A STUDY OF THE GAS-PHASE REACTIVITY OF NEUTRAL ALKOXY RADICALS BY MASS-SPECTROMETRY - ALPHA-CLEAVAGES AND BARTON-TYPE HYDROGEN MIGRATIONS

The reactivity of neutral alkoxy radicals in the absence of any interf ering intermolecular interactions is investigated by means of the rece ntly introduced method of neutral and ion decomposition difference (NI DD) spectra. These are obtained from quantitative analysis of the corr esponding neutralization - reionization (NR) and charge reversal (CR) mass spectra. The following trends emerge: alkoxy radicals with short (C-1 - C-3) or branched alkyl chains give rise to alpha-cleav age prod ucts, whereas longer-chained alkoxy radicals undergo 1,5-hydrogen migr ations from carbon to oxygen, that is, Barton-type chemistry. This fac ile rearrangement has been studied in detail for n-pentoxy radicals by isotopic labeling experiments and computation at the Becke 3LYP/6-31G level of theory. Further, the NIDD spectra of 3-methylpentoxy radica ls permit for the first time the identification of the diastereoselect ivity of the gas-phase hydrogen migrations. The results from the NIDD method are compared to those from earlier studies in the condensed pha se. This new mass spectrometric approach is suggested as a tool for th e examination of intramolecular reactions of free alkoxy radicals whic h can usefully complement theoretical studies.