A new planar chiral bipyridine-ligand: pyrid-2-yl[2](1,4)benzeno[2](5,8)quinolino-phane

Abstract

445In recent years, homogeneous asymmetric catalysis with metal complexes has been a very fast developing field of research [1]. Most of the organic reactions in which prostereogenic compounds can be used, are now feasible under metal-complex-catalysis. The ligands used in these reactions, for the most part, possess phosphorous donor groups as coordination sites. Nitrogen ligands, not as numerous as their phosphorous counterparts, find more and more interest in the last few years, too [2]. Planar chirality generated by making use of the unique [2.2]paracyclophane skeleton, however, is very rare both in the field of phosphorous and nitrogen ligands [3]. Recently, we reported the synthesis and enantiomeric resolution of the first planar chiral 2,2'-bipyridine with [2.2]paracyclophane skeleton (8) and its use in asymmetric catalytic reactions [4].To find out more about the opportunities, which this new lead structure for the design of chiral ligands offers, we now synthesized pyrid-2-yl With this key-compound in hand, we could synthesize the hitherto unknown title compound 1 in a one-stepprocedure, by cobalt-catalyzed cyclisation with acetylene (Bönnemann-reaction) [8].recorded. The enantiomerically pure title compound 1 is used in copper-catalyzed asymmetric cyclopropanation of styrene and in asymmetric iridium-catalyzed transfer-hydrogenation of acetophenone.The synthesis of 1 proceeded via the NBS-bromination of 5,8-dimethylquinoline [6] (3) leading to the dibromide 4. Cyclisation of 4 under high-dilution conditions with 1,4-bis(mercaptomethyl)benzene generated the new dithiaphane 5, which was irradiated by UV (Hg, 180 W) in a thiophilic solvent (P(OMe) 3 ). By cyana-

Chiral 2,2‘-Bipyridines, 1,10-Phenanthrolines, and 2,2‘:6‘,2‘ ‘-Terpyridines: Syntheses and Applications in Asymmetric Homogeneous Catalysis

“…69 The [Ir(235)] complex showed good catalytic activity, and the reaction could be carried out at room temperature in contrast to many other ligands. 128 However, the observed stereoselectivity was low (31% ee).…”

[2.2]Paracyclophane derivatives in asymmetric catalysis

“…Ligands 77 and 78 have also been employed in the iridium catalysed asymmetric transfer hydrogenation of acetophenone 48 (Scheme 27). 65, 66 Use of ligand 77 for 2 h at room temperature gave the product 49 in 8% yield and with no enantioselection. Raising the Scheme 24 temperature to 80 ЊC for 1 h gave 49 in 93% yield and 23% ee.…”

“…The racemic synthesis of quinolinophane 78 proceeded via the NBS-bromination of 80 as shown in Scheme 29. 66 It too was resolved by chiral HPLC.…”

Synthesis of Planar Chiral N‐Heterocyclic‐Substituted Pyridinophanes

“…For new chiral N-heterocyclic ligands, the use of a planar chiral scaffold is a promising opportunity. The combination of the coordinating properties of 2,2Ј-bipyridines with the planar chirality of para-bridged cyclophanes was first investigated by Vögtle et al in the syntheses of pyrid-2-yl[2]- (1,4)benzene [2] (5,8)quinolinophane (1) [6] and 13-pyridinyl [2](1,4)benzene- [2](2,5)pyridinophane (4, see Fig-inophanes. Additionally, the known synthesis of the pyridinophane scaffold was simplified considerably.…”