1,5-Hexadiene hydroalumination

Bis(diamino)alanes (R2N)2A1H were used for the hydroalumination of terminal and internal alkenes [18, 19]. TiCb and CpjTiCb are suitable catalysts for these reactions, whereas CpjZrCb exhibits low catalytic activity. The hydroaluminations are carried out in benzene or THF soluhon at elevated temperatures (60°C). Internal linear cis- and trans-alkenes are converted into n-alkylalanes via an isomerization process. Cycloalkenes give only moderate yields tri- and tetrasubstituted double bonds are inert. Hydroaluminahon of conjugated dienes like butadiene and 1,3-hexa-diene proceeds with only poor selechvity. The structure of the hydroaluminahon product of 1,5-hexadiene depends on the solvent used. While in benzene cyclization is observed, the reaction carried out in THF yields linear products (Scheme 2-10). 

Nickel catalysts promote the hydroalumination of alkenes using trialkylalanes R3AI and dialkylalanes such as BU2AIH as the aluminum hydride sources [9, 29, 30, 33]. However, exhaustive studies of the range of substrates capable of hydroalumination with these reagents has not been carried out. Linear terminal alkenes like 1-octene react quantitatively with BU3AI at 0°C within 1-2 h in the presence of catalytic amounts of Ni(COD)2 [30]. Internal double bonds are inert under these conditions, whereas with 1,5-hexadiene cycHzation occurs. 

A scattered but tantalizing array of reactions have been observed, in which hydroalumination brings about the closure of open-chain compounds to carbon rings. Perhaps the most typical of such reactions is the monohydroalumination of 1,5-hexadiene at - 0 °C, which is followed by an intramolecular carb-alumination (equation 72). 5,i66... 

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