Theoretical and spectroscopic investigations of indigo dyes, part 24. Synthesis, crystal structure and light absorption of vinylogous N,N '-dialkylindigos

The base catalyzed N,N'-dialkylation of the vinylogous indigo dye cis-cis-2 a proceeds with concomitant isomerization and yields cis-trans-2b,c as mino r, and trans-trans-2b,c,d as major products. Upon exposure to light, cis-tr ans-2b,c isomerizes to trans-trans-2b,c. According to an X-ray analysis, tr ans-trans-2c and trans-trans-2d are planar. However, due to the different c onfigurations of 2a and 2b-d, the light absorption data of 2a-d cannot be u sed to quantify the inductive effects of the alkyl groups. (C) 1999 Elsevie r Science Ltd. All rights reserved.