Substituent effects on the barrier to carbamate C–N rotation

Bis(trimethylsilyl)ketene Acetals as C,O‐Dinucleophiles: One‐Pot Formation of Polycyclic γ‐ and δ‐Lactones from Pyridines and Pyrazines

“…Five types of reactions have been examined: (1) those involving the simplest ketene acetal 7a (R 1 = R 2 = H) and pyridine and affording C-2 and C-4 regioisomers, each as a mixture of rotamers; [34,35] (2) those involving symmetrically substituted ketene acetals [e.g., 7b, R 1 = R 2 = Me or 7c, R 1 R 2 = (CH 2 ) 5 ], which gave single isomers, again as mixtures of rotamers; (3) those involving monosubstituted ketene acetals, such as 7h (R 1 = H, R 2 = Ph), which also gave single regioisomers but could give pairs of observable diastereoisomers (for substituted pyridines) through the formation of two adjacent stereogenic centres; (4) those involving substituted pyridines (e.g., 2-methylpyridine), which in the case of R 1 = R 2 = Me gave single isomers as apparently unique rotamers but which for R 1 ϶ R 2 might give pairs of observable diastereoisomers due to the fact that two adjacent stereogenic centres are formed; and (5) those involving pyridines bearing an electron-attracting substituent, which gave single isomers, in most cases with no discernible rotamers.…”

“…The bromoiodolactone 76b, which shows the presence of two rotamers in the NMR spectra, clearly displays two broad singlets for the HC=N proton and a signal for the quaternary CBr= carbon atom at δ = 99. 35 www.eurjoc.org be isolated as white crystals (m.p. 161°C) and its structure firmly established by X-ray diffraction crystallography (Figure 7).…”

Formation of azarhodacyclobutanes with varying N-substitution

“…The observation of sharp 1 H NMR signals and the absence of line broadening for both rotamers indicates that the rotational barrier is sufficiently high at room temperature, presumably due to steric constraints. In terms of rotamer interconversion, variable temperature 1 H NMR spectroscopy (DMSO-d 6 ) showed the ratio of syn/ anti-14 to fluctuate from z1:1 at 25 C to z 0.7:1 at 95 C giving preference for the more stable anti rotamer at elevated temperature [16]. During this experiment, signal broadening was observed, but coalescence of signals for syn/anti-14 was not noted.…”

Experimental and theoretical investigation of the IR spectra and thermochemistry of four isomers of 2-N,N-dimethylaminecyclohexyl 1-N',N'-dimethylcarbamate

“…Much of this flexibility is dictated by a hindered rotation around chemical bonds, as is the case of conjugated C-N linkages [9][10][11][12][13]. Thus, recently carbamates attracted much attention with respect to their molecular structure [14][15][16][17][18][19].…”