Alkylaluminums exchange reactions

Early studies [103-110] on spectroscopies and kinetics predicted the formation of (alkyl) titanocene complexes (Cp2Ti(R)Cl) (R=Me or Et) by ligand exchange reaction of alkylaluminum cocatalyst... 

It is anticipated that all of the complex reactions observed in the boron system and in the studies on indium and gallium derivatives to be discussed below will be found in alkylaluminum systems. There also will be some additional complications which arise from the fact that the A1R3 moiety often occurs in a dimeric form which will alter the energetics of the reaction and may in some instances govern the mechanism of exchange as observed for alkyl group transfer. 

Mole (11) found that the reaction of AlEt3 with AlMe3 to form mixed alkylaluminums was slower in the coordinating solvent pyridine than in benzene. He hypothesized that at least one of the two aluminums participating in the exchange must be pentacoordinated in the transition state. He stated that in the absence of a coordinating solvent the inter-... 

XX is generally considered as the active species formed from titanium chloride and alkyla-luminum components. The in the structure represents an unoccupied (vacant) site of the octahedral titanium complex. XX represents an active titanium site at the surface of a TiCls crystal after modification by reaction with the alkylaluminum component. The titanium atom shares four chloride ligands with its neighboring titanium atoms and has an alkyl ligand (incorporated via exchange of alkyl from the alkylaluminum for chloride) and a vacant orbital. There are molecular mechanics calculations that indicate dimeric Ti2Cl6 may be the active species instead of monomeric TiCls [Monaco et al., 2000]. Other proposals for the active species include bimetallic species that contain both titanium and aluminum [Liu et al., 2002]. To simplify matters, our discussions will center on the monomeric and monometallic titanium species, especially since the mechanistic details of stereoselectivity and activity are essentially the same for both monomeric and dimeric titanium species as well as titanium-aluminum species. 

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