Theoretical Studies on the Mechanism of the Michael Addition Reaction Catalyzed by a Thiourea-Cinchona-Amine: Triple Activation

Theoretical free energy profile and benchmarking of functionals for amino-thiourea organocatalyzed nitro-Michael addition reaction

Amino-thiourea organocatalysis is an important catalytic process for enantioselective conjugate addition reactions. The interaction of the reactants with the catalyst has a substantial effect of dispersion forces and is a challenge for a reliable description when applying density functional theory. In this report, the classical addition of acetylacetone to β-nitro-styrene catalyzed by Takemoto's catalyst in toluene was studied using the PBE functional for geometry optimization and the DLPNO-CCSD(T) benchmark method for single point energy. The complete free energy profile calculated for the reaction was able to explain all experimental observations, including the fact that the carbon-carbon bond formation step is rate-determining. The overall barrier was calculated to be 22.8 kcal mol^-1 (experimental value approximately 20 kcal mol^-1), and the enantiomeric excess was calculated to be 88% (experimental value in the range of 84 to 92%). Some functionals were tested for single point energy. The hybrid B3LYP presented a high mean absolute deviation (MAD) from the DLPNO-CCSD(T) benchmark method by approximately 20 kcal mol^-1. The inclusion of empirical dispersion correction in the B3LYP method decreased the MAD to 6 kcal mol^-1. Even the double-hybrid mPW2-PLYP and B2GP-PLYP methods had MAD values of approximately 5 kcal mol^-1. The inclusion of the dispersion correction decreased the MAD to 3.6 kcal mol^-1. M06-2X and ωB97X-D3 were the most accurate among the tested functionals, with MADs of 2.5 kcal mol^-1 and 1.8 kcal mol^-1, respectively. Additivity approximation of the correlation energy was also tested and presented a MAD of only 0.6 kcal mol^-1.

Theoretical study of the mechanism behind the site- and enantio-selectivity of C-H functionalization catalysed by chiral dirhodium catalyst

The C-H functionalization is very important for the synthesis of pharmaceuticals and complex natural products. Rhodium carbenoids, obtained when a dirhodium(ii) catalyst containing a crown formed by chiral ligands reacts with diazo compounds with both an electron donating group and an electron withdrawing group, play an important part in controlling site- and enantio-selectivity for functionalization of non-activated C-H bonds. It has earlier been demonstrated that the tertiary C-H bond is more favored to be functionalized inside the crown of the dirhodium catalyst with S-configuration ligands compared with the secondary and primary C-H bonds although the latter possess weaker steric effects. We argue that the higher site- and enantio-selectivity for some types of C-H bond functionalization can be related to intermolecular hydrogen bonding, steric hindrance, and weak interactions when the dirhodium catalyst is interacting with the chiral ligands.

How Dihalogens Catalyze Michael Addition Reactions

We have quantum chemically analyzed the catalytic effect of dihalogen molecules (X2 =F2 , Cl2 , Br2 , and I2 ) on the aza-Michael addition of pyrrolidine and methyl acrylate using relativistic density functional theory and coupled-cluster theory. Our state-of-the-art computations reveal that activation barriers systematically decrease as one goes to heavier dihalogens, from 9.4 kcal mol-1 for F2 to 5.7 kcal mol-1 for I2 . Activation strain and bonding analyses identify an unexpected physical factor that controls the computed reactivity trends, namely, Pauli repulsion between the nucleophile and Michael acceptor. Thus, dihalogens do not accelerate Michael additions by the commonly accepted mechanism of an enhanced donor-acceptor [HOMO(nucleophile)-LUMO(Michael acceptor)] interaction, but instead through a diminished Pauli repulsion between the lone-pair of the nucleophile and the Michael acceptor's π-electron system.