Secondary amide dimers

In IR spectra of secondary amides, which exist mainly in the trans conformation, the free NH stretching vibration observed in dilute solutions occurs near 3500-3400 cm-1. In more concentrated solutions and in solid samples, the free NH band is replaced by multiple bands in the 3330-3060 cm-1 region. Multiple bands are observed since the amide group can bond to produce dimers with an s-cis conformation and polymers with an s-trans conformation. 

Fig. 8 Common hydrogen bonding patterns observed for amides a the tape structure formed by primary amides involving i (8) and C(4) motifs b chains formed by non-cyclic secondary amides c the dimer formed by cyclic amides [62]...

The simplest networks are one-dimensional a-networks which may be composed of secondary amides, primary amide dimers or nucleophospholipids. In chapter 5, such structures were discussed as micellar rods and tubules in bulk aqueous solutions. Two-dimensional materials such as copper oxide superconductors, molybdenum sulfide lubricants and intercalated graphites are mostly inorganic. The anisotropic properties are a result of covalent bonds in two dimensions and weak interactions in the third dimension. One may, however, also envision strong hydrogen-bond interactions within an organic layer, whereas adjacent layers are held together only by van de Waals interactions. The two-dimensional, or p-network may form spontaneously from an... 

Other common supramolecular synthons for solid-state supramolecular construction are formed via hydrogen bonding of amide groups [64]. Primary and secondary amides [65], pyridones [66] and 2-aminopyrimidines [67] form cyclic hydrogen-bonded dimers, as shown in Figure 5. 

N-methy lacetamide is known to exist almost exclusively in the irons form whereas N-phenylurethane is found to be 95 % cis and 5 % irons (Russell and Thompson, 1956). Bhaskar and Rao (1967) studied the dimerization equilibria of N-methylaceta-mide and A -phenylurethane by measuring the concentration dependence of the first N—H overtone band. The procedure for determining the dimerization equilibrium constant was similar to that of Liddel and Becker (1957). The enthalpy of dimerization was evaluated from values of at different temperatures. The details of the procedure and uncertainties in the determination of Kj have been described in the paper by Singh and Rao (1967). The values of (liters/mole) at 26 C were 5.4 for N-methylacetamide in CCI4. and 1.5 for N-phenylurethane. The values of —AH were 4.7 and <3.0 kcal/mole, respectively. The values of and — A// for each of these compounds with various bases, e.g., pyridine and benzophenone, were also determined by a method similar to Becker s (1961). The and A// values did not show a clear-cut dependence on the configuration of the N—H bond of the secondary amide. 

Krikorian, S.E., Determination of dimerization constants of cis- and trans-configured secondary amides using near-infrared spectrophotometry, J. Phys. Chem., 86, 1875-1881, 1982. 

On the other hand, the preparation of secondary amides from aldoximes has been recently achieved through the one-pot sequential rearrangement/A -alkylation process depicted in Scheme 18 [93]. The reactions were carried out heating a toluene solution of the aldoxime with a mixture of dimers [ RuCl(p-Cl)(r -/7-cymene) 2]... 

It has been known for a long time that organozinc amides derived from secondary or primary amines usually exist as dimers in solution . However, depending on the particular groups bound to zinc and/or nitrogen, examples of higher aggregates are also known. 

The analysis of these non-pairwise effects allowed us to estimate that the repulsive secondary interactions are twice as large as the attractive ones (+9 vs. -4 kJ mol ). This model has been used to rationalize the case of 2-oxo-7-hydroxy-l,3-dihydro- l //- 1,8-naphthyridine, a compound existing in the solid state probably as the 101 dimer with four intermolecular HBs and having the possibility to sustain SSPT. Unfortunately, the amide derivatives do not exist as amino/imino tautomers but as the diamino (diacylamino) tautomers and, therefore, cannot present the four HBs necessary for SSPT. We have extended these studies to other difunctional naphthyridines [114],... 

Koga and coworkers30-32 have studied the lithium amides of several chiral l-phenyl-2-(l-piperidino)ethylamines with various substituents on the secondary nitrogen (3). NMR studies of the 15N and 6Li labelled lithium amides showed that they exist as monomers in THF, Li-3 2T1I1. and dimethoxyethane solvents, respectively. In Et20 and in toluene solution, symmetrically coordinated dimers, (Li-3)2, were observed. Addition of HMPA to either of the solutions resulted in monomers. 

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