Diastereoselective Formation and Photophysical Behavior of a Chiral Copper(I) Phenanthroline Complex

Renovation of Optically Active Phenanthrolines as Powerful Chiral Ligands for Versatile Asymmetric Metal Catalysis

In the field of asymmetric synthesis, the development of new chiral ligands has been regarded as an attractive challenge for decades. Novel chiral ligands can often have a great impact on synthetic protocols. In this context, we are currently interested in the application of 1,10-phenanthroline (phen) as an entirely new class of chiral ligand. To handle this issue, we designed a chiral phen ligand that provides the N,N,O-tridentate coordination of the phen moiety and an additional phenolic hydroxyl group. As phen possesses greater coordination ability with various ions, our chiral phen ligand would be valuable as one of the "privileged" chiral ligands applied to a broad range of metal catalysts and new reactions. This account summarizes the results of the application of the chiral phen ligand to various kinds of metal catalysis.

Synthesis and optical resolution of a Cu(I) double-stranded helicate with ketimine-bridged tris(bipyridine) ligands

A tetranuclear Cu(I) double-stranded helicate was synthesized from ketimine-bridged tris(bipyridine) ligands and Cu(I) ions, and the racemate was successfully resolved by diastereomeric salt formation using an optically pure phosphate anion followed by anion exchange with NaPF(6) without racemization.

Asymmetric Transformation of a Double-Stranded, Dicopper(I) Helicate Containing Achiral Bis(bidentate) Schiff Bases

Reactions of the bis(bidentate) Schiff-bases N,N'-bis(6-alkyl-2-pyridylmethylene)ethane-1,2-diamine (where alkyl = H, Me, iPr) (L) with tetrakis(acetonitrile)copper(I) hexafluorophosphate and silver(I) hexafluorophosphate afforded, respectively, the double-stranded, dinuclear metal helicates [T-4-(R,R)]-(+/-)-[M2L2](PF6)2 (M = Cu, Ag). The helicates were characterized by 1H and 13C NMR spectroscopy, conductivity, microanalysis, and single-crystal X-ray structure determinations on selected compounds. Intermolecular ligand exchange and intramolecular inversion rates for the complexes were investigated by 1H NMR spectroscopy. Reversible intermolecular ligand exchange between two differently substituted helicates followed first-order kinetics. The rate constants (k) and corresponding half-lives (t(1/2)) for ligand exchange for the dicopper(I) helicates were k = (1.6-1.8) x 10(-6) s(-1) (t(1/2) = 110-120 h) in acetone-d6, k = 4.9 x 10(-6) s(-1) (t(1/2) = 40 h) in dichloromethane-d2, and k > 2 x 10(-3) s(-1) (t(1/2) < 5 min) in acetonitrile-d3. Ligand exchange for the disilver(I) helicates occurred with k > 2 x 10(-3) s(-1) (t(1/2) < 5 min). Racemization of the dicopper(I) helicate by an intramolecular mechanism was investigated by determination of the coalescence temperature for the diastereotopic isopropyl-Me groups in the appropriate complex, and DeltaG() >> 76 kJ mol(-1) was calculated for the process in acetone-d6, nitromethane-d3, and dichloromethane-d2 with DeltaG() = 75 kJ mol(-1) in acetonitrile-d3. Complete anion exchange of the hexafluorophosphate salt of a dicopper(I) helicate with the enantiomerically pure Delta-(-)-tris(catecholato)arsenate(V) ([As(cat)3]-) in the presence of Dabco gave the two diastereomers (R,R)-[Cu2L2][Delta-(-)-[As(cat)3]]2 and (S,S)-[Cu2L2][Delta-(-)-[As(cat)3]]2 in up to 54% diastereomeric excess, as determined by (1)H NMR spectroscopy. The diastereomerically and enantiomerically pure salt (R,R)-[Cu(2)L2][Delta-(-)-[As(cat)3]]2 crystallized from the solution in a typical second-order asymmetric transformation. The asymmetric transformation of the dicopper(I) helicate is the first synthesis of a diastereomerically and enantiomerically pure dicopper(I) helicate containing achiral ligands.

