Free radicals bridgehead

Many free-radical reactions have been observed at bridgehead carbons (e.g., see 14-37), ... 

An intramolecular free radical addition was used to prepare the 1,2-thiazepine derivatives 30 and 31 from 29 (Equation 6). A further elegant intramolecular radical cyclization was then used to convert 30 to a new aza-bicyclic system with a bridgehead nitrogen <2001JOC3564>. 

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. 

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. 

A variety of bridgehead free radical reactions, including aldehyde decarbon-ylations 31°) and f-butyl perester 3I1) and diazo decompositions312)of ada-... 

Many free-radical reactions have been observed at bridgehead carbons, as in formation of bromide 18 (see 14-30), ° demonstrating that the free radical need not be planar. However, treatment of norbomane with sulfuryl chloride and benzoyl... 

The observed control of regioselectivity points to operation of a direct 1,2-aryl shift pathway without the intervention of a cyclopropylmethyl diradical species. The aromatic sextet is not transiently interrupted and the product composition reflects the relative ability of bridgehead substituents to stabilize a free radical center. ... 

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. 

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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