Diastereoselective Synthesis of P-Chirogenic and Atropisomeric 2,2'-Bisphosphino-1,1'-binaphthyls Enabled by Internal Phosphine Oxide Directing Groups

Enantiopure Turbo Chirality Targets in Tri-Propeller Blades: Design, Asymmetric Synthesis, and Computational Analysis

Enantiopure turbo chirality in small organic molecules, without other chiral elements, is a fascinating topic that has garnered significant interest within the chemical and materials science community. However, further research into and application of this concept have been severely limited by the lack of effective asymmetric tools. To date, only a few enantiomers of turbo chiral targets have been isolated, and these were obtained through physical separation using chiral HPLC, typically on milligram scales. In this work, we report the first asymmetric approach to enantiopure turbo chirality in the absence of other chiral elements such as central and axial chirality. This is demonstrated by assembling aromatic phosphine oxides, where three propeller-like groups are anchored to a P(O) center via three axes. Asymmetric induction was successfully carried out using a chiral sulfonimine auxiliary, with absolute configurations and conformations unambiguously determined by X-ray diffraction analysis. The resulting turbo frameworks exhibit three propellers arranged in either a clockwise (P,P,P) or counterclockwise (M,M,M) configuration. In these arrangements, the bulkier sides of the aromatic rings are oriented toward the oxygen atom of the P=O bond rather than in the opposite direction. Additionally, the orientational configuration is controlled by the sulfonimine auxiliary as well, showing that one of the Naph rings is pushed away from the auxiliary group (-CH2-NHSO2-tBu) of the phenyl ring. Computational studies were conducted on relative energies for the rotational barriers of a turbo target along the P=O axis and the transition pathway between two enantiomers, meeting our expectations. This work is expected to have a significant impact on the fields of chemistry, biomedicine, and materials science in the future.

Asymmetric Synthesis of P-Chirogenic Ferrocenyl BiphePFc* Diphosphine by Ephedrine-Aryne Methods and Application in Rhodium-Catalyzed Hydrogenation

Chiral diphosphines with a biphenyl bridge and the chirality borne by the phosphorus atoms and not due to the atropoisomery of the biaryl backbone have been scarcely studied. Herein, we report the asymmetric synthesis of the (S,S)-2,2'-bis(ferrocenylphenylphosphino)biphenyl (BiphePFc*) ligand and its application in Rh-catalyzed hydrogenation. The synthesis was based on the enantioselective preparation of P-chirogenic ferrocenyl(o-bromophenyl)phenylphosphine by the reaction of sec-phosphine-borane with 1,2-dibromobenzene and its homocoupling into BiphePFc*. Hydrogenations catalyzed by the Rh/BiphePFc* complex led to enantioselectivities of ≤96%.

Amino Turbo Chirality and Its Asymmetric Control

A series of new targets containing 3 chiral elements of central, orientational, and turbo chirality have been designed and synthesized asymmetrically. The absolute configurations and conformations of these types of chirality were concurrently controlled by using chiral sulfonimine auxiliary and unambiguously determined by x-ray diffraction analysis. These targets include alpha unnatural amino acid derivatives, which may play an important role for drug design, discovery, and development. Three propellers of turbo framework are covalently connected to a chiral C(sp3) center via C(sp2)-C(sp3) bonding along with a C-N axis, while one of them is orientated away from the same carbon chiral center. The turbo or propeller chirality is characterized by 2 types of molecular arrangements of propellers, clockwise (PPP) and counterclockwise (MMM), respectively. The turbo stereogenicity was found to depend on the center chirality of sulfonimine auxiliary instead of the chiral C(sp3) center, i.e., (S)- and (R)-sulfinyl centers led to the asymmetric formation of PPP- and MMM-configurations, respectively. Computational studies were conducted on relative energies for rotational barriers of a turbo target along the C-N anchor and the transition pathway between 2 enantiomers meeting our experimental observations. This work is anticipated to have a broad impact on chemical, biomedical, and materials sciences in the future.

Phosphonium-Catalyzed Monoreduction of Bisphosphine Dioxides: Origin of Selectivity and Synthetic Applications

Controlling product selectivity in successive reactions of the same type is challenging owing to the comparable thermodynamic and kinetic properties of the reactions involved. Here, the synergistic interaction of the two phosphoryl groups in bisphosphine dioxides (BPDOs) with a bromo-phosphonium cation was studied experimentally to provide a practical tool for substrate-catalyst recognition. As the eventual result, we have developed a phosphonium-catalyzed monoreduction of chiral BPDOs to access an array of synthetically useful bisphosphine monoxides (BPMOs) with axial, spiro, and planar chirality, which are otherwise challenging to synthesize before. The reaction features excellent selectivity and impressive reactivity. It proceeds under mild conditions, avoiding the use of superstoichiometric amounts of additives and metal catalysts to simplify the synthetic procedure. The accessibility and scalability of the reaction allowed for the rapid construction of a ligand library for optimization of asymmetric Heck-type cyclization, laying the foundation for a broad range of applications of chiral BPMOs in catalysis.

Stereochemical Stability of Planar-Chiral Benzazepine Tricyclics: Inversion Energies of P- and S-Alkene Ligands

Inversion barriers ΔG‡ for planar chiral phosphine-alkene and sulfonamide-alkene hybrid ligands based on phenyl-dibenz[b,f]azepine have been determined by density-functional theory calculations. Analysis of the structural and electronic characteristics of the minima and transition states explains the magnitudes of ΔG‡ and the geometrical changes during the inversion process. The steric repulsion caused by bulky substituents attached to the azepine nitrogen atom has a pronounced effect on the ΔG‡ value, explaining, inter alia, the stereochemical stability of the P- and S-alkene ligands when compared to the fluxional parent compound where X = H.

The stereoselective conversion of epimerized alkoxyl phosphine-borane to P,C, axial-stereogenic tertiary phosphine via cleavage of P-O bond

The P-O bond of epimerized alkoxyl phosphine-borane was cleaved by naphthalene-lithium, to form two diastereomers of P-anions in a ratio of 86 : 14, which was then converted to secondary phosphine-borane via acidification, and to tertiary phosphines with alkyl halides with enhanced 96 : 4 dr. The isolated tertiary phosphine containing hydroxyl (in >99 : 1 dr) was converted to multi-stereogenic tertiary phosphines via O-alkylation with alkylene dihalides.