Experimental and theoretical investigation of the relative catalytic activities of triphenylphosphine and N-heterocyclic carbene in oxa–Michael addition of phenols

The catalytic activities of triphenylphosphine (PPh3) and N-heterocyclic carbene, namely 3-benzyl-4,5-dimethylthiazolylidene (NHC) in oxa–Michael reactions have been compared by reacting 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 after refluxing at 82°C for 8 h, but in the presence of NHC, the reaction occurred at room temperature (~30°C). The reaction of a phenol with DMAD in the presence of PPh3 did not occur even after refluxing for 72 h in acetonitrile, however, in the presence of NHC, the reaction was complete after refluxing in acetonitrile for 24 h to afford a mixture of trans and cis isomers in a 68 to 32% ratio. Salicylaldehyde showed similar behaviour, but afforded only the trans isomer. 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 rationalised 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 of 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-catalysed and the NHC-catalysed reactions are 27.14 and 2.08 kcal mol−1 respectively. This indicates that the catalytic activity of NHC is much greater than that of PPh3 in an oxa–Michael reaction.