R 3-allyltitanium

A similar result obtained with nopadiene is presented above (Scheme 13.9). Linear and cyclic trienes furnish alkenyl-r 3-allyltitanium complexes, which can react with aldehydes to produce dienyl alcohols (Scheme 13.13) [24],... 

Efforts have been made to apply r 3-allyltitanium chemistry to the asymmetric synthesis of homoallylic alcohols and carboxylic acids. The synthesis and reactions of chiral r 3 -allyl-titanocenes with planar chirality, or containing Cp ligands with chiral substituents, have been reported [6c,15,30—32]. The enantiofacial selectivity in the allyltitanation reactions has been examined for the complexes 12 [15], 13 [30], 14 [31], 15, 16, and 17 [32] depicted in Figure 13.2. 

These c-complexes have been less extensively studied than the r 3-allyltitanium derivatives. r 1 -Allyltitanocenes can readily be prepared from the corresponding magnesium compounds by reaction with Cp2TiCl2 or by reaction of preformed r 3-allyltitanium complexes with but-2-enyl halides [36]. Crotyl-type reagents, which are accessible only in the E-iso-meric form, add to aldehydes with an anti selectivity (Scheme 13.17). 

A series of q3-allyltitanium compounds has been prepared, and their jr-structures have been confirmed by spectroscopic methods and X-ray analysis [9]. In these complexes, the alkyl substituents at C-l and C-3 preferably occupy the syn position with respect to the H (or R) at C-2 due to steric reasons. The proposed mechanism for their formation is outlined in Scheme 13.4 [8]. 

As far as the stereoselectivity of the addition is concerned, allyltitanium reagents, unlike their alkyl analogs (Section 1.5.3.1.3), give preferentially axial addition to 4-r-butylcyclohexanone. The selectivity... 

Allylation. Allyl halides are converted to allyltitanium compounds by r-PrMgBr/jr-PrOj Ti, which can be used for allylation of carbonyl compounds and aldimines. With imines derived from a chiral a-phenethylamine, the reaction is highly diastereoselective following the Cram pattern exhibiting a 1,3-asymmetric induction. 

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