Allyltitanium species

Titanocene-catalyzed cycloisomerization of dienynes 181 provided terminal allenes 182 in moderate yields (Scheme 3.91) [128]. The reaction proceeded via the titanacycle 183 and the allene 184 was formed via loss of a /3-hydrogen atom from the allyltitanium species. 

Variable enantioselectivities are obserx presence of ligand 35. - P-Hydroxyaldehydi with a chiral allyltitanium species 36. " wh pared from 37 by various organometallic n mercially available and inexpensive, are mediated allylation, even though they iiv Allylation such as that effected by 38 affon lation. ... 

Allyltitanium species are also obtained from a diene and bis(T -indenyl)titanium dichloride in the presence of i-PrMgCl. ... 

Several substituents can be present in the carbon skeleton of 88. The carbonate of 3,5-dienyl alcohol 89 also underwent similar cydization to lead to the corresponding allyltitanium species 90 also having a lactone moiety. This allylic moiety exists most probably as an internal titanium derivative stabilized by intermolecu-lar coordination. The addition of aldehydes leads to the corresponding alcohol via a srx-membered transition state (Scheme 12.60) [78]. 

In contrast to the allyltitaniums derived from acrolein cyclic acetals, such as 1,2-dicyclo-hexylethylene acetal shown in Scheme 9.8, those derived from acrolein acyclic acetals react with ketones and imines exclusively at the y-position. As shown in Eq. 9.29, the reaction with chiral imines having an optically active 1-phenylethylamine moiety proceeds with high diastereoselectivity, thus providing a new method for preparing optically active 1-vinyl-2-amino alcohol derivatives with syn stereochemistry [53], The intermediate allyltita-nium species has also found use as a starting material for a carbozincation reaction [54],... 

The versatile Ti(II) chemistry available using preformed (alkene)Ti(OiPr)2 species was opened up by the discovery of the Kulinkovich cyclopropanation reaction [55]. Since 1995, Sato and collaborators have developed a wide range of elegant and synthetically useful reactions based on the Ti(OiPr)4/iPrMgCl reagent [56]. In particular, it was reported that the Ti(II) complex (q2-propene)Ti(OiPr)2, preformed from Ti(OiPr)4 and 2 equivalents of iPrMgCl, reacts with allylic compounds, such as halide, acetate, carbonate, phosphate, sulfonate, and aryl ether derivatives, to afford allyltitanium compounds as depicted in Scheme 13.27 [57]. 

Allylation of imines.1 A low-valent Ti(0) species generated by reduction of TiCl4 with aluminum foil in THF can effect allylation of imines with allyl bromide, even when used in catalytic amounts (0.05 equiv.). This combination of a catalytic amount of TiCU with 1 equiv. of aluminum presumably generates Al(III) and Ti(0), which reacts with the allyl halide to form an allyltitanium, the reactive species. 

The preparation of allyltitanium compounds including those having functional groups is described by reaction of allylic halides or allylic alcohol derivatives with the system Ti(OPr1)4/MgXPr1 (X = C1, Br) (Scheme 7).24 Analogous allyltitanium complexes have also been reported by treatment of Ti(n) species with allylic alcohol derivatives, which proceeds via an oxidative addition pathway. Their reactions have been studied.25-27 These compounds are used to promote efficient syntheses of alkylidenecyclopropane and cycloalkane derivatives by regioselective reactions with carbonyl compounds,28,29 the stereoselective syntheses of optically active substituted piperidines and pyrrolidines... 

---