Inner- and Outer-Sphere Roles of Ruthenium Phosphido Complexes in the Hydrophosphination of Alkenes

An inner-sphere synthetic cycle for
the hydrophosphination of alkenes
is proposed, based on observed [2 + 2] cycloaddition of a wide range
of alkenes at a coordinatively unsaturated RuPR2 complex. Key intermediates in the cycle were prepared, and their
reactions with various organic acid/base pairs were examined to identify
both new ruthenium precursors and base cocatalysts that allow turnover
of the proposed cycle. Two new cationic ruthenium indenyl phosphine
complexes were isolated and structurally characterized. Although preliminary
screening studies show the moderate activity of these and related
neutral phosphido complexes for catalytic hydrophosphination of acrylonitrile
by both HPPh2 and HPCy2, and comparable activity
for the hydrophosphination of tert-butyl acrylate
by HPPh2, no activity was observed for the analogous hydrophosphination
of 1-hexene. This is attributed to strong binding of the substrate
phosphine to the unsaturated, planar RuPR2 fragment
generated in situ, which inhibits the inner-sphere, alkene cycloaddition
mechanism. An alternative, outer-sphere Michael addition process,
involving a saturated complex with a strongly nucleophilic pyramidal
Ru–PR2 ligand, is proposed to rationalize the observed
selectivity for catalytic hydrophosphination of activated, but not
simple, alkenes. Implications for further catalyst development are
discussed.