Microwave spectroscopy 1,2,4-trioxolanes

Significant advances in the chemistry of these ring systems over the past 10 years include the first unambiguous detection, and characterization by microwave spectroscopy as 1,2,3-trioxolane, of the primary ozonide from ethene and ozone (cf. Section 4.15.3.2), and the introduction of 1,3,2-dioxathiolane 2,2-dioxides as epoxide equivalents in organic synthesis (cf. Section 4.15.5.3). Advances have also been made in the synthesis and characterization of the chemistry of 1,2,3-trithiolanes and 1,2,3-trithioles. 

Table 2 Rotational constants, bond lengths and bond angles of 1,2,3-trioxolane, obtained by microwave spectroscopy.

Ab initio calculations have been carried out on several simple 1,2,4-trioxolanes in conjunction with other experimental methods, notably microwave spectroscopy, in order to provide additional insight into the conformations arising from experiments (Section 4.16.3.2). 

Trioxolanes remain the most studied ring system by microwave spectroscopy and recently, 1,2,4-trithiolane also became the subject of attention. In all cases, isotopically labelled derivatives were made which have very different rotational constants. These aid assignment of structures and also provide useful tools for looking at the mechanism of the ozonolysis reaction. Rotational constants for the parent compounds and their calculated dipole moments are given in Table 3. 

TABLE 5. Comparison of the structural parameters of gaseous 1,2,4-trioxolane determined by microwave spectroscopy and electron diffraction ... 

JPC1545). With all the 1,2,4-trioxolanes studied by microwave spectroscopy, no effects from pseudorotation or internal rotation were observed. 

Relatively few such heterocyclic systems have been studied by microwave spectroscopy some data are included in Table 7. In 1,3-dioxolane the twist conformation (see Table 41) is more stable than the envelope conformation, and pseudorotation occurs. In 1,2,4-trioxolane the equilibrium conformation is the twist form, in which the peroxide bond straddles the plane of the other three atoms, and there is a barrier of 6.3 kj mol 1 opposing pseudorotation <1974PM H (6)53>. Preference for the twist conformation is also observed in the crystal structures of some 1,2,4-trioxolane derivatives (see Table 41, Section 2.4.4.4) . 

In the case of 1,2,4-trioxolane (2), where both the normal species and various isotopic modifications (3,3-D2, D4,1803) were also studied (76JPC1238), all the IR frequencies could be assigned (Table 5) as a result of a normal coordinate analysis. Starting from the molecular geometry determined by microwave spectroscopy (cf. Section 4.33.2.2.2), this analysis led to an excellent agreement between observed and calculated IR frequencies, thus confirming the O—O half-chair conformation of the 1,2,4-trioxolane ring. For a series of substituted... 

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