A library of 19 chiral tropos phosphorus ligands, based on a free-torotate (tropos) biphenol unit and a chiral P-bonded alcohol (11 phosphites, 1-P(O)2O to 11-P(O)2O) or secondary amine (8 phosphoramidites, 12-P(O)2N to 19-P(O)2N), were screened, individually and in combinations of two, in the rhodium-catalyzed asymmetric conjugate addition of arylboronic acids to enones and enoates. High enantioselectivities (up to 99% ee) and excellent yields were obtained in the addition to either cyclic or acyclic substrates. The flexible biphenolic P ligands outperformed the analogous rigid binaphtholic P ligands. Variable-temperature 31P NMR studies revealed that the biphenolic ligands are tropos even at low temperature. Only below 190 K was a coalescence observed; upon further cooling, two atropisomers were detected. The Rh homocomplexes ([Rh(La)2]+) were also studied: in general, a single doublet (P–Rh coupling) was observed in the case of the biphenolic phosphite ligands, over the temperature range 380–230 K, demonstrating their tropos nature in the rhodium complexes even at low temperatures. On the other hand, the phosphoramidites showed different behaviors depending on the structure of the ligand and on the nature of the rhodium source. The spectrum at 230 K of the mixture of [Rh(acac)(eth)2] (eth=C2H4) with phosphite 6-P(O)2O and phosphoramidite 19-P(O)2N (the most enantioselective ligand combination in the conjugate addition reaction) revealed the presence of four homocomplexes (total approximately 40%: [Rh- {6-P(O)2O}2], [Rh{(aR)-19-P(O)2N}2], [Rh{(aS)-19-P(O)2N}2], [Rh{(aR)-19-P(O)2N}{(aS)-19-P(O)2N}]) and one heterocomplex, [Rh{6-P(O)2O}{(aR)-19-P(O)2N}] (approximately 60%) In the heterocomplex, the biphenol-derived phosphite is free to rotate (tropos) while the biphenol-derived phosphoramidite shows a temperaturedependent tropos/atropos behavior (coalescence temperature=310 K).

Rh-Catalyzed Enantioselective Conjugate Addition of Arylboronic Acids with a Dynamic Library of Chiral Tropos Phosphorus Ligands / C. Monti, C. Gennari, U. Piarulli. - In: CHEMISTRY-A EUROPEAN JOURNAL. - ISSN 0947-6539. - 13:5(2007), pp. 1547-1558.

Rh-Catalyzed Enantioselective Conjugate Addition of Arylboronic Acids with a Dynamic Library of Chiral Tropos Phosphorus Ligands

C. Monti
Primo
;
C. Gennari
Secondo
;
2007

Abstract

A library of 19 chiral tropos phosphorus ligands, based on a free-torotate (tropos) biphenol unit and a chiral P-bonded alcohol (11 phosphites, 1-P(O)2O to 11-P(O)2O) or secondary amine (8 phosphoramidites, 12-P(O)2N to 19-P(O)2N), were screened, individually and in combinations of two, in the rhodium-catalyzed asymmetric conjugate addition of arylboronic acids to enones and enoates. High enantioselectivities (up to 99% ee) and excellent yields were obtained in the addition to either cyclic or acyclic substrates. The flexible biphenolic P ligands outperformed the analogous rigid binaphtholic P ligands. Variable-temperature 31P NMR studies revealed that the biphenolic ligands are tropos even at low temperature. Only below 190 K was a coalescence observed; upon further cooling, two atropisomers were detected. The Rh homocomplexes ([Rh(La)2]+) were also studied: in general, a single doublet (P–Rh coupling) was observed in the case of the biphenolic phosphite ligands, over the temperature range 380–230 K, demonstrating their tropos nature in the rhodium complexes even at low temperatures. On the other hand, the phosphoramidites showed different behaviors depending on the structure of the ligand and on the nature of the rhodium source. The spectrum at 230 K of the mixture of [Rh(acac)(eth)2] (eth=C2H4) with phosphite 6-P(O)2O and phosphoramidite 19-P(O)2N (the most enantioselective ligand combination in the conjugate addition reaction) revealed the presence of four homocomplexes (total approximately 40%: [Rh- {6-P(O)2O}2], [Rh{(aR)-19-P(O)2N}2], [Rh{(aS)-19-P(O)2N}2], [Rh{(aR)-19-P(O)2N}{(aS)-19-P(O)2N}]) and one heterocomplex, [Rh{6-P(O)2O}{(aR)-19-P(O)2N}] (approximately 60%) In the heterocomplex, the biphenol-derived phosphite is free to rotate (tropos) while the biphenol-derived phosphoramidite shows a temperaturedependent tropos/atropos behavior (coalescence temperature=310 K).
asymmetric catalysis ; atropisomerism ; enones; NMR spectroscopy ; P ligands; rhodium
Settore CHIM/06 - Chimica Organica
2007
Article (author)
File in questo prodotto:
Non ci sono file associati a questo prodotto.
Pubblicazioni consigliate

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/36202
Citazioni
  • ???jsp.display-item.citation.pmc??? 4
  • Scopus 64
  • ???jsp.display-item.citation.isi??? 59
social impact