NMR and theoretical study of the (CO)–N rotational barrier in the isomers cis- and trans- 2-N,N-dimethylaminecyclohexyl 1-N′,N′-dimethylcarbamate

Hindered Rotation in Some Organic Dithiocarbamates

“…Such an effect has also been observed for G = when Me is replaced by iPr or iBu with slight difference for formamides (Me 2 NCHO: 87.5, iPr 2 NCHO: 86.1) 23-29 and more strongly for acetamides (Me 2 N-CO-CH 3 : 72.7, iPr 2 N-CO-CH 3 : 67.7) 23,26,29,30 or dithiocarbamates (Me 2 N-CS-SMe: 66.5 ± 2, iBu 2 N-CS-SMe: 63.7 ± 0.02). 4a, 8,39,40 Interestingly, Table 2 shows also that the free energies of activation in the range 58-70 kJmol −1 obtained for the dithiocarbamates 1-5 are nearly similar to or slightly higher than those reported for carbamates, 41 but lower than that in structurally related thioamides, which is in fair agreement with the results already reported by Kalikhman et al for dithiocarbamate 1 and its derivatives. 40 The 1 H NMR spectrum of 5 in CDCl 3 , at 50 • C, shows two signals at 3.77 and 4.15 ppm with an integration of 4H for each, corresponding to O(CH 2 ) 2 and N(CH 2 ) 2 , respectively ( Table 2).…”

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

“…In particular the rotational barrier of the conjugated C-N bond in 2-N,N-dimethylaminecyclohexyl 1-N',Ní-dimethylcarbamate has been studied for two isomers. In isomer 1 the carbamate group lies at the axial position, whereas in isomer 2 it adopts an equatorial arrangement [20]. It is, however, interesting to notice that from the theoretical point of view there may be four different isomers of this compound.…”

Conformational preferences of N‐methoxycarbonyl proline dipeptide

“…Carbamates, O(CO)N<, have the similar structural unit to amides but the additional oxygen of carbamates is expected to cause the structural and electronic perturbations in the amide unit, (CO)N<, different from amides. In particular, the barriers to rotation (Δ G ‡ ) of the CN bond for N , N ‐dimethylcarbamates are lower by up to 4 kcal/mol than those of the corresponding amides20 and are almost insensitive to the solvent polarity,20–24 whereas the barriers to rotation for the amides increase as the solvent polarity increases 25–28. The behaviors for the barriers of N , N ‐dimethylcarbamates also hold for those of prolylcarbamates 20, 21.…”

“…From NMR kinetics experiments, it has been known that the barriers to rotation of MDMC20, 24 and 2‐ N , N ‐dimethylaminecyclohexyl‐1‐ N , N ‐dimethylcarbamates23 in aprotic solvents are almost enthalpy‐driven without a significant contribution of entropy change (Δ S ‡ ), as found for amides and proline‐containg peptides in aprotic solvents and in water 28, 31–34. However, the rotations of N , N ‐dimethylcarbamates occur with a negative Δ S ‡ in water,20, 23, 24 which was ascribed to the stronger solvation of the more polar transition state relative to the less polar ground state 20. With the increase of solvent polarity, these insensitive solvent effects on rotational barriers for N , N ‐dimethylcarbamates are resulted by contributing to the decrease in enthalpy change (Δ H ‡ ) that offsets the change in Δ S ‡ ,20, 23, 24 which also hold for N , N ‐dimethylthiocarbamates 24…”

“…However, the rotations of N , N ‐dimethylcarbamates occur with a negative Δ S ‡ in water,20, 23, 24 which was ascribed to the stronger solvation of the more polar transition state relative to the less polar ground state 20. With the increase of solvent polarity, these insensitive solvent effects on rotational barriers for N , N ‐dimethylcarbamates are resulted by contributing to the decrease in enthalpy change (Δ H ‡ ) that offsets the change in Δ S ‡ ,20, 23, 24 which also hold for N , N ‐dimethylthiocarbamates 24…”