Kinetics of the intramolecular four-center elimination of isobutylene from triisobutylaluminum in the gas phase

Abstract

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 ± 0.7)/#, where equals 4.58 X 10-3 (° ). Experiments with /3-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.It is generally accepted that reactions involving aluminum alkyls and their derivatives are largely controlled by molecular mechanisms.1 Radical reactions only occur at elevated temperatures, concurrent with the molecular reaction paths.16The preference for molecular reactions (in contrast to most other metal alkyls) originates from the relatively high Al-C and Al-H bond strength involved and from the electron deficiency of the aluminum atom with the valence coordination of three. This leads to the ready formation of electron-deficient or "half" bonds.The majority of the reactions (both intraand intermolecular) involving aluminum alkyls and their derivatives can be rationalized with a four-center transition state. In a few exceptional cases six-membered transition states are operative2-3 as in the addition of butadiene to diisobutylaluminum hydride, where both 1,2 and 1,4 addition have been observed.23 Six-center transition states are possible with a 1,3diene, allowing for six "half" bonds. With 1,4-or 1,5-dienes, two consecutive four-center additions take place.16 With longer chains, the steric effects inhibit the intramolecular addition of the second olefinic bond to the Al-C bond already formed.4 A general concept of four-center and six-center transition states in chemical reactions has been outlined in view of the predictability of the kinetic parameters of such reactions.5 (1) (a) K. Ziegler in "Organometallic Chemistry," . H. Zeiss, Ed.,

Hydroalumination of a Dinuclear Tantalum Dinitrogen Complex: N−N Bond Cleavage and Ancillary Ligand Rearrangement

“…The cleaved N-N bond of both 4 and 5 was confirmed by 15 N NMR spectroscopy using ([NPN]Ta) 2 (µ-η 1 :η 2 -15 N 2 )(µ-H) 2 , 15 N 2 -1, in the reaction with DIBAL. The resulting 15 N-labeled materials both showed resonances that did not display 25,26 and has only one isobutyl group at the Al center (eq 2). Migration of a ligand amide from Ta to Al, reductive elimination of H 2 , and N-N bond cleavage complete the overall scheme.…”

“…Diisobutylaluminum deuteride was prepared by literature methods. 26 Ethylene, propylene (Praxair), and acetylene (Matheson) were used as obtained from commercial gas suppliers. Phenylacetylene was distilled prior to preparation of a toluene solution of known concentration, which was used in synthesis.…”

Hydroalumination of Cyclopropane: A Transition State Study

“…It has been suggested that the reaction involves formation of a symmetric π-complex intermediate leading to the formation of addition product via a four-centered transition state. Experimental work by Egger supports these results 17 . The alternative π-complex transition state proposed by Eisch 18 ; however, is inconsistent with both the theoretical studies and the experimental work of Egger 17 .…”

“…Experimental work by Egger supports these results 17 . The alternative π-complex transition state proposed by Eisch 18 ; however, is inconsistent with both the theoretical studies and the experimental work of Egger 17 . In 1981, Grophen and Haaland characterized the transition structure for the addition of acetylene into the Al-H bond of alane 19 .…”

Theoretical Study of the Hydroalumination Reaction of Cyclopropane with Alane

“…Exploratory work by Egger supports these results. 17 The alternative π-complex transition state proposed by Eisch;…”