Titanium silyl complexes

It is also possible to carry out a substrate-controlled reaction with aldehydes in an asymmetric way by starting with an acetylene bearing an optically active ester group, as shown in Eq. 9.8 [22]. The titanium—acetylene complexes derived from silyl propiolates having a camphor-derived auxiliary react with aldehydes with excellent diastereoselectivity. The reaction thus offers a convenient entry to optically active Baylis—Hillman-type allyl alcohols bearing a substituent (3 to the acrylate group, which have hitherto proved difficult to prepare by the Baylis—Hillman reaction itself. 

A few dinuclear complexes are known, in which the a-CH bond of a metal-bonded alkyl group is tj2 coordinated to another metal moiety, similar to the previously discussed dinuclear silyl complexes. In the iron complex 22 (75) both metal moieties are the same, and therefore the Fe—C distances [202.5(3) and 211.3(3) pm] can be directly compared without correction. The relative lengthening of the M—C distance of the Fe—H—C three-center bond (4.5%) is comparable to that in the silane-bridged titanium complex 13 (6.5%), corresponding to a similar stage of the oxidative addition. 

The reaction has also been used to make cyclopentenes by adding silylated propargyl cations to alkenes addition to a titanium imide complex has also been reported. ... 

Binaphthol Lewis Acids. In search of Lewis acids which might provide better stereocontrol in these cycloadditions, we turned our attention to l,l -bi-2-naphthol as a potentially useful ligand for the titanium promoter complex. For synthetic reasons, we continued to use benzylic vinyl ethers and silyl vinyl ethers so that these protecting groups could be cleaved to provide the free alcohol later. 

Another way to prepare a reusable catalyst was proposed by Venkataraman. The ligand 25, bearing a linear polyethyleneglycol (PEG) moiety (MW 5 kDa) was used in the titanium-catalysed addition of tri-methylsilyl cyanide to benzaldehyde (Figure 7.1). The silylated cyanohydrin was obtained in more than 95% yield and 86% enantiomeric excess after 24 h at room temperature, with only 0.1 mol% of catalyst. The titanium-salen complex was separated from the reaction mixture by dialysis by means of a Soxhlet apparatus. 

Scheme 5.63 Vinylogous aldol addition of silyl dienolate 203a mediated by titanium-BINOL complex enantioselective synthesis of phorbaside A building block 205.

A combination of diethylzinc with sulfonamides 18 or 19 offers another possibility for the enantioselective acetate aldol reaction39,41. The addition of silyl enol ethers to glyoxylates can be directed in a highly enantioselective manner when mediated by the binaphthol derived titanium complex 2040. 

Addition of the (l-silylalkyne)titanium complex to carbonyl compounds and imines occurs at the (3-position to the silyl group, as shown in Fig. 9.2. However, the reaction with sBuOH takes place exclusively at the carbon—titanium bond a to the silyl group to give the (P-silylalkenyl)titanium species, as in Eq. 9.5 (values in square brackets denote the regioselectivity) [24], where the vinyl—titanium bond is visualized by the outcome of the iodi-nolysis. The overall reaction can therefore be regarded as the hydrotitanation of silylace-... 

The aldol condensation of benzaldehyde with the thioacetamide carbanion (RCHCSNRV), derived from the desilylation of the silyl-thioether with tetra-/i-buty-lammonium fluoride, is stereoselective at—80°C producing the erythro isomer of the p-hydroxy thioamide preferentially (Scheme 6.18, R = Me, erythro threo 95 5) via a conformationally mobile intermediate. However, when R is bulky, a greater amount of the threo isomer is formed. Predictably, as the reaction temperature is raised, so the stereoselectively decreases. This procedure contrasts with the corresponding condensation catalysed by titanium salts, where the complexed intermediate produces the threo isomer [47, 48],... 

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... 

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