Radicals bridgehead

The EPR spectra of a number of bridgehead radicals have been measured and the hyperfine couplings measured (see Section 12.2.3). Both the and couplings are sensitive to the pyramidal geometry of the radical." " The reactivity of bridgehead radicals increases with increased pyramidal character." ... 

When there is a choice of more than one group that might cleave, the more stable radical fragments preferentially.170 A strained bridgehead radical (compare Equations 9.97 and 9.98) is much less susceptible to cleavage than would be expected for an unstrained tertiary radical,171 a result that may reflect difficulty... 

Side Note 1.3. Pyramidalized Bridgehead Radicals—and/or Less Substituted Planar Radicals as Intermediates of Radical Chlorinations... 

In the full paper on [3.2.1]propellane several additional results were reported. Bromination in CH2CI2 at — 50°C afforded not only the dibromide 72. This was accompanied by a chloro-bromo derivative (73) indicating that a bromo-bridgehead radical or cation intermediate then either reacts with bromine or abstracts chlorine from the solvent. The Wiberg group also reported a new approach, from the bridgehead dibromide to the parent propellane as shown in the accompanying scheme. 

When the Cristol procedure was applied to the bridgehead acid (1), bicyclo[2.2.2]octane-l-caiboxylic acid, with bromine and mercuric oxide in carbon tetrachloride, the product was a mixture of the expected bromide (2) with an even larger proportion of the unexpected chloride (3). The chloride evidently arises by abstraction of chlorine from the solvent by an intermediate bridgehead radical. The pure bromide was obtained with use as solvent of either bromotrichloromethane or 1,2-dibromoethane. 

Conclusions about radical structure can also be drawn from analysis of ESR spectra. The ESR spectra of the bridgehead radicals A and B are consistent with pyramidal geometry at the bridgehead carbon atoms... 

The free radical additions common to small propellanes take place particularly readily with 177. Aldehydes add spontaneously and give both 1 1 and 1 2 adducts. Butyraldehyde, for example, furnishes a 1 2 mixture of ketone 208 and ketol 215 by way of the bridgehead radical 217. Radical addition to an aldehyde carbonyl in successful competition with abstraction of the aldehydic hydrogen is exceptional. Here it occurs preferentially, and both acetaldehyde and benzaldehyde yield only ketol adducts. From acetaldehyde this ketol is 216, and haloform oxidation of this adduct with iodine in base provides a convenient route to dicarboxylic acid 178. The formerly difficult-of-access precursor to propellane 177 is thus now readily available in two steps from the propellane itself. 

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