Mechanism and Scope of Phosphinidene Transfer from Dibenzo-7-phosphanorbornadiene Compounds

Dibenzo-7-phosphanorbornadiene
compounds, RPA (A = C14H10 or anthracene), are investigated
as phosphinidene sources upon thermally induced (70–90 °C)
anthracene elimination. Analysis of substituent effects reveals that
π-donating dialkylamide groups are paramount to successful phosphinidene
transfer; poorer π-donors give reduced or no transfer. Substituent
steric bulk is also implicated in successful transfer. Molecular beam
mass spectrometry (MBMS) studies of each derivative reveal dialkylamide
derivatives to be promising precursors for further gas-phase spectroscopic
studies of phosphinidenes; in particular, we present evidence of direct
detection of the dimethylamide derivative, [Me2NP].
Kinetic investigations of iPr2NPA thermolysis in 1,3-cyclohexadiene and/or benzene-d6 are consistent with a model of unimolecular
fragmentation to yield free phosphinidene [iPr2NP] as a transient reactive intermediate.
This conclusion is probed by density functional theory (DFT) calculations,
which favored a mechanistic model featuring free singlet aminophosphinidenes.
The breadth of phosphinidene acceptors is expanded to unsaturated
substrates beyond 1,3-dienes to include olefins and alkynes; this
provides a new synthetic route to valuable amino-substituted phosphiranes
and phosphirenes, respectively. Stereoselective phosphinidene transfer
to olefins is consistent with singlet phosphinidene reactivity by
analogy with the Skell hypothesis for singlet carbene addition to
olefins.