Titanium allyltitanium complexes

The titanium reagent 1 reacts with allyl alcohol derivatives, such as halides, acetates, carbonates, phosphates, or sulfonates to afford allyltitanium complexes of the type (r 1-al-lyl)TiX(OiPr)2, as shown in Eq. 9.21 [42], As a variety of allylic alcohols are easily obtained,... 

Allyltitanium complexes (22) readily add to carbonyl compounds with high regio- and stereo-selection. They are prepared by reaction of a chlorotitanium complex (21) with an allyl-magnesium or -lithium derivative (equation 13). Some of these unsaturated Ti complexes, like (23)-(25) in Scheme 2, obtained from allylmagnesium halides or allyllithium by reaction with titanium tetraisopropoxide or titanium tetramides, are known as titanium ate complexes . The structure of these ate complexes, at least from a formal point of view, can be written with a pentacoordinate Ti atom. Some ate complexes have synthetic interest, as is the case of (allyl)Ti(OPr )4MgBr which shows sharply enhanced selectivity towards aldehydes in comparison with the simple (allyl)Ti(OPr )3. ... 

A few examples of chemoselective additions of allyltitanium reagents to aliphatic and aromatic carbonyl compounds are reported in Table 7. Appreciable chemoselectivity toward the aldehydic function is achieved by the titanium ate complex (23), whereas the reverse chemoselectivity toward ketones is realized using aminotitanium complex (104) and the analogous ate complexes (24) and (25), as is shown in Table 7. This is very interesting since it represents a rare case of chemoselectivity in favor of carbanion addition to ketones. A tentative explanation of this inverse chemoselection considers a fast transfer of the aminyl ligand onto the aldehyde faction which becomes protected , as in (105), and thus unreactive in respect to the keto group. Ketones react also selectively compared with esters, as is shown by the reaction of ethyl levulinate (1) with the ate complex (23 equation 40). ... 

Allyltitanium ate complexes.8 In contrast to organotitanium compounds, which are aldehyde selective, the chemoselectivity of allyltitanium ate complexes depends on the ligands attached to titanium. The most aldehyde-selective allyltitanium ate complex prepared to date is CH2=CHCH2Ti[OCH(CH3) >].jMgCl (1) which reacts... 

Allyltitanium compounds and Ti enolates derived from mono-Gp chloro titanium complexes with two chiral alkoxo ligands add to aldehydes with high enantioface discrimination.973... 

The chiral allyltitanium reagents prepared from the reaction of allyl Grignard or lithium reagents with chiral titanium complexes have been used stoichiomet-rically for the allylic addition to aldehydes with excellent enantioselectivities [71] (Scheme 14.16). Various applications of this methodology in the synthesis of natural products have been reviewed in several articles [72]. 

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