Development of Cationic Transition Metal Complex-Catalyzed Asymmetric [2+2] Cycloaddition Reaction

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1.[2+2] Cycloaddition reaction of carbonyl compounds with ketene The [2+2] cycloaddition reaction was examined by using some cationic group 8-10 transition metal complexes as the catalysts, among which palladiun and platinum complexes showed good perfonnance. It was found that the catalytic activity was greatly affected not only by the metal center and the ligand but also by the counter anion. Attempts on asymmetric induction by using chiral ligands failed but chiral counterions, tetraalkoxy borates derived from substituted BINOLs, were found to serve as a chirality-recognizing element in the asymmetric reaction.2.Tandem [242] cycloaddition-allylic rearrangement or α,β-unsaturatead carbonyl compounds with ketene In the reaction of α,β-unsaturated carbonyl compounds, 4-vinyloxetan-2-ones generated by the cycloaddition oflen rearrange under the conditions to give 3,6-dihydro-2H-pyran-2-ones. Thus, the reaction of a series of α,β-unsaturated aldehydes and ketones with ketene was examined to deduce the origin of the rearrangement. It was found that the reaction occurs if 4-vinyloxetan-2-ones bearing an alkyl substituent at the 4-position or at the 2-position of the vinyl moiety are generated in the cycloaddition. Based on the observations, a reacfion mechanism was proposed, which includes a zwitterion generated by heterolytic cleavage of the C(4)-0 bond of the 4-vinyloxetan-2-one as an intermediate. Asymmetric induction with up to 57% de has been achieved by using α,β-unsaturated aldehydes bearing an asymmetric carbon center at the β-position.3.Activation of CO_2 by Lewis acids and its chemical fixation to carbon-carbon double bonds As an extension of this research project, a possibility was examined to activate C0_2 by aluminum-based Lewis acids and fix it electrophilically to aromatic compounds and organosilanes, which might lead to an alternative method to prepare oxetan-2-ones from olefins and CO_2 in the future.