Zirconium alkoxy complexes

Figure 4 shows the remarkable structural similarity between the bimetallic carbene (1 2) and alkoxy complexes formed from diverse paths 1,2 addition of Zr-H to a carbonyl bound to tungsten (eq. 1) and 1,1 addition of Re-H to a zirconium-bound acetyl (eq. 2). 

In nonpolar solvents, for example alcohols, the hydroxyls of the support can also be used to anchor alkoxy compounds to the surface in a condensation reaction, in which one alkoxy ligand reacts with the proton of the surface OH to give the corresponding alcohol, and the complex binds to the support. An example is the anchoring of zirconium ethoxide, Zr(OC2H5)4, to silica by means of the reaction... 

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. 

Less acidic than Ti and Zi chloroderivatives, MeTi(OPr )3 perfoims chelation-controlled addition to chiral alkoxy ketones as well as or better than organomagnesium compounds, but fails to chelate to aldehydes or hindered ketones. Should the formation of a cyclic chelation intermediate be forbidden, the reaction is subject to nonchelation control, according to Ae Felkin-Anh (or Comforth) model. Under these circumstances, the ratio of the diastereomeric products is inverted in favor of the anti-Cram product(s). In the case of benzil (83 Scheme 7) this can be accounted for by the unlikely formation of a cyclic intermediate such as (85), and thus the preferential intermediacy of the open chain intermediate (86) that leads to the threo compound (88). This view is substantiated by the fact that replacement of titanium with zirconium, which is characterized by longer M—O bonds, restores the possibility of having a cyclic intermediate and, as a consequence, leads to the erythro meso) compound (87) thus paralleling the action of Mg and Li complexes. 

The molecular complexity of metal alkoxides also depends on the steric hindrance of alkoxy groups. Bulky secondary or tertiary alkoxy groups tend to prevent oligomerization. Trimeric species [Ti(OEt)4]3 have been evidenced in pure liquid titanium ethoxide (Fig. lb) whereas titanium iso-propoxide Ti(OPr )4 remains monomeric (Fig.la). This is no more the case for zirconium iso-propoxide which is dimeric because of the larger size of Zr(rV). Moreover solvent molecules can also be used for coordination expansion leading to solvated dimers [Zr(OPri)4(Pr OH)]2 when the alkoxide is dissolved in its parent alcohol (Fig.lc). 

Cationic zirconium complexes (88) having diketonato ligands were examined for selective epoxide ring opening (Equation 40) [45]. The catalysts showed higher activity than simple metal halides such as TiCU and ZrCU, and various epoxides (87) were converted into (89) and/or (90) effectively. When R and R" in (87) were alkyl groups, the product selectivity of the catalyst (88) were moderate to low. But with styrene oxide (R =Ph, R" =H), alkoxy alcohol (89) was obtained with high selectivity. 

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