Efficient Synthetic Protocols in Glycerol under Heterogeneous Catalysis

Abstract

The massive increase in glycerol production from the transesterification of vegetable oils has stimulated a large effort to find novel uses for this compound. Hence, the use of glycerol as a solvent for organic synthesis has drawn particular interest. Drawbacks of this green and renewable solvent are a low solubility of highly hydrophobic molecules and a high viscosity, which often requires the use of a fluidifying co-solvent. These limitations can be easily overcome by performing reactions under high-intensity ultrasound and microwaves in a standalone or combined manner. These non-conventional techniques facilitate and widen the use of glycerol as a solvent in organic synthesis. Glycerol allows excellent acoustic cavitation even at high temperatures (70-100 °C), which is otherwise negligible in water.Herein, we describe three different types of applications: 1) the catalytic transfer hydrogenation of benzaldehyde to benzyl alcohol in which glycerol plays the dual role of the solvent and hydrogen donor ; 2) the palladium-catalyzed Suzuki cross-coupling; and (3) the Barbier reaction. In all cases glycerol proved to be a greener, less expensive, and safer alter-native to the classic volatile organic solvents.

Glycerol: a solvent and a building block of choice for microwave and ultrasound irradiation procedures

“…Cravotto et al () 14 were the first to describe the application of both US and MW irradiation (stand-alone or combined techniques) to the carrying out of different types of organic transformations in glycerol: 1) the catalytic transfer hydrogenation of benzaldehyde to benzyl alcohol in which glycerol plays the dual role of the solvent and hydrogen donor (Scheme 10 A); 2) palladium-catalyzed Suzuki cross-coupling (Scheme 10 B); and (3) the Barbier reaction (Scheme 10 C). In all cases glycerol showed itself to be a greener, less expensive and safer alternative to classic volatile organic solvents.…”

Recent Advances in the Use of Glycerol as Green Solvent for Synthetic Organic Chemistry

“…Using PdCl 2 , Pd(OAc) 2 and Pd/C as the palladium sources and the coupling of 4-iodoanisole with PhB(OH) 2 as model reaction, the same group has recently found that the effectiveness of the Suzuki-Miyaura C-C coupling can be strongly improved under sonochemical (US) conditions. The excellent acoustic cavitation provided by glycerol, and a more effective dispersion of the reactants in the medium, was evoked to explain the shorter reaction times and higher yields observed [66]. In a related study, Mata, Colacino and co-workers also showed the beneficial effect exerted by the ultrasounds in this C-C coupling process catalyzed by the palladium N-heterocyclic carbene (NHC) catalysts 25-28 [67].…”

“…Since the pioneering works by Crotti´s [73] and Wolfson´s groups [74] in 2009, glycerol has emerged as an appealing alternative to replace propan-2-ol as solvent and hydrogen donor in metal-catalyzed transfer hydrogenation (TH) reactions of unsaturated organic compounds [75]. In this context, using as model the reduction of benzaldehyde to benzyl alcohol catalyzed by [{RuCl(µ-Cl)(η 6 -p-cymene)} 2 ], Wolfson and co-workers demonstrated recently that, as observed in the Suzuki-Miyaura C-C couplings described above and also in Barbier-type reactions [66], the use of ultrasounds dramatically accelerates the TH reactions [66]. Thus, using a combination of potassium and sodium hydroxide (1-2 mol%) as the base, benzyl alcohol was quantitatively generated in 3 h under sonochemical conditions, at 60 ºC and with only 1 mol% of [{RuCl(µ-Cl)(η 6 -p-cymene)} 2 ]…”

Glycerochemistry Topics: Use of Glycerol as a Green Solvent in Microwave-assisted Reactions

“…Os autores exemplificaram da literatura, que os procedimentos sintéticos para esses produtos envolvem o uso de reagentes extremamente tóxicos e perigosos como dissulfeto de carbono, sendo que em alguns casos, ainda são necessários longos tempos reacionais (4 a 24 h), aquecimento em altas temperaturas (140°C) com atmosfera inerte e tudo isso para obter rendimentos de 60 a 70%. 2011 Lenardão e col. 80 2012 Um protocolo verde para a oxidação assistida por micro-ondas de tióis para dissulfetos em glicerol foi relatada por Cabrera e col. 84 A reação foi realizada em intervalos de temperaturas de 30 até 120°C, na presença de vários catalisadores básicos. Os catalisadores que apresentaram melhores rendimentos para essa reação foram o Na 2 CO 3 e Cs 2 CO 3 (Esquema 12).…”