Role of Regioisomeric Bicyclo[3.3.0]octa-2,5-diene Ligands in Rh Catalysis: Synthesis, Structural Analysis, Theoretical Study, and Application in Asymmetric 1,2- and 1,4-Additions

Abstract

In order to study the impact of regioisomeric diene ligands on the formation and catalytic activity of Rh complexes, a series of C- and C-symmetric 2,5-disubstituted bicyclo[3.3.0]octa-2,5-dienes C-L and C-L, respectively, were synthesized from Weiss diketone by simultaneous deprotonation/electrophilic trapping of both oxo functions, and the catalytic behavior was studied in the presence of [RhCl(CH)]. Complexes [RhCl(C-L)] bearing C-symmetric ligands catalyzed effectively the asymmetric arylation of N-tosylaldimines to (S)-diarylamines with yields and ee values up to 99%. In Hayashi-Miyaura reactions, however, the complexes showed poor catalytic activity. When complexes [RhCl(C-L)] with C-symmetric ligand or mixtures of [RhCl(C-L)] and [RhCl(C-L)] were employed in 1,2-additions, racemic addition products were observed, suggesting a C═C isomerization of the diene ligands. X-ray crystal structure analysis of both Rh complexes formed from the [RhCl(CH)] precursor and ligands C-L and C-L revealed that only the C-symmetric ligand C-L coordinated to the Rh, whereas C-L underwent a Rh-catalyzed C═C isomerization to rac-C-L, which then gave the racemic [RhCl(rac-C-L)] complex. DFT calculations of the relative stabilities of the Rh complexes and the proposed intermediates provided a mechanistic rationale via Rh-mediated hydride transfer.

Asymmetric Catalysis in Liquid Confinement: Probing the Performance of Novel Chiral Rhodium–Diene Complexes in Microemulsions and Conventional Solvents

“…Although the SuperQuat auxiliary‐derived ligand 6 b formed a binuclear μ‐chloro‐bridged Rh complex [RhCl( 6 b )] 2 (Figure a) with bond lengths and angles, which are in good agreement with literature data, the corresponding Evans auxiliary‐derived ligand 5 b formed a mononuclear complex [RhCl( 5 b )] (Figure b), in which the carbonyl group of the oxazolidinone unit coordinates the Rh center, resulting in an elongation of the C1=O1 double bond to 1.227(4) Å (see the Supporting Information).…”

“…Rh‐ and Ir‐complexes carrying chiral diene ligands turned out to be particularly successful for asymmetric C−C, C−O, and C−N bond formations, but also cycloadditions, reductions, and other more complex transformations have been realized. Mechanistic insight was obtained by several theoretical studies . Although asymmetric catalytic reactions in micellar systems were successfully performed, asymmetric catalysis with Rh–diene complexes in bicontinuous microemulsions have not been reported so far.…”

“…Therefore, we probed for the first time the influence of microemulsions as organized reaction media for the Rh‐catalyzed asymmetric arylation of N ‐tosylimines 1 with the organoboron reagent 2 to the corresponding secondary amines 3 as benchmark reaction. Moreover, this benchmark system contains multiple components as for example, aqueous/organic solvent mixtures, bases and/or salts, which often requires tedious reaction optimization due to sluggish reaction rates and poor yields . Thus, we anticipated that particularly bicontinuously structured microemulsions should overcome limitations of conventional solvents caused by mass transport due to the simultaneous existence of polar and nonpolar nano‐domains controlling the catalyst–substrate interactions.…”

Adventures and Detours in the Synthesis of Hydropentalenes

“…It should be emphasized that Strand recently solved this problem of racemic and meso ligands in an elegant way 101 by selective complexation of the rac-diene with [Rh(ethene) 2 Cl] 2 , while the meso-diene did not form a stable Rh complex, in agreement with our own observations. 100 Strand was able to obtain diastereomerically pure monomeric Rh diene binaphthylamine complexes by treatment of the rac-Rh diene complex with (R)-binaphthyldiamine. 101 The enantiomerically pure diene was finally isolated after decomplexation with COD.…”

Chiral Discrimination in Rhodium(I) Catalysis by 2,5-Disubstituted 1,3a,4,6a-Tetrahydropenatalene Ligands—More Than Just a Twist of the Olefins?

“…During the preparation of this manuscript, an independent study on this topic was reported by Laschat and co-workers. 18 In light of this study, we here present our approach to enantiomerically pure di-μ-chlorobis rhodium(I) complexes of 3a and 3b , and the evaluation of these in asymmetric catalysis. Using a well-defined rhodium complex of 3a as catalyst, up to 99% ee was achieved in a 1,4-arylation reaction, which is higher than what was previously reported.…”