Nachweis innermolekularer beweglichkeit durch NMR-spektroskopie XVI

The Bond‐Rotational Mobility of Guanidinium Ion

“…-Inserting the value K (bond rotation) 2 2 x 103 s-1 into the Eyving equation yields an upper bound for the corresponding free energy of activation : AG+ < 13 kcal/mol. This value is close to that found in [3] for the substrate 2, in particular with respect to that C-N bond which is least affected by steric crowding (AG* = 12-13 kcal/mol).…”

“…Insight into the resonance stabilization of 1, the 'leader' among the Y-aromatics, could be obtained if barriers to rotation about the C-N bonds were accessible. In an elegant study using N, N'-tetramethyl-N"-phenyl-guanidinium ions 2 carrying various substituents R in the aromatic ring, Kessler & Leibfritz [3] provided NMR. evidence that the free energy of activation for rotation of the (non-equivalent!)…”

Derivatives of the Triaminoguanidinium Ion, 2. Prototropic Tautomerism, Crystal and Molecular Structure of N,N´,N´´-Tris(propan- 2-iminyl)guanidine [1, 2]

“…Considering the rationalization of these substituent effects, it is likely that the iminyl moiety as a substituent at the imino nitrogen of 2 is not able to stabilize a negative charge by resonance and therefore resembles more a methoxy substituent, corresponding to a ∆G = value that would be significantly higher than the value determined here for guanidine 2. It has also been reported that the geometrical isomerization of 1,1,3,3-tetramethyl-2-phenylguanidine, which proceeds via the N-inversion mechanism, is independent of solvent polarity but decreases in protic solvents such as CD 3 OD [15]. Prototropic tautomerism, which is well known for the guanidine system, is likely to be the dynamic process that accounts for the temperature-dependent NMR spectra described above.…”

“…The last-mentioned process is likely not to take place in the temperature range of the present study; it has been reported that geometrical isomerization of several N-substituted acetone hydrazones requires much higher temperatures in inert solvents (e. g., ∆G = = 23.9 kcal mol −1 for acetone N,N-dimethylhydrazone in 1,2,4-trichlorobenzene at 170 • C [14]). The barrier to rotation around the guanidine C−N single bond, which have partial double bond character due to n − π conjugation, is difficult to estimate; based on available data for ureas and pentasubstituted amidinium salts [13,15], one may assume that the barrier is significantly lower than the one measured for 2. In addition, the observed solvent dependency of the NMR spectra excludes this process as the rate-limiting one.…”

E/Z‐Photoisomerization of N,N′‐Bis(4‐dimethylaminobenzylidene)1,2‐diaminoethane and N,N′‐Bis(4‐dimethylaminobenzylidene)1,3‐diaminopropane in Chloroform

“…The therma1 [2][3][4] and photo [5][6][7] interconversions of the syn and anti isomers of imines are a subject of long-standing interest. The mechanism for the thermal interconversion of imine diastereomers is currently the subject of considerable debate [8][9][10] and has been considered in terms of either a planar inversion mechanism or a rotation mechanism. The rotation or torsion mechanism involves a twisting about the C-N double bond.…”