Aim and Objective:
Sustainable production of fine chemicals both in industries and
pharmaceuticals heavily depends on the application of solid-phase synthesis route coupled with microwave
technologies due to their environmentally benign nature. In this report, a microwave-assisted esterification
reaction using polymer-bound triphenylphosphine and 4,4′-dinitroazobenzene reagent system was
investigated.
Materials and Methods:
The solvents were obtained from Merck India. Polymer-bound triphenylphosphine
(~3 mmol triphenylphosphine moiety/g) was acquired from Sigma-Aldrich. The progress of the reaction was
observed by thin-layer chromatography. All the reactions were performed in Milestones StartSYNTH
microwave. The NMR spectra were recorded on Bruker Avance III 300, 400, and 500 MHz FT NMR
Spectrometers. Using azo compound and polymer-bound triphenyl phosphine as a coupling reagent,
esterification of different carboxylic acids with alcohols was performed under microwave irradiation.
Results:
Esterification of benzoic acid with 1-propanol under microwave irradiation gave a high yield of 92%
propyl benzoate in 60 minutes only. Isolation of the ester products was relatively simple as both the
byproducts polymer-bound triphenylphosphine oxide and hydrazine could be removed by simple filtration.
The rates of reactions were found to be directly proportional to the pKa of the benzoic acids.
Conclusion:
4,4′-Dinitroazobenzene was introduced as a novel coupling reagent, in conjugation with
polymer-bound triphenylphosphine, for esterification reactions under microwave irradiation. The low
moisture sensitivity of the reaction system, easy separation of the byproducts, and column chromatographyfree
isolation of esters help our methods with application significance, particularly from the ‘Sustainable
Chemistry’ perspective.
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