Zirconium heterocyclic complexes

Amino alcohol-N-oxides as precursors of chiral oxazolidines synthesis of (R)-(-)-cryp-tostyline I. Heterocycles 1993, 36, 1763-1769. Blum, S. A. Walsh, P. J. Bergman, R. G. Epoxide-opening and group-transfer reactions mediated by monomeric zirconium imido complexes./. Am. Chem. Soc. 2003, 325, 14276-14277. 

Sulfur and selenium heterocycles have also been prepared by reaction of zirconium-benzyne complexes with the elemental chalcogens. In these reactions, chalcogens insert into both of the zirconium-carbon bonds (Scheme 7).53-55 Interestingly, neither para-bromo norpara-dimethylamino substituents in 65 interfere with their conversion to 66 (60-80% yields).54 Complexes 66 react further with mono- and bifunctionalized electrophiles (Scheme 7) to yield ort/io-dichalcogenated benzenic compounds 67-69. 

Zirconium complexes of a new five-membered BN heterocycle, the 3a,7a-azaborindenyl ligand (Figure 1), have been reported.149... 

Table 4 Complexes of Zirconium(IV) and Hafnium(IV) Halides with N-heterocyclic Ligands...

Zirconium and hafnium cyanate, thiocyanate, and selenocyanate complexes that contain N-heterocyclic ligands are discussed in Sections 32.4.2.5.iii, 32.4.2.5.iv and 32.4.2.5.V. 

In 1997, the first truly catalytic enantioselective Mannich reactions of imines with silicon enolates using a novel zirconium catalyst was reported [9, 10]. To solve the above problems, various metal salts were first screened in achiral reactions of imines with silylated nucleophiles, and then, a chiral Lewis acid based on Zr(IV) was designed. On the other hand, as for the problem of the conformation of the imine-Lewis acid complex, utilization of a bidentate chelation was planned imines prepared from 2-aminophenol were used [(Eq. (1)]. This moiety was readily removed after reactions under oxidative conditions. Imines derived from heterocyclic aldehydes worked well in this reaction, and good to high yields and enantiomeric excesses were attained. As for aliphatic aldehydes, similarly high levels of enantiomeric excesses were also obtained by using the imines prepared from the aldehydes and 2-amino-3-methylphenol. The present Mannich reactions were applied to the synthesis of chiral (3-amino alcohols from a-alkoxy enolates and imines [11], and anti-cc-methyl-p-amino acid derivatives from propionate enolates and imines [12] via diastereo- and enantioselective processes [(Eq. (2)]. Moreover, this catalyst system can be utilized in Mannich reactions using hydrazone derivatives [13] [(Eq. (3)] as well as the aza-Diels-Alder reaction [14-16], Strecker reaction [17-19], allylation of imines [20], etc. 

Metal rf-inline complexes with various transition metals [1-10] and lanthanides [11,12] are well known in the literature. Early transition metal if-imine complexes have attracted attention as a-amino carbanion equivalents. Zirconium rf-imine complexes, or zirconaaziridines (the names describe different resonance structures), are readily accessible and have been applied in organic synthesis in view of the umpolung [13] of their carbons whereas imines readily react with nucleophiles, zirconaaziridines undergo the insertion of electrophilic reagents. Accessible compounds include heterocycles and nitrogen-containing products such as allylic amines, diamines, amino alcohols, amino amides, amino am-idines, and amino acid esters. Asymmetric syntheses of allylic amines and a-amino acid esters have even been carried out. The mechanism of such transformations has implications not only for imine complexes, but also for the related aldehyde and ketone complexes [14-16]. The synthesis and properties of zirconaaziridines and their applications toward organic transformations will be discussed in this chapter. 

Edworthy, I.S., Blake, A.J., Wilson, C. etal. (2007) Synthesis and NHClability of d° lithium, yttrium, titanium, and zirconium amido bis(N-heterocyclic carbene) complexes. Organometallics, 26, 3684. 

