Dimethylamino groups, rotational barriers

Hence the activation energy barrier to dimethylamino group rotation in dimethylacetamide (41) is calculated from equation 9 with k,. = 17.8 at the coalescence point 353 K (Fig. 2.26) ... 

Older literature has been reviewed <84CHEC-1(4)545>. The energy barrier to internal rotation of the dimethylamino group in 1,3,4-thiadiazoles is greater by 1.7-2.9 kJ mol than for the corresponding... 

Particularly interesting are the barriers to rotation of the dimethylamino group in heteroaromatic amides and thioamides which can be used as a sensor for the following effects (Scheme 73). 

The internal rotation barriers of the dimethylamino groups in substituted azoles including 5-nitro-2-dimethylamino-l,3,4-oxadiazole (AG =9 kcal/mol, 133°C) were also defined by NMR spectroscopy [523],... 

Dahlqvist42 has performed a complete bandshape study of the enamine 10b. He observed different barriers to rotation of the dimethylamino group in the E and Z forms with respect to the double bond and a quite low barrier to rotation about the double bond (Table 3). Inspection of the data for 10b and lib reveal the different effects of the... 

However, it is to be expected that the rotation of the dimethylamino groups out of the plane should diminish their donor capacity. This is supported by the observation that the C=C barriers are lower in acceptor-substituted 1-dimethylamino-1-methyl-thioethenes (ketene N,5-acetals, 26) than in the l,l-bis(dimethylamino)ethenes (24) with the same acceptor combination (Table 6), in spite of the fact that dimethylamino groups in general are much better donors than methylthio groups33. However, the situation is not quite simple, since in a crystallographic study the ketene A S-acetal 26a was found to have the dimethylamino group twisted 25° out of the plane with a C1=C2 bond twist of ca 20°67. 

Heptafulvenes assume a push-pull character when the exocyclic carbon atom (C8) is substituted by acceptor groups90. Daub and coworkers91 have studied this type of compound with dimethylamino groups and other donor groups on C8. NMR spectra of the bis(dimethylamino) compound 41a show that the compound is boat-shaped with alternating single and double bonds, and a barrier to rotation about the exocyclic C=C bond >27 kcal mol-1 is reported for the 8-dimethylamino-8-triethylsilyloxy compound 41b. No low-temperature spectra are reported, but the C—N barrier is likely to be low. 

In the al -trans polymethine dyes, hindered rotation of the dimethylamino groups is observed with quite high barriers. Dale and coworkers99 have determined this barrier... 

A comparison, by variable temperature NMR measurements, of the energy barrier to internal rotation of the dimethylamino group in five-membered heterocyclic structures has included the study of 5-dimethylamino-1,2,4-thiadiazole.260 The compound is iso-7t-electronic with 4-dimethylamino-pyrimidine, and is comparable in the positions of its nitrogen atoms. The observed closeness of their energy barriers reflects the analogies between cyclic isosteric compounds in which a sulfur atom replaces a —CH=CH— group. A variety of other aspects opened up by these measurements have been discussed.260... 

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