Zhen-Feng Diao

Homogeneous hydrogenation of carbon dioxide to methanol

Carbon dioxide, a greenhouse gas mainly from the consumption of fossil fuel, is regarded as an attractive feedstock in view of synthetic chemistry. Great efforts have been devoted to developing catalytic processes for converting CO2 into value-added compounds with reduced carbon footprint. Among versatile applications in organic synthesis, CO2 can serve as a promising raw material for fuel production, especially methanol. ‘Coming from fuel and returning to fuel’ is an appealing objective in terms of sustainable development associated with circumventing the energy shortage and CO2 issue. To date, metal complexes and organocatalysts for CO2 hydrogenation to methanol have been developed along with the reaction mechanistic insight. Understanding the interaction of active catalytic species with CO2 or hydrogen could account for development of efficient homogeneous catalysts. In this context, homogeneous catalytic hydrogenation of CO2 and its derivatives into methanol is highlighted in this article in combination with mechanistic understanding on a molecular level.

Efficient chemical fixation of CO2 promoted by a bifunctional Ag2WO4/Ph3P system

An efficient heterogeneous silver-catalyzed reaction for construction of the α-methylene cyclic carbonate motif was developed through carboxylative assembly of propargyl alcohols and CO2. Such a CO2 fixation protocol proceeded smoothly with only 1 mol% of Ag2WO4 and 2 mol% of PPh3 as well as atmospheric CO2 at room temperature under solvent-free conditions, in an environmentally benign and low energy manner along with an easy operating procedure. Notably, up to 98% isolated yields of carbonates could be attained with exclusive chemo-selectivity. In addition, the dual activation capacity of Ag2WO4 towards both the propargylic substrate and CO2 is based on which cooperative catalytic mechanism by the silver cation and the tungstate anion is proposed. Recycling trials on carboxylative cyclization of propargyl alcohols and CO2 illustrate that the catalyst can be reused at least 4 times with retention of high catalytic activity and selectivity. Especially, it allows the direct and effective application in the one-pot synthesis of various oxazolidinones bearing exocyclic alkenes and carbamates in moderate to high yields upon the alternative introduction of primary or secondary amines.

Catalyst-free approach for solvent-dependent selective oxidation of organic sulfides with oxone

Selective oxidation of sulfides was successfully performed by employing oxone (2KHSO5·KHSO4·K2SO4) as oxidant without utilization of any catalyst/additive under mild reaction conditions. Notably, the reaction can be controlled by the chosen solvent. When ethanol was used as the solvent, sulfoxides were obtained in excellent yield; the reaction almost exclusively gave the sulfone in water. Furthermore, this protocol worked well for various sulfides to the corresponding sulfoxides in ethanol or sulfones in water.

Carboxylation of terminal alkynes at ambient CO2 pressure in ethylene carbonate

The CuI-catalyzed carboxylation of terminal alkynes with CO2 and alkyl halides using ethylene carbonate as the solvent under mild conditions was studied. DFT calculations reveal that the energy barrier for CO2 insertion into the sp-hybridized Cu-C bond could be reduced by employing ethylene carbonate as the solvent. Notably, the procedure was conducted under ambient CO2 pressure without any external ligands. A broad range of substrates with electron-withdrawing groups or electron-donating groups gave the corresponding products in reasonable yields.

Chapter Nine – Carbon Capture with Simultaneous Activation and Its Subsequent Transformation

Abstract Carbon capture and storage/sequestration (CCS) is now being considered as a potential option to mitigate global warming associated with carbon accumulation. The chemical absorption technique employing efficient amino-containing absorbents has been widely developed. Nevertheless, extensive energy consumption in desorption–compression process would be a crucial barrier to realize practical CCS. On the other hand, CO2 is very attractive as a typical renewable feedstock for manufacturing commodity chemicals and fuels. However, the reactions involving CO2 are commonly carried out at high pressure, which may not be economically suitable and also pose safety concerns. Consequently, we have proposed a carbon capture and utilization (CCU) strategy as an alternative approach to addressing energy issue in CCS. This crucial point of CCU could be simultaneous activation of CO2 upon its capture (e.g., formation of carbamate/alkyl carbonate) and thus in situ catalytic transformation into value-added chemicals under mild conditions, avoiding additional desorption step. This chapter is intended to discuss carbon capture and in situ transformation of CO2 to oxazolidinones, carbonates, quinazolines, urea derivatives, isocyanates, and carbamates via the formation of C O and C N bond.

