Titanocenes allyl complexes

Recently Moise and coworkers reported the first catalytic allyltitanation of aldehydes with dienes using only 5 mol% of titanocene complexes at the expense of employing stoichiometric amount of polymethylhydrosiloxane (PMHS) as the reductant [76]. The catalytic cycle includes the cleavage of the titanium alkoxide with a concomitant regeneration of titanocene(III) hydride species with the aid of silyl hydride. The very sensitive and reactive complex [Cp2TiH] was trapped by diene to give Ti-allylic complex (Scheme 14.19). 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

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. 

In both GOMC (1982) and COMC (1995), the syntheses of a variety of different titanocene complexes with allyl-supporting ligands were described.54,59 In the last 10 years, research into these species has focused on their reactivity. 

Cross-metathesis of the allyl-Cp substituted titanocene dichloride leads to the formation of dinuclear titanium complexes (Scheme 12.25, M = Ti, n= 1) [33]. The composition of the reaction mixture depends on the catalyst used. Treatment of the substrate with Ru-I (3 mol%) in benzene, toluene, or dichloromethane gave a mixture of Z and E isomers in a 1 1 ratio, while application of the Ru-II catalyst resulted with the formation of the pure E isomer. 

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