Giorgio Chelucci

Synthesis and metal-catalyzed reactions of gem-dihalovinyl systems.

1,1-Dihalo-1-alkenes are valuable synthetic tools in organic chemistry and serve as interesting synthetic intermediates in a variety of non-metal-assisted chemical transformations. Moreover, the vinyl dihalide functionality is an attractive and versatile bidentate electrophile for organometallic chemistry.

Osmium(II) CNN pincer complexes as efficient catalysts for both asymmetric transfer and H2 Hydrogenation of Ketones

We describe the isolation of the first CNN pincer osmium complexes [OsX(CNN)P 2 ] to display high catalytic activity and productivity in the reduction of ketones by either TH or HY (TOFand TON up to 10 6 h 1 and 10 5 , respectively). A high enantioselectivity (up to 98% ee ) is possible in both reactions with a remarkably low catalyst loading (0.005–0.002 mol%). To the best of our knowledge, this is the first example of an osmium complex that catalyzes the asymmetric HY of ketones. Evidence for an Os-OR vs. Os-H equilibrium suggests that both these species are involved in the catalytic pathways. Mechanistic studies as well as the preparation of new CNN pincer osmium catalysts are currently underway.

Asymmetrische katalysen: XLVI. Enantioselektive Hydrosilylierung von Ketonen mit [Rh(COD)Cl]2 und optisch aktiven Stickstoff-Liganden☆

The synthesis and characterisation of 16 optically active nitrogen ligands, namely, pyridinethiazolidones, pyridinethiazolines, pyridinimidazolines, Schiff bases and bipyridines, are described. These ligands are used as cocatalysts together with the procatalyst [Rh(COD)Cl]2 in the catalytic hydrosilylation of prochiral ketones with diphenylsilane. With these homogeneous in situ catalysts, optically active 1-phenylethanol is produced from acetophenone after hydrolysis of the silyl ether. The diastereomers of N-(1-phenylethyl)-2-(2-pyridinyl)-thiazolidine-4-one give opposite optical inductions. The best optical purity of 71.6% ee is obtained with a pinanyl-substituted bipyridine.

Highly Productive CNN Pincer Ruthenium Catalysts for the Asymmetric Reduction of Alkyl Aryl Ketones

Chiral pincer ruthenium complexes of formula [RuCl(CNN)(Josiphos)] (2-7; Josiphos = 1-[1-(dicyclohexylphosphano)ethyl]-2-(diarylphosphano)ferrocene) have been prepared by treating [RuCl(2)(PPh(3))(3)] with (S,R)-Josiphos diphosphanes and 1-substituted-1-(6-arylpyridin-2-yl)methanamines (HCNN; substituent = H (1 a), Me (1 b), and tBu (1 c)) with NEt(3). By using 1 b and 1 c as a racemic mixture, complexes 4-7 were obtained through a diastereoselective synthesis promoted by acetic acid. These pincer complexes, which display correctly matched chiral PP and CNN ligands, are remarkably active catalysts for the asymmetric reduction of alkyl aryl ketones in basic alcohol media by both transfer hydrogenation (TH) and hydrogenation (HY), achieving enantioselectivities of up to 99 %. In 2-propanol, the enantioselective TH of ketones was accomplished by using a catalyst loading as low as 0.002 mol % and afforded a turnover frequency (TOF) of 10(5)-10(6) h(-1) (60 and 82 degrees C). In methanol/ethanol mixtures, the CNN pincer complexes catalyzed the asymmetric HY of ketones with H(2) (5 atm) at 0.01 mol % relative to the complex with a TOF of approximately 10(4) h(-1) at 40 degrees C.

Chiral ligands containing heteroatoms. 8. 2-[(2s)-2-pyrrolidinyl]pyridine as a novel catalyst in the enantioselective addition of diethylzinc to aldehydes

Abstract The title compound has been demonstrated to be an effective enantioselective catalyst in the addition of diethyizinc to aldehydes: optically active secondary alcohols in up to 100% ee were obtained in high yields. The temperature dependence of the stercodynamic course of the reaction was examined and the nature of the possible catalytic complex was discussed on the basis of NMR data too.

Diastereoisomeric pure 2-(1-hydroxyalkyl)pyridines as catalysts in the enantioselective addition of diethylzinc to aldehydes

Abstract Diastereoisomeric pure 2-(1-hydroxyalkyl)pyridines have been prepared from chiral ketones and checked as enantioselective catalysts in the addition of diethylzinc to aldehydes: enantioselectivities up to 82% were obtained.

