Platinum(0)-Catalyzed Intramolecular Addition of a C−H Bond onto the P═C Bond of a Phosphaalkene

Abstract

The phosphaalkene E-Mes*P═CHPh (1) reacts with a stoichiometric amount of [Ni(COD)2] at room temperature for 15 h in hexanes to afford the 1-P-benzyl-benzophospholane 2, which results from the formal addition of the C−H bond of one tert-butyl group onto the P═C double bond. The formulation of benzophospholane 2 was confirmed by an X-ray diffraction study of the corresponding trans [Pd(2)Cl2] complex 3a (meso complex). Formation of 2 was also achieved by using a catalytic amount of the [Pt(PCy3)2] catalyst (20 mol %) in hexanes at 65 °C for 15 h. Extension of this procedure to the bis-phosphaalkene [(E)-Mes*P═CH]2-(m-C6H4) (4) afforded the corresponding bis-benzophospholane, which was recovered as a mixture of diastereomers 5a and 5b. On the basis of DFT calculations a complete mechanistic pathway was proposed in the case of nickel. The overall mechanism involves a preliminary complexation of 1 at Ni to afford a 12 VE η1-complex, which then evolves to yield the 14 VE η2-species featuring coordination of one C−H of a tert-butyl group. Insertion of nickel into this C−H bond followed by the formation of a P−C bond between phosphorus and the methyl group affords a 14-VE hydrido complex featuring a η3-benzyl ligand. Isomerization of this species through complexation of the phosphorus atom of the benzophospholane unit and decomplexation of the phenyl group affords a 14-VE species, which undergoes a reductive elimination to finally yield the 12-VE Ni complex of 2. A similar mechanism was proposed in the case of platinum considering that the catalytically active species is the 12-VE [Pt(PMe3)] complex. In good agreement with NMR data this mechanism involves, as the most stable intermediate, a 16-VE complex featuring the benzophospholane ligand, which is coordinated to the [Pt(H)(PMe3)] fragment through the phosphorus atom lone pair and the benzylic carbon atom. NMR data and DFT calculations confirm that, in this intermediate, the hydride is located trans to the phosphorus atom of the benzophospholane ligand.