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CHAPTER 3 Radical Reactions: Part 1 A. J. CLARK, J. V. GEDEN, and N. P. MURPHY Department of Chemistry, University of Warwick Introduction Rearrangements Group Migration β-Scission (Ring Opening) Ring Expansion Intramolecular Addition Cyclization Tandem Reactions Radical Annulation Fragmentation, Recombination, and Homolysis Atom Abstraction Reactions Hydrogen abstraction by Carbon-centred Radicals Hydrogen Abstraction by Heteroatom-centred Radicals Halogen Abstraction Halogenation Addition Reactions Addition to Carbon-Carbon Multiple Bonds Addition to Oxygen-containing Multiple Bonds Addition to Nitrogen-containing Multiple Bonds 1 Homolytic Substitution Aromatic Substitution SH2 and Related Reactions Reactivity Effects Polarity and Philicity Stability of Radicals Stereoselectivity in Radical Reactions Stereoselectivity in Cyclization Stereoselectivity in Addition Reactions Stereoselectivity in Atom Transfer Redox Reactions Radical Ions Anion Radicals Cation Radicals Peroxides, Peroxyl, and Hydroxyl Radicals Peroxides Peroxyl Radicals Hydroxyl Radical References 2 Introduction Free radical chemistry continues to be a focus for research with a number of reviews being published in 2000. Green aspects of chemistry have attracted a lot of attention with most work conducted investigating atmospheric chemistry, however on a review the use of supercritical fluids in radical reactions dealing solvent effects on chemical reactivity has appeared.1A The kinetics and thermochemistry of a range of free radical reactions has been reviewed. The review includes descriptions of the kinetics of a variety of unimolecular decomposition and isomerisation reactions as well as bimolecular hydrogen atom abstraction, additions, combinations and disproportionations. 204. In synthetic applications a review on the hydroxylation of benzene to phenol using Fenton’s reagent has appeared 254 as well as reviews on the synthesis of heterocycles using SRN1 mechansims279, and radical reactions controlled by Lewis Acids, 265. This latter review deals with effects of diastereoselectivity of reactions based upon size and chelation effects of the Lewis acids. The effect on radical cascade and rearrangement (Surzur-Tanner rearrangement) is also reported. A range of symposia have appeared including the theoretical study of hydrogen atom substitution in aromatics, 245, kinetic and mechanistic studies on the reaction between hydroxyl radicals with trichloroethylene, 246, the kinetics of radical radical combination reactions of ethyl radicals, 247and a kinetic study on vinyl radical addition to chloroethene 248. Two general reviews on the use of free radicals in organic synthesis have appeared with particular emphasis on inter- and intra-molecular addition reactions as well as stereoselectivity of reactions. 101,102. Continuing the theme of stereoselectivity the electronic factors governing the diastereofacial selectivity of the reactions of 5-substituted -2-adamantyl radicals has been reviewed. 103. Other reviews exploring the use of radicals in synthesis have included a review of the chemistry of acyl radicals (highlighting the properties, structure, decarbonylation, decarboxylation and synthetic utility of 3 acyl radicals) 104 and a comprehensive account of the use of radicals to prepare medium sized rings (including cyclisation and ring expansion approaches). 105 The use of radical addition-cyclisation strategies towards the synthesis of ring systems has also been reviewed, with 106, 107 particular attention on the rearrangement and cyclisation of vinyl radicals promoted by sulfanyl radical addition to alkynes.107. The radical chemistry of sulphur has attracted more attention this year with reviews on the chemistry of aromatic thiyl radicals 108 and sulphur-centered reactive intermediates appearing 109. Accounts of empirical correlational solvent effects 110 in radical reactions as well as radical reactions in supercritical fluids have appeared. 111, The important contempory areas of copper mediated atom transfer polymerisation 112 and polarity reversal catalysis 113 have been reviewed by Matyjaszewski and Roberts respectively. Other reviews that have appeared this year include those on combination and disproportionation reactions, 114, theoretical models for addition and abstraction reactions, 115, homolytic addition reactions of dialkylaminyl radicals 116 and the chemistry of nitroarene and aromatic N-oxide radicals 117. Rearrangements Group Migration The rearrangement of a range of β-(phosphatoxy)alkyl and β-(acyloxy)alkyl radicals (generated by laser flash photolysis) have been studies and the rate constants and kinetic parameters determined in a range of solvents. Migrations were found to be accelerated in polar solvents. Migrations were also accelerated when electron donating groups were substituted in the alkyl group and electron withdrawing groups substituted in the migrating group. 