Using a Tripod as a Chiral Chelating Ligand: Chemical Exchange Between Equivalent Molecular Structures in Palladium Catalysis with 1,1,1‐Tris(oxazolinyl)ethane (“Trisox”)

Abstract

Threefold symmetrical chiral podands may simplify the stereochemistry of key catalytic intermediates for cases in which they only act as bidentate ligands. This applies to systems in which chemical exchange between the different kappa2-coordinated forms takes place and in which the non-coordinated sidearm may play a direct or indirect role at some earlier or later stage in the catalytic cycle. Palladium(II)-catalysed allylic substitutions provide appropriate test reactions along these lines. A series of neutral dichloropalladium(II) complexes, [PdCl2(iPr-trisox)] (1a), [PdCl2(Ph-trisox)] (1b), [PdCl2(Bn-trisox)] (1c) and [PdCl2(Ind-trisox)] (1d) (trisox=1,1,1-tris(oxazolinyl)ethane) were synthesised by reaction of the respective trisox derivative with [PdCl2(PhCN)2] and characterised inter alia by 15N NMR spectroscopy. Direct detection of the heteronuclei without isotope enrichment and with "normal" sample concentrations was achieved with the aid of a cryogenically cooled NMR probe on a 600 MHz NMR spectrometer. Whereas the 15N nuclei of the coordinated oxazoline rings resonate at delta=160-167 ppm and appear as two singlets due to their diastereotopicity, the signal assigned to the dangling oxazoline "arm" is observed at delta=238-240 ppm. Variable-temperature NMR studies along with a systematic series of magnetisation transfer experiments established exchange between ligating and non-ligating oxazoline rings. Reaction of [Pd(allyl)(cod)]BF4 (cod=cyclooctadiene) with Ph-trisox in CH(2)Cl(2) gave the corresponding allyl complex 2, for which fast exchange between the three oxazoline heterocycles as well as between the exo and endo diastereomers was observed along with a very slow eta3-eta1-eta3 process of the allyl fragment (magnetisation transfer). Palladium(0) complexes were prepared by reaction of trisox derivatives or sidearm-functionalised BOX (BOX=bis(oxazolinyl)dimethylmethane) ligands with [Pd(nbd)(alkene)] (nbd=norbornadiene, alkene=maleic anhydride or tetracyanoethylene). X-ray diffraction studies of the iPr-trisox and Ph-trisox complexes (3a and 3b) established Y-shaped trigonal planar coordination geometries with the trisox ligand coordinated in a bidentate fashion, whilst the pi-coordinated maleic anhydride ligand adopts one of the two possible diastereotopic orientations. As the catalytic test reaction, the allylic alkylation of 1,3-diphenylprop-2-enyl acetate substrate with dimethyl malonate as nucleophile (in the presence of N,O-bis(trimethylsilyl)acetamide) was investigated for the trisox derivatives, their BOX analogues, and a series of less symmetric "sidearm" functionalised bisoxazolines. The trisoxazoline-based catalysts generally induce a better enantioselectivity compared to their bisoxazoline analogues and display significant reduction of the induction period as well as rate enhancement.

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…For the sake of completeness it may be interesting to cite the works of Gade, Spek, and Privalov in which ligands or olefins not described in Schemes 1 and 2 were used. However, no particular structural news arose from those studies [87][88][89].…”

Easily Prepared Chiral Scorpionates: Tris(2-oxazolinyl)boratoiridium(I) Compounds and Their Interactions with MeOTf

“…Clear advantages of 15 N NMR spectroscopy include direct observation of the coordinated versus dissociated oxazolines, an identical spectroscopic handle for solution and solid-state measurements, and potential application in the characterization of catalytically important oxazoline-coordinated compounds. [13][14][15]24 Three sets of resonances corresponding to inequivalent oxazoline groups are observed in the 1 H NMR spectra of 6 and 7. The 1 H-15 N HMBC spectra contain three 15 N NMR chemical shifts that correlate to 1 H NMR resonances of the three inequivalent oxazoline rings (see Table 1).…”

Ligand spheres in asymmetric hetero Diels–Alder reactions catalyzed by Cu(ii) box complexes: experiment and modeling

“…pentacoordination) 7 show higher conversion and stereoselectivity even in the strongly coordinating solvent MeNO 2 . 74 Efficient penta-coordinate arrangements were shown for Cu(II)-containing tripodal bis(oxazoline) ("trisox") derivatives 75 and enzymes, 51,72,73 and recent NMR investigations show that solvents and counterions substantially influence the geometry of the transition-metal complexes in solution. 76 Establishing the transition state responsible for the formation of the Diels-Alder adduct in the presence of solvent and counterion molecules will be the next step of our investigations.…”