Silyl-zirconium complex

A zirconium-silene complex was obtained during the course of our study on the synthesis of a zirconium-silyl complex. The zirconium-silene complex is formed from disilylzirconocene 2a, generated from Cp2ZrCl2 and 2 equivalents of Me2PhSiLi 8b. Zirconium-silene complex 3 should be in a state of equilibrium with zirconacyclopropane 3. The insertion of diaryl alkyne 14 or vinyl alkyne... 

Eq.3. Synthesis of hafnium and zirconium trichlorotris(tiimethylsilyl)silyl complexes (3,4). 

The first observation of CO insertion into a transition metal-silicon bond was made recently83,249. Zirconium silyl Cp2Zr(SiMe3)Cl reacts rapidly with carbon monoxide with precipitation of a bright pink complex that has been identified as the silaacyl compound 46 (equation 94). An X-ray crystal structure of 46 confirmed the formation of a... 

Although in the recent years the stereochemical control of aldol condensations has reached a level of efficiency which allows enantioselective syntheses of very complex compounds containing many asymmetric centres, the situation is still far from what one would consider "ideal". In the first place, the requirement of a substituent at the a-position of the enolate in order to achieve good stereoselection is a limitation which, however, can be overcome by using temporary bulky groups (such as alkylthio ethers, for instance). On the other hand, the ( )-enolates, which are necessary for the preparation of 2,3-anti aldols, are not so easily prepared as the (Z)-enolates and furthermore, they do not show selectivities as good as in the case of the (Z)-enolates. Finally, although elements other than boron -such as zirconium [30] and titanium [31]- have been also used succesfully much work remains to be done in the area of catalysis. In this context, the work of Mukaiyama and Kobayashi [32a,b,c] on asymmetric aldol reactions of silyl enol ethers with aldehydes promoted by tributyltin fluoride and a chiral diamine coordinated to tin(II) triflate... 

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. 

The unsymmetrical silyl-hydrido zirconium dimer, Cp2Zr(SiPhMeH) (p-H)2(SiPhMe2) ZrCp2, similar to the above-mentioned complex 6, was isolated from the dehydrogenative coupling reaction of phenylmethylsilane with dimethylzirconocene. The X-ray analysis of the dimer shows a disorder which is produced by the ordered superposition of two... 

Friedel-Crafts alkylation has been used in an important synthesis of aryl C-glycosides, which are potent anti-tumor agents, from glycosyl fluorides (equation 99)65 661. The reaction takes place rapidly in dichloromethane, at room temperature using a novel zirconium complex and silver perchlorate combination catalyst. A similar alkylation has been performed by replacing the aromatic compound with either a silyl enol ether or an allylic compound using silver triflate as the catalyst662,663. 

The reactivity of a silyl-zirconium complex is interesting because an unsaturated bond would be inserted into the silyl-zirconium bond to provide an alternative zirconium complex. It has zirconium-carbon and silyl-carbon... 

Notably, phospholes with controlled regioselectivity can also be obtained from zirconium complexes using silylated alkynes. For example, sequential treatment of Schwartz reagent 167 with 2-butyne, MeLi, and silylated alkynes... 

Kobayashi and co-workers. used zirconium-based bromo-BINOL complex for the catalytic enantioselective Mannich-type reaction. The o-hydroxyphenyl imine 3.36 chelates the Zr(IV)(BrBINOL)2 to form the activated chiral Lewis acid complex A. The ketone acetal 3.37 reacts with the Lewis acid complex A to give the complex B. The silyl group is then transferred to the 3-amino ester to form the product 3.38 and the catalyst Zr(BrBINOL)2 is regenerated, which is ready for binding with another imine molecule (Scheme 3.16). 

---