Three-Membered Ring Systems

BY T. J. MASON

1 Oxirans

The interest of Eastern European chemists in oxirans has been evident for many years, and from such sources have come a number of reviews on the production of these important raw materials. While concentrating mainly on ethene and propene oxides, one review deals more specifically with the types of heterogeneous catalysts employed in the processes. The latest edition of the Kirk-Othmer Encyclopedia contains an entry on epoxidation.

An extensive article on the importance of chalcone oxides in ftavonoid chemistry contains 346 references. Other specialised reviews have appeared on the chemistry of juvenile hormones (up to 1974), epoxy-sulphones, and epoxy-derivatives of dicyclopentadiene.

The important safety aspects of the use of oxiran in high-pressure bench-scale experiments have been considered.

Preparation. — Catalytic Oxidation of Alkenes to Oxirans, using Oxygen or Oxygen-containing Gases. The mechanism for the conversion of the phenol (1) into the epoxy-quinol (4) by molecular oxygen in ButOK-ButOH has been investigated (Scheme 1). The first step is the formation of hydroperoxide (2), which is converted into hydroperoxide (3) via a π-complex intermediate; the yield of (4) increases with temperature.

Photo-oxygenation of cyclohepta-3,5-dienone that is sensitized with tetraphenylporphyrin leads initially to the 3,6-endoperoxide; this, on reduction with di-imide, gives (5), from which (6) and (7) were obtained, via photolysis at 300 nm in CDCl3 and subsequent deoxygenation with PPh3 in CH2Cl2, at 0°C, respectively. Photo-oxidation of 3-methylfuran in CH2Cl2 at 0°C, with methylene blue as the sensitizer, led directly to the epoxy-ketones (8) and (9), together with some diepoxide.

Dye-sensitized oxygenation of vinylsilanes yields β-silylated allylic alcohols via epoxysilane intermediates and provides a new synthesis of allylic alcohols from saturated ketones (Scheme 2). The vinylsilanes (10; R = H) and (10; R = Me), prepared from the corresponding cyclohexanone benzenesulphonylhydrazone, react with singlet O2 to give the intermediates (11) (not isolated); these open regiospecificall y to (12), from which (13) may be obtained on reaction with Bun4N+F-. Calculations indicate the possible role of silicon in directing the opening of the oxiran ring.

The kinetics of oxidation of ethene at 490 — 620 K, using Ag catalysts supported on pumice, have been reported. 15 Total oxidation to CO2 and H2O accompanies formation of oxiran, apparently with similar activation energies. Ethene has also been oxidized, using complex catalysts derived from silver carboxylates and heterocyclic compounds, 78.5% selectivity being obtained using the catalyst from silver acetate and 2-methyl-2-oxazoline. Epichlorohydrin may, however, be obtained directly from allyl chloride in the liquid phase, using simply silver nitrate in dimethyl phthalate.

The epoxy-alcohol (15) (76%) was obtained from (14), using the cyclopentadienyl-vanadium catalyst [(C5H5)V(CO)4] in solution at 50°C. Under similar reaction conditions, a catalyst prepared by exchange of V4+ and Cu2+ with an Na–X–zeolite resin furnished a mixture of (15) and (16) in 46% and 40% yields respectively.

The 1 : 1 complex formed between ZnCl2 and tetramethyl-2-tetrazene reacts with cyclo-octene and oxygen to give solely the oxiran, whereas with styrene the reaction goes further, the intermediate oxiran ring being opened regiospecifically to produce the amino-alcohol PhCH(OH)CH2NMe2.

Oxidation of Alkenes to Oxirans by Peroxy-acids. The mechanism of the reaction of m-chloroperbenzoic acid with double bonds has been investigated through a study of the epoxidation of a series of cycloal kenes (of ring sizes 5, 6, 7, 8, and 12) and substituted cyclohexenes. The second-order rate constants were determined in CHCl3 at 0 — 30°C, and the data support a 1,3-dipolar cycloaddition reaction.

Factors that affect the stereoselectivity of epoxidations with peroxy-acids have been probed. A remarkable change in selectivity has been observed when a β-carbonyl group is introduced into the lanost-9(11)-ene skeleton. While (17; R = H) yields only α-epoxide on treatment with 3-ClC6H4 CO3H, the presence of the carbonyl group in (17; R2 = O) produces a ratio of β- : α-epoxides of 3:1 in CHCl3, which falls to 1:1 in more polar solvents. Undoubtedly there must be considerable steric influence in this reaction, but it is thought that the carbonyl group acts largely through a polar effect, reducing the rate of epoxidation of (17; R = H) by a factor of 300. Mono-epoxidation of the substituted cyclohexa-1,4-dienes (18; R1 = Me, R2 = CO2Me or CH2OH) and (18; R1 = H, R2 = CO2 Me) with 3-ClC6H4CO3H demonstrates a predominant steric control of epoxidation rather than, as previously suggested, a cis-directing eff ect by allylic methoxycarbonyl groups. 24 The diene (18; R1 = Me, R2 = CO2Me) gave a 55:45 mixture of epoxides, with the predominant isomer having the oxiran ring cis to the methoxycarbonyl group. The product distribution reflects the slightly smaller bulk of this group compared with methyl. The steric argument is reinforced with the observation that monoepoxidation of (18; R1 = H, R2 = CO2 Me) yields a 35 : 65 ratio of products in favour of oxiran trans to the more bulky CO2 Me group. High stereoselectivity was obtained from the epoxidation of (19) to a 9:1 ratio of epoxides in which (20) predominated.

The presence of base in the oxidizing media dramatically affects the distribution of epoxide products. In the oxidation of cholesterol acetate with the new reagent pentafluoroperbenzoic acid, a 65:35 mixture of α- : β-epoxides was obtained (almost quantitative yield) in CH2Cl2 at 25°C; this changed to a quantitative yield of solely the α-epoxide when Na2CO3 was added to the reaction mixture. The sesquiterpene lactone (21; R1R2 = a bond) gave the cyclized product (22) on oxidation with 3-ClC6H4CO3H in CH2Cl2, but when the same reaction was run under biphasic conditions, using CH2Cl2 and aqueous NaHCO3, it gave the epoxide (21; R1R2 = O) that is the precursor to (22).

A biphasic system has also been used for the generation of the new reagent O-ethylperoxycarbonic acid (23). Ethyl chloroformate in CH2Cl2 that is in contact with an aqueous phase containing 30% H2O2 generates (23) at the interface. The reaction has the advantages that the conditions may be maintained by buffering within the pH ranges 6.8 — 4.5 (Na2HPO4) and 9.5 — 8.8 (Na3PO4) and also yields high-purity products. Cyclo-octene oxide...