Twisted amide

It is noteworthy that the Cp/amide twist angle is large most likely due to the involvement of the carbonyl group in H bonding. The H bonding in 31 is similar to that of 28 in that it involves the interaction of two Fc-amides [Fc-C=0- -H-N-C (O)-Fc ti(01 NIB) = 2.977A] and two peptide amides [C=Opi,e H-Nphg ... 

A more recent example is the twisted amide (2) devised by Kirby, which despite the lack of electron-withdrawing groups (other than nitrogen) is completely hydrated upon protonation on nitrogen here the amide is unable to delocalize the nitrogen electrons onto the carbonyl, which means there is none of the usual amide stabilization. 

A reaction of the first type, in a broad sense, uses the mixture obtained from n-BuLi and the phosphinic acid (mes)2P(H)=0 (mes = 2,4,6-trimethylphenyl) in thf/hexane to which the Cd amide Cd[N(SiMe3)2]2 is added a mixed amide-phosphinite is obtained, namely [ MeSi)2N Cd (mes)2PO 2Li-2 thf]. The structure analysis (153 K) reveals that the two phosphinite ligands act as a double bridge between Cd and Li to give a six-membered, twisted ring with two independent Cd—P bonds (rav(Cd—P) 259.4 pm) the exocyclic Cd—N bond is rather short (213.6 pm).226... 

Although the ROA spectra of typical /1-sheet proteins share some of the features observed in /3-sheet poly-L-lysine, there are also some differences, especially in the amide I region. This is because the /1-sheet in proteins tends to be twisted and irregular, whereas that in polypeptides tends to be extended, multistranded and relatively flat. 

Fig. 5. Comparison of ab initio, DFT/BPW91/6-31G -computed IR and VCD spectra over the amide I, II, and III regions for model peptides (of the generic sequence Ac-Alaw-NHCH3). These are designed to reproduce the major structural features of an o -helix (top left, n— 6, in which the center residue is fully H-bonded), a 3i helix (PLP Il-like, top right, n— 4), and an antiparallel /1-sheet (n= 2, 3 strands, central residue fully H-bonded) in planar (bottom left) and twisted (bottom right) conformations. The computations also encompass all the other vibrations in these molecules, but those from the CH3 side chains were shifted by H/D exchange (CH3) to reduce interference with the amide modes.

Methodical details of crystal calculations have been published by several authors (8, 95-98). As applications we only mention the calculation of the crystal structure of diphenyl (98, 99) which has a planar structure in the solid state (100) whereas gaseous diphenyl is twisted around the CC-bond, which links both benzene rings, by about 40° (101). The calculations reproduce satisfactorily the experimental findings. A further application is the calculation of the crystal structures of a series of amides (102), with the object of deriving suitable functions for the description of hydrogen bonds (cf. previous Section). 

Kirby s configurationally rigid l-azaadamantan-2-one 1 is the extreme of this class.21 23,25 The nitrogen properties of this lactam are clearly amine-like while the carbonyl is ketonic in all respects. Twisted amides undergo rapid hydrolysis or reduction and exhibit enhanced reactivity. 

Fig. 2 (a) A sterically twisted amide (b) an angularly constrained amide (c) a twisted... 

Fig. 6 Amide C = 0 and C-N vibrational frequencies (cm ) and bond lengths (A in parentheses), twist angles (t), average angles at nitrogen computed at HF/6-31G level for ground state and orthogonal conformations of (a) A-formyloxy- /V-mcthoxyformamidc 40, (b) iV-methoxyfomiamidc 41 and (c) formamide 42. Energies (Hartrees) at B3LYP/6-31G // HF/6-31G level.

Fig. 9 (a) Winkler-Dunitz parameters for defining twist (t) and pyramidality (xn, Xc) in amides (b) angles at nitrogen. 

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