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Experimental and Theoretical Investigation of the Relative Catalytic Activities of Triphenylphosphine and N-Heterocyclic Carbene in Oxa-Michael Addition of Phenols

Priyanka Suthar, Dr. Ruchi Singh, Rashi Bala Vyas, Raj Bansal

Abstract

The catalytic activities of triphenylphosphine (PPh3) and N-heterocyclic carbene, namely 3-benzyl-4,5-dimethylthiazolylidene (NHC) in oxa-Michael reaction have been compared by carrying out experimentally the reactions of some representative phenols with dimethyl acetylenedicarboxylate (DMAD). The two catalysts exhibit remarkable difference in their catalytic activities. Michael addition of β-naphthol to DMAD in the presence of PPh3 was complete only after refluxing at 82 °C for 8 hrs., but in the presence of NHC, the reaction occurred at room temperature (~ 30 °C). The reaction of phenol with DMAD in the presence of PPh3 did not occur even after refluxing for 72 hrs. in acetonitrile, but in the presence of NHC, the reaction was complete after refluxing in acetonitrile for 24 hrs. to afford a mixture of trans- and cis isomers in 68 and 32% ratio. Salicyladehyde showed similar behavior, but afforded only the trans isomer. p-Cresol and p-nitrophenol, however, did not react with DMAD in the presence of either of the two catalysts. The difference in the catalytic activities of PPh3 and NHC could be rationalized by computing model reactions of β-naphthol with DMAD in the presence of PPh3 and NHC catalysts at the B3LYP/6-31+G(d) level. The calculations reveal that the reaction is initiated by the formation the reactant complex between β-naphthol and the catalyst, which is followed by the transfer of the O-H proton to the catalyst to generate the β-naphthoxide ion. This is the rate-determining step and the activation free energy barriers for the phosphine-catalyzed and for the NHC-catalyzed reactions are 27.14 and 2.08 kcal mol- respectively indicating that the catalytic activity of NHC would be much greater than that of PPh3.

CH25013  Accepted 23 May 2025

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