Iron catalyzed diastereoselective hydrogenation of chiral imines.

Cyclopentadienone-based iron complexes were used for the first time to successfully catalyze the diastereoselective hydrogenation of enantiopure imines. Chiral amines, including valuable biologically active products, were obtained often as enantiomerically pure compounds. Computational studies helped to elucidate the chemical and stereochemical aspects of the iron-catalyzed reaction.

Kinetics versus thermodynamics in the proline catalyzed aldol reaction.

In this paper the equilibrium properties of the proline catalyzed aldol reaction was studied. The use of well-established methodologies, like reaction progress kinetic analysis and linear free energy relationship analysis, led to the quantification of the reaction reversibility and to its correlation with the substrate electronic activation. Due to these experimental observations, common computational approaches based on a one way transition state analysis become unsuitable. Therefore, a computational model based on the integration of a system of kinetic differential equations associated to the multiple equilibrium reactions was proposed. Such a model was found to successfully rationalize the chemical and stereochemical outcomes of this paradigmatic reaction for the first time.

Metal-Free Reduction of Aromatic and Aliphatic Nitro Compounds to Amines: A HSiCl3-Mediated Reaction of Wide General Applicability.

A new, mild, metal-free, HSiCl3-mediated reduction of both aromatic and aliphatic nitro groups to amines that is of wide general applicability, tolerant of many functional groups, and respectful of the stereochemical integrity of stereocenters is reported.

Soluble-polymer-supported synthesis of beta-lactams on a modified poly(ethylene glycol)

A modified poly(ethylene glycol) (PEG) has been developed for the soluble-polymer-supported synthesis of beta-lactams. The monomethylether of PEG (MeOPEG) with an average M(W) of 5000 was used as the support, a 4-(3-propyl)phenyl residue as the spacer, and a 4-oxyphenylamino group as the moiety with the reactive functionality. From this modified PEG representative aromatic, heteroaromatic, unsaturated, and aliphatic imines were obtained in high yields by different procedures. The polymer-supported imines were then employed to prepare several beta-lactams by enolate/imine condensation and ketene/imine cycloaddition. Examples of the control of the absolute stereochemistry during the azetidinone ring formation are also reported. The reactions carried out on the polymer-bound imines showed a remarkable similarity to those performed on nonimmobilized imines, both in terms of yields and stereoselectivities. Removal of the beta-lactams from the polymer has also been accomplished to directly deliver the N-unsubstituted azetidinones.

Synthesis of oligopyridines and their metal complexes as precursors to topological stereoisomers.

[structure: see text]. Palladium-based carbon-carbon coupling reactions in sequence with halogen-exchange chemistry on a series of heterocycles lead to an efficient synthetic strategy for oligopyridines that bind metal ions such as ruthenium to form coordination racks. The particular structures are designed to form terpyridine subunits for octahedral binding. Reaction of 4,6-diiodopyrimidine produces a "double-bay" terpyridine from which binuclear coordination complexes have been formed.

Stereoselective synthesis of new beta-lactams by cyclocondensation of 1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene with 4-formyl substituted azetidinones.

The BF3.OEt2 or LiClO4 catalyzed hetero Diels-Alder reaction of 1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene (Danishefsky's diene) with enantiomerically pure 4-formylazetidin-2-ones affords the corresponding cycloadducts in fair to good yields and in diastereoisomeric ratios of up to 98:2.

Wettability of bare and fluorinated silanes: a combined approach based on surface free energy evaluations and dipole moment calculations.

The assessment of the surface free energy (SFE) of a material permits to control and predict a large number of physicochemical properties of a solid surface and its reactivity. Here, the surface energies of a series of bare and fluorinated silanes are determined by means of different semi-empirical models on the grounds of contact angle determinations for different solvents. Literature data are also considered in order to produce a series of films with increasing SFE. Wetting envelopes (WE) are obtained for the various surfaces in order to predict the wettability of the films by numerous commonly employed solvents. The polar and disperse components of the surface energies are obtained by the Owens-Wendt-Rabel-Kaelbe (OWRK) method; the values of the polar components are compared with gas-phase dipole moments obtained by theoretical calculations employing semi-empirical Hamiltonians. The sequences of the polar components of the SFE and of the calculated dipole moments for the different molecules are strictly the same. The interplay between theoretical and experimental approaches proves efficient in predicting the behavior of different systems and it could be employed in tuning the SFE of a solid surface with respect to its final applications.

Recoverable, soluble polymer-supported organic catalysts.

Enantioselective organic catalysis is currently the subject of an intense research activity aimed to develop 'metal-free' alternatives to established 'metal-based' catalytic processes. In the case of chiral catalysts, which are often expensive, or obtained after a complex synthesis, the immobilization on solid support represents an attractive methodology that allows the recovery and possibly the recycling of the catalytic species. Among the different solid matrixes employed, soluble polymers recently emerged as very convenient supports for the synthesis of a variety of small organic molecules, ligands and catalysts. This chapter highlights the more recent results obtained by our group in the field of poly(ethylene glycol) (PEG)-supported chiral and achiral organic catalysts. A few considerations on the methodologies, the future and the problems related to the immobilization of chiral organic catalysts are also briefly discussed.

An improved methodology for the synthesis of enantiomerically pure (S)-2,3-O-cyclohexylideneglyceraldehyde.

An improved methodology to prepare (S)2,3-O-cyclohexylideneglyceraldehyde is described. Starting from the commercially available (L)-cyclohexylidene protected ascorbic acid the enantiomerically pure aldehyde was synthesized in only two steps in 41% overall yield.

Mechanistic studies of radical-based processes. Use and misuse of EPR spectroscopy

The identification of the species observed by EPR spectroscopy in the SmI(2)-promoted coupling of N-(N', N'-dialkylaminoalkyl)benzotriazoles as substituted benzyl radicals is disputed on the basis of results obtained studying the model radical derived from tribenzylamine.

Poly(ethylene glycol)-supported bisoxazolines as ligands for catalytic enantioselective synthesis.

Two chiral bisoxazolines (box) supported on a modified poly(ethylene glycol) (PEG) have been prepared by a reaction sequence that involved formation of the properly functionalized box and their attachment to the polymer matrix by means of a spacer and a linker. The solubility properties of PEG allowed use of the supported box as ligands in some catalytic asymmetric transformations carried out under homogeneous conditions and to recover the ligands as if bound to an insoluble support. When the supported box were employed in combination with Cu(II) salts in the Diels-Alder cycloaddition between cyclopentadiene and N-acryloyloxazolidinone, low levels of enantioselectivity were observed (up to 45% ee). Much better results were obtained in the cyclopropanation of styrenes carried out in the presence of CuOTf (up to 93% ee) and in the ene-reaction between alpha-methylstyrene or methylenecyclohexane and ethylglyoxalate (up to 95% ee). One of the ligands, readily recovered by precipitation and filtration, was recycled two times in the ene-reaction with marginal loss in the catalytic activity and very limited erosion of the enantioselectivity.

A poly(ethylene glycol)-supported quaternary ammonium salt: An efficient, recoverable, and recyclable phase-transfer catalyst

A quaternary ammonium salt readily immobilized on a soluble poly(ethylene glycol) polymer support efficiently catalyzes different reactions carried out under phase-transfer catalysis conditions; the catalyst, easily recovered by precipitation and filtration, shows no appreciable loss of activity when recycled three times.