Synthesis of phosphiranes via organoiron-catalyzed phosphinidene transfer to electron-deficient olefins

Abstract

Herein is reported the structural characterization and scalable preparation of the elusive iron–phosphido complex FpP(^tBu)(F) (2-F, Fp = (Fe(η⁵-C₅H₅)(CO)₂)) and its precursor FpP(^tBu)(Cl) (2-Cl) in 51% and 71% yields, respectively. These phosphide complexes are proposed to be relevant to an organoiron catalytic cycle for phosphinidene transfer to electron-deficient alkenes. Examination of their properties led to the discovery of a more efficient catalytic system involving the simple, commercially available organoiron catalyst Fp₂. This improved catalysis also enabled the preparation of new phosphiranes with high yields (^tBuPCH₂CHR; R = CO₂Me, 41%; R = CN, 83%; R = 4-biphenyl, 73%; R = SO₂Ph, 71%; R = POPh₂, 70%; R = 4-pyridyl, 82%; R = 2-pyridyl, 67%; R = PPh₃⁺, 64%) and good diastereoselectivity, demonstrating the feasibility of the phosphinidene group-transfer strategy in synthetic chemistry. Experimental and theoretical studies suggest that the original catalysis involves 2-X as the nucleophile, while for the new Fp₂-catalyzed reaction they implicate a diiron–phosphido complex Fp₂(P^tBu), 4, as the nucleophile which attacks the electron-deficient olefin in the key first P–C bond-forming step. In both systems, the initial nucleophilic attack may be accompanied by favorable five-membered ring formation involving a carbonyl ligand, a (reversible) pathway competitive with formation of the three-membered ring found in the phosphirane product. A novel radical mechanism is suggested for the new Fp₂-catalyzed system.