Epoxides formation from alkenes

Oxidations. Aromatic aldehydes are obtained by oxidation catalyzed with a Mn(IV) complex. A procedure for oxidation of alcohols which is organic solvent free and halide free employs 30% H Oj, Na2W04 dihydrate, and a quaternary ammonium hydrogensulfate. Under such conditions secondary alcohols are oxidized 4-5 times faster than primary alcohols. In toluene the conversion of benzylic alcohols to aldehydes or acids (depending on quantities of HjOj) is accomplished. A similar system is also effective for epoxidation of alkenes. Terminal epoxides are obtained in reactions mediated by a Mn(II) complex or Mg-Al-O-t-Bu hydrotalcite. The last catalyst is capable of inducing epoxide formation from other alkenes and enones. 

The formation of epoxides in this way complements their formation from alkenes with m-CPBA because it involves construction of a C-C bond. Epoxide formation from a-halogenated carbonyl compounds is known as the Darzens reaction. 

Evidence for epoxide formation from NO3 reactions with alkenes has been reported (Hjorth, Schindler). These results show that oxirane formation occurs via a radical adduct, in which rotation about the C-C bond can occur before elimination of NO2. Reactions of NO3 with isoprene and 2-butene at low pressure and at low O2 concentrations gave mainly oxiranes, whereas in air at atmospheric pressure, oxirane yields were negligible. However, even at atmospheric pressures, the reaction of NO3 with 2,3-dimethyl-2-butene gave an oxirane yield of around 20 %. Thus, it is apparent that, at least with some alkenes, oxirane formation may be important under tropospheric conditions. 

The formation of vicinal halohydrms from alkenes was described m Section 6 17 Halo hydrins are readily converted to epoxides on treatment with base... 

Sasidharan et al. (258) reported the formation of pinacols from alkenes catalyzed by various titanosilicates. Aluminum-free Ti-beta exhibited better activity and pinacol selectivity than TS-1, TS-2, Ti-MCM-22, or mesoporous Ti-MCM-41 (Table XLVIII). The side products included the pinacolone, alcohol, and oligomers. The epoxide was the initial product, which underwent acid-catalyzed nucleophilic ring-opening by H2Q molecules to give the pinacol (Scheme 25). 

A detailed study271 on the reaction of 2,3-dimethyl-2-butene with ozone revealed that epoxide formation strongly depends on alkene concentration and temperature. Under appropiate reaction conditions (neat alkene, 0°C) the corresponding tetra-methyloxirane was the main product. Dimethyldioxirane formed from energy-rich acetone oxide (a cleavage product of the alkene) was postulated to be responsible for epoxidation. 

Epoxidation of cyclohexene by MovO(TPP)X-Bu 02H has been reported.131 Demethylation of JV,A -dimethylaniKne by Fem(TPP)Cl-PhIO offers a model for P-450-catalyzed JV-dealkylation reactions.132 The aziridine formation from acylimino Mn(Por) and alkene may be taken for the nitrogen analogue of the oxygen transfer reaction (Scheme 32).133... 

Ketones from halohydrins. Palladium acetate complexed with a triarylphos-phine, particularly tri-o-tolylphosphine, converts halohydrins into ketones in the presence of K2C03. Yields are about 70-85% for substrates in which the halogen is secondary or tertiary, but less than 50% when the halogen is primary because of epoxide formation. The reaction is useful for conversion of alkenes to ketones in those instances in which halohydrins are formed regioselectively. 

The rate constants for oxidation of a series of cycloalkenes with ozone have been determined using a relative rate method. The effect of methyl substitution on the oxidation of cycloalkenes and formation of secondary organic aerosols has been analysed.155 Butadiene, styrene, cyclohexene, allyl acetate, methyl methacrylate, and allyl alcohol were epoxidized in a gas-phase reaction with ozone in the absence of a catalyst. With the exception of allyl alcohol, the yield of the corresponding epoxide ranged from 88 to 97%.156 Kinetic control of distereoselection in ozonolytic lactonization has been (g) reported in the reaction of prochiral alkenes.157... 

It is noticed that when Fe3+OOR is formed in aprotic diluters (under strongly basic conditions) at low temperature, degradation products are synthesized by reaction (7.4) [29], However, in hydroxide diluters oxidants dissociate heterolytically by reaction (7.3) [30, 31], which is proved by the formation of the following products epoxides from alkenes and alcohols from ROOH. 

A similar cyclisation of an alkene derived from geranyl acetate 24 by dihydroxylation and formation of the epoxide 26 leads to a substituted cyclohexane 28. The Lewis acid ZrCU is used to open the epoxide and the alkene attacks intramolecularly 27 to give eventually the ryn-compound 28 with both substituents equatorial. The alignment of the alkene and the epoxide in a chair conformation 27a is responsible for the diastereoselectivity Note the regioselectivity the less substituted end of the alkene attacks the more substituted end of the epoxide 27. These are just two examples of the very many ordinary ionic reactions that can be used to make six-membered rings. 

Another argument against the oxo-transfer mechanism in our catalytic aerobic oxidation protocol is the lack of formation of sulfoxides from sulfides, N-oxydes from amines and phosphine oxydes from phosphines. Alkenes also proved to be inert towards oxidation no epoxide formation could be detected under our reaction conditions. 

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