Synthesis of Pt compounds containing chiral (2S,4S) -pentane-2,4-diyl-bis(5H-dibenzo[b]phosphindole) as ligand and their use in asymmetric hydroformylation of styrene derivatives

Unlike bis(diphenyl)phosphine derivatives in general, (2S,4S)-pentane-2,4-diyl-bis(5H-dibenzob]phosphindole), S,S-BDBPP, gives a trans oligomeric compound PtCl₂(S,S-BDBPP)]_n, 1, in reaction with dichloro-Pt precursors such as PtCl₂(PhCN)₂, PtCl₂(CH₃CN)₂ and PtCl₂(COD) at room temperature. Compound 1, which could be readily isolated, slowly rearranges in solutions at room temperature to the expected cis-monomer PtCl₂(S,S-BDBPP), 3. Heating or the presence of PtCl₂(COD) accelerates the transformation of compound 1 to 3. SnCl₂ adducts of both compounds, trans-PtCl(SnCl₃)(S,S-BDBPP)]_n, 2, and cis-PtCl(SnCl₃)(S,S-BDBPP), 4, as well as the known cis-PtCl(SnCl₃)(S,S-BDPP), 5, (S,S-BDPP = (2S,4S)-2,4-bis(diphenylphosphino)pentane) have been tested as catalysts in the asymmetric hydroformylation of p-isobutylstyrene. The phenyl analog 5 provides up to 75% e.e. but moderate yields to chiral 2-(4-isobutylphenyl)-2-propanal. Compared to this, the regioselectivity to the branched aldehyde is remarkably increased; however, the enantioselectivity is drastically decreased by the use of both dibenzophosphole derivatives 2 and 4. The similarities in the selectivities provided by 2 and 4 indicate that the trans oligomer 2 transforms to the cis-monomer 4 during the catalytic process. X-ray crystal structure determination of compound 3 shows a half-chair conformation for the chelate ring with a symmetric arrangement of dibenzophosphole groups. Besides a preference for the latter achiral conformation, the planar structure of the dibenzophosphole groups can also be considered as reason for the moderate enantioselectivities provided by 4.