Enamines, Enaminoaldehydes, Cyanines

Carbon-13 shifts of enamines [342] follow the behavior described for other donor substituted alkenes (Sections 4.4.3 and 4.6.2). Electron release by the dialkylamino group has two consequences The inductive electron withdrawal at the a alkene carbon is reduced (Za 10-15 ppm) compared with the a increments of aliphatic amines (Table 4.43). Further, electron density at the fi olefinic carbon increases, as indicated by considerable shieldings in pyrrolidino- and morpholinoalkenes. 

The configuration of the dialkylamino group is monitored by the shift values of the a alkyl carbon nuclei at the other side of the double bond, similar to the manner described for alkenes (dIE 5(/1) and shown for the ( )- and (Z)-pairs of 1-pyrrolidino- and 1-morpholinopropene. - 13C shifts of 1-dimethylamino-l,3-dienes clearly show that elec- 

Electron withdrawing substituents such as aldehyde or imonium functions at the other end of the alkene or the 1,3-diene transform enamines and 1,3-dienamines into push-pull systems characteristic of dyes. Electron release (push) of the donor group shields the / , S, e. carbons and electron withdrawal (pull) of the acceptor group (carbonyl oxygen or imonium nitrogen) deshields in the a, y. position [343], 

deshieldings and shieldings alternate along the polyene chain, as demonstrated by the carbon-13 shifts of N,N-dimethylaminoacrolein, N,N,N, N -tetramethyltri-methinecyanine and their vinyl analogs [343], 

Due to the partial n character of the CN bonds which are, in fact, vinylogous forma-mide or amidine connections in keeping with the resonance formulae, different methyl shift values are observed for some N,N-dimethylamine groups. Moreover, one-bond carbon-proton coupling constants decrease from the end to the center of the polymethine chain. This is explained by CNDO/2 calculations, in which corrected bond orders and bond angles alternating between 125° (a, y, e) and 117.5° (/5, 5) are taken into account [343], 

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