Acceleration of Synthetic Organic Reactions Using Supercritical Water: Noncatalytic Beckmann and Pinacol Rearrangements

Abstract

A high-pressure and high-temperature FTIR was used to study noncatalytic Beckmann and pinacol rearrangements using supercritical water (scH 2 O). Significant acceleration of Beckmann and pinacol rearrangements can be achieved by using scH 2 O, especially near the critical point, even in the absence of any acid catalysts. It has been demonstrated that scH 2 O acts effectively in the place of conventional acid catalysts for both the rearrangements. The rate of pinacol rearrangement using scH 2 O is significantly larger by a factor of 28200 than that in 0.871 M HCl solution at 46.7 MPa under distillation conditions. The activation energy for the former at 25 MPa was found to be markedly reduced to about one-third of that for the latter. The accelerated rates of reaction may be attributed to a great increase in the local proton concentration around the organic reactants. In addition, the nature of scH 2 O can be adjustable to weak acidity in the near-critical region, and then it does not catalyze the pinacol rearrangement, but opens a new reaction pathway from pinacol through a completely dehydrated product and then to a Diels−Alder adduct between the dehydrated products.