SYNTHETIC REACTIONS WITH DIVALENT TITANIU M COMPLEX

Treatment of Ti(O-i-Pr)(4) with 2 equiv of i-PrMgX (X = Cl or Br) prov ides (eta(2)-propene) Ti(O-i-Pr)(2) in essentially quantitative yield. This new low-valent titanium complex nicely acts as a versatile titan ium (II) equivalent to effect the following unique transformations. Th us, alkynes afforded titanium-alkyne complexes which, in turn, react w ith aldehydes, ketones, or imines to furnish the corresponding additio n products. Allylic or propargylic compounds such as halides and alcoh ol derivatives gave allylic- or allenylic titanium species, which coul d be utilized in chemo-and diastereoselective reactions with carbonyl compounds. While acetylenic esters resulted in an intramolecular nucle ophilic acyl substitution (INAS) reaction to provide unsaturated keton es, 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 ea rly transition metal complexes. A few applications to natural product synthesis by taking advantage of these methods are also illustrated.