Abstract
Treatment of Ti (O-i-Pr) 4 with 2 equiv of i-PrMgX (X =Cl or Br) provides (η2-propene) Ti (O-i-Pr) 2 in essentially quantitative yield. This new low-valent titanium complex nicely acts as a versatile titanium (II) equivalent to effect the following unique transformations. Thus, alkynes afforded titaniumalkyne complexes which, in turn, react with aldehydes, ketones, or imines to furnish the corresponding addition products. Allylic or propargylic compounds such as halides and alcohol derivatives gave allylic- or allenylic titanium species, which could be utilized in chemo- and diastereoselective reactions with carbonyl compounds. While acetylenic esters resulted in an intramolecular nucleophilic acyl substitution (INAS) reaction to provide unsaturated ketones, olefinic ones furnished cyclopropanols. 1, 6- or 1, 7-Dienes, enynes, and diynes underwent cyclometallation to give dialkoxytitanacycles, which show some distinctive reactivities among the metallacycles of early transition metal complexes. A few applications to natural product synthesis by taking advantage of these methods are also illustrated.
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