Bridgehead reactivity, radical

Following the extensive investigation of bridgehead carbonium ion reactivities, which provides the most conclusive experimental evidence available for the preferred planarity of carbonium ions 187), similar studies of bridgehead free radical reactivity have been initiated. The results are equally instructive. 

To study bridgehead reactivity, our group (89) prepared a cyclophane of C3 symmetry 74 a comparison with triphenylmethyl chloride indicated that the bridgehead chloride of 74 is conspicuous in its striking lack of reactivity in SnI displacement reactions and in free radical formation. 

While a mixture of products was observed in bicyclic systems, the product ratios were different, favoring the secondary bromide over the primary. Substrate 30 was brominated to afford a 3 2 ratio of the secondary and primary bromo products 31 and 32, respectively, in high yield. These terminal alkenes show that the delocalization of the radical can occur to provide reactive radicals 33 and 34. No bridgehead bromination occurred. ... 

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." ... 

Substitutions by the SRn 1 mechanism (substitution, radical-nucleophilic, unimolecular) are a well-studied group of reactions which involve SET steps and radical anion intermediates (see Scheme 10.4). They have been elucidated for a range of precursors which include aryl, vinyl and bridgehead halides (i.e. halides which cannot undergo SN1 or SN2 mechanisms), and substituted nitro compounds. Studies of aryl halide reactions are discussed in Chapter 2. The methods used to determine the mechanisms of these reactions include inhibition and trapping studies, ESR spectroscopy, variation of the functional group and nucleophile reactivity coupled with product analysis, and the effect of solvent. We exemplify SRN1 mechanistic studies with the reactions of o -substituted nitroalkanes (Scheme 10.29) [23,24]. 

More recently, other approaches to this interesting ring system have also been developed. These are illustrated in Eqs. (41), 29< 13°) and (42) 131 As indicated, photochlorination of the parent hydrocarbon occurs only at the methylene positions 13°). The correspondence between free radical and car-bonium ion reactivities at the bridgehead positions of polycyclic hydrocarbons suggests that the bridgehead position of bisnoradamantane should also be highly unreactive in carbonium ion processes 1321. 

Both radicals and electrophiles react at the bridgehead bond of [l.l.l]propellane. The reactivity toward radicals is comparable to that of alkenes, with rates in the range of 10 to 10 M s depending on the particular radical. For example, [l.l.l]propellane reacts with thiophenol at room temperature. ... 

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