Low-temperature NMR analysis

Cyclization of a mixture of d t and meso di hydroxy ketones 3 and under mild acid conditions gave a mixture of the three isomeric spiroketals 15, 16, and Low temperature nmr analysis confirmed that isomers 15 and 16 are conformational 1y rigid and that they exist in the conformations 15A and 16A respectively. Using the same technique, isomer 17 was shown to exist as a mixture of conformers 17A and 17B as predicted. Furthermore, acid equilibration of 5 (or 6) gave a =97 3 mixture of isomers 15 and 16, and when isomer V7 was treated under the same conditions it was converted into a 97 3 mixture of 1 5 and 16. These results are completely consistent with the analysis made above. 

Proton NMR (sometimes low temperature NMR or NOE) is the major reliable method for conformational analysis of isoxazolidines which undergo pyramidal inversion at the ring nitrogen atom (82JOC4397, 840MR(22)399>. 

In this account we restrict the definition of phosphoranes to systems where five ligands are bonded to the phosphorus atom. Physical methods such as electron diffraction, X-ray analysis and low-temperature NMR of 31P, H, 13C and 19F are the most important aids in the study of phosphoranes, which are one of the most fascinating systems in the chemistry of phosphorus. 

The first precise evaluation (2A, 25) of both the anomeric and the exo-anomeric effects was obtained by studying 1,7-dioxaspiro[5.5]undecane (9) (Fig. 2). With this system, conformational analysis by low temperature nmr spectroscopy was possible because each conformational change involves a chair inversion which has a relatively high energy barrier. The steric effect could also be easily evaluated, and by adding appropriate alkyl substituents, it was theoretically possible to isolate isomeric compounds which would exist in different conformations. 

Moreover, low temperature NMR spectra indicate a pyramidal structure of the carbanion in triphenylphosphonium cyclopropylid 6, and an X-ray diffraction analysis of the crystalline solid confirms this result (14). Contrary to earlier predictions, cyclo-propylids are thus found to be the first class of ylids to contain non-planar carbanions with an unusual ylidic bonding (Fig.2). They form stable metal complexes, as illustrated by a gold complex (7) +... 

Spectral investigations of daphnezomines F (42) and G (43), whose molecular formulas are C27H35NO8 and C27H35NO7, respectively, revealed that they are structurally related and possess a l-azabicydo[5.2.2]undecane moiety. The conformation of the l-azabicyclo[5.2.2]undecane ring in 43 was elucidated by a low-temperature NMR study and computational analysis [46],... 

Effectiveness of the shift reagent analysis may be increased by the use of low temperature NMR. Actinides show higher oxidation states than lanthanides reason ... 

Glueck [116-121] came to the conclusion that the racemic secondary phosphines 197 form, with a platinum complex Pt(Me-Duphos)(Ph)(Br) and NaOSiMes in toluene, an adduct 198, which interconvert rapidly by P-inversion (5p)-198t (Pp)-198 [118]. Adduct 198 was isolated and studied by low-temperature NMR and X-ray monocrystal analysis. The crystal structure of the adduct showed that the major enantiomer of 198 has an (Pp)-absolute configuration [112],... 

If the steric bulk is further increased, stable rotamers can be isolated, when two adamantyl groups and two tert-butyl groups are attached to the ethane skeleton (X, X = tert-butyl, Y, Y = adamantyl) The sym.-tetra-tert-butylethane (X,X, Y,Y = tert-butyl) shows non-equivalent methyl groups which is attributed to an equilibrium of distorted conformers. The conformational analysis of tri-tert-butylethane has just recently been published Other examples are triisopropylmethane, and tetramethyl-pentane and -hexane With modern low-temperature NMR techniques even more complicated conformational equilibria can be assessed. As an example, the spectrum of 3-isopropyl-2,3,4-trimethylpentane (1,1,1-triisopropylethane) 30 is shown in Figure 8 for which a detailed analysis of the conformational equilibria was given. ... 

R 603 H. Fukaya, Analysis of Structure and Dynamics of HF Complexes by Low-Temperature NMR , Kagaku to Kogyo (Tokyo, Japan), 2004,57,441... 

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