| Research Abstract |
This research was undertaken to study solvent effects on chiral oxazaborolidinone-promoted asymmetric aldol reaction, followed by detection of cationic boron species when the B-H bond of the oxazaborolidinone was changed to the B-X. Using the B-Br compound related to (R,R)-stien, existence of the boron species and its utilization to organic syntheses were investigated in detail. It turned out to be quite difficult to control two vacant orbitals of the hypothetical chiral boron cation in asymmetric reactions. In addition, the complete boron cations lead easily to neutral trivalent boronic compounds by attack of nucleophiles because of the extensively high electrophilicity and it is also difficult to catalytically recycle the boron cation. Thus, the reaction system should be necessarily restricted to some concerted reactions.
Physical organic researches on boron cations were interestingly developed. Diphenylboron perchlorate, which was considered as a conventional standard compound of boron cations, was found to be a trivalent boronic compound having a covalent B-O bond by ^<11>B-NMR and calculation studies. Boron cation species in solution were suggested to be solvated tetravalent species, not divalent species. The true boron cationic species were detected for the first time by using laser flash-photolysis.
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