Hydrogen Activation by an Aromatic Triphosphabenzene

Aromatic hydrogenation is a challenging
transformation typically
requiring alkali or transition metal reagents and/or harsh conditions
to facilitate the process. In sharp contrast, the aromatic heterocycle
2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene is shown
to be reduced under 4 atm of H2 to give [3.1.0]­bicylo reduction
products, with the structure of the major isomer being confirmed by
X-ray crystallography. NMR studies show this reaction proceeds via
a reversible 1,4-H2 addition to generate an intermediate
species, which undergoes an irreversible suprafacial hydride shift
concurrent with P–P bond formation to give the isolated products.
Further, para-hydrogen experiments confirmed the
addition of H2 to triphosphabenzene is a bimolecular process.
Density functional theory (DFT) calculations show that facile distortion
of the planar triphosphabenzene toward a boat-conformation provides
a suprafacial combination of vacant acceptor and donor orbitals that
permits this direct and uncatalyzed reduction of the aromatic molecule.