Search

The new phase-separation and immobilization technique known as FBS (fluorous biphase system) is becoming more and more popular among researchers in both industry and academia. The immiscibility of perfluorocarbons with most organic solvents and water is of great advantage, in that it allows the easy recovery and recycling of the catalysts and reagents. The major results so far obtained in this field are discussed.

Fluorous soluble bipyridines bearing two perfluoroalkylated side chains in the 6,6′- or 4,4′-positions have been prepared in good yields via etherification of 6,6′-bis(chloromethyl)-2,2′-bipyridine or C -alkylation of 6,6′-dimethyl-2,2′-bipyridine. The new ligands L exhibit amphiphilic behaviour with respect to certain fluorous-organic biphasic systems. Nevertheless, their ruthenium complexes (RuL n )X generated in situ are efficient catalysts for the epoxidation of trans -stilbene in a fluorous triphasic system CH 2 Cl 2 /H 2 O/C 8 F 18 in the presence of NaIO 4 . The fluorous phase, where (RuL n )X is trapped, can be used up to four times without major loss of catalytic activity.

Four tetraarylporphyrins bearing one n-C8F17 chain on each meso-aryl group have been synthesized. The Mn(III)-complexes of these new compounds (Mn-1 - Mn-4) were used as catalysts in alkene epoxidations carried out under aqueous-organic biphasic conditions. High epoxide yields were obtained with catalysts in which, along with perfluoroalkyl chains, bulky substituents were present at appropriate positions. The expected general enhancement of stability and catalytic activity due to the electron-withdrawing effect of n-C8F17 substituents was not observed. However, Mn-4 was found to be an exceptionally active catalyst for NaOCl promoted epoxidation of poorly reactive linear α-alkenes. © 1997 Elsevier Science Ltd.

The synthesis of Mn(III)-complexes of new chiral porphyrins 1a and 1b prepared by condensation of enantiomerically pure [2.2]- p -cyclophane-4-carbaldehyde and pyrrole are described. These compounds were used as catalysts in epoxidation reactions of prochiral alkenes, carried out in the presence of aqueous NaOCl at pH = 10.0 as oxygen donor and small amounts of 4-tert-butylpyridine as axial ligand, in CH 2 Cl 2 H 2 O two-phase conditions at 0°C. Results indicate a satisfactory catalytic efficiency (up to 700 overall turnovers), with enantiomeric excesses in the range 22–31%.

the reactivities of five MnIII(Cl) - porphinoids were compared in the catalytic alkene epoxidations in CH3CN solution with peracetic acid as primary oxidant. Porphyrins 1 and 2 bearing Cl and NO2 substituents showed the best efficiency while tetraazaporphyrin 5 was found to be an interesting catalyst for terminal alkene epoxidations.

The first example of dimeric Mn(III)-tetraarylporphyrins able to catalyze olefin epoxidation is reported. In the presence of one equivalent of lipophilic bidentate ligand 3 the efficiency of some of these catalysts is among the highest reported to now in 30% H2O2-promoted epoxidation (up to 1500 overall catalytic cycles). Slightly better results were obtained replacing 3 with two equivalents of N-hexylimidazole 10.

Two new Mn(III)-tetraarylporphyrins Mn-1 and Mn-2 bearing one amido-bonded n-C7F15 chain on each meso-aryl group have been synthesized. The presence of four perfluoroalkyl tails strongly influences the solubility of these compounds in common organic solvents, but it is not sufficient to impart solubility in fluorocarbons. The catalytic activity of the new complexes was tested in alkene epoxidations employing aqueous NaOCl as oxygen donor. Results show that Mn-2 is more active than Mn(III)-5,10,15,20-tetrakis-(2,6-dichlorophenyl)porphyrin, one of the most efficient porphyrinic catalysts for hydrocarbon oxygenation.

Four tetraarylporphyrins bearing one n-C8F17 chain on each meso-aryl group have been synthesized. The Mn(III)-complexes of these new compounds (Mn-1 - Mn-4) were used as catalysts in alkene epoxidations carried out under aqueous-organic biphasic conditions. High epoxide yields were obtained with catalysts in which, along with perfluoroalkyl chains, bulky substituents were present at appropriate positions. The expected general enhancement of stability and catalytic activity due to the electron-withdrawing effect of n-C8F17 substituents was not observed. However, Mn-4 was found to be an exceptionally active catalyst for NaOCl promoted epoxidation of poorly reactive linear α-alkenes. © 1997 Elsevier Science Ltd.

The BF3 · OEt2 or LiClO4 catalyzed hetero Diels–Alder reaction of 1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene (Danishefsky’s diene) with enantiomerically pure 4-formylazetidin-2-ones affords the corresponding cycloadducts in fair to good yields and in diastereoisomeric ratios of up to 98:2.

The new phase-separation and immobilization technique known as FBS (fluorous biphase system) is becoming more and more popular among researchers in both industry and academia. The immiscibility of perfluorocarbons with most organic solvents and water is of great advantage, in that it allows the easy recovery and recycling of the catalysts and reagents. The major results so far obtained in this field are discussed.

