Stoichiometric model reactions in olefin hydroformylation by platinum-tin systems
作者:Montserrat Gomez、Guillermo Muller、Daniel Sainz、Joaquim Sales、Xavier Solans
DOI:10.1021/om00058a019
日期:1991.12
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) angstrom, b = 12.716 (3) angstrom, c = 18.135 (3) angstrom, beta = 96.5 (2)-degrees, Z = 8, and R (R(w)) = 0.056 (0.060) for 3235 independent reflections having I > 2.56-sigma(I). The large Pt-Sn bond (2.601 (1) angstrom) 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(mu-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.