Low-temperature NMR

I still did not have suitable low-temperature instrumentation of my own to carry out the low-temperature NMR studies, but Martin Saunders at Yale did. Thus our samples now traveled the Massachusetts Turnpike from Boston to New Haven, where with Marty we were able to study solutions of the norbornyl cation at increasingly lower temperatures using his home-built variable-temperature NMR instrumentation housed in the basement of the old Yale chemistry building. We... 

Note 2. Traces of oxygen will induce polymerization of the cumulene. The NMR tube must therefore be filled with nitrogen before putting the sample in it. Low-temperature NMR gives the most representative results. 

For cations 74-75 (Fig. 27), low temperature NMR experiments were necessary to reveal stereodynamical behaviors and allow the observation of split signals for the enantiomers [38,144]. Stereoselective recognition between the chiral cations and anions was observed in essentially all cases as integration of the split signals revealed the preferential occurrence of one diastereomeric salt over the other. 

The difference in chemical shift can be attributed to a significant change to the local environment of the phosphorus where the O-P-O angle may change from 95° to 117° and changes in P-N bond lengths can be expected. Low-temperature NMR has indicated the presence of more than two isomers for related compound 117 (R = Me and Et) when in solution <20040L145>. 

The isolated platinacyclobutane, (I), has the neopentane skeleton but the rearrangement products (II) and (III) have the isopentane skeleton. Low temperature NMR experiments, when L ... 

Only one process was observed in these derivatives by low-temperature NMR (see, e.g., Ref. 417). As a rule, the barrier of this process was very high (133,203, 346) although, for unknown reasons, the introduction of two nitro groups at the C-3 atom leads to a noticeable decrease in the barrier height (see Scheme 3.166). 

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). 

Attempts to directly observe the cations A and A by low-temperature NMR, developed in the studies 274 and 478, made it possible to detect both types of cations (293) (Scheme 3.219). 

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],... 

In summary, we have shown that stable cationic charge centers can significantly enhance the reactivities of adjacent electrophilic centers. Most of the studied systems involve reactive dicationic electrophiles. A number of the reactive dications have been directly observed by low temperature NMR. Along with their clear structural similarities to superelectrophiles, these dicationic systems are likewise capable of reacting with very weak nucleophiles. Utilization of these reactive intermediates has led to the development of several new synthetic methodologies, while studies of their reactivities have revealed interesting structure-activity relationships. Based on the results from our work and that of others, it seems likely that similar modes of activation will be discovered in biochemical systems (perhaps in biocatalytic roles) in the years to come. 

The distinctive feature of these cations is a very high field chemical shift for the bridging (p-) hydrogen, typically found from ca. 6 -4 to -7 ppm. Using moderately low temperature NMR techniques these cations are not difficult to characterize. 

Since the yield of 161 is only 20%, the overall yields of the cycloadducts with reference to 1,3-cyclopentadiene are rather modest. Therefore, it was tested whether or not 6,6-dibromobicyclo[3.1.0]hex-2-ene (168) is after all stable enough to serve as progenitor of 162. To that end, dibromocarbene was generated from tetrabromo-methane by methyllithium [90] at -60 °C in the presence of 1,3-cyclopentadiene. Low-temperature NMR spectra revealed that 168 remains intact in the solution up to 0 °C. On the basis of this observation, a one-pot procedure was developed for the synthesis of the trapping products of 162 from 1,3-cyclopentadiene. As illustrated in Scheme 6.37, 168 was prepared at -60 °C, then an allenophile and methyllithium in succession were added to the mixture at -30 °C. In this way, the adducts of 162 to... 

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

---