Hydrogen abstraction from spiro[2.n]alkanes

Abstract

Hydrogen abstraction from spiro[2.3] hexane by t-butoxyl radicals gave spiro[2.3] hex-2-yl radicals; their rearrangement to cyclobutenylethyl radicals was followed by kinetic e.s.r. spectroscopy. Hydrogen abstraction at the methylene groups adjacent to the cyclopropyl rings [cyclopropylmethyl (cpm) sites] in higher spiro[2.n]alkanes gave spiro[2.n]alk-2-yl radicals, which rearranged to cycloalkenylethyl radicals too rapidly for detection, together with secondary radicals from abstraction at the other methylene groups in the larger ring. From the measured concentrations of the cycloalkenylethyl and secondary radicals the rate of hydrogen abstraction at the cpm sites relative to the rate of hydrogen abstraction at the secondary sites was determined; significant activation of the cpm hydrogens was found. This activation was attributed to a pseudo-allyl type of effect, i.e., to delocalisation of the unpaired electron into the Walsh orbitals of the cyclopropane ring of the spiro[2.n]alk-2-yl radicals; semi-empirical SCF- MO calculations supported this explanation. Photobromination of spiro[2.3] hexane occurred mainly by S_H2 attack of bromine atoms at the cyclopropane methylene carbons with fission of either C–C bond.