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

Abstract

Dibenzo-7-phosphanorbornadiene compounds, RPA (A = CH 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, [MeN═P]. Kinetic investigations of PrNPA thermolysis in 1,3-cyclohexadiene and/or benzene-d are consistent with a model of unimolecular fragmentation to yield free phosphinidene [PrN═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.

Transition‐Metal‐Like Reversible Cycloadditions of [tBuSP‐W(CO)5] with Alkenes and Alkynes

“…As has been noted, the chemistry of singlet phosphinidenes with a π‐donating substituent such as RO, R 2 N, R 2 P, with or without coordinated metal, were disclosed at a certain level. However, to the best of our knowledge the only example of an RSP‐type complex was reported by Terlouw, that is, (thiohydroxy)phosphinidene (HSP) .…”

“…Bertrand reported thermodynamically stabilized phosphinophosphinidenes by the lone pair of the adjacent phosphorus moiety . Cummins’ group revealed that π‐donating dialkylamide groups were paramount to successful phosphinidene transfer, poorer π‐donors gave reduced or no transfer . Bulky substituents were employed in both Bertrand and Cummins’ work to increase the stability of transient phosphinidenes.…”

“…The stronger donation from a bulky and electron‐rich t BuS group into the P−C σ* antibonding orbital facilitates the fragmentation of phosphiranes and phosphirenes. This correlates with stronger π‐donation to stabilize the phosphinidene intermediate.…”

“…[3] Cummins' group revealed that p-donating dialkylamide groups were paramount to successful phosphinidene transfer,p oorer p-donors gave reduced or no transfer. [4] Bulky substituents were employed in both Bertranda nd Cummins' work to increase the stability of transientp hosphinidenes. The stabilizing effect from the strong p-donor substituent could preventphosphinidene oligomerization.…”

“…[1a-c, e, 6] Since their discoveryb yM athey in 1982, ah uge amount of work has been devoted to the chemistry of terminal phosphinidenec omplexesa nd their synthetici mportance in organophosphorus chemistry has been demonstrated by cycloadditions andb ond insertions. [1a-c, e, 7] In particular, Lammertsma described ap hosphinidene complex [iPr 2 NP-Fe(CO) 4 ], which is stabilized by an amino substituent and effective Fe!Pb ack-bonding, that undergoes facile and reversible cycloadditions with alkenes anda lkynes. [7f] As has been noted, the chemistry of singlet phosphinidenes with a p-donating substituent such as RO, [8] R 2 N, [4, 7f,9] R 2 P, [3] with or withoutc oordinated metal, were disclosed at ac ertain level.…”