Palladium(II) and platinum(II) derivatives with chiral 2,2′-bipyridines. X-ray structure of C18H15ClN2Pd; C- and N-intramolecular coordination in a six-membered metallacycle

Abstract The reactions of (+)-( S )-5-s,butyl-2,2′-bipyridine (LH) and 6-(1-methylbenzyl)-2,2′-bipyridine (L′H) with Na 2 [PdCl 4 ] and K 2 [PtCl 4 ] give the 1 : 1 adducts (LH)MCl 2 (M = Pd, 1 ; M = Pt, 2 ) and the metallated species (L′)MCl (M = Pd, 3 ; M = Pt, 4 ), respectively. In complexes 3 and 4 , the deprotonated bipyridine behaves as a tridentate ligand; there is coordination to the metal through the two nitrogen atoms and the ortho -carbon atom of the phenyl ring to give a six-membered metallacycle. The X-ray structure of complex 3 shows that the coordination around the palladium atom is distorted towards tetrahedral from the usual square-planar geometry.

Recent Advances in Osmium-Catalyzed Hydrogenation and Dehydrogenation Reactions

CONSPECTUS: A current issue in metal-catalyzed reactions is the search for highly efficient transition-metal complexes affording high productivity and selectivity in a variety of processes. Moreover, there is also a great interest in multitasking catalysts that are able to efficiently promote different organic transformations by careful switching of the reaction parameters, such as temperature, solvent, and cocatalyst. In this context, osmium complexes have shown the ability to catalyze efficiently different types of reactions involving hydrogen, proving at the same time high thermal stability and simple synthesis. In the catalytic reduction of C═X (X = O, N) bonds by both hydrogenation (HY) and transfer hydrogenation (TH) reactions, the most interest has been focused on homogeneous systems based on rhodium, iridium, and in particular ruthenium catalysts, which have proved to catalyze chemo- and stereoselective hydrogenations with remarkable efficiency. By contrast, osmium catalysts have received much less attention because they are considered less active on account of their slower ligand exchange kinetics. Thus, this area remained almost neglected until recent studies refuted these prejudices. The aim of this Account is to highlight the impressive developments achieved over the past few years by our and other groups on the design of new classes of osmium complexes and their applications in homogeneous catalytic reactions involving the hydrogenation of carbon-oxygen and carbon-nitrogen bonds by both HY and TH reactions as well as in alcohol deydrogenation (DHY) reactions. The work described in this Account demonstrates that osmium complexes are emerging as powerful catalysts for asymmetric and non-asymmetric syntheses, showing a remarkably high catalytic activity in HY and TH reactions of ketones, aldehydes, imines, and esters as well in DHY reactions of alcohols. Thus, for instance, the introduction of ligands with an NH function, possibly in combination with a pyridine ring, led to a new family of [OsCl2(PP)(NN)] (NN = diamine, 2-aminomethylpyridine; PP = diphosphine) and pincer [OsCl(CNN)(PP)] (HCNN = 6-aryl-2-aminomethylpyridine, 2-aminomethylbenzo[h]quinoline) complexes, which are outstanding catalysts for (asymmetric) HY and TH of carbonyl compounds and DHY of alcohols with turnover numbers and turnover frequencies up to 10(5) and 10(6) h(-1), respectively. In addition, PNN osmium complexes containing the 2-aminomethylpyridine motif have been found to be among the most active catalysts for HY of esters. These complexes have shown catalytic activities that are comparable and in some cases superior to those reported for analogous ruthenium systems. These results give an idea of the potential of Os complexes for the design of new highly productive and robust catalysts for the synthesis of chiral and nonchiral alcohols and amines as well as ketones from alcohols. Thus, we hope that this report will promote increased interest in the chemistry of these metal complexes, opening novel opportunities for new catalytic processes as well as the improvement of existing ones.

CHIRAL SULFOXIDES AND SULFIDES TETHERED TO PYRIDINES AS LIGANDS FOR ENANTIOSELECTIVE CATALYSIS : PALLADIUM CATALYZED ALLYLIC SUBSTITUTION AND ADDITION OF DIETHYLZINC TO BENZALDEHYDE

Abstract Chiral 2-(1- p -tolylsulfinyl)alkylpyridines and the corresponding sulfide-pyridines were prepared and assessed in the enantioselective palladium catalyzed allylic substitution of 1,3-diphenylprop-2-enyl acetate with dimethylmalonate and addition of diethylzinc to benzaldehyde.

Chiral ligands with pyridine donors in transition metal catalyzed enantioselective cyclopropanation and hydrosilylation reactions

Abstract Copper(I) and rhodium(I) complexes prepared in situ from [Cu(OTf)(C6H6)0.5] and [Rh(cod)Cl]2 with a range of chiral 2,2′-bipyridines, 5,6-dihydro-1,10-phenanthrolines, 1,10-phenanthrolines and 2,2′:6′,2′′-terpyridines were assessed as chiral catalysts for the enantioselective cyclopropanation of styrene with diazoacetates and for the hydrosilylation of acetophenone with diphenylsilane. Enantioselectivities up to 68% in the cyclopropanation and up to 32% in the hydrosilylation were obtained.