Chiral N-Heterocyclic Carbene Ligands Derived from 2,2‘-Bipiperidine and Partially Reduced Biisoquinoline:  Rhodium and Iridium Complexes in Asymmetric Catalysis

Abstract

New and reliable procedures for the preparation of both enantiomers of 2,2‘-bipiperidine have been developed. Chiral imidazolinium salts derived from 2,2‘-bipiperidine and partially reduced biisoquinoline were prepared in high yields. Rhodium and iridium complexes of new chiral N-heterocyclic carbenes were obtained by transmetalation from corresponding silver(I) complexes. The structures of these complexes were verified by X-ray diffraction. The novel carbenes seem to range between the very electron-rich bis(amido)carbenes and the imidazole-derived carbenes with regard to the electronic ligand properties (IR evidence). Rhodium and iridium complexes were applied to a variety of catalytic asymmetric reactions. Modest enantioselectivities of up to 28% ee were observed.

Synthesis and Characterization of N‐Heterocyclic Carbene Substituted Phosphine and Phosphite Rhodium Complexes and their Catalytic Properties in Hydrogenation Reactions

“…[19,20,45] However, no systematic optimization of the conditions has been reported. We attempted to optimize this reaction under economically viable conditions, for example, a relatively low H 2 pressure and a low quantity of phosphine.…”

“…[16][17][18] Based on these findings and on our continuing work heading towards more efficient and stable catalysts, we examined whether the catalytic activity of RhA C H T U N G T R E N N U N G (PPh 3 ) 3 Cl (3) for olefin hydrogenation can be improved when two phosphine ligands are replaced by the stronger s-donor NHC ligands. [19][20][21][22] We now report a systematic optimization of the catalytic conditions of a newly synthesized RhA C H T U N G T R E N N U N G (PPh 3 ) 2 A C H T U N G T R E N N U N G (ICy)Cl (4) complex in olefin hydrogenation.…”

“…2.023 ], [37] [ [38] } and phosphine rhodium complexes {[RhClA C H T U N G T R E N N U N G (IMes)A C H T U N G T R E N N U N G (PPh 3 ) 2 ; 2.051 , [39] [trans-(1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene)hydrido-bis(triphenylphosphine)rhodium(I): 2.068 ] [20] } is slightly shorter, [40] a result of the electron-accepting phosphite ligands. A much shorter rhodium-carbene bond length of 1.919 was published by Lappert in 1985 using the stronger s-donor imidazolidin-2-ylidene ligand of chloro-trans-(4-isobutyl-1,3-dimethylimidazolidin-2-ylidene)bis(triphenylphosphine)rhodium(I).…”

{1,1′-Bis[(pyridin-2-yl)methyl]-2,2′-bipiperidyl}(perchlorato)copper(II) perchlorate

“…A detailed synthetic procedure for (2R,2 0 S)-2,2 0 -bipiperidine-1,1 0 -diium dibromide (mesoBPÁ2HBr) via reductive hydrogenation of 2,2 0 -dipyridyl was reported by Herrmann et al (2006) and Yang et al (2013). 1.81 g mesoBPÁ2HBr was dissolved in 8 mL H 2 O, and 8 mL of 5 M NaOH solution was added, followed by addition of 10 mL of CH 2 Cl 2 .…”

Asymmetric transfer hydrogenation catalyzed by a novel planar chiral N‐heterocyclic carbene–rhodium(I) complex

“…Their complexes with rhodium and iridium were employed in ATH, but very low ees were obtained (<24% ee). [13] Therefore, the challenge remains to develop more efficient chiral NHC ligands for ATH. Herein, we report the synthesis of a chelated ferrocenebased planar chiral carbene-Rh (I) complex 1 and its application in ATH of ketones (Fig.…”