Chang-Woo Cho

LITHIATION AND PHOSPHORYLATION OF CHIRAL 1,1'-BIS(OXAZOLINYL)FERROCENES

Two chiral 1,1′-bis[(S-2-(4-R-oxazolinyl)]ferrocenes (3) were prepared by condensation of 1,1′-bis(chlorocarbonyl)ferrocene with (S)-valinol and (S)-tert-leucinol, respectively. Lithiation of 3 with n-BuLi followed by coupling with ClPPh2 gave the diphosphorylated product 5 of C2 symmetry as a minor product and the monophosphorylated product 4 as the major product even with using more than 2 equiv of n-BuLi. Use of t-BuLi led to the non-C2 symmetric diphosphorylated product 6, which acted as an N,P-chelating ligand in the palladium complex 7.

Synthesis of chiral 1′-substituted oxazolinylferrocenes as chiral ligands for Pd-catalyzed allylic substitution reactions

Chiral 1-oxazolinyl-1′-(diphenylphosphino)ferrocenes 4 and 1-oxazolinyl-1′-(phenylthio)ferrocenes 5 were prepared from 1,1′-dibromoferrocene through 1-oxazolinyl-1′-bromoferrocenes 3. The compounds 4 and 5 were employed as chiral ligands in Pd-catalyzed asymmetric allylic substitution reactions. High enantioselectivities (82–99% ee) and high yields (96–99%) were observed in the substitution reactions of 1,3-diphenylprop-2-enyl acetate and dimethyl malonate with the catalysts generated from [(π-allyl)PdCl]2 and 4, while the use of 5 led to inferior results (20–75% ee and 25–28% yields). 1H-, 13C-, and 31P-NMR data are analyzed for the complexes obtained from the reactions of [(η3-allyl)PdCl]2 and [(η3-1,3-diphenylallyl)PdCl]2 with 4a and 4b.

Synthesis of chiral bis(oxazolinyl)biferrocene ligands and their application to Cu(I)-catalyzed asymmetric cyclopropanation

Abstract Homochiral bis(oxazolinyl)biferrocene ligands, which have both planar and central chirality, are synthesized from oxazolinylferrocenes through a diastereoselective directed lithiation followed by an oxidative dimerization. Asymmetric cyclopropanation of styrene with diazoacetates in the presence of 5 mol% CuOTf-bis(oxazolinyl)biferrocene complexes gives 2-(phenyl)cyclopropane carboxylates in up to 99% ee.

Organocatalytic asymmetric synthesis of chiral pyrrolizines by cascade conjugate addition-aldol reactions.

As the first N-centered heteroaromatic nucleophile for organocatalytic cascade reactions, pyrroles underwent the enantio- and diastereoselective organocatalytic cascade conjugate addition-aldol reactions of α,β-unsaturated aldehydes that afford the highly functionalized chiral pyrrolizines bearing three consecutive stereocenters in good yields, high enantioselectivities (90-98% ee), and excellent diastereoselectivities (>20:1 dr in all cases).

A Substrate-Based Methodology That Allows the Regioselective Control of the Catalytic Aminohydroxylation Reaction

Structure I is proposed for the catalytically active species in the bis-cinchona alkaloid based osmium-catalyzed aminohydroxylation (AA) reaction of unsymmetrical olefins. Based on this model, the regiochemistry for this reaction can be controlled through a substrate binding mode wherein steric, electronic, and/or shape complimentarity between the olefin and catalyst direct the regiochemical outcome along the reaction coordinate.

CHIRAL BIFERROCENE-BASED BIS(OXAZOLINES) : LIGANDS FOR CU(I)-CATALYZED ASYMMETRIC CYCLOPROPANATIONS OF ENE-DIAZOACETATES

Abstract Chiral biferrocene-based bis(oxazoline) compounds, which have both planar and central chirality, are synthesized. The Cu(I)-complexes of the bis(oxazolinyl)biferrocenes were found to be conformationally flexible and have a wide bite angle (136°) by NMR and molecular modeling studies. The complexes catalyzed the intramolecular cyclopropanation reaction of ene-diazoacetates, and low to moderate enantioselectivities and decreased reactivity were observed, compared to known malonate-derived bis(oxazoline) systems.

Effects of solvent and lithiating agent on stereoselectivity in lithiation of chiral 1,1′-bis(oxazolinyl)ferrocenes

Abstract The analysis of phosphorylation products was carried out to investigate the stereoselectivity change in lithiation of 1,1′-bis[(S)-2-(4-R-oxazolinyl)]ferrocene (1) with varying reaction conditions such as solvents, lithiating agents, and temperatures. Monolithiation with butyllithiums in Et2O led to the configuration corresponding to (R)-2, while use of THF favors (S)-2. Dilithiation with sec-BuLi or t-BuLi in Et2O led to the configuration corresponding to (R,S)-3. However, the step-by-step lithiation at different temperatures with sec-BuLi in Et2O provided the dilithiated species corresponding to (R,R)-3. sec-BuLi in THF was special to give (S,S)-3 as the major product.

Solution and soluble polymer syntheses of 3-aminoimidazoline-2,4-diones

3-Aminoimidazoline-2,4-dione derivatives have been synthesized from a combination of α- and aza-amino acids by both solution phase and soluble polymer supported approaches; this soluble polymer methodology combines clean product isolation with recycling of the original matrix.