Relationship between intramolecular chemical exchange and NMR-observed rate constants

Green, Malcolm L. H.; Wong, Luet‐Lok; Sella, Andrea

doi:10.1021/om00043a059

Cited by 118 publications

(87 citation statements)

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“…[18] Numerical nonlinear least-squares fits of the theoretical curves to the experimental data gave the NMR spectroscopic rate constants k NMR (T) for the fluxional exchange. Taking into account the statistical factors for the fluxional exchange of the three sites, [19] k NMR (T) was then converted into the chemical rate constant k chem (T) ( Table 1).…”

Section: Resultsmentioning

confidence: 99%

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”)

Foltz

Enders

Bellemin‐Laponnaz

et al. 2007

Chemistry A European J

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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.

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Section: Resultsmentioning

confidence: 99%

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”)

Foltz

Enders

Bellemin‐Laponnaz

et al. 2007

Chemistry A European J

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show abstract

“…It is the responsibility of the user of the program to deconvolute the 'rate' produced by the program into the rate for leaving a particular site or for the rate of the reaction expressed in chemical terms. This problem has been discussed for some complicated cases by Green et al 39 and was covered in a previous review for simpler systems. 16 Green's approach focuses on an overall rate constant, k chem , and there are alternative choices to that approach; nevertheless, a reliable check is provided by comparing results from computer calculations with those from equation (1) for an elementary step in the rearrangement.…”

Section: Measurement Of Barriers To Interconversionmentioning

confidence: 99%

“…186 Propeller chirality, which is designated as P and M, has also been studied in triphenylsilane and triphenylstannane. Generally one would expect the propeller chirality to be controlled by the chirality at the metal center, such that (39) or (40) would be preferred depending upon the interactions with the other ligands on the metal. There is, however, a strong influence of the conformations and configurations of the other ligands.…”

Section: 163166-179mentioning

confidence: 99%

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Stereochemical Nonrigidity of Organometallic Complexes

Faller¹

2005

Encyclopedia of Inorganic Chemistry

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Compounds that undergo intramolecular rearrangements at rates that affect the NMR line shapes in generally accessible temperature ranges are described as stereochemically nonrigid. Analysis of 1D and 2D NMR spectra as a function of temperature often allow the rates, as well as the mechanisms of interconversion of conformers or isomers, to be determined. Examples were selected to illustrate the importance of stereochemical nonrigidity in reactivity and the potential for affecting stereochemistry in the products of reactions.

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“…[19] The general equation for relating k obs to k chem for an exchange from site A with a population p A to site B with a population p B was given by Green et al [19] as:…”

Section: Introductionmentioning

confidence: 99%

Expanding Cavitand Chemistry: The Preparation and Characterization of [n]Cavitands withn≥4

et al. 2001

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The preparation of cavitands composed of 4, 5, 6, and 7 aromatic subunits ([n]cavitands, n=4-7) is described. The simple, two-step synthetic procedure utilized readily available starting materials (2-methylresorcinol and diethoxymethane). The two cavitand products having 4 and 5 aromatic subunits exhibited highly symmetric cone conformations, while the larger cavitands (n = 6 and 7) adopt conformations of lower symmetry. 1H NMR spectroscopic studies of [6]cavitand and [7]cavitand revealed that these hosts undergo exchange between equivalent conformations at room temperature. The departure of these two cavitands from cone conformations is related to steric crowding on their Ar-O-CH2-OAr bridges and is predicted by simple molecular mechanics calculations (MM2 force field). X-ray diffraction studies on single crystals of the [4]cavitand, [5]cavitand, and [6]cavitand hosts afforded additional experimental support for these conclusions.

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Relationship between intramolecular chemical exchange and NMR-observed rate constants

Cited by 118 publications

References 0 publications

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”)

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”)

Stereochemical Nonrigidity of Organometallic Complexes

Expanding Cavitand Chemistry: The Preparation and Characterization of [n]Cavitands withn≥4

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