Bulky Monodentate Phosphoramidites in Palladium-Catalyzed Allylic Alkylation Reactions: Aspects of Regioselectivity and Enantioselectivity

MDK Boele, Paulus Clemens Jozef Kamer, M Lutz, AL Spek, J G de Vries, PWNM van Leeuwen, GPF van Strijdonck

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91 Citations (Scopus)

Abstract

A series of bulky monodentate phosphoramidite ligands, based on biphenol, BINOL and TADDOL backbones, have been employed in the Pd-catalysed allylic alkylation reaction. Reaction of disodium diethyl 2-methyl malonate with monosubstituted allylic substrates in the presence of palladium complexes of the phosphoramidite ligands proceeds smoothly at room temperature. The regioselectivities observed depend strongly on the leaving group and the geometry of the allylic starting compounds. Mono-coordination occurs when these ligands are ligated in [Pd(allyl)(X)] complexes (allyl=C3H5, 1-CH3C3H4, I-C6H5C3H4, 1,3-(C6H5)(2)C3H3; X-Cl, OAc). The solid-state structure determined by X-ray diffraction of [Pd(C3H5)(1)(Cl)] reveals a non-symmetric coordination of the allyl moiety, caused by the stronger trans influence of the phosphoramidite ligand relative to X-. In all of these complexes, the syn,trans isomer is the major species present in solution. Because of fast isomerisation and high reactivity of the syn,cis complex, the major product formed upon alkylation is the linear product, especially for monosubstituted phenylallyl substrates in the presence of halide counterions. In the case of biphenol- and BINOL-based phosphoramidites, however, a strong memory effect is observed when 1-phenyl-2-propenyl acetate is employed as the substrate. In this case, nucleophilic attack competes effectively with the isomerisation of the transient cinnamylpalladium complexes. The asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate afforded the chiral product in up to 93% ee. Substrates with smaller substituents gave lower enantioselectivities. The observed stereoselectivity is explained in terms of a preferential rotation mechanism, in which the product is formed by attack on one of the isomers of the intermediate [Pd{1,3-(C6H5)(2)C3H3}(L)(OAc)] complex.

Original languageEnglish
Pages (from-to)6232-6246
Number of pages15
JournalChemistry - A European Journal
Volume10
Issue number24
DOIs
Publication statusPublished - 17 Dec 2004

Keywords

  • alkylation
  • asymmetric catalysis
  • homogeneous catalysis
  • ligand design
  • palladium
  • ASYMMETRIC CATALYSIS
  • CONJUGATE ADDITION
  • OXIDATIVE ADDITION
  • PHOSPHORUS LIGANDS
  • PHOSPHINE-LIGANDS
  • CHIRAL PHOSPHINE
  • NUCLEOPHILIC-ATTACK
  • IRIDIUM COMPLEX
  • PD COMPLEXES
  • BITE ANGLE

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