Aliphatic epoxidation

Prepared by epoxidation of styrene with per-oxyelhanoic acid. Reactions are similar to those of aliphatic epoxides (s e, e.g. ethylene oxide). Reacts with alcohols to give mono-ethers, e g. PhCH(0Me)CH20H. Phenols give resins. 

Those which have an essentially linear structure on to which are attached epoxide groups—the acylic aliphatic epoxide resins. 

Cyclic aliphatic epoxide resins" were first introduced in the United States. Some typical examples of commercial materials are shown in Table 26.6. 

Miscellaneous Epoxide Resins 765 Table 26.6 Some commercially available cyclic-aliphatic epoxide resins... 

Jacobsen has utilized [(salen)Co]-catalyzed kinetic resolutions of tenninal epoxides to prepare N-nosyl aziridines with high levels of enantioselectivity [72], A range of racemic aryl and aliphatic epoxides are thus converted into aziridines in a four-step process, by sequential treatment with water (0.55 equivalents), Ns-NH-BOC, TFA, Ms20, and carbonate (Scheme 4.49). Despite the apparently lengthy procedure, overall yields of the product aziridines are excellent and only one chromatographic purification is required in the entire sequence. 

Nakajima reported the use of a chiral bipyridine N,N -dioxide 18 in the desym-metrization of acyclic meso epoxides (Figure 7.3). Although the enantioselectivity was not as high as in the method developed by Fu for meso-stilbene oxide (90% ee vs. 94% ee), it was higher for the same aliphatic epoxide (74% ee vs. 50% ee) [57]. Nakajima showed that mono-N-oxide derivatives 19 and 20 were much less effective than 18 in tenns of both yield and enantioselectivity, and accordingly proposed a unique mechanism for 18 involving a hexacoordinate silicon intermediate coordinated to both N-oxides of the catalyst. 

Methyl cyclohexanols Propyl ketones Terminal aliphatic epoxides,... 

Ensign SA, FJ Smakk, JR Allen, MK Sluis (1998) New roles for COj in the microbial metabolism of aliphatic epoxides and ketones. Arch Microbiol 169 179-187. 

Allen JR, DD Clark, JG Krumn, SA Ensign (1999) A role for coenzyme M (2-mercaptoethanesulfonic acid) in a bacterial pathway of aliphatic epoxide carboxylation. Proc Natl Acad Sci USA 96 8432-8437. 

Boyd JM, A Ellsworth, SA Ensign (2006) Characterization of 2-bromoethanesulfonate as a selective inhibitor of the coenzyme M-dependent pathway and enzymes of bacterial aliphatic epoxide metabolism. J Bacteriol 188 8062-8069. 

When simple aliphatic epoxides such as propylene oxide react with hydrogen halides, the dominant product has the halide at the less-substituted primary carbon.111... 

In epoxy alkanols that carry two functional groups separated by a further C-atom, the OH group did not interfere significantly with EH-catalyzed hydrolysis. This was seen with l,2-epoxyheptan-4-ol and 3,4-epoxyhexan-l-ol [128]. The OH group, however, did enhance the bioalkylating potential of even aliphatic epoxides. 

A. R. Jones, W. C. Mackrodt, Structure-Genotoxicity Relationship for Aliphatic Epoxides , Biochem. Pharmacol. 1983, 32, 2359 - 2362 A. R. Jones, W. C. Mackrodt, Structure-Mutagenicity Relationships of Chlorinated Ethylenes A Model Based on the Stability of the Metabolically Derived Epoxides , Biochem. Pharmacol. 1982, 31, 3710 -3712. 

Coenzyme M was shown to function as the central cofactor of aliphatic epoxide carboxylation in Xanthobacter strain Py2, an aerobe from the Bacteria domain (AUen et al. 1999). The organism metabolizes short-chain aliphatic alkenes via oxidation to epoxyalkanes, followed by carboxylation to p-ketoacids. An enzyme in the pathway catalyzes the addition of coenzyme M to epoxypropane to form 2-(2-hydroxypropylthio)ethanesulfonate. This intermediate is oxidized to 2-(2-ketopropylthio)ethanesulfonate, followed by a NADPH-dependent cleavage and carboxylation of the P-ketothioether to form acetoacetate and coenzyme M. This is the only known function for coenzyme M outside the methanoarchaea. 

However, disappointingly low selectivity was observed with monosubstituted aliphatic epoxides such as 1-epoxyoctane (la, E<5) or benzyl glycidyl ether (le, <2) [51,55]. On the other hand, sterically more demanding 2,2-disubsti-tuted oxiranes (2) turned out to be much better substrates (Scheme 9, Table 2). 

Canter DA, Ziege E, Haworth S, et al Comparative mutagenicity of aliphatic epoxides in Salmonella. Mutat Res 172(2) 105-138, 1986... 

During polymer chain growth, a back-biting process can lead to cyclic carbonate formation. In general, this process is more facile for aliphatic epoxides than for alicyclic epoxides and when the growing polymer chain dissociates from the metal center (Scheme 3). 

Based on the landmark studies of Jacobsen and coworkers, who employed chiral (salen)CoX complexes for the asymmetric ring opening and kinetic resolution of aliphatic epoxides [18-20], Lu and coworkers synthesized highly isotactic copolymer from rac-propylene oxide and carbon dioxide (Scheme 5) [21]. 

Canter, D.A., Zeiger, E., Haworth, S., Lawlor, T., Mortelmans, K. Speck, W. (1986) Comparative mutagenicity of aliphatic epoxides in Salmonella. Mutat. Res., 172, 105-138 Chemical Information Services (1999) Directory of World Chemical Producers, Version 99.1.0 [CD-ROM], Dallas, TX... 

Jones, R.B. and Mackrodt, W.C. (1983) Structure-genotoxicity relationship for aliphatic epoxides. Biochem. Pharmacol., 32 (15), 2359-2362. 

Most often, the extent of completely alternating copolymer formation, expressed as 100% C02 linkages or 50% C02 content, is very high. With regards to the selectivity of the coupling reaction for copolymer versus cyclic carbonate production, two observations are consistently found, regardless of the catalyst. First, aliphatic epoxides are more prone to cyclic carbonate formation than alicyclic epoxides for example, PO affords propylene carbonate more readily than CHO provides cyclohexene carbonate. Second, in either instance, since it has been shown that the activation barriers for cyclic carbonate production are higher... 

In closely related studies, Lee and coworkers obtained even more dramatic results in attempts to alleviate the generally observed selectivity for the formation of cyclic carbonates from aliphatic epoxides and C02 [53, 54], This was achieved by adding cationic charge to the salen ligands of cobalt(III) complexes, as illustrated in Figure 8.11. 

Epoxidation and Aromatic Hydroxylation. Epoxidation is an extremely important microsomal reaction because not only can stable and environmentally persistent epoxides be formed (see aliphatic epoxidations, below), but highly reactive intermediates of aromatic hydroxylations, such as arene oxides, can also be produced. These highly reactive intermediates are known to be involved in chemical carcinogenesis as well as chemically induced cellular and tissue necrosis. 

Aliphatic Epoxidation. Many aliphatic and alicylcic compounds containing unsaturated carbon atoms are thought to be metabolized to epoxide intermediates (Figure 7.4). In the case of aldrin the product, dieldrin, is an extremely stable epoxide and represents the principle residue found in animals exposed to aldrin. Epoxide formation in the case of aflatoxin is believed to be the final step in formation of the ultimate carcinogenic species and is, therefore, an activation reaction. 

Figure 7.4 Examples of aliphatic epoxidation. denote Cl atoms.

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