KINETICS OF INTRAMOLECULAR 4-CENTER ELIMINATION OF ISOBUTYLENE FROM TRIISOBUTYLALUMINUM IN GAS PHASE

The gas-phase kinetics and the mechanistics of the unimolecular elimination of isobutylene from triisobutylaluminum have been studied in the presence of excess ethylene or 1-butene for temperatures ranging from 107 to 173°. The 1-alkenes reacted very fast with the diisobutylaluminum hydride formed in the elimination process, thus avoiding complications from back and side reactions. The reaction is apparently homogeneous, when carried out in an all-Teflon reaction vessel. The computed least-squares analysis of the measured rate constants for the unimolecular elimination process yields (with standard errors) log k (sec-1) = (11.2 ± 0.4) - (26.6 ° 0.1)/θ, where θ equals 4.58 × 10-3T(°K). Experiments with β-D-triisobutylaluminum show that the deuterium is transferred to the aluminum atom in the elimination process. It can be concluded that the reaction involves a relatively tight polar four-center transition state. The observed preexponential factor indicates a loss of entropy of ~12 cal/(deg mole) in forming the cyclic transition state. The general applicability of the concept of four-center reaction mechanisms in the chemistry of aluminum alkyls and their derivatives is outlined. The activation energy of the back-reaction, the addition of isobutylene to the diisobutylaluminum hydride, is estimated at 6 ± 3 kcal/mole. The pertinent thermodynamic data have been reviewed. © 1969, American Chemical Society. All rights reserved.