Cycloalkenes 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). 

The hydroalumination of alkenes with BujAlCl catalyzed by Cp2ZrCl2 produces higher dialkylaluminum chlorides, which cannot be prepared by non-catalytic hydroalumination (Scheme 2-12) [63-65]. Terminal alkenes, internal linear alkenes and cycloalkenes can serve as substrates at reaction temperatures increasing in this order. 1,5-Dienes react to give cyclized products. 

The hydroalumination of cycloalkenes exhibits two interesting minima in relative rates toward diethyl-aluminum hydride. The half-life (in min) of a 3 1 ratio of Et2AlH and cycloalkene at 78 °C varied thus with ring size C4, 40 C5, 200 Ce, 1880 C7, 110 Cg, 86 C9, 280 Cio, 2500 Cn, 1350. The reactivity of cycloundecene is approximately that of an open-chain internal alkene. The low reactivity of cyclohexene can be ascribed to the eclipsing repulsions generated in the transition state by the syn attack of Et2AlH on the C=C bond (equation 17). ... 

In contrast with the impracticality of hydroaluminating open-chain internal alkenes, all the unsubstituted cycloalkenes can be usefully hydroaluminated, with titanium catalysis if need be, since isomerization of the C=C bond is of no significance. 

The extent and types of interfering reactions during hydroalumination of alkynes were examined in Section 3.11.2.2 (equations 18-24). To a lesser degree two of these reactions are also encountered in A1—H additions to alkenes and cycloalkenes. Analogous to the reductive dimerization of alkynes (equation 23) is the dimerization of a-alkenes by Bu 2AlH, which can be conducted catalytically in hydride to give high yields of dimer (equation 30). ... 

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