Synthesis and thermolysis of 2,2-dioxy-5,5-dimethyl-delta(3)-1,3,4-oxadiazolines: Dioxycarbenes and their reactions

Abstract

<p>Oxadiazolines (I-III), which are novel thermal sources of dioxycarbenes, were synthesized by the exchange method using the acetoxy analogue. The thermal fragmentation of 2,2-dialkoxy oxadiazolines was established to be unidirectional. At $100\sp\circ$C, oxadiazolines (I) decomposed forming nitrogen, acetone, and alkoxymethoxycarbenes as deduced from the identification of the carbene dimers MeO(OR)C=C(OR)OMe.* On the other hand, thermolysis of the aryloxymethoxy oxadiazolines (II) was a multidirectional fragmentation process, with the aryloxymethoxycarbene formation as the major course. A competitive fragmentation afforded the methyl aryl carbonate and diazopropane. The thermal chemistry of the benzyloxymethoxy oxadiazolines (III) is similar to that of the aryloxymethoxy analogue (II). The oxadiazoline (III) loses N$\sb2$ thermally, and also produces acetone and the benzyloxymethoxycarbene via the carbonyl ylide. In addition, the carbene rearranges producing the corresponding ester via 1,2-benzyl group migration to the carbene centre. This rearrangement is a substituent dependent process which is enhanced by electron donating groups. The intermolecular reactions of aryloxymethoxycarbene (IV) with alkynes resulted in a regioselective generation of a 3,3-dioxyvinylcarbene (V or VI). This carbene can either undergo an intramolecular nucleophilic aromatic substitution and/or further reaction with the alkyne. The reaction pathway of (V or VI) is strongly dependent on the nature of the substituent $\rm(R\sp\prime).$* Finally, the aryloxymethoxycarbenes were trapped inter- and intramolecularly via OH insertion reactions. ftn*Please refer to dissertation for diagrams.</p>