Trichlorotitanium enolates

Trichlorotitanium enolates. These Ti enolatcs are formed generally in high yield by reaction of enol silyl ethers with TiCE at 20-35° in CHiCE." The corresponding reaction with SnCE results in a-trichlorostannyl ketones." The stability of these Ti enolates varies from a few seconds to a few days, depending on the structure. As in the case of... 

Other titanium enolates, these trichlorotitanium enolates are Avn-selective in aldol reactions. 

Trichlorotitanium enolates are directly prepared from a ketone, TiCU, and a tertiary amine [122,123] and undergo aldol reactions with aldehydes [124-129], ketones [129], and imines [130,131], Intramolecular condensation with esters is also known [132-137], Although these reactions, based on a titanium enolate [16], which often results in high diastereoselectivity in aldol and related reactions [122], will not be discussed in detail in this article, the success of the alkylation of this titanium enolate with SNl-active electrophiles should be discussed owing to the high Lewis acidity of the metal center [123], Equation (37) shows stereoselective alkylation with an orthoacetate, which is usually inert to alkali metal enolates [138], Aminoalkylation of trichlorotitanium enolates with (a-chloroalkyl)amine has been performed analogously [139,140],... 

Enolate formation. Trichlorotitanium enolates of ketones and aldehydes are... 

Trichlorotitanium enolates are formed in variable yield from trimethylsilyl enol ethers and an equivalent of TiCU in dichloromethane at 20-35 C. These highly Lewis acidic preformed enolates then undergo aldol reactions at -70 C to give moderate levels of syn selectivity, as in equation (13). Trichlorotitanium enolates have also been used by Reetz et al. in their studies on diastereofacially selective aldol additions to a-alkoxy aldehydes.Trichlorotitanium enolates are formed in situ in the aldol reaction of aromatic ketones and aldehydes using TiCU and EtsN. ... 

Although the mechanism of the Mukaiyama reaction is not yet fully understood, several points have now been firmly established (a) a Lewis acid enolate is not involved (b) the Lewis acid activates the carbonyl group for the nucleophilic addition and (c) the Si—O bond is cleaved by nucleophilic attack of the anionic species, generally halide, on silicon. Point (a) has been established by the use of INEPT- Si NMR spectroscopy. Moreover, trichlorotitanium enolates have been synthesized, characterized and shown to give a completely different stereochemical outcome than the TiCU-mediated reactions of silyl enol ethers. Complexes between Lewis acids and carbonyl compounds have been isolated and characterized by X-ray crystallography and recently by NMR spectrometry. On the basis of these observations closed transition structures will not be considered here open transition structures with no intimate involvement between the silyl enol ether and the Lewis acid offer the best rationale for the after the fact interpretation of the stereochemical results and the best model for stereochemical predictions. 

---