Stoichiometric model reactions in olefin hydroformylation by platinum-tin systems

Article abstract of DOI:10.1021/om00058a019

The three independent steps involved in the hydroformylation process, insertion of the olefin, insertion of carbon monoxide, and hydrogenolysis, have been investigated with use of platinum-tin catalysts and 1-pentene as olefin at low pressure and temperature in CH₂Cl₂. In the temperature range 198-308 K, the three reactions can be studied consecutively. All the intermediates observed were prepared and characterized separately. The complex trans-[PtH(SnCl₃) (PPh₃)₂] was used as the initial compound for this sequence. The hydrido complex crystallized in the monoclinic space group C2/c, with a = 31.345 (5) ?, b = 12.716 (3) ?, c = 18.135 (3) ?, β = 96.5 (2)°, Z = 8, and R (R_w) = 0.056 (0.060) for 3235 independent reflections having I > 2.56σ(I). The large Pt-Sn bond (2.601 (1) ?) distance correlates satisfactorily with the low ¹J(Pt-Sn) value. The Pt-Sn bond is necessary for the insertion of 1-pentene in the hydrido-platinum complex and for the hydrogenolysis of the acyl compounds under these mild conditions. The insertion of 1-pentene was observed at 198 K, giving the cis-alkyl complex; CO insertion took place after isomerization to the trans-alkyl complex. The instability of Pt-Sn and Pt-C bonds in the trans-acyl complex favors easy decarbonylation or loss of SnCl₂, so any other platinum complex without tin accepts SnCl₂ from the acyl complex. The hydrogenolysis of trans-[Pt(SnCl₃) (COC₅H₁₁) (PPh₃)₂] under 1.5 bar of H₂-CO (1:1) did not yield n-hexanal quantitatively; only 12% of n-hexanal was formed. Thus, decarbonylation was the main process observed. From the reactions studied, it is possible to propose the following order of Pt-Sn bond stability: trans-[Pt(SnCl₃)(COC₅H₁₁)(PPh₃) ₂] < [Pt(SnCl₃)(C₅H₁₁)(PPh₃)₂] < trans-[PtH-(SnCl₃)(PPh₃)₂] < [PtH(SnCl₃)(CO)(PPh₃)₂] < [Pt(μ-Cl)(SnCl₃)(PPh₃)]₂ < [PtCl(SnCl₃)(PPh₃)]^- < [PtCl₂(SnCl₃)₂]^2-. The insertion reactions studied with cis-[PtCl₂(olefin)(PR₃)] as an olefin carrier and the hydrido-platinum complexes trans-[PtHCl(PPh₃)₂], trans-[PtH(SnCl₃)(PPh₃)₂], and [PtH(SnCl₃)-(CO)(PPh₃)₂] as hydrogen carriers exclude the participation of intermolecular steps by reaction of two different platinum complexes under the experimental conditions described.