1.3.4- Oxadiazolines, fragmentations

The synthesis and properties of heat-resistant polyazomethines containing 2,5-disubstituted oxadiazole fragments, being insulators convertible into semiconductors by doping with iodine, have been described. The radical copolymerization of alkenes with the fluorescent co-monomer 2-/-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole has resulted in useful macromolecular scintillators. Anionic polymerization of 2-phenyl-l,3,4-oxadiazolin-5-one has produced a nylon-type product <1996CHEC-II(4)268>. 

Thermolysis of the oxadiazoline (123) gives rise to the corresponding dialkoxy-carbene, which can be trapped by reaction with f-butanol to form orthoesters. The formation of a regioisomeric mixture of esters was explained by fragmentation of the carbene to radicals (124) which recombine at either end of the allyl system. 

As part of a mechanistic and synthetic study of nucleophihc carbenes the spirocyclic 4(5/l)-oxazolone 18 has been obtained from benzoyl isocyanate (Scheme 6.1) Thermal extrusion of nitrogen from the 1,3,4-oxadiazoline 14 produced the carbonyl ylide 15 that fragmented via loss of acetone to the aminooxycarbene 16. Spectroscopic data [gas chromatography-mass spectrometry (GC-MS), infrared (IR), proton and C-13 nuclear magnetic resonance ( H and NMR)] of the crude thermolysate was consistent with 18. The formation of 18 was rationalized to result from nucleophihc addition of 16 to benzoyl isocyanate followed by cyclization of the dipolar intermediate 17. Thermolysis of 19 and 21 under similar reaction conditions afforded 20 and 22 respectively, also identified spectroscopically as the major products in the thermolysate. 

As another important mechanistic type, fragmentation into at least three products that are formed more or less simultaneously has to be mentioned. Such [5 - 2 + 2 + 1] reactions were encountered in 2-pyrrolidinones (65, 67), certain pyrazolidine derivatives (68, 72, 73), l,2,4-triazolidine-3-ones (69) and l,2,4-triazolidine-3,5-diones (70), 2//-pyrrol-2-ones (71), 1,3-dithiolane 1-oxides (78), 1,2,3,4-thiatriazoles (74), 1,2,4-oxadiazolines (75), and in several rings containing sulfur in an oxidized state (76-79,82). Most of these molecules have an exocyclic double bond. 

Sometimes it is difficult to classify the fragmentation unequivocally, in particular when short-lived, reactive intermediates that readily decompose into smaller fragments may or may not be encountered depending on reaction conditions. As examples, the oxadiazolines (20,75) can be mentioned (see Sections V.A and VI). Many reactions classified as [5 - 2 + 2 + 1] fragmentations (Section VI) are probably initiated as a [5 —> 3 + 2] 1,3-dipolar cycloreversion. 

In recent years there has been a growing interest in the use of carbonyl ylides as 1,3-dipoles for total synthesis.127-130 Their dipolar cycloaddition to alkenic, alkynic and hetero multiple bonded dipolaro-philes has been well documented.6 Methods for the generation of carbonyl ylides include the thermal and photochemical opening of oxiranes,131 the thermal fragmentation of certain heterocyclic structures such as A3-l,3,4-oxadiazolines (141) or l,3-dioxolan-4-ones132-134 (142) and the reaction of carbenes or car-benoids with carbonyl derivatives.133-138 Formation of a carbonyl ylide by attack of a rhodium carbenoid... 

Evidence in support of a stepwise thermal fragmentation of oxadiazolines in benzene at 110 °C has implicated carbonyl ylide intermediates and oxiranes as unstable precursors of the elimination product, ketene acetals, rather than concerted fragmentation to N2, acetone and carbene.26... 

Similar to internal alkenes, carboxidation of the rubber involves intermediate formation of an oxadiazoline cycle (Figure 7.7). Decomposition of the cycle without cleavage of the C=C bond (route 1) is accompanied by the formation of a ketone and does not lead to change in the molecular weight. Decomposition with cleavage (route 2) leads to fragmentation of the macromolecule with the formation of two smaller fragments a linear aldehyde R2-CH2-CHO and a carbene R -CH2-CH , which further isomerizes into the terminal alkene Rj-CH=CH2. 

Thermolysis of 2-acetoxy-2-methoxy-5,5-dimethyl-A3-l,3,4-oxadiazoline affords acetoxy(methoxy) carbene.60 The thermal rearrangement of acetoxy(methoxy) car-bene to methyl pyruvate was studied by DFT at the B3PW91/6-31G(d,p) level. The conformation of the carbene was considered, as were competing fragmentations to radical pairs. The authors concluded that the reaction is a concerted 1,2-migration rather than a fragmentation-recombination process. 

Boyer, J.H. and Ellis, P.S. (1979) Fragmentation-rearrangement of A3-oxadiazolin-5- and 3-ones. Journal of the Chemical Society, Perkin Transactions , (2), 483-487. 

Fragmentation of 2-phenylimino-A3-l,3,4-oxadiazoline 49 includes break of five a-bonds in the substrate and formation of five new n-bonds.93... 

Oxadiazolines (Scheme 16) show a more complex fragmentation pattern, although as expected the cleavage involves the weak N—O bond (patterns a and c in 93) more frequently than the C—O bond (pattern b). However, the 4,5-dihydro derivative (94) loses hydrogen concurrently with an ethyl group in accord with the sequences (94) -> (95) and (94) -> (96) (780MS14). 

---