Nitrogen-14 shifts

In solution, the phenyl derivative 12.29 (R = Ph) is fluxional. The mechanism of the fluxional process has been shown by an N NMR investigation of a partially N-labelled sample, Le., PhCN2 N3S3 ( N = 99% N), to involve a series of 1,3-nitrogen shifts (Section 4.8.4). Thermolysis or photolysis of 12.29 generates the corresponding 1,2,3,5-dithiadiazolyl radicals [RCNaSa]". ... 

N NMR spectroscopy has been applied to the identification of complex mixtures of S N rings and cages formed from the air oxidation of S3 Ns in acetonitrile and to investigations of the dynamic behavior of the bicyclic compound PhCNsSs (11), which undergoes 1,3-nitrogen shifts. ... 

MesSiNSNSiMes. N NMR studies have demonstrated that (45 R = Ph) is a fluxional molecule that undergoes a series of degenerate 1,3-nitrogen shifts. ... 

A pyrrole-like nitrogen in a ring iV-methyl group gives a peak around -220 p.p.m. if adjacent to two carbon atoms one or two neighboring nitrogens shift the peak downfield by about 40 and 90 p.p.m., respectively. 

The use of mass spectrometry is generally limited to structural confirmation and measurement of molecular ion. However, a more detailed fragmentation pattern was proposed for [l,2,3]triazolo[l,5- ]pyridines, which exhibit a propensity for nitrogen shift (Scheme 4). This can occur before or after the loss of other substituents, according to their initial structures <1996T10519>. 

As an example of nonaromatic triazolopyridine, 84 underwent a nitrogen shift upon heating in (sealed tube... 

The similarity observed for the correlation of the carbon shifts para to the substituent for 1 and 2 indicates that the nitrogen shift is a better probe of the transmission of the substituent effect to nitrogen. This conclusion can also be reached from a study of the polyfluorinated anilines. 

Nitrogen shifts in these compounds behave similarly (Table 16). 

In some very early work the conformation of a-methyl groups in cyclohexanone oximes (76) was assigned from solvent shifts. Results are summarized in Table 17. In solutes bearing a lone pair of electrons on nitrogen, the benzene-solute collision complex is likely to occur at a site as far as possible from the nitrogen. Shifts have been summarized for aziridines, oximes and imines, and for the latter a complex of type 77 was proposed. 

This substance is best represented as the diazonium zwitterion 26 since its chemistry is analogous to diazonium ion chemistry rather than to diazoalkane chemistry. Because 26 is a diazonium compound without a counterion, it allows one to study pure diazonium ion chemistry. The diazonium nature of 26 is confirmed by its N- and C-NMR spectra compared to those of p-nitrophenyldiazonium tetrafluoroborate (27) and diazocyclopentadiene (28) (78JA4974). Chemical shifts of the carbon attached to the central nitrogen are in 26,5112.2 from Mc4Si in 27,6 121.8 and in 28,6 72.2. The central nitrogen shift is 6 146.0 from HNO3 for 26,152.2 for 27, and 106.2 for 28. The terminal nitrogen is d 59.4 for 26, 57.1 for Tl, and — 8.8 for 28. 

Protonation shifts of pyridine (Table 4.67) are much stronger but follow the trend of dilution shifts portrayed in Fig. 4.12. Shielding in the a position is attributed to a change of the N —C-a bond order. Deshieldings at C-3 and particularly at C-4 arise from an increased electron withdrawal of the positively charged nitrogen. Shift changes induced... 

The chemical shifts of the three alkaloids were quite similar. The N-9 amide shifts were observed toward the downfield end of the normal range for amide i N chemical shifts. The N-19 aliphatic nitrogen shifts were observed in the vicinity of 37 ppm, which is typical of secondary and tertiary aliphatic shifts. Chemical shifts for the three compounds are collected in Table 14.3. 

Methoxycarbonylsulfenyl chloride (SMoc-Q)P is a valuable reagent for the synthesis of asynunetric disulfides via reasonably stable, isolable 5-(methoxycarbonylsulfanyl) derivatives. This procedure has been successfully transferred to cysteine peptides for selective intrachain and interchain disulfide formation (see Vol.E22b, Section 6.1.3.1). Due to the high reactivity of 5-(methoxycarbonylsulfanyl)cysteine derivatives 9 (Scheme 21), and complications arising from their base lability and the risk of S N nnigration, this type of protection is not suitable for multistep peptide synthesis. The more recently proposed AT-methyl-AT-phenylcarbamoylsulfanyl (SMpc) derivative of cysteine (Scheme 21) is apparently less prone to sulfur to nitrogen shift. ... 

One of the main purposes of the present review is to bring all the nitrogen shift data published since 1972 onto a common screening constant scale. These are collected in Tables VII-XXXI and the methods of recalculation of the shifts, based mostly on the data from Table VII, are given in footnotes. Table VII may also be used for introducing minor corrections to the older data referred to internal CH3N02 as well as for recalculating those referred directly to other standards (Id). 

Studies of a supramolecular complex of silver(I) and a cholaphane. Virtanen and co-workers reported the results of a study involving a supramolecular complex of a cholaphane analogue (244) with Ag(I) The authors used a 50 ms optimized HMBC spectrum to assign the piperazine nitrogen shifts with and without Ag(I). 

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