Influence of Ring Strain and Bond Polarization on the Ring Expansion of Phosphorus Homocycles

Heterolytic cleavage of homoatomic bonds is a challenge as it requires separation of opposite charges. Even highly strained homoatomic rings (e.g. cyclopropane and cyclobutane) are kinetically stable and do not react with nucleophiles or electrophiles. In contrast, cycloalkanes bearing electron donating/withdrawing substituents on adjacent carbons have polarized C–C bonds and undergo numerous heterolytic ring-opening and expansion reactions. Here we show that upon electrophile activation phosphorus homocycles exhibit analogous reactivity, which is modulated by the amount of ring strain and extent of bond polarization. Neutral rings (tBuP)3, 1, or (tBuP)4, 2, show no reactivity towards nitriles but the cyclo-phosphinophosphonium derivative [(tBuP)3Me]+, [3Me]+, undergoes addition to nitriles giving five-membered P3CN heterocycles. Due to its lower ring-strain, the analogous four-membered ring, [(tBuP)4Me]+, [4Me]+, is thermodynamically stable with respect to cycloaddition with nitriles, despite similar P–P bond polarization. We also report the first example of isonitrile insertion into cyclophosphines, which is facile for polarized derivatives [3Me]+ and [4Me]+, but does not proceed for neutral 1 or 2, despite the calculated exothermicity of the process. Finally, we have assessed the reactions of [4R]+ towards 4-dimethylaminopyridine (dmap), which suggest that the site of nucleophilic attack varies with the extent of P–P bond polarization. These results deconvolute the influence of ring-strain and bond-polarization on the chemistry of inorganic homocycles and unlock new synthetic possibilities.