Giuseppe Faita

Update 1 of: C2-Symmetric Chiral Bis(oxazoline) Ligands in Asymmetric Catalysis

Update 1 of: C2-Symmetric Chiral Bis(oxazoline) Ligands in Asymmetric Catalysis Giovanni Desimoni,* Giuseppe Faita, and Karl Anker Jørgensen* Department of Organic Chemistry, University of Pavia, Viale Taramelli 10, 27100 Pavia, Italy Danish National Research Foundation: Center for Catalysis, Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark This is a Chemical Reviews Perennial Review. The root paper of this title was published in Chem. Rev. 2006, 106 (9), 3561 3651, DOI: 10.1021/cr0505324; Published (Web) August 2, 2006. Updates to the text appear in red type

A new and highly efficient catalyst for the enantioselective Mukaiyama–Michael reaction between ( E )-3-crotonoyl-1,3-oxazolidin-2-one and 2-trimethylsilyloxyfuran

Abstract The Mukaiyama–Michael reaction between 2-trimethylsilyloxyfuran and (E)-3-crotonoyl-1,3-oxazolidin-2-one has been stereoselectively catalysed by several optically active complexes based on bis(oxazoline) (box) or pyridine bis(oxazoline) (pybox) chiral ligands and metal cations. The catalysts derived from the newly synthesised 2,6-bis[(4′R,5′R)-diphenyl-1,3-oxazolin-2′-yl]pyridine and the triflates of EuIII, LaIII, CeIV were highly efficient: the diastereoselectivity was entirely anti and the enantioselectivity was excellent (ranging from 98 to >99%). A mechanistic insight into the nature of the activated substrate–catalyst complex was inferred studying the lanthanum complexes with 1H and 13C NMR spectroscopy. Based on these results and on the crystallographic structure of the complex between pybox and La(OTf)3, a stereochemical model is proposed to rationalise the crucial role of the substituent in position 5, suitably placed to blind the Si-face of the coordinated reagent.

Copper (II) in organic synthesis. XI. Evaluation of the ligand architecture on the efficiency of a copper (II) catalyst for enantioselective Michael reactions

Abstract Several bis-copper (II) complexes with chiral ligands derived from 2-substituted 2-(salicylideneamino) ethanols have been tested as catalysts of enantioselective Michael reactions. The degree of enantioselection is strongly affected by the architecture of the ligand. The best result (75% e.e.) was obtained for a ligand having a substituent potentially suitable to induce the formation of a bis-tetradentate copper (II) complex with a square pyramidal coordination.

CHIRAL AMPLIFICATION IN DIELS-ALDER AND 1,3-DIPOLAR CYCLOADDITIONS CATALYZED BY BIS(OXAZOLINE)-ZN(II)-BASED CHIRAL COMPLEXES

Abstract A Zn(II)-bis(oxazoline) chiral complex catalyzes the cycloaddition reactions of alkenoyl-oxazolidinones with both cyclopentadiene and diphenyl nitrone. The 1,3-dipolar cycloaddition becomes exo selective with an ee of up to 86%. While the Mg(II)-based catalysts give a linear relationship between the ligands optical purity and the ee of the reaction product, a remarkable positive nonlinear effect was observed in both cycloadditions by using Zn(II) as the cationic core of the catalyst. The result may be rationalized by the formation of a stable, insoluble, and catalytically inactive heterochiral complex.

Can a chiral catalyst containing the same ligand/metal components promote the formation of both enantiomers enantioselectively? The bis(oxazoline)-magnesium perchlorate-catalyzed asymmetric Diels-Alder reaction

Abstract The Diels-Alder (DA) reaction between cyclopentadiene (1) and 3-acryloyl-1,3-oxazolidin-2-one (2), to give the endo adduct 5 as the main product, has been found to occur enantiosectively in the presence of catalytic amounts of magnesium perchlorate (MP) and 2,2-bis{2-[(4R)-phenyl-, [(4R,5S)- and [(4R,5R)-diphenyl-1,3-oxazolinyl]}propanes (3a-c respectively). The intermedia are the tetrahedral complexes 4a-c, with 2 and 3 coordinated around Mg(II), and their structures were investigated by NMR. Whereas 3a and 3b gave ee of (S)- 5 in the range 28–73%, 3c was an excellent ligand and 97% ee of (S)- 5 was obtained. When two equivalents of hydroxylic ligands (water or alcohols) are added, all these complexes can expand the coordination number of Mg(II) from 4 to 6. The octahedral complexes 6a-c are again good catalysts of the DA reaction but, with 3a and 3b, the enantioselectivity is reversed and (R)- 5 is obtained with ee up to 89%. For the first time, achiral auxiliary ligands can be used to invert the enantioselectivity induced by a chiral bis(oxazoline), so that both enantiomers can be synthetized with the same metal cation and chiral ligand. The catalytic enantioselective processes are discussed and a rationalization of the experimental results is proposed.

