Titanium complexes, reaction with carbon alkyls

A mechanistically obscure transformation occurs upon treatment of the tetramethylfulvene titanium complex 161 with methallyl Grignard, producing bridged titanacyclobutane complex 162. This reaction is proposed to proceed by intramolecular alkylation at the central carbon of an 7]4-fulvene, 7]3-methallyl intermediate, but with due consideration... 

The chemical reactivities of such titanium homoenolates are similar to those of ordinary titanium alkyls (Scheme 2). Oxidation of the metal-carbon bond with bromine or oxygen occurs readily. Transmetalations with other metal halides such as SnCl4, SbClj, TeCl4, and NbCls proceed cleanly. Reaction with benzaldehyde gives a 4-chloroester as the result of carbon-carbon bond formation followed by chlorination [9]. Acetone forms an addition complex. No reaction takes place with acid chloride and tm-alkyl chlorides. 

Titanium acetylides react with 3-benzyl-tetrahydro-l,3-oxazines and 1,3-oxazolidines to give the corresponding / -aminoacetylenes in modest to good yield.296 Vinyl Ti(iv) species prepared by the alkylation of vinylcarbene complexes with BTCl react with aldehydes to give allylic alcohols. Reaction with terminal alkynes produces conjugated dienes, in which a vinyl group regioselectively bonds to the unsubstituted side of carbon-carbon triple bond.297... 

Alkyl complexes of titanium have been extensively studied both from the point of view of the nature of the metal-carbon bond as well as their application as alkylating reagents in organic reactions [4-6,20,21], The simplest methyl complex, MeTiCl3 (8), is readily obtained by the reaction of TiCl4 with half an equivalent of ZnMe2 [22,23]. An ether adduct of 8 is synthesized by treatment of TiCl with an ethereal solution of MeLi [24]. Alkoxy substitution on titanium stabilizes the methyl complexes. Reaction of TiCl(0 Pr)3 (9) with MeLi affords MeTi(O Pr)3(10), which is thermally very stable and can be distilled under reduced pressure (48-53 °C/0.01 mmHg) [25], The phenyl derivative 11 is prepared by the similar reaction of 9 with PhLi. 

Teuben et al. reported a related reaction, in which the thermolysis of alkyl-bis(pentamethylcyclopentadienyl)titanium(III) caused a dissociation of alkane with formation of 64. further heating at 150°C resulted in hydrogen dissociation and formation of 70 as a diamagnetic material. Subsequent reaction with acetophenone gave chelate complex 71 in 60% yield, which was fully characterized spectroscopically as well as by an X-ray structure analysis (Scheme 10.24). Interestingly, the authors do not mention the presence of two diasteromers, although the alkyl chain bears an asymmetric carbon atom in addition to the planar chirality of the metallocene. Similar reactions were observed with pyridine derivatives [72]. 

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

The following two examples from our recent work also illustrate the limitations of our systematic approach to Lewis acid promoted alkylations of aliphatic tt systems. The reaction of cumyl chloride 26 with tetramethyl-ethylene in presence of various Lewis acids gave complex mixtures of products, probably because of the strain generated during the formation of the regular addition product. Titanium tetrachloride, however, induces a rapid consecutive cyclization, and the TiCl catalyzed reaction of 26 with tetramethylethylene yields hexamethylindan in 72% yield. Since the aromatic ring can be oxidized under Ru(VIII) catalysis, the reaction sequence shown in Figure 16 allows the construction of acyclic compounds with adjacent quaternary carbon atoms. 

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