118 The first example of a conformationally contstrained β-(acyloxy)alkyl radical undergoing rearrangement by a pure 1,2-shift mechanism has 17 -3 been reported. Reaction of the the O labelled esters (1a-c) with Bu3SnH in the presence of 10 M PhSeH led to an inseparable mixture of the reduced (2a-c) and the ring contracted (3a-c) products (Scheme 1). No scrambling of the O17 label was detected indicating that the rearrangement occurs via a 1,2-shift process.119. The study of intramolecular aromatic 1,5-hydrogen transfer reactions in range of systems has been reported. The activation energy and the rate of H-abstraction between the ortho position of ortho- benzophenone has been calculated using molecular orbital theory and transition-state theory. 4 Stabilisation of the transition state appears to be the result of a C-H…O, H-bond between the oxygen atom and the ortho-hydrogens that are not involved in the H-transfer. 120. In other work products derived from the photolysis of iodobenzophenone (4) indicarted that the initially formed radical is in equilibrium with that derived from a 1,5-hydrogen transfer to the other aromatic ring (Scheme 2). 121. The rates of 1,5-hydrogen abstraction is a range of fluorinated radicals containing differing substituents at the site of C-H abstraction (X) have been measured. For 1,1,2,2-tetrafluoro-n-pentyl radicals (XCH2CH2CH2CF2CF2·) and 1,1,2,2,3,3-hexafluoro-n-pentyl radicals (XCH2CH2CF2CF2CF2·), where X = MeO, MeS, Me, Ph there was little observed difference in rates for differing X groups. 122 The energy barrier for the 1,2-shift of a H-atom in a range of aryl radicals has been calculated to be in the order of 60 kcalmol-1 precluding these type of rearrangements from occurring under normal laboratory conditions. However, rearrangements such as these may become important in combustion or flash vacuum pyrolysis. 123 Two new radical clocks have been calibrated, (notably the rearrangement of (5a → 6a) and (5b → 6b)). Rates were found to be 6.4 x 103 and 1.4 x 104 M-1s-1 respectively. These two new clocks are intermediate between those of the neophyl and 1-hexenyl clocks and thus should be valuable new additions as tools to investigate reactions with intermediate rates, (Scheme 3). 124. ΙΝSERT SCHEME 1-3 β-Scission and Ring Opening The accurate kinetic measurement of both 3-exo cyclisations and the reverse ring openings continue to attract much research. The kinetics of ring opening of a series of (trans-2-arylcyclopropyl)methyl radicals (where a range of substituents located at the m- and the p-postions of the aryl group) have been measured by indirect methods. The radicals were generated from PTOC esters and trapping with PhSeH was used as a competition reaction. Rate constants at 25 °C ranged from 1.0 x 1011 s-1 (for p- 11 -1 Me) to 4.1 x 10 s (for p-CF3). These new radical rearrangements have potential to be used as ultra fast mechanistic probes. 125. PTOC esters have also been used to prepare Eand Zisomers of the 4-(2,2- diphenylcyclopropyl)-3-butenyl radical (7) by laser flash photolysis. These radicals cyclise in a 3-exo fashion to give the intermediate radical (8) which undergoes rapid ring opening, (Scheme 4). The initial rate constants for the 3-exo cyclisations were determined and it was found that Z-(7) cyclised 5 faster than the E-(7) isomer as a result of a more favourable entropy of activation. 126. The ring opening of fluorinated cyclopropylcarbinyl radicals has been investigated using DFT calculations and the remarkable impact of the fluorine atoms on the structure and reactivity of these radicals was proposed with the stereoelectroinc influences thought to be largely responsible for the predicted reactivity and regiospecificity of ring opening. 127 A range of 3-arylaziridinylcarbinyl radicals (9) have been prepared from the Bu3SnH mediated reduction of thiocarbamates and the products arising from their ring opening reactions determine, (Scheme 6). Only products arising from specific C-N bond cleavage were detected which is in contrast to the mode of ring opening in the related oxygen analogues (oxiranylcarbinyl radical). 128. The approximate transition structures for the ring opening of a range of cyclopropylcarbinyl radicals have been determined using a specific mathematical model of the Bell-Evans-Polanyi principal. 129. The factors effecting the ring cleavage of the alkoxy radical (10) have been explored computationally in order to understand why cleavage occurs to give the less thermodynamically stable ring expanded product (11) instead of the allylic radical (12) (Scheme 6). Two factors influencing the kinetic preference
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