Flow Technology for the Genesis and Use of (Highly) Reactive Organometallic Reagents

In the field of organic synthesis, the advent of flow chemistry and flow microreactor technology represented a tremendous novelty in the way of thinking and performing chemical reactions, opening the doors to poorly explored or even impossible transformations using batch methods. In this Concept article, we would like to highlight the impact of flow chemistry for exploiting highly reactive organometallic reagents, and how, alongside the well-known advantages concerning safety, scalability, and productivity, flow chemistry makes possible processes that are impossible to control by using the traditional batch approach.     

Stereoselective synthesis of novel beta,gamma-epoxyhydroxylamines and 4-hydroxyalkyl-1,2-oxazetidines

A simple and efficient stereoselective synthesis of polysubstituted beta,gamma-epoxyhydroxylamines and 4-hydroxyalkyl-1,2-oxazetidines, based on the addition of alpha-lithiated aryloxiranes to nitrones and subsequent cyclization of the corresponding intermediates in a 4-exo-tet mode, is described.     

Solvent- and temperature-dependent functionalisation of enantioenriched aziridines

A highly stereo‐ and regioselective functionalisation of chiral non‐racemic aziridines is reported. By starting from a parent enantioenriched aziridine and finely tuning the reaction conditions, it is possible to address the regio‐ and stereoselectivity of the lithiation/electrophile trapping sequence, thereby allowing the preparation of highly enantioenriched functionalised aziridines. From chiral N‐alkyl trans‐2,3‐diphenylaziridines (S,S)‐ 1 a , b , two differently configured chiral aziridinyllithiums could be generated (trans‐ 1 a , b‐Li in toluene and cis‐ 1 a , b‐Li in THF), thus disclosing a solvent‐dependent reactivity that is useful for the synthesis of chiral tri‐substituted aziridines with different stereochemistry. In contrast, chiral aziridine (S,S)‐ 1 c showed a temperature‐dependent reactivity to give chiral ortho‐lithiated aziridine 1 c‐ ortho ‐Li at ?78 °C and α‐lithiated aziridine 1 c‐α‐Li at 0 °C. Both lithiated intermediates react with electrophiles to give enantioenriched ortho‐ and α‐functionalised aziridines. The reaction of all the lithiated aziridines with carbonyl compounds furnished useful chiral hydroxyalkylated derivatives, the stereochemistry of which was ascertained by X‐ray and NMR spectroscopic analysis. The usefulness of chiral non‐racemic functionalised aziridines has been demonstrated by reductive ring‐opening reactions furnishing chiral amines that bear quaternary stereogenic centres and chiral 1,2‐, 1,3‐ and 1,5‐aminoalcohols. It is remarkable that the solvent‐dependent reactivity observed with (S,S)‐ 1 a , b permits the preparation of both the enantiomers of amines ( 11 and ent‐ 11 ) and 1,2‐aminoalcohols ( 13 and ent‐ 13 ) starting from the same parent aziridine. Interestingly, for the first time, a configurationally stable chiral α‐lithiated aziridine ( 1 c‐α‐Li ) has been generated at 0 °C. In addition, ortho‐hydroxyalkylated aziridines have been easily converted into chiral aminoalkyl phthalans, which are useful building blocks in medicinal chemistry.     

Synthesis of [Ir3Rh(CO)12] and Fluxional Behaviour of Some of Its Substituted Derivatives

