Reactions of Group 3 Metal Alkyls in the Gas Phase. Part 10 : The addition of olefins to the monomeric diisobutylaluminiumhydride

The relative rate constants for adding ethylene (k 1 ), propylene (k 2 ) and 2‐methyl‐but‐1‐ene (k 3 ) to gaseous diisobutylaluminium hydride produced in situ from Al i Bu 3 have been measured in the temperature range 104–169° in the presence of an excess of equimolar olefin mixtures. The following temperature dependences of the relative rate constants have been obtained:\documentclass{article}\pagestyle{empty}\begin{document}$$ \begin{array}{*{20}c} {\log \left({{{{\rm k}_1 } \mathord{\left/ {\vphantom {{{\rm k}_1 } {{\rm k}_2 }}} \right. \kern-\nulldelimiterspace} {{\rm k}_2 }}} \right) = {{0.6 - 0.8} \mathord{\left/ {\vphantom {{0.6 - 0.8} {4.58 \times 10^{ - 3} }}} \right. \kern-\nulldelimiterspace} {4.58 \times 10^{ - 3} }}{\rm T}\left({^\circ {\rm K}} \right)} \\ {\log \left({{{{\rm k}_1 } \mathord{\left/ {\vphantom {{{\rm k}_1 } {{\rm k}_3 }}} \right. \kern-\nulldelimiterspace} {{\rm k}_3 }}} \right) = {{1.2 - 2.2} \mathord{\left/ {\vphantom {{1.2 - 2.2} {4.58 \times 10^{ - 3} }}} \right. \kern-\nulldelimiterspace} {4.58 \times 10^{ - 3} }}{\rm T}\left({^\circ {\rm K}} \right)} \\ \end{array} $$\end{document}Two compensating factors determine the rate of addition of olefins to AlH and AlC bonds: (a) the steric effect, reflected in the differences in the preexponential factors and (b) the polar effects, reflected in differences in the activation energies. In the addition of olefins to R 2 AlH bonds in contrast to R 2 AlC bonds, the steric effect (a) does not always overrule the opposing energy effect. At temperatures below 125° e.g., isobutene adds slightly faster to HAl i Bu 2 , than ethylene. These results are in perfect agreement with expectations based on a reaction mechanism involving a tight asymmetric quadrupolar 4‐centre transition state similar to that postulated earlier for the addition of olefins to AlC bonds.