Substrate biphasic conditions

The biphasic method, like the triphasic method, allows the epoxidation of a broad range of substrates accommodating both aliphatic and aromatic substituents at either end of the enone moiety. However, unlike the triphasic method, the biphasic conditions allow reaction of hydroxide-sensitive systems. 

Using a similar approach, Chechik and Crooks (73), modified the PAMAM dendrimer-encapsulated palladium nanoparticles with perfluoropolyether tails utilizing non-covalent ion-pair interactions. The catalytic hydrogenation of six substrates under biphasic conditions (toluene/ perfluoro-2-butyltetrahydrofuran FC-75) was investigated. Allyl alcohol, methyl acrylate, vinyl isopropenyl ether, and... 

Nickel-containing polyfluorometalated [Ni(H20)H2FeNaWi7055] was capable of catalytic activation of hydrogen peroxide for allylic alcohol oxidation in biphasic conditions (catalyst substrate ratio 1 1000). Homoallylic alcohols are significantly less reactive than allylic homologues and cis > trans reactivity is found also in this case. 

Their utility has been demonstrated in various Friedel-Crafts reactions,44-47 halogen addition,48 49 electrophilic nitration of aromatics,50 and various hydrogenation processes51-53 including the Ru-catalyzed hydrogenation of C02 to /V,/V-dipropyl formamide in supercritical C02 under biphasic conditions 54 The use of in situ IR spectroscopy allowed Horvath and coworkers to demonstrate that the same substrate-catalyst and product-catalyst complexes are produced in Friedel-Crafts acetylation of benzene in ionic liquids as in CH2C12 55... 

Behr et al. [83] also tested the linear diols 1,3-propanediol and 1,4-butanediol under the aqueous biphasic conditions described above. Yields of up to 60% could be obtained for 1,3-propanediol, but conversion dropped considerably as a yield of 31% was recorded for 1,4-butanediol. The TOFs for ethylene glycol (339 h ) and 1,4-butanediol (134 h-1) obtained under identical conditions clearly reflect the drop in activity upon elongation of the carbon chain. Palkovits et al. observed a different trend when they tested various diols under monophasic conditions (Figs. 6 and 7). Reaction conditions were optimized for 1,3-propanediol (butadiene/substrate ratio of 2 1, 353 K, 0.01 mol% Pd, TOMPP/Pd 5 1) giving the mono-telomer in 61% yield and 81% selectivity at 76% conversion with a TOF of around 21,000 h. ... 

The use of isolated fatty acids as substrates for epoxidations has already been reported in 1990 with the epoxidation of methyl oleate and methyl linoleate by MT0/H202 in ferf-butanol. After respectively 24 and 2 h, good yields of either the corresponding diol (methyl oleate) or the monoepoxide (methyl linoleate) are obtained (respectively 92% and 80%) [48]. Under biphasic conditions, the MTO-catalyzed epoxidation of methyl linoleate yields a mixture of mono- and diepoxide (approximately 1 1) at complete conversion after 6 h [79]. Finally, a conjugated methyl linoleate is treated with the MT0/H202 system in biphasic conditions, but here, poor results are obtained after 24 h at room temperature, yielding 26% of the 11,12-monoepoxide and 20% of the 9,10-epoxide [80]. 

Biphasic conditions also circumvent the salt solubility problem but classic phase transfer conditions using aqueous sodium hydroxide suffer from competing hydrolysis of the phosphorus(III) esters. However, Kem and co-workers found that butyl esters (their substrate was dibutyl phosphonate) are sufficiently stable to provide Michaelis-Becker products in high yields dialkylphosphine oxides were also suitable substrates.67 Salvatore and co-workers have recently developed an efficient biphasic solid-liquid procedure utilising the cesium cation effect Protocol 7. 

Phase transfer conditions can be used for substrates with only limited aqueous solubility. Phase transfer agents also facilitate the preparation of aldehydes under biphasic conditions. 

A similar triarylphosphane was introduced by Leitner for the performance of transition metal catalyzed reactions in supercritical CO2 [24]. Recently, this phosphane was used for palladium catalyzed substitutions of allylic substrates in perfluorinated solvents under fluorous biphase conditions [25], Therefore, the reaction of cinnamyl methylcarbonate (36) with several nucleophiles (Nu-... 

The first water-soluble system specifically designed to combine both functions of a ligand and a surfactant in one molecule and applied in transition-metal-cata-lyzed conversions of highly water-insoluble substrates in micellar systems is the zwitterionic tenside trisulfoalkylated tris(2-pyridyl)phosphine, 2 (n = 0, 3, 5, 7, 9, 11) [4, 14, 72, 74]. Turnover frequencies (TOF) up to 340 h-1 were achieved in the micellar hydroformylation of 1-tetradecene to pentadecanals, according to Eq. (1), using Rh/2 catalysts at 125 °C, by fine tuning of the hydrophilic/lipophilic properties of the tenside system 2 [4, 14]. In sharp contrast, Rh/TPPTS catalysts gave only traces of pentadecanals under the same biphasic conditions. 

Highly soluble cationic ruthenium-dusters complexes have also been used for hydrogenation of aromatic substrates [31]. Compared with water these ionic liquids dissolve higher concentration of hydrogen gas which leads to increased reaction rate in biphasic conditions. 

The importance of the partitioning of substrates, catalysts, and intermediates between the liquid (mainly organic) and compressed gas (mainly C02) phase for selectivity and conversion in catalytic processes has been noted in several studies [65-67]. Even substrates with melting points higher than the actual reaction temperature can be converted if the melting point depression under C02 pressure is large enough to render them liquid under the biphasic conditions [68],... 

Catalytic enantio- and diastereoselective nitroaldol reactions were explored by using designed guanidine-thiourea brfunctional organocatalysts like 15 (Figure 4.4) under mUd and operationally simple biphasic conditions. These catalytic asymmetric reactions have a broad substrate generality with respect to the variety of aldehydes and nitroalkanes [43]. On the basis of studies of structure and catalytic activity relationships, a plausible guanidine-thiourea cooperative mechanism and a transition state of the catalytic reactions are proposed. 

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