The redox condensation of [Ir(CO)₄]^–, [Ir(cod)(THF)₂]⁺, and [Rh(cod)(THF)₂]⁺ (cod = cycloocta-1,5-diene) followed by saturation with CO (1 atm) in THF afforded the first synthetic route to pure [Ir₃Rh(CO)₁₂] ( 1 ). Substitution of CO by monodentate ligands gave [Ir₃Rh(CO)₈(μ₂-CO)₃L] (L = Br^–, 2 ; I^–, 3 ; bicyclo[2.2.1]hept-2-ene, 4 ; PPh₃, 5 ). Clusters 2 – 5 have C_s symmetry with the ligand L bound to the basal Rh-atom in axial position. They are fluxional in solution at the NMR time scale due to two CO scrambling processes: the merry-go-round of basal CO's and changes of basal face. An additional process takes place in 5 above room temperature: the intramolecular migration of PPh₃ from the Rh- to a basal Ir-atom. Substitution of CO by polydentate ligands gave [Ir₃Rh(CO)_7–x(μ₂-CO)₃(η⁴-L)x] (L = bicyclo[2.2.1]hepta-2,5-diene (= norbornadiene; nbd), x = 1, 6 ; L = nbd, x = 2, 13 ; L = cod, x = 1, 7 ; L = cod x = 2, 15 ), [Ir₃Rh(CO)₇(μ₂-CO)₃(η²-diars)] (diars = 1,2-phenylenebis-(dimethylarsine); 8 ), [Ir₃Rh(CO)₇(μ₂-CO)₃(η⁴-L)] (L = methylenebis(diphenylphosphine), bonded to 2 basal Ir-atom ( 9a ) or one Ir- and one Rh-atom ( 9b )), [Ir₃Rh(CO)₆(μ₂-CO)₃(η⁴-nbd)PPh₃] ( 12 ), and [Ir₃Rh(CO)₆(μ₂-CO)₃(μ₃-L)] (L = 1,3,5-trithiane, 10 ; L = CH(PPh₂)₃, 11 ). Complexes 6 – 8 , 9a , 10 , and 11 have C_s symmetry, the others C₁ symmetry. They are fluxional in solution due to CO scrambling processes involving 1, 3, or 4 metal centres as deduced from 2D-EXSY spectra. Comparison of the activation energies of these processes with those of the isostructural Ir₄ and Ir₂Rh₂ compounds showed that substitution of Ir by Rh in the basal face of an Ir₄ compound slows the processes involving 3 or 4 metal centres (merry-go-round and change of basal face), but increases the rate of carbonyl rotation about an Ir-atom.     

Comparative study of water dissociation on Rh(111) and Ni(111) studied with first principles calculations

The dissociation and formation of water on the Rh(111) and Ni(111) surfaces have been studied using density functional theory with generalized gradient approximation and ultrasoft pseudopotentials. Calculations have been performed on 2x2 surface unit cells, corresponding to coverages of 0.25 ML, with spot checks on 3x3 surface unit cells (0.11 ML). On both surfaces, the authors find that water adsorbs flat on top of a surface atom, with binding energies of 0.35 and 0.25 eV, respectively, on Rh(111) and Ni(111), and is free to rotate in the surface plane. Barriers of 0.92 and 0.89 eV have to be overcome to dissociate the molecule into OH and H on the Rh(111) and Ni(111) surfaces, respectively. Further barriers of 1.03 and 0.97 eV need to be overcome to dissociate OH into O and H. The barriers for the formation of the OH molecule from isolated adsorbed O and H are found to be 1.1 and 1.3 eV, and the barriers for the formation of the water molecule from isolated adsorbed OH and H are 0.82 and 1.05 eV on the two surfaces. These barriers are found to vary very little as coverage is changed from 0.25 to 0.11 ML. The authors have also studied the dissociation of OH in the presence of coadsorbed H or O. The presence of a coadsorbed H atom only weakly affects the energy barriers, but the effect of O is significant, changing the dissociation barrier from 1.03 to 1.37 and 1.15 eV at 0.25 or 0.11 ML coverage on the Rh(111) surface. Finally, the authors have studied the dissociation of water in the presence of one O atom on Rh(111), at 0.11 ML coverage, and the authors find a barrier of 0.56 eV to dissociate the molecule into OH+OH.     

