Structures of N,N-dialkoxyamides: pyramidal anomeric amides with low amidicity.
The first X-ray structures of two anomeric N,N-dialkoxyamides (2 and 3) have been obtained, which confirm that they are highly pyramidalized at nitrogen and have long N-CO bonds, a characteristic of other anomeric amides and a consequence of drastically reduced amidicity. The crystals also demonstrate chirality at the amide nitrogen in the solid state. The structures are well-predicted by density functional calculations using N,N-dimethoxyacetamide as a model. The amidicity of N,N-dimethoxyacetamide has been estimated by two independent methods, COSNAR and a new transamidation method, which give almost identical resonance stabilization energies of -8.6 kcal mol(-1) and only 47% that of N,N-dimethylacetamide computed at the same level. The total destabilization is composed of a resonance and an inductive contribution, which we have evaluated separately. The electronegative oxygens at nitrogen are responsible for localization of the nitrogen lone pair on the amide nitrogen, a factor that contributes to a loss of resonance over and above the impact of pyramidalization at nitrogen, as well as the fact that N,N-dimethoxyacetamide is predicted to protonate on the carbonyl oxygen in preference to nitrogen.
Department of Chemistry, School of Science and Technology, University of New England, Armidale, NSW 2351, Australia. sglover@une.edu.au
Stephen A
SA
Glover
Jonathan M
JM
White
Adam A
AA
Rosser
Katherine M
KM
Digianantonio
D000577:Amides / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure
D000577:Amides