N-inversion

For oxaziridines the N-inversion barrier is considerably higher than that for similar aziridines. Af-Alkyl-3,3-dialkyloxaziridines are resolvable and absolute configurations have been determined (Section 5.08.2.3.1). 

N-Inversion in azetidine and azetidin-2-one is rapid, even at —77 and -40 °C, respectively (B-73NMR144). Again, halo substituents on nitrogen drastically slow the inversion rate, so that Af-chloro-2-methylazetidine can be separated into two diastereomers (b-77SH(1)54). Substituent effects on N-inversion are much the same as in the aziridines Af-aryl and N- acyl... 

This unusually high stereoselectivity is due to the high barrier to N-inversion and the presence of the electron-withdrawing phthalimido group. It can be demonstrated that the reaction is under kinetic control when the reaction mixtures described above are allowed to warm to temperatures near or above 0 °C, whereupon a partial or complete inversion of configuration at nitrogen can be seen (by NMR spectroscopy). 

Nitrobenzenediazoate can be considered as an azo compound comparable to an azobenzene having one electron acceptor and one donor on each side of the azo group the acceptor-donor relationship is more dominant in the (Z) -> (E) diazoate pair than in the diazohydroxide pair. The N=N rotation mechanism of the diazoate pair is therefore the favored process (E = 84 kJ mol-1 Lewis and Hanson, 1967). On the other hand, 4-C1 is not a substituent with a —M effect therefore it does not reduce the double-bond character of the N = N bond and the mechanism involving inversion at the N((3)-atom becomes dominant. The activation energy of the latter process (E = 104 kJ mol-1 Schwarz and Zollinger, 1981) is higher than that of the N = N rotation mechanism for the 4-nitro derivative, but it is reasonable to assume that it is lower than that for N = N rotation in the 4-chloro derivative. Furthermore, one can conclude that N-inversion is more favorable in the diazohydroxide than in the diazoate. ... 

Frequency-selective REDOR (fsREDOR) is a very powerful technique developed for the study of 13C and 15N uniformly labeled peptides or proteins [92]. The basic idea of this technique is to combine REDOR and soft n pulses to recouple a selected 13C-15N dipole-dipole interaction in a multiple-spin system. Usually one could use Gaussian shaped pulses to achieve the required selective n inversions. Other band selective shaped pulses have been developed for a more uniform excitation profile [93]. In its original implementation, fsREDOR was used to extract the intemuclear distances of several model crystalline compounds [92], In the past few years, this technique has proven to be very useful for the study of amyloid fibrils as well. For the Ure2p10 39 fibril samples containing 13C and 15N uniformly... 

In STARTMAS, SQ STs of order q (with q = 2 and p = —1) are initially excited as in STMAS (Fig. 14). The ST coherences then evolve during the first time period r/[2(l + k)], before being converted to a DQ transition by a selective n inversion pulse on the CT (as in DQF-STMAS [161]) and allowed to evolve during kx/(l + k). A second selective n pulse brings the system back to an SQ transition for the last part of the ST evolution. At time t = x, the phase of the detected SQ ST coherence is thus... 

Bemarques X 9 a) Soient S un schema et G un S-schdma en groupes, commutatif, plat, s pare et de presentation finie sur S. Alors pour tout entier n inversible sur S, le morphisme n est un homomorphisme etale, s6par6, de type fini, done quasi-affine,... 

In the conformational analysis of six-membered rings, we deal with ring reversal equilibria and kinetics. To this is added in derivatives of piperidine the need to understand the equilibrium and kinetics of N inversion. Obviously, the sign of AG° is important for it determines the conformer favored at equilibrium. Less obviously, but of equal importance is the fact that the energy barrier between two conformations of N substituents consists of two half barriers AG x ts and AG q ls where AG° is the difference between the half barriers. It is important that in any precise discussion of the kinetics of N inversion that the half barrier under discussion should be clearly defined.2 Neglect of this in the past has led to much confusion and controversy the concept of half barriers is explained in detail in Ref. 2. 

The ring-inversion process in Af,N-dimethylhexahyd ropyrimidine has been estimated by H-NMR spectral studies as AG. 3r 11.6kcal mol-1 297 and AGi29 4" 11-3 kcal mol"1 298 Variable-temperature 13C-NMR work287 permits the estimation of AG values (Table XXVII) for the N-inversion process in the /V,/V-dimethyl- and JV,JV-diethylhexahydropyrimidines. 

Equilibrium and Kinetic Parameters for the N-Inversion Process in Hexahydropyrimidines287... 

For l,3-diethyl-2-methyl-l,3-hexahydropyrimidine, the ring inversion equilibrium favors the 2(ax)-Me conformer, and the data for the N-inversion equilibria (Table XXVII) refer to the set with 2(ax)-Me.287... 

From the H-NMR spectra the barrier to ring inversion was estimated as AG1 13.2 0.2 kcal mol-1 at — 5°C and the barrier to /V-inversion as AG1 7.2 + 0.1 kcal moP1 at — 123.5°C.363 Barriers to ring inversion in 1,3,5-trialkyl derivatives 470 decrease with increasing size of substituent,297 365-366 and AG values for nitrogen inversion (determined by 13C-NMR spectroscopy355) follow a similar trend (Table XXIX). The barriers to N-inversion... 

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