1,721,074 research outputs found
Asymmetric reduction by transition metals complexes and biocatalysts
No full textIn the last years the importance of α- and -amnimoacids is increased since these molecules are interesting synthons for the preparation of unnatural peptides and bioactive compounds, in particular antibiotics or anticancer.
They can be easily obtained in optically pure form by reduction using metal catalyzed asymmetric hydrogenation1(chemocatalysis) or by enzyme mediated reduction2 (biocatalysis). In particular with the biocatalytical approach we used whole cells of different microorganisms endowed with enantioselective dehydrogenases, in chemocatalysis we used essentially Ir(I) and Ru(II) complexes of different atropoisomeric diphosphines.
In both cases our research efforts let us to obtain chiral enriched unnatural aminoacids by reduction of prochiral ketones3 and by reduction of tetra substituted double bonds4, which are generally very low active. Our research will show that the process robustness and stereochemical outcome is strongly dependent on thermodynamic parameters but also on the presence of additives.
We will discuss in detail nature and function of such compounds and we will demonstrate that it is possible to combine metal enantioselective reduction with stereoselective biocatalytic one.
1. E. Cesarotti; I. Rimoldi, P. Spalluto, F. Demartin Tetrahedron Asymmetry. 2007, 18, 1278-1283.
2. R. Gandolfi, E. Cesarotti, F. Molinari; D. Romano; I. Rimoldi Tetrahedron Asymmetry. 2009, 20, 411-414.
3. I. Rimoldi, E. Cesarotti, D. Zerla, F. Molinari, D. Albanese, R. Gandolfi submitted
4. R. Gandolfi, E. Cesarotti, D. Zerla, F. Molinari, I. Rimoldi submitte
Stereochemistry of sulfur dioxide insertion at a chiral metal center: synthesis and crystal structures of Ru[C5H4(neomenthyl)](CO)(PPh3)R (R = Me, SO2Me)
No full textA facile synthesis of complexes of the type Ru(h5-C5H5)L2H (L2 = chelating phosphine
No full textRu(C5H5)(CO)2H, prepared in situ from Ru3CO)12, reacts with bisphosphines L2 to give Ru(C5H5)L2H quantitatively [L2 = Ph2P(CH2)nPPh2 n = 2 or 4; L2 = (R)-Ph2PCH2CH(Me)PPh2]
Chiral cyclopentadienyl hydrogenation catalysts: crystal structures of rhodium complexes [Rh(C5R4R*)Cl2]2 (R = H, R* = neomenthyl; R = Me, R* menthyl)
No full textThe synthesis and crystal structure of the title compounds are reported. The corresponding tetramethylmenthyl derivative was co-crystallized with triphenylphosphineoxide, ie. as [Rh{C5Me4-(menthyl)}Cl2]2·2[(C6H5)3PO]. Both compounds are active hydrogenation catalysts in the presence of Et3N but with prochiral alkenes the enantioselectivity is low (i.e. ≤ 13% ee)
Stereochemistry of substitution reactions at a chiral ruthenium atom: the crystal structure of (R)-[Ru(h5- C5H4R*)(CO)(PPh3)NCCH3]PF6(R* = neomenthyl)
No full textThe stereochemistry of substitution reactions of complexes of the type [Ru(η5-C5H4R*)(CO)(PPh3)L]n+ (R* = menthyl or R = neomenth n = 1, L = NCCH3 or NCCD3 has been studied by X-ray crystallography. Circular Dichroism and 1H, 13C and 31P NMR spectroscopy. All the reactions investigated have been unequivocably proved to occur with ≥ 94% retention of configuration at the ruthenium atom. Thus (S)-Ru(η5-C5H4R*)(CO)(PPh3)Cl reacts with NaI to give (R)-Ru(η5-C5H4R*)(CO)(PPh3)I (R* = menthyl or neomenthyl) and reaction of Ru(η5-C5H4R*)CO(PPh3)X (R*=menthyl, (S)Ru, X = Cl; R* = neomenthyl, (R)Ru, X = I) with AgY in acetonitrile yields the corresponding complex (R)-[Ru(η5- C5H4R*)(CO)(PPh3)(NCCH3)]Y (Y = PF6 or BF4). An X-ray structure determination of [Ru(η5-C5H4R*)(CO)(PPh3)NCCH3]PF6 (R* = neomenthyl) has confirmed that the absolute configuration of the ruthenium centre is R. The crystals are orthorhombic, space group P212121 (No. 19) with a 10.400(2), b 15.850(4), c 24.740(5) Å and Z = 4. The structure was solved via the heavy-atom method and refined to R = 0.036 using 4183 diffractometer data with I ≥ 1.56(I). CD3CN undergoes exchange with the coordinated acetonitrile in (R)-[Ru(C5H4R*)(CO)(PPh3)(NCCH3)]BF4 (R* = neomenthyl) with retention of configuration at ruthenium and reaction of (R)-[Ru(η5-C5H4R*)(CO)(PPh3)NCCD3]BF4 with NaI regenerates (R)-[Ru(η5-C5H4R*)(CO)(PPh3)I] (R* = neomenthyl)
Natural and unnatural aminoacids by catalytic reduction with Rh(I)-diphosphine complexes; role of strong acids in the synthesis of deuterium labelled histidine.
No full text
The additivity of the substituents effects on the polarographic potentials of ferrocene derivatives
No full textSynthesis and stereochemical studies of eta-3-allyl palladium(II) complexes containing a chiral chelating ligand
No full textA complex of the type [Pd(η3-CH2CMeCH2)(P-P′)]+ [where P-P′ = the chiral chelating ligand (S-)-(N-diphenylphosphino)(2-diphenylphosphinoxymethyl) pyrrolidine, not having C2v symmetry] has been prepared and its 1H, 13C, 31P, and two-dimensional H-X (X = 13C or 31P) correlation spectra recorded. On the basis of n.m.r. data the absolute configurations in solution of the major and minor diastereoisomers have been assigned. Proton and 13C n.m.r. data have been discussed in terms of trans influence of the chelating ligand P-P′. Correlations between the n.m.r. parameters of the intermediate η3-allyl diastereoisomers, donor properties of the ligand, and regiochemistry of nucleophilic addition to the allyl termini have been proposed
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