On the Neophyl-like Rearrangement of 2-(9-Anthryl)ethyl Radicals

Article abstract of DOI:10.1021/ja00202a010

The 1,2 migration of the aryl group in deuterated 2-(9-anthryl)ethyl and 2-<9-(10-phenylanthryl)>ethyl radicals has been studied by product analyses with the use of the competing reaction ArCH2CD2(.) + Bu3SnH.The rearrangement was so fast that equimolecular mixtures of the unrearranged (ArCH2CD2H) and rearranged (ArCD2CH3) hydrocarbons were obtained even in the presence of a large excess of Bu3SnH (ca. 1 M).The lower limit for the rate constant of the intramolecular process, k_r, could be estimated as 5E7 s^-1.Ab initio MO computations, performed on the2-phenylethyl radical, chosen as the simplest model for 2-arylethyls, showed that the aryl migration may occur either through the intermediacy of the spirocyclopropanecyclohexadienyl radical or via a three-membered transition state having geometrical and electronic properties different from those of the potential cyclic intermediate.The spectroscopic and chemical results are consistent with the rearrangement taking place via the intermediacy of spirocyclopropane radicals, which, as both thermochemical calculations and experimental evidence indicate, are characterized by a considerable thermodynamic stability.In fact, the spiro radicals could be easily observed by EPR spectroscopy when generated by hydrogen abstraction from the corresponding cyclic hydrocarbons, and the kinetics of their decay could be studied.The diamagnetic dimer of the 9,10-dihydro-10-spiro-9-yl radical was also isolated, and its structure determined by X-ray crystallographic analysis.The same cyclized radicals were detected when producing the two arylethyl radicals within the EPR cavity.Chemical generation of the 2-(9-anthryl)ethyl radical in the presence of low concentrations of hydrogen donors led to the isolation of remarkable amounts of the dimer of its cyclic structural isomer.These results provide evidence that the 2-anthrylethyl radical and the phenyl-substituted analogue may exist in either of the two rapidly equilibrating open-chain and cyclized forms and react preferentially in one of them depending on the experimental conditions.