A serieso f chiral biaryl cyclic monophosphoniteli gands of type RP(OAr)2 (R = Ph, Me, t-Bu, o-anisyl and Cy; (OAr)2 = 1,1'-binaphthyl-2,2'-diyl, 9,9'- biphenanthryl- 10,1 0'-diyl) have been prepared. The ligands have been synthesised by reaction of the dichlorophosphine precursor with the corresponding biphenol (HOAr)2. Chiral biaryl diphosphonite ligand (ArO)2PCH2CH2(OAr)2 derived from binaphthol has also been prepared. Platinum(II), palladium(II) and rhodium(I) complexes of these ligands are described and the X-ray crystal structures of five of these complexes are reported and discussed.T he electronic propertieso f the phosphonitesa re betweent hose of phosphite and phosphine, and this is reflected in their coordination chemistry. The new chiral biaryl monophosphites of type (Ar'O)P(OAr)2 have been synthesised straightforwardly by condensation of a phenol and the corresponding chlorophosphites in the presence of NEt3. Preliminary studies of their coordination chemistry with platinum(II) and rhodium(I) have been carried out. The twist on the seven membered P(OCCCCO) ring leads to a deshielding of the 31P signal for the biaryl ligands and in the complexes studied here. The asymmetric hydrogenation of a-enamides with rhodium(I) complexes of the phosphonites and diphosphonites as catalysts is discussed. The ligands have been screened in the asymmetric hydrogenation of two substrates: methyl-2-acetamido acrylate and methyl-(Z)-2-acetamido cinnamate. Monodentate phosphonite systems show, in some cases, higher enantioselectivity (up to 92% ee) than the analogous bidentate. The results presented here challenge the long-accepted wisdom that chelating ligands are necessary to achieve high enantioselectivities in asymmetric hydrogenation. NMR studies and X-ray crystallographic data show that the asymmetric ligand profile caused by the biaryl units in these phosphonites has three consequences: (i) rotation about the M-P bond in monodentate phosphonites is inhibited; (ii) a different rotamer for the monodentate from that in the chelate analogues is favoured; (iii) the favoured rotamer in the monodentate causes more effective chiral induction in the hydrogenation catalyses. The generality of these new concepts has been probed with some preliminary catalytic studiesw ith monodentatep hosphitel igands. The phosphonite ligands have also been tested in the copper(I) catalysed 1,4- addition of diethylzinc to enones and nitro-olefins in an attempt to extend their potential use in asymmetric catalysis. The phosphonite ligands show moderate to high ee's for the ethyl transfer to enones (cyclic and acyclic). The monophosphonite ligands induce higher enantioselectivity for acyclic enones: ee's up to 82% are the highest obtained with a monodentate phosphonite ligand in asymmetric 1,4-addition of diethylzinc to chalcone.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:340157 |
| Date | January 2001 |
| Creators | Martorell Colomer, Aina |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1983/c3c79146-8c6a-4ba1-9eb6-6d5779456813 |
Page generated in 0.0018 seconds