PEG400-enhanced synthesis of gem-dichloroaziridines and gem-dichlorocyclopropanes via in situ generated dichlorocarbene

PEG400 is employed as an efficient phase transfer catalyst for the cycloaddition reaction of imines with dichlorocarbene, which is generated in situ from chloroform and sodium hydroxide, to give gem-dichloroaziridines in moderate to excellent yields at ambient temperature. This protocol is also extended to the synthesis of cyclopropanes from a variety of alkenes. In this study, PEG400 behaves as a phase transfer reagent thanks to its ability to coordinate with alkali metal cations. Notably, the one-pot synthesis of gem-dichloroaziridines from benzaldehyde and aromatic amines has also been successfully performed. The in situ generated acid, derived from CO2 and H2O, can also be effectively applied to promote the amide synthesis via the gem-dichloroaziridine pathway. The application of the gem-dichlorocyclopropane as a platform chemical is also briefly demonstrated, to afford the 2-phenylacrylaldehyde derivative via a ring-opening reaction.

Propylene oxide as a dehydrating agent: potassium carbonate-catalyzed carboxylative cyclization of propylene glycol with CO2 in a polyethylene glycol/CO2 biphasic system

The synthesis of propylene carbonate (PC) from 1,2-propylene glycol (PG) and CO2 was smoothly performed in a PEG800 (polyethylene glycol)/CO2 biphasic system with K2CO3 as a catalyst and propylene oxide (PO) as a dehydrating agent. In the reaction of PG with CO2, PO presumably removes the water produced, and simultaneously generates more PG, both of which shift the thermodynamic control process and thus accelerate the PC synthesis. The PC yield directly from PG and CO2 reached 78% under relatively mild reaction conditions (4 MPa, 120 °C, 10 h). Notably, no additional by-product was detected in this process, resulting in economic benefits and the ease of workup procedure.

Polyethylene glycol radical-initiated aerobic propargylic oxidation in dense carbon dioxide

Abstract The selective aerobic oxidation of alkynes to corresponding α,β-acetylenic ketones was achieved in polyethylene glycol/dense CO 2 /O 2 biphasic system without any catalyst or additive. The effects of reaction parameters, e.g. temperature, CO 2 pressure, PEG molecular weight and loading on the reaction were carefully examined. Moreover, various substrates worked well in the presence of PEG1000 under 5 MPa of CO 2 and 2 MPa of O 2 at 100 °C for 12 to 24 h and acceptable yield and selectivity could be obtained in most cases. Preliminary mechanistic investigations were also discussed.

Homogeneous Catalysis Promoted by Carbon Dioxide

Supercritical CO2 (scCO2), which is recognized as CO2 heated and pressurized beyond its critical point (T c = 31.06 °C, P c = 7.38 MPa), is considered to be a suitable candidate for the replacement of conventional organic solvents, owing to its unique physical properties, such as abundantly available and cheap, nontoxic and environmentally benign, nonflammable and nonreactive even under oxidative conditions, high gaseous miscibility, effective mass transfer, easily tunable properties with subtle variation of pressure or temperature, weakening of the solvent interactions around the reacting species, and easy separation and recycling. In particular, smart use of dense CO2 would pronouncedly enhance the selectivity of target products and improve catalytic efficiency and lifetime of the catalyst. In this chapter, utilization of scCO2 as innovative and environmentally friendly reaction medium for chemical syntheses and especially for the metal-catalyzed reactions, including hydrogenation (asymmetric hydrogenation, hydrogenation, and hydroboration of styrene), carbonylation (hydroformylation, hydroesterification), C–C forming reaction (Diels–Alder cycloaddition, coupling reaction, olefin metathesis, Aldol reaction, miscellaneous reactions), oxidation reaction (oxidation reaction of alcohols, aldehydes, hydrocarbon and olefins, Baeyer–Villiger reaction), and polymerization (free radical polymerization, cationic polymerization, metal-catalyzed polymerization) is summarized.