Dioxiranes rearrangement

The dioxirane 8a is much less labile than the carbonyl oxide 7a, and UV irradiation (A > 400 nm) is required to induce the rearrangement to lactone 9a. With 70 kcal/mol this is the most exothermic step in the whole reaction sequence from la to 9a. All of these reaction intermediates were generated in subsequent steps in high yields and characterized by matrix IR spectroscopy. 

Halogen substitution is expected to increase the electrophilicity of the carbenes, and in particular lh with four fluorine substituents is expected to be highly electrophilic and of unusual reactivity. All the carbenes le-g could be matrix-isolated by irradiation of their corresponding quinone diazides 2 in argon at 8-10 K.24 68,62 Again, the thermal reaction in (Vdoped matrices results in the formation of quinone oxides 7, which show the expected photochemical rearrangement to the spiro dioxiranes 8 and finally lactones 9. 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

The oxidative cyclization of allenyl alcohol 135 with a small excess of dimethyl-dioxirane leads to an intermediate diepoxide that rearranges to hydroxyfuranone 136 in 55% yield (Eq. 13.44) [52]. If the oxidative cyclization is conducted in the presence of 0.5 equiv. of toluenesulfonic acid, the major product is the furanone lacking the a-hydroxy group of 136. Hydroxyfuranones or pyranones are available from the same kinds of reactions of 5-methylhexa-3,4-dien-l-ol. 

The rearrangement 12 13 is not straightforward and two alternatives routes have been suggested. One of them involves a 1,3 transfer from silicon to oxygen to give the silyl radical 15 followed by Sui reaction on the peroxide moiety [15]. The other indicated by PM3 calculations is a dioxirane-like three-membered intermediate 16 having a pentacoordinated central silicon followed by a 1,2 silyl migration to afford radical 13 [19]. 

FIGURE 15. Predicted geometries of the transition state for rearrangement of the carbonyl oxide to the dioxirane at the DZP CCSD(T) level of theory... 

Steinfatt proposed an alternative mechanism for the formation of excited aminophth-alate, based on the concept of dioxirane-carbene mediated chemiexcitation, which is also attributed to other chemiluminescent systems ° °. After the attack of hydrogen peroxide on the diazaquinone 27 carbonyl carbon, a perhydrolysis step is postulated to result in the intramolecular dioxirane-carbene system (32) in the excited state ° ° . This species presumably rearranges to 3-aminophthalate dianion while still in the singlet-excited state (Scheme 23). Although this is a very interesting mechanistic proposal, it is based on experimental evidence obtained with indirect phthaloyl peroxide chemiluminescence and no further evidence corroborates this proposal. 

Singlet oxygen reacts with binaphthylphosphine derivatives such as 1, l -binaphthyldi-/-butylphosphine to form the corresponding binaphthyl-2-oxide phosphine oxides. This intramolecular arene epoxidation reaction proceeds with complete retention of stereochemistry. The binaphthyl-2-oxide di-/-butylphosphine oxide undergoes a slow NIH rearrangement to form the corresponding hydroxylated product. A transient phospha-dioxirane intermediate has been directly observed by low-temperature NMR.241... 

When the O-methyloxime of acetone was co-ozonided with diacetyl, the known stereoisomers of the ct-diozonide (the achiral meso and the racemic) were obtained. A similar result was obtained when the O-methyloxime of cyclohexanone was co-ozonided with diacetyl. Reduction with Ph3P afforded the expected products plus acetic anhydride, whose formation may be explained by the formation of a diradical or the corresponding dioxirane 82 that rearranged to an anhydride (Scheme 24) <1997T5463>. 

A very unusual, but efficient, rearrangement of the dioxirane 8.56A to a diether 8.56B occurs on heating, but the mechanism of the process is uncertain [186] (Figure 8.56). 

Irrespective of these interesting queries on the mechanistic details of the oxygen transfer by dioxiranes, the even more puzzling question must be raised as to why the readily accessible dioxyl diradical does not rearrange into the corresponding ester [Eq. (4)], especially if it is realized that this reaction channel is exothermic... 

Electronic reasons appear to be responsible for dioxiranes not being rearranged into ester products on heating, despite the fact that such a rearrangement constitutes a rather exothermic process [45]. To understand the thermal persistence of these most strained cyclic peroxides, we must take a closer look at the electronic configurations of these diradicals. 

Scheme 11 summarizes the various unimolecular transformations of the [ct, ct]-, [ct, 7t]-, and [%, 7i]-type dioxyl diradicals, which have been disclosed by our qualitative analysis of the orbital orientations in these electronic isomers. The answer to the original question as to the reasons for the persistence of dioxiranes rests on the appreciable electronic barrier towards rearrangement into the ester for the thermally produced [ct, ct] diradical instead, the [ct, ct] dioxyl species recloses to the dioxirane Consequently, these most highly strained cyclic peroxides can be prepared, isolated, handled, and utilized for synthetic purposes even under ambient conditions. On photochemical activation (n, CT excitation), however, the [ct, it] diradical results, which prefers P-scission into a carboxy-alkyl radical pair rather than rearranging into the ester product. For the latter process,... 

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