Evidence for the structure of the enantioactive ligand in the phosphine-copper-catalyzed addition of diorganozinc reagents to imines

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.

Boosting the Enantioselectivity of Conjugate Borylation of α,β-Disubstituted Cyclobutenones with Monooxides of Chiral C2 -Symmetric Bis(phosphine) Ligands

Chiral bis(phosphine) monooxides (BPMOs) derived from C₂ -symmetric bis(phosphines) have been found to induce superior levels of enantioselection in copper-catalyzed conjugate borylation of α,β-disubstituted cyclobutenones. More precisely, enantiomeric excesses as well as chemical yields are exceedingly high with (R,R)-Bozphos as the chiral ligand while these values are low with parent (R,R)-Me-Duphos. A similar yet less pronounced effect was seen in the corresponding 1,6-addition to para-quinone methides.

Utilization of BozPhos as an Effective Ligand in Enantioselective C-H Functionalization of Cyclopropanes: Synthesis of Dihydroisoquinolones and Dihydroquinolones

The bisphosphine monoxide ( R, R)-BozPhos enables enantioselective C-H functionalization of cyclopropanes in a palladium-catalyzed cyclization. The synthesis of a broad spectrum of dihydroisoquinolones and dihydroquinolones in good yields and high enantiomeric excess was achieved through the use of this hemilabile ligand. Furthermore, the isolation of an intermediary palladium(II)-BozPhos complex after oxidative addition was successful and a second complex provided further insight into bond length and angles through a crystal structure.

A rational pre-catalyst design for bis-phosphine mono-oxide palladium catalyzed reactions

Detailed mechanistic studies of a Pd-catalyzed asymmetric C–N coupling led to a rational design of a new series of bis-phosphine mono-oxides ligated Pd(ii) pre-catalysts that allow for reliable and complete catalyst activation.

Significant catalyst loading reduction and increased reaction robustness have been achieved for a Pd-catalyzed asymmetric intramolecular C–N coupling through comprehensive mechanistic studies. Detailed kinetic, spectroscopic, and crystallographic analyses revealed that the mono-oxidation of the bis-phosphine ligand is critical for a successful transformation. ³¹P NMR studies provided an understanding of the inefficient activation of the Pd(OAc)₂/(R,R)-QuinoxP* pre-catalyst to form the active bis-phosphine mono-oxide–Pd(0) catalyst with competitive formation of a less active (R,R)-QuinoxP*·PdBr₂ complex. Based on these detailed mechanistic studies, a new series of bis-phosphine mono-oxides (BPMO)-ligated Pd(ii) pre-catalysts have been rationally developed that allow for reliable and complete catalyst activation which should have general utility in academic and industrial settings.

Access to Cyclopropyl-Fused Azacycles via a Palladium-Catalyzed Direct Alkenylation Strategy

Palladium-catalyzed direct alkenylation of cyclopropyl C-H bonds proceeds in high efficiency. This transformation provides access to novel cyclopropyl-fused azacycles. Ligand studies suggest that bisphosphine monoxide analogues of dppf and rac-BINAP are the active ligand species. Preliminary results support that both BozPhos and IPrMonophos ligands can achieve high enantioinduction for this novel direct alkenylation reaction. To date, this represents the first example of enantioselective C-H functionalization employing a bisphosphine monoxide ligand.

Enantioselective addition of dialkylzinc to aromatic aldimines mediated by camphor-derived chiral β-amino alcohols

The enantioselective addition of diethylzinc or dimethylzinc to N-(diphenylphosphinoyl)imines mediated by 1 or 2 could be achieved in high yields (70-97 %) and enantioselectivities (85-98 % ee). The catalytic loading of 1 or 2 a could be reduced to 10 mol % for methylation or ethylation of imines in high yields and enantioselectivities (79-96 %) when the reaction was conducted in the presence of 1.8 equiv of methanol. N-Monosubstituted amino alcohols induced higher enantioselectivity than their N,N-disubstituted congener in our catalytic system.

[4+2+2] Cycloaddition catalyzed by a new cationic rhodium-bisphosphine monooxide complex

A rhodium-BozPHOS based complex is reported. This complex is competent in catalyzing the [4+2+2] cycloisomerization of cyclooctatrienes in moderate to good yields. The X-ray crystal structure of this complex is reported, along with formation of both bicyclic and tricyclic cyclooctatrienes.

