Keto ⇄ enol equilibria for and oxidative cyclization to benzofurans of 2,2-ditipyl-1-R-ethenols. Comparison with dimesityl analogs

2,2-Ditipyl-1-R-ethenols (R = H, Me, t-Bu; tipyl (Tip) = 2,4,6-triisopropylphenyl) were equilibrated in hexane with their carbonyl isomers. The equilibration was accompanied by an oxidative cyclization of the enols to substituted 2-R-3-tipyI-4,6-diisopropylbenzofurans, which became more facile with the increase in the bulk of R. At 77.5°C in hexane the K_Enol values were 90, 0.11, and 0.0011 for R = H, Me, t-Bu, respectively. Comparison with K_Enol values for the 2,2-dimesityl analogs indicate a higher value for the 2,2-ditipyl derivative with R = H, but lower values for R = Me and t-Bu. The factors affecting this difference are discussed.