Hydrazones titanated

Reactions of titanated hydrazones with aldehydes occur cleanly at —20 °C (Equation 69). It is not clear whether the observed erythro-selectivity (Table 8) depends upon the geometry of the double bond, since attempts to prepare Z configurated analogs were not rewarding115). The lithiated precursors themselves are unsuitable for selective additions. Titanation not only increases stereo-differentiation, but also chemoselectivity 115). The assumption of a pericyclic transition state means that chair orientations 213 and 214 must be considered, the latter being of higher energy. However, boat transition states also explain the results U5). 

Table 8. Erythro-selective Addition of Titanated Hydrazones 209 to Aldehydes...

Syn selective aldol additions of titanated aldehyde hydrazones and ketone hydrazones have been reported by Reetz. The observed syn selectivity parallels the syn selectivity seen in titanium ketone eno-iates, and the intermediate titanium aldehyde hydrazone derivatives were seen to have ( )c—c geometry (equation 17). 

Cleavage of 2,4-dinitrophenylhydrazones. Carbonyl compounds can be regenerated in high yield (80-95%) from the 2,4-DNP derivatives by treatment in DME with a 20% aqueous solution of TiCls at reflux temperature (N2). McMurry and Sylvestri consider that titanous ion reduces the nitro groups to amino groups and then cleaves the hydrazone N—N bond to generate an imine, which is then hydrolyzed readily to the carbonyl compound. 

Benzeneseleninic anhydride (PhSe0)20 is highly recommended for the regeneration of ketones from hydrazones, oximes, and semicarbazones. Ketones have been regenerated from the same range of derivatives using a stoicheiometric amount of titanous chloride in buffered acetic acid and dioxan. ... 

Titanated aldehyde hydrazones add to aldehydes to give cryrAroselective products (Scheme 24). ... 

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