In recent years, monodentate phosphorus ligands (e.g. phosphites, phosphonites, phosphoramidites and phosphinamines) have held the stage in asymmetric catalysis. The modular nature of all these ligands allows the synthesis of a wide variety of representatives, thereby making a combinatorial approach possible. An important breakthrough in this area was made independently by the groups of Reetz and Feringa, who used a binary mixture of chiral monodentate P-ligands in several asymmetric rhodium catalysed reactions. By mixing two ligands (La and Lb) in the presence of Rh, three species can be formed: RhLaLa, RhLbLb (homocomplexes), and RhLaLb (heterocomplex). The heterocombination is often more reactive and more (regio-, diastero- and enantio-) selective than either of the two homocombinations. The ideal case would constitute an equilibrium completely in favour of the heterocomplex [RhLaLb] because then only a single well-defined catalyst would exist in the reaction, and the undesired competition of the less selective homocomplexes would be avoided. The 1:1 mixture of a chiral phosphite with an achiral phosphine was reported to induce reversal of the enantioselectivity in the Rh-catalysed hydrogenation of N-acetamido acrylate (compared to the chiral phosphite alone). The only possible explanation for this peculiar behaviour is the selective formation of the phosphite-phosphine Rh-heterocomplex, favoured by electronically matching one sigma-donor ligand (phosphine) and one pigreca-acceptor ligand (phosphite). For this reason we investigated the use of combinations of phosphites with phosphinamines for the selective formation of rhodium heterocomplexes. DFT calculations showed that the phosphite-phosphinamine rhodium heterocomplex is more stable than the two homocomplexes by 11.29 kcal/mol. A small library of enantiomerically pure phosphites and phosphinamines was prepared and complexation studies were performed by means of 31PNMR, using Rh(acac)(C2H4)2 as the rhodium source. When C1-symmetric phosphinamine ligands were employed, the cis-heterocomplexes were formed with selectivities ranging from moderate (70%) to excellent (≥99%). The homo- and heterocombinations of phosphites and of the C1-symmetric phosphinamines were then screened in the rhodium-catalysed hydrogenation of methyl 2-acetamidoacrylate and in the palladium-catalysed asymmetric allylic substitution of rac-1,3-diphenyl-3-acetoxyprop-1-ene with dimethyl malonate. Remarkably, the 1:1 combination of a binol-derived phosphite and a phosphinamine induced reversal of the enantioselectivity and, in the case of the palladium-catalysed asymmetric allylic substitution, also a modest increase of the enantioselectivity.
|Titolo:||Combinations of a binaphthol-derived phosphite and a C1-symmetric phosphinamine generates heteroleptic catalysts in Rh- and Pd-mediated reactions|
PIGNATARO, LUCA LUIGI (Secondo)
GENNARI, CESARE MARIO ARTURO (Ultimo)
|Data di pubblicazione:||5-set-2009|
|Settore Scientifico Disciplinare:||Settore CHIM/06 - Chimica Organica|
|Citazione:||Combinations of a binaphthol-derived phosphite and a C1-symmetric phosphinamine generates heteroleptic catalysts in Rh- and Pd-mediated reactions / S. Carboni, L. L. Pignataro, B. Lynikaite, R. Colombo, M. Krupička, U. Piarulli, C. M. A. Gennari. ((Intervento presentato al 7. convegno International school of organometallic chemistry tenutosi a Camerino, Italy nel 2009.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|