Enrico Angeletti

Silica gel functionalized with amino groups as a new catalyst for Knoevenagel condensation under heterogeneous catalysis conditions

Abstract Knoevenagel condensation has been carried out under heterogeneous catalysis conditions using amino groups immobilized on silica gel: the results show that the catalytic support is recoverable without regeneration, and that the matrix probably participates to the catalysis mechanism.

Amino groups immobilized on silica gel: an efficient and reusable heterogeneous catalyst for the Knoevenagel condensation

Silica gel functionalized with amino groups is a useful insoluble catalyst for the Knoevenagel condensation: the reaction can be carried out under continuous-flow conditions and good yields are obtained when aromatic aldehydes, cyclohexanone, and acetophenone react with ethyl acetoacetate, ethyl cyanoacetate, and malononitrile. Lower yields are obtained in the case of ethyl benzoylacetate and acetylacetone; this fact and the easy dehydration of the aldol intermediate strongly suggest the participation of the residual free silanol groups of the matrix in the catalysis mechanism.

GAS-LIQUID PHASE-TRANSFER CATALYSIS - WITTIG-HORNER REACTION IN HETEROGENEOUS CONDITIONS

The Wittig–Horner synthesis of alkenes has been carried out under gas–liquid phase-transfer catalysis (g.l.–p.t.c.) conditions. With this technique the carbonyl compound and the phosphonate flow, under pressure, through solid potassium carbonate contained in a thermostatted column where the conditions (temperature and pressure) are such as to ensure that the reactants are gaseous. The alkene so produced was cooled and collected at the column outlet. Reaction yields were higher when the solid potassium carbonate was coated with poly(ethylene glycol)–Carbowax 6000. The function of the poly(ethylene glycol) is discussed. Aromatic aldehydes react successfully with triethyl phosphonoacetate and diethyl (cyanomethyl)phosphonate; ketones react only with diethyl (cyanomethyl)phosphonate. Reaction conversions (based on the purity of the recovered alkene) are always high, while yields (measured on the basis of the actual amount of pure alkene recovered, with respect to the reacting carbonyl compound) are lower.

Continuous conversion of alcohols into alkyl halides by gas–liquid phase-transfer catalysis (G.L.–P.T.C.)

A solution of a primary alcohol (propan-1-ol, butan-1-ol, and pentan-1-ol) in an aqueous halogen acid (hydrochloric or hydrobromic acid) when passed in the gaseous state over a solid porous bed (silica gel) supporting a catalyst, affords the corresponding alkyl halide. The reagent mixture is introduced continuously into a column maintained at 170 °C and the product is collected at the outlet. Since the process is catalytic, the bed is not consumed during the reaction. The reaction by-products depend on the type of catalyst used: Lewis acids (ZnCl2, AlCl3) lead to large quantities of alkenes and isomeric halides, while under typical g.l.–p.t.c. conditions, with a phosphonium salt as the catalyst, only the corresponding dialkyl ether, in addition to the primary alkyl halide, is obtained. The yield of the ether can be reduced progessively to zero by increasing the contact time between the reaction mixture and the catalytic bed.

Effect of lithium complexation by 12-crown-4 on the regioselectivity of the attack of gem-dichloroallyl-lithium on some carbonyl compounds

Abstract Substituted benzaldehydes and acetophenone preferentially attack the γ position of gem-dichloro allyl anion, but show a significantly increased α-selectivity if 12-crown-4 is present. This behaviour is discussed on the basis of theoretical computations.

Gas–liquid phase-transfer synthesis of phenyl ethers and sulphides with carbonate as base and Carbowax as catalyst

When a mixture of a phenol (or thiol) and an alkyl halide is passed, in the gaseous state, through a solid bed of potassium carbonate (or sodium hydrogencarbonate) and catalytic amounts of Carbowax 6000, contained in a glass column at 170 °C, the corresponding ethers (or sulphides) may be collected at the outlet. The Carbowax acts in a similar manner to the crown ethers used in solid–liquid phase-transfer catalysis. The potassium carbonate–Carbowax combination allows the generation of anions up to a pKa of ca. 12. The catalysis mechanism is discussed and the synthesis of several ethers and thioethers is reported, some of which are obtained only with difficulty by normal liquid–liquid phase-transfer catalysis.

Synthetic and mechanistic aspects of gas–liquid phase-transfer catalysis: carboxylate esters

The synthesis of esters from solid alkyl- and aryl-carboxylates and gaseous alkyl halides (RX), that is gas–liquid phase-transfer catalysis (g.l.p.t.c.) is promoted by the classical phase-transfer catalysts, quaternary phosphonium salts. The reaction is run under continuous-flow conditions in a column and the product is collected at the outlet. An investigation of the mechanism of the reaction shows the great importance of the liquid film of melted catalyst in promoting anion exchange between the underlying solid carboxylate salt and the gaseous alkyl halide which diffuses and reacts. This technique does not require the use of organic or aqueous solvents, leads to high conversion into esters for low carboxylate : RX molar ratios, and transforms large quantities of reagents with small volumes of solid bed.

Alkylation reactions of ethyl malonate, ethyl acetoacetate, and acetylacetone by gas–liquid phase-transfer catalysis (G.L.–P.T.C.)

Malonyl, acetoacetyl, and acetylacetonyl alkylation reactions have been carried out in the gaseous state under gas–liquid phase-transfer catalysis (g.l.–p.t.c.) conditions using potassium carbonate or sodium hydrogen carbonate as the base. No alkali metal was used to generate the reactive anion. No solvents are used in the process, and the reaction mixture requires no stirring. The phase-transfer catalysts which promote the reaction: phosphonium salts, crown ethers, and poly(ethylene glycol) are also the medium in which the reaction occurs; they direct C- or O-alkylation, as a function of the methylene compound and the alkylating agent used. The malonic ester alkylation using poly(ethylene glycol) gives rise to improved selectivity for C-monoalkylation compared with the other catalysts.

Catalytic interconversion of alkyl halides by gas-liquid phase-transfer catalysis

High halogen exchange conversions are achieved when a gaseous mixture of alkyl halides (chlorides, bromides, iodides) is passed over a solid bed consisting of porous inorganic supports bearing a phasetransfer catalyst under gas-liquid phase-transfer catalysis (g.l.-p.t.c.) conditions. The process is catalytic since the bed undergoes no changes once it reaches operating conditions. For example, a methylene dichloride and bromoethane mixture is converted into all the halogen-exchange products, and their statistical distribution at equilibrium depends on the original ratio of the halogens in the organic reagents. Catalytic activity is high: 200 ml of such a mixture can be converted in 1 h by passage through 200 g of alumina coated with 10% tetrabutylphosphonium bromide. The catalytic process is promoted by the halide anions present as Q+X– in the liquid phase constituted by the molten catalyst and as Na+X– in the solid inorganic support; the halide anions partition themselves between the liquid and solid phases as a function of their respective affinities. This catalysis depends on the diffusion, partition, and adsorption of the alkyl halides between the gaseous, liquid, and solid phases, as well as on their intrinsic nucleophilic reactivity. Mechanistic and industrial applicability are discussed.

Crystal growth of alkali-metal halides during gas-liquid phase-transfer catalysis

The alkali-metal halide crystals produced during an organic synthesis carried out under gas-liquid phase-transfer catalysis (g.l.-p.t.c.) conditions grow in an unusual liquid medium by consuming the nucleophile salt; their habit depends on the catalyst used and on the reaction considered. The particular crystal habit, showing a cavity on one face only of the cube or on one corner only of the octahedron, illustrates some aspects of the crystal growth and gives information on the g.l.-p.t.c. mechanism.