Pinacol rearrangement is a reaction in which 1,2-diol undergoes intramolecular rearrangement and dehydration under acid catalysis to form a carbonyl compound (aldehyde or ketone). The following pinacol rearrangement reactions under different conditions are in practice.
1.Pinacol rearrangement in high temperature liquid water
High temperature liquid water usually refers to compressed liquid water at 180-350 ° C. It not only has the functions of acid catalysis and alkali catalysis, but also has the characteristics of dissolving organic and inorganic substances at the same time. In recent years, as an environmentally friendly medium, research on the application of high-temperature liquid water in organic synthesis has received widespread attention and has become one of the green synthesis methods.
Pinacol can rearrange in the presence of no catalyst in high-temperature liquid water. The temperature has a great effect on the conversion rate of pinacol. At 180 ° C for 4 hours, the conversion rate of pinacol is only 46%, and 220 ° C. When the reaction time is 40min, the conversion rate of pinacol can reach 98%.
2.Pinacol rearrangement reaction catalyzed by heteropolyacids
Heteropolyacids (HPA) are a type of oxygen-containing polyacids composed of heteroatoms (such as P, Si, Fe, etc.) and polyatoms (such as Mo, W, V, etc.) according to a certain structure through oxygen atom coordination bridges. , Has a high catalytic activity, it not only has acidity, but also has redox properties, is a new type of multifunctional catalyst. Heteropoly acids have good stability and can be used as catalysts for homogeneous and heterogeneous reactions. They can even be used as phase transfer catalysts without pollution to the environment. Therefore, they are a promising green catalyst.
Torok et al. Reported that the pinacol rearrangement reaction catalyzed by heteropolyacids gave almost the same amount of pinacolone for 1,2-diols. For 1,4-1,5, and 1,6-diols, cyclization dehydration is the main reaction, and the conversion of the corresponding alcohol increases as the heteropoly acid increases in acidity, whether in a homogeneous or heterogeneous system Heteropoly acids all show high efficiency and stability.
3.Pinacol rearrangement catalyzed by molecular sieves
Molecular sieves are crystalline aluminosilicates with clear pore distribution, high internal surface area, good thermal stability, and tunable acid site centers. Therefore, molecular sieves have received widespread attention as catalysts.
Hsien et al. Reported a pinacol rearrangement reaction catalyzed by a porous molecular sieve in toluene. Porous molecular sieves can be either microporous AIPO4-5 or mesoporous MCM-41. The active center of the molecular sieve framework is ferric ions, and the catalyst activity is related to acidity. They used 10 kinds of alkyl or aryl substituted o-diols as substrates. The experimental results showed that AIPO4-5 molecular sieve containing 0.5% -2% trivalent iron ions had the largest catalytic activity for rearrangement reaction, pinazone The highest yield.
Through the rearrangement reaction, the yield of pinacol can reach a relatively high state. But it is the least harmful to the environment.