Cycloaddition to Epoxide

For the homogeneously catalyzed cycloaddition in SCCO2, the catalysts and the starting materials, epoxides and scCOj, form a homogeneous mixture, but the resultant carbonate is not soluble in scCOj. Hence, while running the reaction, the product is automatically separated from the reaction mixture. Thus, the development of an scC02-soluble catalyst is necessary. In this case, the SCCO2-soluble catalysts remain in the CO2 phase and can be recycled [72]. 

Using the system of tetradentate Schiff-base aluminum complexes coupled with a quaternary ammonium or phosphonium salt as the catalyst, ethylene carbonate can be synthesized rapidly in SCCO2 [73]. Under the employed conditions, the rapid diffusion and high miscibility of ethylene oxide and SCCO2 are favorable to the high rate of reaction. 

Recently, Song et al. [74] provided a simple way for performing the homogeneous catalyzed cycloaddition in SCCO2 with the direct and spontaneous separation of the carbonate. The solubility of the catalysts could be enhanced by the incorporation of the fluorinated chain in the polymer, and phosphonium salts acted as a homogeneous C02-soluble catalyst and promoted the cycloaddition very well. Through the release of CO2, the catalyst could be recovered and reused and kept satisfactory catalytic activity. 

In scCOj/l-octyl-S-methylimidazolium tetrafluoroborate ([CginimJBF ) reaction media, the cycloaddition of COj with PO could be promoted effectively with nearly 100% yield at 14MPa and 100°C for propylene carbonate (PC) production at a reaction time shorter than 5 min [75]. The scCO /IL reaction medium could also be apphed to the synthesis of various carbonates in satisfactory yields, in which [C8mim][BF4] acted as the catalyst 

In the absence of a catalyst, styrene carbonate was formed from COj and styrene oxide in the SCCO2/DMF system, although substrates were rather limited and PO did not react under similar conditions [76]. The yield of styrene carbonate could be enhanced just by pressure manipulation of the scCOj/DMF system. 

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Aral and co-workers [432] reported also a direct route for the preparation of cyclic styrene carbonate from styrene, which avoids the preliminary synthesis and isolation of styrene oxide. A catalyst system consisting of Au/Si02, zinc bromide and tetrabutylammonium bromide (Bu4NBr) was applied to the one-pot synthesis of styrene carbonate from styrene, organic peroxide and CO2. Au/Si02 is active for the epoxidation of styrene, and zinc bromide and Bu4NBr cooperatively catalyse the subsequent CO2 cycloaddition to epoxide. 

Simple functionalities have also been used to perform catalytic transformations. Triazine networks have been used to perform cycloadditions to epoxides either using just the basic triazine units or via the incorporation of pyridines. ... 

A plausible reaction mechanism for CO2 cycloaddition to epoxide catalyzed by IL is illustrated in Scheme 2. The reaction is initiated by the ring opening of epoxide that is made by a nucleophilic attack of the anion of IL to the less hindered carbon atom of the epoxide ring then, an oxy anion species 2 is formed. The carbon atom of CO2 interacts with the oxy... 

Scheme 2. A plausible mechanism for CO2 cycloaddition to epoxide catalyzed by IL.

Scheme 3. A proposed mechanism involving an anion species of [Y-C02]" for CO2 cycloaddition to epoxide [11].

Scheme 8. CO2 cycloaddition to epoxide with hydroxyl-functionalized IL. (H-0-[LJ = [HEMIm], [HETBJ. Y = Br, Cl.)...

Currently cyclic carbonates are extensively synthesized by CO2 cycloaddition to epoxides, which has been commercialized. Although the above synthetic approach is quite atom-efficient, such a reaction usually requires the initial synthesis of epoxides an additional step sometimes involves the use of expensive or toxic reagents and requires chemical separations... 

