Carbene rearrangements : intramolecular interaction of a triple bond with a carbene center Public Deposited

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  • The tosylhydrazones of 2-heptanone, 4,4-dimethy1-2- heptanone, 6-heptyn-2-one and 4,4-dimethy1-6-heptyn-2- one were synthesized and decomposed under a variety of reaction conditions: dry lithium and sodium salt pyrolyses, sodium methoxide thermolyses in diglyme and photolyses of the lithium salt in tetrahydrofuran. The saturated analogues 2-heptanone tosylhydrazone and its 4,4-dimethyl isomer afforded the alkenes arising from β-hydrogen insertion. It was determined that product distribution in the dry salt pyrolyses of 2-heptanone tosylhydrazone was different for the lithium and the sodium salts. However, the product distribution of the dry sodium salt was very similar to product distribution obtained on thermolysis in diglyme with sodium methoxide. This difference was explained by a reaction of lithium bromide (present as an impurity in all the lithium salts) with the intermediate diazo compound to afford an organolithium intermediate that behaves in a somewhat different fashion than the free carbene. The unsaturated analogues were found to produce a cyclic product in addition to the expected acyclic alkenes arising from β-hydrogen insertion. By comparison of the acyclic alkene distribution obtained in the saturated analogues with those in the unsaturated analogues, it was concluded that at least some cyclization was occurring via addition of the diazo moiety to the triple bond. It was determined that the organolithium intermediate resulting from lithium bromide catalyzed decomposition of the diazo compound was incapable of cyclization. Alkene distribution upon pyrolyses of the lithium salt of 4,4-dimethy1-6-heptyn-2-one tosylhydrazone at different temperatures provides evidence for cyclization to be occurring by a carbenic pathway in competition with the diazo pathway. Dry salt pyrolyses of 3,5,5-trimethy1-2-cyclohexenone tosylhydrazone showed that 1-carbena-3,5,5-trimethy1-2- cyclohexene does not open up to the acyclic carbena 2- carbena-4,4-dimethy1-6-hexyne. Direct photolysis of 1-diazo-5-hexyn-2-one afforded only the ketene arising from a Wolff rearrangement which was allowed to react with methanol to afford methyl 5- hexynoate before isolation. Sensitized photolysis afforded only the product arising from hydrogen abstraction by the triplet carbene: 5-hexyn-2-one. No cyclization products were detected under these reaction conditions.
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