Amines cyclic ring strains

As with the amine oxides, mixtures of olefins are obtained when more than one type of jS -hydrogen is present. In noncyclic compounds, the olefin composition often approaches that expected on a statistical basis from the number of each type of hydrogen. The tram olefin usually predominates over the cis for a given isomeric pair. In cyclic systems, conformational features, ring strain, and related factors usually distort the mixture from that expected on a statistical basis. Elimination in a direction in which the syn mechanism can operate is strongly preferred over elimination in a direction where this is impossible. 

Poly(ethylene imine). Poly(ethylene amine) (PEI) is the simplest polybase. It can be prepared directly via the acid-catalyzed polymerization of ethyleneimine (aziridine, 24C) (Fig. 40). 24C may be prepared via a number of routes with ethanolamine being a convenient precursor (223). The cationic polymerization of 24C is very rapid due to the release of the ring strain associated with the monomer. However, the synthesis of PEI under these conditions leads to highly branched structnres dne to chain transfer reactions involving the —NH— species in the polymer backbone. Cyclic products can also be prodnced nnder these conditions. Copol5nnerization with an appropriate comonomer can rednce the degree of chain branching (224). 

The protonation of amines causes a shielding of carbons adjacent to the nitrogen except for branched systems, where some deshielding is often seen. The effect of protonation can be about -2 ppm at an a-carbon, -3 ppm at a P-carbon and up to -1 ppm at a y-carbon. For example, the shifts of ethylamine are 36.9 ppm (CH2) and 19.0 (CH3) and these are shifted by -0.2 and -5.0 ppm, respectively, on protonation. Again, ring strain effects in cyclic amines are mirrored by large shift effects. 

We observed that cyclic esters (lactones) may be formed when the carboxyl electrophile and hydroxyl nucleophile are in the same molecule (see Section 7.9.1). Similarly, cyclic amides are produced when carboxyl and amine groups are in the same molecule, and are again most favoured when this results in the generation of strain-free five- or six-membered rings. Cyclic esters are termed lactones, whereas cyclic amides are in turn called lactams. The nomenclature of lactams is similar... 

Azetidine can be considered as a fairly typical cyclic amine. Strain in the four-membered ring is less than that in the three-membered aziridine system, and as a result azetidines show few, if any, of the exceptional properties associated with aziridines. Thus, ring cleavage reactions occur with greater ease than in larger ring cyclic amines, but much less readily than with aziridine for example, unlike aziridines, azetidines do not function as alkylating... 

Related to B-strain. but less well understood, is "l-strain (for internal strain . In cyclic amines and ethers, such as (CH2) 0. the basicity varies with ring size. In such compounds the hybridization (and hence the overlapping ability and electronegativity) of not only the basic center (N, 0, etc.), but also of the carbon atoms in the ring will vary with ring size, and there are no simple rules for predicting the results. 

Cationic polymerization of cyclic amines proceeds usually with a high activation energy and often requires relatively high temperatures, even for the strained four-membered rings, e.g. for 1,3,3-trimethylazetidine AH = 73 kJ mol-1, kp = 1.4 xl0 4mor1 -1-s"1 at 78 °C 5). 

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