Low-temperature NMR spectroscopy

For this purpose, the reactions of various types of nitronates (348) with trialkylsi-lyl triflates in CD2CI2 were studied by low-temperature NMR spectroscopy. More than 10 iminium cationic intermediates (349) were detected (Scheme 3.204) (478). 

In the case of the a-dehydroamino acid (Fig. 10.23, right), it could be shown by using low-temperature NMR spectroscopy that the isolated crystals correspond to the major substrate complex in solution. However, according to the major-minor concept (see Scheme 10.2), it does not lead to the main enantiomer [63]. On the contrary, it could be proven unequivocally for various substrate complexes with yS-dehydroamino acids that the isolated substrate complexes are major-substrate complexes. Surprisingly, they also gave the main enantiomer of the asymmetric hydrogenation, which would not be expected on the basis of... 

The most common activator for the glycosyl sulfoxides is trifluoromethanesulfonic anhydride (triflic anhydride), which, in the absence of nucleophiles, rapidly and cleanly converts most sulfoxides into the corresponding glycosyl triflates in a matter of minutes at —78 °C in dichloromethane solution [86,280,315,316]. In the more extensively studied mannopyranose series, only the a-mannosyl triflate is observed by low-temperature NMR spectroscopy (Scheme 4.35) [280]. In the glucopyranose series, mixtures of a- and (1-triflates are observed, in which the a-anomer nevertheless predominates (Scheme 4.36) [280],... 

An intermediate formed on 1,6-addition of a cuprate to a dienone has recently been examined by low-temperature NMR spectroscopy. This reaction passes though a Cu/olefin re-complex intermediate A, in which cuprate binds to the a- and the j5-carbon. Further 1,3-rearrangement from another intermediate (B) to still another (C) is proposed (Eq. 10.7) [76]. 

Grobe, Auner and coworkers studied the thermal decomposition of 22,1 and methylsilacyclobutane 14 under low pressure flow pyrolysis conditions26. They characterized the transient silenes 2, 25 and 26 by mass spectrometric methods and by low temperature NMR spectroscopy of the adducts 27-29 of the silenes with hexadeuteriomethyl ether (equation 6)27. 

Sulfonium ions are possible important reactive intermediates in stereoselective gly-cosylations, although their role has recently been disputed.191A number of ions including 74,19275,193 and 76194 have been observed by low-temperature NMR spectroscopy, whereas the structure of 77 was determined by X-ray crystallography.195... 

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. 

The molecular structure and the conformational equilibrium of 1-methyl-1-silacyclohexane 174 were determined by gas electron diffraction, in solution by low temperature NMR spectroscopy and by ab initio quantum-chemical calculations (02JOC3827, 03JPC243). The equatorial methyl conformation is preferred and the ring interconversion barrier is less than 6 kcal/mol (cf. Table XXII). A similar barrier to ring interconversion was found for U,4,4-tetramethyl-l,4-disilacyclohexane 175 (98T13181) (cf. Scheme 57 and Table XXII). 

Complex 14 is red-brown and stable at room temperature as a solid. In solution, however, 14 rapidly isomerizes to a violet isomer (6). This isomerization may also be followed by low temperature- NMR spectroscopy which shows that a new Cp singlet at 65.51 grows in as the 65.57 signal of 14 diminishes. The isomerization may be simply trans+cis, but may also be the bridge ter-minal transformation indicated in eq. 37. Repeated attempts to... 

The first dibora[2] ferrocenophane (94) was prepared from l,L-dilithioferrocene and l,2-dichlorobis(dimefhylamino)di-borane. A dynamic process due to motion of the cyclopenfa-dienyl rings between staggered and eclipsed conformations was revealed by low-temperature NMR spectroscopy. A number of l,3-dibora[3]ferrocenophanes with B-E-B bridges (95 E=0, S, Se, Te, NR) were reported. Moreover, Wagner used diborylated ferrocenes for the synthesis of doubly bridged switchable ania-metallocenes (96) and (97), in which the bridges are created and... 

Because of the high discriminating capacity of MAD for sterically and/or electronically similar ethers, Yamamoto and Maruoka examined the affinity of the compound toward other substrates with oxygen-containing functional groups, for example various carbonyl compounds, including both aliphatic and aromatic aldehydes, amides, esters, ethers, and ketones with similar structural substituents. Binding behavior was monitored by low-temperature NMR spectroscopy of these substrates and their... 

Some insight into the mechanisms of the iodine-promoted carbonylation has been obtained by radioactive tracer techniques [17] and low-temperature NMR spectroscopy [18]. The mechanism involves the formation of HI, which in a series of reactions forms with rhodium a hydrido iodo complex which reacts with ethylene to give an ethyl complex. Carbonylation and reductive elimination yield propionic acid iodide. The acid itself is then obtained after hydrolysis. The rate of carboxylation was reported to be accelerated by the addition of minor amounts of iron, cobalt, or manganese iodide [19]. The rhodium catalyst can be stabilized by triphenyl phosphite [20]. However, it is doubtful whether the ligand itself would meet the requirements of an industrial-scale process. 

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