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

Abstract

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.

Keywords: catalyst recycling; chiral ligands; ferrocene; oxazoline; planar chirality.

Figure 1

Modified ligands from oxazoline metallocene skeleton.

Scheme 1

Synthesis of siloxane-substituted oxazoline ferrocene by Aït-Haddou’s group.

Scheme 2

Synthesis of siloxane-substituted oxazoline ferrocene by Sammakia’s group.

Scheme 3

Synthesis of siloxane-substituted oxazoline ferrocene by Zhou’s procedure.

Scheme 4

DoM procedures of oxazoline ferrocene with two stereogenic centers.

Scheme 5

DoM procedures of oxazoline ferrocenes with the addition of TMEDA.

Scheme 6

DoM procedure of oxazoline ferrocene with a bulky substituent.

Scheme 7

Synthetic approach of chiral oxazoline 1,2′-bisphosphine ferrocene.

Scheme 8

Stepwise lithiation of chiral oxazoline ferrocene.

Figure 2

Structures of siloxane-substituted hydroxyl oxazoline ferrocenes.

Scheme 9

Praparation of N,O,O-tridentate ferrocenes via a desilylation reaction. The structure of R₃ in (S,Sp)-25 is presented in Figure 2.

Scheme 10

Palladium-catalyzed asymmetric allylic alkylation with siloxane-substituted oxazoline ferrocene ligands.

Scheme 11

Cu(I)-catalyzed [3 + 2] 1,3-dipolar cycloaddition with nitroalkenes.

Scheme 12

Cu(OAc)₂-catalyzed [3 + 2] 1,3-dipolar cycloaddition with alkylidene malonates.

Figure 3

Catalytic result and X-ray structure of exo-30a. Copyright 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.

Scheme 13

Cu(I)-catalyzed [3 + 2] 1,3-dipolar cycloaddition with β-trifluoromethyl nitroalkenes.

Scheme 14

Cu(II)-catalyzed [3 + 2] 1,3-dipolar cycloaddition with maleate esters.

Scheme 15

Asymmetric self-Michael addition of 2-[(3-cyano-2H-chromen-2-ylidene)amino]acetates.

Scheme 16

Synthetic schemes for establishing ionic-tagged oxazoline phosphine ferrocene ligands.

Scheme 17

Asymmetric hydrogenation of aromatic ketones with quaternary ammonium tagged oxazoline phosphine ferrocene ligand.

Scheme 18

Cu(I)-catalyzed asymmetric [3 + 2] 1,3-dipolar cycloaddition with FimiOXAP ligands.