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| - Effect of Chain Length on Radical to Carbanion Cyclo-Coupling ofBromoaryl Alkyl-Linked Oxazolines: 1,3-Areneotropic Migration ofOxazolines
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| - 2-Halophenylalkyl-2-oxazolines with alkyl chain spacers of two to six C atoms (n = 0−4) were preparedand their SRN1-type reactions with several base systems examined. The best conditions to promote cyclo-coupling to the corresponding benzocycloalkane derivatives involved use of LDA in THF. The precursorswith 3-C-atom and 4-C-atom spacers gave good yields of 2-(1‘-phenylindan-1‘-yl)-2-oxazolines and 2-(1-phenyl-1,2,3,4-tetrahydronaphthalen-1-yl)-2-oxazoline, respectively. The major products from the precursorwith a 5-C-atom spacer were derivatives of benzocycloheptane in which the oxazoline group had undergonea novel areneotropic migration from the end of the spacer to the benzo ring. The product from reactionof the corresponding 2-C-atom precursor was a 9-oxazolinophenanthrene derivative. EPR spectroscopyshowed the intermediates of the LDA-promoted reactions to be radical anions of the productbenzocycloalkanes. This supported an SRN1-type chain mechanism involving initial production of arylradicals connected to azaenolate ions via the spacer groups. Intramolecular radical to carbanion couplingthen generated ring-closed benzocycloalkane radical anions that transferred an electron to more precursor.Diastereoselective radical to carbanion cyclo-coupling reactions were carried out with 2-bromophenylpropylprecursors containing chiral 2-oxazolines. The diastereoselectivity achievable was modest, but the productdiastereoisomeric Indane derivatives were easily separable by chromatography.
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