Carbenes and radicals from benzyloxy delta(3)-1,3,4-oxadiazolines

Abstract

<p>Recently, dialkoxycarbenes bearing the appropriate carbene substituents have been reported to fragment to radical pairs at moderate temperatures (110°C). This dissertation investigates the fragmentation of dialkoxycarbenes bearing one or two benzyloxy groups to a radical pair or a diradical. Δ 3 -1,3,4-Oxadiazolines, which are established precursors of dialkoxycarbenes, were utilized to generate the benzyloxycarbenes. Dialkoxycarbenes bearing one of two benzyloxy substituents fragment to an alkoxycarbonyl and a benzyl radical and subsequent radical coupling afford phenylacetates, Scheme I. The rate constant for this fragmentation is estimated to be 107 s-1 . Utilizing benzyloxy( p -substituted-benzyloxy)carbenes the substituent effects on the fragmentation were studied. A preference for the cleavage to the benzyl group bearing the electron-withdrawing group was observed and this suggests that the transition state for fragmentation of benzyloxycarbenes is quite polar.* After establishing the homolysis of benzyloxycarbenes to radical pairs in solution, carbene traps were used to determine if these carbenes underwent intermolecular reactions with alkenes in competition with carbene fragmentation. Alkenes bearing two cyano groups (methylidenemalononitriles) were cyclopropanated by the benzyloxycarbenes followed by rearrangement of the cyclopropanes, Scheme II. Alkenes bearing substituents that are less electron withdrawing, such as phenyl and ester groups did not react with benzyloxycarbenes. Fragmentation of these carbenes was faster than their addition of the carbene to the alkenes and products from radical coupling and radical additions to the double bonds were detected.* The chemistry of a six-membered-ring dioxycarbene, 1,3-dioxa-4-phenyl-cyclohex-2-ylidene, was also studied. Fragmentation of 1,3-dioxa-4-phenyl-cyclohex-2-ylidene affords a diradical and some unique chemistry of this diradical was observed. Along with typical diradical chemistry, such as coupling, novel products from the intermolecular homolytic aromatic substitution of the diradical onto benzene and reaction of the diradical with acetone were isolated, Scheme III. Intermolecular reactions of diradicals are rare and coupling and disproportionation dominate. The unusual reactivity of this diradical is assumed to be related to the conformation in which it is born.* *Please refer to dissertation for diagrams.</p>