Theoretical study on structures and internal rotations of methylN,N-dimethylcarbamate and its sulphur, selenium, and tellurium homologues (Me2NC(O)YMe, Y = O,S,Se, Te)

Abstract

A theoretical study on the structures and internal rotations of methyl N,N-dimethylcarbamate and its sulphur, selenium and tellurium homologues [Me,NC(O)YMe, 1 (Y = O ) , 2 (Y =S), 3 (Y =Se), 4 (Y =Te)] was performed by means of ab initio molecular orbital calculations at the MP2/3-21G(*)//HF/3-21G(*)level. These calculations indicate that 1-4 are all planar with Z-conformation with respect to the central bonds of their 0-C-Y-Me units, whereas the corresponding E-forms are transition states for rotation about Y-C(0) bonds which have energies higher than the E-forms by 20.6, 15.4, 13-9, and 9.6 kcalmol-I, respectively. The energy of 1 increases monotonically from the Z-form to the E-form with rotation about the Y-C(0) bond, but in 2-4 a transition state and a local minimum were found between the two forms. This different phenomenon for 1 compared with its homologues 2-4 arise mainly from the large steric repulsion between a methyl group on the nitrogen and that on the oxygen in E-1. Optimization of the transition states (TS, and TS,) for rotation about N-C(0) bonds showed that TS, is favoured by 2-4 but disfavoured by 1 owing to the repulsion between lone pairs on the nitrogen and oxygen atoms in its TS,. The barriers for rotation about N-C(0) bonds were estimated to be 16.1,14.7, 14.7, and 15.7 kcal mol-' for 1,2,3,4, respectively.

Spectral evidence for generic charge → acceptor interactions in carbamates and esters

“…The electronic charge at the carbonyl oxygen of carbamates and esters interacts back with the hyperconjugative resonance (HCR*) along C a -C b or with s* along C a -H a , as substantiated by the NMR and FT-IR spectral shis (compared to those for the corresponding alcohols). Such an interaction regulates the rotational states along 51 and drug design, [52][53][54] where carbamates play an important role.…”

Rotational barriers of carbamate‐protected amine crosslinkers for hydrogels: A combined experimental and computational study

“…Theoretical studies on the rotational barriers of carbamates reported two possible transition states TS 1 and TS 2 , where the lone pair on the pyramidalized nitrogen is anti or syn to the carbonyl oxygen (Scheme ) . For methyl N , N ‐dimethylcarbamate, representing a typical carbamate, all quantum chemical calculations involving basis sets beyond 3‐21G* agree with each other in terms of the relative stability of the two transition states, i.e., that TS 1 is lower in energy than TS 2 , whereas the barrier heights depend slightly on the details of the level of theory and basis set used.…”

“…For comparison, the rotational barriers associated with the acyl-OtBu bond (OC-OC plane) differ from the barriers associated with the C-N bond: for methyl N,Ndimethylcarbamate, a value of around 86 kJ/mol was obtained at the MP2/3-21G* level. [25] For simple esters, smaller rotational barriers for the OC-OC bond were reported, such as 53 kJ/mol for tert-butyl acetate. [27] The barrier of rotation around the C-S bond in methyl vinyl sulfone was calculated to 22 kJ/mol at the HF/6-31G** level.…”