Insertion of the Al atom into alkyl ethers: Semiempirical SCF MO and matrix isolation ESR study

Abstract

The insertion process of the Al atom into trimethylene oxide (TMO) and that into dimethyl ether (DME) were examined by a semiempirical SCF molecular orbital method (AM1) and by matrix isolation ESR spectroscopy. The MO study revealed the three step process: (1) an approach of the Al atom toward the ethereal oxygen along the C2v axis of the molecule driven by a favorable overlap between the aluminum SOMO (the 3pz orbital) and the LUMO of the molecule of A1 symmetry, (2) formation of the Al-O dative bond with concurrent scission of one of the C-O bonds, and (3) completion of the process by the three-electron bonding scheme between the unpaired electron on the alkyl terminus and the lone pair electrons on Al. The MO study also revealed that the process would be spontaneous for the Al/TMO system, but for the Al/DME system, there existed a barrier of ∼8 kcal/mol for the onset of the methyl group cleavage from the dative complex Al:O(CH3)2. The MO study also revealed that, in the Al/DME case, the insertion product CH3-Al-O-CH3 of the cis conformation was initially formed but it might readily convert to the more stable trans form. The ESR study revealed that the Al atom insertion into TMO occurred spontaneously. Well-resolved spectra of the expected insertion radical CH2-Al-O-CH2-CH2 were observed. In the case of DME, the ESR study revealed the presence of both the intermediate dative complex and the final insertion product in the as-prepared matrix. When the matrix was subsequently irradiated with red light (λ = 700 ± 50 nm), the dative complex resumed the process and converted to the insertion product. The insertion radicals of both the cis and trans forms were observed in the argon system, but only the more stable trans radical was observed in the neon system.