Dedicated to Professor Hisashi Yamamoto
Abstract
For the mechanism of decarboxylation reactions, all textbooks state that electrons move from the πC=O bond. However, the most electron-donating bond orbital in the carbonyl group is expected to be the lone pair(s) on the oxygen. Thus, orbital theory with delocalization from a lone pair might be more appropriate than delocalization from the πC=O orbital. We confirmed this idea by theoretical calculations. In the transition state of 2-substituted β-keto acids, a boat conformation is expected to result in the exclusive generation of (E)-enols. Normally, the decarboxylation reaction occurs in polar solvents, so the resulting enols should be transformed into the corresponding ketones by tautomerization. The use of heteroatoms to obtain enolate or enol ethers without tautomerization would offer a diastereoselective enol(ate) synthesis with regioselectivity because the C=C double bond would always be introduced between two carbonyl groups. After screening heteroatoms by theoretical calculations, we found that boron is suitable for this purpose. We confirmed our idea by theoretical calculations, offering a new boradecarboxylation reaction for the diastereo- and regioselective production of enolates.
Key words
decarboxylation - transition states - diastereoselectivity - regioselectivity - boradecarboxylation - enolates
