Reactivity of Aryl and Vinyl Radicals: Abstraction of Hydrogen Atom or Reaction with a Nucleophile

Abstract

The reactivities of aryl and vinyl radicals, two fundamental transient intermediates, have been investigated with respect to two elementary processes: H-atom abstraction and reaction with a nucleophile (Y−, in the SRN1 reaction). The radicals of interest were generated from haloarene or haloethene precursors, either by use of the Bu3SnH/AIBN system or by photostimulated electron transfer from a nucleophile, and the partition of the intermediate radical between competing pathways was investigated. Use both of indirect methods (such as the study of the reaction products in competition experiments; use of a radical-clock probe) and of direct ones (such as the detection of the radicals by flash photolysis experiments) enabled the following rate constants to be obtained (all values in M−1·s−1 at 25 °C). For phenyl-type radicals, the rate constants for H abstraction (kH) from the solvents Me2SO (2.8·106) and CH3CN (6.7·106) and the rate constant for combination with a nucleophile (kY) such as Me3CCOCH2− ion (3.3·109) were determined. For vinyl radical Ph2C=C(·)Ph (7·), the kH values from Me2SO (1.1·105), CH3CN (1.2·105), Bu3SnH (7.5·108), and (Me3Si)3SiH (1.6·109) and the kY values with Me3CCOCH2− (3.9·107), (EtO)2PO− (2.8·106), and PhS− (1.9·107) ions were determined. Semiempirical calculations confirmed a stabilization of radical 7· by the α-Ph substituent (ca. 8 kcal/mol), and provided the BDE of the C−Y bond for the vinylic substitution products of 7·. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)