Structure dioxiranes

The ozonolysis of ethylene in the liquid phase (without a solvent) was shown to take place by the Criegee mechanism.This reaction has been used to study the structure of the intermediate 16 or 17. The compound dioxirane (21) was identified in the reaetion mixture at low temperatures and is probably in equilibrium with the biradical 17 (R = H). Dioxirane has been produced in solution but it oxidatively cleaves dialky] ethers (such as Et—O—Et) via a chain radical process, so the choice of solvent is important. 

Various computational models agree that the reaction occurs by a concerted mechanism.91 Comparison between epoxidation by peroxy acids and dioxiranes suggests that they have similar transition structures. 

A number of chiral ketones have been developed that are capable of enantiose-lective epoxidation via dioxirane intermediates.104 Scheme 12.13 shows the structures of some chiral ketones that have been used as catalysts for enantioselective epoxidation. The BINAP-derived ketone shown in Entry 1, as well as its halogenated derivatives, have shown good enantioselectivity toward di- and trisubstituted alkenes. 

An important finding is that all peroxo compounds with d° configuration of the TM center exhibit essentially the same epoxidation mechanism [51, 61, 67-72] which is also valid for organic peroxo compounds such as dioxiranes and peracids [73-79], The calculations revealed that direct nucleophilic attack of the olefin at an electrophilic peroxo oxygen center (via a TS of spiro structure) is preferred because of significantly lower activation barriers compared to the multi-step insertion mechanism [51, 61-67]. A recent computational study of epoxidation by Mo peroxo complexes showed that the metallacycle intermediate of the insertion mechanism leads to an aldehyde instead of an epoxide product [62],... 

The similarity of olefin epoxidation by TM peroxo and hydroperoxo complexes with epoxidation by dioxirane derivatives R2CO2 and percar-boxylic acids RCO(OOH) was confirmed by computational studies [73-79]. This similarity holds in particular for the spiro-type transition structure. 

Hoffmann and coworkers have presented structural data on a series of dioxiranes substituted with electronegative substituents including difluorodioxirane. Difluorodioxirane is of particular interest because it has been described as potentially the most stable of the series of dioxiranes we have described. Cremer and coworkers have predicted on the basis of ab initio calculations that in contrast to the parent dioxirane (CO2H2), difluorodioxirane (CO2F2) should possess considerable thermodynamic stability. Difluorodioxirane is the only known dioxirane that is stable in the gas phase at room temperature. 

The transition structures for the epoxidation of ethylene and propylene with peroxyformic acid and of ethylene with dioxirane and dimethyldioxirane calculated at the B3LYP, QCISD and CCSD levels are symmetrical with a spiro orientation of the electrophilic oxygen, whereas the MP2 calculations favor unsymmetrical transition structures. The geometries of the transition structures calculated using the B3LYP functional are close to those found at QCISD, CCSD, CCSD(T) levels as well as those found at the CASSCF(10,9) and CASSCF(10,10) levels for the transition structure of the epoxidation of ethylene. 

In summary, transition structures with dioxirane and dimethyldioxirane are unsymmet-rical at the MP2/6-31G level, but are symmetrical at the QCISD/6-31G and B3LYP/6-31G levels. The transition states for oxidation of ethylene by carbonyl oxides do not suffer from the same difficulties as those for dioxirane and peroxyforaiic acid. Even at the MP2/6-31G level, they are symmetrical (Figure 17). The barriers at the MP2 and MP4 levels are similar and solvent has relatively little effect. The calculated barriers agree well with experiment . In a similar fashion, the oxidation of ethylene by peroxyformic acid has been studied at the MP2/6-31G, MP4/6-31G, QCISD/6-31G and CCSD(T)/6-31G and B3LYP levels of theory. The MP2/6-31G level of theory calculations lead to an unsymmetrical transition structure for peracid epoxidation that, as noted above, is an artifact of the method. However, QCISD/6-31G and B3LYP/6-31G calculations both result in symmetrical transition structures with essentially equal C—O bonds. 

A typical closed-shell transition structure for DMDO epoxidation is exemplified by the epoxidation of E- and Z-2-butene. Baumstark and Vasquez have reported experimental studies that demonstrate the greater reactivity of Z-alkenes in the DMDO epoxidation of E/Z-pairs of alkenes . As anticipated, approach of the dioxirane ring to the Z-double bond in the less hindered manner, away from the methyl groups of DMDO,... 

This controversy concerning the use of MP2 calculations for epoxidation reactions was rather short-lived since more efficient density functional calculations (DFT) came into general use and generally produced symmetrical spiro transition structures. Consequently, the use of MP2 theory for 0—0 bond cleavage reactions has been largely discontinued. Most have assumed that the question of symmetrical versus asymmetrical approach of the peracid had been resolved. Recall that this same problem with MP2 calculations existed for the early calculations for dioxirane epoxidation (see Section V.D). 

Dimesityldioxirane, the first isolated crystalline derivative, offered the opportnnity to conduct a structure determination by means of X-ray analysis . Clearly, the structural data of the X-ray analysis (Figure 2) match well those obtained by microwave spectroscopy (Figure 1)". One striking feature of the dioxirane structure is the long oxygen-oxygen... 

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. 

With this brief preamble on the more important current theoretical results for the general structural and electronic characteristics of dioxiranes, we shall now examine the computed transition structures of the oxygen transfer in epoxidations, heteroatom oxidations and CFI insertions. Since each reaction type exhibits its individual mechanistic features, these oxyfunctionalizations shall be presented separately. 

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

As already mentioned, the dioxirane epoxidation of an alkene is a stereoselective process, which proceeds with complete retention of the original substrate configuration. The dioxirane epoxidation of chiral alkenes leads to diastereomeric epoxides, for which the diastereoselectivity depends on the alkene and on the dioxirane structure. A comparative study on the diastereoselectivity for the electrophihc epoxidants DMD versus mCPBA has revealed that DMD exhibits consistently a higher diastereoselectivity than mCPBA however, the difference is usually small. An exception is 3-hydroxycyclohexene, which displays a high cis selectivity for mCPBA, but is unselective for DMD . ... 

The results of the dioxirane epoxidation of some 3-alkyl-substituted cyclohexenes and of 2-menthene indicate that the diastereoselectivity control is subject to the steric interactions of the dioxirane with the substituents of the substrate, while the size of the dioxirane substituents has only a minimal effect . In the favored transition structure, the alkyl groups of the dioxirane cannot interact effectively with the substituents at the stereogenic center of the chiral alkene . ... 

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