Dynamic ¹H NMR Study of the Barrier to Rotation about the C−N Bond in Primary Carbamates and Its Solvent Dependence

Abstract

The dynamic 1H NMR study of some primary carbamates in the solvents CDCl3 and CD3COCD3 between 183 and 298 K is reported. The free energies of activation, thus obtained (12.4 to 14.3 kcal mol-1), were attributed to the conformational isomerization about the N-C bond. These barriers to rotation show solvent dependence in contrast to the tertiary analogues and are lower in free energy by ca. 2-3 kcal mol-1.

Solvent-free preparation of primary carbamates using silica sulfuric acid as an efficient reagent

“…The present feasible reaction mechanism resembles the recently reported one 23,25 which is illustrated in Scheme 3. The reaction of sodium cyanate 3 with an acid (SSA) to produce isocyanic acid 5 could be the first step.…”

“…Indeed, the amount of remained starting material after long times and/or at high temperatures was really considerable. Moreover, regarding removal of trichloroacetic acid and reaction time, the present procedure was more effective than what recently reported, 23,24 because the time was shortened from 12 hr to 1 hr (Table 1).…”

“…Moreover, we found that the reaction conditions could tolerate such moieties as O-i-propyl (1h, entry 8) which often undergoes cleavage in strongly acidic media. In addition, it is remarkable to note that reaction of α-and β-naphthol (entries 14 and 15) proceeded effectively in the present experimental conditions, while it did not work completely in trichloroacetic acid (the latest reported procedure 23,24 ). Indeed, the amount of remained starting material after long times and/or at high temperatures was really considerable.…”

“…By comparing the IR and physical properties of the products with those of authentic samples, 23,25,[32][33][34][35][36] carbamates 1a-o were easily identified. Also; they were characterized by 1 H-NMR (500 MHz) and 13 …”

“…In attempts to synthesize primary carbamates from phenols and alcohols under solvent-free conditions, we have recently reported a method for the conversion of compounds containing hydroxyl group to primary carbamates at room temperature in the absence of solvent using trichloroacetic acid as well as spectra data such as IR, NMR and their dynamic NMR. 23,24 Since this acid is relatively toxic and corrosive, we were interested in developing methods for the synthesis of carbamates utilizing solid acids such as HClO 4 -SiO 2 , 25 as they are industrially important due to their potential at replacing conventional acid/base catalysts.…”