Alkene oxygen transfer

Peroxy acid and alkene Transition state for oxygen transfer from the OH group of the peroxy acid to the alkene Acetic acid and epoxide ... 

The carbonyl complex [Ru(EDTAH)(CO)] has been reported to be a very good catalyst for reactions like hydroformylation of alkenes, carbonylation of ammonia and ammines as well as a very active catalyst for the water gas shift reaction. The nitrosyl [Ru(EDTA)(NO)] is an oxygen-transfer agent for the oxidation of hex-l-ene to hexan-2-one, and cyclohexane to the corresponding epoxide. 

Although the complete mechanism for each of the previously described reactions is not known, substantial details have been worked out. First, it is clear that Ti is incorporated into the framework of the silicalite structure. Too much Ti (more than about 2.5%) in the preparation steps forms nonframework TiOz crystallites, which decompose H202. Second, the rate enhancement due to methanol suggests a tight association at the Ti active site as shown in Fig. 6.8.37,38 This is supported by the fact that methanol oxidizes much more slowly than other alcohols.47 This tight coordination of methanol is proposed to increase the electrophilicity of the Ti-coordinated H202 and facilitate oxygen transfer to the alkene.31... 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

Several theoretical investigations described the interaction of a d° metal center with a peroxide in order to understand oxygen transfer from a TM center to an alkene [2, 39-42] or a sulfide [43], These studies, using semiempirical methods or the ab initio Hartree-Fock SCI method, were limited to an orbital analysis ofthe ground state of metal peroxides. 

Alkenes strained by twist or r-bond torsion, such as E-cyclooctene, exhibit much lower barriers due to relief of strain in the TS for the oxygen transfer step. While the epoxidation of symmetrically substituted alkenes normally involve a symmetrical approach to the TT-bond, the TSs for epoxidation of E-cyclooctene and E-l-methylcyclooctene exhibit highly asymmetric transition structures. The AAE = 3.3 kcalmol" for E- versus Z-cyclooctene is clearly a reflection of the relative SE of these two medium ring alkenes (16.4 vs 4.2 kcalmol ) ". The classical activation barrier (AE ) for the highly strained bicyclo[3.3.1]non-l-ene is also quite low (Table 10, Figure 26). In these twist-strain alkenes, the approach of the peracid deviates markedly from the idealized spiro approach suggesting fliat this part of the potential energy surface is quite soft. 

SCHEME 71. Proposed mechanisms for titanium(IV)-catalyzed oxygen transfer to alkenes... 

Chromium complexes in general are poor catalysts for the epoxidation of alkenes with TBHP due to the decomposition of the oxygen donor with formation of molecnlar oxygen . Epoxidation reactions with this metal are known with other oxygen transfer agents than peroxides (e.g. iodosylbenzene) and will not be discnssed here. 

Bis(trimethylsilyl)monoperoxysulfate 6 is also an excellent agent for oxygen transfer to nucleophilic substrates such as alkenes and heteroatoms. Compound 6 could oxidize alkenes such as 1-methylcyclohexene and fraw5-/3-methylstyrene, producing 2-methyl-cyclohexanone and benzyl methyl ketone, respectively, in high yield, most likely via the... 

One mechanistic matter that has caused quite a bit of general consternation about a decade ago concerns the experimental evidence for the involvement of diradical intermediates (proposed as sources for the observed radical products) in dioxirane epoxidations, which were thought to be formed through induced peroxide-bond homolysis by the alkene. Nonetheless, rigorous experimental and high-level theoretical work disposed such radical chemistry in the epoxidation of alkenic substrates. The latter computations unequivocally confirm the established concerted mechanism, in which both CO single bonds in the incipient epoxide are concurrently formed by way of an asynchronous, spiro-structured transition state for the oxygen transfer. 

The mechanistic quintessence of the most recent study by Bach s group, in which the oxygen-transfer trajectories for the DMD and TFD epoxidations of the largest set of alkene substrates ever to have been computed are compared with the theoretical results... 

FIGURE 3. Preferred transition structures for the oxygen transfer in the reaction of alkenes, sulfides and alkanes with dioxiranes... 

The breakthrough came already in 1996, one year after Curd s prediction, when Yang and coworkers reported the C2-symmetric binaphthalene-derived ketone catalyst 6, with which ee values of up to 87% were achieved. A few months later, Shi and coworkers reported the fructose-derived ketone 7, which is to date still one of the best and most widely employed chiral ketone catalysts for the asymmetric epoxidation of nonactivated alkenes. Routinely, epoxide products with ee values of over 90% may be obtained for trans- and trisubstituted alkenes. Later on, a catalytic version of this oxygen-transfer reaction was developed by increasing the pH value of the buffer. The shortcoming of such fructose-based dioxirane precursors is that they are prone to undergo oxidative decomposition, which curtails their catalytic activity. 

Electron-deficient alkenes generally require the use of some other epoxidation procedure, owing to their low reactivity under electrophilic addition conditions. Within this categoiy, o,P-unsaturated ketones tend to be the substrates of interest, and basic oxygen transfer reagents are fiequently encountered, such as HjOj/NaOH, t-BuOOH/NaOH, and NaOCl. Much activity has centered around the modification of these traditional conditions to accommodate asymmetric induction. In this regard, variously substituted Cinchona alkaloids (e.g., 39 - 41) have received a fair amount of attention over the past year. 

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