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 C 2V axis of the molecule driven by a favorable overlap between the aluminum SOMO (the 3p z orbital) and the LUMO of the molecule of A 1 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 threeelectron 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(CH 3 ) 2 . The MO study also revealed that, in the Al/DME case, the insertion product CH 3 -Al-O-CH 3 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 CH 2 -Al-O-CH 2 -CH 2 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.

What can we learn from two‐center three‐electron bonding with the topological analysis of ELF?

“…The competitive formation of σ (i.e., 2c-3e)-type or π-type bonding has also been evidenced in the photochemical oneelectron oxidation of aromatic sulfides [40] and has been shown to be sensitive to the steric and electronic influence of substituents. Finally, although it is formally out of the scope of heteroatom chemistry, 3e bonds involving metal atoms exist: the ferryl group Fe O in di-iron enzymes core models [41], the Al І N or the Al І C bonds that are formed spontaneously after insertion of the Al atom into alkyl ethers [42] or into the NH 3 molecule [43], and the Rh І Rh bond [44] or Rh І Ir bond [45] in bridged complexes, which have been evidenced by X-ray structural studies.…”

Activation of C–Cl by ground-state aluminum atoms: an EPR and DFT investigation

“…Interaction of the Al atoms with the substrate promotes bond cleavage and the resulting radicals recombine with the lone pair of electrons on the Al atom to form the insertion product. In the case of CH 3 AlOCH 3 , Kasai 7 proposed that Al atoms interact with a lone pair of electrons on the oxygen atom of dimethyl ether. Next, the C-O bond cleaves producing methyl radicals.…”

“…1,2 Ground-state Al ( 2 P 1/2 ) atoms have been shown to activate a number of different types of bonds resulting in the formation of insertion products. Experiments involving NH 3 , 3,4 H 2 O, 5 H 2 S, H 2 Se, 6 CH 3 OCH 3 , 7,8 and CH 4 9 yielded HAlNH 2 , HAlOH, HAlSH, HAlSeH, CH 3 AlOCH 3 and CH 3 AlH, respectively, indicating that Al atoms can activate N-H, O-H, S-H, Se-H, C-O and C-H bonds. Al atoms have also been reported to activate the C-C bond of cyclohexanol, 10 1-methylcyclohexanol 10 and diethyl ether.…”

Chemistry in low-temperature matrices

“…The insertion process of the Al atom into trimethylene oxide (TMO) and dimethylether (DME) has been investigated. 124 An ESR study shows that Al atoms insert spontaneously into TMO. Experimental results are backed up by calculations.…”