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.

Stable Vapor-phase Catalytic Conversion of Pinacolone into 2,3-Dimethyl-1,3-butadiene

“…It is well known that ketones are synthesized in the pinacol rearrangement of vicinal diols: for example, 2,3-dimethyl-2,3-butanediol, so-called pinacol, is converted into 3,3-dimethyl-2-butanone, pinacolone, through dehydration. Homogeneous acids such as SbCl 5 AgSbF 6 1 and BF 3 2 catalyze the rearrangement. It is reported that the rearrangement proceeds under supercritical conditions without catalyst.…”

“…It is reported that the rearrangement proceeds under supercritical conditions without catalyst. 3 It is also reported that solid acids such as aluminum phosphate, 4,5 sulfated zirconia, 6 and zeolite 7 catalyze the rearrangement in the vapor phase. In the vapor-phase catalytic rearrangement, however, DMB can be produced as a by-product, into which pinacolone is transformed and dehydrated.…”

Hydrogen production by supercritical water gasification of food waste using nickel and alkali catalysts

“…However, Ni is widely used due to its affordability. Also at SCW conditions, H + and OH -ions are liberated at high concentrations which creates a perfect condition favorable to acid-base catalysts during the WGSR [32][33][34]. Therefore, alkaline homogeneous catalysts such as KOH, NaOH, K 2 CO 3 and Na 2 CO 3 can promote biomass gasification for effective H 2 production [15].…”

Simple, non-catalytic permethylation of catechol derivatives in subcritical and supercritical water

“…Many important organic reactions such as secondary alcohol oxidation without an oxidant or a catalyst to generate carbonyl compounds and hydrogen gas [19], efficient non-catalytic Oppenauer oxidation of alcohols [20], and 4 the Beckmann rearrangement [21] have been performed under non-catalytic conditions in sub-CW and SCW. Regarding 2,6-and 4-alkylation of phenol in SCW, a pioneering work using aldehydes as a source of alkyl groups was reported by Adschiri et al; 2,6-and 4-alkylation of phenol showed much higher reactivity than methylation of phenol using supercritical methanol [22][23][24][25][26].…”