Spiro[3.3]alkyl and spiro[3.3]alkylmethyl radicals

Abstract

Spiro[3.3]heptan-3-yl radicals were generated by bromine abstraction from 3-bromospiro-[3.3]heptane and observed by e.s.r. spectroscopy. Both spiro[3.3]heptan-3-yl and spiro[3.3]heptan-2-yl radicals were observed on hydrogen abstraction from spiro[3.3]heptane. The measured proportions of the two radicals indicated that hydrogen abstraction occurs at roughly the same rate from both C(2) and C(3). The e.s.r. parameters of both radicals show that they are structurally very similar to cyclobutyl radicals. The spiro[3.3]heptan-2-yl radical rearranges to the cyclobutenylpropyl radical at temperatures above ca. 290 K. The photochemical reactions of spiro[3.3]heptane with bromotrichloromethane and bromine were investigated.

Spiro[3.3]heptylmethyl radicals were generated from the corresponding bromo compound; their rearrangement to 1-allylcyclobutylmethyl radicals was monitored by e.s.r. spectroscopy. Indirect evidence suggested a further rearrangement of 1-allylcyclobutylmethyl radicals to 4-methylenehept-6-enyl radicals. From the activation energy for β-scission in spiro[3.3]heptylmethyl radicals it was possible to show that the ring strain in spiro[3.3]heptane cannot exceed twice the ring strain in cyclobutane by more than ca. 3 kcal mol^–1.