Mechanistic and exploratory investigations into the synthesis of 1,3,5-triaroylbenzenes from 1-aryl-2-propyn-1-ones and 1,3,5-triacetylbenzene from 4-methoxy-3-buten-2-one by cyclotrimerization in hot water in the absence of added acid or base.

Neat 1-phenyl- and 1-(p-tolyl)-2-propyn-1-ones (1 and 1', respectively) were heated in water without any additive at 150 °C for 2 h to give 1,3,5-tribenzoyl- and 1,3,5-tri-(p-toluoyl)benzenes (2 and 2', respectively) in 74 and 52% yields, respectively. The crossed reactions of 1 with the enolate of p-toluoylacetaldehyde (3') and 1' with the enolate of benzoylacetaldehyde (3) were carried out to give unsymmetrically substituted 1-toluoyl-3,5-dibenzoylbenzene (Ph2Tol) and 1,3-ditoluoyl-5-benzoylbenzene (PhTol2), respectively, corroborating the previously proposed reaction mechanism in which 3 and 3' that are formed by rate-determining nucleophilic attack of HO(-) on 1 and 1' or its conjugate acids formed by subsequent protonation would serve as a common intermediate for the formation of 2, 2' and the acetophenone derivatives as byproducts. When 4-methoxy-3-buten-2-one (4) was heated in hot pure water without any additive at 150 °C for 30 min, 1,3,5-triacetylbenzene (5) was obtained in an isolated yield of 77% just by removing water by filtering the crystalline product from the cooled reaction mixture.

One-pot synthesis of 1,3,5-tribenzoylbenzenes by three consecutive Michael addition reactions of 1-phenyl-2-propyn-1-ones in pressurized hot water in the absence of added catalysts.

Cyclotrimerization of 1-phenyl-2-propyn-1-one in pressurized hot water gave 1,3,5-tribenzoylbenzene in one pot in 65 % yield after 7 min at 200 °C, or in 74 % yield after 60 min at 150 °C. The reaction did not take place in the absence of water, and added base promoted the reaction at 250 °C, suggesting a mechanism of three-consecutive Michael addition reactions. The reaction rates increased with temperature, but the yield of 1,3,5-tribenzoylbenzene decreased at the expense of formation of acetophenone as a side product at higher temperatures.