Propellanes free radical additions

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. 

The strained hydrocarbon [1,1,1] propellane is of special interest because of the thermodynamic and kinetic ease of addition of free radicals (R ) to it. The resulting R-substituted [ 1.1.1]pent-1-yl radicals (Eq. 3, Scheme 26) have attracted attention because of their highly pyramidal structure and consequent potentially increased reactivity. R-substituted [1.1.1]pent-1-yl radicals have a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radicals. They can add irreversibly to phosphines or alkylphosphinites to afford new alkylphosphonites or alkylphosphonates via radical chain processes (Scheme 26) [63]. The high propensity of a R-substituted [1.1.1] pent-1-yl radical to react with three-coordinate phosphorus molecules reflects its highly pyramidal structure, which is accompanied by the increased s-character of its SOMO orbital and the strength of the P-C bond in the intermediate phosphoranyl radical. 

McGarry and Scaiano have measured the absolute kinetics for the addition of free radicals to the propellane (203). The photodecomposition of ethylene oxide has been examined. 

Homoquinones are also prepared by a variety of methods that involve the free-radical reactions of propellanes with benzoquinones, ° the Michael addition of suUbxonium methyhde" and ethyl cyanoac-etate anion to naphthoquinone and of sulfonium methyhdes to benzoquinones, the photooxygenation of norcaradienes, the reaction of diethoxyphthalans with quinones, and the Diels-Alder reactions of siloxy- and alkoxy-substituted furans with cyclopropenes. ... 

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