Kinetics and a mechanistic investigation of 2H-thiopyrano [2, 3-b] quinoline-2, 3-dicarboxylates from 2-mercaptoquinoline-3-carbaldehydes, dialkyl acetylenedicarboxylates and triphenylphosphine: Empirical approach

Abstract

The main aim of full chemical-kinetics investigation in this work, was recognition of a different role of triphenylphosphine (TPP) in the reaction with dialkyl acetylenedicarboxylates [di-tert-butyl acetylenedicarboxylate (DTBAD), diethyl acetylenedicarboxylate (DEAD) or dimethyl acetylenedicarboxylate (DMAD)] in the presence of SH-acid such as 2-mercaptoquinoline-3-carbaldehyde for the generation of 2H-thiopyrano [2, 3-b] quinoline-2, 3-dicarboxylates. The unexpected and different kinetic results (in comparison with previous work in the presence of TPP) is confirmed by the UV/vis spectrophotometry technique which is accompanied by these results. (1) The reaction followed third-order kinetics. (2) Step1, containing TPP and DTBAD (DEAD or DMAD), was a fast step (kobs = 84.61 M−1.min−1 ). (3) The rate of overall reaction (TPP, S-H acid and DTBAD) in the presence of dichloromethane solvent (DCM) at 13 °C is (5.87 M−1.min−1 ), approximately 14.4 times lesser than the fast step. (4) The proposed mechanism is consist of six steps, amongst them, intermediate I3 in step4 having a well sensitive part to any changes in the reaction environment because it is near the reaction center. Any changes in dialkyl acetylenedicarboxylates’ structures can be influenced their inductive and steric effects, corresponding to remarkable changes in the rate of this step 4 (k4 ). (4) For this reason, step4 was recognized as a rate determining-step.