Controlled Reversal of Chemoselectivity Using Titanium Ate Complexes

VII Controlled Reversal of Chemoselectivity using Titanium Ate Complexes 

6 The chemoselectivity studies involving 6 (Sect. C.II) were performed with distilled reagent. In situ procedures were used for ethyl and n-butyl derivatives (9 and 10 plus Li salts), the reagent being completely aldehydeselective. 

Upon testing the effect of other ligands at titanium, we came across some unexpected results 90). The amino derivative 102, prepared from 96 and IS, reacts in situ preferentially with ketones. In case of 26 and 82 the 100 101 ratio is 22 78 (Table 3). The in situ reaction of the related compound 104 leads to a 13 87 ratio. By using the ate complexes 106, 108 and 110, the ketone-selectivity turned out to be 4 96, 2 98 and 14 86, respectively  

Although a few observations relevant to the mechanism have been made, it is difficult to offer a clear explanation at this time. Initial control experiments indicate that the reactions are kinetically controlled, i.e., they are irreversible under the conditions used. Furthermore, a salt effect is involved, because the distilled amino-titanium compound 102 is essentially non-selective (Table 3). Upon adding MgX2 salts, ketone selectivity increases to 70%. 

Intrigued by the above results, we tested the generality of the above phenomena. Indeed, upon using benzaldehyde 19 and acetophenone 20 (eq. 39), very similar results were obtained (Table 4). 

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