Therefore, the direct synthesis of cyclic carbonates from olefins instead of epoxides, a so-called one-pot "oxidative carboxylation" of olefins, would be appealing. The oxidative carboxylation synthesis from olefins can be roughly viewed as the coupling of two sequential processes of epoxidation of olefins and CO2 cycloaddition to epoxides formed (Scheme 18). The reaction uses easily available and low-priced chemicals of olefins as substrates and, moreover, preliminary synthesis and separation of epoxides would be avoided. So, the oxidative carboxylation would be a simpler and cheaper carbonate synthesis process with industrial potential from environmental and economic points of view. Although the three-component couplings have been known at least since 1%2 [66], up to date, only a few works have been made on these reactions in contrast to extensive studies on the addition reactions of CO2 to epoxides in ILs as catalyst/or solvent. 

As mentioned in section 2.1., there can be some synergistic effect by the combination of IL and zinc halide for the CO2 cycloaddition to epoxide. Park et al. also investigated the influence of the co-presence of ZnBr2 on the copolymerization of CO2 and PGE with [BMIm]Cl [124]. ZnBr2 alone showed a low activity and a low Mn, but enhanced the activity of [BMImjCl. By the co-presence of ZnBr2, Mn of the copolymer produced was also increased however, its f(C02) was smaller than that of the copolymer produced with the IL alone. For the enhancement of the activity, they proposed the cooperative activation of epoxide by Zn and halide anion of the IL, which was very similar to that proposed for the CO2 cycloaddition reaction (Scheme 6 in section 2.1.2.). 

A related series of cycloadducts has been produced employing a 1,3-dipolar cycloaddition approach <2000EJ03363, 2001TL465>. Thus, the 1,3-dipole formed from bis-epoxides on heating at high temperatures in a sealed tube undergoes cycloaddition to benzonorbornadienes to produce cycloadducts in a highly stereoselective exo,aco-fashion (Equation 101). 

Irradiation of carbene complexes in CO atmosphere generates the ketene 305 and its [2+2] cycloaddition to alkene gives the cyclobutanone 306 [93], Total synthesis of (+)-cerulenin (310) has been carried out by the formation of cyclobutanone 309 by cycloaddition of 307 to the double bond of 308 as the key reaction without attacking the triple bond. Then cyclobutanone 309 was converted to (+)-cerulenin (310) via regioselective Bayer-Villiger reaction of 309, and side-chain elongation using n-methallylnickel bromide, epoxidation and hydrolysis [94],... 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

It has been shown that allylic azides can be trapped, using either phenylacetylene cycloaddition to the azide, or alkene epoxidation, and that [3,3]-sigmatropic equilibration of the possible allylic azides is generally faster than the trapping reactions 42 Nucleoside-derived azide (46) has been shown to undergo reversible [3,3]-sigmatropic... 

Recently, PEG-functionalized basic ILs have been proved to be highly efficient and stable catalysts for the cycloaddition reaction of C02 to epoxides without utilization of any organic solvent or additive under modest reaction conditions as depicted in Scheme 5.5 [15]. 

Tn previous work it has been shown that a competition exists during - ozonation of olefins between ozonolysis and epoxide formation (I). As steric hindrance increases around the double bond, the yield of epoxide or subsequent rearrangement products increases. This is illustrated with both old (1) and new examples in Table I for purely aliphatic olefins and in Table II for aryl substituted ethylenes. It was suggested that the initial attack of ozone on an olefinic double bond involves w (pi) complex formation for which there were two fates (a) entrance into 1,3-dipolar cycloaddition (to a 1,2,3-trioxolane adduct), resulting in ozonolysis products (b) conversion to a o- (sigma) complex followed by loss of molecular oxygen and epoxide formation (Scheme 1). As the bulk... 

Although there are doubts about the existence of Brpnsted acid sites on TS-1 and related materials, there is strong evidence that Lewis acid sites are present on the surface of dehydrated TS-1. The significant activity of TS-1 and of Ti-MCM-41 in the cycloaddition of CO2 to epoxides to give cyclic carbonates (140), a reaction typically catalyzed by Lewis acids such as AICI3, SbFs, etc., lends strong support to the inference of the existence of Lewis acid sites on their surfaces. 

