Zirconium alkene complexes

Table 1 Selected bond distances, 1H NMR and 13C NMR spectroscopic data for zirconium alkene complexes... 

Addition of strong-field ligands such as carbon monoxide or alkynes to zirconium alkene complexes can also result in olefin displacement. Treatment of the monocyclopentadienyl complexes, [775-C5H3-(l,3-(SiMe2CH2PR2)2)]Zr( 72-G2H4)Br (R = Pr1, 87 R = Me, 88), with CO or alkynes results in ethylene loss and the formation of the corresponding dicarbonyl (R = Pf, 166 R = Me, 167) and alkyne (R = Pr 168 R = Me, 169) complexes (Scheme 26). For the alkyne addition, no metallacycle is observed, presumably due to the sterics of the ligand array. 

The 7r-back donation stabilizes the alkene-metal 7c-bonding and therefore this is the reason why alkene complexes of the low-valent early transition metals so far isolated did not catalyze any polymerization. Some of them catalyze the oligomerization of olefins via metallocyclic mechanism [25,30,37-39]. For example, a zirconium-alkyl complex, CpZrn(CH2CH3)(7/4-butadiene)(dmpe) (dmpe = l,2-bis(dimethylphosphino)ethane) (24), catalyzed the selective dimerization of ethylene to 1-butene (Scheme I) [37, 38]. 

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 first indication of the likely geometry of zirconium(IV) alkene complexes was found in the crystallographically characterised alkyne insertion product 32 [72],... 

A variety of new ligand designs and ligand combinations were used in attempts to mimic some properties of the ubiquitous bent metallocene environment at the early metal centers consequently, some of these systems were used in the further development of butadiene zirconium chemistry. The pyridine based chelate zirconium dichloride complex 43 cleanly formed the butadiene complex 44 upon treatment with butadiene-magnesium. Its structure shows that the C4H6 is arranged perpendicular to the chelate ligand plane. Complex 44 inserts one equivalent of an alkene or alkyne to form the metallacyclic 7i-allyl system 4545 (Scheme 13). 

Zirconium-benzyne complexes have been used rather extensively in organic synthesis.8 45 For this purpose, one particularly important characteristic of zirconium-aryne complexes is that olefin insertion into the Zr—C bond occurs stereospecifically. Thus, when generated in situ, the zirconium-benzyne complex (45) reacts with cyclic alkenes to give exclusively the cis-zirconaindanes (46), which upon treatment with electrophiles provide access to a variety of m-difunctionalized cycloalkanes (47-49) (Scheme 5).46 For example, carbonylation of intermediate 46 affords tricyclic ketone 49, reaction with sulfur dichloride gives thiophene 48, and reaction of 46 with tert-butylisocyanide followed by I2 gives 47 via 50 and, presumably, intermediate 51 [Eq. (12)]. 

Azaborolyl Zirconium(iv) Complexes as Alkene Polymerization Catalysts... 

The regio- and stereoselective zirconocene-catalyzed addition of alkylmagnesium halides to alkenes, a process which has been described previously (see Section 7.5.2, Scheme 7-79) was investigated with ethylene-l,2-bis( M,5,6,7-tetrahydroind-l-enyl)zirconium dichloride [(EBTHI)ZrCl2l) [118] as chiral zirconocene. Thus, treatment of the latter with alkylmagnesium halides leads to the formation of the derived zirconocene-alkene complex 88, characterized by NMR [119], which reacts with cyclic ethers or amines to lead to the corresponding homoallylic alcohol or amine, respectively, in > 95% ee and good overall yield [120] (Scheme 7-103). 

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... 

As was seen in the previous section, addition of excess olefin to formally divalent zirconium or hafnium alkene complexes usually induces coupling to form the corresponding metallacyclopentane. Interestingly, metallacycle... 

The [2+2] reactions of the zirconium-imido compounds with alkynes and alkenes occurs by a mechanism similar to that for the [2+2] reactions of carbenes with alkynes and alkenes. The alkene or alkyne first binds to an open coordination site at the metal, and this coordination is followed by conversion of the alkyne or alkene complex to the metallacyhc product (e.g. Equation 13.76). Thus, the [2+2] reaction requires a 16-electron intermediate to bind the olefin or alkyne, even though the metallacyHc product and the imido complex have the same overall electron count. In support of the coordination of alkyne or alkene, albeit weakly, to the d° metal center, the rate of the reaction of alkynes with the 18-electron zirconocene-imido compound containing bound pyridine-N-oxide was inhibited by added pyridine-N-oxide (Equation 13.76). ... 

Wood MC, Leitch DC, Yeung CS, Kozak JA, Schafer LL. Chiral neutral zirconium amidate complexes for the asymmetric hydroamination of alkenes. Angew. Chem. Int. Ed. 2007 46(3) 354-358. 

Irrespective of the electron count, coordinative unsaturation may result from easy dissociation of a ligand. The zirconium complex 2.61 and the rhodium complex 2.59 illustrate this point. The zirconium compound is electronically unsaturated, but its reactivity in catalytic alkene polymerization is due to the easy displacement of THF by alkene. Complex 2.59 is electronically saturated but undergoes PPhj dissociation to generate coordinative and electronic unsaturation. 

In their early studies, Schwartz and co-workers [5, 80] reported the zirconocene hydrido chloride [Cp2Zr(H)Cl] (1) as a reagent capable of reacting under mild conditions with a variey of non-functionalized alkenes to form isolable alkylzirconi-um(lV) complexes Cp2Zr(R)Cl in which the zirconium is attached to the least-hindered terminal primary carbon, irrespective of the original location of the double bond in the olefin chain. As an example, at room temperature in benzene, 1-octene, cis-4-octene and trows-4-octene all yield the n-octylzirconocene derivative (Scheme 8-6) [80]. 

The regioselective hydrozirconahon of internal unsymmetrical alkenes remains a challenge, as it could considerably expand the use of zirconocene complexes. Little is known about the mechanism of zirconium migration along an alkyl chain. 

---