Computational Studies on Reaction Mechanism of the Catalyst-Controlled Selective Insertion of Metal Carbenoids into C-C and C-H Bonds of 1,3-Dicarbonyl Compounds

Density functional theory (DFT) calculations were carried out to investigate the mechanism and chemoselectivity of silver- or scandium-catalyzed insertion of diazo compounds into C-C or C-H bonds of 1,3-dicarbonyl compounds. The results show that silver and scandium carbenes are readily generated by metal-induced extrusion of nitrogen from diazo compounds. When low-coordinated silver(I) is used as the catalyst, carbene insertion into the C-C bond of 1,3-dicarbonyls leads to 1,4-dicarbonyl product containing an all-carbon a-quaternary center, through a cascade sequence of electrophilic addition, intramolecular cyclization, selective ring-opening and enol isomerization. When highly coordinated scandium(III) is used, carbene insertion into C-H bond of 1,3-dicarbonyls leads to 1,3-dicarbonyl product containing a-tertiary center, through a cascade sequence of electrophilic addition and protonation. Computational studies show that the chemoselectivity results from the cooperative effect of ring tension and the difference in coordination number of metal centers, which provides useful insight into the development of transition metal-catalyzed carbene transfer reactions.