A well known reaction for the synthesis of oxazolidinones, cycloaddition of isocyanates to epoxides, was applied to resin linked substrates for the synthesis of libraries of isoxazolidinones 198 <03JCC789>. 

Many arene oxides are in dynamic equilibrium with their oxepin forms. The parent molecules, benzene oxide la and oxepin lb, are related as valence tautomers that interconvert by an allowed disrotatory electrocyclic reaction. Structural identification of la and lb was based initially upon spectroscopic evidence and chemical transformation to stable products of known structure. Thus the arene-oxide structure was inferred from its typical dienoid (4 + 27t cycloaddition) and epoxide (ring-opening, aromatization) reactions, while the oxepin structure was deduced by catalytic hydrogenation of the triene oxepin to form oxepane. 

D-Glucose ([52], Fig. 9) has served as an intriguing educt for preparation (31) of the Corey lactone equivalent [59] (32). The iodo compound [53] was readily available from glucose in four steps. Reductive fragmentation, induced by zinc in ethanol, gave the unsaturated aldehyde [54]. Reaction with N-methylhydroxylamine was followed by a spontaneous nitrone cycloaddition to provide the oxazolidine [55]. Catalytic reduction of the N-O bond was accompanied by the unexpected loss of tosylate and aziridine formation. Olefin formation from [56] via the N-oxide and chain extension gave acid [57]. lodolactonization and tri-n-butyltin hydride reduction in the standard fashion led to lactone [58]. After saponification of the benzoates, stereoselective epoxide formation gave epoxy lactone [59]. 

More recently an alternative biosynthetic pathway has been proposed by Townsend and Basak (Scheme 22) [83]. This alternative model involves the iterative syn-oxidative cyclization of a (Z,Z,Z)-triene intermediate 44 in contrast to the (E,E,E)-triene intermediate 42 of the epoxide cascade mechanism. The putative, alkoxy-linked, non-heme metal-oxo species 45 undergoes a [2-1-2] cycloaddition to yield the metalloxetane intermediate 46. Reductive elimination then results in an overall syn-addition of the two oxygen atoms 47. Oxidation of the metal intermediate 47 to 48 and two further [2 + 2] cycloaddition-reductive elimination-metal oxidation cycles would result, after final hydroxylation, in monensin A 41. 

The required chain extension of 12 was accomplished via deprotonation with NaH and condensation with aldehyde 7 to afford the Diels-Alder precursor 13 in 50% yield. Thermolysis of triene 13 and lactam 3 in xylene at 170 C for four days resulted in the desired cycloaddition to 14. Chromatographic purification permitted isolation of pure 14 in addition to a small amount of an exo isomer (>4 1 ratio). Acid treatment induced cleavage of both the silyl ether and acetonide. Reprotection of the diol and selective epoxidation of the A olefin produced 15 in 64% yield from 12. Epoxide 12 was then transformed to the isomeric allylic alcohol 16 by conversion of the alcohol to the bromide followed by reductive elimination. Protecting-group manipulation and subsequent oxidation the gave aldehyde 17, which was homologated and hydrolyzed to give seco acid 18 in 32% overall yield from 16. 

The synthesis of cyclic carbonates in RTILs, via cycloaddition of cathodically activated carbon dioxide to epoxide, has been reported by Deng et al. [139]. Ionic liquids, saturated with CO by bubbling at normal pressure and containing the epoxidic substrate, were electrolyzed in an undivided cell (Cu as cathode. Mg or Al as sacrificial anode). The electrolyses were carried out under potentiostatic conditions at a potential negative enough to the selective reduction of CO to CO (E=-2.4 V vs. 

Scheme 16.17 Synthesis of cyclic carbonates in RTILs, via cycloaddition of cathodiceilly activated carbon dioxide to epoxide (Reprinted from Ref. [139] with kind permission of The Royal Society of Chemistry)...

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