Isomerization, cracking and dehydrogenation of protonated mesityl oxide

Abstract

Abstract Protonated mesityl oxide (1,3,3‐trimethyl‐1‐hydroxyallyl cation, 1 ) in strong acids has been found to give three reactions, resulting in a large variety of products depending on the acid used as the solvent. (1) Isomerization, followed by cyclization, gives the 2,5‐dimethyltetrahydrofuryl cation, which is about 5 × 10 3 times stabler than 1 . The isomerization, which proceeds via intermediate dications, is acid‐catalysed; its rate decreases strongly in the order 1: 1 HF‐SbF 5 〉〉 9: 1 HF‐SbF 5 〉 1: 1 FSO 3 H‐SbF 5 〉〉〉 5: 1 FSO 3 H‐SbF 5 . (2) In FSO 3 H‐SbF 5 , dehydrogenation with subsequent ring closure to give the 2,4‐dihydrofuryl cation. (3) Cracking produces the acetyl cation and isobutene, which are formed by β‐scission of C ‐protonated mesityl oxide. The isobutene fragment reacts further to give, depending on the acid and the rate at which it is formed, t ‐butyl cation, or the disulfonic acids of isobutene, or products thereof, and alkylcyclopentenyl ions. Rate constants of the three reactions in the different solvents have been measured. The 1‐ethyl‐3,3‐dimethyl‐1‐hydroxyallyl cation ( 2 ) reacts in a similar way, but different product selectivities are obtained.