4-Ferrocenyl-1,3-oxazoline derivatives as ligands for catalytic asymmetric allylation reactions

Enhancing the Efficacy of Chiral Ligands and Catalysts: Siloxane-Substituted Oxazoline Ferrocenes as Next-Generation Candidates

Since the discovery of classical chiral oxazoline ferrocene ligands in 1995, they have become pivotal in transition metal-catalyzed asymmetric transformations. Over the past decade, a notable evolution has been observed with the emergence of siloxane-substituted oxazoline ferrocenes, demonstrating significant potential as chiral ligands and catalysts. These compounds have consistently delivered exceptional results in diverse and mechanistically distinct transformations, surpassing the capabilities of classical oxazoline ferrocene ligands. This review meticulously delineates the research progress on siloxane-substituted oxazoline ferrocene compounds. It encompasses the synthesis of crucial precursors and desired products, highlights their achievements in asymmetric catalysis reactions, and delves into the exploration of the derivatization of these compounds, emphasizing the introduction of ionophilic groups and their impact on the recovery of transition metal catalysts. In addition to presenting the current state of knowledge, this review propels future research directions by identifying potential topics for further investigation concerning the siloxane-tagged derivatives. These derivatives are poised to be promising candidates for the next generation of highly efficient ligands and catalysts.

Transition-Metal-Free Multicomponent Approach to Stereoenriched Cyclopentyl-isoxazoles through C-C Bond Cleavage

An efficient multicomponent reaction for the synthesis of stereoenriched cyclopentyl-isoxazoles from camphor-derived α-oximes, alkynes, and MeOH is reported. Our method involved a series of cascade transformations, including the in situ generation of an IIII catalyst, which catalyzed the addition of MeOH to a sterically hindered ketone. Oxidation of the oxime, and rearrangement of the α-hydroxyiminium ion generated a nitrile oxide in situ, which, upon [3+2] cycloaddition reaction with an alkyne, delivered the regioselective product. This reaction was very selective for the syn-oxime. This multicomponent approach was also extended to the synthesis of a new glycoconjugate, camphoric ester-isoxazole C-galactoside.

Ferrocene phosphane-heteroatom/carbon bidentate ligands in asymmetric catalysis

A review of chiral ferrocene ligands featuring a combination of phosphorus and other elements as donor atoms in the asymmetric catalysis.

Metal-catalyzed enantioselective allylation in asymmetric synthesis

Metal-catalyzed enantioselective allylation, which involves the substitution of allylic metal intermediates with a diverse range of different nucleophiles or S(N)2'-type allylic substitution, leads to the formation of C-H, -C, -O, -N, -S, and other bonds with very high levels of asymmetric induction. The reaction may tolerate a broad range of functional groups and has been applied successfully to the synthesis of many natural products and new chiral compounds.

Catalytic Asymmetric Dihydroxylation of 1-Substituted-1-ferrocenylethenes: An Enantioselective Entry to Chiral Tertiary Ferrocenylcarbinols and Ferrocenylalkylamines

The Sharpless asymmetric dihydroxylation (AD) of 1-substituted-1-ferrocenylethenes takes place with good yields and with moderate to good enantioselectivities. The resulting 1-substituted-1-ferrocenyl-1,2-ethanediols are the first alpha-chiral tertiary ferrocenylcarbinols that have been prepared in optically active form. Their absolute configuration, ascertained by Induced Circular Dichroism (ICD), shows that in all cases the ferrocenyl moiety has the highest affinity for the SW binding pocket of the AD ligands. Depending upon the reaction conditions, significant racemization takes place during a side-chain azide nucleophilic substitution on 2-ferrocenyl-1,2-propanediol.

Stereocontrol in Alkyne Cyclocarbonylation Reactions Promoted by a Bioxazoline Palladium(ii) Complex

Insertion of 1,2-disubstituted alkynes into [Pd(CH3)(CO)(BIOX)]+[B{3,5-(CF3)2C6H3}4]- (1), where BIOX=(4S,4'S)-(-)-4,4',5,5'-tetrahydro-4,4'-bis(1-methylethyl)-2,2'-bioxazole, leads to the formation of five-membered palladacycles, which, by reaction with carbon monoxide, produce a mixture of two diastereomeric forms of a palladium complex containing an eta3-allylic gamma-lactone ligand. On leaving the mixture in solution, one isomer was converted into the other, reaching a diastereomeric excess of up to 94 %. The steric and electronic factors responsible for the epimerization process were investigated by theoretical methods. Cleavage of the eta3-allyl--palladium bond by nucleophiles allowed highly substituted chiral butenolides to be synthesized in good enantiomeric excess.

Development of Chiral (S)-Prolinol-Derived Ligands for Palladium-Catalyzed Asymmetric Allylic Alkylation: Effect of a Siloxymethyl Group on the Pyrrolidine Backbone

A series of novel chiral aminophosphine ligands are designed and readily prepared from (S)-prolinol. The reactivity and selectivity in the palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with a dimethyl malonate-BSA-LiOAc system using these chiral ligands are evaluated, and the structural elucidation of ligands and palladium complex is also conducted. Moreover, a series of trialkylsilylated chiral aminophosphine ligands are prepared and applied to palladium-catalyzed asymmetric allylic alkylation (up to 98% ee).

Synthesis and Catalytic Activity of Spaced Ferrocene Oxazolines

Chiral 2-[{N-aryl-N-(ferrocenylmethyl)amino}methyl]-4-(1-methylethyl)-4,5-dihydroxazoles with various substituents at the aryl ring were prepared by alkylation of N-(ferrocenylmethyl)anilines, FcCH2NHC6H4R (Fc = ferrocenyl), with (S)-2-(chloromethyl)-4-(1-methylethyl)-4,5-dihydrooxazole. The oxazoles, substituted anilines, and the precursors of the latter, the respective Schiff bases FcCH=NC6H4R, were characterized by standard methods and further studied by mass spectrometry. The oxazoles were further tested as chiral auxiliaries in the addition of diethylzinc to benzaldehyde but showed only negligible asymmetric induction (ee ca 10%), most likely due to steric hindrance of the nitrogen donor centres. This steric restriction seems to be lowered upon replacement of the substituted phenyl group with a benzyl substituent; compounds FcCH2NHCH2Ph and (R)-FcCH2NHCH(Me)Ph are easily alkylated yielding [FcCH2NMe2(CH2Ph)]I (9) and 2-[{N-(1-phenylethyl)-N-(ferrocenylmethyl)amino}methyl]-4-(1-methylethyl)-4,5-dihydroxazole (10), respectively. Solid-state structures of FcCH2NHC6H4R (R = 2-Me and 4-Cl), 9, and 10 have been determined by single-crystal X-ray diffraction.