The effect of CF3 functional group substituent on bifunctional activation model and enantioselectivity for BINOL N-triflylphosphoramides catalyzed rearrangement reaction

Origin of Ligand and Acid Effects on the Pd-Catalyzed Regiodivergent Coupling Reaction of Indazoles and Isoprene: A DFT Study

Comprehensive computational studies were carried out to explore the mechanisms and origin of regioselectivity in the Pd-catalyzed regiodivergent coupling reaction of indazoles and isoprene. Three different insertion models were investigated for regioselectivity 1,2- or 4,3-insertion with respect to the electrophilic sites on isoprene under two different ligands (L1 and L2) and acids ((PhO)2PO2H, (nBuO)2PO2H) via PdII-H species, allyl-π-PdII-O, and indazoles-acid-assisted PdII-π-allyl. The calculated results show that the PdII-π-allyl coordinated mechanism is the most preferred one. The noncovalent interactions between the less bulky ligand L1, substrates, and (PhO)2PO2- are found to be key factors for 1,2-insertion. The 4,3-insertion selectivity is primarily controlled by the steric repulsion of the t-Bu group of bulky ligand L2 and substrate, as well as the geometry distortion. Therefore, the regioselectivity difference of the 1,2- and 4,3-insertion on electrophilic sites is controlled by the synergistic effect of ligands and acids instead of the size of the ligand. Similarly, nucleophilic site selectivity at N1 or N2 of indazoles is governed by cooperative acid BF3 and solvent iPrOH rather than BF3 alone. Overall, our findings might open a new avenue for designing more efficient regioselective 1,2- or 4,3-addition or N1-/N2-selective nucleophilic reactions.

Identifying the true origins of selectivity in chiral phosphoric acid catalyzed N-acyl-azetidine desymmetrizations

The first catalytic intermolecular desymmetrization of azetidines was reported by Sun and coworkers in 2015 using a BINOL-derived phosphoric acid catalyst (J. Am. Chem. Soc. 2015, 137, 5895-5898). To uncover the mechanism of the reaction and the origins of the high enantioselectivity, Density Functional Theory (DFT) calculations were performed at the B97D3/6-311+G(2d,2p)/SMD(toluene)//B97D3/6-31G(d,p)/CPCM(toluene) level of theory. Comparison of four possible activation modes confirms that this reaction proceeds through the bifunctional activation of the azetidine nitrogen and the thione tautomer of the 2-mercaptobenzothiazole nucleophile. Upon thorough conformational sampling of the enantiodetermining transition structures (TSs), a free energy difference of 2.0 kcal mol-1 is obtained, accurately reproducing the experimentally measured 88% e.e. at 80 °C. This energy difference is due to both decreased distortion and increased non-covalent interactions in the pro-(S) TS. To uncover the true origins of selectivity, the TSs optimized with the full catalyst were compared to those optimized with a model catalyst through steric maps. It is found that the arrangements displayed by the substrates are controlled by strict primary orbital interaction requirements at the transition complex, and their ability to fit into the catalyst pocket drives the selectivity. A general model of selectivity for phosphoric acid-catalyzed azetidine desymmetrizations is proposed, which is based on the preference of the nucleophile and benzoyl group to occupy empty quadrants of the chiral catalyst pocket.

N-Triflylphosphoramides: highly acidic catalysts for asymmetric transformations

N-Triflylphosphoramides (NTPA), have become increasingly popular catalysts in the development of enantioselective transformations as they are stronger Brønsted acids than the corresponding phosphoric acids (PA). Their highly acidic, asymmetric active site can activate difficult, unreactive substrates. In this review, we present an account of asymmetric transformations using this type of catalyst that have been reported in the past ten years and we classify these reactions using the enantio-determining step as the key criterion. This compendium of NTPA-catalysed reactions is organised into the following categories: (1) cycloadditions, (2) electrocyclisations, polyene and related cyclisations, (3) addition reactions to imines, (4) electrophilic aromatic substitutions, (5) addition reactions to carbocations, (6) aldol and related reactions, (7) addition reactions to double bonds, and (8) rearrangements and desymmetrisations. We highlight the use of NTPA in total synthesis and suggest mnemonics which account for their enantioselectivity.

Enantioselective synthesis of chiral tetrasubstituted allenes: harnessing electrostatic and noncovalent interactions in a bifunctional activation model for N-triflylphosphoramide catalysis

DFT calculation reveals that the oxygen activation model is preferred than the nitrogen activation model due to the preferred chiral electrostatic environment.