Downing, S.P., Guadano, S.C., Pugh, D. et al. (2007) Indenyl- and fluorenyl-functionahzed N-heterocyclic carbene complexes of titanium, zirconium, vanadium, chromium, and yttrium. Organometallics, 26, 3762. 

The use of the boratabenzene heterocycle as a ligand for transition metal complexes dates back to 1970 with the synthesis of (C H5B-Ph)CpCo+ (1) (Cp = cyclopentadienyl).1 Since boratabenzene and Cp are 6 it electron donors, 1 can be considered isoelectronic to cobaltocenium. Many other transition metal compounds have been prepared that take advantage of the relationship between Cp and boratabenzene.2 In 1996, the synthesis of bis(diisopropylaminoboratabenzene)zirconium dichloride (CsHsB-NPr ZrCh (2) was reported Of particular interest is that 2 can be activated with methylaluminoxane (MAO) to produce ethylene polymerization catalysts with activities similar to those characteristic of group 4 metallocenes.4 Subsequent efforts showed that, under similar reaction conditions, (CsHjB-Ph ZrCh/MAO (3/MAO) gave predominantly 2-alkyl-1-alkenes5 while (CsHsB-OEt ZrCh/MAO (4/MAO) produced exclusively 1-alkenes.6 Therefore, as shown in Scheme 1, it is possible to modulate the specificity of the catalytic species by choice of the exocyclic group on boron. 

Rearrangement processes of alkyltitanocene dichlorides that occur under electron impact have been investigated using deuterium labelling. A novel type of zirconium-mediated coupling reaction of alkynes with vinyl bromide to afford 2,3-disubstituted dienes has been reported (see Scheme 105), and an inter-intramolecular reaction sequence has been proposed for the observed formation of vinylcyclohexadienes and/or methylenecycloheptadienes from the copper-catalysed reaction of zirconacyclo-pentadienes with allylic dichlorides. The essential step in these processes appears to be transmetallation of the zirconium-carbon bond of the zirconacyclopentadiene to produce a more reactive copper-carbon bond. New phosphorus heterocycles, e.g. (417), have been constructed by the thermal rearrangement of a [l,4-bis(trimethylsilyl)->/ -cyclooctatetraene]- ,3,5-triphospha-7-hafhanorbomadiene complex (416). 

S. A. Blum, V. A. Rivera, R. T. Ruck, F. E. Michael, R. G. Bergman, Synthetic and mechanistic studies of strained heterocycle opening reactions mediated by zirconium(IV) imido complexes, Organometallics 24 (2005) 1647. 

A thriving and highly important field is the construction of coordination complexes from metallic species and heterocycles. These complexes can be useful as reaction catalysts and have other uses as well. To illustrate the catalyst area (which is large), the zirconium complex formed from the anion of indenyhndoyl anion (11.9), and ZrCl4 is offered as an example. The complex has the formula Zr(l 1.9)2CI2 and is an excellent catalyst for the polymerization of olefins. ... 

In the few examples of zirconium-catalyzed domino reactions reported in the literature to date [15], the metal primarily appears to act as a Lewis acid, though mechanistic details have not been established. The reaction of hydrazines with alkynes reported in this chapter proceeds along a complex pathway in which the metal stabilizes a range of reactive intermediates, distinct from those found for titanium-catalyzed hydrohydrazinations. This capabihty merits the development of further applications of the heavier group 4 metals in the synthesis of N-heterocycles [16]. 

Zirconium Schiff-base complex modified SBA-15 (Zr-SBA-15) was used as catalyst in the synthesis of quinoxaline-based heterocycles such as pyridopyrazine, pyrazine, and quinoxaline derivatives [156]. 2,3-Disubstituted quinoxalines among others were prepared, in water media, in good-to-excellent yields. The Zr/SBA-15 catalyst showed an excellent reusability over seven successive runs xmder optimized conditions. 

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