C-N rotation

Fig. 3.5 The plots of steric and conjugative constituents of C,N-rotation potential for CH2=CHN(OSiH3)2 (426)...

Compound Melting point, °C. [ ]n Rotation solvent Refer- ences... 

One of the compounds from the previous study (10, R = R, = R2 = Me) was subjected to a careful band-shape study over a wide temperature range (34). The CN barriers obtained in the two rotamers with respect to the C=C bond were quite different from those obtained by the coalescence approximation. It was also found that the activation entropy for the C=C rotation was 10 to 18 e.u. more negative than that for the C—N rotation, in agreement with results to be discussed later. 

P-Diacylenamines (11) have continued to be the subject of considerable interest. Barriers to C=C and C—N rotation have been reported from systems where the carbonyl groups are held more or less rigidly in the plane by ring... 

R—C —OMe in the transition state, whereas the larger substituents are assumed to lower the barrier further by increasing the ground state strain. However, as discussed for compounds 9, Sect. II-B-2, the effect on both die C=C and theC—N barriers of going from R = HtoR = Me clearly indicates that already Me makes a sizable contribution to the ground state strain, since R = Me can hardly stabilize the transition state for the C—N rotation. This interpretation is... 

The stereochemistry of the interesting group of compounds known as push-pull dienes has not been much studied, although a fair number are available. The earliest studies stem from the interest in the influence of the number of intervening double bonds in vinylogous amides (76) on the barrier to C—N rotation. It was... 

One system with (SCC13)2 as the acceptor has been studied. In 22, the NMR spectra can be interpreted in terms of a Case 3 system (Sect. II-E) with the steric barrier and the C—N barrier both being ca. 14 kcal/mol, and the ir barrier considerably higher. The C—N rotation and the passage past the steric barrier may in that case be correlated. This problem could probably be solved by a combined H and, 3C NMR study. 

The transition state to C—N rotation is less polar than the ground state, and therefore barriers to this rotation are increased by increased solvent polarity (20,83). For similar reasons, the barriers to passage through the planar state in Case 2 push-pull ethylenes increase moderately with increasing solvent polarity (143). 

In Case 2 systems, the ground state is more polar than the transition state to C=C rotation, and positive AS values would be expected, as is indeed observed (Table 22). The situation is similar for the C—N rotations, and here a positive value (16 3 e.u.) was found for 77a (84). However, Hobson and Reeves found values near zero (2.7 1.1 and —3.9 2.0 e.u.) for 124a and b (24). 

Table 12 C—N Rotational barriers (kcal mol ) of substituted aminoalkyl radicals HjNCHR."...

Nitromethane, CH -NOf. The equilibrium structure of singlet nitromethane has been studied at several levels of theory [3,60,64-71]. Two conformations are possible for nitromethane, staggered (Is) and eclipsed (le), but the eclipsed form has been characterized as a transition structure at MP2/6-31G with an imaginary frequency of 30 cm 1 [3]. Rotation around the H3C-NO2 bond occurs essentially without barrier the estimated value is only 0.01 kcal/mol. This is in accordance with a microwave study, which reports a C-N rotation barrier of only 6 cal/mol [72,73]. The C-N bond length of nitromethane has been estimated with X-ray single crystal diffraction [74], neutron diffraction [46,75], microwave spectroscopy [72,73], MP2/6-31G [3], and B3LYP/6-31+G [71] at respectively 1.449, 1.486, 1.489, 1.485, and 1.491 A, showing that the theoretical estimates compare very well with those determined by experimental methods. The experimentally reported vibrational frequencies of nitromethane... 

W One might argue that, if the aryl group and the YZC=N group were coplanar in YZC N—Ar (so as to permit conjugation between the C=N n bond and the aryl system), one could expect similar substituent effects on the C=N rotation barrier (especially for compounds of type L). 

Experimental determinations of the rotational barriers around C=C and C—N bonds have been performed by several research groups212,228-232, leading to the same magnitude of values. The influence of solvents was simulated in calculations225, which leads to a strong reduction of barriers of rotation around the C=C bond and an increase in the values for the C—N rotation. 

TABLE 3. Free-energy barriers (kcal mol 1 at T K) to C=C and C—N rotations and E conformer populations (pE) for compounds of the general type A1A2C=C(R)NR1R2... 

The earliest studies in this field were concerned with the effect of the number of double bonds on the barrier to C—N rotation in the series dimethylformamide, 3-dimethylami-noacrolein and 5-dimethylamino-2,4-pentadienal. The barrier was found to diminish from 20.8 to ca 15 to ca 13 kcal mol-1 92,93, and for each compound it increased with the polarity of the solvent92. 

Dale and coworkers103 have studied the Y-shaped dication 49 as its diperchlorate and found it to exist both in the solid state and in solution as a propeller-shaped entity with C3 symmetry. The W-methyl doublet in the NMR spectrum showed coalescence at +170°C in DMSO-<56 (300 MHz) corresponding to a barrier to C—N rotation of 22.5 kcal mol-1. 

Kleinpeter and Pulst122 have found that the barrier to C—N rotation in N,N-dimethyl-2-thiobenzoylenamine in nitrobenzene first decreases, and then increases on addition of trifluoroacetic acid. The effects, which are quite small, are interpreted as N-protonation at low and S-protonation at higher acid concentrations. 

In dimethyl sulfoxide (DMSO) solution, both C-C and C-N rotational isomerism was observed in 3-aminofuran-2-carbaldehyde 35 (EjZ ratio 57 43), 3-aminofuran-2-thiocarbaldehyde 36 (EjZ ratio 78 22), 3-aminobenzo[ ]furan-2-thiocarbaldehyde 37 (EjZ ratio 72 28), and 2-aminobenzo[7]furan-3-thiocarbaldehyde 38 (EjZ ratio 55 45). Depending on the solvent the C-C rotational barriers of 35-38 were found at 16.2-22.2 kcalmoP and the corresponding C-N rotational barriers at 8.4-13.0 kcalmoP <1997JOC2263>. 

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