenous donor ligand is a common strategy for stabilizing alkene complexes. Several classes of these compounds have been prepared those... 

Hydrozirconation occurs with yyn-addition of the Zr-H bond across a C=C or C=C bond (equation 8.16). Due to lower steric hindrance, the addition also tends to be regiospecific, with the zirconium attached to the less substituted position (just as in hydroboration). Internal alkenes and alkynes isomerize to 1-alkyl and 1-alkenyl complexes, respectively—presumably by alternating reactions of insertion and deinsertion—until the complex with the least steric hindrance is formed. 

Group 4 bis(amidate)bis(amido) complexes have also been identified as precatalysts for the more challenging hydroamination of alkenes. The majority of investigations in this field focus on the intramolecular cychzation of aminoalkenes with zirconium-based catalysts. [64e] Neutral group 4 bis(amidate) zirconium amido or imido complexes are efficient precatalysts for the intramolecular cychzation of primary amines to form pyrrolidine and piperidine products (Scheme 12). The monomeric imido complex can be generated by reaction of the bis(amido) complex with 2,6-dimethylaniline and trapped with triphenylphosphine oxide. [64e] The bis(amido) and imido complexes... 

Alkyl zirconium complexes such as 67 react with CO to give an unstable 18e complex 82 that transfers the alkyl group from the metal to the CO n orbital to give the metal acyl complex 83. This is a Zr(IV) complex of an acyl anion 83a and can be protonated to give the aldehyde 84 in excellent yield. These zirconium complexes are usually made from alkenes so that any hexene or mixture of hexenes gives 85 again in excellent yield.26... 

Zirconium complexes, generated in situ by addition of HZrCp2Cl to alkenes, can be animated with <9-(mesitylenesulfonyl)hydroxylamine an example is shown in Eq. 41.116 When the initial hydrozirconation is not regioselective, as with styrene, mixtures of amines are formed. A reaction that permits animation at the tertiary carbon in a similar substrate is discussed below (Eq. 49). 

Racemic [1,r-ethylenebis(4,5,6,7-tetrahydro-l-indenyl)]zirconium dichloride and methylalu-moxane as cocatalyst promote the formation of highly isotactic polymers from propene and longer a-alkenes1 3. While with a pure enantiomer of this zirconium complex the polymerization of linear terminal alkenes cannot be expected to give optically active products (see Section 1.5.8.3.2.), the oligomers should display measurable optical activities4. 

Tetracarbonylnickel and other nickel(O) compounds, as well as palladium complexes, catalyze the [2 + 2 + 1] cycloaddition of allylic systems with alkenes or alkynes and carbon monoxide to form cyclopentanones or cyclopentenones. This reaction type resembles stoichiometric zirconium- and cobalt-mediated [2 + 2 + 1] cycloadditions (vide supra), mechanistically, however, it proceeds via transition metal 7r-allyl complexes. 

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]. 

Previously reported bis(amidate)- and tethered-amidate-supported zirconium complexes can be used for alkene hydroamination catalysis, and all substrate scope and mechanistic investigations of these systems are consistent with the [2+2] cycloaddition mechanistic profile [61, 62). However, more recent catalyst systems that can be used with secondary amines show broader substrate scope, similar to that attained by rare earth elements and suggest a mechanistic similarity to that observed for previously intensely investigated rare earth hydroamination catalyst systems [7j. Such complexes are proposed to achieve ring closure via o-bond insertion, and thus, consideration of such a mechanistic profile in this case demanded further investigation. 

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