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 toward nitriles, but the cyclo-phosphinophosphonium
derivative [(tBuP)3Me]+, [3Me]+, undergoes addition to nitriles giving five-membered
P3CN heterocycles. Because of 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 assessed
the reactions of [4R]+ R = H, Cl, F toward
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.