On the Conformation of a Sterically Congested Amide Group in the Crystalline State and in Solution

✍ Scribed by Andreas Mühlebach; Gian Paolo Lorenzi; Volker Gramlich

• Publisher: John Wiley and Sons
• Year: 1986
• Tongue: German
• Weight: 386 KB
• Volume: 69
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19860690218

✦ Synopsis

The conformation in the crystdine state and in solution of the sterically congested tetramethylpiperidinederived arnidc group of the symmetric diamide I formed from 2,2,6,6-tetramethylpiperidine (b) and 1,2,5-thiadiarole-3,4-dicarbonyl dichloride (c), and of the mixed diamide 11 derived from b, c, and piperidine (a) has been investigated. In crystals, as observed with 11, this group is strongly bent out-of-plane at both the N-atom and the carbonyl C-atom, and there is also a sizable twisting around the amide bond. Furthermore, the amide bond is abnormally long (1.37 A). In CD2Cl2 (or CDC1,) solution, the group is apparently planar in its ground-state conformation, but the energy barrier to rotation around the amide bond is low. This conclusion is based on low-temperature 'H-NMR measurements on I, 11, and on the symmetric diamide 111 derived from a and c. N(l)-C(6")-C(3")-N(Y) 51.1 51.8 N(I)-C(6")-C(3")-C(4") -138.0 -140.0 O(8'3 -C(6") -C(33 -C(4") 48.3 46.3 0(8")-C(6")-C(3")-N(2") -122.7 ~ 12l.Y ThiadiaLole ring C(6'7 -C(33 -C(4") -C(7") C(6")-C(3")-C(4'1-N(5") N(2")-C(3")-C(4'7-C(7") N(2")-C(3'7-C(4")-N(S'')

") 13.9 -173.4 -174.6 -2.0 1.8 173.7 --1.o 4 . 1 1.1 174.1 Amide group derived from piperidine C(3')-C(2')-N( I')-C(7") -I 18.7 --119.6 C(6'-N( l')-C(7'')-C(4") 9.1 C(S')-C(6')-N(I')-C(7") 118.7 118.7 N(l')-C(7'7-C(4")-C(3") -125.8 C(2')-N( l')-C(7'7-0(9") 1.9 2.4 N( 1 ')-C(7'7-C(4") -N(5") 62.0 C(2')-N(l')-C(7'1-C(4'7 -177.1 -177.9 0(9")-C(7")-C(4")-C(3") 55.0 C(6')-N( l')-C(7")-0(9") -1 7 1.8 -I 72.3 0(9'7-C(7")-C(4")-N(5") -117.1 ")