Rotation about the C-N bond

First order rate constant k, for the rotation about the C-N bond in N, N-dimediylnicotinamide (3) measured at different temperatures by nuclear magnetic resonance (NMR) are ... 

Fig. 8-6. Conformation of phenyldiazenyl radicals with the N=N group rotating about the C-N bond axis (after Suehiro et al., 1986).

Under conditions where the rotation about the C-N bond of dimethylformamide is slow relative to the NMR time scale, the two methyl resonances will be separate singlets. Conversely, if the rotation is made to be very fast, the two methyl groups will be chemically equivalent. Their resonance will then appear as a sharp singlet. In between these extremes, kinetic information can be extracted from the line shapes. In most systems the parameter that is changed to go between these limits is the temperature. In some systems, pH or pressure has the same effect. 

However, a quite different situation is observed for ene nitroso acetal (423). First, a stable conformation and, correspondingly, new transition states TSR1 and TSR1 with a low barrier (8.8 kJ/mol) appear on the rotation coordinate about the C,N bond at the place of the transition state of enamine (422). Therefore, the barrier to rotation about the C,N bond decreases so that the process is fast on the NMR time scale and cannot be detected by this method. 

In contrast, the opposite situation is observed for standard enamines, in which the introduction of electron-withdrawing substituents at the (3-C atom leads to an increase in the barrier to rotation about the C,N bond (505). It should be noted that the experimental inversion barrier in BENA (423b) (52 kJ/mol, entry 4, Table 3.28) is rather similar to the calculated barrier for its very close analog (423a) (65.3 kJ/mol, entry 9, Table 3.27). As can be seen in Table 3.28, steric... 

The stereodynamics of /V-siloxy-ene- nitroso acetals related to 3-alkylen-substituted 4//-tetrahydro-1,2-oxazines (e.g., see (416 d)) differs from that of BENA in that free rotation about the C,N bond in oxazines cannot occur, but the ring inversion should take place (process Ir) (see Scheme 3.228). 

Restricted rotation about the C—N bond of amides was studied by Gutowsky and Holm in the earliest days of NMR spectroscopy (31). A number of papers have been published since then. However, due to various difficulties, the barriers... 

Barriers to rotation about the C—N bond of /V,/V-dimethylformamide are known to be affected by concentration and the nature of the solvent. As expected, polar solvents tend to increase the barrier by stabilizing the polar structure (2). Therefore, it is not surprising that, whereas the barrier to rotation of N,N-dimethylformamide is about 21 kcal/mol in solution, the barrier becomes as low as 15.6 kcal/mol in the gas phase (32). In the practical question of isolating atropisomers, it is the magnitude of the barrier in solution that matters. 

Barriers to Rotation about the C—N Bond of Enamines Derived from Malonhydrazide... 

Taguchi and associates (117) treated 3,3-dimethyl-3H-indole with p-chloroben-zoyl chloride in pyridine, and obtained two crystalline compounds in addition to l-(p-chlorobenzoyl)-3,3-dimethylindolin-2-ol. These two products had the molecular formula C27H25ON2CI, and a tricyclic structure with two benzo moieties was assigned. Dave and co-workers (118) questioned the structure on the basis of mechanistic considerations, and presented evidence that the products are atropisomers of 1 -(p-chlorobenzoyl)-2-(2,3-dimethyl-1 -indolyl)-3,3-dimethylin-doline (75) about the C—N axis. The barrier to rotation about the C—N bond... 

In /V,/V-dialkylated members of this group (1, A = NR, R2, B = H or alkyl), several authors have observed fast thermal isomerizations at the double bond, using the DNMR technique. Most compounds of this type also display hindered rotation about the C—N bond with substantial barriers (see, e.g., ref. 30). Compounds of the general type 9 and 10 (R = H or Me) with a variety of... 

For convenience the imine is drawn here with a Z configuration, but in reality the favoured stereochemistry is thus, inversion of double bond geometry is necessary prior to ring closure. This requirement contributes to the relative severity o1 the conditions needed for cyclization compared to those needed (or the ring closure of the corresponding benzylamines. where there is free rotation about the C-N bond. 

The second-order rate constants for the hydroxide-ion catalysed breakdown of compounds [124] and [126] (Table 19) are very much less than those for the breakdown of hemiorthoesters (cf. Tables 13-15) presumably as a result of the difficulty of expelling a nitrogen anion. Nevertheless both compounds break down more rapidly than rotation about the C—N bond of the amide product, since in both reactions rotational isomers of the products have been detected in non-equilibrium proportions by nmr spectroscopy, and their subsequent equilibration has been followed (Capon el al., 1981a Tee et al., 1982). With [126] the rate constants for breakdown into both rotational isomers were measured, although their structures were uncertain. 

Chlorinated pesticide impurity. The two rings are perpendicular to each other. The mirror images are not superimposable, because there is no free rotation about the C-N bond between the rings. 

The two structures pictured are extreme forms, the true structure being something in between. In the lower form, rotation about the C-N bond would be permitted but then the charge separation present in... 

Unsymmetrically substituted dithiocarbamate complexes of platinum(II) can also be synthesized. Using NMR techniques, the barrier to rotation about the C=N bond is found to be in the region of 85 kJ mol-1. 809... 

Variable-temperature PMR studies on the stereochemically nonrigid, (Cl4-n)Ti(R2Dtc) (n = 2, 3, or 4) complexes show the metal-centered rearrangements to be fast on the PMR time scale at temperatures higher than —90°. Hindered rotation about the C-N bonds was observed for R = r -Pr, and activation parameters were determined for this process. 

The temperature dependence of the PMR spectra of the 00(111X 2 Dtc)3 complexes has been studied and certain unusual aspects of the spectra are accounted for by a total line-shape analysis in terms of a general two-proton-exchange model. In similar studies Siddall (573) assigned certain of the complexities of the PMR spectra of Co(RR Dtc)3 complexes to rotation about the C -N bond. 

Acidic iminium ions (385, R = H) are usually prepared by the protonation of compounds with C—N double bond such as imines542 696 [Eq. (3.110)] and ketox-imes.542 The nonequivalence of the two C-methyl groups below —20°C indicated that in cation 388 the rotation about the C=N bond is slow on the NMR time scale.696... 

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