Peter Hammar

Enantioselective Organocatalytic Conjugate Addition of Fluorocarbon Nucleophiles to α,β-Unsaturated Aldehydes

A highly chemo- and enantioselective organocatalytic addition of fluorocarbon nucleophiles, such as 1-fluoro-bis(phenylsulfonyl)methane, toα,β-unsaturated aldehydes is presented (see scheme). The reactions are catalyzed by simple chiral amines and give access to optically active fluorine derivatives in good yields and up to 95 % ee. Notably, the methodology can be applied to the formation of a chiral quaternary carbon center bearing a fluorine atom with high enantioselectivity.

Theoretical mechanistic study of the TBD-catalyzed intramolecular aldol reaction of ketoaldehydes.

The intramolecular aldol reaction of acyclic ketoaldehydes catalyzed by 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) is investigated using density functional theory calculations. Compared to the proline-catalyzed aldol reaction, the use of TBD provides a unique and unusual complete switch of product selectivity. Three mechanistic pathways are proposed and evaluated. The calculations provide new insights into the activation mode of bifunctional guanidine catalysts. In the favored mechanism, TBD first catalyzes the enolization of the substrate and then the C-C bond formation through two concerted proton transfers. In addition, the computationally predicted stereochemical outcome of the reaction is in agreement with the experimental findings.

Theoretical Study of Mechanism and Stereoselectivity of Catalytic Kinugasa Reaction

The mechanism of the catalytic Kinugasa reaction is investigated by means of density functional theory calculations. Different possible mechanistic scenarios are presented using phenanthroline as a ligand, and it is shown that the most reasonable one in terms of energy barriers involves two copper ions. The reaction starts with the formation of a dicopper-acetylide that undergoes a stepwise cycloaddition with the nitrone, generating a five-membered ring intermediate. Protonation of the nitrogen of the metalated isoxazoline intermediate results in ring opening and the formation of a ketene intermediate. This then undergoes a copper-catalyzed cyclization by an intramolecular nucleophilic attack of the nitrogen on the ketene, affording a cyclic copper enolate. Catalyst release and tautomerization gives the final β-lactamic product. A comprehensive study of the enantioselective reaction was also performed with a chiral bis(azaferrocene) ligand. In this case, two different reaction mechanisms, involving either the scenario with the two copper ions or a direct cycloaddition of the parent alkyne using one copper ion, were found to have quite similar barriers. Both mechanisms reproduced the experimental enantioselectivity, and the current calculations can therefore not distinguish between the two possibilities.

Structure–Reactivity Relationship Studies for Guanidine‐Organocatalyzed Direct Intramolecular Aldolization of Ketoaldehydes

Structure–reactivity studies are performed to explore the reaction mechanism of the guanidine‐catalyzed intramolecular aldol reaction of ketoaldehydes. A large number of guanidine and guanidine‐like catalysts are synthesized and their properties studied. Kinetic profiles and pKa values of the catalysts are measured and correlated to reaction barriers calculated using density functional theory (DFT). The DFT calculations show that structural rigidity influences the pKa of the guanidines. Although the basicity is a very important factor in the catalysis, it is not sufficient to fully account for its catalytic efficiency. The availability of two aligned nitrogen reaction sites for proton shuttling in the transition state is an essential feature that helps to rationalize the reactivity pattern and the activation mode for this family of organocatalysts.