Epoxidation solvent-free condition

Jacobsen subsequently reported a practical and efficient method for promoting the highly enantioselective addition of TMSN3 to meso-epoxides (Scheme 7.3) [4]. The chiral (salen)Cl-Cl catalyst 2 is available commercially and is bench-stable. Other practical advantages of the system include the mild reaction conditions, tolerance of some Lewis basic functional groups, catalyst recyclability (up to 10 times at 1 mol% with no loss in activity or enantioselectivity), and amenability to use under solvent-free conditions. Song later demonstrated that the reaction could be performed in room temperature ionic liquids, such as l-butyl-3-methylimidazo-lium salts. Extraction of the product mixture with hexane allowed catalyst recycling and product isolation without recourse to distillation (Scheme 7.4) [5]. 

The catalytic activiti of the dimer catalyst lb for HKR of the diverse and valuable racemic terminal epoxides are shown in Table 1. Due to high selectivity factor (krei values) every epoxide underwent resolution in excellent yield and high ee % employing 0.2-0.5 mol% of catalyst in solvent free condition in most of the cases. In a similar condition, catalyst la loading per [Co] basis gives < 25% ee and < 10% isolated products. 

Epoxides have been found to cleanly react with acetic anhydride to provide the diacetate under solvent-free conditions <06TL6865>. Treatment of epoxides with ammonium-12-molybdophosphate and a slight excess of acetic anhydride (1.2 equivalents) provides the corresponding diacetate in excellent yields. A number of epoxides were examined and all worked quite well. It was also found that /V-losyl aziridines participate in this reaction providing the corresponding acetoxysulfonamides. 

Functional groups were selectively introduced at the C-2 position of isophorone by epoxide ring-opening by several nucleophiles with active methylene groups. Different behavior was observed depending on the reaction conditions and the nature of the nucleophilic agents [57]. The best experimental systems involved PTC or KF-alumina under solvent-free conditions and MW irradiation (Eq. 37 and Tab. 5.15). 

For such reasons, the following section considers in more detail some of the most significant results obtained by our team on the epoxidation with TBHP of unsaturated FAMEs over mesoporous titanium-grafted silicates. In these examples, the epoxidation tests were carried out either in ethyl acetate, which could be even obtained, in principle, from renewable sources and which is relatively less harmful than other polar non-protic solvents, or under solvent-free conditions. 

Because of the relevant results obtained in ethyl acetate, Ti-MCM-41 was tested in the epoxidation of mixture of FAMEs under solvent-free conditions (Table 12.4). High conversion (particularly of castor oil FAMEs) and very high selectivity values were achieved, albeit the oxidizing agent was added in deficit with respect to the unsaturated substrates. Thus, thanks to the use of no solvent, and no acid reactants at all, and to the simple removal of the solid catalyst by filtration,... 

Table 12.4 Catalytic performances of Ti-MCM-41 in the epoxidation of vegetable FAME mixtures under solvent-free conditions.

Gold-catalyzed oxidation of styrene was firstly reported by Choudhary and coworkers for Au NPs supported on metal oxides in the presence of an excess amount of radical initiator, t-butyl hydroperoxide (TBHP), to afford styrene oxide, while benzaldehyde and benzoic acid were formed in the presence of supports without Au NPs [199]. Subsequently, Hutchings and coworkers demonstrated the selective oxidation of cyclohexene over Au/C with a catalytic amount of TBHP to yield cyclohexene oxide with a selectivity of 50% and cyclohexenone (26%) as a by-product [2]. Product selectivity was significantly changed by solvents. Cyclohexene oxide was obtained as a major product with a selectivity of 50% in 1,2,3,5-tetramethylbenzene while cyclohexenone and cyclohexenol were formed with selectivities of 35 and 25%, respectively, in toluene. A promoting effect of Bi addition to Au was also reported for the epoxidation of cyclooctene under solvent-free conditions. 

Dou X, Wang J, Du Y et al (2007) Guanidinium salt functionalized PEG an effective and recyclable homo-geneous catalyst for the synthesis of cyclic carbonates from C02 and epoxides under solvent-free conditions. Synlett 18(19) 3058—3062... 

One recent publication from the group of Abu-Omar reports on a condensation reaction involving glycerol and furfural, both renewables, to produce dioxolanes, formally a dehydration reaction. Here, a cationic oxorhenium(V) oxazoline species is used as the catalyst for the formation of various 1,3-dioxalanes from furfural with diols or epoxides under mild conditions (Scheme 21). Especially interesting is the reaction of furfural with glycerol to obtain a 70 30 mixture of the corresponding 1,3-dioxolane and 1,3-dioxane in solvent-free conditions [125]. 

In 1997, Tokunaga et al. reported the hy-drolytie kinetie resolution of raeemie terminal epoxides using a Co(III)-Salen eatalyst (164). This remarkably general proeess uses only water as the nucleophile and provides the synthetically useful chiral epoxides and diols in highly enantioenriched form. The catalyst can be recycled and the reactions conducted under solvent-free conditions. 

