Synthesis and reactivity of a P–H functionalized benzazaphosphole

Abstract

Compound 1 was prepared from N-Dipp (Dipp = 2,6-diisopropylphenyl) substituted, 10π-electron benzazaphosphole 2 via a two-step protonolysis/substitution protocol using HCl and LiAlH4. As opposed to structurally related N-heterocyclic phosphines, the P–H unit in 1 acted as a weak hydride. A sluggish insertion reaction between 1 and electron-poor CF3C(O)Ph was observed, but dehydrocouplings with weakly acidic alcohols like MeOH and HFIP (hexafluoroisopropanol) did not proceed, as predicted by DFT computations showing high activation energies. However, a rapid and essentially barrierless reaction with: CF2 occurred, yielding P–CF2H derivative 6, which was fully characterized by NMR spectroscopy, elemental analysis (EA), HRMS, and X-ray crystallography. The complex splitting pattern of the diastereotopic fluorines of the –CF2H group observed in the 19F NMR spectrum was successfully simulated using MestReNova. Finally, 1 underwent hydrophosphination in the presence of phenylacetylene, generating a mixture of anti- (E/Z-7) and Markovnikov products (8). Computations indicated that anti-Markovnikov E-7 (P–CH=CHPh) formed via highly strained phosphirene intermediate 9, while Z-7 crystallized out of solution and was characterized by NMR spectroscopy, EA, HRMS, and X-ray crystallography.