Structural and Stereochemical Properties of Radical Intermediates

EPR studies and other physical methods have provided the basis for some insight into the detailed geometry of radical species. Deductions about structure can also be drawn from the study of the stereochemistry of reactions involving radical intermediates. Several structural possibilities must be considered. If discussion is limited to alkyl radicals, the possibilities include a rigid pyramidal structure, rapidly inverting pyramidal structures, or a planar trigonal structure. 

Precise description of the pyramidal structures would also require that the bond angles be specified. The EPR spectrum of the methyl radical leads to the conclusion 

Alkyl substituents apparently cause carbon radicals to adopt a pyramidal geometry. The butyl radical has been studied particularly extensively, and while experimental results have been interpreted both in terms of planar and slightly pyramidal structures, theoretical results favor the pyramidal structure. Both theory and experiment agree that the barrier to inversion is very low, leading to rapid inversion. 

Cyclic molecules also permit deductions about stereochemistry without the necessity for optically active compounds, and the stereochemistry of a number of reactions of 4-substituted cyclohexyl radicals has been investigated. These reactions give mixtures of cis and trans isomers, indicating that the radical intermediates in these reactions do not retain the stereochemistry of the radical precursor. Such reactions are not usually very stereoselective, but some show a preference for 

Studies of chemical reactivity have revealed no particularly strong resistance to formation of bridgehead radicals. In an early study, the decarbonylation of bridgehead aldehydes was found to proceed without difficulty 

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