White, D. E., Jacobsen, E. N. New oligomeric catalyst for the hydrolytic kinetic resolution of terminal epoxides under solvent-free conditions. 

The hydrolysis of epoxides is a well-known reaction which can be exploited for various synthetically useful outcomes. Chiral nonracemic epoxides can be prepared from their racemates through the salen-mediated hydrolytic kinetic resolution (HKR). Racemic epichlorohydrin 53 was resolved in the presence of catalyst 52 and a slight excess of water under solvent-free conditions. The catalyst counterion exerts a significant effect on the course of the reaction, presumably due to competitive addition onto the epoxide, an effect which is evident in apparent reaction rates, but not enantioselectivities. Less nucleophilic counterions, such as tosylate, lead to more rapid resolution and lower catalyst loading requirements <04JA1360>. 

The search for improved catalytic systems for epoxide opening reactions has yielded a number of methods to improve this reaction. The use of 10 mol% of Cu(BF )2 has been found to catalyze the ring-opening of epoxides with amines under solvent free conditions <05TL2675>. Both aliphatic and aromatic amines provide excellent yields of the P-amino alcohols. Scandium triflate has also been found to catalyze the ring-opening of epoxides with both aromatic and aliphatic amines in the absence of solvent <05TL9029>. 

Because the epoxide hydrolysis reaction is exothermic, isothermal conditions were critical to obtain good data in this parameter study. A solvent (1,2-dichloro-benzene) was used to dilute the reaction mixture in order to help maintain a constant reaction temperature. This proved especially useful when exploring higher catalyst concentrations that accelerate the rate of reaction. It will be demonstrated in Section 2.3.3.3 that the use of solvent did not alter the kinetic parameters of the HKR reaction, and thus the estimated kinetic parameters remained valid under solvent-free conditions. Water retained sufficient solubility in the mixed system to maintain homogeneity at the desired rate of addition. A slight excess of water was used to ensure complete conversion of (S)-epichlorohydrin and to compensate for any competitive hydrolysis of the (R)-enantiomer. 

Alkylation. Indoles are alkylated by epoxides at C-3 under solvent-free conditions in the presence of RuCls xH20. ... 

P-Azido alcohols. Opening of epoxides with the title reagent at room temperature is catalyzed by BU4NF under solvent-free conditions. 

A simple method has been developed for synthesis of thiiranes from epoxides via a one-pot reaction of epoxides with diethyl phosphite in the presence of ammonium acetate or ammonium hydrogen carbonate, sulfur, and acidic alumina under solvent-free conditions using microwave irradiation (Scheme 8.68) [178]. 

The PS-TBD catalyst has been shown to be effective for epoxide ring opening reactions with several nucleophiles such as thiols under solvent free conditions [37,78] (Scheme 6.21). In this case, the reusabihty of the catalyst was also established without a significant loss of reactivity and selectivity. As a related work, the utility of mesoporous silica-supported TBD catalysts was demonstrated in the reaction of propylene oxide with carbon dioxide to prepare the corresponding carbonate derivative under the ultrasonic activation [79]. 

FringuelU, F., Pizzo, F., Vittoriani, C. and Vaccaro, L. (2006) Polystyrene-supported 1,5,7-triazabicyclo[4.4.0]dec-5-ene as an efficient and reusable catalyst for the thiolysis of 1,2-epoxides under solvent-free conditions. European Journal of Organic Chemistry, 1231-1236. 

Since the introduction of the first peptide organocatalyst in the 1980s, a considerable number of new peptide frameworks have been developed that are able to effectively catalyse several important transformations including alcohol esterifications, 1,4-conjugate additions, aldol reactions, Strecker synthesis, asymmetric cyanohydrin synthesis and alkene epoxidation are discussed. A few successful examples of solid-supported peptides and reactions in ball milling under solvent-free conditions have been demonstrated. These methods combine the advantages of being economically and environmentally friendly processes. 

A few examples of base promoted domino ring opening lactonization of epoxides with active methylene compounds (malonate, cyanoacetate, acetoacetates) are known earlier in the literature [42]. Pizzo s group has recently described domino nucleophilic ring opening of epoxides by active methylenes, catalyzed by polymer-supported bases, under solvent-free conditions (SolFCs) (Scheme 4.24)... 

With respect to sulfiir-based nucleophiles, a rapid and efficient method for opening various epoxides with thiophenol derivatives was reported using solvent-free conditions and (C4Hi2N2)2[BiCl6]Cl H20 as a catalyst (13T11174). Also, a one-pot tandem synthesis of P-trimethylsilyloxy thioes-ters was reported by reacting epoxides with thioacids under solvent-free silica gel conditions (13SC1759). 

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