Stereoselective formation of amines by nucleophilic addition to azomethine derivatives

This chapter describes state-of-the-art methods to prepare α-chiral amines by the addition of nonstabilized nucleophiles to imine derivatives. The first part of the chapter illustrates the most effective diastereoselective addition reaction (substrate controlled and chiral auxiliary based methods) whereas the second part focuses on catalytic asymmetric methods.

Formation of C-C Bonds via Iridium-Catalyzed Hydrogenation and Transfer Hydrogenation

The formation of C-C bonds via catalytic hydrogenation and transfer hydrogenation enables carbonyl and imine addition in the absence of stoichiometric organometallic reagents. In this review, iridium-catalyzed C-C bond-forming hydrogenations and transfer hydrogenations are surveyed. These processes encompass selective, atom-economic methods for the vinylation and allylation of carbonyl compounds and imines. Notably, under transfer hydrogenation conditions, alcohol dehydrogenation drives reductive generation of organoiridium nucleophiles, enabling carbonyl addition from the aldehyde or alcohol oxidation level. In the latter case, hydrogen exchange between alcohols and π-unsaturated reactants generates electrophile-nucleophile pairs en route to products of hydro-hydroxyalkylation, representing a direct method for the functionalization of carbinol C-H bonds.

Probing the importance of the hemilabile site of bis(phosphine) monoxide ligands in the copper-catalyzed addition of diethylzinc to N-phosphinoylimines: discovery of new effective chiral ligands

The hemilabile ligand Me-DuPHOS(O) 2 has proven to be a successful ligand for the copper-catalyzed addition of diethylzinc to N-phosphinoylimines. The corresponding alpha-chiral amines were obtained in high yields (80-98%) and enantiomeric ratios (19.0:1 to 99.0:1 er). Furthermore, this Cu* 2 catalytic system has been shown to be effective in the addition of diethylzinc to nitroalkenes and in the reduction of beta,beta-disubstituted vinyl phenyl sulfones. This paper describes a general structure/selectivity study in which the three ligand subunits (chiral phospholane-linker-labile coordinating group (Z)) are systematically modified and tested in the copper-catalyzed addition of diethylzinc to the N-phosphinoylimine 1 derived from benzaldehyde. This study led to the discovery of a new class of effective chiral ligands that combine a chiral phospholane unit and an achiral phosphine oxide.

Application of the chiral bis(phosphine) monoxide ligand to catalytic enantioselective addition of dialkylzinc reagents to beta-nitroalkenes

[reaction: see text] Me-DuPHOS monoxide is shown to be a very effective ligand in the enantioselective copper-catalyzed addition of dialkylzinc reagents to beta-nitroalkenes providing access to chiral nitroalkanes. The major advantages of this process are high yields, broad and complementary substrate scope, and high enantioselectivities. The effect of achiral dummy ligands as an additive has also been documented.

A Highly Tunable Family of Chiral Bisphospholanes for Rh-Catalyzed Enantioselective Hydrogenation Reactions

A set of 16 new and closely related bisphospholane ligands have been prepared by using a highly flexible and convergent approach. Each synthesis can be performed on an industrially relevant scale. The bisphosphines differ in the nature of the bridge connecting both phospholane units. Bridges are formed by three-, four-, five- and six-membered heterocyclic or alicyclic rings. Bisphospholanes and their Rh-precatalysts have been investigated by using results of theoretical calculations (DFT) and analytic measurements ((31)P and (103)Rh NMR spectroscopy, X-ray structure analysis). The studies showed that catalysts based on ligands with maleic anhydride or maleimide bridges give constantly superior enantioselectivities in methanol as the solvent. This may account for optimised steric and electronic effects. However, by changing the solvent catalysts with other backbones can give rise to excellent results. This gives proof that simple correlations between steric and electronic properties and results in the enantioselective hydrogenation frequently claimed in literature are not general.

Asymmetric Addition of Dimethylzinc to N-Tosylarylimines Catalyzed by a Rhodium−Diene Complex toward the Synthesis of Chiral 1-Arylethylamines

A rhodium complex coordinated with a chiral diene, (R,R)-2,5-diphenylbicyclo[2.2.2]octa-2,5-diene (Ph-bod), catalyzed the asymmetric addition of dimethylzinc to N-tosylarylimines to give high yields of chiral 1-aryl-1-ethylamines with high enantioselectivity (94-98% ee).