TANDEM CYCLIZATION CYCLOADDITION REACTION OF RHODIUM CARBENOIDS - SCOPE AND MECHANISTIC DETAILS OF THE PROCESS

Treatment of 1-diazo-2,5-pentanediones with rhodium(II) carboxylates affords cyclic six-ring carbonyl ylide dipoles. These species undergo facile 1,3-dipolar cycloaddition with both electron-deficient and electron-rich dipolarophiles. In certain cases 2:1 cycloadducts are formed. The higher order cycloadducts are derived by further dipolar cycloaddition of the carbonyl ylide across the keto group of the initially formed 1:1 cycloadduct. Attempts to obtain a cycloadduct from the reaction of the diazo dione with nonactivated olefins led to 6-substituted 2H-pyran-3(4H)-ones. The formation of this ring system proceeds via a 1,4-hydrogen shift from the intermediate carbonyl ylide dipole. The observed regioselectivity in these cycloadditions can be nicely accommodated in terms of frontier molecular orbital theory. A type II FMO interaction is suggested since carbonyl ylides possess one of the smallest HOMO-LUMO energy gaps of the common 1,3-dipoles. The HOMO of the dipole is dominant for reactions with electron-deficient dipolarophiles such as methyl propicíate, while the LUMO becomes important for cycloaddition to more electron-rich species such as propargyl ethers. MNDO calculations indicate that the largest coefficient in the HOMO resides on the enolate carbon, whereas the γ-carbon bears the largest coefficient in the LUMO. © 1990, American Chemical Society. All rights reserved.