Reactions of a Hydroxy Phosphonite Ligand in the Coordination Sphere of Rhodium(I)

The complexation behavior of 6-(3,3‘-di-tert-butyl-5,5‘-dimethoxy-2-hydroxy-2‘-oxybiphenyl)-6H-[c,e]-1,2-oxaphosphorine, which generates an active and n-regioselective rhodium(I) catalyst for the isomerizing hydroformylation of internal octenes, was studied. Investigations in the absence of CO/H₂ revealed that coordination of the phenolate moiety of the hydroxy phosphonite on the rhodium center is possible. Interestingly, under conditions related to the hydroformylation (syngas, higher temperature and P:Rh ratios) the ligand suffers two transformations. The first is based on a transesterification reaction involving 2 equiv of the hydroxy phosphonite, giving rise to a substituted biphenol and a symmetric bidentate phosphorus ligand of a heretofore uncertain structure. The second transformation is concerned with a selective Rh(I)-catalyzed P−C bond cleavage of the initial phosphonite structure under the formation of a phosphite. X-ray structural analyses will illustrate the structures of rhodium(I) complexes bearing the original hydroxy phosphonite ligand, a phenoxy phosphonite chelate, and a phosphite formed by selective P−C bond cleavage, respectively.