A New Planar Chiral Bipyridine Ligand

Abstract

Combined methods of cyclophane and bipyridine synthesis open the way to the new planar-chiral 2,2'-bipyridine 1, whose CD spectrum is strongly dependent on metal salts. The absolute configuration of the chiral precursor 2 could be assigned by comparison of experimental and theoretical CD spectra. The usefulness of planar chirality in heterocyclic transition metal ligands is revealed in experiments towards stereoselective catalysis using 1.Chiral ligands are the key compounds in asymmetric catalysis. 2 A ligand playing a major role in transition metal complexation is 2,2'-bipyridine, which has been frequently provided with stereogenic centres. Chiral bipyridines are not only used in asymmetric synthesis, 3 but also for the organization of stereochemically defined supramolecular 4 structures. 5 We combined intramolecularly two structural features that have always been important in stereochemistry as well as in coordination chemistry, the [2.2]paracyclophane system and the 2,2'-bipyridine complex ligand, resulting in a planar chiral ligand of a new type, 13-pyridinyl [2](1,4)benzeno[2](2,5)pyridinophane (1).The number of planar chiral ligands based on ferrocenes in catalytic asymmetric synthesis is very high. 6 A [2.2]paracyclophane skeleton, however, has rarely been described so far in this field. 7 We now make use of the special attributes of the [2.2]paracyclophane type scaffold as chiral unit. It possesses a high sterical demand that is particularly effective because of its rigidity. Furthermore, the chiral information is located near to the metal ion, one side being shielded by the benzene ring.To obtain the key educt for 1, the [2](1,4)benzeno[2](2,5)pyridinophane (2), which is once mentioned in literature, but, concerning synthesis and precursors, was insufficiently described, 8 we proceeded from commercially available 2,5-pyridinedicarboxylic acid, which was converted into the diethyl ester via the acid chloride.The diol 3, generated by reduction of the ester through sodium borohydride/calcium chloride in ethanol, 9,10 could be isolated exclusively by using a strongly acidic cation exchange resin (e.g. Amberlite IR 200 C) 11 . After bromination of the diol, 12 we obtained the dibromide 4. Under high-dilution conditions 13 and by taking advantage of the cesium effect, 14 4 was coupled with 1,4-bis(sulfanylmethyl)benzene to yield 2,11-dithia [3](1,4)benzeno[3](2,5)pyridinophane 5.Irradiation of 5 with ultraviolet light (Hg, 180 W) in a thiophilic solvent generated the [2.2]phane 2, the N-oxide Reagents and conditions: i, SOCl 2 , reflux, 8 h, recryst. of acid chloride (petroleum ether 40/60), then EtOH, reflux, 2 h, 82%; ii, NaBH 4 (2 equiv), CaCl 2 (1 equiv), EtOH, r.t., 16 h, 67%; iii, HBr/HOAc (30%), r.t., 6d; iv, sol. A: 4 in EtOH, sol. B: 1,4-bis(sulfanylmethyl)benzene (1 equiv), KOt-Bu (2.3 equiv) in EtOH (85%); Cs 2 CO 3 in mixture, reflux, 16 h, 62%; v, P(OMe) 3 , hν (Hg, 180 W), r.t., 18 h, 84%; vi, MCPBA (2 equiv), CH 2 Cl 2 , r.t., 20h, then N,Ndimethylcarbamoyl chloride (1.3 equiv.), TMSCN (1.3 equ...

Recent Theoretical and Experimental Advances in the Electronic Circular Dichroisms of Planar Chiral Cyclophanes

“…While the |De| value for the main band of compound 18 (20-30 M -1 cm -1 ) was reduced to half that of compound 17 (30-50 M -1 cm -1 ), the spectrum of planar chiral bipyridine 18 was very sensitive to the added metal salts. The absolute configuration was determined by comparison with the theoretical spectrum at the DFT/SCI level [31].…”

Synthesis of planar chiral pseudo–ortho-substituted aryl[2.2]paracyclophanes by stepwise successive palladium-catalyzed coupling reactions

“…3 Some pseudo-ortho-disubstituted [2.2]paracyclophanes bearing two different functional groups have also proven to be effective ligands in asymmetric catalysis. 4 However, there have not yet been many examples of the use of pseudo-ortho-type cyclophane ligands compared to those bearing other substitution patterns (mono-, 5 orthodi-, 6,7 or pseudo-gem-di-substitution), 8-11 probably due to a lack of their efficient synthetic methodology. Their preparation usually starts with the dibromination of [2.2]paracyclophane.…”

Synthesis of planar-chiral bridged bipyridines and terpyridines by metal-mediated coupling reactions of pyridinophanes

“…[1][2][3][4][5] Most of these bipyridines are based on central-or axial-chirality and only limited examples are known of planar-chiral bipyridines. Although Vögtle and co-workers synthesized 13-pyridyl[2](1,4)benzeno[2](2,5)pyridinophane and 2-pyridyl[2]-(1,4) benzeno[2] (5,8)quinolinophane, planar-chiral bipyridine and pyridylquinoline incorporating [2,2]parapyridinophane and quinolinophane skeletons, 6,7 yields of those racemic molecules are not satisfactory (23% and 36%, respectively) in the final pyridine-forming steps. Recently, Fu et al reported the reductive coupling of ferrocene-type pyridine derivative for synthesizing racemic planar-chiral bipyridine with C 2 -symmetry and also demonstrated effectiveness of the ligand in Cu(I)-catalyzed asymmetric cyclopropanation.…”