Rhodium(II)-Catalyzed Equilibration of Push-Pull Carbonyl and Ammonium Ylides. A Computationally Based Understanding of the Reaction Pathway

Abstract

α-Diazo esters containing an amido group in the γ-position have been found to undergo a rhodium(II)-catalyzed transformation, producing five-membered ammonium or carbonyl ylides depending on the reaction conditions used. In the absence of an external dipolarophile, ammonium ylides are the exclusive products formed. In most cases these ylides cannot be isolated as they readily undergo sigmatropic rearrangement or fragmentation reactions. In the presence of typical dipolarophiles such as DMAD or N-phenylmaleimide, cycloaddition products derived from cyclic carbonyl ylide dipoles are formed as the major products. The rhodium carbenoid intermediate generated in these reactions can either attack the lone pair of electrons on the amide nitrogen (ammonium ylide formation) or the lone pair of electrons on the carbonyl oxygen (carbonyl ylide formation). The experimental observations reflect a catalyst-promoted system of equilibria with a clear-cut thermodynamic bias. To examine the underlying mechanism in detail, density functional theory (DFT) calculations were performed on all plausible intermediates, including the full dirhodium tetracarboxylate functionality. A semiquantitative energy manifold is developed that rationalizes the empirical observations and provides a detailed picture of the role of the dirhodium(II) catalyst.

DFT investigation on mechanism of dirhodium tetracarboxylate-catalyzed O-H insertion of diazo compounds with H2O

“…1 and 2 show the reaction course and the energy profile of the dirhodium tetraformatecatalyzed O-H insertion reaction of diazomethane with H 2 O. Rh 2 (O 2 CH) 4 (1) and diazomethane first couple to form a Rh/diazomethane complex 2 and then proceed via transition structure 3 to give a complex intermediate 4. Calculated activation barrier of this process is 16.5 kcal mol -1 , which is in good agreement with the results in previous reports [10][11][12]. The intermediate 4 is a weak complex and extrudes nitrogen to form a rhodium-methylene carbene complex 5.…”

“…The mechanisms of Rh(II) carbene inserting into C-H and Si-H bond have been studied by many experiments [3][4][5][6][7][8][9]. Theoretical research on the Rh(II) carbene insertion into C-H bond has been performed in detail [10][11][12]. Both experimental and theoretical studies support the concerted insertion mechanism.…”

“…Moreover, the stepwise way was more preferable than the concerted one. In all calculations, the method and basis sets were employed which have been proved to be sufficient to give reasonable results in good agreement with the experimental data [10][11][12]. All density functional calculations were carried out with the Gaussian 98 program [31].…”

Highly Enantioselective Cyclopropenation Reaction of 1‐Alkynes with α‐Alkyl‐α‐Diazoesters Catalyzed by Dirhodium(II) Carboxylates

“…[7b, 11a, 14,29] Recently, Davies and co-workers disclosed that the most favored transition state for the cyclopropenation of propyne with methyl styryldiazoacetate involves a tilted end-on approach that also displays a favorable hydrogen-bonding interaction with a carboxylate ligand on the catalyst.…”

Catalytic enantioselective intermolecular cycloadditions of 2-diazo-3,6-diketoester-derived carbonyl ylides with alkene dipolarophiles

“…Lower enantioselectivity will be observed if the catalyst (reversibly) dissociates from the ylide and cycloaddition occurs (competitively) from the achiral ylide. Energy is necessary to effect catalyst dissociation, however (13), and part of that energy could be supplied by that produced in bond formation to the dipolarophile. Cycloaddition then would occur on the opposite face of the ylide to the catalyst as the catalyst dissociates.…”