Effect of Chain Length on Radical to Carbanion Cyclo-Coupling of Bromoaryl Alkyl-Linked Oxazolines: 1,3-Areneotropic Migration of Oxazolines

2-Halophenylalkyl-2-oxazolines with alkyl chain spacers of two to six C atoms (n = 0−4) were prepared
and 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 precursors
with 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 precursor
with a 5-C-atom spacer were derivatives of benzocycloheptane in which the oxazoline group had undergone
a novel areneotropic migration from the end of the spacer to the benzo ring. The product from reaction
of the corresponding 2-C-atom precursor was a 9-oxazolinophenanthrene derivative. EPR spectroscopy
showed the intermediates of the LDA-promoted reactions to be radical anions of the product
benzocycloalkanes. This supported an SRN1-type chain mechanism involving initial production of aryl
radicals connected to azaenolate ions via the spacer groups. Intramolecular radical to carbanion coupling
then 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-bromophenylpropyl
precursors containing chiral 2-oxazolines. The diastereoselectivity achievable was modest, but the product
diastereoisomeric Indane derivatives were easily separable by chromatography.