Ester-group conformations in carbamates, allophanates, and methacrylates

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

“…Electronic interactions at the R-O-C]O groups in carbamates and esters are less understood than that at the R-N-C]O groups in amides. Carbamates in which both these groups are fused at the carbonyl C]O bond, have several ester-like rather than amide-like features 1,2 including comparable C-O bond lengths in crystals, [3][4][5][6] favourable interactions between the dipoles of the C]O and O-R bonds in the predominant s-trans rotamers of the R-O-C]O groups and unfavourable lone pair repulsion between the two ester oxygens in their trace s-cis rotamers [7][8][9][10] (Fig. 1a).…”

“…Fig.2 (a and b) Incremental downfield shifts in the13 C and 1 H signals of H a and C a in carbamates (1-8) and esters(9)(10)(11)(12)(13)(14)(15)(16) relative to the corresponding homologous alcohols (17-20), indicating the stabilization of positive charge polarization at C a by hyperconjugative resonance from the C b H 3 groups; (c and d) inverse correlation between 13 C and 1 H NMR signals of C a and H a and FT-IR stretching frequencies at C]O in carbamates (1-7) and esters (9-15) with increasing number of methyl (C b H 3 ) substituents (0-2) on C a (for isopropyl N-phenyl carbamate (7), the FT-IR value is not inserted); (e) upfield shifts in the C b signals of carbamates (1-8) and esters (9-16) compared to corresponding alcohols (17-20), indicating electronic back donation towards C b ; (f) shifts in the H b signals of carbamates (1-8) and esters (9-16) compared to corresponding alcohols (17-20), indicating the dominance of charge / HCR* interactions. The lines merely indicate the trends.…”