Fluorous soluble bipyridines bearing two perfluoroalkylated side chains in the 6,6′- or 4,4′-positions have been prepared in good yields via etherification of 6,6′-bis(chloromethyl)-2,2′-bipyridine or C -alkylation of 6,6′-dimethyl-2,2′-bipyridine. The new ligands L exhibit amphiphilic behaviour with respect to certain fluorous-organic biphasic systems. Nevertheless, their ruthenium complexes (RuL n )X generated in situ are efficient catalysts for the epoxidation of trans -stilbene in a fluorous triphasic system CH 2 Cl 2 /H 2 O/C 8 F 18 in the presence of NaIO 4 . The fluorous phase, where (RuL n )X is trapped, can be used up to four times without major loss of catalytic activity.

Sterically hindered chiral (salen)manganese complexes bearing long perfluoroalkyl substituents are synthesized and successfully employed as catalysts in the enantioselective (ee = 50–87%) epoxidation of alkenes under fluorous biphasic conditions.

MnIII–porphyrin 1, bearing an imidazole axial ligand and a carboxylic group juxtaposed on opposite sides of the porphyrin plane, is an extremely efficient catalyst for the activation of 30% H2O2 in hydrocarbon oxygenations carried out in CH2Cl2–H2O; initial rates at 1 min up to 500 turnovers min–1 at 0 °C in the epoxidation of cyclooctene are obtained.

The BF3 · OEt2 or LiClO4 catalyzed hetero Diels–Alder reaction of 1-methoxy-3-(trimethylsilyloxy)-1,3-butadiene (Danishefsky’s diene) with enantiomerically pure 4-formylazetidin-2-ones affords the corresponding cycloadducts in fair to good yields and in diastereoisomeric ratios of up to 98:2.

Two new Mn(III)-tetraarylporphyrins Mn-1 and Mn-2 bearing one amido-bonded n-C7F15 chain on each meso-aryl group have been synthesized. The presence of four perfluoroalkyl tails strongly influences the solubility of these compounds in common organic solvents, but it is not sufficient to impart solubility in fluorocarbons. The catalytic activity of the new complexes was tested in alkene epoxidations employing aqueous NaOCl as oxygen donor. Results show that Mn-2 is more active than Mn(III)-5,10,15,20-tetrakis-(2,6-dichlorophenyl)porphyrin, one of the most efficient porphyrinic catalysts for hydrocarbon oxygenation.

Mn(III)-complexes of tetraarylporphyrins 1–4 bearing a crown-ether covalently linked through a single flexible chain have been synthesized. Their basic frame is that of the robust tetra-(2,6-dichlorophenyl)porphyrin 5 and the chain is connected by ether linkage either to the ortho ( 1–2 ) or to the meta positions ( 3–4 ) of one meso -aryl group. Catalytic efficiency was tested in the epoxidation of the poorly reactive 1-dodecene at 0°C under CH 2 Cl 2 H 2 O two-phase conditions in the presence of NaOCl (pH 9.5–10.0) as oxygen donor. The results obtained led us to propose a general acid/base catalysis as an explanation for the positive effect of crown-ethers in the alkene epoxidations with this catalytic system.

The aerobic epoxidation of alkenes with 2-methylpropanal can be carried out under fluorous biphasic conditions, in the presence of the perfluorocarbon soluble cobalt complex of tetraarylporphyrin 3 as catalyst.

MnIII tetraarylporphyrins bearing either a heterocyclic nitrogen base axial ligand or a carboxylic acid group, or both, covalently bonded to the porphyrin through a single flexible chain, have been synthesized. Their basic frame is that of the robust tetrakis(2, 6-dichlorophenyl)porphyrin, and the chains are connected by ether or amido linkages to the ortho-positions of one meso-aryl group. Catalytic efficiency was tested in alkene epoxidations at O °C under aqueous CH2Cl2 two-phase conditions in the presence of NaOCl (pH 10. 5) or 30% H2O2(pH 4. 5) as oxygen donors (ODs). Compound 4 bearing an imidazole ligand showed satisfactory catalytic activity in the presence of both ODs, whereas MnIII-porphyrins 5a–d bearing a covalently bonded carboxylate group are suitable for the activation of H2O2, and proved to be very efficient in the presence of an externally added axial ligand. Catalyst 6, featuring an imidazole ligand and a carboxylate group bonded on the 2, 6-positions of the same meso-aryl group, is particularly efficient in alkene epoxidations promoted by 30% H2O2(initial rates up to 500 turnovers/min, overall turnovers up to 1200). A possible reaction mechanism for epoxidation catalysed by the bis-tailed Mniii-porphyrins is discussed.

Oxygenation of alkanes promoted by 30% H2O2 and catalysed by chemically robust manganese(III)-tetraarylporphyrins 2–4 is strongly accelerated by addition of small amounts of lipophilic carboxylic acids and lipophilic heterocyclic bases; cycloalkanes are converted into mixtures of alcohols and ketones at a rate of up to 125 turnovers min–1 in CH2Cl2–H2O solution at 0 °C.