Chiral spiro Cu(i) complexes. Supramolecular stereocontrol and isomerisation dynamics by the use of TRISPHAT anions

Association of enantiopure TRISPHAT anion (1) with chiral spiro [Cu(LL')2] complexes (LL' = 2-R-phen, 2, 6-R-bpy, 3, and 2-iminopyridine, 4) leads to an efficient NMR enantiodifferentiation. Variable temperature 1H NMR spectroscopy has been used to determine the isomerisation kinetics of these pseudo-tetrahedral complexes and to evaluate their configurational stability; the latter depending on the structure of the diimine ligands. In the case of the 2-anthracenyl-phen derivative, a decent level of supramolecular stereocontrol was noted (d.e. up to 45%); the configuration of the complex being determined by electronic circular dichroism (ECD).

Solvent-tunable inversion of chirality transfer from carbon to copper

A change of solvent causes an inversion of the stereochemistry at copper of the chiral Cu(I) complex described herein.

NMR evaluation of the configurational stability of Cu(I) complexes

Association of chiral [CuL2]+ complexes (L = 2-R-phen, 6-R-bpy, and 2-iminopyridine) with TRISPHAT (tris(tetrachlorobenzenediolato)phosphate(V)) anion leads to NMR enantiodifferentiation, which can be used to determine the kinetics of racemization of the complexes.

Evaluation of Diimine Ligand Exchange on Cu(I)

Four ligands have been prepared, 8,8-dimethyl-6,7,9-trihydropyrido[1,2-b]acridine and three 4,4',6,6'-tetrasubstituted derivatives of 2,2'-bipyrimidine where the substituents are methyl, phenyl, and p-tolyl. The corresponding [CuL(2)](+) salts of these ligands evidence nonequivalent NMR signals that allow an estimation of the ligand exchange barrier in both acetonitrile and chloroform solution. Lower barriers are found in the former solvent and attributed to solvent participation in the exchange process. Corresponding differences in the oxidation potentials of the complexes are explained in a similar manner. The electronic absorption properties of the complexes are also consistent with the steric and electronic properties of the ligands. [Cu(2c)(2)](PF(6)), where 2c = 4,4',6,6'-tetraphenyl-2,2'-bipyrimidine, was analyzed by X-ray diffraction and found to crystallize in the space group Pccn with a = 14.761(2) A, b = 15.007(2) A, c = 24.407(4) A, and Z = 4. The internal and external phenyl rings are disposed quite differently, with the internal rings interacting strongly with the orthogonal ligand.

Friedländer synthesis of chiral alkyl-substituted 1,10-phenanthrolines

The synthetic scope of the Friedländer condensation in the preparation of chiral alkyl-substituted 1,10-phenanthrolines has been investigated. A range of chiral [x,y-b]-cycloalkeno-condensed phenanthrolines has been prepared in one step from steroidal or other cyclic ketones from the chiral pool and 8-amino-7-quinolinecarbaldehyde (1) via base-catalyzed condensation. Phenanthroline derivatives are formed in good yields with unhindered ketones, but the reaction proceeds even with sterically congested substrates such as camphor, albeit in low yield. The utility of the Friedländer condensation has been extended to the synthesis of chiral 3-alkyl-substituted phenanthrolines from monoalkyl-substituted acetaldehydes.

Facile synthesis of substituted phenanthroline ligands by samarium-promoted coupling of phenanthroline with ketones

[formula: see text] 1,10-Phenanthroline undergoes coupling with ketones promoted by samarium diiodide to produce 2-(1-hydroxyalkyl)-1,10-phenanthrolines. O-Methylation of these derivatives provides the corresponding 2-(1-methoxyalkyl)phenanthrolines. Demethoxylation with samarium diiodide then affords 2-alkylphenanthrolines. This process may be repeated to obtain 2,9-disubstituted phenanthrolines. A variety of new, substituted phenanthrolines are thus obtained. These compounds have numerous potential applications as ligands in metal-promoted reactions, including asymmetric catalysis.