Synthesis of micelle templated silico-aluminas with different alumina contents

The computer aided analysis of the EPR spectra of radical surfactant probes inserted in cetyltrimethylammonium bromide micelles provided information on the kinetics of formation of micelle templated silico-aluminas (MTSA) at 343 K, obtained by means of silica and alumina alkaline solutions at different Si/Al ratios (from infinity to 4). Mainly two spectral components were analyzed and relatively quantified in the EPR spectra: (1) the micellar component, due to probes inserted in the surfactant aggregates, whose mobility decreases over the synthesis time, thus reporting on the progressive modification of the micelle structure and the solid condensation; (2) the interacting component, mainly arising from the electrostatic interactions between the surfactant heads and the charged surface sites. This last component increases its relative intensity over the synthesis time, informing about condensation and structuration of the silico-alumina at the micelle surface. X-ray diffraction (XRD), nitrogen sorption isotherms at 77 K, thermogravimetric analysis, TEM and chemical analysis were performed to characterize both as-synthesized and calcined MTSA materials. Nitrogen sorption isotherms allowed us to evaluate the pore diameter, the specific surface area and the pore volume. At Si/Al<15 a decrease in pore volume and specific surface area was interpreted as due to the contemporaneous presence of a hexagonal MTSA and an amorphous material, which was ascertained by means of XRD as the only present at Si/Al=4. The amorphous structure at Si/Al<15 used Na+ as contraions, whereas the surfactants are no more needed to neutralize the negatively charged groups at the solid surface. The hypothesis of a "break" at Si/Al=15 was supported by EPR: the interactions between the surfactant probe heads and the negatively charged surface groups are drastically reduced at Si/Al<15. On the contrary, at Si/Al>15, increasing amounts of alumina slow the kinetics of the synthesis but enhance electrostatic interactions between the surfactant heads and the negatively charged surface groups. Dilution of the synthesis mixture decreased the extent of the interactions, due to partial protonation of the silanol groups, and slowed the synthesis process.     

A boundary-integral equation method for free surface viscous flows

Summary A boundary integral equation method is proposed for the solution of viscous recirculating flows with free surfaces. In particular the method is applied to thermocapillary convection and to drop formation, both in micro-gravity conditions, the latter to test its capability to handle real unsteady problems.The presence of non linear terms in Navier-Stokes equations leads to a volume integral, which has to be approximated by a linearization procedure.Several numerical results for thermocapillary flows, both with fixed and moving free surface, are discussed in comparison with previously obtained finite difference solutions. Some preliminary results, and in particular the time evolution of the free surface shape, are also presented for the drop formation problem. Only plane two dimensional fields are considered for both problems.

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Sommario Si propone un metodo basato sulla soluzione di equazioni integrali di contorno per flussi viscosi con superficie libera. Tale metodo è applicato allo studio della convezione termocapillare ed al processo di formazione di una goccia, entrambi in condizioni di microgravità. La presenza dei termini non lineari nell'equazione di Navier-Stokes comporta un integrale di volume che viene approssimato mediante un processo di linearizzazione.Risultati numerici per flussi termocapillari con superficie libera sia fissa che mobile sono confrontati con altri ottenuti in precedenza con un metodo alle differenze finite. Si presentano inoltre alcuni risultati preliminari sul problema della formazione della goccia ed in particolare l'evoluzione nel tempo della configurazione geometrica della superficie libera. Nei due casi si analizzano solo campi bidimensionali.

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Presented at the VII National Conference AIDAA, Naples, September 1983.

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In leave of absence from Tianjin University, China.     

Vector green's function method for unsteady Navier-Stokes equations

Summary The Green's function method is reformulated in general terms to treat vector unsteady and nonlinear equations. The particular expressions of the adjoint linear operator, the Green's formula and the integral representation of the solution are derived for unsteady Navier Stokes equations. The appropriate fundamental solutions for incompressible and for certain compressible flows have been obtained in closed form. Both the positive features and the possible limits of the method are briefly outlined.

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Sommario Si riformula in termini generali il metodo della funzione di Green per estenderlo al caso di equazioni vettoriali e non lineari. In particolare si ricavano le espressioni della formula di Green e della rappresentazione integrale della soluzione per le equazioni di Navier Stokes non stazionarie. Si ottengono le soluzioni fondamentali in forma chiusa sia per il caso di fluidi comprimibili che incomprimibili. Si discutono infine brevemente i lati positivi ed i possibili limiti della metodologia illustrata.