The presence of the hydroxymethyl group allows facile functionalisation of 18-crown-6 and of [2.2.2]cryptand. A variety of lipophilic and polymer-supported macrocyclic polyethers is thus available. They are efficient and easily recyclable phase-transfer catalysts.

Anion-promoted nucleophilic substitutions carried out in aqueous–organic two-phase systems in the presence of catalytic amounts of perhydrodibenzo-18-crown-6 follow the classic mechanism of phase-transfer catalysis. The observed pseudo-first-order rate constants are linearly related to the concentration of complexed crown ether in the organic phase. The narrow reactivity range and the sequence found in the reaction between n-octyl methanesulphonate and a homogeneous series of anions in the PhCl–H2O two-phase system (N3– > I–∼ Br– > CN– > Cl– > SCN–) remain largely unaltered in anhydrous PhCl. From this point of view complexed crown ethers differ substantially from lipophilic quaternary salts and cryptates. Indeed, removal of the hydration sphere of the anions in going from two-phase to anhydrous conditions is balanced by a larger cation–anion interaction, resulting in a very small variation of anion reactivity. This indicates that, unlike cryptates, complexed crown ethers can hardly be considered as a source of ‘naked anions.’ A comparison is also reported among lipophilic crown ethers, quaternary salts, and cryptands as phase-transfer catalysts.

Concentrated aqueous NaOH and KOH solutions affect the hydration and hence the reactivity of anions (Cl−, Br−, I−, SCN−, N−3) in aliphatic nucleophilic substitutions catalyzed by lipophilic quaternary onium salts, cryptands and crown ethers under phase-transfer (PTC) condtions. In the presence of aqueous 50% NaOH or 53% KOH non-hydrated anions are transferred from the aqueous into the organic phase by quaternary salts. The anionic reactivity thus becomes identical to that found under anhydrous homogeneous conditions in non-polar organic media. Unlike quaternary salts, in the presence of cryptates water is not completely removed even at the highest KOH concentration, i.e. conditions in which aH2O ∼ 0. Residual hydration depends on the nature of the anion and is the highest for anions with localized and/or less polarizable charge, such as Cl−, Br− and N−3. As a consequence, rate constants noticeably increase in comparison with those found under conventional PTC conditions, but don't reach those of anhydrous solutions. Behavior of crown ethers is in between that of quaternary salts and cryptates, since residual hydration in the presence of 53% aqueous KOH is lower than that of cryptates, whereas anionic reactivity become practically identical to that found under anhydrous conditions. A comparison of the catalytic efficiency of lipophilic quaternary salts, cryptands and crown ethers under various reaction conditions is also included.

Preparation de la meso-tetrakis(dichloro-2,6 phenyl) porphyrine, de la meso-tetrakis(dimethyl-3,5 trichloro-2,4,6 phenyl) porphyrine, de la meso-tetrakis(dimethyl-3,5 tribromo-2,4,6 phenyl) porphyrine, de la meso-tetrakis(dichloro-3,5 phenyl) porphyrine et des complexes de manganese(III) respectifs. On etudie la stabilite chimique de ces diverses porphyrines metalliques comme catalyseur dans l'epoxydation d'olefines

Influence de la concentration de KOH aqueuse sur l'hydratation et donc la reactivite des anions (Cl − , Br − , I − , SCN − et N 3 − ) dans les substitutions nucleophiles aliphatiques catalysees par le cryptand [2•2•2] ou le dicyclohexyl-18-crown-6

In the displacement of the methanesulphonate group by a series of nucleophiles, anionic activation promoted by perhydrotribenzo[2.2.2]cryptand (2) is practically identical to that observed in the presence of the less hindered [2.2.2,C14]cryptand (1), both under phase-transfer catalysis conditions and in homogeneous chlorobenzene solution. However, the stability constants of cryptates noticeably decrease on passing from (1) to (2), especially in anhydrous solutions. The latter behaviour is best explained on the basis of the greater rigidity of cryptand (2) compared with (1), whereas the reactivity data seem to indicate that an optimal ionic separation, and therefore a maximum anionic activation, is already reached with the [2.2.2]ligand.

Crown ethers having aliphatic chains in the molecule, e.g. dicyclohexyl-18-crown-6, can be used as phase-transfer catalysts in anion promoted two-phase reactions.

Influence de la concentration de KOH aqueuse sur l'hydratation et donc la reactivite des anions (Cl − , Br − , I − , SCN − et N 3 − ) dans les substitutions nucleophiles aliphatiques catalysees par le cryptand [2•2•2] ou le dicyclohexyl-18-crown-6

Heterogeneous phase-transfer catalysts, such as phosphonium salts and crown-ethers, immobilized on a polystyrene matrix, show an increase in activity approaching that of the corresponding non-immobilized catalysts, when they are bonded with a long spacer chain to the polymeric support.

Typical phase-transfer catalysts, namely organophilic onium salts, crown ethers, and cryptands, when immobilized on a polymer matrix retain most of their catalytic activity, thus permitting catalyst recycling by simple filtration from the reaction medium.