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Reactivity of Aryl and Vinyl Radicals: Abstraction of Hydrogen Atom or Reaction with a Nucleophile
Author(s) -
Branchi Barbara,
Galli Carlo,
Gentili Patrizia
Publication year - 2002
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200208)2002:16<2844::aid-ejoc2844>3.0.co;2-9
Subject(s) - chemistry , radical , nucleophile , hydrogen atom abstraction , reaction rate constant , substituent , aryl radical , reactivity (psychology) , aryl , photochemistry , flash photolysis , medicinal chemistry , radical substitution , hydrogen atom , computational chemistry , organic chemistry , kinetics , catalysis , medicine , physics , alkyl , alternative medicine , pathology , quantum mechanics
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 S RN 1 reaction). The radicals of interest were generated from haloarene or haloethene precursors, either by use of the Bu 3 SnH/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 ( k H ) from the solvents Me 2 SO (2.8·10 6 ) and CH 3 CN (6.7·10 6 ) and the rate constant for combination with a nucleophile ( k Y ) such as Me 3 CCOCH 2 − ion (3.3·10 9 ) were determined. For vinyl radical Ph 2 C=C( · )Ph ( 7 · ), the k H values from Me 2 SO (1.1·10 5 ), CH 3 CN (1.2·10 5 ), Bu 3 SnH (7.5·10 8 ), and (Me 3 Si) 3 SiH (1.6·10 9 ) and the k Y values with Me 3 CCOCH 2 − (3.9·10 7 ), (EtO) 2 PO − (2.8·10 6 ), and PhS − (1.9·10 7 ) 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)