Solvent effect in pericyclic reactions. IX: The ene reaction

The solvent effect on the ene reactions of diethylazodicarboxylate and triazolindione was determined in 18 and 16 solvents respectively. The first enophile gives a solvent effect that depends on solvent acidity (type a solvent effect), while in the second reaction the significant solvent effect is the result of the nucleophilic character of the solvent (type b solvent effect). These are the same solvent behaviours observed in the analogous Diels-Alder cycloadditions, and evidence of the close parallel between these two pericyclic reactions was produced by determining the effect of inorganic perchlorates on their rates.

The first enantioselective synthesis of both Diels-Alder enantiomers with the same bis(oxazoline)-magnesium perchlorate chiral catalyst

Abstract the Diels-Alder reaction between cyclopentadiene and 3-acryloyl-1,3-oxazolin-2-one has been found to occur enantioselectively in the presence of catalytic amounts of ( R )-2,2′-isopropylidene-bis(4-phenyl-2-oxazoline) and magnesium perchlorate. From a tetrahedral coordination of catalyst and dienophile the ( S )-enantiomer is obtained. If two equivalents of water are added, the coordination occurs with an octahedral geometry and the same catalyst gives the ( R )-enantiomer selectively.

1,3-Dipolar cycloadditions catalyzed by bis(oxazoline)-magnesium-based chiral complexes. The importance of a Mg(II) counterion

Abstract The 1,3-dipolar cycloaddition between N -acryloyl-oxazolidinone and diphenyl nitrone is catalyzed by chiral catalysts derived from bis(oxazoline) (R) - 7 and Mg(II) perchlorate or triflate. In addition to complete regioselectivity, the endo cycloadduct can be obtained with up to 86% ee . The stereochemical results are due either to the specific counterion or to the presence of molecular sieves (as well as to the relative reagent concentrations). As a consequence, the enantioselecvity is opposite to that induced by the catalyst derived from Mg(II) iodide with the same chiral ligand.

Solvent effect as the result of frontier molecular orbital interaction. V. Diels-Alder with heterodienophiles: a unified approach to the solvent effect of the Diels-Alder reactions☆

Abstract The solvent effect of Diels-Alder (D.A.) reactions with heterodienophiles was measured in several solvents. The rate of the reaction between 2,3-dimethylbutadiene (DMB) and diethylazodicarboxylate increases with the increases in the Acceptor Number of the solvent which behaves as an electrophile. The rate of the reaction between DMB and p. bromonitrosobenzene shows a small solvent effect which correlates with the cohesive pressure (δH2) of the solvent. The rate of the reactions between DMB and tetrachloro- or tetrabromo-o.benzoquinones give an inverse linear relationship with Dπ basicity parameters, thus rate decreases with increases in the nucleophilic character of the solvent..p]These monoparametric correlations (alternatively obtained with electrophilic, nucleophilic or cohesive pressure parameters of the solvent) rationalize the solvent effect of D.A. reactions reported in the literature, which fit one of the above reported three classes. The same result is obtained if the Kamlet-Taft multiparametric equation is applied to the same set of reactions. The main contribution remains that outlined with the monoparametric approach and some secondary effects or some borderline cases are usefully focussed

Chiral Lewis acid controlled Diels-Alder reaction: Factors affecting the stereoselective formation of both enantiomers in the bis(oxazoline)-magnesium catalyzed process

Abstract The chiral Lewis acid controlled Diels-Alder (CLAC-DA) reaction between cyclopentadiene and 3-acryloyl- or (E)-3-crotonyl-1,3-oxazolidin-2-ones (2a,b), to give the endo adducts 3a,b has been studied. The use of magnesium perchlorate (MP) or magnesium triflate (MT), and three chiral bis(oxazolines) (5–7) and, eventually, 2 equiv of achiral auxiliary ligands X [water or tetramethylurea (TMU)], induces a strong change of the enantioface selectivity. The intermediates are either tetrahedral or octahedral. The MP-based intermediates can be either tetrahedral, giving rise to the endo (S) adducts enantioselectively, or, by addition of 2 equiv X, octahedral giving rise to the endo (R) adducts. The MT-based intermediates always have an octahedral configuration in the presence of two triflate ligands, and the endo (R) adducts are obtained enantioselectively. The controlled use of the different reagents to assemble the catalysts gives either (S)- 3 a in 94% ee from a tetrahedral complex, or (R)- 3 a in 93% ee from an octahedral one, whereas (R)- 3 b can be prepared with ee up to 95%.