A Combined Synthetic And Theoretical Study Of Benzimidazole Squaraine Dyes

In this study, a regioselective method of synthesis of1,2-benzimidazole SQs inthe presence of strong base has been firstly proposed, and the yield is up89%. Thelaboratory findings are listed as blow:(1) the reaction between benzimidazolium salt and diethyl squarate can beenpromoted efficiently in the presence of strong bases (organic base: EtONa、DBU,inorganic base: CsCO3、KOH and NaOH)，and the product is1,2-substituted SQs(3a-3b), being found to be highly regioselective。(2) The overall process of the reaction between1,2,3-trimethyl-benzimida-zolium（1b） and diethyl squarate have been simulated employing theoreticalcalculation. Results indicate that nucleophile benzimidazole methylene (PRa) is easierto form by deprotonating in the condition of strong base; Also among the twosimulated reaction pathways, the highest activation barrier of I-A pathway forming1,2-SQs is low than that of I-B forming1,3-SQs, meaning that I-A is the optimalpathway of this reaction.(3) Unsymmetrical1,2-SQ(3c) has been synthetized from1,3-dibenzyl-2-methyl-benzimidazolium chloride(1a) and the separated PRb intermediate in thepresence of strong base EtONa as catalyst, which demonstrates that the I-A pathwayis of great rationality.