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Zusammenfassung:
The conversion of classical carbanions such as RMgX, RLi, or deprotonated nitriles, sulfones, and carboxylic esters into titanium analogs results which add chemoselectively to carbonyl compounds in the presence of other functional groups. The standard titanating agent is chlorotrisopropoxytitanium (1). Grignard-type reactions and aldol additions are aldehyde-selective in the presence of ketones. Other functional groups such as alkyl and aryl halides, esters, amides as well as nitro and cyano moieties are tolerated. Discrimination between two aldehydes or two ketones is also possible. Replacing alkoxy ligands by methyl groups at titanium increases reactivity dramatically, relatives rates increasing in the series CH3Ti(OCHMe2)3 < (CH3)2Ti-(OCHMe2)2 < (CH3)4Ti. The latter reagent and its zirconium analog methylate sterically hindered and/or enolizable ketones which normally fail to undergo Grignard reactions. The ate complex H2C=CHCH2Ti(OCHMe2)4MgCl (63) is aldehyde-selective, while the amino analog H2C=CHCH2Ti(NMe2)4MgCl (64) adds selectively to ketones in the presence of aldehydes.
