Oxiranes from alkenes

Usually, various quantities of alkane are found during the hydrogenolyses of oxacycloalkanes. This is especially so for the oxiranes on platinum metals in the presence of hydrogen. These alkanes presumably form from alkenes, which are formed from deoxygenation (Scheme 4.67). 

The alkene (5 mmol) and the chiral trifluoroketone, e.g. (+)-3-trifluoroacetylcamphor (4-6 mmol) in CH2C12 (50 ml) are added to aqueous phosphate buffer (pH 7.5, 50 ml), TBA-HS04 (15 mg, 0.04 mmol) and EDTA-Na2 (15 mg). The mixture is stirred at room temperature and aqueous KHSOs (0.6 M, 20 ml) is added slowly over 30-60 min while the pH 7.5 is maintained by the addition of aqueous KOH (1 M). Oxone (15 g) is added over 24 h at room temperature and pH 7.5. The reaction is monitored by GLC and more Oxone is added, if necessary. When complete, the reaction mixture is filtered and the aqueous phase is separated and extracted with CH2C12 (2 x 25 ml) and Et20 (2 x 20 ml). The combined organic solutions are dried (MgS04) and evaporated to yield the oxirane [e.g. 82% l( ,2/ -oxirane from /ran.v-PhCH=CHMe (ee 13%)]. 

The NMR spectroscopy of oxiranes has attracted considerable interest because the oxiranes can be easily prepared from alkenes under mild conditions. Thus, it is worthwhile investigating so-called epoxidation-induced shifts (E1S) by comparing the observed 13C chemical shifts of an oxirane to the shifts in the corresponding alkene440 441. However, a differentiation of diastereomers using these shifts is not always reliable. The situation is more favorable in confor-... 

Reactions with sulfur ylides proceed differently. The products are oxacyclo-propanes (oxiranes) —not alkenes. The addition step proceeds as with the phosphorus ylides, but the negatively charged oxygen of the dipolar adduct then displaces the sulfonium group as a neutral sulfide. This is an intramolecular Sn2 reaction similar to the formation of oxacyclopropanes from vicinal chloroalcohols (Section 15-11C) ... 

Epoxides (oxiranes) are three-membered cyclic ethers. The simplest and commercially most important example is ethylene oxide, manufactured from ethylene, air, and a silver catalyst. In the laboratory, epoxides are most commonly prepared from alkenes and organic peroxy acids. 

Oxiranes may be transformed into 1,2-dioxolanes, 1,3-dioxolanes and 1,3-oxathiolanes. In Section 4.30.3.1.2 the intramolecular addition of a hydroperoxide to an activated double bond was discussed. Activation may also be achieved using an oxirane. Thus the hydroperoxide (113), obtained from alkene (111) by peroxidation, undergoes cyclization under acidic conditions to generate the 1,2-dioxolane as shown in Scheme 32 (78JOC4048). However, the mercury(II) ion catalyzed cyclization of (111) discussed in Section 4.30.3.1.2 appears to be a more versatile and efficient process. 

The most convenient route to lluorinatcd epoxides is the direct epoxidation of alkenes. Since first reports of a general method using alkaline hydrogen peroxide at low temperature,- several alternative synthetic approache.s have been developed. Molecular oxygen under free-radical conditions has been used to oxidize alkenes such as tetrafluoroethene or hexafluoro-propene however, internal epoxides are formed most conveniently using hypochlorites. The products, e. g. oxirane 14 from alkene 13, are usually obtained in high... 

The Schwartz s reagent see Schwartz s Reagent) allows ring formation from substrates possessing both oxirane and alkene functionalities (equation 48). Based on in situ NMR analyses, it has been shown that the mechanism proceeds by... 

Allylic alcohols may be derived from alkenes by metallation to give the allylpotassium species, followed by treatment with fluorodimethoxyborane. Oxidation of the resultant boronic ester with hydrogen peroxide gives the allylic alcohol (Scheme 15). ° - ° Some allylic rearrangement may be observed for example, metallation of a-pinene with potassium r-butoxide in petroleum ether solution and subsequent boration and oxidation gave myrtenol (42%) and /rons-pinocarveol (1%) (equation 24), while treatment of the allylpotassium with oxirane gave the alkylated prc ucts in a ratio of ca. 2 1. ° ... 

Secondly, from studies of the yields of the conjugate oxiran and alkene found during the oxidation of 3-ethylpentane, see Sect. 3.2.2(a)(ii), Cullis and co-workers [86] have shown that if the values of the Arrhenius parameters for reaction (15)... 

Oxiranes are usually prepared from halohydrins. A general method of obtaining chloroalcohols from olefins is by the addition of hypochlorous acid formed in situ form -chloramides or fert-butyl hypochlorite. Bromoalcohols are produced from alkenes with Ai-bromosuccinimide or iV-bromoacetamide, while iodoalcohols are prepared with iodine in the presence of oxidants (iodic acid, oxygen, and nitrite). 

A method for the production of chiral oxirans from simple unfunctionalized alkenes employs the molybdenum peroxo-complex (51), which has a pentagonal-bipyramidal structure. The oxidations of propene, but-l-ene, and but-2-ene yield e.e. s of around 30% [all of (R) configuration] in PhN02 at 20°C. 

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

Starting materials, like 141, which are accessible from alkenes by azido-selenenylation, afforded only in a few cases vinyl azides exclusively as shown by the example 141 142 (Scheme 5.19). In most cases, after oxidation of the vicinal phenylseleno azides and elimination reaction on the intermediate selenoxide, mixtures of allyl and vinyl azides were obtained. Ring opening of trialkylsilyl-substituted epoxides was utilized several times for stereoselective synthesis of vinyl azides. Thus, treatment of the frani -contigured oxiranes 143 with azidotrimethylsilane and boron trifluoride etherate yielded cis-conflgured products 145. This result was explained by the intermediate 144 which should undergo anti-elimination. On the other hand, when subjected to sodium azide in dimethylformamide, epoxides 146 were transformed into vinyl azides 148 via... 

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