Selectivity in the Chemistry of Oxygen-Centered Radicals - The Formation of Carbon-Oxygen Bonds

Jens  Hartung; Thomas  Gottwald; Kristina  Špehar

doi:10.1055/s-2002-33335

REVIEW

Selectivity in the Chemistry of Oxygen-Centered Radicals - The Formation of Carbon-Oxygen Bonds

Jens Hartung*, Thomas Gottwald, Kristina Špehar
Institut für Organische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany
Fax: +49(931)8884606; e-Mail: hartung@chemie.uni-wuerzburg.de;

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Abstract

In recent years, powerful new methods for the generation of alkoxyl radicals under mild and neutral conditions have been developed. This progress has led to a thorough investigation of most O-radical elementary reactions. Today, sufficiently reliable thermodynamic and kinetic data are available either from experimental or from theoretical studies in order to predict alkoxyl radical reactivities and selectivities in synthesis. For instance, alkoxyl radicals readily add to carbon-carbon and carbon-nitrogen double bonds. Due to generally low activation barriers and strongly negative reaction enthalpies, inter- and intramolecular addition reactions proceed under kinetic control and are associated with high rate constants. Nevertheless, intramolecular 5-exo-trig additions, i.e. cyclizations, proceed with an astonishing degree of diastereoselectivity and often provide complementary selectivities if compared to commonly used methods such as the bromine cyclization of alkenols. Therefore, several useful applications of O-radical cyclizations in the synthesis of functionalized tetrahydrofurans have been discovered in the last few years. A second major reaction channel of alkoxyl radicals is associated with the homolysis of a β-C-C bond. This fragmentation proceeds under thermodynamic control and affords a carbonyl compound besides an alkyl radical from the starting alkoxyl radical. Regioselectivities for C-C bond homolysis may be predicted by considering strain release (cyclic carbon framework) and the stability of the newly formed carbon radical (cyclic and open chain carbon skeletons). The third major group of alkoxyl radical-based transformations are connected with homolytic substitutions such as intramolecular 1,5-hydrogen shifts which have been applied with considerable success to remote functionalization reactions. In view of the diversity of alkoxyl radical reactions it is the aim of this review to organize basic principles of this type of chemistry and to present its latest useful application in organic synthesis.

1 Introduction
2 General Aspects
3 Elementary Reactions
4 Generation of Oxygen-Centered Radicals and their
Application in Chain Reactions
5 Addition of O-Radicals to Multiple Bonds
5.1 Intramolecular Addition to Carbon Carbon Double Bonds
5.2 Intermolecular Addition to Double Bonds
6 Formation of Carbonyl Compounds (β-C-C Cleavage)
7 Rearrangements
8 Hydrogen Abstractions: Remote Functionalizations and
Subsequent Nucleophilic Transformations
9 Summary and Perspectives

Key words

addition - alkoxyl radical - cyclization - fragmentation - hydrogen atom transfer - natural products - rearrangement - stereoselective synthesis

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Geometrical data which are mentioned in the text were obtained from an UB3LYP/6-31+G**//UB3LYP/6-31+G**-optimized transition structure 84 For comparison with data obtained from earlier UHF calculations (see. ref. [109] ), transition structure 84 was in addition optimized using UHF/6-31+G**//UHF/6-31+G**-calculations: E = -231361265 Hartree 〈S ²〉 = 1.003; i = -689 cm^-1; transition structure geometry: d_C1-O = 1.903 Å, α_O-C1-C2 = 108.07°; sum of Mulliken charges: methoxyl radical substructure: -0.12, propene moiety:+0.12

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Hartung, J., unpublished results. The photoreaction of thione 162 [74] in benzene was performed as described in ref. [120] R _f 02 (petroleum ether-EtOAc, 3:1); ¹H NMR (CDCl₃) δ = 1.38 (d, J = 7.0 Hz, 3 H), 1.65-1.80 (m, 3 H), 1.80-2.01 (m, 1 H), 2.52 (br s, 1 H), 3.71 (m_c, 1 H), 3.94 (m_c, 1 H), 6.96 (ddd, J = 0.9, 4.9, 7.3 Hz, 1 H), 7.16 (dt, J = 0.91, 8.2, 1 Hz), 7.46 (ddd, J = 1.8, 7.3, 8.1 Hz, 1 H), 8.38 (ddd J = 0.9, 1.8, 4.9 Hz, 1 H). ¹³C NMR (CDCl₃): δ = 20.5, 29.6, 33.9, 62.3, 77.3, 119.6, 123.2, 136.0, 149.5, 159.7 Anal. Calcd for C₁₀H₁₅NOS: C, 60.88; H, 7.66, N, 7.10; S, 16.25. Found: C, 60.48; H, 7.30, N, 7.28; S, 16.66

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