Free Radical Chain Reactions of [1.1.1]Propellane with Three-Coordinate Phosphorus Molecules. Evidence for the High Reactivity of the Bicyclo[1.1.1]pent-1-yl Radical

Abstract

Three-substituted bicyclo[1.1.1]pent-1-yl radicals (5), generated from additions of radicals to [1.1.1]propellane (1), are found to have high propensities to react with three-coordinate phosphorus molecules. For example, the 3-ethylbicyclo[1.1.1]pent-1-yl radical (5d) reacts with (EtO)3P in a free-radical Arbuzov process to yield dimethyl 3-ethylbicyclo[1.1.1]pent-1-ylphosphonate (13). By contrast, the ethyl radical does not react with (EtO)3P to yield EtP(O)(OEt)2. However, the highly reactive phenyl radical yields PhP(O)(OEt)2. Moreover, attack of the n-pentylbicyclo[1.1.1]pent-1-yl radical (5b) on n-C5H11P(OMe)2 results in a free-radical substitution process that gives dimethyl 3-n-pentyl[1.1.1]bicyclopent-1-ylphosphonite (7b) and the primary alkyl radical n-C5H11•. Thus, 5 has a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radical. Reaction of 2-benzyl-4-methyl-1,3,2-dioxaphospholane (9) with 5a is found to proceed with predominant inversion of configuration at phosphorus. Furthermore, 3-phenylmethylbicyclo[1.1.1]pent-1-yl radical, 5a (from reaction of benzyl radical with 1), participates in reasonably efficient radical chain reactions with PhCH2OP(OMe)2 (11) and PhCH2P(OMe)2 (6a) (chain lengths 30−50) to provide structurally novel products of new radical-Arbuzov and radical-substitution reactions. Dimethyl 3-phenylmethylbicyclo[1.1.1]pent-1-ylphosphonate (12) from 11 and dimethyl 3-phenylmethylbicyclo[1.1.1]pent-1-ylphosphonite (7a) from 6a incorporate the bicyclo[1.1.1]pent-1-yl moiety. The high propensity of various 5 to react with three-coordinate phosphorus molecules reflects the highly pyramidal nature of 5 which is accompanied by the increased s-character of the SOMO orbital of 5 and the strength of the phosphorus−carbon bond in the presumed phosphoranyl radical intermediates (3 and 4) formed. Figure 1 depicts the thermodynamics of three classes of free radical substitution and Arbuzov reactions with three-coordinate phosphorus molecules.