Trichlorotitanium homoenolate

Although two possible mechanisms are conceivable (A and B), the authors have been able to isolate trichlorotitanium homoenolates as intermediates, indicating that the second one is really operating (Scheme 5.11). 

Trichlorotitanium homoenolate 2 smoothly adds to aldehydes at 0 °C [9, 10]. Due to the strongly acidic reaction conditions, however, addition products of aromatic aldehydes tend to undergo further transformations Eq. (29). The trichlorotitanium homoenolate does not react with ketones (Scheme 2). 

Ligand exchange provided a simple and effective solution to these problems. Addition of 0.5 eq. of Ti(0 Pr)4 to the trichlorotitanium homoenolate produces an alkoxytitanium species 19 which is more reactive than the original homoenolate, Eq. (30). This allows the addition to proceed under nearly neutral conditions. Preparation of 4-hydroxy esters by this method is summarized in Table 5. The structure of 19 has been studied by H NMR spectroscopy [19]. 

Trichlorotitanium homoenolates (10), which can be generated by the reaction of 1-alkoxy-l-silyl-oxycyclopropanes (8) with TiCU, ° smoothly add to aldehydes at 0 °C (Scheme 11). They are not, however, sufficiently reactive to add to ketones. 

Addition of 0.S equiv. of Ti(OPi )4 or Ti(OBu )4 to the trichlorotitanium homoenolate (10) produces an alkoxytitanium species (11 Scheme 12) which readily adds to ketones. The basic trend of the chemical behavior, stereo- and chemo-selectivities of (11) is similar to that of simple titanium alkyls. ... 

Two groups independently reported the formation of titanium homoenolates by the transmetalation reaction of 3-stannyl-propionate esters with TiCl, Eq. (48) [45, 46]. Amide homoenolates become available along this route [47], The trichlorotitanium species thus obtained have been shown ( H NMR) to be similar to that generated along the siloxycyclopropane route and indeed exhibit very similar reactivities. This method does provide a conventient alternative to the siloxycyclopropane route. 

Titanium homoenolates can be prepared by transmetallation of 3-stannyl esters wiA TiCU (Scheme 13) 30,31 trichlorotitanium species (10), shows reactivity very similar to that obtained by the silyl-oxycyclopropane route. 

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